Computational design of concomitant type-I and type-II porphyrin sensitized solar cells

Structures and electronic properties of porphyrins with various Donor (D)-Acceptor (A) functionalization adsorbed on TiO2 (anatase) nanoparticles are studied using DFT calculations. Adsorption of porphyrin leads to a substantial loss of planarity (puckering) for the porphyrin rings due to the stabilization of the system by the interaction of the lone-pair of electrons of the N-atoms of porphyrin with the Ti-atoms of anatase. For free porphyrin, the mode of binding to anatase is from the top-site, while for the binding of D-A functionalized porphyrin, side-wise interactions are stabilized via anchoring groups on the A (-SO3H, -PO3H2 and -CO2H). Adsorption of porphyrin on TiO2 changes the relative ordering of HOMO and LUMO levels compared to that of the free molecule and bare TiO2 nanoparticles which critically effects their performance for dye sensitized solar cells (DSSCs). The relative energy differences between the LUMO of the free molecule and LUMO of molecule...TiO2 complex (epsilon(2)) and LUMO of molecule...TiO2 complex and CB (conduction band) of bare TiO2 (epsilon(3)) are proposed as two key parameters for determining the suitability of the material for functioning as a DSSC material. Coupling of the porphyrin ring with nanoparticles leads to the appearance of additional optically-active states due to dye -> TiO2 charge-transfer (CT) transitions. This leads to a possibility for these dyes to act as type-II DSSC materials as well. We suggest that there exists no such general rule that only different sets of molecules are suitable for type-I and type-II DSSCs. Concentration dependent UV-Vis absorption spectra measurements can be a simple experimental test to detect a mechanistic switch-over between type-I and type-II DSSC processes in dyes.