Showing papers on "Organic semiconductor published in 1995"

Charge separation and photovoltaic conversion in polymer composites with internal donor/acceptor heterojunctions

Abstract: The photosensitivity of semiconducting polymers can be enhanced by blending donor and acceptor polymers to optimize photoinduced charge separation. We describe a novel phase‐separated polymer blend (composite) made with poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene], MEH‐PPV, as donor and cyano‐PPV, CN‐PPV, as acceptor. The photoluminescence and electroluminescence of both component polymers are quenched in the blend, indicative of rapid and efficient separation of photogenerated electron‐hole pairs with electrons on the acceptor and holes on the donor. Diodes made with such a composite semiconducting polymer as the photosensitive medium show promising photovoltaic characteristics with carrier collection efficiency of 5% electrons/photon and energy conversion efficiency of 0.9%, ∼20 times larger than in diodes made with pure MEH‐PPV and ∼100 times larger than in diodes made with CN‐PPV. The photosensitivity and the quantum yield increase with reverse bias voltage, to 0.3 A/W and 80% electrons/photon respectively at −10 V, comparable to results obtained from photodiodes made with inorganic semiconductors.

Field effect measurements in doped conjugated polymer films: Assessment of charge carrier mobilities

Abstract: Conductivity and field‐effect mobility measurements using metal‐insulator‐semiconductor field‐effect‐transistor devices and acceptor density measurements using metal‐insulator‐ semiconductor (MIS) diodes are presented. The measurements were made on thin polymer films of the organic semiconductor, poly(β’‐dodecyloxy‐α,α’,‐α’,α‘terthienyl), which were doped to different conductivities using 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) as an oxidizing agent. It is found that both the field‐effect mobility and the conductivity of these films increases superlinearly upon doping while the transistor amplification, the on/off ratio, decreases. Acceptor densities as obtained from MIS diode measurements are in close agreement with the bulk charge density as calculated from the DDQ content. However, the product of this bulk charge density, field‐effect mobility, and the elementary charge e is a factor of 100 larger than the polymer conductivity. This indicates that the average mobility for charge carriers in the...

Band structures of organic thin-film transistor materials

Abstract: We report extended Huckel theory (EHT) band structure calculations for the high-temperature polymorph of α-6T (α-sexithiophene) and C60, two organic materials that have shown promising device characteristics, as the active element in thin-film transistors (TFTs). We also report calculations for α-3T and κ-(ET)2Cu(NCS)2[where ET = bis(ethylenedithio)tetrathiafulvalene] for comparison purposes. Both α-6T/HT and C60 show well developed band dispersions. Whereas C60 is an isotropic three-dimensional (3D) molecular solid, α-6T/HT shows a strongly 2D electronic structure, and the band structure is quite similar to that of the ET organic superconductors such as κ-(ET)2Cu(NCS)2. Factors important in determining the mobilities of organic semiconductors are reviewed.

Semiconductor device having an organic semiconductor material

Abstract: A semiconductor device is provided with an organic material which is formed by a solid-state mixture of organic donor and organic acceptor molecules. A semiconducting solid-state mixture is known with molar ratios between donor and acceptor molecules of 1.3:2 and 1.66:2. The known solid-state mixture has the disadvantage that its electrical conductivity is comparatively high, so that it is not possible to manufacture switchable devices from the mixture. Here the material includes an n- or p-type semiconductor material, the n-type semiconductor material having a molar ratio between the donor and acceptor molecules below 0.05, and the p-type semiconductor material having this ratio above 20. These solid-state mixtures may be used for manufacturing switchable semiconductor devices. The n- and p-type organic solid-state mixtures can be used for manufacturing transistors, diodes, and field effect transistors in a same manner as, for example, doped silicon or germanium.

Conduction type control from n to p type for organic pigment films purified by reactive sublimation

Abstract: The effects of purification by reactive sublimation technique and bromine doping on the Fermi level and the photovoltaic properties of n‐type perylene pigment films were investigated. Photovoltage arisen from the Schottky junction between n‐type perylene pigment film and Au increased significantly by repeating the train sublimation under methylamine gas atmosphere. This phenomenon was revealed to be due to the negative shift of the Fermi level resulting from the effective removal of unknown but specific impurity acting as an acceptor by reactive sublimation. On the other hand, by bromine doping, Fermi level of the pigment film shifted largely to a positive direction and reached the nearby valence band, while the direction of photocurrent flow arising from the Schottky junction with Au was reversed. This result is a clear demonstration of alternating the type of conduction from n type to p type. This means that the pn control of organic semiconductors is possible.

