Fanny Meillaud

In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells

We show that SiO-based intermediate reflectors (SOIRs) can be fabricated in the same reactor and with the same process gases as used for thin-film silicon solar cells. By varying input gas ratios, SOIR layers with a wide range of optical and electrical properties are obtained. The influence of the SOIR thickness in the micromorph cell is studied and current gain and losses are discussed. Initial micromorph cell efficiency of 12.2% (Voc=1.40V, fill factor=71.9%, and Jsc=12.1mA∕cm2) is achieved with top cell, SOIR, and bottom cell thicknesses of 270, 95, and 1800nm, respectively.

Multiscale transparent electrode architecture for efficient light management and carrier collection in solar cells

The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 μm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm(2)/V/s), and the unequaled light-scattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.

Thin-film silicon triple-junction solar cell with 12.5% stable efficiency on innovative flat light-scattering substrate

Karin Soderstrom, Gregory Bugnon, Remi Biron, Celine Pahud, Fanny Meillaud et al. Citation: J. Appl. Phys. 112, 114503 (2012); doi: 10.1063/1.4768272 View online: http://dx.doi.org/10.1063/1.4768272 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i11 Published by the American Institute of Physics.

High-Stable-Efficiency Tandem Thin-Film Silicon Solar Cell With Low-Refractive-Index Silicon-Oxide Interlayer

We report the recent advances and key requirements for high-efficiency “micromorph” tandem thin-film silicon solar cells composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell. The impact of inserting a low-refractive-index silicon-oxide (SiOx) film as intermediate reflecting layer (IRL) is highlighted. We show that refractive indexes as low as 1.75 can be obtained for layers still conducting enough to be implemented in solar cells, and without no additional degradation. This allows for high top-cell current densities with thin top cells, enabling low degradation rates. A micromorph cell with a certified efficiency of 12.63% (short-circuit current density of 12.8 mA/cm2) is obtained for an optimized stack. Furthermore, short-circuit current densities as high as 15.9 mA/cm2 are reported in the amorphous silicon top-cell of micromorph devices by combining a 150-nm-thick SiOx-based IRL and a textured antireflecting coating at the air-glass interface.

Optimization of ZnO Front Electrodes for High-Efficiency Micromorph Thin-Film Si Solar Cells

The quest for increased performances in thin-film silicon micromorph tandem devices nowadays requires an increase of current density. This can be achieved with thin cells by combining both robust cell design and efficient light management schemes. In this paper, we identify three key requirements for the transparent conductive oxide electrodes. First, strong light scattering into large angles is needed on the entire useful wavelength range: A front electrode texture with large enough features is shown to grant a high total current (typically >26 mA/cm2 with a 2.4-μm-thick absorber material), while sharp features are reported to allow for high top cell current (>13 mA/cm2) and reduced reflection at the ZnO/Si interface. Second, sufficiently smooth substrate features are needed to guarantee a high quality of the silicon active material, ensuring good and stable electrical properties (typically Voc around 1.4 V). Third, conduction and transparency of electrodes must be cleverly balanced, requiring high transparent conductive oxide mobility (∼50 cm

Thin-Film Silicon Triple-Junction Solar Cells on Highly Transparent Front Electrodes With Stabilized Efficiencies up to 12.8%

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On the Interplay Between Microstructure and Interfaces in High-Efficiency Microcrystalline Silicon Solar Cells

/V/s) to maintain the sheet resistance below 30 Ω/sq while keeping absorption as low as possible. Optimization of these three key requirements using ZnO electrodes allowed us to realize high-efficiency micromorph devices with 13.5% initial and 11.5% stabilized efficiency.

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

High-efficiency thin-film silicon triple-junction solar cells in p-i-n configuration have been fabricated using amorphous silicon top cell absorber layers, as well as microcrystalline silicon middle and bottom cell absorbers. The triple-junction cells were fabricated on boron doped zinc oxide (ZnO) films with different surface morphologies. To this end, the naturally grown rough ZnO surfaces were flattened using an Ar plasma for three different treatment times. For the shortest time, we achieved a summed current density over 30 mA/cm2 and initial and stabilized conversion efficiencies of 13.5% and 12.5%, respectively. For the medium treatment time, we obtained the highest efficiencies (13.7% initial and 12.8% stable), whereas the longest treatment time led to the highest open-circuit voltage (VOC) of 1.91 V but lower current densities, leading to efficiencies of 12.9% initial and 12.2% stable, respectively. These results were obtained by combining various recently developed features and approaches: first of all, we implemented high-quality μc-Si:H cells with novel buffer layers, leading to very high efficiencies. Second, we applied randomly textured pyramids on the front glass to improve light in-coupling, and finally, we used very thin (~140 nm) top cells that led to a low light-induced degradation (5%-7% relative loss in efficiency).

ZnO Transparent conductive oxide for thin film silicon solar cells

This paper gives new insights into the role of both the microstructure and the interfaces in microcrystalline silicon (μc-Si) single-junction solar cells. A 3-D tomographic reconstruction of a μc-Si solar cell reveals the 2-D nature of the porous zones, which can be present within the absorber layer. Tomography thus appears as a valuable technique to provide insights into the μc-Si microstructure. Variable illumination measurements enable to study the negative impact of such porous zones on solar cells performance. The influence of such defective material can be mitigated by suitable cell design, as discussed here. Finally, a hydrogen plasma cell post-deposition treatment is demonstrated to improve solar cells performance, especially on rough superstrates, enabling us to reach an outstanding 10.9% efficiency microcrystalline single-junction solar cell.

Internal electric field and fill factor of amorphous silicon solar cells

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.