Fullerene-free polymer solar cell based on a polythiophene derivative with an unprecedented energy loss of less than 0.5 eV

The fine alignment of molecular energy levels can efficiently enhance the open-circuit voltage (VOC) and improve the photovoltaic performance of polymer solar cells (PSCs). In this work, a novel polythiophene derivative donor with a low HOMO level of -5.6 eV was synthesized by copolymerizing carboxylate- and fluorine-substituted thiophene alternately. The introduction of fluorine downshifted the HOMO level of the polymer. On the other hand, a methyl-end-capped ITIC derivative, with elevated HOMO and LUMO levels, was selected as the acceptor in the pursuit of a higher V-OC. A best power conversion efficiency (PCE) of 6.6% with an extremely high V-OC of 1.13 V can be obtained after careful morphology optimization. Besides, an unprecedented energy loss of 0.46 eV is seen in this device, which is comparable to perovskite solar cells and has been rarely achieved before in bulk heterojunction PSCs.