Luminescence quantum yield of molecular aggregates and excitons in α-sexithienyl thin films at variable temperature

The energy transport and photoluminescence quantum yield in vacuum sublimed thin films of the alpha-sexithienyl (T-6) model system, are investigated as a function of temperature in the range 30-300 K. The emission from the intrinsic bulk excitons and from the molecular aggregate states, is identified in the photoluminescence spectrum at each temperature. The corresponding absolute quantum yield of photoluminescence is measured with a home-built experimental apparatus based on an integrating sphere, which allows photoluminescence and electroluminescence quantum yield measurements in the temperature range 5-400 K. The photoluminescence quantum yield of the molecular aggregates placed below the exciton band span from 0.1% to 5% in the temperature range 300-30 K. In the same temperature range, the quantum efficiency of the intrinsic excitons increases by a factor of 2 from 0.4% to 1%. Therefore, in organic thin films both the spectral and quantum yield properties depend on the relative concentration of molecular aggregates formed during the growth process. We reconcile in this way the wealth of photoluminescence spectra and quantum yield values reported in the literature for T-6 films grown under different conditions and with different thickness, i.e., containing different concentration of aggregate states. (C) 2000 American Institute of Physics. [S0021-8979(00)07621-0].