Development of 8-Hydroxyquinoline Metal Based Organic Light- Emitting Diodes. Xiaodong Feng

Transcription

1 Development of 8-Hydroxyquinoline Metal Based Organic Light- Emitting Diodes By Xiaodong Feng A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Materials Science and Engineering University of Toronto Copyright by Xiaodong Feng 2008

2 Abstract Development of 8-Hydroxyquinoline Metal Based Organic Light-Emitting Diodes Xiaodong Feng Doctor of Philosophy Graduate Department of Materials Science and Engineering University of Toronto 2008 Because of its potential application for flat panel displays, solid-state lighting and 1.5 µm emitter for fiber optical communications, organic light-emitting diodes (OLEDs) have been intensively researched. One of the major problems with current OLED technology relates to inefficient electron injection at the cathode interface, which causes high driving voltage and poor device stability. Making a low resistance cathode contact for electron injection is critical to device performance. This work mainly focuses on cathode interface design and engineering. The Ohmic contact using a structure of C60/LiF/Al has been developed in electron only devices. It is found that application of the C60/LiF/Al contact to Alq based OLEDs leads to a dramatic reduction in driving voltages, a significant improvement in power efficiency, and a much slower aging process. A new cathode structure based on metal-organic-metal (MOM) tri-layer films has been developed. It is found that MOM cathodes reduce reflection by deconstructive optical interference from two metal films. The absolute reflectance from the MOM trilayer films can be reduced to as low as 7% in the visible light spectrum. In actual working devices, the reflectance can be reduced from ~80% to ~ 20%. MOM cathodes provide a potential low-cost solution for high contrast full-color OLED displays. Low voltage Erq based OLEDs at 1.5 µm emission have been developed. The Erq/Ag cathode interface has been found to be efficient for electron injection. Dramatic improvement in driving voltage and power efficiency has been realized by implementing ii

3 Bphen and C60 into Erq devices as an electron transport layer. Integration of Erq devices on Si wafers has also been demonstrated. iii

4 Acknowledgement There are many people whose support made this thesis possible. First of all, I would like to express my sincere thanks to my supervisor, Professor Zheng-Hong Lu. Without his continuous encouragement and support, I might abandon my graduate studies in the University of Toronto. I cannot thank him enough. I would like to thank my doctor, Yun Huang, who found right medication for me and always encouraged me to finish my studies. I would also like to take this opportunity to express my deep appreciation to Professor Wai Tung Ng, Professor Doug Perovic, and Professor Edward Sargent for serving in my committee and their constructive advice. I have had a wonderful time at the University of Toronto. This is largely due to all my friends the Lu Group with whom I ve been able to work: Hongyu, Antoine, Robert, Daniel, Bee Ling, Ayse, Ranjit, Edward, Marian, Sijin, Yanyan and Changjun. I would also like to thank my long-time friends: Hong Jin, Lu Gan, Jinghao Sun, Helan Xiao, and Zhong Zheng. They have stood by me through thick and thin for all the sorrows and joys I have faced, managing to tolerate my faults, and helped me to reach this point. Finally, there is my family: Mom, Father and Jiandong. Words can never express my love. iv

5 Contents List of Figures.. ix List of Tables...xiii List of Acronyms.xiv List of Symbols xvi 1 Introduction Small molecule organic light emitting-diodes (OLEDs) Basic OLED structure and materials Device operation and efficiency RGB color Device stability Dark spots Alq degradation µm emission from Erbium Erbium Erbium in optical amplifiers IR emission from erbium containing molecules Objective References.19 2 Experimental ITO patterning (Photolithographic method) ITO surface treatment...27 v

6 2.3 OLED cluster tool Organic chamber Metallization chamber Calibration Calibration of film density Calibration of tooling factor Uniformity check.33 3 Characterization techniques X-ray photoelectron spectroscopy Working principles Surface sensitivity Compositional analysis Chemical state analysis Luminance Concept of luminance Measurement of luminance Photoluminescence measurement Visible photoluminescence Infrared photoluminescence Electroluminescence measurement Visible spectrum Infrared spectrum.42 4 Development of fullerene for OLED application Introduction C60/LiF/Al interface Improved power efficiency of OLED by a layer of C60 as ETL LiF thickness dependence C60 thickness dependence Alq thickness dependence 54 vi

7 4.3.4 HTL thickness dependence Different ITO surfaces Al deposition Further improvement by LiF sandwiched between Alq and C60 layer Preliminary lifetime test Summary References.65 5 Development of OLED structure with a metal-organic-metal cathode for contrast enhancement Introduction Experiment Results and discussion Summary References.74 6 Development of Erq devices for infrared emitter Introduction Erq Erq device Electroluminescence spectra Relationship between visible and IR emission Energy transfer in Erq device Cathode interface characterization in Erq devices Erq devices with different cathodes Cathode interface characterization by XPS Enhanced electron injection and transport in Erq devices Bphen as ETL C60 as ETL Erq devices on silicon wafer Thermal oxide on p + -Si 99 vii

8 6.6.2 UV ozone oxide on p + -Si Summary References Summary and future work C60 as ETL in Alq based devices Metal-organic-metal cathode for contrast enhancement Erq device for IR emitter 109 Appendix A List of publications and patents 110 viii

9 List of Figures Figure 1.1 OLED double heterostructure..2 Figure 1.2 Structures of some molecular semiconductors that have been used in OLEDs 3 Figure 1.3 Schematic energy level diagram of OLEDs under forward bias..4 Figure 1.4 OLED working principles.5 Figure 1.5 Measured µ Vs T for three α-6t TFT s A, B, and C.9 Figure 1.6 Variation of the hole mobility of a 6T poly-crystalline film as a function of gate bias...9 Figure 1.7 Electroluminescence spectra at normal directions in ITO/TPD(50nm)/Alq/ MgAg devices 11 Figure 1.8 Schematic layer structure of a patterned planar microcavity in which the Si 3 N 4 filler layer is etched to three different thicknesses to change the optical thickness.12 Figure 1.9 Electroluminescence spectrum from a three-mode microcavity LED, in which the three peaks are at 488, 543, and 610 nm...12 Figure1.10. Schematic of the energy levels of the doped mixed emitting layer and the hole and electron transport layers..16 Figure 1.11 (a) Schematic energy level diagram of the Stark-split Er 3+ energy levels, showing excitation at µm, followed by rapid non-radiative relaxation and emission at 1.54 µm, (b) the 1.54 µm emission by the pump light of 1.48 µm and 0.98 µm respectively, (c) the process of cooperative upconversion, where interaction between two excited Er 3+ ions leads to the population of higher lying energy levels, (d) and (e) the process of excited state absorption of a 1.48 µm or a 0.98 µm pump photon respectively...18 Figure 2.1 A Kurt J. Lesker OLED cluster tool with six chambers...28 ix

10 Figure 2.2 A sample holder (left) and a mask holder (right) 29 Figure 2.3 A crucible heater with a boron nitride crucible inside.30 Figure 2.4 Schematic of sample arrangement in 4 4 inch 2 area for calibration (A, B, and C are Al foils, D is a piece of silicon wafer with a shadow mask.) 31 Figure 2.5. Uniformity check in (a) organic chamber and (b) metallization chamber No1 to 10 is from the center to the edge of deposition area..33 Figure 3.1 Schematic of a XPS system..35 Figure 3.2 Illustration of projected area.38 Figure 3.3 Schematic of a Minolta Luminance meter LS Figure 3.4 The configuration of the spectroflurometerfor visible PL 40 Figure 3.5 The configuration of the spectroflurometer for infrared PL.41 Figure 3.6 The configuration of EL in the infrared range.42 Figure 4.1 IV characteristics of Al/C60/Al devices with and without LiF interlayer (0.5 nm). The bias was applied to the bottom electrode in reference to the top electrode.46 Figure 4.2. Schematic diagram showing the energy-level alignment for a C60 device. All the values shown in the diagram are in units of ev 47 Figur e4.3. J-V and L-V characteristics of OLED using Alq and C60 as the ETL...48 Figure 4.4. Power efficiency and current efficiency as a function of luminance of C60 device and the control 49 Figure 4.5 (a) J-V and (b) L-V characteristics of C60 devices with variable LiF thickness.50 Figure 4.6 (a) J-V and (b) L-J characteristics of C60 devices with variable C60 Thickness...52 Figure 4.7 EL spectra of C60 devices with variable C60 thickness..53 Figure 4.8. C60 devices current efficiency and driving voltage as a function of Alq thickness at a constant current density of 20 ma/cm x

11 Figure 4.9 (a) J-V, (b) η i -L, and (c) η p -L characteristics of C60 devices and the control with variable NPB thickness..55 Figure 4.10 (a) J-V, (b) η i -L, and (c) η p -L characteristics of C60 devices and the control on different ITO surfaces...58 Figure 4.11 (a) J-V, (b) L-V characteristics of C60 devices and the control under different Al deposition conditions.60 Figure 4.12 (a) J-V, (b) η i -L characteristics of C60 devices with a structure of ITO/TPD (60 nm)/alq (25 nm)/lif (1 nm)/c60 (20 nm)/lif/al.62 Figure 4.13 (a) relative luminance and (b) driving voltage versus operation time of C60 device 63 Figure 5.1 Schematic of the working principle of a black layer 67 Figure 5.2 Schematic structure of an OLED device with a MOM cathode...68 Figure 5.3 (a) L-J-V, and (b) L-J characteristics of 2 2 mm 2 OLED devices with MOM cathodes. The control device has a structure of TPD (60 nm)/alq (68 nm)/ LiF (0.5 nm)/al (100 nm)..71 Figure 5.4L-J-V characteristics of 1 2 mm 2 OLED devices with different MOM cathodes. Control 1: ITO/TPD(60 nm)/alq(68 nm)/lif (0.5 nm)/al (100 nm) and control 2 : ITO/TPD(60 nm)/alq(148 nm)/lif(0.5 nm)/al (100 nm) are the referenced devices.72 Figure 5.5 Absolute reflectance spectra measured at 7 73 Figure 6.1. Molecular structure of Erq...77 Figure 6.2. XPS survey of Erq thin film 77 Figure 6.3. Optical absorption of 100 nm Erq and Alq thin solid film..78 Figure 6.4. Optical absorption of 0.05 wt% Erq in tetrahydrofuran..79 Figure 6.5. (a) Visible PL spectrum of Erq powder at the excitation wavelength of 350 nm, (b) PL spectra of 500 nm Erq film in the IR range at the excitation wavelength of 488 nm 80 Figure 6.6. EL spectrum of the Erq device with a structure of ITO/TPD(60 nm)/ Erq (40 nm)/ag, (a) in the visible range, and (b) in the IR range..81 Figure 6.7 The relationship between IR and visible emission...82 xi

12 Figure 6.8. EL and PL from an ITO/TPD(60nm)/Erq(48nm)/LiF(0.5nm)/Al OLED with an active area of 2 mm Figure 6.9. The EL from the OLED devices with Ag cathode..84 Figure 6.10 (a) Current voltage characteristics of Erq devices with the structure of ITO / TPD (60 nm)/ Erq (40 nm)/metal cathode, (b) the relationship between light output and current density...86 Figure 6.11 Schematic illustration of the peeling-off approach...88 Figure 6.12 XPS spectra on the Erq film and the buried cathode interfaces of Erq/Ag and Erq/Al. (a) C 1s, (b) O 1s, (c) N 1s, (d) ) Er 4d,(e) Al 2p, and (f) Ag 3d.90 Figure 6.13 Valance band of the Erq/Ag and Erq/Al interface..91 Figure 6.14 Molecular structure of Bphen.92 Figure 6.15 Optical absorption of 150 nm Bphen film..93 Figure 6.16 (a) IV characteristics, (b) Current efficiency vs light output, (c) power efficiency vs light output...93 Figure 6.17 Optical absorption of 100 nm C60 film.95 Figure 6.18 (a) IV characteristics, (b) Current efficiency vs light output, (c) power efficiency vs light output...96 Figure 6.19 Si 2p core level of different oxides on silicon..100 Figure 6.20 (a) IV characteristics from thermal oxide samples, (b) light output as a function of current density Figure 6.21 Si 2p core level from UV ozone oxide and native oxide. The spectra are referenced to C1s at ev..102 Figure 6.22 (a) IV characteristics from UV and native oxide samples, (b) light output as a function of current density 103 xii

13 List of Tables Table 1.1 The basic properties of OLED materials 3 Table 1.2 Device performance with fluorescent emitters.13 Table 1.3 Device performance with phosphorescent emitters..14 Table 2.1 Procedures for ITO patterning...26 Table 2.2 Density and tooling factor of each deposited film.32 Table 6.1 Calculated oxide thickness based on the peak area ratio of I oxide and I Si.100 xiii

14 List of Acronyms Alq 8-hydroxyquinoline aluminium Btp 2 Ir(acac) bis(2-(2 -benzo[4,5-a]thienyl)pyridinato-n,c) iridium (acetylacetonate) CBP 4,4 -N,N -dicarbazole-biphenyl CDC Central distribution chamber CTL Charge transport layer CuPc Copper phthalocyanine C545T 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro- 1H,5H,11H-[l]benzo-pyrano[6,7,8-ij]quinolizin-11-one DCJTB 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)- 4H-pyran DMQ Quinacridone EDFA Erbium doped fiber amplifier EL Electroluminescence EML Emissive layer Er(acac) 3 (phen) Erbium tris(acetylacetonato) (1,10-phenanthroline) Erq 8-hydroxyquinoline erbium ETL Electron transport layer ETM Electron transport material FIr6 Iridium (III) bis(4,6 -difluorophenylpyridinato)tetrakis(1- pyrazolyl)borate FWHM Full width at half maximum HOMO Highest occupied molecular orbital HTE High temperate evaporation HTL Hole transport layer xiv

