Optical Properties of Organic/Inorganic Perovskite Microcrystals through the Characterization of Fabry–Pérot Resonances

F. Ramiro-Manzano; R. García-Aboal; R. Fenollosa; S. Biasi; I. Rodriguez; P. Atienzar; F. Meseguer

doi:10.1039/D0DT02254C

Research · Publication & note

August 17, 2020 August 26, 2026

Optical Properties of Organic/Inorganic Perovskite Microcrystals through the Characterization of Fabry–Pérot Resonances

Journal article

F. Ramiro-Manzano, R. García-Aboal, R. Fenollosa, S. Biasi, I. Rodriguez, P. Atienzar and F. Meseguer

Dalton Transactions Volume 49 Issue 36 12798–12804 2020

Abstract

A precise knowledge of the optical properties, specifically the refractive index, of organic/inorganic perovskites, is essential for pushing forward the performance of the current photovoltaic devices that are being developed from these materials. Here we show a robust method for determining the real and the imaginary part of the refractive index of MAPbBr₃ thin films and micrometer size single crystals with planar geometry. The simultaneous fit of both the optical transmittance and the photoluminescence spectra to theoretical models defines unambiguously the refractive index and the crystal thickness. Because the method relies on the optical resonance phenomenon occurring in these microstructures, it can be used to further develop optical microcavities from perovskites or from other optical materials.

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Figures

Figure 1 from Optical Properties of Organic/Inorganic Perovskite Microcrystals — **Figure 1.** Schematic representation of the OT (a) and PL (b) experiments. (b1) Effect of the pump and PL propagation losses over the forward PL response. Here, the contributions to the emission of two slices of the active material, excited by P1 and P2 pump intensities, are highlighted. (c) Calculated transmittance (black curve) of an asymmetrical thin film cavity (r₁₂ = 0.2, r₂₃ = 0.6, nᵣ = 2.4). Three spectral regions corresponding to three different scenarios, one with moderate propagation losses (a = 0.44, red colour background) and another without losses (a = 1, blue colour background) and a transition region between them with intermediate losses (white colour background) have been considered. The transmittance T is limited by T+ (blue line) and T− (green line). These two limits comprise an average transmission T_av (dashed red line).

Figure 2 from Optical Properties of Organic/Inorganic Perovskite Microcrystals — **Figure 2.** Transmittance (blue line) and Normalized PL (red line) spectra for different objectives: N.A. 0 (a), N.A. 0.26 (b), N.A. 0.4 (c), N.A. 0.5 (d) for a perovskite layer of 3.96 μm thick. The refractive index and extinction dispersion have been extracted from the fit of experimental data (see Fig. 4).

Figure 3 from Optical Properties of Organic/Inorganic Perovskite Microcrystals — **Figure 3.** Calculated (a) Transmittance and (b) Normalized PL spectra of a MAPbBr₃ thin film as a function of the layer thickness for a collection objective of N.A. = 0.26 where (a1)–(a4) and (b1)–(b4) represent respectively 1, 2.5, 4 and 20.85 μm sample thickness. The refractive index and extinction dispersion have been extracted from the fit of experimental data (see Fig. 4).

Figure 4 from Optical Properties of Organic/Inorganic Perovskite Microcrystals — **Figure 4.** (a) Optical microscopy image of the measured MAPbBr₃ crystal microcavity (square structure at the center of the image). (b) and (c) Experimental data and fitted curves for the OT and the PL spectra respectively. Resonant modes are indicated on top of their resonance peak by their mode-order, m. (d) Real and (e) imaginary part of the fitted refractive index and that of ref. 27 for comparison. (e) Finesse and (f) Quality Factor for all of the identified modes. The lines act as a guide to the eye.

Research fields

Bottom-up

Solution-grown MAPbBr₃ microcrystals act simultaneously as the material under study and as naturally formed planar cavities. Their parallel crystal faces generate Fabry–Pérot resonances without adding a separately fabricated resonator.

Simulations & fits

Transmittance and photoluminescence are fitted together with analytical cavity models that include absorption, dispersion, sample thickness and the numerical aperture of the collection optics. This joint treatment extracts both parts of the refractive index around the electronic band edge.

Characterization

Angle-collecting microscope objectives record complementary transmission and emission spectra from the same microcrystal. Tracking how their resonance envelopes respond differently to numerical aperture provides an internal check on the fitted thickness and optical constants.

RESEARCH TOPICS

EmissionPhotoluminescence is not fitted as an isolated material band. Light generated at different depths inside the MAPbBr₃ crystal is attenuated by the absorbing perovskite and undergoes repeated reflections before leaving the cavity. Consequently, PL and optical transmission contain the same Fabry–Pérot phase term but have different spectral envelopes. Figure 4 shows that one refractive-index model can reproduce both signals from a 3.96 μm crystal, which is more restrictive than fitting either spectrum alone and helps distinguish cavity modulation from the intrinsic emission profile.

InterferenceParallel crystal faces form a Fabry–Pérot cavity whose fringes depend on wavelength, thickness, absorption and collection angle. Figure 3 maps the calculated transmission and PL as thickness varies, showing why some crystals display clear resonances while others with the same material do not. A finite numerical aperture averages contributions from different wave-vector angles and progressively reduces fringe contrast; the optical-path length also suppresses the blue side of the PL. The analysis therefore treats interference visibility as a coupled geometrical and optical problem rather than a simple quality label.

MaterialsThe method retrieves both real and imaginary parts of the refractive index from an individual hybrid perovskite microcrystal, avoiding the grain boundaries and orientational averaging of a polycrystalline film. The measured sample in Figure 4 is a planar MAPbBr₃ crystal, 3.96 μm thick on glass. Its transmission fixes absorption and phase accumulation, while PL tests how the same constants act on internally generated light. This single-crystal approach exposes resonant behaviour that can disappear when many differently shaped grains contribute to a macroscopic spectrum.

ResonatorsThe crystal itself is the resonator: no deposited mirrors are required because reflections at the glass–perovskite and perovskite–air interfaces provide optical feedback. Thickness selects the longitudinal mode spacing, and absorption controls how many round trips remain visible. By fitting transmission and luminescence simultaneously, the study determines crystal thickness and complex refractive index without assigning arbitrary peaks. The resulting model is also a design tool: it predicts which thickness and collection aperture will preserve useful cavity contrast in future perovskite microcavities.

ChemistryMAPbBr₃ microcrystals are grown from 1 M methylammonium bromide and 1 M lead bromide in DMF, using N-cyclohexyl-2-pyrrolidone as an additive, and spin-coated onto quartz at 800 rpm. Those chemical and processing choices produce isolated crystals with sufficiently planar, parallel faces for the optical model. The chemistry is therefore relevant not as a generic perovskite label but because solvent coordination, additive-assisted crystallisation and stoichiometry determine the morphology on which the Fabry–Pérot analysis depends.