C 60 :LiF hole blocking layer for Bulk-Heterojunction solar cells

Transcription

1 C 60 :LiF hole blocking layer for Bulk-Heterojunction solar cells By Dong Gao A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Materials Science and Engineering University of Toronto Copyright by Dong Gao (2010)

2 C 60 :LiF hole blocking buffer layer for Bulk-Heterojunction solar cells By Dong Gao Master of Applied Science Graduate Department of Materials Science and Engineering University of Toronto 2010 Abstract A standard procedure for P3HT:PCBM bulk-heterojunction solar cells has been developed. Fabrication conditions, such as environment; solution concentration, thickness of active layer or post-treatment methods are systematically optimized. The best device performance is obtained by slow-drying spin-coated P3HT:PCBM (1:0.8) blend layer with DCB as solvent. C 60 :LiF composite films with up to 80% LiF concentration as hole blocking layer have been developed to significantly increase power conversion efficiencies of OPV devices. The short-circuit current density is greatly enhanced, without sacrificing open-circuit voltage and fill factor. Due to its superior oxygen diffusion blocking effect, the C 60 :LiF composite layer also can provide a more effective passivation film than a thin LiF layer, resulting in an impressive enhancement in air stability of devices. ii

3 Acknowledgements Firstly, I would like to express my sincere thanks to my supervisor Zheng-Hong Lu from Department of Materials Science and Engineering, University of Toronto, for his kindly guidance and encouragement throughout this project. I would like to thank supports from the members of Lu Group, Zhibin Wang, Michael Helander, Mark Greiner, Jacky Qiu, Stella Tang, Danny Puzzo, Jeffrey Castrucci, Yilu Chang, especially for the excellent XPS and UPS analysis by Michael and Mark. I am also very grateful to Dr. Dan Grozea and Dr. Sai-Wing Tsang. Thanks to all my friends for their mental supports. Finally, I d like to give my thanks to my parents, without them I can not finish this work. iii

4 Contents List of Tables... vii List of Figures... viii Abbreviations and symbols... xi Chapter 1 Introduction... 1 Chapter 2 Fundamentals of polymer-fullerene solar cells Charge generation in active layer Characteristics of OPV devices Power conversion efficiency (PCE, η) Short circuit current density J sc Open circuit voltage (V oc ) Electrodes and buffer layers Low work function metal as cathode ITO as anode Other possible advantages of buffer layer Chapter 3 Experimental methods and standard device Materials Donor and acceptor for active layer...11 iv

5 3.1.2 Buffer layer materials Sample preparation Substrate treatment Method of deposition of active layer: spin-coating Optimization to active layer Method of deposition of buffer layer and cathode: thermal evaporation Structure for standard device Characterization Chapter 4 C 60 :LiF nano composite layer as hole blocking layer Introduction I-V characteristics Enhancement on air stability Conclusion Chapter 5 Summary and future work References Appendix A Recalibration to testing system Appendix B Nickel oxide nano particles as hole buffer layer B.1 Introduction B.2 Material: NiO nano particles B.3 SEM pictures B.4 Oxidation of Ni particles v

6 B.5 Performance of OPV devices B.6 Conclusion Appendix C Support data vi

7 List of Tables Table 3.1 Summary of major materials used as buffer layer in OPV devices Table 4.1 Device characteristics of solar cells with different thickness (d) LiF:C 60 composite layer and different LiF concentrations vii

8 List of Figures Figure 2.1 Schematic energy diagram of a bulk heterojuncition active layer under bias V... 4 Figure 2.2 Equivalent circuit diagram of solar cell... 5 Figure2.3 Schematic energy diagrams for bulk heterojuncition active layers in (a) short circuit condition or (b) open circuit condition Figure 2.4 Schematic energy diagram for metal/semiconductor contact after interfacial dipoles are formed Figure 3.1 Chemical structures of (a) PCBM and (b) P3HT Figure 3.2 A short P3HT fragment with a non head-to-tail defect Figure 3.3 C 60 molecule Figure 3.4 (a) Schematic energy diagram for metal/semiconductor contact with interfacial dipoles formed with LiF; (b) the orderly arrangement of LiF Figure 3.5 Schematic illustration of the optimizing process of standard OSC devices Firgure 3.6 I-V characteristics of solar cells with active layer prepared in air or in nitrogen-filled glove box Figure 3.7 I-V characteristics of solar cells with P3HT:PCBM active layer thickness of 70nm and 150nm viii

9 Figure 3.8 I-V characteristics of solar cells with P3HT:PCBM active layer prepared by different methods: slow drying, fast drying or thermo annealing Figure 3.9 Kurt J. Lesker LUMINOS OLED cluster tool Figure 3.10 I-V characteristics of solar cells with different thickness of MoO Figure 3.11 Schematic structure of the standard P3HT:PCBM OSC device Figure 3.12 Solar illumination and substrate holder Figure 4.1 I-V characteristics of solar cells with 30 nm C 60 :LiF (75 wt.%) composite compared to a reference device with 1 nm LiF Figure 4.2 (a) UPS spectra of P3HT:PCBM blend film with and without a 3nm thick layer of C 60 :LiF composite, (b) schematic device structure showing P3HT-rich region near the top of the active layer, and (c) schematic energy-level diagram based on UPS measurements showing the hole-blocking effect of the C 60 :LiF composite layer Figure 4.3 Power conversion efficiencies and open circuit voltages of solar cells with different thickness C 60 :LiF (83 wt.%) composite layers. The lines are guides for the eye Figure 4.4 Decline in device performance as a function of time for devices with 30 nm thick C 60 :LiF (83 wt.%) composite layer compared to a reference device with 1 nm thick LiF under different storage conditions: (a) un-encapsulated and stored in dark, (b) un-encapsulated and stored under constant illumination, and (c) encapsulated with SiO and stored under constant illumination ix

