Abstract

Self-assembled monolayer (SAM)-based hole-selective layers (HSLs) offer a promising route to defect-passivated and energy-aligned interfaces in perovskite organic tandem solar cells (POTSCs). However, their practical implementation remains hindered by weak anchoring to transparent conductive oxides (TCOs), leading to desorption during perovskite deposition and poor interfacial durability under polar solvent exposure. Here, we present a chemical interfacial stabilization strategy in which potassium carbonate (K2CO3) mediates the controlled deprotonation of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz), forming mixed mono- and di-deprotonated species (2PACz-K) that bind strongly to indium tin oxide (ITO). The resulting SAM exhibits superior solvent resistance, improved energy-level alignment, and enhanced buried interface quality. POTSCs incorporating 2PACz-K achieve 25.10% power conversion efficiency (PCE) with a high open-circuit voltage (VOC) of 2.230 V, while retaining 80% of their initial PCE after 220 h of maximum power point (MPP) tracking under simulated 1-sun illumination. Beyond photovoltaics, the robust 2PACz-K interface is further integrated into a perovskite/organic tandem photocathode (POT-PEC), representing the first transparent, metal-free tandem PEC architecture capable of stable operation in aqueous electrolyte, delivering a photovoltage (Vph) of 2.16 V and achieving a solar-to-hydrogen (STH) conversion efficiency of 7.7%. This work establishes a versatile interfacial design paradigm that bridges photovoltaic and photoelectrochemical energy conversion.

Researchers at UNIST have unveiled a novel interface engineering technique that significantly improves both the performance and durability of perovskite/organic tandem solar cells (POTSCs). Published in the February 2026 issue of Energy & Environmental Science, their study demonstrates how precise control of self-assembled monolayers (SAMs) at the molecular level can lead to more stable, high-efficiency solar devices and open new pathways for solar-driven hydrogen production.

POTSCs are among the most promising photovoltaic technologies, combining different light-absorbing materials to maximize energy conversion. However, instability at the interface between the transparent electrode and the perovskite layer has long hampered their long-term reliability, especially under operational conditions.

Led by Professors Jin-Young Kim and Dong-Seok Kim from the Graduate School of Carbon Neutrality, along with Professor Seung-Jae Shin from the School of Energy and Chemical Engineering, the team developed a method to chemically stabilize this critical interface. They focused on a self-assembled monolayer (SAM) known as 2PACz, which facilitates hole extraction in solar cells.

Figure 1. Schematic illustration of the device structure of POTSCs with 2PACz-K and their performance.

By introducing potassium carbonate (K₂CO₃), the team induced a controlled deprotonation of 2PACz molecules—partially removing hydrogen ions from their phosphonic acid groups. This process creates a mixture of mono- and di-deprotonated species (referred to as 2PACz-K), which acquire a negative charge and form stronger, more stable bonds with indium tin oxide (ITO) electrodes. This enhanced bonding results in an interface that is more resistant to solvents used during device fabrication, maintaining uniformity and stability.

Devices fabricated with this chemically tailored interface demonstrated remarkable performance: perovskite solar cells achieved a power conversion efficiency (PCE) of 25.1% and an open-circuit voltage (VOC) of 2.23 V. Moreover, these cells retained over 80% of their initial efficiency after 220 hours of continuous operation under simulated sunlight, indicating substantial improvements in operational stability.

The team also applied this interface engineering approach to photoelectrochemical (PEC) cells designed for water splitting. The resulting tandem PEC device exhibited a high photovoltage of 2.16 V and achieved a solar-to-hydrogen (STH) conversion efficiency of 7.7% without the need for external bias—marking a significant step toward practical, solar-driven hydrogen generation.

Professor Jin-Young Kim stated, "Controlling the chemical state of interfaces at the molecular level allows us to dramatically improve both the efficiency and long-term stability of solar energy devices. This strategy offers a promising avenue for developing integrated systems that convert sunlight directly into electricity and hydrogen, supporting a sustainable energy future."

The study has been supported by the National Research Foundation of Korea (NRF) and the Ministry of Science and ICT (MSIT).

Journal Reference
Jung Geon Son, Ha-eun Koo, Woojin Lee, et al., "Deprotonated self-assembled molecules as robust hole-selective layers for perovskite/organic tandem solar cells and photocathodes," Energy Environ. Sci., (2026).