Modeling of a high performance bandgap graded Pb-free HTM-free perovskite solar cell Davoud Jalalian, Abbas Ghadimi and Azadeh Kiani Eur. Phys. J. Appl. Phys., 87 1 (2019) 10101 Published online: 13 September 2019 DOI: 10.1051/epjap/2019190095