Temperature-Dependent Photoluminescence and Carrier Dynamics of CsPbBr3 Quantum Dots: Ligand-Mediated Electron-Phonon Coupling and Trap State Energetics in Solution versus Film
Temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy were employed to quantify the divergent optoelectronic behavior of identical CsPbBr3 quantum dots (QDs) in colloidal solution and thin-film states. The electron-phonon coupling strength in solution is approximately twice that of the film, with average phonon energies extracted from one-photon absorption (OPA) and two-photon absorption (TPA) reaching ~38 meV and ~32 meV in solution, respectively, versus ~22 meV and ~16 meV in the film. Given that the dominant intrinsic phonon mode of CsPbBr3 resides at 18 meV, these elevated energies implicate organic ligand phonons in the radiative recombination pathway of the solution phase. TRPL measurements reveal room-temperature luminescence lifetimes of 22.5 ns (solution) and 5 ns (film), both exhibiting anomalous increases with rising temperature, consistent with thermally activated trap-state carrier release. Fitting yields trap energy levels of ~20 meV in the film and ~4 meV in solution. The deeper traps and reduced electron-phonon coupling in the film are attributed to exciton localization and diminished wavefunction overlap with ligand phonons, a consequence of ligand detachment and inter-QD interactions during film formation. These findings establish ligand morphology as a critical regulator of electron-phonon interactions and non-radiative pathways in CsPbBr3 QDs, providing quantitative design rules for solution-processed optoelectronic devices.