Examinando por Autor "Romero, Beatriz"

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Analysing impact of oxygen and water exposure on roll-coated organic solar cell performance using impedance spectroscopy
(Elsevier, 2018-12-28) Arredondo, Belén; Romero, Beatriz; Beliatis, MJ; del Pozo, Gonzalo; Martin-Martin, Diego; Blakesley, JC; Dibb, G; Krebs, FC; Gevorgyan, SA; Castro, Fernando
In this work we study the degradation of roll-coated flexible inverted organic solar cells in different atmospheres. We demonstrate that impedance spectroscopy is a powerful tool for elucidating degradation mechanisms; it is used here to distinguish the different degradation mechanisms due to water and oxygen. Identical cells were exposed to different accelerated degradation environments using water only, oxygen only, and both water and oxygen simultaneously, all of them enhanced with UV light. The photocurrent is dramatically reduced in the oxygen-degraded samples. Impedance measurements indicate that this phenomenon is attributed to defects introduced by absorption of oxygen, which results in an increase of the acceptor impurity (NA) at the cathode interface obtained from a Mott-Schottky analysis. Simultaneously, at the anode interface where PEDOT:PSS is not shielded by the substrate, the nature of degradation differs for the water and oxygen degraded samples. While oxygen + UV light decreases the conductivity of the PEDOT:PSS layer, water + UV light changes the PEDOT:PSS work function inducing a depletion region at the anode.
analytical evaluation of the ratio between injection and space charge limited currents in single carrier organic diodes
(IEEE, 2008) ALvarez, Angel Luis; Arredondo, Belén; Romero, Beatriz; Quintana, Xabier; Gutiérrez, Araceli; Mallavia, Ricardo; Otón, José Manuel
An analytical, complete framework to describe the current-voltage (I-V) characteristics of organic diodes without the use of previous approaches, such as injection or bulk-limited conduction is proposed. Analytical expressions to quantify the ratio between injection and space-charge-limited current from experimental I-V characteristics in organic diodes have been derived. These are used to propose a numerical model in which both bulk transport and injection mechanisms are considered simultaneously. This procedure leads to a significant reduction in computing time with respect to previous rigorous numerical models. In order to test the model, different diode structures based on two different polymers: poly(2-methoxy-5-{3',7'-dimethyloetyloxy}-p-phenylenevinylene) (MDMO-PPV) and a derivative of the poly(2,7-fluorene phenylidene) [PFP:(CN)(2)], have been fabricated. The present model is excellently fitted to experimental curves and yields the microscopic parameters that characterize the active layer.
Carrier Capture and Escape Processes in In Ga As-GaAs Quantum-Well Lasers
(IEEE, 1999) Romero, Beatriz; Esquivias, Ignacio; Weisser, Stephan; Larkings, Eric; Rosen, Joseph
We have extracted the ratio between the carrier capture and escape times, eta, for In0.25Gao0.75As-GaAs lasers containing one, two, or three quantum wells, from high-frequency subthreshold impedance measurements at different temperatures. Our results show that the carrier capture process dominates over the diffusion along the confinement region in the overall transport/capture process. The obtained value for eta is comparable to unity, and this fact has to be taken into account to obtain real material parameters, such as the carrier lifetime and the radiative recombination coefficient.
Carrier capture and escape times in In0.35Ga0.65As-GaAs multiquantum-well lasers determined from high-frequency electrical impedance measurements
(IEEE, 1996-10) Esquivias, Ignacio; Weisser, Stephan; Romero, Beatriz; Ralston, John
We present experimental results on the high-frequency electrical impedance of In0.35Ga0.65As-GaAs multiquantum-well lasers with varied p-doping levels in the active region, The analysis of the data, using a simple three rate equation model, provides information about the dynamical time constants (the carrier lifetime, the effective carrier capture and escape times) under the laser operation conditions, The addition of p-doping increases the carrier escape time at threshold from 0.7 ns, extracted for the undoped devices, up to a value higher than 2 ns for the p-doped lasers, The effective capture time is estimated to be between 2 and 5 ps.
