Organic photovoltaic cells with high open circuit voltages based on pentacene derivatives

Heterojunction organic photovoltaic devices were fabricated using C₆₀ as the electron acceptor and several pentacene derivatives with triisopropylsilylethynyl functional groups as the electron donor. The open circuit voltage (V_oc) of functionalized pentacene-based cells is significantly higher (0.57–0.90 V) than for cells based on unsubstituted pentacene (0.24 V), due to the higher oxidation potentials of these pentacene derivatives. The performance of pentacene derivative cells is limited by lower current densities than the reference pentacene/C₆₀ cell. The absorption spectra of films and solutions of pentacene derivatives closely resemble one another, leading us to conclude that these films are amorphous in nature. Weak intermolecular coupling in the derivative films results in lower charge mobility and shorter exciton diffusion lengths relative to pentacene.     

High-performance n-type organic field-effect transistors fabricated by ink-jet printing using a C60 derivative

We report on the performance of ink-jet-printed n-type organic thin-film transistors (OTFTs) based on a C₆₀ derivative, namely, C₆₀-fused N-methyl-2-(3-hexylthiophen-2-yl)pyrrolidine (C₆₀TH-Hx). The new devices exhibit excellent n-channel performance, with a highest mobility of 2.8 × 10^?2 cm² V^?1 s^?1, an I_On/I_Off ratio of about 1 × 10⁶, and a threshold voltage of 7 V. The C₆₀TH-Hx films show large crystalline domains that result from the influence of an evaporation-induced flow, thus leading to high electron mobility in the ink-jet-printed devices.     

Study of blocking effect of Cu-phthalocyanine layer in zinc oxide/pentacene/CuPc/C60/Al organic solar cells by electric field-induced optical second harmonic generation measurement

By employing electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the blocking effect of sandwiched Cu-phthalocyanine (CuPc) layer on IZO/pentacene/Cu-phthalocyanine (CuPc)/C₆₀/Al organic solar cells (OSCs). Results evidently showed that the Maxwell–Wagner type interfacial charging appears at both pentacene/CuPc and CuPc/C₆₀ interfaces, depending on the CuPc layer thickness. When the thickness of CuPc layer was 8 nm, the charging at the two interfaces was significantly suppressed and the I–V characteristic was improved. CuPc layer is available as a blocking layer for the regulation of interfacial charging induced in IZO/pentacene/C₆₀/Al organic solar cells (OSCs). The blocking effect of the CuPc layer was discussed on the basis of Maxwell–Wagner model analysis.     

Enhanced performance of PbPc photosensitive organic field effect transistors by inserting different-thickness pentacene inducing layers

Lead phthalocyanine (PbPc) based photosensitive organic field effect transistors (PhOFETs) with different-thickness pentacene inducing layers (INLs) inserted between SiO₂ and PbPc layer were fabricated and characterized. The photoelectric measurements demonstrate that the device with 2-nm-thick pentacene INL exhibits the largest photoresponsivity of 505.75 mA/W and maximum photo/dark current ratio of 405.35 in all devices. For this, we give an overall explanation that different-thickness INLs display different continuity and crystallinity and thus produce strong or weak template inducing. Especially, when the INL thickness (δ) is 2 nm a quasi-continuous and highly crystalline approximate-monolayer INL forms on SiO₂ surface, which may play a strong role of template inducing, thus causing its upper PbPc film to demonstrate the strongest triclinic (3 0 0) line and the strongest NIR absorption in series PbPc films. When δ = 1 nm, pentacene does not form a continuous film. And when δ = 5 or 10 nm, a continuous multilayer INL with a declined crystallinity due to possible lattice mismatch forms on SiO₂ surface and gives a weakened template inducing. Thereby, it can be recognized that inserting a pentacene INL can markedly enhance the performance of single layer PbPc PhOFET and the optimum INL thickness is proved ∼2 nm in present conditions.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Organic field-effect transistors based on single-crystalline active layer and top-gate insulator consistently fabricated by electrostatic spray deposition

