Structural, electrical, and optical properties of ITO thin films and their influence on performance of CdS/CdTe thin-film solar cells

摘要

ITO was prepared by mixing gradient In_2O_3 and SnO_2 powders using solid-phase reaction manner. Using electron beam gun tool, ITO films with varied thicknesses were fabricated. The structures and electrical and optical parameters of the prepared films were studied. XRD patterns were used to establish the micro-structural parameters (lattice strain and crystallite size). The SEM shows improvement of grain size with the increase of the film thickness. The electrical parameters of ITO films were measured by means of the standard four-point probe method. It was found that when the film thickness increases from 75 to 375 nm, the resistivity decreases to lower value of 1.65 × 10~(-4) Ω cm and slightly increases to 1.93 × 10~(-4) Ω cm at thickness of 375 nm. The ITO films with lower electrical properties are appropriate for high-efficiency CdTe solar cells. In terms of spectral ellipsometry, three optical layer models (adhesive layer of the sub-strate/B-spline layer of ITO film/surface roughness layer) were applied to estimate the film thickness with high accuracy. The absorption coefficient and energy gap were calculated from the transmission and reflection spectra in the strong absorption region. As the film thickness increases, the optical energy gap was found to increase from 3.56 to 3.69 eV. In terms of Hall effect measurements, both carrier concentration and hall mobility were determined. In addition, influences of ITO layers with various thicknesses on the performance of CdS/CdTe solar cells were checked. When the ITO window layer thickness is 325 nm, J_(sc) = 17 mA/cm~2, V_(oc) = 0.82 V, and FF = 57.4%, the calculated highest power conversion efficiency (PCE) is 8.6%.