• 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.

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.

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.

We present an overview of the project NASCEnT and the first results we were able to achieve after one year of project duration. The overall objective of the project is to develop new nano-materials with new production technologies and to fabricate silicon quantum dot (Si QD) materials for all-silicon tandem solar cells to achieve increased efficiencies. The understanding of electrical transport and recombination mechanisms in these newly developed nano-materials will enable us to design novel solar cell structures that help to overcome the efficiency limits of conventional solar cell concepts. So far we developed simple device structures and process sequences for both material systems (Si QDs in silicon oxide and silicon carbide) and tested a pin solar cell structure. Another objective of the project is to ensure the compatibility of the newly developed technologies with high-throughput processing leading to further cost-reduction. Within the scope of this project the novel concept of band gap-tuneable quantum dot superlattices is to be exploited in combination with a wafer-based high efficiency silicon solar cell as well as with a thin-film solar cell structure. The outcome of the project will be significant progress in solar cell evolution and will lead to higher efficiencies for solar cells and to ongoing cost-reductions from both a short-term and a long-term perspective. 26th European Photovoltaic Solar Energy Conference and Exhibition; 22-27

Amorphous hydrogenated silicon nitrogen alloys (a-Si1-xNx:H), with energy gap in the range 1.9-2.7 eV, were deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) using silane and ammonia gas mixture. The film structure and composition, as well as the neutral defect content and electrical behaviour were measured. A comparison with high quality a-SiC1-xCx:H films, deposited with the same PECVD system, is also reported. A good quality material, with photoconductive gain of about 105, and thus suitable for photovoltaic applications, can be obtained with a higher deposition rate with respect to a-SiC1-xCx:H films having the same energy gap.

The influence of the chamber residual pressure level in the radio frequency magnetron sputtering process on the electrical, optical and structural properties of indium thin oxide (ITO) is investigated. Several ITO films were deposited at various residual pressure levels on Coming glass using In2O3:SnO2 target in argon atmosphere and without the addition of oxygen partial pressure. It is found that a very good vacuum is associated to metallic films and results in less transparent ITO films, with some powder formation on the surface. On the contrary highly transparent and conducting films are produced at a higher residual pressure. The best deposition conditions are addressed for ITO films as transparent conducting oxide layers in silicon heterojunction solar cells. Using the optimal vacuum level for ITO fabrication, a maximum short circuit current of 36.6 mA/cm(2) and a fill-factor of 0.78 are obtained for solar cells on textured substrates with a device conversion efficiency of 16.2%.

Abstract Indium tin oxide (ITO) thin films were deposited by magnetron sputtering from a ceramic target, and annealed at temperatures up to 1000°C in N2 atmosphere. Some samples were capped in a-Si:H or spin-on glass to prevent residual oxygen contamination from the annealing ambient, and annihilation of oxygen vacancies. The electrical and optical properties were measured before and after annealing. It is found that the protecting layer effectively limits the ITO degradation up to 900°C, whereas high transparency is preserved in all cases. The results indicate the applicability of the procedure to high temperature devices, and opens the way to the application of a variety of nanostructured materials in advanced photovoltaic devices.