Organic Semiconductor Films Research Articles

New tailored organic semiconductors thin films for optoelectronic applications

In this study, we have investigated new tailored organic semiconductor materials for optoelectronic application, such as organic solar cells. The carbon-based organic semiconductor material has promising advantages in organic thin-film form. Moreover, due to its low cost, organic thin films are suitable and cheaper than inorganic thin-film. The bandgap of organic semiconductors materials can be tuned and mostly lies between 2.0 eV and 4 eV and the optical absorption edge of organic semiconductors typically lies in between 1.7 eV and 3 eV. They can be easily tailored by modifying the carbon chain and legends and looks promising for engineering the bandgap to harness the solar spectrum. In this work, with new tailored organic semiconductors, the solution route is explored which is a low-cost processing method. (Anthracen-9-yl) methylene naphthalene-1-amine; 4-(anthracen-9-ylmethyleneamino)-1,5dimethyl-2-phenyl-1H-pyrazol-3-one and N-(anthracen-9-ylmethyl)-3, 4-dimethoxyaniline thin-films are processed by spin coating method with changing concentration such as 0.05 wt.% and 0.08 wt.%. Thin films of organic semiconductors were prepared on the glass substrate and annealed at 55 °C. The structural and optical behavior of (Anthracen-9-yl) methylene naphthalene-1-amine, 4-(anthracen-9-ylmethyleneamino)-1,5dimethyl-2-phenyl-1H-pyrazol-3-one, and N-(anthracen-9-ylmethyl)-3, 4-dimethoxyaniline organic semiconductors thin films is studied by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and UV-Visible spectroscopy technique. The XRD data of the synthesized sample suggests the nano crystallinity of the organic layers. And, the SEM micrographs show the dense packing when we increase the wt.% 0.05 to 0.08. Additionally, analysis of the optical absorption measurements found that the engineered bandgap of synthesized thin films are 2.18 eV, 2.35 eV, 2.36eV, 2.52eV, and 2.65eV which suggest suitability for applications of optoelectronic devices such as solar cell. Such lightweight, eco-friendly and disposable new carbon-based materials seem to have the potential to replace other traditional hazardous heavy materials for future eco-friendly flat fast electronics.

Relation between Spherulitic Growth, Molecular Organization, and Charge Carrier Transport in Meniscus‐Guided Coated Organic Semiconducting Films

AbstractMeniscus‐guided coating (MGC) is an efficient and promising route to grow small molecule and polymer organic semiconductors (OSCs) into highly ordered and uniaxially orientated thin films for electronic applications. In this work, the impact of domain size and molecular order on the charge carrier transport in field‐effect transistors for a molecular organic semiconductor 4‐tolyl‐bithiophenyl‐diketopyrrolopyrrole (DPP(Th2Bn)2) is investigated. The spherulitic domain growth of DPP(Th2Bn)2 in thin films is controlled in the evaporative regime of zone‐casting by varying the substrate velocity. The decrease of coating velocity leads to a lower nucleation density and larger domain size of DPP(Th2Bn)2. At sufficiently low velocity, the spherulitic domains first elongate and then uniaxially grow in the coating direction. Although at the same time the molecular order decreases due to higher film thickness, the charge carrier transport improves for larger domain size and reduced density of boundaries in the transistor channel. These results provide insight on the relation between domain growth, molecular organization, and charge carrier transport in zone‐cast OSC thin films that are important for the upscaling of the technique for practical applications.

Electronic excitation spectra of organic semiconductor/ionic liquid interface by electrochemical attenuated total reflectance spectroscopy

The interface of organic semiconductor films is of particular importance with respect to various electrochemical devices such as transistors and solar cells. In this study, we developed a new spectroscopic system, namely electrochemical attenuated total reflectance ultraviolet (EC-ATR-UV) spectroscopy, which can access the interfacial area. Ionic liquid-gated organic field-effect transistors (IL-gated OFETs) were successfully fabricated on the ATR prism. Spectral changes of the organic semiconductor were then investigated in relation to the gate voltage application and IL species, and the magnitude of spectral changes was found to correlate positively with the drain current. Additionally, the Stark shifts of not only the organic semiconductor, but also of the IL on the organic semiconductor films were detected. This new method can be applied to other electrochemical devices such as organic thin film solar cells, in which the interfacial region is crucial to their functioning.

