SHCl Crystallization Control Enables Stable MA-Free Tandem Perovskites

• SHCl enables MA-free all-perovskite tandems with uniform, Cs-rich crystallization—boosting blade-coated films and delivering 24.3% certified efficiency plus strong damp-heat and thermal-cycling stability.

Researchers from Nanjing University, the Australian National University and others report thermally stable, MA-free all-perovskite tandem modules using a crystallization-control additive, semicarbazide hydrochloride (SHCl). The team targets FA/Cs Pb–Sn (FACs Pb–Sn) subcells that typically need methylammonium (MA) for stability; replacing MA with Cs-rich compositions can cause rapid, uneven crystallization that harms large-area film quality.

SHCl introduces cooperative SH+ and Cl− chemistry: it reacts with CsI to form CsCl and SHI and also strongly coordinates Cs+ via a carbonyl group, lowering Cs solubility and triggering a brief, uniform nucleation “burst” followed by slower, more even growth. Benefits scale with Cs content, with FA0.8Cs0.2 yielding the best films. Blade-coated single-junction cells reach 21.9% efficiency and retain 85% after 700 hours at 85°C; 20.25 cm² tandem modules deliver 24.3% certified PCE and show strong damp-heat and thermal-cycling stability.

How does SHCl enable MA-free all-perovskite tandems via controlled Cs crystallization?

• SHCl is used as a crystallization-control additive to replace the conventional reliance on methylammonium (MA) in FA/Cs Pb–Sn perovskites, enabling MA-free all-perovskite tandem subcells.
• The key issue addressed is that Cs-rich perovskite compositions (used to stabilize FA-based Pb–Sn layers without MA) often crystallize too fast and unevenly during coating, which leads to grain-size nonuniformity, defects, and poorer large-area film performance.
  
• SHCl provides “cooperative” ionic chemistry that changes how Cs species transform and assemble into the perovskite lattice during film formation.
• Upon processing, SHCl can react with CsI–derived precursors to generate CsCl in situ (via the Cl− component), altering the halide environment near growing crystals and promoting more favorable perovskite nucleation/growth behavior.
  
• Simultaneously, SHCl contains a functional group that strongly coordinates Cs+ through a carbonyl moiety, effectively regulating how much free Cs+ remains available in solution.
  
• By lowering Cs+ solubility/availability, SHCl slows the uncontrolled buildup of Cs–halide complexes that would otherwise trigger rapid, localized crystallization.
  
• This controlled availability leads to a two-stage crystallization profile: an initial, short “nucleation burst” produces many nuclei fairly uniformly across the film.
• After that burst, the slower release/consumption of Cs+ supports more orderly crystal growth, allowing grains to coarsen more evenly rather than competing chaotically across the coated area.
  
• The result is improved film uniformity and quality at scale (relevant for blade-coated and tandem-active-area layers), reducing pinholes and suppressing defect-forming pathways associated with uneven crystallization.
  
• The performance trend with Cs content indicates that SHCl’s moderation of Cs crystallization becomes more beneficial as the composition shifts toward higher-Cs regimes, where MA-free operation would otherwise be most unstable.
  
• For FA0.8Cs0.2—an optimized Cs fraction reported in this approach—SHCl-mediated control yields the best balance of nucleation density and growth kinetics, producing higher-quality perovskite films.
• In tandem architectures, better-controlled FA/Cs Pb–Sn subcells translate into lower recombination losses and more stable charge extraction, supporting high-efficiency MA-free all-perovskite tandems.
• Overall, SHCl enables MA-free tandems by tuning solution chemistry (Cs complexation + in situ halide transformation) to impose uniform nucleation followed by moderated growth, preventing the rapid/uneven Cs crystallization that would otherwise degrade large-area perovskite layers.