103 research outputs found
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Challenge in Cu-rich CuInSe2 thin film solar cells: Defect caused by etching
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OPTICAL ANALYSIS OF EFFICIENCY LIMITATIONS OF CU(IN,GA)SE2 GROWN UNDER COPPER EXCESS
No full textSolar 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.Curi-K and CuR
Optical Absorption‐Based In Situ Characterization of Halide Perovskites
No full textAbstract Halide perovskites have emerged as materials for high‐performance optoelectronic devices. Often, progress made to date in terms of higher efficiency and stability is based on increasing material complexity, i.e., formation of multicomponent halide perovskites with multiple cations and anions. In this review article, the use of in situ optical methods, namely, photoluminescence (PL) and UV‐vis, that provide access to the relevant time and length scales to ascertain chemistry–property relationships by monitoring evolving properties is discussed. Additionally, because halide perovskites are electron/ion conductors and prone to solid‐state ion transport under various external stimuli, application of these optical methods in the context of ionic movement is described to reveal mechanistic insights. Finally, examples of using in situ PL and UV‐vis to study degradation and phase transitions are reviewed to demonstrate the wealth of information that can be obtained regarding many different aspects of ongoing research activities in the field of halide perovskites
grain boundaries
No full textEffects related to alkali metal doping and grain boundaries have puzzled the chalcopyrite photovoltaics community for a long time. This study is the first to report reversible oxidation of grain boundaries in CuInSe 2 thin films. The phenomenon is observed in sodium-doped films, but not in undoped ones. Cathodoluminescence imaging, secondary ion mass spectrometry and Kelvin probe force microscopy analyses are performed on CuInSe 2 thin films before and after exposure to vacuum. The findings suggest the existence of yet unidentified solid-gas equilibria. Resolving the nature of such reactions will provide new insights into the mechanism of alkali metal doping and passivation in chalcopyrite solar cells
Quasi Fermi level splitting of Cu-rich and Cu-poor Cu(In,Ga)Se2 absorber layers
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The Optical Diode Ideality Factor Enables Fast Screening of Semiconductors for Solar Cells
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Study on the quasi Fermi level splitting of Cu(In,Ga)Se2 absorber layers with Cu-rich and Cu-poor composition
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