Synthesizing Polymers

Here's a summary of the design criteria for organic/polymer solar cells: a good absorption of sunlight using low band gap polymers that provides high current and efficiency, a good interaction between donor and acceptor units in the polymer that provides high mobility and efficient charge transfer, a good match between the energy levels of the donor and acceptor that provides efficient charge transfer and a higher Voc, a stable polymer backbone with side chains that interact to ensure cell morphology, thermal and photochemical stability and processability by solution, and finally, a good scalability of polymer synthesis.

There are 3 more important polymerisation methods for polymer synthesis of low band gap polymers: the Stille cross coupling polymerisation, the Suzuki cross coupling polymerisation, and the direct arylation polymerisation (DARP).  These 3 are palladium catalysed polymerisations.

The Stille cross coupling polymerisation (see diagram above) involves a halogen activated monomer which is usually the acceptor, coupled to an organo-tin activated monomer which is usually the donor.  The halogen is usually bromine.

The Suzuki cross coupling polymerisation (see diagram above) also involves a halogen (usually bromine) activated acceptor monomer, but its donor is a boronic ester activated monomer.  The boronic ester can vary in sizes in the ester chain, and can be cyclic too.

The Stille and Suzuki cross coupling polymerisations are most common.  The DARP method (see diagram above) is a recent development.  This method couples the halogen activated monomer with a non-activated monomer, just by using hydrogen.  This method is useful, because there is one less step in the synthesis of the polymer, and is more environmentally friendly, because the toxic tin activation group is not used.  However, it gives lower efficiencies, for example, for P3HT preparation.

Polymerisation requires activation of monomers.  The acceptor unit can be treated with NBS and THF (see diagrams above).  This reaction is simple and occurs usually in high yield.

For the donor unit activated with boronic ester (see diagram above), the boronic ester cannot react directly on the donor units, so a halogen activating group is required.  So the monomer is first treated with NBS, and then reacted with nBuLi and the boronic ester to create the diboronic ester monomer unit.  For stannylation (activation by organo-tin), the monomer is treated with nBuLi and reacted with trimethyl tin chloride.

To achieve scalable synthesis, cheap starting materials are required.  There should also be fewer steps in production with high yield.  To purify the monomers, flash chromatography, solid state chromatography, or recrystallisation can be used.  After purification, the monomers are mixed in dry toluene, degassed, and the catalyst is added (see diagram above).  These reactions happen quite quickly, with the colours of the polymer solution changing from reddish to purple and bluish as the length of the polymers increase.

When the polymer is ready, Soxhlet extraction is used to purify it.  Different solvents are used to ensure a separation of the polymer with its different molecular weights.  First, methanol is used to remove the catalyst and other low molecular weight molecules.  Then hexane is used to remove the oligomers.  Lastly, chloroform is used to extract the polymer.

After polymerisation, characterisation is done, where the polymer is analysed in depth.



Reference:
Synthesizing Polymers, https://www.coursera.org/learn/solar-cell/lecture/U9Ht2/synthesis