15 HTM IR Ir(ppy) 3 ITO J-V L-I-V L-J-V LUMO L-V MIM MOM NPB OLEDs PL (ppy) 2 Ir(acac) RPM RS RT SCCM SCLC TAZ TCLC TPBA TPD TSC TSL UGH2 UHV UV XPS α-6t Hole transport material Infrared fac tris(2-phenylpyridine)iridium Indium tin oxide Current density-voltage Luminance-current voltage Luminance-current density -voltage Lowest unoccupied molecular orbital Luminance versus voltage Metal-inorganic-metal Metal-organic-metal N,N -di (naphthalene-1-yl)-n,n -diphenyl-benzidine Organic light-emitting diodes Photoluminescence bis(2-phenylpyridine) iridium (III) acetylacetonate Revolutions per minute Richardson- Schottky Room temperature Standard cubic centimeters per minute Space charge limited current 3-phenyl-4-(1 -naphthyl)-5-phenyl-1,2,4-triazole Trapped charge limited current 9,9,10,10 -tetraphenyl-2,2 -bianthracene. N, N -diphenyl-n, N -bis(3-methylphenyl)-1,1 biphenyl-4,4 -diamine Thermally stimulated currents Thermally stimulated luminescence p-bis(triphenylsilyly)benzene Ultrahigh vacuum Ultraviolet X-ray photoelectron spectroscopy α-sexithiophene xv

16 List of Symbols α photoelectron take-off angle, β the Si 2p core level intensity ratio of infinitive thick Si oxide film and substrate silicon. ε material permittivity ε 0 Φ γ cap η EL η Ext η Int η i η p η PL η φ ϕ B λ λ oxide vacuum permittivity work function of the spectrometer factor of the charge balance quantum efficiency of electroluminescence external coupling efficiency internal quantum EL efficiency current efficiency power efficiency quantum efficiency of photolumiescence external quantum EL efficiency Schottky energy barrier. mean free path of photoelectrons, photoelectron effective attenuation length in the silicon oxide film µ mobility θ instrumental angular efficiency factor σ photoelectric cross section Ψ a slowly varying function of electric field, A effective sample area from which photoelectrons are detected, d distance between the contacts xvi

17 d a d n d oxide D a D n e E E B E k f F hν I I 0 I EL I PL I oxide I si J e J e J h J h J r J SCLC n N 0 Ne N photon r st S T actual thickness nominal thickness from a thickness monitor thickness of silicon oxide actual density initial density electron charge electric field binding energy kinetic energy of emitted photoelectrons usual Schottky barrier lowering effect. x-ray flux, energy of X-ray emergent intensity of photoelectrons incident intensity of photoelectrons electroluminescence intensity photoluminescence intensity intensity of Si 2p core level from silicon oxide film intensity of Si 2p core level from silicon substrate electron current electron leakage current hole current hole leakage current current used for charge recombination space charge limited current refractive index of the emissive medium density of charge hopping sites, number of electrons per second input in EL measurement, number of photons per second of the pump beam in PL measurement. fraction of excitons which are formed as singlets, atomic sensitivity factor, detection efficiency for electrons emitted from the sample. xvii

18 TF 1 V y z initial tooling factor, applied voltage photoelectron process efficiency path length for photoelectrons traveling along the normal direction xviii

19 Chapter 1 Introduction Made from nature s building blocks of carbon, hydrogen, oxygen and nitrogen, organic materials with novel electronic and optical characteristics show high conductivity [1], photovoltaic effect [2], photoluminescence (PL) [3], and electroluminescence (EL) [4], which have been intensively studied in the last two decades. Compared with their inorganic counterparts, organic semiconductors offer several advantages. Unlike the extreme high vacuum and temperatures required for inorganic semiconductor fabrication, the process for making organic devices could be much simpler and cheaper. The existing powerful synthetic methods in organic chemistry can provide us with tremendous room to engineer the electronic and vibrational properties of molecules to meet our purposes. Dramatic progress has been made in organic light-emitting diodes (OLEDs) [5], organic transistors [6], organic complementary circuits [7] and organic memory [8]. Those organic based devices can satisfy the pressing need for very low cost displays and circuits used in the consumer market. The successful use of OLED displays, recently seen in the cellular phone and car audio markets, has made believable the possibility that organic optoelectronics will play an important role in our daily life in the near future. 1.1 Small molecule organic light emitting-diodes (OLEDs) Electroluminescence is a phenomenon, converting electrical energy into light that has been seen in a wide range of inorganic semiconductors, and for organic semiconductors was first reported for anthracene single crystals in the 1960s [4]. The 1

20 Chapter 1 Introduction early attempts to make organic electroluminescent devices suffered from several problems such as high turn-on voltage, poor stability, and low efficiency [9-13]. Significant progress has been achieved since Tang and Van Slyke demonstrated the first efficient small molecule organic light-emitting diodes consisting of an aromatic diamine as a hole transport material (HTM) and 8-hydroxyquinoline aluminium (Alq) as both emitting material and electron transport material (ETM) [14]. Improvements in materials and device architecture have resulted in devices with luminance up to 140,000 cd/m 2 [15] and external quantum efficiencies in excess of 18% [16]. Low operating voltages around 3-5 V at a luminance of more than 1000 cd/m 2 [17] and operating lifetimes exceeding 35,000 h [18] have been reported. A variety of luminescent organic materials have been synthesized, providing colors spanning the entire visible spectrum [19 21]. Furthermore, white-light OLEDs have been fabricated using florescent dye doped multilayers and microcavity structures [22-24] Basic OLED structure and materials Cathode ETL/EML HTL Anode (ITO) Glass substrate Figure 1.1 OLED double heterostructure. The total organic thin film thickness is typically ~1000 Å. hν OLEDs are essentially several thin organic film semiconductors sandwiched between two electrodes. A schematic cross-section of an OLED with two organic layers is shown in Figure 1.1. The cathode, top electrode, consists of a low work function metal, typically Al, or Mg: Ag deposited by thermal evaporation. The anode, bottom electrode, is a thin film of the transparent semiconductor indium tin oxide (ITO) deposited onto the 2

21 Chapter 1 Introduction glass substrate by sputtering. The most commonly used organic materials in OLEDs are: TPD or NPB, as a hole transport layer (HTL), and Alq as an emissive layer (EML) and electron transport layer (ETL). Their molecular structures are shown in Figure 1.2. Light is emitted through the ITO if the diode is operated under a sufficient forward bias. Alq TPD NPB Figure 1.2 Structures of some molecular semiconductors that have been used in OLEDs. Alq is used as an electron transport and emissive layer, TPD or NPB is used as a hole transport layer. Table 1.1 The basic properties of OLED materials. Material Glass transition temperature ( C) Optical gap (LUMO- HOMO) (ev) Ionization potential (ev) Electron mobility (cm 2 v -1 s -1 ) Hole mobility (cm 2 v -1 s -1 ) Alq 172 [25] 2.8 [28] 5.7 [28] [31] [33] NPB 95 [26] 3.3 [29] 5.2 [28] NA (8.8 ±2) 10-4 [34] TPD 63 [27] 3.1 [30] 5.4 [30] NA >10-4 [35] Device functionality is based on the properties of materials. The understanding of materials is of great importance to the further improvement of OLEDs. Considerable research efforts have been aimed at studying the basic properties of those molecules [25-35], especially Alq. Some well-documented data are summarized in Table 1.1. Basically, there are two major concerns. Low glass transition temperature of HTMs such as TPD and NPB is an issue to device reliability. The crystallization of HTM, which can occur at 3

22 Chapter 1 Introduction room temperature and can be accelerated by the Joule heat generated during device operation, is one of the important factors resulting in OLED degradation. [36]. The other concern is that the electron mobility of Alq is ~ 2 orders of magnitude lower than the hole mobility of HTM. That can lead to unbalanced electron and hole current. Heidenhain et al synthesized a new n type molecule, perfluorinated phenylene dendrimers, which is a better ETM than Alq due to its higher electron mobility [37] Device operation and efficiency LUMO High work function anode E F + HTL hν + _ + _ ETL/EML Low work function cathode _ HOMO E F Figure 1.3 Schematic energy level diagram of OLEDs under forward bias. Figure 1.3 illustrates the energy level diagram between interfaces in OLEDs under a bias, corresponding to the OLED configuration as shown in Figure 1.1. Under a forward bias, holes from the anode are injected into the highest occupied molecular orbital (HOMO) of the HTL, and in the meantime, electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the ETL. The recombination of holes and electrons results in photon emission. The radiative recombination occurs at the organic/organic interface, which generally is within the ETL. The organic/electrode and organic/organic interfaces play a very important role in the performance of OLEDs. Therefore, the understanding of the mechanisms controlling the energy barriers at those interfaces by alignment of the energy levels is critical for a good device design. 4

23 Chapter 1 Introduction h + injection e injection h + transport e transport J e γ cap J h Exciton J e J h r st Singlet exciton Triplet exciton (Phosphorescent dye) η PL Internal emission Non-radiative decay η Int = η PL r st γ cap η ext External emission η φ = η ext η Int Internal absorption& Dissipation Figure 1.4 OLED working principles. J h, J e stand for leakage current in ETL and HTL respectively [38]. The external quantum EL efficiency η φ is defined by the ratio of the number of photons released from the device to the number of charges injected. η φ can be approximately expressed by η φ = η ext η Int, where η ext = 1/2n 2 if based on classical ray optics (n is the refractive index of the emissive medium), internal quantum efficiency η Int is defined by the number of photons produced within a device divided by the number of charge injected. γ cap is the factor of the charge balance, defined by γ = J r /J, where J r is the current used for charge recombination. The meaning of J r can be explained by Figure 1.4 based on the mass balance and charge neutrality of hole and electrons. So two equations can be derived as follows: J=J h +J e =J e +J h, and J r =J h -J h =J e -J e. If all the holes and electrons are consumed for recombination within an emissive layer, γ cap will be 1.0. If J h >>J e or J e >>J h, γ can be much less than 1.0. Internal quantum EL efficiency can be given by η Int = γ cap r st η PL, where r st is the fraction of excitons which are formed as singlets, and η PL is the efficiency of radiative decay of these singlet excitons. 5

24 Chapter 1 Introduction (1) Electron and hole injection Carrier injection is determined by interfacial electronic properties. The understanding of the formation of energy barriers at cathode and anode interfaces is very challenging. The energy barrier at the interfaces is not exactly the difference between the Fermi level of cathode (anode) and the LUMO (HOMO) of the organic layer since there is an ultra-thin dipolar layer at the interface [39]. However, low work function metals such as Mg, Li and Ca can enhance device performance. Electron injection can play a dominant role in the quantum efficiency of OLEDs if there is a large barrier at the ETL/cathode interface. Carrier injection into a semiconductor can be treated in terms of either Fowler- Nordheim tunneling or Richardson- Schottky (RS) thermionic emission [40]. Both concepts are appropriate in inorganic semiconductors with extended band states and large mean free paths, which is not the case in organic semiconductors. However, Monte Carlo simulation has shown this injection mechanism resembles RS thermionic emission even though quantitative differences exist [41]. The mobility dependence of the thermionic injection rate was first predicted by Emtage and O Dwyer [42] and extended by Scott and Malliaras [43], introducing the field-dependent factor. The rate of injection at a contact limited electrode is proportional to the charge mobility in the organic material [44]. Specifically, the net injected current into the film is the difference between the injected flux and a surface recombination rate. The injection current can be expressed as the following: j INJ = 4Ψ 2 N eµ E exp( ϕ / kt )exp( f 0 where Ψ is a slowly varying function of electric field, N 0 is the density of charge hopping sites, and ϕ B is the Schottky energy barrier. The exponential in the square root of the electric field, f =e 3 E / [4πεε 0 (KT) 2 ], represents the usual Schottky barrier lowering effect. Recent intensive studies on cathode interface engineering show that electron injection can be enhanced by introducing less than 2 nm ionic compound films such as LiF [45], CaF 2 [46], Li 2 O, Cs 2 O, NaCl, KCl [47], or by doping the Al cathode in the near interface region with LiF or CsF [48]. The change in image force, determined by the f B 1/ 2 ) 6

