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.