Direct Characterization of Type‐I Band Alignment in 2D Ruddlesden–Popper Perovskites

Abstract

2D Ruddlesden–Popper halide perovskites have attracted considerable attention due to their desirable optoelectronic properties, high chemical and structural tunability, and improved environmental stability. However, the understanding of their structure–properties relationships is still limited. In particular, the energy level positions and band alignments at interfaces involving these materials, which are important features to control in the context of any applications, are still under debate. Here, the electronic structure of high‐purity films of BA2MAn−1PbnI3n+1 for n = 1–5 (where BA stands for butylammonium and MA for methylammonium) is investigated, using optical absorption, ultraviolet, and inverse photoemission spectroscopies, and density functional theory calculations. This study determines the ionization energy and electron affinity of each compound and demonstrates a type‐I band alignment for the BA2MAn−1PbnI3n+1 series. This study further describes the evolution of the exciton binding energy as a function of the thickness of the inorganic layers.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

“…The establishment of protocols for efficient synthesis of phase-pure 2D HPs 133,134 could open more opportunities in this regard. 135 For a broad overview of the excitonic properties of the archetypal RP perovskites, readers interested are referred to a comprehensive review by Righetto et al on the subject. 136 One aspect of excitons that is frequently overlooked is the influence of the polar lattice on the exciton itself.…”

“…Compared to phase-pure RP perovskite with n = 1, excitonic properties of 2D HPs with n > 1 layers are challenging to characterize directly by PL and absorption techniques due to the lack of phase-purity. The establishment of protocols for efficient synthesis of phase-pure 2D HPs , could open more opportunities in this regard . For a broad overview of the excitonic properties of the archetypal RP perovskites, readers interested are referred to a comprehensive review by Righetto et al on the subject …”

High Layer Number (n = 1–6) 2D Ruddlesden–Popper Lead Bromide Perovskites: Nanosheets, Crystal Structure, and Optoelectronic Properties

“…Homologous n series of RP perovskites have been shown to have type-I band alignment, which induces carrier funneling and photon recycling from low to high n phases in phase-impure RP perovskite mixtures. 37,38 This effect likely explains why the PL spectra imply a pure or majority high n phase, in contradiction to the absorbance spectra. These results attest to the superior accuracy and sensitivity of absorbance in the spectroscopic determination of the compositions of mixed phase RP perovskite samples.…”

“…47,48 It has been previously reported that the increase from n = 1 to n = ∞ for lead iodide perovskites results in a type-I, nested bandgap alignment. 37,38 A type-I bandgap alignment is practical for LED applications because carriers can funnel from RP perovskites with the largest energy (smallest n) to lower energy (highest n). 4 Here, we confirm the same type-I alignment trend with the bromide-based RP perovskites (Figure 4c), which provides further support for our previous spectroscopic arguments.…”

“…Despite indication of phase impurity in PL spectra, absorbance spectra of n = 4, 5, and 6 samples display a clearly visible peak from n = 3 phase impurity (marked with * in Figure b), which formed as a minor layer component in the nanosheets during the layer-by-layer growth process. Homologous n series of RP perovskites have been shown to have type-I band alignment, which induces carrier funneling and photon recycling from low to high n phases in phase-impure RP perovskite mixtures. , This effect likely explains why the PL spectra imply a pure or majority high n phase, in contradiction to the absorbance spectra. These results attest to the superior accuracy and sensitivity of absorbance in the spectroscopic determination of the compositions of mixed phase RP perovskite samples .…”