12 research outputs found
Degradation through Directional Self-Doping and Homogeneous Density of Recombination Centers Hindered by 1,8-Diiodooctane Additive in Non-Fullerene Organic Solar Cells
Non-fullerene-based organic solar cells (OSCs) have recently proven to perform
with efficiencies above 18%. This is an important milestone for one of the most
promising technologies in the fields of flexible and transparent/semitransparent
photovoltaics. However, the stability of OSCs is still a challenging issue to meet the
industry requirements. Herein, several devices with IT-4F:PM6 as the active layer
with and without 1,8-Diiodooctane (DIO) additive are characterized before and after
a 1400 h degradation test under 1 sun white light-emitting diode (LED) illumination
intensity. The optoelectronic study via impedance spectroscopy under illumination
at quasi-open-circuit correlates the use of DIO as an additive with a retarded
degradation behavior and an overall improved device performance. In dark conditions,
theMott–Schottky analysis suggests that samples without DIO develop selfdoping
during degradation, changing the p-i-n doping profile into a p–n type, most
likely related to the evolution of the blend demixing. These mechanisms are further
confirmed by drift-diffusion simulations. Space-oriented redistribution of shallow
trap levels (self-doping) and homogeneous increase in deep-trap levels (nonradiative
recombination) are shown to be hindered by the use of the DIO additive
Compact modeling of organic thin film transistors with solution processed octadecyl substituted tetrabenzotriazaporphyrin as an active layer
Using 70nm thick spin-coated film of newly synthesized octadecyl substituted copper tetrabenzotriazaporphyrin (10CuTBTAP) as an active layer on a highly doped silicon (110) gate electrode substrates, output characteristics and transfer characteristics of bottom-gate bottom-contact organic thin film transistors have been measured at room temperature. A compact model for thin film transistors has been employed as a part of circuit design tool to extract device parameters such as the charge carrier mobility μ, the threshold voltage VT and the contact resistances. Parallel measurements and analysis were performed on similarly constructed devices with a copper phthalocyanine analogue (10CuPc). The results reveal that the 10CuPc layer is relatively more susceptible to trapping degradation near the gate region than a 10CuTBTAP layer, which is significant in order to achieve stability in these transistors. The application of the simple square law in the classical MOS model provides a quicker but approximate interpretation of the transistor performance without providing information on the gate voltage dependence of mobility and the effects of the contact regions. In this comparative study, the analysis of the contact regions is found to be very important for determining the difference in the performance of two transistors
Ionic Dipolar Switching Hinders Charge Collection in Perovskite Solar Cells with Normal and Inverted Hysteresis
Drift-diffusion modeling of the ionic dipole switching from the measurement of fast
scanned and long pre-biased electrical response is proposed as a novel protocol for
evaluation of limit hysteretic effects in perovskite solar cells. Up to eight systems were
measured including CH3NH3PbI3, Cs0.1FA0.74MA0.13PbI2.48Br0.39 and
FA0.83MA0.17Pb1.1Br0.22I2.98 3D perovskite absorbers, as well as 2D capping layers
towards the selective contacts. We show systematic hysteretic patterns, even among
typical hysteresis-free devices, including normal and inverted hysteresis as general
dissimilar trend between CH3NH3PbI3 and mixed perovskite cells, respectively.
