Over the past six years, the solar power conversion efficiencies of organic solar cells (OSC) have greatly improved from 12% to 19%, closing the gap with inorganic and hybrid solar cells. The major breakthrough behind the rapid efficiency improvement is the development of non-fullerene acceptor molecules, replacing the traditional fullerene molecules as electron-accepting materials. Understanding the photophysical processes underlying these high-performance materials is crucial to OSC research. In this talk, I will present transient optical spectroscopy results on non-fullerene OSC blends with small interfacial energy offsets. By optically probing the time evolution of excited states, we show that free charges are generated via thermal activation of interfacial charge-transfer (CT) states on a hundred picosecond timescale. Reduced charge separation rate is observed at lower temperatures, leading to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OSC blends can achieve high photocurrent generation yields even at reduced temperatures given that the thermal dissociation rate of charges outcompetes the recombination rate.
Organic solar cells (OSCs) based on non-fullerene acceptors can achieve high charge generation yields despite near-zero donor-acceptor energy offsets to drive electron-hole separation. In this talk I will present experimental data to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer excitons (CTEs) that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron-hole encounters at later times, CTEs and emissive excitons are regenerated, thus setting up an equilibrium between excitons, CTEs and free charges. This endothermic charge separation process enables these systems to operate close to quasi-thermodynamic equilibrium conditions with no requirement for energy offsets to drive charge separation and achieve greatly suppressed non-radiative recombination.
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