Under 3 min of illumination, the CsPbBr3 NCs produced a significant number of electrons trapped by BMPO, resulting in a maximum EPR signal in both solutions. The signal intensities are 22.8 and 19.1 times higher than the reference measurement in the dark for Sol. 1 and 2, respectively (Figure ). After 6 min of illumination, the EPR signal is changed, and its intensity has decreased by ~45%. This significant signal decay may be caused by the decomposition of the NCs.