The temperature dependence of photoluminescence and absorption spectra of vacuum-sublimed magnesium phthalocyanine thin films

A temperature dependence study of photoluminescence (PL) and absorption (A) spectra of vacuum-deposited magnesium phthalocyanine (MgPc) thin films are presented. The investigated layers were thermally sublimed on quartz glass substrates. PL and A studies were carried out in the temperature range from 13 K to 320 K. Absorption spectra measured in the wavelength range from 200 to 1200 nm show intense B, N and C bands in the UV region followed by a Q band in the visible region. The values of important transitions parameters (intensity, broadening, energy) were determined using the mixed Lorentz–Lorenz model for fitting the absorption spectrum. We have noticed that the positions and intensities of the observed bands depend on temperature. All the investigated samples exhibit weak luminescence in the near-IR region, which decreases with increasing temperature. We have found that the strong temperature dependence of the MgPc film’s PL spectra can be attributed to the weakening of the exciton–phonon interaction with decreasing temperature. The observed PL bands were fitted using a superposition of simple Gaussian functions. The changes in PL intensity, band position and broadening were investigated as a function of temperature. The temperature dependences have revealed mechanisms of energy transfer between different excited states and changes in molecular symmetry.