Influence of the Ethylhexyl Side-Chain Content on the Open-Circuit Voltage in rr-Poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) Copolymers

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Abstract

Although recently considerable attention has been paid to the impact of polymer alkyl side chains on conjugated-polymer:fullerene solar cell performance, and especially the Voc and Jsc, a clear and comprehensive picture of the effect of side-chain positioning, length, and branching has yet to evolve. In order to address some of these questions, we designed a simple and modular model system of random copolymers based on rr-P3HT. The influence of increasing amounts of branched 2-ethylhexyl side chains (10, 25, and 50%) in rr-poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) copolymers on properties such as UV–vis absorption, polymer crystallinity, HOMO energy levels, polymer:PC61BM solar cell performance, and especially the Voc was studied and compared to the corresponding homopolymers P3HT and poly(3-(2-ethylhexyl)thiophene) (P3EHT). Polymers with 50% or less 2-ethylhexyl side chains (P3HT90-co-EHT10, P3HT75-co-EHT25, P3HT50-co-EHT50) have the same band gap and similar absorption properties and also retain the semicrystalline nature of P3HT, whereas P3EHT has a higher band gap and lower absorption coefficient. Polymer HOMO levels were determined by electrochemistry in solution and thin film and are virtually identical for all polymers in solution, whereas in the solid state an increase in the amount of 2-ethylhexyl side chains leads to marked and correlated decrease in the HOMO levels. This decrease is directly reflected in the Voc measured in polymer:PC61BM solar cells which increases with increasing 2-ethylhexyl side-chain content, indicating a relatively straightforward HOMODONOR–LUMOACCEPTOR dependence of the Voc for this family of polymers. P3HT75-co-EHT25 benefits from an increased Voc (0.69 V), a Jsc (9.85 mA/cm2) on the same order of P3HT, and a high FF and ultimately achieves an efficiency of 3.85% exceeding that measured for P3HT (Voc = 0.60 V, Jsc = 9.67 mA/cm2, efficiency = 3.48%). The observed efficiency increase suggests that the random incorporation of branched alkyl side chains could also be successfully used in other polymers to maximize the Voc while maintaining the band gap and improve the overall polymer:fullerene solar cell performance.

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