Polycrystalline silicon passivated tunneling contacts for high efficiency silicon solar cells
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We apply n- and p-type polycrystalline silicon (poly-Si) films on tunneling SiOx to form passivated contacts to n-type Si wafers. The resulting induced emitter and n1/n back surface field junctions of high carrier selectivity and low contact resistivity enable high efficiency Si solar cells. This work addresses the materials science of their performance governed by the properties of the individual layers (poly-Si, tunneling oxide) and more importantly, by the process history of the cell as a whole. Tunneling SiOx layers (,2 nm) are grown thermally or chemically, followed by a plasma enhanced chemical vapor deposition growth of p1 or n1 doped a-Si:H. The latter is thermally crystallized into poly-Si, resulting in grain nucleation and growth as well as dopant diffusion within the poly-Si and penetration through the tunneling oxide into the Si base wafer. The cell process is designed to improve the passivation of both oxide interfaces and tunneling transport through the oxide. A novel passivation technique involves coating of the passivated contact and whole cell with atomic layer deposited Al2O3 and activating them at 400 °C. The resulting excellent passivation persists after subsequent chemical removal of the Al2O3. The preceding cell process steps must be carefully tailored to avoid structural and morphological defects, as well as to maintain or improve passivation, and carrier selective transport. Furthermore, passivated contact metallization presents significant challenges, often resulting in passivation loss. Suggested remedies include improved Si cell wafer surface morphology (without micropyramids) and postdeposited a-Si:H capping layers over the poly-Si.
I. INTRODUCTION
Doped polysilicon (poly-Si) films have long been integral to various Si electronic devices as interlayers between active device layers and metal contacts, and have contributed to high gain in bipolar junction transistors by lowering the base current and emitter resistance.1 In these structures, the presence of an intermediate tunneling SiOx layer ,2 nm between the poly-Si and the single crystal silicon wafer advantageously provides wafer surface passivation without the degradation of transport.2 Shallow emitters are formed by diffusing dopants from the poly-Si through the SiOx into the wafer via post deposition anneals.3 This must be carefully optimized due to possible detrimental side effects such as oxide break-up, secondary phase formation due to heavy doping, and blistering. Advantageously, dopants tend to segregate along grain boundaries and pile-up at the poly-Si/SiOx interface. This has been shown to increase passivation by lowering carrier Contributing Editor: Don W. Shaw a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2016.77
mobility along grain boundaries in the former, and to chemically bond to dangling bonds in the latter.4 When doped poly-Si films on SiOx stacks are applied to solar cells, they form p-n or high-low junctions. These
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