Today, organic solar cells are alternative energy sources, which are now competitive for an introduction into a new generation photovoltaic market due to their high performance, processing on a large surface, flexibility and low price. There has been considerable research that allowed very recently to achieve certified power conversion efficiency over 17% in a tandem organic solar cells [1]. This tremendous increase for organic photovoltaics during the last two years was enabled by the development of novel non-fullerene acceptor (NFA) materials that outperform fullerene-based acceptors. However, the long-term stability of these new photovoltaic materials and the corresponding devices is a key factor to figure out the commercial viability and must be addressed in detail.

   While some works on new NFAs mentioned improved stability of NFA based solar cells under specific conditions (storage in air or under LED light soaking), the work of Brabec and coll. [2] has very recently provided a study dedicated to one of the most important new NFA family, namely the ITIC derivatives (ITIC, ITIC-4F, ITIC-M, ITIC-DM, ITIC-Th) indicating the fluorinated acceptor are most stable. However, the stability of the solar cells was only studied under LED light, while standard tests such as ISOS-D-2 High-temperature storage as well as ISOS-L-1 (Laboratory weathering under continuous illumination at AM 1.5) [3] were missing.

   Previously, we investigated the stability of solar cells based on PTB7 and fullerene derivatives depending on interfacial layers [4], [5] and thermal treatments. The aim of this work is to study in detail the stability of high efficiency organic solar cells using different ITIC derivatives by applying the three standard tests and compare the degradation processes of NFA based solar cells under illumination at AM 1.5 and LED.  Furthermore, in order to compare the NFA to fullerene acceptors, we developed a PCE10:PC₇₀BM blend system that is fully stable [6] under ISOS-L-1 conditions. This demonstrates the stability of our device structure including interfacial layers and allows to evaluate the NFA related degradation processes in the corresponding solar cells. The stabilized PCE-10:PC₇₀BM solar cells are compared to PCE-12:ITIC, PBDBT-F:ITIC-4F, Furthermore, detailed characterizations of the device degradation in relation to the morphology will be discussed using atomic force microscopy and spectral imaging analyses from analytical scanning transmission electron microscopy. Whilst PCE-10:PC₇₀BM solar cells are found to be stable with an efficiency of 7%, we show that both ITIC:PCE-12 and ITIC-4F:PBDBT-F solar cells with performances of 10% and 11%, respectively, degrade very fast under simulated AM 1.5 illumination. LED light clearly reduced device degradation. We also discuss thermal treatment techniques aiming to improve the stability of ITIC based solar cells by improving the crystallinity of the polymer blend.

Figure 1 The stability of encapsulated device under continuous illumination at 1.5 AM lamp in an air