Montaje y optimización de un sistema de pulverización catódica de alta presión de silicio amorfo hidrogenado (Assembly and optimizatión of a high pressure sputtering system for hydrogenated amorphous silicon)

El mapa energético actual se presenta complicado. Frente al crecimiento actual de la demanda de energía y los recursos energéticos, la conversión fotovoltaica aparece como una energía limpia, renovable y realista para dar solución a los problemas energéticos futuros. En la actualidad se está trabajando en la fabricación de una tercera generación de células fotovoltaicas. La integración de materiales con una banda de estados permitidos dentro del gap de energías prohibidas (Banda intermedia) es uno de los caminos posibles. En el presente trabajo de investigación se depositarán y caracterizarán láminas delgadas de silicio amorfo tipo p crecidas mediante la técnica de pulverización catódica para su posterior aplicación en células solares de estructura HIT. Concretamente se montará un sistema de pulverización catódica y se depositarán láminas delgadas de silicio amorfo hidrogenado sobre dos tipos de sustrato, vidrio y silicio, con el objeto de caracterizar el material depositado. Mediante las medidas realizadas se estudiarán los parámetros de depósito de nuestras láminas, así como comprobar sus características y compatibilidades con la estructura HIT. [ABSTRACT] The new energy map shows complex. Faced with the growth of energy demand and energy resources, photovoltaic conversion is presented as a clean energy, renewable and realistic for solving future energy problems. Currently we are working on making a third generation photovoltaic cells. The integration of materials with a band of allowed states in the forbidden energy band gap (intermediated band) is one of the possible paths. This research will study thin films of p-type amorphous silicon grown by sputtering technique for further application in HIT structure solar cells. Specifically we will assemble a sputtering system and will growth thin films on two types of substrate, glass and silicon, in order to characterize the material deposited. By the measurements we will study growth parameters of our films and their features and compatibility with the HIT structure

Mecanismos de transporte en células solares basadas en heterouniones con silicio

Esta tesis doctoral se enmarca dentro de la tecnología fotovoltaica basada en silicio. El silicio es un material semiconductor con unas propiedades y un desarrollo tecnológico tal que le han convertido en el material más importante de la industria fotovoltaica. En la actualidad, las células solares basadas en silicio suponen el 95 % de toda la potencia fotovoltaica instalada en el mundo. De las células solares basadas en silicio, las que han demostrado tener una mayor eficiencia han sido las células de heterounión. El proceso clave por el cual este tipo de estructuras ostentan dicho récord es por lograr una buena pasivación de la superficie del silicio cristalino..

Insulator-to-metal transition in vanadium supersaturated silicon: variable-range hopping and Kondo effect signatures

We report the observation of the insulator-to-metal transition in crystalline silicon samples supersaturated with vanadium. Ion implantation followed by pulsed laser melting and rapid resolidification produce high quality single-crystalline silicon samples with vanadium concentrations that exceed equilibrium values in more than 5 orders of magnitude. Temperature-dependent analysis of the conductivity and Hall mobility values for temperatures from 10K to 300K indicate that a transition from an insulating to a metallic phase is obtained at a vanadium concentration between 1.1 × 10^(20) and 1.3 × 10^(21) cm^(−3) . Samples in the insulating phase present a variable-range hopping transport mechanism with a Coulomb gap at the Fermi energy level. Electron wave function localization length increases from 61 to 82 nm as the vanadium concentration increases in the films, supporting the theory of impurity band merging from delocalization of levels states. On the metallic phase, electronic transport present a dispersion mechanism related with the Kondo effect, suggesting the presence of local magnetic moments in the vanadium supersaturated silicon material

Double ion implantation and pulsed laser melting processes for third generation solar cells

In the framework of the third generation of photovoltaic devices, the intermediate band solar cell is one of the possible candidates to reach higher efficiencies with a lower processing cost. In this work, we introduce a novel processing method based on a double ion implantation and, subsequently, a pulsed laser melting (PLM) process to obtain thicker layers of Ti supersaturated Si. We perform ab initio theoretical calculations of Si impurified with Ti showing that Ti in Si is a good candidate to theoretically form an intermediate band material in the Ti supersaturated Si. From time-of-flight secondary ion mass spectroscopy measurements, we confirm that we have obtained a Ti implanted and PLM thicker layer of 135 nm. Transmission electron microscopy reveals a single crystalline structure whilst the electrical characterization confirms the transport properties of an intermediate band material/Si substrate junction. High subbandgap absorption has been measured, obtaining an approximate value of 10 4 cm(-1) in the photons energy range from 1.1 to 0.6 eV

Electrical characterization of amorphous silicon MIS-based structures for HIT solar cell applications

