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Micro-organic single crystalline phototransistors of 7,7,8,8-tetracyanoquinodimethane and tetrathiafulvalene
Abstract
Classical p-type and n-type organic single crystals, tetrathiafulvalene TTF and 7,7,8,8-tetracyanoquinodimethane TCNQ , are introduced to investigate photoswitch and phototransistor. High photoresponsivity, low persistent conductivity, and response reversibility can be found in single crystalline TCNQ, while TTF has large persistent conductivity when the light is switched on and off. It is probably attributed to different band gaps and the compactness of molecular packing. Single crystalline TCNQ combines light detection, switching, signal amplification in a single device and realization of multiple functions which exhibit a very promising potential for the fabrication of organic photoelectric devices.
... Firstly, solvents used in drop-casting methods should possess suitable boiling points and volatility. 30,67,94,[300][301][302][303][304] Usually, solvents with a high boiling point facilitate a slow self-assembly process, which is helpful in forming ordered molecular arrangements. Secondly, the semiconductor should have suitable solubility in the solvent. ...
... Methods such as PVT, 101 drop-casting, 44,79,87,301,538 dipcoating 385 or ink-jet printing 431 can all be utilized to grow single crystals on the pre-deposited channels for producing devices with BG-BC mode. It should be emphasized that damage on pre-deposited layers (the insulators and the electrodes) should be avoided during the crystal growth. ...
... In this situation, the device is defined as a phototransistor, which can display an even higher on/off ratio (as high as 10 4 -10 6 ) than that of a photoswitch. 109,301,402,403,502,639,641,646 The photoresponsivity is gate dependent and increases with the increase in the gate bias. This in turn confirms that the gate voltage (the photovoltaic effect 567,636 ) can facilitate the dissociation of the photogenerated excitons. ...
Organic semiconductors have attracted a lot of attention since the discovery of highly doped conductive polymers, due to the potential application in field-effect transistors (OFETs), light-emitting diodes (OLEDs) and photovoltaic cells (OPVs). Single crystals of organic semiconductors are particularly intriguing because they are free of grain boundaries and have long-range periodic order as well as minimal traps and defects. Hence, organic semiconductor crystals provide a powerful tool for revealing the intrinsic properties, examining the structure–property relationships, demonstrating the important factors for high performance devices and uncovering fundamental physics in organic semiconductors. This review provides a comprehensive overview of the molecular packing, morphology and charge transport features of organic semiconductor crystals, the control of crystallization for achieving high quality crystals and the device physics in the three main applications. We hope that this comprehensive summary can give a clear picture of the state-of-art status and guide future work in this area.
... Although cocrystals usually exhibit properties that are different from those of their components, they do reflect the individual properties of the donor and acceptor to some degree. For instance, the three most common acceptors used in organic cocrystals are 7,7,8,8-tetracyanoquinodimethane (TCNQ), tetracyanobenzene (TCNB), and fullerene (C 60 ) [48][49][50][51][52]. TCNQ is a strong acceptor that exhibits a high n-type charge-transfer mobility of 1.6 cm 2 V -1 s -1 [53] as well as good photoresponse behavior, with an on/off ratio of 160 [48]. ...
... Although cocrystals usually exhibit properties that are different from those of their components, they do reflect the individual properties of the donor and acceptor to some degree. For instance, the three most common acceptors used in organic cocrystals are 7,7,8,8-tetracyanoquinodimethane (TCNQ), tetracyanobenzene (TCNB), and fullerene (C 60 ) [48][49][50][51][52]. TCNQ is a strong acceptor that exhibits a high n-type charge-transfer mobility of 1.6 cm 2 V -1 s -1 [53] as well as good photoresponse behavior, with an on/off ratio of 160 [48]. TCNB is a molecule that produces bright blue fluorescence [54], and C 60 is utilized to form complexes with various p-conjugated molecules [55,56]. ...
Organic cocrystals that are composed of two or more components usually exhibit novel, unpredictable, and even unique properties rather than a simple combination of the properties of their components, such as white-light emission, ambipolar charge transport, nonlinear optics, and ferroelectricity. Since cocrystal engineering represents a novel strategy for synthesizing multifunctional materials, which opens the door for molecular collaborative innovation, it has aroused much attention in recent years. However, as it is also a relatively new research field, it is only in its early stages of development. In order to provide readers with an understanding of the future design of cocrystals for potential applications, a brief review of organic cocrystals is presented here, including an introduction to organic cocrystals as well as discussions of cocrystal preparation, methods and techniques of characterization, and multifunctional applications of cocrystals. Moreover, the outlook for further studies and applications of cocrystal engineering is considered.
... Organic materials have been successfully utilized for the fabrication of high mobility organic field-effect transistors (OFET) (5), high efficiency (exceeding 4%) organic solar cells (6)(7)(8), organic/polymer light-emitting diodes (9,10), and light-emitting OFET with ambipolar transport properties (11,12). Organic phototransistors (OPT), a new addition to the family of OFETs, has been the recent research interest (13)(14)(15)(16)(17)(18)(19)(20). It is a three-terminal optoelectronic device where light can play the role as one of the external electrodes, just as gate electrode in a typical OFET for the generation of photocarriers, in addition to those induced by the gate bias. ...
... It is a three-terminal optoelectronic device where light can play the role as one of the external electrodes, just as gate electrode in a typical OFET for the generation of photocarriers, in addition to those induced by the gate bias. The other advantages of OPT device include good response time with high current gain (14)(15)(16)(17), cheap fabrication method, low processing temperatures, and 3D stacking. Additionally, using the OPT device a high-speed photocontrolled memory device can be realized from efficient trans-port of charge carriers, which can be achieved through the effective control of the gate bias amplitude and the external light intensity (15,18,19). ...
Highly stable, reproducible, photosensitive organic field-effect transistors based on an n-type organic material, copper hexadecafluorophthalocyanine, and two different polymeric gate dielectrics has been reported and their performances have been compared by evaluating the surface/interface properties. The devices produced a maximum photocurrent gain (I(light)/I(dark)) of 79 at V(G) = 7 V and showed the potentiality as multifunctional optoelectronic switching applications depending upon the external pulses. The switching time of the transistor upon irradiation of light pulse, i.e., the photoswitching time of the device, was measured to be approximately 10 ms. On the basis of optical or combination of optical and electrical pulses, the electronic/optoelectronic properties of the device can be tuned efficiently. The multifunctions achieved by the single device can ensure very promising material for high density RAM and other optoelectronic applications. Furthermore, as the device geometry in the present work is not limited to rigid substrate only, it will lead to the development of flexible organic optoelectronic switch compatible with plastic substrates.
... (d) Chemical structure of TFT-CN 2D single crystal and transfer curves of resulting OSCPTs based on micrometer-sized single crystal under different power light illumination. Reproduced with permission from Ref. [48] (a), Ref. [49] (b), Ref. [50] (c) and Ref. [51].Jiang et al.53 used typical p-type and n-type organic single crystals, TTF and TCNQ to fabricate micro-structured single crystal phototransistor. Upon switching the light on and off, TCNQ rapidly responded within 500 ms, while the response period of TTF exceeded 60 s. ...
