Effect of solution concentration on the morphology, electrical, and optical properties of MEH-PPV thin films

Polymer and organic light-emitting diodes (OLEDs), photovoltaic cells and field effect transistor are being focused towards commercialization. Poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]or known as MEH-PPV, have been extensively studied for OLED applications. In this study, MEH-PPV powders were dissolved in toluene and 1, 2-dichlorobenzene (1, 2-DCB) at different solutions concentration and deposited using spin coating technique. Surface morphologies of the films were obtained using Atom Force Microscope (AFM) and thickness using surface profiler, while the electrical properties were investigated using 2-point probe. The optical properties were characterized using UV-Visible-NIR (UV-VIS-NIR), and photoluminescence (PL) spectrometer. The surface roughness, Ra of the films increased with the concentration for both types of solvent, however for films in toluene, as the concentration increased above 5mlmg^-1, the roughness decreased which is caused by the aggregation of the MEH-PPV. Films' thicknesses increased as well as the resistivity for both types of solvents. However, for MEH-PPV thin films in 1, 2-DCB show better conductivity compared to toluene. PL spectra of films using toluene are red-shifted due to longer conjugation length, and also show higher intensity compared to films using 1,2 DCB.