Toward high efficiency organic photovoltaic devices with enhanced thermal stability utilizing P3HT-b-P3PHT block copolymer additives

Organic photovoltaics (OPVs) have drawn an extensive amount of attention due to their low cost, processibility and flexibility. However, a cell based on a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) has a limited power conversion efficiency (PCE) due to the short exciton diffusion length of ∼10 nm. We address this issue by designing a series of all-conjugated diblock copolymers, poly(3-hexylthiophene)-b-poly(3-(6-diethylphosphonatohexyl)thiophene) (P3HT-b-P3PHT), intended for use as additives to improve the performance of P3HT:PC₆₁BM-based photovoltaic devices. The PCE of the devices improved from 3.30% to 4.03% with the addition of P3HT-b-P3PHT (3:1). The thermal stability of devices with P3HT-b-P3PHT additives improved significantly relative to that of the P3HT:PC₆₁BM reference device, where the devices including a copolymer with a higher P3PHT content exhibited a better thermal stability. It was found that the fill factor (FF) could be regulated by simply varying the block ratio of P3HT-b-P3PHT and played a crucial role in improving both the PCE and the thermal stability. The P3HT-b-P3PHT diffused at the P3HT:PC₆₁BM interface, improved the miscibility between P3HT and PC₆₁BM, optimized the nanoscale morphology of the photoactive layer, and reduced the active layer roughness, all of which improved the FF and thus contributed to an improvement in device performance.