Organic phototransistors (OPTs), compared to traditional inorganic counterparts, have attracted a great deal of interest because of their inherent flexibility, light-weight, easy and low-cost fabrication, and are considered as potential candidates for next-generation wearable electronics. Currently, significant advances have been made in OPTs with the development of new organic semiconductors and optimization of device fabrication protocols. Among various types of OPTs, small molecule organic single crystal phototransistors (OSCPTs) standout because of their exciting features, such as long exciton diffusion length and high charge carrier mobility relative to organic thinfilm phototransistors. In this review, a brief introduction to device architectures, working mechanisms and figure of merits for OPTs is presented. We then overview recent approaches employed and achievements made for the development of OSCPTs. Finally, we spotlight potential future directions to tackle the existing challenges in this field and accelerate the advancement of OSCPTs towards practical applications.
... P-Type organic semiconductors have been extensively used in organic thin film transistors applications and furthermore, high performance OTFTs can be improved by use of derivatives of pentacene, rubrene, anthracene, or thiophene [3,4,8,9]. On the other hand, organic phototransistors which are type of optical transducer in which light detection and signal amplification are one of three terminal optoelectronic devices in which light can be used as an external stimulus to create photogenerated carriers in addition to the carriers induced by the gate voltage [10][11][12][13]. Organic phototransistors (OPTs) are considered to be one of the feasible applications of OTFTs because of their large absorption properties in visible light and the excellent photo current generation [14][15][16][17]. ...
In this paper, Pentacene thin film phototransistors were fabricated and characterized. The magnitude of the photocurrent induced by illumination was found to be the result of two distinct effects: photovoltaic effect, which the photocurrent related to electron-hole pair generation, and photoconductor effect, that the current enhancement caused by a threshold voltage shift. The electrical parameters such as mobility µ, threshold voltage Vth, the Ion/Ioff ratio, the subthreshold S, the interface density of trap Dit, the total trap density Ntrap, and the photoresponse R value were determined. The effect of white light illumination where investigated.
... Specifically, OFETs with photosensitive organic semiconductor materials can be used as organic phototransistors (OPTs) [17][18][19][20][21][22][23][24][25][26][27][28], which are fundamental components in optoelectronic circuits. Moreover, they can be applied in several more complex systems and used as chemical or biological sensors. ...
Organic thin film phototransistor (OPTs) devices in bottom-gate/top-contact configuration were fabricated and used as analytic system to study the electrical and optical properties of pentacene. The channel of the OTFT devices was illuminated by laser radiation of wavelength 670 nm and the effect of irradiation on the electrical responses of the devices was investigated at different temperatures and incident optical powers. The photoresponse and the electrical parameters of the devices (mobility, threshold voltages and on/off ratios – ION/IOFF) were evaluated in order to investigate the relationship between the light sensing behavior of the phototransistors and their electrical performances. Moreover, the OPT's time-resolved electrical response to light irradiation was modelled to decouple the fast-varying photoexcitation effects from slow bias stress decays in order to investigate the reversibility properties, the time stability of electrical responses and the photocurrent.
... The TCNQ nanoparticles exhibited a strong diffraction peak at 10.9°, which corresponded to the (002) diffraction of the TCNQ crystal. [33][34][35] In the nanorods of the TTF-TCNQ CTC obtained by the TCNQ-to-TTF process, the diffractions were observed at 9.7°and 19.4°. These diffractions were assigned to (002) and (004) of the TTF-TCNQ CTCs, respectively. ...
... Jiang et al. [53] used typical p-type and n-type organic single crystals, TTF and TCNQ to fabricate microstructured single crystal phototransistor. Upon switch- ing the light on and off, TCNQ rapidly responded within 500 ms, while the response period of TTF exceeded 60 s. ...
Organic phototransistors (OPTs), compared to traditional inorganic counterparts, have attracted a great deal of interest because of their inherent flexibility, light-weight, easy and low-cost fabrication, and are considered as potential candidates for next-generation wearable electronics. Currently, significant advances have been made in OPTs with the development of new organic semiconductors and optimization of device fabrication protocols. Among various types of OPTs, small molecule organic single crystal phototransistors (OSCPTs) standout because of their exciting features, such as long exciton diffusion length and high charge carrier mobility relative to organic thinfilm phototransistors. In this review, a brief introduction to device architectures, working mechanisms and figure of merits for OPTs is presented. We then overview recent approaches employed and achievements made for the development of OSCPTs. Finally, we spotlight potential future directions to tackle the existing challenges in this field and accelerate the advancement of OSCPTs towards practical applications.
... 15 By combining two types of molecules, a binary compound could be formed in which charge exchanged between different types of molecules, resulting in optical and electrical properties that differed from those of the individual components. 16,17 The binary compound of tetrathiafulvalene-7,7,8,8-tetracyanoquinodimethane (TTF-TCNQ), 18 consisting of one TTF molecule (donor) and one TCNQ molecule (acceptor) per unit cell, exhibits metallic electrical conductivity in contrast to a single component of TTF 19 or TCNQ 20,21 crystal that exhibits low-conductivity semiconducting properties. The first organic superconductor detected below 0.9 K and under a pressure of 12 kbar was di-(tetramethyltetraselenafulvalene)-hexafluorophosphate ((TMTSF) 2 PF 6 ), in which three donors submit electrons for one acceptor. ...
Organic charge transfer compounds have been received great attention because of their tunable electronic properties ranging from insulators to superconductors. It has been demonstrated that these compounds can be applied to both organic semiconducting active materials and organic conductors by appropriate molecular design. 7,7,8,8-Tetracyanoquinodimethane (TCNQ) and FxTCNQ (x=1, 2, 4) as acceptors and aromatic hydrocarbons form a variety of compounds in which the degree of charge transfer (DCT) is adjustable. The donor, acceptor, and stoichiometry of organic charge transfer compounds are the main factors for tuning of the DCT. Tuning the DCT by crystal engineering allows control of delocalized electrons and thus the physical properties of materials in a range that is not available in one-component organic solids.
... p-Type organic semiconductors, pentacene, rubrene, anthracene, or thiophene organic semiconductors have been extensively used in OTFT applications [8][9][10][11][12][13]1,[14][15][16] and furthermore, high performance OTFTs can be improved by use of derivatives of pentacene, rubrene, anthracene, or thiophene. On the other hand, organic phototransistors (OPT) which are type of optical transducer in which light detection and signal amplification are one of three terminal optoelectronic devices in which light can be used as an external stimulus to create photogenerated carriers in addition to the carriers induced by the gate voltage [17,20,18,19,21]. Organic phototransistors (OPTs) are considered to be one of the feasible applications of OTFTs because of their large absorption properties in ultraviolet (UV) and visible light and the excellent photo current generation efficiency of organic semiconductors [22][23][24][25]. ...
