Luminous Transmittance and Color Rendering Characteristics of Evaporated Chalcopyrite Thin Films for Semitransparent Photovoltaics.

The luminous transmittance and the color rendering index of daylight through semitransparent photovoltaic glazing are essential parameters for visual comfort indoors, and they must be considered for different absorber materials that were traditionally developed for opaque solar cells, such as those of the chalcopyrite type. With this aim, various chalcopyrite compounds (CuInSe₂, CuInS₂ and CuGaS₂) were prepared by means of evaporation and then measured to obtain their optical absorption spectra. These experimental data are used here to calculate the solar absorptance (α_S), luminous transmittance (τ_L) and color rendering index (R_a) as a function of the chalcopyrite film thickness. The comparative analysis of the different factors indicates that 70 nm thick CuInSe₂ is optimal to guarantee excellent visual comfort (τ_L = 50% and R_a = 93%) while absorbing as much solar irradiance (α_S = 37%) as 130 nm thick CuInS₂ or 900 nm thick CuGaS₂. The second option (130 nm thick CuInS₂) is also considered good (τ_L = 40% and R_a = 80%), but for CuGaS₂, the thickness should be kept below 250 nm in order to obtain a suitable color rendering R_a ≥ 60%. [ABSTRACT FROM AUTHOR]