Orientation management of α-sexithiophene layer for the application in organic photovoltaic devices

Management of α-sexithiophene (6T) molecular orientation is a key issue to improve the performance of organic photovoltaic (OPV) devices based on 6T thin films, which act as p-type semiconductor layers; however, it is difficult to produce 6T layers with the molecules lying on smooth surfaces, including oxide covered substrates. We have succeeded in orienting the 6T molecules on oriented conductive polythiophene (PT) films, parallel to the oriented PT molecular axis; i.e. parallel to the substrate, by evaporating 6T in vacuum. Here, we reported planar heterojunction (PHJ) OPV devices consisting of 6T films with molecular orientation parallel to the substrate. The orientation of 6T led to improvement of the power conversion efficiency (PCE) in these PHJ OPVs. The average PCE of PHJ OPVs with oriented PT was 2.8 times higher than that without PT. The PCE and the maximum value of the short-circuit photocurrent action spectra of the OPV with the PT was larger under irradiation of polarized light parallel to the 6T molecular axis than that orthogonal to the axis. On the other hand, the PCE and the action spectra of the OPV without the PT did not show the response to polarized light.     

Efficient and color-stable solid-state white light-emitting electrochemical cells employing red color conversion layers

We report efficient and color-stable white light-emitting electrochemical cells (LECs) by combining single-layered blue-emitting LECs with red-emitting color conversion layers (CCLs) on the inverse side of the glass substrate. By judicious choosing of the red-emitting dye doped in CCLs, good spectral overlap between the absorption spectrum of the red-emitting dye and the emission spectrum of the blue-emitting emissive material results in efficient energy transfer and thus sufficient down-converted red emission at low doping concentrations of the red-emitting dye in the CCLs. Low doping concentration is beneficial in reducing self-quenching of the red-emitting dye, rendering efficient red emission. Electroluminescent (EL) measurements show that the peak external quantum efficiency and the peak power efficiency of the white LECs employing red CCLs reach 5.93% and 15.34 lm W⁻¹, respectively, which are among the highest reported for white LECs. Furthermore, these devices exhibit bias-insensitive white EL spectra, which are required for practical applications, due to nondoped emissive layers. These results reveal that single-layered blue-emitting LECs combined with red-emitting CCLs are one of the potential candidates for efficient and color-stable white light-emitting devices.     

High Hole Mobility and Thickness‐Dependent Crystal Structure in α,ω‐Dihexylsexithiophene Single‐Monolayer Field‐Effect Transistors

Monolayer‐thickness two‐dimensional layers of α,ω‐dihexylsexithiophene (α,ω‐DH6T) exhibit field‐effect hole mobility of up to 0.032 cm² V^?1 s^?1, higher than previously reported for monolayers of other small‐molecule organic semiconductors. In situ measurements during deposition show that the source‐drain current saturates rapidly after the percolation of monolayer‐high islands, indicating that the electrical properties of α,ω‐DH6T transistors are largely determined by the first molecular monolayer. The α,ω‐DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In‐plane lattice constants measured using synchrotron grazing‐incidence diffraction are larger in monolayer‐thickness films than the in‐plane lattice constants of several‐monolayer films and of previously reported thick‐film structures. Near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in‐plane lattice constant of single‐monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω‐DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains.     

Enhancement of carrier mobility along with anisotropic transport in non-regiocontrolled poly (3-hexylthiophene) films processed by floating film transfer method

A newly developed floating film transfer method (FTM) has been successfully utilized to fabricate oriented thin films of non-regiocontrolled poly (3-hexyl thiophene) (NR-P3HT) followed by the fabrication of organic field effect transistors (OFETs). AFM microstructural investigations demonstrate the facile molecular alignment of NR-P3HT by FTM leading to highly oriented macromolecular assemblies like fibrous domains with considerably enhanced π-conjugation length. FTM thin films of NR-P3HT not only show enhanced optical anisotropy (dichroic ratio >8) but also significantly improved FET characteristics. FTM films in its parallel orientation exhibited a significant improvement (>2 orders) in the FET mobility as compared to its spin-coated device counterparts.     

