Comparative analysis of search approaches to discover donor molecules for organic solar cells

Identifying organic molecules with desirable properties from the extensive chemical space can be challenging, particularly when property evaluation methods are time-consuming and resource-intensive. In this study, we illustrate this challenge by exploring the chemical space of large oligomers, constructed from monomeric building blocks, for potential use in organic photovoltaics (OPV). For this purpose, we developed a python package to search the chemical space using a building block approach: <i>stk-search</i>. We use <i>stk-search</i> (GitHub link: STK_search) to compare a variety of search algorithms, including those based upon Bayesian optimisation and evolutionary approaches. Initially, we evaluated and compared the performance of different search algorithms within a precomputed search space. We then extended our investigation to the vast chemical space of molecules formed of 6 building blocks (6-mers), comprising over 10¹⁴ molecules. Notably, while some algorithms show only marginal improvements over a random search approach in a relatively small, precomputed, search space, their performance in the larger chemical space is orders of magnitude better. Specifically, Bayesianoptimisation identified a thousand times more promising molecules with the desired properties compared to random search, using the same computational resources.