The organic field-effect transistor having 2,3 benzanthracene as active layer grown on SiO2 dielectric layer was fabricated. AFM results indicate that 2,3 benzanthracene thin film is formed from homogeneous small crystal grains with an average diameter about 200nm. The electrical parameters such as mobility, threshold voltage, Ion/Ioff ratio and SS value were determined. The obtained Ion/Ioff and S values increase with visible light illumination due to more photogenerated charges. The mobility and threshold values were changed by visible light illumination. The organic thin film transistor exhibits a photovoltaic effect in turn-on state. The control of the threshold voltage of 2,3 benzanthracene-based organic thin film transistor was achieved by visible light illumination. The change in the threshold voltage is enough for the electronic technology applications of organic thin film transistors.
Significant advances have been made recently in the area of organic electronics and optoelectronics based on small molecules as a result of the synthesis of new soluble and air-stable molecules. First reported 20 years ago, organic transistors quickly became a focus of intense research and development in academic and industrial laboratories. The great progress achieved thus far offers an opportunity for the production of new small electro-active molecules and the implementation of low-cost device fabrication technologies. This review focuses on recently synthesized p- or n-type organic semiconductors, particularly those suitable for fabrication of solution-processed and/or air-stable field effect transistors with an emphasis on low-cost wet processes. The numerous recent efforts realized in optoelectronics, particularly on phototransistors based on small molecules, offer various opportunities in applications for such organic compounds. Copyright © 2011 Society of Chemical Industry
Organic single crystals are an established part of the emerging field of organic optoelectronics, because they provide an ideal platform for the studies of the intrinsic physical properties of organic semiconductors. As organic crystals have low melting temperatures and high vapor pressures and are soluble in numerous organic solvents, both solution and gas-phase methods can be used for crystal growth. The nature of the individual molecules and the interactions between molecules determine which growth method is preferred for particular materials. Organic semiconductors with very low decomposition or melting temperatures can be grown from solutions, whereas semiconductors with high vapor pressures can be grown using physical vapor transport methods. High-quality crystals can be obtained using both methods. Crystal growth and crystal engineering of multicomponent organic compounds are emerging fields that can provide a variety of new materials with different physical properties. The growth of large crystals from the melt by zone melting, the Bridgman, or the Czochralski methods has been used to produce stable materials used in wafer manufacturing or large scintillator detectors. In this article, single-crystal growth methods for organic semiconductors are discussed with the aim of preparing high-quality specimens for determination of the basic properties of organic semiconductors.
Thienyl-substituted tetrathiafulvalene (DTh-TTF, 1a) and a t-butyl derivative (1b) are prepared, and the remarkable photoresponse and memory effect in the transistors are investigated. Upon irradiation of light with a wavelength shorter than the absorption edge (535 nm), the threshold voltage of 1b shifts by as large as 57 V, and the drain current increases by 103 times. The photo generated conducting state is maintained for several tens of minutes but erased by the application of a large negative gate voltage.
We report the fabrication of a photoresponsive organic field-effect transistor (OFET) based on a stable, n-type organic semiconductor (F16CuPc) and low-temperature processable polymer gate dielectric. The device exhibited a photoswitching speed of much less than 10 ms and a photosensitivity of 1.5 mA/W at low optical power. Under illumination, the device produced a current gain (Ilight/Idark) of 22 at VG = 4 V. The drain current increased gradually with an increase in the illumination intensity, resulting in typical output FET characteristics. The multifunctions (photodetection, photoswitching, signal amplification) achieved by the single device can ensure very promising material for future optoelectronic applications.
Functional organic field-effect transistors (OFETs) have attracted increasing attention in the past few years due to their wide variety of potential applications. Research on functional OFETs underpins future advances in organic electronics. In this review, different types of functional OFETs including organic phototransistors, organic memory FETs, organic light emitting FETs, sensors based on OFETs and other functional OFETs are introduced. In order to provide a comprehensive overview of this field, the history, current status of research, main challenges and prospects for functional OFETs are all discussed.
High quality, single crystalline, ordered arrays of a π-conjugated organic molecule, N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), were grown by solution processing and used to fabricate a low-cost, high-performance organic phototransistor (OPT). The single crystalline nature of the microstructure was investigated using 2D-GIXD measurement. The organic field-effect transistor fabricated using periodic arrays of elongated crystals exhibited a photoresponsivity (P) of ca. 1 A W−1 and a photo to dark current ratio (Ion/Ioff) of 2.5 × 103 at VG = 12 V and a maximum P of ca. 7 A W−1 at the high gate bias regime (VG = 50 V) with an optical power of ca. 7.5 mW cm−2. With polymeric gate dielectric, the OPT exhibited very stable n-type characteristics both in the dark and under light illumination and showed reproducible photo-switching behavior. The dependence of the photocurrent on the gate/drain voltage and on illumination intensity provided an effective way to control the number of photo-carriers generated in the active material, enabling the precise tuning of the device's performance. Performance comparison between OPTs with ordered crystal arrays and thin films of PTCDI-C8 confirmed that the material's intrinsic properties were better realized in the crystalline device, presumably because of higher charge carrier mobility and better charge transport capability. This one-step, solution-based, self-assembly fabrication of multifunctional (photodetection, photoswitching, signal amplification) optoelectronic devices has potential to aid the development of organic semiconductors with high-quality micro/nanostructures for large-scale application and low-cost optoelectronic devices.
Tetrathiafulvalene (TTF) was one of the most widely studied heterocyclic systems. However, TTF itself was easily oxidized, which induced the low stability and limited its potential applications. Here, a TTF derivative, 2,3,6,7-tetrakis(2-cyanoethylthio)tetrathiafulvalene (TCE-TTF), was synthesized. It was found that single crystalline micro- and nanowires of TCE-TTF were easily obtained by simple casting due to the enhanced π–π overlapping and S⋯S interaction. The thermal and vacuum stability analyses revealed TCE-TTF was much better than TTF. Single crystalline micro- and nanowires field-effect transistors were also fabricated by in situ dropping method. The typical mobility and on/off ratio were ∼0.02 cm2/V s and ∼103, which predicted great potential applications of organic nanowires electronics.
An enhanced photoinduced reversible switching of graphene oxide-azobenzene (GO-AZO) hybrid was investigated as a highly sensitive photoswitch. The internal short-range ordered crystalline structure of GO-AZO hybrid was advantageous to charge transfer. The AZO moieties on GO underwent a rapid trans-cis photoisomerization upon ultraviolet irradiation due to the electron interaction between AZO and GO. The GO-AZO hybrid film showed an enhanced reversible photoswitching performance with high on/off ratio of 8 and fast response time less than 500 ms. The high sensitivity of GO-AZO switch arises from the intramolecular donor-acceptor architecture with efficient charge transfer.
Cocrystal nanofibres of cobalt octaethylporphyrin and tetracyanoquinodimethane were prepared by a facile solution method and fully characterized by SEM, AFM, XRD, Raman, EDX, and UV-vis-NIR. The as-prepared cocrystal nanofibres had smooth surfaces and uniform dimension. When incorporated into prototype devices, they exhibited good photoresponse at ambient conditions. Additionally, the phototransistor characteristics with a maximum I(on)/I(off) ratio of -460 was demonstrated. The facile synthesis and good photoresponse may boost the potential applications of cocrystal-based nanostructures in future miniaturized devices.
