Study report of OLED by ePn0RMft


									    Study report of OLED

        Sheng-Horng Yen
C. R. Acad. Sci. Paris, t. 1, Serie IV, p.
           381-402, 2000.

                S-H Yen 2004/4/27            1

Organic materials
Operation steps of an OLED

             S-H Yen 2004/4/27   2
Molecule (left) and Polymer (right)

            S-H Yen 2004/4/27     3
     Different types of emitting materials

 Conjugated small molecule:
  1.Small, 0-D, weakly anisotropic.
  2.Deposited by evaporate (熱蒸鍍), multilayers
  with sharp interfaces are easily prepared.
 Conjugated polymer chains:
  1.large, 1-D, amorphous, highly anisotropic.
  2.Large area homogeneous polymer thin films are
  easily deposited by spin-coating (旋轉塗佈).

                    S-H Yen 2004/4/27               4
Operation steps of an OLED

        S-H Yen 2004/4/27    5

 Thermoionic emission
 Tunneling
 Combination

                S-H Yen 2004/4/27   6
            Tailored interface

 Attaching substituent to the organic
 Inserting a dipolar layer at the interface.
 Insertion at the interface of insulating
  barriers thin enough to allow tunneling.

                 S-H Yen 2004/4/27              7
               Carrier transport

 Mobility (µ):
  In low µ case, dispersive transport is observed.
  (different current transient sharps)
 Fast increase of µ with electric field:
  Particularly in polymer, µ will start to increase
  rapidly at very large fields
  >1 MV/cm. (This property can be used to produce
  laser light)

                    S-H Yen 2004/4/27                 8
     Langevin-type recombination

 Capture distance:

 Recombination rate:

 The capture results from a process of
  diffusion in a field treated by Langevin.

                   S-H Yen 2004/4/27          9
     Space-Charge Limited (SCL)
 High EL yields require:
  1.Efficient recombination
  2.Reach SCL conditions (no trapping carrier)
  In non SCL condition requires large applied
  voltage, hence smaller energetic yields (the
  factor h/eV decrease).

                  S-H Yen 2004/4/27          10
               Photo emission

 Electroluminescence quantum yield is
  proportional to                 .
 s: Singlet emission, fluorescence.
 t: Triplet emission, phosphorescence.
 Currently favored materials in OLED have
  s  0.15-0.6; t  0.

                    S-H Yen 2004/4/27        11
            Photo extraction

 The probability P0, that the photons are
  emitted outside the device is small.
 Improvement:
  1.Reflecting mirrors

                 S-H Yen 2004/4/27           12
          Multilayer diodes (I)
                                              Lowest unoccupied
 Organic material should have:               molecule orbital
1.Large electron affinity
  Ac (Lumo)  3 eV.
2.Small ionization potential
  Ic (Homo)  5 eV.
  Eg = Ic- Ac
                                      Highest occupied molecule
                                      orbital (Homo)
                  S-H Yen 2004/4/27                          13
          Multilayer diodes (II)

 The device would increase yield and shift the
                  S-H Yen 2004/4/27           14

 Much work remains to be done on
  physical problems:
  1.Understanding interfaces and injection.
  2.High voltage/high current operation.
 Optimization depends on improved

                S-H Yen 2004/4/27        15

To top