Growth Of Organic Semiconductors Research Articles

Wafer‐Scale, Highly Uniform Surface Functionalization from Vapor Phase and Applications to Organic Transistors

AbstractThe surface functionalization by self‐assembled monolayers (SAMs) favors well‐packed organic semiconductor growth and reduces interfacial traps, which assists in developing high‐performance organic thin‐film transistors (OTFTs). Herein, the conventional SAM growth from the vapor phase is ameliorated and systematically studied. With 1H,1H,2H,2H‐Perfluorodecyltrichlorosilane as an example, it is found that deposition temperature of no less than 120 °C and deposition pressure of up to 0.02 bar is preferred for SAM deposition without morphological defects. The optimized SAMs are ultrasmooth with a surface roughness of 0.09 nm and can be escalated to wafer scale. It is verified that the growth condition is universal for other trichlorosilane species. Finally, it is shown that the OTFTs with defect‐free SAMs can achieve an average mobility of 1.79 cm2 V−1 s−1 using dinaphtho(2,3‐b:2′,3′‐f)thieno(3,2‐b)thiophene as the active layer, which is 2.06 times to the devices with defective SAMs and paces up the large‐area and high‐performance organic electronics.

Disordered Phase-Assisted Growth of Organic Semiconductor Crystals on Self-Assembled Monolayer Templates.

Achieving high mobility and bias stability is a challenging obstacle in the advancement of organic thin-film transistors (OTFTs). To this end, the fabrication of high-quality organic semiconductor (OSC) thin films is critical for OTFTs. Self-assembled monolayers (SAMs) have been used as growth templates for high-crystalline OSC thin films. Despite significant research progress in the growth of OSC on SAMs, a detailed understanding of the growth mechanism of the OSC thin films on a SAM template is lacking, which has limited its use. In this study, the effects of the structure (thickness and molecular packing) of SAM on the nucleation and growth behavior of the OSC thin films were investigated. We found that disordered SAM molecules assisted in the surface diffusion of the OSC molecules and resulted in a small nucleation density and large grain size of the OSC thin films. Moreover, a thick SAM with disordered SAM molecules on the top was found to be beneficial for the high mobility and bias stability of the OTFTs.

Enhanced performance of room temperature ammonia sensors using morphology-controlled organic field-effect transistors

Developing electronic sensors for ammonia (NH3) is very useful for environmental monitoring and diagnostic purposes. In this work, a highly sensitive, organic field-effect transistor (OFET) based, room temperature sensor for NH3 has been fabricated using dinaphtho [2,3-b:2′,3′-f]thieno [3,2-b]thiophene (DNTT), which showed a fast response to low concentration of the analyte down to 100 ppb. A thin film of solution-processed polymethyl methacrylate (PMMA) has been used as the gate dielectric material and its hydrophobic surface promoted structured growth of organic semiconductor, DNTT, by inducing mass transfer. By controlling the thickness and thereby exploiting the growth dynamics of the semiconductor film, the sensor performance was improved. The sensitivity of the device towards 1 ppm of NH3 was almost doubled with a thinner and porous film of DNTT as compared to that with a thick film. Morphological studies of the sensing layers, using atomic force microscopy (AFM), have established this structure-property relation. The variations in different transistor parameters have been studied with respect to different analyte concentrations. The p-channel devices in the enhancement mode showed depletion upon exposure to NH3. The devices exhibited a fast response and good recovery to the initial state within 2 min.

Crystallization Control of N,N'-Dioctyl Perylene Diimide by Amphiphilic Block Copolymers Containing poly(3-Hexylthiophene) and Polyethylene Glycol.

