Researchers at Nanjing University, the Australian National University, North China Electric Power University and Beijing Institute of Technology have demonstrated thermally stable, MA‑free all‑perovskite tandem solar modules by using a p‑π conjugated additive, semicarbazide hydrochloride (SHCl), to control the crystallization of FACs (FA‑Cs) Pb‑Sn perovskites.
(A) Photograph of the all-perovskite tandem module. (B) Cross-sectional SEM image of the tandem solar device. Image from: Science Advances
All‑perovskite tandems already reach ~25% PCE at module scale, but narrow‑bandgap Pb‑Sn subcells typically rely on thermally unstable methylammonium (MA), and simply replacing MA with Cs‑rich FACs compositions causes rapid, nonuniform crystallization that degrades large‑area film quality. The team tackles this by introducing SHCl into FACs Pb‑Sn precursor solutions, where SH+ and Cl− ions cooperatively regulate nucleation and growth: SHCl reacts with CsI (SHCl+ CsI → SHI + CsCl), forming CsCl and SHI, and strongly coordinates with Cs+ via its carbonyl group, which lowers Cs solubility, increases supersaturation, and drives a short “burst” of homogeneous nucleation followed by slower crystal growth.
In FA1−xCsxPb0.5Sn0.5I3, the benefit of SHCl scales with Cs content: PCE gains of 2%, 6%, and 10% are observed for x=0.05, 0.1, and 0.2, with FA0.8Cs0.2 delivering the smoothest, highest‑crystallinity films. SHCl‑containing precursors form many small, narrowly distributed colloidal clusters, while control solutions show fewer, larger aggregates; importantly, SHCl has negligible effect in FAMA Pb‑Sn systems, confirming its Cs‑specific action. In situ microscopy during gas‑quenching blade coating shows that control FACs films undergo prolonged, spatially heterogeneous nucleation and fast growth, whereas SHCl‑modified films exhibit a high density of uniformly distributed nuclei at the onset and a prolonged, more uniform growth period, yielding larger, vertically oriented grains and uniform Cs+ distribution through the film thickness.
The molecular design of SHCl is key: its strong p‑π conjugation gives higher polarity and stronger binding than other tested p‑π additives (GlyHCl, AAH), and XPS confirms coordination to both Pb and Sn, which slows grain growth and helps passivate bulk defects. Neither SHI alone nor simple Cl‑salts (FACl, MACl, NH4Cl, CsCl) can simultaneously achieve homogeneous nucleation and retarded growth; only the combined action of SH+ and Cl− in SHCl (or an equivalent SHI+CsCl co‑additive system) reproduces the full crystallization control and device benefits.
Blade‑coated, SHCl‑modified narrow‑bandgap single‑junction cells reach a PCE of 21.9% with a VOC of 0.88 V and maintain 85% of their initial efficiency after 700 hours at 85°C. All‑perovskite tandem modules (20.25 cm²) based on these absorbers achieve a certified PCE of 24.3% (24.7% in‑lab), and encapsulated devices retain 90% and 92% of their initial efficiency under ISOS‑D‑3 damp‑heat (200 h) and ISOS‑T‑3 thermal‑cycling (200 cycles) tests, respectively, representing excellent stability for MA‑free all‑perovskite tandem modules.
