Organic light-emitting diodes (OLEDs) are used in a number of commercial applications including small area consumer product displays, military applications, and emergency response equipment. In many display applications, OLEDs respond to a time varying bias as pixels are continuously cycled. Consequently, strategies that optimize frequency dependent charge transport in OLED displays will likely have broad technological
impact.
In previous work, negative capacitance (NC) was identified as an important parameter that influences frequency dependent charge transport in electronic and optoelectronic devices. In components exhibiting NC, the reactance is positive at low frequencies, and the resistance initially increases with frequency. NU-MRSEC researchers have characterized NC in OLED heterostructures using impedance spectroscopy.
Although similar inductive behavior has been previously reported for transient electroluminescence in OLEDs, definitive identification of negative capacitance in impedance spectroscopy data has been elusive due to the high concentration of distributed traps at the anode-organic interface. The addition of a layer of TPD-Si2 (4,4’ bis[(ptrichlorosilylpropylphenyl) phenylamino]-biphenyl) at this interface minimizes these trapping sites, thus enabling the inductive nature of charge transport in OLEDs to be directly observable. By quantitatively correlating the resulting impedance spectroscopy data with equivalent circuit models, a detailed description of charge transport in OLEDs as a function of heterostructure composition has been developed.
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