Physics of Organic Semiconducting Devices
Prof. Paul W. M. Blom (Max Planck Institut für Polymerforschung, Mainz)

Charge transport and charge recombination are recognized as key ingredients in the performance of polymer light emitting diodes (PLEDs) and solar cells. In the last two decades a large effort has been put on the characterization of the transport of the dominant charge carrier, holes. It has been demonstrated that the hole transport is governed by hopping between localized states, characterized by a mobility that depends on density, electric field and temperature. The strongly reduced electron currents are generally attributed to the immobile trapping of electrons. Remarkably, we show that the electron trap distribution is identical for a large variety of polymers, hinting at a common origin for electron traps. Photogenerated current measurements reveal that next to the known bimolecular recombination also trap-assisted recombination is an important recombination channel in organic semiconductors.
Ferroelectric polarisation is an attractive physical property as the mechanism for non-volatile memories, since the two polarisations can be used as two binary levels. The challenge is to develop a storage medium in which the favourable properties of ferroelectrics such as bistability and non-volatility can be combined with the beneficial properties provided by semiconductors such as conductivity and rectification. We developed a new integrated solution by blending semiconducting and ferroelectric polymers into phase separated networks. The polarisation field of the ferroelectric modulates the injection barrier at the semiconductor–metal contact. As a result, charge transport through the semiconductor is switched from injection limited in the Off-state to space charge limited in the On-state.
Self-assembly—the autonomous organization of components into patterns and structures—is a promising technology for the mass production of organic electronics. For diodes based on self-assembled monolayers (SAM) electrical shorts are often formed upon vapour deposition of the top electrode. We developed a method to manufacture molecular junctions with diameters up to 100 mm with high yields (>95 per cent). The junctions show excellent stability and reproducibility. The basic building block of an integrated circuit is the self-assembled-monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. We have demonstrated SAMFETs by using liquid-crystalline molecules consisting of a p-conjugated mesogenic core separated by a long aliphatic chain from a monofunctionalized anchor group. The resulting SAMFETs exhibit a bulk-like carrier mobility, large current modulation and high reproducibility. This allows us to demonstrate real logic functionality by constructing a 15-bit code generator in which hundreds of SAMFETs are addressed simultaneously. Bridging the gap between discrete monolayer transistors and functional self-assembled integrated circuits puts bottom-up electronics in a new perspective.