Capture of non-equilibrium carriers in polymer-nanocrystal composites

A. Shik

Centre for Advanced Nanotechnology, University of Toronto


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)