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Showing papers on "Field effect published in 2004"


Journal ArticleDOI
TL;DR: In this paper, a detailed review of fabrication methods for obtaining device functionality from single ZnO nanorods is presented, where a key aspect is the use of sonication to facilitate transfer of the nanorod from the initial substrate on which they are grown to another substrate for device fabrication.
Abstract: The large surface area of ZnO nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for micro-lasers or memory arrays. In addition, they might be doped with transition metal (TM) ions to make spin-polarized light sources. To date, most of the work on ZnO nanostructures has focused on the synthesis methods and there have been only a few reports of the electrical characteristics. We review fabrication methods for obtaining device functionality from single ZnO nanorods. A key aspect is the use of sonication to facilitate transfer of the nanorods from the initial substrate on which they are grown to another substrate for device fabrication. Examples of devices fabricated using this method are briefly described, including metal-oxide semiconductor field effect depletion-mode transistors with good saturation behavior, a threshold voltage of ∼−3 V and a maximum transconductance of order 0.3 mS/mm and Pt Schottky diodes with excellent ideality factors of 1.1 at 25 °C and very low (1.5 × 10 −10 A, equivalent to 2.35 A cm −2 , at −10 V) reverse currents. The photoresponse showed only a minor component with long decay times (tens of seconds) thought to originate from surface states. These results show the ability to manipulate the electron transport in nanoscale ZnO devices.

562 citations


Journal ArticleDOI
TL;DR: In this article, the intrinsic charge transport properties of organic semiconductors by using organic single-crystal field-effect transistors were investigated, and new aspects that influence charge transport in organic semiconductor FETs, and exploratory measurements in the charge density regime approaching one carrier per molecule.
Abstract: Organic electronics constitute an innovative field, with interesting applications complementary to the silicon semiconductor technology. From a scientific perspective, there is large interest in the fundamental understanding of electrical transport in organic semiconductors. However, a well-developed microscopic description is still lacking, due to the complicated character of the many-body polaronic-type of charge carriers in molecular compounds. In this Thesis, we have experimentally studied the intrinsic charge transport properties of organic semiconductors by using organic single-crystal field-effect transistors. The electric field-effect has been frequently used to investigate thin films of organic compounds. Unfortunately, thin-film transistors are not suitable for the study of intrinsic electronic properties of organic conductors, because their characteristics are often strongly affected by imperfections of the film structure and by insufficient purity of organic materials. Thus, for a higher degree of molecular ordering and an improved quality of the FET, we fabricate devices on the surface of a free-standing single crystal of organic molecules. In short, in this work we have achieved successful fabrication of high-quality single-crystal FETs, exhibiting high mobilities and signs of intrinsic transport. Herewith, we have identified new aspects that influence charge transport in organic semiconductor FETs, and we have performed exploratory measurements in the charge density regime approaching one carrier per molecule.

549 citations


01 Mar 2004
TL;DR: In this paper, a self-aligned regioregular poly(3-hexylthiophene) (P3HT) has been used to control the intermolecular interaction at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with various groups (NH2, NH2, OH, and CH3).
Abstract: With the aim of enhancing the field-effect mobility by promoting surface-mediated two-dimensional molecular ordering in self-aligned regioregular poly(3-hexylthiophene) (P3HT) we have controlled the intermolecular interaction at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with various groups (–NH2, –OH, and –CH3). We have found that, depending on the properties of the substrate surface, the P3HT nanocrystals adopt two different orientations—parallel and perpendicular to the insulator substrate—which have field-effect mobilities that differ by more than a factor of 4, and that are as high as 0.28 cm2 V–1 s–1. This surprising increase in field-effect mobility arises in particular for the perpendicular orientation of the nanocrystals with respect to the insulator substrate. Further, the perpendicular orientation of P3HT nanocrystals can be explained by the following factors: the unshared electron pairs of the SAM end groups, the π–H interactions between the thienyl-backbone bearing π-systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs.

