3.3. Luminance — current density characteristics
Performance of the devices can be viewed best when luminance
is plotted against the device current (Fig. 3b). While the
turn-on current did not differ in the devices, EL at any device
current depended strongly on the nanoparticles in the active layer.
The devices that yielded a lower device current have acted as an
efficient LED. That is, the internal barriers that assist in forming
excitons determine the EL of the devices. The devices based on
manganese-doped AIZS nanoparticles have been the most efficient
ones primarily due to a large bandgap and more importantly dueto an involvement of d-states of the dopants during the radiative
decay process of the electro-generated excitons.
3.4. External quantum efficiency
For further analysis of the devices, we have measured the
external quantum efficiency (EQE) of the devices. The EQE of the
LEDs has been calculated from the internal quantum efficiency
(ηint), which is defined as the ratio of the number of photons
produced in the device to the number of electrons injected to it.
The EQE is related to the internal quantum efficiency as:
EQE¼ηint Re, where Re is the extraction efficiency representing
the number of photons transmitted out of the structure per
number of photons generated. The EQE versus current density
plots, as presented in Fig. 4, show that the EQE for the devices
increases with the device current. The efficiency depended on the
active material, that is, the AIZS nanoparticles. The device with
manganese-doped nanoparticles yielded the highest EQE at any
current. This should be due to the involvement of d-states of the
dopants in the PL emission of the nanoparticles and hence the EL
emission of the devices. At a higher device current, the EQE of the
devices saturate with a trend of a decline. This could be due to the
effect of the heat generated in the devices and also due to an
enhanced Auger loss or imbalance in electron and hole injections
leading to a space-charge build-up in the devices.
3.5. Electroluminescence spectra
We have also recorded the EL spectrum of the devices.
The normalized EL spectra, as presented in Fig. 5, show that color
tuning in the visible spectral range could be achieved in these
LEDs. The EL spectrum of a device matches the PL emission of thecorresponding nanoparticles. The results further show that there is
no other passive EL emission from the TPD layer. This implies that
the EL emission appeared only from the nanoparticles. Excitons
generated in the TPD layer must have transferred their energies to
the nanoparticles that subsequently decayed radiatively.