Correlation between energy level alignment and device performance in planar heterojunction organic photovoltaics

Information on the interfacial electronic structure in organic photovoltaics (OPVs) is essential for fully understanding features of device operation such as the photocurrent generation and relative energy band offsets at the donor/acceptor interface, which directly affect the open circuit voltage (V_oc). Kelvin probe (KP) measurements fully reveal the energy level alignment in a prototype OPV with a copper phthalocyanine (CuPc)/fullerene (C₆₀) planar heterojunction. Energy level pinning at the CuPc/C₆₀ junction fixes the energy band offsets of C₆₀. A downward energy shift of about 0.9 eV appears at the C₆₀/bathocuproine junction, which may act as a hole-blocking barrier. A combination of KP and current density–voltage measurements indicates that photocurrent generation depends strongly on the magnitude of the upward energy shift at the CuPc/C₆₀ junction. The dependence of V_oc on the substrate work function is also discussed in terms of the energy level alignment at indium tin oxide/CuPc/C₆₀ junctions.     

Electronic structure and surface morphology of [6,6]-phenyl-C71-butyric acid methyl ester films

We investigate the electronic structure of [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) films by ultraviolet photoelectron and inverse photoemission spectroscopy. We discuss the electronic structure of PC₇₀BM in comparison with C₇₀ and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM). The molecular orbitals around the energy gap of PC₇₀BM are broadly distributed due to the low symmetry of the molecular structure. Consequently, the energy gap of PC₇₀BM is smaller than that of C₇₀ and PC₆₀BM. The film deposition method affects the polarization energy between the PC₇₀BM molecules, and thus affects the electronic structure of the films.     

New compound upper bound on MIMO channel capacity

MIMO channel capacity may be severely degraded due to correlation between individual sub-channels of the matrix channel. Several models, which are limited to some specific scenarios, have been developed to date to account for this effect. In this letter, we derive a new upper bound on the mean (ergodic) MIMO capacity, which is not limited to a particular scenario and accounts for both transmit (Tx) and receive (Rx) end correlations in such a way that their impact can be estimated separately and compared. Thus, a conclusion can be made as to which end contributes more to capacity reduction. In general, the higher correlated end has a dominant effect on the capacity     

A performance analysis of a four‐leg inverter in two AC motor drives with independent vector control

This paper presents a performance analysis of a four‐leg inverter (FLI) in driving two motors with vector control. We show that an expanded two‐arm modulation (ETAM) known as a modulation method of a five‐leg inverter is applicable for the four‐leg inverter and we theoretically analyze a performance analysis of the four‐leg inverter in driving two motors with vector control. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

An accurate and compact large signal model for III–V HBT devices

An accurate and compact large signal model is proposed for modeling heterojunction bipolar transistors (HBTs) based on III–V materials. In DC mode, the model includes self-heating, Kirk and Early effects, as well as the temperature dependence of the model parameters. In small signal mode, the model captures the variation of various AC parameters with bias. The procedure of extracting the model parameters uses DC and multiple bias S-parameter measurements. The model is compiled in the HP–ADS circuit simulator as user-compiled model and is verified by comparing its simulations to measurements in all modes of operation for an AlGaAs/GaAs transistor with an emitter area of 2 × 25 μm².     

Self-assembled monolayer effect on the characteristics of organic diodes

In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior of organic diodes based on sexithiophene (6T) sandwiched between indium tin oxide (ITO) and aluminum. We have used molecules of SAMs based on a thiol with functional groups of oligothiophene (3T(CH₂)₆SH). Wettability measurements have been performed to characterize ITO surface energy and its modification upon deposition of SAMs. The results of contact angle measurements and surface energies demonstrate the homogeneity and rigidity of grafting surface. The current vs. applied voltage characteristics of devices show that conduction in weak biasing follow Richardson–Schottky behavior. Beyond 1.5 V, J–V characteristics can be successfully modeled by space-charge limited current (SCLC) theory followed by a trap charge limited current (TCLC). The electrical as well as optical characteristics of 6T layer are clearly affected by the presence of the SAM. The differences between ITO/SAM and bare ITO samples are interpreted in terms of structural effect induced by the self-assembled monolayer of 3T(CH₂)₆SH.     

Double liquid crystalline side-chain type block copolymers for hierarchically ordered nanostructures: Synthesis and morphologies in the bulk and thin film

Well-defined double liquid crystalline side-chain type block copolymers (DLC-BCPs) were synthesized by nitroxide-mediated living free radical polymerization (NMP) of two different types of styrene monomers containing liquid crystalline moieties. The molecular weights and compositions were controlled by changing the polymerization time and/or the feed amount of the monomers. The bulk morphologies were studied using wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). DLC-BCPs were favored to form microphase-separated lamellar structures over a wide range of volume fractions. By controlling the annealing conditions, further higher-ordered hierarchical nanostructures were obtained with a long-range alignment, in which the semi-crystalline side-chains were oriented parallel to the interface of microphase-separated lamellar structure. Thin film morphology was investigated using atomic force micrography (AFM). The formation of lamellar nanostructure with a long-range alignment vertically oriented to the silicon wafer substrate was achieved by simple thermal annealing.