Polymerelectronics

Semiconducting Polymers for Electronics Applications

The research group of Prof. Dr. Ullrich Scherf aims to develop and optimize synthetic procedures towards organic semiconductor materials. Main focus is the design of tailor-made materials of high purity and with optimum electronic properties. The materials are made for various application fields including organic light emitting diodes (OLED), organic solid-state polymer lasers, organic photovoltaics, and polymer electronics (polymer transistors).

Main target of the group is an intense interdisciplinary research with leading partners from Physical Chemistry, Physics, Electrical Engineering, Printing Technology etc.
The first presentation of an organic light emitting diode (OLED) based on organic semiconducting polymers in 1990 (Burroughes et al., 1990) has initiated broad research activities in the field of polymer electronics. One advantage of such polymer electronics devices is their simple fabrication process. In addition, large area devices can be designed leading to novel applications which are impossible with conventional devices based on rigid, small area substrates. The efficiency of OLED devices now reaches up to 15% for green or red emitting devices. Moreover, the construction of flexible OLEDs on plastic substrates (polymer foils) becomes possible.

In the step from pure polymer synthesis to research activities which are focussed on an active function of the materials, from structure oriented polymer chemistry to the solid state properties of molecular ensembles, the problem of structural regularity and purity of the materials comes into the focus. Many important properties of such materials as thermal stability, photo- and electroluminescence, electrical and photoconductivity etc are dramatically influenced by structural defects and impurities, the resulting properties are often dominated by defects and traps. Reliable structure-property relations are coupled to the availability of structurally defined materials with a reproducible profile of electronic properties. Furthermore, such structure-property relation can form the base for a synthetic tailoring of polymeric materials with optimum materials properties.

Conjugated ladder polymers of the poly(para-phenylene)-type (LPPP) represent one of the most intensively investigated classes of organic semiconducting polymers. Such LPPP materials have been first prepared in 1991 (Scherf et al., 1991). Until now more then 200 scientific papers describing synthesis, physical properties and applications in photonics and electronics have been published.

LPPP ladder structures display a unique set of optical properties as a narrow, structured absorption band (λ_max 450 nm), an intense blue photoluminescence with mirror symmetry of absorption and emission, and a very small Stokes loss (≥ 150 cm^-1). The PL quantum efficiency reaches > 90% in dilute solution and up to 50% in the solid state. Efficient OLEDs could be designed based on these LPPP-type emitters; the OLEDs showed high external EL quantum efficiencies of up to 4% (at driving voltages of 4-9 V) (Tasch et al., 1996).

The observation of so-called gain narrowing in conjugated semiconducting polymers has initiated a novel research field focussed on the fabrication of injection lasers based on semiconducting polymers (Hide et al., 1996). With this goal LPPP-type ladder polymers have been tested as active component in such collective emission processes („amplified spontaneous emission“ -ASE-). High quality, spin coated LPPP films display a spectral narrowing of the emission for energies of the femtosecond pump pulses of <5 nJ/pulse. Application of a „distributed feedback“ (DFB) device configuration results in a blue-green monomials laser emission already at low threshold energies of the femtosecond pump pulses of ca. 2 nJ/pulse (Kallinger et. al, 1998). The substrate used for the DFB laser devices was a periodically modulated poly(ethyleneterephthalate) (PET) foil (thickness 125 μm, period Λ ca. 300 m) or a quartz substrate which have been structured by electron beam lithography/etching. The active layer of the semiconducting polymer is finally spin coated on top of the grating (thickness ca. 400 nm). The solid state DFB polymer lasers show a remarkable stability. No degradation was detected after 10⁷ pulses. The stability is comparable to that of commercially available laser dyes. Moreover, also metallic gratings have been successfully testes as efficient DFB resonators (Stehr et al., 2003). The results define an important step towards a realization of electrically pumped plastics injection lasers.

Nanostructured polymer materials are increasingly attractive as active component of electronic devices especially for photovoltaic applications (photodiodes, solar cells). Hereby, novel strategies to design such nanoscale materials are of growing importance. The variety of methods includes nanoimprinting techniques, self assembly processes, the use of semiconducting polymer nanoparticles (Landfester et al., 2002), and the synthesis and processing of novel semiconducting block copolymers into nanostructured materials with defined dimensions of phase separation (Asawapirom et al., 2004). Two examples of such approaches are depicted in the Figures 4 and 5.

Figure 4: Spontaneous Nanostructure Formation in Semiconducting Block Copolymer Materials for Photovoltaic Devices

Figure 5: Semiconducting Polymer Nanoparticles as active layer for novel light emitting and photovoltaic devices

The availability of heterophase materials with a very large internal interface is seen as one key for an improvement of the power efficiency of organic solar cells. Hereby, so-called bulk heterojunction materials with a scale length of nanostructure formation near the diffusion length of free charge carriers in organic materials below 20 nm seem to be of outstanding importance.