2, Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland, United States
7, Department of Physics, University of Maryland, College Park, Maryland, United States
3, Institute of Chemistry, University of Campinas, Campinas, SP, Brazil
4, Department of Electrical and Computer Engineering, University of Maryland, College Park, Maryland, United States
5, Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California, United States
6, Department of Physics, Federal University of Minas Gerais, Belo Horizonte, MG, Brazil
Metal halide perovskites show great potential for a wide variety of optoelectronic devices, ranging from light-emitting diodes to photovoltaics. While the prototypical MAPbI3 has relatively poor stability under ambient conditions, the addition of small amounts of Cs (10-20%) to FAPbI3 has been shown to increase thermal, optical, and structural stability across the entire range of possible I/Br ratios. However, the influence of moisture, oxygen, and temperature on the optoelectronic properties of these Cs-FA perovskites remains unknown. To explore the individual and combined contributions of each of these parameters onto perovskites’ optical behavior, we use micro-photoluminescence (micro-PL) with in situ environmental control on four relevant CsxFA1-xPb(IyBr1-y) compositions [1, 2]. We subject each sample to temporally identical humidity loops (< 5%, 15%, 35%, 55%, and < 5% rH) and identify that humidity levels up to 35% rH increase the PL emission of all compositions considered by removing surface trap states. By contrast, we find that 55% rH reduces the overall PL emission for 38%-Br films, but sustains the PL enhancement from the prior 35% rH soak for the 17%-Br perovskites. The same 38%-Br films also show an appreciable and partially reversible red shift in their PL peak, correlated with the relative humidity level. Finally, upon completion of the humidity loop, all compositions except Cs-17%/Br-38% exhibit luminescence hysteresis; the extent of hysteresis is predominantly influenced by the Cs-Br ratio. We this attribute the PL hysteresis to surface-limited degradation occurring throughout the 55% rH condition, where sufficient intercalation of the water into the perovskite lattice leads to the formation of FAI, FABr, PbI2, and PbBr2. This degradation process leads to the formation of new sites for nonradiative recombination. Our environmental micro-PL method can be expanded to a range of emerging perovskite compositions and extended to include additional degradation factors. Finally, we will discuss how the control of each environmental parameter on perovskites degradation and recovery can be tackled by a machine-learning paradigm .
 J. M. Howard, et al., Journal of Physical Chemistry Letters, DOI: 10.1021/acs.jpclett.8b01127 (2018)
 E. M. Tennyson, J. M. Howard, et al., ACS Energy Letters 2, 1825 (2017) - Invited Perspective.
 J. M. Howard, et al., Joule, in preparation (2018) - Invited Perspective.