25 Chapter 1 Introduction item in the above formula and caused by inserting the above mentioned high dielectric constant buffer layers, seems to have been overlooked in much of the recent literature. The nature of charge injection is determined by interfacial chemistry that is more complicated than previously assumed [49-51]. Also, it is difficult to quantitatively explain the device electrical characteristics on the basis of injection barrier. The understanding of carrier injection also needs a proper knowledge of transport. (2) Carrier transport in amorphous organic semiconductors OLED materials are conjugated (the bonds between the carbon atoms are alternately single and double). The π electrons are completely delocalized along the conjugated carbons. Hence, their charge transport properties can be affected by the π - π * molecular orbital overlap. However, the overlap of molecular orbitals for intermolecular electron exchange is much smaller than their inorganic counterparts due to weak van der Walls bonding between molecules. Physically, carrier transport in amorphous organic materials depends on the electron-phonon and electron-exchange interactions [52]. It is very challenging to disentangle their intrinsic transport properties directly from the electrical characteristics because of a complicated interfacial situation and unknown trap states. In OLEDs, carrier transport strongly depends on the electric field distribution. There is no universal transport mechanism in a wide temperature range. Phonon assisted thermionic emission might be a dominant mechanism at room temperature (RT). At low temperatures, tunnelling can be dominant for carrier transport between molecules. Carrier transport in OLEDs has been described by space charge limited current (SCLC) theories at RT if charge injection is not a limiting factor. SCLC obeys the Mott-Gurney equation [53]: j SCLC 9 V = µεε 0 8 d 2 3 where µ and ε are mobility and dielectric constant of the material, ε 0 is the permittivity of vacuum, d is the distance between the contacts, and V is the applied voltage. 7

26 Chapter 1 Introduction The space charges can be either free carriers or trapped charges. The early observation of the temperature-dependent power law relation of current-voltage suggested a trapped charge limited current (TCLC) model [54], which employed band models with a distribution of trapping levels below the conduction band. It was developed for bandlike transport rather than hopping transport. Furthermore, a constant charge carrier mobility is required, which is contrary to the field-dependence of mobilities [33]. The nature of trapping needs to be clarified for proper device modeling. Thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) have been used to investigate the trap properties of Alq [55, 56]. The results derived from those spectra are not in good agreement. A trap depth from 0.05 to 0.7 ev and a distribution of trap states from 0.13 to 0.25 ev are suggested by TSC and TSL spectra respectively. TSC spectra vary strongly for Alq of different suppliers [56]. Trap states are more like an extrinsic factor. Very recently, non-dispersive electron transport in well-purified Alq film at nitrogen atmosphere also indicates the trap states might be attributed to impurities and oxygen [57]. Trapping may be seen at very low fields since the free carrier density is lower and may become comparable to the trap concentration. However, at a typical driving voltage of OLEDs, trap-free transport is more like an intrinsic behavior. The mobility of charge carriers, in amorphous organic materials, is strongly electric field and temperature dependent. Electron transport in amorphous organic films takes place by hopping in the LUMO of each molecule. The energy of LUMO can be assumed to have a Gaussian distribution, with a width of the order of 0.1 ev [58]. A packet of carriers propagating in such a system can quickly reach thermal quasiequilibrium at room temperature [58]. Very limited information about the temperature dependence of OLED material mobility has been provided in the temperature range from 10 to 70k. In field effect transistors, the mobility in α-sexithiophene (α-6t) displays a nonmonotonic temperature dependence [59]. Above 50 K, the transport is thermally activated, whereas below 40K, the field-effect mobility is approximately temperature independent, as shown in Figure

27 Chapter 1 Introduction Figure 1.5 Measured µ Vs T for three α-6t TFT s A, B, and C [59]. Figure 1.6 Variation of the hole mobility of a 6T poly-crystalline film as a function of gate bias. Data were recorded at 300 K. Closed circles correspond to uncorrected data, and open circles to data corrected for the contact series resistance [60]. The hole mobility is also found to increase quasilinearly with gate voltage at room temperature, and that dependence becomes superlinear at low temperatures [60], as shown in Figure 1.6. In silicon field effect transistors, more scattering events can lower 9

28 Chapter 1 Introduction the mobility a bit when the carrier density is increased by raising the gate voltage. This abnormal behaviour might indicate some transport coherence in a high carrier density if grain boundaries and defects in thin films are negligible. The picture of charge transport is not very clear. However, a lot of effective experimental work aimed at better electron transport has been done. At driving voltages for a luminance of over 1, 000 cd/m 2, usually there is a substantial hole leakage current in ETL, resulting in low current efficiency. N-type doping such as Li [17, 61], LiF [62] can improve the conductivity of ETL and keep a better balance of hole and electron current. The introduction of a hole block layer, which is still a good ETM, can also benefit the device performance at high driving voltages [17]. (3) Pairing of electrons and holes to form excitons Spin ½ electron and spin ½ hole can form 4 spin combinations, 3 triplets and 1 singlet. If singlet and triplet capture cross-sections are equal, it implies only 25% efficiency for generation of singlet excitons. This factor is assumed to be insensitive to both devices and materials. Since the ground state of fluorescent molecules is typically singlet, the transition from triplet to singlet is usually forbidden by spin conservation. Thus the maximum internal quantum efficiency is expected to be 25% for fluorescent OLEDs. Recent studies showed that this is not the case in conjugated polymer [63-67]. Singlet to triplet ratio depends on the conjugated length [67]. Recombination is spinindependent for the monomer, but that spin-dependent process favoring singlet formation is effective in the polymer as a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius [65]. Making use of the triplet excitons can greatly improve quantum efficiency. One approach is to introduce species that will allow efficient triplet luminescence (phosphorescence). This can be provided by high-atomic-number elements with strong spin-orbit coupling. A platinum-containing porphyrin (PtOEP) has been successfully used as a dopant in molecular OLEDs [16,68]. The internal quantum efficiency can be as high as 87%[16]. 10

29 Chapter 1 Introduction Under typical working conditions, a potential of order 5V might be applied across a 100 nm thick device. This corresponds to an average applied field of V/m. The local electric field is strongly affected by the presence of space charge and the precise details of the device structure. It may be significantly smaller than this value over the recombination zone. In OLEDs, hopping is the dominant carrier transport mechanism so that the mean free path of charge carriers is roughly the molecular separation. This is about one order of magnitude shorter than the range of the Coulomb interaction. Thus at low fields, recombination can be treated using the carrier motion-controlled Langevin approximation [69, 70]. Nevertheless, at a high-applied field, the size of the carrier capture surface will be greatly reduced. The simple Langevin model is no longer valid. This behaviour can be explained by the Thomson-like recombination controlled by the carrier capture [71]. (4) Photons released out of OLED The coupling of electronic excitations to photon states is affected by the physical structure of OLEDs. The presence of the metallic cathode provides a mirror that modifies the pattern of the electromagnetic modes near the cathode. The variation of Alq film thickness in a two-layer structure will change the distance between the emissive zone and the cathode, therefore emission spectra, as shown in Figure 1.7, and spectral emission patterns will change accordingly. The emission Figure 1.7 Electroluminescence spectra at normal directions in ITO/TPD(50nm)/Alq/MgAg devices [72]. 11

30 Chapter 1 Introduction pattern could deviate from the ideal Lambertian pattern [72]. This dependence of emission pattern on Alq thickness cannot be explained by classical optics. Due to the refractive index mismatch between the emitting layer and air, a large fraction of the light is trapped in the glass, ITO, and organic layers. The external coupling efficiency is estimated by classical theory to be 1/2n 2 for large n [73]. The external coupling efficiency in planar organic light-emitting devices can be modeled based on a quantum mechanical microcavity theory [74] and measured by examining both the farfield emission pattern and the edge emission of light trapped in the glass substrate. The external coupling efficiency is dependent upon the thickness of the ITO layer and the refractive index of the substrate. The coupling efficiency ranges from 24% to 52%, but in general it is much larger than the 18.9% expected from classical ray optics [75]. It is clear that the results obtained from ray optics can overestimate the internal quantum efficiency. Figure 1.8 Schematic layer structure of a patterned planar microcavity in which the Si 3 N 4 filler layer is etched to three different thicknesses to change the optical thickness [77]. Figure 1.9 Electroluminescence spectrum from a three-mode microcavity LED, in which the three peaks are at 488, 543, and 610 nm [77]. After adding a multilayer dielectric stack on the other side of the ITO from the organic layer, this stack, along with the metal cathode, forms a resonant cavity with a narrow bandpass that can be selected by varying the optical thickness of the cavity [76, 77]. With an emitting layer that has a white spectrum, the three primary colors can be 12

31 Chapter 1 Introduction obtained by patterning different dielectric thicknesses as indicated in Figures 1.8 and 1.9. This technology is promising for RGB colors even though close control of the etching process is required RGB color Three primary colors, red, green and blue, are needed for display applications. The main advantage of using organic materials as emitters is the availability of highly luminescent molecules throughout the entire visible region, making possible the fabrication of full-color displays. Many color dopants have been developed since Tang et al introduced guest-host doped emitter system [78]. Basically there are two types of dopants: fluorescent dopants harvesting singlet excitons, and phosphorescent emitters harvesting triplet excitons. Some important results are summarized in the following tables. Table 1.2 Device performance with fluorescent emitters. Color Dopant Host Current efficiency (cd/a) CIE coordinate Red RD-2* RH-1* 11.4 cd/a[79] (0.67, 0.33) DCJTB Alq 2.58 cd/a [80] (0.609, 0.383) Green C545T TPBA 29.8 cd/a [81] (0.24, 0.62) Blue BD-3* NBH* 7.2 cd/a[79] (0.14, 0.16) DCJTB is 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran C545T is 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H- [l]benzo-pyrano[6,7,8-ij]quinolizin-11-one TPBA is 9,9,10,10 -tetraphenyl-2,2 -bianthracene. *The chemical structures of RD-2, RH-1, BD-3 and NBH are not disclosed. 13

32 Chapter 1 Introduction Table 1.3 Device performance with phosphorescent emitters. Color Dopant Host Power efficiency CIE coordinate (lm/w) Red Btp 2 Ir(acac) CBP 4.6±0.5 lm/w [82] (0.68, 0.32) Green (ppy) 2 Ir(acac) TAZ 60±5 lm/w [16] NA Ir(ppy) 3 TAZ 40±2 lm/w [83] NA Blue FIr6 UGH2 13.9±1.4 lm/w [84] (0.16, 0.26) Btp 2 Ir(acac) is bis(2-(2 -benzo[4,5-a]thienyl)pyridinato-n,c) iridium (acetylacetonate) CBP is 4,4 -N,N -dicarbazole-biphenyl (ppy) 2 Ir(acac) is bis(2-phenylpyridine) iridium (III) acetylacetonate TAZ is 3-phenyl-4-(1 -naphthyl)-5-phenyl-1,2,4-triazole Ir(ppy) 3 is fac tris(2-phenylpyridine)iridium FIr6 is Iridium (III) bis(4,6 -difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate UGH2 is p-bis(triphenylsilyly)benzene Device stability The reliability of OLEDs has been improved significantly during the last decade as a deeper understanding has been achieved on the fundamental processes of charge injection, transport, and recombination. In general, there are two independent mechanisms in OLED degradation. The first, associated primarily with degradation at the device electrode interfaces, occurs through the formation of non-emissive regions, usually referred to as dark spots, which lead to a rapid decrease in device luminance [85], [86]. The second one is reflected in a long-term intrinsic decrease in the quantum efficiency of the emissive material in a device [87] Dark spots Dark spots have been identified as electrical broken-circuits that can reduce the active area, and hence the luminous output of the device. The growth of dark spots is 14

33 Chapter 1 Introduction known to occur even under storage conditions (the devices are not being operated), but is accelerated by device operation. From earlier studies, a number of phenomena that lead to the growth of dark spots have been identified. It could be caused by the delamination at the cathode/alq interface [86,88] the crystallization of the HTMs [89], and crystallization of the Alq layers [90]. The development of dark spots is also attributed to cathode oxidation [91] or to electrochemical reactions at the organic/electrode interfaces [92]. It has been confirmed that the growth of dark spots occurs primarily due to cathode delamination. The growth of dark spots is also associated with changes in the organic layers, especially at the organic/cathode interface. Recent results suggest that the nucleation of dark spots takes place at the organic/cathode interface and originates during the deposition of the cathode [93]. The anode/organic interface might not play a critical role in the formation of the dark spots. The presence of water and/or oxygen is believed to be hazardous for the performance of OLEDs. Water and/or oxygen-activated photo-degradation have been found in organic materials [94,95]. Humidity encourages the growth of crystalline Alq, delamination of the cathode and hence the formation of dark spots [93,96]. Processing in a standard clean room (e. g. Class 1000) and proper encapsulations could avoid the presence of dark spots. The insertion of an ultra-thin dielectric layer such as LiF, can also benefit device lifetime since it can improve interfacial adhesion between Alq and Al cathode [43] Alq degradation Recently, a more balanced electron and hole at the emissive zone was reported to greatly improve device lifetime [97]. After long-term device operation, the cationic state of Alq is not stable, which eventually causes Alq molecules to become fluorescent quenchers. As we know, the electrons are minority carrier in small molecule OLEDs. Therefore, the buffer interlayer of CuPc between anode and NPB can impede too many holes from injecting into the Alq, resulting in a smaller number of Alq molecules in cationic states. The device lifetime can thus be greatly increased. The mixing of NPB, 15

34 Chapter 1 Introduction Alq and DMQ at the emissive zone can also avoid the cationic state of Alq during the formation of exciton, as shown in Figure It is a promising way to make high stable OLEDs. Figure1.10. Schematic of the energy levels of the doped mixed emitting layer and the hole and electron transport layers. Direct recombination of NPB + and Alq -, without creation of the Alq + intermediate, leads to formation of Alq * excited state [97] µm emission from Erbium Erbium Er 3+ Erbium is a rare earth element belonging to the group of the Lanthanides. The ion has an incompletely filled 4f-shell, allowing for different electronic configurations with different energies due to spin-spin and spin-orbit interactions [98]. Radiative transitions between most of these energy levels are parity forbidden for free Er 3+ ions. Since the 4f shell is well shielded from the host matrix by the outer 5s and 5p orbitals, the energy of this transition is relatively independent of the host material and ambient temperature [99]. However, the surrounding material might perturb the 4f wave functions. This has two important consequences. Firstly, the host material can introduce odd-parity character in the Er 4f wave functions, making radiative transitions weakly 16

