Synthesis, characterization and photovoltaic applications of noble metal—doped ZnS quantum dots

Abstract

Un-doped zinc sulphide (ZnS) and noble metal-doped ZnS quantum dots were synthesized by chemical method. The synthesized nanomaterial was characterized by UV-Visible, Fourier Transformed Infra-red Spectrometer (FTIR), Fluorescence, High Resolution Transmission Electron Microscope (HRTEM) and X-ray Diffraction (XRD). Scanning electron microscope complemented with energy dispersive X-ray was used to study particle size and chemical composition of un-doped and metal-doped ZnS QDs. A significant red shift in absorption band was observed in metal doped ZnS sample having concentration of impurity 0.05 M with respect to the un-doped ZnS with increasing the size of nanocrystals. Such red shift of the absorption bands are attributed due to quantum confinement effect. As un-doped and noble metal-doped ZnS QDs are direct band gap materials. The band gap energy of ZnS, Ag-doped, and Au-doped ZnS quantum dots is found to 2.5 eV, 2.4 eV and 2.2 eV, respectively. PL spectrum of the un-doped zinc sulphide nanocrytals witnessed two bands, first at 418 nm and second at 468 nm. However, PL spectrum of Ag-doped ZnS nanocrystals shows no green light emission band while a broad emission ranging up to 700 nm in Au-doped ZnS sample. XRD spectra of synthesized QDs exhibited that the material was in cubic phase. X-ray Photoelectron Spectroscopy (XPS) was used for comparative study of surfaces of zinc sulphide and noble metal zinc sulphide nanocrystals. The effects of inserting ZnS and noble metal doped ZnS nanocrystals in the active layer on the performance of organic solar cells were studied. The active layer mainly comprises of electron acceptor [6,6] phenyl-C61-butyric acid methyl ester (PCBM) and organic electron donor poly (3-hexylthiophene (P3HT) with quantum dots dissolved in dichlorobenzene. The cell conversion efficiency improved by embedding the doped QDs in the active layer blend. The cells fabricated with ZnS QDs exhibited cell conversion efficiency of 1.46%. By doping with silver and gold, the cell conversion efficiency was increased about 1.85% and 2.01%, respectively. The morphology of blend of nanocrystals with P3HT:PCBM were studied using atomic force microscopy.