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