Recent Advances in Green-Solvent-Processable Organic Photovoltaics

Over the last four years, tremendous progress has occurred in the field of organic photovoltaics (OPVs) and the champion power conversion efficiency (PCE) under AM1.5G conditions, as certified by the National Renewable Energy Laboratory (NREL), is currently 18.2%. However, these champion state-of-the-art devices were fabricated at lab-scale using highly toxic halogenated solvents which are harmful to human health and to the environment. The transition of OPVs from the lab to large-scale production and commercialization requires the transition from halogenated-solvent-processing to green-solvent-processing without compromising the device’s performance. This review focuses on the most recent research efforts, performed since the year 2018 onwards, in the development of green-solvent-processable OPVs and discusses the three main strategies that are being pursued to achieve the proposed goal, namely, (i) molecular engineering of novel donors and acceptors, (ii) solvent selection, and (iii) nanoparticle ink technology.

Diversity of N-triphenylacetyl-L-tyrosine solvates with halogenated solvents

The structure of N-triphenylacetyl-L-tyrosine (C29H25NO4, L-TrCOTyr) is characterized by the presence of both donors and acceptors of classical hydrogen bonds. At the same time, the molecule contains a sterically demanding and hydrophobic trityl group capable of participating in π-electron interactions. Due to its large volume, the trityl group may favour the formation of structural voids in the crystals, which can be filled with guest molecules. In this article, we present the crystal structures of a series of N-triphenylacetyl-L-tyrosine solvates with chloroform, namely, L-TrCOTyr·CHCl3 (I) and L-TrCOTyr·1.5CHCl3 (III), and dichloromethane, namely, L-TrCOTyr·CH2Cl2 (II) and L-TrCOTyr·0.1CH2Cl2 (IV). To complement the topic, we also decided to use the racemic amide N-triphenylacetyl-DL-tyrosine (rac-TrCOTyr) and recrystallized it from a mixture of chloroform and dichloromethane. As a result, rac-TrCOTyr·1.5CHCl3 (V) was obtained. In the crystal structures, the amide molecules interact with each other via O—H...O hydrogen bonds. Noticeably, the amide N—H group does not participate in the formation of intermolecular hydrogen bonds. Channels are formed between the TrCOTyr molecules and these are filled with solvent molecules. Additionally, in the crystals of III and V, there are structural voids that are occupied by chloroform molecules. Structure analysis has shown that solvates I and II are isostructural. Upon loss of solvent, the solvates transform into the solvent-free form of TrCOTyr, as confirmed by thermogravimetric analysis, differential scanning calorimetry and powder X-ray diffraction.

Binding Between Cyclohexanohemicucurbit[n]urils and Polar Organic Guests

Inherently chiral, barrel-shaped, macrocyclic hosts such as cyclohexanohemicucurbit[n]urils (cycHC[n]) bind zinc porphyrins and trifluoroacetic acid externally in halogenated solvents. In the current study, we tested a set of eighteen organic guests with various functional groups and polarity, namely, thiophenols, phenols, and carboxylic and sulfonic acids, to identify a preference toward hydrogen bond–donating molecules for homologous cycHC[6] and cycHC[8]. Guests were characterized by Hirshfeld partial charges on acidic hydrogens and their binding by 1H and 19F NMR titrations. Evaluation of association constants revealed the complexity of the system and indirectly proved an external binding with stoichiometry over 2:1 for both homologs. It was found that overall binding strength is influenced by the stoichiometry of the formed complexes, the partial atomic charge on the hydrogen atom of the hydrogen bond donor, and the bulkiness of the guest. Additionally, a study on the formation of complexes with halogen anions (Cl− and Br−) in methanol and chloroform, analyzed by 1H NMR, did not confirm complexation. The current study widens the scope of potential applications for host molecules by demonstrating the formation of hydrogen-bonded complexes with multisite hydrogen bond acceptors such as cycHC[6] and cycHC[8].