http://hdl.handle.net/2117/2413 2013-05-24T23:55:32Z http://hdl.handle.net/2117/17686 Title: Emissive properties of SiO2 thin films through photonic windows Authors: Hernández García, David; Garin Escriva, Moises; Todorov Trifonov, Trifon; Rodríguez Martínez, Ángel; Alcubilla González, Ramón 2013-02-12T18:01:52Z http://hdl.handle.net/2117/17606 Title: Evidence of intrinsic ambipolar charge transport in a high band gap organic semiconductor Authors: Moreno Sierra, César; Pfattner, Raphael; Mas Torrent, Marta; Puigdollers i González, Joaquim; Bromley, Stephan; Rovira, Concepcio; Veciana, Jaume; Alcubilla González, Ramón Abstract: Theoretical and experimental investigations combining in situ Kelvin probe microscopy (KPM) and macroscopic electrical studies are employed to explore the intrinsic transport in dithiophene-tetrathiafulvalene (DT-TTF) single crystal organic field-effect transistors. Our work demonstrates that ambipolar behavior is not restricted only to materials possessing a high electron affinity and thus may be a more general phenomenon. 2013-02-08T11:54:37Z http://hdl.handle.net/2117/17605 Title: Colloidal crystals by electrospraying polystyrene nanofluids Authors: Coll Valenti, Arnau; Bermejo Broto, Sandra; Hernández, David; Castañer Muñoz, Luis María 2013-02-08T11:30:11Z http://hdl.handle.net/2117/16705 Title: Optical absorption of radio frequency sputtered GaAs(Ti) films Authors: Boronat, A; Silvestre Bergés, Santiago; Fuertes Marrón, D.; Castañer Muñoz, Luis María; Martí, A.; Luque, Antonio Abstract: Composition and optical absorption of thin films of GaAs(Ti) and GaAs, deposited by sputtering on glass substrates under different process conditions, have been investigated. The thin films obtained are typically 200 nm thick. ToF–SIMS measurements show a quite constant concentration and good uniformity of Ti profiles along the GaAs(Ti) layers in all cases and EPMA results indicate that Ti content increases with the substrate temperature in the sputtering process. Measurements of the transmittance and reflectance spectra of the GaAs and GaAs(Ti) thin films have been carried out. In the optical characterization of the films it is found that optical absorption is enhanced in all samples containing Ti. The determination of the optical gap from the optical absorption, shows optical gap variations from 1.15 to 1.29 eV in the GaAs thin films, and from 0.83 to 1.13 eV in the GaAs(Ti) thin films. The differences in absorption and EgTAUC observed between samples of GaAs and GaAs(Ti) are consistent with the presence of an intermediate band. 2012-10-10T12:52:41Z http://hdl.handle.net/2117/16137 Title: Dielectric charge control in electrostatic MEMS positioners/varactors Authors: Blokhina, Elena; Gorreta, Sergi; Lopez, David; Molinero Giles, David; Feely, Orla; Pons Nin, Joan; Domínguez Pumar, Manuel Abstract: A new dynamical closed-loop method is proposed to control dielectric charging in capacitive microelectromechanical systems (MEMS) positioners/varactors for enhanced reliability and robustness. Instead of adjusting the magnitude of the control voltage to compensate the drift caused by the dielectric charge, the method uses a feedback loop to maintain it at a desired level: the device capacitance is periodically sampled, and bipolar pulses of constant magnitude are applied. Specific models describing the dynamics of charge and a control map are introduced. Validation of the proposed method is accomplished both through discrete-time simulations and with experiments using MEMS devices that suffer from dielectric charging. 2012-06-25T16:24:07Z http://hdl.handle.net/2117/16135 Title: Laser processing of Al2O3/a-SiCx:H stacks: a feasible solution for the rear surface of high-efficiency p-type c-Si solar cells Authors: Martín García, Isidro; Ortega Villasclaras, Pablo Rafael; Colina, Monica; Orpella García, Alberto; López, Gema; Alcubilla González, Ramón Abstract: We explore the potential of laser processing aluminium oxide (Al2O3)/amorphous silicon carbide (a-SiCx:H) stacks to be used at the rear surface of p-type crystalline silicon (c-Si) solar cells. For this stack, excellent quality surface passivation is measured with effective surface recombination velocities as low as 2 cm/s. By means of an infrared laser, the dielectric film is locally opened. Simultaneously, part of the aluminium in the Al2O3 film is introduced into the c-Si, creating p+ regions that allow ohmic contacts with low-surface recombination velocities. At optimum pitch, high-efficiency solar cells are achievable for substrates of 0.5–2.5 Ω cm. 2012-06-25T14:53:26Z http://hdl.handle.net/2117/16134 Title: Accurate and wide field of view MEMS-based sun sensor for industrial applications Authors: Delgado, Francisco; Quero, J.M.; Garcia Ortega, Juan; López Tarrida, Cristina; Ortega Villasclaras, Pablo Rafael; Bermejo Broto, Sandra Abstract: This paper describes the design, fabrication, simulation, and experimental results of an improved miniaturized two-axis sun sensor for industrial applications, created by adapting a technology used previously in satellite applications. The sensor for each axis is composed of six photodiodes integrated in a crystalline silicon substrate, and a layer of cover glass, which is used to protect the silicon and to hold the windows. The high precision is obtained by the subdivision of the field of view (FOV), which is ±60º, with a resolution of 0.1º. Each region is controlled by an independent high precision solar sensor. The sensor can be used for sun tracking applications in photovoltaic system, heliostat concentration plants and lighting applications. 