Charge recombination losses in non-fullerene acceptor based organic solar cells

Abstract

This thesis focuses on the factors determining the photocurrent generation efficiency of organic solar cells utilizing non-fullerene acceptors as opposed to the generally employed PCBM. This work explicates the photophysical properties of organic blends and devices incorporating such acceptors with transient absorption spectroscopy to understand their charge dynamics impact upon device performance and strategies to inhibit recombination detrimental to the photocurrent generation of such devices. The first results chapter introduces transient kinetic studies of a couple of promising blends with rhodanine based non-fullerene acceptors. These results show that the photocurrent generation in these acceptor blends is limited primarily by geminate recombination losses following efficient exciton separation and the losses are assigned to the difference in the nanomorphology of the blends. The next chapter reports an approach to suppress these losses in the device with non-fullerene acceptors: 1) An energetic offset increase is shown to suppress geminate recombination in blends with barbiturate acceptors, leading to a maximum EQE of ~84%. The third results chapter demonstrates a trade-off using a polymeric acceptor, N2200 which exhibits superior device thermal stability to PCBM based devices due to aggregation of PCBM upon thermal stress, but its initial device performance is limited by a geminate recombination and exciton decay to ground. The forth results chapter reports a comparison of charge generation for a series of non- fullerene acceptors blended with P3HT and PTB7-Th donors. A formation of an intermediate state prior to free dissociated charges is demonstrated to be bound polaron pair state and a high yield of such state is originated from the amorphous blend system. Geminate recombination losses suffered in the blend systems are suppressed by increasing the energy offset which in turn leads to an efficient photocurrent generation. The dissociation efficiency of bound-polaron pairs is shown to correlate with photocurrent generation efficiency. The last chapter provides concluding remarks and recommendations for further work.