Over the years, the efficiency of PSCs has increased at an unprecedented pace. However, many reports have revealed significant irreversible decomposition of the organic component under high humidity and high temperature conditions, implying the instability of organic-inorganic hybrid perovskite solar cell in real applications. Normally, inorganic materials exhibit better stability compared to organic materials, especially at elevated temperatures. However, the size of the Cs+ cation is too small to hold the PbI62? octahedron. Therefore, the photoactive α-phase (cubic phase) is unstable and the CsPbBrI2 and CsPbI3 materials easily convert to the undesired δ-phase (orthorhombic phase) at room temperature. In addition, a main limiting factor in the photoelectric performance of all-inorganic PSCs is the energy loss (a large Eloss ca. 0.7 to 0.9 eV). In brief, a large Eloss reflects inhomogeneous energy landscape, large trap density and significant energy disorder in the device, which generate a nonradiative energy loss channel and a Voc reduction. Therefore, enhancing the Voc to reduce Eloss is crucial for high performance in all-inorganic PSCs.
Generally, Voc is related to the band gap of perovskite, the highest occupied molecular orbital (HOMO) of the hole-transporting layer (HTL) and the lowest unoccupied molecular orbital (LUMO) of the electron-transporting layer (ETL). Meanwhile, the Voc of devices is also related to the quality of film (grain size). Such as, Kim et al. analyzed an intensity-dependent Voc on the basis of Shockley-Read-Hall (SRH) model and confirmed that decrease in grain size is accompanied by a downturn in optoelectronic performance of PSCs, due to the increase in trap density.
In this work, we demonstrate a secondary grain growth functionalization with ammonium oxalate ((NH4)2C2O4* H2O) to improve the optoelectronic performance of all-inorganic PSCs, wherein (NH4)2C2O4* H2O can effectively promote the secondary growth of the perovskite crystal to a few microns. The resulting high-quality perovskite film exhibited higher carrier mobility and lower trap density and eventually achieved ultra-low energy loss (0.64 eV). The CsPbBrI2:(NH4)2C2O4* H2O-based device exhibits a highest Voc of 1.24 V and PCE of 16.55% under AM 1.5 G, and a record PCE is 28.48% under under a fluorescent lamp of 1000 lux.
This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the "111" Project of The State Administration of Foreign Experts Affairs of China), and the Open Fund of the State Key Laboratory of Integrated Optoelectronics (IOSKL2018KF07).
See the article:
Kai-Li Wang, Xiao-Mei Li, Yan-Hui Lou, Meng Li, Zhao-Kui Wang, CsPbBrI2 Perovskites with Low Energy Loss for High-Performance Indoor and Outdoor Photovoltaics, Science Bulletin, 2020, doi: 10.1016/j.scib.2020.09.017