35 Chapter 1 Introduction allowed. Secondly, the host material causes Stark-splitting of the different energy levels, which results in a broadening of the optical transitions. Figure 5 (a) shows a schematic level diagram of the Stark-split Er 3+ energy levels. When Er is excited in one of its higher lying levels it rapidly relaxes to lower energy levels via multi-phonon emission. This results in typical excited state lifetimes ranging from 1 ns to 100 ms. The transition from the first excited state ( 4 I 13/2 ) to the ground state ( 4 I 15/2 ) is an exception to this rule. Due to the large transition energy (0.8 ev) multi-phonon emission is unlikely, resulting in relatively efficient emission Erbium in optical amplifiers Trivalent erbium ions have long played an important role in optical communication technology [100,101], due to their intra-4 f emission from the first excited state 4 I 13/2 to the ground state 4 I 15/2 at 1.54 µm. Since radiative transitions in Er 3+ at 1.54µm are only weakly allowed, the cross sections for optical excitation and stimulated emission are quite small, typically on the order of cm 2, and the radiative lifetimes of the excited states are long, up to several milliseconds [102]. Consequently, it needs a long structure to excite (such as an Er doped fiber) to get efficient excitation by optical pumping. In an erbium doped fiber amplifier (EDFA), erbium is incorporated into the core of a silica fiber. Erbium can be pumped directly into the first excited manifold using a 1.48 µm diode laser, or via one of the higher levels, for example using a 0.98 µm diode laser, as indicated in Figure 1.11b. In the latter case, the Er relaxes rapidly into the first excited state. When sufficient pump power is applied, this leads to population inversion between the first excited state and the ground state. A 1.54 µm signal traveling through the EDFA will then induce stimulated emission from the first excited state to the ground state, resulting in signal amplification. At high Er concentrations, interaction between Er ions is an important gainlimiting factor. One such process is cooperative upconversion [ ]. In this process, an excited Er ion de-excites by transferring its energy to a neighboring excited ion, promoting it into the 4 I 9/2 level, as depicted in Figure This lowers the amount of excited Er, or conversely, increases the pump power needed to obtain a certain degree of 17

36 Chapter 1 Introduction inversion. Cooperative upconversion is possible due to the presence of a resonant level at twice the energy of the first excited state. In practice, cooperative upconversion is an important gain-limiting factor for Er concentrations above Er/cm 3.The effect 0.53 µm 0.55 µm 2 H 11/2 4 S 3/ µm 4 F 9/ µm 0.98 µm 1.54 µm (d) (e) 4 I 9/2 4 I 11/2 4 I 13/2 (a) (b) (c) 4 I 15/2 Figure 1.11 (a) Schematic energy level diagram of the Stark-split Er 3+ energy levels, showing excitation at µm, followed by rapid non-radiative relaxation and emission at 1.54 µm, (b) the 1.54 µm emission by the pump light of 1.48 µm and 0.98 µm respectively, (c) the process of cooperative upconversion, where interaction between two excited Er 3+ ions leads to the population of higher lying energy levels, (d) and (e) the process of excited state absorption of a 1.48 µm or a 0.98 µm pump photon respectively. of cooperative upconversion not only depends on the average Er concentration, but also on the microscopic distribution of the Er ions in the host material. In order to fabricate an efficient amplifier, the fabrication method should be optimized to obtain a homogeneous distribution of Er ions. 18

37 Chapter 1 Introduction IR emission from erbium containing molecules Recently amorphous OLEDs [ ] have provided one possibility to make infrared emitters that can be easily integrated into silicon wafers. Room-temperature electroluminescence of erbium tris(8-hydroxyquinoline) [Erq] or erbium tris(acetylacetonato) (1,10-phenanthroline) [Er(acac) 3 (phen)] as the emitting layer was observed at 1.54 µm. This suggests a possible route to producing a silicon-compatible 1.54 µm source technology. The studies on IR emission from organic materials are still at a very early stage. Current studies reported high operating voltage (> 25V), and the physical picture of IR emission from erbium related compounds is not clear. A systematic investigation of IR OLEDs is of great importance for potentially extending the current OLED technology to the area of optical communications. 1.3 Objectives There are three main objectives in this thesis. The first objective is to improve device performance such as the driving voltage, power efficiency and lifetime in Alq based OLEDs by using fullerene as an ETL. The second objective is to enhance the contrast of Alq based OLEDs by using a metal-organic-metal cathode. The third objective is to develop a device fabrication technology for Erq based OLEDs that operate at low voltages, achieve a fundamental understanding of energy transfer processes in IR emission, and realize efficient IR emission. 1.4 References 1. C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and A. G. MacDiarmid, Phys. Rev. Lett. 39, 1098 (1977). 2. G. A. Chamberlain, Solar Cell 8, 47 (1983). 3. M. Furst and H. Kallman, Phys. Rev. 85, 816 (1952). 4. M. Pope, H. Kallmann, and P. Magnante, J. Chem. Phys. 38, 2042 (1962). 19

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43 Chapter 1 Introduction 100. P. J. Mears, L. Reekie, I. M. Jauncey, and D. N. Payne, Electron. Lett. 23,1026 (1987) E. Desurvire, R. J. Simpson, and P. C. Becker, Opt. Lett. 12, 888 (1987) A. Polman, J. Appl. Phys. 82, 1 (1997) W. J. Miniscalco, J. Lightwave Technol. 9, 234 (1991) G. N. van den Hoven, E. Snoeks, A. Polman, C. van Dam, J. W. M. van Uffelen, and M. K. Smit, J. Appl. Phys. 79, 1258 (1996) E. Snoeks, G. N. van den Hoven, A. Polman, B. Hendriksen, M. B. J. Diemeer, and F. Priolo, J. Opt. Soc. Am. B 23, 1468 (1995) W. P. Gillin and R. J. Curry, Appl. Phys. Lett. 74, 798 (1999) R. G. Sun, Y. Z. Wang, Q. B. Zheng, H. J. Zhang, and A. J. Epstein J. Appl. Phys. 87, 7589 (2000) H. Suzuki, Appl. Phys. Lett. 80, 3256 (2002) N. Tessler, V. Medvedev, M. Kazes, S. H. Kan, and U. Banin, Science 295, 1506 (2002). 25

44 Chapter 2 Experimental The fabrication of OLED devices involves several steps including ITO patterning, ITO surface cleaning and treatment, thermal evaporation of organic and metal films and mask changing. In this chapter, device processing will first be described, and then calibration of organic cluster tool will be discussed. Table 2.1 Procedures for ITO patterning. Cleaning Pre-baking Spin coating Softbaking Mask alignment Exposure Developer Hardbaking 80 o C Primer 30 Photoresist o C UV illumination for 45 s 30 s 5 mins@50 o C 15 mins@120 o C Etching 20 mins in the solution of HCl: H 2 O: HNO 3 =10:10:1 Cleaning Acetone and De-ionized water 26

45 Chapter 2 Experimental 2.1 ITO patterning (Photolithographic method) Photolithography is used to pattern the ITO for defining the exact dimensions of devices. It is the process of transferring a pattern on a mask to the surface of a substrate. There are a lot of steps involving in the photolithographic process for ITO patterning, There are two types of photoresist: positive and negative. Only positive photoresist was used for ITO patterning. In positive resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. The mask, therefore, contains an exact copy of the pattern, which is to remain on the substrate. The detailed procedures are described in Table ITO surface treatment ITO coated glass substrates were ultrasonically cleaned in acetone, methanol and de-ionized water for 5 mins, respectively. After they were baked for 15 mins under 150 C, ITO substrates were treated by UV ozone for another 15 mins. The UV ozone treatment was carried out in a UV ozone cleaning system with a low-pressure quartz mercury vapor lamp that generates 185 nm and 254 nm UV light. Atmospheric oxygen absorbs the 185-nm UV light, dissociates to atomic oxygen, and forms ozone. The organic contaminants are first excited by the 254 nm UV light. Then they easily react with ozone to form volatile compounds such as CO 2 and H 2 O vapor. This process takes place at room temperature in several minutes. 2.3 OLED cluster tool A Kurt J. Lesker OLED cluster tool is capable of plasma treatment, sputtering, and evaporation of organic and metallic layers. It can process 2 or 4 inch square substrates with good layer uniformity, repeatability, and reliability. As shown in Figure 27

46 Chapter 2 Experimental 2.1, five chambers are arranged around a central distribution chamber (CDC). After ITO surface treatment, the patterned ITO coated glass substrate mounted in the sample holder together with the mask holder, as seen in Figure 2.2, is loaded into the entry/lock chamber to pump down. Subsequently the sample is transferred to the organic chamber via CDC. After deposition, the sample can be transferred to the entry/lock chamber to receive another mask, then to the metallization chamber, or to any other chamber until it has received all specified layers. At the completion of the process of device fabrication the sample can be loaded out from the entry/lock chamber. Generally, evaporation of thin films for OLEDs only involves in the organic chamber and metallization chamber. Plasma chamber Sputtering chamber Organic chamber Entry/lock chamber Central distribution chamber Metallization chamber Figure 2.1 A Kurt J. Lesker OLED cluster tool with six chambers. 28

47 Chapter 2 Experimental Figure 2.2 A sample holder (left) and a mask holder (right). The sample holder can be put on the top of the mask holder Organic chamber The organic chamber is the heart of the OLED cluster system. The chamber is pumped by a 1500 L/sec CTI cryo-pump. The base pressure of the chamber is better than torr. There are six organic sources, which are controlled by two separate power supplies and have pneumatically actuated shutters. The organic sources are designed for low temperature evaporation, up to 500ºC, and have an excellent temperature controller, Eurotherm, which temperature resolution is 0.1 C. Heating is realized by an electrical current passing through a resistance coil surrounding high pure Al 2 O 3 crucible where the organic material is deposited. At high vacuum, the mean free path of vapor molecules is the same order as the vacuum chamber dimensions, so these molecules travel in straight lines from the evaporation source towards the substrate. Deposition rates are monitored by one quartz crystal sensor in the same height as the substrate holder, ~30 cm above the organic sources. To improve the uniformity of the film, the substrate holder can rotate around its center point at a controllable speed. Except six organic sources, there are two high temperature evaporation (HTE) sources in the bottom of the organic chamber for inorganic materials such as LiF. 29

48 Chapter 2 Experimental Metallization chamber Metallization is a crucial process for the performance of OLEDs. Similar to the organic chamber, the base pressure of the metallization chamber is ~10-8 torr. There are only two HTE sources in the bottom of the chamber. Heating is realized by an electrical current Figure 2.3 A crucible heater with a boron passing through a crucible heater nitride crucible inside. made of molybdenum, as seen in Figure 2.3, where the boron nitride crucible is put inside. For Al evaporation, the amount of Al is limited to one gram to avoid overflow. The distance between the HTE source and the sample holder is quite long about 70 cm. Such long distance reduces the thermal radiation that can potentially damage organic molecules, and also it can guarantee good uniformity of evaporated metal films. The HTE sources are controlled by Eurotherm in current mode. To evaporate films, the current should be raised in steps (~40A per step) until the desired current is reached. 3 5 mins is needed to wait between steps. When the desired deposition rate is reached, open the substrate shutter. After the deposition, the current should also be reduced in steps (~40 A per step) Calibration Proper information on film thickness is critical for device fabrication because the performance of OLEDs is sensitive to the thickness of each deposited layer except metallic cathode. On a thickness monitor, three parameters: density, tooling factor and Z- ratio are relevant to thickness reading. Basically Z-ratio depends on the density and shear modulus of deposited film. It can get values from the manual since the Z-ratio values of materials in thin film form are very close to the bulk values. For organic materials, the Z- 30

49 Chapter 2 Experimental ration is generally set as 1.0. Therefore only density and tooling factor are needed for calibration Calibration of film density Determination of density is quite simple. It needs information on the mass and volume of deposited films. Three pieces of flat Al foil A, B, C and a piece of silicon wafer with a shadow mask are fixed on a sample holder using copper tape as shown in Figure2.4. The shadow mask on silicon wafer is used to create steps after film deposition. Information on actual thickness can be acquired by measuring the height (H) of each step on silicon wafer by a stylus profilometer. The film mass can be determined by the mass difference on Al foil between before and after film deposition. Since the area of Al foils (S) is known, the density of each film is simply calculated by the mass divided by the volume of the film, which is S H. The density of each deposited film is summarized in Table 2.2. A B D C Figure 2.4 Schematic of sample arrangement in 4 4 inch 2 area for calibration (A, B, and C are Al foils, D is a piece of silicon wafer with a shadow mask.) 31

