Metal-Containing Functional Materials for OLEDs and Solar Cells

Transcription

1 Metal-Containing Functional Materials for OLEDs and olar Cells Prof. Wai-Yeung Wong (Raymond) Department of Chemistry Centre for Advanced Luminescence Materials Hong Kong Baptist University Chemistry Enriching Knowledge Course HK Chemists in Action: 9 July 2009

2 Contents I. Introduction II. Organic Light-Emitting Devices (OLEDs) III. Electrophosphorescent OLEDs IV. Organic Photovoltaics (OPVs) V. Metal-based Materials for olar Cells VI. Conclusions and Future Prospects

3 Cathode ETL Host: Guest HTL ITO

4 Energy Interconversions Photovoltaic Cells (olar Cells): Electricity from Light (unlight) Organic Light-Emitting Diodes (OLEDs): Light from electricity

5 History of Light Thomas Alva Edison ( )

6 Principles and Classifications of Luminescence Energy Molecular ystems Light Emission Light is a form of energy To create light, another form of energy must be supplied. The two common ways are incandescence and luminescence

7 Incandescence Incandescence refers to the emission of light from heat energy e.g. An electric stove s heater or metal in a flame glows red hot The tungsten filament of an ordinary incandescent light bulb glows white hot

8 The un and stars Incandescence Direct sunlight 5400 K 100 W Tungsten bulb 2865 K Candle flame 1930 K

9 Luminescence Luminescence refers to the emission of light from other sources of energy, which can take place at normal and lower temperatures. It is commonly termed cold light emission everal types of luminescence can be distinguished depending on the excitation source

10 Different Forms of Luminescence Luminescence type Catholuminescence Photoluminescence Chemiluminescence Bioluminescence Electroluminescence Triboluminescence Excitation ource Electrons (UV) Photons Chemical energy Biochemical energy Electric field Mechanical energy Application TV sets, monitors Fluorescent lamps, plasma displays Lightsticks, environmental analysis Analytical chemistry, maintenance of life LEDs, EL displays

11 Photoluminescence (PL) The emission of light triggered by absorption of photons hv A* A* (excited state) hv Absorption (Excitation) A Emission A hv < hv

12 Fluorescent liquids

13 Lighting and Electricity Lighting consumes about 30% of all electricity used in a building. In UA, that translates into 18% of the total energy being used in a building. In HK, the highest energy consumer is airconditioning. The next highest is lighting. This is especially true in commercial buildings. This trend is expected to permeate and propagate into mainland.

14 Lighting accounts for 19% of all Electrical Energy Used World-wide

15 Lighting Incandescent light is still responsible for about 40% of all the lighting. Its power efficiency is lm/w. olid state light sources (LEDs or OLEDs) can easily be 4-5 times higher. For OLED, laboratory demonstration of lm/w has been achieved.

16 Electroluminescence (EL) The emission of light by electric current Historical development a (Holonyak and Bevacqua) Inorganic semiconductors for light-emitting diodes (LEDs) e.g. p-n junction diodes The energy of the light emitted can be changed by adjusting the composition of the material

17 Electroluminescence (EL) p-type semiconductors : the added material has fewer electrons than the host and adds positive holes e.g. i doped with B n-type semiconductors : the added material has one more electrons in the valence shell than the host material e.g. i doped with As

18 Electroluminescence (EL)

19 Electroluminescence (EL) + Light output p-type n-type Crystal substrate (transparent) Device structure for common LEDs

20 Electroluminescence (EL) 1987 (Tang and Vanlyke in Kodak) Fluorescent organic dyes as electroluminescent materials for organic light-emitting devices (OLEDs)

21 Electroluminescence (EL) 1990 (Richard Friend in Cambridge University) Luminescent conjugated polymers for polymer lightemitting devices (PLEDs) e.g. poly(1,4-phenylene vinylene) (PPV) is a famous light-emitting polymer PPV n

22 Features of OLEDs elf-emitting property High luminous efficiency Full color capability Wide viewing angle High contrast, low power consumption, low weight Large-area color displays and flexibility