A complete electrical characterization of hydrogenated amorphous silicon layers (a-Si:H) deposited on crystalline silicon (c-Si) substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) was carried out. These structures are of interest for photovoltaic applications. Different growth temperatures between 30 and 200 °C were used. A rapid thermal annealing in forming gas atmosphere at 200 °C during 10 min was applied after the metallization process. The evolution of interfacial state density with the deposition temperature indicates a better interface passivation at higher growth temperatures. However, in these cases, an important contribution of slow states is detected as well. Thus, using intermediate growth temperatures (100–150 °C) might be the best choice

Far infrared photoconductivity in a silicon based material: vanadium supersaturated silicon

We have analyzed the spectral sub-bandgap photoresponse of silicon (Si) samples implanted with vanadium (V) at different doses and subsequently processed by pulsed-laser melting. Samples with V concentration clearly above the insulator-metal transition limit show an important increase of the photoresponse with respect to a Si reference sample. Their photoresponse extends into the far infrared region and presents a sharp photoconductivity edge that moves towards lower photon energies as the temperature decreases. The increase of the value of the photoresponse is contrary to the classic understanding of recombination centers action and supports the predictions of the insulator-metal transition theory

Electrical decoupling effect on intermediate band Ti-implanted silicon layers

We investigated the electrical transport properties of ultraheavily Ti-implanted silicon layers subsequently pulsed laser melted (PLM). After PLM, the samples exhibit anomalous electrical behaviour in sheet resistance and Hall mobility measurements, which is associated with the formation of an intermediate band (IB) in the implanted layer. An analytical model that assumes IB formation and a current limitation effect between the implanted layer and the substrate was developed to analyse this anomalous behaviour. This model also describes the behaviour of the function V/Delta V and the electrical function F that can be extracted from the electrical measurements in the bilayer. After chemical etching of the implanted layer, the anomalous electrical behaviour observed in sheet resistance and Hall mobility measurements vanishes, recovering the unimplanted Si behaviour, in agreement with the analytical model. The behaviour of V/Delta V and the electrical function F can also be successfully described in terms of the analytical model in the bilayer structure with the implanted layer entirely stripped

Room temperature photo-response of titanium supersaturated silicon at energies over the bandgap

Silicon samples were implanted with high Ti doses and subsequently processed with the pulsed-laser melting technique. The electronic transport properties in the 15–300 K range and the room temperature spectral photoresponse at energies over the bandgap were measured. Samples with Ti concentration below the insulator-metal (I-M) transition limit showed a progressive reduction of the carrier lifetime in the implanted layer as Ti dose is increased. However, when the Ti concentration exceeded this limit, an extraordinary recovery of the photoresponse was measured. This result supports the theory of intermediate band materials and is of utmost relevance for photovoltaic cells and Si-based detectors

Study of the Electrical Behavior in Intermediate Band-Si Junctions

In this study we analyze the electrical behavior of a junction formed by an ultraheavily Ti implanted Si layer processed by a Pulsed Laser Melting (PLM) and the non implanted Si substrate. This electrical behavior exhibits an electrical decoupling effect in this bilayer that we have associated to an Intermediate Band (IB) formation in the Ti supersaturated Si layer. Time-of-flight secondary ion mass spectrometry (ToFSIMS) measurements show a Ti depth profile with concentrations well above the theoretical limit required to the IB formation. Sheet resistance and Hall mobility measurements in the van der Pauw configuration of these bilayers exhibit a clear dependence with the different measurement currents introduced (1menor queA-1mA). We find that the electrical transport properties measured present an electrical decoupling effect in the bilayer as function of the temperature. The dependence of this effect with the injected current could be explained in terms of an additional current flow in the junction from the substrate to the IB layer and in terms of the voltage dependence in the junction with the measurement current

Deposition of Intrinsic a-Si:H by ECR-CVD to Passivate the Crystalline Silicon Heterointerface in HIT Solar Cells

We have deposited intrinsic amorphous silicon (a-Si:H) using the electron cyclotron resonance (ECR) chemical vapor deposition technique in order to analyze the a-Si:H/c-Si heterointerface and assess the possible application in heterojunction with intrinsic thin layer (HIT) solar cells. Physical characterization of the deposited films shows that the hydrogen content is in the 15-30% range, depending on deposition temperature. The optical bandgap value is always comprised within the range 1.9- 2.2 eV. Minority carrier lifetime measurements performed on the heterostructures reach high values up to 1.3 ms, indicating a well-passivated a-Si:H/c-Si heterointerface for deposition temperatures as low as 100°C. In addition, we prove that the metal-oxide- semiconductor conductance method to obtain interface trap distribution can be applied to the a-Si:H/c-Si heterointerface, since the intrinsic a-Si:H layer behaves as an insulator at low or negative bias. Values for the minimum of D_it as low as 8 × 10^10 cm^2 · eV^-1 were obtained for our samples, pointing to good surface passivation properties of ECR-deposited a-Si:H for HIT solar cell applications