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Journal ArticleDOI

Study of fluorescence quenching due to 2, 3, 5, 6-tetrafluoro-7, 7′, 8, 8′-tetracyano quinodimethane and its solid state diffusion analysis using photoluminescence spectroscopy

TL;DR: The quenching mechanism was found to be of static type, which was inferred by the independent nature of excited state life time from the F4-TCNQ thickness, and PL intensity was found for the first time to increase with the increase in this thickness.
Abstract: In this work, we have studied the fluorescence quenching and solid state diffusion of 2, 3, 5, 6-tetrafluoro-7, 7', 8, 8'-tetracyano quinodimethane (F4-TCNQ) using photoluminescence (PL) spectroscopy. Quenching studies were performed with tris (8-hydroxyquinolinato) aluminum (Alq3) in solid state samples. Thickness of F4-TCNQ was varied in order to realize different concentrations and study the effect of concentration. PL intensity has reduced with the increase in F4-TCNQ thicknesses. Stern-Volmer and bimolecular quenching constants were evaluated to be 13.8 M(-1) and 8.7 × 10(8) M(-1) s(-1), respectively. The quenching mechanism was found to be of static type, which was inferred by the independent nature of excited state life time from the F4-TCNQ thickness. Further, solid state diffusion of F4-TCNQ was studied by placing a spacing layer of α-NPD between F4-TCNQ and Alq3, and its thickness was varied to probe the diffusion length. PL intensity was found to increase with the increase in this thickness. Quenching efficiency was evaluated as a function of distance between F4-TCNQ and Alq3. These studies were performed for the samples having 1, 2.5, and 5.5 nm thicknesses of F4-TCNQ to study the thickness dependence of diffusion length. Diffusion lengths were evaluated to be 12.5, 15, and 20 nm for 1, 2.5, and 5.5 nm thicknesses of F4-TCNQ. These diffusion lengths were found to be very close to that of determined by secondary ion mass spectroscopy technique.
Citations
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Journal ArticleDOI
TL;DR: Recent breakthroughs in molecular doping of organic semiconductors suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.
Abstract: The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

363 citations

Journal ArticleDOI
TL;DR: In this article, optical, electrical, and morphological properties of P3HT films doped with F4TCNQ, both from mixed solutions and using sequential solution processing with orthogonal solvents.
Abstract: Doping polymeric semiconductors often drastically reduces the solubility of the polymer, leading to difficulties in processing doped films. Here, we compare optical, electrical, and morphological properties of P3HT films doped with F4TCNQ, both from mixed solutions and using sequential solution processing with orthogonal solvents. We demonstrate that sequential doping occurs rapidly (<1 s), and that the film doping level can be precisely controlled by varying the concentration of the doping solution. Furthermore, the choice of sequential doping solvent controls whether dopant anions are included or excluded from polymer crystallites. Atomic force microscopy (AFM) reveals that sequential doping produces significantly more uniform films on the nanoscale than the mixed-solution method. In addition, we show that mixed-solution doping induces the formation of aggregates even at low doping levels, resulting in drastic changes to film morphology. Sequentially coated films show 3–15 times higher conductivities at a given doping level than solution-doped films, with sequentially doped films processed to exclude dopant anions from polymer crystallites showing the highest conductivities. We propose a mechanism for doping induced aggregation in which the shift of the polymer HOMO level upon aggregation couples ionization and solvation energies. To show that the methodology is widely applicable, we demonstrate that several different polymer:dopant systems can be prepared by sequential doping.

239 citations

Journal ArticleDOI
TL;DR: In this article, the authors used quasi-elastic neutron scattering (QENS) and fluorescence quenching techniques to develop a comprehensive picture of both the microscopic and macroscopic dynamics of the soluble p-type molecular dopant tetrafluoromethyloxycarbonyltricyanoquinodimethane (F4MCTCNQ) in the conductive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT).
Abstract: Understanding the nature of dopant dynamics in the solid state is critical for improving the longevity and stability of organic electronic devices and for optimizing the doping-induced solubility control (DISC) patterning method. In this work, we use quasi-elastic neutron scattering (QENS) and fluorescence quenching techniques to develop a comprehensive picture of both the microscopic and macroscopic dynamics of the soluble p-type molecular dopant tetrafluoromethyloxycarbonyltricyanoquinodimethane (F4MCTCNQ) in the conductive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT). Specifically, fast dynamics (ps–ns) of the dopant, such as the methyl and the methoxycarbonyl group rotations, are observed in QENS experiments. From confocal fluorescence microscope experiments, longer-range/slower dopant diffusion (ms–days) is captured. However, in order to fit these data, it is necessary to incorporate a Langmuir isotherm equilibrium between the neutral and ionized dopant molecules. Ionized F4MCTCNQ is strongly favor...

