Organic Light-Emitting Diodes (LEDs)
Electrically conducting or semiconducting organic polymers have been known for some time.
Since the beginning of the 90s it is known that some semiconducting organic polymers show
electroluminescence when used as an emissive layer sandwiched between a positive and a
negative electrode respectively. Thus, the construction of an organic LED is possible.
Compared with classic inorganic LEDs, more flexible devices of larger area are accessible.
Besides that, changing the functional groups easily allows the tuning of the emitted lights' colour
in the whole range of the visible spectrum, which is hardly possible in classic LEDs.
Construction and operation of organic LEDs
A thin film of an organic compound (e. g. a conjugated organic polymer) is deposited onto a
transparent substrate precoated with indium tin oxide (ITO). In the next step a low work
function metal electrode (e. g. aluminium or magnesium) is evaporated onto the organic
layer by vacuum metal vapour deposition (Fig. 1).
Fig.1:
Structural principle of an organic light-emitting diode
Thus, electrons and holes can be injected via the Al- or Mg-polymer contact and the
ITO-polymer contact, respectively. They combine inside the polymer layer to form electron-hole
pairs (excitons) which can decay with emission of light (electroluminescence).
Own research fields
In our research group we work on poly(p-phenylenevinylene) analogous systems
(PPV analogous systems, Fig. 2) exhibiting the following special properties:
-
Solubility - this leads to better processability and facilitates the production
of thin films
-
Electron-withdrawing groups at the double bonds of PPV - thus, the barrier
for electron injection into the polymer is reduced and the emission colour can be changed
-
Substitution of the phenylene units by more electron rich or electron poor
systems - by this the colour of emission can also be varied
Fig.2:
General scheme for the synthesis of PPV - analogous systems
Our aim is to evaluate the influences of various functional groups on emission colour and
intensity and thus to optimize the LEDs.
Top of the Page