Effect of process optimization on electronic properties of conjugated small molecules

Abstract

In this work, powders of a crystalline conjugated small molecule are thermally evaporated and electronic devices are fabricated to understand process property relationship. The conjugated small molecule possessing donor-acceptor-donor structure is deposited at various rates ranging from very low; 0.5 Å s−1 to as high as 10 Å s−1. It is observed that the chemical structure of 2,5-dithienyl-3,4-(1,8-naphthylene) cyclopentadienone (DTCPA) remains unaltered after thermal evaporation. It is also observed that there is a change in morphology and crystal orientation of the deposited films with a change in deposition rate. A growth mechanism concerning morphology is proposed based on the morphological and structural studies. As the deposition rate increases, there appears to be a gradual change in physical structure and molecular alignment. Optical properties such as UV-Visible absorption suggests that majority of the thermally evaporated films have considerable absorption spectra in the visible range. While photoluminescence studies suggest that 0.5 Å s−1 film showed large emission at 700 nm whereas, all other films showed large emission at 540 nm. Current density versus voltage characteristics of DTCPA showed that at higher evaporation rate the curve no longer acts as rectifying diode, which is further explained by an increase in ideality factor. The photoelectric responses of these films indicate that deposition rates and therefore the film morphology has a profound effect on the energy band level of the film as indicated by the extracted device parameters. The DTCPA films deposited at 2 Å s−1 exhibit the best response with an ideality factor of 3.8 and highest mobility of 0.27 cm2 V−1 s−1 suggesting optimum deposition rate is one of the critical parameters for device fabrication.