Hybrid halide perovskites are excellent materials for next generation photovoltaics, demonstrating outstanding power conversion efficiencies over 25% measured in lab-scale devices.[1,2] Despite the extraordinary progresses, such record efficiencies are obtained by perovskite processing in a controlled glove-box environment, by means of non-scalable techniques (i.e. spin-coating often associated with solvent dripping) and with the use of highly toxic solvents.[3] The inherent limitations interfering with large-scale production of perovskite solar cells are related to the critical material deposition/reproducibility, which relies on film formation occurring throughout a complex self-assembly process driven by weak interactions.[4-6]  That being said, there has been significant and exciting developments in recent years to bring viability to the large scale manufacturing of perovskite photovoltaics in roll-to-roll facilities.[7] To this purpose herein we propose a simple, yet, effective, material preparation allowing a facile and scalable process at mild-temperature and ambient air conditions, with the use of low toxicity dimethyl sulfoxide (DMSO). .

To elegantly simplify the solvent based deposition, we exploit polysaccharides as rheological modifiers to tune the viscosity of perovskite-polymer formulation, which positively influences the formation of perovskite films via single step coating. The hydroxyl groups on the biopolymer chains establish hydrogen interactions with organic cations and at the same time, with the DMSO solvent leading to solution gelation that allows for a convenient and finely tuned viscosity modulation.[8] Moreover, due to the organic polymeric nature and to the non-covalent interactions between adjacent chains, they confer superior flexibility, moisture/stability to the perovskite-polymer films, enabling the nanocomposite material to accommodate a strain, whilst maintaining transport properties suitable for devices, thus very attractive for flexible photovoltaics.

Herein, we demonstrate that, thanks to the easily tuneable viscosity, such perovskite-polymer inks can be adapted to the requirements of scalable slot-die and gravure printing techniques. The superior film forming properties of polymeric materials guarantees the deposition of perovskites on large area flexible substrates without the use of the antisolvent-bath, thus significantly simplifying the large-scale processing that is a mandatory prerequisite in view of the large-scale manufacturing of perovskite solar cells at low-cost.

Recently, we have demonstrated using VTT’s pilot manufacturing lines the roll-to-roll gravure printing of flexible solar cell devices by depositing the aforementioned perovskite-polymer inks in ambient conditions via a single step printing method. The flexible and fully printed, except the electrodes, solar cells, were fabricated on 50-meter-long rolls featuring promising power conversion efficiencies near 10%.