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).

We fabricated inverted polymer solar cells (PSCs) using an organic–inorganic hybrid interlayer for electron extraction. The surface energy and surface defects of an organic–inorganic ZnO–PFN hybrid film, which was prepared by dissolving the conjugated polymer electrolyte poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) in a ZnO solution, were reduced, compared to ZnO film. By introducing the ZnO–PFN electron extraction layer, the interfacial contact between the active and electron extraction layers was improved and the series resistance of the PSC device was decreased. As a result, electron extraction from the active layer to the electrode was enhanced. The highest power conversion efficiency (PCE) of the inverted PSCs was 9.2%. Moreover, the ZnO–PFN-based inverted PSCs showed improved long-term stability compared to ZnO-based devices. The ZnO–PFN interlayer aimed to overcome the drawbacks of the conventional hydrophilic surface of ZnO, based on the properties of the conjugated polymer (PFN) without the need for additional processes. It was therefore simple to fabricate the inverted PSCs, making the devices commercially viable.