An electrostatic spray deposition (ESD) method was applied to prepare both crystalline domains of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and insulating films of poly(methyl methacrylate) (PMMA) for fabricating top-gate single-crystal organic field-effect transistors (OFETs). The electrical characteristics of the top-gate device were compared to those of the bottom-gate one (SiO₂ bottom-gate insulator) with the same active layer, and the lower charge-trap density at the interface between the top-gate insulator and single-crystalline active layer was demonstrated. The drain current compression in the output characteristics of the top-gate device, however, occurred due to the large parasitic resistance between the source/drain electrodes and accumulation channel. Reducing the thickness of the single-crystalline active layer resulted in a high charge-carrier mobility of 0.29 cm²/V s (channel length of 5 μm).     

Structural and optoelectronic properties of nanostructured TiO2 thin films with annealing

About 480 nm thick titanium oxide (TiO₂) thin films have been deposited by electron beam evaporation followed by annealing in air at 300–600 °C with a step of 100 °C for a period of 2 h. Optical, electrical and structural properties are studied as a function of annealing temperature. All the films are crystalline (having tetragonal anatase structure) with small amount of amorphous phase. Crystallinity of the films improves with annealing at elevated temperatures. XRD and FESEM results suggest that the films are composed of nanoparticles of 25–35 nm. Raman analysis and optical measurements suggest quantum confinement effects since Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk TiO₂ Optical band gap energy of the as-deposited TiO₂ film is 3.24 eV, which decreases to about 3.09 eV after annealing at 600 °C. Refractive index of the as-deposited TiO₂ film is 2.26, which increases to about 2.32 after annealing at 600 °C. However the films annealed at 500 °C present peculiar behavior as their band gap increases to the highest value of 3.27 eV whereas refractive index, RMS roughness and dc-resistance illustrate a drop as compared to all other films. Illumination to sunlight decreases the dc-resistance of the as-deposited and annealed films as compared to dark measurements possibly due to charge carrier enhancement by photon absorption.     

Effect of AlN buffer layer thickness on the properties of GaN films grown by pulsed laser deposition

The GaN films are grown by pulsed laser deposition (PLD) on sapphire, AlN(30 nm)/Al₂O₃ and AlN(150 nm)/Al₂O₃, respectively. The effect of AlN buffer layer thickness on the properties of GaN films grown by PLD is investigated systematically. The characterizations reveal that as AlN buffer layer thickness increases, the surface root-mean-square (RMS) roughness of GaN film decreases from 11.5 nm to 2.3 nm, while the FWHM value of GaN film rises up from 20.28 arcmin to 84.6 arcmin and then drops to 31.8 arcmin. These results are different from the GaN films deposited by metal organic chemical vapor deposition (MOCVD) with AlN buffer layers, which shows the improvement of crystalline qualities and surface morphologies with the thickening of AlN buffer layer. The mechanism of the effect of AlN buffer layer on the growth of GaN films by PLD is hence proposed.     

Dry growth of n-octylphosphonic acid monolayer for low-voltage organic thin-film transistors

Dry method for monolayer deposition of n-octylphosphonic acid (C₈PA) on the surface of aluminium oxide (AlO_x) is presented. Vacuum thermal evaporation is employed to deposit initial thickness corresponding to several C₈PA monolayers, followed by a thermal desorption of the physisorbed C₈PA molecules. AlO_x functionalized with such C₈PA monolayer exhibits leakage current density of ～10^?7 A/cm² at 3 V, electric breakdown field of ～6 MV/cm, and a root-mean-square surface roughness of 0.36 nm. The performance of low-voltage pentacene thin-film transistors that implement this dry AlO_x/C₈PA gate dielectric depends on C₈PA desorption time. When the desorption time rises from 25 to 210 min, the field-effect mobility increases from ～0.02 to ～0.04 cm²/V s, threshold voltage rises from ～?1.2 to ～?1.4 V, sub-threshold slope decreases from ～120 to ～80 mV/decade, off-current decreases from ～5 × 10^?12 to ～1 × 10^?12 A, on/off current ratio rises from ～3.8 × 10⁴ to ～2.5 × 10⁵, and the transistor hysteresis decreases from 61 to 26 mV. These results collectively support a two stage model of the desorption process where the removal of the physisorbed C₈PA molecules is followed by the annealing of the defect sites in the remaining C₈PA monolayer.     

