Perovskite solar cells technology are approaching enough maturity regarding their high power conversion efficiency and temporal stability. Moreover, these devices can be fabricated using large-scale solution-based methods to achieve mass fabrication. However, the application of printing technologies for deposition of perovskite films require processing adaptation and optimization towards reproducibility. Slot-die coating technology is regarded as the most suitable processing technique for printing perovskite layers due to high production velocity and large area printing capability. Nevertheless, recent investigations concerning perovskite solar cells (PSCs) fabricated by slot-die coating were mainly focused on the optoelectronics properties of the resulting layer, including film drying kinetics and crystal growth, neglecting the technical conditions to reach highly reproducible layers. The path from laboratory to industrial scale fabrication begin with the stabilization of the fluid inside the Slot-die head in order to obtain films with low or bearable defects. In an ideal slot die device, the fluid should fill all the head, reach a laminar regime and have a uniform flux through the lips. Commonly, this condition is achieved by fine tuning of pumping parameters, printing position, and ink rheological properties. The flow dynamics of perovskite inks inside a Slot die head has not been analyzed to date, remarking the gap between industrial and laboratory processes. In this work, we introduce the concept of volume uniformity as a figure of merit for the analysis of volumetric flow through the slot die head. In particular, the flow regimes inside the head have been analyzed, showing their dependence on printing position and ink viscosity, tracing two paths of work and optimization; geometric modification of the head or rheological modification. This latter was finally pursued in this report. By using water as model fluid of perovskite ink, we found optimal parameters of nine o’clock angle (completely horizontal) and 3mL/min inlet flow contrasting twelve o’clock angle (completely vertical) of the majority of laboratory fabrication machines, reaching a maximum of 0.85 volume uniformity. Those results were validated using an actual 28wt% perovskite ink achieving an average 0.82 volume uniformity. Additionally, we found that non-uniform flow in near vertical printing positions can be overcome by increasing the ink viscosity, achieving an ideal flow regimen and a volume uniformity over 0.95. These results represent a benchmark to obtain uniform and reproducible large area perovskite films.

Keywords: uniformity, volume distribution, laminar flow, slot-die coating, perovskite ink.