Molecular semiconductors have the potential to enable new possibilities in the fields of radiation detection and space applications, but they need to prove resilience against the ionizing radiation present in these harsh environments. The lack of molecular oxygen in space requires the challenging task of performing the degradation studies at inert conditions. In this work, a strategy is presented to investigate the inert radiation hardness of molecular semiconductors using total ionizing dose (TID) tests based on gamma radiation from a cobalt-60 (Co-60) source - a traditional proxy for the space environment. For the first time, a large-scale gamma stability screening of 46 structurally diverse organic semiconductors was performed at inert conditions, deriving a stability target from the UV-visible (UV-vis) evolutions during degradation. The resulting stability ranking of the small-molecule hole transport materials (HTMs) designed for use in perovskite solar cells spans more than two orders of magnitude and shows that molecular structure - rather than atomic composition alone - governs gamma stability. On average, the ionizing dose tolerance exceeds 10 kGy, corresponding to a calculated lifetime of over two years in the Van Allen belt at ∼1000 km altitude in low Earth orbit (LEO). Derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) stand out, even showing seemingly infinite stability targets. Using the ranking, a predictive model could be trained, which implies that the number of boron atoms – or the BODIPY unit in which they are embedded – outperforms more than 1900 other structural and semi-empirical descriptors. Overall, this work lays the groundwork for future gamma stability studies of molecular semiconductors and thin-film technologies in general. With more efforts targeted at understanding the structure-stability relationships and structure-dependent degradation mechanisms, including up to complete recovery of the UV-vis spectra, this class of materials could become a competitive option for ionizing radiation detectors as well as for organic and perovskite space solar cells.
Bornschlegl et al. (Wed,) studied this question.