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

Abstract

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 (V_OC) of 0.71V, a short circuit current (J_SC) 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).