Polymeric Light Emitting Diodes(PLED)

Electroluminescence in conjugated polymersR. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley,

D. A. Dos Santos, J. L. Bre¬ das, M. Lo» gdlund & W. R. Salaneck Nature 1999 397 121

Wessling Approach

Figure 6 Energy levels for electroluminescent diodes. a±c, An ITO-PPV-Ca diode before contact between the three layers, illustrating the energies expected, a, from the metal Fermi energies, assuming no chemical interactions at the interface, b, after some `doping' of the interfacial layer of PPV by Ca, setting upbipolaron' bands within the PPV semiconductor gap (note that the Fermi energy for the `doped' PPV lies between the upper bipolaron level and the conduction band), and c, after interfacial chemistry which sets up a blocking layer at the interface (as expected in the presence of oxygen). d, Energy levels for the components of a two-layer heterojunction diode fabricated with PPVand CN-PPV.

Unexpectedly high efficiency

Red

Solubilizing Groups

Red

Green Blue

Cacialli F. etal Chem. Commun. 2001, 1778

Brightness 200 cd/m2 with efficiencies of 0.5 cd/A

Light Emitting Cell (LEC)

Anode ITO

PPV + CF3SO3-

PPV+ / CF3SO3-

[O]

Cathode Metal

PPV + Li+

PPV - / Li+

[Re]

PPV - + PPV+ hν

SCl-

200 OCCF3SO3Li/

O On(PEO)

Spin Coat on ITO

heat

CF3SO3Li/

O On(PEO)

Film

Greeen 1000 cd/m2O O

OO

Blue 3.5V 1000 cd/m2

Heeger, A. AM 1998, 917

Using R(OCH2CH2O)nOSO3Lito replace CF3SO3Li/PEO

Bilayer Polymeric Light Emitting Diodes

Bradley, APL 1998, 629

Molecular-scale interface engineering for polymer light-emitting diodes R. H. Friends etal. Nature 2000 404, 481

Achieving balanced electron±hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (100 A) charge-injection interfacial layers in polymer LEDs. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turnon leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15±20 percent) at a luminance of 1,600 candelas per m2 at 5 V.

Figure 1 Schematic diagram of the hole injection interlayer between ITO and the LEP, and chemical structure of some of the polymers. The interlayer depicted is a graded (PEDT:PSS/PXT)5(PSS/PXT) film assembled layer-by-layer on silylated ITO via alternate polyanion±polycationdeposition. The LEP film is formed by spin-casting. The upper panel indicates how the sub-layers (10±15 A thin) of the interlayer are stratified and yet highly interpenetrated to facilitate carrier hopping between neighboring PEDT layers in the direction of the arrows. The reduced shading of the PEDT chains to the right indicates a decreasing doping level across the graded interlayer. Thermal baking at 70±80 8C eliminates tetrahydrothiophene to formsulphonate crosslinks between PXT and PSS. The PXT contains short conjugatedoligophenylenevinylene sequences.

He Y. etal APL 1999, 2265

High performance organic polymer light-emitting heterostructure devices

Ca

BTDF

Poly-TBPITO

10000 cd/m2, efficiency 14.5 cd/A

Barrier to electron extraction in polymer light-emitting diodes

Murata, K. etal APL 2001, 79, 1193

ITOBTDF

Ca-

+

General Device

ITOBTDF

Au

Hole-only devices

+

Polarized Electrolumescence from an oriented substituted polythiophene in PLED

Inganäs, O AM 1996, 43