Two-dimensional π-conjugated molecules based-on 2,6,9,10-tetra(prop-1-yn-1-yl)anthracene and their application to solution-processed photovoltaic cells

Novel two-dimensional π-conjugated molecules, i.e., 5′,5″′-((9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (2,6-HBT) and 5′,5″′-((2,6-bis((4-hexylphenyl)ethynyl)anthracene-9,10-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (9,10-THB) were successfully synthesized and utilized as highly soluble p-type organic semiconductors for organic thin film transistors (TFTs) and solar cells. From the TFTs, the high hole mobility of the pristine film of 9,10-THB was measured to be 0.07 cm² V⁻¹ s⁻¹ (I_on/off = 10⁶–10⁷), which is mainly attributed to slip-stacked charge-transport behavior in J-aggregation-induced crystallites. Further, a solution-processed solar cell made of 9,10-THB and PC₆₁BM exhibited very promising and reproducible power conversion efficiencies of 3.30% and 2.53% with composition 1:1 and 1:2 w/w ratio, respectively.     

Defect modification in ZnInSnO transistor with solution-processed Al2O3 dielectric by annealing

The properties of solution-processed Al₂O₃ thin films annealed at different temperatures were thoroughly studied through thermogravimetry–differential thermal analysis, UV–vis-NIR spectrophotometer measurements, scanning electron microscopy, X-ray diffraction, atomic force microscopy and a series of electrical measurements. The solution-processed ZnInSnO thin films transistors (TFTs) with the prepared Al₂O₃ dielectric were annealed at different temperatures. The TFTs annealed at 600 °C have displayed excellent electrical performance such as the field-effect mobility of 116.9 cm² V⁻¹ s⁻¹ and a subthreshold slope of 93.3 mV/dec. The performance of TFT device could be controlled by adjusting the annealing temperature. The results of two-dimensional device simulations demonstrate that the improvement of device performance are closely related with the reduction of interface defects between channel and dielectric and subgap density of stats (DOS) in the channel layer.     

High performance electric-double-layer amorphous IGZO thin-film transistors gated with hydrated bovine serum albumin protein

Device performance of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) has been improved greatly by using bovine serum albumin (BSA) as the top gate dielectric. BSA is a natural protein with acidic and basic amino acid residues, which is easily hydrated in air ambient. A typical a-IGZO TFT with hydrated BSA as the top gate dielectric exhibits a field-effect mobility (μ_FE) value of 113.5 cm² V⁻¹ s⁻¹ in saturation regime and a threshold voltage (V_TH) value of 0.25 V in air ambient. The excellent device performance can be well explained by the formation of electric double layers (EDLs) near the interfaces of a-IGZO/hydrated BSA and hydrated BSA/gate electrode. The reliability issue of a-IGZO TFTs gated with hydrated BSA has been also investigated by using the life time test without encapsulation. The V_TH value increases and μ_FE,sat value reduces slightly for the a-IGZO TFT and remain stabilized over 60 days.     

Role of Schottky barrier height at source/drain contact for electrical improvement in high carrier concentration amorphous InGaZnO thin film transistors

We report the fabrication of bottom-gate thin film transistors (TFTs) at various carrier concentrations of an amorphous InGaZnO (a-IGZO) active layer from ~10¹⁶ to ~10¹⁹ cm⁻³, which exceeds the limit of the concentration range for a conventional active layer in a TFT. Using the Schottky TFTs configuration yielded high TFT performance with saturation mobility (μ_sat), threshold voltage (V_TH), and on off current ratio (I_ON/I_OFF) of 16.1 cm²/V s, −1.22 V, and 1.3×10⁸, respectively, at the highest carrier concentration active layer of 10¹⁹ cm⁻³. Other carrier concentrations (<10¹⁹ cm⁻³) of IGZO resulted in a decrease of its work function and increase in activation energy, which changes the source/drain (S/D) contact with the active layer behavior from Schottky to quasi Ohmic, resulting in achieving conventional TFT. Hence, we successfully manipulate the barrier height between the active layer and the S/D contact by changing the carrier concentration of the active layer. Since the performance of this Schottky type TFT yielded favorable results, it is feasible to explore other high carrier concentration ternary and quaternary materials as active layers.     

