Dresden 2014 – wissenschaftliches Programm

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HL: Fachverband Halbleiterphysik

HL 71: Energy materials: Silicon-based photovoltaics

HL 71.1: Vortrag

Mittwoch, 2. April 2014, 15:00–15:15, POT 151

Strained Lonsdaleite Silicon as a Potential Absorber for Photovoltaics — •Claudia Rödl^1,2,3, Tobias Sander^2,3, and Friedhelm Bechstedt^2,3 — ¹Laboratoire des Solides Irradiés, Ecole Polytechnique, CNRS, CEA-DSM, 91128 Palaiseau cedex, France — ²Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität, Max-Wien-Platz 1, 07743 Jena, Germany — ³European Theoretical Spectroscopy Facility (ETSF)

Si in the diamond structure is one of the the key materials in photovoltaics, since it is abundant in nature and can be grown in extremely good quality. However, its fundamental band gap of 1.2 eV is indirect and only phonon-assisted absorption occurs in the visible spectral range. The direct absorption edge at 3.2 eV lies in the ultraviolet spectral region. Today a large variety of Si polymorphs is known, whereof some are semiconductors with direct gaps smaller than 3.2 eV.

Here, we calculate the optical absorption spectrum of Si in the lonsdaleite structure, also known as wurtzite Si, one of the most promising candidates for strong absorption in the visible spectral range. It features a direct band gap of 1.6 eV matching ideally with the solar spectrum. The quasiparticle band gap and the dielectric function including excitonic effects are calculated from first principles by many-body theory (GW method, Bethe-Salpeter equation). The optical absorption at the onset is compared to the absorption of diamond Si and discussed in terms of dipole-allowed and forbidden transitions. We study in detail the influence of hydrostatic pressure, uniaxial, and biaxial strain on the band gap and the optical absorption.