Charge separation and transport in conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity

TLDR

The authors investigate charge separation and transport in composites of cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer MEH‑PPV, aiming to understand and control these processes. They employ a simple model that links recombination, absorption, charge separation, and transport to nanocrystal size, material, and surface ligands, allowing tuning of composite properties. Removing surface ligands quenches polymer photoluminescence, TEM reveals 10–200 nm phase segregation providing extensive interfaces, and thin‑film photovoltaic devices achieve up to 12 % quantum efficiency, demonstrating enhanced charge separation.

Abstract

We study the processes of charge separation and transport in composite materials formed by mixing cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer poly(2-methoxy,5-(2\ensuremath{'}-ethyl)-hexyloxy-$p$-phenylenevinylene) (MEH-PPV). When the surface of the nanocrystals is treated so as to remove the surface ligand, we find that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nanocrystal interface. Transmission electron microscopy of these quantum-dot/conjugated-polymer composites shows clear evidence for phase segregation with length scales in the range 10-200 nm, providing a large area of interface for charge separation to occur. Thin-film photovoltaic devices using the composite materials show quantum efficiencies that are significantly improved over those for pure polymer devices, consistent with improved charge separation. At high concentrations of nanocrystals, where both the nanocrystal and polymer components provide continuous pathways to the electrodes, we find quantum efficiencies of up to 12%. We describe a simple model to explain the recombination in these devices, and show how the absorption, charge separation, and transport properties of the composites can be controlled by changing the size, material, and surface ligands of the nanocrystals.