Large surface polarization buildup and charge dynamics in lead-halide hybrid perovskite films under very intense optical irradiance

Solution-processed hybrid lead halide perovskites are most highly-performant and cost-effective material systems for advanced thin film solar cells also extremely appealing for high concentrated photovoltaic (HCPV) applications, at extremely high levels of optical irradiance [1,2] and as light emitting devices [3]. In this perspective, the assessment of the impact of intense illumination on the performances and long-term device reliability of hybrid perovskites is still an open research challenge. We report on the comparative characterization of long-term dynamical evolution of polarization and charge dynamics, photoinduced by intense optical irradiance at 500suns in methylammonium lead iodide (MAPbI3). Polycrystalline MAPbI3 films were deposited by the two step interdiffusion method on charge selective contacts, namely electron-transport-layer (ETL) compact TiO2 and hole-transport-layer (HTL) PEDOT:PSS, respectively as parts of standard and inverted solar cells structures. The analysis was performed in vacuum conditions and in the absence of any electrical bias, by Laser-Assisted Time-Resolved Scanning Electron Microscopy (TR-SEM) technique [3], supported by in-situ time-integrated PL measurements. A large surface positive photovoltage builds up in both systems with a turn-on time of seconds and a much slower decay time of tens of minutes and up to hours. Mixed electron-ionic conduction and defect formation and migration -triggered and driven by strong optical absorption- can be at the origin of the phenomenon. However, the evolution of photoinduced polarization is strongly different in the two cases. Both TR-SEM and PL confirm that the specific nature of the charge selective contact and related interface crucially affect the optoelectronic response of MAPbI3 thin films. Specifically, MAPbI3/PEDOT:PSS films show a p-type semiconducting behavior, with reversible photo-excited polarization effects, fast and reversible quenching of PL and some degree of persistent luminescence. MAPbI3/TiO2 films behave as n-type, featuring much slower but non-reversible PL quenching dynamics, no residual luminescence, photo-induced inhomogenous space-charge fields and semi-permanent modifications. The results of jointly performed TR-SEM and PL characterizations suggest that PEDOT:PSS/MAPbI3 films are more robust in view of high-irradiation applications, while in the case of TiO2/MAPbI3 films, even in the absence of morphological surface damage, physico-chemical degradation may locally occur in the bulk and at the contact interface, definitely altering the photo-physical response of the material and deserving further investigations. [1] Wang, Z., et al., Nat. Energy 2018, 3, 855-861. [2] Lin, Q., et al., Adv. Sci. 2018, 5 (4), 1–8. [3] Kim, Y.-H., Cho, H.,Lee, T.-W., Proc. Natl. Acad. Sci. 2016, 113 (42), 11694–11702 [4] Irde, G. et al., Micron 2019, 121, 53–65.