F. Karg

Determination of the band gap depth profile of the penternary Cu(In(1-X)GaX)(SYSe(1-Y))2 chalcopyrite from its composition gradient

A simple model is introduced which determines the optical band-gap energy Eg for penternary Cu(In(1−X)GaX)(SYSe(1−Y))2 (CIGSSe) alloys from its Ga∕(Ga+In) ratio as well as from its S∕(S+Se) ratio. In order to verify the model the depth dependent composition of a CIGSSe sample was revealed by elastic recoil detection analysis. Applying the model, the concentration profiles were transferred in an Eg profile. Finally, these values were compared with optical band-gap energies, which were obtained directly by independent characterization methods.

Highly efficient Cu(Ga,In)(S,Se)2 thin film solar cells with zinc-compound buffer layers

Abstract Chemical bath deposited Zn-compound buffer layers have been applied as an alternative to the CdS buffer layer in the development of Cu(In,Ga)(S,Se)2 (CIGSSe) thin film solar cells. We used CIGSSe absorbers developed by Shell Solar for large-scale production. When ZnO is sputtered directly on such absorbers, very poor performances are obtained. In contrast, when the CIGSSe films are treated in electrolyte containing Zn-ions before sputtering, device efficiency of 12% is achieved. Including a sulfur or selenium source, we have developed a process to fabricate Cd-free CIGSSe devices with over 14% efficiency, certified at NREL. The structure and composition of the CBD-ZnSe on CIGSSe surface were investigated. The growth mechanism of chemical bath deposited ZnSe and ZnS on CIGSSe are discussed.

Band alignment at the i-ZnO/CdS interface in Cu(In,Ga)(S,Se)2 thin-film solar cells

The interface between the i-ZnO layer and the CdS buffer in Cu(In,Ga)(S,Se)2 thin-film solar cells from the Shell Solar baseline process has been investigated using ultraviolet- and x-ray photoelectron spectroscopy and inverse photoemission. Combining both techniques, a direct determination of the conduction and valence band offsets at the interface is possible. Different from existing models, we find a flat conduction band alignment (i.e., a conduction band offset of 0.10±0.15 eV), ∼0.5 eV above the Fermi level, and a valence band offset of 0.96±0.15 eV.

Monitoring chemical reactions at a liquid–solid interface: Water on CuIn(S,Se)2 thin film solar cell absorbers

The chemical and electronic structure of the interface between liquid water and a CuIn(S,Se)2 thin film surface was studied with synchrotron-based, high energy-resolution soft x-ray emission spectroscopy (XES). By probing the local environment around the sulfur atoms, an x-ray-induced sulfate formation at the CuIn(S,Se)2 surface can be monitored, correlated with a substantial enhancement of sodium impurity atoms from the CuIn(S,Se)2 film and its glass substrate. The results demonstrate that, with XES, an experimental probe is available to in situ study chemical reactions at liquid–solid interfaces or at surfaces in a high-pressure gas environment in a chemically sensitive and atom-specific way.

Inducing and monitoring photoelectrochemical reactions at surfaces and buried interfaces in Cu(In,Ga)(S,Se)2 thin-film solar cells

We report the direct observation of a photoinduced oxidation process at the buried buffer/absorber interface in high-efficiency Zn(O,OH)∕Cu(In,Ga)(S,Se)2 thin-film solar cell structures by means of x-ray emission and photoelectron spectroscopy. We propose a reaction mechanism that involves the decomposition of a hydroxide compound in the buffer layer into water and an oxide and present evidence that this process also occurs with visible light excitation and after accelerated lifetime tests of nonencapsulated devices. This suggests a possible photoinduced aging effect in solar cell devices with other hydroxide containing buffer layers or under humid conditions.