Band gap engineering in blended organic semiconductor films based on dielectric interactions.

Blending organic molecules to tune their energy levels is currently being investigated as an approach to engineer the bulk and interfacial optoelectronic properties of organic semiconductors. It has been proven that the ionization energy and electron affinity can be equally shifted in the same direction by electrostatic effects controlled by blending similar halogenated derivatives with different energetics. Here we show that the energy gap of organic semiconductors can also be tuned by blending. We use oligothiophenes with different numbers of thiophene rings as an example and investigate their structure and electronic properties. Photoelectron spectroscopy and inverse photoelectron spectroscopy show tunability of the single-particle gap, with the optical gaps showing similar, but smaller, effects. Theoretical analysis shows that this tuning is mainly caused by a change in the dielectric constant with blend ratio. Further studies will explore the practical impact of this energy-level engineering strategy for optoelectronic devices.

Solutal‐Marangoni‐Flow‐Mediated Growth of Patterned Highly Crystalline Organic Semiconductor Thin Film Via Gap‐Controlled Bar Coating

AbstractApplication‐oriented patterned growth of organic semiconductor (OSC) thin films with single crystalline domains is crucial for fabricating sophisticated high‐performance organic‐electronic and optoelectronic devices; however, fabricating these patterned nanometer‐thick crystals in a simple, fast, and effective manner is a difficult task with a roll‐to‐roll printing process. Here, a simple bar‐coating approach to form an array of single‐crystal‐like OSC thin‐film patterns at a rate of a few millimeters per second is introduced. To this end, the processing parameters of a gap‐controlled bar‐coating method is optimized, including coating speed, crystal nucleation, and solution fluidics, which allow a high degree of morphological control of bar‐coated OSC films in an area of several centimeters. In particular, it is demonstrated that the solutal‐Marangoni flow induced by a suitable solvent additive can considerably improve molecular mass transport and induce favorable vertical phase separation. Thus, organic transistors based on the OSC patterns fabricated with the additive‐assisted bar coating show a field‐effect mobility of up to 20 cm2 V−1 s−1 and superior operational stability. The proposed bar coating method will facilitate an industry‐level application of organic electronics.

Polarization Raman Imaging of Organic Monolayer Islands for Crystal Orientation Analysis.

An organic semiconductor film made of diphenyl derivative dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DPh-DNTT) has high carrier mobility. However, this mobility may be greatly affected by the crystal orientation of the DPh-DNTT’s first layer. Polarization Raman microscopy is widely used to quantitatively analyze the molecular orientation, and thus holds great potential as a powerful tool to investigate the crystal orientation of monolayer DPh-DNTT with high spatial resolution. In this study, we demonstrate polarization Raman imaging of monolayer DPh-DNTT islands for crystal orientation analysis. We found that the DPh-DNTT sample indicated a strong dependence of the Raman intensity on the incident polarization direction. Based on the polarization dependence, we developed an analytical method of determining the crystal orientation of the monolayer DPh-DNTT islands and experimentally confirmed that our technique was highly effective at imaging the islands’ crystal orientation with a spatial resolution of a few hundred nanometers.

Use of doping to achieve low contact resistance in bottom-gate top-contact type organic transistor with liquid-crystalline organic semiconductor, Ph-BTBT-10

We have investigated the characteristics of bottom-gate and top-contact type field effect transistors fabricated with polycrystalline thin films of a liquid-crystalline organic semiconductor, 2-decyl-7-phenyl-benzothienobenzothiophene (Ph-BTBT-10), with a p-type dopant, tetrafluoro-tetracyano-quinodimethane (F4-TCNQ). We found that the contact resistance between the semiconductor and electrode was reduced from 3.0 kΩ cm to 1.2 kΩ cm by contact doping with F4-TCNQ, and to 0.9 kΩcm by subsequent thermal annealing of the films, in which the F4-TCNQ dopant diffused from the surface to the interior of the Ph-BTBT-10 thin film. In addition, we found that contact-doped and thermally annealed devices showed higher mobility and smaller threshold voltage in short-channel devices compared to pristine devices. We conclude that thermal diffusion of dopants to improve FET performance is an important technique.

