Introducing back-surface field for efficient inverted CsPbI3 perovskite solar cells

Energy loss at perovskite/electron transporting layer (ETL) interface is one key reason limiting the efficiency of inverted CsPbI3 perovskite solar cells (PSCs). Here we introduce a back-surface field in inverted PSCs through 4-Imidazoleethylamine (4-IEA) treatment to mitigate such interfacial energy loss. 4-IEA treatment will upshift the Fermi level of CsPbI3 surface and thus induce an extra back-surface field aligning with the built-in potential of inverted PSCs, thus reducing energy loss and facilitating electrons extraction at the CsPbI3/ETL interface. In addition, 4-IEA can passivate interfacial defects owing to its Lewis base-acid interaction with CsPbI3. As a result, power conversion efficiency (PCE) of 20.22% is achieved in inverted CsPbI3 PSCs. Furthermore, PSCs with 4-IEA also exhibit good operational stability, retaining over 70% of initial efficiency after maximum power point (MPP) tracking at high temperature of 65 ℃ for 200 h.