Constructing Type-II Band Alignment of 3D/2D Heterojunction for Improved Carrier Transport in Carbon-Based Perovskite Solar Cells.

High defect density and low stability remain significant obstacles in the development of efficient and stable 3D perovskite solar cells (PSCs), while 3D/2D hybrid PSCs have emerged as promising candidates due to their excellent environmental tolerance. However, the performance of 3D/2D PSCs might be further limited by carrier transport within the films caused by quasi-2D phases with multiple n-values. Herein, precrystallized 2D phenylethylamine lead iodide (PEAPbI) crystals and methylammonium chloride (MACl) were introduced into the bulk phase of methylammonium lead iodide (MAPbI) to improve crystal quality and growth orientation. During the film formation process, 2D PEAPbI delayed crystal nucleation and formed a 3D/2D mixed film. The as-formed type-II band alignment in the 2D/3D heterojunction film restricts the movement of charge carriers within the 3D phase lattice, avoiding the risk of being trapped by defects at the grain boundaries. Additionally, MACl facilitates vertical crystal growth, enlarging grain size and the thickness of perovskite films. Under the combined effect of 2D PEAPbI and MACl, the power conversion efficiency (PCE) of the carbon-based PSCs increased from 10.53% to 16.07%. The unpackaged devices retained 90% of their initial PCE after storage for 30 days at room temperature and 40% relative humidity. These findings not only provide a facile way for the crystal tuning and defect passivation by low dimensional perovskites, but also give new insights into constructing efficient 3D/2D types of carbon-based PSCs.