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 (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 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 E_g 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.