The preparation of micron- to nanometer-sized functional materials with well-defined shapes and packing is a key process to their applications. There are many ways to control the crystal growth of organic semiconductors. Adding polymer additives has been proven a robust strategy to optimize semiconductor crystal structure and the corresponding optoelectronic properties. We have found that poly(3-hexylthiophene) (P3HT) can effectively regulate the crystallization behavior of N,N′-dioctyl perylene diimide (C8PDI). In this study, we combined P3HT and polyethylene glycol (PEG) to amphiphilic block copolymers and studied the crystallization modification effect of these block copolymers. It is found that the crystallization modification effect of the block copolymers is retained and gradually enhanced with P3HT content. The length of C8PDI crystals were well controlled from 2 to 0.4 μm, and the width from 210 to 35 nm. On the other hand, due to the water solubility of PEG block, crystalline PEG-b-P3HT/C8PDI micelles in water were successfully prepared, and this water phase colloid could be stable for more than 2 weeks, which provides a new way to prepare pollution-free aqueous organic semiconductor inks for printing electronic 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.

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.

Comparison of semiconductor growth and charge transport on hydrophobic polymer dielectrics of organic field-effect transistors: Cytop vs. polystyrene

Abstract Organic field-effect transistors (OFETs) have been fabricated by deposition of an organic semiconductor on inorganic dielectric layers like SiO2. However, inorganic oxide dielectrics have hydrophilic surface properties, which interrupts the growth of organic semiconductors with highly aligned and π-extended domains, and provide many absorbing sites for moisture that work as trap sites for charge carriers. To overcome this problem, a proper surface-treatment method is required to improve the morphology of organic semiconductors. In this study, we treat SiO2 dielectrics with polystyrene (PS) and Cytop (a fluorinated cross-linkable polymer) to tune the surface properties when fabricating pentacene-based bottom-gate top-contact OFETs, and analyzed the influence of surface functionalization on the growth of the pentacene crystals under different deposition conditions and electrical device characteristics. By comparing the morphological features of pentacene with the device characteristics, the effect of hydrophobicity on the morphology of the pentacene polymorph, strongly related to the charge-transport mechanism, could be described.

The Direct Solution‐Process Crystallization of π‐Conjugated Small Molecules In‐Situ Integrated Planar Electrodes

Organic electronic devices have seen a new enthusiasm in flexible electronics. Indeed, they know new applications and improved performances even fabricated at low temperatures by low‐cost solution‐process and printing technologies. Nonetheless, some critical aspects remain to be improved, such as organic semiconductor (OSC) patterning and the control and alignment of organic crystal growth in the desired location and direction. Herein, a new controlled OSC solution‐growth processing, which directly occurs within planar electrodes patterning, is reported. A capillary‐driven flow occurs in air‐opened microchannels and then confines the OSC solution before growing these crystals in the desired region and direction. 1) The modeling, 2) fabrication, 3) experimental functionality of such capillary‐driven flow techniques, and 4) growth of OSC single‐colored crystals in situ integrated planar device electrodes are reported. Polarized optical images show large crystal growth (i.e., >100 μm) of typical π‐conjugated small molecules (diF‐TES‐ADT and C8‐BTBT). By such a technology, benefits from both 1) the meniscus‐guided coating mechanisms and 2) advantages of maskless and noncontact digital printing technologies are taken. Finally, these results visualize the directional control of OSC single‐crystal growth in a printing‐compatible (dispensing, inkjet) route in a reliable and reproducible way.

Analysis of Initial Stage in Thin Film Growth of Organic Semiconductor Using Quartz Crystal Microbalance

Analysis of Initial Stage in Thin Film Growth of Organic Semiconductor Using Quartz Crystal Microbalance

Gas Blow Coating: A Deposition Technique To Control the Crystal Morphology in Thin Films of Organic Semiconductors.

Rapid, large-scale, and low-cost coating methods that enable precise control of the crystal growth of organic semiconductors are essential to deliver high-performance devices that are robust and reproducible. In this work, a novel method is presented based on a gas blow coating technique, enabling the deposition of thin films of organic semiconductors, whose morphology can be optimized by adjusting the deposition parameters. We demonstrate the deposition of aligned single crystals of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) by gas blow coating and their use as active layers in organic field-effect transistor (OFET) devices. The OFETs of TIPS-pentacene and C8-BTBT have charge mobilities of 0.15 and 1.4 cm2 V–1 s–1, respectively, with low threshold voltages and on/off ratios exceeding 105. This coating method can also be extended to polymeric semiconductors: films based on poly(3-hexylthiophene) and poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] are realized, establishing gas blow coating as a novel and efficient technique for the deposition of thin films of organic semiconductors.

