Analyzing Open-Voltage of Double-Layer Organic Solar Cells Using Optical Electric-Field-Induced Second-Harmonic Generati
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Analyzing Open-Voltage of Double-Layer Organic Solar Cells Using Optical Electric-Field-Induced Second-Harmonic Generation Dai Taguchi, Xiangyu Chen, Takaaki Manaka and Mitsumasa Iwamoto Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 S3-33 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan. ABSTRACT By using electric-field-induced second-harmonic generation (EFISHG) measurement, we analyzed photovoltaic effect of two-layer solar cells (indium zinc oxide/pentacene/C60/Al). Results evidently showed that negative and excessive charges Q s accumulated at the two-layer interface under illumination, e.g., Qs =-1.7×10-9 C/cm2 at 0.05 mW/cm2 and –3.6×10-9 C/cm2 at 0.5 mW/cm2, while a short-circuit current flowed. The open-circuit voltage changed in accordance with accumulation charge Qs , and finally saturated. Modeling that accumulated negative charge is a source of space charge field and directly effects on the electrostatic energy stored in OSCs, dependence of the open voltage on the accumulated charge Qs was explained. INTRODUCTION Organic solar cells (OSCs) are promising candidates for the next generation device, where low cost and printable methods are available in the device fabrication [1]. The energy conversion efficiency of OSCs has been improved year after year [2], but it is still not sufficient for use in electronics. One of the main reasons lies in inadequate understanding of carrier behaviors in OSCs. It is well-known that donor-acceptor interface plays a key role for efficient electron-hole dissociation. Upon illumination, excitons are created in donor (D) and/or acceptor (A) layers, and subsequently they move to the D-A interface by diffusion. At the interface, these excitons are separated into electrons and holes by electron-hole dissociation, and then transported to opposite electrodes through the donor and acceptor layers, respectively. This is a basic process of photovoltaic effect. However, owing to ambiguous interfacial properties and complex charge transfer routes after the electron-hole dissociation, understanding carrier behaviors in OSCs is not so easy. To overcome this situation, experimental method that is capable of directly probing carrier motion is of great help. Accordingly, we have been developing an optical electric-fieldinduced second–harmonic generation (EFISHG) method that can directly probe carrier dynamics in organic devices, and have visualized carrier transport in organic field-effect transistors [3,4], and charge accumulation and decay processes at the interface in organic light-emitting diodes [57]. The EFISHG measurement was found very effective for tracing carrier behaviors in organic devices [8-10]. This motivated us to study carrier mechanism in OSCs in terms of photovoltaic effects [11-13]. In this paper, we directly probed charge generation process in pentacene/C60 solar cells by using the EFISHG measurement. Results showed that negative and excessive charges were accumulated at the double-layer interface by illumination. Accordingly, the open-circuit vo
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