Enhancement of structure and optical dispersion properties of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine thin films: Impact of UV irradiation

Abstract

Owing to the great importance of hole transport layer (HTL) in the performance of many optoelectronic and microelectronic applications, this report represents the investigations on the influence UV irradiation for 1, 3, and 5 min on some physical properties of a significant HTL known as N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) in the form of thin films. A stable amorphous structure with non-detectable IR variations in the molecular structure of the deposited films are observed in the XRD and FTIR results even under the irradiation for 5 min. But, the 1 min UV exposure caused a significant enhancement in the photoluminescence of TPD film with a conserved peak position at 437.5 nm. Furthermore, the surface morphology features are inspected by using the field emission electron microscopy technique (FESEM). The UV irradiation till 3 min induced stabilization of the film morphology and decreased the roughness of the films. The transmission, reflection, and absorption spectra of the pristine and irradiated TPD films are measured via spectrophotometric method from UV to NIR band. The spectral distribution of transmission is analyzed using the Swanepoel method to estimate the linear refractive index and film thickness. Moreover, the dispersion parameters of the films are estimated and interpreted in terms of the single oscillator model. The energy gap of the prepared films showed a little increase upon irradiation for 1 min (3.02 eV) and then decreased to 2.97 eV for 5 min irradiation. The values of molecular polarizability, plasmon frequency, Penn's energy, and Fermi energy are calculated for the pristine and irradiated films. Finally, the nonlinear refractive index and the third-order nonlinear optical susceptibility are estimated and the static value of both are found to be 9.667 × 10−13 esu, and 4.105 × 10−14 esu, respectively.