50 Chapter 2 Experimental Calibration of tooling factor Table 2.2 Density and tooling factor of each deposited film. Sourc e No. CH1 CH2 CH3 CH4 CH5 CH6 HT2 Materia l TPD CuPc Erq NPB Alq C60 Al Sampl e No. Mass (µg) Area (cm 2 ) A B C A B C A B C A B C A B C A B C A B C Nominal thicknes s (nm) Measured thickness (nm) Average density (g/cm 3 ) Tooling factor (%) Because there is a lateral distance between the crystal sensor used for in-situ monitoring of the deposited films and the substrate, it is necessary to determine the ratio of respective amounts of deposit between these two surfaces. This ratio is known as tooling factor. Once the actual density is known, the tooling factor can be calculated by the formula: Tooling (%) = TF 1 (d a /d n ) (D a /D n ), where TF 1 is initial tooling factor, d a and d n are actual thickness and nominal thickness from a thickness monitor, respectively, D a and D n are actual density and initial density, respectively. For organic materials, the initial tooling factor and initial density are set as 100 and 1g/cm 3, respectively. The tooling factor for each source position is also summarized in Table 2.2. The tooling factor 32

51 Chapter 2 Experimental is only relevant to source position, and it has nothing to with the type of deposited materials Stylus from Si to TPD Stylus from TPD to Si Stylus from Si to Al Stylus from Al to Si Thickness (nm) Thickness (nm) Position No Position No. Figure 2.5. Uniformity check in (a) organic chamber and (b) metallization chamber No1 to 10 is from the center to the edge of deposition area Uniformity It is necessary to check the film uniformity across the area of 4 4 inch 2 in the cluster tool. A stylus profilometer was used to scan across the deposited film on silicon wafer where has 10 steps. No 1 step on the silicon wafer is located in the center of the deposition area, and No 10 step is close to the edge of the deposition area. The length between each step is ~5 mm. As shown in Figure 2.5a, the thickness variation of the TPD film is within 7% across 4 4 inch 2 for organic deposition. For metallization, as shown in Figure 2.5b, the thickness variation of the Al film across 4 4 inch 2 area is within 5%. Since only 2 2 inch 2 ITO coated substrates were used in this thesis work, the variation of film thickness should be significantly lower than the abovementioned values. 33

52 Chapter 3 Characterization techniques 3.1 X-ray photoelectron spectroscopy Working principles X-ray photoelectron spectroscopy (XPS) is a surface sensitive analytic tool to study the surface composition and chemical state of a sample. Its physical basis is photoelectric effect, involving a photo-ionization process of inner shell electrons emitted from a surface excited by X-rays. Since the energy is conserved during the ionization process, and the kinetic energy (E k ) of emitted photoelectrons is measured, therefore the binding energy (E B ) of the atomic orbital from which the electron originate can be given by: E B = hν-e K -Φ where hν is the energy of X-ray, and Φ is the work function of the spectrometer. A XPS system is schematically illustrated in Figure 3.1. X-rays are produced from an anode by bombardment of hot electrons created by a filament. The most widely used anodes are Al ( ev), and Mg ( ev). A testing sample is placed in ultra high vacuum (UHV) environment after it is mounted on a sample holder. Due to photoelectric effect, photoelectrons can be emitted from the surface of the sample after x- ray irradiation. The binding energy distribution of these emitted photoelectrons (i.e. the number of emitted photoelectrons as a function of their binding energy) can be measured using any appropriate electron energy analyzer and detector. A photoelectron spectrum 34

53 Chapter 3 Characterization techniques can thus be recorded. Because the energy of core electrons is very specific for the element that the atom belongs to, the spectrum gives information on the elemental composition of the surface region. Furthermore, the binding energy of an electron is influenced by its chemical environment and hence can be used to identify its chemical state of a given atom. Figure 3.1 Schematic of a XPS system Surface sensitivity XPS is a surface analytical technique. As a result of ionization processes, electrons may be created both near the surface and deep within a solid, but only those electrons that originate within tens of angstroms of the surface can escape without energy loss. These electrons that leave the surface without energy loss produce the peaks in the spectra and are the most useful. The electrons that undergo inelastic loss processes before emerging from the sample surface form the background. The electron escape depth is 35

54 Chapter 3 Characterization techniques defined as the distance normal to the surface at which the probability of an electron escaping without significant energy loss drops to 1/e. Note that the escape depth depends not only on the matrix composition and the energy of the transition, but also on the analytical geometry. By assuming a process of homogeneous attenuation, the electron flux reduction can be described by the equation: I = I 0 z exp λ cosα where I 0 and I are the incident and emergent intensities, respectively, λ is the mean free path of photoelectrons, and z/cosα is the path length for electrons traveling α off-normal through a material of depth z along the normal Compositional analysis XPS provides a quantitative analysis of the surface composition and is sometimes known by the alternative acronym, electron spectroscopy for chemical analysis (ESCA). For each element, there will be a characteristic binding energy associated with each core atomic orbital. The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample, and the intensity I of the peaks is proportional to the number of atoms n of the particular element present: I = n S= n(fσθyλat) -1 where S defined as the atomic sensitivity factor, F is the x-ray flux (photon/cm 2 sec), σ is the photoelectric cross section (cm 2 ), θ is the instrumental angular efficiency factor based on the angle between the photon path and detected electron, y is the photoelectron process efficiency for formation of photoelectrons of the normal photoelectron energy, λ is the photoelectron mean free path, A is effective sample area from which photoelectrons are detected, T is detection efficiency for electrons emitted from the sample. The atom fraction of any constituent in a sample, C x, can be calculated as: 36

55 Chapter 3 Characterization techniques Cx = Ix Sx Ii Si i Chemical state analysis The exact binding energy depends not only upon the energy level from which photoemission is occurring, but also upon the local chemical environment. Generally variations in the binding energies, also called the chemical shift, arise from differences in the chemical potential of compounds. Different chemical states resulting from compound formation are reflected in the photoelectron peak positions and shapes. These chemical shifts can be used to identify the chemical state of the material by comparing the binding energies of the same atom in various reference compounds. 3.2 Luminance measurement The luminance-current -voltage (L-I-V) characteristics of OLEDs were measured in ambient atmosphere using a HP 4140B pa meter and a Minolta LS-110 meter. Since the measurement of IV is routine, it is important to introduce the measurement mechanism of a luminance meter Concept of luminance The concept of luminance is challenging and deserves detailed discussion. The concepts of luminous flux, and luminous intensity are fundamental to the understanding of luminance Luminous flux is visible power, or light energy per unit of time. It is measured in lumens. The lumen is the product of luminous intensity and solid angle, cd sr. It is analogous to the unit of radiant flux (watt), differing only in the eye response weighting. If a light source is isotropic, the relationship between lumens and candelas is 1 cd = 4π 37

56 Chapter 3 Characterization techniques lm. In other words, an isotropic source having a luminous intensity of 1 candela emits 4π lumens into space, which just happens to be 4π steradians. One watt of radiant power at 555 nm - the wavelength at which the typical human eye is most sensitive is equivalent to a luminous flux of 683 lumens. Luminous intensity is the luminous flux per solid angle emitted or reflected from a point. The unit is the lumen per steradian, or candela (cd). Luminance is the luminous intensity per unit area projected in a given direction. The unit is the cd/m 2. First, let's look at what is meant by "projected area." As seen in Figure 3.2, think of a slide projector containing a slide that is opaque except for a small clear spot at the center. When d1, and d2 are correctly related to the focal length of the lens, light passing from the lamp through the clear spot in the slide is focused by the lens onto the receiving surface. This in-focus image of the spot is the projected area. The size of the projected area can be adjusted by changing the focal length of the lens, d1 and d2, and/or the size of the spot (the aperture) on the slide. Replacing the projection lamp with a photodetector and the projected area with a source of light provides the basic elements of a luminance meter. Aperture slide Lens Screen Lamp Projected area d1 d2 Figure 3.2 Illustration of projected area 38

57 Chapter 3 Characterization techniques Figure 3.3 Schematic of a Minolta Luminance meter LS Measurement of luminance Most luminance meters have special optics that allows the user to view the source and bring the projected area into focus, as shown Figure 3.3. Any luminous flux that leaves the source - as defined by the projected area - and passes through the lens will also pass through the Aperture. That luminous flux will first go through spectral response correction filer that simulates the sensitivity of human being s eyes, then enter the silicon photocell and permit a luminance measurement. What is being measured is power - the rate at which energy is being transferred from source to detector. This power is related to the sensitivity of human being s eyes. 39

58 Chapter 3 Characterization techniques 3.3 Photoluminescence measurement Visible photoluminescence Monochromator Lamp Focusing lens Spectrometer PMT Sample holder Sample compartment Detection filter Figure 3.4 The configuration of the spectroflurometerfor visible PL. The spectrofluorometer configuration used for the visible PL measurement is shown in Figure 3.4 The tungsten-halogen lamp is used as a broadband source. The monochormator is fixed as 350 nm. The spectral bandwidth at the output of the monochromator is set to ~ 15 nm. A grating spectrometer which can be scanned from 100 nm to 1000 nm is mounted on the other side of the sample compartment.. The PL from the sample is focused onto the entrance of this spectrometer. There is a longpass filter placed in front of the entrance of the spectrometer to prevent the excitation light from 40

59 Chapter 3 Characterization techniques entering into the spectrometer. The light from the output of the spectrometer is collected by a photomultiplier tube (PMT) Infrared photoluminescence Argon laser 488 nm Focusing lens Spectrometer Sample holder InGaAs detector Detection filter Figure 3.5 The configuration of the spectroflurometer for infrared PL. The configuration of the spectroflurometer used for the infrared PL measurement is shown in Figure 3.5. The argon plasma laser is used as the exciation source. The output of the laser is centered at 488 nm with a controllable power up to 100 mw. There is a longpass filter placed in front of the entrance of the spectrometer to prevent the excitation light from entering into the spectrometer. The light from the output of the spectrometer is collected by a thermo-electrically cooled InGaAs detector which has high sensitivity in the spectral range of nm. 41

60 Chapter 3 Characterization techniques 3.4 Electroluminescence measurement Visible spectrum The EL spectra in the visible range were recorded by a USB2000 miniature fiber optic spectrometer. The USB2000 spectrometer couples a low-cost, high-performance 2048-element linear CCD-array detector The USB2000 works the same way as other spectrometers in that it accepts light energy transmitted through single-strand optical fiber and disperses it via a fixed grating across the linear CCD array detector, which is responsive from nm Infrared spectrum Electrical connection Focusing lens Power source Spectrometer Device holder Detection filter Detector Figure 3.6 The configuration of EL in the infrared range The configuration used for the infrared EL measurement is shown in Figure 3.6.The OLEDs were driven by a power source to emit infrared light, which is foucesd onto the entrance of the spectrometer. To prevent the visible light from the Erq device to 42

61 Chapter 3 Characterization techniques enter into the spectrometer, a longpass filter is place in front of the entrance of the spectrometer. There are two detectors used in this configuration: liquid-nitorgen cooled gemanium detector and thermo-electrically cooled InGaAs detector. 43

62 Chapter 4 Development of fullerene for OLED application 4.1 Introduction Organic light-emitting diodes (OLEDs) have gained tremendous progress through improvements in both OLED materials and device structure in the last decade. However, efficient power conversion from electricity to light is still one of the main goals in the development of organic electroluminescent displays. A dominant factor in determining power efficiency is injection and transport of carriers. In other words, the majority of device impedance during operation is related to charge injection and transport process. There have been many experimental works on seeking more conductive charge transport layer (CTL) [1-3]. It is generally known that hole mobility is 2 to 3 orders of magnitude higher than that of electron in organic semiconductors. This large disparity results in a more voltage drop across the ETL. A desirable attempt is to acquire a more conductive ETL. The approach of doping of Li is one of successful examples. However, device lifetime is potentially an issue. As a strong electron acceptor, C60 has received a lot of attention in n-channel field effect transistors [4], photodetectors [5], and solar cells [6], but relatively little attention for its use in organic electroluminescent devices. It might be due to two difficulties. Fullerene molecules have been generally regarded as emission quenchers because radiative transitions of C60 are exceedingly slow and its intersystem crossing from the excited singlet to the triplet state is very fast and efficient (with the triplet yield close to unity). Another difficulty arises from unavailability of efficient electron injection 44

63 Chapter 4 Development of fullerene for OLED application at the C60/metal interface due to fabrication conditions. Here, we report the practical use of C60 as an ETL in OLEDs with a LiF/Al bi-layer cathode. This robust C60/LiF/Al interface has been confirmed to behave as an ohmic-like contact for electron injection. The high electron mobility of C60 in combination with the low resistance of the C60/LiF/Al cathode interface results in a significant improvement in power efficiency mainly through a reduction in operation voltage. The excellent chemical stability of C60 can further increase the lifetime of OLEDs. 4.2 C60/LiF/Al interface In order to establish a cathode structure that may form ohmic contact to C60, we have designed and made a series of electron only devices where the C60 film was sandwiched between two metal electrodes. The devices were made on 2 in. 2 in. Si(100) wafers with 200 nm silicon dioxide on top. A first metal electrode (1 mm wide, 50 mm long, and 60 nm thick) was deposited through a shadow mask, and is referred to as bottom electrode. The C60 film (180 nm thick) was then deposited over the bottom electrode. A second metal electrode (1mm wide, 50 nm long, and 100 nm thick), referred to as top electrode, was deposited over the C60 film. The top electrode lines are orthogonal to the bottom electrode lines so that each intersection of these two lines produces one metal/c60/metal device. There are 20 devices on each wafer. A final silicon oxide film (SiO x ) of ~300 nm was used to encapsulate the device. The final encapsulation is essential to produce consistent and reproducible results. All devices including OLEDs were made using a K.J. Lesker OLED cluster Tool. All metals were deposited in the metallization chamber having a base pressure of ~10 7 Torr. C60 was deposited in the organic chamber having a base pressure of ~10 8 Torr. The transfer of samples between the metallization chamber and the organic chamber was via a central distribution chamber, which has a base pressure of ~10 9 Torr. Figure 4.1 compares current density-voltage (J-V) characteristics of a Al/C60(180 nm)/al device and a Al/LiF(0.5 nm)/c60(180 nm)/lif(0.5 nm)/al device. The J-V characteristics were measured using a HP4140B pa meter at room temperature. It is found that the J-V relationship from the Al/LiF/C60/LiF/Al device exhibits a perfectly 45

