Boosting solar cell performance during highly thermo- and photo-stable asymmetric perylene diimide dimeric acceptors by selenium-annulation at the outside bay position†

Asymmetric perylene diimide (PDI)-based dimeric electron acceptors have received relatively little attention. Herein, two asymmetric fused PDI dimers, namely PDI-TPDI and PDI-SePDI, linked through one single bond (σ bond), were synthesized by S/Se-annulation at the outside bay position. Both of them exhibited outstanding thermal-stability with a 5% decomposition temperature of higher than 400 °C and good photostability. After replacing S-annulation with Se-annulation, a blue-shifted absorption peak, weakened aggregation, deepened E_LUMO, but almost unchanged molecular twisting angle between two aromatic rings were observed. Accordingly, the Se-annulated PDI-SePDI-based device achieved a 56.64% elevated PCE as high as 5.31% with a slightly decreased V_OC of 0.70 V, a 55.91% increased J_SC of 14.64 mA cm⁻², a 3.45% enhanced FF of 51.84%, and the S-annulated PDI-TPDI-based device obtained the PCE of 3.39%. The PCE enhancement was mainly due to the enhanced exciton dissociation, suppressed charge recombination, and increased charge mobility, benefiting from the beneficial microstructural morphology as the result of larger heteroatom Se-annulation. This work revealed the regulating mechanism of Se-annulation of PDI at the outside position on molecular distortion, morphology and photovoltaic performance.