S. H. Lee, T. S. Lee, K. S. Lee, B. Cheong, Y. D. Kim and W. M. Kim.

Journal of Physics D, Applied Physics, Vol. 41, No. 9, 7 May 2008, pp. 095303 (7pp).

ZnO films co-doped with H and Al (HAZO) were prepared by sputtering ZnO targets containing 1 wt% Al2O3 on Corning glass at a substrate temperature of 150 deg C with Ar and H2/Ar gas mixtures. The effects of hydrogen addition to Al-doped ZnO (AZO) films with low Al content on the electrical, the optical and the structural properties of the as-grown films as well as the vacuum- and air-annealed films were examined. Secondary ion mass spectroscopy analysis showed that the hydrogen concentration increased with increasing H2 in sputter gas. For the as-deposited films, the free carrier number increased with increasing H2. The Hall mobility increased at low hydrogen content, reaching a maximum before decreasing with a further increase of H2 content in sputter gas. Annealing at 300 deg C resulted in the removal of hydrogen, causing a decrease in the carrier concentration. It was shown that hydrogen might exist as single isolated interstitial hydrogen bound with oxygen, thereby acting like an anionic dopant. Also, it was shown that the addition of hydrogen to ZnO films doped with low metallic dopant concentration could yield transparent conducting films with very low absorption loss as well as with proper electrical properties, which is suitable for thin film solar cell applications.

Axel Eicke, Stefanie Spiering, Ariane Dreael and Michael Powalla.

Surface and Interface Analysis, Vol. 40, No. 3-4, Mar.-Apr. 2008, pp. 830-833.

For the series production of Cu(In,Ga)Se2 (CIGS)-based thin-film solar cells it is desirable to replace the thin CdS buffer layer between absorber and transparent front contact by non-toxic, low-absorbing semiconductors. In2S3, deposited by atomic layer deposition, has already been qualified as an alternative buffer material. In this work, results of indium sulphide buffer layers deposited by thermal evaporation are presented. Pressed powders with different compositions and morphology were used for evaporation at about 720 deg C, resulting in different layer compositions and cell performances. The composition of the initial powder material and of the pellets after the deposition steps was determined by XRF. The deposited In2SxOy buffer layers and the buffer/absorber interface region were analysed by SIMS and sputtered neutral mass spectrometry (SNMS) depth profiling. Fine-grained pressed In2S3 powder evaporates rather homogeneously during the entire deposition run, resulting in nearly stoichiometric In2S3 layers. S and Cl are evaporated preferentially from pellets of coarse-grained, S-poor In2S2.4 powder containing 2 at% Cl, leading to excess S and high Cl concentrations in the first deposited layer. The subsequent layers are S-poor and the In/S ratios continue to increase. In all interface regions additional amounts of Se, In, and Ga are detected, which could be attributed to the Cu-poor defect layer on top of the CIGS absorbers. The best solar cell performance with efficiences of about 13% was achieved with the S-rich buffer layers. Stoichiometric In2S3 or S-poor layers yield lower efficiences between 8 and 11%.

A. Drici, M. Mekhnache, A. Bouraoui, et al.

Materials Chemistry and Physics, Vol. 110, No. 1, 15 July 2008, pp. 76-82.

Cu(In1-xGax)Se2 (CIGSe) thin films with 0 < = x < = 1 are grown by co-evaporation. Cu, In and Ga elements are evaporated from simple tungsten baskets, while Se is evaporated from broad Ta basket. Different combinations of the metal sources have been tested using three and two tungsten baskets. It is shown that, when deposited on a substrate heated at 500 deg C, the Ga is present throughout the thickness of the films whatever the technique used. X-ray photoelectron spectroscopy (XPS measurements have shown that Ga depth profile is more reliable in that case. X-ray diffraction shows that the films crystallize in the expected chalcopyrite structure. The lattice parameters decrease with increasing Ga atomic percentage. It is revealed that the optical band gap increases with the Ga content and yielded a bowing parameter around 0.28. The best results have been obtained with the four sources technique. Thin film solar cells, Mo/CIGSe/In2S3-xO3x/i-ZnO/ZnO:Al/Ni-Al grid, have been fabricated and probed. The efficiency of the cells depends strongly in the film composition but also in the metal source number.

Melinda M. Valencia.

Star, Vol. 45, No. 26, 5 Jan. 2008

The objective of this thesis is to contribute to the development of p-type materials for transparent electronics applications. Thin films of ?-BaCu2S2, a p-type semi-transparent semiconductor, are fabricated and characterized. ?-BaCu2S2 has a transmittance of 60% to 80 % in the visible portion of the electromagnetic spectrum. The mobility, conductivity, and carrier concentration of ?-BaCu2S2 are 3.5 cm2/V-s 17 S/cm, and 1019 cm?3, respectively. The potential use of BaCu2S2 in thin-film solar cells is described. A number of p-channel transparent thin-film transistors 'p-TTFTs' based on BaCu2S2, NiO, NiO:Li, and CuScO2 are fabricated and characterized. None of these p-TTFTs are operational. The key issues in these transistors are as follows. BaCu2S2 p-TTFTs exhibit excessively large gate leakage current caused by the interaction of BaCu2S2 with the gate insulator. In undoped NiO p-TTFTs and in CuScO2 p-TTFTs, the injected carriers are trapped in the transistor channel layer thin film. CuScO2 p-TTFTs also suffer from gate leakage due to interaction of CuScO2 with the gate insulator. In this work it is found that having Cu containing materials in contact with gate insulators leads to enhanced gate leakage current. In NiO:Li p-TTFTs, the bulk channel layer is too conductive to modulate with the transistor gate; thus, the transistors do not work. Information obtained from the characterization of these p-TTFTs is used to identify and explore important considerations in making a functional p-TTFT. These considerations include efficient injecting contacts to wide-bandgap p-type insulators, and the conductivity of materials used for the transistor channel in p-TTFTs. The topic of injecting contacts to wide-bandgap insulators and the topic of channel layer conductivity are explored and quantified.

Th Glatzel, M. Rusu, S. Sadewasser and M. Ch Lux-Steiner.

Nanotechnology, Vol. 19, No. 14, 9 Apr. 2008, pp. 145705 (7pp).

An extensive Kelvin probe force microscopy study in an ultrahigh vacuum has been undertaken to examine the influence of growth modifications of a few nm thick CdS buffer layers in thin film chalcopyrite solar cells. In regions around the grain boundaries of the polycrystalline Cu(In,Ga)Se2 substrate a lowering of the work function extending to about 200 nm away from this vertical interface was observed. This electrical inhomogeneity depends strongly on the Cu(In,Ga)Se2 surface condition and is interpreted by a diffusion process along the substrate grain boundaries. Our results contribute to the understanding of the crucial role of the several nm thick CdS layer for improving the photovoltaic performance of chalcopyrite thin film solar cells.