This paper reports the development of a VHF PECVD process at 40.68 MHz for deposition of device-grade nc-Si:H. It
further reports the evaluation of textured ZnO:Al films produced by hollow cathode sputtering as regards their suitability
to serve as a TCO substrate for a-Si:H / nc-Si:H tandem device fabrication. The tandem devices were produced using an
established VHF PECVD process at 100 MHz. Both VHF processes are capable of producing similar nc-Si:H material
based on their analysis using micro-Raman spectroscopy. For the tandem junction devices, a peak in device efficiency
was obtained at a Raman crystalline fraction of 50-52 % and a microstructure parameter of 0.60-0.68. A best tandem
cell efficiency of 9.9% was achieved on HC ZnO compared to 11.3% on a reference Type-U SnO2 substrate.
We have deposited textured ZnO:Al films over large areas using a reactive-environment hollow cathode sputtering (RE-HCS)
system developed in house, and have achieved excellent carrier mobilities (up to 49.5 cm2/Vs at a carrier
concentration of 4.42 x 1020/cm3). Both the electrical properties and optical properties (total transmission and haze) are
superior to those exhibited by commercially available SnO2:F. Using these textured ZnO:Al films, we have achieved an
a-Si:H solar cell efficiency boost of 8% relative to commercial SnO2:F superstrates which resulted from improvements
in all three PV parameters, namely Voc, Jsc, and FF. We have also determined the dependence of cell performance on the
degree of haze in the ZnO:Al films. Electrical, physical, and optical properties of ZnO:Al and SnO2:F, as determined by
four-point probe, Hall effect, SEM, AFM, ICP, transmission (total and diffuse), and work function measurements are
presented and correlated to the observed differences in a-Si solar cell performance. We have also developed a refractive
index matching layer that, when inserted between the TCO and the a-Si:H layers, resulted in an increase in Jsc of 3%.
Finally, we present some experiments on the effect of TCO type on nc-Si:H solar cell performance. From these
experiments, we confirmed that SnO2:F by itself is not a suitable TCO for nc-Si:H cells, but found that SnO2:F
overcoated with TiO2 followed by ZnO was the most effective superstrate for this type of cell.
The properties of a transparent conductive oxide (TCO) used as a front electrode for thin-film solar cells and modules
play a major role in determining the maximum attainable conversion efficiency. Doped ZnO is an important TCO that is
widely used in amorphous/nanocrystalline silicon (a-Si/nc-Si) and CIGS thin-film solar cells. In the case of a-Si/nc-Si
cells, the ZnO thin film should be textured to promote light trapping to increase the short-circuit current density Jsc. In
this work, textured, aluminum-doped ZnO (ZnO:Al) thin films have been directly deposited by a sputtering-based
method and without the need for post-deposition etching. The morphology, optical properties and electrical properties of
the films have been studied. SEM micrographs show that feature sizes around 0.2 - 0.4μm have been achieved at a film
thickness of 1μm, and that the morphology can be controlled by the deposition conditions. AFM images were analyzed
to extract a set of topographic parameters (amplitude, spatial, and hybrid). The optical transmission, haze, and angle-resolved
light scattering of the textured ZnO:Al films were measured and compared to properties of commercially-available
textured SnO2:F thin films on glass. Higher haze and reduced absorption could be obtained with the textured
ZnO:Al films. Hall effect measurements on these films yielded a carrier concentration and mobility of 2.75 x 1020cm-3
and 24.1cm2/Vs, respectively. We also report that the use of these textured ZnO:Al films as the top TCO for CIGS solar
cells results in reduced cell reflectance and increased Jsc. The novel deposition method provides a potential pathway to
large area and cost effective production of a textured ZnO TCO for thin-film PV manufacturing operations.
Proceedings Volume Editor (4)
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