10 Figure B1 SEM pictures for NiO particles separated on ITO substrates. Ni particles are spin-coated under 3000 rpm Figure B2 Ni 2p core level spectra for NiO x particles on ITO substrates. Ni particles is spin-coated uner 1000 rpm for device(a) and (b) and 3000 rpm for device (c), device (b) is annealed at 140 for 1 hour Figure B3 I-V characteristics of solar cells with NiO particle without annealing and with annealing Figure S1 Adsorption spectra of P3HT:PCBM solar cells with 1 nm thick LiF and 30 nm thick C 60 :LiF composite layers Figure S2 O 1s and S 2p core level spectra from PCBM and P3HT, respectively, used to determine the PCBM:P3HT ratio in the blend film Figure S3 O 1s and S 2p core level from PCBM in pristine PCBM and PCBM:P3HT films, used to determine the energy-levels of PCBM in the blend film Figure S4 Normalized PCE as a function of time of glass gap packaged C 60 :LiF device x

11 Abbreviations and symbols c CB DCB e EQE ETL FF FRET h HOMO HTL IPCE IQE J Speed of light in vacuum Chlorobenzene 1, 2-Dichlorobenzene Electron charge External quantum efficiency Electron transport layer Fill factor Förster resonance energy transfer Plank s constant Highest occupied molecular orbital Hole transport layer Incident photon to current efficiency Internal quantum efficiency Current density J 0 Saturation (dark) current J light Photocurrent J sc Short circuit current density xi

12 k B Boltzmann constant LUMO MW n OPV OSC PCBM PCE P3HT Lowest unoccupied molecular orbital Molecular weight Dipole ideality factor Organic photovoltaic Organic solar cell [6,6]-phenyl-C61-butyric acid methyl ester Power conversion efficiency Poly(3-hexylthiophene) P in Incident light intensity. R P Parallel resistance rr Regioregular R S Serial resistance T V Temperature External applied voltage V oc Open circuit voltage λ 1 and λ 2 Limits of the active spectrum of the active layer xii

13 Chapter 1 Introduction Chapter 1 Introduction As an attractive new technology, organic solar cells (OSCs) have received rapid development in the past few years.[1-3] Compared with traditional inorganic solar cells, most of which are built on Si or GaAs, organic semiconductors have the potential advantage of low cost fabrication using spin-coating or roll-to-roll printing.[4] Over 7% power conversion efficiencies (PCEs) have been achieved based on bulk-heterojunctions structure.[3, 5, 6] Modification on the electrode/organic interface is an important method to improve OSC performance.[7-10] Buffer layers inserted at the interface of electrode and organic active layer have been demonstrated for enhancing electrode organic interfacial physical and electrical contact, resulting in increasing open circuit voltage, blocking of unwanted carriers, enhancing air stability, and finally boosting power conversion efficiency. In this study, we build up a standard procedure for OSC fabrication and then investigate the possibility of C 60 :LiF composite as hole blocking buffer layers. 1

14 Chapter 1 Introduction This thesis is organized as follows: Chapter 2 provides a brief overview of polymer organic solar cells, and explanation of the role of buffer layer at the electrode/organic interface. Chapter 3 describes materials and experimental methods involved in this study. Process optimization for standard OSC devices will also be included in this chapter. Chapter 4 present results for applying C 60 :LiF composite as hole blocking buffer layer. The last chapter provides a summary and future work. 2

15 Chapter 2 Fundamentals of polymer-fullerene solar cells Chapter 2 Fundamentals of polymer-fullerene solar cells 2.1 Charge generation in active layer The commonly accepted mechanism[11] of charge generation in polymer active layer is shown in Figure 2.1. Firstly, a conjugated polymer absorbs incident photons to generate an electron-hole pair otherwise known as excitons in a donor polymer. Excitons diffuse to the interface of donor and acceptor, where electrons transfer from the LUMO level of donor to the LUMO level of acceptor. This step is also called the dissociation of exciton or generation of charge. Subsequently, driven by an internal electric field, electrons or holes as charge carriers are transported toward the cathode or the anode, where they are extracted, respectively. Another possible mechanism involves a Förster resonance energy transfer (FRET) from the donor to the acceptor. In this mechanism excitons are generated in acceptor instead of donor, followed by exciton dissociation via hole transfer to donor. [12, 13] 3

