Author John Sullivan (Princeton Institute of Technology Communications Office)
Compiled Lin Xin (paper first author)
The discovery by an international team of Princeton University, Georgia Institute of Technology and Humboldt-Universität Berlin has opened the door to more widespread use of high-tech organic electronics.
The research, focused on organic semiconductors, was published in the November 13 issue of Nature Materials, which has gained prominence for its use in emerging technologies such as flexible electronics, solar energy conversion, and smartphones and The high-quality color screen of the TV, which is simply a huge help in launching organic light-emitting diodes that emit high photon energies such as green and blue light.
'Organic semiconductors are ideal materials for flexible devices with low power consumption and low process temperature,' said Lin Xin, a Ph.D. student in Princeton's Department of Electrical Engineering who is also the lead author of the study. 'One of their major drawbacks is the relative Weak electrical conductivity, which in some applications leads to troublesome and inefficient devices, and we are looking for new ways to improve the electrical properties of organic semiconductors.
Semiconductors, commonly silicon, are the cornerstone of modern electronics because engineers can take advantage of their unique properties to control current, and in many applications semiconductor devices are used for computing, signal processing and switching. They are also used in energy-saving devices , Such as light-emitting diodes, and energy conversion devices, such as solar cells.
Doping is the most essential nature of these functions, and refers to the chemical composition of semiconductors being adjusted by adding small amounts of other chemicals or impurities By selecting the type and amount of dopant, the researchers are relatively free to adjust the semiconductor's electrons Band structure and electrical properties.
In their article, the researchers described a new way to greatly enhance the conductivity of organic semiconductors, made of carbon molecules rather than silicon atoms, which is a ruthenium-containing compound used as a reducing agent , In other words, introducing additional electrons into the organic semiconductors during part of the doping process.These additional electrons are the key to enhancing the conductivity of the semiconductors, which belong to the newly developed dimer organometallic dopants and other strong Unlike the reductant, these dopants are stable when exposed to the air and become strong electron donors upon reaction with other semiconductors in a solvent or film.
Seth Marder and Stephen Barlow from Georgia Tech dominate the development of this new dopant and call the ruthenium-containing compound 'super-reduced dopant.' They say it's not unusual for it to combine with giving Electrons and the ability to stably exist in the air and because they function in a class of organic semiconductors that have previously been difficult to be doped.The Princeton researchers found that this new type of dopant can improve the conductivity of these semiconductors by hundreds Wan times.
This ruthenium-containing compound is a dimer, that is, it consists of two identical molecules, or monomers, linked by a chemical bond. Just because the compound is relatively stable, when added to those difficult to be Doped semiconductor, it does not spontaneously react but remains in equilibrium, which raises the issue of enhancing the conductivity of semiconductors that react with semiconductors and then split into two single body.
Lin Xin said they were looking for different ways to separate the ruthenium dimer in order to activate the doping. Eventually he and Berthold Wegner, a visiting graduate student from the Norbert Koch group at Humboldt University, found out on how photochemical systems work They irradiated the system with UV light because UV light excites the molecules in the semiconductor and then directs the start of the entire reaction so the dimer can dope this hard-to-doped semiconductor under illumination and produce 100,000 or even more Million times the improvement of conductivity.
Next, the researchers conducted interesting observations.
'Once the lighting stops, one might think simply that the reverse reaction will take place, which leads to the loss of enhanced conductance,' Marder said. 'But that's not true.'
The researchers found that the ruthenium monomer can remain isolated in the semiconductor such that the enhanced conductance does not disappear, even though the thermodynamic principles allow the molecules to tend to return to their original dimeric structure.
Antoine Kahn, Stephen C. Macaleer, engineering and applied science chair, led the entire research team and said that the location distribution of the molecules in the doped semiconductor provides a possible answer to the puzzle: They assume that the monomer is in the semiconductor Scattered within the distribution, making it difficult for them to return to the original layout and then re-dimeric.He said that because the reorganization requires the monomer must have the correct orientation, but in this hybrid system, the monomer is always skewed.Therefore, Even though thermodynamics allow the monomer to be reorganized, it does not happen very quickly on most cells.
