The environment sensitivity of organic semiconductors may change their molecular structure and hence optical properties. Exploiting this concept, experiments were performed on a green light emitting material bis(8-hydroxy quinoline)Zinc, (Znq2) used in organic light emitting diodes (OLEDs). Thin films were deposited at varying deposition parameters, and their properties were compared. We investigated that as deposited films have a significant component of Znq2 tetramer out of two known forms, that is, dihydrate and anhydrous tetramer (Znq2)4, the films deposited at lower deposition rates have higher anhydrous content. The degradation of thin film is shown, that changes the optical properties of film from green emission to blue which may be due to water adsorption and crystallization. 1. Introduction Small molecules such as metal based quinoline derivatives like Tris-(8-hydroxyquinoline) aluminum (Alq3) have been shown to have high efficiency as well as stability in organic light emitting devices, OLEDs [1]. Potential of Zinc(II) bis(8-hydroxyquinoline) (Znq2) has been amply recognized in the literature [2–5]. Znq2 devices have shown advantages over Alq3 in electron transport and have higher quantum yields in device performance which results in lower operating voltages [2]. Further, the devices have been shown to be comparatively more stable under influence of high operating voltages [3–5]. It is shown that electroluminescence (EL) of Znq2 devices does not shift with operating voltage [2]. As Znq2 is a symmetric molecule, it does not have variegated isomers as found in tris-quinalato structures [6]. Thermal analysis of Znq2 powders by differential scanning calorimetry (DSC) demonstrates a sole oligomeric species and no polymorphism [7]. It has also been shown that thermal stability of zinc complexes is higher than other transport layers [8]. In order to take advantage of various properties of this molecule, it is important to understand and correlate its properties in powders with thin films. While the lack of polymorphism is an important attribute in device application, the structure of the molecules in thin film forms is poorly understood. However, no clear study has yet been done on morphology and properties of Znq2 as thin film. In solution or powder form Znq2 is known to exist as monomer with H2O group attached to it. It has been shown that oligomers, in particular, tetramer of Znq2, are energetically favorable as well in solid state [7, 9]. Though many reports have suggested that tetramers may be present in thin films and may be responsible for
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