Reference : OPTICAL ANALYSIS OF EFFICIENCY LIMITATIONS OF CU(IN,GA)SE2 GROWN UNDER COPPER EXCESS
Dissertations and theses : Doctoral thesis
Physical, chemical, mathematical & earth Sciences : Physics
Physics and Materials Science
http://hdl.handle.net/10993/38677
OPTICAL ANALYSIS OF EFFICIENCY LIMITATIONS OF CU(IN,GA)SE2 GROWN UNDER COPPER EXCESS
English
Babbe, Finn mailto [University of Luxembourg > Faculty of Science, Technology and Communication (FSTC) > Physics and Materials Science Research Unit >]
22-Feb-2019
University of Luxembourg, ​Esch-sur-Alzette, ​​Luxembourg
DOCTEUR DE L’UNIVERSITÉ DU LUXEMBOURG EN PHYSIQUE
159
Siebentritt, Susanne mailto
Michels, Andreas mailto
Christen, Jürgen mailto
Bücheler, Stephan mailto
Brida, Daniele mailto
[en] Chalcopyrites ; Cu-Rich ; Photoluminescence ; CuInSe2 ; CIGS ; Post depostion treatment ; KF PDT
[en] Solar cells made from the compound semiconductor Cu(In,Ga)Se2 reach efficiencies of
22:9 % and are thus even better than multi crystalline silicon solar cells. All world records
are achieved using absorber layers with an overall copper deficient composition, but Cu-rich
grown samples have multiple favourable properties. However, especially losses in the open
circuit voltage limit the device performance. Within this work these efficiency limitations
of chalcopyrites grown with copper excess are investigated. The work has been divided
into four chapters addressing different scientific questions.
(i) Do alkali treatments improve Cu-rich absorber layers?
The alkali treatment, which lead to the recent improvements of the efficiency world record,
is adapted to CuInSe2 samples with Cu-rich composition. The treatment leads to an
improvement of the VOC which originates roughly equally from an improvement of the
bulk and the removal of a defect close to the interface. The treatment also improves the
VOC of Cu-poor samples. In both cases, the treatment increases the fill factor (FF) and
leads to a reduction of copper content at the surface.
(ii) Is the VOC limited by deep defects in Cu-rich Cu(In,Ga)Se2?
A deep defect, which likely limits the VOC, is observed in photoluminescence measurements
(PL) independent of a surface treatment. The defect level is proposed to originate from the
second charge transition of the CuIn antisite defect (CuIn(-1/-2)). During the investigation
also a peak at 0:9 eV is detected and attributed to a DA-transition involving a third
acceptor situated (135 ± 10) meV above the valence band. The A3 proposed to originate
from the indium vacancy (VIn). Furthermore the defect was detected in admittance
measurements and in Cu(In,Ga)Se2 samples with low gallium content.
(iii) Is the diode factor intrinsically higher in Cu-rich chalcopyrites?
Cu-rich solar cells exhibit larger diode ideality factors which reduce the FF. A direct link
between the power law exponent from intensity dependent PL measurements of absorbers
and the diode factor of devices is derived and verified using Cu-poor Cu(In,Ga)Se2 samples.
This optical diode factor is the same in Cu-rich and Cu-poor samples.
(iv) Is the quasi Fermi level splitting (qFLs) of Cu-rich Cu(In,Ga)Se2 absorber layers
comparable to Cu-poor samples?
Measuring the qFLs of passivated Cu-rich and Cu-poor Cu(In,Ga)Se2 samples, on average
a 120 meV lower splitting is determined for Cu-rich samples. This difference increases with
gallium content and is likely linked to a defect moving deeper into the bandgap, possibly
related to the second charge transition of the CuIn antisite defect.
Overall, samples with Cu-rich composition are not limited by the diode factor. However,
a deep defect band causes recombination lowering the qFLs and thus the VOC. This defect
is not removed by alkali treatments. A key component to improve Cu-rich solar cells in
the future, especially Cu(In,Ga)Se2, will be to remove or passivate this defect level.
Laboratory for Photovoltaics
Fonds National de la Recherche - FnR
Researchers ; Professionals
http://hdl.handle.net/10993/38677
FnR ; FNR8267152 > Susanne Siebentritt > CURI-K > Cu rich CIS - the effect of potassium treatment > 01/02/2015 > 31/01/2018 > 2014

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