Bulk-heterojunction solar cells were fabricated using ternary blend dichlorobenzene solutions of poly [4,8-bis[(2-ethylhexyl)poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) with different weight ratios between PTB7 and P3HT on an indium-tin-oxide-coated glass substrate. The UV-vis absorption spectra of these ternary blend films show that the photon absorptions at the wave lengths of 500 and 700 nm can be adjusted depending on the P3HT polymer weight fraction in the PTB7 and P3HT total weight. The measurement results of the solar cell performance showed that the open circuit voltage V oc continuously increased from 0.62 to 0.79 V as the P3HT fraction increased from 0 to 20 %, while it gradually decreased above 30%. The short circuit current J sc slightly decreased from 10.4 to 9.0 A/cm 2 up to 20%, and it suddenly dropped above 30%. The power conversion efficiency of 4% decreased to 1.8% when the P3HT fraction increased to 30%. These results may suggest that the transports of photo excited electrons and holes between the PTB7 nano domains and P3HT nano domains are disturbed by the differences between the HOMO and LUMO levels of the PTB7 and P3HT.

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