2012-06-25T14:44:19Z http://hdl.handle.net/2117/15677 Title: Simulation of organic inverter Authors: Papadopoulos, N. P.; Marsal, Albert; Picos, Rodrigo; Puigdollers i González, Joaquim; Hatzopoulos, A. A. Abstract: Pentacene and N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27) have been used as organic semiconductors for the fabrication of p-type and n-type organic thin-film transistors (OTFTs). Both types of semiconductors are well-established and demonstrate good performance in single devices, but few competitive results have been reported in complementary circuits. In this manuscript, we show the fabrication, electrical characterization and simulation of an organic complementary inverter using pentacene and PTCDI-C13 as active semiconductors. Simulation was done using a model with physical aspects. We report good fitting of p-type and n-type parameters for the OTFT model and good results for DC transfer characteristics of the organic complementary inverter. The fitting of the parameters of the OTFT model is performed using an optimized parameter extraction technique which is using fuzzy logic to adjust the parameters to its optimal value. 2012-03-27T17:39:26Z http://hdl.handle.net/2117/14665 Title: Evidence of intrinsic ambipolar charge transport in a high band gap organic semiconductor Authors: Moreno Sierra, César; Pfattner, Raphael; Mas Torrent, Marta; Puigdollers i González, Joaquim; Bromley, Stephan; Rovira, Concepció; Alcubilla González, Ramón; Veciana, Jaume Abstract: Theoretical and experimental investigations combining in situ Kelvin probe microscopy (KPM) and macroscopic electrical studies are employed to explore the intrinsic transport in dithiophene-tetrathiafulvalene (DT-TTF) single crystal organic field-effect transistors. Our work demonstrates that ambipolar behavior is not restricted only to materials possessing a high electron affinity and thus may be a more general phenomenon. 2012-01-18T19:32:53Z http://hdl.handle.net/2117/14608 Title: Comparison between the density-of-states of picene transistors measured in air and under vacuum Authors: Voz Sánchez, Cristóbal; Marsal, Albert; Moreno Sierra, César; Puigdollers i González, Joaquim; Alcubilla González, Ramón Abstract: Picene has recently attracted much attention as the active layer in organic thin-film transistors because of its good performance in air. In this work, we have fabricated picene thin-film transistors that exhibit field-effect mobilities up to 1.3 cm2 V−1 s−1 and on/off ratios above 105 in ambient conditions. These devices have been electrically characterized over the temperature range 300–360 K in air and also under vacuum conditions. In particular, the thermal activation energy of the channel conductance as a function of the gate bias has been measured. The dependence of the activation energy on the gate bias corresponds to a gradual shift of the Fermi level towards the HOMO level as more gap states are filled by trapped holes. The density-of-states can be estimated from the derivative of the activation energy with respect to gate bias. The calculated density-of-states is compared for devices measured in air and under vacuum conditions. These results can help to understand the gas sensing capability of picene, together with its enhanced electrical performance after air exposure. 2012-01-17T17:41:02Z http://hdl.handle.net/2117/14475 Title: Células fotovoltaicas con un rendimiento del 20.5% Authors: Alcubilla González, Ramón; García Molina, Francisco Miguel; Ortega Villasclaras, Pablo Rafael; López Rodríguez, Gema; Orpella García, Alberto; Martín García, Isidro; Colina, M.; Voz Sánchez, Cristóbal; Bermejo Broto, Sandra; Puigdollers i González, Joaquim Abstract: Un grupo de investigadores de la Universitat Politècnica de Catalunya (UPC) han desarrollado un proceso de fabricación que permite obtener rendimientos de conversión fotovoltaicos del 20,5%. Esto supone una mejora sustancial con respecto a los rendimientos habituales en células comerciales que sitúa en un 15%, y la cuota más elevada a la que se ha llegado hasta ahora en España. En estos momentos trabajan en mejoras que las hagan accesibles industrialmente. El avance, tras años de investigación, consiste en minimizar los diferentes mecanismos de pérdidas que influyen en el cómputo de la eficiencia de conversión fotovoltaica. 