An Automated Workflow to Discover the Structure–Stability Relations for Radiation Hard Molecular Semiconductors

Emerging photovoltaics for outer space applications are
one of
the many examples where radiation hard molecular semiconductors are
essential. However, due to a lack of general design principles, their
resilience against extra-terrestrial high-energy radiation can currently
not be predicted. In this work, the discovery of radiation hard materials
is accelerated by combining the strengths of high-throughput, lab
automation and machine learning. This way, a large material library
of more than 130 organic hole transport materials is automatically
processed, degraded, and measured. The materials are degraded under
ultraviolet-C (UVC) light in a nitrogen atmosphere, serving as the
conditions for electromagnetic radiation hardness tests. A value closely
related to the differential quantum yield for photodegradation is
extracted from the evolution of the UV–visible (UV–vis)
spectra over time and used as a stability target. Following this procedure,
a stability ranking spanning over 3 orders of magnitude was obtained.
Combining Gaussian Process Regression based on predictors from structural
fingerprints and manual filtering of the materials by features, structure–stability
relations for UVC stable materials could be found: Fused aromatic
ring clusters are beneficial, whereas thiophene, methoxy and vinylene
groups are detrimental. Comparing the UV–vis spectra of the
degraded material in film and solution, bond cleavage could be made
out as the leading degradation mechanism. Even though UVC light can
in principle break most organic bonds, the stable materials are able
to distribute and dissipate the energy well enough so that the chemical
structures remain stable. The established predictive model quantifies
the effect of specific molecular features on UVC stability, allowing
chemists to consider UVC stability in their molecular design strategy.
In the future, a larger data set will allow to inversely design molecular
semiconductors which show high performance and radiation hardness
at the same time.