A printing technology combining screen-printing with a wet-etching process for the gate electrodes of organic thin film transistors on a plastic substrate

We have developed a practical printing technology for the gate electrode of organic thin film transistors (OTFTs) by combining screen-printing with a wet-etching process using nano-silver (Ag) ink as a conducting material. An Ag film was deposited onto a PVP (polyvinylphenol)-coated PC (polycarbonate) plastic substrate by screen-printing with nano-Ag ink, where Ag content of 20 wt.% was mixed using a terpineol solvent. Subsequently, the film was cured at 200 °C for 60 min, and then finally wet-etched through patterned positive photo-resist masks. The screen-printed Ag electrode exhibited a minimum line width of ∼5 μm, a thickness of ∼65 nm, and a resistivity of ∼10⁻⁶ Ω cm, producing good geometrical and electrical characteristics for a gate electrode. Additionally, it also provided good step coverage with the PVP dielectric layer, and consequently leakage current between the gate and source/drain electrodes was eliminated. Moreover, the electrical characteristic of the screen-printed Ag electrode was not significantly changed even after a bending test in which the Ag electrodes were bent with a bending radius of 6 mm and 2500 iterations of cyclic bending. OTFTs with the screen-printed Ag electrode produced a saturation mobility of 0.13 cm²/Vs and a current on/off ratio of 1.79 × 10⁶, being comparable to those of an OTFT with a thermally evaporated Al gate electrode.     

Surface analysis of oligothiophene films using HREELS: molecular orientation effects

The surface of evaporated films of α-oligothiophenes—quaterthiophene (4T), quinquethiophene (5T) and sexithiophene (6T)—was studied by high-resolution electron energy loss spectroscopy (HREELS), extending the common vibrational domain of energy loss to electronic excitation and secondary emission. Vibrational spectra show that the molecular orientation is not the same on graphite and gold substrates. HREELS results corroborate those obtained by reflection–absorption infrared spectroscopy (RAIRS), confirming that graphite substrates induce films where molecules lie flat on the surface, whereas gold substrates generate a molecular orientation close to perpendicularity. Using these two differently oriented samples, we demonstrate the existence of two relaxation channels for the electron surface interaction: electronic excitation and ionisation. HREELS spectra recorded with incident electron energy between 2 and 7 eV reveal that secondary electron emission originating from ionisations is always more efficient for escape directions perpendicular to the plane of the molecule. This is here associated with the π orbital origin of this emission. Copyright © 2000 John Wiley & Sons, Ltd.     

Photomodulated transmittance of GaBiAs layers grown on (0 0 1) and (3 1 1)B GaAs substrates

In this work, photomodulated transmittance (PT) has been applied to investigate the energy gap of GaBiAs layers grown on (0 0 1) and (3 1 1)B GaAs substrates. In PT spectra, a clear resonance has been observed below the GaAs edge. This resonance has been attributed to the energy gap-related absorption in GaBiAs. The energy and broadening of PT resonances have been determined using a standard approach in electromodulation spectroscopy. It has been found that the crystallographic orientation of GaAs substrate influences on the incorporation of Bi atoms into GaAs and quality of GaBiAs layers. The Bi-related energy gap reduction has been determined to be ∼90 meV per percent of Bi. In addition to PT spectra, common transmittance spectra have been measured and the energy gap of GaBiAs has been determined from the square of the absorption coefficient α² around the band-gap edge. It has been found that the tail of density of states is significant for GaBiAs and influences the accuracy of energy gap determination from the α² plot. In the case of PT spectra, the energy gap is determined unambiguously since this technique is directly sensitive to singularities in the density of states.     

Langmuir–Blodgett films of indandione-1,3 pyridinium betaine. III: Linear dichroism and non-linear optical properties

Linear dichroism and second-harmonic generation (SHG) have been measured in Langmuir-Blodgett (LB) multilayers of Z-type structure of amphiphilic indandione-1,3 pyridinium betaine (IPB) with an aliphatic tail containing 17 carbon atoms (C17IPB). The dichroic ratio A_s/A_p of electronic absorption of s- and p-polarised light and the SHG intensity show that the ‘heads’ of C17IPB molecules are predominantly oriented along the deposition direction of monolayers on a quartz substrate. Orientation parameters and mean statistical azimuthal and tilt angles are evaluated as a function of the number of monolayers. The SHG efficiency has been measured in C17IPB monolayers on a water surface as a function of surface pressure. The effective hyperpolarisability β of the IPB molecule has been determined experimentally in a chloroform solution by the hyper-Rayleigh-scattering method. The obtained value of β = (138 ± 16) × 10⁻³⁰ esu is in good agreement with theoretical estimates.     