391 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report the electrical characterization of field effect transistors based on regioregular poly(3-hexylthiophene) (RRP3HT) nanofibers fabricated using nanostencil shadow masks.
Abstract: We report the electrical characterization of field effect transistors based on regioregular poly(3-hexylthiophene) (RRP3HT) nanofibers fabricated using nanostencil shadow masks. Mobility values were 0.02 cm2/Vs with on/off current ratios of 106. Current densities of ∼700 A/cm2 were achieved in single nanofibers. A series of Soxhlet extractions was employed to separate RRP3HT into narrow molecular weight fractions. Nanofibers made from the THF fraction exhibited superior electrical properties in terms of increased current levels and decreased activation energy. The lowest activation energies in the nanofibers were achieved by using top contacts (i.e., vapor-deposited metal on top of the nanofibers) and material purified by Soxhlet extraction. Contact effects were eliminated from bottom contact devices (i.e., nanofibers on top of metal electrodes on a dielectric substrate) with a four-probe geometry. Temperature-dependent measurements reveal two distinct regimes of transport. The high-temperature regime (35...

234 citations


Journal ArticleDOI
TL;DR: It is confirmed that DNA sequence can be detected by measuring the variation of the drain current due to the variationof DNA charge and the proposed FET-type DNA charge sensor might be useful in the development for DNA chips.

205 citations


Journal ArticleDOI
TL;DR: In this article, single crystal high mobility organic field effect transistors (OFETs) were prepared on prefabricated substrates using a "flip-crystal" approach.
Abstract: We report on single crystal high mobility organic field-effect transistors (OFETs) prepared on prefabricated substrates using a "flip-crystal" approach. This method minimizes crystal handling and avoids direct processing of the crystal that may degrade the FET electrical characteristics. A chemical treatment process for the substrate ensures a reproducible device quality. With limited purification of the starting materials, hole mobilities of 10.7, 1.3, and 1.4 cm^2/Vs have been measured on rubrene, tetracene, and pentacene single crystals, respectively. Four-terminal measurements allow for the extraction of the "intrinsic" transistor channel resistance and the parasitic series contact resistances. The technique employed in this study shows potential as a general method for studying charge transport in field-accumulated carrier channels near the surface of organic single crystals.

188 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented the first results of thin film transistors produced completely at room temperature using ZnO as the active channel and silicon oxynitride as the gate dielectric.
Abstract: In this paper we present the first results of thin film transistors produced completely at room temperature using ZnO as the active channel and silicon oxynitride as the gate dielectric. The ZnO-based thin film transistors (ZnO-TFT) present an average optical transmission (including the glass substrate) of 84% in the visible part of the spectrum. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 1.8 V. A field effect mobility of 70 cm 2 /V s, a gate voltage swing of 0.68 V/decade and an on-off ratio of 5 × 10 5 were obtained. The combination of transparency, high field-effect mobility and room temperature processing makes the ZnO-TFT very promising for the next generation of invisible and flexible electronics.

150 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the Schottky barrier heights in the perspective of integration of metal-oxide-semiconductor field effect transistors (MOSFETs) with a metallic source/drain.
Abstract: This article investigates the extraction of low Schottky barrier heights in the perspective of integration of metal–oxide–semiconductor field effect transistors (MOSFET) with a metallic source/drain. A test structure composed of two back-to-back junctions is proposed to characterize materials with a low Schottky barrier. To complete the proposed measurement setup, particular attention is placed on a Schottky transport model that continuously combines thermionic emission, field emission, and barrier lowering due to image charge. In the case of platinum silicide (PtSi) contact, it is shown that Arrhenius plots can be accurately reproduced over a wide range of temperature and applied bias. A consolidation of the measurement strategy and of the associated transport model is also performed through measurements and simulations on a long channel p-type Schottky barrier silicon-on-insulator MOSFET with PtSi source/drain. A excellent agreement between simulated and experimental current-voltage characteristics is o...