64 Chapter 4 Development of fullerene for OLED application linear line, characteristics of an ohmic contact at both the top and the bottom interfaces. A strong rectifying J-V characteristics was observed on the Al/C60/Al device. The data suggest that a strong potential barrier was formed at the Al/C60 interfaces. As the top interface formation involves hot vaporized Al atoms landing on the C60 surface, it is quite possible that atomic Al vapor has reacted with C60 forming a carbide species, as suggested by a previous report using Raman scattering technique [7]. The bottom electrode formation involves molecular C60 depositing on the Al surface, and is a less disruptive process. This difference in interface formation processes leads to the asymmetric J-V characteristics observed on the Al/C60/Al devices. It is an intriguing phenomenon that a thin LiF interlayer between the C60 film and the Al electrode changes the electrical characteristics fundamentally. There are two possible mechanisms contributing to the formation of such an ohmic contact. First, it is possible that as a strong electron acceptor, C60 gains partial electron charge from F ( Li + ) at the cathode interface. This n-type doping was found by XPS analysis of LiF-doped C60 films [8]. Second, a recent investigation suggested that the LiF interlayer gives effective passivation for Al electrode by preventing oxidation of Al and suppresses the oxygen penetration into C60 film to deteriorate its electrical properties [9]. 500 Current density (ma/cm 2 ) C60/LiF/Al C60/Al Voltage (V) Figure 4.1 J-V characteristics of Al/C60/Al devices with and without LiF interlayer (0.5 nm). The bias was applied to the bottom electrode in reference to the top electrode. 46

65 Chapter 4 Development of fullerene for OLED application 4.3 Improved power efficiency of OLED by a layer of C60 as ETL The testing devices have a structure of ITO (120 nm)/cupc (25 nm)/npb (45 nm)/alq (25nm)/ETL (20 nm)/lif (1 nm)/al, where the ETL is C60 for C60 device and Alq for the control, respectively. Both devices were made on the same ITO substrate thus eliminated process variables, such as ITO surface conditions, organic layer thicknesses, etc. The details of device fabrication can be found elsewhere [10]. The luminance-current -voltage (L-I-V) characteristics of OLEDs were carried out in ambient atmosphere using a HP 4140B pa meter and a Minolta LS-110 meter. Figure 4.2 shows a schematic diagram of energy level alignment for C60 devices. The work function of the LiF/Al bi-layer cathode is 0.6 ev lower than Al (4.2 ev) [11]. There is no energy barrier at the C60/LiF/Al cathode interface. Electrons can easily be injected and then are transported into the Alq layer. Simultaneously, holes are injected from the CuPc/ITO interface and then are transported into the NPB layer. The excitons are formed at the NPB/Alq interface. It should be noted that there is a significant energy barrier at the Alq/C60 interface. 4.7 ITO 2.8 CuPc NPB Alq C LiF/Al Figure 4.2. Schematic diagram showing the energy-level alignment for a C60 device. All the values shown in the diagram are in units of ev.. Figure 4.3 shows J-V and luminance versus voltage (L-V) characteristics of the C60 device and the control. The driving voltage of the C60 device is significantly lower than that of the control. There are two factors contributing to the low driving voltage 47

66 Chapter 4 Development of fullerene for OLED application characteristics of the C60 device; (1) the formation of an ohmic contact at the C60/LiF/Al interface and (2) the high electron mobility in C60 [7]. It is well known in semiconductor device physics that interface impedance plays a critical role in electrical-field distribution within the various layers of a diode-type device [12]. For a p-n junction-type diode, removal of metal-semiconductor rectifying interface is essential to reduce driving voltage. These two factors are not possible to realize using conventional organic semiconductors, such as Alq which is by far the most widely used ETM with an electron mobility of ~10 6 cm 2 /Vs. A combination of charge injection limited (by a potential mismatch between the LUMO of ETL and the work function of metal cathode) and Coulomb blockade caused by electron accumulation near the cathode interface leads to a strong Schottky-type contact which is the fundamental limitation of conventional organic molecules as an ETL [13]. Current density (ma/cm 2 ) Alq C Voltage (V) Figure 4.3. J-V and L-V characteristics of OLED using Alq and C60 as the ETL. 1 Luminance (Cd/m 2 ) 48

67 Chapter 4 Development of fullerene for OLED application Power efficiency (lm/w) Alq C Current efficiency (cd/a) Luminance (Cd/m 2 ) Figure 4.4. Power efficiency and current efficiency as a function of luminance of C60 device and the control. Figure 4.4 shows the current efficiency and power efficiency as a function of luminance from the C60 device and the control. It is noted that the current efficiency of the C60 device is higher than that of the control. Because of its lower driving voltage and higher current efficiency, the power efficiency of the C60 device is much better than that of the control. This increase in the power efficiency is more significant at ~1,000 cd/m 2 (a value typical for designing active matrix displays) than that at ~10,000 cd/m 2 (a value typical for designing passive matrix displays) LiF thickness dependence As an interlayer in the cathode interface, LiF plays an interesting role in electron injection at the cathode interface. In general, it reduces injection barrier. As a consequence, the efficiency of electron injection and luminous efficiency are greatly improved. The LiF layer plays a critical role in C60 devices. As abovementioned, there is a substantial energy barrier at the C60/Al interface, and the current density is dramatically reduced in the electron-only devices. 49

68 Chapter 4 Development of fullerene for OLED application Current density (ma/cm 2 ) (a) 0 nm 0.5 nm 1 nm 3 nm 5 nm 10 nm Voltage (V) 10 4 (b) Luminance (Cd/m 2 ) nm 0.5 nm 1 nm 3 nm 5 nm 10 nm Voltage (V) Figure 4.5 (a) J-V and (b) L-V characteristics of C60 devices with variable LiF thickness 50

69 Chapter 4 Development of fullerene for OLED application Figure 4.5 shows J-V and L-V characteristics of C60 devices with variable LiF thickness. C60 devices have a structure of ITO/NPB (60 nm)/alq (25 nm)/c60 (20 nm)/lif(x nm)/al. All the devices were made on the same substrate. Without LiF, only dim light (~10-2 cd/m 2 ) was detected when the driving voltage is over 9.4 V. With LiF, the devices show excellent L-J-V characteristics. The devices with the LiF thickness of 5Å, 10Å and 30Å show identical performance. There is an obvious shift in J-V curve to higher voltages when the LiF thickness is increased to 5 nm. However, the change of the current efficiency is not significant. This suggests that the LiF thickness within a range of 50Å doesn t affect the hole/electron population ratio significantly at the NPB/Alq interface. The J-V curve of the device with 10 nm LiF further shifts to the right compared to the device without LiF. It can be concluded that the C60/LiF (10 nm)/al interface demonstrates a larger resistance than the C60/Al interface. But its turn-on voltage is still 2.2 V and its current efficiency is reasonably high. This indicates that electrical field distribution at the NPB/Alq interface is changed dramatically in the device without LiF interlayer, resulting in a high hole/electron population ratio at the NPB/Alq interface that leads to a huge hole leakage current in the Alq side C60 thickness dependence Figure 4.6 shows J-V and luminance versus current density (L-J) characteristics of C60 devices with different C60 thicknesses. The devices have a structure of ITO/TPD (60 nm)/alq (25 nm)/c60 (x nm)/lif (0.5 nm)/al. All the OLEDs were fabricated on the same substrate. TPD, Alq, LiF and Al were deposited on the same vacuum cycle ensuring the same chemical conditions on anode and cathode interfaces. The variation of C60 thickness was realized by changing the shadow mask. The device with 30 nm C60 requires 3V to generate cd/m 2 at a current density of 4.25 ma/cm 2, indicating an excellent injection behavior at both cathode and anode interfaces. Because the shift to higher voltages in J-V characteristics is quite small when the C60 thickness is increased from 30 nm to 120 nm, it can be concluded that a small voltage drop is across the C60 51

70 Chapter 4 Development of fullerene for OLED application layer during device operation. The high electron mobility in C60 is undoubtedly the key factor attributing to such a small voltage drop. The current efficiency of these devices is different. The device with 30 nm C60 demonstrates the highest value of 2.7 cd/a, and the devices with 60 nm and 120 nm C60 show a similar current efficiency of 1.2 cd/a. Interestingly, the device with 90 nm C60 demonstrates the lowest current efficiency of 0.28 cd/a. The coupling of electronic excitations to photon states is affected by the physical structure of OLEDs. The presence of the metallic cathode provides a mirror that modifies the pattern of the electromagnetic modes near the cathode. The variation of C60 film thickness in a two-layer structure will change the distance between the emissive zone and the reflective cathode, therefore emission spectra, and spectral emission patterns will change accordingly. As seen in Figure 4.7, the EL spectrum from the C60 device changes when increasing the C60 thickness from 30 nm to 120 nm. To a first approximation, surface emission of an OLED can be simplified as the interference between forward emission and back emission reflected forward by a cathode. The maximum deconstructive interference occurs when the C60 layer reaches 90 nm, resulting in the lowest luminance at the same current density. 600 (a) 30 nm Current density (ma/cm 2 ) nm 90 nm 120 nm Voltage(V) 52

71 Chapter 4 Development of fullerene for OLED application Luminance (Cd/m 2 ) (b) 30 nm 60 nm 90 nm 120 nm Current density (ma/cm 2 ) Figure 4.6 (a) J-V and (b) L-J characteristics of C60 devices with variable C60 thickness. Normalized intensity (a.u.) 30 nm C60 60 nm C60 90 nm C nm C Wavelength (nm) Figure 4.7 EL spectra of C60 devices with variable C60 thickness. 53

72 Chapter 4 Development of fullerene for OLED application Alq thickness dependence We have made various simple devices with a structure of ITO/NPB(60 nm)/alq(d Alq nm)/c60(20 nm)/lif(1 nm)/al, where d Alq is varied. Figure 4.8 plots the driving voltage and current efficiency as a function of the Alq thickness at a constant current of 20 ma/cm 2. It is interesting to note that the driving voltage scales linearly with the Alq thickness. This indicates the resistance from the Alq layer plays a dominant role in the current-voltage characteristics of the OLED. The current efficiency, however, is found to be reduced dramatically when the Alq thickness is reduced to below 15 nm. This steep reduction in current efficiency indicates that the Alq/C60 interface acts as a quenching pathway for excitons. The slope of the curve suggests that excitons distribute mostly within 10 nm from the NPB/Alq interface. 7 Voltage (V) Alq thickness (nm) Current efficiency (cd/a) Figure 4.8. C60 devices current efficiency and driving voltage as a function of Alq thickness at a constant current density of 20 ma/cm 2. 54

73 Chapter 4 Development of fullerene for OLED application HTL thickness dependence Current density (ma/cm 2 ) 600 (a) nm: Control C60 40 nm: Control C60 50 nm: Control C60 60 nm: Control C Voltage (V) 12 (b) 30 nm: Control C60 Current efficiency (Cd/A) nm: Control C60 50 nm: Control C60 60 nm: Control C Luminance (Cd/m 2 ) 55

74 Chapter 4 Development of fullerene for OLED application 12 (c) 30 nm: Control C60 Power efficiency (lm/w) nm: Control C60 50 nm: Control C60 60 nm: Control C Luminance (Cd/m 2 ) Figure 4.9 (a) J-V, (b) η i -L, and (c) η p -L characteristics of C60 devices and the control with variable NPB thickness. Figure 4.9a shows J-V characteristics of different NPB thicknesses. The devices have a structure of ITO/NPB/Alq (25 nm)/etl (20 nm)/lif (0.5 nm)/al. The J-V curve of the C60 device or the control shifts to higher voltages when increasing the NPB thickness. Compared to the controls, all the C60 devices show a reduction of ~2 V in driving voltages, which is independent of the NPB thickness. Current efficiency-luminance (η i -L) characteristics of all the C60 devices and the controls are shown in Figure 4.9b. All the C60 devices show a significant improvement in current efficiency. This is not a universal phenomenon seen in all the ITO coated substrates, which will be discussed in the section. There are 3 regimes that we should discuss here. Before 100 cd/m 2, the C60 devices show a current efficiency of ~5.5 cd/a and the controls is ~4 cd/a. There is no correlation between the current efficiency and the NPB thickness. Between 100 cd/m 2 and 300 cd/m 2, the current efficiency shows a peak value in each of the eight devices. The highest current efficiency is ~8.0 cd/a for the C60 devices and ~5.5 cd/a for the controls. A hole/electron population ratio in C60 56

