Assessing the Performance of Eumelanin/Si Interface for Photovoltaic Applications

Among the possible applications of eumelanin (biolelectronics, biointerfaces, etc.), photovoltaics is a promising field which could highly benefit from its intrinsic features (broadband UV-vis absorption, etc.), with such a pigment employed as photoactive layer in hybrid solar cells. The envisioned application poses the need for a careful theoretical analysis on the adhesion properties of eumelanin on a substrate and the characterization of the hybrid system electronic features. In this work, we investigate a eumelanin/Si interface, where Si plays the role of inorganic layer. By means of ab initio calculations, we study the feasibility of the experimental formation process of eumelanin protomolecular structures in methanol ambient, evaluating the corresponding formation energy. Then, we explore the adhesion properties of eumelanin molecules on a silicon surface and extract the electronic structure of the resulting system. The corresponding band alignment is then used to address the overall photoconversion efficiency. Adopting the scheme of chemical disorder, which has been proved to successfully capture the variety of eumelanin protomolecules, we show that (1) the formation process of eumelanin protomolecules from the constituting monomers is generally hindered in a solvent environment with respect to vacuum and (2) key factors in improving the adhesion properties and band lineup of the molecules on an inorganic interface are the molecular electronic state and the planarity of their structures. Protomolecular models with a large number of nonterminated oxygen atoms and endowed with an intrinsically planar character tend to bind more strongly to the surface. In addition, they are more likely to produce a favorable band alignment for photoconversion applications.