S. Lehmann

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

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) 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∕Zn(S,O) bilayer buffer in former investigations [M. Bar et al., J. Appl. Phys. 99, 123503 (2006)], 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 (PES) measurements were also conducted using tunable monochromat...

Three-dimensional structure of the buffer/absorber interface in CdS/CuGaSe2 based thin film solar cells

The chemical structure of the CdS/CuGaSe2 chalcopyrite solar cell buffer/absorber interface is investigated by combining element depth profiling using elastic recoil detection analysis and surface-near bulk sensitive x-ray emission spectroscopy. Significant Cd and S concentrations (≥0.1 at. %) are found deep in the absorber bulk. The determined high Cd and S diffusion coefficient values at 333 K of 3.6 and 3.4×10−12 cm2/s, respectively, are attributed to diffusion along CuGaSe2 grain boundaries. As a result, a three-dimensional buffer/absorber interface geometry is proposed.

Cd2+∕NH3 treatment-induced formation of a CdSe surface layer on CuGaSe2 thin-film solar cell absorbers

CuGaSe2 (CGSe)-based high-gap thin-film solar cells have to date not reached their potential level of electrical performance. In order to elucidate possible shortcomings of the electronic interface structure, we have studied the initial stage of the CdS∕CGSe interface formation by use of a simple Cd2+∕NH3 treatment. As in the case of low-gap chalcopyrites, we find a Cd-containing surface layer, in the present case comprised of approximately one monolayer of CdSe. The results indicate that the CdS∕CGSe interface is not abrupt, but contains intermediate layers. Furthermore, they shed light on possible surface modification schemes to enhance the overall performance of high-gap CGSe chalcopyrite solar cells.

The chemical and electronic surface and interface structure of CuGaSe2 thin-film solar cell absorbers

The chemical and electronic surface and interface structure of CuGaSe2 thin films was investigated by photoelectron spectroscopy. With bulk [Ga]/[Cu] ratios increasing from 0.94 to 1.39 a transition of the Cu:Ga:Se surface composition from 1:1:2 to 1:3:5 and a downward shift of the valence band maximum with respect to the Fermi energy were observed. The comparison with the conduction band minimum (CBM) of CdS reveals that at the CdS/CuGaSe2 interface the recombination barrier height simultaneously increases and a “clifflike” offset is formed to the CBM of CuGaSe2.

Long-range structure of Cu(InxGa1−x)3Se5: A complementary neutron and anomalous x-ray diffraction study

Distinguishing the scattering contributions of isoelectronic atomic species by means of conventional x-ray- and/or electron diffraction techniques is a difficult task. Such a problem occurs when determining the crystal structure of compounds containing different types of atoms with equal number of electrons. We propose a new structural model of Cu(InxGa1−x)3Se5 which is valid for the entire compositional range of the CuIn3Se5–CuGa3Se5 solid solution. Our model is based on neutron and anomalous x-ray diffraction experiments. These complementary techniques allow the separation of scattering contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly identically in monoenergetic x-ray diffraction experiments. We have found that CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification exhibits a modified stannite structure (space group I4¯2m). Different occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies have been found at three different cationic pos...

Cd/sup 2+//NH/sub 3/-treatment of high-gap CuGaSe/sub 2/ thin film solar cell absorbers

For chalcopyrite-based solar cells with conventional CdS buffers (prepared in a chemical bath), the Cd/sup 2+//NH/sub 3/-treatment can be used as a tool to visualize the processes during the initial stages of interface formation in the chemical bath. For high-gap CuGaSe/sub 2/ (CGSe) - an absorber material which has not yet shown its full potential in terms of solar cell performance - a CdSe surface compound was recently identified at the CGSe absorber surface after this kind of simple surface conditioning. In this contribution, we will clarify whether the [Ga]/[Cu]-ratio of the CGSe absorber has an impact on the formation of this non-abrupt interface. In addition, the lateral variation of the observed absorber surface modification will be investigated. Thus, as-deposited and Cd/sup 2+//NH/sub 3/-treated CGSe surfaces were characterized by X-ray excited Auger and photoelectron spectroscopy using Mg K/spl alpha/ and/or tuneable monochromatized synchrotron radiation.

Improved CuGaSe/sub 2/-based solar cell performance by In-S surface treatments

CuGaSe/sub 2/ (CGSe) thin films for photovoltaic applications have been subjected to surface treatments based on In-S by means of chemical vapor deposition. Structural and electronic characterization of as-grown films and processed devices show the effective incorporation of In and S in the near-surface region of CGSe thin films and a positive impact on the solar cell performance.