No abstract

No abstract

Organic light-emitting diodes (OLEDs) using the red phosphorescent emitter iridium(III)bis(2methyldibenzo[f,h]quinoxaline) (acetylacetonate) [Ir(MDQ) 2 (acac)] are studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off is found, which is attributed to electron accumulation on Ir(MDQ) 2 (acac) molecules. The mechanism is verified via impedance spectroscopy and by application of positive and negative off-voltages. We calculate the density of accumulated electrons and find that it scales linearly with the doping concentration of the emitter. Using thin quenching layers, we locate the position of the emission zone during normal OLED operation and after voltage turn-off. In addition, the transient overshoot is also observed in three-color white-emitting OLEDs. By time-and spectrally resolved measurements using a streak camera, we directly attribute the overshoot to electron accumulation on Ir(MDQ) 2 (acac). We propose that similar processes are present in many state-of-the-art OLEDs and believe that the quantification of charge carrier storage will help to improve the efficiency of OLEDs.

During the last decade interest in dye-sensitized solar cells (DSC) has grown enormously. Electrical impedance spectroscopy (EIS) is an electrochemical technique commonly used for investigation of charge carrier dynamics in these photovoltaic devices. We used EIS for characterization of our DSC; moreover, symmetric cells with counter-counter or photo-photo electrodes were realized and measured in order to simplify impedance cell analysis by separating the contribution of counter and photoelectrode, respectively. In particular, we provided very accurate illumination of the photoelectrode symmetric cell, aiming to approach the experimental conditions of a complete cell. By fitting of experimental data, we obtained values for the charge transfer resistance at the counter-electrode and the electron percolation time at the photoelectrode. Moreover, the simulation of a whole cell, combining the data from the fitting procedures above, was in good agreement with experimental data

No abstract