Atomic-Scale Imaging and Nano-Scale Mapping of Cubic α-CsPbI3 Perovskite Nanocrystals for Inverted Perovskite Solar Cells

Colloidal synthesized cubic alpha-CsPbI3 perovskite nanocrystals having a smaller lattice constant (a = 6.2315 angstrom) compared to the standard structure, and nanoscale mapping of their surfaces are reported to achieve superior photovoltaic performance under 45-55% humidity conditions. Atomic scale transmission electron microscopic images have been utilized to probe the precise arrangement of Cs, Pb, and I atoms in a unit cell of alpha-CsPbI3 NCs, which is well supported by the VESTA structure. Theoretical calculation using density functional theory of our experimental structure reveals the realization of direct band to band transition with a lower band gap, a higher absorption coefficient, and stronger covalent bonding between the Pb and I atoms in the [PbI6](4-) octahedral, as compared to reported standard structure. Nanoscale surface mapping using Kelvin probe force microscopy yielding contact potential difference (CPD) and conductive atomic force microscopy for current mapping have been employed on alpha-CsPbI3 NCs films deposited on different DMSO doped PEDOT:PSS layers. The difference of CPD value under dark and light illumination suggests that the hole injection strongly depends on the interfaces with PEDOT:PSS layer. The carrier transport through grain interiors and grain boundaries in alpha-CsPbI3 probed by the single-point c-AFM measurements reveal the excellent photosensitivity under the light conditions. Finally, inverted perovskite solar cells, employing alpha-CsPbI3 NCs film as an absorber layer and PEDOT:PSS layer as a hole transport layer, have been optimized to achieve the highest power conversion efficiency of 10.6%, showing their potential for future earth abundant, low cost, and air stable inverted perovskite photovoltaic devices.

Automated summary

Colloidal synthesized cubic alpha-CsPbI3 perovskite nanocrystals with smaller lattice constant and their nanoscale surface mapping have shown superior photovoltaic performance. Atomic scale transmission electron microscopy has been used to probe the precise arrangement of atoms in the nanocrystals, and theoretical calculations reveal the realization of direct band to band transition with a lower band gap. Nanoscale surface mapping and conductive atomic force microscopy have been employed to study the films deposited on different layers, and the carrier transport through grain interiors and boundaries has been investigated. Inverted perovskite solar cells using alpha-CsPbI3 as an absorber layer have achieved high power conversion efficiency, showing potential for future photovoltaic devices.