| Abstract: | Organic solar cells (OSCs) have attracted huge attention because of their unique merits[1-3].In last few years,thanks to the design of new materials and device engineering,the power conversion efficiencies (PCEs) of OSCs have surpassed 18％[4-8].The PM6:Y6 cells are efficient binary cells,offering high PCEs over 16％[9-11].The high performance originates from the efficient free charge generation and the ground state dipole field at the donor-acceptor interface that pro-motes the exciton dissociation[12].To further boost the per-formance of PM6:Y6 cells,ternary architectures were adop-ted.Significant improvements in short-circuit current density(Jsc) and open-circuit voltage (Voc) were realized.However,most of the ternary cells still suffer from low fill factor (FF) (gen-erally ＜78％) (Table S1).The FF is generally determined by the competition between recombination and extraction of charge carriers[13-15].The interfacial electronic structures have nonnegligible impacts on charge transport in OSCs,also influ-encing the FF[16,17].Previously,Bao et al.demonstrated that the energy of positive integer charge transfer (ICT) states(EICT+) of PM6 is equal to the energy of negative ICT states(EICT-) of Y6,leading to no potential step at the PM6:Y6 inter-face and an ideal binary host system[18].To avoid the forma-tion of potential step in the ternary system,the EICT-of the second acceptor should be lower than (or equal to) EICT+ of the donor,thus suppressing the ICT trap-assisted recombina-tion[1].In this work,we carefully incorporate a second accept-or EH-IDTBR into the host PM6:Y6 blend (Fig.1(a)).From the ultraviolet photoelectron spectroscopy (UPS)-derived energy levels (Fig.1(b)),the negative ICT states of EH-IDTBR (EICT-=4.25 eV) is smaller than the positive ICT states of PM6 (EICr+ =4.5 eV),suggesting no ground state charge transfer at the PM6:EH-IDTBR interface,thus avoiding interfacial ICT trap-assisted recombination. |
