Optical and electrical properties of nanostructured N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine organic thin films

In this paper, we introduce an extensive study on nanostructured thin films of an organic small molecule N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine, (NTD). Further, the possibility of using NTD as a hole transport layer in optoelectronic devices is reported for the first time to the best of our knowledge. In this context, the thermal behaviour, crystal structure, optical absorption in ultraviolet-visible regions, and DC electrical conductivity of the as-deposited NTD thin films are investigated. The differential scanning calorimetry (DSC) investigations show that NTD has a phase transition at a high temperature of 190 A degrees C which may not affect its morphological stability. Further, the XRD patterns reveal that NTD thin films have an as-amorphous nature with some crystals. Additionally, optical investigations indicate that the indirect electronic transition from valence to conduction band is the most probable transition which agrees well with the amorphous structure of NTD thin films. The value of the mobility gap decreases from 2.74 to 2.51 eV when the thickness of the films increases from 80 to 200 nm. Furthermore, the absorbance lies below 370 nm and the thinnest film of thickness 80 nm achieves the highest absorbance. Further, the DC electrical measurements show that the NTD film has an activation energy of 378 meV. The DC conductivity is interpreted in terms of the variable range hopping (VRH) model. Based on our studies, NTD thin films are proposed with thickness range 50-80 nm to serve as a hole transport layer in white organic light emitting diodes (OLEDs).