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Commercialization of dye sensitized solar cells: Present status and future researchneeds to improve efficiency, stability, and manufacturingJason B. Baxter
Citation: Journal of Vacuum Science & Technology A 30, 020801 (2012); doi: 10.1116/1.3676433 View online: http://dx.doi.org/10.1116/1.3676433 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/30/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Improved efficiency of organic dye sensitized solar cells through acid treatment AIP Conf. Proc. 1512, 774 (2013); 10.1063/1.4791267 Molecular modification of coumarin dyes for more efficient dye sensitized solar cells J. Chem. Phys. 136, 194702 (2012); 10.1063/1.4711049 Photovoltaic manufacturing: Present status, future prospects, and research needs J. Vac. Sci. Technol. A 29, 030801 (2011); 10.1116/1.3569757 Optical description of solid-state dye-sensitized solar cells. II. Device optical modeling with implications forimproving efficiency J. Appl. Phys. 106, 073112 (2009); 10.1063/1.3204985 Tandem dye-sensitized solar cell for improved power conversion efficiencies Appl. Phys. Lett. 84, 3397 (2004); 10.1063/1.1723685
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 77.53.185.83 On: Tue, 20 May 2014 23:20:34
REVIEW ARTICLE
Commercialization of dye sensitized solar cells: Present status and futureresearch needs to improve efficiency, stability, and manufacturing
Jason B. Baxtera)
Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA 19104
(Received 23 August 2011; accepted 7 December 2011; published 15 February 2012)
Dye sensitized solar cells (DSSCs) have received a tremendous amount of attention since the first
report of a 7% efficient cell in 1991. Confirmed record efficiencies are now 11.2% for small cells
and 9.9% for submodules, and low-cost production methods are enabling manufacturing of DSSC
products for a variety of markets. This review describes the present status of DSSC devices and
manufacturing as well as research challenges that must be addressed to continue the rapid commer-
cialization of DSSC technology. These challenges fall into the categories of improving efficiency,
stability, and manufacturability. Efficiency improvements will hinge on the development of new
combinations of dyes, redox couples, and photoanodes. Best-case lifetimes are determined by the
kinetics of various molecular-level processes, and realization of these lifetimes will require
improved encapsulation of cells and modules. Low-cost and sustainable manufacturing of DSSC
modules depends on use of high-throughput roll-to-roll processing and inexpensive, abundant
materials. Prospects for simultaneous improvement of efficiency, stability, and manufacturing are
discussed. VC 2012 American Vacuum Society. [DOI: 10.1116/1.3676433]
I. INTRODUCTION
The dye sensitized solar cell (DSSC) captured the attention
of the international research community in 1991 with the report
of a 7% efficient cell by O’Regan and Gratzel.1 DSSCs can be
fabricated from inexpensive oxide nanoparticles and coordina-
tion complexes or organic dyes without the expensive vacuum
processing or high temperatures required for single crystal or
thin film solar cell production. Not only did the DSSC have
potential for inexpensive and efficient conversion of sunlight to
electricity, but it was also relatively easy for research groups to
enter the field and contribute in many areas. As a result, the
amount of work on DSSCs has literally grown exponentially
over the past two decades, surpassing 10 publications per year
in 1992, 100 in 2001, and 1000 in 2010.2
The performance of photovoltaics should be quantified
not only by efficiency, but by a figure of merit that Fonash
defines as
energy conversion efficiency x lifetime
true costs;
where true costs contains manufacturing and installation
costs as well as environmental impact.3 Other quantities
such as levelized cost of electricity can be calculated from
this figure of merit and a set of further assumptions. Wolden
et al. recently reviewed manufacturing challenges for a vari-
ety of different photovoltaic (PV) technologies, including a
brief section on DSSCs.4 This review article greatly expands
on the DSSC section of that work with specific consideration
for the status and challenges of all three relevant quantities
in Fonash’s figure of merit: efficiency, lifetime, and cost.
The efficiencies of DSSCs have increased considerably in
the last 20 years, with the confirmed record now standing
at 11.2%.5,6 Typical DSSCs employ a monolayer of dye
adsorbed to a mesoporous oxide with pores filled by an elec-
trolyte. Dye molecules absorb light and inject electrons into
the oxide, where they are transported to the substrate. The
dye is regenerated by a redox couple in the electrolyte,
which then diffuses to the platinized counterelectrode to
complete the circuit. Dye sensitization of semiconductors
has been investigated for many decades.7–11 However, the
optical density of a single monolayer of dye molecules is
very small and inter-dye transport in multilayer stacks is of-
ten poor, preventing efficient energy conversion in planar
devices. The major breakthrough by the Gratzel group came
in coupling a stable dye and electrolyte to TiO2 nanoparticle
films with very large surface areas, such that a monolayer of
dye on the mesoporous support could both absorb nearly all
visible light and efficiently inject excited photoelectrons into
the oxide. An I�/I3� redox couple in liquid electrolyte pro-
vides the necessary combination of fast dye regeneration and
extremely slow recombination with electrons in the TiO2.
This combination of materials and architecture improved
efficiencies by orders of magnitude compared to planar var-
iations of the dye sensitized semiconductor concept.12
Since the first major jump in efficiency in 1991, advances
have been made in relation to many different facets of the cell.
New sensitizers have extended the absorption spectrum further
into the red,13–15 addition of light scattering layers has
improved light harvesting,16 and new electrolyte formulations
have increased the photovoltage.17,18 Small increases in effi-
ciency are still being gained by tailoring components individu-
ally, but further advances may soon require changing multiplea)Electronic mail: [email protected]
020801-1 J. Vac. Sci. Technol. A 30(2), Mar/Apr 2012 0734-2101/2012/30(2)/020801/19/$30.00 VC 2012 American Vacuum Society 020801-1
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components simultaneously. Fortunately, much work has been
undertaken to understand the physical and chemical mecha-
nisms underlying DSSC performance.19–23 Additionally, a
suite of analytical tools has been developed to characterize
DSSCs and identify major loss mechanisms.24–26 Computa-
tional efforts to identify new and improved sensitizers are
becoming more common.27 This theoretical and analytical
underpinning will expedite the search for materials and archi-
tectures with improved performance.
Efforts to improve the stability, lifetime, and robustness of
DSSCs were carried out in parallel with studies to increase
efficiency. “Lifetime” refers specifically to the expected time
for which solar cells retain some measure of performance, of-
ten 80% of their initial efficiency. “Stability” refers to the
performance of individual processes or the entire solar cell at
any time relative to the initial time. Good stability leads to
long lifetimes. Stability testing enables prediction of life-
times. Achieving DSSC module lifetimes of more than 20
years requires 108 turnovers for dye molecules12 and high-
quality encapsulation to prevent leakage of the electrolyte
and ingress of water. Excellent stability has been demon-
strated through both accelerated aging tests and outdoor test-
ing. For example, Harikisun and Desilvestro at Dyesol have
reported stability over 25 000 h of continuous 1-sun light
soaking at 55 �C with only 17% relative loss of efficiency.28
Outdoor testing from several groups has shown good stability
on the order of years.29,30 These results indicate promise for
outdoor modules with lifetimes of decades.
