Phenothiazine‐Modified PTAA Hole Transporting Materials for Flexible Perovskite Solar Cells: A Trade‐Off Between Performance and Sustainability

Hole Transport Materials (HTMs) are one of the key elements in Perovskite Solar Cells (PSCs) and specifically polymeric HTMs have recently emerged as one of the most viable options to couple excellent performance and good stability. However, most are processed only in aromatic solvents (e.g., toluene or chlorobenzene), thus negatively impacting the overall sustainability of the device. In this contribution, four novel polymers are synthesized specifically designed to be processable in less harsh, non-aromatic, and non-chlorinated solvent (i.e., Tetrahydrofuran – THF): the conventional PTAA scaffold is modified by the insertion of a phenothiazine (PTZ) and by the modulation of the methyl moieties on the peripheral benzene. Alternatively, a benzothiadiazole moiety is also added. The polymers are then implemented in flexible PSCs (F-PSCs) that have recently attracted increased attention due to their high power-to-weight ratio. The THF-processed P1 (a PTZ-PTAA copolymer with one methyl group substituted) reaches an overall efficiency of 9.10%, outperforming THF-processed PTAA (PCE = 8.25%) and approaching the one of toluene-processed reference (PCE = 9.30%). Furthermore, P1 shows better stability under light soaking conditions. To rationalize these results, different characterizations are presented, including optoelectronic techniques, thermal and surface analyses, and GWAXS measurements.