Charge photogeneration dynamics in random terpolymer donor-based non-fullerene polymer solar cells

Abstract

Random ternary polymerization is a strategy for tuning the energy levels and improving the batch-to-batch reproducibility of polymer semiconductors for the application in polymer solar cells (PSCs). However, the influence of third component incorporation on exciton properties and charge photogeneration processes in terpolymer-based solar cells is still unclear. In this work, time-resolved spectroscopies were employed to study exciton properties and charge photogeneration processes in PSCs based on a series of terpolymers, PM1 and PM2, which have 20% and 50% of thiophene-thiazolothiazole (TTz) building blocks on the PM6 backbone, respectively. For neat terpolymer films, we found that the small amount (20%) of TTz incorporation in PM6 slightly reduces the exciton diffusion coefficient, but the exciton lifetime is significantly increased, resulting in a significant increase in exciton diffusion length. However, further increasing the TTz component (50%) in the PM6 backbone decreases exciton lifetime, the diffusion coefficient, and consequently exciton diffusion length. We found that a small amount of acceptor (Y6) addition can efficiently dissociate terpolymer excitons due to the weak molecular stacking of terpolymers in the blend films. For terpolymer:Y6-based blend films, we find that the small amount of TTz incorporation (PM1) could reduce the phase size of the donor and suppress bimolecular carrier recombination in blend films. Furthermore, we find that the energy level offset plays a critical role in charge photogeneration processes, and a HOMO energy level offset of 0.06 eV can dissociate acceptor excitons in terpolymer-based organic solar cells effectively.