A density functional theory study of dye-sensitized solar cells with graphene quantum dots: only a matter of size?

Abstract

In this work, we investigate the electronic interaction between graphene quantum dots (GQDs) of varying sizes, organic dye sensitizers and a TiO₂-cluster. The dyes included here have been previously investigated by means of density functional theory in models for dye-sensitized solar cells (DSSCs). It was shown that local hybrid functionals are highly suitable for the calculation of spectra and level alignments at the dye-semiconductor interface. Here, their assessment is extended to GQDs and their combination with dyes and TiO₂-clusters. The focus of this work lies in understanding the electron-transfer mechanism of GQD implementation in a typical DSSC. Our systematic approach includes investigating individual GQDs, their compatibility with dyes and a semiconductor (TiO₂) and at the end, the more complex system dye@GQD@TiO₂. The overall charge transfer mechanism depends crucially on the graphene sheet sizes, and the dyes can have versatile roles. That is, even without direct participation in the charge injection, they are vital for light absorption and exciton generation and thus facilitate charge injection into the semiconductor.