Solution Processable Inorganic–Organic Double‐Layered Hole Transport Layer for Highly Stable Planar Perovskite Solar Cells

Abstract Perovskite solar cells (PSCs) have reached their highest efficiency with 2,2′,7,7′‐tetrakis( N , N′ ‐di‐ p ‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). However, this material can cause problems with respect to reproducibility and stability. Herein, a solution‐processable inorganic–organic double layer based on tungsten oxide (WO 3 ) and spiro‐OMeTAD is reported as a hole transport layer in PSCs. The device equipped with a WO 3 /spiro‐OMeTAD layer achieves the highest efficiency (21.44%) in the tin (IV) oxide planar structure. The electronic properties of the double layer are thoroughly analyzed using photoluminescence, space‐charge–limited current, and electrochemical impedance spectroscopy. The WO 3 /spiro‐OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO 3 layer particularly improves the open‐circuit voltage ( V OC ) by lowering the quasi‐Fermi energy level for holes and reducing charge recombination, resulting in high V OC (1.17 V in the champion cell). In addition, the WO 3 layer as a scaffold layer promotes the formation of a uniform and pinhole‐free spiro‐OMeTAD overlayer in the WO 3 /spiro‐OMeTAD layer. High stability under thermal and humid conditions stems from this property. The study presents a facile approach for improving the efficiency and stability of PSCs by stacking an organic layer on an inorganic layer.