Fermi Level Shift in Photoconductive Organic Pigment Films Measured by Kelvin Vibrating Capacitor Method

Abstract: Fermi level shift in n-type perylene pigment film was measured by the Kelvin vibrating capacitor method. Purification of pigment by reactive sublimation under methylamine atmosphere caused negative shift of the Fermi level due to the effective removal of unknown but specific impurity acting as an acceptor. On the other hand, bromine doping of pigment film caused marked positive shift of Fermi level to near the valence band, resulting in reversal of the photocurrent flow arising from the Schottky junction with Au. This is a clear demonstration of the control of conduction type from n-type to p-type, which was observed for the first time in pigment-based organic semiconductors. This result enabled construction of an organic homojunction composed of p- and n-type perylene pigment films.

Photonic Devices Based on Crystalline Organic Semiconductors for Optoelectronic Integrated Circuits

Abstract: Crystalline organic thin films are deposited under ultrahigh vacuum conditions at substrate temperatures from 77 K to 300 K. Due to the weak van der Waals bonding forces between the organic molecules, lattice matching is not required for layer growth. Structures can be prepared both on dielectric or amorphous substrates and on top of completely processed III-V or Si wafers. In this paper, we discuss the fabrication and performance of organic semiconductor electroluminescent devices, waveguides, and photodetectors.

Picosecond time-resolved luminescence of tetracene thin films

Abstract: The energy relaxation of excitons in the organic semiconductor tetracene (film thickness 160 nm) is studied by photoluminescence from 15 to 300 K using time‐correlated single‐photon counting. The well‐known relaxation of triplet excitons with time constant ∼7 ns is accompanied by a faster relaxation process with time constants ranging from 260 ps to 1.2 ns, depending on temperature and wavelength.

Light-emitting diodes based on copolymer organic semiconductors

Abstract: Light-emitting diodes (LED5) based on organic semiconductors have received much attention recently due to their promise as cheap, novel light sources for electro-optical applications. Unlike conventional diodes, light emission from these organic LEDs is achieved by double injection of electrons from a low-work-function electrode and holes from a high-work-function electrode into the organic polymer light-emitting layer. By appropriately engineering the polymer backbone, emission of various visible colors has been demonstrated. Our work in this field has concentrated on organic block copolymers as the active light-emitting materials. We demonstrate that the use of copolymer systems leads to large enhancement of device performance.

Thin film transistor and fabrication thereof

Abstract: PROBLEM TO BE SOLVED: To control the conditions of an organic semiconductor layer, employed as an active layer, in the conducting direction of carrier by arranging the skeletal chain of polymer in the organic semiconductor layer in random direction using a simple method. SOLUTION: A source electrode 4 and a drain electrode 5 are formed on a gate insulation layer 2 which is then subjected to orientation between the source electrode 4 and drain electrode 5 by rubbing the surface in one direction with a dust-free cloth. Consequently, liquid crystal substituents introduced into a polymer composing an organic semiconductor layer are oriented in parallel with the rubbing direction. Since the skeletal chain of polymer is not arranged necessarily in parallel with the source/drain direction of semiconductor element but in a random direction for ensuring good characteristics of element, the characteristics of organic semiconductor layer can be enhanced structurally and a high performance thin film transistor can be obtained.

