Berlin 2012 – wissenschaftliches Programm

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HL: Fachverband Halbleiterphysik

HL 55: Photovoltaics: Silicon-based Systems II

HL 55.6: Vortrag

Mittwoch, 28. März 2012, 12:30–12:45, ER 270

Determination of quasi-Fermi level separations and characteristic tail-state energies of microcrystalline silicon by photoluminescence — •Sven Burdorf, Rudolf Brüggemann, and Gottfried Heinrich Bauer — Institut für Physik, Carl von Ossietzky Universität, D-26111 Oldenburg, Germany

Hydrogenated microcrystalline silicon (µc-Si:H) is used as the bottom cell of amorphous silicon/microcrystalline silicon tandem cells. The density of band-tail states is one of the factors limiting the performance of the microcrystalline absorber layer; in particular tails limit the splitting of the quasi-Fermi levels and thus the open-circuit voltage. The band-tail profile can be measured by electronic methods like modulated photocurrent (MPC), thermally stimulated currents (TSC) etc; the straightforward interpretation of the results however, requires samples on insulating substrates with coplanar contacts, which are in contradiction to solar cell architectures. Thus, a contactless method, like photoluminescence, for the analysis of the absorber properties in diode structures is much more appropriate. In this contribution, we present a luminescence approach based on Kirchhoff’s generalized law to evaluate photoluminescence (PL) spectra of µc-Si:H in terms of the separation of quasi-Fermi levels and the characteristic tail state energies by assuming an exponential band-tail distribution and PL originating from tail-tail transitions. We extract from the PL-spectra of HWCVD-prepared µc-Si:H layers characteristic tail-state energies of about 50 meV and an energetic splitting of quasi-Fermi level at 20 K of about 1 eV.