Mixed phase silicon thin films grown at high rate using 60 MHz assisted VHF-PECVD technique

Mixed phase amorphous and nanocrystalline silicon (a-Si:H and nc-Si:H) thin films were deposited by VHF-PECVD (60 MHz) using Argon (Ar) as the diluent of silane. These amorphous and crystalline silicon thin films were deposited by varying the argon dilution (f_Ar) from 10–97.5% while keeping other process parameters constant. The effects of argon dilution on deposition rate, structural and optical properties of micro/nanocrystalline silicon thin films are studied. It has been observed that the films deposited from f_Ar 10–70% showed the deposition rate >20 Å/s with the highest deposition rate achieved of ~25 Å/s. Structural characterization has been performed by micro-Raman analysis and Atomic force microscopy. Raman shift towards higher wave number (515 cm⁻¹) with increase of f_Ar indicates variation in crystallinity of silicon films. HRTEM studies revealed the distribution of grain size and the degree of crystallinity. Optical absorption spectroscopy confirmed the increase in band gap of the materials from 1.5 to 2.1 eV.     

Highly ordered C60 films on epitaxial Fe/MgO(0 0 1) surfaces for organic spintronics

Hybrid interfaces between ferromagnetic surfaces and carbon-based molecules play an important role in organic spintronics. The fabrication of devices with well defined interfaces remains challenging, however, hampering microscopic understanding of their operation mechanisms. We have studied the crystallinity and molecular ordering of C₆₀ films on epitaxial Fe/MgO(0 0 1) surfaces, using X-ray diffraction and scanning tunneling microscopy (STM). Both techniques confirm that fcc molecular C₆₀ films with a (1 1 1)-texture can be fabricated on epitaxial bcc-Fe(0 0 1) surfaces at elevated growth temperatures (100–130 °C). STM measurements show that C₆₀ monolayers deposited at 130 °C are highly ordered, exhibiting quasi-hexagonal arrangements on the Fe(0 0 1) surface oriented along the [1 0 0] and [0 1 0] directions. The mismatch between the surface lattice of the monolayer and the bulk fcc C₆₀ lattice prevents epitaxial overgrowth of multilayers.     

Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 Ω/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current–voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine exhibits a high transmission rate of ∼75% in the range of visible light from 400 to 550 nm, a threshold voltage of −6.0 V, an on–off drain current ratio of 8.4 × 10⁵, and a field-effect mobility of 1.71 cm²/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers.     

Effect of rapid thermal annealing on properties of thermally evaporated nanostructured CdTe thin film treated with CdCl2

The effects of rapid thermal annealing on properties of crystalline nanostructured CdTe films treated with CdCl₂ and prepared by vacuum evaporation are described. X-ray diffraction confirmed the crystalline nature of post-treated films with high preferential orientation around 23.7°, corresponding to a (1 1 1) diffracted plane of cubic phase. Optical band gap of CdTe films increased from 1.4 eV to 1.48 eV after annealing at 500 °C for 90 s. Atomic force microscopy of annealed films revealed an increase in root mean square roughness and grain size with increased annealing time. Electrical measurements of as-grown and annealed films are consistent with p-type; film resistivity has decreased significantly with increased annealing time.     

