To study the optical properties of CsPbBr3 and Cs4PbBr6 NCs, absorption and photoluminescence (PL) measurements were performed on NCs dispersed in toluene (Figure ). The absorption edge of CsPbBr3 NCs appears at 505 nm, whereas for Cs4PbBr6 NCs, we observe absorption edges at 314 and 506 nm. According to the previous reports, the edge at 505 and 314 nm can be ascribed to the optical band gap absorption of CsPbBr3 and Cs4PbBr6 NCs. The additional absorption edge at 506 nm for Cs4PbBr6 NCs can be ascribed to the coexisting phase of the CsPbBr3. This coexisting phase of CsPbBr3 is also supported by the XRD analysis, as presented in Figure . For both NCs, a weak shoulder-like peak can be observed around 360 nm and is assigned to the lead bromide complex in the CsPbBr3 and Cs4PbBr6 lattice. This lead bromide complex was not detected in XRD analysis, possibly due to its low concentration. The band gap was calculated using a Tauc -equation, and the results are presented in Figures and. The calculated band gap for CsPbBr3 and Cs4PbBr6 are 2.38 and 3.53eV, respectively, which coincides with the previous report. The PL emission peak is at 517 and 520 nm for CsPbBr3 and Cs4PbBr6 NCs, respectively (Figures and). The CsPbBr3 and Cs4PbBr6 NCs exhibit an intense and narrower PL emission peak. The CsPbBr3 exhibits an emission peak with the fullwidth half maxima (FWHM) of ~17 nm, indicating a homogeneous size distribution of NCs. The broader FWHM of Cs4PbBr6 (~21 nm) is due to the larger particle sizes and wider size distribution than the CsPbBr3 NCs. The impact of the solvent on the optoelectronic properties of the NCs was studied by the absorption and PL measurement for the NCs dispersed in Sol. 1 and 2 (Figures ). The results of absorption and PL for CsPbBr3 and Cs4PbBr6 NCs in Sol. 1 and 2 are summarized in Table . The absorption edge and PL peak of CsPbBr3 and Cs4PbBr6 NCs in Sol. 1 and 2 exhibited slight shifts compared to toluene. These shifts can be attributed to the change in the excitonic binding energy, which is induced by the change in the dielectric constant of the solution when the EtOH concentration is increased. The change in the polarity of the solution also induces FWHM broadening and a significant decrease in PL intensity, indicating the decomposition of the NCs. Indeed, as the polarity of the solvent increases, the ligands are detached from the NCs more easily, compromising their structural integrity. Hence, when the polarity increases, the NCs are destroyed much quicker.