High current, high efficiency graded band gap perovskite solar cells
  Organic-inorganic halide perovskite materials have emerged as attractive alternatives to conventional solar cell building blocks. Their high light absorption coefficients and long diffusion lengths suggest high power conversion efficiencies (PCE),1-5 and indeed perovskite-based single band gap and tandem solar cell designs have yielded impressive performances.1-16 One approach to further enhance solar spectrum utilization is the graded band gap, but this has not been previously achieved for perovskites. In this study, we demonstrate graded band gap perovskite solar cells with steady-state conversion efficiencies averaging 18.4%, with a best of 21.7%, all without reflective coatings. An analysis of the experimental data yields high fill factors of ~75% and high short circuit current densities up to 42.1 mA/cm2. These cells, which are based on a novel architecture of two perovskite layers (MASnI3 and MAPbI3-xBrx), incorporating GaN, monolayer hexagonal boron nitride, and graphene aerogel, display the highest efficiency ever reported for perovskite solar cells. 
Onur Ergen, S. Matt Gilbert, Thang Pham, Sally J. Turner, Mark Tian   Zhi Tan, Marcus A. Worsley, and Alex Zettl
cond-mat.mtrl-sci