(193e) Virus-Templated Three-Dimensional Photoanodes for Dye-Sensitized Solar Cells With Efficient Electron Collection and Plasmon-Enhanced Light Absorption

Virus-Templated Three-Dimensional Photoanodes for
Dye-Sensitized Solar Cells with Efficient Electron Collection and
Plasmon-Enhanced Light Absorption

Presenter:
Po-Yen Chen

Principal
Authors: Prof. Paula T. Hammond and Prof. Angela M. Belcher

Session Selection: Nanostructured Photovoltaics

Keywords: M13 bacteriophage; Three-dimensional
network; Localized surface plasmon; Dye-sensitized solar cells.

Abstract

M13 virus is a filamentous bacteriophage
of 880 nm in length and 6.5 nm in diameter, and has been demonstrated as a
versatile bio-template to assemble and to nucleate a broad range of materials. The
targeted materials can be assembled along the phage surface, and used to
assemble functional materials into two- and three-dimensional structures for various
device applications,
including photovoltaics. Dye-sensitized solar
cells (DSSCs) are a promising solution-processed photovoltaic technology. The
power conversion efficiency (PCE) of DSSCs is mainly determined by light
harvesting and electron collection. In general, to efficiently harvest light
and collect electrons, different design criteria must be considered. For
efficient electron collection, nanostructures of titanium dioxide (TiO₂)
with high aspect ratio have been proven to be effective, compared to the
commonly used architecture of photoanodes composed of randomly-packed
nanoparticles (NPs). However, the reduced surface area of one-dimensional
structures leads to insufficient dye adsorption and thus restricts light
harvesting. As a result, achieving sufficient light harvesting requires
developing strong-absorbing dye-molecules or various mechanisms to increase
optical absorption, e.g. localized surface plasmon (LSP)-enhanced
optical absorption by using metal NPs. Therefore, different functional materials
are required for electron collection and light harvesting, which makes it
attractive to develop an approach to assembling different functional components
together into a homogeneously distributed composite with a three-dimensional
(3-D) structure.

Here, we report a novel 3-D viral network as a
versatile platform for DSSCs which can template TiO₂ nanowires for
improved electron transport, and can also bind Au NPs and then incorporate them
uniformly into the photoanodes for the LSP-enhanced light harvesting of
dye-molecules. The virus-templated TiO₂
nanowires promote electron transport from the photo-excited dyes to the current
collector (Figure 1) in the photoanodes, resulting in an
increased electron diffusion length compared
to conventional NP-based photoanodes. In addition,
the M13 virus can bind Au NPs prior to the templated synthesis of TiO₂;
the resulting TiO₂-coated gold nanoparticles generate LSP effects
that improve the photo-absorption of dye-molecules adsorbed on the Au@TiO₂
nanocomposites (Figure 1). The 3-D thin film is
composed of tightly packed TiO₂ crystallites forming interconnected
nanowires (scanning electron microscopy image in Figure 1). Compared to
the size of individual viral particles, the virus-templated TiO₂
nanowires are approximately 100 nm in diameter and 2-3 mm in
length, which suggests that each nanowire in the photoanode is templated by a
bundle of viruses. In addition, the mesoporous structure of the
interconnected nanowire network with interconnected pores maximizes interfacial
contact with the electrolyte, which increases the mobility of redox couples and
also minimizes back recombination.

The ability of M13 viruses to
bind with Au NPs can fabricate the virus-templated Au@TiO₂
photoanodes to improve the optical absorption. The
resulting Au@TiO₂ nanocomposite results in the LSP-improved light
harvesting of dye molecules in the DSSC photoanodes. The device performance of
the virus-templated Au@TiO₂ photoanodes is optimized by varying the
concentration of the plasmonic Au NPs and the thickness of the photoanodes (Figure
1). The virus-templated Au@TiO₂ DSSCs (concentrations of Au NPs,
~0.24 wt.% and ~0.8 wt.%) always show higher PCE than the virus-templated TiO₂-only
DSSCs with similar thicknesses of the photoanodes. However, PCE decreases when
the concentration of Au NPs is increased to 2.4 wt.%, probably due to the
increased trapping of photo-excited electrons by Au NPs and competing light
absorption of Au NPs with dye-molecules. Moreover, the LSP from Au NPs in nanowire-based
photoanodes has the ability to increase the light harvesting without affecting
the high diffusion length achieved by the virus-templated nanowire network, and
thus maintain the efficiency of electron collection at 99.4%. Thus, the optimal
concentration of incorporated Au NPs was found to be 0.8 wt.%, and the best virus-templated
plasmon-enhanced DSSC achieves an increased efficiency of 8.46% due to the
efficient electron collection and the LSP-improved light harvesting
simultaneously.

Figure 1.