Hans-Werner Schock

Texture etched ZnO:Al coated glass substrates for silicon based thin film solar cells

ZnO:Al films were r.f.- and d.c.-magnetron sputtered on glass substrates from ceramic (ZnO:Al2O3) and metallic (Zn:Al) targets, respectively. The initially smooth films exhibit high transparencies (T≥83% for visible light including all reflection losses) and excellent electrical properties (ρ=2.7–6×10−4 Ω cm). Depending on their structural properties these films develop different surface textures upon post deposition etching in diluted HCl. The light scattering properties of suitable films can be controlled over a wide range simply by varying the etching time. Moreover, the electrical properties are not affected by the etching process. Thus, within certain limits a separate optimization of the electro-optical and light scattering properties is possible. Amorphous silicon (a-Si:H) based solar cells prepared on these new texture etched ZnO-substrates show high quantum efficiencies in the long wavelength range demonstrating an effective light trapping. First a-Si/a-Si stacked solar cells were realized with initial efficiencies exceeding 10%.

ZnO/CdS/CuInSe2 thin‐film solar cells with improved performance

An important milestone in the development of photovoltaic thin‐film solar cells is the achievement of 15% conversion efficiency. This letter describes the highest efficiency single junction thin‐film cell reported to date. An active area efficiency of 14.8% is obtained with the cell structure n‐ZnO/n‐CdS/p‐CuInSe2 deposited on a soda‐lime glass substrate. The current achievements are due to improved properties of the CuInSe2 layer and the heterojunctions compared to previously reported results. The rate and substrate temperature profiles used during the coevaporation process yield a relatively large‐grained material with very strong 〈112〉 orientation and low porosity. This results in reduced recombination rates, hence higher open circuit voltage and fill factor.

On the Sn loss from thin films of the material system Cu–Zn–Sn–S in high vacuum

In this paper the Sn loss from thin films of the material system Cu–Zn–Sn–S and the subsystems Cu–Sn–S and Sn–S in high vacuum is investigated. A combination of in situ x-ray diffractometry and x-ray fluorescence (XRF) at a synchrotron light source allowed identifying phases, which tend to decompose and evaporate a Sn-containing compound. On the basis of the XRF results a quantification of the Sn loss from the films during annealing experiments is presented. It can be shown that the evaporation rate from the different phases decreases according to the order SnS→Cu2SnS3→Cu4SnS4→Cu2ZnSnS4. The phase SnS is assigned as the evaporating compound. The influence of an additional inert gas component on the Sn loss and on the formation of Cu2ZnSnS4 thin films is discussed.

Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(In,Ga)Se2 thin film solar cells

A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a ...

Influence of sodium on the growth of polycrystalline Cu(In,Ga)Se2 thin films

We investigate the influence of Na on the growth of Cu(In,Ga)Se2 thin-films by three model experiments. First, we examine the influence of Na on the Se activity during selenisation of Mo films on soda-lime and borosilicate glass after growth and after thermal treatments. Second, we analyse the location and possible binding partners of Na in polycrystalline Cu(In,Ga)Se2 prior to and after air-exposure. The final experiment focuses on the identification of the chemical state of Na on the surface of as grown and air-exposed films. Our experiments demonstrate that Na influences the growth of CIGS Cu(In,Ga)Se2 due to its interaction with Se. In non-air-exposed films Na is mainly localised in the form of sodium-polyselenides (Na2Sex) at the grain boundaries. We conclude that Na2Sex acts as Se-reservoir during film formation and oxidation.

A novel cadmium free buffer layer for Cu(In,Ga)Se2 based solar cells

Abstract Solar cells based on Cu(In,Ga)Se 2 were prepared replacing the “standard buffer layer” CdS with a In x (OH,S) y thin film. The film is deposited in a chemical bath (CBD) process using an aqueous solution containing InCl 3 and thioacetamide. X-ray photoemission spectroscopy measurements were performed in order to characterize the growth kinetics and the chemical composition. The influence of different concentrations of InCl 3 and thioacetamide in the solution on the electrical properties of the solar cells was studied by measuring the j-V characteristics and the spectral quantum efficiencies. Capacitance-voltage ( C-V ) measurements indicate that the high V ∞ values of devices with the novel buffer layer are correlated with narrower space charge widths and higher effective carrier concentrations in the absorber materials. The achieved conversion efficiency of 15.7% (active area) using the cadmium free In x (OH,S) y buffer demonstrates the potential of this process as an alternative to the standard chemical bath deposition of CdS.

CIGS‐based solar cells for the next millennium

Thin-film photovoltaic (PV) solar cells based on Cu(In,Ga)Se2 (CIGS) have two key distinctive features: highest performance of any true thin-film solar cell (18·8% efficient) and leading performance on the module level. There is no evidence of limits to further improvement of the efficiency. Device stability is not curtailed by intrinsic material properties. The obstacles to large-scale production and commercialization of Cu(In,Ga)Se2-based modules are the complexity of the material and the manufacturing processes. Published in 2000 by John Wiley & Sons, Ltd.

Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu2ZnSnS4 thin-film solar cell heterojunction

The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface.

Interface redox engineering of Cu(In,Ga)Se2 – based solar cells: oxygen, sodium, and chemical bath effects

Abstract The chemical effects of oxygenation of Cu(In,Ga)Se 2 (CIGS) interfaces are analyzed and are shown to involve passivation of Se deficiencies and Cu removal. The former effect is beneficial at grain boundaries, but detrimental at the CdS/CIGS interface. The latter effect is purely detrimental. Na and chemical bath deposition (CBD) treatments are shown to isolate the ‘good’ oxygenation effect from the ‘bad’ ones. Na is shown to promote oxygenation already before the deposition of the buffer and window layers, which allows a maximization of the benefits of Se deficiency passivation and a minimization of Cu removal. Next, the CBD of the CdS buffer layer restores the interface charge, due to creation of Cd Cu interface donors and possibly a removal of O Se interface acceptors. This highlights the crucial role that interface redox engineering plays in optimizing the performance of CIGS-based solar cells.

Thin film photovoltaics

Abstract The different technologies for thin film photovoltaics are compared. Three main candidates namely a-Si, CdTe and CuInSe 2 exhibit specific material properties which require different deposition processes and determine the design of photovoltaic devices. The properties of these material are discussed in detail in view of their advantages, drawbacks and potential for future developments. The present state of the art of thin film solar cell manufacturing is reviewed and the different deposition processes are analyzed in view of material quality and scalability. Specific differences of the semiconductors and their potential for high efficiency solar cells are evaluated. The formation and analysis of these junctions is described and ways for new developments and material combinations are shown. Recent results on the manufacturing issues and performance of thin film modules are reported.