Bandgap tuning in iodide perovskites for silicon–perovskite tandem applications is commonly achieved through either A-site cation alloying or halide mixing. While halide alloying enables wide bandgap tunability, it suffers from phase segregation and long-term instability. A-site alloying offers a more stable alternative that avoids halide segregation; however, conventional antisolvent-based fabrication methods face severe miscibility constraints, particularly at high cesium concentrations. Although CsPbI₃ provides an ideal wide bandgap and fully inorganic composition, its poor thermal and phase stability prevent its use as a standalone absorber, necessitating alloying with formamidinium (FA). Consequently, accessing silicon-tandem-relevant bandgaps through A-site alloying alone has remained a significant challenge.

Here, we adopt an A-site alloying strategy based on an AX intermediate phase method adapted from inorganic materials synthesis. This approach enables stable and continuous Cs–FA mixing across the entire compositional range from FAPbI₃ to CsPbI₃, achieving bandgap tuning from 1.48 to 1.72 eV without halide alloying. This previously underexplored compositional space exhibits crystallization dynamics fundamentally distinct from antisolvent processing, requiring systematic optimization of additives, charge transport layers, passivation strategies, and fabrication parameters.

Our laboratory has previously developed automation and high-throughput workflows to accelerate the screening of perovskite materials, as presented by some of us at TandemPV 2025. However, cell fabrication was a significant bottleneck with our previous methods. To overcome this limitation, we have since developed a unique high-throughput blade-coating methodology that enables the fabrication of more than 500 fully completed devices spanning a wide range of material and fabrication parameters within the timeframe of conventional device batch of 20-30 samples. Importantly, absorbers optimized using this approach are directly scalable to large-area substrates without further optimization, in contrast to conventional antisolvent-processed absorbers.

During the workshop, I will introduce the as-yet relatively unexplored A-site alloying approach that unlocks the full compositional range of Cs–FA perovskite absorbers suitable for Si-pk tandems. I will also share our novel high-throughput blade-coating methodology that successfully closes the automation loop in high-throughput experimentation, enabling rapid materials optimization and direct scalability to large-area devices.