CNMS User Research
Spin injection in conjugated polymer for enhanced solid-state
Bin Hu and Yue Wu (CNMS users), University of Tennessee; An-Ping Li
and Jian Shen (CNMS Staff), and Jane Howe (ORNL)
work, we have explored the introduction of spin polarization in p-conjugated
polymer MEHPPV [Poly(2-methoxy-5-(2’-methylhexyloxy)-1,4
phenylenevinylene] by using spin injection from ferromagnetic materials.
The approach uses thermal deposition to prepare Co nanodots on
polymer thin films via Volmer-Weber growth. These Co nanodots form
Co/organic heterojunctions and consequently overcomes the common
difficulty of spin injection by providing significantly improved
surface contact and reducing the conductivity mismatch. The spin-polarized
hole injection from the Co nanodots results in a significant magnetic
field-dependent electroluminescence (EL) as compared to light-emitting
diodes (LEDs) with a nonmagnetic gold (Au) nanodot electrode of
similar workfunction. The introduction of spin polarization breaks
limit of spin singlet/triplet exciton ratio of 1/3, increasing
the singlet fraction from 25 % to 30 %. Furthermore, the Co nanodots
are significantly more efficient than the Co continuous film for
spin injection, and a spin diffusion length ~ 60 nm is revealed
this polymer material.
polymers, as soluble organic semiconductors, have been the
subject of intense research for the last decade with the objective
of low-cost and/or large-area LED and photovoltaic device applications.
A new “spin” in this research is to utilize the electron
spin degree of freedom in these polymer materials to provide a new
and extremely tantalizing route towards organic spin electronics/optoelectronics.
The promise is clear: these materials are composed of light elements,
principally carbon and hydrogen, the spin–orbit interaction
is small and spin-polarization lifetimes of charge carriers are expected
to be comparatively long. The promise of this new “spin” can
only be explored based on the following achievements: to inject
spin polarized carriers across an electrode/polymer interface;
and maintain the spin polarization over distances comparable to
device feature lengths; and to tune the spin polarization in the
by e.g., using a magnetic field. In this work, these three essential
capabilities have been demonstrated for the first time in a high
molecular-weight organic polymer LED device. The application of
magnetic nanomaterials is proven to be critical for this success,
the contact interface and circumvents the conductivity mismatch
problem. As a result, the ferromagnetic nanodot electrode presents
way to achieve spin-polarized charge injection in organic semiconducting
materials for improving the optoelectronic properties of organic
Spin injection from ferromagnetic Co nanoclusters into organic semiconducting
polymers”, Y Wu, B. Hu, J. Howe, A.P. Li, and J. Shen, Phys.
Rev. B 75, 075413 (2007).
This research was conducted in part at the Center for Nanophase Materials
Sciences, which is sponsored at Oak Ridge National Laboratory by the
Division of Scientific User Facilities, U.S. Department of Energy.
(a) Schematic device structure and band diagram.
(b) EL-voltage-current characteristics are given for different
magnetic fields at reverse bias. (c) The reverse EL (REL), forward
EL (FEL), and PL spectra were taken at 0, 200 Oe, and 450 Oe. The
REL spectral intensity shows magnetic field dependence while the
FEL and the PL are independent of the magnetic field. (d) The EL
enhancement is plotted as a function of the MEHPPV film thickness.
of EL enhancement using magnetic Co and non-magnetic Au electrodes.
Only Co electrodes give EL enhancement under magnetic field.