![]() 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. The solution viscosity is also easier to control since the polymers can be prepared to be much more soluble than the PCBM. 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. Besides the small molecules and the fullerene derivates, we can also use polymers as acceptors in our OPVs. Like with the bis adduct ICBA where the LUMO level is 0.17 electron volts higher than the LUMO level of PCBM. 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. It's very easy and cheap materials which are used in the synthesis. The synthesis of the fullerene, soluble fullerene is a four-step synthesis. 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. 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. The acceptors in the OPV, as I said, accept the electron from the donor material. These two types of materials, the donor material and the acceptor material, is what we're gonna look at this week. 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. 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. And of course, it has to implement a high stability of the finished solar cell. It has to be processed by solution in large scale, so it also needs to be prepared in large quantities with cheap starting materials. So it has to be a polymer which give high efficiency. So, when we want to look at the design of the polymer, we want to have a polymer which fulfills all these criterias. Then we have the processing, and of course, we need to go very large scale and this include coating and printing. 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. Efficiencies, we have the 3-4% of P3HT/PCBM mixture. 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. 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. We will focus on low band gap polymers and what kind of acceptors we can use and the synthesis of both. 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. You have learned how these functions and what kind of layers are in a cell. You have also learned why we are focusing on organic photovoltaics or polymer solar cells. The past two weeks, you have learned about solar cells, solar energy, and the three types of solar cells which exist. ![]()
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