Laser printing of air-stable high performing organic thin film transistors

Active layers involved in top contact organic thin film transistors (TC-OTFTs) have been printed using the laser induced forward transfer (LIFT) technique. Bis(2-phenylethynyl) end-substituted terthiophene (diPhAc-3T) as a p-type organic semiconductor was vacuum evaporated on a quartz substrate prior to the transfer by laser onto an acceptor substrate to form an organic active layer for charge transport. The resulting printed diPhAc-3T pixels on the receiver substrates have a homogeneous morphology as shown by optical microscopy and atomic force microscopy (AFM). Electrical characterizations demonstrated that these transistors are fully functional with hole mobilities up to 0.04 cm²/V s, threshold voltage V_t near 0 V and I_on/I_off ratio up to 2.8 × 10⁵. The efficient cohesion of diPhAc-3T vacuum evaporated thin films induced by 3-dimensional growth offers an exceptionally high physical resistance to laser pulses. The large intermolecular interaction involved in such growth mechanism makes the thin films less sensitive to the mechanical damages induced by the laser. Due to the optical properties of diPhAc-3T, the use of a protecting layer deposited on the donor substrate prior to the diPhAc-3T active layer to trap the incident radiation during the LIFT was not required.     

Growth of rubrene crystalline thin films using thermal annealing on DPPC LB monolayer

High crystalline thin films of 5,6,11,12-tetraphenylnaphthacene (rubrene) can be obtained after in situ thermal post annealing using SiO₂ gate dielectric modified with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer obtained via Langmuir–Blodgett transfer. Such formed rubrene crystalline films are interconnected and highly ordered with defined molecular orientation. Organic thin film transistors (OTFTs) with high performance are reproducibly demonstrated with the mobility of 0.98 cm²/V s, the threshold voltage of −8 V and the on–off current ratio of higher than 10⁷. The results indicate that our approach is a promising one for preparing high quality rubrene crystalline films.     

A maskless laser-write lithography processing of thin-film transistors on a hemispherical surface

We report on the design and fabrication methods for a hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) on a non-planar substrate using laser-write lithography (LWL). Level-to-level alignment with a high accuracy is demonstrated using LWL method. The fabricated a-Si:H TFT exhibits a field-effect mobility of 0.27 cm²/V s, threshold voltage of 4.9 V and on/off current ratio of ∼6 × 10⁶ in a saturation regime. The obtained results demonstrate that it is possible to fabricate the a-Si:H TFTs and complex circuitry on a curved surface, using a well-established a-Si:H TFT technology in combination with the maskless lithography, for hemispherical or non-planar sensor arrays.     

Charge transfer-induced horizontal orientation of organic molecules near transition metal oxide surfaces

The influence of charge transfer from organic molecules to transition metal oxide on molecular orientation characteristics of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) was investigated. Absorption peaks originating from neutral and cationic states of α-NPD increased in absorbance when α-NPD was deposited on metal oxides (MoO₂, MoO₃, and WO₃). Photoluminescence from this α-NPD was directional normal to the film plane. These results indicate that α-NPD is horizontally oriented near the metal oxide surfaces so that charge transfer from α-NPD to metal oxide occurs efficiently. Such horizontal orientation of α-NPD enhanced current density of hole-only α-NPD devices because of improvement of wave function overlap and charge transfer degree at the metal oxide/α-NPD interface.     

High performance lead phthalocyanine films and its effect on the field-effect transistors

The film morphology, structure, and electrical properties of lead phthalocyanine (PbPc) epitaxially grown on 5,5″-bis(3′-fluoro-biphenyl-4-yl)-2,2′:5′,2″-terthiophene (m-F2BP3T) inducing layer substrates were systematic investigated. The morphologies of PbPc films sensitively depend on the thickness of the inducing layer and substrate temperature. All the epitaxial PbPc films with high quality presented the triclinic form with a variation of the out-of-plane orientation. The field-effect mobility of the epitaxial PbPc films was 0.05–0.31 cm²/V s, which was significantly improved by 1–2 orders of magnitude compared to the traditional films. The evolution of the device performance is the synergistic effect of the morphology and out-of-plane orientation of the triclinic form of PbPc. The higher quality of the films and the smaller ratio of (1 0 0)/(0 0 1), the higher device performance is. A clear relationship between the morphology, structure, and the performance of epitaxial PbPc-based organic field-effect transistors was reported.     

