Peter K. H. Ho

General observation of n-type field-effect behaviour in organic semiconductors

Organic semiconductors have been the subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. One characteristic feature of these materials is the strong trapping of electrons but not holes: organic field-effect transistors (FETs) typically show p-type, but not n-type, conduction even with the appropriate low-work-function electrodes, except for a few special high-electron-affinity or low-bandgap organic semiconductors. Here we demonstrate that the use of an appropriate hydroxyl-free gate dielectric—such as a divinyltetramethylsiloxane-bis(benzocyclobutene) derivative (BCB; ref. 6)—can yield n-channel FET conduction in most conjugated polymers. The FET electron mobilities thus obtained reveal that electrons are considerably more mobile in these materials than previously thought. Electron mobilities of the order of 10-3 to 10-2 cm2 V-1 s-1 have been measured in a number of polyfluorene copolymers and in a dialkyl-substituted poly(p-phenylenevinylene), all in the unaligned state. We further show that the reason why n-type behaviour has previously been so elusive is the trapping of electrons at the semiconductor–dielectric interface by hydroxyl groups, present in the form of silanols in the case of the commonly used SiO2 dielectric. These findings should therefore open up new opportunities for organic complementary metal-oxide semiconductor (CMOS) circuits, in which both p-type and n-type behaviours are harnessed.

Molecular-scale interface engineering for polymer light-emitting diodes

Achieving balanced electron–hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (<100 Å) charge-injection interfacial layers in polymer LEDs. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15–20 per cent) at a luminance of 1,600 candelas per m2 at 5 V.

Electroluminescence emission pattern of organic light-emitting diodes: Implications for device efficiency calculations

The electroluminescence (EL) pattern emitted through the surface and edge of the glass substrate of two efficient polymer light-emitting diodes (LEDs) has been characterized. The surface emission is nearly Lambertian, while the edge emission comprises discrete substrate reflection and leaky waveguide modes. A simple “half-space” optical model that accounts for optical interference effects of the metal cathode–reflector is developed to extract the location and orientation of the emitting dipoles from these patterns. Numerical simulations for a range of polymer and metal refractive indices show that the surface out-coupling efficiency ξ of the internally generated photons can be greater than the 0.5 n−2 relation (where n is the refractive index of the emitter layer) valid for isotropic emitters that are not subjected to optical interference effects. When the emitting dipoles are optimally located for maximum rate of surface emission, the model predicts ξ to vary as 0.75 n−2 for the isotropic case, and as 1....

High-stability ultrathin spin-on benzocyclobutene gate dielectric for polymer field-effect transistors

Using a thermal-crosslinkable siloxane bisbenzocyclobutene, high quality spin-on (solutionprocessable) gate dielectric layers as thin as 50 nm have been fabricated over the semiconductor layer for polymer field-effect transistors. This was demonstrated on a poly(9,9-dialkylfluorene-alt-triarylamine) as p-channel semiconductor, with a surfactantion-exchanged poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate complex as top-gate electrode. The devices operate at a low voltage with a field-effect mobility of few 10−4 cm2/Vs, and can be continuously operated at 120 °C.

High-performance polymer semiconducting heterostructure devices by nitrene-mediated photocrosslinking of alkyl side chains

Heterostructures are central to the efficient manipulation of charge carriers, excitons and photons for high-performance semiconductor devices. Although these can be formed by stepwise evaporation of molecular semiconductors, they are a considerable challenge for polymers owing to re-dissolution of the underlying layers. Here we demonstrate a simple and versatile photocrosslinking methodology based on sterically hindered bis(fluorophenyl azide)s. The photocrosslinking efficiency is high and dominated by alkyl side-chain insertion reactions, which do not degrade semiconductor properties. We demonstrate two new back-infiltrated and contiguous interpenetrating donor-acceptor heterostructures for photovoltaic applications that inherently overcome internal recombination losses by ensuring path continuity to give high carrier-collection efficiency. This provides the appropriate morphology for high-efficiency polymer-based photovoltaics. We also demonstrate photopatternable polymer-based field-effect transistors and light-emitting diodes, and highly efficient separate-confinement-heterostructure light-emitting diodes. These results open the way to the general development of high-performance polymer semiconductor heterostructures that have not previously been thought possible.

