CuInSe2 Surface Research Articles

Chemical interaction of Na with cleaved (011) surfaces of CuInSe2

To study the beneficial effect of sodium-containing substrate material on the photovoltaic properties of thin-film CuInSe2 solar cells the chemical interaction of Na with CuInSe2 has been investigated by synchrotron excited photoelectron spectroscopy. A clean CuInSe2 (011) surface was prepared by cleaving an oriented single crystal in UHV. The cleaved surface exhibits an electron affinity of χ≊4.6 eV. The Se 3d level shows a surface core level shift of −0.4 eV. Na was sequentially deposited in UHV in small steps from a commercial dispenser source. Initially ionized Na adsorbs on the surface leading to a shift of the surface Fermi level by 0.3 eV closer to the conduction band. Removal of Cu from the surface is observed. For higher Na deposition a chemical reaction of Na with the CuInSe2 surface occurs leading to the formation of metallic indium and Na2Se.

XPS Analysis of Chemically Treated I-III-VI Semiconductor Surfaces and the Relation to II-VI/I-III-VI Heterojunction Formation

AbstractDevice-grade thin-film CuInSe2 was subjected to various chemical treatments commonly used in photovoltaic device fabrication to determine the resulting microscopic surface composition/morphology and the effect on II-VI/CuInSe2 heterojunction formation. HCI (38%), Br-MeOH (<1% Br), (NH4)2S, and NH4OH/thiourea solutions were used separately to modify the surface chemistry of the CuInSe2 polycrystalline films. Scanning electron microscopy was used to evaluate the resultant surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and Cu 2p, In 3d, Ga 2p and Se 3d core lines were used to evaluate the chemistry of the chemically treated surfaces. CdS overlayers were then deposited in steps on these chemically treated surfaces. Photoemission measurements were acquired after each growth to determine the resultant heterojunction valence-band discontinuity between the CdS and the chemically modified CuInSe2 surface.

Surface type conversion of CuInSe2 with H2S plasma exposure: A photoemission investigation

Surface type conversion of CuInSe2 by H2S plasma exposure was studied by synchrotron radiation soft x‐ray photoemission spectroscopy. The low power H2S plasma was generated with a commercial electron cyclotron resonance plasma source using pure H2S with the plasma exposure being performed at 400 °C. In situ photoemission measurements were acquired after each plasma exposure in order to observe changes in the valence band electronic structure as well as changes in the In 4d and Se 3d core lines. The results were correlated in order to relate changes in surface chemistry to the electronic structure. These measurements indicate that the H2S plasma exposure type converts the n‐type CuInSe2 surface to a p‐type surface at this elevated temperature and that the magnitude of the band bending is 0.5 eV, resulting in a homojunction interface.

Valency and type conversion in CuInSe2 with H2 plasma exposure: A photoemission investigation

The effect of H2 plasma exposure on CuInSe2 was studied by synchrotron radiation soft-x-ray photoemission spectroscopy. The low-power H2 plasma was generated with a commercial electron cyclotron resonance plasma source using pure H2 with the plasma exposure being performed at 200 °C. In situ photoemission measurements were acquired after each plasma exposure in order to observe changes in the valence-band electronic structure as well as changes in the In 4d and Se 3d core lines. The results were correlated in order to relate changes in surface chemistry to the electronic structure. These measurements indicate that the H2 plasma exposure type converts the CuInSe2 surface to an n-type surface as well as converting the In+3 valency state to an In+1 valency state.

Novel approach to CuInSe2-based thin-film photovoltaic cells

A new n-CuInSe2-based cell has been constructed with a concerted effort at the synthesis of n-CuInSe2 thin-film absorber and the electrochemical conversion of the CuInSe2 surface to a semi-insulating, lattice-matched CuInxSeyIz transition layer and a semiconducting p-CuISe3 window. The CuISe3 layer can be anodically formed on n-type or p-type CuInSe2; it forms a rectifying p-n junction with n-CuInSe2 and an ohmic contact with p-CuInSe2. This cell obviates some of the prevailing concerns with the p-CuInSe2/n-CdS cell while retaining the thin-film attributes of the radiation-tolerant CuInSe2.

ChemInform Abstract: Site‐Specific Surface Interaction of Adsorbed H2O and Halogens on CuInSe2 Surfaces.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Site-specific surface interaction of adsorbed water and halogens on copper indium selenide (CuInSe2) surfaces

The interaction of adsoorbed H 2 O, I 2 , and Cl 2 on cleaved CuInSe 2 surfaces has been investigated in ultrahigh vacuum to model semiconducteur/electrolyte interfaces. The semiconducteur/adsorbate system has been characterized by UV and X-ray photoelectron spectroscopy (UPS,XPS), low-energy electron diffraction (LEED), low-energy ion scattering spectroscopy (LEISS) and surface photovoltage measurements (SPV).

