Abstract
The optical and electrical properties of Si rich SiC (SRSC) solar cell absorber layers will strongly depend on interfacial layers between the Si and the SiC matrix and in this work, we analyze hitherto undiscovered interfacial layers. The SRSC thin films were deposited using a plasma-enhanced chemical vapor deposition (PECVD) technique and annealed in a nitrogen environment at 1100 °C. The thermal treatment leads to metastable SRSC films spinodally decomposed into a Si-SiC nanocomposite. After the thermal treatment, the coexistence of crystalline Si and SiC nanostructures was analysed by high resolution transmission electron microscopy (HRTEM) and electron diffraction. From the quantitative extraction of the different plasmon signals from electron energy-loss spectra, an additional structure, amorphous SiC (a-SiC) was found. Quantitative spectroscopic electron tomography was developed to obtain three dimensional (3D) plasmonic maps. In these 3D spectroscopic maps, the Si regions appear as network structures inside the SiC matrix where the a-SiC appears as an interfacial layer separating the matrix and Si network. The presence of the a-SiC interface can be explained in the framework of the nucleation and growth model.
Highlights
Insulating or semiconducting films containing Si nanoparticles (NPs), fabricated by thermal annealing from a Si rich matrix material have emerged as highly absorbing layers for third-generation solar cells.[1]
In order to improve the transport through the absorber film by hopping, the use of Si rich SiC (SRSC) instead of Si rich SiO2 (SRSO) films is motivated by the fact that the band gap of SiC is much lower than the one of SiO2, which in turn facilitates carrier transport between two adjacent Si NPs in SRSC as compared to SRSO.[3,4]
A complete 3D characterization of the percolated Si and SiC nanostructures embedded in the SRSC film has been performed by using TEM-energy loss spectrum (EELS) related techniques
Summary
It has turned out that the interfacial properties of the Si NPs play a decisive role in determining the optical absorption and the optical activity of the Si NPs as concluded on both experimental and theoretical grounds.[9]. Whereas electron tomography has shown Si nanostructures in the SRSO films,[11,12] a direct three dimensional (3D) visualization of the interfacial structure between the precipitated NPs and their surrounding matrix is still lacking This is due to the fact that the plasmon peaks of the amorphous matrix SiOx and the interfacial layer SiOy, that have different oxygen concentrations, would be at very similar energy and these layers could not be distinguished from plasmon spectra for SRSO films.[13] The situation is different in the SRSC films, where the SiC matrix is crystalline and we can make use of distinct plasmons of different components in the SRSC system to analyze 3D interfacial layers. The growth mechanism of the observed structures is discussed in the framework of spinodal decomposition and the nucleation and growth model
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