Fabrication of White Light-emitting Electrochemical Cells with Stable Emission from Exciplexes

Summary

The authors present a method for fabricating stable white-light-emitting electrochemical cells utilizing emission from exciplexes formed between a blue-emitting fluorene polymer and aromatic amines.

Abstract

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active layer which consists of blue-fluorescent poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and π-conjugated triphenylamine molecules. This white light emission originates from exciplexes formed between PFD and amines in electronically excited states. A device containing PFD, 4,4',4''-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), Poly(ethylene oxide) and K₂CF₃SO₃ showed white light emission with Commission internationale de l'éclairage (CIE) coordinates of (0.33, 0.43) and a Color Rendering Index (CRI) of Ra = 73 at an applied voltage of 3.5 V. Constant voltage measurements showed that the CIE coordinates of (0.27, 0.37), Ra of 67, and the emission color observed immediately after application of a voltage of 5 V were nearly unchanged and stable after 300 sec.

Introduction

Research and development of polymer light-emitting electrochemical cells (PLECs) have expanded in recent years.^1-15 PLECs are similar to organic light-emitting diodes (OLEDs) in that both are surface emitting organic devices and are expected to find their way into future lighting applications. OLEDs are already on the market, but the cost is still high, one reason being that OLEDs need a complicated device structure with multiple layers. In contrast, PLECs have a very simple device structure which consists of a single active layer (emitting layer) between a pair of electrodes. This means that PLECs are suited to mass production processes such as roll-to-rol....

Protocol

1. Preparation of Active Layer Solutions

1. Active layer solution for the amine doped PFD devices
   NOTE: The PFD, 4,4',4''-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), 9,9-dimethyl-N,N'-di(1-naphthyl)-N,N'-diphenyl-9H-fluorene-2,7-diamine (DMFL-NPB), Poly(ethylene oxide) (PEO), were used as received. The potassium trifluoromethanesulfonate (K₂CF₃SO₃) was dried under vacuum at 200 °C for 1 hr prior to use.
   1. For the devices having a PFD:amine ratio of 1:0.25, dissolve 10 mg of PFD and 2.5 mg of the aromatic amine in 1 ml of chloroform and stir....

Results

The electroluminescence (EL) spectra were used to calculate the CIE coordinates and CRI values (Figures 2, 4, 5). Photographic images of the emitting devices were collected to verify the whiteness of the emission (Figure 3).

The EL spectra of the amine doped PFD devices and the undoped PFD device are shown in Figure 2. The undoped PFD device showed blue emission that corresponds.......

Discussion

The LEC has an active layer containing hydrophobic PFD and aromatic amines, and hydrophilic polyethylene oxide and KCF₃SO₃. Because these materials have very different solubilities, careful preparation of the spin coating solution is critical to avoid incomplete solvation. Each must be first dissolved separately and completely in solvents with sufficient solvating ability, then the solutions are mixed together to form a uniform mixture. Balancing the exciton and exciplex emissions is key to obtainin.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD)	Aldrich	571660
4,4’,4’’-Tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA)	Aldrich	768669
9,9-Dimethyl-N,N’-di(1-naphthyl)-N,N’-diphenyl-9H-fluorene-2,7-diamine (DMFL-NPB)	Aldrich
Poly(ethylene oxide) (PEO)	Aldrich	182028
Potassium tirifluoromethansulfonate (KCF3SO3)	Aldrich	422843	dried under vacuum at 200 °C for 2 hr prior to use
Chloroform	Kanto Chemical Co.	08097-25	dehydrated
Cyclohexanone	Kanto Chemical Co.	07555-00
SCAT 20-X (detergent)	Daiichi Kogyo Seiyaku		diluted with water
Acetone	Kanto Chemical Co.	01866-25	Electronic grage
2-propanol	Kanto Chemical Co.	32439-75	Electronic grage
13 mm GD/X Disposable Filter Device PVDF Filter Media, Polypropylene Housing	Whatman	6872-1304
UV/O3 Treating Unit	SEN Lights Co.	SSP16-110
Spectral Photo Detector	Otsuka Electronics	MCPD 9800
Voltage Current Source Monitor	ADCMT	6241A
Evaporation Mask	Tokyo Process Service Co., Ltd.	NA	The evaporation mask was wet-etched to create openings for patterned deposition of aluminum. The size of the mask is 100 mm x 100 mm x 0.2 mm-thick.