The great difference of electron mobility between TiO2 electron transport layers and perovskite layers in planar perovskite solar cells may lead to serious electron stacking in their interfaces, which further results in high electron recombination rates and photocurrent density-voltage hysteresis. The surface modification of TiO2 electron transport layers is an effective way to overcome the problem. Here, we report the successful fabrication of high efficiency and negligible hysteresis planar perovskite solar cells through modifying the TiO2 electron transport layers with p-type NiO nanocrystals. We find that the NiO nanocrystals form large numbers of NiO islands rather than homogeneous NiO films on the TiO2 electron transport layers. This special island-like morphology is propitious to maintain high light transmittance and induce large crystal grains and high crystallinity of perovskite layers. Furthermore, the device with NiO nanocrystals modification shows more effective electron extraction and lower charge accumulation than the pristine one. The champion device can achieve forward scanned and reverse-scanned efficiencies of 19.22% and 19.42%, respectively. Hence, it is a novel guideline toward facile surface modification of TiO2 electron transport layers for efficient and hysteresis-free planar perovskite solar cells.