Weierstraß-Institut für Angewandte Analysis und Stochastik Self-heating, bistability, and thermal switching in organic semiconductors (original) (raw)
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Self-Heating, Bistability, and Thermal Switching in Organic Semiconductors
Physical Review Letters, 2013
We demonstrate electric bistability induced by the positive feedback of self-heating onto the thermally activated conductivity in a two-terminal device based on the organic semiconductor C 60. The central undoped layer with a thickness of 200 nm is embedded between thinner n-doped layers adjacent to the contacts minimizing injection barriers. The observed currentvoltage characteristics follow the general theory for thermistors described by an Arrhenius-like conductivity law. Our findings including hysteresis phenomena are of general relevance for the entire material class since most organic semiconductors can be described by a thermally activated conductivity.
Self-heating effects in organic semiconductor crossbar structures with small active area
Organic Electronics, 2012
We studied the influence of heating effects in an organic device containing a layer sequence of n-doped / intrinsic / n-doped C 60 between crossbar metal electrodes. A strong positive feedback between current and temperature occurs at high current densities beyond 100 A/cm 2 , as predicted by the extended Gaussian disorder model (EGDM) applicable to organic semiconductors. These devices give a perfect setting for studying the heat transport at high power densities because C 60 can withstand temperatures above 200 • C. Infrared images of the device and detailed numerical simulations of the heat transport demonstrate that the electrical circuit produces a superposition of a homogeneous power dissipation in the active volume and strong heat sources localized at the contact edges. Hence, close to the contact edges, the current density is significantly enhanced with respect to the central region of the device, demonstrating that three-dimensional effects have a strong impact on a device with seemingly one-dimensional transport.
52.1: Analysis of Self‐Heating and Negative Capacitance in Organic Semiconductors Devices
SID Symposium Digest of Technical Papers, 2015
A numerical model for charge transport in organic semiconductor devices that accounts for self‐heating is presented. In admittance spectroscopy this model reproduces the negative capacitance in bipolar, and more importantly, in single carrier devices. We show that self‐heating is crucial not only in large‐area OLEDs, but also in small‐area devices.
Analysis of self-heating and trapping in organic semiconductor devices
Organic Light Emitting Materials and Devices XIX, 2015
So far self-heating has only been of concern in large-area devices where the resistive transparent anode leads to a potential drop over the device resulting in inhomogeneous current, brightness and temperature distributions. In this work, we show that even small lab devices suffer from self-heating effects originating from the organic semiconductor layer. In admittance spectroscopy of organic semiconductor devices, negative capacitance values often arise at low frequency and high voltages. In this study we demonstrate the influence of self-heating on organic semiconductor devices with the aid of a numerical 1D drift-diffusion model that is extended by Joule heating and heat conduction. Furthermore the impact of trap states on the capacitance in combination with self-heating is demonstrated. The typical signature of self-heating might be overshadowed depending on the trapping dynamics. In a next step, we compare the negative capacitance vs. frequency for uni-and bipolar devices to quantify the different processes. We emphasize the impact of self-heating and trapping on OLEDs and organic solar cells. To ease the interpretation of the results we investigate simulations in the time domain as well as in the frequency domain. We have provided clear evidence of self-heating of organic semiconductor devices and conclude that a comprehensive model requires the inclusion of heat conduction and heat generation in the drift-diffusion model.
Nature Communications, 2014
The crystalline structure of organic materials dictates their physical properties, but while significant research effort is geared towards understanding structure-property relationships in such materials, the details remain unclear. Many organic crystals exhibit transitions in their electrical properties as a function of temperature. One example is the 1:1 chargetransfer complex trans-stilbene-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. Here we show that the mobility and resistivity of this material undergo a transition from being thermally activated at temperatures above 235 K to being temperature independent at low temperatures. On the basis of our experimental and theoretical results, we attribute this behaviour to the presence of a glass-like transition and the accompanied freezing-in of orientational disorder of the stilbene molecule.
Proceedings of the National Academy of Sciences, 2017
Significance The operation of organic field-effect transistors is governed by the processes taking place at the device interfaces. The mismatch in the coefficients of thermal expansion of the consecutive layers can induce inhomogeneous strain in the organic semiconductor layer and reduce performance by increasing the electronic trap density. We show that a high-quality organic semiconductor layer is necessary, but not sufficient, to obtain efficient charge-carrier transport, and we propose a device design strategy that allows us to achieve the intrinsic performance limits of a given organic semiconductor regardless of the relative thermal expansions of the constituent layers.
