Organic Bulk Heterojunction Solar Cells: Materials Properties Device Stability And Performance

In the field of Photovoltaic technologies the organic solar cells are particularly attractive because of their ease of processing, mechanical flexibility and potential low cost production techniques. So far, the reported efficiencies are not high enough to allow to be competitive in the market, however with the introduction of new photoactive materials, device architectures and light management structures, the power conversion efficiencies, at laboratory scale, has rapidly reached the 12%, showing a great potential and a bright future for organic solar cells. Nevertheless, in view of commercial products, two main problems are still unresolved: the relatively low performance of the modules and their lifetime. In sight of this, the present Ph.D thesis has a double goal: 1) a better understanding of the relationship between devices performance and photoactive materials structures 2) a deep investigation on device degradation processes, with particular attention on the effects induced by temperature and incident light. As a result, promising approaches to further optimize the polymer’s optoelectrial properties, and thus the corresponding device performance, are proposed. About the lifetime, first the thermal degradation mechanisms involved on the active layer was investigated and it has been demonstrated the role of the other layers and interfaces in the solar cell thermal stability. In this contest an innovative fast capacitance based thermal test has been developed in order to obtain information regarding the limit operating temperature above which the device becomes thermally unstable. In the end, a preliminary study on the photostability issue was carried out demostrating that the photodegradation of organic solar cells not depends just on the photostability of the donor polymer, but is connected also with the composition of the active layer solution and on the interaction with the adjacent layers. Solutions to limit or prevent the devices degradation processes are proposed.