High Efficiency P3HT/PCBM Solar Cell
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High Efficiency P3HT/PCBM Solar Cell Kanzan Inoueab, Ross Ulbrichta, Pallavi C. Madakasiraab, Miaoxin Zhouc, Sergey B. Leea, John Ferrarisac, Anvar A. Zakhidovab a) NanoTech Institute, The University of Texas at Dallas, 2601 N. Floyd Rd. Richardson, TX 75083, USA. b) Department of Physics, The University of Texas at Dallas, 2601 N. Floyd Rd. Richardson, TX 75083, USA. c) Department of Chemistry, The University of Texas at Dallas, 2601 N. Floyd Rd. Richardson, TX 75083, USA.
ABSTRACT We report a nearly twofold increase of short circuit current: from Isc ~ 10 mA/cm2 to Isc = 16-20 mA/cm2 in P3HT/PCBM solar cells (SC) employing freshly prepared regio-regular poly(3hexylthiophene) (RR-P3HT) without special purification . The power conversion efficiency is enhanced to η≥ 4% as compared to our best η=3.8% in SC with commercial polymer despite the decreased filling factor (FF= 0.42, as compared to best FF = 0.59). We used our earlier found [1] procedures with optimal post heat treatment temperatures and time for our polymer SC. We also discovered a strong correlation between the device preparation procedures and performance. The optimal phase separation of PCBM and RR-P3HT into a bi-continuous network structure occurs after quite long solution stirring times (enhanced homogenization) and surprisingly very short annealing time at optimal temperature. We also found that the optimal concentration of PCBM in a RR-P3HT matrix is rather low, only c~35 wt%, contrary to high c~80 wt% in PPV based SC. 1 INTRODUCTION Efficiencies of organic solar cells have improved dramatically since the introduction of the concepts of a bulk-heterojunction and bi-continuous interpenetrating network of acceptor and donor materials. The optimal nanoscale phase separation of the donor and acceptor into a bicontinuous network is critical for the performance of solar cells because of the short exciton diffusion length (~10 nm) in organic materials. The effect of the postproduction treatment on polymer/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) organic solar cells has been studied by several groups [2-5]. However, the mechanism behind this effect is yet to be understood clearly. In our earlier papers [1] we demonstrated the effects of the postproduction heat treatment and solution homogenization time on the performance of bulk heterojunction solar cell based on commercial RR-P3HT and PCBM over a range of different temperatures and durations, as well as, explored the physical phenomenon behind the improvement during the treatment. In the present paper, we demonstrate, that “freshly prepared” polymer even without special purification shows much better photocurrent in optimally processed solar cells, as compared to the RR-P3HT from commercial sources (e.g. American Dye Source Co.). Here ”fresh” means stored in argon glove box to avoid oxidation and used within a few days after synthesis. The effect of the solution homogenization time on the performance of the device is also reported in this paper. Long stirring of freshly synthesized, but i
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