DSSCs possess a number of advantages compared to c-Si,
CdTe, and CIGS devices, even though these other technolo-
gies have higher cell and module efficiencies. DSSCs can be
manufactured using roll-to-roll processing without vacuum or
high temperatures. This processing results in low embodied
energy, with expected energy payback period of less than one
year.31 DSSCs can be made lightweight and flexible
by deposition on plastic substrates or metal foils. They contain
primarily nontoxic, earth-abundant materials, with the excep-
tion of very small amounts of Pt and Ru. DSSCs exhibit good
performance in diverse lighting conditions including high
angle of incidence, low light intensity, and partial shadow-
ing.31 They also perform as well at 50 �C as at room tempera-
ture. Under the course of real outdoor conditions, these
features combine to allow DSSCs to produce 10–20% more
electricity per year than c-Si rated at the same peak power,32
although DSSCs require larger area. DSSCs can also be semi-
transparent, selected colors, and bifacial. DSSCs’ good per-
formance under a wide range of lighting conditions and
diversity in appearance and form factor make them good can-
didates for building-integrated applications.33
With cell efficiencies surpassing 11%, demonstrations of
stable performance over many years, and inexpensive produc-
tion pathways, manufacturing of DSSC products has begun in
earnest. Dozens of companies and industrial research laborato-
ries are now involved in development, commercialization and
manufacturing of DSSC technology and products, mostly in
Europe, Asia, and Australia. Many products have been demon-
strated, as shown in Fig. 1. In 2009, G24 Innovations (G24i),
Wales, was the first to commercialize a DSSC product. Its flexi-
ble modules are integrated into items like bags, backpacks, and
wireless keyboards for portable recharging of consumer elec-
tronics. 3GSolar, Israel, is focused on DSSC modules for off-
grid rural applications to provide power for lighting and irriga-
tion pumps. While G24i and 3GSolar are exclusively working
on DSSCs, many large companies also have branches devoted
to DSSCs for both large area panels and indoor electronics.
These include Aisin Seiki (in collaboration with Toyota Central
R&D Laboratories), Sharp, and Sony in Japan. Sharp has the
official confirmed record for solar cell efficiencies, 10.4% for a
1 cm2 cell34 and 11.2 % for a 0.2 cm2 cell.5,6 Sony has pro-
duced the record submodule, 9.9% for 17 cm2.5,35 Dyesol, Aus-
tralia, is a leading developer and distributor of DSSC materials
and equipment, and it also is actively partnering to address the
building-integrated photovoltaics (BIPV) market. Its pilotline
with Tata Steel has produced the largest DSSC modules, over 1
m2, on steel strips for use as roofing panels.31 Further informa-
tion on over 25 different companies involved in DSSC
research, development, and commercialization has been com-
piled by Kalyanasundaram et al.36 Presentations from the 4thInternational Conference on the Industrialisation of DSC(DSC-IC 2010), which took place Nov. 2010 in Colorado
Springs USA, can be found from the Dyesol website.37
This article reviews recent progress in improving the effi-
ciency, stability, and manufacturing of DSSCs and provides
perspective on potentially fruitful research directions to over-
come remaining challenges in these areas. These research
directions include fundamental scientific studies of DSSC
materials and their interactions, materials development and
discovery, and efforts to advance integration of DSSC cells
into modules and panels using inline manufacturing. Improv-
ing efficiency frequently requires sacrificing stability and
manufacturability, and vice versa. The compromises that are
struck will depend on the specific requirements of the DSSC
application.
FIG. 1. (Color online) DSSC modules produced by (a) Fraunhofer ISE, Ger-
many, (b) G24 Innovations, Ltd., Wales, (c) Dyesol Ltd., Australia, and (d)
3GSolar Ltd., Israel. (a)–(c) are copyrighted images reprinted with permis-
sion from the respective companies. (d) Reprinted from Ref. 29 by permis-
sion of Elsevier, copyright 2011.
020801-2 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-2
J. Vac. Sci. Technol. A, Vol. 30, No. 2, Mar/Apr 2012
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II. OVERVIEW OF DSSC STRUCTURE ANDOPERATING PRINCIPLES
This section presents the essential structure and operating
principles of DSSCs, focusing on the most commonly used
materials. Details on how to make high efficiency DSSCs
and the importance of each component have recently been
published.38 Many further details on the underlying science of
DSSCs can be found elsewhere,39–42 but enough information
is presented here to provide context for subsequent sections
on challenges in efficiency, stability, and manufacturing.
A schematic of a conventional DSSC is shown in Fig. 2,
along with a schematic showing the energy levels of the cell
components and characteristic times for different charge trans-
fer steps. Optimal DSSC materials are chosen such that the rel-
ative energetics and kinetics of the elementary processes
allow it to function efficiently. DSSCs typically comprise a
mesoporous TiO2 nanoparticle film coated with a monolayer
of polypyridyl ruthenium dye, and with pores filled by an elec-
trolyte containing I�/I3� redox couple. This structure sepa-
rates the functions of light absorption and charge transport
into separate materials and ensures near-quantitative charge
separation because all light absorption occurs at the interface
between electron- and hole-transport materials.
When the cell is illuminated, a dye molecule can absorb an
incident photon and promote an electron from the highest occu-
pied molecular orbital (HOMO) to the lowest unoccupied mo-
lecular orbital (LUMO), whose energy level is above the
conduction band edge of the semiconductor. The excited elec-
tron is injected into the semiconductor where it can be trans-
ported to the substrate. Oxidized dye molecules are regenerated
by the reduced redox species I�, returning the dye to its ground
state and allowing it to absorb another photon. When many
electrons are injected into the conduction band, the quasi-Fermi
level in the semiconductor rises. At open circuit under solar
light intensities, large electron concentrations accumulate and
the quasi-Fermi level can approach the conduction band level
of the semiconductor. The maximum photovoltage possible
with the DSSC is then given by the difference between the
semiconductor conduction band edge and the electrolyte redox
potential. When the I�/I3� redox couple is used with TiO2, the
maximum possible photovoltage is approximately 0.9 V. When
the circuit is closed, electrons flow through the semiconductor
to the conducting substrate and through the load. At the same
time, holes are transported, as the oxidized species I3�, through
the electrolyte to the counterelectrode. The I3� is reduced by a
redox reaction with an electron from the Pt electrocatalyst at
the counterelectrode to complete the circuit.
The semiconductor must be mesostructured to achieve large
enough surface area so that there is sufficient dye to harvest the
incident light. For typical ruthenium-based dyes that have
absorption spectra that peak in the visible region of the spec-
trum, photocurrents can exceed 20 mA/cm2.15 A flat semicon-
ductor film with a monolayer of dye will not absorb enough
light to produce practical currents, while multilayers of dye are
inefficient at injecting electrons into the wide band gap semi-
conductor. In addition to providing high surface area, the semi-
conductor must also enable electron transport to the substrate
that is fast relative to recombination processes that may take
place at the semiconductor-electrolyte interface. In the course
of moving through the semiconductor, an electron can recom-
bine with either an oxidized dye molecule adsorbed on the
semiconductor surface or with the oxidized redox species (e.g.,
I3�) in the electrolyte near or adsorbed to the surface.43
FIG. 2. (Color online) (a) Schematic of a DSSC. (b) Energy diagram of
DSSC with conventional components including N3 dye. Favorable electron
transfer processes (1-3) are shown in green and recombination processes
(4-6) are shown in red. (c) Typical time constants for charge transfer proc-
esses in conventional DSSC under 1 sun illumination.
020801-3 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-3
JVST A - Vacuum, Surfaces, and Films
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A 10 lm thick, �50% porous film made up of sintered
�10 nm diameter TiO2 nanoparticles is the conventional sys-
tem that has been used to provide high surface area for dye
adsorption and light harvesting.12 It is estimated that the surface
area of such a mesoporous film is approximately 1000 times
that of a flat film.12 This enhancement enables dye loadings of
more than 7� 10�5 mol/cm3 for a 10lm film, which is suffi-
cient to harvest nearly all visible light.44 The nanoparticle film
must also transport electrons to the substrate before they can
recombine with oxidized dye molecules or redox species.
Charge collection typically occurs on millisecond time scales.
