Improving efficiency of non-fullerene polymer solar cells by using non-halogenated solvents

Abstract

How processing solvents influence the overall photovoltaic performances of polymer solar cells (PSCs) remains unclear in various recently emerged new material systems. Here we systematically studied this issue by integrating an extensively used poly (3-hexylthiophene) (P3HT), or a recently developed conjugated polymer P2F-EHp, with different non-fullerene acceptors. The P3HT based devices processed with the non-halogenated solvent, 2-methylanisole in presence of 1-methylnaphthalene as solvent additive, exhibit reduced bimolecular and trap-assisted monomolecular recombination, facile charge extraction and enhanced charge carrier mobilities. Morphological investigation reveals that the optimizing crystallites, phase purity as well as nanofibrous structure is effective to the enhancement of charge generation and transport. Note that P3HT:O-IDTBR based devices processed with these non-halogenated solvents exhibit an impressive power conversion efficiency of 7.1% with a high fill factor of 75.09% on a device area of 0.05 cm2, and the efficiency remained 6.89% even in a device with large active layer area of 1 cm2 with promising thermal stability. It is also noted that efficient PSCs consisting of P2F-EHp and non-fullerene acceptors of IT-4F and IT-4Cl were developed via optimizing non-halogenated toluene:o-xylene co-solvent. The detailed investigation of film morphology demonstrated that the co-solvent appeared to assist the manipulation of crystal coherent lengths and effectively decrease the phase separation of the corresponding blend films. Of particular importance is that this material system is compatible with the low-cost blade-coating technique and can be processed under ambient conditions without post-treatment. A remarkable power conversion efficiency of 10.1% was achieved by blade-coating the P2F-EHp:IT-4F:IT-4Cl blends in air. The results indicated that using non-halogenated solvents is a promising candidate for constructing efficient PSCs toward practical applications.