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Register a new userTuning the Electronic Levels of NiO with Alkali Halides Surface Modifiers for Perovskite Solar Cells
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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.
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We revisit the well-known Mollwo-Ivey relation that describes the universal dependence of the absorption energies of F-type color centers on the lattice constant $a$ of the alkali-halide crystals, $E_{mbox{abs}}propto a^{-n}.$ We perform both state-of-the-art ab-initio Quantum Chemistry and post-DFT calculations of F-center absorption spectra. By tuning independently the lattice constant and the atomic species we show that the scaling of the lattice constant alone (keeping the elements fixed) would yield $n=2$ in agreement with the particle-in-the-box model. Keeping the lattice constant fixed and changing the atomic species enables us to quantify the ion-size effects which are shown to be responsible for the exponent $n approx 1.8$.
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