Investigation of Rare Earth - Doped Silicon Nitride Layers for Solar Cell Applications

Nowadays the application of thin SiNx layers as bulk passivating and antireflection coatings for Si-based solar cells applications is considered to be a successful solution for the increase in efficiency1. A new concept to further increase the efficiency of the solar cell is based on the light conversion mechanism: according to this approach the solar spectrum can be efficiently modified by shifting the photons towards a wavelength range where the solar cell has a better or higher response2. Recently, a novel class of rare earth (RE)- doped SiNx layers has been demonstrated to be a highly promising red-emitting conversion phosphor for white-LED applications. These materials have allowed the shifting of the emission wavelength by tuning the concentration of a specific RE element in a SiNx based crystalline matrix3. The investigation of RE-doped amorphous silicon nitride (SiNx) compounds, where the electronic properties of Si are combined with the optical properties of RE3+ ions, have been shown already potential in optoelectronics4. Therefore, parallel studies on the incorporation of a RE material in amorphous SiNx host lattices, which could be implemented in solar cells to increase the efficiency, are considered to be presently a challenge. In this contribution the properties of europium- and samarium- doped amorphous SiNx layers are investigated. The RE-doped SiNx layers are deposited using a remote PECVD expanding thermal plasma fed with Ar/SiH4/NH3 mixtures in combination with a RE magnetron sputtering source implemented in the proximity of the substrate holder. Growth rates of the RE doped layers obtained from Spectroscopic Ellipsometry (SE) measurements were in the range 0.6-2.2 nm/s. The successful incorporation of RE in the SiNx matrix has been demonstrated by means of Rutherford Back Scattering (RBS) and X-ray Photoelectron Spectroscopy (XPS) analysis, i.e. up to 2%. Preliminary photoluminescence results point out a broad band emission in the region of 500-800 nm when excitation wavelengths of 270 nm and 320 nm have been used. The emission band observed can be attributed to Sm2+. [1] J. Hong et al., J. Vac. Sci. Technol. B. 21 (5). [2] C. Strümpel et al., Sol. Energ. Mat. Sol. C. 91 (2007) 238 – 249. [3] Y. Q. Li et al., J. Alloys. and Comp. 417, 273 – 279. [4] A. R. Zanatta, et al., J. Phys.:Condens. Matter 19 (2007) 436230