Physics and Applications of Doping in Molecular Semiconductor Films

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Technical: This project addresses fundamental issues in organic semiconductor materials; dopant selection, impact of doping on the electrical behavior of molecular films and interfaces, and features of doping particularly relevant to device performance. The approach encompasses: a search for strongly reducing agents of the organometallic 'sandwich' type. Cr- or Fe-based organometallics such as Fe(C5H5)(C6Me6) are predicted to have an ionization energy 200-300 meV smaller than the strongest molecular n-dopant investigated to date. Similarly, strongly oxidizing agents in the transition metal dithiolene family and transition metal oxides will be studied, along with the stability of the dopant vs. diffusion or contamination as a function of operating conditions (temperature, environment). In the second area of research the relative benefits of interface doping vs. interface engineering (electrode modification) in terms of carrier injection will be addressed. The impact of doping on the mobility of carriers in bulk films, and whether the benefit of doping-induced filling deep traps outweighs additional scattering and trapping caused by these impurities will be assessed. Scanning tunneling microscopy/ spectroscopy (STM/STS) will be used to investigate dopants and related potentials with molecular resolution. The third general theme of the research, application of doping to devices, will involve investigation of power conversion efficiencies in organic photovoltaic cells doped on both sides, i.e., p-doped donor and n-doped acceptor layers, to improve carrier extraction, contacts and electron-hole pair separation. The use of transition metal oxide molecules (as dopants) and films (as high work function electrodes) for charge generation layers and key structures of high efficiency, multi-layer stacked devices will also be studied. These activities will involve a multi-technique approach to fully characterize the electronic, chemical and charge transport properties of these materials (ultra-violet, X-ray and inverse photoemission spectroscopy, STM/STS, current- and capacitance-voltage measurements). Collaborations with chemists, device physicists and theoreticians are established, and will be an integral feature of the research approach.Non-technical: The project addresses basic research issues in electronics/photonics materials science with high technological relevance, and will provide important opportunities for student training. The project is expected to improve our understanding and facilitate the generalization of chemical doping in organic devices, with potentially important industrial applications. It will expose students (graduate and undergraduate) to a range of fundamental materials physics and electronic structure problems, as well as to practical device issues, to a variety of experimental techniques, and to different research environments through collaborations in chemistry and theory of electronic structure. Through his participation in outreach programs at Princeton University, short courses given to industry, and involvement in scientific societies, the PI has been and will remain extremely active in the teaching and dissemination of knowledge in the field of organic electronics.