A. Shik
Centre
for Advanced Nanotechnology,
Semiconductor nanocrystals embedded in a
conducting polymer matrix are now actively investigated as promising materials
for light-emitting and photovoltaic devices. The capture of non-equilibrium
carriers (created in polymer by injection or optical excitation) by nanocrystals play the central role in operation of these
devices. In conducting polymers optical excitation exist usually in the form of
Frenkel excitons. A model
for their capture by semiconductor nanocrystals via
dipole-dipole interactions is developed. The transition probability W is found by calculating Joule energy
losses in the nanocrystal induced by an external
oscillating dipole. To find the net capture rate, the problem is solved
simultaneously with the diffusion equation describing generation and
recombination of excitons in the polymer matrix and
their transport to the nanocrystal interface. In real
conducting polymers the capture rate is often limited not by dipole interactions
but by diffusion transport so that the key condition for high efficiency of
light-emitting devices based on organic-inorganic nanocomposites
is that the diffusion length of excitons in the polymer
matrix exceeds the average inter-nanocrystal spacing.
In the case of electroluminescence, non-equilibrium electrons and holes
are injected and captured by nanocrystals separately
and, due to their different concentrations and mobilities,
result in charging of nanocrystals. Theoretical
description of this process requires analysis of the electron and hole drift towards nanocrystals in
a resulting electric field. Due to very low carrier mobility in polymers, this
drift delivery of carriers plays the role of “bottleneck” in capture processes
and details of energy relaxation inside nanocrystals
remain irrelevant.
Relevant references:
1. L.Bakueva, S.Musikhin, E.H.Sargent, H.E.Ruda, A.Shik “Luminescence
and photovoltaic effects in polymer-based nanocomposites”
– In: Handbook of Organic-Inorganic
Hybrid Materials and Nanocomposites (ed.H.S.Nalwa), American Scientific Publishers 2003, v.2,
p.181-215
2. A.Shik, S.Yu, E. Johnson, H.Ruda, E.H.Sargent “Carrier transport and luminescence in
composite organic-inorganic light-emitting devices” – Solis-State Electr. 46, 61-68 (2002)
3. A.Shik, G.Konstantatos, E.H.Sargent, H.E.Ruda, “Exciton capture by nanocrystals in a polymer matrix” – J.Appl.Phys. 94, 4066-4069 (2003)