Interface engineering for highly efficient polymer solar cells

    INTERFACE ENGINEERING FOR HIGHLY EFFICIENT POLYMER SOLAR CELLS         SUN KUAN  B.Appl.Sc.(Hons.), National University of Singapore         A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MATERIALS SCIENCE & ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 Declaration I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. Sun Kuan 31 July 2012 i ii Acknowledgements  One of the great lessons learnt from my Ph.D. candidature is this: Most of the great and exciting achievements in research are not so much stories of individual endeavor, but are stories of a unified, talented, aspiring team that learns from each other, assists each other and stays loyally committed to each other for a shared vision. I could not achieve what I have achieved without the support and encouragement from many people. So it is with deep gratitude that I express my appreciation to the following lovely individuals. My first and foremost praise and tribute goes to my supervisor, Prof. Ouyang Jianyong. I am very fortunate and proud to be one of his students. I will always remember the lessons he taught me, the questions he challenged me, the encouragement he gave to me, the way he trained me to find out a problem, understand it and solve it. And I will never forget the value and strength he has passed to me ------ “Stay hungry, stay foolish”, which is going to support me in my future endeavor. My appreciation also goes to my collaborators and group members: Prof. Zeng Kaiyang, Dr. Amit Kumar, Dr. David John Jones, Dr. Wallace Wong, Dr. Jegadesh Subbiah, Dr. Zhang Hongmei, Dr. Wu Zhonglian, Dr. Li Aiyuan, Dr. Zhou Dan, Dr. Zhao Baomin, Dr. Vajjiravel Murugesan, Dr. Anil Suri, Dr. Bhatia Ravi, Dr. Xia Yijie, Dr. Mei Xiaoguang, Mr. Fan Benhu, Ms. Cho Swee Jen, Mr. Neo Chin Yong and Ms. iii Zheng Huiqin. Their invaluable suggestions, persistent assistance and unfailing support are beneficial to my research. More importantly, the friendship and comradeship we have built throughout the years are going to last a life long. I am grateful to all the department staffs, including professors, teaching assistants, lab technologists and administrative officers. I really learnt and benefited a lot from them. They passed their knowledge unreservedly to me so that I could grow to become a better researcher. They also supported my research work in a number of ways. I owe a big gratitude to their hard work. I would like to give my recognition to the lab mates who have worked together under the same roof. The time we spent together in lecture theatres, tutorial rooms, libraries, laboratories and canteens is going to be a memorable chapter in my life. I want to express my thankfulness to National University of Singapore and Ministry of Education, Singapore for the generous financial support and scholarship. Last but not the least, I am indebted to my parents for their unconditional love and to my wife for her endless support and meticulous care. Sun Kuan December 2012 in Singapore iv v vi Table of Contents Declaration .i Acknowledgements iii Table of Contents .vii Summary xi List of Tables .xv List of Figures .xvii List of Abbreviations xxiii List of Publications xxv Chapter Introduction .1 1.1 A brief overview of polymer solar cells (PSCs) 1.1.1 Historical background of polymer solar cells .2 1.1.2 Device physics of bulk-heterojunction PSC .5 1.1.3 Important parameters in PSC characterization .10 1.2 Background of interface engineering in PSC 15 1.2.1 Roles of interfacial layer .15 1.2.2 Integer charge transfer model .18 1.2.3 Study the interface by photoemission spectroscopy .22 1.3 Objectives and outline of the thesis 24 Chapter Experimental .27 2.1 Materials .27 2.2 Experimental procedures 29 2.2.1 Fabrication of polymer solar cells .29 2.2.2 Surface modification of indium tin oxide .31 2.2.3 Treatment of PEDOT:PSS buffer layer .31 2.2.4 Chlorination of ITO 31 2.3 Characterization techniques 32 2.3.1 J-V curve measurement .32 2.3.2 Incident photon to current efficiency (IPCE) 32 2.3.3 Morphology characterization 33 vii 2.3.4 Optical spectroscopy .34 2.3.5 Photoemission spectroscopy .34 2.3.6 Film conductivity measurement 35 Chapter ITO modified with sodium compounds as cathode of inverted PSCs 37 3.1 Introduction .37 3.2 Results and discussion 39 3.2.1 Inverted PSCs with NaOH-treated ITO 39 3.2.2 Photovoltaic performance of inverted PSCs with sodium compoundtreated ITO cathodes 49 3.2.3 Mechanism for reduction of the work function of ITO by sodium compounds .51 3.3 Conclusions .59 Chapter ITO modified with solution-processed