A Mixed Heterojunction Layer for High Performance and Stability P-I-N-Based Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted a lot of attention due to high efficiency and low-cost fabrication processing. For planar structure PSCs, the electron transport layer (ETL) plays an important role to enhance the electron extraction capacity, device stability and balance the energy level between perovskite materials and metal electrode, which can effectively optimize the performance of PSCs. However, traditional ETL materials are not comfortable for commercialization of PSCs due to their inherent characteristics like strong aggregate, relatively low electrical conductivity, and poor ohmic contact with the metal electrodes. Here, a mixed heterojunction layer (MHL) combined an amphiphilic polymer material of poly[(9, 9-bis(3′-(N, N-dimethylamino)propyl)-2, 7-fluo-rene)-alt-2, 7-(9, 9-dioctylfluorene)] with conventional ETL materials [6, 6] phenyl C61-butyric acid methyl ester (PC61BM) was developed to optimize the interface characteristics between perovskite and metal electrode. Atomic force microscopy, the space charge limited current, photoluminescence measurements, and impedance spectroscopy have been measured to reveal the interface contact, electronic, and optical properties of the MHL modified PSCs device. As a result, the power conversion efficiency (PCE) of PSCs with MHL has dramatically increased from 15.17% to 18.39%. More importantly, the stability of the MHL based PSCs was improved significantly, with reduced PCE degradation (from 29% to 18%) after 160 h under ambient conditions. This research provides a simple but effective method to improve the efficiency and stability of PSCs.