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 (ε₂) and LUMO of molecule⋯TiO₂ complex and CB (conduction band) of bare TiO₂ (ε₃) 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.