In the framework of the one-dimensional mean-field (MF) drift-diffusion approach the well-defined boundary conditions far away from the metal/insulator contacts of a planar metal/insulator/metal system are used to determine the boundary condition at the interface itself. The novel self-consistent boundary condition linking the carrier density and the electric field at the interface enables a straightforward description of the current voltage (IV) characteristics in forward and reverse bias bridging space charge and injection-limited regimes and accounting for barrier lowering from the potential drop in the used contact materials. Yet, because of the low carrier density in the insulator under injection limitation, single-particle phenomena, such as the Schottky effect, must be considered. We reconsider the validity of the MF approach, depending on the external bias and the prevailing injection barriers. For the crucial parameter window where the MF approach fails and single-particle phenomena become important, a modification of the boundary conditions at the insulator/metal interface is proposed to account for the discrete nature of carriers. The difference between the thus modified MF and the unmodified MF approach is illustrated by several examples.
In this contribution we emphasise the ambipolar organic field-effect transistors (OFETs) as the prime element for the
realization of various OFET types. It will be shown that ambipolar OFETs can be used to produce on the one hand
complementary unipolar OFETs and thus CMOS elements and on the other hand light-emitting OFETs. Some ambipolar
light-emitting OFETs will be presented and the impact of the contact formation at the source and drain electrodes on the
device characteristics will be discussed. In general, the investigation of ambipolar OFETs provides a deep understanding
of the OFET operation and guides the way to novel aspects of the OFET applicability.
The charge carrier transport in organic field effect transistors (OFETs) is determined by the
transport properties of the insulator / organic semiconductor interface. We demonstrate that an
adequate treatment of this interface results in a polarity change of the OFET charge carrier
transport properties, without further altering the device structure. Illuminating the utilized
PMMA polymer dielectric, by using UV radiation, leads to the introduction of mainly
electron traps at the dielectric interface. This results in the suppression of the electron
transport for an otherwise n-type pentacene OFET. However, as a consequence of trapped
electrons in the near surface layer of the PMMA dielectric, the hole transport of the device is
enabled though a hole blocking source/drain metallization. This effect, as well as the impact
of the UV irradiation on the PMMA dielectric will be discussed in detail. The UV treatment
yields a PMMA interface rich on polar groups. The influence of these groups on the OFET
characteristics is investigated by studying several polymer dielectrics with varying content of
the emerging groups.
In the present paper a new concept towards O-CMOS technology is presented substantiating the importance of the semiconductor/dielectric interface for charge carrier transport in organic semiconductors. It will be demonstrated that by controlling the interface properties of either SiO2 or PMMA, unipolar p- and n-type OFETs can be realized using a single organic semiconductor and even a single metal for source and drain contacts. Two dielectric/semiconductor interface modifications are considered for the realization of complementary OFETs on the basis of pentacene, otherwise known for its exclusive hole transporting properties. Selective modification of the SiO2 dielectric interface with traces of vacuum deposited Ca, allows for electron transport in pentacene and the realization of complementary pentacene OFETs on a single substrate. By this technique electron traps are removed due to a reaction of atomic Ca with oxygen from available hydroxide groups, resulting in the formation of an oxidized Ca layer. In a second approach, it is demonstrated that by selective UV treatment of a PMMA dielectric surface, unipolar n-type pentacene OFETs can be converted to unipolar p-type by the introduction of electron traps in the form of -OH and -COOH groups at the PMMA interface. Both methods allow for the realization of CMOS organic inverter stages with decent electrical properties.
KEYWORDS: Electrodes, Electron transport, Solar cells, Fullerenes, Silver, Heterojunctions, Temperature metrology, Organic photovoltaics, Polymers, Metals
We have measured the electron and hole mobility in blends of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) with varying MDMO-PPV/PCBM composition. It is shown that the electron mobility in the PCBM-rich phase gradually increases up to 80 wt.% PCBM, due to an increased number of percolated pathways from bottom to top electrode. In contrast to the expectations the hole mobility in the MDMO-PPV phase shows a similar behavior as a function of fullerene concentration; Starting at 40 wt.% with the value of pristine MDMO-PPV the hole mobility strongly increases and saturates beyond 67 wt.% at a value which is more than two order of magnitude higher. The large enhancement of the hole mobility and its saturation is related to recent findings on the film morphology of this material system.
Push-pull molecules are key components for doped low glass transition temperature photorefractive polymers. In this paper we report on the optimization of the factor of merit of photorefractive polymers doped by molecules belonging two different classes of push-pull molecules, on one hand molecules with a dominant neutral resonance form and, on the other hand, zwitterionic or charge separated molecules. The theoretical analysis is illustrated by experimental result obtained both on a molecular level and on polymeric materials of the interest of the analysis developed.
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