23 Basic Principle of OLEDs

24 Theory of OLEDs or PLEDs The simplest OLED or PLED configuration consists of an electroluminescent layer sandwiched between an anode and a cathode, one of which has to be semi-transparent Under an applied bias, injection of holes takes place at the anode whereas injection of electrons occurs at the cathode

25 Theory of OLEDs or PLEDs ome of the electrons and holes combine within the emissive material to form singlet (25%) and triplet excited states (75%). uch an electron-hole pair (exciton) may then result in the emission of a photon. chematic drawing of single-layer EL device

26 Theory of OLEDs or PLEDs Unfortunately, the mobility of electrons and holes in most organic materials are considerably different, leading to exciton formation in the vicinity of one of the two contacts (For example, since holes migrate much more easily through PPV than electrons, electron-hole recombination takes place near the cathode)

27 Theory of OLEDs or PLEDs ince non-radiative exciton recombination, or quenching, is enhanced at the electrodeorganic interface, the single-layer structure typically exhibits a low quantum efficiency Heterojunction OLEDs

28 Layers in OLEDs Hole-Transporting Layer (HTL) Transports holes from the anode to the EML or ETL Electron-Transporting Layer (ETL) Transports electrons from the metal cathode to the EML or HTL

29 Emission Layer (EML) Transports both holes and electrons The layer where the recombination of holes and electrons takes place Double layer membrane Layers in OLEDs Cathode ETL/EML HTL hv Cathode ETL HTL/EML hv Anode ubstrate + Anode ubstrate + Either ETL or HTL behaves as EML

30 Triple-layer device Layers in OLEDs Cathode ETL EML HTL Anode ubstrate + hv The recombination of holes and electrons occurs in an independent EML

31 Common Layer Materials in OLEDs HTL materials e.g. PPV, TPD, PD CH 3 H 3 C TPD α-pd

32 ETL materials, e.g. PBD, Alq 3 O PBD O Oxadiazoles are electron-deficient

33 Polymer LEDs Polymers have bigger molecules and therefore cannot be thermally deposited. Instead, thin films are spin coated onto the substrate. Although it has advantage of fabricating device in ambient atmosphere, it has solvent problem when depositing multi-layer thin films.

34 Fluorescence vs Phosphorescence Internal Conversion 1 Intersystem Crossing Absorption Fluorescence 25% T 1 Phosphorescence 75% 0 Energy level diagram for a photoluminescence system

35 Ir Iridium(III) Complexes Ir(ppy) 3 One of the best phosphorescent dyes at present Effective intersystem crossing by spin-orbit coupling High phosphorescence efficiency at room temperature

36 Literature examples for color tuning Ir PF 6 Ir PF 6 Inorg. Chem. 2006, 45, Ir PF 6 2 Ir PF 6 F Ir Ir Ir O O Ir O O Ir O O Ir O O F red shift of emissions

37 CH 3 2 Ir O O CH 3 CH 3 F Ir O O 2 CH 3 CH Ir O O 2 CF 3 CH 3 Ir O O Me 2 CH 3 CH 3 PL intensity (a.u.) Wavelength (nm) Ir O O 2 CH 3 CH 3

38 CIE 1931 CHROMATICITY DIAGRAM Ir F F Ir F F 3 C Ir CF 3 CF 3 Cathode ETL Host: Guest Ir HTL ITO

39 [Ir(ppy-X) 2 (acac)] with main group elements O O O O Ir Ir Ir O O O O O Ir 2 O O Ir-O 2 Ir- Ir-O Ir- O O O O Ir O Ir O Ir O Ir O P O Ge i B Ir-PO Ir-Ge Ir-i Ir-B

40 Multi-color OLEDs at 8 V EL Intensity (a.u.) Ir-B (8 wt.-%) Ir-i (10 wt.-%) Ir-Ge (8 wt.-%) Ir- (6 wt.-%) Ir-PO (10 wt.-%) Ir-O (10 wt.-%) Ir- (8 wt.-%) Ir-O 2 (8 wt.-%) Wavelength (nm) W. Y. Wong et al., Adv. Funct. Mater., 2008, 18, 499.

