Molecular energy level modulation by changing the position of electron-donating side groups

An investigation on the opto-electronic and photovoltaic properties of a pair of alkoxy substituted quinoxaline-based copolymers PTTQx is performed in order to describe the effect of changing the position of alkoxy substituents on the peripheral phenyl rings. The copolymer with meta-positioned alkoxy showed lower HOMO and LUMO levels and a higher Voc of 0.73 V, while the copolymer with para-positioned alkoxy displayed higher HOMO and LUMO levels and lower Voc of 0.60 V when a polymer/PC71BM blend film was used as the active layer in polymer solar cells (PSCs) under AM 1.5 G irradiation (100 mW cm−2). With the good agreement between theoretical calculation and experimental observation, it has been observed that the effect of the substituents depends on the position of the alkoxy group which exhibits a stronger electron donating effect in the para-position than in the meta-position. The resonance electron donating effect of the alkoxy group on the para-position can elevate the HOMO and LUMO levels simultaneously, while this effect is not obviously reflected on the meta-position. Therefore, PTTQx-m exhibits lower HOMO level, higher Voc correspondingly and thereby higher PCE of the PSCs based on it.