Analysis of Zinc Compound Buffer Layers in Cu(In, Ga)(S, Se) 2 Thin Film Solar Cells by Synchrotron-Based Soft X-Ray Spectroscopy

Zinc-based buffer layers like ZnSe, ZnS, or wet-chemically deposited ZnO on Cu(In, Ga)(S, Se) 2 absorber materials (CIGSSe) have yielded thin film solar cell efficiencies comparable to or even higher than standard CdS/CIGSSe cells. However, little is known about surface and interface properties of these novel buffer layers. In this contribution we characterize the specific chemical environment at the absorber/buffer-interface using X-ray Emission Spectroscopy (XES) and Photoelectron Spectroscopy (PES) in a complementary way. Evidence of intermixing and chemical reactions is found for different buffer materials and deposition methods.

Sulfur gradient-driven Se diffusion at the CdS/CuIn(S,Se)2 solar cell interface

The diffusion behavior of Se at the CdS/Cu(In,Ga)(S,Se)2 thin film solar cell interface was investigated by x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. Buffer/absorber structures with S/Se ratios between zero and three at the initial Cu(In,Ga)(S,Se)2 surface were analyzed. Samples from a high-efficiency laboratory process (NREL) as well as from an industrial large-area process (AVANCIS) were investigated. We find selenium diffusion into the CdS buffer layer, the magnitude of which strongly depends on the S content at the absorber surface. The associated modification of the heterojunction partners has significant impact on the electronic structure at the interface.

Cd2+∕NH3-treatment of Cu(In,Ga)(S,Se)2: Impact on the properties of ZnO layers deposited by the ion layer gas reaction method

Cu(In,Ga)(S,Se)2- (“CIGSSe”) based solar cells with a ZnO layer deposited by the ion layer gas reaction (ILGAR) method yield superior efficiencies (15.0%) than the references with a chemical bath-deposited CdS buffer (14.1%). However, this high performance is only reached if the absorber is pretreated in a Cd2+- and aqueous ammonia-containing bath prior to the ILGAR-ZnO deposition. The photovoltaic as well as the dark device parameters are strongly influenced by this treatment. Scanning and transmission electron microscopy (TEM) as well as x-ray diffraction measurements reveal a different morphology and structure of ILGAR-ZnO layers on top of Cd2+∕NH3-treated and on as-deposited absorbers, indicating a considerably modified absorber surface. By energy dispersive x-ray analysis in the TEM, Cd could only be identified at the ILGAR-ZnO∕Cd2+∕NH3-treated-CIGSSe interface of the respective cross sections, if the absorber was treated in a bath with an atypically high Cd2+-concentration. In this case a Cd-contain...

Spectroscopic investigation of the deeply buried Cu(In,Ga)(S,Se)2∕Mo interface in thin-film solar cells

The Cu(In,Ga)(S,Se)(2)Mo interface in thin-film solar cells has been investigated by surface-sensitive photoelectron spectroscopy, bulk-sensitive x-ray emission spectroscopy, and atomic force microscopy. It is possible to access this deeply buried interface by using a suitable lift-off technique, which allows us to investigate the back side of the absorber layer as well as the front side of the Mo back contact. We find a layer of Mo(S,Se)(2) on the surface of the Mo back contact and a copper-poor stoichiometry at the back side of the Cu(In,Ga)(S,Se)(2) absorber. Furthermore, we observe that the Na content at the Cu(In,Ga)(S,Se)(2)Mo interface as well as at the inner grain boundaries in the back contact region is significantly lower than at the absorber front surface.

Comparison of Band Alignments at Various CdS/Cu(In,Ga)(S,Se)2 Inter-Faces in Thin Film Solar Cells

The band alignment at the CdS/Cu(In,Ga)(S,Se)2 interface, as derived in our earlier publications, are compared for different absorber compositions. The discussed band alignments were directly determined using a combination of UV- and X-ray photoemission and inverse photoemission. While a flat conduction band alignment can be found for low-gap material, the cell structure with a high-gap absorber shows a cliff-like alignment. The different alignments can be correlated with the respective cell parameters, explaining why the expected linear gain in open circuit voltage for the high-gap absorbers has not yet been achieved