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42 unlight Duplicator Pt O O Pt O O O Ge 1.0 EL intensity (a.u.) Wavelength (nm)

43 RGB WOLEDs Activate spot: 6.25 cm 8V

44 EL spectrum Two-Color WOLEDs F Ir F EL Intensity (a.u.) Wavelength (nm) F F Ir O O 30:1 Maximum efficiencies η L = 42.9 cd/a η P = 20.3 lm/w η ext = 19.1% O O 3

45 Efficient White Polymer Light-emitting Devices for olid- tate Lighting Ir Ir O O O O Al (100 nm) Ba (4 nm) Emission layer (70-80 nm) PEDOT:P (40 nm) 3 n 2 O O ITO O O Glass O O PVK OXD n O O O O O 3 H O 3 O 3 HO 3 O 3 H n PEDOT:P F F Ir F O O FIrpic F W.Y. Wong et al., Adv. Mater., in press.

46 WOLEDs vs Incandescent/Fluorescent Bulbs

47 Energy Crisis: Renewable Energy ources The goals of developing renewable energy sources can only be achieved by solutions from the chemical sciences in all steps. Photovoltaic Cells (olar Cells): Electricity from unlight olar Energy: The Clean Energy for the 21 st Century

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49 Why solar energy? olar energy: renewable, environmentally friendly, abundant and free

50 olar pectrum U Dept. of Energy Image: REL

51 Renewable Energy vs Fossil Fuels Fossil fuels including coal, oil, and natural gas provide more than 90% of the energy consumed in a modern society. Fuels are substances that when burned release significant amounts of energy together with CO 2 emission. Fuels are not unlimited.

52 Harvesting olar Energy Harnessing the un: olar Energy It has been noted that nearly all of the energy available on Earth comes from the un. Energy from the un is diffuse and must be concentrated to make it useful. Chemistry can develop more efficient materials for photovoltaics to transform sunlight into electricity.

53 (PVET-European Roadmap for PV R&D, JRC-EUR21087 E,2004) Primary Energy Use in The World

54 Photovoltaics-Projections ource: REL

55 Traditional olar Cell Technology Photovoltaic cells (solar cells) can be used to convert solar energy into electricity. These devices can be made from a variety of substances e.g. elemental i, light-absorbing dyes, organic semiconductors (small-molecules or polymers).

56 Biomass: Photosynthesis for Fuel Burning plant material by-products is one means of harvesting energy from the un. H 2 O O 2 CO 2 sugar atural Photosynthesis: The olar Cell Model

57 Inorganic emiconductor (ilicon) olar Cell Individual cell Modules

58 Advantages Lifetime years for systems, up to 30 years for modules o pollution, no waste products Energy from the un is free and abundant It is cleaner, safer, and renewable Disadvantages High initial cost (although it is recovered by fuel savings over the lifetime of the product) Area intensive (can be solved by utilizing roofs and surfaces in urban areas, deserts etc.) Intermittence and seasonality of sunlight need for storage of energy

59 Applications olar cells are useful in a wide range of applications. pace olar cell modules in calculators and wrist watches one of the key technologies towards a sustainable energy supply.

60 teckborn, witzerland. First church in the world with solar power

61 Organic solar cells Advantages Low cost Large area Flexible substrates and devices Improved coverage of solar spectrum Disadvantages Low efficiency Low exciton diffusion length Low charge carrier mobility Inferior stability compared to inorganic materials

62 Why organic solar cells? Flexible photovoltaic diodes ilicon solar cell (left) and plastic film solar cell (right)

63 chematic representation of flexible and easy produce organic materials Polymer photovoltaic cells with flexible substrate

64 Dye-sensitized solar cells (DCs) COOH HOOC HOOC Ru C C COOH

65 Dye-sensitized solar cells (DCs) The ruthenium-based dye collects incident light in the same way as green chlorophyll molecules harvest sunlight in plants. The dye is absorbed onto the surface of a semiconducting electrode of TiO 2. Once excited by light, the dye molecules lose electrons and are oxidized by TiO 2. They are then reduced by iodide ions present in the electrolyte solution. The resulting iodine produced is then itself reduced at a second electrode to complete the cycle. The electrons injected into TiO 2 travel around the external circuit and do the work for the user. Relatively high efficiency (> 10%), cheap, extremely promising as clean energy sources.

