Towards highly stable and efficient planar perovskite solar cells: Materials development, defect control and interfacial engineering

Planar perovskite solar cells (p-PSCs) have attracted increasing attention recently due to their advantageous features of simple fabrication process, low cost and superior flexibility. Referring to the stability issue and relatively lower power conversion efficiencies (PCEs) than mesoporous PSCs, three main strategies, i.e., materials design, defect control and interfacial engineering, have been extensively applied on purpose to enhance both stability and PCEs of p-PSCs at the same time. In this review paper, a comprehensive summary of the recent advances in developing stable and efficient p-PSCs will be provided, focusing on adopting these three strategies. Firstly, the underlying origins of stability issue induced by the internal factors brought by the key materials in p-PSCs including electron transporting layers (ETLs), hole transporting layers (HTLs) and perovskite light absorbers are discussed, and then the progress in materials development towards improved stability and efficiency are summarized. Secondly, the energy loss, energy level mismatch and ionic migration problems in p-PSCs caused by the defects are discussed and recent advances in interfacial engineering and defect control are emphasized. In addition, some new and superior PSC configurations that can further enhance the stability of p-PSCs, such as ETL-free and HTL-free structures, are briefly discussed. Finally, conclusions and perspectives are provided to illustrate the opportunities and the challenges in this dynamic and frontier research field. This review will provide some useful insights for the rational design of highly stable and efficient PSCs.