Fullerene as an additive for increasing the efficiency of organic solar cells to more than 17%

In this work, we introduced a fullerene acceptor (PC71BM) into the binary photo-active layer based on a polymer donor (PM6) and a non-fullerene small molecular acceptor (BTP-BO-4Cl), and as a consequence, the ternary organic solar cells realized a high-power conversion efficiency of 17.39% compared to 16.65% in binary solar cells. The performance enhancement was found to be due to the optimized morphology and hence balanced hole and electron mobilities, which is responsible for the suppressed charge recombination and hence high photocurrent in solar cells. In addition, PC71BM shows the complementary absorption with PM6 and BTP-BO-4Cl, which can broaden the absorption range of the photo-active layer and hence more photons from the sunlight can be utilized. Besides, PC71BM shows the cascade energy level alignment between PM6 and BTP-BO-4Cl, which is helpful for charge transfer from donor to acceptor. All these merits explain the high performance in ternary solar cells, and also demonstrate that ternary photovoltaics adopting non-fullerene acceptor with the fullerene acceptor as small amount of additive is an efficient strategy to gain high performing organic solar cells.