16 Chapter 2 Fundamentals of polymer-fullerene solar cells Figure 2.1 Schematic energy diagram of a bulk heterojuncition active layer under bias V. Red arrows indicate the transfer directions of electrons and blue arrows are transfer directions of holes. By blending conjugated polymers with high-electron affinity molecules, for instance fullerenes, charge generation can be dramatically improved. This structure is normally called bulk-heterojunction structure. Photon-induced charge transfer from the conjugated polymer to fullerene is suggested as an ultrafast process (<100fs).[14, 15] In bulk-heterojunction structures, the enlarged donor-acceptor interface effectively reduces 4

17 Chapter 2 Fundamentals of polymer-fullerene solar cells excitons quenching. 2.2 Characteristics of OPV devices According to a standard equivalent circuit of solar cell devices (figure 2.2) [16, 17], the current density-voltage characteristics can be described by V JR S V JRS J = J0 exp e 1 + Jlight nkbt RP, where J is the current density, J 0 is the saturation (dark) current, e is the electron charge, V is the external applied voltage, n is the dipole ideality factor, k B is the Boltzmann constant, T is temperature, R S and R P are the serial and parallel resistance, and J light is the photocurrent. Figure 2.2 Equivalent circuit diagram of solar cell [16, 17] Power conversion efficiency (PCE, η) Power conversion efficiency, η, is given by 5

18 Chapter 2 Fundamentals of polymer-fullerene solar cells P J V η = = FF P P out sc oc in in J sc is the short circuit current density, V oc is the open circuit voltage, FF is the fill factor, P in is the incident light intensity Short circuit current density J sc J sc is the current density under short circuit conditions, seen in Figure 2.3-a. It can be expressed by:[18] λ 2 e Jsc = Pin ( ) EQE( ) d hc λ λ λ λ λ1, where h is Plank s constant, c is the speed of light in vacuum, and λ 1 and λ 2 are the limits of the active spectrum of the active layer. Figure2.3 Schematic energy diagrams for bulk heterojuncition active layers in (a) short circuit condition or (b) open circuit condition 6

19 Chapter 2 Fundamentals of polymer-fullerene solar cells The external quantum efficiency (EQE, or IPCE: incident photon to current efficiency) is defined by the ratio of collected charges (electrons or holes) to the number of incident photons at a single wavelength. Two reasons can lower EQE: incomplete incident photon absorption of active layer, and exciton quenching before dissociation. Base on the calculation of Dennler et al., Assuming the optical gap of polymer-fullerene composite is determined by the band gap of conjugated polymer, under Air Mass 1.5G illumination (see note in Appendix A), P3HT:PCBM blend can absorb up to 27% of the available photons and 44.3% of the available power at best.[18] Photon absorption can be increased by increasing the thickness of the active layer, but meanwhile decreasing the fill factor due to the larger serial resistance. Device thickness will also affect light interference when incident light is reflected by the reflective metal electrode, and thus changes optical distributions within device and photon absorption. Exciton quenching can be evaluated by internal quantum efficiency (IQE), which is the ratio of the collected charges to the number of photons absorbed by devices. Because the diffusion length of exciton in organic material is very short, typically just about 5~10 nm,[1, 19] the generation of excitons has to take place nearby the Donor-Acceptor interface to prevent exciton quenching before harvesting. For P3HT:PCBM active layer, the yield of excitons is strongly depending on phase separated morphology with crystalline P3HT and PCBM domains, which can be controlled by varying deposition procedures. [20] 7

20 Chapter 2 Fundamentals of polymer-fullerene solar cells Open circuit voltage (V oc ) V oc is the voltage under open circuit conditions, see Figure 2.3-b. For Ohmic contacted electrodes, because of the formation of Fermi level pinning, V oc is determined by the energy difference between the HOMO level of donor and the LUMO level of acceptor: [21, 22] 1 Voc = ( EHOMO donor ELUMO acceptor ) + C e, where C is an empirical constant that concerned to the dark I-V curve of the diode. 2.3 Electrodes and buffer layers Low work function metal as cathode Most of OPV devices employed low work function metals like Ca, Mg or Al as cathode. Al is the most common cathode material, because it is relatively stable in air compared with other low work function metals. V oc is not dependent on the work function of cathode, which supports Fermi level pinning being formed between cathode and active layer. Since electrons in organic molecule are highly localized, Fermi level pinning is most likely caused by forming an interfacial dipole,[23] which is formed at the metal-organic semiconductor interface (see figure 2.4) when the cathode contacts with the 8

21 Chapter 2 Fundamentals of polymer-fullerene solar cells active layer and an equilibrium condition is achieved. Formation of dipole lowers the energy barrier between the Fermi level of metal and LUMO level of acceptor, thereby helping electron transfer. Interfacial dipole also can be introduced by inserting a thin layer of LiF,[24] which will be discussed in next chapter. Figure 2.4 Schematic energy diagram for metal/semiconductor contact after interfacial dipole is formed ITO as anode Indium tin oxide (ITO) is one of the major electrode materials because of its high transparency and high conductivity. Because of its mid-range Fermi level, ITO can work as either cathode or anode when modified by different buffer layers.[25, 26] For the anode case, employing a polymer or metal oxide hole transport buffer layer (HTL) between ITO and active layer will greatly improve device performance. Buffer layers can not only reduce hole transport barrier between ITO and donor, but also block electron 9

