Spin and density longitudinal response of quantum dots in the time-dependent local-spin-density approximation

The longitudinal dipole response of a quantum dot has been calculated in the far-infrared regime using local-spin-density-functional theory. We have studied the coupling between the collective spin and density modes as a function of the magnetic field. We have found that the spin dipole mode and single-particle excitations have a sizable overlap, and that the magnetoplasmon modes can be excited by the dipole spin operator if the dot is spin polarized. The frequency of the dipole spin edge mode presents an oscillation which is clearly filling factor (v) related. We have found that the spin dipole mode is especially soft for even-n values. Results for selected numbers of electrons and confining potentials are discussed.

Transverse dipole spin modes in quantum dots

We have carried out a systematic analysis of the transverse dipole spin response of a large-size quantum dot within time-dependent current density functional theory. Results for magnetic fields corresponding to integer filling factors are reported, as well as a comparison with the longitudinal dipole spin response. As in the two-dimensional electron gas, the spin response at high-spin magnetization is dominated by a low-energy transverse mode.

Optoelectronic properties of CuPc thin films deposited at different substrate temperatures

Structural and optical characterization of copper phthalocyanine thin film thermally deposited at different substrate temperatures was the aim of this work. The morphology of the films shows strong dependence on temperature, as can be observed by atomic force microscopy and x-ray diffraction spectroscopy, specifically in the grain size and features of the grains. The increase in the crystal phase with substrate temperature is shown by x-ray diffractometry. Optical absorption coefficient measured by photothermal deflection spectroscopy and optical transmittance reveal a weak dependence on the substrate temperature. Besides, the electro-optical response measured by the external quantum efficiency of Schottky ITO/CuPc/Al diodes shows an optimized response for samples deposited at a substrate temperature of 60 °C, in correspondence to the I-V diode characteristics.

Progress in a-Si:H/c-Si heterojunction emitters obtained by Hot-Wire CVD at 200°C

In this work, we investigate heterojunction emitters deposited by Hot-Wire CVD on p-type crystalline silicon. The emitter structure consists of an n-doped film (20 nm) combined with a thin intrinsic hydrogenated amorphous silicon buffer layer (5 nm). The microstructure of these films has been studied by spectroscopic ellipsometry in the UV-visible range. These measurements reveal that the microstructure of the n-doped film is strongly influenced by the amorphous silicon buffer. The Quasy-Steady-State Photoconductance (QSS-PC) technique allows us to estimate implicit open-circuit voltages near 700 mV for heterojunction emitters on p-type (0.8 Ω·cm) FZ silicon wafers. Finally, 1 cm 2 heterojunction solar cells with 15.4% conversion efficiencies (total area) have been fabricated on flat p-type (14 Ω·cm) CZ silicon wafers with aluminum back-surface-field contact.

PEN as substrate for new solar cell technologies

The possible use of polyethylene naphthalate as substrate for low-temperature deposited solar cells has been studied in this paper. The transparency of this polymer makes it a candidate to be used in both substrate and superstrate configurations. ZnO:Al has been deposited at room temperature on top of PEN. The resulting structure PEN/ZnO:Al presented good optical and electrical properties. PEN has been successfully textured (nanometer and micrometer random roughness) using hot-embossing lithography. Reflector structures have been built depositing Ag and ZnO:Al on top of the stamped polymer. The deposition of these layers did not affect the final roughness of the whole. The reflector structure has been morphologically and optically analysed to verify its suitability to be used in solar cells.

Optoelectronic studies in nanocrystalline silicon Schottky diodes obtained by Hot-Wire Chemical Vapour Deposition

The very usual columnar growth of nanocrystalline silicon leads to electronic transport anisotropies. Whereas electrical measurements with coplanar electrodes only provide information about the electronic transport parallel to the substrate, it is the transverse transport which determines the collection efficiency in thin film solar cells. Hence, Schottky diodes on transparent electrodes were obtained by hot-wire CVD in order to perform external quantum efficiency and surface photovoltage studies in sandwich configuration. These measurements allowed to calculate a transverse collection length, which must correlate with the photovoltaic performance of thin film solar cells. Furthermore, the density of charge trapped at localized states in the bandgap was estimated from the voltage dependence of the depletion capacitance of these rectifying contacts.