Ultrathin Self‐Assembled Layers at the ITO Interface to Control Charge Injection and Electroluminescence Efficiency in Polymer Light‐Emitting Diodes

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X-ray photoelectron spectroscopy of surface-treated indium-tin oxide thin films

Angle-resolved X-ray photoelectron spectroscopy and photothermal deflection spectroscopy are used to study the oxygen-plasma or aquaregia treated indium-tin oxide (ITO) anodes for organic light-emitting diodes. Detailed analysis of the Ols core-level spectra and their dependence on photoemission angle was performed. The results indicate the presence of different chemical forms of oxygen atoms (two types of O2-, OH-, organic oxygens and H2O) which evolve with surface treatment. We find that the treatments lead to a modification of the surface chemical states and therefore of the physico-chemical properties of ITO, which in turn control the performance of organic light-emitting diodes

Improved photoinduced charge carriers separation in organic-inorganic hybrid photovoltaic devices

We demonstrate enhanced performance of a hybrid photovoltaic device, where poly[3-hexylthiophene] (P3HT) is used as active material and a solution-processed thin flat film of ZnO modified by a self-assembled monolayer (SAM) of phenyl-C61-butyric acid (PCBA) is used as electron extracting electrode. Ultraviolet photoemission spectroscopy measurements reveal an increase in the substrate work function from 3.6 to 4.1 eV upon PCBA SAM deposition due to an interfacial dipole pointing away from the ZnO. External quantum efficiency (EQE) of the SAM modified devices reached 9%, greatly improved over the 3% EQE of the unmodified devices. This corresponds to full charge separation of all photoexcitations generated in the P3HT within an exciton diffusion range from the interface.

Preparation of polyanilines doped in mixed protonic acids: Their characterization by X-ray photoelectron spectroscopy and thermogravimetry

Abstract A comparative study of polyaniline chemically prepared and simultaneously doped in protonic acid mixtures is presented. As the resultant polyaniline salts (PANI salts) are insoluble, solid-state techniques such as X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), derivative thermogravimetry (DTG) and elemental analysis have been applied to study the dopant environment as well as the electronic and chemical structure of the polymers. Both XPS and TG/DTG indicate that in the presence of competing acids, the stronger acid component is preferentially incorporated over the others. No advantage can be gained by the use of mixed-acid systems. XPS confirms that ring substitution by the dopant and the degree of protonation can be accurately measured based on the percentage of charged nitrogen in the N 1s envelope. Previous methods based on the total halogen content can lead to serious errors. TG results suggest an upper application temperature limit of about 150°C, at which point elimination of dopant occurs with loss of conductivity.

Photoluminescence of poly(p-phenylenevinylene)–silica nanocomposites: Evidence for dual emission by Franck–Condon analysis

The vibronic mode intensity pattern of the photoluminescence (PL) spectra of poly(p-phenylenevinylene) (PPV) nanocomposites dispersed with 5-nm-diam silica particles shows an apparent redistribution toward the nominal 0–0 mode with increasing silica volume fraction. Franck–Condon analysis of this variation, corrected for refractive index dispersion, reveals the presence of overlapping emission from two excited electronic states separated by 180 meV. The principal emission arises from the molecular exciton while the lower-lying one is assigned to a dipole–dipole coupled two-chain aggregate exciton. The quantum yield of the aggregate emission decreases monotonically with silica loading up to 50 vol %, whereas that of the molecular state exhibits a maximum at 15 vol %. When the samples are photoexcited below the π-π* localization edge, both of these emissions jointly redshift without a change in their relative intensities. When cooled below a transition temperature centered at 120 K, the yield of the aggrega...