Electron injection into organic semiconductor devices from high work function cathodes

Abstract

We show that polymer light-emitting diodes with high workfunction cathodes and conjugated polyelectrolyte injection/transport layers exhibit excellent efficiencies despite large electroninjection barriers. Correlation of device response times with structure provides evidence that the electron-injection mechanism involves redistribution of the ions within the polyelectrolyte electron-transport layer and hole accumulation at the interface between the emissive and electron-transport layers. Both processes lead to screening of the internal electric field and a lowering of the electron-injection barrier. The hole and electron currents are therefore diffusion currents rather than drift currents. The response time and the device performance are influenced by the type of counterion used.conjugated polyelectrolytes ͉ ion motion ͉ polymer light-emitting diodes ͉ electron transporting layer ͉ charge injection L ight-emitting diodes and thin-film transistors fabricated with semiconducting (conjugated) polymers are examples of an emerging technology with potential impact in low-cost displays and solid-state lighting (1). Balanced charge injection (holes into the -band from the anode and electrons into the *-band from the cathode) is a basic requirement of high-efficiency polymer light-emitting diodes (PLEDs) (2). In the absence of interfacial effects, the barrier for electron injection is determined by the difference between the energy of the bottom of the *-band (lowest occupied molecular orbit, LUMO) of the polymer and the Fermi energy of the metal used as the cathode; similarly, the barrier for hole injection is determined by the difference between the energy of the top of the -band (highest occupied molecular orbit, HOMO) and the Fermi energy of the anode. Because the charge injection is described (in first approximation) by a combination of Fowler-Nordheim tunneling and thermionic emission mechanisms, these barriers limit the device performance (3). Large and unequal barriers reduce power and optical output efficiencies by increasing the turn-on voltages and creating unbalanced injection of charge carriers. Electron injection continues to be an important problem because low workfunction metals such as Ca or Ba, with Fermi energies that match the *-bands of organic semiconductors, are unstable and decrease device operational lifetimes.Inserting injection/transport layers (TLs) between the emissive layer (EL) and the electrodes can improve charge injection into organic LEDs via different mechanisms. For example, a favorable dipole can be introduced that shifts the vacuum level at the electrode/TL interface (4). Charge-carrier blocking and accumulation at the EL/TL interface can also lead to improved injection by redistributing the field toward the TL and thereby reducing the charge tunneling distance (5-8). Efficient electron injection from stable metals into PLEDs incorporating a conjugated polyelectrolyte (CPE) electron TL (ETL) was recently demonstrated. CPE materials are characterized by a -delocalized backbone with p...

High-Efficiency Polymer LEDs with Fast Response Times Fabricated via Selection of Electron-Injecting Conjugated Polyelectrolyte Backbone Structure

“…16,17 This situation is similar to the behaviour observed for PLEDs with deep charge traps. 18 In an attempt to address such issues, Fang et al fabricated PLEDs with zwitterionic CPEs in which both anions and cations are covalently attached to the polymer side chains; 19 the resulting PLED transient response was then ~1-100 μs.…”

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

“…[ 1,5,6 ] A recent study on conjugated polymer zwitterions revealed a similar lowering in work function, attributed to electrostatic realignment of the dipolar zwitterionic side-chains near the metal interface. [ 18 ] The formation of interfacial dipoles and electric fi eld-induced reorientation of molecules has also been reported.…”

“…[ 20 ] The relatively low ionic mobility can result in slow transient behavior during device operation. [ 5 ] Modifi cation of the work function can also occur due to dipole formation induced by integer or partial charge transfer between the substrate and the WML (Figure 5 b). Charge transfer can take place in a large variety of circumstances as reviewed by Braun et al in Ref.…”

“…A new generation of WMLs fi rst introduced by Cao et al is based on polyelectrolytes or tertiary aliphatic amines, such as poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN) and the corresponding ethyl ammonium bromide. [ 1 ] Although these materials improve carrier injection or extraction in different organic diode devices, [1][2][3][4][5][6][7] the mechanism behind this improvement is not completely clear. This ambiguity hinders design, selection and optimization of new WMLs.…”

Conjugated Zwitterionic Polyelectrolyte as the Charge Injection Layer for High-Performance Polymer Light-Emitting Diodes

“…[6][7][8][9][10][11][12] A range of CPEs with different counterions (anion/cation) and backbones 13,14 have been synthesized in attempts to understand the role of the ions in the charge injection process. 15,16 The promising electronic properties of CPEs in combination with their solubility in polar solvents offer a unique route to the realization of high-performance polymer light-emitting diodes (PLEDs) without the use of reactive Ca or Ba cathodes.…”