Presenter:Prof. Antoine Kahn (Princeton University)
Topic: Beating the Thermodynamic Limit: n-Doping of Very Low Electron Affinity Organic Semiconductors
Time: 10:00 AM, Mar. 26th ( Tuesday)
Doping is one of the keys to controlling the electronic and electrical properties of organic semiconductors, lower contact resistance, enhance bulk conductivity and carrier mobility, and create higher performance devices. The talk starts with a rapid overview of doping mechanisms, of the reducing or oxidizing power of several n- and p-type molecular dopants, and of their impact on the conductivity of both vacuum- and solution-processed organic semiconductor films. We then turn to very low electron affinity materials (EA ~ 2.0-2.1 eV), which are central to electron transport layers in modern green and blue OLEDs, but are notably difficult to n-dope. We look at phenyldi(pyren-2-yl)phosphineoxide (POPy2) with an EA = 2.1 eV doped with the air-stable dimer of (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium ([RuCp*Mes]2). We demonstrate that photo-activation of the cleavable dimeric dopant results in kinetically stable and efficient n-doping of the host semiconductor, whose reduction potential is beyond the thermodynamic reach of the dimer’s effective reducing strength. This stability arises from the photo-assisted cleavage of the dopant, which ultimately leads to the resulting monomeric cationic organometallic species for which the reverse reaction sequence is kinetically hindered on a time scale of at least thousands of hours. We show that the electron transport material doped in this manner is used to fabricate high-efficiency organic light-emitting diodes with significantly improved performance relative to un-doped devices.
Antoine Kahn, native of France, joined the Princeton faculty in 1979. He is currently the Stephen C. Macaleer ’63 Professor in Engineering and Applied Science, Professor in Electrical Engineering, and the vice-dean of the School of Engineering and Applied Science. He has done extensive work on atomic and electronic structures of surfaces and interfaces of semiconductors. Over the past twenty years, he focused on the structural, electronic and chemical properties of surfaces and interfaces of intrinsic and doped organic molecular and polymer films. Recent work includes (i) the physics and applications of n- and ptype molecular dopants in organic thin films, (ii) the physics of organic/inorganic and organic/organic heterojunctions, (iii) the use of transition metal oxides in organic photovoltaics, and (iv) the electronic structure of hybrid organic inorganic perovskite systems. Kahn has co-authored over 400 refereed regular and review articles. He holds 4 US patents. He was the recipient of a Presidential Young Investigator Award (1984-85), and was elected Fellow of the American Vacuum Society (AVS) (1999), American Physical Society (APS) (2002) and Fellow of the School of Engineering of the University of Tokyo, Japan (2015). He was listed among the “World’s Most Influential Scientific Minds”, Thomson Reuters 2014. He currently holds a Weston Visiting Professorship (2019-21) at the Weizmann Institute of Science, Israel, and a Visiting Professorship at Chiba University, Japan (2007-present).