The most commonly used dyes are transition metal Ru-
based dyes such as cis-bis(isothiocyanato)bis(2,20-bipyridyl-
4,40-dicarboxylato)-ruthenium(II), also known as N3. The
dye must adsorb to the oxide surface and must have a
LUMO level that is properly aligned with the conduction
band edge of TiO2 and a HOMO level below the redox level
of the redox mediator. The electron injection rate into the
semiconductor conduction band must be much faster than
electron relaxation in the dye. Furthermore, the dye must
have an absorption spectrum that overlaps well with the solar
irradiance spectrum and must be stable over many redox
cycles. Figure 3(a) shows the absorption spectrum of N719,
the bis-tetrabutylammonium salt of N3, overlaid on the solar
irradiance spectrum.45 The N3 and N719 dyes have the
ability to harvest most photons between 400 and 700 nm,
although a significant fraction of the energy in the infrared is
lost. Electron injection typically occurs in less than one pico-
second,46 which is orders of magnitude faster than the
�50 ns relaxation time of typical Ru dyes,39 resulting in
injection quantum efficiencies near unity. Recently subnano-
second injection in working DSSCs was reported by Durrant
et al.47 This injection is significantly slower than that meas-
ured previously, but it is still fast compared to relaxation.
The electrolyte contains an I�/I3� redox couple in nona-
queous solvent. A common electrolyte formulation consists
of 0.6 M iodide salt (such as butylmethylimidazolium iodide)
and 0.03 M I2 in acetonitrile.48 Additives such as 0.1 M gua-
nidinium thiocyanate and 0.5 M 4-tert-butylpyridine are of-
ten included to modify the TiO2/dye/electrolyte interface.
The dye regeneration reaction by I� is much faster than back
electron transfer from the semiconductor to the dye, micro-
seconds compared to hundreds of microseconds.49–51 The
I3� diffuses through the solvent to the platinized countere-
lectrode, where triiodide is reduced to I� to complete the cir-
cuit. Dye regeneration must be fast, but recombination of
electrons in TiO2 with I3� must be very slow. Recombina-
tion occurs on time scales of tens of milliseconds, which is
an order of magnitude slower than charge transport and ena-
bles efficient charge collection.52
In a conventional lab-scale sandwich cell, both electrodes
are transparent and conducting, typically made of glass
coated with a �300 nm film of fluorine-doped tin oxide,
F:SnO2. Transmission decreases as conductivity increases
because of free carrier absorption in the film. The best solar
cell performance for current densities on the order of 10–20
mA/cm2 is usually achieved with film sheet resistances of
8–15 X/h, which has visible transmission of over 80%. The
counterelectrode is coated with either a thin Pt film or Pt
clusters to catalyze the redox reaction between the electron
from the substrate and the I3� in the electrolyte.
Solar cell performance is primarily evaluated by overall
solar to electric energy conversion efficiency, g, given by
g ¼ JSCVOCFF
Pin
; (1)
where Jsc is short circuit current, Voc is open circuit voltage,
FF is fill factor, and Pin in the incident light intensity. For
context, the 11.2% confirmed record cell had Jsc¼ 21 mA/cm2,
Voc¼ 0.736 V, and FF¼ 72.2% under AM1.5 spectrum
(1000 W/m2).5 Another important performance metric is inci-
dent photon to current conversion efficiency (IPCE), which is
often called external quantum efficiency in other photovol-
taics (PV) literature. IPCE is given by
IPCEðkÞ ¼ LHEðkÞ � /inj � gcoll; (2)
where LHE is light harvesting efficiency, /inj is injection
efficiency, and gcoll is charge collection efficiency. Convolu-
tion of IPCE with the solar spectrum allows calculation of
Jsc. IPCE spectra of DSSCs with N3 and black dye are
FIG. 3. (a) Solar irradiance spectrum and absorption spectrum of N719
diluted in ethanol. (b) IPCE of DSSCs using N3 and black dye (N749), as
well as bare TiO2. (a) Reprinted from Ref. 45 with permission of University
of California, copyright 2005. (b) Reprinted from Ref. 15 with permission
of American Chemical Society, copyright 2001.
020801-4 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-4
J. Vac. Sci. Technol. A, Vol. 30, No. 2, Mar/Apr 2012
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shown in Fig. 3(b). Causes of low current density can be
identified by considering the different components of IPCE.
The best DSSCs already have nearly unity /inj and gcoll, so
broadening the spectral range of light harvesting is the best
way to increase photocurrent.
III. CHALLENGES IN IMPROVING EFFICIENCY
DSSCs have already achieved sufficiently high efficiency,
above 11% confirmed6 and above 12% in the literature,53
that several companies have begun to manufacture commer-
cial products. However, development of new combinations
of materials could improve DSSC efficiencies to values as
high as 17%.54 The three main areas for materials develop-
ment are dyes, redox couples, and anodes. Examining the
energy level diagram of a conventional DSSC, Fig. 2(b), it is
clear that the major source of losses is the poor alignment
between the dye LUMO level and the redox potential of
I�/I3�. The overpotential for this transition is �550 mV,
significantly greater than the �200 mV necessary for fast
kinetics. Two options exist for better matching and increased
efficiency. First, a different redox couple with more positive
redox potential would increase the maximum photovoltage.
Second, a dye with less positive HOMO level would increase
the photocurrent. However, new materials cannot be arbitra-
rily chosen solely on the basis of energetics because kinetics
of charge transfer processes will also be affected. For exam-
ple, changing the redox couple is likely to result in signifi-
cantly faster recombination that would reduce the current
collection efficiency. This faster recombination might be tol-
erated if charge transport is also faster, as could be achieved
using nanowire arrays. However, nanowire arrays have
much lower surface area than nanoparticle films, resulting in
less absorption.
The interplay between different cell components is com-
plex, and the number of conditions that must be satisfied
for efficient energy conversion is large. It is likely that small
increases in efficiency can continue to be gained by modify-
ing one component at a time, but larger increases in effi-
ciency will require simultaneously changing multiple cell
components. Next we review some strategies and recent
developments related to improving DSSC efficiency by mod-
ifying the dye, redox couple, and anode. For each compo-
nent, we highlight the requirements for efficient energy
conversion, status of conventional materials, general oppor-
tunities for improvement, and specific alternative solutions.
Further detail on these topics can be found in the review by
Hamann et al.,54 which followed a similar approach, as well
as other reviews.40,41
A. Dyes and other sensitizers
The sensitizing dye must satisfy many requirements for
efficient energy conversion. The dye should broadly absorb
across the visible and near-infrared portions of the solar
spectrum with high molar absorptivity. High absorptivity
over a broad spectral range will increase LHE and photocur-
rent. Additionally, higher absorptivity also allows reducing
the surface area, which will result in smaller dark current
and increased photovoltage. Not only must the dye’s optical
gap be the right energy for light harvesting, but it must also
have HOMO and LUMO positions appropriate for charge
transfer. Overpotentials should be sufficient for fast kinetics
but not excessive such that absorption of red photons is
unnecessarily reduced. Electron injection into the oxide
must be fast compared to relaxation back to the dye ground
state. After injection, dye regeneration by the reduced redox
species should be much faster than recombination with elec-
trons in the oxide. The dye must bind strongly to the oxide
and should also be stable for �108 turnovers in order to
achieve sufficiently long lifetimes for commercial use.
The most common high-performing dyes are the
ruthenium-centered polypyridyls such as N3, N719, and N749
(black dye). Chemical structures of N3, N749, and some other
dyes are shown in Fig. 4. The Ru dyes can be used to obtain
DSSC efficiencies greater than 10%. Several reviews on this
family of dyes are available.55,56 DSSCs using N3 or N719 can
achieve reflection-limited IPCEs of over 80% for wavelengths
less than 650 nm but have very small response beyond 750 nm
due to low absorption, Fig. 3(b). High IPCE indicates very effi-
cient light harvesting and injection, as well as efficient charge
transport through the TiO2 film. The N719 LUMO level lies
�200 mV above the TiO2 conduction band and injection into
TiO2 occurs on ultrafast time scales.46,47 This interfacial
charge transfer is orders of magnitude faster than relaxation to
the ground state; the excited state lifetime is �50 ns.39 Regen-
eration of the N719 by I� occurs in microseconds, which is
much faster than recombination with electrons in the oxide.
