Dye-sensitized quantum dots (QDs) are promising candidates for dye-sensitized solar cells (DSSCs). Here, we report steady state (absorption and photoluminescence) optical measurements on several sizes of CdS QDs ligated with Coumarin 343 dye (C-343) and two different solvents, viz., chloroform and toluene. We further report detailed first principles density functional theory and time-dependent density functional theory studies of the geometric, electronic and optical (absorption and emission) properties of three different sized capped QDs, ligated with C-343 dye. The absorption spectrum shows a QD-size-independent peak, and another peak which shifts to blue with decrease in QD size. The first peak is found to arise from the dye molecule and the second one from the QD. Charge transfer using natural transition orbitals (NTOs) is found to occur from dye-to-QDs and is solvent-dependent. In the emission spectra, the luminescence intensity of the dye is quenched by the addition of the QD indicating a strong interaction between the QD and the dye.

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.

Direct and indirect charge transfer mechanisms in dye–QD composites.

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.

A set of eight heteroleptic bis-diimine copper dye complexes with two different ancillary ligands are investigated for their potential use as sensitizers in dye-sensitized solar cells using first principles density functional theory.