'The question is why these monomers are not reorganized into equilibrium,' Kahn said. 'The answer is that they are thermodynamically restricted.'
In fact, the researchers observed these doped semiconductors for more than a year and found only a slight decrease in conductivity, and at the same time, on the LEDs made from these materials, they found that the doping was emitted by the device Light continues to be activated. These devices were fabricated in partnership with Barry Rand, an assistant professor in the Princeton Department of Electrical Engineering and at the Anglinger Center for Energy and Environment.
'With every step of the system, light produces more light for further activation until it is fully activated,' said Mader, Georgia Power's chief energy efficiency professor and professor of chemistry at the University. This is a very novel and surprising finding. '
Other authors include Princeton graduate students Kyung Min Lee, Michael A. Fusella and Zhang Fengyu, and Georgia Tech's Karttikay Moudgil.
The National Science Foundation and the U.S. Department of Energy provided partial support for the study.
Princeton Antoine Kahn Group Profile: Focusing on the electronic, chemical, structural and electrical properties of materials in thin-film electronic devices, the research interests include a wide range of semiconductor materials (both simple and compound) and are currently focused on organic and molecular electronics Small molecules and polymer semiconductors, metals and metal oxides, and dielectrics, and is particularly interested in processing materials and interfaces with a view to improving OLEDs, FETs, organic photovoltaic cells and other thin film devices for large-scale flexible electronics Nearly infinite possibilities for chemical synthesis of new molecular compounds, coupled with the unparalleled simplicity of film deposition on a variety of substrates by vacuum evaporation, solution processes or printing, make organic semiconductors a key advantage over other semiconducting materials, and There are countless possibilities for innovation in device structure.
http://www.ee.princeton.edu/research/kahn/
Doping and Related Directions Recently published (in part)
Beating the thermodynamic limit withphoto-activation of n-doping in organic semiconductors, Xin Lin, Berthold Wegner, Kyung Min Lee, Michael A. Fusella, Fengyu Zhang, Karttikay Moudgil,Barry P. Rand, Stephen Barlow, Seth R. Marder, Norbert Koch and Antoine Kahn.Nat. Mater. DOI: 10.1038/NMAT5027 (2017)
Investigation of the High Electron AffinityMolecular Dopant F6-TCNNQ for Hole-Transport Materials, Fengyu Zhang andAntoine Kahn. Adv. Funct. Mater. 1703780 (2017)
Pairing of near-ultraviolet solar cellswith electrochromic windows for smart management of the solar spectrum,Nicholas C. Davy, Melda Sezen, Jia Gao, Xin Lin, Amy Liu, Antoine Kahn andYueh-Lin Loo, Nature Energy, 2, 17104 (2017)
Morphological Tuning of the Energetics inSinglet Fission Organic Solar Cells, YunHui L. Lin, Michael A. Fusella, Oleg V.Kozlov, Xin Lin, Antoine Kahn, Maxim S. Pshenichnikov, and Barry P. Rand, Adv.Func. Mat., 26, 6489 (2016)
Impact of a Low Dopant Concentration on theDistribution of Gap States in a Molecular Semiconductor, Xin Lin, Geoffrey E.Purdum, Swagat K. Mohapatra, Stephen Barlow, Seth R. Marder, Yueh-Lin Loo andAntoine Kahn, Chem. Mat. 28, 2677 (2016)
Experimental Characterization of Interfacesof Relevance to Organic Electronics, Gabriel Man, James Endres, Xin Lin andAntoine Kahn, in WSPC Reference on Organic Electronics, Jean-Luc Brédas andSeth R. Marder, edts., World Scientific, chapt. 6, p. 159-191