Recently, flexible Perovskite Solar Cells (f-PSC) have been a growing niche within the research on PSC with the aim of broadening its range of applications toward light-weight Building Integrated Photovoltaics (BIPV), Internet of Things (IoT), portable electronics and even space applications. [1] This is mainly related to the higher power-to-weight (PWR) ratio of flexible devices compared to glass-based counterparts. [2] However, to unlock f-PSC full potential, research must focus on boosting the sustainability of the cell, besides its performance and stability. Sustainability concerns mainly deal with the use of organic Hole Transport Materials, whose synthesis has a huge impact on the overall score of the final device, especially when polymeric HTMs are considered.

Indeed, state of the art polymeric HTMs, such as poly(triarylamine) (PTAA), are still processed with toxic solvents such as chlorobenzene (CB), dichlorobenzene (DCB) or toluene, [3] which pose an obvious barrier to the upscaling of this technology. As the literature in green-solvent-deposition charge transport layers is still scarce, we present a new set of HTMs processed using Tetrahydrofuran (THF) that is a non-aromatic, non-halogenated, cheap low environmental risk solvent and has low human health toxicity. [4] We find that the choice of green solvent processable HTM’s is a defining parameter for polymers when compared to small molecules, due to their limited solubility resulting from prioritising extended 𝜋-𝜋 conjugation frameworks, [5] yielding this study even more promising to the sustainable upscaling of f-PSCs.

Throughout this work, the four newly synthetized HTMs incorporate additional scaffolds to modify the conventional triphenylamine moiety characterizing the PTAA, with the scope of improving their solubility in THF. Aiming at this, the phenothiazine moiety was opted due to its good solubility in common organic solvents, high chemical stability, high tunability, high hole mobilities and extremely low cost. [6] The methyl substitution of the TPA phenyl unit was also altered to evaluate the trade-off effect on solubility and polymeric chain packing. As an alternative to PTZ-PTAA polymers, we also considered the benzothiadiazole unit due to its electron deficient system, [7] giving rise to the ability to tune the electronic properties of the final HTM. To the best of our knowledge, the coupling of these three scaffolds in a polymeric system has never been reported.

The novel polymers P1-4 were successfully synthesized using specially optimized protocols, adhering to Green Chemistry Principles, [8] and thoroughly characterized in terms of structural, optoelectronic, and thermal properties. All the polymers have appropriate band gaps and a HOMO energy levels aligned with the valence band of the active layer, assuring fast and efficient charge extraction. Additionally, they are thermally stable well above operating standard temperatures and could withstand high annealing temperatures.

After their characterization, P1-4 were implemented as HTMs on flexible n-i-p devices using PTAA as the reference. Results reveal that P1-4 can achieve competitive efficiencies compared to PTAA when the latter is processed with toluene, and even outperform the reference processed with THF.

This work will serve as a baseline for the pursuit of increasing environmentally friendly, solution processable materials (e.g BioRenewable sources, 2-MeTHF), paving the way toward highly efficient, stable, and sustainable f-PSCs to enter the market.