Optimizing the plasma oxidation of aluminum gate electrodes for ultrathin gate oxides in organic transistors

A critical requirement for the application of organic thin-film transistors (TFTs) in mobile or wearable applications is low-voltage operation, which can be achieved by employing ultrathin, high-capacitance gate dielectrics. One option is a hybrid dielectric composed of a thin film of aluminum oxide and a molecular self-assembled monolayer in which the aluminum oxide is formed by exposure of the surface of the aluminum gate electrode to a radio-frequency-generated oxygen plasma. This work investigates how the properties of such dielectrics are affected by the plasma power and the duration of the plasma exposure. For various combinations of plasma power and duration, the thickness and the capacitance of the dielectrics, the leakage-current density through the dielectrics, and the current–voltage characteristics of organic TFTs in which these dielectrics serve as the gate insulator have been evaluated. The influence of the plasma parameters on the surface properties of the dielectrics, the thin-film morphology of the vacuum-deposited organic-semiconductor films, and the resulting TFT characteristics has also been investigated.

Charge generation efficiency of electrically doped organic semiconductors

Organic semiconductors (OSCs) have been a significant focus of research for electronic devices over the years exclusively due to their superior mechanical and optical properties as opposed to conventional inorganic semiconductors (ISCs) such as silicon or germanium. These unique materials have smoothened the path for developing extremely light-weight and ultrathin electronic devices with built-in flexibility and transparency for a considerably low cost. However, the commercial application of these organic materials is limited by their inferior electrical conductivity and charge carrier mobility compared with inorganic counterparts. This review article presents an overview of the works published on how to control and adjust the electrical conductivity of OSCs, focusing on electrical doping. The main point of this review is related to the principles and fundamental mechanisms of charge generation efficiency (CGE) in OSCs, which are significantly different from those of ISCs. The reported CGE of OSCs, defined as the ratio of the generated charge carriers to the dopants, is found to be in the range of a few percent and is significantly small compared with the ISCs. The origin of this lies in the lower dissociation rate of the charge transfer complexes (CTCs) formed between the tightly bound ionized dopants and generated charges. The CTCs induce localized electron-hole pairs, and the dissociation of such CTCs into free charge carriers requires large activation energies to overcome the strong Coulombic forces exerted on the generated charges by the dopant ions. Therefore, high doping concentrations are required to generate a large number of free charge carriers in doped OSC films. In this review, the CGE of OSCs is discussed in detail from the point of view of CTC formation and charge separation efficiencies.

A Triple Axial Chirality, Racemic Molecular Semiconductor Based on Thiahelicene and Ethylenedioxythiophene for Perovskite Solar Cells: Microscopic Insights on Performance Enhancement

AbstractFor the low‐cost fabrication of large‐area, durable perovskite solar cells, it is of pivotal importance to engineer organic semiconducting films with a combined property of matched energy level, sufficiently large conductivity, high glass transition temperature, and excellent solution processability. Toward this goal, herein an in silico tailored molecular semiconductor (T5HE‐OMeTPA) with triple axial chirality, by joint use of thia[5]helicene and ethylenedioxythiophene, is reported. T5HE‐OMeTPA with a reduced reorganization energy of hole transfer can be exploited as the hole transport layer for perovskite solar cells with 21% efficiency, which also display excellent long‐term stability at 60 °C. The doped, sufficiently conductive T5HE‐OMeTPA composite with a glass transition temperature of 121 °C not only exhibits persistent film morphology under thermal stress, but also surprisingly damps the motion of diffusive components of perovskite for a better control of the degradation of photoactive layer. The translational motion of both ions and molecules is intrinsically associated with the glass transition of a doped molecular semiconductor composite, which is in stark contrast to the microscopic fashion for the glass transition of an undoped molecular semiconductor, that is, thermally activated rotation of diphenylamine.

