Three sources of energy lossesexist

Three sources of energy lossesexist in OSCs, the first part comes from radiative recombinationoriginating from absorption above the bandgap, which isunavoidable for all OSCs and is typically between 0.25 and0.30 eV. The second part is additional radiative recombinationfrom absorption below the bandgap due to the presence ofcharge-transfer state at the donor–acceptor heterojunction. InOSCs, an energy offset (typically >0.2 eV) between the electronaffinity of the donor and acceptor is always required to overcomethe binding energy of the excitons and provide driving force togenerate charge carriers, which leads to significant energy loss.Thus, to reduce the energy difference between the singlet excitonon donor and/or acceptor and the charge transfer state withoutaffecting charge generation efficiency is necessary to minimizethe second part of energy loss. Recent studies found that someblends of novel polymers and nonfullerene acceptors that showsteep absorption edges exhibit highly efficient charge generationefficiencies though the donor–acceptor energy offset is small.The third term is from nonradiative recombination, which is correlated to the electroluminescence (EL) quantum efficiency gEL(typically 10
6–10
8) of the bulk heterojunction blends.Increase the gEL of the blends can reduce the voltage loss inducedby nonradiative recombination.