Block copolymer compatibilized polymer: fullerene blend morphology and properties

Abstract

Recent studies have shown the role of block copolymer as compatibilizer in tuning the phase separated morphology of the active layer so as to improve the overall photovoltaic efficiency of organic photovoltaic (OPV) devices. Here, we substantiate this observation by investigating the role of a rod-coil block copolymer poly-(3-hexylthiophene)-b-polystyrene (P3HT-b-PS) as compatibilizer in influencing the blend morphology and device performance of several polymer:fullerene blend systems. Fullerene derivatives N-(3-methoxypropyl)-2-(carboxyethyl)-5-(4-cyanophenyl) fulleropyrrolidine (NCPF) and N-(3-methoxypropyl)-2-(carboxyethyl)-5-(5, 5-difluorobenzo-dioxole) fulleropyrrolidine (FFNCPF) were synthesized using Prato reaction, while P3HT-b-PS copolymer was synthesized using the combination of Grignard metathesis, ATRP, and click chemistry. The addition of P3HT-b-PS in P3HT:PCBM blend led to the formation of more homogenous structure compared to the pure P3HT:PCBM blend. Also, a reduction in domain size was observed in P3HT: FFNCPF system upon P3HT-b-PS addition which can be attributed to the compatibilization effect of BCP. Incorporation of P3HT-b-PS block copolymer was found to effectively alter the thin film nanostructure of polymer/fullerene derivative blends and polymer crystalline structure. Maximum enhancement in power conversion efficiency (PCE) of compatabilized P3HT:PCBM blend system was noticed followed by P3HT:FFNCPF and P3HT:NCPF. The moderate improvement in photovoltaic properties can be correlated with face-on orientation of P3HT crystallites and the segregation of the fullerene domains at the bulk heterojunction (BHJ) cathode interface which facilitates the efficient charge collection as respective electrodes.