Pulsed Electroluminescence from Quantum Dot Light-Emitting Diodes

We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell layer and characterize the devices under dc and ac voltages. By applying an ac voltage, we achieve pulsed electroluminescence (EL) from these quantum dot LEDs (QDLEDs). As frequency of the applied ac voltage increases, we observe a phase-lag between the pulsed EL and applied sinusoidal voltage. From frequency response of the pulsed emission, we determine -3 dB frequency of the ac-QDLEDs. The frequency response of EL also provides a route to study mechanism of the LEDs. The EL spectra under dc and ac voltages provide information on carrier transport and exciton generation in these devices. From the resemblance between EL and photoluminescence spectra, which appear due to transitions between d-states of Mn-ions in doped nanocrystals, we infer that excitons form directly in the quantum dots in these QDLEDs.