Publication date: 15th December 2025
Multi-junction solar cells are a key materials strategy for increasing photovoltaic energy yield beyond the Detailed balance limit. In this context, mixed tin–lead (Sn–Pb) perovskites are promising absorbers for triple-junction architectures due to their bandgap bowing, which enables narrow band gaps near 1.2 eV [1 - 3]. From a material perspective, however, Sn–Pb perovskites remain limited by poor film quality and operational stability, mainly driven by Sn(II) oxidation and fast crystallization kinetics. Although PEDOT:PSS is widely used as a hole transport layer (HTL), its acidic and hygroscopic nature accelerates degradation and introduces parasitic optical losses, motivating the development of alternative interfacial materials such as self-assembled molecules (SAMs) [4-5]. In this work, we show that conventional carbazole-based SAMs with phosphonic acid anchoring require adaptation when implemented in Sn–Pb perovskite devices. For instance, we show that (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz), a carbazole-based hole-selective SAM with phosphonic acid anchoring, exhibits reduced performance, which is further amplified in the presence of GlyHCl, a commonly used precursor additive for the passivation of the buried interface in p-i-n device architecture [6]. We demonstrate that (2-(9H-Carbazol-9-yl)acetic acid (9CAA), a carbazole-based hole-selective SAM with carboxylic acid anchoring, enables improved device performance under traditional processing conditions that include GlyHCl. Preliminary data show a PCE increase from 14.5% to 20% using 2PACz and 9CAA as HTL, respectively, with GlyHCl as precursor additive. Moreover, we suggest a practical approach to fit X-ray photoelectron spectroscopy data of carboxylic-acid-based SAMs to characterize their attachment on transparent conducting oxides.
The authors thank J. Beckedahl, M. Choyne., C. Ferber, M. Wittig, T. Lußky, and H. Heinz for the technical support in HySPRINT Helmholtz Innovation Lab. The authors thank the Helmholtz Association for funding the HySPRINT Helmholtz Innovation Lab as well as the “Zeitenwende” project. In addition, we acknowledge the research school “HyPerCells”. S. Mariotti acknowledges the Initiative and Networking Fund of the Helmholtz Association for the funding of the Helmholtz Investigator Group.
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