These kinetics, as well as a very stable carboxylate linkage to
the oxide, lead to stability over many millions of turnovers.
While polypyridyl Ru dyes can be used to achieve effi-
ciencies above 10%, there is significant room for improve-
ment if spectral coverage and absorptivity can be improved
without negatively affecting the other required characteris-
tics. Reducing the optical gap of the dye would allow
absorption of red and near-IR photons. However, the N3 dye
LUMO is already ideally positioned with respect to the TiO2
conduction band energy. Z907 offers similar absorption to
N3, but its hydrophobic ligands offer improved stability.57
Shifting the dye HOMO from 550 mV below the iodide re-
dox potential to 200 mV below would increase the capture of
red photons and improve Jsc from�16 mA/cm2 to 27 mA/cm2.
Black dye, N749,14 extends absorption further into the near-
IR compared to N3 and resulted in current densities of
20.5 mA/cm2 and efficiencies of 10.4%.15 However, further
increases are still possible. Increasing the molar absorptivity
of the dye would allow the same light harvesting with films of
smaller surface area. N719 has emax �1.4� 104 M�1 cm�1,
which requires hundreds of monolayers of dye to absorb
more than 90% of the incident light. Multilayers of dye are
not effective since intermolecular charge transport is poor.
Therefore, surface areas approximately 1000� larger than a
flat film are required. Dyes with higher molar absorptivity
could employ much thinner nanoparticle films; thus reducing
dark current and increasing photovoltage. Photovoltage could
also be increased if smaller transport lengths, and conse-
quently shorter charge collection times, enable used of redox
020801-5 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-5
JVST A - Vacuum, Surfaces, and Films
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couples with more favorable redox potentials. Such redox
couples have faster recombination rates that inhibit their use
in conventional DSSCs. A huge number of possible dye mol-
ecules can be imagined, and computational studies are
becoming an important method to screen for the most promis-
ing candidates.27
Many organic dyes offer much larger molar absorptivities
than polypyridyl Ru dyes with relatively broad spectral cov-
erage and suitable kinetics. For example, an indoline dye has
been used to achieve a 9% efficient cell.58 While this effi-
ciency is lower than with Ru dyes, much less effort has
been given to optimizing the cell around organic dyes. The
indoline has emax �7� 104 M�1 cm�1, five times larger than
N719, and yields IPCE of over 80% from 410 – 670 nm.
Coumarins, hemicyanines, and squaraines have also been
used in cells of at least 4.5% efficiency.59–61 However, there
is not an obvious way to extend the spectral coverage of
these four families of dyes further into the red.54
Porphyrins and phthalocyanines present interesting
alternatives,62–64 possessing two strong absorption bands and
emax �4� 105 M�1 cm�1. Both of these types of dyes are
widely tunable by changing ligands or metal centers, allowing
minimization of the dip between bands and extension into the
red. Bands can also be broadened through the creation of
oligomers, which are fully conjugated and allow electron
transfer into the oxide.65 Orientation of porphyrins, phthalo-
cyanines, and their oligomers and method of attachment to the
substrate can dramatically affect DSSC performance.64,66–69
Metal-free, organic donor-p-acceptor dyes have also
shown great promise. These dyes have high extinction
coefficients and can be easily tailored to tune their properties.
Cells made with donor-p-acceptor dyes have shown efficien-
cies of �10% with liquid electrolytes with I�/I3�.70–72
Recently the cyclopentadithiophene-bridged dye in Fig. 4(f)
was used in conjunction with a Co polypyridyl redox couples
to achieve 9.6% efficiency.73 This dye, coded Y123, has emax
�5 � 104 M�1 cm�1 and spectral coverage to 700 nm. The
Gratzel group has recently reported DSSCs with record effi-
ciencies of 12.3% at 1 sun, and up to 13.1% at 0.5 sun, that
utilized cosensitization with Y123 and another donor-
p-acceptor dye to extend absorption further into the red.53
Additionally, record efficiencies of 6% with solid state hole
conductors have been achieved with donor-p-acceptor dyes.74
Avoiding dye aggregation and subsequent excited state
quenching is essential,75 and strategies have been devised to
modify the molecular structure to avoid these losses.76
Semiconductors can also be used instead of dye mole-
cules to sensitize the oxide. The semiconductor, typically
chalcogenides such as CdSe or Sb2S3, can be in the form of
quantum dots77–79 or thin continuous coatings.80–84 These
cells are typically called quantum dot sensitized solar cells
(QDSSCs) and extremely thin absorber solar cells (ETA
cells), respectively. Quantum dots have widely tunable band
positions through a combination of composition and size.
They can be grown on or attached directly to the mesoporous
oxide or they can be attached through a linker molecule.
Continuous semiconductor coatings can be made thick
enough to match photoexcited carrier transport lengths and
thereby require much smaller surface areas than a monolayer
of dye does.83 However, the solid-solid interface may lead to
FIG. 4. Chemical structures of some important dye molecules for DSSCs. (a) N3 (Ref. 13), (b) black dye N749 (Ref. 14), (c) Z907 (Ref. 57), (d) indoline (Ref.
58), (e) porphyrin (Ref. 62), (f) donor-p-acceptor organic dye Y123 (Ref. 73).
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faster interfacial recombination than the liquid junction with
iodide redox couple.85 Iodide electrolytes are corrosive to
chalcogenides, and alternative redox couples or solid state
hole conductors are necessary. Efficiencies higher than 5%
have been reported for semiconductor sensitized solar cells,
for instance, using a solid state TiO2 nanoparticles/Sb2S3/
P3HT structure.86
B. Redox couples
Redox couples must satisfy both energetic and kinetic
requirements to enable efficient DSSCs. The redox potential
should be positioned slightly more negative than the dye
HOMO level in order to regenerate the dye. Kinetics of two
charge transfer steps are critical to device performance. First,
the oxidized dye molecule must be regenerated by the
reduced redox species much faster than dye recombination
with an electron in the oxide. Second, recombination
between the oxidized redox species and an electron in the
oxide must be slow compared to electron transport to the
substrate. Charge transport between electrodes must also be
sufficiently fast that it does not contribute significant series
resistance.
The I�/I3� redox couple in organic solvent such as aceto-
nitrile is by far the most ubiquitous and most efficient in
DSSCs to date. Fast dye regeneration and slow recombina-
tion leads to internal quantum efficiencies of nearly 100%.
Although the kinetics of the I�/I3� redox couple are essen-
tially perfect for DSSCs with N3-type dyes, energetics are
not. The redox potential is 550 mV higher than the N3
HOMO level, an excessively large overpotential that wastes
a significant fraction of the incident photon energy. Addi-
tionally, the electrochemistry of the I�/I3� redox couple is
complicated, and dye regeneration is a multielectron process
that is not completely understood.42,52,87 Furthermore, while
iodide is fairly stable when used with conventional materials
and small cells, it is corrosive to many metals used as inter-
connects in modules and to alternative semiconductor sensi-
tizers. Finally, the use of high vapor pressure, low viscosity
solvents requires careful sealing of cells and modules to pre-
vent leakage.
The drawbacks of iodide redox couples in organic sol-
vents present great opportunities for the use of alternative re-
dox couples and solid state hole conductors. As discussed in
Sec. III A, shifting the dye HOMO level to better match the
redox potential would increase the photocurrent. Alterna-
tively, shifting the redox potential to match the dye HOMO
would increase the photovoltage. However, efforts to shift
the redox potential using other redox couples have not
improved efficiencies because of inferior charge transfer
kinetics. There has also been significant recent effort on the
use of ionic liquids and solid state hole conductors to
improve stability. Progress and challenges in these two areas
will be discussed below.
After almost 20 years of investigation, I�/I3� is still the
champion redox couple because of its fast dye regeneration,
slow recombination, high solubility, and fast diffusion.
