Optics and Light Trapping for Tandem Solar Cells on Silicon

The rapid advancement of thin-film photovoltaic (PV) technology increases the real possibility of large-area Si-based tandems reaching 30% efficiency, although light in these devices must be managed carefully. We identify the optical requirements to reach high efficiencies. Strict conditions are pla...

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Veröffentlicht in:IEEE journal of photovoltaics 2014-11, Vol.4 (6), p.1380-1386
Hauptverfasser: Lal, Niraj N., White, Thomas P., Catchpole, Kylie R.
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Sprache:eng
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Zusammenfassung:The rapid advancement of thin-film photovoltaic (PV) technology increases the real possibility of large-area Si-based tandems reaching 30% efficiency, although light in these devices must be managed carefully. We identify the optical requirements to reach high efficiencies. Strict conditions are placed on material parasitic absorption and transmission of contacts: Absorption of 20% of sub-bandgap light leads to the required top-cell efficiencies of 18% at a bandgap of 1.5 eV to break even and 23% to reach tandem efficiencies of 30%. Perovskite-silicon tandem cells present the first low-cost devices capable of improving standalone 25% efficiencies and we quantify the efficiency gains and reduced thickness afforded by wavelength-selective light trapping. An analytical formalism for Lambertian tandem light trapping is introduced, yielding stringent requirements for wavelength selectivity. Applying these principles to a perovskite-based top cell characterized by strong absorption and high luminescence efficiency we show that tandem efficiencies greater than 30% are possible with a bandgap of E g = 1.55 eV and carrier diffusion lengths less than 100 nm. At an optimal top-cell bandgap of 1.7 eV, with diffusion lengths of current vapor-deposited CH 3 NH 3 PbI x Cl 1-x perovskites, we show that tandem efficiencies beyond 35% are achievable with careful light management.
ISSN:2156-3381
2156-3403
DOI:10.1109/JPHOTOV.2014.2342491