(193c) Layer-By-Layer/M13 Virus Assembled Porous Photoanodes for Efficient Electron Collection in Dye-Sensitized Solar Cells

Chen, P. Y., Massachusetts Institute of Technology
Belcher, A. M., Massachusetts Institute of Technology

Layer-by-Layer/M13 Virus Assembled Porous Photoanodes
Facilitating Electron Collection in Dye-Sensitized Solar Cells

Presenter: Po-Yen Chen

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

Selection: Thin Film and Organic Photovoltaics

Keywords: Layer-by-layer assembly; M13
bacteriophage; Electron transport; Electron diffusion length; Dye-sensitized
solar cells.


Dye-sensitized solar cells (DSSCs)
represent a promising solar technology because of their ease of fabrication and
high power conversion efficiency (PCE). Two important factors determining the
device performance, light harvesting and electron collection, are influenced by
the architecture of the titanium dioxide
(TiO2) photoanode. However, the
widely-used architecture of randomly-packed nanoparticles, made via screen
printing or doctor blading, are optimized for light harvesting but not
necessarily for electron collection. Controlling the nanoscale
morphology of the TiO2 photoanode is
required to minimize electron recombination; however, this control is
not necessarily accessible via doctor blading or screen printing. To address
this challenge, strategies have been developed to create oriented
nanostructures and process them into photoanodes to form directional electron
pathways and to improve the electron collection. While these methods improve the electron collection of
DSSC, complicated processes limit their application in mass production. Layer-by-layer
(LbL) assembly is a widely-used thin film deposition method, which offers
several advantages:it is a simple and cheap
process with strong potential for scalability and a high degree of nanoscale
control over thickness, morphology, and chemical composition of films. Nano-
and micro-porous LbL assemblies have been formed using a simple pH
post-treatment developed by Rubner et al, and these
porous films have been used to template DSSC photoanodes.

Herein, we present a facile process to generate an oriented porous TiO2
photoanode via dip and spray LbL assemblies, using alternating layers of
positively charged linear poly(ethylenimine)
(LPEI), negatively charged poly(acrylic acid) (PAA) and phage. In this study,
bilayer architectures without phage (LPEI/PAA)n
and tetralayer architectures with phage (LPEI/PAA/LPEI/phage)m are
both assembled via dip and spray LbL. A porous transition occurs to the
nonporous (LPEI/PAA)n or
(LPEI/PAA/LPEI/phage)m films when they are exposed to a low pH environment. A liquid phase deposition reaction is then used to
achieve a conformal surface coating of TiO2. The subsequent
calcination step removes the sacrificial polymer and phage template to generate
an anatase TiO2 photoanode. Moreover, the presence of the high
aspect ratio M13 bacteriophage
(phage) further optimizes electron collection by incorporating into the
LbL-assembled porous films to generate
interconnections among the TiO2 crystallites inside the oriented

The DSSCs employing the LbL-templated photoanodes
show excellent photocurrent and high PCE with relatively low dye loading
compared to nanoparticle DSSC. The dip and spray LbL-templated DSSC show ~98%
and ~106% of photocurrent with only ~52% and ~64% of the adsorbed dye compared
to the nanoparticle device, respectively. This is due to the efficient electron
collection in both interconnected porous LbL-templated photoanodes, while the
randomly packed nanoparticle photoanode is designed for sufficient light harvesting
but is not ideal for efficient electron collection. To further optimize the
electron pathway inside the TiO2 pore structure, the high aspect
ratio phage nanowire as a biomineralization template is incorporated within the
LbL templates. The phage templates form a percolative
network to generate interconnected TiO2 crystallite chains
throughout the photoanode. To realize the effect of the interconnected TiO2
wires embedded in the architecture, the phage-containing LbL-templated device
performance was investigated. Both phage-containing dip and spray LbL-templated
devices can further achieve 114% of the photocurrent compared to the
nanoparticle device, while the dye loadings (and thus light harvesting) almost
remain the same. Also, the inclusion of phage-templated nanowires further
enhances the electron diffusion length (Ln) of LbL-templated
devices. The Ln between the LbL-templated devices without and with
phage templates was improved by a factor of from ~4-5 and ~7-8, respectively.
These results shows that the inclusion of phage-templated nanowires in
LbL-templated photoanodes can help the crystallite interconnection and optimize
the electron pathway, which further reduces interfacial recombination and thus
further enhances electron collection.

Figure 1.