Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in a... more Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in achieving good energy-conversion efficiency and integrating light-management into the solar cell design. In this report a technique to circumvent these obstacles is presented: transferability and an efficient light-harvesting scheme are combined for thin-film silicon solar cells by the incorporation of a NaCl layer. Amorphous silicon solar cells in p-i-n configuration are fabricated on reusable glass substrates coated with an interlayer of NaCl. Subsequently, the solar cells are detached from the substrate by dissolution of the sacrificial NaCl layer in water and then transferred onto a plastic sheet, with a resultant post-transfer efficiency of 9%. The light-trapping effect of the surface nanotextures originating from the NaCl layer on the overlying solar cell is studied theoretically and experimentally. The enhanced light absorption in the solar cells on NaCl-coated substrates leads to ...
A new back-reflector architecture for light-management in thin-film solar cells is proposed that ... more A new back-reflector architecture for light-management in thin-film solar cells is proposed that includes a morphologically smooth top surface with light-scattering microstructures buried within. The microstructures are pyramid shaped, fabricated on a planar reflector using TiO2 nanoparticles and subsequently covered with a layer of Si nanoparticles to obtain a flattened top surface, thus enabling growth of good quality thin-film solar cells. The optical properties of this back-reflector show high broadband haze parameter and wide angular distribution of diffuse light-scattering. The n-i-p amorphous silicon thin-film solar cells grown on such a back-reflector show enhanced light absorption resulting in improved external quantum efficiency. The benefit of the light trapping in those solar cells is evidenced by the gains in short-circuit current density and efficiency up to 15.6% and 19.3% respectively, compared to the reference flat solar cells. This improvement in the current generation in the solar cells grown on the flat-topped (buried pyramid) back-reflector is observed even when the irradiation takes place at large oblique angles of incidence. Finite-difference-time-domain simulation results of optical absorption and ideal short-circuit current density values agree well with the experimental findings. The proposed approach uses a low cost and simple fabrication technique and allows effective light manipulation by utilizing the optical properties of micro-scale structures and nanoscale constituent particles.
The purpose of this study is a systematic description of the properties of small deposited cluste... more The purpose of this study is a systematic description of the properties of small deposited clusters in dependence of the cluster geometry and composition and of the shape of the adsorbing surface. Therefore hydrocarbon molecules deposited onto the dimerized Si(100) surface and onto monolayer steps of this surface, are considered and the properties of these systems are determined using the Density Functional and scattering theories. It has been found that, though the step is a weaker sink than the flat surface, the molecules are bonded to the step and the adsorption geometries reproduce the ones of the flat surface. The transmission function depends on the type of molecule and of the substrate and on the transport channels available to the deposited system. However the contact potential has a paramount importance and deep resonances are produced by a proper tuning of this quantity.
Physica E-low-dimensional Systems & Nanostructures, 2007
The purpose of this study is the assessment of the properties of the conductance of deposited ato... more The purpose of this study is the assessment of the properties of the conductance of deposited atomic chains. Therefore, linear chains of covalent and metallic atoms, i.e. As and Ag, deposited onto monolayer steps of the Si(100) surface have been considered. The study is based on the extended Hückel theory, used for the evaluation of both the electronic structure and the conductance, and the calculations analyze the binding and adsorption energies of chains of variable length deposited onto SA steps in the light of the similar properties of free standing chains and of chains deposited onto the flat Si(100) surface. This comparison shows that the stability of the chain depends on its composition, rather than on its length, and increases in the order: free standing, deposited onto SA, deposited onto Si(100). The central result of the calculations of the conductance is that the dependence of this quantity on the chain length and composition and on the type of substrate parallels the one of the characteristic energies.
