The aim of the present contribution is to give a review on the recent work concerning Cd-free buffer and window layers in chalcopyrite solar cells using various deposition techniques as well as on their adaptation to chalcopyrite-type absorbers such as Cu(In,Ga)Se 2 , CuInS 2 , or Cu(In,Ga)(S,Se) 2 . The corresponding solar-cell performances, the expected technological problems, and current attempts for their commercialization will be discussed. The most important deposition techniques developed in this paper are chemical bath deposition, atomic layer deposition, ILGAR deposition, evaporation, and spray deposition. These deposition methods were employed essentially for buffers based on the following three materials: In 2 S 3 , ZnS, Zn 1 À x Mg x O.

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Solar cells with an extremely thin light absorber were realized by wet chemical preparation on arrays of ZnO nanorods. The absorber consisted of an In2S3 layer (∼20nm thickness) and its interface region with a transparent CuSCN hole conductor. By changing the length of the nanorods (0–3.3μm) and keeping the In2S3 layer thickness constant at ∼20nm, the short circuit current increased from about 2–10mA∕cm2. A marked increase of the external quantum efficiency at longer wavelengths is attributed to light scattering and a solar energy conversion efficiency of 2.5% has been demonstrated.

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The preparation of specifically passivated silver quantum dots is reported. Investigations into the surfacecapping agents have highlighted the importance of both the chain length and bonding strength of the individual ligands. Capping agents traditionally utilised to stabilise semiconductor quantum dots cannot always be used to effectively passivate metal structures, and a dual surfactant system was chosen. Long chain amines, usually weakly binding to a bulk silver surface, were found to be effective passivating agents. The steric properties of weakly bound trialkylphosphine oxides also appear to affect the particle stability. The choice of capping agents highlight the importance of both the Lewis base functionality and steric factors and these may play a key part in the design of future nanostructured materials. It also emphasizes that effective capping agents for nanodispersed metals are not necessarily the optimum ligands for other systems such as semiconductors, metal oxides, etc.

Single crystalline and polycrystalline CuInS2 samples prepared by different methods are characterized by Raman spectroscopy. The measured spectra are fitted according to the phonon confinement model. Correlation lengths were obtained, which correspond to the size of domains of perfect crystallinity. These correlation lengths are in good agreement with distances between twin defects observed by transmission electron microscopy in polycrystalline CuInS2. Additionally, the strain present in the samples was determined from the Raman spectra. A tensile strain was obtained for the polycrystalline CuInS2 thin films, which agrees well with published values for the same material.

Chalcopyrite thin film solar cells with an In2S3 buffer showed high efficiencies above 15%, but only after annealing at 200 °C. One possible explanation is a Cu interdiffusion at the absorber/buffer interface. We were able to directly measure a Cu interdiffusion with a new tool at the BESSY synchrotron facility: HIKE (high kinetic energy X‐ray photoelectron spectroscopy). Due to its increased information depth of up to 10–20 nm HIKE is able to provide chemical information even through thin closed overlayers. The HIKE endstation also allows in‐situ heating. A layer stack of In2S3/ Cu(In,Ga)Se2 was prepared and heated in‐situ up to a temperature of 300 °C while continuously measuring HIKE spectra. A strong increase of the Cu signal above temperatures around 200 °C was found. With this setup we were able to directly monitor the Cu diffusion from the absorber into the buffer layer. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)