Semitransparent inverted bulk-heterojunction solar cells were fabricated on an ITO/glass substrate utilizing oxide/metal/oxide transparent anode. The device structure was glass/ITO/PFN/PTB7:PC 71 BM/MoO 3 /Au/MoO 3 . The semitransparent solar cell showed a transparency of 44% at 550 nm and bifacial power generation at both front and backside illumination. The power conversion efficiencies (PCEs) with light illuminated from ITO or Au/MoO 3 side were 2.8% and 2.3%, respectively. Fabrication of stacking structure of two separated solar cells was further demonstrated to harvest an increased amount of photons and enhance the PCEs. The stacked cells showed a transmittance of 17 % at 550 nm and improved PCE of 3.3% when connecting two cells in parallel. Keyword: semitransparent organic solar cells, bulk-heterojunction, PTB7, PC71BM, oxide/metal/oxide electrode

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Bulk-heterojunction solar cells were fabricated using ternary blend dichlorobenzene solutions of poly [4,8-bis[(2-ethylhexyl)]-phenyl-C61-butyric acid methyl ester (PC 61 BM):[6,6]-phenyl C71 butyric acid methyl ester (PC 71 BM) with different weight ratios between PC 61 BM and PC 71 BM. In all the solar cells, the overall weight ratio of polymer to fullerene was maintained at 1:1.5, while the composition of the fullerene component (PC 61 BM:PC 71 BM) was varied. The ultraviolet-visible absorption spectra of these ternary blend films showed that the photon absorptions at wavelengths between 300 and 800 nm continuously decreased with the increase of the PC 61 BM weight fraction in the PC 61 BM and PC 71 BM total weight. The measurement results of the solar cell performance showed that the open-circuit voltage notably increased for PC 61 BM weight fractions between 10% and 90%, while it decreased at 100%. The short-circuit current showed the most significant increase in the PC 61 BM weight fraction range between 50% and 60%. A power conversion efficiency of 3.4% was achieved when the PC 61 BM weight fraction was between 50% and 60%. These results may suggest that the transport of the photoexcited electrons between the cathode and the PC 61 BM/PC 71 BM nanodomains was enhanced.