Optoelectronics with Carbon Nanotubes

Let us finally comment on the efficiency of the CNT LEDs. Measurements of the PL
efficiency of SWNTs yielded values up to 77, 79 ~1 × 10 -2 , whereas in our devices we obtain at
most ~10 -4 photons per injected electron-hole pair. This difference of about two orders of
magnitude can be understood by taking the following two factors into account. First, only a
fraction of electrically induced electron-hole pairs possesses the right spin to populate radiative
singlet exciton states; [1 3 exp( /kT)] times as many populate non-radiative triplet states (k is
the Boltzmann constant, T = 300 K, and Δ is the singlet-triplet splitting). Using literature values
33, 34
for Δ, we estimate that this effect reduces the efficiency by about an order of magnitude.
Second, the short non-radiative lifetime leads to an efficiency reduction by another order of
magnitude. Possible routes for improving the efficiency hence would be brightening of the
triplet states – for example, by adding magnetic nanoparticles 143 – and/or suspending the CNT to
increase the non-radiative lifetime.
4. Conclusions
P-n junction devices were created with single carbon nanotubes using top split gates that
electrostatically control spatially-separated p- and n- regions. The transport shows a clear
rectifying behavior when the gates are biased in opposite voltages. Compared to conventional
ambipolar devices as seen in devices with a global back gate 73, 74 , the p-n junction thus created
and controlled emits light much more efficiently and at low input power, leading to a narrow
linewidth that approaches that of photoluminescence.
The narrow spectra allow us to analyze emission characteristics in greater detail than was
previously possible. In some devices, two peaks associated with the E11 transition were
observed, separated by ~65 meV. We assign the higher-energy peak to free excitons and the
lower-energy peak to weakly localized excitons. The latter may be bound to local potential
fluctuations, such as defects. As we increase current, we observe a saturation of emission
intensity from such localized excitons, suggesting a higher exciton density and a local
annihilation mechanism by exciton-exciton interaction. We also observe no shift in the free
exciton peak, which indicates a constant local charge density as expected in the recombination
region of the p-i-n structure. Conversely, the lower-energy peak red-shifts with increased
80