Increased Below-Bandgap Absorption in CsPbBr₃/Cs₄PbBr₆ Through Post-Annealing and Regrowth

Lead halide perovskites have attracted much attention for optical devices such as light-emitting diodes, solar cells, and photodetectors due to the high light-absorption coefficient and the tunable bandgaps by adjusting the halide ratios or changing the compositions. Among many kinds of lead halide perovskites, CsPbBr₃ has exhibited high external quantum efficiency (EQE) and remarkably narrow line-width of photoluminescence (PL). Similarly to other lead halide perovskites, however, the optical properties of CsPbBr₃ are also affected by the fragility against moisture, illumination, and heating, which limit further application in photonics and optoelectronics. To overcome the instability, CsPbBr₃/Cs₄PbBr₆ structures, where CsPbBr₃ nanocrystals embedded in Cs₄PbBr₆, have recently become a research hotspot. In addition, anti-Stokes light emission that occurs at higher energy than the excitation energy has been confirmed in the CsPbBr₃/Cs₄PbBr₆ composites, and it suggests a possibility to realize optical refrigeration[1]. Although anti-Stokes PL (AS-PL) is essential to achieve the optical refrigeration, the absorption coefficient decreases exponentially with energies below the bandgap energy. Assuming the EQE of the AS-PL is almost 100%, the optimal excitation energy for cooling is reported to be 2.33 eV[2], and increased light absorption at the excitation energy is critical to obtain large amount of AS-PL. In this study, we report on enhanced light absorption of CsPbBr₃/Cs₄PbBr₆ structures under below-bandgap excitation by increasing the crystal sizes and thermal annealing technique.<br/>CsPbBr₃/Cs₄PbBr₆ crystals were grown by following procedures, CsBr powders and PbBr₂ powders (molar ratio 3:1) were dissolved in a fixed mixture of <i>N</i>, <i>N</i>-dimethylformamide (DMF) and aqueous solution of HBr (volume ratio 5:4) at 100°C. At first, the solution was cooled down to 50°C with 12 hours, and micro-sized crystals were precipitated at the bottom of a glass vial. To grow seed CsPbBr₃/Cs₄PbBr₆ crystals, we decreased the temperature of the solution to 40°C with the rate of 2.5°C per day, and subsequently cooled it to room temperature with the rate of 5°C per day. After the precipitated CsPbBr₃/Cs₄PbBr₆ crystals were dissolved in a mixture of DMF and HBr at 80°C to prepare a supersaturated solution, and a CsPbBr₃/Cs₄PbBr₆ seed crystal was placed at the bottom of the glass vial. The solution was then cooled to room temperature with a rate of 10°C per day. A large CsPbBr₃/Cs₄PbBr₆ crystal (0.725 g) with more than four times as heavy as the seed crystal (0.179 g) was successfully grown after the twice regrowth-process. AS-PLs were clearly observed in both seed and regrown crystals, and the PL spectra and the EQEs indicated that the regrown crystal maintained crystal quality same as the seed crystal. Furthermore, it was found that the light absorption under 2.33 eV excitation increased from 4.4% to 15.6% (around 4-fold) reflecting the sample sizes. In addition, the size of CsPbBr₃ nanocrystals embedded in Cs₄PbBr₆ can be controlled by proper annealing process[3], and it would improve the absorption coefficient and control the optoelectronic and photovoltaic properties. We conducted thermal annealing for the regrown CsPbBr₃/Cs₄PbBr₆ crystals at different annealing temperature (ranging from 125°C to 300°C) for a minute in ambient air atmosphere. The PL peaks after the annealing processes exhibited red-shifts as the annealing temperature increased. Consequently, the optimal annealing condition for the material was determined to be 200°C, where the light-absorption was enhanced maintaining high EQEs. We concluded that the regrowth technique and annealing treatment for CsPbBr₃/Cs₄PbBr₆ crystals were quite effective to achieve large amount of AS-PLs towards optical refrigeration.<br/>[1] Y. Kajino <i>et al., Phys. Rev. Mater.</i> <b>6</b>, L043001 (2022).<br/>[2] M. Fukuda <i>et al.,</i> The 42nd Electronic Materials Symposium, <b>Th1-22</b> (2023).<br/>[3] T. Bai <i>et al., RSC Adv. </i><b>13</b>, 24419 (2023).