Abstract
M.Sc. (Physics)
Amorhous silicon is one of the most promising materials for large area solar cells for
terestrial photovoltaic applications. Unfortunately these cells suffer from two serious
problems: the efficiencies drop when laboratory processes are scaled up and the cells
degrade after some exposure to sunlight. The exact causes of these two problems
are still unknown. In this project some aspects of the above two problems where
investigated.
The drop in efficiency due to scaling up of laboratory processes can be ascribed to
macroscopic inhomogeneities in the film. An investigation was done by changing the
chamber geometry and gas flow pattern to establish empirical conditions to obtain films
with maximum macroscopic homogeneity. It was found that a uniform electric field
above the substrate was the most important factor determining the macroscopic homogeneity
of the film. The hydronamic gas flow pattern was of secondary importance.
Some techniques to obtain a uniform electric field has been devised.
The photo-degradation was investigated by illuminating films of o-Si.H with simulated
sunlight for different lenghts of time. The change in the electrical and optical
properties of the intrinsic films were determined as function of total photon flux. No
change in the optical properties could be detected.
The effect of the photo-degradation manifests itself in a drop in the the dark conductivity
and photoconductivity. The observed phenomena is explained in terms of
photo-induced deep levels in the gap. The Fermi level shifts to the middle of the gap
due to these defect states, causing a drop in the free carrier concentration and conductivity.
These defect levels increase the absorptiom coefficient in the long wavelength
region, but they also decrease the lifetime of the photo-generated carriers.
The photo-induced defects were investigated with the CPM-technique. A large part
of this project involved the construction and commissioning of the CPM-apparatus.
It was found that the light introduced two types of defects at energies 0.5 eV and 0.75
eV below the conduction band edge. The concentration of the defects increases with
illumination, but saturates after about 24 hours of illumination. The defects could
almost completely be annealed at ISOaC.
The photo-degradation of o-Si.H solar cells is ascribed to the reduction in the carrier
lifetimes of photo-generated carriers due to recombination at these defect centers.