Interface Defect Elimination with Modified ALD Layer ELT in p-i-n Perovskite Solar Cells

Metal halide perovskite solar cells exhibit remarkable efficiencies (26.1% for single junctions and 33.9% for tandem cells)[1]. However, commercialization faces hurdles related to intrinsic perovskite film instability, interfacial chemical reactions, and contact layer detachment, leading to overall performance decline. Notably, in tandem cells with a p-i-n configuration top cell, delamination of the electron transport layer (ETL) from the perovskite absorber significantly reduces device performance [2]. Researchers have addressed instability issues through ultra-thin, low-temperature processed atomic layer deposition (ALD) of Al₂O₃, effectively controlling perovskite layer instability and interfacial chemical reactions [2, 3]. However, achieving robust mechanical stability at the interface remains a challenge. While groups have explored ALD SnO₂ on perovskite in p-i-n cells [4,5], high-temperature processing of metal oxides like SnO₂ and TiO₂ often alters perovskite surface chemistry, hindering device efficiency [6]. Consequently, ALD SnO₂ necessitates an interlayer (C₆₀ or PCBM), introducing a risk of entire ETL stack delamination.<br/>To circumvent the temperature and chemical reaction induced surface chemistry changes in perovskite, we implemented a modified ALD approach for ETL deposition directly onto the perovskite, aiming to minimize processing-induced interfacial defects. This approach yielded significantly improved device performance (11% efficiency) compared to the standard direct ALD method (6-7% efficiency).<br/>Hard X-ray photoelectron spectroscopy (HAXPES) revealed that standard ALD ETL deposition triggers Pb-O bond formation and perovskite surface degradation through nitrogen dissociation and halide migration due to processing temperature. Conversely, our modified ALD ETL deposition minimized perovskite surface degradation and halide migration, interestingly preventing Pb-O formation. The superior performance observed in p-i-n cells using our modified approach stems from a smoother and more intact interface between the perovskite and ETL. Furthermore, solar cells fabricated with the modified ALD process exhibited enhanced shelf-life stability compared to those using standard ALD. While the performance and stability of p-i-n cells with our ALD ETL approach fall short of C60/ETL combinations, it offers a promising avenue for further development of direct ETL deposition in p-i-n cells for tandem cell applications.<br/><br/>1. https://www.nrel.gov/pv/cell-efficiency.html<br/>2. M. De Bastiani, G. Armaroli, R. Jalmood, L. Ferlauto, X. Li, R. Tao, G. T. Harrison, M. K. Eswaran, R. Azmi, M. Babics, A. S. Subbiah, E. Aydin, T. G. Allen, C. Combe, T. Cramer, D. Baran, U. Schwingenschlögl, G. Lubineau, D. Cavalcoli, S. De Wolf, Cite This: ACS Energy Lett 2022, 2022, 833.<br/>3. M. Kot, C. Das, Z. Wang, K. Henkel, Z. Rouissi, K. Wojciechowski, H. J. Snaith, D. Schmeisser, ChemSusChem 2016, 9.<br/>4. C. Das, M. Kot, T. Hellmann, C. Wittich, E. Mankel, I. Zimmermann, D. Schmeisser, M. Khaja Nazeeruddin, W. Jaegermann, Cell Rep Phys Sci 2020, 1.<br/>5. A. F. Palmstrom, J. A. Raiford, R. Prasanna, K. A. Bush, M. Sponseller, R. Cheacharoen, M. C. Minichetti, D. S. Bergsman, T. Leijtens, H. P. Wang, V. Bulović, M. D. McGehee, S. F. Bent, Adv Energy Mater 2018, 8, 1.