Femtosecond photoemission study of excited state dynamics in organic photoreceptors

The charge transfer (CT) process in organic semiconductor thin film structures is an important problem for applications such as photoreceptors and light-emitting devices. The operation of a photoreceptor structure is based on a CT process between a donor molecule and an acceptor transport molecule. We have investigated such a structure formed by vacuum-grown thin films of two organic molecules, N,N'-diphenethyl-3,4,9,10-perylenetetracarboxylic-diimide (DPEP) and N,N'-diphenyl-N,N'(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD), with femtosecond time-resolved photoemission spectroscopy (TR-PES). By measuring the lifetimes of the excited electron states in the mixtures of these molecules we are able to determine the timescale for the electron transfer (ET) between the excited states of the TPD and DPEP molecules. We show that the biexponential ET dynamics consist of a short component of less than 100 fs and a long component of several hundreds fs in length.