Impact of gamma-ray irradiation on the electronic structures of PCBM and P3HT organic semiconductor films

Gamma-ray irradiation alters the material properties of organic semiconductors, especially the electronic structure. These changes due to ionization can be useful in dosimetry. Although there are some device-based reports, the effect of gamma-ray irradiation on the electronic structure of organic semiconductors is still unclear. In this study, we investigated the electronic structures of representative organic semiconductor films, namely, n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and p-type poly(3-hexylthiophene-2,5-diyl) (P3HT), after gamma-ray irradiation generated by a Cs-137 source. The X-ray and ultraviolet photoelectron spectra of the PCBM and P3HT films were measured for various gamma-ray doses. In both PCBM and P3HT, chemical interaction with atmospheric oxygen, assisted by high-energy photons, led to significant oxidation. However, the degree of oxidation of PCBM was considerably higher than that of P3HT. The oxidation also affects the valence electronic structures. The possible chemical structures of the oxidized PCBM and P3HT are estimated using density functional theory calculations.

New charge-transfer states in blends of ZnPC with F8ZnPC

With the aim of pushing the knowledge and understanding on mixed films of organic semiconductors forward, blends of ZnPC and F8ZnPC in different ratios are manufactured. The films have a polycrystalline structure, as indicated by electron diffraction profiles and infrared-spectroscopy. Photoluminescence data show completely different spectra for the blends, compared to the pure materials, which can be ascribed to the suppressing of excimer formation and the appearance of a new charge-transfer excitation between the two different molecules in the blends. This new excitation can also be seen in optical absorption. Momentum dependent measurements of the electronic excitations by electron energy-loss spectroscopy confirm the localized character of the new charge-transfer excitation in the blends. Our experimental data help understand the important issue of donor/acceptor coupling in organic semiconductors.

Theoretical Insight into Molecular Orientation for Thermally Activated Delayed Fluorescence Emitters in Vacuum Deposition

The molecular orientation of thermally activated delayed fluorescence (TADF) emitters in vapor-deposited organic semiconductor films significantly affects the light out-coupling efficiency and the ...

Investigation of Structural and Optoelectronic Properties of Organic Semiconductor Film Based on 8-Hydroxyquinoline Zinc

In this work, we investigated an organic semiconductor based on zinc 8-hydroxyquinoline (ZnQ2) and tetracyanoquinodimethane (TCNQ), which can be used as a photoactive layer in organic devices. The semiconductor was optimized by applying density-functional theory (DFT) methods, and four hydrogen bridges were formed between ZnQ2 and TCNQ. Later, thin films of ZnQ2-TCNQ were successfully deposited. The films were structurally and morphologically characterized, and the optical characteristics of the photoactive layer were investigated using ultraviolet–visible spectroscopy and time-dependent density-functional theory (TDDFT) calculations. The comparison and analysis of the experimental and theoretical absorption spectra indicate that the optical bandgap of the photoactive layer is 2.4 eV. Additionally, a flexible photo device was manufactured with the active layer ZnQ2-TCNQ, and its electrical behavior was evaluated under dark and light conditions. The results show a significant change in the behavior of the device when radiation is eliminated; the layer is light sensitive. The electrical resistance in the flexible photo device is associated with the optical behavior of the materials that constitute the active layer.

Collective orientation barrier in growth of organic thin films revealed by pMAIRS

We used $p$-polarized multiple-angle incidence resolution spectrometry (pMAIRS) to investigate a collective orientation barrier (COB) in the growth of organic semiconductor (OSC) films. We demonstrate a temperature-dependent variation in the growth of pentacene (PEN) on $\mathrm{Si}{\mathrm{O}}_{2}$ films as a model system. The molecular orientation varied from lying to standing as the growth temperature increased. This change suggests that the formation of a standing orientation is thermally activated compared with the lying state. The nucleation of standing-oriented islands occurs by molecular self-assembly at sufficiently high temperatures. Conversely, molecules deposit in a flat-lying state at low temperatures owing to hindrance of the kinetic barrier to reorientation from lying to standing states. The COB is defined as a collective energy barrier for the reorientation processes throughout the growth. The COB is quantitatively estimated from an Arrhenius plot of the probability to form a standing orientation, derived from the dichroic ratio measured by pMAIRS. We found that the COB in the growth of PEN on the $\mathrm{Si}{\mathrm{O}}_{2}$ system was approximately 0.02 eV, which is a key parameter for determining the molecular orientation. A quantitative evaluation of the COB will be applicable to other systems and enable effective control over the molecular orientation of OSC films.