45 citations

Journal ArticleDOI
TL;DR: In this paper, the diffusion of a bulky sequentially deposited p-dopant in poly(3-hexylthiophene) (P3HT) thin films using non-destructive in situ infrared (IR) spectroscopy and photoelectron spectrography (PES) was investigated.
Abstract: The diffusivity of dopants in semiconducting polymers is of high interest as it enables methods of sequential doping but also affects device stability. In this study, we investigate the diffusion of a bulky sequentially deposited p-dopant in poly(3-hexylthiophene) (P3HT) thin films using nondestructive in situ infrared (IR) spectroscopy and photoelectron spectroscopy (PES). We probe dopant diffusion into the polymer film at varying coverage by differentially evaluating electron transfer in the bulk and at the surface. Thereby it is possible to determine dopant coverages at which both electron transfer and incorporation of dopants are saturated. By use of PES, neutral and charged dopants can be distinguished, revealing that charged dopants are less mobile in the diffusion process than neutral molecules. We further compare the diffusivity in semicrystalline and fully amorphous P3HT. We find that at high coverage semicrystalline P3HT seems to yield a higher capacity for dopants than fully amorphous P3HT. A t...

32 citations

Journal ArticleDOI
TL;DR: The results emphasize the importance of dynamic processes under operating conditions that must be considered even for single doped layers, and a novel memory device is presented to illustrate the applicability of this effect.
Abstract: Stable electrical doping of organic semiconductors is fundamental for the functionality of high performance devices. It is known that dopants can be subjected to strong diffusion in certain organic semiconductors. This work studies the impact of operating conditions on thin films of the polymer poly(3-hexylthiophene) (P3HT) and the small molecule Spiro-MeOTAD, doped with two differently sized p-type dopants. The negatively charged dopants can drift upon application of an electric field in thin films of doped P3HT over surprisingly large distances. This drift is not observed in the small molecule Spiro-MeOTAD. Upon the dopants' directional movement in P3HT, a dedoped region forms at the negatively biased electrode, increasing the overall resistance of the thin film. In addition to electrical measurements, optical microscopy, spatially resolved infrared spectroscopy, and scanning Kelvin probe microscopy are used to investigate the drift of dopants. Dopant mobilities of 10-9 to 10-8 cm2 V-1 s-1 are estimated. This drift over several micrometers is reversible and can be controlled. Furthermore, this study presents a novel memory device to illustrate the applicability of this effect. The results emphasize the importance of dynamic processes under operating conditions that must be considered even for single doped layers.

25 citations

References
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Journal ArticleDOI
TL;DR: In this article, a double-layer structure of organic thin films was prepared by vapor deposition, and efficient injection of holes and electrons was provided from an indium-tinoxide anode and an alloyed Mg:Ag cathode.
Abstract: A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V.

13,185 citations

Journal ArticleDOI
TL;DR: In this paper, a multilayer-doped EL was constructed using a hole-transport layer and a luminescent layer, and the electron-hole recombination and emission zones can be confined to about 50 A near the hole.
Abstract: Electroluminescent (EL)devices are constructed using multilayer organic thin films. The basic structure consists of a hole‐transport layer and a luminescent layer. The hole‐transport layer is an amorphous diamine film in which the only mobile carrier is the hole. The luminescent layer consists of a host material, 8‐hydroxyquinoline aluminum (Alq), which predominantly transports electrons. High radiance has been achieved at an operating voltage of less than 10 V. By doping the Alq layer with highly fluorescent molecules, the EL efficiency has been improved by about a factor of 2 in comparison with the undoped cell. Representative dopants are coumarins and DCMs. The ELquantum efficiency of the doped system is about 2.5%, photon/electron. The EL colors can be readily tuned from the blue‐green to orange‐red by a suitable choice of dopants as well as by changing the concentration of the dopant. In the doped system the electron‐hole recombination and emission zones can be confined to about 50 A near the hole‐transport interface. In the undoped Alq, the EL emission zone is considerably larger due to excitondiffusion. The multilayerdopedEL structure offers a simple means for the direct determination of excitondiffusion length.

3,009 citations

Journal ArticleDOI
TL;DR: In this article, a brief overview of organic electroluminescence and electrophosphorescence is provided, and a more detailed consideration of ways in which electron transport in these systems has been enhanced by the incorporation of electron-deficient small molecules and polymers into the devices, either as blends or by covalent attachment of sub-units to the luminophore or as an additional electron-transporting, hole-blocking (ETHB) layer adjacent to the cathode.
Abstract: One of the requirements for efficient organic electroluminescent devices (OLEDs) is balanced charge injection from the two electrodes and efficient transport of both holes and electrons within the luminescent layer in the device structure. Many of the common luminescent conjugated polymers, e.g. derivatives of poly(phenylenevinylene) and poly(fluorene), are predominantly hole transporters (i.e. p-dopable). This article gives a brief overview of organic electroluminescence and electrophosphorescence and provides a more detailed consideration of ways in which electron transport in these systems has been enhanced by the incorporation of electron-deficient (i.e. n-dopable) small molecules and polymers into the devices, either as blends or by covalent attachment of sub-units to the luminophore or as an additional electron-transporting, hole-blocking (ETHB) layer adjacent to the cathode. The chemical structures of these systems are presented and their roles are assessed. Most of these ETHB molecules are electron-deficient aromatic nitrogen-containing heterocycles, e.g. derivatives of 1,3,4-oxadiazole, pyridine, pyrimidine, pyrazine, quinoline, etc. Non-aromatic thiophene-S,S-dioxide derivatives are also discussed. The article is written from an organic chemist's perspective.

599 citations

Journal ArticleDOI
TL;DR: In this article, a study on p-doping of organic wide band gap materials with Molybdenum trioxide using current transport measurements, ultraviolet photo-electron spectroscopy and inverse photo electrophoresis was presented.

422 citations