Publication date: 5th November 2025
Wide-bandgap (WBG) perovskites are promising absorbers for perovskite-silicon and all-perovskite tandem solar cells due to their tunable bandgap (1.7-1.8 eV) and high open-circuit voltage potential. However, their performance and long-term stability are limited by persistent issues such as halide segregation, formation of hexagonal polytypes, and defect-induced nonradiative losses. [1] These instabilities mainly arise from unbalanced nucleation-growth dynamics governed by strong coordination between polar aprotic solvents and perovskite precursors, which promote the formation of undesirable intermediate phases and stacking defects during crystallization. [2] While long-chain alkyl ammonium chlorides have been employed to regulate crystallization, the role of volatile ammonium chloride (AC) in tuning precursor chemistry and crystallization pathways in WBG perovskites remains unexplored. In this work, we demonstrate that AC weakens precursor-solvent coordination and destabilizes hexagonal intermediate polytypes. In-situ characterization reveals that AC promotes the formation of high-valence, de-intercalated iodoplumbate complexes that suppress the sol-gel state and balance nucleation-growth kinetics. Simultaneously, Cl-rich intermediates act as heterogeneous nucleation sites and, through cation exchange between NH4+ and Cs⁺/FA⁺ ions, regulate crystal growth and minimize defect formation. The combined effects facilitate phase-pure cubic perovskite formation with improved grain uniformity and defect self-elimination during crystallization. As a result, AC-treated films yield high-quality 1.73 eV WBG perovskite solar cells exhibiting a power conversion efficiency of ~18%, a high Voc of 1.22 V, and remarkable photostability, demonstrating an effective and scalable route for achieving stable and efficient WBG perovskites for tandem applications.
Authors thank financial support from the Department of Science and Technology (DST), India, through Grant No. DST/TMD/CERI/C140(G) and DST/TMD/IC-MAP/2K20/03C under the Clean Energy Research Initiative; the Science and Engineering Research Board (SERB), India, through Grant No. IPA/2021/000096, IPA/2021/000031, and SPR/2021/000700; and the UKRI Global Challenge Research Fund through project SUNRISE (EP/P032591/1). We also acknowledge the funding support from the Chandrakanta Kesavan Center for Energy Policy and Climate Solutions, IIT Kanpur.
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