Energy efficiency with organic electronics: Ching W. Tang revisits his days at Kodak
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Energy efficiency with organic electronics: Ching W. Tang revisits his days at Kodak Interviewed by Stephen Forrest and Nicole Casal Moore
Ching W. Tang, widely considered the father of organic electronics, built the first organic solar cell and the first organic light-emitting diode (OLED) in the 1980s. The enabling innovation for both devices was made possible by an organic heterojunction—a bilayer structure of an electron donor and an electron acceptor. This structure can be used to fabricate exceptionally efficient OLEDs and organic solar cells. Tang’s insights unleashed a torrent of activity in the field and eventually sparked an industry. His seminal articles on organic photovoltaics and OLEDs were published in Applied Physics Letters 48 (1986) p. 183 and 51 (1987) p. 913, respectively. Now a professor at the University of Rochester, Tang discussed with MRS Bulletin his days at Kodak.
MRS BULLETIN: How did you get interested in organic electronic devices? CHING W. TANG: I took an organic chemistry lab as a junior at the University of British Columbia. I liked the class, but I was really bad in organic synthesis. I clearly remember these words the professor said: “Hold this beaker like this and use the stirrer like this, and make sure you don’t punch a hole through your beaker.” Before he finished the sentence, I got a hole in the beaker and tried desperately to collect my material with my bare hands. It dawned on me then that organic chemistry was not for me, so I concentrated on physical chemistry. I went to Cornell University to do my PhD, where I studied the photoconductive properties of chlorophyll. That was the beginning of my work in organic electronics. There was a lot of interest in photovoltaics then, and I started working in that area. I
constructed photovoltaic cells with chlorophyll and learned that they were really inefficient, like 0.01%. Although I didn’t make any breakthroughs, I learned a great deal about organic/ metal contacts and how to manipulate them, which I found very useful in my future work on OLEDs. Kodak hired me as a postdoc to work on organic solar cells. After exploring a few classes using dyes without much success, I learned about a technology using aggregate photoconductors, which was being developed at Kodak for electrophotography applications. Basically, the aggregate photoconductor is a mixture of a polymer, an absorbing dye, and a holetransport molecule that can be induced to segregate to form two separate and interpenetrating phases—one capable of transporting holes and the other electrons. I used the aggregate photoconductor as an absorber in a solar cell. It worked much better, but funda-
Stephen Forrest, University of Michigan Nicole Casal Moore, [email protected]
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MRS BULLETIN
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VOLUME 37 • JUNE 2012
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www.mrs.org/bulletin • Energy Quarterly
mentally the carrier mobility was too low for the aggregate photoconductor to be useful in solar cells. Nevertheless, the idea of using a heterogeneous mixture of two phases to enhance the charge generation and transpor
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