Correlation between the PL and EL emissions of polyfluorene-based diodes using bilayers or polymer blends

The electroluminescent (EL) properties of two polyfluorene-based (poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT)) organic light-emitting diodes (OLEDs) were investigated. Two diode configurations were tested: blends with emissive layers of PFO (higher weight percentage, from 98 to 99.8 wt.%) and F8BT (lower weight percentage, from 0.2 to 2 wt.%) and bilayers of PFO/F8BT. To understand the diode performances, we compared the photoluminescence (PL) and the EL emissions of both configurations. PL emission decays showed that the fluorescence resonant energy transfer (FRET) efficiency is higher for blends that have less of the F8BT component than in bilayers. For the bilayer systems, however, the FRET efficiency is approximately 50%, confirming the significant inter-penetration of these two polymers. Upon decreasing the F8BT concentration in the blend or the F8BT film thickness in the bilayer, the EL emission shifted toward the blue color due to a more significant contribution from the PFO emission. The EL color can be tuned between green (from the F8BT) and blue (from the PFO) shades by adjusting the polymer composition. The differences between the PL and EL band profiles were discussed.