A ternary strategy could combine the advantages of incorporated materials as an encouraging approach to achieve high power conversion efficiency (PCE) polymer solar cells (PSCs). In this work, ternary PSCs based on PTBTz-2:IT-M:PC₇₁BM with different weight ratios were systematically investigated. With 20 wt% PC₇₁BM incorporated into the accepters, the PSCs achieved the best PCE as high as 12.28% with an open-circuit voltage (V_OC) of 0.928 V, a short-circuit current density (J_SC) of 18.70 mA cm⁻² and a fill factor (FF) of 70.78%, which is undoubtedly superior to those of binary devices based on PTBTz-2:IT-M (10.39%) or PTBTz-2:PC₇₁BM (9.20%). The energy transfer mechanism between PC₇₁BM and IT-M was investigated in depth by femtosecond pump–probe transient absorption spectroscopy and time-correlated single-photon counting. The Förster resonance energy transfer mechanism between PC₇₁BM and IT-M has been powerfully verified. Morphology characterization analysis, density of states theory and density functional theory calculations indicated that the excellent combination of PC₇₁BM and IT-M has a synergistic effect on the V_OC values of the devices. Our results suggested that the two acceptors with drastically different structures could synergistically combine the merits of each to optimize crystallinity and charge generation, separation, and collection, which simultaneously enhanced the PCEs.

Self-doped polymer cathode interface materials for organic solar cells have been widely investigated to enhance the ohmic contact between the electrode and the photoactive layer. Herein, a novel polymer named PBTA-FN with self-doping effect was successfully synthesized by incorporating benzotriazole (BTA) as an electron-deficient group and a fluorene containing an amino group. In favor of the n-type backbone and amine-based groups, an obvious n-type doping was obtained, resulting in the dramatically improved conductivity of PBTA-FN. Subsequently, PBTA-FN and PFN as cathode interface layers (CILs) were successfully applied in the organic solar cells based on PBDB-T-2F:IT-4F. A notable power conversion efficiency of 12.18% and 11.03% could be achieved with PBTA-FN and PFN as CILs, respectively. PBTA-FN showed better planarity than PFN as observed from the results obtained via density functional theory. The self-doping behaviour of PBTA-FN was determined by electron paramagnetic resonance, which exhibited a higher mobility and carrier density. The water contact angle results on the surface of the active-layer/PBTA-FN bilayer suggested that the PBTA-FN surface polarity was improved, which was attributed to the larger interface dipole. Thus, PBTA-FN can reduce the work function of an Al electrode and enhance the built-in electric potential, which were further confirmed by ultraviolet photoelectron spectroscopy and Mott Schottky curves, and the related device produced a higher V_oc (0.88 V) than PFN (0.86 V). This work provides a deeper understanding of the PBTA-FN interlayer mechanism and has a potential application in optoelectronic devices.

A diketopyrrolopyrrole-based small bandgap polymer (DPPT-TT) with high mobility is introduced as an additive to D–A1–D–A2 type thieno[3,4-b]thiophene-based random copolymer (P3):(6,6)-phenyl-C₇₀-butyric acid methyl ester (PC₇₁BM) polymer solar cells (PSCs).