Probing Phase Transitions and Stability of Organic Semiconductor Single Crystals by Dielectric Investigation (original) (raw)
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Crystal-to-crystal phase transition in α-quaterthiophene: An optical and structural study
Journal of Applied Physics, 2007
At 191°C the organic semiconductor ␣-quaterthiophene is demonstrated to undergo a phase transition from the low temperature to the high temperature polymorph. Calorimetry, x-ray diffraction, optical reflectivity, and ellipsometry measurements were employed to analyze this transition and to show that this can be thermally induced on single crystals preserving the single crystalline nature of the sample over large domains.
Optical characterization of organic semiconducting single crystals
Over the last decade interest in the possibility of "plastic electronics," semi-conducting organic materials that hold the potential for display devices with improved characteristics, has increased. To date, the majority of research has focused on material development and device design. However, if scientists are able to understand the connection between the structural and electronic properties of molecules, they can tailor new compounds with desired physical characteristics. Optical characterization is the first step to understanding the structural properties of materials. This work focuses on characterizing single crystals of two specific molecules: α-hexathiophene and 5,6,11,12-tetraphenyl tetracene (rubrene). While there is plenty of information currently available in the literature about α-hexathiophene, the resonant Raman experiments I describe in Chapter 3 have uncovered the lowest experimentally-recorded excited electronic levels. I have tentatively identified thes...
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.
Nanoscale Research Letters
A combination of in situ electrical and grazing-incidence X-ray diffraction (GIXD) is a powerful tool for studies of correlations between the microstructure and charge transport in thin organic films. The information provided by such experimental approach can help optimizing the performance of the films as active layers of organic electronic devices. In this work, such combination of techniques was used to investigate the phase transitions in vacuum-deposited thin films of a common organic semiconductor dihexyl-quarterthiophene (DH4T). A transition from the initial highly crystalline phase to a mesophase was detected upon heating, while only a partial backward transition was observed upon cooling to room temperature. In situ electrical conductivity measurements revealed the impact of both transitions on charge transport. This is partly accounted for by the fact that the initial crystalline phase is characterized by inclination of molecules in the plane perpendicular to the π-π stacking direction, whereas the mesophase is built of molecules tilted in the direction of π-π stacking. Importantly, in addition to the two phases of DH4T characteristic of the bulk, a third interfacial substrate-stabilized monolayer-type phase was observed. The existence of such interfacial structure can have important implications for the charge mobility, being especially favorable for lateral two-dimensional charge transport in the organic field-effect transistors geometry.
The Journal of Physical Chemistry C, 2017
The influence of annealing temperature on the structural and electrical properties of conjugated poly(dodecyl-quaterthiophene) (PQT-12) polymer films is exploited. The temperature induced changes of structural parameters are monitored by in situ grazing incident x-ray diffraction (GIXD) and the conductivity. They are complemented by studies of the dielectric properties using variable angle spectroscopic ellipsometry (VASE). An increase of the scattered intensity, the size of the crystalline domains and the current response is observed for a first thermal cycle with step-wise heating up to 90 °C, which revealed two polymorphs with different degrees of interdigitation in PQT-12. Irreversible changes are observed for the second cycle with a higher thermal budget up to 140 °C and are connected with a transition from the highly ordered to powder-like disordered phase for the main PQT-12 form whereas the second polymorph with stronger interdigitation completely vanished. In agreement with these observations hightemperature VASE studies demonstrated a blueshift of the transitions with a reduction in the conjugation length caused by an increase in the twist and torsion of the backbone. Combined GIXD, VASE and electrical characterizations show that PQT-12 exhibit a complex interplay between two polymorphs with a strong influence on the charge carrier transport depending on the thermal budget employed.
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.
2010
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.
Microscopic calculation of dielectric tensor of quaterthiophene crystals
Physical Review B, 2005
We present a microscopic calculation of the dielectric tensor for a quaterthiophene crystal near resonances of Frenkel excitons in a vibronic coupling regime. We show how the intermolecular interactions modify the intensity and energy of each vibronic level from the isolated molecule to the solid state after transverse excitation. In particular, a strong redistribution of oscillator strength is found between the vibronic levels of the upper Davydov state. On the basis of the calculated dielectric tensor, we reproduce the measured absorbance spectra as recorded in different experimental configurations taking into consideration the additional role of the longitudinal macroscopic field. Some replicas in the optical spectra are clearly attributable to transitions of Frenkel origin while the role of other types of excitons, such as those of charge-transfer parentage is also discussed.
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.