22 Chapter 2 Fundamentals of polymer-fullerene solar cells diffusion to ITO then recombine with hole Other possible advantages of buffer layer Besides those advantages mentioned above, buffer layers also be used to modify surface morphology of substrate; prevent interlayer reaction; improve device stability; and help optimizing optical distribution. 10

23 Chapter 3 Experimental methods and standard device Chapter 3 Experimental methods and standard device 3.1 Materials Donor and acceptor for active layer [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM, structure see Figure 3.1-a) is purchased from American Dye Sources Inc.. PCBM is a soluble fullerene derivative and has been widely used as acceptor for OPV devices. Because the electron mobility of organic materials is normally several orders of magnitude lower than the hole mobility, fullerene and its soluble derivatives are considered as an ideal acceptor for organic solar cell base on their high electron mobility. In order to increase solubility of C 60 in most organic solvents, chemical modification is necessary. 11

24 Chapter 3 Experimental methods and standard device Figure 3.1 Chemical structures of (a) [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) and (b) poly(3-hexylthiophene) (P3HT) High-regioregular (90-95%) poly(3-hexylthiophene) (P3HT, structure see Figure 3.1-b) with average molecular weight (MW) 50,000 is purchased from Rieke Metals, Inc.. The degree of regioregularity is defined as the percentage of monomers linked between the 2- and the 5-position or head-to-tail coupled monomers (Figure 3.2). Regioregularity is an important variable that affects polymer crystallinity and blend layer morphology.[27] A higher regioregularity P3HT (approximately 98%, also purchased from Rieke Metals, Inc.) was not used due to its poor wetting property on ITO substrates. 12

25 Chapter 3 Experimental methods and standard device S S S S Figure 3.2 A short P3HT fragment with a non head-to-tail defect (indicated in red part) Buffer layer materials A number of materials have been investigated as buffer layer materials for OPVs (see Table 3.1). In this thesis we will investigate using C 60 :LiF nanocomposite as the buffer layer, whose property, as well as the individual properties of C 60 and LiF, will be introduced. Table 3.1 Summary of major materials used as buffer layer in OPV devices P-Type (For hole transport) PEDOT:PSS N-Type (For electron transport) LiF MoO 3 Cs 2 CO 3 V 2 O 5 WO 3 TiO x ZnO 13

26 Chapter 3 Experimental methods and standard device NiO C 60 C 60 : C 60 is the most well-known fullerene molecule. Its unique structure (See Figure 3.3) is assembled by 60 carbon atoms building a highly symmetric cage with 20 hexagons faces and 12 pentagons faces. The LUMO level of C 60 is 3.7 ev while its HOMO level is 6.1 ev. The triply degenerate LUMO level allows the additions of up to six electrons. C 60 shows a high electron mobility of up to 1 cm 2 /Vs as well as high thermo stability.[28] The application of C 60 as electron transport material for OLED has been reported.[29] However, C 60, just like any other organic semiconductors, is sensitive to oxygen and moisture, and due to its poor contact with Al cathode, C 60 cannot be applied as the buffer layer directly. Figure 3.3 C 60 molecule 14

27 Chapter 3 Experimental methods and standard device LiF: LiF/Al bilayer electrode was reported as early as 1997.[30] By inserting a thin layer of LiF, electron injection barrier is significantly reduced. Several mechanisms have been suggested to explain how LiF works.[24, 30-33] The formation of a dipole layer at cathode surface is believed as the critical one. Because LiF has a very strong dipole moment, even a very thin layer, normally just 0.5~1 nm, can form an effective interfacial dipole between aluminum and organic semiconductor, thereby reducing the barrier between Fermi level of metal and LUMO of organic semiconductor (see Figure 3.4). For polymer solar cells, LiF has been proved to enhance both the fill factor and the open circuit voltage.[10] LiF also can help improve the formation of an ohmic contact with C 60.[34]. (a) E vac (b) F - -Li + E F LUMO F - -Li + HOMO F - -Li + LiF Figure 3.4 (a) Schematic energy diagram for metal/semiconductor contact with interfacial dipoles formed with LiF; [24] (b) the orderly arrangement of LiF 15

28 Chapter 3 Experimental methods and standard device Furthermore, LiF is an excellent passivation barrier against oxygen.[35] Grozea et al. have demonstrated a 20 nm LiF layer will effectively block oxygen diffusion from the exterior surface of Al cathode into the organic film.[36] But due to its poor conductivity, LiF with such thickness will result in OPV device insulating. C 60 :LiF composite: C 60 :LiF nanocomposite was deposited by co-evaporating C 60 and LiF molecules. Zhao et al. demonstrated that C 60 :LiF composite is extremely conductive even when the concentration of LiF is up to 75wt%.[37] The C 60 :LiF nanocomposite thin film has a high transparency, high thermal conductivity, high environmental stability, and high suitability as an ohmic contact with Al electrode. 3.2 Sample preparation Substrate treatment Commercial ITO coated glass substrates with a sheet resistance of 15Ω/ are purchased from Colorado Concept Coatings Inc. Substrates were first cut into 1inch 1inch pieces so they can fit in the sample holder. Ultrasonicated in Alconox, DI water, acetone and methanol are subsequently applied to clean the substrate. Then substrates are UV-Ozone treated for 11min before the deposition of other layers. 16

