Random Poly(3-hexylthiophene-co-3-cyanothiophene-co-3-(2-ethylhexyl)thiophene) Copolymers with High Open-Circuit Voltage in Polymer Solar Cells

For the purpose of developing poly(3-hexylthiophene) (P3HT) based copolymers with deep-lying highest occupied molecular orbital (HOMO) levels for polymer solar cells with high open-circuit voltage (V-oc), we report a combined approach of random incorporation of 3-cyanothiophene (CNT) and 3-(2-ethylhexyl)thiophene (EHT) units into the P3HT backbone. This strategy is designed to overcome CNT content limitations in recently reported P3HT-CNT copolymers, where incorporation of more than 15% of CNT into the polymer backbone leads to impaired polymer solubility and raises the HOMO level. This new approach allows incorporation of a larger CNT content, reaching even lower-lying HOMO levels. Importantly, a very low HOMO level of -5.78 eV was obtained, representing one of the lowest HOMO values for exclusively thiophene-based polymers. Lower HOMO levels result in higher V-oc and higher power conversion efficiencies (PCE) compared to the previously reported P3HT-CNT copolymers containing only 3-hexylthiophene and CNT units. As a result, solar cells based on P3HT-CNT-EHT(15:15), which contains 70% of P3HT, 15% of CNT and 15% of EHT, yield a V-oc of 0.83 V in blends with PC61BM while preserving high fill factor (FF) and high short-circuit current density (J(sc)), resulting in 3.6% PCE. Additionally, we explored the effect of polymer number-average molecular weight (M-n) on the optoelectronic properties and solar cell performance for the example of P3HT-CNT-EHT(15:15). The organic photovoltaic (OPV) performance improves with polymer M-n increasing from 3.4 to 6.7 to 9.6 kDa and then it declines as M-n further increases to 9.9 and to 16.2 kDa. The molecular weight study highlights the importance of not only the solar cell optimization, but also the significance of individual polymer properties optimization, in order to fully explore the potential of any given polymer in OPVs. The broader ramification of this study lies in potential application of these high band gap copolymers with low-lying HOMO level in the development of ternary blend photovoltaics as well as tandem OPV. (C) 2015 Wiley Periodicals, Inc.