Differentiating the 2D passivation from amorphous passivation in perovskite solar cells

As the certified record for single-junction perovskite solar cells (PSCs) closes in on 27 %, the stability gap between n-i-p and p-i-n architectures remains a commercial bottleneck. Now, a multi-centre team led by Prof. Zhixin Liu (SUSTech), Prof. Danish Khan (SZTU) and Prof. Xingzhu Wang (SUSTech) has delivered a definitive study that differentiates true 2D passivation from amorphous capping with only a single-atom fluorine substitution. Using 4-fluoro-benzamidine (4F-BA) they create an n-type crystalline 2D layer that pushes inverted PSCs to 25.02 % efficiency while surviving 1 200 h damp-heat—offering a design rule for the next wave of >25 % flexible and tandem modules.

Why Crystalline 2D Capping Matters
   • Energy-Level Matching: 4F-BA lowers the work-function of the 3D film, forming an n-type 3D/2D junction that accelerates electron extraction and suppresses interface recombination.
   • Hydrophobic Seal: The oriented (4F-BA)₂Pb(I,Cl)₄ over-layer raises the water contact angle to 53.2°, blocking moisture ingress without extra encapsulants.
   • Universal Applicability: The same molecule boosts Cs_0.05(FA_0.95MA_0.05)_0.95Pb(I_0.95Br_0.05)₃ absorbers, validating its robustness across mixed-halide bandgaps for tandem stacks.

Innovative Design and Features
   • Electronic-Tuning Principle: Balanced intramolecular charge polarization (Lowdin charge –0.51 e on –NH₃⁺) favours strong –NH₃···I– hydrogen bonding and a formation energy of –2.18 eV—twice that of the trifluoromethyl analogue.
   • In-Situ Characterisation: GIWAXS, XRD and SEM distinguish a sharp 7.18° 2D peak for 4F-BA versus a faint amorphous hump for 4TF-BA, while AIMD at 300 K confirms shorter H···I bond lengths and stable Pb–I frameworks.
   • Device Architecture: p-i-n stack (FTO/NiOx/Me-4PACz/3D/2D/PCBM/BCP/Ag) fabricated at ≤150 °C, compatible with flexible PET and tandem bottom cells.

Applications and Future Outlook
   • Certified Performance: Champion cell delivers 25.02 % PCE (Voc = 1.143 V, FF = 84.5 %) and retains 93 % of initial efficiency after 1 200 h at 65 °C in N₂—outperforming amorphous-capped (89 %) and control (65 %) devices.
   • UV & Moist Resilience: 88 % retention after 400 h continuous 1-sun illumination and 4-h open-air water-droplet test, paving the way for outdoor-qualified modules.
   • Challenges & Opportunities: Scaling uniform 2D layers over >100 cm² and adapting the concept to wide-bandgap perovskites for all-perovskite tandems are next targets.

This work provides a molecular-level roadmap for selecting spacer cations that guarantee crystalline 2D formation, merging efficiency and longevity in inverted perovskite photovoltaics. Stay tuned for further advances from Prof. Liu, Prof. Khan and Prof. Wang!