New dithieno[3,2-b:2',3'-d]pyrrole-based organic precursors of hole-transporting self-assembled monolayers for highly efficient Perovskite Solar Cells

Thanks to the very high efficiencies in the sunlight-to-electricity conversion gained in the last ten years, Perovskite Solar Cells (PSCs) have attracted tremendous interest, stating as one of the leading technologies in the next-generation photovoltaic field . Within them, the hole-transporting material (HTM), capable of transporting the holes formed after photo-excitation of the perovskite to the counter-electrode, plays an essential role in improving the performance of the device. A breakthrough has been represented by the introduction of small organic molecules as HTMs capable of forming a very thin self-assembled monolayer (SAM) onto the ITO electrode and extracting the photogenerated hole from the perovskite layer. This work aimed to design, synthesize and characterize spectroscopically and electrochemically a new family of six organic precursors of SAMs based on the tricyclic structure of the dithieno[3,2-b:2',3'-d]pyrrole and bearing a phosphonic acid as the anchoring group onto the ITO electrode. Different aryl substituents in 2- and 4-positions have been introduced to modulate the photo-electrochemical properties of the final SAMs, while a common and straightforward synthetic procedure based on Pd-catalyzed cross-coupling reactions has been optimized for all the compounds. All the new molecules have been successfully tested for the realization of PSC devices and the most promising results have been obtained with QM06 and QM10, outperforming the PCE values of the standard literature reference 2-MeOPACz. Optimization of the PSC construction and full characterization of the devices is ongoing.