Tailoring the surface morphology of zinc oxide films for high-performance micromorph solar cells

Self-textured zinc oxide polycrystalline films prepared by metalorganic low-pressure chemical vapor deposition combine excellent transparency, conductivity and light-scattering ability when used as electrodes for high-efficiency thin-film silicon solar cells. However, the growth of silicon layers with low defect density, which are necessary for high-performance solar cells, often requires the rough surface morphology of as-deposited zinc oxide films to be smoothened. This is usually achieved by a post-deposition argon plasma-etching treatment. We investigate here an alternative method to modify the surface morphology by changing the zinc oxide growth conditions over only the last hundreds of nanometers of the total film thickness. We discuss two types of zinc oxide cap layers, one grown with the addition of ethanol and the other with an enhanced diethylzinc (DEZ) precursor flow. We show that the presence of either type of cap layer does not alter the layer's electrical properties, but does slightly diminish the layer's light-scattering ability. The cap layer grown with ethanol leads to a more pronounced leveling of the film texture, while the high-DEZ-grown layer better preserves the sharp features of the underlying ZnO. Finally, zinc oxide films with cap layer are used as front electrodes for tandem amorphous/microcrystalline silicon solar cells, and are compared to rough films treated with an argon plasma. With rising thickness of the capping layers, the efficiency of tandem cells increases reaching values over 12%, and approaches that of cells on Ar plasma treated ZnO films. (C) 2014 Elsevier B.V. All rights reserved.

Amorphous silicon-germanium for triple and quadruple junction thin-film silicon based solar cells

Christophe Ballif, Mathieu Gérard Boccard, Maximilien Joseph Marie Bonnet-Eymard, Grégory Bugnon, Matthieu Despeisse, Franz-Josef Haug, Björn Niesen, Michael Elias Stückelberger

We study amorphous silicon-germanium (a-SiGe:H) as intrinsic absorber material for thin-film silicon-based triple and quadruple junction solar cells. First, we present the development of a-SiGe:H single junction devices, in particular the Ge-content grading in the absorber layer, the influence of the Ge-content on electrical properties and (infra)red-response, and the influence of using different types of players. We subsequently show the incorporation of optimized single-junction devices in triple junction cells and discuss the interplay between Ge-content and intermediate reflector thickness. For triple junction devices with amorphous silicon (a-Si:H) top cells, a-SiGe:H middle cells and microcrystalline silicion (mu c-Si:H) bottom cells, we obtained an initial efficiency of 13.6% and an efficiency of 11.3% after light-soaking. We also present a quadruple junction device with an a-Si:H top cell, a low Ge-content a-SiGe:H second cell, and mu c-Si:H third and bottom cells. In this device configuration, we obtained an open-circuit voltage as high as 2.57 V. The performance of these cells was limited by not yet optimized current matching, leading nevertheless to an initial efficiency of 10.1%. A brief roadmap towards quadruple-junction devices with stabilized efficiencies of 14% is also outlined. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier2015

Potential of Amorphous and Microcrystalline Silicon Solar Cells

Cédric Bucher, Sylvie Faÿ, Arvind Shah, Joël Alexandre Spitznagel

Low pressure chemical vapour deposition (LP-CVD) ZnO as front transparent conductive oxide (TCO), developed at IMT, has excellent light-trapping properties for a-Si:H p-i-n single-junction and 'micromorph' (amorphous/microcrystalline silicon) tandem solar cells. A stabilized record efficiency of 9.47% has independently been confirmed by NREL for an amorphous silicon single-junction p-i-n cell (∼1 cm2) deposited on LP-CVD ZnO coated glass. Micromorph tandem cells with an initial efficiency of 12.3% show after light-soaking a stable performance of 10.8%. The monolithic series connection by laser-scribing for module fabrication has been developed at IMT as well, for both amorphous single-junction and micromorph tandem cells in combination with the LP-CVD ZnO technique. Mini-modules (areas between 22 and 24 cm2) with an aperture efficiency of 8.7% in the case of amorphous single-junction p-i-n cells (independently confirmed by NREL), and of 9.8% in the case of micromorph tandem cells, have been obtained. Micromorph tandem cells with an intermediate TCO reflector between the amorphous top and the microcrystalline bottom cell show an almost stable performance (η=10.7%) with respect to light-soaking. © 2003 Elsevier B.V. All rights reserved.

2004

Towards Very Low-Cost Mass Production of Thin-film Silicon Photovoltaic (PV) Solar Modules on Glass

Arvind Shah

Production volume of PV modules increases at > 35% per year, but one is yet far from making a global impact on energy supply. One of the obstacles is given by the present high production costs of PV modules. A possibility to reduce costs are thin-film PV modules on glass. The specific option of thin-film silicon is considered. The combination of amorphous and microcrystalline silicon thin films within a tandem solar cell corresponds to a theoretical optimum. In practice, stabilized efficiencies of 10% to 12% have so far been obtained in the laboratory with such tandem solar cells. Silicon being a material with an indirect band gap, its absorption coefficient is relatively low, and therefore light management in the solar cell has to be further optimized. Thin-film silicon can be deposited by plasma-enhanced CVD, as used for AM-LCD displays. The use of modified fabrication equipment from the AM-LCD Display Industry is therefore a promising way to implement low-cost mass production. © 2005 Elsevier B.V. All rights reserved.

2006