From fragile lab cells to 22 % durable performers

CsPbI3

1. The CsPbI₃ Story: From fragile lab cells to 22 % durable performers

2. ACT I – Context All‑inorganic CsPbI₃ has the ‘dream’ 1.7 eV band‑gap for tandem‑PV top cells. No volatile organics → survives heat & UV better than hybrid perovskites. But: record efficiencies stalled below the Shockley‑Queisser promise; early devices died quickly.

3. ACT II – The Problem Buried TiO₂/CsPbI₃ interface is riddled with iodide vacancies (V_I) & under‑coordinated Pb²⁺. These traps fuel Shockley–Read–Hall recombination → big V_oc loss (≥0.5 V). Band‑offset mismatch blocks charges → low fill–factor. Ion migration & moisture trigger phase flip to non‑photonic δ‑phase → rapid decay.

4. What Hasn’t Worked (Enough) Simple DMAI additive tunes crystallisation but leaves a defect‑rich hidden interface (Tan 2025). Long‑chain 2D cappings stop water, yet hinder carrier transport. Passivators rarely fix BOTH deep traps AND band alignment.

5. ACT III – Solution #1 (Tan et al., ACS Energy Lett. 2025) Spin‑coat 18‑crown‑6 + CsBr on TiO₂ before perovskite. In‑situ exchange → 18C6/Pb²⁺ molecular barrier; free Br⁻ seeps 2–3 nm into film. Effects: fills V_I, passivates Pb²⁺, creates gentle Br‑rich front‑surface field.

6. Solution #2 (Tan et al., Adv. Mater. 2023) Post‑treat finished film with benzyl‑trimethyl‑ammonium bromide (BTABr). Forms BTA⁺‑terminated surface + Br‑gradient ∼8 %→1 % into bulk. Dual role: heals traps AND nudges valence band to match spiro‑OMeTAD.

7. Alternative View (Wang et al., Angew. Chem. 2019) DMAI is just a volatile growth aid; residues hurt devices → must be removed. Agrees with Tan’s interface focus, but argues bulk DMA⁺ incorporation is a myth. Common ground: clean crystals + targeted passivation key.

8. ACT IV – Results Lab cell with 18C6/CsBr interface: 22.14 % PCE, V_oc 1.235 V, FF 0.841. BTABr gradient: 21.31 % PCE, FF 0.854; printed 12 cm² minimodule 16.6 %. Trap density slashed 50 %; carrier lifetime ↑4×; built‑in field ↑.

9. Stability – The Happy Ending 95 % of initial PCE after 1500 h at MPP & 55 °C (Tan 2025). Unencapsulated films keep black phase >30 min @45 %RH, 45 °C; thermal 85 °C endurance. TOPO‑dipole tweak (Iqbal 2023) adds +17 % energy yield over 664 h cycling.

10. ACT V – Take‑aways Interfaces, not just bulk, decide the fate of CsPbI₃ cells. Gradient halide fields + molecular barriers can heal AND realign in one step. Story so far: 3 % absolute gain in three years, mini‑modules on way. Next chapter: pair short hydrophobic cations with inorganic shells for 25 % & 30 000 h.