Tuning the Electronic Levels of NiO with Alkali Halides Surface Modifiers for Perovskite Solar Cells

TL;DR

This study uses density functional theory to show that alkali halide surface modifiers can significantly tune the electronic levels of NiO, enhancing its compatibility as a hole transport layer in perovskite solar cells.

Contribution

The paper demonstrates, through DFT calculations, that alkali halides can effectively and predictably shift NiO's valence band maximum, offering a new method for optimizing perovskite solar cell interfaces.

Findings

Alkali halides can shift NiO's VBM from -3.10 eV to +1.59 eV.

Surface dipoles formed by adsorbed ions determine the direction and magnitude of the energy level shifts.

The VBM shift correlates monotonically with surface coverage of the alkali halides.

Abstract

Favorable optoelectronic properties and ease of fabrication make NiO a promising hole transport layer for perovskite solar cells. To achieve maximum efficiency, the electronic levels of NiO need to be optimally aligned with those of the perovskite absorber. Applying surface modifiers by adsorbing species on the NiO surface, is one of the most widespread strategies to tune its energy levels. Alkali halides are simple inorganic surface modifiers that have been extensively used in organic optoelectronics, however, rarely studied in perovskite solar cells. Using density functional theory (DFT) calculations, we investigate the effect of single layer adsorption of twenty different alkali halides on the electronic levels of NiO. Our results show that alkali halides can shift the position of the valence band maximum (VBM) of NiO to a surprisingly large extend in both directions, from -3:10 eV to +1:59 eV. We interpret the direction and magnitude of the shift in terms of the surface dipoles, formed by the adsorbed cations and anions, where the magnitude of the VBM shift is a monotonic function of the surface coverage. Our results indicate that with alkali halide surface modifiers, the electronic levels of NiO can be tuned robustly and potentially match those of many perovskite compositions in perovskite solar cells.