Electronic properties of the interface between the organic semiconductor α-sexithiophene and polycrystalline palladium

The interface properties of α-sexithiophene (α-6T) and polycrystalline, atomically clean and contaminated palladium have been studied by combined core level X-ray photoemission spectroscopy and valence band ultraviolet photoemission spectroscopy. Our data indicate for the atomically clean palladium substrate a chemical reaction between the α-sexithiophene molecules and the substrate during the formation of a monolayer of flat lying α-6T molecules. We find an interface dipole of −1.2 eV and an injection barrier for holes of about 0.7 eV. In the case of ex-situ treated, contaminated palladium as metal electrode material we find a reduced interface dipole −0.4 eV and hole injection barrier 0.5 eV. By the comparison to the results of α-sexithiophene on gold we demonstrate the importance of the strength of chemical reactions at the interface.     

Interface optimization using diindenoperylene for C60 thin film transistors with high electron mobility and stability

C₆₀-based organic thin film transistors (OTFTs) with high electron mobility and high operational stability are achieved with (1 1 1) oriented C₆₀ films grown by using template effects of diindenoperylene (DIP) under layer on the SiO₂ gate insulator. The electron mobility of the C₆₀ transistor is significantly increased from 0.21 cm² V⁻¹ s⁻¹ to 2.92 cm² V⁻¹ s⁻¹ by inserting the template-DIP layer. Moreover much higher operational stability is also observed for the DIP-template C₆₀ OTFTs. A grazing incidence X-ray diffraction and ultrahigh-sensitivity photoelectron spectroscopy measurements indicate that the improved electron mobility and stability arise from the decreased density of trap states in the C₆₀ film due to increased (1 1 1) orientation of C₆₀-grains and their crystallinity on the DIP template.     

Molecular order,charge injection efficiency and the role of intramolecular polar bonds at organic/organic heterointerfaces

The effect of orientational changes in thin films of the non-crystalline hole transport material α-N-N′-diphenyl N-N″-bis(1 naphthayl)-1,1′-biphenyl-4,4′-diamine (α-NPD) on the energy level alignment and the film electronic structure has been investigated by angle-resolved ultraviolet photoelectron spectroscopy and related to the transport characteristics of hole-only devices. Changes in the anisotropic α-sexithiophene (α-6T) substrate from a “standing” to a “flat” molecular orientation induced by mechanical rubbing lead to molecular order and a preferential orientation in subsequently deposited thin α-NPD films and cause a reduction of the charge injection barrier at the organic/organic interface. The results show that the height of this barrier is determined by the surface dipoles of the individual organic films that relate to the orientation of intramolecular polar bonds at the interface.     

AP-MOVPE of InGaAs on GaAs (0 0 1): Analysis of in situ reflectivity response

We have investigated in situ monitoring of growth rate and refractive index by laser reflectometry during InGaAs on GaAs (0 0 1) substrate growth in atmospheric pressure metalorganic vapour-phase epitaxy (AP-MOVPE). The indium solid composition (x_In^s) was varied by changing the substrate temperature or the indium vapour composition (x_In^v). The refractive index of InGaAs alloys as a function of temperature and composition was quantified and compared which that of GaAs for 632.8 nm wavelength by simulation of experimental reflectivity responses. Composition analyses were carried out by high-resolution X-ray diffraction (HRXRD) and optical absorption (OA). The layers thicknesses were estimated by scanning electron microscopy (SEM) observations. The temperature dependence of InGaAs growth rate has been investigated in the temperature range 420-680 °C using trimethylgallium (TMGa), trimethylindium (TMIn) and arsine (AsH₃) sources. It shows Arrhenius-type behaviour with an apparent activation energy E_a of 0.62 eV (14.26 kcal/mol). This value is close to that determinate in the AP-MOVPE of GaAs.     

High-mobility organic thin-film transistors based on a small-molecule semiconductor deposited in vacuum and by solution shearing

