Picosecond luminescence approach to charge transfer in a tetraphenylporphyrin/SnO 2 interface

Abstract

The decay time of the luminescence from mono-molecular and excimer states in very thin tetraphenylporphyrin (H 2TPP) films evaporated on SnO 2 substrates has been measured in situ in a pressure of 2 × 10 −6 Pa using a picosecond dye laser at 100 K. The decay time of the 665 nm luminescence in H 2TPP on a highly conductive SnO 2 substrate (σ = 77 Ω −1 cm −1 is 800 ps in films with a thickness above 25 Å and decreases with decreasing thickness to 170 ps at 7.5 Å. Contrarily, the decay time for films on low conductive SnO 2 (σ = 4.5 × 10 −3 Ω −1 cm −1) is independent of film thickness. The luminescence quenching ratio calculated from the decay time coincides well with that from the relative luminescence intensity. This coincidence leads to the conclusion that luminescence quenching occurs through the mono-molecular excited state or the excimer state. We propose the following mechanism to explain these experimental results. First, a conduction electron of the highly conductive SnO 2 substrate recombines with a hole of the H 2TPP excimers resulting in a charge transfer state. Second, an electron is transferred from the excimer state of H 2TPP to the conduction band.