Contact resistance between pentacene and indium–tin oxide (ITO) electrode with surface treatment

Contact resistance between indium–tin oxide (ITO) electrode and pentacene was studied by transmission line method (TLM). Organic solvent cleaned, inorganic alkali cleaned, and self-assembled monolayer (with OTS: octadecyltrichlorosilane) modified ITO electrode structures were compared. Pentacene layer of 300 Å thickness was vacuum deposited on patterned ITO layer at 70 °C with a deposition rate of 0.3 Å/s. Alkali cleaned and SAM modified ITO gave a lower contact resistance of about 6.34 × 10⁴ Ω cm² and 1.88 × 10³ Ω cm², respectively than organic solvent cleaned ITO of about 6.58 × 10⁵ Ω cm². Especially with the SAM treatment, the work function of ITO increased closer to the highest occupied molecular orbital (HOMO) level of pentacene, which lowers the injection barrier between ITO and pentacene. It was also believed that pentacene morphology was improved on SAM modified ITO surface due to the lowering of the surface energy. We could obtain the low contact resistance with SAM treatment which is comparable to the measured value of gold–pentacene contact, 1.86 × 10³ Ω cm². This specific contact resistance is still much higher than that of amorphous silicon thin film transistor (0.1–30 Ω cm²).     

Green and blue-green phosphorescent heteroleptic iridium complexes containing carbazole-functionalized β-diketonate for non-doped organic light-emitting diodes

Two novel iridium complexes both containing carbazole-functionalized β-diketonate, Ir(ppy)₂(CBDK) [bis(2-phenylpyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)], Ir(dfppy)₂(CBDK) [bis(2-(2,4-difluorophenyl)pyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)] and two reported complexes, Ir(ppy)₂(acac) (acac = acetylacetonate), Ir(dfppy)₂(acac) were synthesized and characterized. The electrophosphorescent properties of non-doped device using the four complexes as emitter, respectively, with a configuration of ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) (20 nm)/iridium complex (20 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (5 nm)/tris(8-hydroxyquinoline)aluminum (AlQ) (45 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were examined. In addition, a most simplest device, ITO/Ir(ppy)₂(CBDK) (80 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm), and two double-layer devices with configurations of ITO/NPB (30 nm)/Ir(ppy)₂(CBDK) (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) and ITO/Ir(ppy)₂(CBDK) (30 nm)/AlQ (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were also fabricated and examined. The results show that the non-doped four-layer device for Ir(ppy)₂(CBDK) achieves maximum lumen efficiency of 4.54 lm/W and which is far higher than that of Ir(ppy)₂(acac), 0.53 lm/W, the device for Ir(dfppy)₂(CBDK) achieves maximum lumen efficiency of 0.51 lm/W and which is also far higher than that of Ir(dfppy)₂(acac), 0.06 lm/W. The results of simple devices involved Ir(ppy)₂(CBDK) show that the designed complex not only has a good hole transporting ability, but also has a good electron transporting ability. The improved performance of Ir(ppy)₂(CBDK) and Ir(dfppy)₂(CBDK) can be attributed to that the bulky carbazole-functionalized β-diketonate was introduced, therefore the carrier transporting property was improved and the triplet–triplet annihilation was reduced.     

High-performance pentacene field-effect transistors using Al2O3 gate dielectrics prepared by atomic layer deposition (ALD)

High-performance pentacene field-effect transistors have been fabricated using Al₂O₃ as a gate dielectric material grown by atomic layer deposition (ALD). Hole mobility values of 1.5 ± 0.2 cm²/V s and 0.9 ± 0.1 cm²/V s were obtained when using heavily n-doped silicon (n⁺-Si) and ITO-coated glass as gate electrodes, respectively. These transistors were operated in enhancement mode with a zero turn-on voltage and exhibited a low threshold voltage (< −10 V) as well as a low sub-threshold slope (<1 V/decade) and an on/off current ratio larger than 10⁶. Atomic force microscopy (AFM) images of pentacene films on Al₂O₃ treated with octadecyltrichlorosilane (OTS) revealed well-ordered island formation, and X-ray diffraction patterns showed characteristics of a “thin film” phase. Low surface trap density and high capacitance density of Al₂O₃ gate insulators also contributed to the high performance of pentacene field-effect transistors.     