The small-molecule organic semiconductor 2,9-di-decyl-dinaphtho-[2,3-b:2′,3′-f]-thieno-[3,2-b]-thiophene (C₁₀-DNTT) was used to fabricate bottom-gate, top-contact thin-film transistors (TFTs) in which the semiconductor layer was prepared either by vacuum deposition or by solution shearing. The maximum effective charge-carrier mobility of TFTs with vacuum-deposited C₁₀-DNTT is 8.5 cm²/V s for a nominal semiconductor thickness of 10 nm and a substrate temperature during the semiconductor deposition of 80 °C. Scanning electron microscopy analysis reveals the growth of small, isolated islands that begin to coalesce into a flat conducting layer when the nominal thickness exceeds 4 nm. The morphology of the vacuum-deposited semiconductor layers is dominated by tall lamellae that are formed during the deposition, except at very high substrate temperatures. Atomic force microscopy and X-ray diffraction measurements indicate that the C₁₀-DNTT molecules stand approximately upright with respect to the substrate surface, both in the flat conducting layer near the surface and within the lamellae. Using the transmission line method on TFTs with channel lengths ranging from 10 to 100 μm, a relatively small contact resistance of 0.33 kΩ cm was determined. TFTs with the C₁₀-DNTT layer prepared by solution shearing exhibit a pronounced anisotropy of the electrical performance: TFTs with the channel oriented parallel to the shearing direction have an average carrier mobility of (2.8 ± 0.3) cm²/V s, while TFTs with the channel oriented perpendicular to the shearing direction have a somewhat smaller average mobility of (1.3 ± 0.1) cm²/V s.     

Synthesis and characterization of triphenylamine flanked thiazole-based small molecules for high performance solution processed organic solar cells

Two new small molecules, 5,5-bis(2-triphenylamino-3-decylthiophen-2-yl)-2,2-bithiazole (M1) and 2,5-bis(2-triphenylamino-3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole (M2) based on an electron-donor triphenylamine unit and electron-acceptor thiophene-thiazolothiazole or thiophene-bithiazole units were synthesized by a palladium(0)-catalyzed Suzuki coupling reaction and examined as donor materials for application in organic solar cells. The small molecules had an absorption band in the range of 300-560 nm, with an optical band gap of 2.22 and 2.25 for M1 and M2, respectively_. As determined by cyclic voltammetry, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of M1 were −5.27 eV and −3.05 eV, respectively, which were 0.05 eV and 0.02 eV greater than that of M2. Photovoltaic properties of the small molecules were investigated by constructing bulk-heterojunction organic solar cell (OSC) devices using M1 and M2 as donors and fullerene derivatives, 6,6-phenyl-C61-butyric acid methyl ester (PC₆₁BM) and 6,6-phenyl-C71-butyric acid methyl ester (PC₇₁BM) as acceptors with the device architecture ITO/PEDOT:PSS/M1 or M2:PCBM/LiF/Al. The effect of the small molecule/fullerene weight ratio, active layer thickness, and processing solvent were carefully investigated to improve the performance of the OSCs. Under AM 1.5 G 100 mW/cm² illumination, the optimized OSC device with M1 and PC₇₁BM at a weight ratio of 1:3 delivered a power conversion efficiency (PCE) of 1.30%, with a short circuit current of 4.63 mA/cm², an open circuit voltage of 0.97 V, and a fill factor of 0.29. In contrast, M2 produced a better performance under identical device conditions. A PCE as high as 2.39% was recorded, with a short circuit current of 6.49 mA/cm², an open circuit voltage of 0.94 V, and a fill factor of 0.39.     

Solution-processable tandem solid-state light-emitting electrochemical cells

Compared to organic light-emitting diodes (OLEDs), solid-state light-emitting electrochemical cells (LECs) exhibit simple single-layered structure and low operating voltages due to in situ electrochemical doped layers. However, device efficiencies of LECs are usually lower than those of sophisticatedly designed OLEDs. Furthermore, device efficiencies and lifetimes of LECs degrade significantly as brightness increases. In this work, we demonstrate tandem LECs to obtain nearly doubled light outputs (μW cm⁻²) in comparison with single-layered LECs under similar current densities. Since the output EL emission is modified by microcavity effect of the device structure, the EL spectra of tandem LECs exhibit EL emission peak at ca. 625 nm while the EL spectra of single-layered LECs center at ca. 660 nm. Better spectral overlap between the EL spectrum of tandem LECs and the luminosity function results in further enhanced candela values, rendering a tripled brightness (cd m⁻²). The device efficiencies can be optimized by adjusting the thickness of the connecting layer between the two emitting units of the tandem devices. The peak external quantum efficiency achieved in tandem LECs is up to 5.83%, which is higher than twice of that obtained in single-layered LECs due to improved carrier balance. When single-layered and tandem LECs are biased under higher voltages to reach similarly higher brightness, tandem LECs show higher device efficiencies and longer lifetimes simultaneously. These results indicate that device efficiencies and lifetimes of LECs can be improved by employing a tandem device structure.