Organic Light Emitting Diodes

2 The physics behind OLEDs
2.1 Organic semiconductors
SMOLEDs are constructionally more similar to LEDs than PLEDs are, thus we first take a look at
conductivity properties of polymer materials used in PLEDs. The question arrising is: How polymer
becomes conductive? How can plastic conduct electricity?
Polymers are made of long chain molecules entangled between each other. The polymer chains are
formed by connecting many small molecular units called monomers (Fig. 2)[9].
Figure 2: polymer chain building
Most polymers are organic compounds, which means they are composed mostly of carbon chains with
hydrogen, oxygen and nitrogen atoms. These atoms form covalent bonds where the electrons are
localized in the low energy bonding orbitals of the chain molecule. Hence polymers typically do not
conduct electricity and are used in electronic application as insulator.
The prototypical conducting polymer is polyacetylene (PA), (CH)n (Fig. 3). Every bond contains a
localized “sigma” (σ) bond which forms a strong chemical bond. In addition, every double bond also
contains a less strongly localised “pi” (π) bond which is weaker. A π molecular orbital is thus formed
when two carbon atoms form a double bond and the 2pz orbitals have the same symmetry. The
electrons in this π orbital have equal probability of being around each carbon nucleus.
Figure 3: 2pz orbitals interact and form a “conductive” electron cloud
Morover, π-bonding, in which the carbon orbitals are in the sp²pz configuration and in which the
orbitals of successive carbon atoms along the backbone overlap, leads to electron delocalization along
the backbone of the polymer. (Delocalized electrons are electrons that are shared by more than two
atoms). This electronic delocalization provides the “highway” for charge mobility along the backbone
of the polymer chain. [2]
As a result, therefore, the electronic structure in conducting polymers is determined by the chain
symmetry (i.e. the number and kind of atoms within the repeat unit), with the result that such polymers
can exhibit semiconducting or even metallic properties.
Actually because of the Peierls Instability with two carbon atoms in the repeat unit, the π-band is
divided into π- and π* bands (Fig. 4) [10]. What Peierls showed is that due to the coupling between
electronic and elastic properties the polymer develops a structural distortion such as to open a gap in
the electronic excitation spectrum.
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