## 3 Nanocomposite Properties
### Nanocomposites for energy conversion  
Systems based on semiconductor NCs such as CdSe, ZnO, PbS or TiO\({}_{2}\) incorporated in conjugated polymers have recently interested the scientist community for their potential application in the field of energy conversion [120]. Semiconductor NCs can be coupled with conducting polymers for the fabrication of heterojunctions in which the high surface to volume ratio and high conductivity of NPs combine with the flexibility and versatility of the polymeric medium [122]. The resulting hybrid heterojunctions can exhibit high electron mobility or improved spectral coverage and can be therefore exploited for photovoltaic applications.  
[PERSON] _et al._ recently developed hybrid heterojunctions by blending nanocrystalline TiO\({}_{2}\) nanorods and high hole mobility polymer (poly(3-hexylthiophene), P3 HT) [123]. Anisotropic shaped NCs combined with the polymer properties can reduce the number of interparticle hopping events and therefore improve both electron and hole transport in the nanocomposite, resulting in enhanced performances of the photovoltaic device. The authors have studied the key parameters for enhancingheterojunction photovoltaic efficiencies, such as light absorption, charge separation, charge recombination, morphology and transport.  
Application of P3 HT in organic-inorganic hybrid as heterojunction-type containing II-VI semiconductor NCs as electron acceptors is at the basis of some recent papers of [PERSON] and co-authors [124; 125]. The synthesis of a functionalized P3 HT based polymer strongly interacting with the surface of 1-(6-mercaptohexyl)thymine capped CdSe NCs (CdSe(MHT)) has been described. In particular, a supramolecular association is formed between diaminopyrimidine functionalities of the polymer and thymine groups of the CdSe ligands, due to molecular recognition directed by the formation of hydrogen bond [124].  
A layer-by-layer (LbL) processing technique has been utilized to obtain nanocomposite film with adjustable thickness [125] as electrochemically active materials suitable for applications in optoelectronic and electrochemical devices. The investigated red-ox and spectro-electrochemical properties of the prepared thin films have demonstrated an appropriate band alignment for charge separation at the NCs/polymer interface. Preliminary tests of the use of such hybrids in organic/inorganic photovoltaic cells have been carried out, and morphological studies have pointed out the formation of a quasi-interpenetrating network which can assist their application in solar cells.  
A photoconductivity study performed on hybrid system formed by CdTe NPs randomly distributed in a thin poly[2-methoxy-5-(2'-ethyl-hexyloxy)]-1,4-phenylene vinylene (MEH-PPV) film has been performed by [PERSON] _et al._[126]. Highly sensitive interference grating technique has revealed that charge carriers are effectively separated across the interface between inorganic organic components, due to combined effects of band offset and band bending. Semiconductor NCs behave as efficient sensitizer when the CdTe content in the polymer is low, a photocurrent response has been measured with photon energy normally absorbed by the polymer. The increase of NC content in the matrix results in a second transport path that is spatially separated and electrically decoupled from the transport path in MEH-PPV. Transport in the polymer matrix has been found to be no longer ambipolar, and therefore long lifetimes, larger electrochemical potentials and higher mobilities can be achieved. Photocarriers can thus be efficiently separated and transported in such hybrid systems, as required for the fabrication of detector, sensor and solar cells.