Localized Transformation of 2D Perovskite by Protonated Metformin for Efficient Carbon‐Based Perovskite Photovoltaic and Photo‐Charging Applications

Dimensional engineering is promising for achieving a high power conversion efficiency (PCE) and long‐term stability of perovskite solar cells (PSCs). However, insulated organic spacers in 2D perovskites often severely hinder carrier transport between the internal layers of devices. Herein, the “protonation‐induced localized transformation of 2D perovskites” is proposed to overcome the low carrier transport and conductivity of 2D/3D perovskite heterojunctions. Metformin, with its multiple amine groups and a substantial difference between its pKa value and perovskites, is protonated in an acidic environment or directly converted into the hydrochloride salt for the surface passivation of methylammonium lead iodide. This leads to the transformation of disorderedly oriented layered 2D perovskite into vertically oriented ones at grain boundaries. Consequently, the PCE of a carbon‐based PSC treated by protonated metformin increased considerably, reaching an optimal level of 14.13%. Additionally, applying this passivation strategy to a planar device (ITO/4PADCP/perovskite/PCBM/BCP/Ag) increased PCE from 20.82% to 22.09%, confirming the applicability of the strategy. To demonstrate the practical stability, an integrated PSC–supercapacitor device is assembled, which shows good cycling stability. This article introduces a novel method to improve carrier transport in 2D/3D perovskite heterojunctions, promoting the extensive utilization of dimensional engineering.