New Physical and Chemical Treatments to Improve the Quantum Efficiency in Polymer Solar Cells
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New Physical and Chemical Treatments to Improve the Quantum Efficiency in Polymer Solar Cells Osamu Yoshikawa1, Taro Sonobe2, Takashi Sagawa1, and Susumu Yoshikawa1 1 Institute of Advanced Energy, Kyoto University, Gokasho, Uji, 611-0011, Japan 2 Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, 611-0011, Japan ABSTRACT The performance of the devices of bulk heterojunction polymer-based solar cells were investigated by using poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) as light absorption (viz. active) layer, with TiOx as interlayer as follows: ITO/PEDOT:PSS/P3HT-PCBM/TiOx/Al [1] through the treatment of microwave irradiation (single mode of 2.45 GHz, 800 W for 1, 2.5, or 5 min). Such treatments enabled to increase the short-circuit current density Jsc (from 4.53 mA cm-2 to 7.27 mA cm-2) and fill factor FF (from 0.41 to 0.66) of the cell, though the open circuit voltage Voc was decreased (from 0.61 V to 0.57 V) along the irradiation. Absorption spectra of P3HT-PCBM blended film before and after the microwave treatment were observed. Shoulders at 550 nm and 600 nm appeared after the irradiation. This result implies that the microcrystallization of P3HT was slightly promoted through the microwave treatment. INTRODUCTION Organic solar cells have attracted a lot of attention due to a low cost device and high potential for printable process compared with silicon-based solar cells. Much efforts have been improved the efficiency of organic solar cells by modifying the cell architecture and development of new materials for active layer. Recently, Heeger et al reported power conversion efficiency of 5% in organic solar cells based on regioregular P3HT in the combination with PCBM [2]. Crystallinity of the P3HT is attributed to the high quantum efficiency of the cell. Annealing is one of the promising methods to improve the crystallinity of P3HT [3]. In addition, solvent-annealing [4] and slow drying method [5] are similar to the thermal annealing treatment. Every method requires warming process in each procedure. The disadvantageous points of such heat treatment are hard to regulate the heating conditions, timeconsuming, and some extent of the loss of the energy. While a lot of attention has now been focused on microwave annealing as homogeneous treatment and saving the energy compared to the heating method, because microwave annealing is non contact, rapid heating and regulated and selective heating for the materials which have rotation of the dipoles. For example, in the case of the no resistance to heat or curve substrates like plastic films, microwave can anneal only the organic materials selectively and noncontactly. Thus this annealing procedure is promising method for roll to roll processes and flexible organic solar cell. In this context, microwave annealing polymer photovoltaic device was reported recently [6]. They succeeded to improve the power conversion efficiency by multimode microwave annealing. However, since the electric inte
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