It was investigated the influence of thickness on the photophysical and optoeletronic properties of poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)], F8BT. The active layer films were prepared by spin-coating solutions of the polymer, using glass as substrate. The thickness was varied from 50 nm to 600 nm. The photophysical properties of F8BT were studied in terms of emission spectra, the full width at half maximum (FWHM) of the spectra, the temperature dependence of the peak position of the 0-0 emission band and the vibronic progression of the emission spectra. The comparison of the photophysical results with optoeletronic device parameters enables us to optimize the active layer thickness. We noted that the power efficiency is lower for the thicker devices due to the higher operating voltages, and lower for thinner OLEDs due to the low luminance. As a consequence of this interplay between luminance and turn-on voltages, it was observed an optimum overall device performance in the intermediated thickness regime, 165-185 nm (considering the range of studied thicknesses), which can be explained in terms of mobility. Analyzing the photophysics we classify the F8BT films into two groups. Those with thicknesses between 52 nm to 185 nm have shown greater inhomogeneous broadening and very heterogeneous cybotatic environments for the fluorophores, while those with film thicknesses greater than 450 nm have shown lower inhomogeneous broadening (inom). The homogeneous broadening follows an inverse relationship with inom, that can be explained by the biaxial constraint generated by the substrate, causing a decrease in the rates of the processes of rotational or conformational relaxation in thinner films. This biaxial constraint is confirmed by the electron-phonon interactions parameter, that are much smaller for films with thinner thicknesses than those found for thick films up to 450 nm , indicating lower coupling efficiency. Since the magnitude of the electron-phonon interaction is related to transversal optical modes, we can conclude that, as it was proposed by homogeneous broadening results, thinner films has less freedom due to rigidity imposed by the biaxial constraint generated by the substrate. As a result, the charge carrier mobility increases and a better output performance can be derived. Even if in films with thickness below 165 nm the rigidity remains, we suggest power efficiency towards thinner layers is low because quenching becomes more pronounced since electrons and holes recombine closer to electrodes, which will lead to higher probability of excitons undergoing non-radiative decay near the cathode [1].
1. K. Narayan et al, Current Aplied Physics, 13 (2013), 18-25.