Quantifying Parasitic Losses From Metal Scattering Structures in Solar Cells: How Uncertainty in Optical Constants Affects Simulation Results

Abstract

The optical constants of many metals commonly used in solar cells, e.g. as contacts or for light trapping structures, are not documented consistently in the literature, with different sources giving different values. In the case of metallic structures designed to improve absorption in a solar cell junction, the use of data from different sources can give strongly varying results for the effectiveness of nanophotonic light-trapping structures. The trade-off between diffraction into more oblique orders in the junction, enhancing absorption in the photovoltaic material, and the number of photons absorbed parasitically in the metal means small differences in the optical constants can lead to different very conclusions about the EQE and JSC. This work documents the different optical constants for silver, aluminium, gold and titanium from several sources, the effect this has on plasmon quality factors, and quantifies the effect on modelling outcomes by considering the optimization of a test structure using a grid of metal nanodisks on the front surface of a thinned-down GaAs cell. Finally, we consider the effect for a structure previously predicted to give a very high JSC for a solar cell with an ultra-thin GaAs layer. Index Terms-light-trapping, ultrathin cells, plasmonics, optical modelling THE METALS CONSIDERED HERE CAN BE FOUND IN EACH REFERENCE, AND A BRIEF SUMMARY OF THE METHOD BY WHICH THE OPTICAL CONSTANTS WERE MEASURED OR MODELLED. Reference Label Metals Method Hagemann et al. [19] Hag Al, Ag, Au Transmittance measurements of vacuum-evaporated films/Kramers-Kronig analysis Jiang et al. [11] Jia Ag Variable-angle spectroscopic ellipsometry of overlayer-protected films Johnson & Christy [12] J&C Ag, Au, Ti Reflection and transmission measurements on vacuum evaporated films Lynch et al. [20] Lyn Ti Absorptivity/reflectivity measurements of polycrystalline Ti/Kramers-Kronig analysis McPeak et al. [10] McP Ag, Al, Au Variable-angle spectroscopic ellipsometry of template-stripped samples Olmon et al. [21] Olm Au* Variable-angle spectroscopic ellipsometry of evaporated gold Palik [15] Pal Ag, Al, Au, Ti Various Rakić et al. [22] Rak-BB Ag, Al, Au, Ti Brendel-Bormann model fit to available experimental data Rakić et al. [22] Rak-LD Ag, Al, Au, Ti Lorentz-Drude model fit to available experimental data Werner et al. [23] Wer-R Ag, Au, Ti Reflection electron energy-loss spectroscopy (REELS) Werner et al. [23] Wer-D Ag, Au, Ti Density functional theory (DFT) calculations * Olmon et al. [21] measured the optical constants of evaporated gold, template-stripped gold, and single-crystal gold; the values used here are for evaporated gold.