Functional layers in efficient and stable inverted tin-based perovskite solar cells

Perovskite solar cells (PSCs) are promising among various photovoltaic technologies due to their high efficiencies, low costs, and simple preparation routes. Owing to the lead toxicity issue in lead-based PSCs, tin (Sn)-based PSCs (TPSCs) have attracted tremendous attention. Especially, the TPSCs with an inverted structure exhibit a power conversion efficiency (PCE) exceeding 14% in the past few years. Although such advances have been achieved, the functional layers in inverted TPSCs still face two major challenges, i.e., the easy oxidation issue from Sn2+ to Sn4+ in the Sn-based perovskite layers and the mismatching energy levels between these functional layers, which result in the easy formation of vacancy defects and undesirable nonradiative recombination. To address these challenges, significant efforts on the functional layers in TPSCs have been devoted to passivating deep-level defects and optimizing energy level alignments. Herein, we review the recent progress of functional layers in TPSCs. We first focus on the structures of TPSCs, especially the inverted structure. Then, we summarize the developments of the functional layers in TPSCs systematically, including electron transport layers, hole transport layers, and Sn-based perovskite layers. We mainly emphasize the corresponding principles and their effects on the performance of the TPSCs. Moreover, we review the latest progress of the large-area TPSCs. Finally, we outline the challenges facing the current TPSC field and some perspectives along with useful guidelines to achieve TPSCs with better performance.