2012-01-11T16:08:34Z http://hdl.handle.net/2117/14362 Title: Laser fired contacts applied to the rear surface of heterojunction silicon solar cells Authors: Martín García, Isidro; Labrune, M.; Salomon, A.; Roca i Cabarrocas, Pere; Alcubilla González, Ramón Abstract: In this work, we fabricate heterojunction silicon solar cells on p-type substrates whose rear surface configuration is based on dielectric passivation and laser fired contacts (LFC cells). This is an alternative to boron-doped amorphous silicon film, with which we also fabricate solar cells for direct comparison (HJ cells). As substrates, 3.5 and 0.8 Ω cm p-type double-side polished FZ c-Si wafers are used. Regarding surface passivation for highly doped substrates, LFC configuration has some advantage due to the higher difficulty in creating an efficient amorphous back surface field. Additionally, those substrates are also more advantageous in terms of carrier injection when the rear surface is locally contacted. Thus LFC cells made on 0.8 Ω cm substrates reach Voc values up to 680 mV, in the same range as that of their HJ cell counterpart, with better FF demonstrating that LFC configuration is a feasible alternative for highly doped substrates. Focusing on the impact of the distance between rear contacts on cell performance, we found a trade-off between open circuit voltage Voc and fill factor FF. Finally electroluminescence characterization and the dependence of Voc on pitch, modeled by Fischer's equation, indicate that the depassivated area due to the laser processing of the contacts is bigger than the contacted area. 2011-12-29T16:04:03Z http://hdl.handle.net/2117/14202 Title: Light harvesting photovoltaic mini-generator Authors: Bermejo Broto, Sandra; Silvestre Bergés, Santiago; Ortega Villasclaras, Pablo Rafael; Herrera, Gerard Abstract: In this work, the design, fabrication and characterisation of a millimetre-size photovoltaic (PV) energy source is described. The fabrication process is based on p-type silicon-on-insulator wafers. It is shown that scaling up the number of cells from 9 to 169 makes the open-circuit voltage to increase from 3.6 to 101.5V with generated power densities ranging from 2.07 to 6.7mW/cm2 under 100mW/cm2 standard AM 1.5 Global Spectrum. A prototype consisting of a PV mini-module connected to a mini-battery has been assembled, modelled and haracterised. The results show that the mini-PV source can be used in low power applications. 2011-12-09T18:25:30Z http://hdl.handle.net/2117/14201 Title: Laser-fired contact optimization in c-Si solar cells Authors: Alcubilla González, Ramón; Ortega Villasclaras, Pablo Rafael; Orpella García, Alberto; Martín García, Isidro; Colina, M.; López Rodríguez, Gema; Voz Sánchez, Cristóbal; Sánchez, Isabel; Molpeceres, Carlos Abstract: In this work we study the optimization of laser-fired contact (LFC) processing parameters, namely laser power and number of pulses, based on the electrical resistance measurement of an aluminum single LFC point. LFC process has been made through four passivation layers that are typically used in c-Si and mc-Si solar cell fabrication: thermally grown silicon oxide (SiO2), deposited phosphorus-doped amorphous silicon carbide (a-SiCx/H(n)), aluminum oxide (Al2O3) and silicon nitride (SiNx/H) films. Values for the LFC resistance normalized by the laser spot area in the range of 0.65–3 mΩ cm2 have been obtained. 2011-12-09T18:11:54Z http://hdl.handle.net/2117/14074 Title: Modeling and simulation of a grid connected PV system based on the evaluation of main PV module parameters Authors: Chouder, Aissa; Silvestre Bergés, Santiago; Sadaoui, Nawel; Rahmani, Lazhar Abstract: In this work we present a new method for the modeling and simulation study of a photovoltaic grid connected system and its experimental validation. This method has been applied in the simulation of a grid connected PV system with a rated power of 3.2 Kwp, composed by a photovoltaic generator and a single phase grid connected inverter. First, a PV module, forming part of the whole PV array is modeled by a single diode lumped circuit and main parameters of the PV module are evaluated. Results obtained for the PV module characteristics have been validated experimentally by carrying out outdoorI–Vcharacteristic measurements. To take into account the power conversion efficiency, the measured AC output power against DC input power is fitted to a second order efficiency model to derive its specific parameters.The simulation results have been performed through Matlab/Simulink environment. Results has shown good agreement with experimental data, whether for theI–Vcharacteristics or for the whole operating system. The significant error indicators are reported in order to show the effectiveness of the simulation model to predict energy generation for such PV system. 2011-11-24T18:58:07Z