Device physics of homojunction perovskite solar cells: a design omitting all the charge transport layers with efficiency exceeding 26.3%

Perovskite solar cells (PSCs) omitting all the charge transport layers with p-n homojunction structure are considered a promising alternative for commercialization owing to their low fabrication cost and simplified structure. Deep understanding of the device physics of these all-free p-n homojunction structured PSCs is of paramount importance. Here, a thorough investigation of all-free perovskite-perovskite p-n homojunction structured PSCs is performed by using a photoelectrical coupling model. Four different configurations including a standard n-i-p cell, electron transport layer-free cell, hole transport layer-free cell, and all-free cell are compared to identify the limiting performance factors, and the results indicate that no extra built-in electric field in the perovskite layer and severe surface recombination occurring at the perovskite interface are the two main factors limiting the power conversion efficiency (PCE) of all-free p-n homojunction structured PSCs. Based on doping engineering, a highly efficient all-free p-n homojunction structure is designed, which consists of an asymmetric p-n junction with both a front surface field layer and a back surface field layer. The effects of optical loss, thickness of the emitter, doping concentration for both the emitter and base, and diffusion length on the performance of p-n homojunction structured PSCs are optimized. After optimization, the PCE of the all-free p-n homojunction structured PSCs reaches 26.33%, which is slightly higher than that of a standard n-i-p heterojunction cell (26.22%). This work demonstrates that all-free p-n homojunction structured PSCs are a promising alternative to standard n-i-p heterojunction structured PSCs for realizing high efficiency, which may pave the way toward commercialization of PSCs in the future.