New pyran-based molecules as both n- and p-type sensitizers in semi-transparent Dye Sensitized Solar Cells

We report here on the synthesis of four novel pyran-based DSSC photosensitizers. The dyes are constituted of a pyran core functionalized with different electron acceptor groups and with peripheral cyano-acrylic groups. The molecules were carefully characterized for what concerns optical and electrochemical properties and a clear dependence from the strength of the electron-acceptor group functionalizing the pyran core was observed. A detailed DFT analysis on the dyes gave a better insight of the electronic properties and showed a good accord with the experimental properties. The dyes were used as photosensitizers both in n-type and p-type DSSCs. In the first case, the stabilization of the LUMO achieved partially prevented a fast kinetics of the electron injection. To overcome this, we used an I-based electrolyte containing an excess of LiI: the photoconversion efficiency significantly increases for all the dye-sensitized devices, up to a maximum of 2.1%. The dyes were successfully employed the as photosensitizers in p-type DSSC, obtaining PCEs approaching 0.1%, a promising result in the field on NiO-based devices. More remarkably, as far as we are aware, this is the first paper in which a single series of dyes is effectively employed as sensitizers in both n and p-type DSSCs.