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, 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-CsPbI3 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 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 ASNPbI(3) 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 degrees C in a N-2 glovebox.