Mechanochemistry for the Sustainable Synthesis of Organic Hole Transport Materials in Perovskite Solar Cells

Mechanochemical coupling reactions are typically single-site events that are thermally driven, require an inert atmosphere, and are kinetically slow under ball milling conditions. Here, we demonstrate the rapid 4-fold single-pot mechanochemical C-N coupling of tetrabromopyrene and phenothiazine leading to a novel pyrene-phenothiazine (PYR-PTZ) molecule that is shown to be an effective hole-transport material (HTM) in a perovskite solar cell (PSC). When compared to previously reported mechanochemical C-N coupling reactions, the mechanosynthesis of PYR-PTZ is achieved in just 99 min of ball-milling under ambient conditions without a glovebox or the need for external heating. This represents an advance over previous methods for the synthesis of HTMs and opens new avenues for exploring the discovery of other organic HTMs for PSC applications. The photophysics, crystal structure, and electron transport properties of the novel HTM have been characterized using a combination of experimental and density functional theory methods. In an encapsulated PSC, the photoconversion efficiency of PYR-PTZ is comparable to that of the widely used spiro-MeOTAD molecule, but the stability of PYR-PTZ is superior in a naked PSC after 4 weeks. This work demonstrates the value of mechanochemistry in the sustainable synthesis of new organic HTMs at significantly reduced costs, opening up new opportunities for mechanochemistry in optoelectronics.