• Energy Research

A 3-layer graded-bandgap solar cell with glass/FTO/ZnS/CdS/CdTe/Au structure has been fabricated using all-electrodeposited ZnS, CdS and CdTe thin layers. The three semiconductor layers were electrodeposited using a two-electrode system for process simplification. The incorporation of a wide bandgap amorphous ZnS as a buffer/window layer to form glass/FTO/ZnS/CdS/CdTe/Au solar cell resulted in the formation of this 3-layer graded-bandgap device structure. This has yielded corresponding improvement in all the solar cell parameters resulting in a conversion efficiency >10% under AM1.5 illumination conditions at room temperature, compared to the 8.0% efficiency of a 2-layer glass/FTO/CdS/CdTe/Au reference solar cell structure. These results demonstrate the advantages of the multi-layer graded-bandgap device architecture over the conventional 2-layer structure. In addition, they demonstrate the effective application of the two-electrode system as a simplification to the conventional three-electrode system in the electrodeposition of semiconductors with the elimination of the reference electrode as a possible impurity source.

Electrochemical deposition and characterization of nanocrystallite-CdS thin films for thin film solar cell application are reported. The two-electrode system used provides a relatively simple and cost-effective approach for large-scale deposition of semiconductors for solar cell and other optoelectronic device application. Five CdS thin films were deposited for 45 minutes each at different cathodic deposition voltages in order to study their properties. X-ray diffraction study reveals that the as-deposited films contain mixed phases of hexagonal and cubic CdS crystallites with large amounts of internal strain and dislocation density. Postdeposition annealing results in phase transformation which leaves the films with only the hexagonal crystal phase and reduced strain and dislocation density while increasing the crystallite sizes from 21.0–42.0 nm to 31.2–63.0 nm. Photoelectrochemical cell study shows that all the CdS films have n-type electrical conductivity. Optical characterization reveals that all samples show similar transmittance and absorbance responses with the transmittance slightly increasing towards higher growth voltages. All the annealed films show energy bandgap of 2.42 eV. Scanning electron microscopy and energy dispersive X-ray analyses show that grains on the surface of the films tend to get cemented together after annealing with prior CdCl2 treatment while all the films are S-rich.

Abstract Electrodeposition of CdTeSe thin films with two-electrode electrodeposition method in potentiostatic mode was performed at different TeO2 concentrations of 0.075, 0.150, 0.225, 0.30, and 0.375 mM. The structural, optical, surface morphology, surface roughness, and compositional properties of as-deposited (AD) and annealed (HT) CdTeSe thin film samples were investigated by using x-ray diffraction (XRD), ultraviolet-visible (UV-Vis) spectrophotometry, scanning electron microscopy, scanning probe microscopy and energy-dispersive x-ray spectroscopy, respectively. The XRD results confirmed that CdSe thin films are of hexagonal structure. After TeO2 was added, the CdTeSe films were found to have mixed hexagonal and cubic phases. The UV-Vis spectrophotometry results confirmed that the absorbance and band gap of the materials varied as the TeO2 concentration changed. For AD samples, the energy band gap was found to be (1.45–1.75) eV; for HT samples, it varied from (1.53–1.86) eV with TeO2 concentration. The average surface roughness was 93.17 nm and 79.59 nm for the AD and HT un-doped CdSe films, respectively. The average surface roughness values for the AD TeO2 doped (CdTeSe) samples were observed to be 23.00, 162.29, and 26.90 nm, and for the HT samples, they were 25.80, 153.10, and 19.35 nm for TeO2-concentrations of 0.075,0.150, and 0.375 mM respectively. Compositional analysis verified the presence of Cd, Te, and Se in the films. The results show that CdSeTe thin films have potential applications in thin-film solar cell device technology.