Structured monocrystalline Si thin-film modules from layer-transfer using the porous Si (PSI) process
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Structured monocrystalline Si thin-film modules from layer-transfer using the porous Si (PSI) process Auer Richard and Brendel Rolf Bavarian Center for Applied Energy Research (ZAE Bayern) Am Weichselgarten 7, D-91058 Erlangen, Germany ABSTRACT We demonstrate a novel technique for fabricating monolithically series connected solar modules from surface structured thin monocrystalline Si films that we prepare by layer transfer using porous Si (PSI process). The novel series connection technique bases on reactive ion etching of the silicon film in a microwave plasma prior and after layer transfer. The module has an area of 25 cm2 and consists of 5 unit cells that have a film thickness of 16 µm. We measure an open-circuit voltage of 3028 mV and a confirmed efficiency of 9.9%. The Si film has a randomly textured surface for light trapping. INTRODUCTION Cost reduction and higher conversion efficiencies are the major objectives of the photovoltaic-R&D work. Monocrystalline silicon wafers of typically 300 µm in thickness achieve high conversion efficiencies, but sawn wafers from ultra-pure single crystals are an energyintensive and costly material. For cost reduction, the fabrication of thin crystalline Si cells with thickness values of 3 to 25 µm is under heavy investigation. Thin-films in this thickness range require a supporting carrier. Unfortunately epitaxial growth on low-cost carriers such as glass is a very difficult task, that generally leads to crystal defects which reduce the conversion efficiency of the device. Layer transfer using porous Si is a technique to fabricate high quality thin-film cells on glass. The ELTRAN process applies epitaxy on porous Si and transfers a planar film to a carrier by bonding [1]. A planar thin Si film does however not absorb the solar light sufficiently. We therefore introduced the porous Si (PSI) process [2], that fabricates monocrystalline thin-film cells with a surface structure by performing epitaxy on structured porous Si. Using layer transfer with porous Si, Tayanaka et al. achieved 12.5% solar cell efficiency [3] with a 12-µm-thick and planar Si film. Rinke et al. reported 14.0% efficiency [4] for a 24.5-µm-thick and planar Si film. Both authors apply sophisticated high-temperature processing steps including photolithography. At ZAE Bayer we develop low-cost processes for thin-film solar cells. Using a simple processing sequence without photolithography, we reported an efficiency of 12.2% for a cell with a thickness of 16 µm that has random pyramids for light trapping [5]. Having introduced light trapping and having simplified the cell process the open question is how to fabricate a solar module using layer transfer techniques. Integrated series connection is required for almost all applications to enhance the output voltage and to reduce the ohmic losses of large area devices. There are several approaches to realize an integrated series connection for thick standard Si wafers. Keller reports an open-circuit voltage of 3.43 V and an efficiency of 10.9% for a I9.2.1
21.0 cm2
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