To a certain extent, the Voc in BHJ

To a certain extent, the Voc in BHJ OSCs is determined roughly bythe difference of the HOMO level of the donor and the LUMOlevel of the acceptor (Ionization energy(D) (IE(D))–Electron affinity(A) (EA(A))) extracted by the exciton binding energy forthis charge-transfer state (corresponding to the Coulombicattraction between D+ and A
) and is limited by the energy ofthe interfacial donor and acceptor charge-transfer excited state[163,164], which provides the driving force for dissociation ofexcitons. In addition, the Voc is reduced by energy losses resultedfrom radiative and non-radiative recombination processes viacharge-transfer state. For a single junction cell, the optimumVoc is 1.1–1.4 V (PCE  34%) in the Shockley–Queisser limit[165], assuming no driving force is required for charge separation, so that there is no energy loss resulted from non-radiativerecombination. Donor and acceptor pairs should present appropriate energy levels, i.e., ionization energy, electron affinity,and the energy of relaxed excitons to ground state, which facilitates efficient charge separation, as well as minimizes energy loss[25,166]. The recent review article by Yan et al. has discussedvoltage loss in OSCs in detail [37].