Air‐Processed Carbon‐Based Cs _0.5 FA _0.5 PbI ₃ –Cs ₄ PbI ₆ Heterostructure Perovskite Solar Cells with Efficiency Over 16%

The preparation of low‐cost, high‐stability cell devices is the primary trend in the future development of perovskite solar cells (PSCs). The hole transport layer (HTL)‐free carbon electrode‐based PSCs (C‐PSCs) based on methylamine (MA) free perovskites prepared in the ambient air environment is expected to realize this goal. In MA‐free perovskites represented by Cs x FA 1– x PbI 3 , although the increase of Cs content increases the bandgap, Cs x FA 1– x PbI 3 with high Cs content has superior stability. Herein, a hot flow‐assisted (HFA) spin‐coating method to prepare Cs 0.5 FA 0.5 PbI 3 films in the ambient air environment with a wide relative humidity operation window is proposed. Compared with the traditional antisolvent method that requires a dry processing environment, the HFA method can easily prepare high‐quality Cs 0.5 FA 0.5 PbI 3 films in a high‐humidity ambient air environment. In addition, it is found that the presence of trace amount of Cs 4 PbI 6 can not only passivate the defects of Cs 0.5 FA 0.5 PbI 3 but also suppress the undesired phase transition. As a result, the in situ formed Cs 0.5 FA 0.5 PbI 3 –Cs 4 PbI 6 heterostructure film corresponds to better stability and higher photovoltaic performance than the plain Cs 0.5 FA 0.5 PbI 3 film. The efficiency of the assembled champion C‐PSCs is up to 16.30%, which is currently the highest efficiency for MA‐ and HTL‐free low‐temperature C‐PSCs.