Thin Films Solar Cells Based on Semiconducting Bismuth Sulphide and Quantum Dots Heterojunction

Abstract

The present research work was aimed to investigate the potential of bismuth sulfide and lead sulphide quantum dots thin films to be employed as n-type and p-type nanomaterials for efficient solar harvesting. Binary and ternary n-type bismuth sulphide and p-type lead sulphide thin films at different dopant concentration levels were deposited by the chemical bath deposition (CBD) and Successive Ionic layer Adsorption Reaction (SILAR) methods, respectively. Nitrate salts were used as cationic precursor, while thioacetamide and sodium sulphide were used as S2- source for deposition of bismuth sulphide and lead sulphide thin films, respectively. The aim of the study was also to improve the optoelectronic properties and reduce the toxicity level of constituent materials particularly, lead sulphide by means of doping. Few earth abundant and environment friendly, bi and tri-valent cations like; Cu2+, Ni2+, Co2+ and Al3+ were used as dopants. Five doped series of Bi2S3 and two series of PbS quantum dots thin films having different dopant content were deposited on microscopic glass slides. CBD and SILAR were found as the suitable and cost effective methods even extendable for the deposition of derivatives of both Bi2S3 and PbS thin films, respectively. Phase composition, optical, electrical, morphological and electronic transport properties were investigated by X-Ray Diffraction, UV-Vis. Spectroscopy, Photoluminescence, Hall Effect Studies, Scanning Electron Microscopy and Atomic Force Microscopy. The film thickness was measured by ellipsometry and was found to be dependent on the composition of bath solutions. Optical parameters i.e. absorption-coefficient, dielectric constants, dispersion and Eurbach energy were investigated. Electronic as well as transport properties including conductivity, type of charge carrier, sheet carrier concentrations and mobility of charges were also studied. The obtained data revealed that all deposited Bi2S3 and PbS quantum dots thin films have direct allowed band gaps energies (Eg). For Bi2S3 thin films, Eg value was 1.6eV which upon dopant addition reduced down to 1.1eV, while in case of PbS quantum dots thin films, Eg was as high up to 2.1eV which was successfully reduced down to 1.8eV. Doping also played a role to enhance the absorption capacity of the materials, especially for Bi2S3 derivatives. In case of Bi2S3 thin films and its derivatives, values of refractive index (n) were found in the range of 2.9 to 1.3 and for extinction co-efficient (k) values were 1.03 to 0.3. While, in case of PbS quantum dots thin films, respective values were in the range of 1.6 to 1.5 and 0.1 to 0.002, respectively. Photoluminescence spectra exhibited by all doped derivatives were also modified with reduced luminescence intensity. Ellipsometry studies revealed the decrease in film thickness for all samples but only in case of Al3+ doped Bi2S3, the thickness increased from 269.99 to 506.04 nm. Structural analysis showed that Bi2S3 conserved its orthorhombic crystal lattice for Ni2+ and Co2+ doped series. While for Cu2+ doped series, emergence of new crystalline phase occurred and for Al+3 doped series, a transition from crystalline to amorphous phase was observed. In case of quantum dots, few other peaks were observed along with PbS cubic phase. Topographical analysis validated the use of most of the synthesized materials in photovoltaic devices due to homogeneous and compact film deposition. Optoelectronic properties suggested that doping is an effective tool to enhance the charge carrier concentration for both the studied materials. All the synthesized binary and ternary materials were studied for fabrication of new heterojunctions in photovoltaic devices. Results showed that efficiency was enhanced from 0.36% to 0.54%, owing to the modification in the characteristic properties of individual n and p layers.