Structural influences on charge carrier dynamics for

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Structural influences on charge carrier dynamics for small-molecule organic photovoltaics Zhiping Wang, Tetsuhiko Miyadera, Akinori Saeki, Ying Zhou, Shu Seki, Yosei Shibata, Toshihiro Yamanari, Koji Matsubara, and Yuji Yoshida Citation: Journal of Applied Physics 116, 013105 (2014); doi: 10.1063/1.4887076 View online: http://dx.doi.org/10.1063/1.4887076 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Relation between charge carrier mobility and lifetime in organic photovoltaics J. Appl. Phys. 114, 184503 (2013); 10.1063/1.4829456 Enhanced charge collection in confined bulk heterojunction organic solar cells Appl. Phys. Lett. 99, 163301 (2011); 10.1063/1.3651509 Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cells J. Appl. Phys. 101, 114503 (2007); 10.1063/1.2737977 Tuning acceptor energy level for efficient charge collection in copper-phthalocyanine-based organic solar cells Appl. Phys. Lett. 88, 153504 (2006); 10.1063/1.2194207 Enhanced carrier collection observed in mechanically structured silicon with small diffusion length J. Appl. Phys. 86, 7179 (1999); 10.1063/1.371809

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JOURNAL OF APPLIED PHYSICS 116, 013105 (2014)

Structural influences on charge carrier dynamics for small-molecule organic photovoltaics Zhiping Wang,1,a) Tetsuhiko Miyadera,1,2,b) Akinori Saeki,3 Ying Zhou,4 Shu Seki,3 Yosei Shibata,1 Toshihiro Yamanari,1 Koji Matsubara,1 and Yuji Yoshida1

1 Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, 305-8565 Tsukuba, Japan 2 JST-PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, 332-0012 Saitama, Japan 3 Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, 565-0871 Suita, Osaka, Japan 4 Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, 305-8565 Tsukuba, Japan

(Received 21 May 2014; accepted 24 June 2014; published online 3 July 2014) We investigated the structural influences on the charge carrier dynamics in zinc phthalocyanine/ fullerene (ZnPc/C60) photovoltaic cells by introducing poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) and 2,5-bis(4-biphenylyl)-bithiophene (BP2T) between indium tin oxide and ZnPc layers. ZnPc films can be tuned to be round, long fiber-like, and short fiber-like structure, respectively. Time-resolved microwave conductivity measurements reveal that charge carrier lifetime in ZnPc/C60 bilayer films is considerably affected by the intra-grain properties. Transient photocurrent of ZnPc single films indicated that the charge carriers can transport for a longer distance in the long fiber-like grains than that in the round grains, due to the greatly lessened grain boundaries. By carefully controlling the structure of ZnPc films, the short-circuit current and fill factor of a ZnPc/C60 heterojunction solar cell with BP2T are significantly improved and the power conversion efficiency is increased to 2.6%, which is 120% larger than the conventional cell without BP2T. C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4887076] V I. INTRODUCTION

Organic photovoltaics (OPVs) have attracted much attention due to their compatibility with flexible substrates and tunable absorption windows.1–3 Significant progresses have been made in improving device performance in the past decade.4–7 Structural control of organic films is one of the most effective routines to improve light absorption, exciton diffusion, exciton dissociation, and charge carrier transport for efficient OPV devices. For solution-processed polymer OPVs, thermal/solvent annealing8–10 and processing addictives11–15 are widely employed to modify the structure of polymers at nanoscale for higher device efficiencies, while for vacuum-processed small-molecule OPVs, heating treatments,16,17 changing the mixing ratio of donor and acceptor18 and introducing buffer layers19–22 are commonly used to realize structural modification. We have recently demonstrated that introduction of nanostructured templates enables the structural control of organic films at molecular level, and realizes an ideal phase separation morphology in bilayer or co-evaporated blend films, which have significantly improved the power conversion efficiency (PCE) to above 4%.23–26 Understanding the parameters which influence device performance and efficiency becomes important for further improvement of OPV cells. The charge carrier transport and collection strongly depend on charge carrier transport a)

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b)

0021-8979/2014/116(1)/013105/7/$30.00

distance, which is given by l ¼ l  s E, where l is the mobility of charge carriers, s their lifetime, and E the electric field in the device.27 In this sense, the charge carrier dynamics involving l and s are the main parameters strongly influencing the current density and charge carrier recombination rate.28,29 However, few study has focused on the correlation between film structures and charge carrier dynamics. Timeresolved microwave conductivity (TRMC)30–33 is a noncontact technique which allows for a microscopic detection of charge carriers dynamics before deactivation by trapping sites, using monochromic light pulse from a nanosecond laser34,35 or white-light pulse from a Xe flashlamp36 as photoexcitation sources. Transient photocurrent (TPC)37–43 is a powerful tool to examine the charge carrier propagation, including charge carrier transport, trapping, de-trapping, and recombination, by applying a short pulse of light and a voltage bias, which yield in-depth knowledge of macroscopic charge carrier dynamics, such as charge carrier mobility, recombination mechanisms, charge carrier density, and the density of state.43 In this work, we perform both of TRMC and TPC measurements on ZnPc films with different structural characteristics to investigate the charge carrier dynamics for ZnPc/C60 planar heterojunction cell. The structural control of ZnPc films was realized by regulating the growth on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and 2,5-bis(4-biphenylyl)-bithiophene (BP2T) layers [molecular structure in Fig. 5(a)]. We found that charge carriers have a longer lifetime on a smoother BP2T layer, and are able to transport a longer distance in larger grains despite a

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changeless charge carrier mobility. The results indicate that the charge carrier dynamics are strongly dependent on the morphology and crystalline structure of ZnPc films. Corresponding impacts on device performance are discussed in detail. By optimizing the film structure and charge carrier dynamics, ZnPc/C60 heterojunction cells showed a best PCE of 2.6% with a high fill factor (FF) of 0.66. II. EXPERIMENTS

All photovoltaic cells were fabricated on commercially available indium tin oxide (ITO) patterned on glass substrates that were pretreated in oxygen plasma for 30 min. A 40 nm PEDOT:PSS layer was spin-coated on ITO, and then annealed at 135  C on a hotplate for 30 min under the ambient atmosphere. Prior to use, BP2T (Aldrich) and ZnPc (Aldrich) were further purified three times by vacuum gradient sublimation, while C60 (Frontier carbon, 99.9%) and bathocuproine (BCP; Dojindo, 98%) were used as-received. These materials were thermally evaporated in a vacuum chamber (

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