Particularly, strong changes in the short-circuit current density ( Jsc ) were identified, in
addition to different trends affecting the fill factor (FF) and the open-circuit voltage (Voc
). The changes in Jsc were analyzed with state-of-the-art numerical drift-diffusion
simulations concluding in an important reduction in the charge collection due to ionic
distribution switching depending on the pre-biasing protocol and the type of absorbing
perovskite. It is shown that mixed perovskites inhibit ionic dipolar switching. In
addition, our calculi signal on the required conditions for the occurrence of inverted
hysteresis and changes in the Voc . Regarding the FF and Voc patterns a new empirical
approach is introduced and corresponding interpretations are proposed
Time trends in leisure time physical activity and physical fitness in elderly people: 20 year follow-up of the Spanish population national health survey (1987-2006)
<p>Abstract</p> <p>Background</p> <p>To estimate trends in leisure time physical activity and physical fitness between 1987-2006 in older Spanish people.</p> <p>Methods</p> <p>We analyzed data collected from the Spanish National Health Surveys conducted in 1987 (n = 29,647), 1993 (n = 20,707), 1995-1997 (n = 12,800), 2001 (n = 21,058), 2003 (n = 21,650), and 2006 (n = 29,478). The number of subjects aged ≥ 65 years included in the current study was 29,263 (1987: n = 4,958-16.7%; 1993: n = 3,751-17.8%; 1995-97: n = 2,229-17.4%; 2001: n = 4,356-20.7%; 2003: 6,134-28.3%; 2006: 7,835-26.5%). Main variables included leisure-time physical activity and physical fitness. We analyzed socio-demographic characteristics, self-rated health status, lifestyle habit and co-morbid conditions using multivariate logistic regression models.</p> <p>Results</p> <p>Women exhibited lower prevalence of leisure time physical activity and physical fitness compared to men (P < 0.05). The multivariate analysis for time trends found that practising leisure time physical activity increased from 1987 to 2006 (P < 0.001). Variables associated with a lower likelihood of practicing leisure time physical activity were: age ≥ 80 years old, ≥ 2 co-morbid chronic conditions, and obesity. Variables associated with lower physical fitness included: age ≥ 80 years, worse self rated health; ≥ 2 medications (only for walking), and obesity.</p> <p>Conclusions</p> <p>We found an increase in leisure time physical activity in the older Spanish population. Older age, married status, co-morbid conditions, obesity, and worse self-perceived health status were associated with lower activity. Identification of these factors can help to identify individuals at risk for physical inactivity.</p
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Modelado compacto de mecanismos físicos en células solares orgánicas
Organic solar cells (OSCs) are promising devices in the field of solar energy. Their many advantages
are intrinsic of the organic/polymeric technology, such as light weight, flexibility and low manufacturing
costs. However, the degradation of OSCs hinders a predictable and a stable performance. In order to
improve the device performance, accurate physics-based models, including boundary conditions, are
needed. In this thesis, a model that relates the charge carrier density at the metal-organic boundaries
with the current density in OSCs is considered for simulation and modeling purposes. The model is
proposed after initial studies on single-carrier and bipolar organic diodes in darkness.
The work begins with the proposal of a model for the current-voltage characteristics of organic and
polymeric single-carrier diodes. The model unifies two different mechanisms in the structure, the injection
and transport of charge, and includes a proper boundary condition for the free charge density at the
metal-organic interface and a temperature and electric-field dependent mobility model. The results of
the model highlight the importance of the boundary condition at the interface, which is used to explain
different trends and their transitions in experimental current–voltage characteristics: linear, quadratic
and a higher than quadratic trend at high electric fields.
Una de las regiones más sensibles de una célula solar orgánica es la zona de contacto entre el electrodo metálico y el material orgánico. Por un lado, los contactos controlan el flujo de la corriente. Por otro lado, la región del contacto es altamente sensible a la degradación. La formación de una capa aislante cerca a la interfaz metal-orgánico o el decrecimiento de la velocidad de recombinación en el contacto son efectos desfavorables que reducen la eficiencia de las células solares [3, 4]. Para optimizar el rendimiento de estos dispositivos es requisito indispensable una detallada descripción de los mecanismos físicos que tienen lugar en la estructura metal-orgánico. El modelado y la simulación de estas estructuras son herramientas muy adecuadas para conseguir este objetivo.Tesis Univ. Granada. Programa Oficial de Doctorado en: Física y Ciencias del EspacioThe research was carried out within the framework of a scholarship supported financially by Ministerio
de Educación y Ciencia under research Grant FPU12/02712 and MINECO under research Project
TEC2013-47283-R
Physical aspects of ferroelectric semiconductors for photovoltaic solar energy conversion