29 Chapter 3 Experimental methods and standard device Method of deposition of active layer: spin-coating Spin-coating technique is one of the major methods to cast a thin film of soluble material. Compare with thermo-evaporation system, spin-coating has several advantages, including high speed, low cost, ease of operation, and suitability to thermall unstable materials. However, is it limited by the solubility of materials, and most of solution will spin out of the substrate as waste materials. A typical spin-coating process includes two stages: a short and low spin-speed stage to obtain uniform solution layer on substrate, followed by a high spin-speed stage to achieve expected thickness. Factors that can influent the thickness of the solution layer are complicated. Generally speaking, thickness will exponentially decrease with rinsing spin speed Optimization to active layer As mentioned in the last chapter, device performance is related to the morphology of the blend layer, which is varying with different deposition conditions. A number of fabrication procedures have been developed to control the packing of the molecules and the formation of domain of different compositions, for instance, solvent choice,[38] drying time,[39, 40] vapor annealing[41] or thermal annealing.[42] Meanwhile, the thickness of the active layer also needs to be carefully controlled to balance incident 17

30 Chapter 3 Experimental methods and standard device photon absorption and device serial resistance. Due to the difference of equipments and other variables, systematically understanding and optimizing fabrication procedures are important for obtaining stable, repeatable and high performance standard OSC devices. A simplified schematic for process optimization in standard OSC devices is described in Figure 3.5. Figure 3.5 Schematic illustration of the optimizing process of standard OSC devices 18

31 Chapter 3 Experimental methods and standard device Solvent: Firstly 1,2-dichlorobenzene was chosen as solvent to prepare P3HT:PCBM blend solution. Common Solvents for P3HT:PCBM blend solution include chloroform, chlorobenzene (CB) and 1,2-dichlorobenzene (DCB). Chloroform is not a good choice for spin-coating because of its low boiling point and high volatility.[43] Compared with CB, DCB has a higher boiling point (181, compare with 131 for CB), which means DCB is evaporated slower than CB. Hence the drying time for the polymer layer prepared from DCB is longer than the layer casted by CB solution, which will help PCBM domains growing. Fabrication environment: Firgure 3.6 shows I-V characteristics for devices fabricated in air or in nitrogen. Compared with device with active layer prepared in air, device with active layer prepared in nitrogen-filled glove box has higher J sc and FF. This result demonstrates oxygen and moisture in air will greatly reduce the activity of polymer-fullerene blend. To minimize the influence from oxygen and moisture and prevent declination in polymer activity, whole active layer fabrication procedure for the following devices is operated in a nitrogen-filled glove box. 19

32 Chapter 3 Experimental methods and standard device 10 Current density (ma/cm 2 ) in air in nitrogen Voltage (V) Figure 3.6 I-V characteristics of solar cells with active layer prepared in air (black square) or in nitrogen-filled glove box (red triangle). The structures of devices are ITO/PEDOT:PSS (not optimized)/p3ht:pcbm (1:1 wt; 70 nm)/lif (1nm)/Al (100nm). Thickness of active layer: The thickness of P3HT:PCBM active layer is targeted at 120~150 nm. Figure 3.7 shows I-V characteristics of devices with the thickness of polymer layer is 70 nm or 150 nm, obtained by using 1000 rpm or 500 rpm spin-speed, respectively. To maintain enough thickness for sufficient photon absorption, corresponding spin-speed is required when solution concentration is changed. For slow-drying process, devices fabricated by a more dilute polymer blend solution have the better performances, indicating low concentration 20

33 Chapter 3 Experimental methods and standard device is in favor of the growth of PCBM domains. Current density(ma/cm 2 ) nm 150nm Voltage(V) Figure 3.7 I-V characteristics of solar cells with P3HT:PCBM active layer thickness of 70nm (black square) and 150nm (blue triangle). The structures of devices are ITO/P3HT:PCBM (1:0.8 wt; x nm)/lif (1 nm)/al (100 nm). Slow drying or thermo-annealing: Similar to slow drying process, thermo-annealing is reported that can also help domains growing. A typically annealing temperature is around 90 to 150 from references. After comparing devices fabricated by different methods, the device with the active layer 21

34 Chapter 3 Experimental methods and standard device casted through slow-drying P3HT:PCBM solution has the best performance (see Figure 3.8). Post-annealing is also expected to enhance the contact between the active layer and the Al cathode. However it is not suitable for LiF/Al bilayer cathode because the arranged LiF dipole may be disturbed. Current density(ma/cm 2 ) Slow drying Fast drying Thermo Annealing Voltage(V) Figure 3.8 I-V characteristics of solar cells with P3HT:PCBM active layer prepared by different methods: slow drying (black square), fast drying (green triangle) or thermo annealing at 140 (red circle). The structures of devices are ITO/MoO 3 (1 nm)/p3ht:pcbm (1:1 wt; 150 nm)/lif (1 nm)/al (100 nm). 22

