Vivek B. Shah

Linker-free deposition and adhesion of Photosystem I onto nanostructured TiO2 for biohybrid photoelectrochemical cells.

Photosystem I (PSI) from oxygenic photosynthetic organisms is an attractive sensitizer for nano-biohybrid solar cells as it has a combined light-harvesting and reaction center in one protein complex and operates at a quantum yield close to one in biological systems. Using a linker-free deposition technique enabled by an electrospray system, PSI was coupled to 1-D nanostructured titanium dioxide thin films to fabricate an electrode for a photoelectrochemical cell. After deposition, the surfactant in the PSI aggregate was dissolved in the surfactant-free electrolyte, ensuring that partly hydrophobic PSI was not resuspended and stayed in contact with titanium dioxide. A maximum current density of 4.15 mA cm(-2) was measured after 10 min of electrospray deposition, and this is the highest current density reported so far for PSI-based photoelectrochemical cells. The high current is attributed to 1D nanostructure of titanium dioxide and orientation of the PSI onto the surface, which allows easy transfer of electrons.

Aerosolized Droplet Mediated Self-Assembly of Photosynthetic Pigment Analogues and Deposition onto Substrates

Self-assembled photosynthetic molecules have a high extinction coefficient and a broad absorption in the infrared region, and these properties can be used to improve the efficiency of solar cells. We have developed a single-step method for the self-assembly of synthetic chlorin molecules (analogues of native bacteriochlorophylls) in aerosolized droplets, containing a single solvent and two solvents, to synthesize biomimetic light-harvesting structures. In the single-solvent approach, assembly is promoted by a concentration-driven process due to evaporation of the solvent. The peak absorbance of Zn(II) 3-(1-hydroxyethyl)-10-phenyl-13(1)-oxophorbine (1) in methanol shifted from 646 nm to 725 nm (∼ 80 nm shift) after assembly, which is comparable to the shift observed in the naturally occurring assembly of bacteriochlorophyll c. Although assembly is thermodynamically favorable, the kinetics of self-assembly play an important role, and this was demonstrated by varying the initial concentration of the pigment monomer. To overcome kinetic limitations, a two-solvent approach using a volatile solvent (tetrahydrofuran) in which the dye is soluble and a less volatile solvent (ethanol) in which the dye is sparingly soluble was demonstrated to be effective. The effect of molecular structure is demonstrated by spraying the sterically hindered Zn(II) 3-(1-hydroxyethyl)-10-mesityl-13(1)-oxophorbine (2), which is an analogue of 1, under similar conditions. The results illustrate a valuable and facile aerosol-based method for the formation of films of supramolecular assemblies.

Supramolecular self-assembly of bacteriochlorophyll c molecules in aerosolized droplets to synthesize biomimetic chlorosomes

The unique properties of chlorosomes, arising out of the self-assembled bateriochlorophyll (BChl) c structure, have made them attractive for use in solar cells. In this work, we have demonstrated the self-assembly of BChl c in aerosolized droplets to mimic naturally occurring chlorosomes. We compare two different methods for self-assembly of BChl c, one using a single-solvent and the other using two-solvents, and demonstrate the superiority of the two-solvent method. Results show that the self-assembled BChl c sprayed at different concentrations resulted in a varying red shift of 69-75 nm in absorption spectrum compared to the solution, which has peak at 668 nm corresponding to the monomeric BChl c. The sample fluoresces at 780 nm indicating a quality of self-assembly comparable to that observed in naturally occurring chlorosomes. In order to mimic chlorosomes, solution containing BChl c, BChl a, lipids and carotenes in same proportion as in chlorosomes is sprayed. The resulting self-assembly has an absorption peak at 750 nm, shifted by 82 nm compared to that of monomers and the fluorescence peak at 790 nm. Thus in presence of lipids and carotenes, both the absorption and fluorescence peaks are red shifted. Further, using grazing incidence small angle X-ray scattering (GISAXS), we characterized the deposited films, and the 2D X-ray scattering patterns of sample clearly indicate the distinct lamellar structure as present in chlorosomes. The results of this work provide new insights into self-assembly in aerosolized droplets, which can be used for assembling a wide range of molecules.

Biomimetic approach to synthesize sensitizers for hybrid solar cells

Natural light harvesting organisms use self-assembled dye structures to absorb light and transfer energy. These self-assembled structures have high extinction coefficient and transfer energy efficiently. Aerosol-based method of electrospray deposition is demonstrated to assemble dyes and also deposit the structures onto a surface. The kinetics of assembly are studied to understand the optimal conditions for assembly. Finally a concept of novel device using self-assembled structures which allows broad absorption spectrum and increased efficiency is explained.

Production and performance of a Photosystem I-based solar cell using nano-columnar TiO 2

To meet the worlds growing energy demands in a renewable and cost-effective manner, Photosystem I has been studied as a model for solar energy capture. Previous work has demonstrated the ability to construct low cost biohybrid solar cells using Photosystem I and specialty surfactants onto metal oxide surfaces. Electrospray deposition has also been used for adsorption of plant proteins onto metal oxide surfaces. Attachment of Photosystem I onto nanostructured materials increases the surface area and activity of biohybrid solar cells. Recent work has shown that Photosystem I can be attached to metal oxide surfaces without using linkers and maintain high current density. However, the current density of the surface decreases over the course of weeks (unpublished data). This work investigated the performance of Photosystem I-based solar cells manufactured using electrospray deposition. Photosystem I was successfully deposited onto metal oxide surfaces, and current densities of up to 6.84 mA/cm2 were measured. Different amounts of protein were deposited and the solar performance was tested for each cell. It was shown that very small and very large amounts of PS I increase the current density, but there is an intermediate regime where performance drops. This intermediate regime was attributed to a shading effect of the protein on the surface and self-assembly effects of the protein on the surface.

Single-crystal semiconductor thin films for biohybrid photovoltaic devices

Although solar energy is freely available, the utility cost for electricity generated from photovoltaic (PV) modules is higher than that from coal or natural gas. Existing silicon (Si)-based PV devices are efficient, but the high manufacturing costs eventually contribute to the high utility cost. Energy generation from artificial photosynthesis, which borrows partial steps from natural photosynthesis, is a promising strategy for future energy supply.1 Photocatalytic metal oxides, such as titanium dioxide (TiO2/ thin films, are used for artificial photosynthesis to harvest solar energy. However, for the wide-scale use of metal oxide solar cells, it is essential to have low production costs and high efficiency. Thin-film morphology is a very important determinant of solar energy conversion efficiency. A 1D structure is particularly advantageous in photoelectrochemical applications. Several methods have been introduced to synthesize 1D metal oxide electrodes,2 but most of them are multistep processes that are hard to scale up. Our lab recently developed a novel thinfilm coating system called aerosol chemical vapor deposition (ACVD)3 that is simple and operates at atmospheric pressure. Conventional chemical vapor deposition takes place at low pressure, and particle formation is a nuisance for homogeneous and uniform deposition. In contrast, ACVD involves intentionally forming particles and depositing them on a substrate to form thin films with high surface area. ACVD involves feeding the precursor, for example, titanium tetra isopropoxide (TTIP) for TiO2 films, into a heated reactor (see Figure 1). The precursor decomposes at high temperatures into monomers that collide to form particles. The concentration gradient results in particles diffusing toward Figure 1. Aerosol chemical vapor deposition (ACVD) (left) and particle deposition in ACVD (right). N2: Nitrogen gas. ITO: Tin-doped indium oxide.