Highly sensitive 2D organic field-effect transistors for the detection of lithium-ion battery electrolyte leakage.

Lithium ion batteries (LIBs) have become indispensable in daily life. Here, we fabricated a broad range of polymer semiconductor films as thin as 2 nm via a standard spin-coating method, and utilized the resulting organic transistors for the detection of LIB electrolytes, which can give an early warning signal before LIB failure.

Completely Solar-Driven Photoelectrochemical Water Splitting Using a Neat Polythiophene Film

Summary π-Conjugated polymers are emerging as appealing photoelectrode materials for the photoelectrochemical hydrogen evolution reaction via water splitting, which has otherwise been extensively explored using inorganic semiconductors. Here, we report the performance of a pure organic semiconductor film as a catalyst for hydrogen production via visible light-driven water splitting. The neat and unsubstituted polythiophene film, characterized with a well-filled grain morphology of the crystalline polymer, is prepared by a facile polymerization method. The photovoltage of 1.38 V versus RHE at pH 12 enables solar-driven one-electron-per-photon water splitting in combination with a traditional water-oxidation catalyst to produce hydrogen and oxygen separately. The high photoelectrocatalytic hydrogen evolution rate of 1.02 mol(H2) h−1 g−1 or 0.75 mA/cm2 at 0 V versus RHE is also achieved with high durability. This study may open a new window for π-conjugated polymers for ultimately sustainable hydrogen production.

Rubbing induced reversible fluorescence switching in thiophene-based organic semiconductor films by mechanical amorphisation

Rubbing is applied on thiophene-based organic semiconductor crystalline thin films to induce a reversible mechanical amorphisation. Rubbing causes fluorescence switching regulated by the polymorphic nature of the film. The process is reversible by a thermal annealing.

Фотофизические свойства тонких пленок перилена, модифицированного функциональными группами диангидрида и диимида тетракарбоновой кислоты

The morphology of organic semiconductor films of perylenetetracarboxylic acid dianhydride (PTCDA) and perylenetetracarboxylic acid dibenzyl-diimide (N, N`-DBPTCDI) formed by thermal vacuum deposition was studied by atomic force microscopy. It was shown that annealing of films at 420 K leads to rearrangement of their structure and crystallization. The optical absorption spectra of the films under study were used to estimate the optical band gap. The temperature dependence of the dark conductivity of PTCDA and N, N-DBPTCDI films before and after annealing (Т = 420 K) was established. The values of the activation energy of charge carrier traps are determined. The computer simulation of the density of localized states in the band gap of the films studied was carried out using the photoconductivity spectra in the constant photocurrent mode. Model photovoltaic cells based on PTCDA / СuPc and N, N-DBPTCDI / СuPc structures were formed. The kinetics of decay of the interfacial photo-voltage of the cells prepared was measured using pulsed light as an excitation source. On the basis of the performed measurements, the charge carrier mobility values in the investigated semiconductor materials were estimated.

Effect of the conformer distribution on the properties of amorphous organic semiconductor films for organic light-emitting diodes.

With the remarkable improvement in the electrical and optical properties of organic light-emitting diodes (OLEDs) in recent years, the details of the higher-order structure of vacuum-deposited amorphous organic films and its formation mechanism need to be understood. In particular, to clarify the effect of the higher-order structure on the film properties, it is necessary to analyze the molecular aggregation states in the vacuum-deposited amorphous films. Toward their deep understanding, the higher-order structure and film properties have often been discussed with relation to the surface diffusion and structural relaxation of the molecules immediately after deposition on the film surface. However, the effect of the variety of conformers, which is specific to amorphous organic materials, on the thermal and electrical properties of the films has not been deeply discussed. In this study, we focused on three structural isomers of OLED materials and discuss the effect of the conformer distribution on the molecular aggregation states and thermal and electrical properties of the vacuum-deposited films. From their comparison, we found that the properties of the film composed of a relatively small number of stable conformers are superior to those of the other two films composed of relatively large numbers of stable conformers. This superiority originates from formation of aggregates of the same conformer, which become the starting points for crystallization when the film is heated. Our detailed comparison and discussion focusing on the variety of conformers will lead to a deeper understanding of the molecular aggregation states and physical properties of amorphous organic films.