Band gap Tuning, Photosensitization and Applications of ZnS and CdS Nanoparticles in Solid State Dye Sensitized Solar Cells

Abstract

In this work an attempt was made to enhance the optical and morphological properties and tune the band gap of ZnS and CdS nanoparticles for application in SSDSSCs (solid state dye sensitized solar cells) as the photoactive materials on blending with a renowned organic polymer P3HT (Poly(3-hexyl thiophene)). In order to achieve this goal engineering of these nanoparticles was done by various ways i.e. varying the synthesis scheme, composite making, photo sensitization and doping. ZnS and CdS nanoparticles were synthesized by two different schemes i.e. simple co-precipitation procedure and hot injection method, their binary composites were prepared using GO (graphene oxide), photo sensitization was done by grafting three bench mark Ruthenium (Ru) dyes i.e. i.e. N3 [cis-Bis (isothiocyanato) bis (2,2’-bipyridyl-4,4’-dicarboxylato ruthenium(II))], N719 [Di-tetrabutyl ammonium cis-bis (isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium (II)] and Z907 [cis-Bis (isothiocyanato) (2,2’-bipyridyl-4,4’-dicarboxylato)(4,4’-di-nonyl-2’ bipyridyl) ruthenium (II)] over the surface of these nanoparticles and their graphene composites. Doping of ZnS and CdS nanoparticles was done with Eu (Europium) using single source molecular precursors i.e. dialkyldithiocarbamates via hot injection technique. ZnS, CdS, ZnS/rGO (reduced graphene oxide ZnS nanocomposites) and CdS/rGO (reduced graphene oxide CdS nanocomposites) with an approximate particle size of 15 ± 1 nm, 26± 1 nm, 5 ± 0.5 nm, 6 ± 0.5 nm were prepared by facile co-precipitation method using DMF (N, N-Dimethyl formamide) as a solvent. It also served as reducing agent in the reaction. These nanoparticles and composites were characterized by X-ray diffraction (XRD), Elemental dispersive X-ray analysis (EDX), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM), Fourier transform infra-red (FT-IR), Raman, Photoluminiscence (PL) and UV-Visible (UV-Vis) spectroscopy. Grafting of the renowned Ruthenium ((Ru) dyes i.e. N3, N719 and Z907) on the surface of ZnS, CdS, ZnS/rGO and CdS/rGO was studied comprehensively including concentration and functional group effects. UV-Vis, Photoluminiscence (PL) and Fourier transform infra-red (FT-IR) spectroscopy confirms the successful grafting of these dyes over ZnS, CdS, ZnS/rGO and CdS/rGO composites via carboxyl moiety. Low-energy metal-to-ligand charge-transfer transition (MLCT) bands of dyes are mainly affected on grafting over the nanoparticle surface. Current voltage (I-V) plots and resulting solar cell parameters of Ru dye grafted ZnS and CdS nanoparticles when applied as photo active material in SSDSSCs confirm the efficiency enhancement owing to effective sensitization of this material with Ru dyes and helps in finding the optimum dye concentration. 0.3mM concentration of the dye was found optimum for sensitization probably due to dye aggregation at higher concentrations. Ru dye grafted nanoparticle reduced GO ternary composites were found as effective photo active materials for application in SSDSSCs. 0.3mM N719/ CdS/rGO-P3HT has shown a maximum output efficiency of 1.01% which is two times in comparison to the reference device i.e. CdS-P3HT under the same experimental conditions. This report also throws light on the charge transfer mechanism in the nanocomposite. Synergic effects of nanoparticles, rGO, Ru dye, fitting of the Ru dyes LUMO in between LUMO of P3HT and the conduction band of nanoparticles and formation of cascaded energy levels for rGO /nanoparticle/ Ru ternary composites resulted in their enhanced efficiencies when employed as photo active materials in SSDSSCs. One of the major challenge in nanomaterial research is the incorporation of dopants with optical and magnetic functionalities into the colloidal nanocrystals. Chalcogenide sulphides such as ZnS and CdS are an important class of host materials for a variety of luminescent dopants including both transition metals and lanthanide ions. Doped ZnS and CdS nanoparticles have a broad spectrum of applications however the major hurdle towards their development is the lack of synthetic methodologies. In the second part of this work we have reported the single source precursor approach using dialkyldithiocarbamate precursors for synthesizing Eu-doped ZnS and CdS nanoparticles. It was aimed to study the enhancement in the optical properties of ZnS and CdS nanoparticles upon doping. All the doped and undoped nanoparticles synthesized using single source precursors were than characterized by XRD, TEM, FT-IR, XPS (X-Ray photo electron spectroscopy) and UV-Vis spectroscopy. Presence of Eu as a dopant in ZnS and CdS nanoparticles was confirmed by XPS. Addition of Eu metal as a dopant effectively tuned the band gap of chalcogenide nanoparticles from 3.5 to 3.2 eV for ZnS and 2.4 to 2.2 eV for CdS. However at higher dopant concentration for CdS slight widening of the band gap is observed probably due to Burstein-Moss phenomenon. Significant enhancement in the absorption spectrum of ZnS and CdS nanoparticles and tuning of their band gap towards visible solar spectrum upon doping with Eu showed their potential applications in SSDSSCs. When applied as the photoactive material in SSDSSCs ZnS-Eu 5-P3HT and CdS-Eu 3-P3HT were found 1.13 times and 2.25 times more efficient than the reference devices i.e. ZnS-P3HT and CdS-P3HT respectively.