Hydrolysis preparation of the compact TiO2 layer using metastable TiCl4 isopropanol/water solution for inorganic–organic hybrid heterojunction perovskite solar cells*Project supported by the National Natural Science Foundation of China (Nos. 51472071, 51272061, 51072043), and the National Basic Research Program of China (No. 2011CBA00700). (July 2015)