Probing Fabry–Perot Interference in Self-Assembled Excitonic Microcrystals with Subgap Light Emission

Abstract

Establishing a fundamental understanding of the range of optical effects possible in self-assembled excitonic materials remains crucial to the utility of these materials in future optoelectronic and photonic technologies. In this study, we report the observation and subsequent analysis of Fabry–Perot interference intrinsic to two types of self-assembled excitonic crystalline materials: the hybrid organic–inorganic perovskite-like quantum well superlattice structure of hexyl ammonium lead iodide and the charge-transfer cocrystal of anthracene and pyromellitic dianhydride. The observation of Fabry–Perot interference stems from strong suboptical gap photoluminescence (PL) from both materials in a spectral region of very low material absorption. We characterize this subgap PL in each material to propose permanent defect trap excitons, stabilize their energy, and cause subgap light emission in both materials. We use a model developed to explain interference in the photoluminescence of vacuum-deposited thin films to estimate the thickness, mid-bandgap index of refraction, and surface roughness of our samples. These results indicate that self-assembled excitonic materials may have use in applications such as laser amplifiers, frequency discriminators, and single photon emitters.

Kinetic Molecular Cationic Control of Defect-Induced Broadband Light Emission in 2D Hybrid Lead Iodide Perovskites

“…For example, various group report different subgap PL spectra depending on the sample structure and morphology of hexyl ammonium lead iodide (HA 2 PbI 4 ). In particular, while Booker et al 20 report a relatively weak subgap PL feature centered at 650 nm and possessing a bandwidth of 150 nm emitted by thin films of HA 2 PbI 4 in its orthorhombic Pbca phase, Sanni et al 18 report a significantly more intense spectrum centered at 750 nm and possessing a nearly 300 nm bandwidth in single crystals of this material in its monoclinic P2 1 c structure.…”

“…12 In addition to this intense light emission assigned to the recombination of the free exciton, several studies report the appearance of PL caused by the recombination of trapped excitons and individual charge carriers. [13][14][15][16][17][18][19] Despite this broad assignment, the characteristics of the spectra emitted by hybrid SAQW samples vary with many conditions, even for the same chemical compound. For example, various group report different subgap PL spectra depending on the sample structure and morphology of hexyl ammonium lead iodide (HA 2 PbI 4 ).…”

“…Researchers propose these spectra can stem from both local chemical and macroscopic, mor-phological differences between SAQW samples. For example, recent studies suggest similar subgap PL spectra can be caused by chemical impurities such as metal and halide interstitials and vacancies, 13,[16][17][18][19]21 solution impurities, 22 and exciton self-trapping induced by molecular cations. 1,3,[23][24][25] In addition, other studies indicate the presence of electronic states on the edges and dislocations of crystals correlate with broadband, subgap light emission from SAQW samples possessing the Ruddlesden-Popper structure.…”

“…Inspection of the panel (a) of Figure 3 shows the intensity of the broadband emission increases by nearly 3 orders of magnitude as we cool samples formed from the 1.5 mM HexA/methanol solution to 78 K, which conforms with similar measurements we made on defective samples formed through alternative synthetic methods. 17,18 In contrast, we only observe the appearance of a weak, yet distinct broadband light emission feature when we cool a HA 2 PbI 4 sample formed from the 0.3 mM HexA/methanol solution to 123 K. Additionally, we find the intensity of this feature only increases by a factor of 2 as we cool the sample to 78 K. Despite the qualitative differences in the temperature-dependent intensity of the subgap PL spectra in panels (a) and (b) of Figure 3, the fact that we observe some subgap light emission from both samples indicates the broadband PL competes with relaxation processes activated by thermal energy, as shown by Figure 3(c). Other authors have shown features similar to the subgap PL we report in this study increase significantly with decreasing temperature due to reductions in the probability of crossing over energy barriers between electronic or excitonic states.…”

Kinetic Molecular Cationic Control of Defect-Induced Broadband Light Emission in 2D Hybrid Lead Iodide Perovskites

“…12 In addition to this intense light emission assigned to the recombination of the free exciton, several studies report the appearance of PL caused by the recombination of trapped excitons and individual charge carriers. [13][14][15][16][17][18][19] Despite this broad assignment, the characteristics of the spectra emitted by hybrid SAQW samples vary with many conditions, even for the same chemical compound. For example, various group report different subgap PL spectra depending on the sample structure and morphology of hexyl ammonium lead iodide (HA 2 PbI 4 ).…”