Simple Molecules as Hole Transport Materials Enabled by Solvothermal Technology: Towards Solution-Processable Carbon Electrode Perovskite Solar Cells

Introduction of hole-transporting materials (HTMs) in perovskite solar cells (PSCs) announced a beautiful blueprint for a performance boost of solar–electricity conversion device. However, using structurally simple and commercially available organic small molecules [e.g., benzothiophene (BT)] as HTMs is hardly possible from energy levels. This is because their highest occupied molecular orbital (HOMO) levels are lower than the valence band of perovskite, inhibiting the hole transport. Here, the solvothermal treatment for the simple BT with unsaturated rigid planar structure and heteroatom has been proposed, which is unusual because of oxygen-containing solvent participated in the carbonization. The emergence of a new functional group C-O-C in the product carbon dot (BT-CD) elevates HOMO value, making BT-CD more closely match the valence band of perovskite. Thus, all-air processed carbon-based PSCs modified with BT-CD show a fairly good power conversion efficiency of 13.22%. Our exploration broadens the choice of HTM and reduces the manufacturing costs of PSCs.