Crystalline-Size Dependence of Dual Emission Peak on Hybrid Organic Lead-Iodide Perovskite Films at Low Temperatures

Abstract

In this work, we have investigated the crystalline-size dependence of optical absorption and photoluminescence emission of CH 3 NH 3 PbI 3 films, which is necessary to identify the potential practical applications of the gadgets based on perovskite films. This study was carried out at low temperatures to minimize the extra complexity induced by thermal effects. The purpose was to clarify the origin of the dual emission peak previously reported in the literature. We found that the grain size is responsible for the appearance or disappearance of this dual emission on CH 3 NH 3 PbI 3 at low temperatures, whereas we have inferred that the thickness of the perovskite layer is a much more important factor than the size of the grains in the location of the energy of the band gap. Moreover, the increase in the grain size allows slowing down the phase transition. Additionally, we evidence a decrease in the effective Rydberg energy of the exciton in several samples, from 23−25 meV at 7 K to 12−13 meV at 165 K, by fitting to Elliott−Toyozawa theory. We have extracted other important physical parameters of perovskites from the photoluminescence-data deconvolution, such as the band gap, exciton−phonon interaction, and exciton binding energy. A new phase transition at 45.5 K was determined by the temperature dependence of full width at half-maximum and the integrated intensity of the photoluminescence, and it was confirmed by the radiative lifetime obtained from the time-resolved photoluminescence emission by means of time-correlated single-photon counting at different temperatures, excitation fluencies, and emission energies.

Short Photoluminescence Lifetimes Linked to Crystallite Dimensions, Connectivity, and Perovskite Crystal Phases

“…It can be recognized as a peculiarity lying in a sharp reversal of the continuous/monotonous variation of the lifetime. These characteristic minima agree with the observed shifts, drops, broadenings, and disappearances of the temperature-dependent data of the PL peak energy of the two bands 4 and could be originated from the phase transition. 44 The different lifetimes obtained for the different morphologies by using the GBSe 30 can be linked to different recombination channels (see Figures 3 and 4 a): The emission with the longest lifetime, τ LG , on the order of several to 10 ns is dominant in the configuration of large grains (352 nm-CL, see Figure 3 and solid green triangles in Figure 4 a).…”

“…Only for 103 nm-IS, there is no coexisting crystallographic phases. The details of the PL spectra can be found in refs ( 1 ) and ( 4 ). Figure 2 a–f shows the representative groups of normalized PL transients registered under relatively high excitation fluencies (370 nJ/cm 2 ) for the three types of morphologies studied: 550 nm-CS, 352 nm-CL, and 103 nm-IS.…”

“…Samples have been grown using a spin-coating solution processing method. The details of the process can be found in refs ( 1 ) and ( 4 ):…”

Mechanisms of Spontaneous and Amplified Spontaneous Emission in CH3NH3PbI3 Perovskite Thin Films Integrated in an Optical Waveguide

“…In the case of our own experimental data in Fig. 4(a), the best fitting parameters are E g = 1.643 eV, (R * y ) ≈ 12.5 meV (with an excitonic linewidth of around 30 meV), which is consistent with a similar fitting procedure to experimental results in the temperature range of 150-300 K (tetragonal phase of MAPI) [45].…”

Halide Perovskite Nanocrystal Emitters

“…[ 54,55 ] In addition, the grain size of conventional CH 3 NH 3 PbBr 3 bulk crystals can also determine which band structure dominates the overall PL spectra in between different structural phases (e.g., tetragonal‐to‐orthorhombic phase transition) at low temperature (e.g., 40–50 K), generating a dual‐color emission when the same weight in both bands is achieved upon an optimum grain size of about 100 nm. [ 56 ] Recently, Wu et al clarified that dual‐color emission is even a common character of all lead halide perovskites, but the relative intensity of the low‐energy peak (i.e., at longer wavelength) strongly depends on the surrounding conditions, such as the environmental temperature, and can lead to a nondistinguishable second peak in some cases. [ 57 ] The authors confirmed the existence of indirect tail states below the direct transition edge due to the dynamic Rashba splitting effect in APbBr 3 (A = CH 3 NH 3 + , CH(NH 2 ) 2 + , Cs + ) single crystals, which was originated from the thermal polar distortions of the PbBr 6 octahedra at increased temperatures.…”