In order to transfer the potential for the high efficiencies seen for Cu(In,Ga)Se 2 (CIGSe) thin films from co-evaporation processes to cheaper large-scale deposition techniques, a more intricate understanding of the CIGSe growth process for high-quality material is required. Hence, the growth mechanism for chalcopyrite-type thin films when varying the Cu content during a multi-stage deposition process is studied. Break-off experiments help to understand the intermediate growth stages of the thin-film formation. The film structure and morphology are studied by X-ray diffraction and scanning electron microscopy. The different phases at the film surface are identified by Raman spectroscopy. Depth-resolved compositional analysis is carried out via glow discharge optical emission spectrometry. The experimental results imply an affinity of Na for material phases with a Cu-poor composition, affirming a possible interaction of sodium with Cu vacancies mainly via In(Ga) Cu antisite defects. An efficiency of 12.7% for vacancy compound-based devices is obtained.

The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

Superstrate solar cells were prepared by thermal evaporation of Cu(In,Ga)Se2 onto ZnO coated glass substrates. For the first time, photo-conversion efficiencies above 11% were reached without the necessity of additional light soaking or forward biasing of the solar cell. This was achieved by modifying the deposition process as well as the sodium doping. Limitations of the superstrate device configuration and possible ways to overcome these were investigated by analyzing the hetero-interface with electron microscopy and X-ray photoemission spectroscopy measurements, combined with capacitance spectroscopy and device simulations. A device model was derived that explains how on the one hand the GaO x , which forms at the CIGSe/ ZnO interface, reduces the interface recombination. On the other hand how it limits the efficiency by acting as an electron barrier at the hetero-interface presumably because of a high density of negatively charged acceptor states like Cu Ga . The addition of sodium enhances the p-type doping of the absorber but also increases the net doping within the GaO x . Hence, a trade-off between these two effects is required. The conversion efficiency was found to decrease over time, which can be explained in our model by field-induced diffusion of sodium cations out from the GaO x layer. The proposed device model is able to explain various effects frequently observed upon light soaking and forward biasing of superstrate devices.

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The electrical properties, in particular the U-I characteristics, current and voltage signal shapes within the pulse, are important parameters for the understanding of the processes taking place in the pulsed glow discharge (PGD). The electrical properties are also closely related to the analytical performance of the PGD such as sputtering rates, crater shapes and emission yields. Moreover, the dependence of the U-I plots on the density of the discharge gas can be used to estimate the gas temperature. This result is relevant for the analysis of thermally fragile samples. Nevertheless, there is a lack of PGD studies where the current and voltage signals are considered in detail. Therefore, this article is dedicated to the electrical properties of PGD. The influence of the PGD parameters (duty cycle and pulse duration) on the electrical properties is examined. The results highlight the optimum parameters for particular analytical applications. The question, whether direct current (dc) and radio frequency (rf) discharges behave similarly is also discussed and all experiments are performed for both modes. The comparative studies reveal strong similarities between dc and rf pulsed discharges.

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