Development of a High Irradiance Setup for Precisely Controlled Accelerated Photo-Degradation of Organic Solar Cells

During the last years a lot of new technologies and materials have been appearing in the field of photovoltaics with plenty of interesting features enabling cheap production and a vast variety of applications. While the efficiencies of these new devices are already on a good way and show a continuous increase, most of them struggle with competitive lifetimes (standard silicon solar modules offer warranties of 20 years and more). Thus, testing materials concerning their reliability has been growing into a very important task. As desired operation lifetimes typically lie in the range of many years, product testing under normal working conditions would be a very protracted process. Therefore, accelerated lifetime testing is a feasible way of assessing material reliabilities. Accelerated degradation is reached by increasing the loads, appearing at a certain device, (far) beyond the stress levels which are reached in normal operation mode. Different ways of lifetime testing are employed to assess the resilience against various influences a device is exposed to during its service lifetime. Most of them address degradation processes caused by external influences like humidity and promote the subsequent development towards better barrier materials and techniques to protect the devices against external influences. Since photovoltaic devices are made for absorbing light the most basic and important feature of a certain device is sufficient light (and temperature) stability as this influence simply never can be suppressed. Accordingly, the current work deals with the design, establishment and startup of a laboratory for accelerated lifetime testing concerning light degradation. Accelerating the degradation process will be achieved by temperature and irradiance variations. The crucial feature of such a laboratory is the precise control of sample temperatures and irradiances independently from each other. This aim is reached by actively cooling the solar cells while being illuminated. Additionally, a new way of temperature determination of the cells under irradiation was developed which provides accurate information about sample temperature. How important this step of accurate temperature measurement is, will be presented by comparing the new way of temperature determination with common ones. This new way of solar cell temperature management enables accurate investigations of degradation kinetics under highly concentrated irradiances (high-C) and accordingly, by using the new setups, degradation kinetics of solar cells under high-C conditions could be precisely studied which was not reported so far. An Arrhenius law was found for the temperature dependence of the solar cell degradation progress under high illumination densities. This underlines the importance of precise temperature determination and control. Concerning the dependence of the degradation progress of the short circuit current on the irradiance itself at a fixed temperature, a linear relation was found in the investigated range of irradiance. In other words, light degradation (in a certain range) is just a question of the light dose or number of photons absorbed by the solar cell as long as the temperature stays constant. As a rule of thumb, the experimental time required for material testing may be reduced by the described setups by a factor of several hundred compared to standard tests and, hence, the light stability of a certain material during typical operation times can be assessed within several days. Besides conducting degradation experiments on opto-electronic devices the laboratory was additionally designed to perform aging experiments on materials and structures intended for concentrated solar power plants (CSP). Typical stress levels as they appear in such a power plant can be simulated with one of the developed ALT setups. Accordingly, ALT experiments with potential new CSP absorber structures were successfully conducted in a cooperative research project and demonstrate the capability and versatility of the new laboratory: fast lifetime testing of various materials based on illumination and temperature degradation.