Carrier Dynamics and microwave characteristics of GaAs-Based Quantum-Well Lasers
(IEEE Journal of quantum electronics, 1999-04) Esquivias, Ignacio; Weisser, Stephan; Romero, Beatriz; Ralston, John; Rosenzweig, Joseph
We investigate the effects of carrier capture and re-emission on the electrical impedance, equivalent circuit, and modulation response of quantum-well (QW) laser diodes. The electrical impedance is shown to be a sensitive function of the time constants associated with carrier capture/transport and carrier re-emission. We compare the theoretical results with measured values of the electrical impedance of high-speed InGaAs-GaAs multiple-quantum-well lasers fabricated using different epilayer structures with a common lateral structure. The experimental results agree well with the theoretical model, allowing us to extract the effective carrier escape time and the effective carrier lifetime in the QW's, and to estimate the effective carrier capture/transport time
Damping limited modulation bandwidths up to 40 GHz in undoped short-cavity InGaAs-GaAs multiple quantum well lasers
(IEEE, 1996) Weisser, stephan; Larkins, Eric; Czotscher, Konrad; Benz, W; Daleiden, J; Esquivias, Ignacio; Fleissner, J; Ralston, John; Romero, Beatriz; Sah, RE; Schönferlder, Alexander; Rosenzweig, Joseph
We demonstrate record direct modulation bandwidths from MBE-grown In0.5Ga0.65As-GaAs multiple-quantum-well lasers with undoped active regions and with the upper and lower cladding layers grown at different growth temperatures, Short-cavity ridge waveguide lasers achieve CW direct modulation bandwidths up to 40 GHz for 6 x 130 mu m(2) devices at a bias current of 155 mA, which is the damping limit for this structure, We further demonstrate large-signal digital modulation up to 20 Gb/s (limited by the measurement setup) and linewidth enhancement factors of 1.4 at the lasing wavelength at threshold of similar to 1.1 mu m for these devices.
Evolution with annealing of solar cell parameters modeling the S-shape of the current-voltage characteristic
(Elsevier Science, 2012) Pozo, Gonzalo del; Romero, Beatriz; Arredondo, B
In this work we use a modified equivalent circuit to simulate the S-shape found in current density-voltage (J-V) characteristics of organic solar cells (OSC) based on P3HT:PCBM. The modification of the traditional model includes a second diode opposite to the main one together with a parallel resistance, R-P2. The transcendental equation derived from the circuit is solved exactly without approximations. The S-shape (so called kink), usually attributed to a poor quality of polymer/cathode interface, can be removed after three slow annealing treatments. The evolution of the J-V curves with the annealing process has been quantified by extracting the equivalent circuit parameters of each curve (pristine and annealed) by fitting the model to the experimental data. Results show that kink annihilation is mostly due to a strong decrease of R-P2, which diminishes the reverse diode effect on the J-V curve.
Exact analytical solution of a two diode circuit model for organic solar cells showing S-shape using Lambert W-functions
(Exact analytical solution of a two diode circuit model for organic solar cells showing S-shape using Lambert W-functions, 2012) Romero, Beatriz; Pozo, Gonzalo del; Arredondo, Belén
Organic solar cells (OSCs) often show a kink, also called S-shape, in the current-voltage (I-V) characteristics, that has been attributed to different physical phenomena such as poor quality of cathode-active layer interface or unbalance charge carrier mobilities. This non-ideal behaviour can be electrically modelled including a second diode, in reverse bias, together with an extra shunt resistance (R-P2) in the traditional solar cell equivalent circuit. In this paper, we solve without approximations the transcendental equation system derived from this modified circuit. We have obtained an analytical expression for I-V curves decoupling the voltage drop in each diode using Lambert W function. This expression has been fitted to experimental data in order to obtain circuital parameters. Simulations varying saturation current of reverse diode (I-02) and R-P2 have been performed in order to study the dependence of S-shape with these parameters.
Extraction of circuital parameters of organic solar cells using the exact solution based on Lambert W-function
(SPIE, 2012) Pozo, Gonzalo del; Romero, Beatriz; Arredondo, Belén
he electrical behavior of organic solar cell (OSC) has been analyzed using a simple circuital model consisting on an ideal diode together with a series and parallel resistances (R-S and R-P respectively). Applying Kirchhoff's Laws to the circuit leads to a transcendental equation that can be solved numerically without approximations using the Lambert W function. Theoretical expression has been fitted to experimental current-voltage (I-V) curves under forward bias, obtaining fairly accurate values for the electrical parameters. This model has been validated comparing the extracted parameters for dark and illumination conditions of different devices. Results show good agreement for R-S, and ideality factor eta. Electrical parameters obtained in this work are also compared to those ones extracted using an approximated method often employed by other authors (1). We conclude that approximated method leads to reasonable good values for R-S, R-P and. However, in the case of Rp the voltage range chosen to fit the data with the exact method must be constrained to the fourth quadrant, where the role of parallel resistance is more critical. To validate the model, a bunch of organic solar cells with structure ITO/ poly(3,4-ethylenedioxythiophene)-poly (4-styrene sulfonate (PEDOT:PSS)/ poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM)/Al has been fabricated in inert atmosphere. Different active layers were deposited varying the P3HT: PCBM ratio (1:0.64, 1:1, 1:1.55) and the active layer thickness ( ranging from 100 to 280 nm). Devices are encapsulated inside the glove-box prior its characterization outside the glove-box. Electro optical characterization has been performed with a halogen lamp. Values extracted for RS range from 142 Omega to 273 Omega, values for R-P range from 25 k Omega to 331 k Omega. Ideality factor ranges from 5 to 17.
