Multivalent anions as universal latent electron donors

Cindy G. Tang, Mazlan Nur Syafiqah, Qi-Mian Koh, Chao Zhao, Jamal Zaini, Qiu-Jing Seah, Michael J. Cass, Martin J. Humphries, Ilaria Grizzi, Jeremy H. Burroughes, Rui-Qi Png (), Lay-Lay Chua () and Peter K. H. Ho
Additional contact information
Cindy G. Tang: National University of Singapore
Mazlan Nur Syafiqah: National University of Singapore
Qi-Mian Koh: National University of Singapore
Chao Zhao: National University of Singapore
Jamal Zaini: National University of Singapore
Qiu-Jing Seah: National University of Singapore
Michael J. Cass: Cambridge Display Technology Limited
Martin J. Humphries: Cambridge Display Technology Limited
Ilaria Grizzi: Cambridge Display Technology Limited
Jeremy H. Burroughes: Cambridge Display Technology Limited
Rui-Qi Png: National University of Singapore
Lay-Lay Chua: National University of Singapore
Peter K. H. Ho: National University of Singapore

Nature, 2019, vol. 573, issue 7775, 519-525

Abstract: Abstract Electrodes with low work functions are required to efficiently inject electrons into semiconductor devices. However, when the work function drops below about 4 electronvolts, the electrode suffers oxidation in air, which prevents its fabrication in ambient conditions. Here we show that multivalent anions such as oxalate, carbonate and sulfite can act as powerful latent electron donors when dispersed as small ion clusters in a matrix, while retaining their ability to be processed in solution in ambient conditions. The anions in these clusters can even n-dope the semiconductor core of π-conjugated polyelectrolytes that have low electron affinities, through a ground-state doping mechanism that is further amplified by a hole-sensitized or photosensitized mechanism in the device. A theoretical analysis of donor levels of these anions reveals that they are favourably upshifted from ionic lattices by a decrease in the Coulomb stabilization of small ion clusters, and by irreversibility effects. We attain an ultralow effective work function of 2.4 electronvolts with the polyfluorene core. We realize high-performance, solution-processed, white-light-emitting diodes and organic solar cells using polymer electron injection layers with these universal anion donors, demonstrating a general approach to chemically designed and ambient-processed Ohmic electron contacts for semiconductor devices.

Date: 2019
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DOI: 10.1038/s41586-019-1575-7

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