ABSTRACT Highly efficient n-type doping of a-Si:H films can be achieved by potassium implantation... more ABSTRACT Highly efficient n-type doping of a-Si:H films can be achieved by potassium implantation. Conductivities in the dark are two to four orders of magnitude larger than in the case of phosphorus doping. Moreover, an increase of up to a factor of four is detected when more hydrogen is introduced in the potassium doped samples. For high dose implantations (≥1020atoms/cm3)1.3 eV spectral photoluminescence, carried out at 77 K, shows peaks of about a factor of three larger in potassium implanted films than in the phosphorus implanted ones at the same dose. If P and K implanted samples with the same electrical conductivity are compared, the same amount of disorder is detected by photothermal deflection spectroscopy measurements, but the photoluminescence of the K implanted samples is more than one order of magnitude higher in the K doped samples than in the P doped ones. The effect is tentatively interpreted in terms of dependence of photoluminescence quenching on total impurity concentration.
Philosophical Magazine B-physics of Condensed Matter Statistical Mechanics Electronic Optical and Magnetic Properties, 2000
Very-high-frequency plasma-enhanced chemical vapour deposition was used to produce p-type microcr... more Very-high-frequency plasma-enhanced chemical vapour deposition was used to produce p-type microcrystalline samples. Spectroscopic ellipsometry measurements and transmission electron microscopy observations on the deposited samples are compared and discussed. Continuous deposition is observed to result in a growth which is initially amorphous and then evolves to microcrystalline. At this stage, the grains are observed to propagate towards the interface with the substrate. In order to obtain very thin layers, a deposition + hydrogen etching + deposition sequence was also used. This technique produces an increase in the microcrystalline fraction by a factor of more than ten with respect to continuous deposition: a crystalline fraction as large as 48% at the film-substrate interface for a 20nm film is detected. Electrical measurements are correlated with the sample structure. The dark conductivity confirms the microcrystalline nature of samples, but is also shown to depend on the distribution of the microcrystalline phase.
High quality amorphous silicon nitrogen films with energy gap in the range of 1.9–3.2 eV have bee... more High quality amorphous silicon nitrogen films with energy gap in the range of 1.9–3.2 eV have been deposited by plasma enhanced chemical vapor deposition in silane and ammonia gas mixtures. Compositional, structural, electrical and optical properties have been investigated revealing good semiconducting features comparable to those of amorphous silicon-carbon films. Advantages such as high deposition rate have been observed making
Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in a... more Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in achieving good energy-conversion efficiency and integrating light-management into the solar cell design. In this report a technique to circumvent these obstacles is presented: transferability and an efficient light-harvesting scheme are combined for thin-film silicon solar cells by the incorporation of a NaCl layer. Amorphous silicon solar cells in p-i-n configuration are fabricated on reusable glass substrates coated with an interlayer of NaCl. Subsequently, the solar cells are detached from the substrate by dissolution of the sacrificial NaCl layer in water and then transferred onto a plastic sheet, with a resultant post-transfer efficiency of 9%. The light-trapping effect of the surface nanotextures originating from the NaCl layer on the overlying solar cell is studied theoretically and experimentally. The enhanced light absorption in the solar cells on NaCl-coated substrates leads to ...
A new back-reflector architecture for light-management in thin-film solar cells is proposed that ... more A new back-reflector architecture for light-management in thin-film solar cells is proposed that includes a morphologically smooth top surface with light-scattering microstructures buried within. The microstructures are pyramid shaped, fabricated on a planar reflector using TiO2 nanoparticles and subsequently covered with a layer of Si nanoparticles to obtain a flattened top surface, thus enabling growth of good quality thin-film solar cells. The optical properties of this back-reflector show high broadband haze parameter and wide angular distribution of diffuse light-scattering. The n-i-p amorphous silicon thin-film solar cells grown on such a back-reflector show enhanced light absorption resulting in improved external quantum efficiency. The benefit of the light trapping in those solar cells is evidenced by the gains in short-circuit current density and efficiency up to 15.6% and 19.3% respectively, compared to the reference flat solar cells. This improvement in the current generation in the solar cells grown on the flat-topped (buried pyramid) back-reflector is observed even when the irradiation takes place at large oblique angles of incidence. Finite-difference-time-domain simulation results of optical absorption and ideal short-circuit current density values agree well with the experimental findings. The proposed approach uses a low cost and simple fabrication technique and allows effective light manipulation by utilizing the optical properties of micro-scale structures and nanoscale constituent particles.
