The potential of perovskite-based photovoltaics is rapidly becoming recognised as a probable and significant contributor towards attaining climate neutrality by 2050. Indeed, perovskites highly tuneable optoelectronic properties coupled to its facile, low-cost and scalable fabrication allow for a wide variety of innovative applications such as tandem configurations (silicon-perovskite or perovskite-perovskite), space applications due to their high power-to-weight ratio and even indoor light harvesting. [1] Whilst most of the attention is being put towards maximizing performance, critical sustainability aspects are often being overlooked. Indeed, the sustainability of these devices should also be at the forefront of development in order for this emerging technology to be commercialized and accepted by the market, allowing for its widespread deployment.

In the last 10 years, Perovskite Solar Cells (PSCs) have experienced a meteoritic rise in performance, arriving most recently at 26.1% power conversion efficiency, (PCE) directly competing with silicon-based photovoltaics. [2] Nevertheless, comprehensive solutions on reducing the environmental impact of the device are still lacking in literature. More specifically, high performing devices are still manufactured with materials that require, out of necessity, toxic and environmentally damaging solvents that could pose a threat towards the ambient-air, large-scale (e.g. Roll-to-Roll) manufacturing aspects. [3] Furthermore, the use of critical raw materials [4] is still highly favoured (i.e dopants), discouraging the technology impact at a larger scale. Finally, materials employed in the cell (e.g. organic hole transport layers (HTL)) are usually very expensive due to their lengthy and inefficient synthetic and purification procedures, resulting in no technology readiness levels (TRL) associated for TW levels of production. [5]

To help fill the research gap between sustainability and performance, we developed an array of cheap, novel HTL layers, based on the state-of-the-art poly(triarylamine) (PTAA) [6], that can be processable in “greener” solvents. This was achieved by the modification of the triphenylamine polymeric backbone via the incorporation of a phenothiazine organic scaffold, which shows good solubility in common organic solvents. [7] Additionally, the methyl substitution of the TPA phenyl unit was modified to evaluate the trade-off effect on solubility and performance. An additional benzothiadiazole unit was also included due to its promising nature in organic semiconductors. [8]

The resulting polymers presented good solubility in our proposed, more sustainable solvent: tetrahydrofuran (THF) that is non-aromatic, non-halogenated, cheap, has a low environmental risk and a low human health toxicity. [9] A full structural, optoelectronic, and thermal characterization resulted in polymeric compounds P1-4 that displayed suitable HTL properties. These were then applied to flexible n-i-p devices with PTAA as the reference. One of the polymers (P1) revealed competitive efficiencies when PTAA is deposited using toluene (conventionally used in literature) and even outperforms the reference when processed with THF. P1 also displayed a remarkable enhancement in unencapsulated light soaking stability, with respect to PTAA devices. A solid-state film analysis allowed us to determine that the synthetic tailoring of structural components is key for performance enhancement.

Following on from this, a doping optimization was also conducted using multi-variate analysis approach (i.e. Design of Experiment), aiming at obtaining the right combination of HTL and dopant concentrations, resulting in enhanced PCEs with a better efficacy in material use. Finally, a thoughtful revisit of the synthetic protocols was considered targeting innovation and more eco-friendly processes. In fact, we were able to substitute conventional organic solvent procedures that are lengthy, generate waste, typically low to moderate yielding and employ high temperatures, to protocols employing more sustainable and recyclable mediums (i.e. water), permitting for reactions that are very high yielding, selective, fast and above all, scalable, simple, low-cost and can progress to higher TRL levels.