3.6. Tunneling mechanism
From the current-voltage characteristics, we have observed
that the turn-on voltage of the LEDs that depended on the active
nanoparticles could be related to the bandgap of the AIZS
nanoparticles. As we tuned the bandgap of the nanoparticles by
decreasing the ratio of the content of silver and zinc, M[Ag]/M[Zn],
the band-edges may shift with an increase in the bandgap. In a
device based on such nanoparticles, this would mean a change in
internal barriers for the carriers.
Considering the fact that holes contribute to device current in
these LEDs, the barrier for hole injection from the electrode can
be determined from the current–voltage characteristics. With
the current in the devices being injection limited, the tunnelingcurrent can be described as
J ¼ ðAβ2E2=φÞexpðBφ3=2=βEÞ
Here, A and B are constants, β the field-enhancement factor, E the
electric field, and φ the barrier height. The equation reduces to ln
(J/E2
)p1/E with a factor of φ3/2 being the proportionality constant.
Plots of ln(J/E2
) versus 1/E (Fowler–Nordheim plots) for the LEDs
have been presented in FigLEDs based on these nanoparticles so that the EL emission appears
from the radiative decay of excitons in the nanoparticles. The
results demonstrated LEDs based on nontoxic quantum dots along
with the ability to tune the EL emission wavelength by choosing
the zinc-content of the nanoparticles.
Acknowledgment
The authors acknowledge financial supprt from the DeitY
project 12(1)/2012-EMCD and the DST project DST/TSG/PT/2008/
46. The work of SB and AG were funded by CSIR – Junior Research
Fellowship Nos. 09/080(0692)/2010-EMR-I (Roll no. 507693) and
09/080(0505)/2006-EMR-I (Roll no. 503974), respectively. AJP
acknowledges Ramanna Fellowship SR/S2/RFCMP-01/2009.
A comparison of the plots for devices with different AIZS
nanoparticles as the active material shows that the slope of the
plots at high-fields that is proportional to φ3/2 remained invariant.
This implies that the hole-injecting barrier, which is dictated
by the work-function of the ITO electrode and the highest
occupied molecular orbitals (HOMO) of the hole-transporting
TPD layer, did not differ in these devices. This is expected since
all the devices had TPD as the hole-transport layer on ITO
electrodes.