A New Class of Solar Cells Based on Stable Protein-Dye Hybrids

Design and operating principle of the prototype of an MspA-
based “hybrid soft cell” consisting of an (Ru-Diad)8MspA 
double layer adsorbed onto nanosized TiO2
Credit: Ayomi Perera.
For decades, the high level of organization in the photosynthetic process has dazzled researchers. Attempts to incorporate the light-harvesting proteins involved into functioning devices with long-term stability, however, have been unsuccessful so far. Now, researchers at Kansas State University and the University of North Texas have reported the development of a new kind of photovoltaic device based on the highly stable protein MspA.

The researchers recognized that the key to functional protein solar cells was thermal stability, so they relied on mycobacterial porin MspA. “MspA is an incredibly stable protein, even surviving temperatures of up to 220°C, and indefinitely stable at room temperature,” explains Stefan Bossmann, professor of chemistry at Kansas State University. The researchers capitalized on this knowledge, and in a highly interdisciplinary effort with Francis D’Souza’s group at the University of North Texas, produced a prototype solar cell with incident-photon-to-current-conversion efficiencies of up to 1%.

In order to achieve this result, MspA mutants expressing cysteine residues were produced and linked to eight ruthenium dyes bearing maleimides via Michael addition reactions. Additionally, MspA self-assembles into a nanostructured supramolecular complex on titanium dioxide, forcing the light-harvesting dyes into a well-defined geometry. This type of hierarchical structure is reminiscent of the protein’s organization in nature.

As reported recently in the Journal  of the American Chemical Society, the two groups then used time-resolved emission measurements to determine that MspA is capable of supporting electron transfer between the dyes and TiO2. Lastly, the group produced Grätzel-type Dye-Sensitized Solar Cells and showed that the ruthenium dye–protein hybrids on TiO2 had a higher photoconductance compared with ruthenium dyes on TiO2 or MspA on TiO2.

The results are promising for the production of a “greener” class of solar cells, since the protein component can be harvested from plants, bacteria, or algae. The protein can be readily isolated from the host organisms since it is thermally stable and the other proteins are quickly denatured at higher temperatures. This type of production would be more environmentally friendly compared with other types of photovoltaics, which generally require costly amounts of energy or hazardous solvent processing.

Although the device efficiency is fairly modest, the researchers stress that this is just a prototype. “MspA conducts current, but not very well, and there are two pathways for improvement. One is introducing mutations, such as more tryptophane residues, which are better for charge transport. A more promising pathway utilizes the empty channels in MspA, and fills them with molecules that are adept at moving charges,” notes Bossmann.

Overall, the incorporation of a stable protein into a functional device demonstrates the power of biomimicry and offers a promising new genre of solar cells with greener production. The authors propose that this new class of photovoltaics be referred to as “hybrid soft” solar cells.

Read the abstract in the Journal of the American Chemical Society.
Found here http://www.materials360online.com/newsDetails/40404