Analysis of thickness-dependent optical parameters of a-Si:H/nc-Si:H multilayer thin films

Ashok Kherodia,Ashish K Panchal

doi:10.1007/s40243-017-0107-3

Abstract

Multilayer thin films with alternate hydrogenated amorphous (a-Si:H) and nanocrystalline silicon (nc-Si:H) layers are deposited in hot wire chemical vapor deposition (HWCVD) chamber using hydrogen (H2) dilution of silane (SiH4). Two sets (two samples for each set) have different nc-Si:H layer thickness with the same a-Si:H thickness. The transmittance spectra, refractive index and absorption coefficient are obtained using UV–Vis spectroscopy. An analysis of Tauc plot suggests thickness-dependent optical band gap shift. The films with thick nc-Si:H layers show a decrease in band gap from 1.78 to 1.68 eV, whereas the films with lower nc-Si:H layer thickness have an increased band gap from 1.64 to 1.72 eV. Further, the slope of the Tauc plot (B) and disorder indicator (Urbach energy, Eu) show that the films with thick nc-Si:H layers are poorly ordered as hydrogen dilution increases. These results illustrate that band gap shift can be attributed to the crystal size effect and film disorder. This shifting of optical band gap with the nc-Si:H layer thickness is useful in tuning of the absorber layer band gap which finds an application in photovoltaics.

Highlights

In the first generation of solar photovoltaics technologies, the crystalline silicon (c-Si) is the leading material for solar cell fabrication. c-Si offers the highest efficiency and longterm stability [1]
The experiment deals with the effect of nanocrystalline silicon layer thickness on the optical properties of a multilayer stack of amorphous silicon (a-Si):H/nc-Si:H
The band gap of the multilayer films varies with the film thickness and the hydrogen dilution

Summary

Introduction

In the first generation of solar photovoltaics technologies, the crystalline silicon (c-Si) is the leading material for solar cell fabrication. c-Si offers the highest efficiency and longterm stability [1]. In the first generation of solar photovoltaics technologies, the crystalline silicon (c-Si) is the leading material for solar cell fabrication. Amorphous silicon (a-Si:H) film can be fabricated at very low temperature. The films have higher light absorption than c-Si and need very small film thickness for solar cell fabrication in comparison with the c-Si wafer. The light-induced instability in the a-Si:H films (Staebler-Wronski effect) causes long-term degradation in efficiency [2]. The nc-Si:H films provide better carrier transport and have a broader absorption range in the visible solar spectrum, tunable band gap, almost complete absence of light-induced degradation [3], higher electrical conductivity and greater doping efficiency [4]. A tandem cell (a-Si:H/ lc-Si:H) and triple junction silicon cell (a-Si:H/lc-Si:H/ lc-Si:H) have been fabricated with efficiencies * 13%

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Conclusion