Organic Semiconductors for Thermoelectric Applications
Journal of Electronic Materials, 2010
The thermoelectric performance of thin films fabricated from two commercially available, highly conductive polymer formulations based on poly (3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated. In order to enhance the electrical conductivity, the high-boiling solvent dimethyl sulfoxide (DMSO) was added. By changing the content of DMSO the electrical conductivity was increased by a factor of two without changing the Seebeck coefficient or the thermal conductivity. We achieved ZT = 9.2 9 10 À3 at room temperature upon the addition of 5 vol.% DMSO to the PEDOT:PSS formulation.
Organic Materials for Electronic and Thermoelectric Applications
B&H Electrical Engineering
In this invited review article, we give a comprehensive account of the existing literature on the electronic properties of organic materials. The main focus of this article is the rich and extensive literature on the electronic transport in organic materials, particularly conjugated polymers, as they offer numerous advantages over inorganic materials. Consequently, they have found widespread application in photovoltaics, light-emitting displays, and even, more recently, in thermoelectric energy conversion. This literature review will be useful to researchers starting in the field of organic electronics as well as experts seeking to broaden their understanding of transport in polymers.
A Robust, High-Temperature Organic Semiconductor
The Journal of Physical Chemistry C, 2014
We introduce a new pentacene-based organic semiconductor, 5,6,7-trithiapentacene-13-one (TTPO). TTPO is a small molecule organic semiconductor that is simple to synthesize and purify, readily crystallizes, melts in air from 386-°C without decomposition, and is indefinitely stable against degradation in acid-free solution. TTPO has a high molar absorptivity, optical and electrochemical HOMO-LUMO gaps of 1.90 and 1.71 eV, respectively, and can be thermally evaporated to produce highly uniform thin films. Its cyclic voltammogram reveals one reversible oxidation and two reversible reductions between +1.5 and-1.5 V. The crystal structure for TTPO has been solved and its unique parallel displaced, head to tail packing arrangement has been examined and explained using high-level density functional theory. Highresolution scanning tunneling microscopy (STM) was used to image individual TTPO molecules upon assembly on a pristine Au(111) surface in ultrahigh vacuum. STM images reveal that vapor-deposited TTPO molecules nucleate in a unique stacked geometry with a small acute angle with respect to Au(111) surface. Preliminary TTPO-based bi-layer photovoltaic devices shows increases in short circuit current density upon heating from 25 to 80 °C with a concomitant 4-fold to 160-fold increase in power conversion efficiencies. TTPO has the potential to be used in thinfilm electronic devices that require operation over a wide range of temperatures such as thin-film transistors, sensors, switches and solar cells.
Thermoelectric properties improvement in quasi-one-dimensional organic crystals
Journal of Applied Physics, 2019
The charge and energy transport in highly conducting quasi-one-dimensional organic crystals of p-type tetrathiotetracene-iodide, TTT 2 I 3 , and of n-type tetrathiotetracene-tetracyanoquinodimethane, TTT(TCNQ) 2 , is studied. Two electron-phonon interactions are considered simultaneously. One interaction is of the acoustic deformation potential type and the other one is of polaronic character. Charge transport along the conducting molecular chains is bandlike, whereas in the transversal directions, it is of the hopping type. It is shown that due to a partial compensation of these interactions for a narrow interval of states in the one-dimensional conduction band, the relaxation time is of Lorentzian shape and shows a distinct dependence on carrier energy with a pronounced maximum. The scattering of charge carriers on adjacent molecular chains and by impurities and structural defects limits the height of this maximum. However, rather high relaxation times might be anticipated in the case of perfect single crystals. As the carriers in these states show an enhanced mobility, this will lead to a simultaneous increase of electrical conductivity and Seebeck coefficient. It is proposed that, if the above-mentioned crystals could be accomplished by means of sufficient purification and by optimization of the carrier concentration, so that the Fermi level is close to energetic states for which the relaxation time has a maximum, one might achieve values for the thermoelectric figure of merit ZT 5 in crystals of tetrathiotetracene-iodide and ZT 1:5 in those of tetrathiotetracene-tetracyanoquinodimethane.