Síntesi de nanopartícules de TiO2 fàcilment processables i el seu ús per a fabricar a baixa temperatura cel·les solars sensibilitzades

La gran demanda energètica i la problemàtica dels combustibles fòssils i d‟altres recursos per a obtenir energia posen de manifest la necessitat de desenvolupar tecnologies netes, sostenibles i econòmicament viables de generació d‟energia. En aquest àmbit, les cel·les solars sensibilitzades amb colorant (Dye Sensitized Solar Cells, DSSC), que transformen l‟energia solar a energia elèctrica, són una solució factible. A més, el desenvolupament de mètodes per a construir aquestes cel·les a baixa temperatura permetria fabricar-les sobre substrats plàstics, fet que els donaria un valor afegit i permetria una producció en continu, ràpida i amb baix cost tant, econòmic, com d‟impacte ambiental. Aquest treball presenta el desenvolupament d‟un mètode de producció a baixa temperatura (140 ºC) de DSSC, amb eficiència de 5,9 % sobre substrats FTO/vidre. Aquest valor és superior a la majoria de les eficiències reportades a la bibliografia de DSSC construïdes a baixa temperatura. Les capes mesoporoses que formen els elèctrodes de les DCCS es dipositen per doctor blade a partir d‟una pasta composta per nanopartícules de TiO2, de 4-8 nm, recobertes d‟àcid 3,6,9-trioxadecanoic, per nanopartícules de Degussa P25, de 20-25 nm, i com a dissolvents només s‟utilitza aigua i etanol. L‟aplicació d‟un tractament a 140 ºC permet eliminar la matèria orgànica de la superfície de les nanopartícules de 4-8 nm i unir-les a les de Degussa P25. Aquest fet permet obtenir capes mesoporoses sinteritzades de 6 μm de gruix. A més, l‟aplicació d‟un post-tractament, en el qual s‟utilitza l‟àcid hexafluoro titànic (IV), produeix un lleuger increment en l‟eficiència. Endemés, l‟obtenció de capes primes de TiO2 sobre substrats plàstics és un tema d‟actualitat a causa de la falta de mètodes de deposició a baixa temperatura. En aquest context, s‟ha sintetitzat, mitjançant processos respectuosos amb el medi ambient nanopartícules de TiO2 cristal·lí modificades superficialment amb lauril gal·lat i àcid 3,6,9-trioxodecanoic. Les nanopartícules poden ser dispersades en dissolvents orgànics i aigua respectivament, fet que permet obtenir suspensions estables i de fàcil manipulació. Aquestes poden ser utilitzades com a precursores per a obtenir capes primes a baixa temperatura de TiO2 cristal·lí. En concret, les capes primes formades per nanopartícules de TiO2 modificades amb àcid 3,6,9-trioxodecanoic s‟han utilitzat com a blocking layer en les DSSC construïdes a baixa temperatura.

Analysis of S-rich CuIn(S,Se)2 layers for photovoltaic applications: Influence of the sulfurization temperature on the crystalline properties of electrodeposited and sulfurized CuInSe2 precursors

This paper reports the microstructural analysis of S-rich CuIn(S,Se)2 layers produced by electrodeposition of CuInSe2 precursors and annealing under sulfurizing conditions as a function of the temperature of sulfurization. The characterization of the layers by Raman scattering, scanning electron microscopy, Auger electron spectroscopy, and XRD techniques has allowed observation of the strong dependence of the crystalline quality of these layers on the sulfurization temperature: Higher sulfurization temperatures lead to films with improved crystallinity, larger average grain size, and lower density of structural defects. However, it also favors the formation of a thicker MoS2 interphase layer between the CuInS2 absorber layer and the Mo back contact. Decreasing the temperature of sulfurization leads to a significant decrease in the thickness of this intermediate layer and is also accompanied by significant changes in the composition of the interface region between the absorber and the MoS2 layer, which becomes Cu rich. The characterization of devices fabricated with these absorbers corroborates the significant impact of all these features on device parameters as the open circuit voltage and fill factor that determine the efficiency of the solar cells.

Silicon nanocrystals as an enabling material for silicon Ppotonics

Silicon nanocrystals (Si-nc) is an enabling material for silicon photonics, which is no longer an emerging field of research but an available technology with the first commercial products available on the market. In this paper, properties and applications of Si-nc in silicon photonics are reviewed. After a brief history of silicon photonics, the limitations of silicon as a light emitter are discussed and the strategies to overcome them are briefly treated, with particular attention to the recent achievements. Emphasis is given to the visible optical gain properties of Si-nc and to its sensitization effect on Er ions to achieve infrared light amplification. The state of the art of Si-nc applied in a few photonic components is reviewed and discussed. The possibility to exploit Si-nc for solar cells is also presented. in addition, nonlinear optical effects, which enable fast all-optical switches, are described.

Integer filling facfor phases and isospin in vertical diatomic artificial molecules

Integer filling factor phases of many-electron vertically coupled diatomic artificial quantum dot molecules are investigated for different values of the interdot coupling. The experimental results are analyzed within local-spin density functional theory for which we have determined a simple lateral confining potential law that can be scaled for the different coupling regimes, and Hartree-Fock theory. Maximum density droplets composed of electrons in both bonding and antibonding or just bonding states are revealed, and interesting isospin-flip physics appears for weak interdot coupling when the systematic depopulation of antibonding states leads to changes in isospin.