Nevertheless, alternative redox couples may yet be discovered
that can maintain these desirable properties while also reduc-
ing the overpotential for regeneration, as recently reviewed
by Hamann.18 Alternative redox couples may be inferior for
N3 dye but offer advantages when coupled with other sensi-
tizers. For instance, Br�/Br3� has redox potential 500 mV
more positive than I�/I3�, and therefore is better matched to
the N3/N719 HOMO. This more positive potential did not
result in improved Voc with N719 dye because of poor
kinetics, but it did give Voc of 813 mV compared to 451 mV
for I�/I3� when paired with Eosin Y.88 Pseudohalogens such
as (SeCN)�/SeCN3� have shown reasonably high efficien-
cies, 7.5% using ionic liquid solvent, but their long term
chemical stability appears to be insufficient for commercial
use.89
One-electron, outer-sphere redox couples, such as poly-
pyridyl cobalt complexes and Cu(dmp)2, offer a more signif-
icantly different alternative compared to halogens and
pseudohalogens. These redox couples are good model sys-
tems whose properties and charge transfer mechanisms are
better understood than the I�/I3� system. Early work on
cobalt complexes showed efficiencies of 7.9% using [CoII/
III(dbbip)2](ClO4)2, where dbbip is 2,6-bis(1-butylbenzimi-
dazol-2-yl)pyridine, at light intensities of 1/10 sun.90 How-
ever, efficiency fell to 3.9% under 1 sun illumination due to
slow regeneration kinetics and mass transport limitations.
Recent work from several groups has shown efficiencies in
the range of 7–8 % at 1 sun illumination by using a combina-
tion of organic dyes and cobalt complexes with tailored
ligands.53,73,91,92 New record cell efficiencies as high as
12.3% using cobalt(II/III) tris-bipyridyl redox couple and
cosensitization with two donor-p-acceptor dyes with com-
plementary absorption spectra have been reported.53 I-V and
IPCE data for the Co-based redox couple compared with the
iodide-based couple are shown in Fig. 5. This new system
containing organic donor-p-acceptor dyes and Co-based re-
dox couples offers significant promise. It is the first system
not including I�/I3� that has shown 10% efficiency, and
ligands of both the dye and the redox couple can be further
tailored to optimize performance.
Replacing volatile organic liquid electrolytes is desirable
for robust and stable performance of DSSC modules. Alter-
natives include solvent-free ionic liquids, gels and polymers,
and solid-state hole conductors. Iodide-containing ionic
liquids have negligible vapor pressure, high ionic conductiv-
ity, and good ability to fill the mesoporous oxide film. Diffu-
sion of I3� is 1–2 orders of magnitude slower than in organic
electrolytes, requiring higher concentrations of I3� to avoid
concentration polarization and voltage loss, particularly at
higher light intensities. DSSCs using solvent-free imidazo-
lium-based ionic liquids have achieved efficiencies of more
than 8%.93 These cells have shown good stability, retaining
93% of their initial efficiency after 1000 h of light soaking at
1 sun and 60 �C. Gels and polymer electrolytes that contain
the redox couple also offer potential for improved stability
compared to organic liquids, although efficiencies have not
typically surpassed �5%.94–96
Solid state hole conductors differ from the aforemen-
tioned electrolytes because they do not contain a redox
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couple that diffuses through a solvent or matrix. Instead,
positive charges move by hopping between neighboring
molecules. Inorganic solid state hole conductors such as
CuSCN and CuI show charge transport rates similar to liquid
electrolytes, but stability remains an issue and creating high
quality interfaces and completely filling the pores has proven
to be difficult.97–99 The most promising organic solid state
hole conductor is 2,20,7,70-tetrakis(N,N-di-p-ethoxyphenyl-
amine)-9,90-spirobifluorene (spiro-OMeTAD).100,101 One of
the major problems is filling the tortuous pore structure of
nanoparticle films that are thick enough to be optically
dense. The promise of spiro-OMeTAD may be realized if it
can be coupled with high absorptivity dyes that allow thinner
films to be used. Very recently, DSSCs based on spiro-
OMeTAD have achieved 6% efficiency when coupled with
specially designed donor-p-acceptor dyes.74 Comparison
cells with Z907 dye had efficiencies of only 3.2%, again
demonstrating the importance of optimizing multiple cell
components simultaneously.
C. Anode materials
The photoanode must satisfy requirements regarding both
morphology and chemical and electronic structure. The an-
ode must present large enough surface area that a monolayer
of dye can harvest visible light. It must be transparent to visi-
ble light and have conduction band position and density of
states to quickly accept electrons from the photoexcited dye.
The anode must allow transport of injected electrons to the
FTO substrate faster than they can recombine with either
oxidized dye molecules or the oxidized redox species. To
achieve long lifetimes, it must also be chemically and
mechanically stable.
All high efficiency DSSCs to date use a mesoporous film
of sintered TiO2 nanoparticles as the photoanode. TiO2 has a
band gap of 3.3 eV and does not absorb visible light. It has
conduction band position �200 mV below the N3 LUMO
level. Thanks to empty d-band orbitals, it has high density of
states to accept electrons from the dye on picosecond time
scales.46 With N3 dye, the surface area required for maxi-
mum light harvesting is approximately 1000� that of a flat
film. This can be achieved using �10 lm thick film of 20 nm
diameter nanoparticles, as in Fig. 6. A scattering layer of
larger particles is often added to increase the photon path-
length and hence the probability of absorption in the red, or
alternatively to reduce film thickness and enhance charge
collection. The scattering layer is typically a �4 lm layer of
400 nm TiO2 particles. The average pore size is on the order
of the particle size, �20 nm. This pore size is sufficient for
diffusion of the iodide redox couple to the counterelectrode
with liquid electrolytes, but it may cause mass transport or
charge transport limitations for alternative hole conductors.
The mesoporous TiO2 nanoparticle films has very low drift
mobility of 10�4 – 10�7 cm2/V s,102 which is at least four
orders of magnitude smaller than bulk TiO2. Electrons dif-
fuse through the nanoparticle film and are collected on milli-
second time scales. While this collection time is quite slow,
recombination with iodide is even slower, such that charge
FIG. 5. (a) I-V curve and (b) IPCE of DSSCs using organic donor-p-
acceptor dye in Fig. 4(f) with electrolytes containing cobalt (II/III) tris-
bypyridine redox couple (triangles) and two different I�/I3� formulations
(circles, squares). Inset in (a) shows structure of Co redox couple. Inset in
(b) shows molar absorptivity of Y123 dye. Reprinted from Ref. 73 with per-
mission from Wiley, Inc., copyright 2011.
FIG. 6. Scanning electron micrographs of TiO2 nanoparticle film. Scale bar
is 5 lm. Inset has side length 150 nm. Image courtesy of Siamak Nejati,
Drexel University.
020801-8 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-8
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collection efficiency at short circuit is still essentially
100%.103 However, slow electron transport inhibits the use
of redox couples that have more favorable redox potentials
but faster recombination kinetics.
The primary opportunities for improvement of the anode
lie in improving the rate of electron transport while maintain-
ing high surface area, appropriate electronic structure, and
stability. Increasing the electron diffusion coefficient, and
thus decreasing collection time, would enable the use of re-
dox couples with proportionately faster recombination rates
without sacrificing charge collection efficiency. It is worth
noting specifically that faster charge transport would have no
effect on collection efficiency with I�/I3�, since it is already
essentially unity at low bias voltages. However, other redox
couples with more favorable redox potential would increase
photovoltage if high charge collection efficiency can be
maintained. Morphologies with more open pore structures, or
better aligned and less tortuous pore structures, would be
advantages for alternative hole conductors. As noted earlier,
cobalt coordination complexes diffuse much slower than
iodide and would benefit from shorter, more direct, and larger
cross-section transport pathways. The same is true of solid
state hole conductors with low hole mobilities. Pore-filling
with solid state hole conductors would also be facilitated with
larger, oriented pores. While morphology can be changed to
improve transport, changes that reduce the surface area would
also reduce photocurrent unless alternative highly absorbing
dyes or semiconductors are also used.
