Crystallization of Organic Materials and its Effect on Device Performance

Abstract

The field of organic devices has rapidly developed over the past couple of decades, with a growing need for light, flexible, and low-cost devices, particularly for displays and biomedical applications. With more applications and higher demand, the performance of these devices has become increasingly important. Normally deposited in bulk, organic devices’ active and transport layers have very low mobility, languishing between 10-4 and 10-2 cm2 /Vs, which is the limiting factor in their performance [1]. Using crystalline organic materials could lead to higher mobilities; however, unlike their inorganic counterparts, little is known about the crystallization process of organic thin film materials. The primary goal of this thesis was to develop large-grain crystal thin films of common organic active and transport layers. This has already been done for rubrene by using a 5 nm underlayer of tris[4-(5- phenylthiophen-2-yl)phenyl]amine (TPTPA), and this project aims to bring the same success observed with rubrene to other organic materials [2]. The secondary goal of the project was to make prototypical devices, specifically an organic light-emitting diode (OLED) and a solar cell, to observe differences between devices with amorphous versus crystallized transport layers. Five materials were tested to determine if they show any promise of forming large-grain crystals. These materials are TPTPA, 1,3,5-Triazo-2,4,6-triphosphorine2,2,4,4,6,6-tetrachloride (TAPC), Bathocuproine (BCP), Bathophenanthroline (BPhen), and N-Bis(naphthalenyl)-bis(phenyl)benzidine (NPB). We were able to crystallize thin films of NPB using a 5 nm underlayer of the Triphenylamine derivative (TPD). After optimizing the crystallization process, we made an OLED and a solar cell and observed differences in the electrical properties of the amorphous and crystalline devices.