SubBand absorption in organic molecular semiconductors

In this paper we present results on subband absorption in two molecular semiconductors - tris (8 hydroxy -quinolato) Aluminium (Alq) and (TPD) in both the solid state and in solution. We show that at low energy, the absorption is governed by surface states in these semiconductors. Using photoluminescence excitation spectroscopy, we show that it is possible to to discriminate against the absorption due to surface states and measure bulk states in thin films. The absorption in the bulk is rdlated to defects absorption that arises due to a strong electron-phonon interaction. We identify the phonon modes responsible for the electron-phonon interaction in these systems.