Organic electronics

Organic semiconductors are attracting considerable research interest as the electrically active materials for low-cost electronic and photonic devices such as organic light emitting diode (OLED) displays, thin film transistors and related applications (e.g. TFT sensors), RF identification tags (RFID), smart cards electronic paper etc.). This is largely because of their ease in fabrication and versatility of property tuning by molecular design and engineering. In the past decade research in this field has progressed to the extent that desirable charge transport in the organic semiconductor film in organic thin film transistors (OTFT) can be achieved through molecular design by selective placement of electron-rich, electron-withdrawing, and aromatic groups in different parts of the molecule. Although the electronic properties are easily tuned by molecular design, the molecular packing within the thin film and the film microstructure have a significant influence on the OTFT performance. Despite this importance, this interrelationship between molecular structure, thin film molecular packing and charge transport are only poorly understood.

For the above applications to become reality, a fundamental understanding of materials requirements and interactions is required. Novel printing methods and fine-tuning of organic materials to be compatible with the desired printing methods are also necessary.

Research Projects
-Rational design and synthesis of high-performance organic semiconductors. (GCEP)
-Design and synthesis of solution-processable and printable organic semiconductors.
-Innovation and development of non-lithographic printing methods.
-Development of fundamental understanding of the growth mechanisms of organic semiconductors from vapor and solution phases.
-Exploration of charge transport in organic single crystals.


Pentacene example:
In the field of organic semiconductor research, the material pentacene has developed into a benchmark material because high-performance thin film transistor (TFT) devices are easily and robustly obtained from vacuum-deposited thin films of pentacene on a variety of substrates. Pentacene thin films on silicon oxide are a particularly interesting case because, despite their polycrystallinity (i.e. structural imperfections and small grains), they outperform single crystal-based pentacene transistors. The key to understanding the electrical performance of pentacene TFTs lies with the first few layers of pentacene. When a TFT device is switched "on", the current flows predominantly in the first few molecular layers and the packing and exact arrangement of molecules in these layers determine the current obtained at an applied voltage. The knowledge of the precise packing in the first monolayer is, therefore, crucial to understanding the charge transport properties of pentacene TFTs.

Figure 1: Chemical structure of the organic semiconductor pentacene.


Grazing incidence X-ray diffraction (GIXD) with synchrotron light offers the unique opportunity to study the diffraction from weakly scattering, ultra-thin films such as a single molecular layer of pentacene. The scattering geometry of the GIXD experiment performed on SSRL's beam line 11-3 is depicted in Fig. 2. The incidence angle α of the incident beam k₀ was set to 0.1°.


Figure 2: Geometry of the GIXD setup. The incident beam k₀ makes an angle α with the sample surface; the scattered beam k is registered by an image plate detector at a distance L. The in-plane component of the momentum transfer vector q is q_XY and the vertical component q_Z.