Xugang Guo
As a scientist of Materials Science and Engineering at the South University of Science and Technology of China (SUSTC), Prof. Guo joined Mark D. Watson’s group at the University of Kentucky in 2006 and obtained his Ph.D. degree in Chemistry in 2009. From 2009 to 2012 he worked with Prof. Tobin J. Marks at Northwestern University, and joined SUSTC immediately thereafter.
Prof. Xugang Guo’s current research interests are novel semiconductor and dielectric materials (organic, inorganic, and hybrid) and their applications in opto-electronic devices: including materials synthesis, processing, and integration, interface (insulator/semiconductor and conductor/semiconductor) modification and functionalization, electronic device fabrication and optimization.
His work Imide- and Amide-Functionalized Polymer Semiconductors was recently published in Chemical Reviews, an internationally well-known journal，having impact factor of 45.661.
Polymeric semiconductors are attractive for the fabrication of cost-effective, large-area, and mechanically flexible electronic devices via solution-based high-throughput patterning techniques such as slot-dye coating or printing. The low temperature processing enables device integration with flexible substrates, such as fabrics or plastics, without compromising substrate functionalities. Therefore, materials development and device applications of polymer semiconductors have been one of the most active research fields in both academia and industry.
Among various organic semiconductors, imide-functionalized small molecule semiconductors play a paramount role in organic electronics. The first imide-functionalized organic semiconductor, a perylene diimide, was used as an n-channel semiconductor for organic thin-film transistors in 1996. Since then, imide-functionalized polycyclic arenes attracted a great amount of attentions (Figure 1a), and highly promising n-channel transistor performance has been achieved for this class of semiconductors.
Polymer semiconductors have several advantages over their small molecule counter parts. First, polymers have better film forming properties. Polymer films are generally smooth, uniform, and isotropic. Therefore the devices fabricated from polymers can show small performance variation. Second, for commercial applications, printing techniques require great control of the rheological properties, which are more easily tuned for polymer solutions than for the small molecule counterparts. Third, polymers are more suitable for fabricating flexible devices than small molecules in terms of their mechanical properties.
The great success of imide-functionalized small molecule semiconductors serves as a test bed for the use of imide-functionalized polymers as semiconductors in organic electronics. Note that depending on the structural connectivity, imide-containing polymers can be insulators (Figure 1b) or semiconductors(Figure 1c). Indeed polyimides (Figure 1b) have been widely used as electrical insulators due to their synthetic accessibility, mechanical property, thermal property, and supramolecular self-assembly. Some characteristics making polyimides superior insulators are also desired for organic semiconductors, such as the high electron-deficiency and the capability to self-assemble into ordered structures.
Figure 1.General chemical structures of rylene diimide small molecule semiconductors (a), polyimide insulators (b), and imide-functionalized π-conjugated polymer semiconductors(c).
Inspired by the superior device performance of rylene diimide-based organic small molecule semiconductors and excellent insulating property of polyimides, Professor Xugang Guo pioneered the studies of the imide-functionalized polymer semiconductors (Figure 1c). His research mainly focuses on the development of imide-functionalized polymer semiconductors and their applications in organic electronics. He reported the first naphthalene diimide-based n-type polymer semiconductors, which result in unprecedented n-channel performance in organic thin-film transistors (http://www.rsc.org/chemistryworld/news/2009/january/21010902.asp). He firstly developed high-performance imide-functionalized am bipolar polymer and integrated the material into complementary inverters with high voltage gains (http://www.sciencedaily.com/releases/2009/08/090817143606.htm).Recently, Prof. Guo reported thiophene imide and bithiophene imide polymers with unprecedented fill factors in polymer solar cells (http://www.sciencedaily.com/releases/2013/08/130814101440.htm). These imide-functionalized polymers lead to unprecedented opportunities for organic electronics.
Figure 2. Cover stories from Prof. Guo’s work on imide-functionalized polymer semiconductors and their applications in opto-electronic devices.
In the recent review published in Chemical Reviews, Prof. Guo summarized this class of polymer semiconductors containing imide-functionalized arenes as π-conjugated electron-withdrawing units. Both diimide and monoimide-functionalized polymer semiconductors are reviewed. Structural analogues, amide-functionalized polymers, are also included. The combination of solubilizing substituent, electron-deficiency, and structure-enforcing geometry enables these polymer semiconductors tosynergistically integrate desirable opto-electronic properties. The arenes include historical molecules produced from natural sources as well astheir newly developed derivatives by modern organic synthetic chemists. The unique combination of the performance-enhancing characteristics of imide (and amide)-functionalized polymer semiconductors results in superior performance inorganic thin-film transistors and solar cells.
Recently I had a great opportunity to discuss with Prof.Guo about his research and the impact of his work on organic electronics.
“Imide-Functionalized small molecules have achieved such superior performance in organic electronics, why no researches on imide-functionalized polymer semiconductors have been done before?”
“This is a very interesting question.” Prof.Guo said: “When I was in Graduate School at the University of Kentucky in 2006, I started this project, imide-functionalized conjugated polymers, which is also the title of my later Ph.D. thesis. I was struggling to brominate the rylene diimides due to their high electron deficiency. Without bromination or iodination, nobody can make polymer semiconductors. I believe this is the major reason for the delay of imide-functionalized polymer semiconductors. Secondly, synthetic materials scientists thought thereis a significant steric hindrance created by the carbonyl group on the imidemoiety. Such hindrance will lead to low degree of conjugation, minimal crystallinity,therefore inferior charge carrier mobility. Fortunately, it turns out that the carbonyl group can lock the conformation by promoting the sulfur oxygen interaction between the carbonyl unit and thiophene ring. Such interaction is unique for imide- and amid-functionalized polymer semiconductors, which results in unprecedented performance in organic electronics.”
“What is the influence of your work on the future of organic electronics?”
“The materials derived from rylene diimide have already shown the most promising performance in many organic electronic devices, including organic thin-film transistors and solar cells. These materials provide a platform for further performance improvement, which have great potentials for commercialization.” Dr.Guo added “Moreover, the fundamental study of materials structure-property-device performance correlation will guide materials scientists to develop polymer semiconductors with precisely controlled property for performance improvement.”
“Any suggestions to students interested in scientific research?”
“We should take a skeptical attitude on what stated in textbooks or in high-profile journals” Prof. Guo smiled.
Reference
1 Xugang Guo, Antonio Facchetti, and Tobin J. Marks.Imide- and Amide-Functionalized Polymer Semiconductors. Chemical Reviews, DOI: 10.1021/cr500225d.
Related web link: http://pubs.acs.org/doi/abs/10.1021/cr500225d
(Source from: Kaizhu Guo)
