Organic PV Technology 1

Organic solar cells (OPV) include polymer cells and dye-sensitised solar cells (DSSC).  The materials used can be considered large conjugated systems, with compounds based on carbon, including P3HT, phthalocyanine, PCBM, and ruthenium dye N3.  A conjugated system is a system that has carbon atoms in a chain with alternating single or double bonds, and every atom in the chain contains a p-orbital that can be delocalised.  Delocalisation is the merging of all individual valence electrons of p-orbitals in the chain in a shared space, such that all electrons belong to the chain of atoms.  The compounds may be cyclic, acyclic, linear or mixed conjugated.  An example is the benzene ring (see diagram below).

Another example is the ethene molecule (see diagram below) consisting of 2 atoms.  It has 3 sp² hybrid bonds with bond angles of 120° per atom, and an electron in the p_z-orbital.  Both electrons in both p_z-orbitals will form a pi bond, making a molecular pi orbital.  The pi bond will consist of bonding and/or anti-bonding states.  Hence, conjugated molecules have similar properties to semiconductor materials.

Most electrons at room temperature will be in the bonding state, also known as the HOMO (highest occupied molecular orbital).  The anti-bonding state is known as the LUMO (lowest unoccupied molecular orbital).  Since conjugated molecules are quite long, the HOMO and LUMO will broaden and become similar to the valence and conduction band respectively.

To distinguish between p- and n-types of an organic material, the vacuum level must be considered.  The vacuum level is the energy of a free stationary electron that is not in any material, which means it's in a vacuum.  It is a reference energy level to align energy levels between different materials.  Ionisation energy is the energy required to excite an electron from the HOMO (valence band) to the vacuum level, and so would be for the formation of positive ions.  Electron affinity is the energy released when an electron moves from the vacuum level to the LUMO (conduction band), and so would be for negative ions.

When an organic material has low ionisation potential, less ionisation energy is required to excite an electron.  Hence, this type of material can be electron donors.  When an organic material has high electron affinity, it can attract additional electrons more easily, which means it can be an electron acceptor.



Reference:

5.5 Organic PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=jCtgMm55nBA

BONDING IN ETHENE, http://www.chemguide.co.uk/basicorg/bonding/ethene.html

ELECTRON AFFINITY, http://www.chemguide.co.uk/atoms/properties/eas.html