Publication date: 2nd November 2020
The power conversion efficiency of organic bulk heterojunction solar cells has crossed 18 % with the advent of novel non-fullerene acceptors (NFA) [1,2]. Particularly, PM6:Y6 based BHJ solar has shown encouraging performance with near-unity photocurrent generation and low voltage loss. Here, we utilise femtosecond transient absorption spectroscopy to understand the charge generation dynamics in these blends via both electron and hole transfer processes at ultrafast timescales. By selectively exciting the PM6 and Y6 molecule in the blends we differentiate the electron and hole transfer kinetics. For the Y6 excitation, the hole transfer rate is observed to be slow and proceed up to 100 ps. This reveals that the hole transfer rate is not detrimental in overall efficiency of free charge generation. The slow hole transfer process is associated with multiple factors such as low offset energy between highest-occupied-molecular-orbital (HOMO) levels, phase pure Y6-domains suppressing non-radiative recombination and weakly bound intermolecular charge-transfer excitons which are delocalised to neighbouring molecules. While the photoexcitation of PM6 proceed by dominant resonant energy transfer to Y6 molecule and then followed by back-hole transfer process from Y6 to PM6 molecule. These results suggest that the overall charge generation process is mainly defined by the hole transfer efficiency from Y6 to PM6 molecule. Correspondingly we observe a balanced electron and hole mobility in the pristine PM6 and Y6 and a low barrier for hole transport even in Y6 which is expected to promote efficient hole transport and contribute to high PCE. We envisage that our study paves a way for designing novel NFA having complementary absorption to alternatively harvest the photons via resonant energy transfer process in the BHJ components.
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