Influence of Cu(In,Ga)Se2 band gap on the valence band offset with CdS

Schulmeyer, T.; Kniese, R.; Hunger, R.; Jaegermann, Wolfram; Powalla, Michael; Klein, Andreas

doi:10.1016/j.tsf.2003.10.122

Cited by 44 publications

(36 citation statements)

References 12 publications

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“…Such a film can be transported in air without oxidation of the burried chalcopyrite and the protective layer can be evaporated by sample heating inside the UHV chamber. Hunger et al [23,24] have shown by PES that the resulting chalcopyrite surface is free of oxides and other contamination and shows a composition similar to UHV-prepared samples. We have followed this route using a co-evaporated Cu(In, Ga)Se 2 thin film.…”

Section: Uhv-clean Surfacesmentioning

confidence: 98%

Surface potential of chalcopyrite films measured by KPFM

Sadewasser

2006

Physica Status Solidi (a)

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Atomic force microscopy is widely used to characterize the surface topography of a variety of samples. Kelvin probe force microscopy (KPFM) additionally allows determining images of the surface potential with nanometer resolution. The KPFM technique will be introduced and studies on surfaces of chalcopyrite semiconductors for solar cell absorbers will be presented. It is shown that operation in ultra-high vacuum (UHV) is required to obtain meaningful work function values. Different methods for obtaining UHV-clean surfaces are presented and KPFM studies on these are compared. Surfaces where prepared by in-vacuum deposition, inert-gas transfer, in-vacuum decapping of a protective Se-cap and a peel-off method. Finally, a sputter-annealing cycle also allows to obtain well-suited surfaces for KPFM studies. Employing KPFM, variations in the local surface potential at grain boundaries of polycrystalline CuGaSe2 films were observed. A potential drop indicates the presence of charged defects at grain boundaries. Furthermore, different electronic activity was found for different grain boundaries, as concluded from studies under illumination. Using laterally resolved surface photovoltage, a Cu2−xSe impurity phase could be observed in CuGaSe2.Copyright line will be provided by the publisher 1 Introduction Photovoltaic energy conversion is one route followed in the quest for a sustainable energy supply in the future. Thin film solar cell technologies offer a high cost reduction potential compared to standard silicon wafer technology. With their high power conversion efficiencies on the laboratory scale, thin film solar cells from the Cu-chalcopyrite materials system are a promising candidate for this technology; in fact, pilot production lines have recently been started. Currently, the highest efficiency of nearly 20% is obtained for Cu(In, Ga)Se 2 containing ≈ 30% of Ga [1]. A typical solar cell consists of a metallic Mo layer on a float glass substrate, a polycrystalline p-type Cu-chalcopyrite absorber layer with a typical thickness of 2 µm, a thin buffer layer (typically CdS) and the n-type double layer window (ZnO). A NiAl grid provides the front contact. A wide variety of growth techniques for the chalcopyrite absorber have been studied, including evaporation, sputtering and subsequent rapid thermal processing in chalcogen atmosphere, chemical vapor deposition, metal organic vapor phase epitaxy and electrochemical deposition. In such multilayered structures the various interfaces play a crucial role, as they are prone to contain electronically active defects causing electronic losses in the device. The polycrystalline character of the materials introduces additional effects, as for example varying composition in different grains and grain boundaries [2]. Recently, also compositional variations on the nanometer scale have been proposed [3]. Therefore, studies of surfaces and interfaces are highly important in order to further the understanding of these materials and eventually increase their efficiency towards the theore...

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Section: Uhv-clean Surfacesmentioning

confidence: 98%

Surface potential of chalcopyrite films measured by KPFM

Sadewasser

2006

Physica Status Solidi (a)

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“…processes such as chemical shifts during junction formation must often be neglected in order to determine an offset value [12,13,14].…”

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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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“…The dependence of band-gap energy on hydrostatic compressive strain generated by Al-dopant incorporation is indirectly accessed using the strain ε h versus Al (at. %) relation of Figure 6b: (25) coefficient S as wavelength independent constant, the sample absorption in Equation (23) can be expressed in terms of the inverse remission function of Equation (22) as follows:…”

Section: Optical Characterization Ofmentioning

confidence: 99%

Optimization of Electrochemically Deposited Highly Doped ZnO Bilayers on Ga-Rich Chalcopyrite Selenide for Cost-Effective Photovoltaic Device Technology

et al. 2016

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High quality polycrystalline bilayers of aluminium doped ZnO (Al:ZnO) were successively electrodeposited in the form of columnar structures preferentially oriented along the 1011 crystallographic direction from aqueous solution of zinc nitrate (Zn(NO 3 ) 2 ) at negative electrochemical potential of E C = (−0.8)-(−1.2) V and moderate temperature of 80 • C on gallium rich (30% Ga) chalcopyrite selenide Cu(In,Ga)Se 2 (CIGS) with chemically deposited ZnSe buffer (ZnSe/Cu(In,Ga)Se 2 /Mo/glass). The aluminium doped ZnO layer properties have initially been probed by deposition of Al:ZnO/i-ZnO bilayers directly on Mo/glass substrates. The band-gap energy of the Al:ZnO/i-ZnO reference layers was found to vary from 3.2 to 3.7 eV by varying the AlCl 3 solute dopant concentration from 1 to 20 mM. The electrical resistivity of indium-pellet contacted highly doped Al:ZnO sheet of In/Al:ZnO/i-ZnO/Mo/glass reference samples was of the order ρ~10 −5 Ω·cm; the respective carrier concentration of the order 10 22 cm −3 is commensurate with that of sputtered Al:ZnO layers. For crystal quality optimization of the bilayers by maintenance of the volatile selenium content of the chalcopyrite, they were subjected to 2-step annealing under successive temperature raise and N 2 flux regulation. The hydrostatic compressive strain due to Al 3+ incorporation in the ZnO lattice of bilayers processed successively with 5 and 12 mM AlCl 3 dopant was ε h = −0.046 and the respective stress σ h = −20 GPa. The surface reflectivity of maximum 5% over the scanned region of 180-900 nm and the (optical) band gap of E g = 3.67 eV were indicative of the high optical quality of the electrochemically deposited (ECD) Al:ZnO bilayers.

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Influence of Cu(In,Ga)Se2 band gap on the valence band offset with CdS

Cited by 44 publications

References 12 publications

Surface potential of chalcopyrite films measured by KPFM

Surface potential of chalcopyrite films measured by KPFM

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

Optimization of Electrochemically Deposited Highly Doped ZnO Bilayers on Ga-Rich Chalcopyrite Selenide for Cost-Effective Photovoltaic Device Technology

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