High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films.

Daniele Di Nuzzo,Richard H Friend,E W Meijer,Baodan Zhao,Stefan C J Meskers,Chidambar Kulkarni,Ron Naaman,Eilam Smolinsky,Francesco Tassinari

doi:10.1021/acsnano.7b07390

Abstract

We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.

Highlights

S ources of circularly polarized light are interesting for a range of applications across different fields such as spinoptoelectronics,[1,2] optical manipulation of information,3,4 3D imaging systems, biology, and medicine.[5−7] Organic LEDs can emit circularly polarized electroluminescence (CPEL) when the emitting organic material is chiral and of one handedness only
By varying the thickness of the emitting layer, we achieve CPEL with up to 40% excess of right-handed polarization, the second highest value for polymer OLEDs and the highest obtained without the use of chiral dopants or alignment layers
These are Article between the highest values of CPEL reported for OLEDs and the highest achieved without the use of chiral dopants or alignment layers

Summary

Introduction

S ources of circularly polarized light are interesting for a range of applications across different fields such as spinoptoelectronics,[1,2] optical manipulation of information,3,4 3D imaging systems, biology, and medicine.[5−7] Organic LEDs can emit circularly polarized electroluminescence (CPEL) when the emitting organic material is chiral and of one handedness only. CPL can originate locally, i.e., at the molecular site of light emission In this case, for the emitted light to be fully circularly polarized, the optical transition has to be magnetic-dipole allowed and electric-dipole forbidden.[17] This requirement poses a challenge for reaching strong circular polarization while maintaining high luminescence efficiency. The only known examples of chiral molecules with high luminescence efficiency and high circular polarization are chiral lanthanide complexes.[18] The second mechanism that can be exploited to obtain CPL is nonlocal: this is the case of luminescent cholesteric liquid crystals, for which, in the limit of thick films (102 nm), circular polarization is largely determined by the anisotropy of the cholesteric dielectric medium and is weakly dependent on the polarization state of light at the site of emission. Our results show that OLEDs based on multidomain cholesteric emitting layers allow strong and broad-band CPEL

Methods

Results

Conclusion