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

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 (WO3) and spiro‐OMeTAD is reported as a hole transport layer in PSCs. The device equipped with a WO3/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 WO3/spiro‐OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO3 layer particularly improves the open‐circuit voltage (VOC) by lowering the quasi‐Fermi energy level for holes and reducing charge recombination, resulting in high VOC (1.17 V in the champion cell). In addition, the WO3 layer as a scaffold layer promotes the formation of a uniform and pinhole‐free spiro‐OMeTAD overlayer in the WO3/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. A tungsten oxide (WO3)/2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD double layer) is developed as a hole transport layer (HTL) in perovskite solar cells. WO3 improves the charge extraction, charge transport, and the quality of the spiro‐OMeTAD overlayer, exhibiting 21.44% efficiency. Particularly, WO3 increases the VOC of devices owing to its valence band energy level. The stabilities of devices are considerably improved by the uniform and pinhole‐free HTL derived from the WO3 scaffold layer.