There has been increasing interest in rational, computationally driven design methods for materials, including organic photovoltaics (OPVs). Our approach focuses on a screening "pipeline", using a genetic algorithm for first stage screening and multiple filtering stages for further refinement. An important step forward is to expand our diversity of candidate compounds, including both synthetic and property-based measures of diversity. For example, top monomer pairs from our screening are all donor-donor (D-D) combinations, in contrast with the typical donor-acceptor (D-A) motif used in organic photovoltaics. We also find a strong "sequence effect", in which the average HOMO-LUMO gap of tetramers changes by ∼0.2 eV as a function of monomer sequence (e.g., ABBA versus BAAB); this has rarely been explored in conjugated polymers. Beyond such optoelectronic optimization, we discuss other properties needed for high-efficiency organic solar cells, and applications of screening methods to other areas, including non-fullerene n-type materials, tandem cells, and improving charge and exciton transport.

No abstract

The preparation of phosphazenes that possess reversible cross-linking groups to control mechanical stability and hydrolysis has been accomplished using cysteine and methionine amino acid side groups. Small molecule models and linear polymeric phosphazenes that contain methionine ethyl ester and cysteine ethyl disulfide ethyl ester side groups were synthesized. Protection of the free thiol groups was carried out to circumvent unwanted cross-linking of the phosphazenes through the cysteine ethyl ester N- and S-termini. Cyclic trimeric cysteine ethyl disulfide ethyl ester model compounds were deprotected by S−S bond cleavage using β-mercaptoethanol, dithiothreitol (DTT), and zinc in aqueous hydrochloric acid. For the high polymeric derivatives, the extent of S−S bond cleavage varied depending on the deprotection method used. With the exception of the Zn/HCl method, the resultant deprotected polymers were soluble in common organic solvents and underwent minimal chain cleavage during the reaction sequence. The protected or deprotected high polymers are candidates for reversible cross-linking in drug delivery systems and for cross-link stabilization of tissue engineering scaffolds.