Printed zinc tin oxide diodes: From combustion synthesis to large-scale manufacturing

Printed metal oxide devices have been widely desired in flexible electronic applications to allow direct integration on foils and to reduce electronic waste and associated costs. Especially, semiconductor devices made from non-critical raw materials, such as Zn, Sn (and not, for example, In), have gained significant interest. Despite considerable progress in the field, the upscale requirements from lab to fab scale to produce these materials and devices remain a challenge. In this work, we report the importance of solution combustion synthesis (SCS) when compared with sol-gel in the production of zinc tin oxide (ZTO) thin films using a solvent (1-methoxypropanol) that has lower environmental impact than the widely used and toxic 2-methoxyethanol. To assure the compatibility with low-cost flexible substrates in high-throughput printing techniques, a low annealing temperature of 140 °C was achieved for these thin films by combining SCS and infrared annealing in a short processing time. These conditions allowed the transition from spin-coating (lab scale) to flexographic printing (fab scale) at a printing speed of 10 m min-1 in a roll-to-roll pilot line. The ZTO (1:1 Zn:Sn-ratio) diodes show a rectification ratio of 103, a low operation voltage (≤ 3 V), promising reproducibility and low variability. The results provide the basis for further optimisation (device size, encapsulation) to meet the requirements of diodes in flexible electronics applications such as passive-matrix addressing, energy harvesting and rectification.