66 Bulk Heterojunction olar cells

67 Bulk Heterojunction olar cells interface donor e LUMO acceptor photon + Donor exciton HOMO h

68 Current-Voltage Curve of olar Cell J ma V ma P max =(J max V max ) x V oc V Pout Pmax Vmax Imax Voc Isc FF η = = = = Pin Pin Pin Pin Vmax Imax FF = V I oc sc I x J sc J sc = short-circuit current V oc = open-circuit voltage FF = fill factor η = power conversion efficiency

69 Research on Conjugated Organic Polymers C 6 H 13 n P3HT C 12 H 25 2 OP1 n R R C 8 H 17 C 8 H 17 n R = H O OP2 OP3 C8 H 17 C 6 H OP4 C 8 H 17 C 6 H 13 n n C 12 H 25 C 12 H 25 OP5 C 12 H 25 O OP6 OC 12 H 25 n C 6 H 13 OP7 OP8 n

70 Optimum photovoltaic performance for some reported organic polymers Polymer E g [ev] E HOMO [ev] E LUMO [ev] V oc [V] J sc [ma cm 2 ] FF PCE [%] EQE [%] P3HT OP1 OP2 OP3 OP4 OP5 OP6 OP7 OP8 3.7 ~ ~ ~ ~40 14 ~37 ~

71 trategies Toward High-Performance Devices ynthesis of low bandgap organic polymers ew electron-acceptors Exploitation of tandem solar cells using multiple layers of different bandgaps Any alternative? Organometallic polymers

72 Organometallic Photovoltaics Literature reports: Dye-sensitized solar cells using smallmolecule organometallic donor materials (Grätzel-type cell) Organic solar cells based on metal phthalocyanines

73 Typical solar cell structure mall molecule OPVs Fullerene (C 60 ) Metal Phthalocyanine

74 Research on Metallopolyynes L M R L n Good solution-processibility Excellent model systems to study the photophysics of excited states in conjugated polymers Effective intersystem crossing into triplet state due to the heavy atom effect (harvesting of triplet state energy)

75 Research tructural on Metallopolyynes Variability L M R Variation of metal centers (M) Variation of spacer groups (R) Variation of auxiliary ligands (L) L n Transition eries Rh Pd Ag Cd Ir Pt Au Hg

76 Applications in Optoelectronics and anotechnology Luminescence PLEDs Liquid Crystals and Chemosensing Metallopolyynes anomaterials Optical limiting Photovoltaics/olar Cells

77 W.-Y. Wong, Dalton Trans (Perspectives)., 2007, Cover page, Hot article

78 OMe O C C Fe H (H 3 C) 3 C C(CH 3 ) 3 X R = H Me OMe Cl F Ph i Ph Ph i Ph Ph i Ph

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82 C 4 H 9 Bu 3 P Pt PBu 3 n PBu 3 C 6 H 13 C 6 H 13 PBu 3 O PBu 3 C C Pt PBu 3 n Pt PBu 3 n Pt PBu 3 n

83 Organometallic Photovoltaics

84 Metallopolyynes as ew Functional Materials for Photovoltaic and olar Cell Applications hν P Pt P e R O OCH 3 n Cathode (Al) Metallopolyyne:PCBM PEDOT:P Anode (ITO) Glass substrate - + W.-Y. Wong, Macromol.. Chem. Phys., 2008, 209,, 14.

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86 9 selected articles out of 1200

87 Light Conversion to Electricity Absorption of photon, leading to the formation of an exciton (bound electron-hole pair) Exciton diffusion to a region where exciton dissociation (charge separation) occurs Charge transport of electrons and holes within the layer to the respective electrodes The resulting photocurrent and photovoltage can charge a battery

88 Current tatus P Pt P R n Ph Ph MP1 MP2 MP3 MP4 MP5 MP6 C 9 H 19 m C 9 H 19 m m = 0 MP7 m = 1 MP8 m = 2 MP9 m = 3 MP10 C 8 H 17 C 8 H 17 m m = 0 MP11 m = 1 MP12 m = 2 MP13 m=3 MP14 m R R R = H MP15 R = C 6 H 13 MP16