Deposition and characterisation of crystalline organic semiconductors for photonic devices

Abstract: The growth of multilayer sequences of the organic semiconductors 3,4,9,1 O-perylenetetracarboxylic dianh ydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, copper phthalocyanine, and 8-hydroxyquinoline aluminiumfor photonic devices by organic molecular beam deposition is investigated. Quasiepitaxial thin films are obtained at low growth rates and low substrate temperatures. Waveguides, photodetectors, and electroluminescent devices are successfully fabricated and characterised with respect to their electrical and optical characteristics and the device performance.MST/3287

Novel Organic Semiconductor BTQBT with High Conductivity and High Mobility

Abstract: We have found a novel organic semiconductor, BTQBT, with a high electrical conductivity of about 10 -3 S cm -1 and a high Hall mobility of about 3 cm2 V1 s -1 at room temperature. We have also succeeded in measuring the temperature dependence of the Hall mobility. It varies with temperature as Τ -1 · 6 , which agrees with the theoretical Τ -1 '5 dependence for the mobility determined by lattice scattering. The characteristic transport property of BTQBT results from strong intermolecular interactions in the crystal, which is convinced by the energy dispersion with a bandwidth of about 0.5 eV from the theoretical and experimental band structures. PACS numbers: 72.20.-i, 79.60.-i

Organic solar battery equipment

Abstract: PURPOSE:To improve power generation efficiency, by installing an organic solar battery in the manner in which the battery surface is inclined to incident ray. CONSTITUTION:In an organic solar battery 01, an N-type organic semiconductor 3, a P-type semiconductor 2, and a metal electrode 5 are formed on an ITO transparent electrode substrate 4 by vacuum evaporation. A glass plate 5 is fixed on the electrode substrate 4. The organic solar battery 01 is fixed on a retaining stand 11 by a fixing plate 12, at an inclination angle alpha=60 deg.. The traveling distance of the ray passing the vicinity of the interface of the P-layer 2 and the N-layer 3 becomes long in the case of oblique ray (b), as compared with vertical ray (a). Hence the absorption efficiency in this part is increased, and power generation efficiency can be improved.

Photoconductivity of iodine-doped single crystals of phthalocyanine

Abstract: Single crystals of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were grown in the presence of iodine vapour. The presence of iodine enhances the spectral dependence of photoconductivity of H2Pc in the visible region but of CuPc in the near-IR region. The dark current is decreased but the photocurrent is increased by one order of magnitude in iodine-doped H2Pc but in the case of iodine-doped CuPc both currents are increased by nearly three orders of magnitude. Introduction of iodine results in about one order of magnitude decrease in response time for both modifications. Thus the introduction of iodine into Pc crystals decreases the energy barrier for conduction and increases the drift mobility of charge carriers thereby enhancing the conductivity of the material.

Bio-sensors with polymeric working layer

Abstract: Biosensors for converting a biochemical signal into an electronic signal are characterised in that biological specificity is achieved by: (1) molecular imprinting of polymers; or (2) using an organic semiconductor having inbuilt organic mols. that specifically recognise the substance to be detected.

X-Ray photoelectron spectroscopy characteristics of a novel organic semiconductor BTQBT and its derivatives

Abstract: X-Ray photoelectron spectra of bis[1,2,5]thiadiazole-p-quinobis(1,3-dithiole)(BTQBT) reveal that the sulfur and carbon atoms in the molecules show four- and two-peak profiles, respectively, and are in mixed-valence states. Substitution of the sulfur atoms linked to nitrogen atoms by selenium atoms removes the mixed valency of sulfur. In systems with asymmetrical molecular skeletons, such as TMBTQBT, TMTBTQBT and DBBTQBT, sulfur and carbon atoms are also in mixed-valence states. TMBTQBT shows the largest energy difference between the C 1s and S 2p peaks, indicating the largest charge distribution. Each of the S 2p and C 1s peaks is accompanied by shake-up satellites located to higher energy at ca. 2.7 eV from the corresponding photoelectron main peak.

Semiconductor device provided with an organic semiconductor material

Abstract: The invention relates to a semiconductor device provided with an organic material which is formed by a solid-state mixture of organic donor and organic acceptor molecules. A semiconducting solid-state mixture is known with molar ratios between donor and acceptor molecules of 1.3:2 and 1.66:2. The known solid-state mixture has the disadvantage that its electrical conductivity is comparatively high, so that it is not possible to manufacture switchable devices from the mixture. According to the invention, the semiconductor device is characterized in that the material comprises an n- or p-type semiconductor material, the n-type semiconductor material having a molar ratio between the donor and acceptor molecules below 0.05, and the p-type semiconductor material having this ratio above 20. The solid-state mixtures according to the invention may be used for manufacturing switchable semiconductor devices. The n- and p-type organic solid-state mixtures can be used for manufacturing transistors, diodes, and field effect transistors in a same manner as, for example, doped silicon or germanium.

