N-doping of organic molecular films by pyronin B Antoine Kahn , Princeton University, DMR-0408589

1. top Au NTCDA bottom Au substrate N-doping of organic molecular films by pyronin BAntoine Kahn, Princeton University, DMR-0408589 N-doping of organic molecular semiconductor films is a potentially important way to improve injection into electron-transport materials via tunneling through a thin depletion layer. While p-doping of technologically relevant hole-transport materials has been achieved with highly electronegative molecules like F4-TCNQ, n-doping remains a challenge, as it requires dopants with very small ionization energy (< 4 eV), which are easily oxidized upon device processing. We show here that n-doping can be achieved from a commercially available, stable, organic salt precursor, pyronin B chloride. The salt decomposes under evaporation in vacuum into two components, the leuco form and the neutral radical form(Fig. a) of pyronin B (PyB). Photoelectron spectroscopy of the evaporated material, complemented with density functional theory calculations of the electronic structure of both species show that the latter has a much lower ionization energy (4.3 eV) than the former (6.0 eV), and is responsible for n-doping in a film of NTCDA (Fig. a). Electrical measurements on electron-only devices (Fig. b) clearly show the impact of doping, due in part to an increase in “free” electron concentration and in part to more efficient injection through a depletion region. (a) (b) NTCDA PyB neutral radical

2. Evac 4.3 4.5 LUMO 1.4 Hg ECB 0.8 EF 5.4 1.1 EVB n-Si Hg 2.9 n-Si HOMO How do electrons cross Si-bound alkyl monolayers?Antoine Kahn, Princeton University, DMR-0408589 Mechanisms of transport of electrons through single molecules sandwiched between electrodes are still elusive. Here, we investigate an “ideal” system consisting of a well ordered monolayer of alkyl chains (CnH2n+1, n=12,14,16, 18) chemisorbed on a Si wafer and contacted with a mercury droplet (Fig. a). The electronic structure of the system is determined by electron spectroscopy (Fig. b). The current-voltage (J-V) characteristics (Fig. c) reveal two dominant transport mechanisms. At low bias (<0.5 V), the current is limited by thermionic emission over the barrier in the Si substrate, yielding a current that is independent of the molecular chain length. The inset shows a temperature-dependent measurements, which confirm this mechanism. At high bias, the current shows an exponential dependence on chain length, which signal a current dominated by tunneling across the molecular layer. The combination of spectroscopic and detailed J-V data allows a realistic investigation of the meaning of charge-carrier effective mass in these systems. (b) (c) (a) CnH2n+1

3. Electrical doping of organic molecular filmsAntoine Kahn, Princeton University, DMR-0408589 Education Outreach The research on electrical doping of organic molecular films in the group of the PI involved two graduate students (Calvin Chan and Wei Zhao) and two undergraduate students (Rachel Cheng and Ying Gao) in 2004-05. Three of these students are women, reflecting increasing success at diversifying our student population. As in previous years, this multi-disciplinary effort put students in close contact with faculty members of the chemistry department at Princeton and Georgia Institute of Technology and with researchers at DuPont Research, General Electric and Intel Corporation. Through this collaborative approach, our students are being exposed to the solid state physics and chemistry of these materials, as well as to the concerns of industrial laboratories working toward commercialization of organic devices. In 2004-05, these students presented their work in international conferences, e.g. the 2004 Fall MRS meeting, the 2005 Electronic Materials Conference and the AVS International Symposium. In 2005, the PI led a group of grade-school teachers in a QUEST follow-up workshop on education and outreach program on Energy. QUEST is a professional development program in science for New Jersey teachers. The program offers a unique opportunity for upper elementary and middle school teachers to enhance their personal knowledge of science by engaging in laboratory experiments led by the faculty and staff of the university. The workshop focused on concepts of energy, energy conservation, and transformation from potential to kinetic energy.