High stable CsFAPbIBr perovskite solar cells with dominant bulk recombination at real operating temperatures
(Royal Society of Chemistry, 2023-05-02) Romero, Beatriz; Delgado, silvia; Glowienka; Chang, Cheng-Tsung; del Pozo, Gonzalo; Arredondo, Belén; Martin-Martin, Diego; Contreras, Pedro; Galagan, Yulia
Mixed-cation mixed-halide perovskite solar cells have been characterized in DC at different temperatures (from -20 degrees C up to 50 degrees C) and the time evolution of the device efficiency has been assessed using different degradation protocols (indoors and outdoors). The completely planar p-i-n structure is ITO/CuNiOx/PTAA/CsFAPbIBr/PCBM/PEI/Ag. Pristine current-voltage characteristics barely show hysteresis, at any temperature. Open circuit voltage decreases with temperature at a rate of -1.5 mV degrees C-1, and the obtained PCE temperature coefficient is lower than -0.001% K-1, which is an outstanding value for this emerging photovoltaic technology. Cells have been degraded under different protocols: indoors using different light/dark cycles and outdoors in a high temperature and high irradiation location. Cells show no significant decrease of the efficiency after more than 350 h of indoor light cycling and the estimated T-80 obtained for the sample degraded outdoors under high irradiation and high temperature conditions is similar to 15 days.
High-bandwidth organic photodetector analyzed by impedance spectroscopy
(IEEE, 2012) Arredondo, Belén; Dios, Cristina de; Vergaz, Ricardo; Pozo, Gonzalo del; Romero, Beatriz
An organic bulk heterojunction photodetector (OPD) is fabricated on the basis of a blend of poly(3-hexyl thiophene):1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61. The OPD responsivity is calculated from current density-voltage characteristic (J-V) under green-LED illumination obtaining 1 A/W. Frequency response at different biases is measured, which shows a high ¿3-dB cutoff frequency of 343 kHz at ¿3 V. Impedance measurements are carried out at different reverse biases, and the results are fitted with a small signal equivalent circuit. The device cutoff frequency (fc) can be estimated with the parameters extracted from this circuit obtaining good agreement between measured and calculated fc.
Identification of Degradation Mechanisms in Slot-Die-Coated Nonfullerene ITO-Free Organic Solar Cells Using Different Illumination Spectra
(ACS, 2020-07-27) Arredondo, Belén; del Pozo, Gonzalo; Hernández-Balaguera, Enrique; Martin-Martin, Diego; López-Gonzalez, Maria del Carmen; Romero, Beatriz; Lopez-Fraguas, Eduardo; Vergaz, Rcardo; Quintana, Xabier; Lamminaho, Jani; Destouesse, Elodie; Ahmadpour, Mehrdad; Turkovic, Vida; Madsen, Morten
In this work, we have studied degradation mechanisms of nonfullerene-based organic solar cells with PET/Ag/ZnO/ PBDTB-T:ITIC/PEDOT:PSS/CPP PEDOT:PSS device structure. We compare pristine and degraded samples that were subjected to outdoor degradation following the standard ISOS-O2 protocol. The ideality factors for different incident wavelengths obtained from open-circuit voltage vs irradiation level and current density-voltage (J-V) measurements at different temperatures indicate that for aged samples recombination is governed by the Shockley-Read-Hall mechanism occurring in a region near the anode. Samples were also characterized using impedance spectroscopy (IS) and fitted to an electrical model. Impedance parameters were used to obtain mobility, indicating a clear degradation of the active layer blend for aged samples. The change in the chemical capacitance also reveals a worsening in carrier extraction. Finally, two-dimensional (2D) numerical simulations and fits to experimental J-V curves confirm the existence of a layer near the anode contact with poorer mobility and a decrease in the anode work function (WF) for the degraded samples.
