Universal
alignment of hydrogen levels in semiconductors, insulators, and solutions
Hydrogen
strongly affects the electronic and structural properties of many materials. It
can bind to defects or to other impurities, often eliminating their electrical
activity: this effect of defect passivation is crucial to the performance of
many photovoltaic and electronic devices. A full understanding of hydrogen in
solids is required to support development of improved hydrogen-storage systems,
proton-exchange membranes for fuel cells, and high-permittivity dielectrics for
integrated circuits. In chemistry and in biological systems, there have also
been many efforts to correlate proton affinity and deprotonation with host
properties. We have performed systematic studies, based on first-principles
methods, of hydrogen in a wide range of hosts.
The study revealed the existence of a universal alignment for the
electronic transition level of hydrogen in semiconductors, insulators and even
aqueous solutions. This alignment allows the prediction of the electrical
activity of hydrogen in any host material once some basic information about the
band structure of that host is known. A physical explanation was presented that
connects the behavior of hydrogen to the lineup of electronic band structures
at heterojunctions.
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