110 citations


Journal ArticleDOI
TL;DR: In this article, a new electrokinetically driven active micro-mixer is presented, which uses localized capacitance effects to induce zeta potential variations along the surface of silica-based microchannels.
Abstract: This paper presents a new electrokinetically driven active micro-mixer which uses localized capacitance effects to induce zeta potential variations along the surface of silica-based microchannels. The mixer is fabricated by etching bulk flow and shielding electrode channels into glass substrates and then depositing Au/Cr thin films within the latter to form capacitor electrodes, which establish localized zeta potential variations near the electrical double layer (EDL) region of the electroosmotic flow (EOF) within the microchannels. The potential variations induce flow velocity changes within a homogeneous fluid and a rapid mixing effect if an alternating electric field is provided. The current experimental data confirm that the fluid velocity can be actively controlled by using the capacitance effect of the buried shielding electrodes to vary the zeta potential along the channel walls. While compared with commonly used planar electrodes across the microchannels, the buried shielding electrodes prevent current leakage caused by bad bonding and allow direct optical observation during operation. It also shows that the buried shielding electrodes can significantly induce the field effect, resulting in higher variations of zeta potential. Computational fluid dynamic simulations are also used to study the fluid characteristics of the developed active mixers. The numerical and experimental results demonstrate that the developed microfluidic device permits a high degree of control over the fluid flow and an efficient mixing effect. Moreover, the developed device could be used as a pumping device as well. The development of the active electrokinetically driven micro-mixer could be crucial for micro-total-analysis-systems.

100 citations



Journal ArticleDOI
TL;DR: In this article, an n-type pentacene field effect transistor based on interface-doped Pentacene is demonstrated, laying a headstone for an organic complementary metal-oxide-semiconductor technology.
Abstract: The realization of an n-type pentacene field-effect transistor based on interface-doped pentacene is demonstrated, laying a headstone for an organic complementary-metal–oxide–semiconductor technology. The doping is performed by depositing traces of calcium onto the gate insulator before applying the organic semiconductor. Electron field-effect mobilities of 0.19cm2V−1s−1 are achieved. The field effect, i.e., the electron accumulation behavior, is studied by impedance spectroscopy and charge measurements on a metal–insulator–semiconductor (MIS) diode. A good correlation between the physical properties of the transistor and the MIS diode can be reported. A temporal dynamics and a hysteresislike accumulation behavior are observed, both explainable by the influence of deep electron traps.

Journal ArticleDOI
TL;DR: In this article, a method of extracting the field-effect mobility from the transfer characteristics of organic polymer thin-film transistors (OP-TFTs), in both the linear and saturation regimes, by accounting for the dependence of the mobility on the gate bias, which translates to a dependence on the accumulated density of majority charge carriers in the channel.
Abstract: We present a method of extracting the field-effect mobility from the transfer characteristics of organic polymer thin-film transistors (OP-TFTs), in both the linear and saturation regimes, by accounting for the dependence of the mobility on the gate bias, which translates to a dependence on the accumulated density of majority charge carriers in the channel. This method is compared to the commonly used extraction methods, which are based on the standard MOSFET square-law drain current equations that do not account for the variation of mobility with the applied gate bias. We show that by using the standard MOSFET equations, the extracted field-effect mobility can be significantly overestimated. We also demonstrate the use of the proposed method to extract the field-effect mobility at different measurement temperatures and present the dependence of the extracted parameters on temperature.

Journal ArticleDOI
TL;DR: In this article, the influence of dip-coating speed and concentration of the polymer solution on the characteristics of field effect transistors (FETs) fabricated in the bottom-contact structure with regioregular poly(3-hexylthiophene) (RR-P3HT) as the active semiconducting material was reported.

Journal ArticleDOI
TL;DR: In this paper, the field effect in pentacene thin-film transistors was studied using bottom-contact devices with channel lengths below 10nm, and a pair of guarding electrodes as close as 20nm to the two sides of the channel was employed to suppress spreading current.
Abstract: The field effect in pentacene thin-film transistors was studied using bottom-contact devices with channel lengths below 10nm. To suppress spreading current in these devices, which have a small channel width-to-length (W-L) ratio, we employed a pair of guarding electrodes as close as 20nm to the two sides of the channel. The responses of these nanometer scale transistors exhibit good gate modulation. Mobilities of 0.046cm2∕Vs and on/off ratios of 97 were achieved in sub-10-nm transistors. We find that the device response is strongly influenced by the nature of the metal-semiconductor contact.

