Researchers at Ludwig-Maximilians-Universität (LMU), Kyungpook National University (KNU), University of the Bundeswehr Munich, University of Konstanz and Nantes Université (CNRS) have developed a simple, solution-based strategy to re-engineer indium tin oxide (ITO) surfaces for self-assembled monolayer (SAM) contacts in perovskite solar cells.
The approach simultaneously tunes surface chemistry, conductivity, and homogeneity, leading to more uniform molecular contacts, improved charge extraction, and enhanced device stability across single-junction and tandem architectures.
Self-assembled monolayers (SAMs) are seen as the benchmark molecular charge-selective contacts in p‑i‑n perovskite solar cells, but standard ITO treatments are designed mainly for cleaning rather than for promoting controlled SAM self-assembly. In this work, the team revisits ITO treatment from the standpoint of molecular anchoring, focusing on how phosphonic-acid-based SAMs interact with the oxide surface. The bonding is governed not only by the density of surface hydroxyl groups, but also by the chemical nature and spatial distribution of lattice oxygen, hydroxyl, and hydroxide species, as well as the Lewis acidity of the ITO surface. Variations in these oxygen species strongly affect SAM packing and coverage, promoting either well-ordered monolayers or, under non-ideal conditions, island growth, multilayer formation, or aggregation. While hydroxyl groups are required for anchoring, an excess of weakly bound hydroxide and adsorbed water hinders uniform bonding, thereby degrading electronic homogeneity at the interface.
The researchers show that the prevailing assumption in the field - that maximizing surface hydroxylation is always beneficial for phosphonic-acid-based SAMs - is incomplete. “We show that maximizing surface hydroxylation is not the key,” explains first author Rik Hooijer. “Rather, a balanced ratio of different oxygen species yields more uniform and electronically favorable interfaces.” Instead of targeting the highest possible hydroxyl density, the work identifies a moderately hydroxylated, electronically homogeneous, and sufficiently Lewis-acidic surface as the optimal condition for SAM anchoring. Under these conditions, condensation reactions involving lattice oxygen and hydroxyl groups lead to more controlled binding configurations, which translate into more continuous monolayers and reduced electronic disorder.
To realize this optimized surface state, the team uses straightforward chemical treatments based on piranha acid and SC‑1 solution. These solution-based steps systematically modify the ITO surface, increasing conductivity and lateral homogeneity while tuning the balance between lattice oxygen, hydroxyl, and hydroxide species. The resulting surfaces provide more favorable sites for SAM anchoring, yielding better-ordered and electronically more uniform SAM layers. This synthetic surface design route is compatible with common phosphonic-acid-based SAMs and can be implemented without altering the molecular structures themselves, effectively turning ITO surface preparation into a powerful and previously underused design parameter.
The improved ITO/SAM interfaces enable more efficient charge extraction at the electrode, contributing to higher fill factors and reduced series losses in perovskite solar cells. The enhanced surface homogeneity decreases device-to-device performance variations, leading to a narrower distribution of power conversion efficiencies across nominally identical cells. These benefits are observed across multiple SAM chemistries, perovskite compositions, and device structures, including both single-junction and tandem solar cells. Beyond the gains in efficiency metrics, the treatment also improves the lifetime of SAM-coated substrates, which is critical for reproducible device fabrication and scale-up.
The researchers subjected the optimized devices to rigorous operational stability testing using maximum power point tracking (MPPT). Under continuous operation, the treated interfaces supported more stable performance, with reduced degradation relative to conventionally prepared references. They further tested thermal robustness by cycling the devices between −80 °C and 80 °C, conditions representative of low-earth-orbit (LEO) environments. The treated interfaces showed enhanced resilience under this extreme thermal cycling, confirming that the improved molecular contact and surface chemistry provide not only better initial performance but also superior mechanical and electronic robustness under stress.
This combination of high efficiency, reproducibility, and operational stability underscores that metal oxide/SAM interfaces must be engineered as coupled chemical and electronic systems, where optimal behavior arises from a carefully balanced interplay of surface chemistry, bonding configuration, and charge transport.
