Nanostructured Materials for Photovoltaic Applications: a Theoretical Study

We present a theoretical investigation of the main aspects that characterize two novel photovoltaic devices: the dye sensitized solar cells and all carbon bulk heterojunction cell. In particular, firstly we studied the attachment of usually employed anchoring groups and a novel and promising one (squaric acid) to a prototype surface (rutile-TiO2(110) surface) to determine the lowest energy adsorption mode and discussing the electronic properties of the resultant hybrid interface by means of density functional theory (DFT) calculations using the hybrid exchange (B3LYP) functional. Secondly, we present a theoretical investigation of the attachment of the hemi-squaraine dye (CT1) to the anatase-TiO2(101) and ZnO(1-100) surfaces. This molecule can be considered as prototypical dye for use in dye sensitized solar cells (DSSCs), which present the squaric acid moiety as anchoring group to determine the lowest energy adsorption mode and discussing the electronic properties of the resultant hybrid interface by means of density functional theory (DFT) calculations. We find that CT1 adsorbs dissociatively at both the TiO2 and ZnO surfaces giving a type II (staggered) heterojunction. Compared to ZnO surface, TiO2, due to the greater hybridization of its conduction band states with the unoccupied molecular orbitals of the dye, is expected to enhance performance when employed with CT1 in DSSCs. Regarding the all-C BHJ, we analize the relation between stochiometry and the opto-electronic properties in amorphous carbon and hydrogenated amorphous carbon thin films to predict their employment as active layer in innovative photovoltaic devices. The electronic and optical properties of a large statistical set of structures are explored by means of firstprinciples molecular dynamics and electronic structure calculations, correlating structural features such as the density, concentration of sp2 and sp3 hybridized C atoms, and H content to the density of states, Tauc and mobility gaps, and optical absorption. Our work suggests strategies to tune and control the bonding geometry and the optical and electronic properties in amorphous carbon, and highlights the promising features of this material as valid substitute to carbon based nanostructured materials.