Ligand-dependent transient absorption studies of hybrid polymer:CdSe quantum dot composites

In this work, fourier transform infra-red (FTIR), photoluminescence (PL), optical and transient absorption (UV-vis & TA) spectroscopies have been used to monitor charge transfer from excited conjugated polymer (MEH-PPV) to smaller sized (5-7 nm) monodispersive CdSe quantum dots (QD) prepared by the chemical route using different capping ligands as trioctyl phosphine oxide (TOPO) and oleic acid (OA). CdSe (OA) QD's (similar to 7 nm) owing to its relatively smaller surface energies compared with CdSe (TOPO) QD's (similar to 5 nm) shows lesser quenching capabilities within the MEH-PPV polymer matrix. These studies demonstrate dominance of respective processes of photoinduced Forster energy transfer between host polymer (donors) and guest CdSe nanocrystals (acceptors) in polymer:CdSe(OA) nanocomposites and charge transfer in polymer:CdSe(TOPO) nanocomposites. Due to limitations of our nanosecond laser flash photolysis apparatus, transient absorption signatures of positive polaron of MEH-PPV could not be detected since it exhibits characteristic absorption in the mid-IR region. However, signatures of transient corresponding to CdSe-* radical in the visible region was seen only when the sample was excited in the presence of polymer (MEH-PPV). A model has been proposed, which elucidate the possibility to modulate charge/energy transfer rate between polymer and semiconductor quantum dots using a suitable ligand-exchange process. The various characterization techniques used in this work provide an insight into the charge separation, charge accumulation and/or trapping of charge carriers for the better understanding of hybrid organic-inorganic photovoltaics