Bulk and contact-sensitized photocarrier generation in single layer organic devices

Abstract

In this paper, we report on the photoelectronic properties of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) studied in sandwich geometry. In particular, we have obtained from both forward and reverse bias measurements the μτ product for holes in TPD. μ is the hole mobility and τ is the carrier trapping time. The μτ product is a measure of the electronic quality of the material and allows a quantitative comparison of different samples. We have carried out numerical simulations to understand the photocurrent in these structures. We show that in reverse bias, the photocurrent (PC) is due to carrier generation in the bulk of the sample. The carrier generation is governed by field assisted exciton dissociation at electric fields greater than 106V∕cm. At lower fields the generation of carriers occurs spontaneously in the bulk of the sample. In forward bias, the photocurrent is due to exciton dissociation at the indium tin oxide contact. We also obtain a μτ product for holes from forward bias PC measurements which is in agreement with the value obtained from reverse bias measurements. Based on our experiments, we demonstrate that TPD in a sandwich structure is a good candidate for cheap large area visible blind ultraviolet detector arrays.