zwitterions as transparent cathode 61 4.1 Introduction .61 4.2 Results and discussion 62 4.2.1 Inverted PSCs with rhodamine-modified ITO as cathode 63 4.2.2 Inverted PSCs with ITO sheets modified by other zwitterions .66 4.2.3 Mechanism for zwitterion-induced reduction in the work function of ITO .73 4.3 Conclusions .78 Chapter Improvement in PCE by treating PEDOT:PSS buffer layer with co-solvents .81 5.1 Introduction .81 5.2 Results and discussion 84 5.2.1 Conductivity enhancement of PEDOT:PSS films through a co-solvent treatment 84 5.2.2 Photovoltaic performance of PSCs .90 5.2.3 Mechanism for the co-solvent treatment-induced improvement in the photovoltaic performance 94 5.3 Conclusions .97 Chapter PSCs using chlorinated ITO electrodes with high work function as the anode .99 6.1 Introduction .99 6.2 Results and discussion 100 6.2.1 Photovoltaic performance .100 6.2.2 Degradation of photovoltaic performance 103 6.3 Conclusions .109 viii 7. Concluding remarks ole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and Poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3, 4-b]thiophenediyl]] (PTB7) can absorb more light from sunshine than P3HT does[56,98,135,136]. Indene-C60 bisadduct (ICBA) is known to improve Voc due to its shallow LUMO level[55,57]. The work function after tuning should be able to form ohmic contact with these materials and deliver better performances. (2) The surface modification described in this work is all carried out on ITO transparent electrode. However, ITO is expensive and brittle, which will limit its wide application. Many new materials are proposed as novel transparent and conducting substrate to replace ITO, notably PEDOT:PSS, carbon nanotube (CNT), graphene and metal meshes[121,137-149]. Since the methods here will induce a localized electric field on substrate surface, the induced work function change is in principle independent of substrate. It is promising to modify the above mentioned transparent electrodes with the novel interfacial materials, such as zwitterions, to bring about truly cheap and flexible organic electronics. (3) Besides the active layer/electrode interfaces, D/A interface (interface between donor and acceptor materials in the active layer) is also an important interface in a polymer solar cell. Many critical processes, for example, charge transfer, exciton dissociation and free charge recombination, all happen at this interface. This thesis has demonstrated local electric field can facilitate electron transfer at the active layer/cathode interface. It is reasonable to believe if the local electric field can be introduced to D/A interface, the electron transfer from donor to acceptor will be 117 7. Concluding remarks promoted; exciton dissociation will be easier; and free charge recombination will be suppressed. (4) Due to the fixed bandgap of conjugated polymer, there is always limitation to single junction solar cells. One can only increase Jsc with the expenses of Voc, or vice versa. The concept of tandem cell can break such limitation. By connecting two or more cells in parallel or in series, tandem cell can harvest more light to produce a larger Jsc or a higher Voc. The challenge to build a tandem cell is to introduce a mechanically robust recombination layer which can form ohmic contact with the top and bottom sub-cells. The novel methods introduced in this thesis provide a set of tools to engineer the recombination layer. Tuning the energy levels and constructing tandem cells will become easier. (5) From the experience with sodium compounds, zwitterions and chlorination of ITOs, it is found surface dipole formation plays a critical role in tuning the interface energy levels. So materials that carry intrinsic dipoles or can induce dipole formation are potential candidate interfacial materials. If they also possess some other interesting properties, such as processibility, surface energy, surface morphology, film integrity, etc., they will be found useful in interface engineering. 118 7. Concluding remarks 119 7. Concluding remarks 120 Bibliography Bibliography [1] http://en.wikipedia.org/wiki/Earth%27s_energy_budget [2] http://www.exxonmobil.com/Corporate/files/news_pub_eo.pdf [3] Peet J., Heeger A.J., Bazan G.C. (2009) Accounts Chem. Res., 42, 1700. [4] Kallmann H., Pope M. (1959) J.Chem. Phys., 30, 585. [5] Chamberlain G.A. (1983) Sol. Cells, 8, 47. [6] Tang C.W. (1986) Appl. Phys. Lett., 48, 183. [7] Sariciftci N.S., Smilowitz L., Heeger A.J., Wudl F. 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Mater., 21, 1709. 132 [...]