Constructing a Cascaded Type Perovskite/Perovskite Heterostructure with Transfer Printing Technique for Efficient Solar Cells

The perovskite/perovskite heterostructures, based on mixed-halide perovskites, manifest solution-processability, low-temperature, and competent efficiency like tandem solar cells. However, the current challenges of limited fabrication routes on solution-based materials, the interaction of the top layer with the underlying layer, solvent compatibility, and unmatched band alignment can cause poor performance and long-term stability issues. Herein, we have designed and demonstrated a sequential spin and transfer printing technique to fabricate a perovskite/perovskite device architecture (p-i-n type). We integrated Br-based perovskites (1.7 eV bandgap) as a bottom layer and Sn-based perovskites (1.2 eV bandgap) as a top layer with a thin interlayer. Due to cascaded-type band alignment between the top and bottom layer, a smooth extraction and transfer of photo-carriers happened, which boosted power conversion efficiency compared to a single-junction counterpart. With further morphology investigation, we found that SEM and AFM analysis confirmed that the crafted perovskite film with a modified PDMS stamp provides a smooth surface, fewer pinholes, and better interface quality. The high crystalline nature with larger grain size, lesser grain boundaries, and defects across interfaces in heterostructure film helps to suppress recombination losses at the interfaces. The printed layer positively impacts morphology, improving the heterostructure's power conversion efficiencies (PCE). Apart from PCE, the well-distinguished top and bottom layer, as investigated with XPS analysis, greatly helps to maintain the stability of ~1000 hours. With our ongoing investigation, we are focusing on the variation of top and bottom layer composition with consideration of active bandgap of 1.7 eV/ 1.6 eV, 1.7 eV/ 1.5 eV, 1.7 eV/1.4 eV, and 1.7 eV/1.3 eV and investigate heterostructures potential to maintain stability and performance of perovskite-based solar cells. Our technique can be adopted for low-temperature processable layer printing, including HTLs, ETLs, or any interlayer, and promotes low-weight, portable, and flexible photovoltaic devices. Additionally, this strategy is advantageous for implementing traditional Si solar cells and multi-junction tandem structures.