Advances in CuInSe2 and CdTe thin film solar cells

Abstract Research on CuInSe2 and CdTe thin film solar cells is discussed. CuInSe2 was deposited by selenization of Cu/In layers and was used to make a 10% efficient CuInSe2/(CdZn)S cell. Characterization of the reaction mechanisms is described. The open-circuit voltage V oc of CuInSe2/(CdZn)S cells is dominated by recombination in the space charge region, so increasing the band gap or decreasing the width of this region should increase V oc . Increasing the band gap with a thin Cu(InGa)Se2 layer at the CuInSe2 surface has demonstrated increased V oc with collection out to the CuInSe2 band gap. A post-deposition treatment and contacting process for evaporated CdS/CdTe cells was developed and high efficiency cells were made. Several steps in the process, including a CdCl2 coating, a 400 °C heat treatment, and a contact containing copper are critical. ZnTe films were deposited from an aqueous solution as a contact to CdTe.

Al/p-CuInSe2 metal-semiconductor contacts

Al/p-CuInSe2 Schottky contacts have been fabricated by vacuum-depositing Al onto Bridgman-grown p-CuInSe2 single crystals. The barrier heights from the temperature-dependent forward I-V characteristics are significantly less than the C−2-V intercepts. This is evidence of the presence of an interfacial layer and surface states in equilibrium with p-CuInSe2. The barrier height, electron affinity, effective acceptor density, and relative Fermi potential as well as diffusion potential were estimated. The surface-state density of the minimum order of 1012 cm−2 eV−1 was obtained for the etched CuInSe2 surface. The transverse doping profile was qualitatively determined.

ChemInform Abstract: Electrolyte‐Induced Changes in Charge Transport Rates Across the CuInSe2 Interface.

AbstractThe role of electrochemical parameters (particular electrolyte pH) in the formation, composition, and photoconductivity of films on the CuInSe2 surface, and their effect on the charge transfer kinetics and on the quantum efficiency, have been investigated by combining a variety of optoelectrochemical methods.

Electron beam effects in the analysis of compound semiconductors and devices

The effects of electron beams on the analysis of CuInSe2 surfaces are examined in this paper. Potential changes in the surface chemistry—including oxidation and desorption—under a range of incident probe conditions, are investigated for possible artifactual information generation. Emphasis is placed on the relationships between beam conditions and oxygen chemisorption and physisorption, since oxygen treatments of devices utilizing this semiconductor are critical to performance. Single crystals and polycrystalline thin films are analyzed and compared to establish the beam-induced phenomena.

Simulation of electrochemical interface properties: UHV‐investigation on CuInSe2 and CuInS2

AbstractThe interaction of water and chlorine with cleaved CuInSe2 and CuInS2 surfaces is studied by XPS and UPS. Whereas water appears to adsorb undissociatively, chlorine interacts chemically with the surfaces. Photoinduced changes of valence band spectra are investigated and their relevance with respect to surface states and band bending is discussed.

Minority-carrier diffusion and recombination in CdZnS/CuInSe2 solar cells

Minority-carrier diffusion has been investigated in as-grown CdZnS/CuInSe2 solar cells. Capacitance-voltage (C-V) and photoconductivity measurements are combined to determine diffusion and interface recombination processes. Diffusion lengths of about 2 μm and a small interface recombination velocity are determined from biased photoconductivity, biased spectral response, and photoluminescence studies. Photoluminescence studies also indicate that a much lower interface recombination velocity occurs at the CdZnS/CuInSe2 interface than that at a bare CuInSe2 surface.

Surface order and stoichiometry of sputter-cleaned and annealed CuInSe2

CuInSe2 surfaces can be cleaned by bombardment with Ar ions at glancing incidence and with energies up to 1.5 keV without change in composition. At near-normal incidence a slight depletion of Cu is observed. The surface crystallinity can be partially restored by annealing to ∼550 °C, without preferential evaporation of an element. On Ar+-cleaned and annealed surfaces, single-crystal low-energy electron diffraction patterns of CuInSe2 have been produced for the first time.

Quantitative studies of cleaved and sputtered CuInSe2 surfaces

CuInSe2 has emerged as an important photovoltaic semiconductor. The critical importance of stoichiometry changes, interfacial chemical reactions, and compositional variations on cell performance has made the acquisition of reliable quantitative data by surface analysis methods mandatory. In this study, single crystals of CuInSe2 were cleaved in UHV, exposing the (110) plane which is representative of the bulk material. Quantitative AES analysis was performed using both N(E) and dN(E)/dE data, yielding concentration results within 2 at. % of stoichiometric values. The method developed is based upon an independent evaluation of matrix effects using elemental standards and actual density considerations. The effects of Ar+ ion sputtering were investigated for beam energies 0.1<Ep<5.0 kV. Quantitative data for sputter-etched and virgin surfaces have been compared. Oxygen adsorption and In oxidation was observed during exposure to O2 and electron beam irradiation. Polycrystalline thin films of CuInSe2 were also analyzed, and quantitative results have been compared to the single-crystal values.