Organic photovoltaics (OPVs) have seen tremendous progress after introduction of non-fullerene acceptors (NFAs) and recently achieved certified power conversion efficiency (PCE) 19.2% for small scale devices and 14.5% for the minimodules.^1&2 OPVs became materials of choice as organic active layers are easy to solution process in green solvents and easy to transition to roll-to-roll (R2R) manufacturing on flexible substrates using different printing techniques like, slot-die coating, blade coating etc. They have enormous potentials to be used in different targeted applications, e.g. indoor photovoltaics to power internet-of-things (IoTs), building integrated photovoltaics and in agrivoltaics by achieving semi-transparency. Zinc Oxide (ZnO) is the most commonly used electron transport layer (ETL) in OPVs but needs high temperature processing which limits its use on flexible substrates that are damaged by the required annealing temperature.

We are working towards development of process for R2R manufacturing of OPVs, and here we report comparison of different ETLs to achieve similar performance at low temperature processability, that can be implemented on ITO coated flexible substrates, specifically Polyethylene Terephthalate (PET). Different metal oxide ETLs compared in the study are Tin Oxide nanoparticles (NPs) from two commercial suppliers and Zinc Oxide from sol-gel recipe. Laboratory scale semi-transparent devices using PTQ10: BTP-4F-12 (Y12) active layer in a green solvent o-xylene were fabricated, elsewhere reported comparing different green solvents.³ We achieved average transmittance above 70% and comparable performance about 7.5% has been achieved using both Zinc Oxide (sol-gel) and Tin Oxide (SnO₂ NPs) processed 150^oC and 120^oC respectively. The process then transferred slot-die coating using progressive layer deposition, where initially just ETL and active layer slot-die coated, and hole transporting layer spin coated and then all three layers. i.e. ETL, active layer and hole transporting layers slot-die coated. Silver back contacts were evaporated to complete the device in both cases. Slot-die coated devices with 60% average transparency showed 9.3% PCE, with short circuit current density (Jsc) reaching to 19.9 mA/cm² with open circuit voltage (Voc) 0.85V and fill factor (FF) close to 55%, as shown in figure 1.

Further research towards translating these S2S printed devices to realise a complete module and translating the process to our state-of-the-art R2R fabrication facility to develop flexible modules is ongoing and will be the part of this talk.