Vacuum-processed polyethylene as a dielectric for low operating voltage organic field effect transistors

We report on the fabrication and performance of vacuum-processed organic field effect transistors utilizing evaporated low-density polyethylene (LD-PE) as a dielectric layer. With C₆₀ as the organic semiconductor, we demonstrate low operating voltage transistors with field effect mobilities in excess of 4 cm²/Vs. Devices with pentacene showed a mobility of 0.16 cm²/Vs. Devices using tyrian Purple as semiconductor show low-voltage ambipolar operation with equal electron and hole mobilities of ～0.3 cm²/Vs. These devices demonstrate low hysteresis and operational stability over at least several months. Grazing-angle infrared spectroscopy of evaporated thin films shows that the structure of the polyethylene is similar to solution-cast films. We report also on the morphological and dielectric properties of these films. Our experiments demonstrate that polyethylene is a stable dielectric supporting both hole and electron channels.     

Memory effects in hybrid silicon-metallic nanoparticle-organic thin film structures

We report on the electrical behaviour of metal–insulator–semiconductor (MIS) structures fabricated on silicon substrates and using organic thin films as the dielectric layers. These insulating thin films were produced by different methods, including spin-coating (polymethylmethacrylate), thermal evaporation (pentacene) and Langmuir–Blodgett deposition (cadmium arachidate). Gold nanoparticles, deposited at room temperature by chemical self-assembly, were used as charge storage elements. In all cases, the MIS devices containing the nanoparticles exhibited hysteresis in their capacitance versus voltage characteristics, with a memory window depending on the range of the voltage sweep. This hysteresis was attributed to the charging and discharging of the nanoparticles from the gate electrode. A maximum memory window of 2.5 V was achieved by scanning the applied voltage of an Al/pentacene/Au nanoparticle/SiO₂/p-Si structure between 9 and −9 V.     

Growth temperature dependence of epitaxial Gd2O3 films on Si(1 1 1)

This article reports on the epitaxy of crystalline high κ oxide Gd₂O₃ layers on Si(1 1 1) for CMOS gate application. Epitaxial Gd₂O₃ thin films have been grown by Molecular Beam Epitaxy (MBE) on Si(1 1 1) substrates between 650 and 750 °C. The structural and electrical properties were investigated depending on the growth temperature. The C–V measurements reveal that equivalent oxide thickness (EOT) equals 0.7 nm for the sample deposited at the optimal temperature of 700 °C with a relatively low leakage current of 3.6 × 10^?2 A/cm² at |V_g ? V_FB| = 1 V.     

Fluorene trimers with various 9,9′-substituents: The synthesis,characteristics, condensed state structures,and electroluminescence properties

The four fluorene-based trimers with various aromatic and alkyl substituents (T1–T4) are synthesized and characterized. These oligomers show the similar electronic absorption and emission characteristics (e.g., absorption peak at 351 nm, and highly efficient deep blue emission at 394 nm in solution), indicating that the major electronic properties of the core chromophore are essentially independent of the substituents. However, the condensed state structures and thermal properties of four trimers are found to be different from each other, from crystalline (full alkyl (T1) or full aromatic (T2) substituted trimers) to amorphous (mixed aromatic and alkyl (T4) substituted trimers). The effect of different condensed state structures on electroluminescence device properties is presented: The blue light-emitting devices with accordant structure of ITO/PEDOT:PSS/TCTA (40 nm)/trimers (40 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF/Al exhibit different EL efficiency (2.9% of T2, 1.8% of T3 and 2.7% of T4). Using amorphous T4, the white light-emitting device of ITO/TCTA (40 nm)/rubrene (0.1 nm)/T4 (8 nm)/Alq₃(52 nm)/LiF/Al is fabricated with high efficiency (6.15 cd A⁻¹), high brightness (9500 cd m⁻²) and good white light CIE coordinates (0.32, 0.37).