ZnO nanowire arrays have been used to provide one-
dimensional electron transport through single-crystal nano-
wires,104–107 as shown schematically in Fig. 7(a). ZnO has
been the primary material used for nanowire DSSCs because
it easily forms anisotropic nanostructures and also has much
higher electron mobility than TiO2, 200 cm2/V s compared to
1 cm2/V s for bulk materials. Proof of concept for nanowire
DSSCs was reported by Baxter et al. and then by Law et al.
FIG. 7. (Color online) (a) Schematic of nanowire DSSC. (b) Lifetime (triangles) and collection time (circles) of photoexcited electrons in ZnO nanowire (open
symbols) and ZnO nanoparticle (closed symbols) DSSCs. (c) Scanning electron micrograph of ZnO nanowire array, with 10 lm scale bar, and (d) I-V curves
for DSSCs with nanowires of different length. (a) Reproduced from Ref. 106 with permission of Institute of Physics Publishing, copyright 2006. (b) Repro-
duced from Ref. 109 with permission of Royal Society of Chemistry, copyright 2006. (c), (d) reproduced from Ref. 110 with permission of American Chemi-
cal Society, copyright 2011.
020801-9 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-9
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in 2005.104,107 Shortly thereafter, ratios of electron collection
time to recombination time were demonstrated to be at least
two orders of magnitude smaller with ZnO nanowires than
TiO2 nanoparticles,108,109 Fig. 7(b), indicating great promise
for improved charge collection when using redox couples
with fast recombination kinetics. Nanowire arrays typically
have had roughness factors of only 10–100, and light harvest-
ing efficiency suffered significantly with Ru dyes.106,107
Therefore, nanowire arrays are probably best used with
extremely thin semiconductor coatings or potentially with
porphyrin multilayers. However, very long ZnO nanowires
with array roughness factor of 500 and DSSC efficiency of
7% have recently been reported, Figs. 7(c) and 7(d).110 Pho-
tocurrent and efficiency increased with nanowire length, con-
firming that energy conversion was limited by light
harvesting. Record efficiency of ZnO nanoparticle cells is
about half that of TiO2 cells,44 so efforts have also been
devoted to synthesizing TiO2 nanowires and nanotubes.111,112
Polycrystalline TiO2 nanotubes did provide somewhat better
charge collection compared to TiO2 nanoparticle films.112–114
Single-crystal TiO2 nanowires have recently been synthesized
and may further improve performance.115
The oxide material can also be changed. For instance, con-
ductivity and flat band potential of TiO2 can be tuned by dop-
ing with Nb. In one study, 5% doping resulted in higher
efficiency than pure TiO2.116 Alternative wide band gap
oxides such as ZnO,44 SnO2,117 and SrTiO3118 have also been
used, but none have shown nearly comparable performance to
TiO2 so far. ZnO is the second most widely investigated mate-
rial, with maximum efficiencies of 6.6%.119 Strategies of
coating more conductive nanoparticles with insulating shells
have shown some promise to increase charge transport rates
and reduce recombination. This strategy has been employed
for both nanoparticles120–122 and nanowires.110,123 The afore-
mentioned 7% cells with ZnO nanowire arrays used TiO2
coatings.110
D. Multicomponent optimization
Tailoring individual components of the DSSC may still
enable small gains in efficiency, but this approach has not
shown large improvements in efficiency for about 10 years.
It appears very likely that major leaps in efficiency will only
be achieved by optimizing multiple components simultane-
ously. Several examples have been given in previous sections.
Polypyridyl cobalt redox couples worked only moderately
well with conventional N3 dye, but they show significant
promise with new organic dyes. Changing anode morphology
to improve charge transport will not gain anything with I�/I3�
redox couple, but it could lead to higher efficiency if com-
bined with redox couples with more positive redox potentials
and strongly absorbing dyes.
Based on a reasonable set of assumptions, Hamann et al.show that efficiencies of 17% can be achieved with the right
combination of materials in a single-layer (not tandem)
DSSC.54 Figure 8 shows that there is a fairly broad maxi-
mum that can be reached using different combinations of
dye and redox couple if the energetics and kinetics are
appropriately designed. The great challenge will be in find-
ing the right combinations of materials to meet all of the
energetic and kinetic requirements of the DSSC simultane-
ously. Considering the vast number of possible materials
combinations, the key to meeting this challenge will be to
continue to improve our fundamental understanding of de-
vice physics and material properties to enable predictive tai-
loring of DSSC components and architectures.
Further work can also be done in the area of advanced
photon management. For example, plasmonic enhancement
of light harvesting has been shown using structured silver
back contacts on solid state DSSCs124 as well as coupling
oxide-coated metal particles in close proximity to the dye.125
Upconversion can be employed to convert near-IR photons,
either at the dye level or by coatings on the glass sub-
strate.126 Tandem cells can also be utilized with different
combinations of dyes and oxides to generate power greater
than either single cell alone.127–130
The key ideas of the DSSC are the separation of light
absorption and charge transport into different materials and
the absorption of light precisely at the interface between
intermixed, bicontinuous electron- and hole-selective materi-
als. There are many manifestations of this theme, including
solid state DSSCs, QDSSCs, and ETA cells with different
combinations of organic and inorganic as well as solid and
liquid components. While each design has its own challenges,
the physics have much in common and lessons learned from
each of these designs can inform the others. Increased
research efforts in all of these areas will be beneficial.
IV. CHALLENGES IN IMPROVING LIFETIME
In terms of the figure of merit define by Fonash, lifetime is
just as important as efficiency. Assuming negligible operating
FIG. 8. (Color online) Estimated efficiency, g, of DSSCs employing dyes
with increased spectral coverage in conjunction with redox couples with
varying redox potentials. Efficiencies of 15 – 17 % are potentially achieva-
ble over a fairly wide range of combinations when there is minimal overpo-
tential (�200 mV) for dye regeneration (dotted line). Reproduced from
Ref. 54 with permission of the Royal Society of Chemistry, copyright 2008.
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costs, a given module will produce electricity at approxi-
mately half the cost per kW-h if its lifetime can be doubled.
However, considering balance of systems and module manu-
facturing costs, it is impractical to commercialize solar cells
with lab scale efficiencies of less than 10%. Therefore, the
majority of past research efforts have been devoted to
improving efficiencies further beyond this threshold. None-
theless, increasing attention has recently been paid to stability
and lifetime.
For practical applications, DSSCs must be stable at the
molecular, cell, and module levels. Each of these scales
presents different scientific and engineering challenges. At
the molecular level, DSSC components must have comple-
mentary kinetics such that desirable electron transfer reac-
tions are much faster than parasitic degradation pathways.
At the cell level, DSSCs must be extremely well sealed to
prevent electrolyte leakage and moisture ingress. This chal-
lenge is even greater at the module level, where dimensions
for sealing are larger and even the flatness of the glass can
become an issue.
A. Stability requirements, testing methods, andstandards
The requirements for DSSC lifetime depend strongly on
the application. Use of DSSCs in building-integrated mod-
ules would require lifetimes of >25 yr to avoid disruption of
the building environment for repair or replacement. Con-
versely, lifetimes of 5 yr may be sufficient for portable elec-
tronics chargers integrated into apparel and accessories.
Indoor environments are much less harsh in terms of temper-
ature, humidity, and light intensity, so less expensive DSSC
construction is possible. The most stable DSSCs are sand-
wiched between two pieces of glass. However, this is also
the most expensive, rigid, and heavy configuration. Flexible
metal foils or plastics are less expensive and can be proc-
essed using roll-to-roll methods, but they require more so-
phisticated encapsulation and may have shorter lifetimes.
