Elsevier

Synthetic Metals

Volume 161, Issues 23–24, January 2012, Pages 2580-2584
Synthetic Metals

Highly efficient solution-processed white organic light-emitting diodes based on novel copolymer single layer

https://doi.org/10.1016/j.synthmet.2011.08.010Get rights and content

Abstract

We fabricate by solution processed methods organic light emitting diodes with single-layer structure (ITO/(PEDOT:PSS)/co-polymer/Ba/Al) and study the transport properties of the final devices. The co-polymer is novel poly(fluorene-alt-phenylene) (PFP) derivatives containing co-monomers, involving red-emitting 1,8-naphthalimide units as pendant groups (0.0005, 0.005, 0.02 and 0.08 wt%) covalently attached. All the devices exhibited emission at very low driving currents in the μA range (47–73 μA). White emission with luminous efficiency of 9.42 Cd/A at 50 μA is obtained for the co-polymer with the smallest amount of chromophore. Commission Internationale de L’Eclairage (CIE) coordinates evolve from almost pure white color (0.26, 0.30) for low currents to stable cool white (0.21, 0.23). Increasing naphthalimide contents leads to stable green and orange emission with 3.07, 19.5 and 6.7 Cd/A efficiencies. The current–voltage response of the devices is analyzed by means of a numerical model that includes an injection mechanism based in the microscopic hopping theory and a field-dependent carrier mobility for the bulk transport regime. The fitting results allow to estimate the dependence of carrier mobility on polymer composition in the diodes.

Highlights

► White-emitting polymer for solid state lighting. ► Single component, single-layer, solution processed devices. ► Blue-emitting poly(fluorine-alt-phenylene) containing co-monomers bearing red-emitting 1,8-naphtalimide pendant groups. ► Efficiency of 9.42 Cd/A for the device with a fraction x = 0.0005 of monomers with chromophore, at low driving current, 47 μA.

Introduction

White organic light emitting diodes (WOLEDs) have been recently considered one of the top 10 technologies of the decade [1] due to their potential ability for energy saving, and the key properties of lightness, optimum heat dissipation and flexibility, seeking to replace conventional white light sources by means of large area panels. Since 2008, WOLED designed lamps are already available [2] and, in the near future, organic solid-state lighting is expected to move from decorative applications to technical lighting and general illumination. This will however require higher efficiency, color purity and lifetime, as well as improvements of processed materials and architectures to reduce production costs.

Since white light emitting polymers are solution-processed, they have become a strategic issue within the solid-state lighting (SSL) context [3]. Currently, white polymer light emitting diode (WPLED) technology is mainly focused in reducing manufacturing costs as well as to improve procedures for mass-production lines. In this context, solution-processed methods are receiving much attention due to their compatibility with profitable manufacturing techniques for large area production, such as roll-to-roll (R2R). In fact, the old dichotomy between molecules and polymers has been replaced by evaporated or solution processed capability of the materials. WPLED efficiency is still a major concern, achieving so far lower scores than those obtained by phosphorescent materials [4], or molecule-based active layers deposited by evaporation [2]. Currently high efficiency values in WPLEDs are considered to be around a few Cd/A [5]. Consequently there is currently a significant need for new organic semiconducting materials that combine the ability to perform good processability and stability with efficient charge transport and light emission.

Recently, we have demonstrated tunable fluorescence emission from novel poly(fluorene-alt-phenylene) (PFP) derivatives containing co-monomers, involving red-emitting 1,8-naphthalimide units as pendant groups (0.0005, 0.005, 0.02 and 0.08 wt%) [6] covalently attached. The integration of chromophore within the polymer structure avoids phase separation and formation of aggregates. In spite of the naphthalimide content all polymers showed similar electrochemical properties, with HOMO–LUMO levels of −4.93 and −2.1 eV respectively. However, variation of naphthalimide chromophore concentration in the polymers induces significant changes in the current–voltage (I–V) response of the devices based on these materials. In this work, we have fabricated solution processed WPLEDs with single-layer structure (ITO/(PEDOT:PSS)/co-polymer/Ba/Al) to study the effect of the naphthalimide amount on the device properties, that is, the electroluminescence emission (EL) and transport properties. For this purpose, we use a numerical model that includes field-dependent carrier mobility to analyze the devices I–V responses, which allows us to estimate the carrier mobility of the novel copolymers.

Section snippets

Material and experimental

We use a blue-emitting poly(fluorine-alt-phenylene) (PFP) containing co-monomers bearing red-emitting 1,8-naphtalimide units as pendant groups. The synthetic route is reported elsewhere [4]. The naphtalimide dopant has been covalently attached to the pendant chain of the host with an alkyl spacer. See structure of the material in Fig. 1.

Results and discussion

A detailed study [4] of the material optical response, that includes absorption, emission and time resolved emission, and electrochemical characterization in diluted solution, revealed that neither the photophysical properties not electrochemical properties of the copolymers in diluted solution are affected by the introduction of the naphthalimide chromophore. Minimal interaction between chromophores (PFP and naphthalimide) in the ground state of the resulting copolymers were observed in

Conclusions

White emission with high luminous efficiency of 9.42 Cd/A is obtained for the x = 0.0005% co-polymer (where x means the fraction of monomers with naphtalimide pendant groups) at very low driving currents (47 μA) in a single-layer, single-component WPLED fabricated by solution methods. It makes these materials promising candidates for being integrated in flat panel displays.

Acknowledgments

Financial support by Comunidad Autónoma de Madrid under project S2009/MAT-1756, S2009/MAT-1467 and by Ministerio de Educación y Ciencia (Spain) under MAT2009-08786, CTQ2010-14982 and TEC2009-13991-C02-02 projects is gratefully acknowledged. Also, we thank to UCM-BSCH join project GR35/10-A-910759.

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