Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction

Morkel, Matthias; Weinhardt, L.; Lohmuller, B.; Heske, C.; Umbach, E.; Riedl, W.; Zweigart, S.; Karg, F.

doi:10.1063/1.1428408

Cited by 229 publications

(183 citation statements)

References 26 publications

(18 reference statements)

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“…First, there is ample evidence that the Cu-poor region providing the internal valence band offset is present at the surface of the absorber. [34][35][36] It is therefore reasonable to conclude that a Cu-poor region at the GBs is more likely to be present toward the surface of the absorber than toward the back contact. Second, measurements of the majority-carrier transport properties of CIGS films in coplanar geometry, i.e., perpendicular to many GBs, exhibit activation energies in a range between 60 and 120 meV, 41 i.e., much smaller than an effective internal band offset.…”

Section: A Excess Barrier Heightmentioning

confidence: 99%

“…[13][14][15] The effect of this internal offset at the GBs could be of similar importance as it is at the surface of the absorber, 33 whereas, there is ample experimental evidence for the Cu-poor surface layer 34,35 and its beneficial consequences for the performance of CIGS solar cells as long as the overall film composition is Cu-poor, 36 the question whether or not such a Cu-poor layer is a general positive feature of GBs in CIGS is still under discussion. [37][38][39][40] The present study concentrates on investigating the effect of such an internal band offset and on finding the conditions that have to be fulfilled if the Cu-poor layer should have a decisive beneficial effect on the performance of polycrystalline CIGS solar cells.…”

Section: A Excess Barrier Heightmentioning

confidence: 99%

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Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

Taretto

Rau

2008

Journal of Applied Physics

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Two-dimensional numerical device simulations investigate the influence of grain boundaries ͑GBs͒ on the performance of Cu͑In, Ga͒Se 2 solar cells. We find that the electronic activity of grain boundaries can reduce the efficiency of Cu͑In, Ga͒Se 2 solar cells from 20% to below 12% making proper passivation of GBs a primary requirement for high efficiency. Cell efficiencies larger than 19% require GB defect densities below 10 11 cm −2 . Also, an internal band offset in the valence band due to a Cu-poor region adjacent to the GBs could effectively passivate grain boundaries that are otherwise very recombination active. It is shown that such a barrier must be more than 300 meV high and at least 3 nm wide to virtually suppress all grain boundary recombination. Contrariwise, such a barrier represents an obstacle for hole transport reducing carrier collection across grain boundaries that are not perpendicular to the cell surface. We further find that inverted grain boundaries lead to an accumulation of the short circuit current along the grain boundary, which in certain situations enhances the total short circuit current. However, we do not find any beneficial effect of any type of grain boundaries on the overall cell efficiency.

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Section: A Excess Barrier Heightmentioning

confidence: 99%

Section: A Excess Barrier Heightmentioning

confidence: 99%

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

Taretto

Rau

2008

Journal of Applied Physics

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“…Na Cu ). 20 As a consequence, the total amount of positive charges close to the CIGSe surface and the related downward band bending toward the CIGSe surface (repeatedly indicated by photoemission experiments, as the Fermi energy is closer to conduction band minimum than to the valence band maximum [56][57][58] ) would be reduced. This reduction of the downward band bending could also be interpreted as a relative upward band bending, and so this reasoning is consistent with our photoemission data.…”

Section: E Binding Energy Shiftmentioning

confidence: 99%

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

Song

Caballero

Félix

et al. 2012

Journal of Applied Physics

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The following article appeared in Journal of Applied Physics 111.3 (2012): 034903 and may be found at http://scitation.aip.org/content/aip/journal/jap/111/3/10.1063/1.3679604Na has deliberately been incorporated into Cu(In,Ga)Se2 (CIGSe) chalcopyrite thin-film solar cell absorbers deposited on Mo-coated polyimide flexible substrates by adding differently thick layers of NaF in-between CIGSe absorber and Mo back contact. The impact of Na on the chemical and electronic surface structure of CIGSe absorbers with various Cu-contents deposited at comparatively low temperature (420 C) has been studied using x-ray photoelectron and x-ray excited Auger electron spectroscopy. We observe a higher Na surface content for the Cu-richer CIGSe samples and can distinguish between two different chemical Na environments, best described as selenide-like and oxidized Na species, respectively. Furthermore, we find a Cu-poor surface composition of the CIGSe samples independent of Na content and - for very high Na contents - indications for the formation of a (Cu,Na)-(In,Ga)-Se like compound. With increasing Na surface content, also a shift of the photoemission lines to lower binding energies could be identified, which we interpret as a reduction of the downward band bending toward the CIGSe surface explained by the Na-induced elimination of In Cu defects.X.S., R.F., D.G., R.G.W., and M.B. are grateful to the Helmholtz-Association for financial support (VH-NG-423). R.F. also acknowledges the support by the German Academic Exchange Agency (DAAD; 331 4 04 002)

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“…hard X-ray [77]) would be required to measure the bulk gap. Ref 22 noted that in a polycrystalline semiconductor a surface gap larger than that of the bulk is not unusual [78], due to possible structural and/or compositional differences between bulk and surface. Further studies are clearly necessary to clarify the difference between surface and bulk WO 3 quasiparticle gaps.…”

Section: Photoemission Gapmentioning

confidence: 99%

Optical properties of tungsten trioxide from first-principles calculations

2013

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Tungsten trioxide (WO3) is an Earth abundant material of potential use as a light absorber for solar energy conversion processes. We carried out ab initio calculations of the band structure and absorption spectrum of WO3 using many body perturbation theory and we present a detailed comparison of our results with photoemission and absorption data. We show that it is necessary to take into account multiple effects, including spin-orbit and electron-phonon interaction, and exciton binding, in order to correctly predict the measured optical gap. The absorption spectrum obtained by solving the Bethe Salpeter equation compares well with experiments over a wide energy range, and our calculations correctly account for the red shift observed experimentally upon N2 intercalation in WO3.

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Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction

Cited by 229 publications

References 26 publications

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

Optical properties of tungsten trioxide from first-principles calculations

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