Performance and Long-Term Stability of Industrially Manufactured Non-Fullerene Based Organic Photovoltaics (OPV)

In the past years, organic photovoltaics (OPV) has experienced noticeable progress in material developments, device optimization, and industrial implementation towards commercialization. From lightweight, flexibility and the ability of solution-processing and large-area production at low-cost, organic solar cells have attracted increasing interests from academic and industrial entities. Initially, conjugated polymers were used as donor and fullerene as acceptor materials. However, the limitations in terms of synthetic flexibility, weak intrinsic absorption capability in the visible and near infrared region and poor coordination of energy levels restrained their further application. The evolution of non-fullerene electron acceptors (NFA) has mainly overcome these drawbacks. Hence, NFAs have been identified as promising materials for future OPV applications, and maximum power conversion efficiencies (PCEs) of over 19% have been reported already. However, such high PCE numbers have so far only been achieved for small-area devices (few mm²), usually fabricated using non-scalable materials (evaporated MoO₃), processing techniques (e. g. spin-coating) and conditions (inert environment), which together with the use of rigid substrates is far away from meeting industrial requirements.<br/>This work focuses on large-scale solution-based manufacturing of NFA devices by means of roll-to-roll processing, taking into consideration industrial standards. Using a commercially available NFA-based material system and optimizing the device architecture, the active layer morphology and the coating process, PCEs of over 10% on cell level and close to 7% on module level were achieved so far, with potential for further improvements. In parallel, the stability of the fabricated devices was carefully investigated and optimized as well with a focus on thermally and photo-induced changes. The NFA-based devices feature very good photostability, with more than 80% of the initial PCE maintained after 2000 h of continuous illumination and under open-circuit conditions. Based on previous experience, even better results are expected at operating conditions, i.e. when holding the devices at the maximum power point. Thermal stability limiting factors, were studied by means of GIWAXS as well as UV-Vis-NIR, PL and impedance spectroscopy and steps were taken to stabilize the device performance under high continuous heat. The results presented in this work confirm the potential of NFAs for the next generation of high performing OPV solar cells and show promise for their commercialization.