Abstract

The degree of order and interaction between molecules is a crucial factor in determining the photophysical and charge-transporting properties of organic materials. Strong intermolecular interactions may be favourable for charge transport but reduce the efficiency of luminescence. We explore this trade-off using a family of conjugated dendrimers in which the generation number (G) provides an elegant way of controlling the degree of interaction between chromophores. For low generation numbers a red component of the electroluminescence (EL) spectrum due to inter-molecular excited states is observed (Fig. 1). As the generation number is increased, reducing interaction between the emissive chromophores, this feature disappears, and the spectrum resembles the solution PL spectrum The effect of the increase in localisation of excitations is clearly seen in the transport properties of LEDs based on these materials: the EL quantum efficiency increases exponentially with generation and the majority carrier current-flow is inhibited. Time-of-flight measurements show highly dispersive transport and a strong increase of transit times and photocurrent decay transients with generation. The increase in hopping distance gives rise to a decrease in mobility by almost 2 orders of magnitude as the generation is increased. As seen in Fig. 2 this gives rise to a scaling of the mobility with dendrimer radius comparable to the concentration dependence of mobility previously observed in doped polymer host systems [1]. The dendrimer generation hence provides a unique method for tuning the intermolecular electronic overlap integral in undiluted systems.

© 2000 IEEE