The used absorber materials are either conductive organic polymers or organic molecules that are based on carbon, which may form a cyclic, a-cyclic, linear or mixed compound structure in ORGANIC PHOTOVOLTAICS.
above Figure shows some example of organic materials that can be used for PV applications: P3HT, Phtalocyanine, PCBM and Ruthenium Dye N3. All these materials can be considered as large conjugated systems, which means that carbon atoms in the chain have an alternating single or a double bond and every atom in the chain has a p-orbital available. In such conjugated compounds, the p-orbitals are delocalised, which means that they can form one big mixed orbital. Hence,the valence electron of the original p-orbital is shared over all the orbitals. A classical example would be the benzene molecule, which is a cyclic conjugated compound.
As we can see in below Figure , This molecule has 6 carbon atoms and six p-orbitals, which mix and from two circular orbitals that contain three electrons each. These electrons do not belong to one single atom but to a group of atoms.
In contrast, a methane molecule (CH4) is tretrahedrally coordinated, which means that it has 4 equivalent sp3 hydride bonds with a bond angle of 109.5°. An ethene (C2H4) molecule has three equivalent sp2 hybrid bonds with a bond angle of 120° plus an electron in a pz orbital.
At room temperature, most electrons are in the bonding state, which is also called the highest occupied molecular orbital (HUMO). The anti-bonding state can be considered as the lowest unoccupied molecular orbital (LUMO). As the conjugated molecules are getting longer, the
HOMO and LUMO will broaden and act similar to valence and conduction band in conventional semiconductors. The energy difference between the HOMO and LUMO levels can be e considered as the band gap of the polymer material in ORGANIC PHOTOVOLTAICS.