Numerical simulation of novel designed HTL-free perovskite solar cells to realize over 25% efficiency based on CH3NH3PbI3/CH3NH3PbI2Br cascade structure

The hole transport layer-free perovskite solar cells ((HTL-free PSCs) have drawn tremendous interest because of their simple structure, superior stability and low production cost. However, the inferior charge separation at the perovskite absorber/back electrode (carbon in most cases) interface limits the solar cell performance. In the research, a novel energy band structure with CH3NH3PbI3/CH3NH3PbI2Br cascade layers is designed and extensively analyzed using the SCAPS-1D simulator to explore its potential for HTL-free PSCs. In order to obtain optimal device performance, several critical device parameters, including the thickness value, doping concentration and defect density for both the CH3NH3PbI3 and CH3NH3PbI2Br layers, are systematically investigated and selected. After optimization, a decent photoelectric conversion efficiency of 25.32% is obtained for this structure, which provides 21.5% relative improvement compared with the 20.84% efficiency for the single-layered CH3NH3PbI3 PSC. The outcome of the current study indicates that the proposed CH3NH3PbI3/CH3NH3PbI2Br cascade energy band structure will provide an innovative route to the fabrication of high performance HTL-free PSCs.