Chemical and structural degradation of CH3NH3PbI3 propagate from PEDOT:PSS interface in the presence of humidity
Understanding interfacial reactions that occur between the active layer and charge-transport layers can extend the stability of perovskite solar cells. In this study, the exposure of methylammonium lead iodide (CH 3 NH 3 PbI 3 ) thin films prepared on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated glass to 70% relative humidity (R.H.) leads to a perovskite crystal structure change from tetragonal to cubic within 2 days. Interface-sensitive photoluminescence measurements indicate that the structural change originates at the PEDOT:PSS/perovskite interface. During exposure to 30% R.H., the same structural change occurs over a much longer time scale (>200 days), and a reflection consistent with the presence of (CH 3 ) 2 NH 2 PbI 3 is detected to coexist with the cubic phase by X-ray diffraction pattern. The authors propose that chemical interactions at the PEDOT:PSS/perovskite interface, facilitated by humidity, promote the formation of dimethylammonium, (CH 3 ) 2 NH 2 + . The partial A-site substitution of CH 3 NH 3 + for (CH 3 ) 2 NH 2 + to produce a cubic (CH 3 NH 3 ) 1− x [(CH 3 ) 2 NH 2 ] x PbI 3 phase explains the structural change from tetragonal to cubic during short-term humidity exposure. When (CH 3 ) 2 NH 2 + content exceeds its solubility limit in the perovskite during longer humidity exposures, a (CH 3 ) 2 NH 2 + -rich, hexagonal phase of (CH 3 NH 3 ) 1− x [(CH 3 ) 2 NH 2 ] x PbI 3 emerges. These interfacial interactions may have consequences for device stability and performance beyond CH 3 NH 3 PbI 3 model systems and merit close attention from the perovskite research community.