35 Chapter 3 Experimental methods and standard device The final fabrication procedure of P3HT:PCBM active layer is as shown below: P3HT:PCBM (17 mg/ml : 13.6 mg/ml) in 1,2-dichlorobenzene (DCB) solution is prepared in nitrogen-filled grove box and stirred on a hot plate at 50 over night, to be sure that all materials are completely dissolved and mixed. Blend solution is subsequently spin-coated on treated substrates at 500 rpm for 45 s. After spin-coating, substrates are placed in closed glass Petri-dishes and naturally dried in dark over 24 hours Method of deposition of buffer layer and cathode: thermal evaporation All thermo-evaporated procedures, including deposition of oxide hole transport layer, electron transport layer and metal cathode, are finished in Kurt J. Lesker LUMINOS OLED cluster tool (Figure 3.9). Kurt J. Lesker LUMINOS OLED cluster tool has multiple deposition chambers around a central distribution chamber. Pressure of central distribution chamber is maintained at about 10-8 torr. The multiple heaters in the organic chamber enable a co-evaporated operation, which is LiF and C 60 co-evaporation in this study. LiF or C 60 :LiF composite layer are deposited in organic chamber at rate of 0.1~0.4 Å/sec. Metallization chamber carries on deposition of Al cathode at rate about 1 Å/sec and MoO 3 hole transport layer at rate of 0.1~0.2 Å/sec. The optimization of MoO 3 thickness is shown in Figure Shadow mask with different patterns was loaded under the sample holder so only designed area is exposed to evaporate sources. By changing the shadow mask, different device structures can be obtained. 23

36 Chapter 3 Experimental methods and standard device Figure 3.9 Kurt J. Lesker LUMINOS OLED cluster tool 24

37 Chapter 3 Experimental methods and standard device Current denstiy (ma/cm 2 ) MoO 3 1nm MoO 3 3nm MoO 3 5nm MoO 3 10nm Voltage (V) Figure 3.10 I-V characteristics of solar cells with different thickness of MoO 3. The structures of devices are ITO/ MoO 3 (x nm)/p3ht:pcbm (1:0.8 wt; 150 nm)/lif (1 nm)/al (100 nm) Structure for standard device The standard device structure is as follows: ITO/ MoO 3 (1 nm)/p3ht:pcbm (1:0.8 wt; 150 nm)/lif (1 nm)/al (100 nm), showed in Figure

38 Chapter 3 Experimental methods and standard device Figure 3.11 Schematic structure of the standard P3HT:PCBM OSC device. 3.3 Characterization I-V characteristics of solar cell devices were measured by using a Keithley 6430 multimeter under 100mW/cm 2 (see Appendix A) simulated AM1.5G solar illumination (figure 3.12). 26

39 Chapter 3 Experimental methods and standard device Figure 3.12 Solar illumination and substrate holder 27

40 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer Chapter 4 C 60 :LiF nano composite layer as hole blocking layer 4.4 Introduction In this work, we demonstrate the use of C 60 :LiF composite as an hole block layer for OSC devices. OSCs fabricated using the C 60 :LiF composite exhibited improvement on PCE and an impressive enhancement in device stability. The excellent environmental stability and high conductivity make the C 60 :LiF nano-composite a versatile buffer layer to enable high performance OPVs with long lifetime. 4.2 I-V characteristics Figure 4.1 shows the I-V characteristics for devices using a 30 nm thick C 60 :LiF (75 wt.%) composite compared to a reference device with 1nm thick LiF interlayer. With the composite layer, the PCE increased to 3.59% with J sc = 9.83 ma/cm 2, V oc = 0.58 V and FF = 63%, while the reference device had a PCE of 2.63%, J sc = 7.58 ma/cm 2, V oc = 0.56 V and FF = 62%. It should not be surprising that device with C 60 :LiF have a similar V oc to devices with 1 nm LiF, due to the high concentration of LiF in the composite film. 28

41 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer Zhao et al. have reported C 60 :LiF composite is extremely conductive, even up to 75% LiF concentration.[37] Due to the high conductivity of the composite a much thicker film can be used to provide better protection to the P3HT:PCBM active layer during cathode evaporation. The significant improvement in J sc is more impressive. Recent research has showed that a vertical composition gradient exists in spin-coated P3HT:PCBM films after casting. Due to the surface energy difference between P3HT and PCBM molecules at the anode and air interfaces, the polymer layer is PCBM-rich near the anode and P3HT-rich near the interface with air (see Figure 4.2-b).[20] It was confirm by X-ray photoelectron spectroscopy (XPS) chemical composition analysis that the P3HT:PCBM weight ratio at the top of the film is 1.35:1, which is slightly higher than the bulk film value of 1.25:1. C 60 is a good electron transport material with similar properties to its derivative PCBM, but its poor solubility in most solvents limits its application in bulk-heterojunction OSCs. The UPS spectra (Figure 4.2-a) shows about 1.5 ev HOMO offset between P3HT in active layer and C 60 in composite, which is sufficient to block hole diffusing to cathode (Figure 4.2-c). 29

42 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer Current density(ma/cm 2 ) Voltage(V) Figure 4.1 I-V characteristics of solar cells with 30 nm C 60 :LiF (75wt.%) composite (solid square, open square for dark) compared to a reference device with 1 nm LiF (solid triangle, open triangle for dark). 30

