‘2D Metal Halide Perovskites: Energy Gap and Exciton Binding Energy vs. Octahedral Twist and Quantum and Dielectric Confinement”‘

Prof. Antoine Khan, Princeton University


Two-dimensional (2D) halide perovskites exhibit remarkable tunability of optoelectronic properties and good environmental stability achieved through the selection of organic cations. In particular, the incorporation of bifunctional ligands featuring non-ammonium terminus and functional groups capable of forming extra bonding motifs within the organic bilayer provides an effective strategy to engineer perovskite structures and introduce additional functionalities. 2D halide perovskites are therefore poised to perform an important role, both active and passive, in the developing halide perovskite device field.

This talk addresses the determination of optoelectronic properties, i.e. single particle gap (EG) and exciton binding energy (EB), in several groups of Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) 2D metal halide perovskites via direct and inverse photoemission spectroscopy (UPS/IPES) aided by density functional theory (DFT). We first determine the electronic gap progression as a function of inorganic layer thickness in high purity films of BA2MAn-1PbnI3n+1 (n = 1-5).[1] We show that this series exhibits a type I, nested band gap heterostructure arrangement. By subtracting the optical from the electronic gap, we show that EB vs. n ranges from 420 meV (n = 1) down to 100 meV (n = 5), well fitted by the empirical scaling law developed by Blancon et al.[2] We then turn to 2D RP perovskites incorporating organic ligands with diverse functional groups (-CN, -OH, -COOH, -Ph, and -CH3), each exhibiting distinct bonding characteristics and dielectric properties, and report on the impact of these bifunctional ligands on the electronic and excitonic properties of these 2D perovskites.[3] These bifunctional ligands featuring non-ammonium terminus and functional groups form extra bonding motifs within the organic bilayer and provide an effective strategy to engineer perovskite structures and introduce additional functionalities. We observe a strong correlation between EG of the -CN, -COOH, -Ph, and -CH3-based perovskites and the in-plane Pb-I-Pb bond angle, aligning with earlier findings regarding the relationship between optical gaps and in-plane Pb-I-Pb bond angle.[4,5] The -OH-based perovskite exhibits a significantly deviation from this correlation, attributed to band dispersion in the  out-of-plane direction caused primarily by interlayer electronic coupling. EB in these 2D layers is found to range from 360 meV for (CH3–PA)2PbI4 to 70 meV for (OH–EA)2PbI4, a variation attributed to specific structural aspects, such as in-plane Pb-I-Pb bond angle, interlayer spacing, and the dielectric constant of the bifunctional ligands. Overall, these results provide deeper insight into the complex impact of organic ligands on the electronic and excitonic properties of 2D perovskites, in particular the substantial role of interlayer electronic coupling. 

[1] X. Zhong et al., Adv. Energy Mater. 12, 2202333 (2022)
[2] J.C. Blancon et al., Nat. Commun. 9, 2254 (2018)
[3] X. Zhong et al., Adv. Energy Mater., 2304345 (2024)
[4] S. Silver et al., Adv. Energy Mater. 10, 1903900 (2020)
[5] X. Zhao et al., Nat. Commun. 13, 3970 (2022)

Antoine Kahn bio.

• Ph.D., Princeton University, 1978
• M.S., Electrical Engineering, Princeton University, 1976
• Diploma of Engineer in Electronics, Institut National Polytechnique de Grenoble, 1974
Stephen C. Macaleer ’63 Professor in Engineering and Applied Science
Vice Dean, School of Engineering and Applied Science
Associated Faculty in the Princeton Materials Institute (PMI)

My research programs center on the electronic, chemical, structural and electrical properties of materials relevant to thin-film electronic devices. My interests span a range of semiconductor materials (elemental and compounds), but my current work focuses specifically on organic molecular and polymer semiconductors, dielectrics developed for applications in organic and molecular electronics, and the hot new class of optoelectronic materials called hybrid metal halide perovskites (MHP). Our group is particularly interested in engineering materials and interfaces that improve the performance of devices, with application to organic light-emitting diodes (OLEDs), field-effect transistors (OFETs), organic photovoltaic cells (OPVs), MHP-based solar cells and light-emitting diodes, and other thin-film devices applicable to large-area, flexible electronics.
On the organic scene, the quasi-infinite possibilities for chemical synthesis of new organic molecular compounds, combined with the unmatched ease of fabrication of organic semiconductor films by vacuum evaporation, liquid processing or printing on a variety of substrates, give organic semiconductors key advantages over other semiconductor materials, and open tremendous opportunities for innovation in device structures. Our research spans fundamental issues of electron-hole interaction in molecular semiconductors; chemistry and electronic structure of metal-organic and organic-organic heterojunctions; physics, implementation and impact of chemical (n- and p-) doping to control conductivity and carrier injection.
On the metal halide perovskite side, our work contributes to a better understanding of surface and interface properties of these fascinating materials. We are investigating their interfaces with metal oxides and organics (small molecules and polymers alike), which are all central to device performance.
Our group is involved in extensive collaborations with synthetic chemists, theoreticians, and device physicists in the US, Asia, and Europe, in academia, national laboratories, and industry. Our approach involves a variety of spectroscopic techniques for determining electronic structures, charge carrier transport measurements, morphological and structural tools, and device fabrication.