The purpose of this study is a systematic description of the properties of small deposited cluste... more The purpose of this study is a systematic description of the properties of small deposited clusters in dependence of the cluster geometry and composition and of the shape of the adsorbing surface. Therefore hydrocarbon molecules deposited onto the dimerized Si(100) surface and onto monolayer steps of this surface, are considered and the properties of these systems are determined using the Density Functional and scattering theories. It has been found that, though the step is a weaker sink than the flat surface, the molecules are bonded to the step and the adsorption geometries reproduce the ones of the flat surface. The transmission function depends on the type of molecule and of the substrate and on the transport channels available to the deposited system. However the contact potential has a paramount importance and deep resonances are produced by a proper tuning of this quantity.
Physica E-low-dimensional Systems & Nanostructures, 2007
The purpose of this study is the assessment of the properties of the conductance of deposited ato... more The purpose of this study is the assessment of the properties of the conductance of deposited atomic chains. Therefore, linear chains of covalent and metallic atoms, i.e. As and Ag, deposited onto monolayer steps of the Si(100) surface have been considered. The study is based on the extended Hückel theory, used for the evaluation of both the electronic structure and the conductance, and the calculations analyze the binding and adsorption energies of chains of variable length deposited onto SA steps in the light of the similar properties of free standing chains and of chains deposited onto the flat Si(100) surface. This comparison shows that the stability of the chain depends on its composition, rather than on its length, and increases in the order: free standing, deposited onto SA, deposited onto Si(100). The central result of the calculations of the conductance is that the dependence of this quantity on the chain length and composition and on the type of substrate parallels the one of the characteristic energies.
ABSTRACT Highly efficient n-type doping of a-Si:H films can be achieved by potassium implantation... more ABSTRACT Highly efficient n-type doping of a-Si:H films can be achieved by potassium implantation. Conductivities in the dark are two to four orders of magnitude larger than in the case of phosphorus doping. Moreover, an increase of up to a factor of four is detected when more hydrogen is introduced in the potassium doped samples. For high dose implantations (≥1020atoms/cm3)1.3 eV spectral photoluminescence, carried out at 77 K, shows peaks of about a factor of three larger in potassium implanted films than in the phosphorus implanted ones at the same dose. If P and K implanted samples with the same electrical conductivity are compared, the same amount of disorder is detected by photothermal deflection spectroscopy measurements, but the photoluminescence of the K implanted samples is more than one order of magnitude higher in the K doped samples than in the P doped ones. The effect is tentatively interpreted in terms of dependence of photoluminescence quenching on total impurity concentration.
Philosophical Magazine B-physics of Condensed Matter Statistical Mechanics Electronic Optical and Magnetic Properties, 2000
Very-high-frequency plasma-enhanced chemical vapour deposition was used to produce p-type microcr... more Very-high-frequency plasma-enhanced chemical vapour deposition was used to produce p-type microcrystalline samples. Spectroscopic ellipsometry measurements and transmission electron microscopy observations on the deposited samples are compared and discussed. Continuous deposition is observed to result in a growth which is initially amorphous and then evolves to microcrystalline. At this stage, the grains are observed to propagate towards the interface with the substrate. In order to obtain very thin layers, a deposition + hydrogen etching + deposition sequence was also used. This technique produces an increase in the microcrystalline fraction by a factor of more than ten with respect to continuous deposition: a crystalline fraction as large as 48% at the film-substrate interface for a 20nm film is detected. Electrical measurements are correlated with the sample structure. The dark conductivity confirms the microcrystalline nature of samples, but is also shown to depend on the distribution of the microcrystalline phase.
High quality amorphous silicon nitrogen films with energy gap in the range of 1.9–3.2 eV have bee... more High quality amorphous silicon nitrogen films with energy gap in the range of 1.9–3.2 eV have been deposited by plasma enhanced chemical vapor deposition in silane and ammonia gas mixtures. Compositional, structural, electrical and optical properties have been investigated revealing good semiconducting features comparable to those of amorphous silicon-carbon films. Advantages such as high deposition rate have been observed making
Uploads
Papers by Rita Rizzoli