SOLUTION PROCESSING OF CADMIUM SULFIDE BUFFER LAYER AND ALUMINUM-DOPED ZINC OXIDE WINDOW LAYER FOR THIN FILMS SOLAR CELLS

Abstract

Cadmium sulfide ( CdS ) and aluminum-doped zinc oxide ( Al : ZnO ) thin films are used as buffer layer and front window layer, respectively, in thin film solar cells. CdS and Al : ZnO thin films were produced using chemical bath deposition (CBD) and sol–gel technique, respectively. For CBD CdS , the effect of bath composition and temperature, dipping time and annealing temperature on film properties was investigated. The CdS films are found to be polycrystalline with metastable cubic crystal structure, dense, crack-free surface morphology and the crystallite size of either few nanometers or 12–17 nm depending on bath composition. In case of CdS films produced with 1:2 ratio of Cd and S precursors, spectrophotometer studies indicate quantum confinement effect, owing to extremely small crystallite size, with an increase in Eg value from 2.42 eV (for bulk CdS ) to ~ 3.76 eV along with a shift in the absorption edge toward ~ 330 nm wavelength. The optimum annealing temperature is 400°C beyond which film properties deteriorate through S evaporation and CdO formation. On the other hand, Al : ZnO films prepared via spin coating of precursor sols containing 0.90–1.10 at.% Al show that, with an increase in Al concentration, the average grain size increases from 28 nm to 131 nm with an associated decrease in root-mean-square roughness. The minimum value of electrical resistivity, measured for the films prepared using 0.95 at.% Al in the precursor sol, is ~ 2.7 × 10-4 Ω ⋅ cm. The electrical resistivity value rises upon further increase in Al doping level due to introduction of lattice defects and Al segregation to the grain boundary area, thus limiting electron transport through it.