89 Physical data and photovoltaic performance of MP6-MP16 /PCBM (1:4) best devices Polyme r E g [ev] a) E HOMO [ev] E LUMO [ev] V oc [V] J sc [ma cm 2 ] FF PCE [%] MP6 MP7 MP8 MP9 MP10 MP11 MP12 MP13 MP14 MP15 MP (0.90) b) 0.88 (0.88) b) 0.77 b) 0.95 b) 0.95 b) 0.89 b) 0.55 (0.55) b) 0.51 (0.51) b) (6.39) b) 6.50 (6.01) b) 1.21 b) 2.73 b) 4.39 b) 7.56 b) 2.10 (2.66) b) 2.03 (3.23) b) (0.46) b) 0.44 (0.43) b) 0.38 b) 0.59 b) 0.60 b) 0.43 b) 0.34 (0.30) b) 0.36 (0.39) b) (2.66) b) 2.50 (2.30) b) 0.36 b) 1.54 b) 2.47 b) 2.88 b) 0.39 (0.43) b) 0.36 (0.63) b) a) Optical bandgaps determined from the onset of absorption in solid state. b) At Pt polyyne/pcbm (1:5, w/w) ratio.

90 Comparison of the optical data of various low-bandgap platinum(ii)-based metallopolyynes with different spacer Ar. PBu 3 Ar Pt PBu 3 n Ar = Color Maximum λ max in solid film / nm E g / ev Yellow Orange-red Purple Deep blue R C C R Deep green Deep bluegreen 681 (R = H) 714 (R = C 6 H 13 ) (R = H) 1.47 (R = C 6 H 13 ) 1.58

91 PBu 3 Pt PBu 3 n W.-Y. Wong et al., ature Materials, 2007,, 6, 521.

92 Donor-Acceptor Concept PBu 3 Dark purple 1.85 ev Pt PBu 3 n PBu 3 Pt PBu 3 n Yellow 2.55 ev PBu 3 Pt PBu 3 n Orange yellow 2.20 ev

93 Absorption and Photophysics Absorbance (a.u.) 1.0 Ligand MP Wavelength (nm) trong absorption in the visible region

94 Photovoltaic Behavior MP6 P3HT J (ma cm 2 ) Voltage (V) EQE (%) MP6:PCBM P3HT:PCBM Absorbance (a.u.) Wavelength (nm) Wavelength (nm)

95 Photovoltaic Behavior and Morphology 5 0 J (ma cm -2 ) MP6:PCBM 1:4 MP6:PCBM 2:3 MP6:PCBM 1: Voltage (V) Phase separation occurs for films with a 1:4 blend ratio, whereas films with a 1:1 ratio are smooth.

96 Factors Influencing the anometer-scale Morphology of Polymer Blends The solvent (affecting the drying time during film formation) The relative blend ratio between polymer and electron acceptor The polymer solution concentration The primary chemical and electronic structures of the metallopolyynes

97 Hole and electron mobilities in MP6:PCBM blends b1e-3 c 1E-3 μ e (cm 2 V 1 s 1 ) 1E-4 1E-5 μ h (cm 2 V 1 s 1 ) 1E-4 1E-5 1E E 1/2 1E E 1/2

98 Effects of Metalation Extending the absorption to longer wavelengths without the need of triplet excitons Involvement of triplet excitons in the highefficiency energy conversion Enhancing conductivity and mobility? Affecting the structural features and packing arrangement?

99 Tuning of Polymer olar Cell Efficiency Br C 9 H 19 C 9 H 19 m m Br TM C 9 H 19 C 9 H 19 m m TM m = 0 3 m = 0 3 C 9 H 19 C 9 H 19 m m = 0 MP7 m = 1 MP8 m = 2 MP9 m = 3 MP10 m PBu 3 Pt PBu 3 n H C 9 H 19 C 9 H 19 m m = 0 3 m H

100 Absorption and Photophysics Absorbance (a.u.) Wavelength (nm) MP7 MP8 MP9 MP10 W.-Y. Wong et al., J. Am. Chem. oc., 2007, 129,,

101 Photovoltaic Behavior 0 Current Density (ma cm 2 ) Voltage (V) MP7 MP8 MP9 MP10 J-V curves of solar cells with MP7-MP10:PCBM (1:4) active layers under simulated AM1.5 solar irradiation

