Green CsSnX3 (X = Cl, Br, I)-Derived Quantum Dots for Photovoltaic Applications: First-Principles Investigations

Photovoltaic (PV) materials with high efficiencies are currently the lead-based perovskites. However, nontoxic, stable, lead-free perovskites are of immense interest as environment-friendly green materials. Hence, using first-principles density functional theory (DFT), we investigate ligated CsSnX3 (X = Cl, Br, I)-derived quantum dots (QDs), to assess their suitability for PV cells. The well-known band gap increase due to quantum confinement effects is observed, with the excitonic energies quite close and exhibiting the same size dependence in all three types of QDs. The choice of ligands has no appreciable effect in altering the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap. Time-dependent DFT simulations show that all the QDs have good absorption in the useful UV–vis region of the spectrum, and the peaks are both size and halide dependent. Natural transition orbital analysis shows that interestingly, in most cases the charge transfer on optical excitation occurs from the halide p orbital to the Sn p orbital. The charge distribution assumes several interesting patterns, which may aid in charge collection. Larger QDs allow for greater charge separation and lower recombination rates, increasing the PV efficiency. Our work shows that good electronic and optical absorption properties, with appropriate band gaps and wide tunability, make H+ ligated CsSnX3-derived QDs (and in particular, CsSnI3-derived QDs) promising candidates for PV applications.