Synthesis and characterization of new organic precursors of hole-transporting self-assembled monolayers for perovskite solar cells

Perovskite solar cells (PSCs) are one of the emerging technologies in the photovoltaic industry. 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. This work aimed to design, synthesize and characterize spectroscopically and electrochemically a new class of compounds capable of forming self-assembling monolayers (SAMs) to be used as organic HTMs for inverted-configuration perovskite solar cells (p-i-n PSCs). Six new SAMs with a common structure containing a dithieno[3,2-b:2',3'-d]pyrrolic (DTP) core acting as a heteroaromatic functional group, a propyl chain as a spacer and a phosphonic acid as an anchor group were synthesized and characterized. A common synthetic procedure based on Pd-catalyzed cross-coupling reactions was developed for all the molecules, enabling them to be obtained in a few steps and with good yields. All the DTP-based derivatives possess the appropriate spectroscopic and electrochemical properties to act properly as HTMs within a PSC. All the new molecules have been successfully tested for the realization of p-i-n PSC devices and the most promising results in terms of the efficiency of conversion of sunlight into electric current (18.42%) have been obtained with QM06, slightly lower than the efficiency obtained with the literature reference SAM, MeO-2PACz (20.4 %).