We report on the formation of photoconductive self-assembled fibres by solvent induced precipitation of a HBC-PMI donor-acceptor dyad. Kelvin Probe Force Microscopy revealed that upon illumination with white light the surface potential of the fibres shifted to negative values due to a build-up of negative charge. When integrated in a field-effect transistor (FET) configuration, the devices can be turned 'on' much more efficiently using light than conventional bias triggered field-effect, suggesting that these structures could be used for the fabrication of light sensing devices. Such a double gating represents an important step towards bi-functional organic FETs, in which the current through the junction can be modulated both optically (by photoexcitation) and electrically (by gate control).
A simple approach is developed to prepare periodic arrays of large, elongated crystals of p-conjugated
organic molecules, viz, 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) through a solution based method.
The ordered crystalline array is successfully used to fabricate low-voltage organic phototransistors
(OPT). The OPT with polymeric gate dielectric exhibits very stable n-type characteristics with a low
threshold voltage (<0.5 V). Under illumination, the devices produce a current gain (Ilight/Idark) of 31 at
VG ¼ 0.3 V. Photoswitching occurs within 10 ms and photosensitivity is greater than 1 mA W�1 at low
driving voltages and low optical powers. The drain current increases gradually with increasing the
illumination intensity resulting in typical output FET characteristics. The one-step, solution-based,
self-assembly method for highly ordered organic crystals in large area could have significant potential
for future large-scale and low-cost optoelectronic devices. In addition, this study aims to aid the
development of organic semiconductor materials with high quality crystalline structures for various
optoelectronic applications.
Organic photoresponse materials and devices are critically important to organic optoelectronics and energy crises. The activities of photoresponse in organic materials can be summarized in three effects, photoconductive, photovoltaic and optical memory effects. Correspondingly, devices based on the three effects can be divided into (i) photoconductive devices such as photodetectors, photoreceptors, photoswitches and phototransistors, (ii) photovoltaic devices such as organic solar cells, and (iii) optical data storage devices. It is expected that this systematic analysis of photoresponse materials and devices could be a guide for the better understanding of structure-property relationships of organic materials and provide key clues for the fabrication of high performance organic optoelectronic devices, the integration of them in circuits and the application of them in renewable green energy strategies (critical review, 452 references).
A study was conducted to demonstrate the performance analysis of π-conjugated systems in organic field-effect transistors (OFET). The performance analysis made it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and captured what is needed for new materials and what was the synthesis orientation of new π-conjugated systems. The investigations revealed that the device configuration played a key role in the device performance. It was found that devices of the type B and D gave better performances than that of devices A and C. This was attributed to the improved contact between the organic semiconductor and the electrodes. It was certain that other factors, such as the semiconductor layer, the materials used for the electrodes and the gate insulator, the interface, and energy alignments between the organic semiconductor and the gate insulator or between the organic semiconductor and the electrodes played crucial roles in the performance of OFETs.
A photoresponsive organic complementary inverter was fabricated and its light sensing characteristics was studied. An organic circuit was fabricated by integrating p-channel pentacene and n-channel copper hexadecafluorophthalocyanine (F16CuPc) organic thin-film transistors (OTFTs) with a polymeric gate dielectric. The F16CuPc OTFT showed typical n-type characteristics and a strong photoresponse under illumination. Whereas under illumination, the pentacene OTFT showed a relatively weak photoresponse with typical p-type characteristics. The characteristics of the organic electro-optical circuit could be controlled by the incident light intensity, a gate bias, or both. The logic threshold (V(M), when V(IN) = V(OUT)) was reduced from 28.6 V without illumination to 19.9 V at 6.94 mW/cm². By using solely optical or a combination of optical and electrical pulse signals, light sensing was demonstrated in this type of organic circuit, suggesting that the circuit can be potentially used in various optoelectronic applications, including optical sensors, photodetectors and electro-optical transceivers.
Recent progress in photoactive organic field-effect transistors (OFETs) is reviewed. Photoactive OFETs are divided into light-emitting (LE) and light-receiving (LR) OFETs. In the first part, LE-OFETs are reviewed from the viewpoint of the evolution of device structures. Device performances have improved in the last decade with the evolution of device structures from single-layer unipolar to multi-layer ambipolar transistors. In the second part, various kinds of LR-OFETs are featured. These are categorized according to their functionalities: phototransistors, non-volatile optical memories, and photochromism-based transistors. For both, various device configurations are introduced: thin-film based transistors for practical applications, single-crystalline transistors to investigate fundamental physics, nanowires, multi-layers, and vertical transistors based on new concepts.
Using vacuum evaporation and magnetron sputtering technology, with copper phthalocyanine (CuPc) as the active layer, a vertical organic thin-film transistor (VOTFT) with a structure of Cu/CuPc/Al/CuPc/Cu was fabricated. The static photoelectric and dynamic photoelectric characteristics of the VOTFT were experimented, and its photocurrent multiplication mechanism was analyzed. The results of the static electrical characteristics experiment show that the base Al and the adjacent two layers of CuPc form good Schottky contact and the output characteristics show unsaturated characteristics. Illumination lowers the Schottky barrier between the active layer and the base, which is conducive to carrier transport. A dynamic light signal with a frequency of 200 Hz is applied to the VOTFT; according to the experiment, its turn-on time ton = 1.5 ms, and its turn-off time toff = 1.8 ms, due to its shorter conductive channel and smaller junction capacitance.Graphic Abstract
A one-step process to generate single-crystal organic nanowire arrays using a direct printing method (liquid-bridge-mediated nanotransfer molding) that enables the simultaneous synthesis, alignment, and patterning of nanowires from molecular ink solutions is reported. Using this method, many single-crystal organic nanowires can easily be synthesized by self-assembly and crystallization of organic molecules within the nanoscale channels of molds, and these nanowires can then be directly transferred to specific positions on substrates to generate nanowire arrays by a direct printing process. The position of the nanowires on complex structures is easy to adjust, because the mold is movable on the substrates before the polar liquid layer, which acts as an adhesive lubricant, is dried. Repeated application of the direct printing process can be used to produce organic nanowire-integrated electronics with two- or three-dimensional complex structures on large-area flexible substrates. This efficient manufacturing method is used to fabricate high-performance organic nanowire field-effect transistors that are integrated into device arrays, inverters, and phototransistors on flexible plastic substrates.
Compared with one-component organic single crystals, organic cocrystals with unique packing structures and aggregate states show a variety of novel optoelectronic properties through multi-component synergistic and collective effects, paving the way to the development of high-performance or multifunctional optoelectronic devices, particularly in electrical conductors, ferroelectricity, ambipolar charge transporting, photoconductivity and luminescence. Moreover, the charge transfer pathway between donor (D) and acceptor (A) in organic cocrystal is also fundamentally interesting. Therefore, studies on organic cocrystals have gained much attentions in recent years. In this paper, firstly, we have classified organic cocrystals into three categories by the driving forces: charge transfer (CT), π-π interaction and halogen/hydrogen bond; secondly, we have introduced common organic donor materials, using 7,7,8, 8-tetracyanoquinodimethane (TCNQ), 1,2,4,5-tetracyanobenzene (TCNB) and fullerene (C60) as the typical acceptors; Thirdly, we have listed eight popular methods used to prepare these organic cocrystals and discussed the relationship between molecule packing and performance of organic cocrystal; Finally, we have covered the applications of these novel cocrystals in organic optoelectronics. We believe that the study on the organic cocrystals is an effective way to achieve organic multi-functional single crystal materials and devices and to promote the development of organic solid state optoelectronics.
