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 (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 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.