Band alignment at the CdS∕Cu(In,Ga)S2 interface in thin-film solar cells

Weinhardt, L.; Fuchs, O.; Gross, D. H. E.; Storch, G.; Umbach, E.; Dhere, Neelkanth G.; Kadam, Ankur A.; Kulkarni, Sachin S.; Heske, Clemens

doi:10.1063/1.1861958

Cited by 89 publications

(91 citation statements)

References 15 publications

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“…Depending on that, the modification to the growth process can be suggested with, in particular, the objective to minimize the influence of the defect at ∼280 meV. Nevertheless, other factors that can give a strong contribution for the reduction of V OC must be also taken into account [14,70,88,91]: (1) in quaternary compounds the presence of both electrostatic and band-gap fluctuating potentials will be expected; (2) the polycrystalline character of the absorber layer will favor the diminishing of the lifetime of photogenerated carriers and, consequently, of their collection;…”

Section: Discussion Of the Resultsmentioning

confidence: 99%

Radiative transitions in highly doped and compensated chalcopyrites and kesterites: The case ofCu2ZnSnS4

Teixeira

Sousa

et al. 2014

Phys. Rev. B

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The theoretical models of radiative recombinations in both CuIn 1−x Ga x Se 2 chalcopyrite and Cu 2 ZnSnS 4 kesterite, and related compounds, were revised. For heavily doped materials, electrons are free or bound to large donor agglomerates which hinders the involvement of single donors in the radiative recombination channels. In this work, we investigated the temperature and excitation power dependencies of the photoluminescence of Cu 2 ZnSnS 4 -based solar cells in which the absorber layer was grown through sulphurization of multiperiod structures of precursor layers. For both samples the luminescence is dominated by an asymmetric band with peak energy at ∼1.22 eV, which is influenced by fluctuating potentials in both conduction and valence bands. A value of ∼60 meV was estimated for the root-mean-square depth of the tails in the conduction band. The radiative transitions involve the recombination of electrons captured by localized states in tails of the conduction band with holes localized in neighboring acceptors that follow the fluctuations in the valence band. The same acceptor level with an ionization energy of ∼280 meV was identified in both absorber layers. The influence of fluctuating potentials in the electrical performance of the solar cells was discussed.

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Section: Discussion Of the Resultsmentioning

confidence: 99%

Radiative transitions in highly doped and compensated chalcopyrites and kesterites: The case ofCu2ZnSnS4

Teixeira

Sousa

et al. 2014

Phys. Rev. B

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“…While for the CdS / Cu 2 S heterojunction the Cu outdiffusion is a wellknown phenomenon, 28,29 for the buffer/CIGSSe heterocontact this was never observed and even ruled out due to the significant built-in potential drop across the space charge region ͑SCR͒ in the CIGSSe absorber. 30 Furthermore, the downward surface band bending observed for state-of-the-art CIGSSe absorbers, [31][32][33][34] which is believed to be caused by positive surface charges, might induce a Cu "electromigration" away from the buffer/absorber interface. 35,36 However, for the CBD-CdS buffer preparation on CIGSSe absorbers it was repeatedly reported in the past that Cu ions were found in used chemical baths, 37 which were obviously leached out of the absorber's surface as long as the buffer was not completely closed and hence must have taken place in the early stages of the CBD.…”

Section: -3mentioning

confidence: 99%

Intermixing at the heterointerface between ZnS∕Zn(S,O) bilayer buffer and CuInS2 thin film solar cell absorber

Bär

Ennaoui

Klaer

et al. 2006

Journal of Applied Physics

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The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS 2 ͑CIS͒ based thin-film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. After identifying the deposited Zn compound, as ZnS / Zn͑S,O͒ bilayer buffer in former investigations ͓M. Bär et al., J. Appl. Phys. 99, 123503 ͑2006͔͒, this time the focus lies on potential diffusion/intermixing processes at the buffer/absorber interface possibly, clarifying the effect of the heat treatment, which drastically enhances the device performance of respective final solar cells. The interface formation was investigated by x-ray photoelectron and x-ray excited Auger electron spectroscopy. In addition, photoelectron spectroscopy ͑PES͒ measurements were also conducted using tunable monochromatized synchrotron radiation in order to gain depth-resolved information. The buffer side of the buffer/absorber heterointerface was investigated by means of the characterization of Zn͑S,O͒ / ZnS / CIS structures where the ZnS / Zn͑S,O͒ bilayer buffer was deposited successively by different deposition times. In order to make the ͑in terms of PES information depth͒ deeply buried absorber side of the buffer/absorber heterointerface accessible for characterization, in these cases the buffer layer was etched away by dilute HCl aq . We found indications that while ͑out-leached͒ Cu from the absorber layer forms together with the educts in the chemical bath a ͓Zn ͑1−Z͒ ,Cu 2Z ͔S-like interlayer between buffer and absorber, Zn is incorporated in the uppermost region of the absorber. Both effects are strongly enhanced by postannealing the Zn͑S,O͒ / ZnS / CIS samples. However, it was determined that the major fraction of the Cu and Zn can be found quite close to the heterointerface in the buffer and absorber layer, respectively. Due to this limited ͑in the range of one monolayer͒ spatial extent, these "diffusion" mechanisms were rather interpreted as a chemical bath deposition induced and heat-treatment promoted Cu-Zn ion exchange at the buffer/absorber interface. Possible impacts of this intermixing on the performance of the final solar cell devices will also be discussed.