Charge-transfer (CT) co-crystals composed of organic donor and acceptor molecules are attractive materials from both basic and applied perspectives. One of the most fascinating protocols for the formation of a CT crystal is the use of donor and acceptor crystals instead of their molecules as starting materials. However, the lack of studies has limited the universality of the method for future applications. In this study, we adapted the mixing of donor and acceptor crystal dispersions to fabricate CT co-crystals, and "phase-separated"crystals, which are a mixture of the donor and acceptor crystals. The aqueous dispersions of each crystal were prepared by the reprecipitation method, and mixed to form the targeted crystals. We controlled the co-crystallization behavior between the donor and acceptor crystals by aging of the dispersion, which was left standing in air for 24 h before mixing. CT co-crystals were formed in the non-aged mixture; in contrast, "phase-separated"crystals were obtained in the aged mixture. Current-voltage measurements using tetrathiafulvalene and tetracyanoquinodimethane revealed that the conventional CT co-crystals exhibited lower sheet resistance (103 to 104 Ω sq-1) than that of the "phase-separated"crystals, whereas the "phase-separated"crystals showed a photoconductive response.
In this research work, the charge transfer complex of 2,2′-bipyridine with picric acid a 1:1 stoichiometry has been investigated by using quantum chemical calculations B3LYP/6-311 ⁺⁺ G(d, p), CAM- B3LYP/6-311 ⁺⁺ G(d, p), PBE0/6-311 ⁺⁺ G(d, p) and M06-2X/6-311 ⁺⁺ G(d, p) level of theory. Binding energies (Δ E int , Δ H int and Δ G int ), reactivity descriptors, HOMO and LUMO orbitals, TD-DFT, NBO and QTAIM analysis have been determined. The results demonstrate that the complex formation is energetically favorable in vacuum and in chloroform. Furthermore, TD-DFT, NBO and QTAIM analysis suggest that a charge transfer occurs by the establishment of H-bond interaction essentially between O(8) of the picric acid and H(19) of 2,2′-bipyridine.
2D molecular crystals (2DMCs) have attracted considerable attention because of their unique optoelectronic properties and potential applications. Taking advantage of the solution processability of organic semiconductors, solution self‐assembly is considered an effective way to grow large‐area 2DMCs. However, this route is largely blocked because a precise molecular design towards 2DMCs is missing and little is known about the relationship between 2D solution self‐assembly and molecular structure. A “phase separation” molecular design strategy towards 2DMCs is proposed and layer‐by‐layer growth of millimeter‐sized monolayer or few‐layer 2DMCs is realized. High‐performance organic phototransistors are constructed based on the 2DMCs with unprecedented photosensitivity (2.58 × 107), high responsivity (1.91 × 104 A W−1), and high detectivity (4.93 × 1015 Jones). This “phase separation” molecular design strategy provides a guide for the design and synthesis of novel organic semiconductors that self‐assemble into large‐area 2DMCs for advanced organic (opto)electronics. A “phase separation” molecular design strategy towards 2D molecular crystals (2DMCs) is proposed. The designed molecule is composed of a π core with alkyl chains above and below. “Phase separation” drives the self‐assembly of millimeter‐sized monolayer and few‐layer 2DMCs. Organic phototransistors are constructed based on the 2DMCs with unprecedented photosensitivity, up to 2.58 × 107.
Organic semiconducting single crystals are ideal candidates both for fundamental and application‐oriented researches due to their advantages of free grain boundaries, few defects, minimal traps and impurities as well as low‐temperature processability, flexibility, and low cost. Mobility over 10 cm2 V‐1 s‐1 in some organic single crystals indicates their promising in electronic devices. Here, we suggest a new branch of electronics, organic single crystal electronics, is emerging. Devices of single crystals such as field‐effect transistors, phototransistors, p‐n heterojunctions, and circuits are summarized. Organic two‐dimensional single crystals, co‐crystals, large‐sized single crystals together with some potential applications are also included.
Keine halben Sachen: Organische halbleitende Einkristalle eignen sich sowohl für die Grundlagen‐ als auch für die angewandte Forschung. Dieser Aufsatz diskutiert die jüngsten Fortschritte in der organischen Einkristallelektronik.
Abstract
Organische halbleitende Einkristalle eignen sich sowohl für die Grundlagenforschung als auch für die anwendungsorientierte Forschung aufgrund der Vorteile von freien Korngrenzen, wenigen Fehlstellen, Ladungsträgerfallen und Verunreinigungen sowie der Tieftemperaturverarbeitbarkeit, der hohen Flexibilität und der niedrigen Herstellungskosten. Ladungsträgerbeweglichkeiten von mehr als 10 cm² V⁻¹ s⁻¹ in einigen organischen Einkristallen sind vielversprechend für die Anwendung in elektronischen Bauteilen. Die bisherigen Ergebnisse, einschließlich der Molekülstrukturen und Techniken zur Herstellung organischer Einkristalle, werden hier vorgestellt. Organisch‐einkristalline elektronische Bauteile, einschließlich Feldeffekttransistoren, Phototransistoren, p‐n‐Heteroübergänge und Schaltungen, werden ebenfalls diskutiert. Des Weiteren werden zweidimensionale organische Einkristalle, Cokristalle sowie große Einkristalle samt einigen möglichen Anwendungen vorgestellt. Damit wird ein aktueller Überblick über die organische Einkristallelektronik mit ihren Herausforderungen und Perspektiven geboten.
Organic phototransistors (OPTs) simultaneously introduce photo-induced hole and electron into the structure of organic field-effect transistor (OFET). The memory effect of minority carrier is the origin of big dark current and slow response during the light on/off switching, which is bad for the OPT performance and cycling stability. In this research, we have reported a novel OPT working mode by photoelectric dual control. After each light switch, the dark current is erased by gate voltage at depletion mode which keeps unchanged during light off; the photocurrent increases by device conversing to accumulation mode when light is on. By this way, the high performance OPTs have been obtained in FBT-Th4(1,4):PC61BM (5:1) composite film with the structure of Si/ SiO2 /OTS /FBT-Th4(1,4):PC61BM /Au electrodes, which show broad spectral response (maximum value at zero gate bias: R 1.2 × 10⁵ A/W, gain 3.7 × 10⁵ and D* 3.18 × 10¹⁶ Jones) from 410 to 740 nm (calculated by transfer curve). The best performance with photoelectric dual control under 0.0031 mW/cm² @ 405 nm have been achieved with on/off current ratio 1.0 × 10⁶, response 1.6 × 10⁴ A/W, gain 5.0 × 10⁴, spectral detectivity 2.3 × 10¹⁷ Jones and response time sub-40 ms, respectively. It makes the phototransistor a very promising component for light sensor application.
