Mechanistic origins of excitonic properties in 2D perovskites: Implications for exciton engineering

As the field of 2D halide perovskites (HPs) matures, state-of-the-art techniques to measure important properties, such as the band gap (Eg) and exciton binding energy (Eb), continue to produce inconsistent values. Here, we tackle this long-standing problem by obtaining direct measurements of Eg and Eb for 31 unique HP structures. The Eb values are lower than in previous literature reports and lower than expected from standard theory that assumes excitons are screened by optical-frequency dielectric constants. These low Eb values are shown to be a consequence of unique screening effects, such as superlattice screening and phonon screening. We find a strikingly strong correlation between Eb and Eg and provide design principles to a priori tune Eg and Eb to their optimal values. As such, this work offers a blueprint for Eg-Eb engineering of low-dimensional semiconductors as an even more useful replacement for simply band-gap engineering. [Display omitted] •Direct measurements of Eb for 31 unique perovskite structures•A strong correlation between Eb and Eg is identified and shown to be useful•We find that a superlattice model accurately describes the exciton properties•Dielectric constants and theory show that excitons are partially screened by phonons The properties of excitons underpin the absorption, emission, and energy transfer properties of quantum well systems. For GaAs-based quantum wells in the 1980s and 1990s, mapping the relationships between the exciton properties and the quantum well structure allowed control of these optoelectronic properties and ultimately led to invention of blue-light-emitting diodes and the 2014 Nobel Prize in Physics. For quantum well systems of today, such as 2D perovskites, there is a large variance in the reported values of the exciton’s properties, both measured and calculated. Here, we solve this issue by obtaining unambiguous measurements of the exciton properties. By repeating these measurements on a large number of structures, we map the relationships between exciton properties and the underlying quantum well structure. This allows us to determine the correct theory for calculating exciton properties and ultimately enables control of these properties. Exciton properties dictate the optical and electronic behavior of low-dimensional semiconductors; however, these properties have remained uncertain for 2D perovskites because of conflicting reports. This study uses spectroscopic tools to precisely measure exciton proper