Currently, there are no standard practices for testing life-
time and stability that are specific to DSSCs. Instead, proto-
cols such as IEC 61646 (United States) and Japan Industrial
Standard C-8938 for thin film photovoltaics can be applied
or adapted. Common treatments include light soaking, ther-
mal cycling between �40 and 90 �C, damp heat (85 �C, 85%
RH for 1000 h), and humidity freeze testing. The relevance
of these tests is highly dependent upon the application of the
DSSC. Cell temperatures would not exceed 70 �C in many
applications, such as in moderate climates or indoors. How-
ever, temperatures of 85 �C could be reached in more tropi-
cal locations.33 Critical need exists to identify which
accelerated aging tests are most relevant to real outdoor con-
ditions in to order to identify common degradation/failure
mechanisms and predict lifetimes.
B. Stability testing and lifetimes of small area DSSCcells
The stability and lifetime of DSSC cells and modules
depends critically upon encapsulation and sealing. Glass
provides the best barrier to electrolyte leakage and water
ingress, but it is also rigid and expensive. Glass can be used
for both electrodes in a sandwich configuration, or a glass
substrate can be used along with a polymer barrier layer.
Alternatively, polymer or metal foil substrates with poly-
meric encapsulation can produce a flexible cell with possibil-
ity for roll-to-roll processing. However, these flexible cells
are more prone to detrimental defects and slow leaks.
The seal must hermetically enclose the cells to minimize
leakage of solvent, prevent ingress of water, prevent electro-
lyte contact with current collectors and other cells, and
mechanically hold the substrates together over the possible
range of operating temperature. Common materials for seal-
ing cells include thermoplastic or elastomeric polymers,
adhesives, and glass frits.33 Seals must be chemically resist-
ant to the electrolyte solvent, redox couple, and any other
additives. DuPont SurlynVR
is the most commonly used hot-
melt seal for laboratory-scale DSSC cells. It can easily be
cured with a hotplate or hot iron at 170 �C. silicone provides
good chemical resistance but can be more porous and less re-
sistant to vapor transport. The best seal is usually provided
by glass frits. However, lead-free glass frits require tempera-
tures greater than 600 �C, which would degrade the dye.
Consequently, glass frits require sealing to be done before
the cell is dyed by injection through fill holes. Glass frit seals
are also more brittle than plastics under high thermal cycling
and are not suitable for flexible devices. Ideally, any sealing
process should not expose dyed TiO2 to temperatures above
80 �C. Cells can be edge-sealed before dyeing, leaving only
small fill holes. Dyeing and electrolyte can be done using the
fill holes, which are then sealed using a only very brief and
localized exposure to high temperatures.
A trade-off exists between efficiency and stability. The
highest performing cells use volatile electrolytes with low
viscosity that allow fast diffusion of the redox couple. How-
ever, these electrolytes are also most prone to evaporation
and leakage. DSSCs that use solvent-free approaches such as
room-temperature ionic liquids (RTILs) have lower efficien-
cies but promise longer lifetimes. Multiple groups have
investigated DSSC stability over 1000 h or more.131–136 Pet-
terson et al. showed good stability at low illumination levels
over 4300 h.137 They also observed improved stability upon
filtering UV light to avoid direct photoexcitation of the
TiO2. Kubo et al. showed no degradation and even slight
improvement over 1000 h at 85 �C in dark when an organo-
gelator was used in conjunction with RTIL.138 By compari-
son, 30% degradation was seen in un-gelled electrolyte.
Goldstein et al. at 3GSolar have demonstrated large area
225 cm2 cells made with two glass substrates and edge seal
that show initial efficiencies of �4.2%.29 At 85 �C, the cell
efficiency quickly dropped to 3.5% after 600 h and then
declined slowly to 3.2% after 3500 h. Outdoor testing on a
Jerusalem rooftop showed declines from initial value of
4.0–4.2 % to efficiencies of 3.0–3.5 % after 7000 h.
Much of the publicly available work on longer term cell
and module stability has been done by Dyesol and/or the
Gratzel group. Bai et al. reported a combination of reason-
ably high efficiency and good stability using a solvent-free
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eutectic melt of RTILs.93 Their best DSSC contained Z907
amphiphilic dye and a liquid electrolyte containing primarily
1,3-dimethylimidazolium iodide (DMII), 1-ethyl-3-methyli-
midazolium iodide (EMII), and 1-ethyl-3-methylimidazo-
lium tetracyanoborate (EMITCB). That device initially
displayed 8.2% efficiency and retained 93% of this initial ef-
ficiency after 1000 h in 1 sun illumination at 60 �C. Molten
salts have low vapor pressure and low permeability through
plastics, giving them significant practical advantages com-
pared to systems requiring organic solvents.
While good sealing is critical, internal components also
have an important role in stability. The dye Z907 with
hydrophobic ligands gave more stable performance than
N719 as a result of improved resistance to water that had
infiltrated the cell. The starting efficiencies were 4.9 and
5.8 %, respectively; and crossover of efficiencies of 4.2%
occurred after 11 000 h of continuous 0.8 sun illumination at
the maximum power point.33 Other Dyesol cells have been
tested for at least 25 000 h under 0.8 sun illumination near
the maximum power point. Figure 9 shows the evolution
of cell parameters over time. After 25 000 h, efficiency
decreased 17% relative to initial condition, from 4.2 to
3.7 %.28 Given the average temperature of the cells, the Dye-
sol accelerated aging tests indicate a potential lifetime of 40
years in Middle Europe and 25 years in Southern Europe.
Determining which components, interfaces, and physical
phenomena cause degradation is essential to improving
performance. In the case of the Dyesol cells, Voc decreased
significantly over the first 1000 h and fill factor began a slow
decline beyond 6000 h.28 They performed electrochemical
impedance spectroscopy periodically to investigate the
source of resistive and capacitive losses. They found that
recombination resistance across the TiO2-electrolyte inter-
face decreased significantly in the first 1000 h. The chemical
capacitance increased during that time because charging of
the TiO2 lead to shifting conduction band potential. Together
these effects lead to lower photovoltage. No degradation of
the Pt counterelectrode was seen. Fill factor decrease is due
to increased series resistance, which was attributed to contact
resistance to FTO. Further studies of loss mechanisms that
result from different outdoor and accelerated aging tests
would be greatly beneficial for designing DSSCs with
improved stability and longer lifetimes.
Stable performance over millions to hundreds of millions
of turnovers is a stringent requirement for a molecular sys-
tem. However, studies mentioned here have shown signifi-
cant promise for the future implementation of DSSCs in
practical applications. Further investigation of degradation
mechanisms and molecular design of components to mitigate
losses will be essential for stability. While broad and inde-
pendent tunability of different DSSC components is poten-
tially beneficial for discovering new high-efficiency
combinations, it is not clear whether stability studies focused
on ruthenium dyes and iodide redox couples will be relevant
to future materials combinations. For example, complexed
cobalt redox couples with organic dyes have recently shown
promising efficiencies of up to 12.3%,53 but much work
remains to investigate their stability and performance with
nonvolatile electrolytes.
C. Additional challenges for DSSC modules andpanels
Stability measurements show promise for small DSSC
cells, but making stable modules brings additional chal-
lenges. Modules simply have much larger areas and there-
fore longer edges to seal than small area cells. Edge seals
and encapsulants must be very high quality to maintain good
hermetic sealing of the entire module. Glass is an excellent
barrier and can easily be used in small area DSSCs to give
high stability. However; in addition to being expensive, it is
difficult to manufacture glass that is flat at the 10 lm length
scale over areas much larger than 30� 30 cm2.33 This
restriction limits the size of all-glass modules. Another chal-
lenge is selection and protection of metal interconnects
between cells. The most common metals in PV modules are
silver, copper, and aluminum, which are chosen on the basis
of cost and resistivity. However, all of these metals are cor-
roded by the iodide electrolyte and must be thoroughly
encapsulated. This requirement adds both cost and potential
new failure mechanisms. Modules perform optimally when
all cells produce the same current and/or voltage, so high
degree of control over cell-to-cell reproducibility is required.
