The past two weeks, you have learned about solar cells, solar energy, and the three types of solar cells which exist. You have also learned why we are focusing on organic photovoltaics or polymer solar cells. You have learned how these functions and what kind of layers are in a cell. This week, we will focus on the active layer, which materials can be used in this area or in this layer, and you will be able to discuss or argue why we need it. We will focus on low band gap polymers and what kind of acceptors we can use and the synthesis of both. When we're looking at the design of the polymers for polymer solar cells, we look at these three main areas: efficiency, process, and stability. If we look at the areas separated, for the stability, it has been documented with a lifetime up to one year outdoor, but there's still a lot to be done in this field for it to be comparable to other types of PV technologies. Efficiencies, we have the 3-4% of P3HT/PCBM mixture. However, we can have it up to 10% for new band gap polymers, and this polymer types we will look more into in the next lesson. Then we have the processing, and of course, we need to go very large scale and this include coating and printing. So, when we want to look at the design of the polymer, we want to have a polymer which fulfills all these criterias. So it has to be a polymer which give high efficiency. It has to be processed by solution in large scale, so it also needs to be prepared in large quantities with cheap starting materials. And of course, it has to implement a high stability of the finished solar cell. The active layer consists of two types of material: a polymer or a small molecule which acts as a donor and then an acceptor material. The polymer or small molecule is a conjugated material which absorb the light, and an electron is then excited from the HOMO to the LUMO and the electron is then transferred to the acceptor, and we have our charge separation. These two types of materials, the donor material and the acceptor material, is what we're gonna look at this week. The acceptors in the OPV, as I said, accept the electron from the donor material. They are normally fullerene. In the beginning, it was the C60, however, this is not very soluble in common organic solids and therefore it was evaporated onto the active layer, and this was the active layer was then a bilayer and not a heterojunction. Development of a soluble C60 derivate, the PCBM, made it possible to prepare a heterojunction where the donor and the acceptor is mixed and therefore the charge transfer is much more efficient. The synthesis of the fullerene, soluble fullerene is a four-step synthesis. It's very easy and cheap materials which are used in the synthesis. The derivate of the PCBM, which can also be prepared with a C70 fullerene, it's a bit more expensive, but we can tune the band gaps or the LUMO levels of the acceptor, and this way we can get a more efficient polymer solar cell. Like with the bis adduct ICBA where the LUMO level is 0.17 electron volts higher than the LUMO level of PCBM. Besides the small molecules and the fullerene derivates, we can also use polymers as acceptors in our OPVs. This has an advantage that you get the absorption from two polymers, and the energy levels of a polymer is easier to tune, so you can get a much easier leveling of the HOMO and LUMO levels of the two polymers. The solution viscosity is also easier to control since the polymers can be prepared to be much more soluble than the PCBM. However, these polymer OPVs are still under development because the efficiency is much lower for these, and this is mainly because the miscibility of the two polymer is much lower than with the PCBM, where it's easy to prepare a interpenetrating layer. This shows us that the morphology of the polymer is much, of the active layer is very important. So what is a polymer? It's a monomer combined together, like thiophene combined in length of chain, so you get a polymer, a polythiophene. For the polymers we need in OPVs and polymer solar cells, we have a polymer backbone exemplified – that's the thiophene – and then we have the side chain to make the polymer soluble, and this is necessary for us to do, prepare the polymer solar cell by coating and printing techniques. The side chains can be either alkyl chains or it can be an ester so you can cleave it off and make a more stable polymer. The most important thing that the polymer needs to fulfill is that it has to be conjugated. Here you see a simple polymer, a polyalkane, which is not conjugated, and below you see a polyacetylene, which is conjugated – it's the switch between single bonds and double bonds. In these double bonds, we have the pi electrons. These pi electrons can diffuse over the entire molecule, and this is what we're interested in because these pi electron are the ones that can be shifted upon absorption of the visible light. They are shifted from a HOMO level up to the LUMO level, and this is called a pi to pi* transfer. The absorption also depends on conjugation length. So, if you have a break in the conjugation length, the absorption will blueshift since the pi orbitals cannot overlap. The absorption, as you can see here, is different from polymer to polymer,and it changes from orange to red and purple, blue and green, and it all represents different absorption spectrums of the different polymers. The absorption depends on polymer back bone, how long is the chain – a long chain can have a higher absorption than compared to a short chain – and if it's a absorption spectrum of a polymer film, it also depends on how the molecules order. For instance, for P3HT, it will order in a lamella structure and this shifts the absorption spectrum when you compare it to a solution. So as I mentioned, the most studied polymer is P3HT together with PCBM in solar cells. It has a reported efficiency of 3-4%, it's relatively stable, and it's processable in large scale by solution. However, the absorption of the polymer blend is very low, it's not that comparable to the solar spectrum, and that's why we want to look more into other polymers.
