Design of hybrid dye/colloidal nanocrystal composites for energy and charge transfer processes

The urgent demand for low-cost green clean energy has stimulated the research for new materials for energy storage and conversion. Colloidal semiconductor nanocrystals (NCs) were demonstrated as promising materials for light harvesting applications, while nanostructured titanium dioxide (TiO2) has been successfully exploited in electrodes for photovoltaics (PV). In particular, NCs with controlled size and shape, coupled and/or functionalized with suitable organic chromophores proved efficient energy transfer (ET) and charge transfer (CT) processes [3]. Understanding the events occurring at organic/inorganic interfaces in both ET and CT processes is crucial for the direct application of the developed materials in real devices. In this work, CdSe NCs of selected size, synthesized by hot-injection methods [4], were coupled with an organic chromophore, based on boron-dipyrromethene (BODIPY) dye, properly functionalized with amino groups to improve its affinity with NC surface. The spectroscopic investigations on such hybrid nanosystems in solution revealed a FRET process occurring between the NCs acting as donor to the conjugated dye molecules. A remarkable increase of the FRET efficiency was obtained for the coupled nanosystems deposited on substrate, with respect to the layered thin films of NCs and dye, indicating a favourable influence of the reduced interparticle separation. In addition, TiO2 NCs and a zinc phthalocyanine functionalized with carboxylic acid groups (PcTC) were selected to investigate CT processes, due to proper band alignment. The hybrid composite was prepared in solution and in solid state, by exploiting both simple physisorption and effective chemical bond formation after activation of the carboxylic acid functionalities. The quenching of the PcTC fluorescence suggested the emergence of CT. In particular, in the solutionphase nano-hybrids, CT efficiency and stability were sensitive to the nature of the solvent, as well as to the preparation strategy.