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Enhanced light trapping in thin amorphous silicon solar cells by directionally selective optical filters

: Ulbrich, C.; Peters, M.; Tayyib, M.; Bläsi, B.; Kirchartz, T.; Gerber, A.; Rau, U.


Wehrspohn, R.B. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Photonics for Solar Energy Systems III : 13.-15.4.2010, Brussels, Belgium
Bellingham, WA: SPIE, 2010 (Proceedings of SPIE 7725)
ISBN: 978-0-8194-8198-6
Paper 77250P
Conference "Photonics for Solar Energy Systems" <3, 2010, Brussels>
Fraunhofer ISE ()

Optical absorption losses limit the efficiency of thin-film solar cells. We demonstrate how to increase the absorption in hydrogenated amorphous silicon solar cells by using a directionally selective optical multilayer filter covering the front glass. The filter transmits perpendicularly incident photons in the wavelength range 350 nm - 770 nm. In the regime of low absorptance, i.e. large optical absorption lengths, however, it blocks those photons impinging under oblique angles. Thus, the incoming radiation is transmitted with almost no loss while the emitted radiation is mostly blocked due to its wider angle distribution. We determine the enhancement in the optical path length from reflectivity measurements. In the weakly absorbing high wavelength range (650 nm - 770 nm) we observe a peak optical path length enhancement of 3.5. The effective path length enhancement calculated from the external quantum efficiency of the solar cell with filter, however, peaks at a lo wer value of only 1.5 in the same wavelength range. Parasitic absorption in the layers adjacent to the photovoltaic absorber limit the increase in the effective light path enhancement. Nonetheless we determine an increase of 0.2 mAcm-2 in the total short circuit current density.