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

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

Materials in the Active Layer in Organic Solar Cells

When a polymer is designed for the active layer, there are 3 factors to consider.  It has to be highly stable, it has to provide high efficiency, and it should be produced cheaply on a large scale (by processing in solution with cheap starting materials).  A P3HT PCBM mixture provides only 3-4% efficiency, but it's easy to process, so it's the choice for this description.  The active layer is made up of a donor polymer, or a donor small molecule, and an acceptor material.

Acceptor materials accept electrons from the donor materials (which are transported to the Cathode) and are usually fullerenes (see diagram above).  The use of fullerenes started with the C60 (see diagram below), which was not very soluble in common organic solvents, so it was evaporated onto the active layer to form bilayers, not a BHJ.  Hence, the PCBM was developed to enable BHJ to be formed with the donor material.

The C70 fullerene can be used to prepare the derivative of PCBM.  The derivative is more expensive, but its band gap (acceptor LUMO level) can be tuned.  Another example is the Bis-adduct (ICBA), which has a higher LUMO level (see diagrams below).

Polymers can also be used as acceptor materials.  This enables absorption in both donor and acceptor materials.  The energy level of polymers is easier to tuned, and there is easier levelling of the HOMO and LUMO levels of both polymers.  Both polymers are also formed with more solubility than PCBM, so there is easier control of solution viscosity.  However, these polymer blends lead to lower efficiencies because the mixability of polymers is poor.

The donor material is the polymer P3HT.  A polymer is formed from a combination of monomers (see diagram above), which are repeated units.  For example, thiophene is combined to form polythiophene.  Side chains (indicated by R in the diagram above) are also included to make the polymer soluble (see diagram below for the side chain of P3HT).  This is required for polymer solar cell production by printing and coating.  The side chains can be either alkyl or ester chains, which can be cleaved off to create more stable polymers.

The most important property of conducting polymers is that they should be conjugating (see diagram below for the difference between an unconjugated polyalkane and a conjugated polyacetylene).  Conjugation refers to the alternating of single and double bonds along a chain of carbon atoms.  The Pi electrons of the double bonds can diffuse (be delocalised) over the entire polymer molecule.  Upon the absorption of light, these Pi electrons are the ones that are excited.

Upon excitation, when electrons are shifted from the HOMO to the LUMO level, a pi to pi* transfer has occurred.  The conjugation length also affects the absorption.  If the conjugation length is shorter, the absorption will blue shift because there is less overlap of pi orbitals.  Hence, the different conjugation lengths will determine the visible polymer colours, which are the complementary of the wavelengths absorbed.  A longer polymer backbone/chain will also have higher absorption.

Lastly, the absorption spectra of a polymer film can be different from that of being in solution, because of the ordering of the molecules.  In the P3HT example, it is ordered in a lamella structure, which results in an absorption spectra different from the spectra in solution.



Reference:
Materials in the Active Layer, https://www.coursera.org/learn/solar-cell/lecture/omqzo/materials-in-the-active-layer

Layers in the Organic/Polymer Solar Cell

There are many types of organic/polymer solar cells, but basically, there are 2 main types: normal geometry devices, and inverted geometry devices.  For normal geometry devices, holes are extracted from the front electrode (anode) where light enters, while electrons are extracted from the back electrode (see diagram above).  This extraction is reversed for inverted geometry devices.  The naming convention is because the inverted geometry was invented after the normal geometry.  The inverted geometry allows the use of other electrode materials.

The diagram above names the layout of a polymer solar cell in sequence from top to bottom.  The main materials for substrates are glass (stiff material) and plastic (flexible material).  Glass is often used in research due to its suitability with techniques such as spin coating and evaporation.  It is very heat stable, and a can be a very good barrier to oxygen and moisture.  However, the advantage of polymer solar cells is that they can be mass produced cheaply by simple printing and coating technologies.  Hence, plastic foil, mainly based on PET (polyethylene terephthalate), is also used in research.  Its barrier properties also had much improvements, resulting in very stable solar cells.

Due to the requirement of the front electrode to be transparent, ITO (indium doped tin oxide) is often used with a good hole conductor PEDOT:PSS.  However, indium is a scarce material and ITO is very brittle, so alternatives such as FTO are being researched into.  An alternative technique to ITO deposition is to use a very thin metal grid.  This has been described in High-Efficiency Concepts of c-Si Wafer Based Solar Cells.

Intermediate layers - HTL (hole transport layer) and ETL (electron transport layer) - are required to facilitate charge carriers transport to the electrodes.  HTL such as PEDOT:PSS, MoOx (molybdenum oxide) and V2O5 (vanadium(V) oxide), and ETL such as TiO2, ZnO, LiF (lithium fluoride) and CeCO3 (caesium carbonate), have been used.

The active layer, a BHJ (Bulk Heterojunction), is created quite randomly by mixing 2 components together in a solvent.  The components have a tendency to microphase separate once the solvent evaporates.  Much research is done into the generation and maintenance of better BHJ structures, such as by using heat treatment or exposure to solvent vapours to help phase separation.

Finally, there is the difficulty of creating a tandem solar cell, where efficiency is possibly doubled due to the addition of voltages from both individual solar cells.  The difficulty arises from the stacking of many layers of materials (see diagram above), the selection of polymer materials with approximately the same current production and with different absorption spectra, and the development of an intermediate layer that can form an ohmic contact to both individual cells to transport charge carriers between them.


Reference:
Layers in the Solar Cell, https://www.coursera.org/learn/solar-cell/lecture/smOjn/layers-in-the-solar-cell