Optoelectronic properties of (Z)-3-(4-(4,5-diphenyl-1H-imidazole-2-yl)phenyl)-2-phenylacrylonitrile films under acid and thermal environments for tuning OLED emission

Abstract

A systematic study of changes on optoelectronic properties of (Z)-3-(4-(4,5-diphenyl-1H-imidazole-2-yl)phenyl)-2-phenylacrylonitrile (DPimdPPA) based films owing to treatments with acid or temperature, is reported. The DPimdPPA films treated with vapors of HNO3, HCl, or CH3COOH exhibited a hypsochromic shift up to 70 nm as well as an enhancement in the fluorescence quantum yield. Interestingly, a similar behavior was observed when films were thermally treated at temperatures between 100 °C and 240 °C. XRD studies showed that DPimdPPA in powder form is a polycrystalline material, while the as-deposited films were amorphous. However, the treatments with temperature or acid, induced crystallinity in the films. Studies by atomic force microscopy showed that the induced crystallinity modified the film morphology leading to the formation of agglomerates with needle like shape. These changes in fluorescence were used to tune the emission in OLEDs. Notably, devices with pristine DPimdPPA film as well as treated with HNO3 vapor or heated at 240 °C were prepared. The emission with pristine films was warm white with CIE coordinates (0.45, 0.43); with HNO3 treatment it was green with CIE coordinates (0.28, 0.40) and, when treated at 240 °C the emission was greenish-blue with CIE coordinates (0.35, 0.48). Besides, with the treatments the turn-on voltage decreased, and luminous parameters were enhanced; the maximum luminance was about 5400 cd/m2 while the highest luminance efficiency 5.2 lm/W.