Improved power conversion efficiency of bulk heterojunction poly(3-hexylthiophene):PCBM photovoltaic devices using small molecule additive

We report that the power conversion efficiency (PCE) of the bulk heterojunction organic photovoltaic device based on poly(3-hexylthiophene):[6,6]-phenyl–C61–butyric acid methyl ester (P3HT:PCBM) blend was improved by incorporating a small molecule SM having absorption band in the longer wavelength region. SM is a small molecule containing thienothiadiazole central unit with terminal cyanovinylene 4-nitrophenyl at both sides, which were connected to the central unit via a thiophene ring. The combination of SM with P3HT and PCBM allows not only a broad band absorption up to longer wavelength, but also tuning the inter-energy level leading to a higher short circuit current (Jsc) and open circuit voltage (Voc). The device based on the as cast P3HT:PCBM:SM exhibits a PCE of 3.69%, which is higher than the device based on P3HT:PCBM and SM:PCBM blends. The overall PCE of the device based on thermally annealed blend is further improved up to 4.1%. The improvement of the PCE has been attributed to a better charge transport in the device, due to the increased crystallinity of the blend through thermal annealing.