Small-molecule organic semiconductor crystals (SMOSCs) combine broadband light absorption (ultraviolet-visible-near infrared) with long exciton diffusion length and high charge carrier mobility. Therefore, they are promising candidates for realizing high-performance photodetectors. Here, after a brief resume of photodetector performance parameters and operation mechanisms, we review the recent advancements in application of SMOSCs.
We demonstrate a novel solution-based assembly method using a writing brush to realize the controllable fabrication of highly-oriented and large-scale TCNQ single-crystal microwire arrays. The arrays can not only be grown on conventional rigid substrates, such as Si, Si/SiO2 and low-cost glass, but also on nonconventional substrates, which include flexible polyethylene terephthalate (PET), curved glass hemispheres and commercially available plastic contact lenses. Their morphology is optimized by tuning solution concentration, substrate temperature, brush type, inclination angles and pressure of the brush. The length of the microwire arrays can extend to the millimeter level, and their preferential orientation is perpendicular to the lengthwise direction of the brush hair. The coverage area of microwire arrays with a consistent orientation can reach 1.5 × 2.0 mm2 and the success ratio is as high as 93%. Based on these microwire arrays, devices on different substrates, including rigid Si/SiO2 and flexible PET, can be easily realized in one step. The anisotropic transport of TCNQ crystals is studied with respect to the concentration controlled morphology. All these results illustrate the broad application prospects of this facile writing-brush method in the growth of large-scale, high-quality organic micro/nanowires for integration into flexible organic semiconductor devices and circuits.
The photoelectronic characteristics of single-crystalline nanowire organic phototransistors (NW-OPTs) are studied using a high-performance n-channel organic semiconductor, N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI), as the photoactive layer. The optoelectronic performances of the NW-OPTs are analyzed by way of their current–voltage (I–V) characteristics on irradiation at different wavelengths, and comparison with corresponding thin-film organic phototransistors (OPTs). Significant enhancement in the charge-carrier mobility of NW-OPTs is observed upon light irradiation as compared with when performed in the dark. A mobility enhancement is observed when the incident optical power density increases and the wavelength of the light source matches the light-absorption range of the photoactive material. The photoswitching ratio is strongly dependent upon the incident optical power density, whereas the photoresponsivity is more dependent on matching the light-source wavelength with the maximum absorption range of the photoactive material. BPE-PTCDI NW-OPTs exhibit much higher external quantum efficiency (EQE) values (≈7900 times larger) than thin-film OPTs, with a maximum EQE of 263 000%. This is attributed to the intrinsically defect-free single-crystalline nature of the BPE-PTCDI NWs. In addition, an approach is devised to analyze the charge-transport behaviors using charge accumulation/release rates from deep traps under on/off switching of external light sources.
Visible-light transparent 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) single crystal ribbons prepared by low-cost solution process are applied as the channel layer of the UV active field-effect transistor. The device exhibits ultrahigh response to visible-blind and deep UV signals because of the good charge transport ability of the C8-BTBT single-crystals. In the case of very weak 365 nm UV lamination (0.2 mW cm−2), the photo to dark current ratio (P) and photoresponsivity (R) reach to 3.0 × 104 and 1200 A W−1 at Vg = −23 V, Vds = −30 V. When 1 mW cm−2 280 nm deep UV light is applied, the P and R value is 8300 and 44 A W−1, respectively. The result is the highest record in the organic UV photodetectors ever reported. This work offers the possibility of fabricating a high-performance, low-energy exhaust UV sensor free of visible-light interference or UV controlled memory device, and opens a practical avenue for the realization of sensitive detection of deep UV signals.
With the rich experience of developing silicon devices over a period of the last six decades, it is easy to assess the suitability of a new material for device applications by examining charge carrier injection, transport, and extraction across a practically realizable architecture; surface passivation; and packaging and reliability issues besides the feasibility of preparing mechanically robust wafer/substrate of single-crystal or polycrystalline/amorphous thin films. For material preparation, parameters such as purification of constituent materials, crystal growth, and thin-film deposition with minimum defects/disorders are equally important. Further, it is relevant to know whether conventional semiconductor processes, already known, would be useable directly or would require completely new technologies. Having found a likely candidate after such a screening, it would be necessary to identify a specific area of application against an existing list of materials available with special reference to cost reduction considerations in large-scale production. Various families of organic semiconductors are reviewed here, especially with the objective of using them in niche areas of large-area electronic displays, flexible organic electronics, and organic photovoltaic solar cells. While doing so, it appears feasible to improve mobility and stability by adjusting π-conjugation and modifying the energy band-gap. Higher conductivity nanocomposites, formed by blending with chemically conjugated C-allotropes and metal nanoparticles, open exciting methods of designing flexible contact/interconnects for organic and flexible electronics as can be seen from the discussion included here.
Tetrathiafulvalene (TTF) has become one of the most studied heterocyclic systems since its discovery. TTF and some of its derivatives have semiconducting properties, providing promising potential for their application in organic electronics. In this review, the molecular structures, devices and optoelectronic properties of TTF are discussed. Single crystals of TTF show phase dependence with a high mobility of up to 1.2 cm2 V−1 s−1. However, TTF itself is not stable under ambient conditions due to its low oxidation potential. The modification of the side chains of TTF is a solution to allow for its application in organic field-effect transistors (OFETs) with low cost, good air stability and high charge carrier mobility. Four types of TTF derivatives are introduced, and their molecules, morphologies and performance are summarized. Organic charge transfer compounds based on TTF and its derivatives are also discussed in this review.
High photosensitivity and high photocurrent gain have been obtained based on dielectric optimized dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2) single crystal microplate transistors. In our experiments, the PMMA dielectric device shows the best operational stability without hysteresis effect. Based on such an optimized device, the photoelectric properties of the Ph5T2 single crystal microplates have been studied for the first time. The Ph5T2 phototransistor has the high photosensitivity at 21 mA W−1 and the extremely high photocurrent gain (Ilight/Idark) at 6.8 × 105. The photocurrent gain is higher than that of the most reported organic phototransistors (OPTs), and is in a class with the highest photocurrent gain for the reported values so far. This confirms that Ph5T2 is a photosensitive material and shows it promising potential in photoswitches and phototransistors.
We developed a drop-casting method to grow the well-aligned single-crystal tetrathiafulvalene (TTF) microwire arrays with success ratio as high as 86%. The key improvement over the earlier study is that the substrate is placed in an apparatus which is filled with the saturated solvent vapor at room temperature. The saturated solvent atmosphere ensures stable environment and adequate time to form the arrays with high success ratio, and to dramatically improve the crystalline quality. Combined with the optimized concentration, the highly ordered single-crystal TTF microwire arrays are obtained. Based on these microwire arrays, the assembly of devices can be easily realized in one step. These results show the potential of this facile method to form the high-quality arrays, and the assemblies of nanoscale circuits for fundamental studies and future applications.
