This section introduces the concepts of stability and degradation of solar cells which are vital for a technology that is exposed to light, oxygen and humidity. This is also one of the areas where huge improvements in performance have been achieved. A distinction will be made between recording the symptoms lifetime under various condition, investigating the causes of these symptoms, and on covering the various degradation mechanisms through scientific research. The stability of polymer solar cells is a very important topic. This is due to the fact that many organic materials exposed to sunlight and oxygen decompose. Including those used in this type of solar cells. Earlier versions of the technology degraded very fast, but fortunately, the lifetime has been increased greatly. It is also important if we ever want to manufacture solar cells commercially, then they must be able to last for a very long time, comparable to other photovoltaic technologies. As you can see in the graph, there has been an exponential growth in the number of scientific articles on polymer solar cells, but only a small fraction of these relates to degradation and stability issues. Also shown is a timeline of this scientific field with a few milestones, such as a 10,000 hour indoor and a one-year outdoor study culminating in a series of conferences dedicated to these stability studies. As we have seen in the previous lessons, polymer power cells are built up of a number of layers composed of different materials. Each of these may degrade in different ways and affect the stability. A number of important degradation mechanisms have been listed with physical or chemical nature that will require dedicated methods to analyze. In order to produce a stable solar cell, it is important to understand these mechanisms and how they interact. Some require changes in the choice of materials, but others can be solved by protecting the cell by encapsulating it to reduce influx of oxygen and water. Polymer solar cells can be printed or coated onto plastic substrates and are therefore flexible. This can be an advantage in many situations but it also means that they must be mechanically stable. They should be able to withstand bending both during manufacture and also in deployment without delamination and internal fracture. The multilayer structure requires a different spatial order with different materials for efficient carrier transport. Defects and internal migration of some complements may upset this order and render the device ineffective. As you saw earlier, the active layer requires an intricate face separated structure called a heterojunction to function properly. Unfortunately the optimum structure may not be the most stable and it can therefore change to a less efficient one over time. Much research has been set on how to stabilize the optimum phase separation, for instance, by freezing it out. This has been achieved by immobilizing the donor and accepted domains in various ways. Most organic material are susceptible to chemical degradation through reaction with oxygen and humidity from the atmosphere. When we add ultra violet light from the sun this is accelerated. Fortunately, the reactions rates for activation of the organic polymers used differ greatly. And so it's plausible to select materials there are less prone to this type of degradation. The great challenge is to find polymers that both have the desired electrooptical properties, and the stability. On each side of the active layer, selective barrier layers direct the carriers so that the negative and positive charges, end up at their respective electrodes. Zinc Oxide is selective for the transport of electrons, but it may also react with oxygen and ultra violet light, changing its properties. PEDOT is a polymer used for positive carriers, and its ionic nature makes it hydrophilic, and it can therefore act as a reservoir for water that can later move around in the device and react with other components. The metals used for electrodes can also react with oxygen and water. One of these is aluminum, which is actually very reactive, and therefore alternative and more stable devices use a silver. Total immunity towards oxygen and water is not possible. So one of the most successful strategies is to encapsulation the cells with transparent materials that block the transport into the cell. Glass is ideal in many respects. But it is not flexible and it is difficult to incorporate in a continuous roll to roll production. Special, clear, plastic films with barrier properties have been developed for this purpose. As you have seen, it is a major challenge to develop stable polymer solar cells. Both chemical and physical degradation mekanisms, conspire to degrade them. We have chemical degradation, photo oxidation of the active layer. Metal electrode oxidation. Reversible oxidation. And photo reduction of zinc oxide. And PEDOT water degradation. In physical degradation we have morphology changes, in the active layer. Delamination of interfaces. And coating and printing defects. The importance of increasing the stability have prompted a greater number of scientists to focus on this area. And an international forum called ISOS has been created to host conferences and oversee this research area. In one of the next lessons we'll focus on this. Generally we distinguish between two ways of investigating the stability of solar cells. The first is focused on testing the stability of a specific solar cell by putting under conditions that may or may not resemble its daily use. The second way is to design experiments directly, to uncover the degradation mechanisms involved. These methods may involve lifetime testing with or without light, or light at different intensities, variable temperature, or humidity studies. And the use of a controlled atmosphere with or without oxygen. During these tests, the performance of the solar cell is monitored, and afterwards destructive tests can be carried out to evaluate chemical and physical changes that have taken place during the different degradation mechanisms.
