Organic (plastic) electronics are expected to impact the future semiconductor industry by low-cost, high diversity and low power consumption. The simplest unit in many of the plastic electronic applications is the organic field-effect transistor (OFET). Our efforts in this area consist of several major lines of interest:
The progress in the field of organic electronics requires a good fundamental understanding of the electronic behavior of organic semiconductors incorporated in devices. A correct insight on the interplay between the different factors that affect the charge carrier transport in this class of materials can only be obtained by investigating single crystals. Single crystals are not meant to be incorporated in applications, but to serve as model systems and to provide a well defined structure, in which the intrinsic electronic properties can be measured. Here, the properties are not masked by grain-boundaries or other microstructure features that can severely localize and trap the charge carriers. For this reason, single crystals are suitable systems for study of fundamental aspects of charge carrier transport in organic semiconductors such as charge injection and collection, transport mechanism, etc.
We grow single crystals of different organic semiconductors, and investigate their structure and electronic properties. Single crystals are also incorporated in organic field-effect transistors. Typical transistor characteristics are shown below. The variation if the drain current ID as a function of gate voltage VGS shows very sharp turn on. This property is important for low power/low voltage applications. The device shows almost no hysteresis between the forward and reverse sweeps. This is a good indication of a low trap density at the interface between the gate insulator and the organic semiconductor. The increase in the drain current ID as a function of drain voltage VDS for different negative gate voltages VGS is typical for the hole conduction
We are particularly interested in single-crystal devices of novel organic semiconductor with potential for high-performance electronic applications. By structure-property correlations, we compare the electronic response to different crystal packing in novel electronic materials. This allows feed-back for design of novel materials with enhanced electrical properties.Collaborators:
The interest in organic semiconductors is motivated by increasing demands of supplementing Si-based electronics with materials that offer lower fabrication cost when scaled to large area applications. Replacing thermally-evaporated organic semiconductors with solution-processable organic semiconductors would reduce the processing costs considerably. Moreover, the moderate thermal budget required for deposition from solution allows compatibility with plastic substrates, which is attracting considerable interest for numerous flexible electronics applications.
In collaboration with John Anthony's chemistry group at University of Kentucky we develop novel electronic materials that are soluble at moderate temperatures and form highly ordered films when deposited from solvent. This allows us to achieve both solution processiblity and good molecular order, leading to high performance organic thin-film transistors (OTFTs). The crystal packing is optimized by appropriate choice of the nature, size and position of the substituent, leading to various stacking motifs. We explore the structure-processing-performance relation for a variety of solution processable organic semiconductors. These studies will allow comparisons between novel organic semiconducting molecules and will promote the discovery of materials with enhanced electronic properties.
Figure: Different crystal packing obtained by variation in the substituent group in the class of anthradithiophene.
Thin film patterning is desired, as it reduces the crosstalk between neighboring devices by minimizing the parasitic current paths. At the same time, it promotes the increase in the on/off ratio, by decrease in the off current, leading to enhanced contrast between the onand off states. Conventional photolithography techniques developed for patterning inorganic semiconductors require the use of solvents, which yields considerable damage on the weak organic semiconducting materials, and device performance degradation. For this reason, the use of traditional patterning methods is not appropriate for organic semiconductors and there is an urgent need for non-destructive patterning methods suitable for organic electronics.
We are working on developing reliable and reproducible methods for efficiently patterning organic semiconductors by using non-traditional patterning methods. The methods explore specific properties of organic semiconductors. For example, by manipulation of interactions between organic materials and surfaces where they are deposited, patterning can be achieved by simple, inexpensive and straightforward procedures.
We investigate organic semiconductors with molecular design tailored to provide targeted interactions with the contacts, dielectric materials or self-assembled monolayers (SAMs) on their surface. These interactions promote self-patterning of the organic film by driving the formation of specific structural features that exhibit distinct properties. We aim to develop and generalize concepts and methods for patterning organic thin film transistors by simple procedures, at low cost, through molecular design and innovative processing.
Deposition of organic semiconductors by ink-jet printing is particularly attractive, as it can provide a route to simultaneously deposit and pattern the semiconducting active layer, at moderate temperatures and low-cost, with micron-level resolution. The low thermal budget required for printing will allow deposition on flexible substrates. This property will enable the use of organic semiconductors in flexible electronics and is the basis of a novel thin-film technology, in which transistor circuits are manufactured by printing.
Organic spintronics is an emerging research field that combines the advantages of organic electronics and spintronics. The use of organic materials for spintronics applications is particularly attractive due to weak spin-orbit coupling, and weak hyperfine interaction that they offer. This gives long spin relaxation times (spin lifetimes), which is of tremendous importance, as it sets a larger length scale for loss of spin polarization, therefore larger dimensions for the devices (an important advantage over inorganic materials). The research on injection, transport and control of spin-polarized signals in organic crystals is performed in collaboration with the group of Dr. Curt Richter at NIST.