Nano/microstructures of copper octaethylporphyrin (CuOEP, donor)–tetracyanoquinodimethane (TCNQ, acceptor) cocrystals with controlled stacking were prepared by a facile solution method and fully characterized. The as-prepared CuOEP–TCNQ cocrystals were either nanoribbons of TCNQ·2CuOEP or microrods of TCNQ·CuOEP, which had DDA(DDA)nDDA and DA(DA)nDA stacking, respectively. For the first time, a phase transformation was found to occur from nanoribbons of TCNQ·2CuOEP to microrods of TCNQ·CuOEP, which was mainly controlled by the concentration of TCNQ. Moreover, prototype devices were fabricated to investigate their photoresponse properties. The results demonstrated that the DDA(DDA)nDDA stacking had much better photoresponse than the DA(DA)nDA stacking.
In this chapter, organic/polymeric field-effect transistors (OFETs) based on organic and polymeric semiconductors are introduced, including the device configurations, their working principles, and technologies for the fabrication of such devices. The chapter analyses the factors that dominate the performance of these transistors, such as the grain boundaries, morphology, interfaces and crystallization. It discusses the way in which organic semiconductor molecules are ordered and the alignment of energy levels of organic semiconductors and electrodes for the fabrication of high-performance transistors. The chapter describes the organic single-crystal transistors, including the controllable growth of organic single crystals, new fabrication techniques, transistor applications, structure-properties dependence and the transport anisotropy of organic single crystals. Solution-processed techniques are widely used to deposit organic thin films, due mainly to the low cost of such processes and the facility of obtaining large-area films by a variety of printing techniques. OFETs; organic semiconductors; Organic single-crystal field effect transistors; organic thin film filed effect transistors; semiconductor doped polymers; thin film transistors
We propose an organic heterojunction phototransistor stacked with organic layers of electron and hole transport materials. The materials used in this study are perylene derivative (td-PTC) as the ultrathin base layer and triphenylamine derivative (TPD) as the emitter and collector layers. The current versus voltage characteristics as a function of incident optical intensity are typical of a phototransistor with current saturation. The bipolar transistor operation is confirmed by a comparison with the diode structure of the TPD/td-PTC device and because of the base thickness dependence of the current. External quantum efficiency is 2.9% under blue light-emitting diode illumination. In the current response, the initial displacement current is superimposed. Obtained response times are 600 μs for both transient turn-on and turn-off currents. © 2002 American Institute of Physics.
The study of high-performance n- and p- type single-crystal organic transistor using free-space gate dielectrics is analyzed. A transistor with a wide range range of channel dimensions and dielectric thickness was built using single crystals of two different molecules. It is observed that the transportation characteristics of the conducting channel depend critically on the properties of the rubrene crystals. It is also observed that the magnitude of the capacitance, which includes contributions from fringing fields that pass through the elastomer, can be accurately predicted by finite element modeling of the electrostatics.
We report highly photosensitive organic phototransistors (OPTs) based on a 2,5-bis-biphenyl-4-yl-thieno[3,2-b]thiophene (BPTT). The measured maximum sensitivity and the ratio of photocurrent to dark current (I<sub> ph </sub>/I<sub> dark </sub>) in BPTT OPTs were 82 A / W and 2.0×10<sup>5</sup> under 380 nm UV light with 1.55 mW / cm <sup>2</sup> , respectively. The prepared OPTs show a photocurrent response similar to the absorption spectrum of BPTT. The major mechanisms for photocurrent amplification in this device were verified from experimental results as photovoltaic (turn-on) and photocurrent effect (turn-off) by a fitting to theoretic equations.
The authors present a strategy to manufacture wavelength-selective field-effect phototransistors by employing dye-doped poly-3-hexylthiophene (P3HT) as a semiconducting layer. The dye doping of the semiconductor P3HT was achieved by blending organic molecules—coumarin 6, oxazine 1, and nile red—into the conjugated organic polymer. Illuminating these transistors with monochromatic light in the range of 400–700 nm resulted in varying conductivities for certain wavelengths in dependence on the particular dye. This effect is attributed to the photogeneration of excitons on the dye molecules, which are subsequently transferred to the conjugated polymer.
We report detailed studies of the slow relaxation of the photoinduced excess charge carriers in organic metal-insulator-semiconductor field effect transistors consisting of poly(3-hexylthiophene) as the active layer. The relaxation process cannot be physically explained by processes, which lead to a simple or a stretched-exponential decay behavior. Models based on serial relaxation dynamics due to a hierarchy of systems with increasing spatial separation of the photo-generated negative and positive charges are used to explain the results. In order to explain the observed trend, the model is further modified by introducing a gate voltage dependent coulombic distribution manifested by the trapped negative charge carriers.
We report on observation of a light-induced switching of the conductance in the back-gated organic field-effect transistors (OFETs) with built-in conduction channel. In the studied devices, the built-in channel is formed owing to the self-sensitized photo-oxidation of rubrene surface. In the dark, the back gate controls the charge injection from metal contacts into the built-in channel: the high-current ON state corresponds to zero or negative back-gate voltage; the low-current OFF state - to a positive back-gate voltage that blocks the Schottky contacts. Illumination of the OFET in the OFF state with a short pulse of light switches the device into the ON state that persists in the dark for days. The OFF state can be restored by cycling the back gate voltage. The observed effect can be explained by screening of the back-gate electric field by the charges photo-generated in the bulk of organic semiconductor. Comment: 3 pages
A general method is developed for preparing multicomponent mixtures of organic semiconductors and conjugated polymers with a high degree of control of the phase separation and morphology. The resulting nanocomposite materials have broad spectral responses and tailorable photoelectronic properties. The method is exemplified by binary nanocomposites of titanylphthalocyanine and poly(benzobisimidazobenzophenanthroline ladder).
We report on photoconductivity in polyparaphenylenevinylene films under continuous wave illumination in the presence of air traces. Near room temperature the photocurrent buildup is extremely slow, steady state not being reached after 1 h illumination. When light is turned off the photocurrent decay kinetics is slower and slower and it takes about 24 h to reach thermal equilibrium. The instantaneous lifetime follows a power-law time dependence τinst(t)=ip(t)/|dip(t)/dt|∝tα, 0≺α≺1 and the decay kinetics can be accurately fitted to a stretched exponential relaxation law ip(t)=ip(0)exp-(t/τ)β where β=1-α. When temperature is decreased down to ≈255K the exponent β and the relaxation time τ are found to be temperature dependent, β being linearly dependent upon T: β=T/T0 with T0=1175K. The nonlinear decrease of β between 255 and 190 K is interpreted as a transport mode transition at Tt≈225K. When temperature is further decreased in the range 160–77 K the slow component is frozen and leaves room to a fast signal that rapidly reaches steady state and is slightly dependent upon temperature. The magnitude of the slow photocurrent is proportional to the square root of the light intensity whereas that of the fast one is proportional to the light intensity. Modulated photocurrent studies allowed us to show that the fast signal exists in the whole temperature range. Examination of the temperature dependence of the fast photocurrent at low temperature led us to interpret the fast signal as being an intrinsic photocurrent due exciton dissociation. Such an interpretation rests on the recent work of Albrecht and Bässler on the yield of geminate pair dissociation in an energetically random hopping system with built-in energetic disorder. The slow component is interpreted as being extrinsic due to dissociation of polaron pairs through interaction with oxygenated defects to create positive polarons. Recombination and the functional form of the extrinsic component decay is attributed to the dispersive diffusion of the positive polarons in a random distribution of negatively charged defects.
