The Importance of Moisture in Hybrid Lead Halide Perovskite Thin Film Fabrication

Abstract

Moisture, in the form of ambient humidity, has a significant impact on methylammonium lead halide perovskite films. In particular, due to the hygroscopic nature of the methylammonium component, moisture plays a significant role during film formation. This issue has so far not been well understood and neither has the impact of moisture on the physical properties of resultant films. Herein, we carry out a comprehensive and well-controlled study of the effect of moisture exposure on methylammonium lead halide perovskite film formation and properties. We find that films formed in higher humidity atmospheres have a less continuous morphology but significantly improved photoluminescence, and that film formation is faster. In photovoltaic devices, we find that exposure to moisture, either in the precursor solution or in the atmosphere during formation, results in significantly improved open-circuit voltages and hence overall device performance. We then find that by post-treating dry films with moisture exposure, we can enhance photovoltaic performance and photoluminescence in a similar way. The enhanced photoluminescence and open-circuit voltage imply that the material quality is improved in films that have been exposed to moisture. We determine that this improvement stems from a reduction in trap density in the films, which we postulate to be due to the partial solvation of the methylammonium component and "self-healing" of the perovskite lattice. This work highlights the importance of controlled moisture exposure when fabricating high-performance perovskite devices and provides guidelines for the optimum environment for fabrication. Moreover, we note that often an unintentional water exposure is likely responsible for the high performance of solar cells produced in some laboratories, whereas careful synthesis and fabrication in a dry environment will lead to lower-performing devices.

Investigation of moisture stability and PL characteristics of terpineol-passivated organic–inorganic hybrid perovskite

“…This effect may also explain why an increase of the PL lifetime is observed when the perovskite films are stored under more humid atmospheres. Typically, the surface modifications of perovskite are performed during the solution preparation or annealing step [29,30]. However, the findings of this work show that when minimal surface modifications occur due to degradation, surface states can become passivated leading to prolonged charge carrier recombination lifetimes.…”

Stability Issues on Perovskite Solar Cells

“…They argued that the transport of chemicals ions would be accelerated because of dissolution of hygroscopic materials in devices. Very recently, Snaith et al reported the similar views that moisture exposure in preparation process resulted in the partial solvation of methylammonium component and reorganization of perovskite lattice, leading to a decrease in trap density and an improvement of Voc and photoluminescence [24]. Park et al reported a water-repellent perovskite based on an anti-reflective lotus leaf-inspired hierarchical pyramidal arrays that were fabricated using polydimethylsiloxane (PDMS) film [25].…”

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…[90,222] As the structure is soluble in water, the presence of humidity during film processing can significantly influence the thin film morphology and can lead to improved solar cell performance. [3,46,223] However, when fully-formed perovskite solar cells are exposed to air with a water-vapor content above 50% at room temperature, a loss in performance is incurred. [224] Understanding the effect of moisture on perovskite materials is thus rather important with a view towards achieving stable performance over a decade.…”

“…[45,223] Petrus et al have recently investigated the influence of moisture on MAPbI 3 films and solar cells derived from nonstoichiometric precursor mixtures. [101] They followed both the structural changes under controlled air humidity (Figure 13) through in situ X-ray diffraction, and the electronic behavior of devices prepared from these films.…”