XRD characterization of graphene-contacted perovskite solar cells: moisture degradation and dark-resting recovery

The instability of emerging perovskite solar cells at moisture (humidity) exposure is now the main target of many research groups. The moisture instability impedes the up-scaling of the solution processing perovskite solar cells for large production lines. Analyzing the crystallinity of perovskite film can reveal the sensitivity against humidity at air exposure under light and the recovery dynamic at dark conditions. In this paper, X-ray diffraction has been deployed as a reliable characterization method to detect the film crystallinity and defect formation in perovskite films. We prepared Perovskite solar cells with reduced graphene oxide as the hole transporting layer aiming to protect the pemvskite layer from direct exposure to air oxygen and decomposition. Graphene has excellent heat conductivity and will accelerate the heat dissipation and light-induced degradation. It can also reduce the PbI2 defective layer formation on the perovskite surface. We have stressed the fabricated cell under light at air exposure for 30, and 45 days to track the degradation rate and then rested them in dark for 45 days to recover the performance and film crystallinity. The results were interesting: the grain size of perovskite layer reduced and the defect layer became thicker under the light of 30 and 45 days. Then a reverse phenomenon occurs at dark rest for 45 days and the grain size recovers slightly whereas the defect layer shrinks somewhat. We also observe peak (200) related to rGO layer which vanishes after 45 days and only a single peak (101) remains and becomes stronger after 45 days' dark rest. The lattice constants of the films were calculated and the phase recovery was analyzed. The dynamic crystallinity is an additional instability source for perovskite photovoltaics.