Highly efficient bulk heterojunction (BHJ) polymer photovoltaic cells with the polymer donor - poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) or 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) – and the soluble C61-fullerene electron-acceptor ([6,6]-phenyl-C61-butyric acid methyl ester, PC61BM) are presented.
Significant enhancement on cells´efficiencies were achieved by thermally evaporating a bilayer of 4,7-Diphenyl-1,10-phenanthrolin (Bphen)/LiF on top of the active layer, reaching power conversion efficiencies of 4.7 % and 8.3 % for cells with F8T2 and PTB7, respectively, under 0.7 sun AM1.5G illumination. Such efficiencies exceed or well-compare with others reported for devices with the same polymers but using the electron-acceptor PC71BM [1,2]. Also, measured efficiencies for cells with PTB7:PC61BM and Bphen/LiF as cathode interlayer compete with most efficient ones reported for BHJ PV cells with the same active blend but with both optimized anode and cathode interlayers (8.42 %) [2]. The role of the cathode interlayer in the cells´improvement will be discussed.
Acknowledgments: This work was supported by Fundação para a Ciência e Tecnologia (Portugal) under the projects PEst-OE/EEI/LA0008/2013 and PTDC/CTM-NAN/1471/2012

1. J.-H. Huang, C.-P. Lee, Z.-Y. Ho, D. Kekuda, C.-W. Chu, and K.-C. Ho, Sol. Energ. Mat. Sol. 94(1), 22 (2010).
2. C. Gu, Y. Chen, Z. Zhang, S. Xue, S. Sun, C. Zhong, H. Zhang, Y. Lv, F. Li, F. Huang, and Y. Ma, Adv. Energy Mater. 4, 1301771 (2014).