... device physics and key parameters of polymer solar cells, followed by emphasis on the importance of interface engineering in PSCs The objective of my PhD research work and thesis outline will be presented at the end of this chapter 1 1 Introduction 1.1 A brief overview of polymer solar cells 1.1.1 Historical background of polymer solar cells The history of polymer solar cells, or organic photovoltaics in... transparent electrode of polymer solar cells, Energy & Environmental Science, 2012, 5, 5325-5332 7 Sun K, Ouyang JY, Polymer solar cells using chlorinated indium tin oxide electrodes with high work function as the anode, Solar Energy Materials & Solar Cells, 2012, 96, 238-243 8 Sun K, Zhang HM, Ouyang JY, Indium tin oxide modified with sodium compounds as cathode of inverted polymer solar cells, Journal of... high-work function metal or conducting glass Since the solar cell only contains one type of semiconductor, this kind of solar cells are also known as homojunction solar cells (Figure 1.1 (a)) The invention of heterojunction solar cell by Tang in 1986 represents a great breakthrough[6] In hetero-junction solar cells (Figure 1.1 (b)), an interface is formed between two different organic semiconductors in... cathode for highly- efficient inverted polymer solar cells, ACS Applied Materials & Interfaces, 2012, 4, 2009-2017 4 Xia YJ, Sun K, Ouyang JY, Solution-processed metallic conducting polymer films as transparent electrode of optoelectronic devices, Advanced Materials, 2012, 24, 2436-2440 (Listed in “Advanced Materials Top 40”) 5 Sun K, Xia YJ, Ouyang JY, Improvement in the photovoltaic efficiency of polymer. .. low-bandgap polymers, morphology control, light management for better light harvesting and charge transport in the active layer Besides the active layer, the interfaces between the active layer and the two electrodes play key roles in improving the photovoltaic performance and device stability This work aims to develop novel interfacial materials and cost-effective methods to modify the interface for highly efficient. .. provides a promising solution, since solar energy received by the earth in an hour is estimated at 174 PetaWh, which is more than enough to fulfill the energy consumption by entire human race for one year (reported to be 154 PetaWh in 2010)[1-3] Polymer solar cells (PSCs) have been actively explored as a promising renewable energy converter due to their potential for energy -efficient, low-effective, large-area... processing additive, the efficiency of the BHJ solar cells comprised of PCPDTBT and PC71BM was improved from 2.8% to 5.5%[14] The latest world records were 10.0% for organic single junction cell and 10.7% for tandem cell[15,16] 1.1.2 Device physics of bulk-heterojunction PSC A fundamental parameter that differentiates a polymer solar cell from a silicon solar cell is the dielectric constant of the semi-conducting... lost before it reaches the D/A interface and gets dissociated Therefore, the bi-layered solar cell (Figure 1.2 (a)) faces a dilemma: if the photoactive layer is too thick, exciton could not reach the D/A interface within its life time; if the photoactive layer is too thin, light absorption is not maximized This dilemma was well resolved by Yu et al who invented the bulk-heterojunction (BHJ) solar cells. .. Ouyang JY, Improvement in the photovoltaic efficiency of polymer solar cells by treating the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) buffer layer with co-solvents of hydrophilic organic solvents and hydrophobic 1,2-dichlorobenzene, Solar Energy Materials & Solar Cells, 2012, 97, 89-96 xxv 6 Xia YJ, Sun K, Ouyang JY, Highly conductive poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)... quantum efficiency for charge transfer is close to unity This shows the potential to use fullerene as electron acceptor material Further improvement was made in 1995 by Wudl[8], who chemically synthesized soluble fullerene derivatives, e.g PCBM, which had become the most popular acceptor used in organic solar cells since then 3 1 Introduction Before 1995, all the heterojunction organic solar cells adopted .   INTERFACE ENGINEERING FOR HIGHLY EFFICIENT POLYMER SOLAR CELLS     SUNKUAN B.Appl.Sc.(Hons.), National University of Singapore     A THESIS SUBMITTED FOR THE DEGREE. photovoltaic performance and device stability. This work aims to develop novel interfacial materials and cost-effective methods to modify the interface for highly efficient polymer solar cells (PSCs). Publications xxv Chapter 1 Introduction 1 1.1 A brief overview of polymer solar cells (PSCs) 2 1.1.1 Historical background of polymer solar cells 2 1.1.2 Device physics of bulk-heterojunction PSC 5