Highly-ordered Triptycene Modifier Layer Based on Blade Coating for Ultraflexible Organic Transistors

We present a highly ordered surface modification layer for polymers based on ambient solution-processed triptycene (Trip) derivatives for high-mobility organic thin-film transistors (OTFTs). The nested packing of Trip molecules results in the formation of 2D hexagonal arrays, which stack one-dimensionally on the surface of polymer dielectrics without anchoring groups. The Trip surface was previously shown to be preferable for the growth of organic semiconductors (OSCs), and hence for enhancing the mobility of OTFTs. However, although the Trip modifier layer has been realized by thermal evaporation in a high-vacuum environment (TVE), it still has grain-boundary disorders that hinder the optimal growth of OSCs. To fabricate OTFTs with higher mobility, a disorder-free Trip layer is needed. We developed highly ordered Trip layers on polymer dielectrics via blade coating. In addition, we clarified that the highly ordered Trip modifier layer enhances the mobility of the OTFTs by more than 40%, relative to the disordered Trip layer prepared by TVE. Finally, we realized a ring oscillator composed of OTFTs with a highly ordered Trip layer.

High-k polymer materials containing cyclic carbonate as gate dielectrics for application in low-voltage operating organic thin-film transistors

Low-voltage operating OTFTs have attracted interest as core components for low power applications and electronics. An ideal insulation material as the OTFT gate dielectric layer should possess high-k, good insulation property, mechanical flexibility and inherent nature suitable for growth of organic semiconductors.

33‐1: Invited Paper: Exploring the Formation and Growth of Organic Semiconductors with mm‐Scale Grains

While record mobilities have been reported for organic semiconductors in their single crystal form, the bulk nature of such crystals prohibit their practical application in devices. Here, we discuss our efforts to realize pinhole free films of organic semiconductors with grains of up to 1 mm in extent. One such material is rubrene, but we will also show our efforts on other materials applicable to organic light emitting diodes, such as the electron transport layer 2,2′,2″‐(1,3,5‐benzinetriyl)‐tris(1‐phenyl‐1‐H‐benzimidazole) (TPBi). For rubrene, we will show our efforts to understand crystal formation, epitaxy, and transistors. Homoepitaxial studies uncover evidence of point and line defect formation in these films, indicating that homoepitaxy is not at equilibrium or strain‐free. Point defects that are resolved as screw dislocations can be eliminated under closer‐to‐equilibrium conditions, whereas we are not able to eliminate the formation of line defects. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result strain built into the thin film template, indicating that, in general, organic crystalline thin films may not adopt the exact lattice of a bulk crystal. Transistors made out of these large‐grained films of rubrene display charge carrier mobility of up to 3.5 cm2 V−1s−1, very close to single crystal values, highlighting their potential for practical application.

新たな有機エレクトロニクスの展開 : フレキシブル・プリンタブル・エレクトロニクス(FPE)の幕開け( 応用を視野においた有機半導体の結晶成長)

新たな有機エレクトロニクスの展開 : フレキシブル・プリンタブル・エレクトロニクス(FPE)の幕開け( 応用を視野においた有機半導体の結晶成長)

有機結晶成長技術を利用した新たな有機電界発光(EL)デバイスの研究( 応用を視野においた有機半導体の結晶成長)

有機結晶成長技術を利用した新たな有機電界発光(EL)デバイスの研究( 応用を視野においた有機半導体の結晶成長)

Interface-controlled growth of organic semiconductors on graphene

We have studied submonolayer coverages of N,N-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2) on mechanically exfoliated graphene transferred onto SiO2 substrates. Our atomic force microscopy (AFM) data show that PDIF-CN2 forms irregularly-shaped 1.4 nm-high islands. From the selected area diffraction performed with transmission electron microscope (TEM) we conclude that this height corresponds to π−π stacks of molecules, which are inclined for 43° relative to the graphene surface. AFM also showed complete absence of PDIF-CN2 on single-layer graphene (SLG). Electric force microscopy revealed a marked difference in surface charge density between a single-layer graphene and bilayer graphene, with a higher surface charge on SLG than on the bilayer graphene. We associate this behavior with p−type doping of graphene due to the electrostatic dipole induced by the molecular water layer present at the graphene/SiO2 interface. The crucial role of the graphene/SiO2 interface in determining growth of PDIF-CN2 was further confirmed by TEM examination of PDIF-CN2 deposited onto unsupported SLG.

