Potential of Amorphous and Microcrystalline Silicon Solar Cells

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.

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

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

Arvind Shah, Joël Alexandre Spitznagel

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design: (1) In 1987, IMT introduced the so-called "very high frequency glow discharge (VHF-GD)" technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies fexc in the range 30-300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment. (2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material - a form of crystalline silicon - opens up the way for a new concept, the so-called "micromorph" tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon. Both the VHF-GD deposition technique and the "micromorph" tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes. ©2004 Elsevier Ltd. All rights reserved.

2004

Silicon oxide buffer layer at the p-i interface in amorphous and microcrystalline silicon solar cells

Christophe Ballif, Grégory Bugnon, Mathieu Charrière, Matthieu Despeisse, Gaetano Parascandolo, Michael Elias Stückelberger

The use of intrinsic silicon oxide as a buffer layer at the p-i interface of thin-film silicon solar cells is shown to provide significant advantages. For microcrystalline silicon solar cells, when associated with highly crystalline i-layers deposited at high rates, all electrical parameters are improved. Larger efficiency gains are achieved with substrates of increased roughness. For cells with an improved i-layer material quality, there is mainly a gain in short-circuit current density. An improvement in carrier collection in the blue region of the spectrum is systematically observed on all the cells. The presence of a silicon oxide buffer layer also promotes the nucleation of the subsequent intrinsic microcrystalline silicon layer. In amorphous silicon solar cells, the silicon oxide buffer layer is proven to act as an efficient barrier to boron cross-contamination, eliminating the need for additional processing steps (e.g. water vapor flush), while providing a wide bandgap material at the interface. The implementation of silicon oxide buffer layers for both types of cells thus provides a decisive improvement, as it allows extremely fast deposition of the full p-i-n stack of layers of the cell in a single-chamber configuration while providing a high-quality substrate-resilient p-i interface. (C) 2013 Elsevier B.V. All rights reserved.

Elsevier2014

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

Arvind Shah, Joël Alexandre Spitznagel

2004