Effects of substrate temperature on properties of Alq3 amorphous layers prepared by vacuum deposition

Abstract

ABSTRACT In this report, various organic Alq3 amorphous layers are prepared by vacuum deposition at different substrate temperatures T sub (from 30 to 180  ). The surface morphology, structural info rmation, electrical and optical properties of these as-deposited layers are investigated by atomic force microscopy, X-ray diffraction, J-E curves, and photoluminescence studies, respectively. Furthermore, a temperature dependence of dark electrical conductivity 1(T ) deduced from J-E curves of these organic amorphous layers is presented. Finally, effects from T sub on the physical properties of these organic Alq3 amorphous layers are discussed and a model based on a thermal interconversion between Alq3 isomers is proposed to explain these experimental results. Keywords : Organic amorphous layer; Vacuum deposition; Electrical conductivity; Surface morphology; Photoluminescence; Isomer ic transformation. 1. INTRODUCTION Due to the processability advantages offered by organic mate rials, an intense research effort has been devoted to the preparation of organic thin-film field-effect transistors (OTFTs) during the last two decades [1-10]. The active layer in an OTFT is fabricated of small organic molecules [1-4], conjugated oligomers and polymers [5-10], or organic-inorganic hybrids. The performance of OTFTs involves the intramolecular and intermolecular charge transport mechanisms which are strongly affected by the structural and morphological characteristics of the as-deposited organic layer. Generally, organic layers from small molecules or conjugated oligomers [1-5] are prepared by vacuum deposition on cold substrate. These organic amorphous layers are not macroscopically homogeneous because of growth morphology defects [11]. In addition, during vacuum depo sition in mass production, the latent heat released from vaporized molecule to its condensed phase may increase the real temperature of the unheated “cold” substrates (especially when a plastic substrate of poor thermal conductivity is used). From an external control and monitoring of the substrate temperature T