In this lecture I will try and explain the working principles of a polymer solar cell. There's a cascade of events that starts with the absorption of photons in the active layer, this creates an exciton bound electron hole pair, which can then diffuse through a phase boundary between the polymer donor and an acceptor. Here the charge separation happens. The charges travel in the two materials to the electrodes. First, I will give you a brief summary of the build-up of the typical solar cell. You have the electrodes, the outer electrodes, one is transparent. You have an electron transport layer, a hole transport layer, and the active material. The active material has two constituents. The polymer which absorbs the light and has a great affinity for the holes and an acceptor material which has a large affinity for electrons. The active material in a solar cell is a semi-conductor. It is halfway between a normal conductor like a metal, which has surfaces inside a conduction band, and a insulator - where there is a big gap between the filled states and the empty states. So with the semiconductor you have a fairly close proximity between the filled band and an empty band - and this way the photon can excite an electron in the filled band to the empty band. Silicon used in normal solar cells is an indirect band gap material. This means that in order to excite an electron from the filled state to the excited states, you also need - lattice vibrations to help. This is also called a phonon. This makes the transition less likely and therefore you need a thicker slab of material. So typically, silicon solar cells are, rather thick and consume a lot of material. Therefore, we would like to use direct band gap materials, which can be much thinner. Thin film inorganic solar cells and, in our case, organic materials fulfill this criteria, so we can make much thinner solar cells. The solar spectrum covers a broad range from ultra violet to the visible range and finely to infrared. And to utilize all this light we could use or think of using semi conductors with a low band gap. The low band gap also limits the voltage output of the solar cell. So the efficiency which is the product of the voltage and current has a limit, a theoretical limit, Which is also called the Shockley-Queisser Limit. The silicon solar cells have a band gap of about 1.3 eV. This means that the maximum power conversion efficiency is about 33%. Most polymer materials, however, have a higher band gap, and this means that they absorb less of the sunlight, and the power conversion efficiency is even lower. You can, in theory, use multiple junctions, and thereby increase the efficiency. If you have an infinite number of, junctions you can, in theory, reach 86% efficiency. When the photon, gets absorbed in the active material it forms an excited state. We will just call it an exciton which is an electron hole pair that is bound in a localized environment. This, exciton can diffuse through the material but the lifetime is limited. So the distance it can travel before it collapses is also very small. Some 10 to 20 nm in general. This means that the optimal size structure in the active material must also be of that order. And to do this we make what is called a bulk hetero junction or a microphase separated material where the donor, the polymer and the acceptor is ordered in several domains where the holes and the electrons can travel. Older types of polymer solar cells was built up in a bi-layer geometry. So you had one layer of the polymer donor and one layer of the acceptor. And this also meant that the exciton had to travel a rather great distance. This is not very effective. The exciton travels to a face boundary between the donor material at the acceptor material which can be a fullerene. Here the charges can get separated and the electron can travel in the acceptor material and the hole can travel in the polymer material and they go through these two materials through to the electrodes. In order to generate electricity, the charge carriers must travel to seperate electrodes rather than moving randomly about. This is achieved by setting up an internal field by using different electrode materials with different energy levels. And also by using intermediate layers that preferentially transport either the electrons or holes thus direct the carrier transport in the right directions. In the remaining part of this week, you learn how to design organic solar cells and measure their properties.
