Luminescence quantum yield of molecular aggregates and excitons in alpha-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 a s a function of temperature in the range 30-300 K. The emission from the in trinsic bulk excitons and from the molecular aggregate states, is identifie d in the photoluminescence spectrum at each temperature. The corresponding absolute quantum yield of photoluminescence is measured with a home-built e xperimental apparatus based on an integrating sphere, which allows photolum inescence and electroluminescence quantum yield measurements in the tempera ture range 5-400 K. The photoluminescence quantum yield of the molecular ag gregates placed below the exciton band span from 0.1% to 5% in the temperat ure range 300-30 K. In the same temperature range, the quantum efficiency o f the intrinsic excitons increases by a factor of 2 from 0.4% to 1%. Theref ore, in organic thin films both the spectral and quantum yield properties d epend on the relative concentration of molecular aggregates formed during t he 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., contai ning different concentration of aggregate states. (C) 2000 American Institu te of Physics. [S0021-8979(00)07621-0].