43 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer (a) Intensity (a.u.) P3HT P3HT:PCBM PCBM C 60 :LiF Binding Energy (ev) (b) (c) Al P3HT C 60 :LiF + P3HT PCBM e - h + C 60 :LiF ITO Glass substrate MoO 3 PCBM Figure 4.2 (a) UPS spectra of P3HT:PCBM blend film with and without a 3nm thick layer of C 60 :LiF composite, (b) schematic device structure showing P3HT-rich region near the top of the active layer, and (c) schematic energy-level diagram based on UPS measurements showing the hole-blocking effect of the C 60 :LiF composite layer. Based on LiF s superior oxygen diffusion blocking effect, a high concentration of LiF in 31

44 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer the composite layer is targeted, so devices with LiF concentrate higher than 75 wt.% were fabricated. Table 4.1 shows the parameters of devices with 30 nm LiF (75 wt.%, 83 wt.%, and 94 wt.%):c 60 composite layer. Theory and experiment have confirmed that LiF is critical to forming an Ohmic contact between C 60 and Al.[44-46] To balance the need for a percolative path of C 60 molecules through the composite, with the need for sufficient LiF at the cathode interface to for an Ohmic contact, the concentration of LiF should therefore be kept around ~ 80%. Table 4.1 Device characteristics of solar cells with different thickness (d) LiF:C 60 composite layer and different LiF concentrations. LiF (%) d (nm) J sc (ma/cm 2 ) V oc (V) FF (%) PCE (%)

45 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer Under the same LiF:C 60 composition, improvement with respect to the reference structure was obtained within a wide thickness range, from 10 nm to 40 nm, with an optimal thickness of 30 nm (see Figure 4.2). However, when the composite thickness was greater than 50 nm, the advantages composite layer are traded off by a decline in V oc, and as a result, a lower PCE compared to the reference device. The decrease in V oc is most likely dependent on the surface morphology of the composite layer, during co-evaporation of LiF and C 60 to obtain composite films, the surface structure of the film will change with thickness. Zhao et al. has found that for C 60 :LiF (75 wt.%) composite films the LiF will tend to crystallize as the film thickness increases.[37] So when the composite film is too thick, the insulated crystallized LiF cannot form a good contact with the polymer active layer or the Al cathode, causing a voltage drop across the interface. 33

46 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer PCE(%) Voc (V) Thickness (nm) 0.50 Figure 4.3 Power conversion efficiencies and open circuit voltages of solar cells with different thickness C 60 :LiF (83 wt.%) composite layers. The lines are guides for the eye. 4.3 Enhancement on air stability Using the optimized LiF concentration and composite layer thickness we investigated the environmental stability of devices with the C 60 :LiF composite using accelerated lifetime testing of devices with various encapsulation schemes. Figure 4.4-a shows the normalized PCE as a function of time of devices stored in the dark and in ambient air. For devices with a 30 nm C 60 :LiF (83 wt.%) composite layer, about 70% of the initial PCE was retained even after 125 hours, while the reference device under the same environment 34

47 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer conditions,had a half-life of only ~ 24 hours, and was completely dead after 72 hours. This finding demonstrates that the superior oxygen diffusion blocking effect of LiF is preserved in the C 60 :LiF composite, thereby greatly improve device life time. Figure 4.4-b shows the normalized PCE as a function of time of devices stored under continuous illumination(~ 50 mw/cm 2 ). The reference device had a half-life of only ~ 12 hours, and was completely dead after 24 hours, which is significantly worse than the device stored in the dark. This finding suggests that photo-oxidation of the active-layer is the primary degradation mechanism in OPVs. Remarkably there is no significant change in the lifetime of the device with C 60 :LiF composite. These findings suggest that the C 60 :LiF composite layer helps to prevent photo-oxidation of the active layer by limiting the ingress of moisture and oxygen into the device. To further examine the roll of the C 60 :LiF composite in blocking moisture and oxygen diffusion into the active layer we tested devices with different packaging schemes. Figure 4.4-c shows the normalized PCE as a function of time of devices packaged using a 500 nm thick SiO layer. The SiO thin film encapsulation layer helps to slow the diffusion of oxygen and moisture into the device, but does not provide a perfect hermetic seal. Surprisingly the baseline device declines equally as quickly with the SiO encapsulation layer as the un-encapsulated device (see Fig. 4.4-b). This finding demonstrates that the P3HT:PCBM active layer is extremely sensitive to photo-oxidation. Even the minimum amount of moisture and oxygen that might have diffused through the SiO layer in the first few hours of operation still managed to rapidly degrade the device. On the other hand 35

48 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer there is no significant degradation in the C 60 :LiF device, demonstrating the superior moisture and oxygen blocking effect of the composite. The slight increase in PCE in the first few hours of testing the C 60 :LiF device is due to the thermal annealing effect and can only be observed in devices with low moisture and oxygen leakage.[47, 48] 1.0 Reference C 60 :LiF Normalized PCE 0.5 (a) (b) Time (hrs) ( ) (c) Figure 4.4 Decline in device performance as a function of time for devices with 30 nm thick C 60 :LiF (83 wt.%) composite layer compared to a reference device with 1 nm thick LiF under different storage conditions: (a) un-encapsulated and stored in dark, (b) un-encapsulated and stored under constant illumination, and (c) encapsulated with SiO and stored under constant illumination. The lines are a guide for the eye. 4.4 Conclusion In conclusion, we have demonstrated that the PCE of bulk heterojunction OPVs can be 36

