| Summary: | We report first-principles calculations, within density functional theory, on the lead halide compounds PbCl<sub>2</sub>, PbBr<sub>2</sub>, and CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3−x</sub>Clx, taking into account spin-orbit coupling. We show that, when the modified Becke-Johnson exchange potential is used with a suitable choice of defining parameters, excellent agreement between calculations and experiment is obtained. The computational model is then used to study the effect of replacing the methylammonium cation in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> with either N<sub>2</sub>H<sub>5</sub><sup>+</sup>or N<sub>2</sub>H<sub>3</sub><sup>+</sup>, which have slightly smaller ionic radii than methylammonium. We predict that a considerable downshift in the values of the band gaps occurs with this replacement. The resulting compounds would extend optical absorption down to the near-infrared region, creating excellent light harvesters for solar cells.
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