The first layer is not supposed to have any effect on the incident sunlight, yet it needs to conduct electricity. Most metals are wonderful conductors, but they are not transparent, so wouldn t work. Some semiconductors, however, do a reasonable job at both. There is a tradeoff between how transparent and how conducting this layer is. The visible region of light is the most important, as most of the light emitted from the sun is in the visible range, so to maximize efficiency the cells are focused on making use of this light.
Several parameters go into the conductivity of a sample.
One is the number of available charge carriers and another is the mobility of these charge carriers. Increasing both of these parameters will help the overall efficiency of the solar cell. Since we don t want the incident light to scatter while going through this first layer, it is made quite thin. The mean free path of light l mfp in a sample is the average distance it can travel before experiencing a collision. The CdTe layer does most of the actual absorbing of the incident light.