2.4 Polymer light emitting diodes  (Page 2/4)

Polymer	Chemical name	Structure	π-π* energy gap (eV)	Emission peak (nm)
PA	trans -polyacetylene		1.5	600
PDA	polydiacetylene		1.7
PPP	poly( para -phenylene)		3.0 (red)	465
PPV	Poly( para -phenylenevinylene)		2.5 (green)	565
RO-PPV	poly(2,5-dialkoxy-p-phenlyenevinylen)		2.2 (blue)	~580
PT	polythiophene		2.0 (red)
P3AT	Poly(3-alkythiophene)		2.0 (red)	690
PTV	Poly(2,5-thiophenevinylene)		1.8
PPy	Polypyrrole		3.1
PAni	Polyaniline			3.2

Approaches to improve the efficiency

Efficiency for any LED is defined:

n _ext = n _esc * n _int

where n _ext is the external quantum efficiency, n _int is the internal efficiency (represents the fraction of injected carrier, usually electron, that is converted to photon), and n _esc is the escape efficiency (represent fraction of photons that can reach to the outside).

The most common way to improve the internal efficiency is to balance the number of electrons and holes which arrives at the polymer layer. Originally, there are more holes than electron that arrive of the polymer layer because conjugated polymers have a higher electron affinity, and as a consequence will favor the transport of hole than electron. There are two ways to maintains the balance:

1. Match the work function of electrode with the electron affinity and ionization potential of the polymer.
2. Tune the polymer’s electron affinity and ionization potential to match the work function of the electrode.

The escape efficiency is also important because a polymer LED is made up of layers of materials that have different refractive index, and some of the photon generated from the excition may be reflected at the boundary and trapped inside the device.

Improvement in internal quantum efficiency using low working function cathode

Conjugated polymer is electron rich, the mobility for hole is higher than electron, and more holes will arrive in the polymer layer than electrons. One way to increase the population of the electron is to use a lower working function metal as cathode. Braun and Heeger have replaced the aluminum cathode with calcium results in improved internal efficiency by a factor of ten, to 0.1%. This approach is direct and fast but low working function electrode like calcium will be oxidized easily and shorten the devices’ life.

Improvement in internal quantum efficiency using multiple polymer layers

A layer of poly[2,5-di(hexyloxy)cyanoterephthalylidene] (CN-PPV, [link] ) is coated on top of PPV to improve the transport and recombination of electron and holes ( [link] ).

The nitrile group in the CN-PPV has two effect on the polymer.

1. It increases the electron affinity so electrons can travel more efficient from the aluminum to the polymer layer. And metal of relative high working function like aluminum and gold can be now be used as cathode instead of calcium.
2. It increases the binding energy of the occupied π and unoccupied π* state but maintain a similar π-π* gap. So when the PPV and CN-PPV is placed together, holes and electron will be confined at the heterojunction.