Dissociation of vertical semiconductor diatomic artificial molecules

We investigate the dissociation of few-electron circular vertical semiconductor double quantum dot artificial molecules at 0 T as a function of interdot distance. A slight mismatch introduced in the fabrication of the artificial molecules from nominally identical constituent quantum wells induces localization by offsetting the energy levels in the quantum dots by up to 2 meV, and this plays a crucial role in the appearance of the addition energy spectra as a function of coupling strength particularly in the weak coupling limit.

Transparent conducting thin films by the co-sputtering of ZnO-ITO targets

Transparent and conductive Zn-In-Sn-O (ZITO) amorphous thin films have been deposited at room temperature by the rf magnetron co-sputtering of ITO and ZnO targets. Co-sputtering gives the possibility to deposit multicomponent oxide thin films with different compositions by varying the power to one of the targets. In order to make ZITO films with different Zn content, a constant rf power of 50 W was used for the ITO target, where as the rf power to ZnO target was varied from 25 W to 150 W. The as deposited films showed an increase in Zn content ratio from 17 to 67 % as the power to ZnO target was increased from 25 to 150 W. The structural, electrical and optical properties of the as deposited films are reported. The films showed an average transmittance over 80% in the visible wavelength range. The electrical resistivity and optical band gap of the ZITO films were found to depend on the Zn content in the film. The ZITO films deposited at room temperature with lower Zn content ratios showed better optical transmission and electrical properties compared to ITO film.

Electrical and optical properties of Zn-In-Sn-O transparent conducting thin films

Indium tin oxide (ITO) is one of the widely used transparent conductive oxides (TCO) for application as transparent electrode in thin film silicon solar cells or thin film transistors owing to its low resistivity and high transparency. Nevertheless, indium is a scarce and expensive element and ITO films require high deposition temperature to achieve good electrical and optical properties. On the other hand, although not competing as ITO, doped Zinc Oxide (ZnO) is a promising and cheaper alternative. Therefore, our strategy has been to deposit ITO and ZnO multicomponent thin films at room temperature by radiofrequency (RF) magnetron co-sputtering in order to achieve TCOs with reduced indium content. Thin films of the quaternary system Zn-In-Sn-O (ZITO) with improved electrical and optical properties have been achieved. The samples were deposited by applying different RF powers to ZnO target while keeping a constant RF power to ITO target. This led to ZITO films with zinc content ratio varying between 0 and 67%. The optical, electrical and morphological properties have been thoroughly studied. The film composition was analysed by X-ray Photoelectron Spectroscopy. The films with 17% zinc content ratio showed the lowest resistivity (6.6 × 10 - 4 Ω cm) and the highest transmittance (above 80% in the visible range). Though X-ray Diffraction studies showed amorphous nature for the films, using High Resolution Transmission Electron Microscopy we found that the microstructure of the films consisted of nanometric crystals embedded in a compact amorphous matrix. The effect of post deposition annealing on the films in both reducing and oxidizing atmospheres were studied. The changes were found to strongly depend on the zinc content ratio in the films.

Development of laser-fired contacts for amorphous silicon layers obtained by Hot-Wire CVD

In this work we study aluminium laser-fired contacts for intrinsic amorphous silicon layers deposited by Hot-Wire CVD. This structure could be used as an alternative low temperature back contact for rear passivated heterojunction solar cells. An infrared Nd:YAG laser (1064 nm) has been used to locally fire the aluminium through the thin amorphous silicon layers. Under optimized laser firing parameters, very low specific contact resistances (ρc ∼ 10 mΩ cm2) have been obtained on 2.8 Ω cm p-type c-Si wafers. This investigation focuses on maintaining the passivation quality of the interface without an excessive increase in the series resistance of the device.

Uniformity Study of Amorphous and Microcrystalline Silicon Thin Films Deposited on 10cmx10cm Glass Substrate using Hot Wire CVD Technique

The scaling up of the Hot Wire Chemical Vapor Deposition (HW-CVD) technique to large deposition area can be done using a catalytic net of equal spaced parallel filaments. The large area deposition limit is defined as the limit whenever a further increment of the catalytic net area does not affect the properties of the deposited film. This is the case when a dense catalytic net is spread on a surface considerably larger than that of the film substrate. To study this limit, a system able to hold a net of twelve wires covering a surface of about 20 cm x 20 cm was used to deposit amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon over a substrate of 10 cm x 10 cm placed at a filament-substrate distance ranging from 1 to 2 cm. The uniformity of the film thickness d and optical constants, n(x, λ) and α(x,¯hω), was studied via transmission measurements. The thin film uniformity as a function of the filament-substrate distance was studied. The experimental thickness profile was compared with the theoretical result obtained solving the diffusion equations. The optimization of the filament-substrate distance allowed obtaining films with inhomogeneities lower than ±2.5% and deposition rates higher than 1 nm/s and 4.5 nm/s for (μc-Si:H) and (a-Si:H), respectively.