A. Grimm

Influence of Na on Cu(In,Ga)Se2 solar cells grown on polyimide substrates at low temperature: Impact on the Cu(In,Ga)Se2/Mo interface

There are still open questions regarding the nature of the positive effect of the presence of Na on the performance of Cu(In,Ga)Se2 based, chalcopyrite thin film solar cells, especially at low processing temperatures. Studying Cu(In,Ga)Se2 thin film devices fabricated from low-temperature coevaporated absorbers on polyimide substrates by admittance and J-V-T measurements, characteristic properties are identified for different amounts of Na present during the growth. A roll-over behavior can be directly correlated with the Na-content. X-ray photoelectron spectroscopy shows the development of a MoSe2 phase at the back contact of the device. Efficiencies of 15.1% with MgF2 antireflection coating are demonstrated.

Zn(O,OH) layers in chalcopyrite thin-film solar cells: Valence-band maximum versus composition

Zn(O,OH) layers deposited by the ion layer gas reaction (ILGAR) technique have the potential to replace the conventionally used CdS buffer layer in Cu(In(1−X)GaX)(SYSe(1−Y))2-based thin-film solar cells. To avoid stability issues, the fraction of metastable Zn(OH)2 should be reduced in the final buffer layer. However, hydroxide-poor or -free ZnO “buffers” result in noncompetitive devices. We have therefore investigated the impact of different oxide/hydroxide ratios on the electronic band alignment at the absorber/buffer heterointerface. The surface composition as well as the position of the valence-band maximum (VBM) of respective ILGAR-Zn(O,OH) samples was determined by photoelectron spectroscopy. The position of the conduction-band minimum (CBM) was estimated using optical band gaps determined from optical reflection/transmission measurements. From the comparison of these VBM and CBM values with the respective values of the absorber surface, predictions are made in terms of valence- and conduction-band ...

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...

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.

Surface Cu-depletion of Cu(In,Ga)Se2 thin films: Further experimental evidence for a defect-induced surface reconstruction

The surface Cu-depletion of chalcopyrite thin films and its influence on the interface properties of related solar cells have been subject of a controversial debate for many years. Although the nature of this Cu-depletion and its extension in depth are crucial for the device physics, there are only a few contradictory experimental results that address this topic. To clarify this issue, we performed depth-dependent compositional analysis by angle dependent soft x-ray emission spectroscopy (AXES) on Cu(In,Ga)Se2 thin films with different integral Cu-contents. By considering depth profiles from literature and by taking the accuracy of AXES into account, our numerical AXES simulations predict a pronounced angle dependence for our samples. However, our experimental data show only a minor angle dependence, which leads to the conclusion that the Cu-depleted surface layer must be restricted to a very thin surface layer, which is not accessible by AXES. This conclusion is consistent with the result from our previo...

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.

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.

ZnO layers deposited by the ion layer gas reaction on Cu(In,Ga)(S,Se)2 thin film solar cell absorbers: Morphology, growth mechanism, and composition

Cu(In,Ga)(S,Se)2 (CIGSSe) based solar cells with a ZnO window extension layer (WEL) deposited by the ion layer gas reaction (ILGAR) reach competitive efficiencies compared to corresponding references with CdS buffer and lead to a simplified device structure. The WEL replaces not only the CdS buffer, but also the undoped part of the usually applied rf-sputtered ZnO window bilayer. Since the performance of corresponding solar cell devices depends strongly on the ILGAR process parameters (number of deposition cycles and process temperature), respective ILGAR-ZnO∕CIGSSe test structures were investigated by means of scanning electron microscopy and x-ray photoelectron spectroscopy. Thereby, the growth mechanism of ILGAR-ZnO on CIGSSe absorbers and its morphology was investigated. In addition, the surface composition was determined, showing that ILGAR-ZnO layers contain a certain amount of metastable hydroxide. Due to the systematic variation of the ILGAR process parameters it could be demonstrated that it is p...

Gallium gradients in chalcopyrite thin films: Depth profile analyses of films grown at different temperatures

The following article appeared in Journal of Applied Physics 110.9 (2011): 093509 and may be found at http://scitation.aip.org/content/aip/journal/jap/110/9/10.1063/1.3656986

Band alignment at Sb2S3/Cu(In,Ga)Se2 heterojunctions and electronic characteristics of solar cell devices based on them

Band offsets at Sb2S3/Cu(In,Ga)Se2 heterojunctions have been studied by x-ray and ultraviolet photoemission spectroscopy. The valence and conduction band offset have been estimated to −(0.6±0.3) eV and (0.2±0.3) eV, respectively. This result suggests Sb2S3 as a potential buffer layer material for chalcopyrite based solar cells. However, Cu(In,Ga)Se2/Sb2S3/ZnO solar cells have been investigated. While the open circuit voltage ranged up to ∼0.4–0.5 V, the short circuit current was limited to ∼1.8–4.9 mA/cm2. A photocurrent of about 30 mA/cm2 was found for negative bias. On the basis of bias dependent quantum efficiency measurements and calculations, limiting mechanisms are discussed.