75 Chapter 4 Development of fullerene for OLED application devices is much closer to the unit than the controls because a more balanced electron and hole injection is achieved in the C60 devices. After 300 cd/m 2, the thickness dependence is shown in current efficiency. The device with thicker NPB layer shows a higher current efficiency. In a high luminance (high electrical field), the hole/electron population ratio at the NPB/Alq interface increase to more than 1, the population of holes at the NPB/Alq interface is reduced for thicker NPB layer, therefore a more balanced hole/electron ratio is reached. It is not surprising that the improvement in current efficiency of the C60 device with 30 nm NPB is marginal at a luminance of over 3,000 cd/m 2. The improvement in power efficiency (η p ) of C60 devices on this ITO substrate is realized through voltage reduction and the increase in current efficiency. As seen in Figure 4.9c, all the C60 devices show almost 100% improvement in power efficiency Different ITO surfaces Indium Tin Oxide (ITO) is essentially formed by substitional doping of In 2 O 3 with Sn, replacing the indium in the cubic bixbyite structure of In 2 O 3 [14]. Sn exists either as SnO or SnO2, having a valence of +2 or +4 accordingly. This valence state, along with oxygen vacancy, contributes to the ultimate conductivity of ITO. The high conductivity of ITO films is said to be due to high carrier concentration, rather than high Hall mobility [15]. Because of its transparency, high conductivity, and high work function, ITO has been widely used as the anode for OLEDs. ITO is not a well-controlled material, and its surface conditions are sensitive to the fabrication process. It is important to explore the effect of ITO surface conditions on C60 devices. ITO1, ITO2 and ITO3 were purchased from Coronado Coating Concept, Kuramoto Seisakusho and Delta technology, respectively. All the ITOs have a thickness of ~120 nm and a sheet resistance of <15 Ω/. ITO surface was cleaned ultrasonically in acetone, methanol and DI water for 5 mins respectively. After 15 mins baking, ITO was further treated by UV ozone for 15 mins. then loaded into the vacuum chamber. The devices have a structure of ITO/CuPc (25 nm)/npb (45 nm)/alq (30 nm)/etl (20 nm)/lif (1 nm)/al. 57

76 Chapter 4 Development of fullerene for OLED application Figure 4.10a shows J-V characteristics of the devices with different ITOs. Improvement in driving voltage is found in the devices on all ITOs. However, there is a significant variation in the degree of improvement. The C60 device on ITO1, ITO2 and ITO3 shows a reduction of 1.8 V, 2.4V and 2.8 V in driving voltages, respectively. It has been noted that the degree of improvement depends on the hole injection behavior of ITO. For all the C60 devices, the cathode interface are identical, the only difference is from the ITO surface. Higher driving voltages from ITO1 to ITO3 suggest a decrease in the efficiency of hole injection. A larger improvement in driving voltages is found in the C60 device on the ITO with less efficient hole injection. Not all the C60 devices demonstrate an improvement in current efficiency as seen in Figure 4.9b. Compared to the control, the C60 device on ITO1, which has the highest efficiency of hole injection, shows a slightly decreased current efficiency. This is different from the C60 devices on ITO2 and ITO3. However, the power efficiency of all the C60 devices is improved through the reduction of driving voltages, as seen in Figure 4.10c. Current density (ma/cm 2 ) (a) ITO1: Alq C60 ITO2: Alq C60 ITO3: Alq C Voltage (V) 58

77 Chapter 4 Development of fullerene for OLED application Current efficiency (Cd/A) 6 (b) ITO1: Alq C ITO2: Alq C60 ITO3: Alq C Luminance (Cd/m 2 ) Power efficiency (lm/w) (c) ITO1: Alq C60 ITO2: Alq C60 ITO3: Alq C Luminance (Cd/m 2 ) Figure 4.10 (a) J-V, (b) η i -L, and (c) η p -L characteristics of C60 devices and the control on different ITO surfaces. 59

78 Chapter 4 Development of fullerene for OLED application Al deposition On the fabrication side, it is not routine to make successful C60 devices. The deposition conditions of the Al layer (particularly first 100Å) have been noted to play an important role in making devices with a low driving voltage. Figure 4.11 shows J-V and L-V characteristics of the devices under different conditions of Al evaporation. The devices have a structure of ITO/NPB (60 nm)/alq (25 nm)/etl (20 nm)/lif(1 nm)/al. The C60 device and the control on the same substrate were fabricated on the same vacuum cycle. For different substrates, the organic materials and LiF were evaporated under the same conditions. The C60 device, which Al evaporation was made under a relatively low vacuum of ~10-6 torr and a low deposition rate of 0.6 Å/s, shows a much higher driving voltage and a much lower luminance. Interestingly, the difference between these two kinds of fabrication conditions has no effect on the Alq devices. This suggests that the Alq/LiF/Al interface is less sensitive on fabrication conditions. A dirty cathode interface in the C60 device not only lowers the efficiency of electron injection, but also changes the hole/electron ratio at the NPB/Alq interface, leading to a large hole leakage current. It can be concluded that cleanness at the cathode interface is more critical for the C60 device than the device with Alq as the ETL. Current density (ma/cm 2 ) (a) 0.2 nm/s, ~10-7 Torr Alq C nm/s, ~10-6 Torr Alq C Voltage (V) 60

79 Chapter 4 Development of fullerene for OLED application 10 4 (b) Luminance (Cd/m 2 ) nm/s, ~10-7 Torr Alq C nm/s, ~10-6 Torr Alq C Voltage (V) Figure 4.11 (a) J-V, (b) L-V characteristics of C60 devices and the control under different Al deposition conditions Further improvement by LiF sandwiched between Alq and C60 layer A serious LUMO mismatch has been found in the Alq (2.9 ev)/c60 (3.6 ev) interface [15, 16 ], leading to a significant energy barrier for electrons at this interface. It is natural that electron carriers tend to accumulate at this Alq/C60 interface. Therefore, a significant voltage is needed to compensate this LUMO mismatch during device operation. In order to reduce this energy barrier, a thin layer of LiF was used to be sandwiched between the Alq and C60 layer. The testing device has a structure of ITO/TPD (60 nm)/alq (25 nm)/lif (1 nm)/ C60 (20 nm) /LiF/Al, and no LiF layer was placed at the Alq/C60 interface in the control device. Figure 4.12a shows there is a voltage drop of ~ 0.5 V in the C60 device with a sandwiched layer of 1 nm LiF. Its current efficiency is also improved, as seen in Figure 4.12b. With this LiF layer, more electrons can be transported into the emission zone, and a more balanced hole/electron 61

80 Chapter 4 Development of fullerene for OLED application 500 (a) Current density (ma/cm 2 ) Alq/C60 Alq/LiF/C Voltage (V) 14 (b) Current efficiency (Cd/A) Alq/C60 Alq/LiF/C Luminance (Cd/m 2 ) Figure 4.12 (a) J-V, (b) η i -L characteristics of C60 devices with a structure of ITO/TPD (60 nm)/alq (25 nm)/lif (1 nm)/c60 (20 nm)/lif/al. 62

81 Chapter 4 Development of fullerene for OLED application ratio at the TPD/Alq interface can be reached. The role of LiF here is not well understood. It is possible that LiF at the Alq/C60 interface can introduce new energy states between the LUMO of Alq and that of C60, resulting in a lower energy barrier for electrons. This understanding is in agreement with a recent investigation that LiF material was found to be n type doping to both Alq and C60 by a XPS analysis [8] Preliminary lifetime test We investigated the enhanced stability of the OLEDs having a structure of NPB (50 nm)/ Alq (25 nm)/ C60 (20 nm)/lif (1 nm)/al. The characteristics of standard NPB(50 nm)/alq(45 nm) devices, fabricated on the same substrates during the same vacuum cycle, are also shown as a reference. Both devices have a capping layer of 200 nm LiF and 500 nm SiOx. No further encapsulation was applied to the devices. Figure 4.13 shows relative luminance and driving voltage versus operation time. The devices Relative luminance (a) Alq C Time (hour) 63

82 Chapter 4 Development of fullerene for OLED application 14 (b) Alq C60 12 Voltage (V) Time (hour) Figure 4.13 (a) Relative luminance and (b) Driving voltage versus operation time of C60 device. were driven at a constant current of 25 ma/cm 2. The initial luminance is 817 cd/m 2 for the C60 device, 802 cd/m 2 for the control. The C60 device shows a much slower aging process in its relative luminance and driving voltage. It is clear that the aging process in both devices is from the aging of cathode interface due to environmental effect. Slow aging process in the C60 device can confirm that the C60/LiF/Al interface is more chemically stable than the Alq/LiF/Al interface. 4.4 Summary The C60/LiF/Al interface has been found to exhibit ohmic type junction characteristics in electron only devices. The OLEDs with the C60/LiF/Al cathode interface show significantly lower driving voltages, regardless of the variation of HTL thickness, and ITO surface conditions. The power efficiency of C60 devices is thus improved mainly through voltage reduction. The LUMO mismatch at the Alq/C60 interface can be modified by an ultra thin layer of LiF, and the further improvement of 64

83 Chapter 4 Development of fullerene for OLED application device performance has been found. The performance of C60 devices is found to be more sensitive to the conditions of Al evaporation than conventional Alq devices. Due to a more stable cathode interface, the C60 device shows a much slower aging process than conventional Alq device. 4.5 References 1. J. Kido and Y. Lizumi, Appl. Phys. Lett. 73, 2721 (1998). 2. W.Y. Gao and A. Kahn, Appl. Phys. Lett. 79, 4040 (2001). 3. J.S. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K.L. Leo and S. Y. Liu, Appl. Phys. Lett. 80, 139 (2002). 4. R. C. Haddon, A. S. Perel, R. C. Morris, T. T. M. Palstra, A. F. Hebard, and R. M. Fleming, Appl. Phys. Lett. 67, 121 (1995). 5. P. Peumans, A. Yakimov, and S. R. Forrest, J. Appl. Phys. 93, 3693 (2004). 6. K. Murata, S. Ito, K. Takahashi, and B. M. Hoffman, Appl. Phys. Lett 68, 427 (1996). 7. A. F. Hebard, C. E. Eom, Y. Iwasa, K. B. Lyons, G. A. Thomas, D. H. Rapkins, R. M. Fleming, R. C. Haddon, J. M. Philips, J. H. Marchall, and R. H. Eick, Phys. Rev. B 50, 403 (1994). 8. Y. Yuan, D. Grozea, S. Han, and Z. H. Lu, Appl. Phys. Lett. 85, 4959 (2004). 9. C. J. Hung, D. Grozea, A. Turak, and Z. H. Lu, Appl. Phys. Lett. 86, (2005). 10. X. D. Feng, R. Khangura, and Z. H. Lu, Appl. Phys. Lett. 85, 497 (2004). 11. T. Mori, H. Fujikawa, S. Tokito, and Y. Taga. Appl. Phys. Lett. 73, 2783 (1998). 12. See, for example, E. F. Schubert, Light-Emitting Diodes (Cambridge University Press, New York, 2003). 13. R.S. Khangura, M.A.Sc. thesis, University of Toronto, Toronto, J. C. C. Fan and J. B. Goodenough, J. Appl. Phys. 48, 3524 (1977). 15. K. L. Chopra, S. Major, and D. K. Pandya, Thin Solid Films 102, 1 (1983). 16. S. T. Lee, X. Y. Hou, M. G. Mason, and C. W. Tang, Appl. Phys. Lett. 72, 1593 (1998). 17. R. Mitsumoto et al, J. Phys. Chem. A 102, 552 (1998). 65

84 Chapter 5 Development of OLED structure with a metalorganic-metal cathode for contrast enhancement 5.1 Introduction A typical organic light-emitting diode (OLED) includes two electrodes. One of them has to be transparent or semi-transparent (e.g. ITO as a transparent anode) while the other one is usually highly reflective (e.g., Mg-Ag [1], Ca/Al [2] or LiF/Al [3], as a reflective cathode). In the case of high ambient illumination, a substantial amount of light is reflected by the reflective electrode, thereby degrading the visually perceived contrast of the emitted light from the device. It is quite important that an OLED display can be easily viewed under all ambient illumination conditions (e.g. full sunlight). In general, light absorbing layers [4], circular polarizers [5], or optical interference layers [6] have been used to reduce ambient light reflection in the inorganic electroluminescent displays. Recently, high contrast OLEDs, have been realized, using a conductive light-absorbing layer made of a mixture of metals and organic materials [7], or based on optical destructive interference, using a black layer [8, 9], which includes a thin semitransparent metal layer, a phase-changing layer (inorganic materials, e.g. SiO-based conductive layer or oxygen-deficient ZnO), and a thick reflective metal layer. As seen in Figure 5.1, the black layer results in cancellation of two light waves reflected from the front thin layer and the rear thick metal layer over a wide bandwidth due to a π phase difference. Before a black layer becomes useful in OLED displays, issues such as the shift in driving voltages, and original color chromaticity have to be resolved. However, 66