Sol–gel derived nanocrystalline ZnO photoanode film for dye sensitized solar cells

Nanocrystalline ZnO was synthesized from zinc (II) acetate/oxalate mixture using a facile sol–gel synthesis and is characterized by techniques such as powder XRD, FTIR and Raman spectroscopy, TEM and SEM. The TEM and SEM study showed that the nanocrystalline ZnO powder and film have an average particle size of 25 nm. This material has been successfully applied as photoanodes in dye sensitized solar cells (DSCs) constructed with standard N719 dye and conventional iodide/triiodide (I⁻/I₃⁻) electrolytes. A systematic investigation of the performance of DSCs with film thickness and dyeing time had also been carried out. Among the five different film thicknesses 4, 8, 12, 16 and 20 μm prepared, the best result was obtained for the film thickness of 16 μm for 2 h dying showing an efficiency of 2.2% with a J_SC of 4.7 mA cm⁻² and a very high fill factor of >73%.     

High-performance C60 and picene thin film field-effect transistors with conducting polymer electrodes in bottom contact structure

C₆₀ and picene thin film field-effect transistors (FETs) in bottom contact structure have been fabricated with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes for a realization of mechanical flexible organic FETs. The C₆₀ thin film FETs showed n-channel enhancement-type characteristics with the field-effect mobility μ value of 0.41 cm² V^?1 s^?1, while the picene thin film FET showed p-channel enhancement-type characteristics with the μ of 0.61 cm² V^?1 s^?1. The μ values recorded for C₆₀ and picene thin film FETs are comparable to those for C₆₀ and picene thin film FETs with Au electrodes.     

Structural,optical and transport properties of SxZnO

In this paper, S-doped ZnO (S_xZnO) was prepared using sol-gel method at different S amounts. The structural, optical and transport properties were investigated. The introduction of S atoms into the ZnO network was found to lower the crystallization level which results in reducing the crystallite size up to x=0.3. The doping process is confirmed by the observed peak at ~610 cm⁻¹ in the ATR spectrum related to the Zn-S linking. EDX mapping shows a homogeneous distribution of S atoms on the particles surface. The best compromise between the band gap (Eg=2.96 eV), the charge carriers (N_A=2.139×10²² cm⁻³), the conductivity (σ=5.56×10⁻⁴ Ω⁻¹ m⁻¹) and the mobility (µ=16.26×10⁻¹⁴ m² V⁻¹ s⁻¹) is obtained for x=0.1. The conduction mechanism is assumed by small hopping polaron. The S-doping has impacted positively the photocatalytic activity of ZnO, with particularly high performance for S_0.2ZnO.     

Natural polyelectrolyte: Major ampullate spider silk for electrolyte organic field-effect transistors

The major ampullate (MA) silk collected from giant wood spiders Nephila pilipes consists of 12% glutamic acid (Glu) and 4% tyrosine (Tyr) acidic amino residues. The MA silk may act as a natural polyelectrolyte for organic field-effect transistors (OFETs). Pentacene and F₁₆CuPc OFETs were fabricated with the MA silk thin film as the gate dielectric. The MA silk thin film with surface roughness of 4 nm and surface energy of 36.1 mJ/m² was formed on glass using a hexafluoroisopropanol (HFIP) organic process. The MA silk gate dielectric in pentacene OFETs may improve the field-effect mobility (μ_FE,sat) value in the saturation regime from 0.11 in vacuum to 4.3 cm² V⁻¹ s⁻¹ in air ambient at ca. 70% RH. The corresponding threshold voltage (V_TH) value reduced from −6 V in vacuum to −0.5 V in air ambient. Similar to other polyelectrolytes, the changes of μ_FE,sat and V_TH may be explained by the generation of electric double layers (EDLs) in the MA silk thin film in air ambient due to water absorption.     

Fabrication of Ag nanoparticle/ZnO thin films using dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with photoreduction method and its application

We report a novel method to grow silver nanoparticle/zinc oxide (Ag NP/ZnO) thin films using a dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with a photoreduction method. The crystalline quality, optical properties, and electrical characteristics of Ag NP/ZnO thin films depend on the AgNO₃ concentration or Ag content and annealing temperature. Optimal Ag NP/ZnO thin films have been grown with a AgNO₃ concentration of 0.12 M or 2.54 at%- Ag content and 500 °C- rapid thermal annealing (RTA); these films show orientation peaks of hexagonal-wurtzite-structured ZnO (002) and face-center-cubic-crystalline Ag (111), respectively. The transmittance and resistivity for optimal Ag NP/ZnO thin films are 85% and 6.9×10⁻⁴ Ω cm. Some Ag NP/ZnO transparent conducting oxide (TCO) films were applied to InGaN/GaN LEDs as transparent conductive layers. The InGaN/GaN LEDs with optimal Ag NP/ZnO TCO films showed electric and optical performance levels similar to those of devices fabricated with indium tin oxide.     

