Sheyu Guo

Influence of TCO type on the performance of amorphous silicon solar cells

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.

Reactive-Environment Hollow Cathode Sputtering: Compound Film Production, and Application to Thin Film Photovoltaics

This paper reviews EPVs development of hardware and process technology to accomplish large area, high rate sputtering from metal targets in a reactive mode without target poisoning. The method is termed reactive-environment hollow cathode sputtering (RE-HCS) and makes use of the intense plasma confined in a hollow cathode. A linearly extended cathode is described, with one or more reactive gases delivered externally to the cathode. The basic operating characteristics of the cathode are described. The method is applied to the development of high performance transparent conducting oxides (TCOs) and other oxides and nitrides. The relevance of RE-HCS to the PV community is further demonstrated through the incorporation of TCOs and other materials produced by the method into thin film PV devices with competitive performance. Versions of the cathode suitable for large width industrial coating have been developed

Deposition of large area, directly textured, ZnO:Al films by reactive-environment, hollow cathode sputtering

Aluminum-doped zinc oxide (ZnO:Al) is a promising transparent conducting oxide (TCO) for the second generation, thin film based solar cells. Moderately large area, directly textured ZnO:Al films were successfully deposited by reactive-environment, hollow cathode sputtering (RE-HCS) using metal targets. The morphology, structural, electrical, and optical properties of the films have been investigated and comparisons are made with the properties of commercially available textured SnO2:F. Higher haze and reduced absorption could be obtained with the textured ZnO:Al films. Besides the textured surface, these films (∼1030nm thick) also have a low sheet resistance of 2.8 ohms/square. Hall effect measurements on these films yielded a record high mobility of 49.5 cm2/V-s and carrier concentration of 4.42 × 1020 cm−3. The use of these textured ZnO:Al films as a TCO for single junction a-Si cells resulted in increased Voc, Jsc, and FF. The novel deposition method of RE-HCS provides a possible and promising pathway to a relatively low cost, large area production process for a textured ZnO TCO for thin-film PV manufacturing.

Hybrid a-Si/nc-Si solar cells fabricated on a directly-deposited textured zinc oxide transparent conductor

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.

CIGS devices with ZIS, In/sub 2/S/sub 3/, and CdS buffer layers

The device performances of Cu(In,Ga)Se/sub 2/ solar cells are compared as a function of various buffer layers applied by thermal evaporation that are considered as candidates to replace the conventional CdS buffer layer applied by chemical bath deposition. The buffer layers include ZnIn/sub 2/Se/sub 4/ (ZIS), In/sub 2/Se/sub 3/ and ZnSe. Devices with CdS and ZIS buffers are also studied by EBIC and cathodoluminescence.