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“…Knowledge of these offsets is, therefore, critical to understanding the performance of the resulting solar cell. While the VB offset, ∆E V B , can be determined with established methods, such as combined XPS/UPS [5,6] or Constant Final State Yield Spectroscopy [7], a determination of CB edge positions and offsets, ∆E CB , has proved more difficult. The most common method is simply the assumption that the CB minimum is the energy of the VB plus the band gap.…”

mentioning

confidence: 99%

Limitations of Near Edge X-ray Absorption Fine Structure as a tool for observing conduction bands in chalcopyrite solar cell heterojunctions

Johnson¹,

Klaer²,

Merdes³

et al. 2013

Journal of Electron Spectroscopy and Related Phenomena

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A non-optimized interface band alignment in a heterojunction-based solar cell can have negative effects on the current and voltage characteristics of the resulting device. To evaluate the use of Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) as a means to measure the conduction band position, Cu(In,Ga)S 2 chalcopyrite thin film surfaces were investigated as these form the absorber layer in solar cells with the structure ZnO/Buffer/Cu(In,Ga)S 2 /Mo/Glass. The composition dependence of the structure of the conduction bands of CuIn x Ga 1−x S 2 has been revealed for x = 0, 0.67 and 1 with both hard and soft NEXAFS and the resulting changes in conduction band offset at the junction with the buffer layer discussed. A comprehensive study of the positions of the absorption edges of all elements was carried out and the development of the conduction band with Ga content was observed, also with respect to calculated densities of states. Knowledge of these offsets is, therefore, critical to understanding the performance of the resulting solar cell. While the VB offset, ∆E V B , can be determined with established methods, such as combined XPS/UPS [5,6] or Constant Final State Yield Spectroscopy [7], a determination of CB edge positions and offsets, ∆E CB , has proved more difficult. The most common method is simply the assumption that the CB minimum is the energy of the VB plus the band gap. However, the determination of the surface band gap, which is relevant for the band offset, is more involved. Two of the main methods for the direct determination of the CB minimum are inverse photoelectron spectroscopy (IPES) and Near Edge X-ray Absorption Fine Structure (NEXAFS). They have given reliable results in some situations [8,9,10,11], although both have unresolved difficulties and the results must be carefully analyzed. IPES requires high intensity electron irradiation of the sample which often leads to charging of less conductive materials. In the case of NEXAFS these include transition probabilities, spectrum broadening and excitonic or core-hole effects. The latter may cause shifts in the measured position of the absorption edges which do not correspond to the ground state of the material. This is because the position of the absorption edge in NEXAFS represents the energy difference between the initial state (core level) and the final empty state (conduction band) in the material's excited state. The attraction between the core-hole and the excited electron may make the energy difference between the core level and conduction band state appear artificially smaller than it is in the ground state of the material. Also, because the absorption edge represents an energy difference, the energy of the initial state (core level) must be considered to determine whether differences in binding energy could influence the calculated energy of the final conduction band state. Here, while considering only the position of the absorption edge, we assume at first a constant initial state (core level binding) energy, a...

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Band alignment at the CdS∕Cu(In,Ga)S2 interface in thin-film solar cells

Cited by 89 publications

References 15 publications

Radiative transitions in highly doped and compensated chalcopyrites and kesterites: The case ofCu2ZnSnS4

Radiative transitions in highly doped and compensated chalcopyrites and kesterites: The case ofCu2ZnSnS4

Intermixing at the heterointerface between ZnS∕Zn(S,O) bilayer buffer and CuInS2 thin film solar cell absorber

Limitations of Near Edge X-ray Absorption Fine Structure as a tool for observing conduction bands in chalcopyrite solar cell heterojunctions

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