Enhanced light trapping in thin-film solar cells by a directionally selective filter 21 June 2010 / Vol. 18, No. 102 / OPTICS EXPRESS Carolin Ulbrich, 1,* Marius Peters, 2 Benedikt Bläsi, 2 Thomas Kirchartz, 1 Andreas Gerber, 1 and Uwe Rau 1 1 IEF5 - Photovoltaik, Forschungszentrum Jülich, 52425Jülich, Germany 2 Fraunhofer-Institut für Solare Energiesysteme,Heidenhofstraße 2, Freiburg, Germany Advisor : H.C. Kuo / C.C. Lin Student : H.W. Han

NCTU IPS STUDENT2010/09/29 2/8 Outline  Introduction  Experiment  Result  Conclusions

NCTU IPS STUDENT2010/09/29 3/8 Introduction  enhancement of the optical path length improving the performance of devices reducing material consumption  optical path length  with threshold value θ th  this paper compare the reflection and the external quantum efficiency before and after filter deposition

NCTU IPS STUDENT2010/09/29 4/8 Experiment  Bragg-like filter 73 alternating layers of SiO 2 and Ta 2 O 5 total thickness of 5.5 µm  threshold wavelength λ th  Superstrate textured SnO 2 :F on AsahiU glass etched ZnO:Al on Corning glass  Absorber layer thickness 175 nm ~ 410 nm

NCTU IPS STUDENT2010/09/29 5/8 Result (1/3)  Thickness : 322 nm ; Corning glass with etched ZnO:Al  650nm < λ < 770nm due to the directional selectivity of the filter suppressing re-emission of non-absorbed light  350nm < λ < 650nm due to the antireflective properties of the filter  short-circuit current densities J sc : mAcm − mAcm −2 - 40%

NCTU IPS STUDENT2010/09/29 6/8 Result (2/3)   κ r represents a factor quantifying the additional light path prolongation in the device.  The large difference between the improvement factor κ EQE and κ r is due to parasitic absorption in the TCO and at the back contact. AsahiU

NCTU IPS STUDENT2010/09/29 7/8 Result (3/3)  the external quantum efficiency EQE fi of a 414nm thick a-Si:H solar cell (prepared on Corning glass)  The dashed line is calculated from Eq. using λ 0 = 767nm and λ 1 = 600nm.

NCTU IPS STUDENT2010/09/29 8/8 Conclusions  a directional selective filter can improve light trapping in solar cell and enhance the overall short-circuit current density  For a-Si:H thin film solar cells the improvement depends on the texture of the front TCO and the thickness of the active absorber layer.  A maximum improvement of ΔJ sc = 0.26mAcm −2 textured ZnO and an absorber thickness of 322 nm 0.06 mAcm −2 due to the antireflective effect of the filter 0.20 mAcm −2 due to its directional selectivity  limited by parasitic absorption

NCTU IPS STUDENT2010/09/29 9/8 Thanks for your attention!!