17.8%-efficient Amorphous Silicon Heterojunction Solar Cells on p -type Silicon Wafers
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0910-A26-05
17.8%-efficient Amorphous Silicon Heterojunction Solar Cells on p-type Silicon Wafers Tihu Wang, Matt P. Page, Eugene Iwancizko, Yueqin Xu, Yanfa Yan, Lorenzo Roybal, Dean Levi, Russell Bauer, Howard M. Branz, and Qi Wang National Renewable Eenergy Laboratory, 1617 Cole Blvd, Golden, CO, 80410 a
Email: [email protected]
ABSTRACT We have achieved an independently-confirmed 17.8% conversion efficiency in a 1-cm , p-type, float-zone silicon (FZ-Si) based heterojunction solar cell. Both the front emitter and back contact are hydrogenated amorphous silicon (a-Si:H) deposited by hotwire chemical vapor deposition (HWCVD). This is the highest reported efficiency for a HWCVD silicon heterojunction (SHJ) solar cell. Two main improvements lead to our most recent increases in efficiency: 1) the use of textured Si wafers, and 2) the application of a-Si:H heterojunctions on both sides of the cell. Despite the use of textured c-Si to increase the short-circuit current, we were able to maintain the same 0.65 V open-circuit voltage as on flat c-Si. This is achieved by coating a-Si:H conformally on the c-Si surfaces, including covering the tips of the anisotropically-etched pyramids. A brief atomic H treatment before emitter deposition is not necessary on the textured wafers, though it was helpful in the flat wafers. It is essential to high efficiency SHJ solar cells that the emitter grows abruptly as amorphous silicon, instead of as microcrystalline or epitaxial Si. The contact on each side of the cell comprises a thin (< 5 nm) low substrate temperature (~100°C) intrinsic a-Si:H layer, followed by a doped layer. Our intrinsic layers are deposited at 0.3-1.2 nm/s. The doped emitter and backcontact layers were deposited at a higher temperature (>200°C) and grown from PH3/SiH4/H2 and B2H6/SiH4/H2 doping gas mixtures, respectively. This combination of low (intrinsic) and high (doped layer) growth temperatures was optimized by lifetime and surface recombination velocity measurements. Our rapid efficiency advance suggests that HWCVD may have advantages over plasma-enhanced (PE) CVD in fabrication of high-efficiency heterojunction c-Si cells; there is no need for process optimization to avoid plasma damage to the delicate, high-quality, Si wafers. 2
INTRODUCTION The silicon heterojunction (SHJ) with a hydrogenated amorphous silicon (a-Si:H) emitter is one of the most successful device structures for manufacturing high-efficiency
crystal silicon solar cells at low temperatures (200°C) from PH3/SiH4/H2 and B2H6/SiH4/H2 doping gas mixtures. Our front-only single-side SHJ has a structure of ITO/a-Si:H/c-Si(p)/Al-BSF and the double-side SHJ is made of ITO/a-Si:H/c-Si/a-Si:H/ITO/Al. The finished cell has 1 cm2 in area with emitter isolation by dry SF6 etching and a 5%-coverage metal grid on top of the ITO. The official JV measurement is done at NREL by the accredited PV Performance Characterization Team. High-resolution transmission electron microscopy (HRTEM) and real-time spectroscopic ellipsometry (RTSE) are also used to diag
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