Growth of Highly Oriented Ultrathin Crystalline Organic Microstripes: Effect of Alkyl Chain Length.

The growth of organic semiconductor with controllable morphology is a crucial issue for achieving high-performance devices. Here we present the systematic study of the effect of the alkyl chain attached to the functional entity on controlling the growth of oriented microcrystals by dip-coating. Alkylated DTBDT-based molecules with variable chain lengths from n-butyl to n-dodecyl formed into one-dimensional micro- or nanostripe crystals at different pulling speeds. The alignment and ordering are significantly varied with alkyl chain length, as is the transistor performance. Highly uniform oriented and higher-molecular-order crystalline stripes with improved field-effect mobility can be achieved with an alkyl-chain length of around 6. We attribute this effect to the alkyl-chain-length-dependent packing, solubility, and self-assembly behavior.

Patterned Growth of Organic Semiconductors: Selective Nucleation of Perylene on Self-Assembled Monolayers.

Organic semiconductors (OSC) have received a large amount of attention because they afford the fabrication of flexible electronic devices. However, the limited resistance to radiation and etching of such materials does not permit their patterning by photolithography, which has been a driving force for the development of integrated circuits and therefore requires alternative structuring techniques. One approach is based on precoating the substrate with self-assembled monolayers (SAMs) to control the nucleation of subsequently deposited OSC layers, but the underlying mechanism is barely understood. Here, we used alkanethiols with different chemical terminations to prepare SAMs on gold substrates serving as model systems to identify the mechanism of selective nucleation for the case of the OSC perylene. Using atomic force microscopy and fluorescence microscopy, we demonstrate that the chemical functionalization of the SAMs determines the adhesion forces for the OSC that are smallest for CF3-terminated and largest for OH-terminated SAMs, hence yielding distinctly different sticking probabilities upon perylene deposition at room temperature. Microcontact printing and immersion were employed to prepare SAM patterns that enable the selective growth of polycrystalline perylene films. A quite different situation is found upon printing long-chain thiols with low vapor pressure, which leads to the transfer of multilayers and favors the growth of perylene single crystallites. In a more abstract scenario, patterns of silicone oil droplets were printed on a gold substrate, which was previously covered with a repelling fluorinated SAM. Such droplets provide nucleation centers for liquid-mediated growth, often yielding platelet-shaped perylene single crystallites without unwanted perylene nucleation on the remaining surface.

One-pot surface modification of poly(ethylene-alt-maleic anhydride) gate insulators for low-voltage DNTT thin-film transistors

This study investigates the one-pot surface modification of poly(ethylene-alt-maleic anhydride) (PEMA) gate insulators crosslinked with 1,5-naphthalenediamine (1,5-NDA) for enhancing the device performance of low-voltage dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) organic thin-film transistors (OTFTs). Surface properties of the PEMA gate insulator could be easily modified by adding poly(maleic anhydride-alt-1-octadecene) (PMAO) to the coating solution. The surface energy of the gate insulator is strongly correlated with the growth of organic semiconductors and the charge carrier transport at the interface between the semiconductor and gate insulator. The results indicate that the device performance of low-voltage DNTT OTFTs can be improved by one-pot surface modification of the PEMA gate insulator.

(Invited) Surface-Mediated Molecular Assembly of Organic Semiconductors for High-Performance Organic Electronics

Microstructure in organic semiconductor thin films has been regarded as the key factor determining the performance of the organic electronics. In case of bottom-contact organic field effect transistors (OFETs) and organic photovoltaics (OPVs), the control of the surface characteristics of the underlying substrates can govern the mesoscale and/or nanoscale ordering of the semiconductor assembled on them. Here, we present various approaches for controlling the growth of organic semiconductors on the gate dielectrics and electrodes to achieve high performance OFETs and OPVs.