Low band-gap polymers based on quinoxaline derivatives and fused thiophene as donor materials for high efficiency bulk-heterojunction photovoltaic cells

In this study we synthesized three low band-gap polymers—poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-2,5-thieno[3,2-b]thiophene] (PQTT), poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-3,6-dipentadecylthieno[3,2-b]thiophene] (PQPDTT) and poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-2,5-bis(thieno-2-yl)-3,6-dipentadecylthieno[3,2-b]thiophene] (PQTPDTT)—that are based on quinoxaline and thieno[3,2-b]thiophene, and examined their photovoltaic properties. The band-gaps of PQTT and PQTPDTT were 1.65eV and 1.7eV, respectively. For PQTPDTT, a HOMO level of −5.12eV, which was lower than that of P3HT, was observed. According to the analysis of the characteristics of the photovoltaic device using PC61BM and PC71BM as acceptors, an open circuit voltage (VOC) of 0.71V, a short circuit current (JSC) of 8.80mA/cm2, a fill factor of 0.36 and a power conversion efficiency (PCE) of 2.27% were observed at 100mW/cm2 (AM 1.5 illumination) when a PQTPDTT/PC71BM blend film was used as the active layer. A very high incident photon to current efficiency (IPCE) was detected (60% at 400nm, 57% at 500nm).