Spectroscopic Studies of Charge Dynamics in Non-Fullerene Organic Solar Cells

Abstract

Organic solar cells (OSCs) are a promising technology for emission-free electricity supply. However, low power conversion efficiencies and poor device stabilities have so far prevented widespread commercial applications. Remarkable progress has been made recently by the development of non-fullerene electron acceptor materials. They benefit from good tunablity of energy levels as well as greater thermal and photochemical stability, and remarkable power conversion efficiencies of up to 16.5 % have been achieved. Furthermore, they have shown efficient charge separation with negligible energy loss, overcoming one of the crucial limitations of fullerene based OSC. Here, we present a study on the charge generation, separation, and recombination dynamics of non-fullerene solar cells. We investigate the model system P3TEA:SF-PDI$_2$ via pump-probe spectroscopy and time-resolved photoluminescence spectroscopy. We find ultrafast charge generation as well as regeneration of singlets via encounters of free electrons and holes. Additionally, we observe slow ($\sim$ 100 ps) and thermally activated charge separation from vibrationally relaxed charge transfer exciton (CTE) states. To directly track the CTE population, we employ pump-push-probe spectroscopy. Our results suggest that singlet excitons, CTE states, and free charges form an equilibrium, with reversible interconversion between them. These systems are therefore not limited by the need to provide excess energy to overcome the Coulomb binding energy. Thus, future efforts to improve efficiencies should focus on removing energy offsets and irreversible processes, such as non-radiative recombination. Furthermore, we study PBDB-T:ITIC, a material that was used in the first non-fullerene cell that exceeded 10 % efficiency. We find that the charge generation occurs on a $\sim$ 10 ps timescale, several orders of magnitude slower than in efficient fullerene-based systems. Additionally, we identify two different pathways for non-radiative decay: triplet generation in the polymer and trap-assisted charge recombination.