López Varo, Pilar et al.Solar energy conversion using semiconductors to fabricate photovoltaic devices relies on efficient light absorption, charge separation of electron-hole pair carriers or excitons, and fast transport and charge extraction to counter recombination processes. Ferroelectric materials are able to host a permanent electrical polarization which provides control over electrical field distribution in bulk and interfacial regions. In this review, we provide a critical overview of the physical principles and mechanisms of solar energy conversion using ferroelectric semiconductors and contact layers, as well as the main achievements reported so far. In a ferroelectric semiconductor film with ideal contacts, the polarization charge would be totally screened by the metal layers and no charge collection field would exist. However, real materials show a depolarization field, smooth termination of polarization, and interfacial energy barriers that do provide the control of interface and bulk electric field by switchable spontaneous polarization. We explore different phenomena as the polarization-modulated Schottky-like barriers at metal/ferroelectric interfaces, depolarization fields, vacancy migration, and the switchable rectifying behavior of ferroelectric thin films. Using a basic physical model of a solar cell, our analysis provides a general picture of the influence of ferroelectric effects on the actual power conversion efficiency of the solar cell device, and we are able to assess whether these effects or their combinations are beneficial or counterproductive. We describe in detail the bulk photovoltaic effect and the contact layers that modify the built-in field and the charge injection and separation in bulk heterojunction organic cells as well as in photocatalytic and water splitting devices. We also review the dominant families of ferroelectric materials that have been most extensively investigated and have provided the best photovoltaic performance.The research leading to these results has received funding from the European Union Seventh Framework Program [FP7/2007–2013] under grant agreement 316494 and from MINECO of Spain under project TEC2013-47283-R and Grant FPU12/02712. F.R. acknowledges funding and partial salary support for work on multiferroic materials from NSERC, through an EWR Steacie Memorial Fellowship.Peer reviewe
Compact modeling of the effects of illumination on the contact region of organic phototransistors
Please cite this article as:
A. Romero, C. Jiménez, J. González, P. López-Varo, M.J. Deen, J.A. Jiménez-Tejada, Compact modeling of the effects of illumination on the contact region of organic phototransistors, Organic Electronics, (2019), 70, 113-121.A good modeling of degrading effects in an electronic device, such as the contact region of organic phototransistors (OPTs), can be favorably used to better describe and optimize the performance of the whole device. Furthermore, a proper design of the contacts can enhance the exciton dissociation and the extraction of photogenerated charge in the device. In this work, a compact model for OPTs is developed. This model is valid for all the operation regimes of the transistors. It includes a model for the contact region of the device that incorporates the effects of illumination. The compact model and the contact region model are validated with published experimental data from several OPTs under different illumination conditions. The tool used to validate the model is an evolutionary parameter extraction procedure developed in a previous work. The results show that both photoconductive and photovoltaic effects impact the intrinsic region of the transistor, as well as the electrical behavior of the contact region. The parameters used in the contact region model are linked to these photovoltaic and photoconductive effects.This work was supported by projects MAT2016-76892-C3-3-R and TIN2015-67020-P funded by the Spanish Government and European Regional Development Funds (ERDF) and by “Beca de Iniciación a la Investigación para estudiantes de Grado” given by the University of Granada
Ionic field screening in MAPbBr3 crystals revealed from remnant sensitivity in X-ray detection
International audienceResearch on metal halide perovskites as absorbers for X-ray detection is an attractive subject due to the optimal optoelectronic properties of these materials for high-sensitivity applications. However, the contact degradation and the long-term instability of the current limit the performance of the devices, in close causality with the dual electronic-ionic conductivity of these perovskites. Herein, millimeter-thick methylammonium-lead bromide (MAPbBr) single and polycrystalline samples are approached by characterizing their long-term dark current and photocurrent under X-ray incidence. It is shown how both the dark current and the sensitivity of the detectors follow similar trends at short-circuit (V = 0 V) after biasing. By performing drift-diffusion numerical simulations, it is revealed how large ionic-related built-in fields not only produce relaxations to equilibrium lasting up to tens of hours but also continue to affect the charge kinetics under homogeneous low photogeneration rates. Furthermore, a method is suggested for estimating the ionic mobility and concentration by analyzing the initial current at short-circuit and the characteristic diffusion times