Johannes Kepler University of Linz

About: Johannes Kepler University of Linz is a education organization based out in Linz, Oberösterreich, Austria. It is known for research contribution in the topics: Thin film & Quantum dot. The organization has 6605 authors who have published 19243 publications receiving 385667 citations.

Charge Transport and Recombination in Low‐Bandgap Bulk Heterojunction Solar Cell using Bis‐adduct Fullerene

Abstract: Charge transport and recombination are studied for organic solar cells fabricated using blends of polymer poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) with [6,6]-phenyl-C61-butyric acid methyl ester (mono-PCBM) and the bis-adduct analogue of mono-PCBM (bis-PCBM). The photocurrent of Si-PCPDTBT:bis-PCBM devices shows a strong square root dependence on the effective applied voltage. From the relationship between the photocurrent and the light intensity, we found that the square-root dependence of the photocurrent is governed by the mobility-lifetime (μτ) product of charge carriers while space-charge field effects are insignificant. The fill factor (FF) and short circuit current density (Jsc) of bis-PCBM solar cells show a considerable increase with temperature as compared to mono-PCBM solar cells. SCLC analysis of single carrier devices proofs that the mobility of both electrons and holes is significantly lowered when replacing mono-PCBM with bis-PCBM. The increased recombination in Si-PCPDTBT:bis-PCBM solar cells is therefore attributed to the low carrier mobilities, as the transient photovoltage measurements show that the carrier lifetime of devices are not significantly altered by using bis-PCBM instead of mono-PCBM.

A Short Measure of the Need for Affect

Abstract: Two studies are presented to introduce a 10-item short form of the Need for Affect Questionnaire (NAQ–S; cf. Maio & Esses, 2001). Study 1 was based on 4 independent samples (German or English language; N total = 2,151) and demonstrated the expected factorial structure of the NAQ–S; its measurement invariance with respect to gender, age, and education; and the predicted associations with relevant personality measures. A latent state-trait analysis conducted in Study 2 (N = 140) suggests that most of the reliable variance of the NAQ–S represents stable individual differences.

Mobile Ionic Impurities in Poly(vinyl alcohol) Gate Dielectric: Possible Source of the Hysteresis in Organic Field-Effect Transistors†

Abstract: Threshold voltage shifts in organic field effect transistors (OFETs) have been reported frequently. Cyclic sweeps of the gate voltage in OFETs reveal a hysteresis in the transfer characteristics (drain-source current versus gate-source voltage) thereby unfolding an electrical instability of the transistor element. On the one hand bistable transistors with a nonvolatile hysteresis in the transfer characteristics may be used in organic memory elements, whereas on the other hand hysteresis free transistors are desired in integrated organic circuits. Therefore understanding the causes of these electrical bior instabilities in organic field-effect devices is of primary interest. OFETs using charged electrets or ferroelectric-like gate dielectrics (e.g., see [7,8,12,14]) show a hysteresis which is normally attributed to the intrinsic properties of these materials. In polymer dielectrics without an intrinsic hysteresis such as poly(vinyl alcohol) (PVA), gate voltage induced hysteresis are often interpreted in terms of electrostatic screening of trapped charge carriers released under the influence of the electric field between the gate and the source/drain electrodes. Experimental results so far revealed a fairly complex picture of this “bias-stress” effect where the detailed mechanisms involved are not yet fully understood. Several publications report evidences of a charge trapping process in the organic semiconductor close to the semiconductor/dielectric interface. Lindner et al. supposed that the origin of the hysteresis in organic devices is a combination of slow transport (polarons or mobile ions) with a reaction other than trap recharging, e.g., a direct polaron-bipolaron reaction or a complex formation reaction of polarons / bipolarons with counterions. Bias-stress experiments with pentacene on various inorganic dielectrics indicate a reversible structural change of the semiconductor. A substantial number of experiments demonstrate an influence of the gate dielectric material on the formation of hysteresis effects. Transistors with a polymer gate dielectric are more likely to show a pronounced hysteresis than transistors with an inorganic gate dielectric. It has been reported that hydroxyl groups in the form of silanols at the SiO2-dielectric/semiconductor interface can work as electron traps which can be eliminated by a thin alkane interlayer. In many organic dielectrics hydroxyl groups are present in large numbers, either as integral part of the chemical structure and/or as impurities remaining from the process of synthesis. Such hydroxyl groups therefore were suspected of being responsible for the hysteresis effect in many OFET configurations. Lee et al. conclude that the increase of hydroxyl groups in polymer dielectrics equally increases electron trapping sites which in turn cause a large hysteresis in OFETs, but as they also discussed, the drain current as well as the gate leakage current increased with increasing hydroxyl density, which may be in contradiction with the proposed idea of immobilized carriers. Polymer dielectrics are often hosts for mobile ions. It has been demonstrated for example that mobile Na ions may diffuse from an underlying substrate into organic semiconductors, like pentacene or poly(3-hexylthiophene) under the influence of an applied voltage. Thereby they cause an increase of the current through the semiconductor and, additionally, a current-voltage hysteresis. The finding that mobile ions in semiconductors alter their electrical performance is not new. p-n junction devices, called flexodes, with a variable current-voltage (I–V) characteristics resulting from a reversible drift of Li ions were suggested in 1963. Two years later, small traces of mobile alkali ions in SiO2 gate dielectrics were reported to cause significant problems in semiconductor devices. In fact, the practical application of MOSFETs was delayed in the early 1960s because of severe gate bias instability problems caused by mobile ionic oxide charges like Na, C O M M U N IC A TI O N