Retraction: Graded bandgap perovskite solar cells

Nature Materials 16, 522–525 (2017); published online 7 November 2016; retracted after print 20 December 2017. The authors of the study are retracting this Letter due to concerns about the reproducibility of the photovoltaic architecture performance presented, and with the interpretation of the dataincluded in the manuscript and Supplementary Information.