Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells

Abstract

Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor–acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. Here, we use time-resolved optical spectroscopy to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states 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, both charge-transfer states and emissive excitons are regenerated, thus setting up an equilibrium between excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination.

Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells

“…In line with previous studies 18,26,28,29,37,48 , this band is attributable to the EA of PM6. When an exciton dissociates to form an electron-hole pair at the D/A interface, the electron-hole pair generates a 16 dipole-like local electric field in the surroundings.…”

Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells

“…In line with previous studies 18,26,28,29,37,48 , we attribute this band to the EA of PM6. When an exciton dissociates to form an electron-hole pair, the electron-hole pair generates a dipole-like local electric field in the surroundings.…”

Adjusting the energy of interfacial states in organic photovoltaics for maximum efficiency

“…This result is counter-intuitive at first sight, given the stronger Coulomb attraction of a CT state as compared to a CS state and suggests that charge separation in low driving force systems should be endothermic because these systems lack excess energy needed for coherent long-range phenomena. Recently, temperature-dependent pump-push-probe transient data were presented in low driving force NFA blends, showing very slow charge separation kinetics in the hundreds of picoseconds time domain, supporting the picture of endothermic charge separation 39 . However, there is an increasing body of evidence that many systems display exothermic charge separation.…”