Electroluminescence and photoluminescence in poly(di-n-octylfluorene) light-emitting diodes operated at elevated temperatures

Both electroluminescence (EL) and photoluminescence from conventional poly(di-n-octylfluorene) (PF8) light emitting diodes (LEDs) has been monitored in situ during thermal cycling at temperatures up to 410 K. Even after the PF8 polymer undergoes a thermotropic phase transition to a liquid crystal phase these LEDs continue to operate although at reduced currents. This behavior is likely caused by changes in the hole transport layer and/or polymer interfaces and not by the phase transition itself. Overall EL output is diminished but PL measurements indicate no evidence of PF8 degradation. The step-wise thermal cycling of a working device produces a decrease in the relative EL efficiency at higher applied bias voltages but results in a pronounced EL efficiency increase at voltages near the device turn-on threshold. EL emission and measured device currents are both time- and temperature-dependent.