In Situ Growth of a Robust 1D Capping Layer for Stable and Efficient CsPbI3 Perovskite Solar Cells Without Hole Transporter

CsPbI3 perovskite is a promising light absorber in perovskite solar cells (PSCs) due to its suitable bandgap (Eg) 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 CsPbI3 has proved effective. However, the energy levels for most of low‐conductivity LD perovskites are not very compatible with those of CsPbI3, which will impair charge separation and device performance. Herein, a new 1D perovskite (5‐Azaspiro[4.4]nonan‐5‐ium lead triiodide, ASNPbI3) with compatible energy levels and a large Eg is reported as a capping layer. The p‐type ASNPbI3 forms a p‐n heterojunction with n‐CsPbI3 with a staggered band alignment, considerably enhancing the carrier separation. Besides, by pre‐forming a ASNPbI3 at an intermediate stage, defects are suppressed in perovskite film. Consequently, the CsPbI3 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 ASNPbI3 capping layer has a high thermal stability that greatly enhances perovskite phase stability. Hence, the CsPbI3 C‐PSCs maintains 68% of its initial PCE after 400 h aging at 85°C in a N2 glovebox.