I. Rimmaudo

Electrical characterization and aging of CdTe thin film solar cells with Bi 2 Te 3 back contact

Despite CdTe solar cells have been already industrialized in large scale and modules are already available in the market, still the research for a stable, low cost, high performing back contact is very hot topic. As a matter of fact, it has been shown that the presence of copper in the CdTe device increases efficiency, not only by improving the back contact but also by doping the absorber and enhancing all the electrical properties. However copper is well known to be a degrading element in the device, diffusing down to the front contact. In this paper we present an electrical study and aging tests of a CdTe devices where copper is encapsulated within a Bi2Te3 buffer layer. The solar cells show a remarkable stability and a good efficiency.

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.

Etching effect of CdTe absorber on the stability of thin film solar cell devices

Cu seems to be necessary to achieve best efficiencies, however it is strongly connected with performance degradation due to its tendency to diffuse. The Cu benefits and drawbacks are still not completely clear. Other studies have shown a direct connection between Cu and defects concentration, however it is still not clear if the Cu induced degradation is due to a compensation or enhanced recombination due to the formation of new defects in bulk CdTe. Within this study many samples with Cu/Au back-contact have been prepared with different etching processes applying thermal, luminous and electrical stresses. We have analyzed the aging effects of those stresses by means of current-voltage, capacitance-voltage, drive level capacitance profiling characterization techniques.

Analysis of the Stability of Ultra-Thin CdTe Absorbers

The application of copper in the CdTe back contact is believed to be crucial to form an ohmic contact with CdTe, indeed the highest conversion efficiencies have been obtained only with copper insertion. However it is well known that the devices performance degradation with time is strongly connected to Cu presence. On the other hand it was actually shown that the carriers collection at the back contact improves as CdTe thickness decreases. We have prepared devices with identical Cu/Au back contact and different CdTe thickness which ranges from 0.7 microns to 6 microns and then we have performed accelerated lifetime stability tests and dark aging at room temperature for very long time (up to 16000 h), in order to study the dependency of the performance degradation with the CdTe thickness. We have analyzed the effects of the aging by current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopy in order to address the physical effects connected with the back contact stability. This study brings out that a possible way to improve the CdTe devices stability is to reduce the absorber thickness.