Optical properties of metal nanoparticles and their influence on silicon solar cells

Abstract

The optical properties of metal nanoparticles have been investigated by simulation and experimental techniques. The aim of this investigation was to identify how to use metal nanoparticles to improve light-trapping in silicon solar cells. To do this we require nanoparticles that exhibit a high scattering efficiency and low absorption (i.e. high radiative efficiency) at near-infrared wavelengths. The simulation results identified Ag, Au, Cu and Al as potential candidates for use with silicon solar cells. The optical properties of Ag, Au and Cu nanoparticles are very similar above 700 nm. Below this wavelength Ag was found to be the preferred choice due to a decreased effect from interband transitions in comparison with Au and Cu. Al nanoparticles were found to exhibit markedly different optical properties to identical noble metal nanoparticles, with broader, weaker resonances that can be excited further into the UV. However, Al nanoparticles were found to exhibit higher absorption than noble metals in the NIR due to a weak interband region centred at around 825 nm.
Tuning of the resonance position into the NIR was demonstrated by many methods, and extinction peaks exceeding 1200 nm can be achieved by all of the metals studied. However, it is important that the method used to red-shift the extinction peak does not also decrease the radiative efficiency. Core-shell nanoparticles, triangular nanoparticles and platelet-type nanoparticles were found to be unsuitable for silicon solar cells applications due their low radiative efficiencies. Instead, we propose the use of large (> 150 nm) Ag spheroids with moderate aspect ratios. A maximum radiative efficiency of 0.98 was found for noble metal nanospheres when the diameter exceeded 150 nm.
The optical properties of Au and Al nanoparticles fabricated by electron-beam lithography were found to be in good agreement with simulations, provided that the substrate and local dielectric environment were accounted for by inclusion of an effective medium in the model. Cr adhesion layers were found to substantially weaken the extinction peaks of Au nanoparticles, and also result in a strong decrease of radiative efficiency. Adhesion layers were not required for Al nanoparticles. The morphological and optical properties of Ag island films were found to be highly dependent on the layer thickness, deposition speed and anneal temperature. Dense arrays containing average particle sizes ranging from 25 nm to 250 nm were achieved using anneal temperatures lower than 200oC. The largest nanoparticles were found to exhibit high extinction from 400 nm to 800 nm.
Depositing Ag nanoparticles onto a-Si:H solar cells was found two have two effects on the spectral response. At short wavelengths the QE was decreased due to absorption by small particles or back-scattering by larger particles. At longer wavelengths large maxima and minima are present in the QE spectra. This latter effect is not due to excitation of surface plasmons, but is instead related to modification of interference effects in the thin-film layer stack.

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