Hybrid Organic-Inorganic Nanocomposite Materials for Energy Harvesting and Energy Conversion Applications

The development of hybrid materials based on a nanoscale combination of organic and inorganic semiconductors is a promising way to enhance the cost-effectiveness of solar cells through a more efficient exploitation of the solar spectrum and a better control of such a critical feature as the topological distribution of the interfaces within the active photovoltaic (PV) layer. The maximum theoretical efficiency of PV devices depends on the selection of components with appropriate electronic levels, while the practically achievable efficiency is strongly related to the architecture of these materials, and therefore on the methodologies available for the preparation of the active PV layer. In the case of hybrid PV layers based on conjugated polymer and CdSe nanocrystals, the latter act as electron acceptors and possibly photoabsorbers, while the conjugated polymer is an efficient photoabsorber (although within a relatively narrow spectral window of solar light), electron donor, and hole (or electron) carrier. An important feature of inorganic semiconducting nanocrystals (or quantum dots, QDs) is that they are characterized by a band gap that can be controlled simply by adjusting the size of the nanoparticle. On its part, conjugated polymers offer the possibility to build devices based on ultrathin flexible and continuous films that require relatively simple processing conditions and would allow the fabrication of devices with a variety of shapes. However, achieving a uniform and topologically well controlled dispersion of QDs in the polymer matrix is still a critical issue, affecting the volume density of the interface, and thus the probability that an exciton (photoexcited hole-electron couple) undergoes the required charge separation upon hitting the interface within the hybrid active PV layer before charge recombination or other exciton deactivation processes take place. In this research, three different routes have been explored to obtain a dispersion of CdSe QDs within a polymer matrix based on the conjugated poly(3-hexylthiohene) (P3HT). These involve either the controlled growth and/or the stabilization of pre-formed CdSe nanoparticles by using templating or stabilizing polymers, as described in Chapter 6. The three different stabilizing systems, separately described in Chapters 3, 4 and 5, are based on, respectively: a)phosphonic acid end-functional regioregular P3HT; b)block copolymers of P3HT with the templating poly(acrylonitrile) (PAN); c)Multi-walled carbon nanotubes (MWNTs) grafted with templating PAN.