Abstract:

Wide bandgap perovskite materials show promising potential as tandem top cells to pair with silicon bottom cells and achieve power conversion efficiencies (PCEs) over 32%, while the fabrication costs are likely to stay low. To date, most of the efficient wide bandgap perovskite layers are fabricated by spin coating, which is difficult to scale up to large area and the crystallization mechanism remains unknown. In this work, we report on slot-die coating for an efficient, wide bandgap triple-halide perovskite, (Cs_0.22FA_0.78)Pb(I_0.85Br_0.15)₃ + 5 mol% MAPbCl₃^{1 2}. A suitable solvent system was designed and optimized specifically for the slot-die coating technique. We demonstrate that with this perovskite, our fabrication route enables a bandgap of 1.68 eV which is suitable for tandem solar cells, and without phase segregation typically observed for high Br loadings. The slot-die coated wet perovskite film was dried using a stream of nitrogen (N₂) from an ''N₂ knife'' with high reproducibility, and avoiding the need to use antisolvents.

We explored varying drying and annealing conditions from 100°C to 170°C and measured absolute as well as transient photoluminescence (PL) to extract information about the perovskite bandgap, quasi Fermi level splitting (QFLS) and charge carrier lifetimes. We find parameters allowing to crystallize the perovskite film into large grains reducing charge collection losses and thus enabling higher current density in solar cells (Fig. 1B). With annealing at 150 °C, an optimized tradeoff between crystallization and the detrimental formation of PbI₂ aggregates on the film’s top surface is found. Insitu Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) investigation of the solution intermediate and film annealing at various stages has also unveiled the perovskite crystallization and PbI₂ formation processes. With the optimized annealing conditions, we improve the cell stability and performance of perovskite single junction cells towards a stabilized power output of up to 19.4 %.

By integrating the optimized perovskite fabrication with commercial saw damage etched Czochralski silicon bottom cells, a two-terminal monolithic tandem solar cell with a PCE of 25.2 % on 1 cm² active area is demonstrated with fully scalable processes. Note that this is reached for a wafer thickness of around 120 µm, not enabling the full photocurrent potential in the NIR wavelength regime compared to thicker wafers (> 250 µm) that are typically used in literature. Furthermore, a 4 cm² tandem solar cell has been developed with this fabrication route and using screen printed silver front grids, yielding PCEs up to 24 %. 

Finally, we show a detailed comparison between spin coated and slot-die coated perovskite films. For the solar cells, we present the loss mechanisms as well as guidelines for further improving the printed films. With that, we highlight the high potential for slot-die coating as fabrication route for scalable and industrially relevant perovskite/silicon tandem solar cells.