Organic solar cells have recently surpassed 19% power conversion efficiency (PCE) [1], becoming a promising technology for future applications. Nonetheless, the still limited lifetime prevents organic photovoltaics from commercialization. Many different factors, such as UV radiation [2] or thermal stress [3], can contribute to the degradation of the solar cell. In particular, inverted solar cells suffer from unwished interactions at the interfaces between the photoactive layer and interlayers due to the oxide-based electron and hole transport layers (ETL and HTL, respectively). It has already been reported that the ZnO ETL is responsible for photocatalytic activity under UV irradiation [4]. Additionally, a study by Ding et al. showed that the commonly used MoO₃ HTL is involved in a redox reaction with the photoactive layer in the presence of a chlorinated acceptor material. This interaction is linked to the degradation of the solar cell and brings to a redistribution of the chlorine content within the layer stack of the device [5]. A similar phenomenon was observed by Hoefler et al. in an inverted organic tandem solar cell containing a fluorinated donor. In this case, the fluorine atoms migrated from the polymeric donor towards the MoO₃ interlayer and accumulated at the recombination layer of the tandem solar cell [6].

On the one hand, halogenation of donor or acceptor molecules has proved to be very beneficial for the solar cell performance, allowing to tune the molecular energy levels and the crystallinity to obtain enhanced charge transport and better film morphology, thus, higher efficiencies [7]. On the other hand, an in-depth research about the behavior of halogenated photoactive materials in a solar cell under operating conditions is still missing.

In this work, we were interested in the role of halogen atoms on the operating state of the popular fluorine-containing PM6:Y6 blend. More specifically, we investigated the distribution of fluorine and chlorine atoms over the whole device structure after a single measurement under illumination. For this, solar cells in conventional and inverted architecture with and without the solvent additive 1-chloronaphthalene were built and characterized via current-voltage analysis, transmission electron microscopic elemental mapping of cross sections based on energy dispersive X-ray (EDX) spectroscopy and dynamic secondary ion mass spectrometry (D-SIMS).

Interestingly, a diffusion of fluorine and, for additive-containing devices, also chlorine from the donor/acceptor blend to the ZnO and the MoO₃ interlayers took place, leaving a significantly lower halogen concentration in the photoactive layer. However, conventional PM6:Y6 solar cells with PEDOT:PSS as HTL and PNDIT-F3N-Br as ETL showed a homogeneous halogen distribution in the active layer and no evident accumulation at both interfaces, suggesting that the rearrangement of fluorine in these halogen-containing absorber layers is triggered by the presence of oxide-based interlayers. This outcome is of great importance, since the observed halogen diffusion, in combination with the photocatalytic effect, might account for the lower photostability of PM6:Y6 inverted solar cells in the first hours under continuous illumination compared to their conventional counterparts.