CH3NH3PbI3 perovskite materials deposited on ZnO electron transport layers (ETLs) undergo decomposition during annealing in N-2 atmosphere. We found that this decomposition can be retarded by restricting sublimation. Moreover, the decomposition upon thermal annealing is highly suppressed by TiCl4 treatment of the ZnO ETLs. Finally, we present a facile method for prolonged annealing in sol-gel ZnO-based perovskite solar cells, which exhibit a power conversion efficiency of 16.5% at a small area of 0.098 cm(2) and 15.1% at a large area of 1 cm(2) (active area) under AM 1.5 simulated sunlight illumination. This study shows that TiCl4 hydrolysis is an effective surface treatment for ZnO ETLs to be used in low-temperature and efficient sol-gel ZnO-based per-ovskite solar cells.