Characterization of charge accumulation on multiple interfaces in phosphorescent organic light-emitting diodes

A series of green phosphorescent organic light emitting diodes (OLED) were studied using I(V), admittance spectroscopy, and capacitance-versus-voltage, C(V), measurements. We found that both the logarithmic derivative of I(V) and C(V) spectra revealed two distinct peaks related to the build-up and consequent dissipation of charge on different interfaces of the device. The first peak is common for many types of OLEDs and is caused by the external built-in potential. The second peak is a feature attributed to the specific stacking sequence of the devices studied. We argue that the secondary charge build-up occurs at the interface between hole transport layer and emission layer due to (i) the strong mismatch of HOMO level of these layers and (ii) restricted direct injection of holes onto the phosphorescent dopant molecules. Consequent dissipation of the charge is caused by greatly-enhanced supply of electrons, which in turn is caused by exponential growth of electron mobility due to the Frankel-Poole effect. Our study shows that C(V), in conjunction with I(V), measurements are not only useful for model devices with two metal-organic interfaces, but can also characterize charge accumulation in complex, multi-interface OLEDs. We also observed a strong negative contribution to the capacitance at low frequency and high biases; the emergence of the negative capacitance correlates with the onset of light emission.