Imaging the Interfaces of Conjugated Polymer Optoelectronic Devices

Steuerman, David W.; García, Alberto Luis García; Dante, Mark; Yang, Renqiang; Löfvander, J. P. A.; Nguyen, Thuc‐Quyen

doi:10.1002/adma.200700887

Cited by 53 publications

(50 citation statements)

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“…1b). Multilayer fabrication is facilitated by the orthogonal solubility of MEH-PPV and PFN-BIm 4 (19,20). Gold or aluminum was used as the cathode.…”

Section: Resultsmentioning

confidence: 99%

Electron injection into organic semiconductor devices from high work function cathodes

Hoven

Yang

García

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

132

139

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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...

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“…1b). Multilayer fabrication is facilitated by the orthogonal solubility of MEH-PPV and PFN-BIm 4 (19,20). Gold or aluminum was used as the cathode.…”

Section: Resultsmentioning

confidence: 99%

Electron injection into organic semiconductor devices from high work function cathodes

Hoven

Yang

García

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

132

139

View full text Add to dashboard Cite

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“…That CPEs are soluble in polar solvents, while neutral conjugated polymers typically used as EMLs are not, allows spin casting multiple layers by alternating solvents of orthogonal polarities. [105,106] For example, Cao and coworkers demonstrated the ETL function of P1 with a Br À and I À counterions as well as the resulting device performance of ETL layers containing neutral precursors cast from a methanol and acetic acid solution. Emissive layers included poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), P-PPV, or poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO).…”

Section: Multilayer Pledsmentioning

confidence: 98%

Recent Applications of Conjugated Polyelectrolytes in Optoelectronic Devices

et al. 2008

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Conjugated polyelectrolytes comprise an electronically delocalized backbone with pendant groups bearing ionic functionalities. Important new developments regarding their use to improve charge injection from metallic electrodes into organic semiconductors, a key requirement in emissive devices, have recently appeared. This article provides an overview of recent studies concerning the basic properties of conjugated polyelectrolytes as a function of molecular structure, and of optoelectronic devices with conjugated polyelectrolytes as essential functional components. Processes where more insightful mechanistic understanding is needed and areas of opportunity are discussed.

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“…[15][16][17][18] However, multilayer fabrication is challenging if one wishes to deposit layers atop of each other when the materials have similar solubility characteristics. [19] Under these circumstances, one observes poorly defined interfaces and a disruption of the underlying polymer layers.…”

mentioning

confidence: 99%