Perylene Diimide-Induced Conformational Modulation in Polyfluorene and Its Impact on OLED Device Performance

Abstract

In this work, a solution-processable approach is presented to tune the electroluminescence (EL) characteristics of blue polymer organic light-emitting diodes (OLEDs) through controlled dopant-polymer interactions. The electroluminescence performance and spectral distribution of a poly(9,9-dioctylfluorenyl-2,7-diyl) end-capped with a polyhedral oligomeric silsesquioxane (BE)-based device are modulated using PVK and PVK/mCP (3:1 wt %) as hole-transporting layers (HTLs) together with trace amounts (0.1 wt %) of perylene diimide (PDI) derivatives with contrasting molecular geometries; i.e., planar PDIref and sterically hindered PDI1. The HTL- and PDI-containing devices exhibit up to 6-fold enhancement in external quantum efficiency (EQE) and a 9-fold increase in luminance compared with pristine BE devices. In addition, the emission spectrum becomes significantly broader, with a full width at half-maximum of 120-132 nm compared with 41 nm for bare BE, accompanied by excitation-induced shoulders in the 440-520 nm region. Raman and optical measurements suggest that these changes originate from PDI-induced modulation of the polyfluorene backbone conformation; the polymer chains evolve toward a more planar geometry without the formation of a distinct beta phase, instead an intermediate conformational state between the alpha and beta forms.