Researchers at the Institute for Molecular Science, National Institutes of Natural Sciences (Japan) have developed a method for high performance doping of organic single crystal. Furthermore, the researchers succeeded in measuring the Hall effect of the crystal - which the researchers claims is the world's first case. The research findings have been published in the Advanced Materials.
According to the announcement the researchers said that controlling "holes" and "electrons" responsible for electric conduction of p-type and n-type semiconductors by doping -- adding a trace amount of impurity -- had been the central technology in the 20th century's inorganic single crystal electronics represented by silicon chips, solar cells, and light emitting diodes.
The researchers continued by saying that the number of carriers (holes and electrons) created by doping and their mobility can be freely evaluated by "Hall effect measurement" using a magnetic field.
However, in the field of organic electronics emerging in the 21st century, no one has ever attempted to dope impurities into an organic single crystal itself nor measure its Hall effect.
Chika Ohashi, a PhD student, SOKENDAI, said, "We have combined the rubrene organic single crystal growth technique with our original ultra-slow deposition technique of one billionth of a nanometer (10-9 nm) per second, which includes a rotating shutter having aperture." Chika continued "For the first time, we have succeeded in producing the 1 ppm doped organic single crystal and have detected its Hall effect signal." The doping efficiency of the organic single crystal was 24%, which is a much higher performance compared to 1% for the vacuum deposited amorphous film of the same material.
Hall Effect in Bulk-Doped Organic Single Crystals
Chika Ohashi | Seiichiro Izawa | Yusuke Shinmura | Mitsuru Kikuchi | Seiji Watase | Masanobu Izaki | Hiroyoshi Naito | Masahiro Hiramoto
First published: 18 April 2017 | DOI: 10.1002/adma.201605619
Abstract
The standard technique to separately and simultaneously determine the carrier concentration per unit volume (N, cm−3) and the mobility (μ) of doped inorganic single crystals is to measure the Hall effect. However, this technique has not been reported for bulk-doped organic single crystals. Here, the Hall effect in bulk-doped single-crystal organic semiconductors is measured. A key feature of this work is the ultraslow co-deposition technique, which reaches as low as 10−9 nm s−1 and enables us to dope homoepitaxial organic single crystals with acceptors at extremely low concentrations of 1 ppm. Both the hole concentration per unit volume (N, cm−3) and the Hall mobility (μH) of bulk-doped rubrene single crystals, which have a band-like nature, are systematically observed. It is found that these rubrene single crystals have (i) a high ionization rate and (ii) scattering effects because of lattice disturbances, which are peculiar to this organic single crystal.