49 Chapter 4 C 60 :LiF nano composite layer as hole blocking layer significantly enhanced by inserting a C 60 :LiF composite film as hole blocking layer between the polymer active layer and Al cathode. The superior oxygen diffusion blocking effect of LiF in the composite provides effective protection of the polymer active layer, thereby significantly extending device lifetime. 37

50 Chapter 5 Summary and future work Chapter 5 Summary and future work A standard P3HT:PCBM bulk-heterojunction solar cells fabrication procedure has been successfully developed and optimized. Fabrication process is found to be critical to control phase separated morphology with crystalline P3HT and PCBM domains. The best active layer condition is supposed to be slow-drying P3HT:PCBM (1:0.8) blend layer with DCB as solvent. On the other hand, the use of C 60 :LiF composite as a hole blocking layer has been developed. Devices using the C 60 :LiF composite exhibits a 37% increase in PCE and an impressive enhancement in device lifetime. Compared with a thin LiF interlayer, the C 60 :LiF composite layer can significantly enhance the short circuit current (J sc ) without impacting the V oc or FF. Furthermore, C 60 :LiF composite is demonstrated that can effectively block oxygen and moisture diffusion, thereby enhancing device environmental stability. It s accepted that buffer layer selection should be considered as an active and critical 38

51 Chapter 5 Summary and future work method to improve the efficiency of the photovoltaic device, especially when the ranges of options for transparent electrodes and acceptor materials are narrow. On the anode side, several materials including metal oxides like MoO 3, WO 3, V 2 O 5 or polymer like PEDOT:PSS, have been reported that can help hole transport. On the cathode side we also have various choices like ZnO or TiO x. However, the interfacial structures are still not clear. Meanwhile, buffer layers also play a critical role in the multijuction structure. Tandem structure, which stacks two subcells in series, has been demonstrated as an effective method to overcome the limitation of organic semiconductors, such as limited absorption and low charge carrier mobility. Several intermediate layers have been investigated for tandem organic solar cells, which exhibit nearly doubled V oc. But so far few works have been conducted on the charge transfer properties within intermediate layers. For future work, the working principal of buffer layer at polymer/electrode interface needs to be investigated, and the knowledge will provide a clear guide for choosing suitable buffer layer material, and moreover, building tandem devices to achieve higher efficiency. 39

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57 Appendix A Appendix A Recalibration to testing system In this thesis, all OSC devices were tested under simulated AM 1.5 G irradiation*. The illumination intensity (P in ) was calibrated to 100 mw/cm 2, which value we used to calculate PCE (η): η = FF JscV P in oc Calibration of light intensity is based on previous calibration curve of single-wavelength (628 nm) intensity - full-spectrum intensity. The light intensity at single wavelength is measured by Newport optical power meter every times before testing. However, very recently recalibration to the solar simulator shows the actual light intensity is about 30~40% lower than previous calibration result. Therefore the values of PCE should be at least 30% higher than shown. * Air Mass 1.5 Global (AM 1.5G) irradiation: The solar spectrum after solar radiation has traveled though 1.5 thicknesses of atmosphere to sea level, corresponding to a solar zenith angle of [Source: 45

58 Appendix B Appendix B Nickel oxide nano particles as hole buffer layer B.1 Introduction Several metal oxides have been demonstrated that can work as hole transport layer for OPV devices,[9] which is more stable compared with polymer hole transport buffer material like poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). However, the thermo deposition of metal oxide requires high vacuum system and is hard to apply on large area substrates. In this chapter, we attempt to obtain effective NiO buffer layer directly from Ni nanoparticles. B.2 Material: NiO nano particles Ni nano particles (Inco ) were spread onto ITO surface by spin-coated. Ni particle suspension is obtained by ultrasonic treating nano size Ni power in methanol 30 min. This suspension is stable after staying over night. Subsequently Ni suspension was spin-coated on cleaned ITO first at 500 rpm for 9 s, then at 1100 rpm or 3000 rpm for 20 s. After deposit Ni substrates were UV-ozone treated for 2 hours to oxides Ni. 46

59 Appendix B Thermo-annealing is taken place in oven at 140 for 1 hour. B.3 SEM pictures The morphology of Ni nano particles on ITO surface is shown in SEM pictures (see Figure B1). Ni particles were in irregular shapes with sizes varied from 30 nm to over 300 nm. Ni particles distribution doesn t have significantly changed under different spin-speed or after thermo annealing, and no significantly aggregation until the very edge area. Directly rinsing by methanol or blow by nitrogen gun can not clean off those particles. 47

60 48 Appendix B

61 Appendix B Figure B1 SEM pictures for NiO particles (white) separated on ITO substrates (dark). Ni particles are spin-coated under 3000 rpm. B.4 Oxidation of Ni particles Figure B2 shows the XPS study on Ni particles coated substrate. After 2 hours UV-Ozone treatment, Ni is oxidized in all samples. However there is a shoulder on the low binding energy side of the main Ni 2p peak, which indicates that there is still some metallic Ni in 49