Solution-processed photocells based on low energy-gap polymer and unmodified C70 composites from halogen-free solvent exceeding 5% power conversion efficiency

In terms of the environmentally-friendliness, it is desirable that the energy and toxicity embodied in a photocell are as low as possible. In this context, a solution-processed organic photocell with unmodified fullerene as electron acceptor coated with halogen-free solvent is an attractive object to study. Here, the dependences of the photovoltaic performance of PTB7-Th:C70 bulk heterojunction photocells prepared from a chlorine-free solvent 1,2,4-trimethylbenzene on composition and annealing temperature have been studied. Similar to the PTB7:C70 bulk heterojunction photocells, the device performance of PTB7-Th:C70 photocells is improved by the thermal annealing up to 175 °C, suggesting that the PTB7-Th:C70 photocells are superior in thermal robustness. The optimal device using PTB7-Th:C70=1:1 annealed at 175 °C showed the power conversion efficiency of 5.1% under 1 sun illumination, which is the highest value reported for solution-processed organic photocells using unmodified fullerene so far.