Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells

Schöppe, Philipp; Schönherr, Sven; Chugh, Manjusha; Mirhosseini, Hossein; Jackson, Philip; Wuerz, Roland; Ritzer, Maurizio; Johannes, Andreas; Martı́nez-Criado, Gema; Wisniewski, Wolfgang; Schwarz, Torsten; Plass, Christian T.; Hafermann, Martin; Kühne, Thomas D.; Schnohr, Claudia; Ronning, Carsten

doi:10.1016/j.nanoen.2020.104622

Cited by 27 publications

(23 citation statements)

References 41 publications

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“…[ 179 ] The strongest passivation was found when placing Cs at the GB. [ 180 ] For the recorded cell with 23.35% with a CsF PDT, the V oc loss was even reduced to 350 mV. [ 6 ] Up to now, we have only considered the impact of alkalis on the GB properties.…”

Section: Designing a Cu(in Ga)se2 Absorbers With Superior Electronic Propertiesmentioning

confidence: 99%

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Cojocaru-Mirédin

Raghuwanshi

Wuerz

et al. 2021

Adv Funct Materials

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Cu(In,Ga)Se 2 thin-film solar cells have attracted significant research interest in recent decades due to their high efficiency in converting solar energy into electricity for enabling a sustainable future. Although the Cu(In,Ga)Se 2 absorber can be grown as a single crystal, its polycrystalline form is dominating the market not only due to its lower costs, but also due to its unexpectedly higher cell efficiency. However, this absorber contains a high fraction of grain boundaries. These are structural defects where deep-trap states can be localized leading to an increase in recombination activity. This controversy is mirrored in the existing literature studies where two main contradictory believes exist: 1) to be crucial grain boundaries in Cu(In,Ga)Se 2 absorber are anomalous, being benign in terms of cell performance, and 2) grain boundaries are regions characterized by an increased recombination activity leading to deteriorated cell performance. Therefore, the present review tackles this issue from a novel perspective unraveling correlations between chemical composition of grain boundaries and their corresponding electronic properties. It is shown that features such as Cu depletion/In enrichment, segregation of 1-2at.% of alkali dopants, and passivation by a wide-bandgap or type inversion at grain boundaries are crucial ingredients for low open-circuit voltage loss and, hence, for superior cell performance.

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Section: Designing a Cu(in Ga)se2 Absorbers With Superior Electronic Propertiesmentioning

confidence: 99%

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Cojocaru-Mirédin

Raghuwanshi

Wuerz

et al. 2021

Adv Funct Materials

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“…In contrast, a more homogenous distribution of Cs at the buffer interface was detected if the sample was not exposed to oxygen before PDT. [35] Even in the case of RbF-PDT on CIGS, some studies report on RbInSe 2 particle [61] or island [34] formation at the buffer interface, whereas others suggest a closed Rb(Na)-In-Se layer. [28] Thus, it cannot be excluded that widely dispersed Rb-containing compounds formed at the CuInS 2 surface, but were not captured by the %10 μm wide TEM lamella.…”

Section: Analysis Of Heavy Alkali Metal Incorporation Intomentioning

confidence: 99%

“…However, it is not clear if this phase always forms and if/when it occurs as a closed layer or rather as distributed clusters. [30,35] On the contrary, a suppression of bulk recombination is discussed. While a reduced concentration of deep defects by the Alk-PDT appears unlikely, [30] it was suggested that a detrimental band-bending at GBs is reduced by the introduction of heavy alkalis (expressed as reduced band-tailing), which may agglomerate in GBs as AlkInSe 2 and reduce/passivate charged defects.…”