Journal ArticleDOI
TL;DR: In this article, the effects of O 2 and H 2 O on the electrical characteristics of pentacene thin film transistors have been investigated using in situ and ex situ I-V measurements.

Journal ArticleDOI
TL;DR: In this article, the effect of octadecyltrimethoxysilane (OTMS) interlayer with gate dielectric surface modification on the field effect mobility of OTFTs has been examined.
Abstract: Organic thin-film transistors (OTFTs) based on pentacene semiconductor are elaborated on the plastic substrates through a four-level mask process without photolithographic patterning to yield a simple fabrication process. Octadecyltrimethoxysilane (OTMS) as an organic molecule for self-assembled monolayers is deposited on the surface of zirconium oxide dielectric layer. The effect of OTMS interlayer with gate dielectric surface modification on the field effect mobility of OTFTs has been examined and these prototype organic transistors showed excellent electrical characteristics with field effect mobility >0.66cm2∕Vs and Ion∕Ioff>10.5

Patent
25 Nov 2004
TL;DR: In this paper, the authors proposed a field effect transistor which can carry out an enhancement type operation in which a current does not flow between a source and a drain when a gate voltage is 0 V.
Abstract: PROBLEM TO BE SOLVED: To provide a field effect transistor which can carry out an enhancement type operation in which a current does not flow between a source and a drain when a gate voltage is 0 V. SOLUTION: The field effect transistor comprises a hetero structure which forms an AlGaN layer 3 composed of a first nitrogen compound film, and a GaN layer 2 composed of a second nitrogen compound film having a composition different from the AlGaN layer 3. Further, one GaN layer 4 or more are formed as a stress relief layer for cancelling a piezo field effect generating at a crystalline interface between the first nitrogen compound film and the second nitrogen compound film. As an FET of the enhancement type operation used for a general-purpose switching electric source or the like, the AlGaN/GaN hetero structure FET technology of a wide gap semiconductor with the high mobility is utilized, thereby intending to decrease an energization loss by a reduction in an operation resistance, in particular, materializing a complete operation in a GaN system FET. COPYRIGHT: (C)2005,JPO&NCIPI

Journal ArticleDOI
TL;DR: In this paper, the possibility of metallic field-effect transistors in one-dimensional systems and particularly in armchair carbon nanotubes was demonstrated, and theoretical arguments were developed that demonstrate that metallic field effect transistors can penetrate the relatively weakly screened carbon nano-tubes and open an energy gap.
Abstract: We develop theoretical arguments that demonstrate the possibility of metallic field-effect transistors in one-dimensional systems and particularly in armchair carbon nanotubes A very inhomogeneous electric field, such as the field of a tunneling tip, can penetrate the relatively weakly screened nanotubes and open an energy gap As a consequence, an energy barrier forms that impedes electron flow and thus permits transistor action This type of metallic field effect is advantageous because of the high conductance of the metallic tubes in the ON state

Journal ArticleDOI
TL;DR: In this paper, the magnetotransport properties of artificial superlattices with ferromagnetic Pr0.85Ca0.15MnO3 insulating layer and ferroelectric Ba0.6Sr0.4TiO3 layer were measured with the current perpendicular to the plane geometry.
Abstract: Artificial superlattices designed with ferromagnetic Pr0.85Ca0.15MnO3 insulating layer and ferroelectric Ba0.6Sr0.4TiO3 layer were grown on (100) SrTiO3 substrates. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low field effect, was observed as the number of ferroelectric Ba0.6Sr0.4TiO3 layer thickness increases even up to 9 unit cells. For example, the superlattice [(Pr0.85Ca0.15MnO3)10(Ba0.6Sr0.4TiO3)9]25 shows 35 % MR at 100 K, though the Pr0.85Ca0.15MnO3 film was a robust insulator with negligible MR even at high applied magnetic field. This observed large MR cannot be explained by simple interfacial ferromagnetism or by the tunneling magnetoresistance. One possible explanation could be the effect due to the ferroelectric spacer layer and the associated magnetoelectric coupling.

