Degradation Mechanisms in Polymer: Fullerene Photovoltaic Devices (Degradatie mechanismen in polymeer-fullereen zonnecellen)

Organic photovoltaic (OPV) devices are one of the most promising applications of organic semiconductors as these materials are compatible with flexible plastic substrates. This enables development of light-weight, inexpensive and decorative products. Current state-of-the-art cells have achieved power conversion efficiencies of over 10 \%, which is commonly considered as one of the critical stepping stones for market introduction. Besides the challenge of transferring these record-efficiency technologies into large-scale modules, the long-term stability of OPVs is indispensable for commercial applications. This doctoral research project focuses on the in-depth investigation of intrinsic degradation mechanisms in polymer:fullerene devices, contributing to the progress in device reliability. Reaction with oxygen, humidity, temperature as well as light induces degradation in both the bulk layers and at the interfaces of the thin organic and metallic layers employed in OPV. This leads to multiple concurrent degradation mechanisms. Therefore, discriminating between the parallel mechanisms is one of the biggest challenges of this research. Complementary device degradation studies in controlled conditions enable us to delineate the main degradation mechanisms in the prototypical polythiophene:fullerene devices with a conventional architecture. These studies, moreover, elucidate how novel materials and device structures can enhance stability. Our investigation in shelf-life conditions proved that the main failure mechanism is the oxidation of the low workfunction metal cathode, which is extensively accelerated by the commonly used hygroscopic polymeric transport layer. Substitution of this hole transport layer by a metal oxide considerably enhances the device stability, even at high humidity levels. The independent impact of solar illumination on the devices activates several degradation mechanisms at different time scales. In the initial stages, the so-called burn-in process is attributed to electrical degradation of the photo-active blend. In contrast the second stage of the failure is induced by the degradation of the metal electrode active layer interface. Another critical stress factor for organic solar cells is elevated temperature that triggers large-scale reorganization of the blend morphology. This process is studied over a broad temperature range to develop the foundation for accelerated testing. Incorporation of novel bis-fullerene derivatives is proposed as a means to suppress the rapid physical degradation within the active layer. The correlation of the fullerenes' material properties with their initial morphology and thermal stability allows us to derive guidelines for the development of stable materials. The research is concluded with a brief investigation of the electrical and mechanical stability of devices with inverted architecture. This architecture has the potential to considerably elongate the device lifetime of OPV and thereby paves the way towards the first commercial product.status: publishe