Effects of Sodium on Polycrystalline Cu(In,Ga)Se2 and Its Solar Cell Performance

Kronik, Leeor; Cahen, David; Schock, H.W.

doi:10.1002/(sici)1521-4095(199801)10:1<31::aid-adma31>3.0.co;2-3

Cited by 339 publications

(271 citation statements)

References 28 publications

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“…However, this barrier cannot be a general feature of all GBs in high-efficiency CIGS absorbers. The generally benign character of GBs must also embrace beneficial crystallographic GB structures 11,12 and defect chemical passivation of GB defects [18][19][20][21] in order to keep the defect density low.…”

Section: Discussionmentioning

confidence: 99%

“…Especially, the question was discussed whether, and if yes, how GBs can be beneficial for CIGS and CdTe solar cells whereas they are detrimental for most other photovoltaic materials. In addition, there are several models that explain the low electronic activity of GBs in CIGS with the help of arguments concerning beneficial crystallography of GBs, 12 extrinsic defect chemistry based on the beneficial effect of oxygen 18,19 and sodium, 20,21 as well as a self-passivation effect by an internal valence band offset. 13,14 The present work uses two-dimensional numerical simulations of polycrystalline CIGS solar cells to investigate the influence of GBs on their electrical performance.…”

Section: Introductionmentioning

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: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

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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“…104,114,115 Various models have been proposed to explain the effects of Na on the electronic and structural properties of layers and influence on solar cells. 97,110,[116][117][118] CdTe layers CdTe thin films have been succesfully grown by a variety of vacuum and non-vacuum deposition methods. Generally, CdTe growth methods such as CSS or close-spaced vapor transport with deposition temperature above 500 C are classified as high-temperature processes, while methods such as electrodeposition, HVE and sputtering with deposition temperature below 450 C are classified as low-temperature processes.…”

Section: Sodium Incorporation In Cigsmentioning

confidence: 99%

Development of thin‐film Cu(In,Ga)Se₂ and CdTe solar cells

Romeo¹,

Terheggen²,

Abou‐Ras³

et al. 2004

Progress in Photovoltaics

353

192

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Cu(In,Ga)Se2 and CdTe heterojunction solar cells grown on rigid (glass) or flexible foil substrates require p‐type absorber layers of optimum optoelectronic properties and n‐type wide‐bandgap partner layers to form the p–n junction. Transparent conducting oxide and specific metal layers are used for front and back electrical contacts. Efficiencies of solar cells depend on various deposition methods as they control the optoelectronic properties of the layers and interfaces. Certain treatments, such as addition of Na in Cu(In,Ga)Se2 and CdCl2 treatment of CdTe have a direct influence on the electronic properties of the absorber layers and efficiency of solar cells. Processes for the development of superstrate and substrate solar cells are reviewed. Copyright © 2004 John Wiley & Sons, Ltd.

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“…Oxides on the surface after air exposure reduce the work function by 690 meV. At the free CIGSe surface the formation of oxides induces surface dipoles which modify the effective electron affinity 13 and thus the surface work function. With increasing CdS thickness ⌽ increases in both cases up to ϳ5.45 eV which corresponds to the work function of the closed ͓confirmed by scanning electron microscopy ͑SEM͒ images 14 ͔ CdS layer with a thickness of ϳ55 nm.…”

mentioning

confidence: 99%

“…In the case of reference devices, the absorber GBs are passivated at first by the oxygen during the air exposure. 13,19 Furthermore, sulfur can passivate GBs, since the absorber surface is cleaned/etched in the chemical bath, 9 thereby eliminating this diffusion barrier.…”

mentioning

confidence: 99%

Formation of the physical vapor deposited CdS∕Cu(In,Ga)Se2 interface in highly efficient thin film solar cells

Rusu

Glatzel

Neisser

et al. 2006

Applied Physics Letters

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We report on the buffer/absorber interface formation in highly efficient ͑14.5%, air mass 1.5͒ ZnO/CdS/Cu͑In, Ga͒Se 2 solar cells with a physical vapor deposited CdS buffer. For Se-decapped Cu͑In, Ga͒Se 2 ͑CIGSe͒ absorbers we observe sulfur passivation of the CIGSe grain boundaries during CdS growth and at the interface a thermally stimulated formation of a region with a higher band gap than that of the absorber bulk, determining the height of the potential barrier at the CdS / CIGSe interface. For air-exposed CIGSe samples the grain boundary passivation is impeded by a native oxide/adsorbate layer at the CIGSe surface determining the thermal stability of the potential barrier height. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2190768͔ Cu͑In, Ga͒Se 2 ͑CIGSe͒ thin film solar cells achieve efficiencies up to 19.5% when using a "wet" chemical bath deposited ͑CBD͒ CdS buffer layer between the p-type absorber and the n-type ZnO window. 1 Surprisingly, CIGSe polycrystalline thin film solar cells exhibit higher internal quantum efficiencies ͑QEs͒ than devices from single crystalline CIGSe films. 2 Recent theoretical 3 and experimental 4 studies propose an explanation based on a model where Cu depleted polar grain boundary ͑GB͒ interfaces lead to a valence band offset between GB and grain interior ͑GI͒, repeling holes from the GB and thus diminishing GB recombination. Other experiments observed light induced changes in the conduction band at GBs. 5 Nevertheless, when a CdS buffer is deposited by a "dry" physical vapor deposition ͑PVD͒ the CI͑G͒Se solar cells show significantly lower efficiency. 6 The highest reported efficiency using a PVD-CdS is ϳ12.4%. 7 This shows that differently deposited CdS buffers provide differences at the related interfaces and finally in the device operation.In this Letter we show that ͑1͒ not only the initial state of the absorber GBs has a decisive role but that additionally a GB passivation process, which occurs during the CdS buffer deposition, enhances the absorber electronic properties; ͑2͒ the condition of the absorber surface strongly influences the passivation process and the potential barrier of recombination at the CdS / CIGSe interface; and ͑3͒ the preparation of highly efficient solar cells with PVD-CdS buffers is possible.CIGSe thin films ͓Ga/ ͑Ga+ In͒ =24%͔ were deposited by a three-stage coevaporation process 8 on Mo-coated soda lime glass substrates. An ϳ300 nm Se cap protection layer was deposited on top of the CIGSe absorber. Se decapping and in situ thermal deposition of a CdS layer from a single source were performed in an ultrahigh vacuum ͑UHV͒ ͑p ϳ 10 −9 mbar͒ system. Samples comparable to those in standard solar cell production were prepared by exposure to air for 40 min prior to CdS deposition. This results in the formation of Na 2 CO 3 and also of In 2 O 3 , Ga 2 O 3 , and SeO x native oxides at the absorber surface. 9 For the deposition of the CdS layers, the CdS source and substrate temperatures were kept constant at 680 and 100°C, respectively, providin...

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Effects of Sodium on Polycrystalline Cu(In,Ga)Se2 and Its Solar Cell Performance

Cited by 339 publications

References 28 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

Development of thin‐film Cu(In,Ga)Se₂ and CdTe solar cells

Formation of the physical vapor deposited CdS∕Cu(In,Ga)Se2 interface in highly efficient thin film solar cells

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Effects of Sodium on Polycrystalline Cu(In,Ga)Se2 and Its Solar Cell Performance

Cited by 339 publications

References 28 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

Development of thin‐film Cu(In,Ga)Se2 and CdTe solar cells

Formation of the physical vapor deposited CdS∕Cu(In,Ga)Se2 interface in highly efficient thin film solar cells

Contact Info

Product

Resources

About

Development of thin‐film Cu(In,Ga)Se₂ and CdTe solar cells