Two-Dimensional 111-TypeIn-Based Halide PerovskiteCs3In2X9(

Despite rapid progress in the power-conversion efficiency of $\mathrm{Pb}$-based perovskite solar cells, both the long-term instability and $\mathrm{Pb}$ toxicity are still the main challenges for their commercial applications. Here, by first-principles $GW$ calculations, we find three kinds of two-dimensional (2D) 111-type $\mathrm{Pb}$-free $\mathrm{In}$-based halide perovskites of the form ${\mathrm{Cs}}_{3}{\mathrm{In}}_{2}{X}_{9}\phantom{\rule{0.2em}{0ex}}(X=\mathrm{Cl},\mathrm{Br},\mathrm{I})$ as promising alternatives to the star material ${{\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}\mathrm{I}}_{3}$ (${\mathrm{MA}\mathrm{Pb}\mathrm{I}}_{3}$) because of the following excellent electronic, optical, and transport properties: (i) The 2D $\mathrm{In}$-based halide perovskites are environmentally friendly lead-free materials. (ii) Compared with $\mathrm{MA}\mathrm{Pb}{X}_{3}$, they have greater structural stability. (iii) As energetic photovoltaic materials, 2D ${\mathrm{Cs}}_{3}{\mathrm{In}}_{2}{\mathrm{I}}_{9}$ perovskites are direct-band-gap semiconductors with optimal band gaps from 1.25 eV (trilayer) to 1.47 eV (monolayer). (iv) The 2D ${\mathrm{Cs}}_{3}{\mathrm{In}}_{2}{X}_{9}$ perovskites have ideal band structures for solid-state lighting with a wide direct-optical-band-gap range (approximately 0.94--3.54 eV), covering the whole visible-light region, and light electron (heavy hole) effective mass, which will directly enhance the defect-insensitive emission efficiency due to the localization of holes. Particularly, ${\mathrm{Cs}}_{3}{\mathrm{In}}_{2}{\mathrm{Br}}_{x}{\mathrm{Cl}}_{9\ensuremath{-}x}$ has a suitable direct optical band gap for highly desired blue emission. (v) The absorption coefficient of ${\mathrm{Cs}}_{3}{\mathrm{In}}_{2}{X}_{9}$ is up to $7\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which is between that of $\mathrm{Ga}\mathrm{As}$ (${10}^{4}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) and that of ${\mathrm{MA}\mathrm{Pb}\mathrm{I}}_{3}$ (${10}^{5}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$). (vi) The estimated power-conversion efficiency in ${\mathrm{Cs}}_{3}{\mathrm{In}}_{2}{\mathrm{I}}_{9}$ reaches 28%, which is close to that of ${\mathrm{MA}\mathrm{Pb}\mathrm{I}}_{3}$ (30%). These findings pave a way for designing nontoxic, stable, and high-performance photovoltaic and light-emitting devices.