Studies of photoexcitations in polymer/non-fullerene blend for high-efficiency organic solar cells

We studied the photoexcitations properties in the blend of PBDB-T-SF, a donor π-conjugated copolymer, and ITIC-2F, a non-fullerene acceptor molecule, which is used as an active layer in high-performance organic photovoltaic (OPV) solar cells. We used several steady-state spectroscopies such as photoinduced absorption (PIA), photoluminescence (PL), magneto-photoinduced absorption (MPA), and magneto-PL in the pristine and blend films. The PIA spectra of the pristine copolymer and acceptor films contain a photoinduced absorption (PA) band due to triplet excitons, which is confirmed by the MPA(B) response, whereas the PIA spectrum of the copolymer/molecule blend contains several PA bands due to polaron pairs at the copolymer chain/molecule interfaces. Interestingly, when exciting the PIA spectrum of the blend at two different photon energies, namely, ℏ ω ≈ 2.39 eV that preferentially excites the copolymer chains and ℏ ω ≈ 1.58 eV that excites only the acceptor molecules, we found that the PIA spectra are the same. This shows that charge photogeneration is possible when exciting either the copolymer chains or the acceptor molecules, which might explain the high power conversion efficiency of OPV cells based on this blend. We also found from the MPA(B) responses of the pristine copolymer and acceptor films and their blend that the back reaction of the photogenerated polaron pairs into the copolymer forming triplet excitons is absent, further explaining the high power conversion efficiency of this blend as the active layer in OPV solar cells.