| dc.description.abstract | Silicon solar cell technology is now a mature technology and competitive with other
energy technologies in many energy markets worldwide. However, to have major
contribution to the electricity generation higher energy conversion efficiencies are
highly desirable. The intermediate band solar cell (IBSC) is one of the solar cell
concepts that have been proposed to increase the efficiency significantly. IBSCs are
based on materials where an additional, intermediate energy band (IB) is present inside
the semiconductor. IBSCs were proposed as late as in 1997, and different research
groups have attempted to realize them using various materials. So far, the device
performance has been affected by losses due to defects in the materials, and in addition
there is a risk that the IB itself will lead to increases losses due to so-called Shockley-
Read-Hall (SRH) recombination.
The objective of this thesis is two-fold: firstly to carry out a theoretical study of the
impact of SRH recombination on the IBSC performance, and secondly to investigate
experimentally the material system that has been proven to result in the desired
improved performance, but only at low temperature and a concentrated light, namely
InAs/GaAs quantum dots. It is now well known that this material is not suitable for use
in IBSCs under normal operating conditions. However, since the next version of QDbased
IBSC most likely will be a modification of InAs/GaAS QDs, we need to study
this material system to establish a starting point for the materials development.
In the first part of the work, the IBSC performance has been calculated using detailed
balance analysis, and adding the effect of SRH recombination. The defect levels causing
the SRH recombination have been placed energetically in the middle of the two subbandgaps
of the IBSC, as well as at the same energy as the IB. The theoretical work is
an extension of the work of a former PhD student in the group, Rune Strandberg, who
developed a detailed balance model for (defect free) IBSCs, where so-called photofilling
effects were included. With photo-filling, the IB is assumed to be empty in
thermal equilibrium, and then the IB will be filled as light is absorbed and carriers are
excited to the IB from the valence band. In many cases the performance of IBSCs is
calculated assuming the IB is half-filled (by pre-filling), but that might be far from the
filling in realistic materials. In the current thesis the performance of IBSCs with SRH
recombination is therefore calculated for both pre-filled and photo-filled IBSCs. The
main finding is that SRH recombination can be detrimental to the IBSC performance,
similar to what has been observed experimentally by many groups, but not in all cases.
Defects above the IB might aid the filling of a photo-filled IBSC, and might lead to
“over-filling” of the IB if it is pre-filled. In many cases the SRH recombination has only
a minor effect, especially for concentrated light. Also the case where states in the IB
itself lead to SRH recombination does not have a dramatic effect on the performance, as
long as the radiative recombination dominates.
In the experimental, second part of the work, InAs/GaAs QDs have been grown using
molecular beam epitaxy (MBE) and studied using atomic force microscopy (AFM),
scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL). QD samples with a wide range of growth parameters have been made and studied, with the
aim to make ultra small, high density quantum dots. Small dots are needed to have only
one confined state for electrons in the QDs, thus limiting the thermal escape out of the
QDs. If thermal escape is possible, the quasi-Fermi level of the IB, made up of the
confined electron states, will be coupled to the quasi-Fermi level of the conduction band
(CB). With coupled quasi-Fermi levels, the cell would behave as a single junction, low
bandgap cell, and not an IBSC. High density is needed to maximize the sub-bandgap
absorption. With too low sub-bandgap absorption, there will be a too small increase in
the photo-generated current, and the IBSC will have poor performance. The
experimental part of the thesis shows, as is known from literature, that it is possible to
grow small QDs of high density, but that the QD properties are very sensitive to all
growth parameters. It also shows that improvement in one QD parameter often is
accompanied with a decrease in another parameter, making the growth optimization
very challenging. An important point that seems to be overlooked by many groups
working with QD based IBSCs, is the high Auger recombination rates that can occur in
QD materials, especially when illuminated with high intensity light. Nevertheless, the
experimental work in this thesis has formed a basis for further development of QD
materials for IBSCs based on the InAs/GaAs system. | nb_NO |
