No abstract

We report evidence for the idendification of the capacitance transients detected at room temperature for thin-film photovoltaic cells with CIGS absorbers as an additional non-ohmic contact in the structure with a time constant larger than that of the solar cell pn-jucntion. The N1 signal was recently interpreted as a back contact barrier for which the RC-like time constant is smaller than the time constant of the junction. In this work we unite these experimental observations in one model. Since for a Mo/CIGS/CdS/ZnO solar cell several interfaces are connected in series, we introduce the idea of modeling capacitance spectroscopy signals based on RC-circuits in series for each interface. It is shown that the distinct features observed in capacitance spectroscopy of CIGS solar cells can be mimicked using this circuit as an electrical model. The differential equation for this structure as a function of time is solved numerically. It is inherent to the model that the transients in such a structure are voltage transients over each of the interfaces and that the transients are coupled. These findings question the practical use of capacitance spectroscopy for direct measurement of defects in the absorber layer.

Sb doping of Cu(In,Ga)Se 2 (CIGS) solar cells has been reported to exhibit a positive effect on the morphology of the absorber layer, offering a possibility to lower manufacturing cost by lowering the annealing temperatures during the CIGS deposition. In this work electron microscopy, energy-dispersive X-ray spectroscopy and photoluminescence experiments have been performed on cells deposited on soda lime glass substrates, adding a thin Sb layer onto the Mo back contact prior to the CIGS absorber deposition. The defect structure of CIGS solar cells doped with Sb in this way has been investigated and is compared with that of undoped reference cells. The influence of substrate temperature during absorber growth has also been evaluated. For all samples the photoluminescence results can be explained by considering three donor-acceptor pair recombination processes involving the same defect pairs.

Upon biological screening of a series of African medicinal plants, substantial phytotoxic activity was found in the leaves of Laggera decurrens (Vahl.) Hepper & Wood (Asteraceae), using a Lemna minor bioassay. Bioassay-guided fractionation of the leaves led to the isolation of two physiologically active compounds: 3-hydroxythymoquinone and 5-acetoxy-2-hydroxythymol, causing death of Lemna minor in the 25-100 microM range. Symptoms were a rapidly developing chlorosis, followed by necrosis of fronds. The compounds also inhibited growth and germination of the grass weed Agrostis capillaris down to 250 microM. The mode of action of both compounds could not be elucidated, but they do not appear to be photosystem II inhibitors.

Cu 2 ZnSnS 4 (CZTS) is a promising absorber for thin film solar cells which is non-toxic and consists of only abundant elements. In order to achieve higher solar cell efficiencies for this material, a better understanding about its defect structure is necessary. In this paper photoluminescence (PL) from sputtered CZTS thin film solar cells which differ in buffer layer thickness was studied. In the PL spectra three broad peaks could be distinguished between 0.9 and 1.3 eV. The PL measurements as a function of temperature and excitation power density revealed the presence of potential fluctuations. The separations between the band edge of the photoluminescence excitation signal and the PL emission peaks are large, which indicates that PL involves trapping of carriers by deep -level defects.