From black pigment to green energy: shedding light on melanin electrochemistry in dye-sensitized solar cells

Abstract

The sustainably obtainable poly indolequinone eumelanin shows exciting properties for green, natural dye sensitized solar cell (DSSC) applications: broad absorption until 800 nm, metal ion chelation, and long conjugated, aromatic structures featuring an abundance of quinone, semiquinone, and some hydroquinone moieties providing rich binding sites. Despite this, there are limited literature works covering the use of this natural dye that typically reported conversion efficiencies of less than 0.10%. Thus, there is room for improving both the performance and understanding of the electrochemical mechanisms behind eumelanin-based DSSC applications. This work fills this gap by first characterizing eumelanin to confirm its potential stability in films on TiO2 substrates, then providing theoretical calculations on the HOMO-LUMO gap and simulating the absorption spectrum, giving promising results for potential use as a dye in DSSCs, and finally covering new ground in the optimization of the fabrication process of eumelanin-sensitized DSSCs. The prepared eumelanin DSSC devices are of high cycling stability and show a maximum performance of 0.24% before and 0.42% after treatment with UV-light. The devices were analyzed in detail to give insights into the microscopic explanation of why eumelanin-based DSSCs differ from other natural dye-based devices. Using intensity modulated photocurrent and photovoltage spectroscopy, the comparatively high recombination rate of eumelanin in relation to other natural dyes is identified as the main inhibitor to overcome in future endeavors of optimizing eumelanin films in DSSCs.