Fast Vacuum Process Achieves 24.3% Perovskite-Silicon Tandem Cell Efficiency

Perovskite-silicon tandem solar cells are widely viewed as a pathway to surpass the efficiency limits of conventional crystalline silicon photovoltaics. While laboratory efficiencies continue to improve, translating these devices into high-volume manufacturing remains challenging. The industry requires deposition processes that are not only efficient but also fast, reproducible, and compatible with large-area production.

Researchers from the Karlsruhe Institute of Technology (KIT) and the University of Valencia have now demonstrated a solvent-free vacuum process that can rapidly deposit uniform perovskite layers on silicon solar cells. The findings were reported in the paper titled ‘Close-space sublimation as a versatile deposition process for efficient perovskite silicon tandem solar cells’, published in Nature Energy. The study shows that the approach can be applied to both flat and textured silicon surfaces while maintaining high device performance.

The process is based on close-space sublimation (CSS), a vacuum deposition technique in which precursor materials travel only a few millimeters before reaching the substrate, where they react directly to form the perovskite absorber. According to the study, the process achieved an effective deposition rate of approximately 47 nm/min, which is approximately one order of magnitude higher than conventional thermal co-evaporation or sequential vacuum deposition approaches. The complete conversion of the precursor stack into the final perovskite absorber was achieved within 10 minutes.

Beyond deposition speed, the researchers also addressed bandgap tuning of the perovskite top cell. Tandem architectures require a wider-bandgap material that selectively captures high-energy photons while transmitting lower-energy light to the silicon bottom cell. The team fabricated a mixed-halide perovskite, MAPb(I₀.₇₉Br₀.₂₁)₃, with a bandgap of 1.64 eV, suitable for use as the top cell material in perovskite-silicon tandem devices.

A key finding of the work was that bromine incorporation could not be controlled through the inorganic precursor layer. During conversion, bromine introduced through the inorganic scaffold was largely removed. Instead, the researchers adjusted the ratio of methylammonium iodide (MAI) and methylammonium bromide (MABr) in the organic source to control bromine incorporation and tune the bandgap of the final perovskite material.

Before tandem integration, the researchers assessed the performance of the wide-bandgap perovskite layer in fully vacuum-processed p-i-n perovskite solar cells. The best-performing device achieved a power conversion efficiency of 18.5%, with a stabilized output of 18.2% under maximum power point tracking.

The devices also retained more than 95% of their initial performance after more than 400 hours of combined thermal and illumination stress testing at both 65°C and 85°C.

They further demonstrated industrial relevance through testing on silicon wafers featuring different surface textures. Textured silicon surfaces are commonly used in commercial solar cells because they enhance light trapping and improve absorption. The CSS process was applied to planar, nano-textured, and micro-textured silicon heterojunction (HJT) bottom cells without changing deposition parameters. The resulting perovskite layers exhibited comparable morphology, crystallinity, and optoelectronic properties across all 3 substrate types, despite the differences in surface texture.

The tandem solar cells achieved efficiencies of 23.5% on planar silicon, 23.7% on nano-textured silicon, and 24.3% on micro-textured silicon. According to the researchers, the textured architectures generated higher current due to improved light trapping and greater light absorption in the silicon bottom cell.

Scanning electron microscopy showed conformal perovskite coverage across all substrate morphologies, while the perovskite films maintained similar thicknesses of approximately 460-480 nm along the optical axis. Unlike many solution-based approaches that fill textured surface features, the CSS process produced conformal coatings that followed the underlying texture without penetrating into it.

The study concludes that CSS combines high deposition rates with efficient precursor utilization and the use of reusable source materials. The researchers also note that the sublimation source can be scaled through wider source geometries or showerhead-type evaporators, providing a potential pathway toward GW-scale manufacturing of perovskite-silicon tandem solar cells.