102 Effect of blend composition on solar cell performance MP9:PCBM 0 Current Density (ma cm 2 ) :1 2:3 1:4 1:5 MP Voltage (V) Current Density (ma cm 2 ) MP10:PCBM 1:1 2:3 1:4 1:5 MP Voltage (V) Current Density (ma cm 2 ) P3HT:PCBM 2:3 1:4 1:5 P3HT Voltage (V)

103 Hole and electron mobilities in MP7-MP10:PCBM blends 1E-3 MP7:PCBM MP8:PCBM MP9:PCBM MP10:PCBM 1E-3 MP7:PCBM MP8:PCBM MP9:PCBM MP10:PCBM μ h (cm 2 V 1 s 1 ) 1E-4 μ e (cm 2 V 1 s 1 ) 1E-4 1E-5 1E-5 a) E 1/2 (V 1/2 cm 1/2 ) b) E 1/2 (V 1/2 cm 1/2 )

104 Effect of Oligothienyl Chain Length on Tuning the olar Cell Performance in Fluorene-Based Polyplatinynes PBu 3 C 8 H 17 C 8 H 17 Pt PBu 3 m m n m = 0 MP11 m = 1 MP12 m = 2 MP13 m =3 MP14 Absorbance (a.u.) MP11 MP12 MP13 MP Wavelength (nm) Absorption spectra in CH 2 Cl 2 at 293 K W.-Y. Wong et al., Adv. Funct.. Mater., 2008, 18,, 2824.

105 J-V curves of solar cells with MP11-MP14:PCBM (1:5) active layers under simulated AM1.5 solar irradiation 2 0 J (ma cm 2 ) MP11 MP12 MP13 MP V (V) The comparison between hole (μ h ) and electron (μ e ) mobilities in MP11-MP14: PCBM blends obtained by the CLC modeling μ h (cm 2 V 1 s 1 ) 1E-3 1E-4 MP11 MP12 MP13 MP14 μ e (cm 2 V 1 s 1 ) 1E-3 1E-4 MP11 MP12 MP13 MP14 1E E 1/2 (V 1/2 cm 1/2 ) 1E E 1/2 (V 1/2 cm 1/2 )

106 Polymer solar cells based on very narrow-bandgap polyplatinynes with photocurrent extended to the near-infrared (IR) region Ph PEt 3 Pt PEt 3 R R R = H R = C 6 H 13 PEt 3 Pt Ph PEt 3 R R R = H MP15 R = C 6 H 13 MP16 PBu 3 Pt PBu 3 n Absorbance (a.u.) Ligand MP15 film MP15 Absorbance (a.u) Ligand MP16 film MP Wavelength (nm) UV-Vis absorption spectra Wavelength (nm)

107 MP3 MP5 MP16 MP3 E g = 2.55 ev MP5 E g = 1.77 ev MP16 E g = 1.47 ev

108 Absorbance (a.u.) MP6 MP7 MP8 MP9 MP Wavelength (nm) Irradiance (W m 2 nm 1 ) Organometallic photovoltaics shed new light on the road to commerical polymer solar cells to solve the energy crisis.

109 Conclusions and Prospects To become more widely used, the production cost of modules must be reduced. Due to relatively large investment costs, government incentives are important. The biggest users of solar energy: U, Europe, Japan. Developing technologies for thin film and organic solar cells may enable production of inexpensive and efficient modules. This area sheds light on the road to commerical polymer solar cells to solve the energy crisis. Projected target is efficiency (>5%), lifetime (>3 years) and cost (<U$0.5/Wp). Currently, there is no commercial OPV yet.

110 Challenges in Organometallic Research Great Opportunity for OLEDs and olar Cells Organometallic molecules have become a field of intense activities in the optoelectronic research. They hold great promise as versatile functional materials for use in energy interconversions. PBu 3 Pt PBu 3 HOOC HOOC COOH Ru C C COOH Ir n

111 WOLEDs for Lighting Alternately, new and environmentally more friendly light sources are being introduced. The development of these light sources required new materials, devices and fabrication conditions. These new light sources allow us to design new types of lamp that can revolutionize illumination.

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