In Situ Growth of a Robust 1D Capping Layer for Stable and Efficient CsPbI 3 Perovskite Solar Cells Without Hole Transporter

CsPbI 3 perovskite is a promising light absorber in perovskite solar cells (PSCs) due to its suitable bandgap ( E g ) and high chemical stability, but the perovskite phase is metastable in ambient atmosphere and prone to defect formation. To enhance phase stability and reduce defects, forming a low dimensional (LD) perovskite on CsPbI 3 has proved effective. However, the energy levels for most of low‐conductivity LD perovskites are not very compatible with those of CsPbI 3 , which will impair charge separation and device performance. Herein, a new 1D perovskite (5‐Azaspiro[4.4]nonan‐5‐ium lead triiodide, ASNPbI 3 ) with compatible energy levels and a large E g is reported as a capping layer. The p‐type ASNPbI 3 forms a p‐n heterojunction with n‐CsPbI 3 with a staggered band alignment, considerably enhancing the carrier separation. Besides, by pre‐forming a ASNPbI 3 at an intermediate stage, defects are suppressed in perovskite film. Consequently, the CsPbI 3 PSCs based on carbon electrode without hole transporter (C‐PSCs) achieves a PCE of 18.34%, which is among the highest‐reported values for inorganic C‐PSCs. Furthermore, the hydrophobic ASNPbI 3 capping layer has a high thermal stability that greatly enhances perovskite phase stability. Hence, the CsPbI 3 C‐PSCs maintains 68% of its initial PCE after 400 h aging at 85°C in a N 2 glovebox.