Photoconductivity in thin films of phthalocyanine

Abstract: The photocurrent and electrolyte electromodulation (EEM) spectra of thin films of metal-free phthalocyanine (H2Pc) and of copper phthalocyanine (CuPc) were investigated. The modulation spectra yielded three distinct features around 1·61, 2·30 and 2·93 eV for H2Pc and around 1·63, 2·04 and 3·20 eV for CuPc. The spectral dependence maxima of photoconductivity correspond to the modulation spectra. These features are interpreted to indicate transitions at critical points, i.e. the existence of transitions between three valence bands, since Pc’s are p-type, and the lowest conduction band in Pc’s.

Heterojunction organic thin-film transistors

Abstract: We have developed a unique organic transistor structure which enables us to realize both p-channel and n-channel operation in a single device. This device also demonstrates the applicability of heterojunction concepts to organic semiconductors. The active material consists of two layers: the first layer, typically 10-20 nm thick, is made up of /spl alpha/-hexathienylene (/spl alpha/-6T), a thiophene oligomer which exhibits good p-channel operation. The second active material is C/sub 60/ and is about 20-30 nm thick. The energy levels of the occupied and unoccupied molecular orbitals of /spl alpha/-6T and C/sub 60/ are such that when the gate is biased negatively with respect to the source, the p-channel material is filled with holes and when the gate is biased positively, the n-channel material (C/sub 60/) is filled with electrons. A detailed analysis of the transistor characteristics provides important insights into transport in organic materials and is discussed.

Electric “Contacts” between Conductors and Protein Active Sites

Abstract: The problems related to the mechanisms of electron transfer between electron-conducting structures (metals, semiconductors or organic conductors) and biopolymer molecules are central and most important in biolelectronics. Our interest in this area was to study the enzyme behavior at the interface “ionic conductor (electrolyte solution or solid electrolyte) - electron conductor (metal, carbon or semiconductor)”. In the experiments with various enzymes, we have shown that the direct electron transport between a conductor and the active site of redox enzyme is feasible. The rates of electron transfer, which may well proceed by the tunneling mechanism, can exceed the rates of subsequent enzymatic steps. In this case, we can think of an electric “contact” between semiconductor and enzyme active site. Experimentally, the processes of electron transfer between enzyme active site and conductor are most convenient to observe in electrochemical systems upon enzyme immobilization on electron conductor surface. In this case, the enzyme can act as electrocatalyst “pumping out” the electrons from conductor or donoring them into the conductor. This phenomenon was first revealed for a blue copper-containing oxidase (lacease) reducing molecular oxygen to water; the source of electrons was the conductor matrix [Berezin I.V et al. 1978; Berezin I.V et al., 1980]. The phenomenon of acceleration of electrode processes in direct “contact” of the enzyme and conductor was termed bioelectrocatalysis. Later, this phenomenon was observed for a number of other enzymes and for the enzymes entrapped in the matrices of organic semiconductors and organic metals [Varfolomeev S.D. et al, 1978; Hill H.A.O. et al, 1981]. This review is an endeavour to sum up these studies and to demonstrate the applicability of direct electron transport for creation of reagentless biosensors.

Optimization of Current-Voltage Characteristics of Organic-on-Inorganic Heterostructure Diodes

Abstract: Low barrier quasi-Schottky diodes for future microwave mixer applications are presented. The diodes are based on a heterojunction between an inorganic III-V semiconductor (GaAs, InP) and the aromatic compound PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) as a crystalline organic semiconductor. Measurements of the static I-V-characteristic with emphasis on the quadratic I-V behavior in forward direction and the use in microwave systems are discussed.

Simultaneous transferring and reserving of solar energy as electrical energy in a single organic semiconductor structure

Abstract: Thin and thick films of a specially doped organic material, here called CHLOH, on a special pair of inorganic substrates are investigated using a trial method. The reached component converts and reserves the radiated energy such as solar energy in the environment simultaneously. The stability and efficiency of the new thick film structure have been tested since 1984 and the results illustrate that is useful as an alternate energy supplier. The other aspect of the material is its organization as a thin CHLOH structure for application as an active element in optoelectronics. >