Perovskite-based photovoltaics (PV) are increasingly recognized as a pivotal technology in the pursuit of climate neutrality by 2050. Their customizable optoelectronic characteristics, combined with straightforward, low-cost manufacturing methods, make them ideal candidates for a wide array of applications. These include tandem solar cells, space applications that leverage their favourable power-to-weight ratio, and even harvesting ambient indoor light. [1] While significant attention is directed toward optimizing their efficiency, the critical issue of environmental sustainability is often ignored. For perovskite-based PV to achieve large-scale commercial deployment, it is crucial that sustainability becomes a central aspect of its development.

Over the last decade, perovskite solar cells (PSCs) have undergone a remarkable increase in efficiency, recently reaching a 26.1% power conversion efficiency (PCE), rivalling traditional silicon-based systems. [2] Despite this progress, comprehensive strategies to mitigate the environmental impact of PSCs are still lacking. High-performance devices are typically produced with solvents that are toxic and potentially harmful to both the environment and the large-scale manufacturing processes, such as Roll-to-Roll production. [3] Additionally, the reliance on scarce raw materials, and the high cost of organic hole transport layers (HTLs) – due to their complex and inefficient synthesis methods – limits the scalability of this technology. [4, 5]

To address the gap between performance and sustainability, we developed a range of affordable, novel HTLs, founded from the well-known poly(triarylamine) (PTAA) [6] that can be processed with more environmentally benign solvents. This was accomplished by modifying the polymer backbone with a phenothiazine scaffold, improving its solubility in common organic solvents. [7] The impact of methyl substitutions on the TPA phenyl group was also evaluated, exploring the trade-offs between solubility and overall device performance. Additionally, a benzothiadiazole unit was considered, given its promising role in organic semiconductors. [8]

These modified polymers demonstrated excellent solubility in tetrahydrofuran (THF), a cost-effective, non-aromatic, halogen-free solvent that is environmentally friendly and poses low toxicity risks. [9] A full set of structural, optoelectronic, and thermal analyses of the resulting polymers (P1-4) confirmed their viability as HTLs. These materials were incorporated into flexible n-i-p devices, using PTAA as a benchmark. Among the polymers, P1 exhibited competitive efficiency when PTAA is processed with conventional toluene, and even surpassing the latter when processed with THF. Additionally, P1 showed notable improvements in light soaking stability in unencapsulated devices compared to PTAA. A solid-state film study revealed that the structural modifications were key to improving device performance.

To further optimize device performance, we used a multivariate analysis approach (Design of Experiment) to fine-tune the balance of HTL and dopant concentrations, resulting in improved PCEs and more efficient material use. Additionally, we revamped the synthetic methods to prioritize sustainability. By replacing traditional solvents with water-based processes, we achieved high-yield, fast, and scalable reactions that are environmentally friendly, cost-effective, and suitable for advancing to higher technology readiness levels (TRLs).