Composition Related Tunability of “Green” Core/Shell Quantum Dots for Photovoltaic Applications from First Principles

Quantum dots (QDs) with core/shell (c/s) type configurations are promising candidates for photovoltaic (PV) applications, as they are known to enhance the QD stability, and are also expected to reduce charge carrier recombination both by reducing the trap states and increasing charge carrier separation. Hence, here we report detailed first-principles studies of different compositions of c/s QDs made from nontoxic materials, namely, CuInSe2/ZnS, CuInSe2/ZnSe, and CuInSe2/CuInS2 and their inverts, namely, ZnS/CuInSe2, ZnSe/CuInSe2, and CuInS2/CuInSe2. The geometric and electronic properties are studied using first-principles density functional theory (DFT). The optimized structures of all the QDs were found to have a defect-free c/s interface, which would reduce charge-carrier recombination rates arising due to charge trapping. The projected density of states (PDOS) of the QDs shows that the highest occupied molecular orbital (HOMO) is mainly composed of either S or Se states, whereas Zn or In states constitute the lowest unoccupied molecular orbital (LUMO). Time-dependent DFT (TDDFT) calculations of the optical transitions show that these systems have a strong absorption in the visible region of the spectrum. Interestingly, the c/s configuration enables the tailoring of the electronic and optical properties compared to the bulk as well as QD systems; as in the c/s QDs, the relative thickness as well as material composition of the core and shell is also a tunable parameter, in addition to the QD size. A natural transition orbital (NTO) analysis of the charge transfer upon light absorption shows, surprisingly, that charge separation occurs only in certain c/s configurations, such as the CuInSe2/ZnS QD, via fast direct electron transfer from core to shell, and CuInSe2/ZnSe QD, via indirect electron transfer, which may be slower. Hence, these compositions are expected to exhibit better efficiencies for PV applications. Thus, our study also highlights the importance of the NTO analysis in giving a detailed insight into local excitations and charge transfer excitations in these promising systems.