Design of Single-Porphyrin Donors toward High Open-Circuit Voltage for Organic Solar Cells via Energy Level Gradient-Distribution Screening Strategy of Fragments: A Theoretical Study

Open circuit voltage (VOC) is a key factor for improving the power conversion efficiency (PCE) of bulk heterojunction (BHJ) organic solar cells (OSCs). At present, more and more attention has been devoted to modification π bridges in single-porphyrin small molecule donors with A-π-D-π-A configuration to reduce the highest occupied molecular orbital (HOMO) levels and improve the VOC of devices. However, how to screen the π bridges is a key issue. In this work, nine π bridges were screened by the HOMO levels gradient-distribution strategy of fragments (electron-donating donor (D), π bridges, and electron-withdrawing acceptor (A)), where fragments meet the requirements were combined into five novel small molecule donors. Meanwhile, in order to test whether the strategy is beneficial to increase VOC, the [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) was selected as acceptor material. The energy levels of all molecules were compared. And photoelectric properties (i.e., energy gap, energy driving force, reorganization energy, intermolecular charge transfer rate, charge recombination rate, VOC) of five small molecules were calculated. The results showed that the HOMO levels of porphyrin donors could be significantly lower via this strategy, VOC was raised without losing of short-circuit current (JSC) and fill factor (FF) of the device. Meanwhile, the designed five molecules could be used as donor candidates to improve the performance of OSCs.