S. Faÿ

Transition between grain boundary and intragrain scattering transport mechanisms in boron-doped zinc oxide thin films

A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3×1019to2×1020cm−3.

Limiting factors in the fabrication of microcrystalline silicon solar cells and microcrystalline/amorphous (‘micromorph’) tandems

This contribution presents the status of amorphous and microcrystalline silicon solar cells on glass, and discusses some material/fabrication factors that presently limit their conversion efficiency. Particular attention is focused on recent results and developments at the Institute of Microtechnology (IMT) in Neuchâtel. The performances and stability of microcrystalline silicon single-junction and amorphous/microcrystalline (‘micromorph’) tandem solar cells are discussed, as a function of material properties. Recent results on the electrical effect of cracks in microcrystalline silicon material are presented. Degradation under the effect of illumination is a well-known limiting factor for amorphous silicon solar cells. As a comparison, studies on the stability of microcrystalline silicon with respect to light-induced degradation are commented upon. The importance of transparent contacts and anti-reflection layers for achieving low electrical and optical losses is discussed. Finally, efforts towards industrialization of micromorph tandem solar cells are highlighted, specifically (i) the development and implementation of an in situ intermediate reflector in a large-area industrial deposition system, and (ii) recent achievements in increasing the growth rate of microcrystalline silicon.

ZnO Transparent conductive oxide for thin film silicon solar cells

There is general agreement that the future production of electric energy has to be renewable and sustainable in the long term. Photovoltaic (PV) is booming with more than 7GW produced in 2008 and will therefore play an important role in the future electricity supply mix. Currently, crystalline silicon (c-Si) dominates the market with a share of about 90%. Reducing the cost per watt peak and energy pay back time of PV was the major concern of the last decade and remains the main challenge today. For that, thin film silicon solar cells has a strong potential because it allies the strength of c-Si (i.e. durability, abundancy, non toxicity) together with reduced material usage, lower temperature processes and monolithic interconnection. One of the technological key points is the transparent conductive oxide (TCO) used for front contact, barrier layer or intermediate reflector. In this paper, we report on the versatility of ZnO grown by low pressure chemical vapor deposition (ZnO LP-CVD) and its application in thin film silicon solar cells. In particular, we focus on the transparency, the morphology of the textured surface and its effects on the light in-coupling for micromorph tandem cells in both the substrate (n-i-p) and superstrate (p-i-n) configurations. The stabilized efficiencies achieved in Neuchâtel are 11.2% and 9.8% for p-i-n (without ARC) and n-i-p (plastic substrate), respectively.

Research and developments in thin-film silicon photovoltaics

The increasing demand for photovoltaic devices and the associated crystalline silicon feedstock demand scenario have led in the past years to the fast growth of the thin film silicon industry. The high potential for cost reduction and the suitability for building integration have initiated both industrial and research laboratories dynamisms for amorphous silicon and micro-crystalline silicon based photovoltaic technologies. The recent progress towards higher efficiencies thin film silicon solar cells obtained at the IMT-EPFL in Neuchatel in small-area laboratory and semi-large-area industrial Plasma Enhanced Chemical Vapor Deposition (PE-CVD) systems are reviewed. Advanced light trapping schemes are fundamental to reach high conversion efficiency and the potential of advanced Transparent Conductive Oxides (TCO) is presented, together with issues associated to the impact of the substrate morphology onto the growth of the silicon films. The recent improvements realized in amorphous-microcrystalline tandem solar cells on glass substrate are then presented, and the latest results on 1 cm2 cells are reported with up to 13.3 % initial efficiency for small-area reactors and up to 12.3 % initial for large-area industrial reactors. Finally, the different strategies to reach an improved light confinement in a thin film solar cell deposited on a flexible substrate are discussed, with the incorporation of asymmetric intermediate reflectors. Results of micromorph solar cells in the n-i-p configuration with up to 9.8 % stabilized efficiency are reported.

Improved interface between front TCO and microcrystalline silicon p-i-n solar cells.

Note: IMT-NE Number: 335 Reference PV-LAB-CONF-2001-002 Record created on 2009-02-10, modified on 2017-05-10

Boron Doping Effects on the Electro-optical Properties of Zinc Oxide Thin Films Deposited by Low-Pressure Chemical Vapor Deposition Process

Note: IMT-NE Number: 453 Reference PV-LAB-CONF-2006-013 Record created on 2009-02-10, modified on 2017-05-10