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Calculation of the Quasi Fermi-Level Splitting in an Ideal Superlattice of Silicon Nanocrystals

: Müller, R.; Löper, P.; Hiller, D.; Gutsch, S.; Keding, R.; Reichel, C.; Bivour, M.; Hermle, M.; Janz, S.; Goldschmidt, J.C.; Zacharias, M.; Glunz, S.W.


European Commission:
26th European Photovoltaic Solar Energy Conference and Exhibition, EU PVSEC. Proceedings : 5th to 9th September 2011 at the CCH - Congress Centre and International Fair Hamburg in Germany
München: WIP-Renewable Energies, 2011
ISBN: 3-936338-27-2
European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC) <26, 2011, Hamburg>
Conference Paper
Fraunhofer ISE ()
Solarzellen - Entwicklung und Charakterisierung; Alternative Photovoltaik-Technologien; Neuartige Konzepte

The splitting of the quasi Fermi-levels under illumination constitutes an upper limit for the open circuit voltage of a silicon nanocrystal solar cell. To determine this splitting, the band structure and the density of states of a superlattice of cubic silicon quantum dots is calculated. Furthermore, the absorption and the minority charge carrier lifetime of a sample with silicon nanocrytsals in a silicon dioxide matrix are measured. From these data the excess carrier density under illumination with the AM1.5G solar spectrum is estimated to be about 1014 cm-3. Based on the density of states and the carrier concentration, the quasi Fermi-levels are calculated. Superlattices of silicon nanocrystals in SiO2, Si3N4 and SiC are compared. It is found that under AM1.5G illumination the splitting of the quasi Fermi-levels follows the calculated band gap with an offset of about 0.6 eV because the integrated density of states in the first miniband is always much higher than the excess carrier density. This is quite independent from the quantum dot size, interdot distance and matrix material. A 100 fold increase of charge carrier generation by e.g. light trapping or concentration enhances the quasi Fermi-level splitting by 0.24 eV.