A free base porphyrin as an effective modifier of the cathode interlayer for organic solar cells

Herein, we demonstrate the co-optimization of power conversion efficiency (PCE) and air stability of the organic solar cells (OSCs) by free porphyrin-based buffer of zinc oxide (ZnO) cathode. By combining first-principle calculations and experimental measurements, we investigated the adsorption behaviors and working mecha-nism of free base porphyrin (TPP) and zinc porphyrin (ZnTPP) on ZnO cathode. The N atoms of TPP are capable of forming Zn-N coordination with the Zn atom of ZnO substrate, thereby generating interfacial dipoles and reducing the work function of ITO/ZnO cathode, which is beneficial for the charge transfer in the OSCs. More importantly, the large adsorption energy between ZnO and TPP, as well as the good compatibility between TPP and the organic layer, enables TPP to act as a "molecular glue" between the inorganic interface and the organic active layers, which improves not only the PCE but also the stability of the devices under high humidity air environment. In contrast, the already coordinated porphyrin ZnTPP has no such positive effects on the device performance when used as ZnO buffer. The results are of great significance for the study of the working mechanism of interlayer buffers in OSCs, and are also of guidance for designing the buffer molecules.