Summary of the Thesis
 
 

Currently, CdTe/CdS solar cells are on the verge of being introduced to the market. It is expected that this type of thin film solar cell with a record efficiency of 16.5% on small areas (1 cm2) will help to lower the price of photovoltaics in the future. A variety of deposition techniques for the preparation of cell efficiencies exceeding 10% can be applied. It has been found that the long term performance of the cells sensitively depends upon the particular processing conditions and the applied materials. After 3 decades of research, the limitations inherent to the devices are still not sufficiently understood. The major concern with regards to the stability of the device is the back contact, which often contains Cu to improve the electronic properties of the CdTe absorber and to enable a quasi ohmic back contact.
The CdTe/CdS solar cell absorbers were processed by either high vacuum evaporation or close spaced sublimation. A variety of buffer layer and metallisation material combinations, as well as variations in processing were used to produce back contacts on which accelerated stability tests were preformed by stressing. A back contacting process consisting of a mild etching, which produces a necessary surface modification, followed by a deposition of a Sb2Te3 buffer layer at a substrate temperature higher than 150°C and a metallisation with Mo, yielded the most stable cells of close to 12% efficiency. Cells with Sb/Mo back contacts exhibited a fair stability, but had a tendency toward a slight degradation in performance. Cells with contact materials such as Cu, Au or Al were found to yield high initial efficiencies but degraded quickly, even to the point of complete failure. The buffer layer cannot fulfil both of the functions desired: to contribute to the doping at the back facilitating quasi ohmic contact, and to act as a diffusion barrier for the metallisation material. For stability, the second function is clearly to be favoured.
The investigation of degradation of the photovoltaic properties, caused by the influences of impurity diffusion, led to the development of an advanced characterisation method, the voltage dependent apparent quantum efficiency characterisation, and to a new model for CdTe/CdS solar cells. The latter consistently explains several electronic features of these cells, especially the behaviour of the Apparent Quantum Efficiency (AQE) of CdTe cells. The AQE was applied to identify the impact of the diffused back contact material upon the degradation of the electronic properties.
The first detailed investigations on electron and proton irradiation hardness of CdTe/CdS cells were conducted. These were performed in order to examine the suitability for space applications and to explain the mechanism by which the particles damage the cells. A displacement damage dose formalism was applied for the first time to CdTe cells. Using this formalism, a comparison of their stability with respect to particle irradiation with other cell technologies was carried out. The polycrystalline thin film CdTe cells exhibited a superior stability when compared to monocrystalline materials, and still showed a higher stability compared to other polycrystalline thin film cells, e.g. Cu(In,Ga)Se2 and CuInSe2. The cell degradation due to particle irradiation was modelled on the basis of changes in the recombination centre density. This allowed the calculation of the variation of open circuit voltage and short circuit current density. Under medium fluences of protons (~ 1012 cm-2), an increase in performance was observed, which is inferred to originate from the passivation of recombination centres by incorporation of the absorbed protons in the lattice.
The cells damaged by irradiation showed a fast recovery even under ‘laboratory’ storage conditions. Doses equivalent to those received during years in space at an orbital altitude of 5000 km were almost completely compensated for within a month.