CHARGE SEPARATION AND TRANSPORT IN CONJUGATED POLYMER CADMIUM SELENIDE NANOCRYSTAL COMPOSITES STUDIED BY PHOTOLUMINESCENCE QUENCHING AND PHOTOCONDUCTIVITY

We study the processes of charge separation and transport in composite materials formed by mixing cadmium selenide nanocrystals with the con jugated polymer poly(2-methoxy, 5-(2'-ethyl)-hexyloxy-p-phenylenevinyl ene (MEH-PPV). When the surface of the nanocrystals is treated so as t o remove the surface ligand, we find that the polymer photoluminescenc e is quenched, consistent with rapid charge separation at the polymer/ nanocrystal interface. Transmission electron microscopy (TEM) of these quantum dot/conjugated polymer composites shows clear evidence for ph ase segregation, providing a large area of interface for charge separa tion to occur. Thin-film photovoltaic devices using the composite mate rials show quantum efficiencies which are significantly improved over those for pure polymer devices, consistent with improved charge separa tion. At high concentrations of nanocrystals, where both the nanocryst al and polymer components provide continuous pathways to the electrode s, we find short circuit quantum efficiencies of up to 12 %.