This work is supported financially by the Joint Research Program between China and the European Union (2016YFE0112000); the Major Research Plan of the National Natural Science Foundation of China (91733302); the National Basic Research Program of China-Fundamental Studies of Perovskite Solar Cells (2015CB932200); the Natural Science Foundation of Jiangsu Province, China (BK20150043, BK20150064, BK20180085); the National Natural Science Foundation of China (11474164, 11474249, 61634001); the National Science Fund for Distinguished Young Scholars (61725502, 61725503); and the Synergetic Innovation Center for Organic Electronics and Information Displays. We thank D. Di and B. Zhao for cross-checking the LED measurement system and Y. Zhao for helpful discussions. We thank M. Winton and N. Greenham for proof reading.

Reviewer information

Nature thanks A. Urban and the other anonymous reviewer(s) for their contribution to the peer review of this work.