Influence of thermal annealing on photoluminescence and structural properties of N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) organic thin films

Photoluminescence (PL) spectra and intensities of thin N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) films have been measured at room temperature, during sample heating with various rates and annealing times at constant temperatures, and after annealing. It was found that the temperature of T = 80 °C being considerably lower than the glass transition temperature of α-NPD is sufficient to cause substantial irreversible changes in PL and PL excitation characteristics. A PL efficiency increase up to 10 times, an emission spectrum short-wavelength shift up to 130 meV and a spectral narrowing from 69 to 39 nm are reached using annealing. The surface roughness of the films annealed at the moderate temperature of 80 °C does not undergo observable changes contrary to films annealed at higher temperatures.