No abstract

Combining scanning electron microscopy (SEM) and electron-beam-induced current (EBIC) imaging with transport measurements, it is shown that the current flowing across a two-terminal oxide-based capacitor-like structure is preferentially confined in areas localized at defects. As the thin-film device switches between two different resistance states, the distribution and intensity of the current paths, appearing as bright spots, change. This implies that switching and memory effects are mainly determined by the conducting properties along such paths. A model based on the storage and release of charge carriers within the insulator seems adequate to explain the observed memory effect. 61.16.Bg, 72.20.Jv, 73.40.Rw

Multijunction stacked (tandem) solar cells can increase the overall photovoltaic conversion efficiency by optimal utilization of the solar spectrum in individual cells. We demonstrate that a photovoltaic tandem cell comprising a nanocrystalline dye-sensitized solar cell as a top cell for high-energy photons and a copper indium gallium selenide thin-film bottom cell for lower-energy photons produces AM 1.5 solar to electric conversion efficiencies greater than 15%.

No abstract

Incorporation of a small amount of sodium into Cu(In,Ga)Se2 (CIGS) absorbers for thin-film solar cells is well known to enhance conversion efficiencies. Usually, Na is added in a way such that it is present during CIGS growth and therewith influences the growth kinetics. We have used post-deposition Na in-diffusion into as-grown, Na-free absorbers and observed typical efficiency improvements. This suggests that in general the main positive Na effect originates from changes in the electronic absorber properties rather than from modification of CIGS growth kinetics. At low substrate temperatures, Na impedes CIGS phase formation. This may explain why absorbers grown at substrate temperatures below 450°C in the presence of Na yield inferior cells compared with post-deposition-treated CIGS. We have developed post-deposition Na incorporation for the processing of flexible CIGS solar cells on polyimide substrates. A conversion efficiency of 14.1% under AM1.5 standard test conditions was independently measured. This represents the highest reported efficiency of any solar cell grown on a polymer substrate to date.