Enhanced light trapping in thin-film solar cells by a directionally selective filter

1. 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,2Benedikt Bläsi,2 Thomas Kirchartz,1 Andreas Gerber,1and Uwe Rau1 1IEF5 - Photovoltaik, ForschungszentrumJülich, 52425Jülich, Germany 2Fraunhofer-Institut fürSolareEnergiesysteme,Heidenhofstraße 2, 79110 Freiburg, Germany Advisor : H.C. Kuo / C.C. Lin Student : H.W. Han

2. Outline Introduction Experiment Result Conclusions

3. Introduction • enhancement of the optical path length • improving the performance of devices • reducing material consumption • optical path length • with threshold value θth • this paper compare the reflection and the external quantum efficiency before and after filter deposition

4. Experiment • Bragg-like filter • 73 alternating layers of SiO2 and Ta2O5 • total thickness of 5.5 µm • threshold wavelength λth • Superstrate • textured SnO2:F on AsahiU glass • etched ZnO:Al on Corning glass • Absorber layer thickness • 175 nm ~ 410 nm

5. Result (1/3) - 40% 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 Jsc : 13.40 mAcm−2 13.66 mAcm−2

6. Result (2/3) AsahiU κr represents a factor quantifying the additional light path prolongation in the device. The large difference between the improvement factor κEQE and κr is due to parasitic absorption in the TCO and at the back contact.

7. Result (3/3) the external quantum efficiency EQEfi of a 414nm thick a-Si:H solar cell (prepared on Corning glass) The dashed line is calculated from Eq. using λ0 = 767nm and λ1 = 600nm.

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 ΔJsc = 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

9. Thanks for your attention!!