A large photocurrent multiplication reaching 130 000‐fold at room temperature has been observed in naphthalene tetracarboxylic anhydride (NTCDA) film sandwiched between metal electrodes. This phenomenon is reasonably interpreted in terms of the tunneling injection of electrons from a metal electrode, which is triggered by the accumulation of photogenerated trapped holes near the metal/organic interface. The combination of an ultrathin NTCDA film with another photoconductive pigment film in a layered structure allowed us to fabricate a photocurrent multiplication device with the desired spectral response. © 1996 American Institute of Physics.
A photocurrent multiplication of up to 200 times has been observed in single crystals of naphthalene tetracarboxylic anhydride sandwiched between metal electrodes. Photocurrent multiplication arises from photoinduced electron injection occurring at the crystal/metal interface. The high-speed response of the multiplied photocurrent reached 500 ms. © 2002 American Institute of Physics.
The fabrication of phototransistors using bifunctional low-molecular glasses with intramolecular charge transfer properties was discussed. The intramolecular charge transfer properties were the underlying principle of phototransistor operation. An asymmetric spiro compound in which two different functional chromophores are linked together by a central spiro atom was used as the photoresponsive material for the OPTs. The amplification effect of the OPTs due to the intramolecular charge separation was also analyzed. The phototransistors were found to exhibit a response of more than 1 AW-1 for ultraviolet light and 370 nm.
The photoresponse of polymer field-effect transistors (PFETs) based on the 2,5-bis(dibutylaminostyryl)-1,4-phenylene-b-alkyne-b-1,4-bis(2-ethylhexyl)benzene terpolymer (BAS-PPE) is investigated. BAS-PPE is a photoluminescent conducting polymer with a band gap of 2.25 eV. The BAS-PPE PFETs were fabricated using an open coplanar configuration and light is illuminated onto the top side of the PFETs with no shadowing present. A sweep of VDS demonstrates that IDS saturation is suppressed during illumination, which suggests that pinch-off cannot be reached since the injected photogenerated carriers continue unabated. Also, with incident light, the channel cannot be turned off, even at high positive gate biases, due to the accumulation of photogenerated carriers. A sweep of VDS shows that BAS-PPE can act as a p-type polymer and favors hole injection and transport. A sweep of VGS shows an increase in IDS with different light intensities. The Ilight∕Idark ratio reaches as high as about 6000 at an incident light intensity of 4 μW and a photoresponsivity of 5 mA∕W is calculated.
Ribbons of an air-stable, n-type organic single-crystal semiconductor, F16CuPc, are used to study photoswitches and phototransistors. High-quality, reversibly switching, fast photoswitches together with strongly photodependent field-effect phototransistors (see figure) of F16CuPc are made. The high light sensitivity and large on/off ratio of the phototransistors realize a new way to detect light and magnify signals in a single organic device, indicating a future in low-cost, ultrahigh density, organic photoelectric integration.
The organic composite thin films, which exhibited electrical bistability, and are used in memory devices were described. The films consisted of polystyrene (PS), methanofullerene 6,6-phenyl C61-butyric acid methyl ester (PCBM) as the organic electron acceptor, and tetrathiafulvalene (TTF) as the organic electron donor. In the film, the electronic transition was due to an electrical-field-induced charge transfer between TTF and PCBM. It was found that the electrically bistable devices exhibited a sharp increase in conductivity after the carrier charge transfer.
(Graph Presented) Photons control number of mobile charge carriers: Organic field-effect transistors based on dithiophene-tetrathiafulvalene (DT-TTF) and dibenzo-tetrathiafulvalene (DB-TTF) show enhanced conductivity under illumination (see figure). A very high ratio of photocurrent to dark current of the order of ≈ 104 was observed for DT-TTF due to the photogeneration of mobile charge carriers (holes). These materials have the potential to be employed as light sensors or optoelectronic memory devices.
We report the effect of light incident on a polymer-based field-effect transistor and demonstrate the utility of light as an additional controlling parameter of the transistor state. The transistor exhibits large photosensitivity indicated by the sizable changes in the drain–source current at low levels of light. The response here is considerably higher than that from existing organic/polymeric planar, two-terminal photodetectors due to an additional process contributing to the enhancement. The light-responsive polymer transistor opens up a device-architecture concept for polymer-based electronics. © 2001 American Institute of Physics.
Graphical Abstract
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Memory operations in polymer phototransistors have been demonstrated. A fraction of light-induced drain current in the depletion mode of a polythiophene-based field-effect transistor persists after switching off the photoexcitation, and can be erased by reversing the gate voltage (Vg) (see Figure). Write, store, read, and erase operations can be performed by applying a combination of gate voltages and incident light over a wide temperature range.
The aim of this study is to establish the structure-properties relationship of tetrathiafulvalene (TTF). After this study (1) TTF crystals with alpha and beta phases were controllably obtained by the cast method with careful selection of the solvents, and (2) organic field-effect transistors (OFETs) of alpha and beta phases both exhibited field-effect performance, and the performance of alpha phase was much better than that of beta phase. The highest mobility of alpha phase reached 1.2 cm(2) V-1 s(-1), while beta phase showed the maximum mobility only about 0.23 cm(2) V-1 s(-1), indicating the strong performance dependence of OFETs performance on the crystal phases. (c) 2007 American Institute of Physics.
We have used tetracyanoquinodimethane (TCNQ) as the active semiconducting material in metal-insulator-semiconductor field-effect transistors (MISFETs). TCNQ behaves as an n-type semiconductor. Differential capacitance measurements on metal-insulator-semiconductor (MIS) devices confirm the n-type behaviour. A maximum field-effect mobility of 3 × 10-5 cm2 V-1 s-1 is observed. On exposure to air the on/off ratio of the FETs improves to in excess of 450, due to oxidative dedoping of the TCNQ and narrowing of the channel.
We report the spectral response and slow decay of the steady-state photoconductivity in poly(p-phenylenevinylene) (PPV) films. The spectral response of the photoconductivity is in good agreement with that calculated from the absorption data with the assumption of rapid recombination at the surface of the film; the results indicate direct photogeneration of free charge carriers via an interband transition. The photoconductivity is, therefore, consistent with a description of the electronic structure of PPV in terms of a semiconductor band model (rather than an exciton model). The very slow stretched-exponential relaxation of the photoinduced conductivity is reminiscent of the persistent photoconductivity observed in inorganic semiconductors. By assuming that the photocurrent is carried predominantly by mobile polarons near the surface, one can construct a model for the persistent photoconductivity in which the recombination of long-lived bipolarons is inhibited in the bulk where bipolarons have a lower free energy than polarons. The persistent photoconductivity, therefore, is caused by the slow dispersive diffusion of photogenerated bipolarons to the surface where they dissociate into polarons and where both polaron transport and recombination occur.
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