Sputtered oxides used for passivation layers of amorphous InGaZnO thin film transistors

Four sputtered oxide films (SiO₂, Al₂O₃, Y₂O₃ and TiO₂) along with their passivating amorphous InGaZnO thin film transistors (a-IGZO TFTs) were comparatively studied in this paper. The device passivated by an Al₂O₃ thin film showed both satisfactory performance (μ_FE=5.3 cm²/V s, I_on/I_off>10⁷) and stability, as was probably related to smooth surface of Al₂O₃ thin films. Although the performance of the a-IGZO TFTs with a TiO₂ passivation layer was also good enough (μ_FE=3.5 cm²/V s, I_on/I_off>10⁷), apparent V_th shift occurred in positive bias-stress tests due to the abnormal interface state between IGZO and TiO₂ thin films. Sputtered Y₂O₃ was proved no potential for passivation layers of a-IGZO TFTs in this study. Despite unsatisfactory performance of the corresponding a-IGZO TFT devices, sputtered SiO₂ passivation layer might still be preferred for its high deposition rate and excellent transparency which benefit the mass production of flat panel displays, especially active-matrix liquid crystal displays.     

Optimization of N2/Ar gas flow ratio for the realization of nitrogen-doped p-type ZnCdO thin films synthesized by RF magnetron sputtering

Nitrogen doped ZnCdO films [ZCO:N] have been grown on quartz substrates by radio frequency (RF) reactive magnetron sputtering technique, and the effect of the ratio of nitrogen to argon gas flow [N₂:Ar] on their electrical, microstructure and optical properties were investigated by Hall effect, energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscope (TEM), optical absorbance and photoluminescence (PL) measurements. The results indicate that all the ZCO:N films are of hexagonal wurtzite structure with highly (002) preferential orientation. As the N₂:Ar increases from 0:1 to 4:1, the absorption edge for the samples exhibits blue shift. Hall effect measurement results indicate that the N₂:Ar exerts an immense influence on the p-type conduction conversion for ZCO:N film. It is found that ZCO:N film deposited at the N₂:Ar of 1:2 shows the optimal p-type behavior, which has a carrier concentration of 1.10×10¹⁷ cm⁻³, a mobility of 3.28 cm²V⁻¹s⁻¹ and a resistivity of 17.3 Ω cm. Compared with the other samples, ZCO:N film fabricated at the relatively lower N₂:Ar possesses the superior crystal quality, luminescent and electrical properties. Additionally, a possible mechanism of p-type conduction for ZCO:N film was discussed in this work.     

Effects of RTA temperatures on conductivity and micro-structures of boron-doped silicon nanocrystals in Si-rich oxide thin films

In this work, the B-doped Si rich oxide (SRO) thin films were deposited and then annealed using rapid thermal annealing (RTA) to form SiO₂-matrix silicon nanocrystals (Si NCs). The effects of the RTA temperatures on the structural properties, conduction mechanisms and electrical properties of B-doped SRO thin films (BSF) were investigated systematically using Hall measurements, Fourier transform infrared spectroscopy and Raman spectroscopy. Results showed that the crystalline fraction of annealed BSF increased from 41.3% to 62.8%, the conductivity was increased from 4.48×10⁻³ S/cm to 0.16 s/cm, the carrier concentration was increased from 8.74×10¹⁷ cm⁻³ to 4.9×10¹⁸ cm⁻³ and the carrier mobility was increased from 0.032 cm² V⁻¹ s⁻¹ to 0.2 cm² V⁻¹ s⁻¹ when the RTA temperatures increased from 1050 °C to 1150 °C. In addition, the fluctuation induced tunneling (FIT) theory was applicable to the conduction mechanisms of SiO₂-matrix boron-doped Si-NC thin films.     

