Important Materials for the Active Layer in Organic Solar Cells - Polymers

The polymers in organic solar cells should also be considered based on the same 3 factors: efficiency, stability and processing costs.  The polymer P3HT has a band gap of about 1.9eV, which means that it absorbs light up to about 600nm, which is about 17% of the photons (see diagram above).  If EQE is 1, there will be a current of about 11mA/cm².  A low band gap polymer with a band gap of 1.2eV can absorb 53% of the photons, which means a current of 34mA/cm² if EQE is 1.

This means that current can be increased by lowering the band gap (donor HOMO and acceptor LUMO level difference), but this will also mean reducing the V_oc of the solar cell (see equation above).  Polymers that absorb light lower than 2eV, which means at wavelengths longer than 600mm, are called low band gap polymers.

The highest reported efficiency for organic/polymer solar cells is about 10%.  The band gap of this polymer is about 0.7eV if PCBM is the acceptor.  This is obtained by ensuring that the solar cell has an efficient inter-chain charge transfer.  A better inter-chain charge transfer is achieved by different combinations of the electron deficient donor and the electron rich acceptor within the polymer backbone.

If the PITN polymer is used (see diagram above), the band gap will also be affected by the quinoid structure, which is very stable.  The band gap of this polymer is 1eV.  However, this polymer is not very soluble, so it has to be combined with other materials, such as thiophene, which will vary the band gap.  See the diagram below for an example of 2 polymers, where the donor unit is the same and the acceptor units are different.  The 2nd polymer is PSBTBT, with acceptor unit benzothiadiazole.  The donor unit is 4,4-di-2-ethylhexyl-dithieno[3,2-b:2',3'-d]silole.

See the diagram below for examples of donor and acceptor units.

With regards to stability, there is photochemical stability, usually accomplished by using different kinds of stable monomers.  For example, benzothiadiazole is more stable than other kinds of acceptor units.  The donor unit thiophene is also more stable than the fluorene monomer.

The side chains of the polymer can also affect its stability by improved morphological stability.  For example, the polymers can be crosslinked, creating a stable grid in the morphology.  Heat treatment can also be used to cleave off the side chains to obtain a stable polymer backbone.

Lastly, there's processing costs, which is lowered if the side chains enable polymer solubility in common solvents, such as chloroform or chlorobenzene.  It is also possible to use side chains to make the polymer soluble in water or ethanol.  This would benefit environmental considerations.  All these can also benefit large scale production, thereby lowering costs.



Reference:
Important Materials for the Active Layer in Organic Solar Cells - Polymers, https://www.coursera.org/learn/solar-cell/lecture/tVuEM/polymers