• Energy Research

Ordered silicon nanocrystals in silicon carbide are produced by Plasma Enhanced Chemical Vapor Deposition by means of the multilayer approach followed by annealing at 1100 °C. The crystallization is verified by Raman scattering, X-ray diffraction, Transmission Electron Microscopy, and UV-Vis spectroscopy. The conditions for the periodic structure to survive the high temperature annealing and for the SiC barrier to confine the Si crystal growth are examined by energy-filtered transmission electron microscopy and X-ray reflection. The final layout appears to be strongly influenced by the structural features of the as-deposited multilayer. Threshold values of Si-rich carbide sublayer thickness and Si-to-C ratio are identified in order to preserve the ordered structure. The crystallized fraction is observed to be correlated with the total silicon volume fraction. The constraints are examined through the use of ab-initio calculations of matrix-embedded silicon nanocrystals, as well as in terms of existing models for nanocrystal formation, in order to establish the role played by the interface energy on nanocrystal outgrowth, residual amorphous fraction, and continuous crystallization. A parameter space of formation of ordered Si nanocrystals is proposed. The diffusivity of carbon in the crystallized material is evaluated, and estimated to be around 10-16 cm2/s at 1100°C.

Ordered silicon nanocrystals in silicon carbide are produced by Plasma Enhanced Chemical Vapor Deposition by means of the multilayer approach followed by annealing at 1100 °C. The crystallization is verified by Raman scattering, X-ray diffraction, Transmission Electron Microscopy, and UV-Vis spectroscopy. The conditions for the periodic structure to survive the high temperature annealing and for the SiC barrier to confine the Si crystal growth are examined by energy-filtered transmission electron microscopy and X-ray reflection. The final layout appears to be strongly influenced by the structural features of the as-deposited multilayer. Threshold values of Si-rich carbide sublayer thickness and Si-to-C ratio are identified in order to preserve the ordered structure. The crystallized fraction is observed to be correlated with the total silicon volume fraction. The constraints are examined through the use of ab-initio calculations of matrix-embedded silicon nanocrystals, as well as in terms of existing models for nanocrystal formation, in order to establish the role played by the interface energy on nanocrystal outgrowth, residual amorphous fraction, and continuous crystallization. A parameter space of formation of ordered Si nanocrystals is proposed. The diffusivity of carbon in the crystallized material is evaluated, and estimated to be around 10-16 cm2/s at 1100°C.

Aim of the present work is to establish the optimum deposition conditions for a pure SiH4 + CH4 plasma in order to obtain a good quality baseline a-SiC:H material. The effects of CH4 flow, substrate temperature, and pressure on the optoelectronic properties of deposited films have been carefully studied by means of a numerical procedure which allows to study a large number of parameters with a small number of experiments. Optimized films of a-SiC:H with energy gap in the range 1.84–1.90 eV, Urbach energies below 60 meV, photoconductive gain higher than 106 cm2V−1 and ημτ product higher than 8.9 × 10−8 have been obtained.

Aim of the present work is to establish the optimum deposition conditions for a pure SiH4 + CH4 plasma in order to obtain a good quality baseline a-SiC:H material. The effects of CH4 flow, substrate temperature, and pressure on the optoelectronic properties of deposited films have been carefully studied by means of a numerical procedure which allows to study a large number of parameters with a small number of experiments. Optimized films of a-SiC:H with energy gap in the range 1.84–1.90 eV, Urbach energies below 60 meV, photoconductive gain higher than 106 cm2V−1 and ημτ product higher than 8.9 × 10−8 have been obtained.

Self-organizing nanopatterns can enable economically competitive, industrially applicable light-harvesting platforms for thin-film solar cells. In this work, we present transparent solar cell substrates having quasiperiodic uniaxial nanowrinkle patterns with high optical haze values. The self-organized nanowrinkle template is created by controlled heat-shrinking of metal-deposited pre-stretched polystyrene sheets. A scalable UV nanoimprinting method is used to transfer the nanopatterns to glass substrates on which single-junction hydrogenated amorphous silicon p-i-n solar cells are subsequently fabricated. The structural and optical analyses of the solar cell show that the nanowrinkle pattern is replicated throughout the solar cell structure leading to enhanced absorption of light. The efficient broadband light-trapping in the nanowrinkle solar cells results in very high 18.2 mA/cm2 short-circuit current density and 9.5% energy-conversion efficiency, which respectively are 35.8% and 39.7% higher than the values obtained in flat-substrate solar cells. The cost- and time-efficient technique introduces a promising new approach to customizable light-management strategies in thin-film solar cells.

Self-organizing nanopatterns can enable economically competitive, industrially applicable light-harvesting platforms for thin-film solar cells. In this work, we present transparent solar cell substrates having quasiperiodic uniaxial nanowrinkle patterns with high optical haze values. The self-organized nanowrinkle template is created by controlled heat-shrinking of metal-deposited pre-stretched polystyrene sheets. A scalable UV nanoimprinting method is used to transfer the nanopatterns to glass substrates on which single-junction hydrogenated amorphous silicon p-i-n solar cells are subsequently fabricated. The structural and optical analyses of the solar cell show that the nanowrinkle pattern is replicated throughout the solar cell structure leading to enhanced absorption of light. The efficient broadband light-trapping in the nanowrinkle solar cells results in very high 18.2 mA/cm2 short-circuit current density and 9.5% energy-conversion efficiency, which respectively are 35.8% and 39.7% higher than the values obtained in flat-substrate solar cells. The cost- and time-efficient technique introduces a promising new approach to customizable light-management strategies in thin-film solar cells.

In the field of nanostructured materials, our research was focussed on the synthesis and applications of silicon based nanostructured films. In particular, we studied nanocrystalline Si and SiC thin films, having film thicknesses in the nanometer range and including nanometric grains, and compositionally modulated nanometric multilayers of a-Si3N4:H / a-SixN1-x:H. The Plasma Enhanced Chemical Vapor Deposition (PECVD) technique was used to deposit these materials at low temperature, which is advantageous for device applications. This paper is centred on our main results on p-type nanocrystalline Si (p nc-Si). The p nc-Si films were deposited by Very High Frequency (VHF) PECVD in high hydrogen dilution of the gas mixture at 170 °C. Our results on the application of these films in nanocrystalline Si / amorphous Si /crystalline Si heterojunction solar cells are discussed in details. The long range effects of plasma H atoms on the heterojunction nanostructure are studied by the simulation of optical spectra and the High Resolution Transmission Electron Microscopy (HRTEM) observations on the p nc-Si / i a-Si:H double layers deposited on c-Si substrates. The heterojunction built-in potential of these double layers is larger than in p a-Si:H / i a-Si:H structures and therefore in the nanocrystalline Si / amorphous Si /crystalline Si solar cell a Voc up to 640 mV can be obtained. The simulation of optical spectra and the HRTEM observations are reported and correlated with the corresponding solar cells characteristics.