Influence of O2 on rectification properties of Nickel Phthalocyanine thin film devices

Thin sandwich film structure devices of Gold/Nickel Phthalocyanine/Lead (Au/NiPc/Pb) were fabricated employing a novel in-situ method. Electrical measurements were performed prior to, and after exposure of the samples to dry air. Under forward bias and for low applied voltages an ohmic conduction was evident, followed by SCLC in the higher voltage range. In the reverse bias, devices were found to exhibit weak rectifying properties originated mainly from the bulk of the NiPc layer. After exposure of the sample to dry air for five days a strong rectifying effect at the NiPc/Pb interface was evident. The phenomenon is believed to be associated with a change of NiPe work function as result of O2 adsorption on the NiPe layer. To verify this a second sample of the type Au/NiPcO2/Pb was fabricated. Electrical characterization of the sample showed stronger rectifying properties providing further experimental evidence on the influence of 02 adsorption on the organic layer. Potential barrier height and diode ideality factor for both NiPc/Pb, and NiPcO2/Pb interfaces after exposure to dry air, were also calculated

Effect of Chalcogens on Electronic and Photophysical Properties of Vinylene-Based Diketopyrrolopyrrole Copolymers

Three vinylene linked diketopyrrolopyrrole based donor-acceptor (D-A) copolymers have been synthesized with phenyl, thienyl, and selenyl units as donors. Optical and electronic properties were investigated with UV-vis absorption spectroscopy, cyclic voltammetry, near edge X-ray absorption spectroscopy, organic field effect transistor (OFET) measurements, and density functional theory (DFT) calculations. Optical and electrochemical band gaps decrease in the order phenyl, thienyl, and selenyl. Only phenyl-based polymers are nonplanar, but the main contributor to the larger band gap is electronic, not structural effects. Thienyl and selenyl polymers exhibit ambipolar charge transport but with higher hole than electron mobility. Experimental and theoretical results predict the selenyl system to have the best transport properties, but OFET measurements prove the thienyl system to be superior with p-channel mobility as high as 0.1 cm2 V-1 s-1. (Chemical Equation Presented). © 2015 American Chemical Society

Polymer Light‐Emitting Transistors With Charge‐Carrier Mobilities Exceeding 1 cm2 V−1 s−1

The vast majority of conjugated polymer-based light emitting field-effect transistors (LEFETs) are characterized by low charge carrier mobilities typically in the range 10-5 to 10-3 cm2 V-1 s-1 range. Fast carrier transport is a highly desirable characteristic for high frequency LEFET operation and, potentially, for use in electrically-pumped lasers. Unfortunately, high mobility organic semiconductors are often characterised by strong intermolecular π-π interactions that reduce luminescence. Development of new materials and/or device concepts that overcome this hurdle are therefore required. We report single organic semiconductor layer, light-emitting transistors that combine the highest hole mobilities reported to date for any polymer-based LEFET, with encouraging light emission characteristics. We achieve this in a single polymer layer LEFET, which was further enhanced through the use of a small-molecule/conjugated polymer blend system that possesses a film microstructure which supports enhanced charge carrier mobility (3.2 cm2 V-1 s-1) and promising light emission characteristics (1600 cd m-2) as compared to polymer-only based LEFETs. This simple approach represents an attractive strategy to further advance the performance of solution-processed LEFETs

Tyrian purple : an ancient natural dye for cross-conjugated n-type charge transport

Herein, we present two novel organic semiconducting polymers synthesised from an ancient dye. By employing cross-conjugation within the polymer backbone as a synthetic strategy, we are able to engineer optical gaps such that the novel materials absorb over the entire visible spectrum. The cross-conjugated polymers exhibited relatively high n-type charge transport performance in organic field-effect transistors, a rare characteristic for this type of polymer. Quantum chemical calculations provide insight into this behaviour, suggesting that, whilst conjugation along the HOMO is indeed inhibited via molecular design, these materials possess highly delocalized LUMOs, facilitating high n-type charge transport

