Effect of PTB7 Properties on the Performance of PTB7:PC71BM Solar Cells

The effect of poly[[4,8-bis [(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) properties on the optical properties, charge transport and photovoltaic performance Of PTB7:[6,6]-phenyl C-71 butyric acid methyl ester (PC71BM) blend films is investigated. We found that the variations in the molecular weight (M-w) and polydispersity index (PDI) of PTB7 mainly affect the phase separation and charge transport (hole mobility) in the blend films. The optical properties are also affected, but the increase in the extinction coefficient does not necessarily imply increased power conversion efficiency. The obtained power conversion efficiency for optimized thickness varied from 4.8% to 7.8% depending on the properties of PTB7. The wide range of obtained power conversion efficiencies illustrates the importance of optimizing the M-w and PDI to optimize bulk heterojunction morphology and achieve high performance solar cells.