420 nm thick CH3NH3PbI3−xBrx capping layers for efficient TiO2 nanorod array perovskite solar cells*Project supported by the National Natural Science Foundation of China (Grant Nos. 51472071 and 51272061) and Talent Project of Hefei University of Technology, China (Grant Nos. 75010-037004 and 75010-037003). (January 2018)