Effect of 100 MeV O7+ ion beam irradiation on radio frequency reactive magnetron sputtered ZnO thin films

Zinc oxide (ZnO) thin films were deposited on sapphire substrates at room temperature by radio frequency (RF) magnetron sputtering. These films were irradiated with 100 MeV O⁷⁺ ions of the fluencies 5×10¹³ ions/cm² at room temperature (RT) and at liquid nitrogen temperature (LNT). Profilometer studies showed that the roughness of pristine and LNT irradiated ZnO thin films were higher than that of the RT irradiated ZnO thin film. The glancing angle X-ray diffraction analysis reveals a reduced intensity and increased full width at half maximum (FWHM) of the (002) diffraction peak in the case of LNT irradiated film indicating disorder. However, the intensity and FWHM of the (002) diffraction peak in the case of RT irradiated ZnO thin films are comparable to those of the pristine film. UV–visible transmission spectra show that the percentage of transmission and band gap energy are different for RT and LNT irradiated films. While the pristine ZnO thin film exhibits two emissions—a broad emission at 403 nm and a sharp emission at 472 nm in its photoluminescence spectrum; the emission at 472 nm was absent for the irradiated films. The atomic concentrations of zinc and oxygen during the irradiation process were obtained using auger electron spectroscopy.     

Structural,morphological, electrical and optical studies of Cr doped SnO2 thin films deposited by the spray pyrolysis technique

Tin oxide (SnO₂) and chromium (Cr) doped tin oxide (Cr:SnO₂) thin films were deposited on the preheated glass substrates at 673 K by spray pyrolysis. Concentration of Cr was varied in the solution by adding chromium (III) chloride hexahydrate from 0 to 3 at%. The effect of Cr doping on the structural, electrical and optical properties of tin oxide films is reported. X-ray diffraction pattern confirms the tetragonal crystal structure for undoped and Cr doped tin oxide films. Scanning electron microscopic photographs show the modification of surface morphology of tin oxide film due to varying concentration of Cr. X-ray photoelectron spectra of Cr:SnO₂ (3 at%) thin film revealed the presence of carbon, tin, oxygen, and chromium. Carrier concentration and mobility of the SnO₂ films decrease with increasing concentration of Cr and 0.5 at% Cr doped tin oxide film acquires a mobility of 70 cm²/V s. Average optical transmittance in the 550–850 nm range varies from 38% to 47% with varying Cr concentration in the solution.     

Solution-processed single-crystal perylene diimide transistors with high electron mobility

We have successfully demonstrated a single-crystal field-effect transistors (FETs) based on an asymmetric perylenetetracarboxylic diimide (a-PDI) compound with polystyrene (PS)/SiO₂ bilayer as gate dielectric. The single crystals are in-situ grown on substrate from simple solution evaporation method, thus may be suitable for large area electronics applications. The PS modified gate dielectric could minimize charge trapping by the hydroxyl groups of the SiO₂ surface. The resulting solution processed single crystals transistors are characterized in ambient air, and exhibited maximum electron mobility of ca. 1.2 cm² V⁻¹ s⁻¹ and high I_on:I_off > 10⁵.     

Star-shaped self-assembly of an organic thin film transistor sensor in the presence of Cu2+ and CN− ions

In this study we developed organic thin film transistors (OTFTs) for the sensing of metal ions and anions through the self-assembly of a pentacene/Schiff base pyrene derivative. Our bilayer OTFTs displayed attractive device parameters: an electron mobility (μ) of 0.12 cm² V⁻¹ s⁻¹, a threshold voltage (V_th) of 22.20 V, and a five-orders-of-magnitude on/off ratio. This device was sensitive toward Cu²⁺ among 13 metal cations and toward CN⁻ among nine anions, as measured through changes in the values of V_th and I_off in the presence of Cu²⁺ cations and a change in the value of I_sat in the presence of CN⁻ anions. We observed selectivity toward both of these ions in mixed ion solutions, with sensitivity over different concentrations (from 20 to 350 μM for Cu²⁺; from 100 to 350 μM for CN⁻) as well as in sea water. The pyrene derivative self-assembled through pyrene–pyrene^* coordination in the presence of Cu²⁺ ions; the rods of the pyrene derivative broke into smaller pieces upon formation of benzoxazole rings in the presence of CN⁻ ions, as confirmed using atomic force microscopy and fourier transform attenuated total reflection spectroscopy.     