Journal ArticleDOI
TL;DR: In this article, the correlation between the evolution of the nc-Si:H structure and TFT electrical characteristics and determine the onset thickness of the continuously crystalline channel is determined.
Abstract: Thin film transistors (TFTs) of nanocrystalline silicon (nc-Si:H) can be made at temperatures as low as 150 °C and are capable of p and n channel operation. High carrier mobility and low off current can be achieved in the staggered top-gate, bottom-source/drain geometry. Beginning the growth of nc-Si:H with a seed layer placed underneath the TFT promotes the structural evolution of the nc-Si:H channel layer and raises the carrier field effect mobilities and current ON/OFF ratio. We study the correlation between the evolution of the nc-Si:H structure and TFT electrical characteristics and determine the onset thickness of the continuously crystalline channel. The nc-Si:H surface topography is characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM), while electrical properties are examined by TFT performance. 50-nm thick channel layers have the smallest surface roughness and yield transistors with the highest hole and electron field effect mobilities of ∼0.25 and ∼40 cm 2 V −1 s −1 , respectively.

Journal ArticleDOI
TL;DR: In this paper, the mechanism behind the current modulation is investigated, and it is shown that the current is modulated through ion-assisted oxidation and reduction of the semiconductor by ions moving vertically in the insulator material to the transistor channel.
Abstract: We have fabricated solution processable polymer transistors with high conductivity, requiring only a few volts for obtaining good current modulation. The devices can be fabricated and operated in air and the operation is greatly enhanced in humid atmosphere. Devices reach an On∕Off ratio of about 600 and a subthreshold swing of 500mV per decade operating on voltages less than 2V. In this letter the mechanism behind the current modulation is investigated, and we show that the current is modulated through ion-assisted oxidation and reduction of the semiconductor by ions moving vertically in the insulator material to the transistor channel.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of annealing on the field effect transistor (MOSFET) interface states and found that the S-factor and mobility of (1) p-MOS-FETs were improved by decreasing the interface states.
Abstract: In this study, the characteristic improvement of Si (111) metal–oxide–semiconductor field-effect transistor (MOSFET) by decreasing the interface states has been investigated. As a process step for this characteristic improvement, long-time annealing in hydrogen ambient was performed after complementary MOS (CMOS) fabrication. It is found from the evaluated results that the interface state density of the Si (111) MOS structure decreases markedly upon increasing the annealing time. Furthermore, the S-factor and mobility of (111) MOSFETs were improved markedly. In particular, the field effect mobility of a (111) p-MOSFET was higher than that of a (100) p-MOSFET in spite of its possessing large interface states. It has been experimentally confirmed for the first time that long-time hydrogen annealing is an essential and effective process for the fabrication of (111) MOSFETs with a standard Si–SiO2 system.

Journal ArticleDOI
TL;DR: In this article, an FET operation is observed without any annealing processes even once the fabricated devices are exposed to air, which has not ever seen in the conventional C 60 FETs.

Journal ArticleDOI
TL;DR: In this article, a two orders of magnitude increase in field effect mobility was observed from electrical data of transistors fabricated using FeCl3 doped RR-P3HT and MEH-PPV.
Abstract: Polymer-based thin film transistors (PTFTs) were fabricated on glass substrates with anodized Al2O3 as gate insulators. RR-P3HT (regioregular poly–3-hexylthiophene) and MEH-PPV [poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene)] were respectively used as semiconducting active layers for the transistors. A two orders of magnitude increase in field effect mobility (from 7.2×10−4 cm2/V s to 7.4×10−2 cm2/V s) deduced from electrical data of transistors fabricated using FeCl3 doped RR-P3HT was observed. This increase is believed to be mainly due to a large reduction in contact resistance (from 108 Ω to 103 Ω) to the source and drain Au contacts. The conductivity of RR-P3HT was found to increase only slightly with the doping. For MEH-PPV, doping with FeCl3 also decreased its contact resistance. However, it (4 GΩ) was still much larger than the channel (polymer) resistance (1 MΩ), leading to a slight improvement in its field effect mobility. Theoretically, contacts between Au and P3HT should have very...

