Two new quinoxaline-based copolymers with different alkylthio side chains show high photovoltaic performance in as-cast devices.

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In order to investigate the effects of fluorine atoms on the photovoltaic performance, three 2-D D-A conjugated copolymers, namely PT-QX (0F), PT-FQX (1F) andPT-DFQX (2F), were designed and synthesized using alkylthienyl substituted quinoxaline with different numbers of F substituents as acceptor unit and thiophene as donor unit. The physicochemical and photovoltaic properties were comparatively studied in details. The results demonstrate that the highest occupied molecular orbital (HOMO) energy levels are gradually lowered from -5.10 eV, to -5.18 eV and then to -5.33 eV for PT-QX (0F), PT-FQX (1F) and PT-DFQX (2F), respectively, while the lowest occupied molecular orbital (LUMO) energy levels nearly kept constant as the increase of F substituents. Introducing F on the polymer backbone widens the energy bandgap and makes the absorption peaks of the polymers blue-shifted. The highest power conversion efficiencies of bulk heterojuncton polymer solar cells increased with the increase of F substituents from 2.82% for PT-QX (0F) to 4.14% for PT-FQX (1F) to 5.19% for PT-DFQX (2F) thanks to the enhanced V oc and J sc , The enhanced V oc and J sc can be mainly ascribed to the lower HOMO energy levels and moderate hole mobility of the fluorinated polymers, as well as better morphology and preferential orientation of face-on structure of the blend films of the fluorinated polymers donor with PC 71 BM acceptor.

Reducing voltage loss, namely, V loss , has been demonstrated to be an effective way to improve the efficiencies of photovoltaic devices, and power conversion efficiencies (PCEs) exceeding 10% have been reported in non-fullerene based polymer solar cells (PSCs) with V loss value lower than 0.6 V. However, for all polymer solar cells (APSCs), the PCEs lag far behind the non-fullerene PSCs with organic small molecular acceptors. And there have been no successful examples of high-efficiency APSCs along with low V loss values so far. Here, we reported APSCs that demonstrated a high efficiency of 6.66% simultaneously with a small voltage loss of 0.56 V by using a new polymer PBDT-DFQX1 as donor and N2200 as acceptor. Notably, when PBDT-DFQX1 is combined with a small molecular acceptor (SMA) O-IDTBR, the relative SMA based PSC exhibited a higher PCE of 8.76% also with a low voltage loss of 0.56 V. These results indicated that PBDT-DFQX1 would be a promising polymer donor material in photovoltaic device application, and the strategy by minimizing the voltage loss to improve the photovoltaic efficiencies is still valid for APSCs.