High Efficiency Ultra-Thin CdTe Absorbers by Physical Vapor Deposition

Cadmium telluride in photovoltaics is a commercial technology that has reached the remarkable efficiency of 21 %. This continuous progress of solar cells performance puts this technology as one of the most interesting for mass production. But possible production increase would face the problem of tellurium scarcity, since it is considered a rare element. It was reported that reduction of CdTe thickness down to 1μm – 1,5 μm could solve this problem. In this work we endeavor to maximize the performance of ultra-thin CdTe solar cells with absorber thickness of 1.5 μm in order to understand how the CdTe amount reduction is limiting the performance. We have studied and optimized a vacuum evaporation process where substrate temperature does not exceed 450 °C. Among many different thicknesses we have concentrated our efforts on 1.5 μm CdTe thin absorbers because it reduces the typical limitations of thin devices such as formation of pin-holes and reduction of light absorption. But, more interesting is that we have observed a dependence of performance from the CdS thickness which can not be attributed only to an enhanced transmission of the buffer layer but probably to a different mechanism in the formation of the junction. An optimized process has delivered an efficiency of more than 13 %, which is the highest for devices made by vacuum evaporation.