Exciton diffusion in light-emitting organic thin films studied by photocurrent spectra

Photocurrent spectroscopy is employed to obtain the exciton diffusion coefficient and diffusion length in two typical light-emitting organic thin films, the hole transport material N,N′-diphenyl-N,N′-bis(3-methyl-phenyl)-1,1′biphenyl-4,4′diamine (TPD) and the electron transport material tris(8-hydroxyquinolinolato) aluminum III (Alq3). The photocurrent spectra of both films at higher biases exhibit either symbatic or antibatic responses. However, complicated responses are observed for smaller biases. At certain small biases, the direction of the photocurrent even alters with the energy of illuminating photons. A model based on exciton diffusion and dissociation at film/electrode interfaces is used to explain these experimental results. Good agreement is achieved between experiments and theory by taking into account both symbatic and antibatic responses, especially at zero or small biases. The diffusion coefficient and the diffusion length of excitons derived are 1.53×10−3 cm2 s−1 and 17 nm for TPD and 4×10−5 cm2 s−1 and 8 nm for Alq3, respectively.