The significantly enhanced Voc of the FA0.91Cs0.09PbI3 device is mainly due to suppressed nonradiative recombination, which can be quantified by measuring the external quantum efficiency of electroluminescence (EQEEL) values . As shown in Fig. , at the injection current densities corresponding to Jsc, the EQEEL value of the FA0.91Cs0.09PbI3 device is 6.38%, while that of the FAPbI3 device is 0.16%. We calculate the voltage losses due to non-radiative recombination (ΔVoc,non-rad) based on the formula :