Section: Introductionmentioning

confidence: 99%

Heavy Alkali Treatment of Post‐Sulfurized Cu(In,Ga)Se₂ Layers: Effect on Absorber Properties and Solar Cell Performance

et al. 2020

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This contribution evaluates a sequential post‐deposition treatment of Cu(In,Ga)Se2 (CIGS) films, consisting of 1) a post‐sulfurization in elemental S‐atmosphere and 2) a subsequent treatment by heavy alkali fluorides (Alk‐PDT). First, the effect of the sulfurization step on the corresponding solar cell performance is investigated and optimum process parameters, leading to an efficiency improvement, are identified. Losses in carrier collection observed after S‐incorporation are attributed to an increased grain boundary (GB) recombination. It is found that the corresponding reduction in short‐circuit current density can be mitigated by a RbF‐ or KF‐PDT, supposedly by depleting GBs in Cu. However, in strong contrast to non‐sulfurized CIGS, the Alk‐PDT results in a lower open‐circuit voltage and distortions in the current–voltage (I–V) characteristics for sulfurized absorbers. Possible explanations are the absence of a wide‐gap surface phase and/or air exposure between the post‐treatment steps. It is further proposed that a back contact barrier may be responsible for the distortions in I–V.

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“…Figure shows the SEM image of the cross section of the same lamella (as shown in Figure 1) but from a slightly different area together with the nano‐XRF maps. The use of a hard X‐ray nanobeam for compositional analysis not only improves the elemental sensitivity at higher energies but also the detection limits, allowing the determination of much lower compositional variations at nanometer scale, such as variations located at grain boundaries [ 30–32 ] or nanoscale secondary phases. [ 13,30 ] The only drawback is that S cannot be detected and quantified, because the experimental setup was under ambient conditions with a significant low‐Z absorption by the air and the detector Be window.…”

Section: Resultsmentioning

confidence: 99%

Interplay of Performance‐Limiting Nanoscale Features in Cu₂ZnSn(S,Se)₄ Solar Cells

Ritzer

Schönherr

Schöppe

et al. 2020

Physica Status Solidi (a)

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Highly performing kesterite‐based Cu2ZnSn(S,Se)4 (CZTSSe) thin‐film solar cells are typically produced under Cu‐poor and Zn‐rich synthesis conditions. However, these processing routes also facilitate the formation of secondary phases as well as deviations from stoichiometry, causing intrinsic point defects. Herein, the local composition of CZTSSe absorbers prepared with different nominal cation concentrations is investigated by applying energy dispersive X‐ray spectroscopy and synchrotron X‐ray fluorescence spectroscopy at the nanoscale to cross‐sectional lamellae. The findings confirm the formation of ZnS(Se) secondary phases, whose presence, number, and dimension strongly increase with the reduction of the nominal Cu and increment of the nominal Zn content. Furthermore, the local compositions of the CZTSSe phase within the absorber reveal strong variations, leading to collateral and multiple off‐stoichiometry types of the kesterite phase in the absorber, which cause different intrinsic point defects. Therefore, the off‐stoichiometry type determined from the integral composition does not represent the complete true picture of this complex material system. Accordingly, the correlation of integral composition with electrical properties or conversion efficiency may be misleading. Overall, the approach provides new experimental insights into the nanoscale relationship among local compositional fluctuations, off‐stoichiometry types, and secondary phases in these promising photovoltaic materials.

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Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells

Cited by 27 publications

References 41 publications

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Heavy Alkali Treatment of Post‐Sulfurized Cu(In,Ga)Se₂ Layers: Effect on Absorber Properties and Solar Cell Performance

Interplay of Performance‐Limiting Nanoscale Features in Cu₂ZnSn(S,Se)₄ Solar Cells

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Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells

Cited by 27 publications

References 41 publications

Grain Boundaries in Cu(In, Ga)Se2: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Grain Boundaries in Cu(In, Ga)Se2: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Heavy Alkali Treatment of Post‐Sulfurized Cu(In,Ga)Se2 Layers: Effect on Absorber Properties and Solar Cell Performance

Interplay of Performance‐Limiting Nanoscale Features in Cu2ZnSn(S,Se)4 Solar Cells

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Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Heavy Alkali Treatment of Post‐Sulfurized Cu(In,Ga)Se₂ Layers: Effect on Absorber Properties and Solar Cell Performance

Interplay of Performance‐Limiting Nanoscale Features in Cu₂ZnSn(S,Se)₄ Solar Cells