where k, T and q are Boltzmann constant, temperature, and elementary electric charge, respectively. The difference in ΔVoc,non-rad (0.09 V) matches well with the difference of device Voc (0.09 V). In addition to improved PV performance, the FA0.91Cs0.09PbI3 device also shows significantly enhanced stability. We firstly measure the shelf life by storing the unencapsulated devices in dark at 25 °C and 20% relative humidity. Fig. shows that the PCE of the FAPbI3 device decreases by about 30% after 3,000 h aging, whereas the FA0.91Cs0.09PbI3 device shows a degradation of only 10% over 4,500 h aging. We then investigate the long-term operational stability of the PSCs by aging the unencapsulated devices under a nitrogen atmosphere, using maximum power point (MPP) tracking under simulated 1-sun conditions. As shown in Figure , the FA0.91Cs0.09PbI3 based PSCs retains over 90% of the initial PCE while the FAPbI3 device maintains only 60% PCE after 1000 h continuous illumination (fig. ).