Transient electroluminescence in heavy metal complex-based phosphorescent organic light-emitting diodes

Abstract

In phosphorescent organic light-emitting diodes (PHOLEDs), both the rise time and decay time decrease with increasing amplitude of the applied voltage pulse. The rise time τ r of the transient electroluminescence (TEL) increases linearly with increasing value of the ratio of voltage V to the current j, that is, with V/ j. Using the equations for the dynamics of charge carriers an expression is derived for the rise time τ r of the TEL in OLEDs. It is shown that τ r should increase with increasing values of the ratio ( V/ j), dielectric constant ε, and area of cross-section of the emission layer, however, it should decrease with the thickness of emission layer. For higher values of the applied voltage nonlinearity occurs in the τ r versus V/ j plot because the increase in mobility of carriers at high electric field causes increase in the current flowing through the OLEDs. In fact, the rise time of TEL is related to the product of capacitance and effective resistance of the OLED. Considering the rate of generation and decay of radiative triplet excitons in the emission layer, an expression is derived for the decay time of TEL in PHOLEDs and it is shown that, for higher values of the time-constant of OLED, the decay time should be equal to the time-constant, however, for lower values of the time-constant, the decay time should be equal to the lifetime of radiative triplet excitons in the emission layer. A good agreement is found between the theoretical and experimental results.