Spin injection in conjugated polymer for enhanced solid-state lighting efficiency

Bin Hu and Yue Wu (CNMS users), University of Tennessee; An-Ping Li
and Jian Shen (CNMS Staff), and Jane Howe (ORNL)

Achievement

In this 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 nanoscale 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 the theoretical 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 in this polymer material.

Significance

Conjugated 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; to transport and maintain the spin polarization over distances comparable to device feature lengths; and to tune the spin polarization in the structure 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, which improves the contact interface and circumvents the conductivity mismatch problem. As a result, the ferromagnetic nanodot electrode presents a practical way to achieve spin-polarized charge injection in organic semiconducting materials for improving the optoelectronic properties of organic semiconductor devices.

Publication

“ 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.

Comparison of EL enhancement using magnetic Co and non-magnetic Au electrodes. Only Co electrodes give EL enhancement under magnetic field.