Strategies to Control Crystal Growth of Highly Ordered Rubrene Thin Films for Application in Organic Photodetectors

Abstract

Organic semiconductors still lag behind their inorganic counterparts in terms of mobility due to their lower structural order, in particular in thin films. Here, the highly ordered phase of triclinic rubrene – characterized by high vertical hole mobility – grown from a vacuum‐deposited thin film is used by post‐annealing and implemented into organic photodetectors. Since the triclinic rubrene exhibits a high roughness with a peak‐to‐valley value of 250 nm, which is detrimental to the dark current, strategies to control the crystal growth are developed. These investigations show that a suppression layer of 20 nm C60 is the most promising approach to successfully reduce the surface roughness while maintaining the triclinic phase, proven by grazing‐incidence wide‐angle X‐ray scattering (GIWAXS). With the smoothened active layer, the dark current density is reduced by three orders of magnitude compared to the neat rubrene layer. It is as low as 2.5 × 10−10 A cm−2 at −0.1 V bias, reflected in an overall specific detectivity of 6 × 1011 Jones at zero bias (based on noise measurements) and a high linear dynamic range of 170 dB. This strategy using a suppression layer thus proves successful and is very promising to be applied to other crystalline materials.