85 Chapter 5 Contrast enhancement limited choices of inorganic materials as phase-changing layers compatible with current OLED processing will be an unfavorable factor in the engineering capability of optical interference layers in OLED displays. Ambient light Optical interference R1 R2 Thin metal layer Phase changing layer Thick metal layer Figure 5.1 Schematic of the working principle of a black layer. R1 = R2, 180 out of phase Figure 5.1 Schematic of the working principle of a black layer. Here, we report an alternative black layer based also on optical interference. Instead of the exiting structure of metal-inorganic-metal (MIM), a metal-organic-metal (MOM) cathode was used. As compared with a MIM structure, a MOM cathode can easily be made from a wide range of available organic semiconductive molecules and thus promises tremendous flexibility in engineering the optical and electrical characteristics of the black-layer cathode. For example, the tuning of absorption across the full spectrum of visible light can be realized by thermal co-evaporation of two or more molecules with proper optical gaps. Molecular doping [10] may also enhance the electrical conductivity of the organic layer within the MOM cathode. 67

86 Chapter 5 Contrast enhancement 5.2 Experiment Al (100 nm) LiF (0.5 nm) ETL Al (5.5 nm) LiF (0.5 nm) Alq (68 nm) MOM TPD (60 nm) ITO Glass substrate Figure 5.2 Schematic structure of an OLED device with a MOM cathode. A typical testing structure of our high-contrast OLEDs includes ITO as anode, TPD as hole transport layer, Alq as emissive and electron transport layer and the MOM cathode, as schematically shown in Figure 5.2. MOM cathode includes a thin 0.5 nm LiF and 5.5 nm Al bilayer as the front semi-transparent mirror, an electron transport layer (e.g. 80 nm Alq) as a phase changing layer, and rear LiF and Al bilayer as the second mirror. The electrical connection is made between the ITO and the rear 100 nm Al layer. The devices were fabricated using a K.J. Lesker OLED cluster tool on 2 2 ITO coated glass substrates. The ITO surface was treated by a routine wet chemical cleaning method [11] and UV ozone. Two types of OLEDs were made. For Type-I OLED, 2 2 mm 2 diode areas were defined by a cross-intersection of 2 mm ITO lines and 2 mm opening lines on an insulating dielectric coating on the ITO. The opening lines, running perpendicular to the ITO lines, were defined by a shadow mask during dielectric film deposition. For type-ii OLED, 2 1 mm 2 active areas were defined by a cross-intersection of 2-mm-wide ITO lines and 1-mm-wide cathode lines deposited through a shadow mask. The base pressure of the organic and metallization chambers is generally lower than Torr, and the pressures during the deposition process in those two chambers are 68

87 Chapter 5 Contrast enhancement normally lower than and Torr, respectively. The growth rates are ~1 Å/s for organic materials and ~2 Å/s for Al. Only the performance of the diodes on the same ITO substrate was compared in the following. The fabrication of different diodes on the same substrate was realized through the changing of shadow masks inside the cluster tool without breaking vacuum. This minimized any unknown factors arising from the variation in ITO surface and growth conditions. Luminance-current density-voltage (L-J- V) characteristics were carried out in ambient atmosphere using a HP 4140B pa meter and a Minolta LS-110 meter. The optical reflectance measurement was made using a Cary 500 UV- VIS-NIR spectrometer with a VW absolute specular reflectance accessory 5.3 Results and discussion The L-J-V characteristic of an OLED device with a MOM cathode is shown in Figure 5.3a. Compared with the control device having a structure of TPD (60 nm)/alq (68 nm)/lif (0.5 nm)/al (100 nm), the MOM device with 80 nm Alq spacer shows a much lower current density under 2 V, indicating the device is at a high impedance state. However, at a critical voltage, 2.6 V, the current has a sharp increase of nearly 3 orders of magnitude, indicating that the device has had a transition from the high impedance state to a low-impedance state. When the bias voltage is further increased, the device shows excellent injection of carriers. It requires 3.2 V to generate a luminance of cd/m 2 at a current density of 10.3 ma/cm 2. The luminance can reach 1,200 cd/m 2 at 4.6 V, which is half of the luminance of the control. To a first approximation, surface emission of a conventional OLED causes from the interference between forward emission and back emission reflected forward by a cathode. A MOM cathode can filter out environmental light and back-emission light by optical interference enhanced absorption. Therefore the current efficiency of an OLED with a MOM cathode is reduced. Figure 5.3b shows the luminance as a function of driving current. A good linearity between the luminance and current density indicates that MOM cathode behaves similarly to conventional cathodes and is therefore compatible with the current-controlled drive circuit used for OLED displays, in which the gray scale of each pixel is controlled by current density. 69

88 Chapter 5 Contrast enhancement We now discuss each constituent MOM layer. On the fabrication side, the deposition conditions of the first thin LiF/Al layer were noted to play an important role in making devices with a low driving voltage [12]. The first thin Al layer can be easily oxidized under poor vacuum, resulting in an increase in driving voltages. In order to examine whether the first thin metal layer in our devices plays a critical role in the injection of electrons, the OLEDs with different MOM cathodes and two control devices were made. Control 1 has a structure of TPD (60 nm)/alq (68 nm)/lif (0.5 nm)/al (100 nm) and control 2 has a structure of TPD (60 nm)/alq (148 nm)/lif (0.5 nm)/al (100 nm). As shown in Fig.5.4a, there is a small voltage shift in J-V characteristics of device with a MOM cathode compared to the device with regular LiF/Al (control 1), while the current density is found to decrease dramatically if the first thin LiF/Al layer in the MOM cathode is removed (control 2). Also shown in Fig.5.4a, if the rear LiF/Al layer is replaced by Ag, the change in the L-J-V characteristics is quite small. To further investigate whether the possible shorting between the first thin metal layer and the second metal layer plays a role here, a MOM device using hole transporting TPD as an organic spacer was made. The current density is 4 to 5 orders lower than other MOM devices with electron transporting spacers and there is no light output. The possible shorting can be safely ruled out. Although the physical mechanism in the MOM cathode needs further investigation, it has been shown experimentally that the front metal layer plays a dominant role for electron injection. It is quite possible that injection of electrons from both the front metal layer and the rear LiF/Al bi-layer occurs simultaneously under a forward bias. The injection rates, however, may differ initially at these two interfaces. Therefore a built-in potential across the organic spacer would be formed, which can eventually help establish a balanced electron flow across the MOM cathode. Compared to the whole device, the voltage drop across the MOM cathode is small. Quite a few electron-transport materials can potentially be chosen as organic spacers, which are under our current investigation. As many semiconductive organic molecules are photoconductors, their conductivities can be improved by light- absorption. This implies that device performance can be further improved by using better organic photoconductors. It has been shown in Fig. 5.4b that device performance is slightly improved when Alq is replaced by CuPc in the MOM cathode. 70

89 Chapter 5 Contrast enhancement Current density (ma/cm 2 ) (a) Control MOM OLED Luminance (Cd/m 2 ) Voltage (V) 5000 (b) Luminance (Cd/m 2 ) Control MOM OLED Current density (ma/cm 2 ) Figure 5.3 (a) L-J-V, and (b) L-J characteristics of 2 2 mm 2 OLED devices with MOM cathodes. The control device has a structure of TPD (60 nm)/alq (68 nm)/lif (0.5 nm)/al (100 nm). 71

90 Chapter 5 Contrast enhancement Current density (ma/cm 2 ) x10-4 1x (a) LiF/Al/Alq(80nm)/LiF/Al LiF/Al/Alq(80nm)/Ag Control 1 Control Voltage (V) Luminance (Cd/m 2 ) Current density (ma/cm 2 ) x10-4 1x (b) LiF/Al/Alq(80 nm)/lif/al LiF/Al/CuPc(90 nm)/lif/al Voltage (V) Luminance (Cd/m 2 ) Figure 5.4 L-J-V characteristics of 1 2 mm 2 OLED devices with different MOM cathodes. Control 1: ITO/TPD(60 nm)/alq(68 nm)/lif (0.5 nm)/al (100 nm) and control 2 : ITO/TPD(60 nm)/alq(148 nm)/lif(0.5 nm)/al (100 nm) are the referenced devices. 72

91 Chapter 5 Contrast enhancement a a - Control OLED b - MOM OLED c - MOM layer Reflectance (%) b c Wavelengrh (nm) Figure 5.5 Absolute reflectance spectra measured at 7. Figure 5.5 shows optical reflectance characteristics of 2 2 mm 2 OLED measured at 7 o off the surface normal. As a reference, identical MOM (LiF/Al-Alq-LiF/Al) layers were deposited on the glass substrate. The reflectance from the reference MOM structure was found to be ~7% in the range of 400 to 650 nm. However, the reflectance from real devices with a MOM cathode was found to be reduced to ~20%, from ~80% on conventional OLED. It should be pointed out that 20% reflectance consists of several reflections from the air/glass, glass/ito, and ITO/organic interfaces where a discontinuity in refractive indices ocuurs. Our eyes are not equally sensitive to all wavelengths. In photopic vision (active for > 3 cd/m 2 ), nm is the most sensitive range for perception. The design of optical interference layers is based on 550 nm, one quarter of which is approximately the optical thickness of the organic spacer. Thus the highest part of reflectivity is expected to be in the red color from such black layer, as shown in curve c of Figure 5.5.This effect can be more pronounced at a larger viewing angle [6]. This phenomenon is known to alter the color chromaticity. Using a MOM design, however, this problem could be solved by the choice of some molecules such as 73

92 Chapter 5 Contrast enhancement phthalocyanine, which has a strong absorption band in the range of nm. It is worthwhile to note that the organic spacer can be made of one or more layers. The absorption band can be easily tuned by the control of the thickness of each layer. Each layer can be further tuned by compounding two or more molecules. Moreover, there is little reflection at the organic/organic interface due to a good matching of refractive indices. In principle, the overall reflection would be reduced to approximately a few percentage in a wide range of visible light if the thicknesses of the first thin metal layer and organic spacer were carefully engineered to ensure that two light waves reflected from two metal layers are equal in amplitude and opposite in phase. 5.4 Summary We have demonstrated contrast enhancement of low operating voltage OLEDs by using a MOM cathode. These devices showed excellent diode characteristics, e.g., a luminance of cd/m 2 at ~3 V. The absolute reflectance from metal-organic-metal multilayer can be as low as 7% in the range of 400 to 650 nm. In real devices, it can be reduced from ~80% to ~ 20%. MOM cathodes are a potential solution for high contrast full-color OLED displays. 5.5 References 1. C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett. 51, 913 (1987). 2. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burn, and A. B. Holmes, Nature (London) 347, 539 (1990). 3. L.S. Hung, C.W. Tang, and M.G. Mason, Appl. Phys. Lett. 70, 152 (1997). 4. M. Takeda, H. Kishishita, H. Kawabata, and K.Isaka, U.S. Pat. No. 4,287,449 (1981). 5. J. Pucilowski, R.Schuman, and J. Velasquez, Appl. Opt. 13, 2248 (1974). 6. J. Dobrowolski, B. Sullivan, and R. Bajcar, Appl. Opt. 31, 5988 (1992). 7. H. Aziz, Y.F. Liew, H.W. Grandin, and Z.D. Popovic, Appl. Phys. Lett. 83, 186 (2003). 74

93 Chapter 5 Contrast enhancement 8. A.N. Krasnov, Appl. Phys. Lett. 80, 3853 (2002). 9. L.S. Hung and J. Madathil, Adv. Mater. 13, 1787 (2001). 10. W.Y. Gao and A. Kahn, Appl. Phys. Lett. 79, 4040 (2001). 11. X.D. Feng, D. Grozea, and Z.H. Lu, MRS Proceedings V 734, X.D. Feng, D. Grozea, A. Turak, and Z.H. Lu, (unpublished). 75

94 Chapter 6 Development of Erq devices for infrared emitter 6.1 Introduction OLEDs have gained tremendous progress in driving voltage, quantum efficiency, color tuning and lifetime [1-4] since Tang and Van Slyke demonstrated the first efficient heterostructure OLED [5]. Technologically, it paves the way to fabricate reliable OLEDs with infrared (IR) emission. Three approaches have been suggested for making IR OLEDs, where the IR emitter could be nanoparticles incorporated with a semiconducting polymer [6, 7], organic ionic dyes [8], or rare-earth containing molecules [9-12]. Erbium containing molecules are of great interest because trivalent erbium ions have long played an important role in optical communication technology [13], due to their intra-4 f emission at 1.5 µm, a standard telecommunication wavelength. Undoubtedly, demonstration of 1.5 µm EL at room temperature from 8-hydroxyquinoline erbium (Erq) molecules indicates its potential application in optical communications. However, the studies on IR emission from organic materials are still at an early stage. Current studies reported high operating voltage (> 25V), and the physical picture of IR emission from erbium related compounds is not clear. An investigation of Erq devices is of great importance for potentially extending the current OLED technology to the area of optical communications. 76

95 Chapter 6 Development of Erq devices for infrared emitter 6.2 Erq Erq was purchased from America Dye Source. It was produced by mixing erbium III chloride in aqueous solution with 8-hydroxyquinoline in methanol and was purified through recrystallization methods, not involving sublimation. The molecular structure is similar to Alq, as indicated in Figure 6.1. Molecular structure of Erq. Fig Very little information on its material properties has been documented. Its properties such as purity, optical absorption and photoluminescence will be discussed in the following. 2.5x x10 5 C1s Intensity (a. u.) 1.5x x x10 4 C KLL O KLL O 1s N 1s Er4p3/2 Cl 2p Er 4d Binding energy (ev) Figure 6.2. XPS survey of Erq thin film. 77