Impedance characteristics of quantum well lasers
(IEEE, 1994) Weisser, Stephan; Esquivias, Ignacio; Tasker, PJ; Ralston, John; Romero, Beatriz; Rosenzweig, Joseph
We derive theoretical expressions for the impedance of quantum-well lasers below and above threshold based on a simple rate equation model, These electrical laser characteristics are shown to be dominated by purely electrical parameters related to carrier capture/transport and carrier re-emission, The results of on-wafer measurements of the impedance of high-speed In0.35Ga0.65As/GaAs multipie-quantum-well lasers are shown to be in good agreement with this simple model, allowing us to extract the effective carrier escape time and the effective carrier lifetime, and to estimate the effective carrier capture/transport time,
Incorporation of carrier capture and escape processes into a self-consistent cw model for Quantum Well lasers
(Elsevier, 2003) Borruel, Luis; Arias, Julia; Romero, Beatriz; Esquivias, Ignacio
We describe a one-dimensional model for the simulation of the cw properties of Quantum Well laser diodes, which incorporates self-consistently the carrier capture/escape processes into the complete semiconductor equations. The approach is applied to simulate some laser structures properties that can only be described considering carrier capture effects. We point out the importance of these processes in the performance of high power laser diodes operating under cw conditions
Influence of Electrical Operating Conditions and Active Layer Thickness
(Elsevier Editorial, 2009) Romero, Beatriz; Arredondo, Belen
We have studied the influence of the electrical working conditions (voltage or current biased), and the active layer thickness on electroluminescence (EL) properties of polymeric light emitting diodes based on poly-[9,9-bis(60-cyanohexyl)-2,7-fluorene-alt-co-1,4-phenylene], [PFP:(CN)2]. Diodes with different active layer thicknesses (55¿140 nm) have been fabricated and characterized. Temporal evolution of the spectra at constant bias and current, as well as the spectral evolution with the current, has been performed. Excitation photoluminescence has been used to discriminate between intrinsic and defectrelated transitions. The relative spectral area arising from defects has been quantified by means of Gaussian deconvolution for different device excitations. Active layer thickness has been observed to play an important role on the emissive spectral shape. In thick samples EL tends to resemble fluorescence from the pristine material. In contrast, thinner samples clearly show two additional bands related to defects: the first one is structured in the range 470¿510 nm, which is proposed to be due to electron accumulation in the active layer, and a second band at 535 nm, arising from on-chain keto defects due to the presence of oxygen. The role of the electron blocking character of the PEDOT:PSS on the spectral shape, as well as the influence of the active layer thickness on the oxygen concentration, are discussed.
Influence of electrical operating conditions and active layer thickness on electroluminescence degradation in polyfluorene-phenylene based light emitting diodes
(Elsevier, 2009) Romero, Beatriz; Arredondo, Belén; Alvarez, Angel Luis; Mallavia, Ricardo; Salinas, A; Quintana, Xabier; O, José Manuel
We have studied the influence of the electrical working conditions (voltage or current biased). and the active layer thickness on electroluminescence (EL) properties of polymeric light emitting diodes based on poly-[9,9-bis(6'-cyanohexyl)-2,7-fluorene-alt-co-1,4-phenylene], [PFP:(CN)(2)]. Diodes with different active layer thicknesses (55-140 nm) have been fabricated and characterized. Temporal evolution of the spectra at constant bias and current, as well as the spectral evolution with the current, has been performed. Excitation photoluminescence has been used to discriminate between intrinsic and defect-related transitions. The relative spectral area arising from defects has been quantified by means of Gaussian deconvolution for different device excitations. Active layer thickness has been observed to play an important role on the emissive spectral shape. In thick samples EL tends to resemble fluorescence from the pristine material. In contrast, thinner samples clearly show two additional bands related to defects: the first one is structured in the range 470-510 nm, which is proposed to be due to electron accumulation in the active layer, and a second band at 535 nm. arising from on-chain keto defects due to the presence of oxygen. The role of the electron blocking character of the PEDOT:PSS on the spectral shape, as well as the influence of the active layer thickness on the oxygen concentration, are discussed
Influence of solvent additive on the performance and aging behavior of non-fullerene organic solar cells
(Elsevier, 2022) Arredondo, Belén; Pérez-Martínez, José Carlos; Muñoz-Díaz, Laura; López-González, Maria del Carmen; Martín-Martín, Diego; Pozo, Gonzalo del; Hernández-Balaguera, Enrique; Romero, Beatriz; Lamminaho, Jani; Turkovic, Vida; Madsen, Morten
The performance of organic solar cells has improved significantly in recent years due to the use of non-fullerene acceptors (NFA). While processing additives are typically added to the active layer blends to enhance device performance in NFA organic solar cells, their impact on device degradation remains unclear. In this work we have compared the performance, in pristine and degraded state, between air-processed slot-die coated NFA ITOfree organic solar cells with and without the processing additive DIO, using a structure of PET/Ag/ZnO/PBDB-T: ITIC/FHC PEDOT:PSS. We observed an improvement in the power conversion efficiency of the devices when adding DIO, from 4.03% up to 4.97%. The evolution of the performance for both devices under ISOS-L1 life testing protocol reveals that the drop in efficiency is mainly due to a decay of JSC for both cells. In the short time scale the efficiency of non-DIO cells decays faster than the DIO cells, whereas in the long time scale the efficiency of non-DIO cells tends to stabilize sooner. Carrier mobilities estimated from impedance measurements decrease with time at similar rate for both degraded samples. Besides, DIO devices present a steep increase of the series resistance with time causing a decrease of the FF and thus of the efficiency. Moreover, in both degraded devices, the open-circuit voltage saturates with increasing illumination intensity. Numerical simulations reveal that a reduced anode work function of 5 eV is needed to fit experimental data.
