Charge trapping and electroluminescence at quantum dots embedded in a polymer matrix

We unambiguously demonstrate that mechanisms of photoluminescence (PL) and electroluminescence (EL) are different for CdSe/ZnS quantum dots (QDs) embedded in a polymer host. With increase in concentration of QDs, EL intensity increases exponentially when the impressed current is kept the same. In contrast, PL intensity shows only a linear dependence on the concentration of quantum dots. In the case of EL, the QDs of 3.2 nm diameter act as giant centers with a nearly temperature independent capture cross-section in the temperature range of 10-300 K. A phenomenological model of carrier capture is proposed in which the hole capture cross-section is exponentially distributed due to non-uniform distribution of QD particles in the host. We also show that EL yield and effective carrier mobility (mu(eff)) share identical non-Arrhenius temperature dependence for each concentration of embedded QDs. Possible origin of hole capture mechanisms are discussed in the light of these experimental observations. (C) 2012 Elsevier B.V. All rights reserved.