Electronic and Morphological Inhomogeneities in Pristine and Deteriorated Perovskite Photovoltaic Films

Abstract

We perform scanning microwave microscopy (SMM) to study the spatially varying electronic properties and related morphology of pristine and degraded methylammonium lead-halide (MAPI) perovskite films fabricated under different ambient humidity. We find that higher processing humidity leads to the emergence of increased conductivity at the grain boundaries but also correlates with the appearance of resistive grains that contain PbI2. Deteriorated films show larger and increasingly insulating grain boundaries as well as spatially localized regions of reduced conductivity within grains. These results suggest that while humidity during film fabrication primarily benefits device properties due to the passivation of traps at the grain boundaries and self-doping, it also results in the emergence of PbI2-containing grains. We further establish that MAPI film deterioration under ambient conditions proceeds via the spatially localized breakdown of film conductivity, both at grain boundaries and within grains, due to local variations in susceptibility to deterioration. These results confirm that PbI2 has both beneficial and adverse effects on device performance and provide new means for device optimization by revealing spatial variations in sample conductivity as well as morphological differences in resistance to sample deterioration.

Impact of H2O on organic–inorganic hybrid perovskite solar cells

“…The presence of small quantities of newly formed PbI 2 were later shown to benefit the electronic properties of grain boundaries in humid annealing, increasing the local conductivity through passivating traps at the grain boundaries and performing a selfdoping role. 63 In situ techniques detected amorphous materials containing hydroxyl ions and the increase of grain size as direct evidence of the effect of water on FAPbI 3 films. 56,64 Besides H 2 O, the annealing process under different solvent vapors such as DMF and DMSO, 65 has resulted in better perovskite crystallinity.…”

“…68,69 However, the high humidity processing results in the appearance of more PbI 2 as insulating regions localized at the grain boundaries and within perovskite grains. 63 To restrain the likely adverse effects, the range of optimized R.H. had better be precisely controlled by considering other parameters like temperature scales, and ingredients. Regardless of the extremely humid conditions, annealing in a moderate humid atmosphere could be a self-improvement method for the perovskite device.…”

“…132 Irreversible decomposed product PbI 2 caused by water exposure is found to spatially contaminate the conductivity within the grain inner regions and grain boundaries. 63…”

Optically coupled methods for microwave impedance microscopy

“…1 d. Local topography can still affect optically modulated MIM measurements, however, both through local geometry (eg. ∇ 2 z) [20] and inhomogeneous lighting. To demonstrate this, optically modulated MIM measurements of a highly textured, photoconductive sample are shown in Appendix B.…”

Determination of the True Lateral Grain Size in Organic–Inorganic Halide Perovskite Thin Films

“…3 Despite the strong interest in these materials, many basic material properties remain elusive. For example, there has been considerable debate about the nature of grain boundaries (GBs) and their impact on the PCE of organic–inorganic perovskite PV devices, 4–7 with the latest work suggesting that GBs exhibit GB-to-GB heterogeneity and depth-dependence in electrical properties. 6,7 Similarly, there has been no study to date that has been able to definitively demonstrate if the apparent grain size, as assessed from top-down imaging techniques, represents a single crystal or a polycrystalline domain.…”