Molecular Tuning Lifts Perovskite-Silicon Tandems to 32.3%

• China-led breakthrough: perovskite–silicon tandem hits 32.12% efficiency via steric-complementary passivation, slashing recombination, edging toward 40%, and retaining 80% after 1,000 hours MPP with HJT.

A China-led consortium reported a perovskite–silicon tandem with a certified 32.12% efficiency, edging toward a 40% target. Teams from Jiangsu University, Soochow, Huaqiao, the Chinese Academy of Sciences, Hong Kong PolyU, GCL System Integration and Suzhou Maxwell used a steric-complementary synergistic strategy to suppress interfacial recombination.

Size-mismatched cations—small piperazinium (PipI) and bulky phenethylammonium (PEAI)—jointly passivate deep defects and form a hydrophobic barrier, cutting trap density and resistive losses. A single-junction perovskite reached 22.26% (Voc 1.270 V, Jsc 21.50 mA/cm², FF 81.52%). The tandem paired with an HJT silicon bottom cell retained 80% performance after 1,000 hours MPP at one sun.

How do size-mismatched cations suppress recombination to reach 32.12% tandem efficiency?

• Dual-cation steric complementarity: the small dication slips into undercoordinated A‑site/GB pockets while the bulky organic sits at the surface, jointly neutralizing deep Pb- and I‑vacancies that drive Shockley–Read–Hall recombination.
  
• Quasi‑2D surface reconstruction: the bulky cation forms an ultrathin Ruddlesden–Popper–like cap that blocks surface traps and creates an energy-selective interface, extracting majority carriers while reflecting minority carriers away from recombination centers.
  
• Interfacial dipole tuning: opposite dipoles from the two cations shift band edges to reduce valence/conduction offsets with the transport layer, cutting interface recombination velocity and contact losses, thus lifting Voc and FF simultaneously.
• Lattice strain relief and lower Urbach energy: size mismatch relaxes microstrain and smooths the band tail, shrinking sub‑gap states to suppress nonradiative pathways.
  
• Grain-boundary sealing: the small cation penetrates GBs to “plug” deep traps; the bulky cation sterically hinders ion migration along GBs, curbing light‑induced halide segregation and recombination hot spots.
  
• Reduced interlayer resistance with maintained transparency: a nanometer‑scale passivation stack adds negligible optical loss for the top subcell while lowering contact resistance, preserving Jsc in the tandem.
  
• Moisture/oxygen barrier: the hydrophobic bulky cation shields the perovskite from ambient ingress, stabilizing passivation over time so recombination does not rebound under MPP stress.
  
• Suppressed interdiffusion with transport layers: the surface cap prevents chemical reactions (e.g., with NiOx/Spiro/LiF), maintaining clean interfaces and low trap densities.
  
• Enhanced dielectric screening: organic layers increase local permittivity, screening Coulombic trap capture and reducing carrier recombination cross‑sections.
  
• Current‑match friendly: higher top‑cell Voc from fewer nonradiative losses raises tandem efficiency without demanding extra thickness (which would hurt current balance with the HJT bottom cell).
  
• Ion‑migration pinning: hydrogen bonding/chelation locks mobile iodide and undercoordinated Pb2+, stabilizing quasi‑Fermi level splitting under illumination and bias.
• Process compatibility: the two-cation treatment self‑limits to a few monolayers, avoiding overpassivation that would impede charge extraction, enabling the measured 32.12% tandem efficiency.