On the electrical degradation and green band formation in alpha- and beta-phase poly(9,9-dioctyfluorene) polymer light-emitting diodes
(Elsevier, 2011) Arredondo, Belén; Romero, Beatriz; Gutiérrez, Araceli; Martínez, Armando; Alvarez, Angel Luis; Quintana, Xabier; Otón, José Manuel
In this work we report a detailed comparison of optical and electrical degradation between alpha- and beta-phase poly(9,9-dioctyfluorene) (PFO) based diodes. Analysis of the EL spectra along continuous operation time in alpha- and beta-PFO based diodes reveals that the unwanted green emission traditionally associated to fluorenone is more likely to occur in a-phase PFO. The relative spectral areas arising from excitonic and vibronic transitions as well as fluorenone defects have been quantified by means of Gaussian deconvolution along the operation time. The relative spectral area associated to the formation of the fluorenone increases 13% for the beta-PFO diode and up to 21% for the alpha-PFO diode only after 35 min of continuous operation. Analysis of the I-V curve before and after electrical stressing has lead to hole mobilities in pristine diodes of 1.4 x 10(-4) cm(2)/Vs and 1.6 x 10(-5) cm(2)/Vs for beta-PFO and alpha-PFO respectively. Both beta-PFO and alpha-PFO degraded samples show a reduction in the hole mobility, as well as an increase in the width of the Gaussian density of states.
Performance of ITO-free inverted organic bulk heterojunction photodetectors: Comparison with standard device architecture
(Elsevier, 2013) Arredondo, Belén; Dios, Cristina de; Vergaz, Ricardo; Romero, Beatriz; Zimmermann, Birger; Wurfel, Uli
We report on the fabrication of Indium Tin Oxide (ITO)-free inverted organic bulk heterojunction (BHJ) photodetectors of poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)propyl-1-1-phenyl-(6,6) C61 (PCBM). The final inverted device structure is Cr/Al/Cr/P3HT:PCBM/poly-3,4-ethylenedioxythiophene:poly-styrenesulfonate (PEDOT:PSS)/Ag (Zimmermann et al., 2009) [1]. The device is top-absorbing with the light entering through the hole contact grid. We have fabricated standard devices with structure ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al in order to carry out a comparison study. Inverted photodetectors show slightly higher quantum efficiency and responsivity compared to standard devices. Frequency responses at different bias voltages were measured showing a maximum -3 dB cut-off frequency of 780 kHz and 700 kHz at -3 V for the standard and inverted structures respectively. Parameters extracted from the fit of a circuital model to the impedance spectroscopy measurements were used to estimate the photodiode cut-off frequency as function of bias. (C) 2013 Elsevier B.V. All rights reserved.
Physical model for the current-voltage hysteresis and impedance of halide perovskite memristors
(ACS, 2022-03-11) Berruet, Mariana; Pérez-Martínez, Jose Carlos; Romero, Beatriz; Gonzales, Cedric; Añ-Mayouf, Abdullah; Guerrero, Antonio; Bisquert, Juan
An investigation of the kinetic behavior of MAPbI(3) memristors shows that the onset voltage to a high conducting state depends strongly on the voltage sweep rate, and the impedance spectra generate complex capacitive and inductive patterns. We develop a dynamic model to describe these features and obtain physical insight into the coupling of ionic and electronic properties that produce the resistive switching behavior. The model separates the memristive response into distinct diffusion and transition-state-formation steps that describe well the experimental current-voltage curves at different scan rates and impedance spectra. The ac impedance analysis shows that the halide perovskite memristor response contains the composition of two inductive processes that provide a huge negative capacitance associated with inverted hysteresis. The results provide a new approach to understand some typical characteristics of halide perovskite devices, such as the inductive behavior and hysteresis effects, according to the time scales of internal processes.