IGZO thin film transistors with Al2O3 gate insulators fabricated at different temperatures

Thin film transistors (TFTs) with bottom gate and staggered electrodes using atomic layer deposited Al₂O₃ as gate insulator and radio frequency sputtered In–Ga–Zn Oxide (IGZO) as channel layer are fabricated in this work. The performances of IGZO TFTs with different deposition temperature of Al₂O₃ are investigated and compared. The experiment results show that the Al₂O₃ deposition temperature play an important role in the field effect mobility, I_on/I_off ratio, sub-threshold swing and bias stability of the devices. The TFT with a 250 °C Al₂O₃ gate insulator shows the best performance; specifically, field effect mobility of 6.3 cm²/Vs, threshold voltage of 5.1 V, I_on/I_off ratio of 4×10⁷, and sub-threshold swing of 0.56 V/dec. The 250 °C Al₂O₃ insulator based device also shows a substantially smaller threshold voltage shift of 1.5 V after a 10 V gate voltage is stressed for 1 h, while the value for the 200, 300 and 350 °C Al₂O₃ insulator based devices are 2.3, 2.6, and 1.64 V, respectively.     

Properties of fluorine-doped SnO2 thin films by a green sol–gel method

Fluorine doped tin oxide (FTO) films were fabricated on a glass substrate by a green sol–gel dip-coating process. Non-toxic SnF₂ was used as fluorine source to replace toxic HF or NH₄F. Effect of SnF₂ content, 0–10 mol%, on structure, electrical resistivity, and optical transmittance of the films were investigated using X-ray diffraction, Hall effect measurements, and UV–vis spectra. Structural analysis revealed that the films are polycrystalline with a tetragonal crystal structure. Grain size varies from 43 to 21 nm with increasing fluorine concentration, which in fact critically impacts resultant electrical and optical properties. The 500 °C-annealed FTO film containing 6 mol% SnF₂ shows the lowest electrical resistivity 7.0×10⁻⁴ Ω cm, carrier concentration 1.1×10²¹ cm⁻³, Hall mobility 8.1 cm²V⁻¹ s⁻¹, optical transmittance 90.1% and optical band-gap 3.91 eV. The 6 mol% SnF₂ added film has the highest figure of merit 2.43×10⁻² Ω⁻¹ which is four times higher than that of un-doped FTO films. Because of the promising electrical and optical properties, F-doped thin films prepared by this green process are well-suited for use in all aspects of transparent conducting oxide.     

Low temperature Solution-Processed ZnO film on flexible substrate

Transparent conductive ZnO films were directly deposited on unseeded polyethersulfone (PES) substrates with a spin-spray method using aqueous solution at a low substrate temperature of 85 °C. All ZnO films were crystalline with wurtzite hexagonal structure and impurity phases were not detected. ZnO films deposited without citrate ions in the reaction solution had a rod array structure. In contrast, ZnO films deposited with citrate ions in the reaction solution had a continuous, dense structure. The transmittance of the ZnO films was improved from 11.9% to 85.3% as their structure changed from rod-like to continuous. After UV irradiation, the ZnO films with a continuous, dense structure had a low resistivity of 9.1×10⁻³ Ω cm, high carrier concentration of 2.7×10²⁰ cm⁻³ and mobility of 2.5 cm² V⁻¹ s⁻¹.     

Solution-processable ambipolar organic field-effect transistor based on Co-planar bisphthalocyaninato copper

A soluble binuclear phthalocyaninato copper (II) complex, Cu₂[Pc(COOC₈H₁₇)₆]₂ (1), with planar molecular structure and extended conjugation system, has been designed and synthesized. By fusing two phthalocyanine rings side by side and introducing electron withdrawing groups at periphery positions, the energy levels of HOMO and LUMO have been tuned successfully into the range of an air-stable ambipolar organic semiconductor required as revealed by the electrochemical studies. With the help of a solution-based quasi-Langmuir–Shäfer (QLS) method, thin solid films of this compound were fabricated and organic field effect transistors (OFETs) based on these QLS thin solid films were constructed. Because of the promising electrochemical properties as well as the high ordered packing structure of the molecules in the thin solid films, the OFETs performed excellent ambipolar operating properties, with the electron and hole mobility in air as high as 1.7 × 10⁻¹ and 2.3 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. For comparison purpose, mononuclear compound Cu[Pc(COOC₈H₁₇)₈] (2) was comparatively studied. The QLS thin solid films of this compound possess similar ordered structure with that of Cu₂[Pc(COOC₈H₁₇)₆]₂ (1), but the OFETs based on the thin solid films of this compound can only show n-type properties under nitrogen atmosphere with an extremely small electron mobility of 1.6 × 10⁻⁴ cm² V⁻¹ s⁻¹. This result suggests that extension on the conjugation system of an aromatic compound with multiple electron withdrawing groups can tune the molecule into an air stable ambipolar organic semiconductor.