Journal ArticleDOI
TL;DR: In this article, the field effect mobility of lateral n-channel Si face 4H-SiC MOSFETs made by gate oxidation in N/sub 2/O ambient was investigated.
Abstract: A report is made on field effect mobility of 150 cm/sup 2//Vs in lateral n-channel Si face 4H-SiC MOSFETs made by gate oxidation in N/sub 2/O ambient. The high mobility is correlated with a two orders of magnitude reduction in density of interface states near the SiC conduction band edge when compared to gate oxides made in a wet or dry oxygen ambient.

Journal ArticleDOI
TL;DR: In this paper, an epitaxial heterostructure composed of ferroelectric Pb(Zr0.2Ti0.8)O3 and superconducting Nb-doped SrTiO3 was used for field effect experiments.
Abstract: We have performed ferroelectric field effect experiments using an epitaxial heterostructure composed of ferroelectric Pb(Zr0.2Ti0.8)O3 and superconducting Nb-doped SrTiO3. The films were prepared on (001) SrTiO3 substrates by off-axis radio-frequency magnetron sputtering and pulsed-laser deposition. By switching the polarization field of the 500-A-thick Pb(Zr0.2Ti0.8)O3 layer, a large change of about 30% in resistivity and a 20% shift of Tc (ΔTc∼0.05 K) were induced in the 400-A-thick epitaxial Nb-doped SrTiO3 layer. The relationship between Tc and the electrostatically modulated average carrier concentration can be mapped onto the phase diagram of chemically doped SrTiO3.


Journal ArticleDOI
TL;DR: In this article, a comparative analysis of a new series of oligothiophene derivatives as active semiconductor in organic field effect transistors (OFETs) is presented, in terms of steric interactions and also on the basis of the influence of the end group on the molecular arrangement on the substrate.

Journal ArticleDOI
TL;DR: In this article, the authors show that the trap concentration of each level is highly sensitive to the applied electric field during the DLTFS measurements, and correlate this effect to the transformation of electrically active charged defects into inactive ones under the influence of the electric field, which is increased in the depletion zone by the applied reverse bias.
Abstract: Deep level transient Fourier spectroscopy (DLTFS) has been performed on p–n diodes of GaN. Typical deep level spectra of the various diodes realized on the same wafer demonstrate three electron trap levels, E1, E2 and E3 with activation energies of 0.59, 0.76 and 0.96 eV, respectively below the conduction band. The traps concentration of each level is highly sensitive to the applied electric field during the DLTFS measurements. As a result of repetitive DLTFS scans on the representative diode, the level E2 disappears after the fourth scan, while the associated peak height of the levels E1 and E3 exhibit an overall reduction by a factor of two. We correlate this effect to the transformation of electrically active charged defects into inactive ones under the influence of the electric field, which is increased in the depletion zone by the applied reverse bias. Possible inactivation mechanisms and origins of the observed levels are discussed.

Journal ArticleDOI
TL;DR: In this article, a correlated quantum-chemical approach coupled with a first-order perturbation was applied to investigate the relationship between the formation ratio and the electric field, and it was found that for p-phenylene-vinylene oligomer, the formation rate of singlet excitons with respect to triplet exciton increases with the external electric field.
Abstract: Recently, both experimental and theoretical evidence indicate that the electrically-generated singlet and triplet exciton formation ratio (r S / T ) in conjugated polymers can exceed the 1:3 spin statistics limit. However,the extent that the electric field influences r S / T is in controversy. By measuring the rates of photo- and electro-phosphorescence and fluorescence, Wilson et al. 6 concluded that the ratio r S / T is independent of the external electric field; Lin et al. 8 found that the ratio increases monotonically with the electric field by measuring the relative densities of singlet and triplet exciton in EL and PL processes; from a quantum-dynamical calculation, Tandon et al. 1 1 observed an abrupt increase in the ratio with respect to the electric field. In this work, we apply a correlated quantum-chemical approach coupled with a first-order perturbation, to investigate the relationship between the formation ratio and the electric field. We have calculated the influences of the electric field on the singlet and triplet exciton states as well as on the electron-hole free pair (charge-transfer) state. It is found that for p-phenylene-vinylene oligomer, the formation ratio of singlet excitons with respect to triplet excitons increases with the electric field.