Air-Stable n-channel Diketopyrrolopyrrole-Diketopyrrolopyrrole Oligomers for High Performance Ambipolar Organic Transistors

n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity, and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin-film X-ray diffraction (XRD), and grazing incidence small-angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show an increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulfur to selenium on the photophysical, optical, electronic, and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ∼0.2 cm2 V-1 s-1 with an on/off ratio exceeding 104. Field-effect behavior was retained for up to ∼4 weeks, which illustrates remarkable air and bias stability. This work paves the way toward the development of n-channel DPP-DPP-based oligomers exhibiting retention of field-effect behavior with superior stability at ambient conditions. © 2016 American Chemical Society

Understanding the degradation of methylenediammonium and its role in phase-stabilizing formamidinium lead triiodide

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl-. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability

Electrical properties of α- nickel phthalocyanine/aluminium interfaces: Effects of oxygen doping and thermal annealing

Sandwich structures devices consisting of gold/nickel phthalocyanine/aluminium (Au/NiPc/Al) were thermally evaporated on borosilicate glass substrates maintained at room temperature under high vacuum. Electrical characterisation was performed under; (i) in situ, (ii) after exposure to dry air and (iii) after annealing at 395K. In all cases a rectifying junction between NiPc/Al was evident. Under forward bias condition, in situ device exhibit two different space charge limited conduction regions, whose density of traps decreases exponentially as a function of increasing energy. Upon exposure to dry air, and within the high voltage range, a transition from exponentially distributed traps to a single dominant trapping level is observed. The effect is attributed to oxygen adsorption close to NiPc/Al interface. Under reverse bias, oxygen is found to enhance Schottky type conduction. A transition to exponential trap distribution mode, in the higher voltage range, is observed upon annealing of the sample at 395K. © 2003 Elsevier Science Ltd. All rights reserved

Junction Properties of Nickel Phthalocyanine Thin Sandwich Film Structures Using Dissimilar Electrodes

Multilayer sandwich structures of Au/NiPc/Pb were fabricated in-situ utilising a sequential deposition technique. Electrical measurements were performed on both in-situ and oxygen-doped samples. Under forward bias conditions, at low voltages, Ohmic conduction, and at higher voltages SCLC were identified. However, in the reverse bias, a transition from electrode limited to bulk limited conduction process was evident. Depletion region width as well as the potential barrier height (φb) at the NiPc/Pb interface were calculated from the reverse J-V characteristics yielding values of 183 nm and 1.03 eV, respectively. After exposure to dry air a strong rectifying effect was observed. The latter is suggested to be associated with the change in the work function of NiPc as a result of oxygen adsorption. The potential barrier height for oxygen-doped samples was calculated yielding a value in the range of 0.955-0.96 eV. Hole and trap parameters, for both in-situ and oxygen-doped sample devices were also evaluated. Derived values suggested that trap concentration associated with higher voltage characteristic is significantly higher for the oxygen-doped sample. This type of behaviour is strongly believed to be due to an oxidisation process occurring near the NiPc/Pb interface

Sequential turret source-masking system for fabrication of multilayer structures

The design of sequential turret source-masking system for used in the fabrication of multilayer structures is studied. The system's component parts include: the masking system, the driving shaft connecting the four position turret source with the support plate, and the four position turret. The system in operation has produced samples with excellent dimensional accuracy operating satisfactorily up to designed temperatures

Low frequency capacitance characterization of α-phase nickel phthalocyanine/lead interfaces: Effects of temperature and oxygen doping

The Schottky barrier characteristics of evaporated α-phase nickel phthalocyanine/lead (α-NiPc/Pb) structures were investigated using the small AC signal capacitance-voltage (C-V) technique. It was established that for junctions fabricated and tested in situ at room temperature detection of the barrier depletion layer is not possible. Voltage-dependent capacitance characteristics are detected only upon heating or after exposure of the devices to dry air for prolonged periods of time. The C-V response is attributed to the increase of carrier concentration, first due to increased temperature and second due to p-type doping induced by oxygen absorption within the α-NiPc layer. The acceptor state density was determined to be in the range 1.10×1022 to 7.15×1022m-3 for devices tested in situ and after exposure to dry air for 120 h, respectively. © 2004 Elsevier Ltd. All rights reserved