Encoding scalar device fabrication features such as energy levels, annealing temperature or the D:A ratio is straightforward. However, encoding categorical features is more challenging. In the case of OPV device fabrication, these are (i) solvent, (ii) solvent additive (if used), (iii) the hole transport layer, HTL, and (iv) the electron transport layer, ETL. Similarly to the donor and acceptor materials, these categorical features could be encoded using one-hot encoding or as labels. However, as seen with OHE of the active layer materials, these encodings are not informative for a ML model. The HTL and ETL were encoded with their energy levels, as determined from literature reports (Table ). The solvent and solvent additive were encoded with physicochemical descriptors obtained from the HSPiP database (Table ). The features beyond molecular structure were split into four broad categories: material properties (as in Figure ), film fabrication parameters (fabrication), device architecture, and electrical characterization. As outlined in Figure First, the identity and physical properties of solvents and solvent additives are known to strongly influence the performance of OPV devices through a number of factors (e.g., material solubility and rate of evaporation). Second, it is possible that there is also a degree of spurious correlation between solvent or solvent additive and PCE because higher-performing materials are more often dissolved in particular solvents.