Two-dimensional (2D) perovskites are being increasingly recognized as promising candidate materials for the next generation of optoelectronic devices. These materials combine key characteristics of both 2D semiconductors and three-dimensional (3D) perovskites. As a result, they can overcome some limitations of conventional 2D materials, such as low light absorption and environmental instability, while offering improved chemical stability and stronger excitonic effects than their 3D counterparts.
The origin of these unique characteristics lies in their structure. 2D perovskites consist of inorganic layers separated by organic spacer layers. The inorganic layers act as quantum wells, while the organic layers function as dielectric barriers, giving rise to quantum and dielectric confinement effects that influence how excitons, bound electron-hole pairs, absorb and emit light. Understanding these effects is crucial for engineering these materials. In particular, the dielectric screening environment surrounding excitons, which is determined by the organic and inorganic layers, has a profound influence on their excitonic properties. However, the exact impact of this relationship is poorly understood, hindering predictive modeling and rational design of 2D perovskites.
To bridge this gap, a research team led by Professor Ki-Ha Hong from the Department of Materials Science and Engineering at Hanbat National University in South Korea conducted a systematic study to isolate the effect of the screening environment. "Our study addresses a long-standing challenge in 2D perovskite research: when the organic spacer is changed, the dielectric environment and the inorganic lattice structure often change at the same time, making it difficult to determine which factor actually controls the excitonic properties," explains Prof. Hong. "To disentangle these effects, we used a homologous series of organic spacers and focused on a structurally consistent set of 2D perovskites in which the Pb–I framework remains nearly unchanged. This allowed us to isolate the role of dielectric screening in modulating the quasiparticle bandgap and exciton binding energy." Their study was made available online in Advanced Functional Materials on December 09, 2025, and published in Volume 36, Issue 30 of the journal on April 13, 2026.
The researchers fabricated a series of high-quality 2D lead-iodide perovskite thin films incorporating organic spacers with identical ammonium functional groups but varying alkyl chain lengths. This approach allows for controlled tuning of the interlayer distance between inorganic layers and consequently the effective dielectric constant, while minimizing structural distortions. As a result, the researchers were able to isolate the effect of the screening environment. The team first examined six spacer molecules and then focused on the structurally consistent even-numbered series, where the inorganic Pb–I framework remained nearly unchanged.
Using ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy, the researchers directly measured the quasiparticle bandgap, while UV–vis absorption spectroscopy provided the exciton energy. The results revealed an intriguing divergence: while the quasiparticle bandgaps increased with increasing organic spacer length, the exciton energies remained nearly constant. These findings indicate that changes in the dielectric environment play a dominant role in modulating the quasiparticle bandgap. The increasing separation between the quasiparticle bandgap and exciton energy leads to a substantial rise in exciton binding energy as the organic spacers become longer, consistent with previous observations.
To explain this phenomenon, the researchers employed the Keldysh model, which is a standard theoretical framework for describing excitonic properties in 2D systems. Although the standard Keldysh model captures dielectric screening effects, it did not fully reproduce the experimental behavior observed in the layered perovskites. To address this, the researchers introduced a phenomenological dielectric function to account for the finite thickness of the organic spacers. The modified model showed close agreement between its predictions and experimental data, providing an experimentally validated basis for predicting excitonic properties.
"Our model offers a practical design rule for predicting how organic spacer length controls excitonic properties of 2D perovskites," concludes Prof. Hong. "This provides a molecular-level design rule for tuning exciton binding energy and energy levels in 2D perovskites, which can guide future design of light-emitting, photovoltaic, and other optoelectronic materials."
