We demonstrate tuning of Schottky energy barriers in organic electronic devices by utilizing chemically tailored electrodes. The Schottky energy barrier of Ag on poly͓2-methoxy, 5-͑2Ј-ethyl-hexyloxy͒-1,4-phenylene was tuned over a range of more than 1 eV by using self-assembled monolayers ͑SAM's͒ to attach oriented dipole layers to the Ag prior to device fabrication. Kelvin probe measurements were used to determine the effect of the SAM's on the Ag surface potential. Ab initio Hartree-Fock calculations of the molecular dipole moments successfully describe the surface potential changes. The chemically tailored electrodes were then incorporated in organic diode structures and changes in the metal/organic Schottky energy barriers were measured using an electroabsorption technique. These results demonstrate the use of selfassembled monolayers to control metal/organic interfacial electronic properties. They establish a physical principle for manipulating the relative energy levels between two materials and demonstrate an approach to improve metal/organic contacts in organic electronic devices.

We show that strained type II superlattices made of InAs-Ga1−xInxSb x∼0.4 have favorable optical properties for infrared detection. By adjusting the layer thicknesses and the alloy composition, a wide range of wavelengths can be reached. Optical absorption calculations for a case where λc∼10 μm show that near threshold the absorption is as good as for the HgCdTe alloy with the same band gap. The electron effective mass is nearly isotropic and equal to 0.04 m. This effective mass should give favorable electrical properties, such as small diode tunneling currents and good mobilities and diffusion lengths.

We present a unified device model for single layer organic light emitting diodes (LEDs) which includes charge injection, transport, and space charge effects in the organic material. The model can describe both injection limited and space charge limited current flow and the transition between them. We specifically considered cases in which the energy barrier to injection of electrons is much larger than that for holes so that holes dominate the current flow in the device. Charge injection into the organic material occurs by thermionic emission and by tunneling. For Schottky energy barriers less than about 0.3–0.4 eV, for typical organic LED device parameters, the current flow is space charge limited and the electric field in the structure is highly nonuniform. For larger energy barriers the current flow is injection limited. In the injection limited regime, the net injected charge is relatively small, the electric field is nearly uniform, and space charge effects are not important. At smaller bias in the injection limited regime, thermionic emission is the dominant injection mechanism. For this case the thermionic emission injection current and a backward flowing interface recombination current, which is the time reversed process of thermionic emission, combine to establish a quasi-equilibrium carrier density. The quasi-equilibrium density is bias dependent because of image force lowering of the injection barrier. The net device current is determined by the drift of these carriers in the nearly constant electric field. The net device current is much smaller than either the thermionic emission or interface recombination current which nearly cancel. At higher bias, injection is dominated by tunneling. The bias at which tunneling exceeds thermionic emission depends on the size of the Schottky energy barrier. When tunneling is the dominant injection mechanism, a combination of tunneling injection current and the backflowing interface recombination current combine to establish the carrier density. We compare the model results with experimental measurements on devices fabricated using the electroluminescent conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] which by changing the contacts can show either injection limited behavior or space charge limited behavior.

No abstract

Strain-induced gradients of local electric fields in semiconductor quantum dots can couple to the quadrupole moments of nuclear spins. We develop a theory describing the influence of this quadrupolar coupling (QC) on the spin correlators of electron and hole "central" spins localized in such dots. We show that when the QC strength is comparable to or larger than the hyperfine coupling strength between nuclei and the central spin, the relaxation rate of the central spin is strongly enhanced and can be exponential. We demonstrate a good agreement with recent experiments on spin relaxation in hole-doped (In,Ga)As self-assembled quantum dots. PACS numbers:The spin of an electron or a hole in a semiconductor quantum dot is the main component of numerous proposed spintronic and quantum computing devices 1 . Spin decoherence and finite spin lifetimes are currently the major factors that limit our ability to control spin states in dots. A single "central" (i.e., electron or hole) spin in a dot interacts via hyperfine coupling with a large number (10 4 − 10 6 ) of nuclear spins. The net effect of this coupling to the nuclear spin bath can be characterized by an effective Overhauser magnetic field B n that acts upon the central spin. Within a quantum dot ensemble, each central spin precesses around a different B n . If B n is time-independent, such precession alone cannot lead to complete relaxation of the central spin polarization. This is evidenced from the observation of spin echoes 2 that can be used to cancel the dephasing of central spins in an ensemble of dots with different constant B n . However, stochastic dynamics of the Overhauser field B n induces irreversible relaxation of the central spin and loss of coherence 3,4 . The physics that leads to changes of B n and its corresponding influence on central spin relaxation are the subject of considerable theoretical debate 1,4-8 .It was suggested that, at microsecond time scales, the dynamics of the Overhauser field is dominated by hyperfine-mediated nuclear co-flips, which originate from unequal strengths of the hyperfine couplings of the central spin to different nuclear spins inside the same dot 4 . Numerical simulations by Al-Hassanieh et al. 1 showed that such co-flips generally lead only to a logarithmically slow central spin relaxation. In contrast, recent experimental studies with hole-doped (In,Ga)As quantum dots reported a nearly ideal Lorentzian shape of the spin noise power spectrum, indicating exponential relaxation of central hole spins rather than a power-law or logarithmic relaxation 10 .Here we show that quadrupolar couplings (QC) of nuclear spins to the strain induced electric field gradients inside typical semiconductor quantum dots can induce relatively fast dynamics of the Overhauser field B n , and consequently accelerated relaxation of electron and hole spins in weak external fields. Our model directly applies to InGaAs self-assembled quantum dot systems, which are among the most popular platforms for spin memories and qubits 11,12 ; however, ...

We report polarized optical transmission and reflection measurements which are used to determine the orientation of polymer chains in spin-cast thin films of soluble, electroluminescent polymers. We find that the polymer chains lie primarily in the plane of the film. This result has three important implications for polymer light emitting diodes: the electroluminescence is preferentially emitted propagating perpendicular to the polymer film; the relevant dielectric and electrical transport properties are those perpendicular to the polymer chains; and large area, uniform devices can be produced by spin casting.

We present time-of-flight mobility measurements and measured and calculated current–voltage (I–V) characteristics of structures fabricated using a soluble poly(p-phenylene vinylene) derivative. Time-of-flight measurements were used to determine the electric field dependent hole mobility. This mobility was then used, without adjustable parameters, to calculate the I–V characteristics of space-charge-limited, hole only devices. The measured and calculated I–V characteristics are in good agreement over five orders of magnitude in current. These results demonstrate that an electric field dependent mobility, without invoking trapping effects, provides an accurate description of hole transport in this polymer.

We report electrical impedance measurements of polymer light-emitting diodes employing the soluble, conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the light-emitting layer. The diode structures were metal-polymer-metal structures utilizing thin gold as the transparent, positive contact, and calcium as the negative contact. The devices were fabricated using undoped, polymer active layers ∼40 nm thick. The polymer light-emitting diodes are accurately modeled as a resistor and capacitor in parallel for frequencies from 100 Hz to 1 MHz and for bias conditions from reverse bias to forward current densities of 0.1 A/cm2. The diode capacitance as a function of bias voltage is qualitatively different from conventional Schottky or p-n junction diodes; in reverse bias, the polymer layer is fully depleted and the capacitance is independent of bias; at small forward bias, traps are charged near the metallic contacts and the capacitance increases; under large forward bias, with significant electron and hole injection, the traps are neutralized and the capacitance decreases. From the magnitude of the initial increase in capacitance with forward bias the trap density is estimated to be only a few times 1016 cm−3.