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Date
2019Type
- Doctoral Thesis
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Abstract
Photovoltaic (PV) energy generation has become one of the key pillars of the shift to a
renewable energy future. Current devices, under favorable conditions, can already undercut
the price per kWh electricity of other technologies on the market. Further reduction
in the cost of installed PV systems and increase in solar module conversion efficiency will
improve the affordability even more and will substantially aid in wider market penetration
and enhance the volume of PV installations.
Currently the PV market is dominated by silicon wafer based solar cells, but alternative
technologies offer some distinctive advantages, making them interesting for numerous
applications. Thin film technologies, as for example based on Cu(In,Ga)Se2 (CIGS) compound
semiconductors with high optical absorption coefficient, are becoming important
due to lower material and energy requirements for processing of high conversion efficiency
solar cells. Inherent advantages are large area depositions with low production costs, and
the possibilities for construction of lightweight, flexible devices with roll-to-roll manufacturing
processes.
The highest efficiency of single-junction CIGS solar cells is approaching the thermodynamic
limit, making the use of alternative concepts such as concentration or multijunction
(tandem-) devices the next logical step for further increase in efficiency beyond
the Shockley-Queisser limit (S-Q limit). Especially the multi-junction technology, in which
the thermodynamic losses are reduced by stacking of solar cells with different band gaps,
decreasing thermalization of charge carriers excited with energies above the band gap, is
a promising approach for enhanced utilization of the solar spectrum, yielding improved
efficiency. Such devices, based on epitaxial layers of III-V compounds have already demonstrated remarkably high efficiencies beyond the S-Q limit. However, these devices grown
on rather expensive single crystal wafers and with small size are prohibitively pricey for
low cost terrestrial solar electricity generation. On the other hand, multi-junction solar
cell technology based on polycrystalline thin films is an attractive option for large area,
low cost production, provided adequately high efficiencies are achieved. In this context,
two-junction tandem devices, developed by stacking a semitransparent large band gap solar
cell of 1.6-1.7 eV on top of a low band gap (~1.0 eV) bottom cell, is a viable option.
Earlier attempts in this direction were not so successful, but with the rise of perovskite
thin film solar cells as a compatible high efficiency wide band gap (>1.6 eV) top cell and
CIGS with a tunable band gap as bottom cell, the prospect for all thin film tandem devices
with efficiencies beyond the single-junction limitations has opened. Such all thin film
devices hold the potential for the low cost production necessary for large scale terrestrial
application.
This thesis focuses on the development of high efficiency narrow bandgap (1.0 eV) CIGS
solar cells for application in all thin film tandem devices.
While for CIGS with band gap of around 1.15 eV efficiencies of over 23 % have been
demonstrated, cells with a narrow band gap close to 1.0 eV only reach 15.0 %. The
efficiency of these narrow band gap cells are limited by charge carrier recombination,
leading to low open circuit voltage (VOC) and reduced fill factor. For solar cell efficiency
enhancement it is necessary to investigate the underlying reasons contributing to the
deficits in PV parameters and develop processes to overcome the limiting factors.
An option to reduce recombination within the solar cell is the implementation of a band
gap grading as discussed in Chapter 3. The increase of the band gap at the location of
highest recombination leads to a reduction in diode current, and therefore an increase in
VOC. To keep the band gap of 1.0 eV a substantial part of the absorber needs to be Ga
free. As the primary source of recombination is not obvious, different gradings (realized
by a change in the Ga to In ratio) are implemented and compared. A single grading
with increased band gap (higher Ga/In ratio) towards the front of the absorber shows no
significant improvement on photovoltaic parameters. Any gain in VOC is offset by losses in current due to reduced charge collection, mainly visible for long wavelength photons
and probably a result of the upwards bending in the conduction band. A single backgrading
(higher Ga/In ratio towards the back electric contact) on the other hand leads
to substantial improvements in performance ( from 12.0 % to 16.1 %). It is shown that
the collection of photo-generated charge carriers improves and recombination is reduced.
Measurements of the effective lifetime by time resolved photo-luminescence are carried
out, showing an increase from approximately 20 ns to 100 ns when comparing ungraded
with back-graded absorbers. By selectively changing the recombination speed at the back
contact, strong differences in the behavior of cells with and without a band gap widening
towards the back are observed. The results support that considerable recombination at
the back contact is present in pure CIS solar cells, and that the single Ga back-grading
approach is effective at suppressing this loss channel.
In Chapter 4 the alkali treatment of CIS based solar cells is investigated. Alkali elements
are known to strongly influence doping and passivation in CIGS solar cells. It is shown
that the amount of sodium necessary to reach sufficient doping levels for high performance
CIS solar cells is not achieved using the processes developed for CIGS. This may be based
on insufficient Na diffusion into the grain, as those cells generally show larger grains than
their CIGS counter parts, and since alkali migration energies in CIS are reported to be
higher compared to those in CGS. If CIS cells are grown on soda lime glass without any
diffusion barrier and additionally receive post deposition treatment (PDT) with NaF they
still show low apparent doping concentration and poor PV performance ( = 10.9 %).
However, additional annealing at ~ 370 C substrate temperature after PDT is shown to
solve this problem, leading to an increase in apparent doping levels close to 1016 cm−3 and
cell efficiency of 15.0 %. The application of an additional heavy alkali PDT, specifically
RbF, is shown to lead to further improvements in cell efficiency. Changes at the front
interface due to the PDT allow a decrease of buffer layer thickness, leading to a higher
photo current (approximately + 1.0 mAcm−2). In addition, reduced recombination and
the resulting increase in lifetime leads to additional gains in VOC, resulting in considerably
improved device performance, up to an efficiency of 18.0 %.
Further efficiency improvement is achieved by investigating the effect of close to stoichiometric compositions of Cu to group III elements as described in Chapter 5. The
sub-stoichiometric Cu composition of state-of-the-art CIGS absorbers leads to a high concentration
of detrimental defects. The defect density within the absorbers is reduced by
approaching a stoichiometric Cu composition. Improvements in the defect density are
identified by the decrease of Urbach energy from 20 to 16 mV and an increase in doping is
observed for cells with almost stoichiometric Cu content. Cells with high, and especially
stoichiometric Cu composition tend to be limited by recombination at the front interface,
leading to a decrease of VOC of about 20 mV. Using the modified absorber surface after
heavy alkali PDT, these losses are suppressed. Based on these improvements, a narrow
band gap cell with record breaking 19.2 % efficiency and an open circuit voltage of 609
mV is achieved.
Throughout the whole thesis the suitability of these cells for tandem devices with semitransparent
perovskite top cells is investigated by 4-terminal tandem measurements. The
improvements achieved in this work led to CIS based solar cells that not only show outstanding
single cell performance, but also enable highly efficient tandem devices up to
25.0 %. They outperform state-of-the-art single junction CIGS and perovskite cells while
showing prospects for further efficiency improvement. Due to the low band gap of the
CIS absorber the current density from the bottom cell is high enough to produce current
matched tandem devices with high efficient perovskite top cells (19.2 to 18.6 mAcm−2 in
4-terminal configuration), and also monolithic two-terminal configurations are feasible in
the future. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000389221Publication status
publishedExternal links
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Contributors
Examiner: Tiwari, Ayodhya
Examiner: Siebentritt, Susanne
Examiner: Leuthold, Juerg
Examiner: Bücheler, Stephan
Publisher
ETH ZurichSubject
Photovoltaics; Solar cells; CuInGaSe2; CIGS; CuInSe2Organisational unit
02140 - Dep. Inf.technologie und Elektrotechnik / Dep. of Inform.Technol. Electrical Eng.
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