Charge generation in organic solar cells: Journey toward 20% power conversion efficiency

The power conversion efficiencies of organic solar cells(OSCs)have routinely lagged far behind those of their inorganic counterparts.However,owing to the enor-mous contributions of many researchers,the power conversion efficiencies of OSCs have rapidly improved and now exceed 19%.The charge generation mechanisms in OSCs have been heavily debated during this period while acquiring valuable knowl-edge.This review highlights fundamental and cutting-edge research that rationalizes why OSCs can generate photocurrent so efficiently.In particular,a photophysi-cist’s views on exciton diffusion to donor:acceptor interfaces,charge transfer at the donor:acceptor interface,and long-range spatial dissociation of charge transfer states are discussed.Although a general consensus in this area has not been reached yet,recent time-resolved spectroscopic measurements provide important photophys-ical insights that can help achieving a better understanding of the charge generation mechanism in OSCs.Based on these observations,future research directions for realizing further improvements in OSC performance are discussed.