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 efficiencies1,2,3,4,5, and indeed perovskite-based single bandgap and tandem solar cell designs have yielded impressive performances1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16. One approach to further enhance solar spectrum utilization is the graded bandgap, but this has not been previously achieved for perovskites. In this study, we demonstrate graded bandgap 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 cm−2. The cells are based on an architecture of two perovskite layers (CH3NH3SnI3 and CH3NH3PbI3−xBrx), incorporating GaN, monolayer hexagonal boron nitride, and graphene aerogel.
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The authors thank B. Lechene (A. Arias group) and D. Hellebusch, S. Hawks and N. Bronstein (P. Alivisatos group) for use of the solar simulator, J. Kim and C. Jin (F. Wang group) for PL measurements and discussions, E. Cardona (O. Dubon group) for XRD measurements, L. Leppert (J. Neaton group) for valuable discussions on investigation of bandgap alignment, and T. Moiai and K. Emery (National Renewable Energy Laboratory) for valuable technical discussions on calibration, J–V measurements, and EQE measurements. This research was supported in part by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the US Department of Energy under Contract No. DE-AC02-05CH11231, which provided for PL measurements under an LDRD award, and, within the sp2-bonded materials program (KC2207), for the design of the experiment and material characterization; the National Science Foundation under Grant 1542741, which provided for photovoltaic response characterization; and by the Office of Naval Research (MURI) under Grant N00014-16-1-2229, which provided for h-BN growth. This work was additionally supported by Lawrence Livermore National Laboratory under the auspices of the US Department of Energy under Contract DE-AC52-07NA27344 through LDRD 13-LW-099, which provided for graphene aerogel synthesis. S.M.G. acknowledges support from the NSF Graduate Fellowship Program.