Cells must also be isolated from each other to prevent leak-
age of electrolyte and interdiffusion of redox species.
FIG. 9. (a) Efficiencies, (b) short circuit currents, (c) open circuit voltages,
and (d) fill factors of solvent-based DSSC cell, periodically assessed at 1
sun (bold lines), 0.33 sun (intermediate linewidth), and 0.1 sun (thin lines),
as a function of light soaking time at >0.8 sun. Cell temperature was main-
tained at 55 – 60 �C and cells were close to the maximum power point for
the duration of the light soaking. Reproduced from Ref. 28 with permission
of Elsevier, copyright 2011.
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Despite these challenges, there does not appear to be any
fundamental roadblock to the development of stable and effi-
cient DSSC modules.
Less stability data is available for modules than for cells.
Pettersson et al. investigated monolithic modules with one
glass electrode and methoxypropionitrile liquid electrolyte
with appropriate encapsulation.139 These modules had effi-
ciencies of over 5% and showed no degradation over the first
1000 h under 1 sun illumination at 50 �C, followed by slight
decrease of �0.3% over the next 1000 h. In contrast, effi-
ciency decreased to less than 3.5% upon storage in darkness
at 80 �C for the same time. Dyesol has shown that after 200
cycles between �40 and 85 �C, modules of size >100 cm2
retained 80% of their initial performance.140 The same mod-
ules maintained stable efficiency, 3.5%, after 1000 h of light
soaking at 0.8 sun and 60 �C. G24 Innovations reported flexi-
ble modules on Ti foil contained between two laminate films
that showed stable efficiencies of 1.7% over 250 days of out-
door testing in Wales.141 They have also produced modules
designed for indoor applications that show double that effi-
ciency due to relaxed constraints on thermal stability. This
comparison illustrates the importance of designing materials
with specific applications in mind. Toyota Central R&D
Laboratories and Aisin Seiki reported 110 cm2 modules with
two glass substrates and solvent-free imidazolium-based
RTIL electrolyte.30 Light soaking at 1 sun and 60 �C showed
remarkable stability, with efficiency retaining more than
80% of its initial value until 15 800 h. This stability test pre-
dicts outdoor lifetimes of at least 15 yr. No degradation and
even slight improvement was seen during outdoor testing of
the modules for 160 days. Unfortunately all data in this
report was referenced to an unspecified initial performance
condition, so absolute efficiencies are not known. Outdoor
testing of modules and panels was also performed by Dai
et al., using cells with both glass electrodes and volatile
nitrile-based electrolyte.142 Even with organic electrolyte,
modules showed minimal degradation after 1 yr outdoors,
with panel efficiencies up to 5.9%.
As mentioned earlier, the most stable formulations are not
the most efficient ones. Additionally, some materials may be
suitable for indoor or low-light applications but not suitable
for outdoor use. The end application should be considered
when developing new DSSC materials and sealing proce-
dures. While accelerated aging and outdoor stability tests
show great promise, future efforts must be devoted to finding
higher efficiency combinations that maintain excellent sta-
bility. Further effort should also be devoted to delineating
more direct connections between accelerated aging tests and
actual device lifetimes and degradation mechanisms.
V. CHALLENGES IN MANUFACTURINGAND COST-REDUCTION
Fonash’s photovoltaic figure of merit and other quantities
such as levelized cost of electricity depend equally on effi-
ciency, lifetime, and cost. Total costs include the module or
panel costs, balance of systems costs, and environmental
impact. Module and panel costs include materials costs and
processing costs. Balance of systems includes inverters and
other electronics, installation, and support systems. Environ-
mental impact includes both positive and negative compo-
nents. For example, carbon is emitted while producing solar
panels, but use of solar panels to produce electricity elimi-
nates the need for electricity production from fossil fuels.
Environmental impact is often accounted for through the
implementation of feed-in tariffs or carbon taxes.
DSSC modules offer similar advantages to thin film pho-
tovoltaics when compared to crystalline Si. They are not lim-
ited in two dimensions by wafer size, and instead can be
continuously processed in one dimension. Continuous proc-
essing, especially on flexible substrates, offers potential cost
advantages because of high speed and low manufacturing
cost. This section reviews several different module designs,
requirements and methods for high volume processing, and
projected materials costs and availability.
A. Module designs
A number of different designs are available that take the
concepts and materials of small area DSSC cells and apply
them to large area cells interconnected in various ways to
form modules. Forming modules from cells connected in
series and parallel allows generation of higher voltage and
current. However, as in all photovoltaics, module area effi-
ciency is always lower than cell efficiency. Module effi-
ciency can be improved by minimizing nonactive areas
required for interconnects and busbars and also by minimiz-
ing resistive losses. These constraints generally oppose each
other and optimization of the module design is necessary.
Several module designs will be briefly reviewed here. Fur-
ther details can be found elsewhere.33,41
Module designs can be divided into two categories:
sandwich designs and monolithic designs. Schematics of
different module designs are shown in Fig. 10. Sandwich
modules use two conductive substrates encapsulating the
TiO2/dye/electrolyte, while monolithic designs use only one.
Sandwich designs can be further divided by the scheme of
interconnecting individual cells, namely parallel-connected,
Z-interconnected, and W-interconnected.
The parallel-connected scheme is similar to crystalline Si
modules, wherein a thin metal grid enhances current collec-
tion by reducing sheet resistance. Metal lines must be care-
fully encapsulated to prevent corrosion by the electrolyte.
This architecture is suitable for glass, metal, or polymer sub-
strates. Other than encapsulation, it is straightforward to
employ. Arakawa et al. have achieved 8.7% module area ef-
ficiency using protected silver grids in 100 cm2 glass mod-
ules under 1000 W/m2 illumination.143 Dai et al. reported
much larger panels of up to 3600 cm2 based on the same
design. This panel maintained relatively high efficiency of
5.9% with negligible degradation during outdoor testing for
one year.142
Z-interconnected modules connect neighboring cells in
series to build up voltage. In this architecture, interconnects
between cells are very short, reducing requirements on con-
ductivity and corrosion resistance. However, unlike the
020801-13 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-13
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parallel connected design, interconnects contact both electro-
des and stable performance without delamination under tem-
perature cycling is critical. Jun et al. have reported 100 cm2
Z-interconnected modules that use glass substrates, silver
interconnects, and polymer encapsulation.144 6.6% active
area efficiency and �3.9% module area efficiency were
obtained at 1000 W/m2.
W-interconnected modules use alternative back and front
side illuminated cells to avoid the need for costly intercon-
nects and to maximize active area. These advantages are
countered by the difficulty in designing cells that are electri-
cally well-matched under diverse lighting conditions when
every alternating cell is illuminated from the back side.
Nonetheless, Han et al. from Sharp Corporation have dem-
onstrated 25 cm2 modules with certified efficiencies as high
as 8.4%145 and ability for larger module areas of at least
625 cm2.146
In contrast to sandwich modules, monolithic modules
use only a single transparent conductive substrate. A porous
counterelectrode layer functions as both electrocatalyst
and current collector and the module is sealed with a non-
conductive hermetic backsheet. The monolithic design,
introduced by Kay et al. in 1996,147 offers potential cost sav-
ings because only one substrate is required. Monolithic mod-
ules had typically used a mixture of catalytic carbon black
and conductive graphite as the counterelectrode. This choice
resulted in opaque modules that lacked flexibility with
regard to coloration, limiting their potential applications.
However, semitransparent monolithic cells were introduced
in 2009 by Aisin-Seiki and Toyota Central R&D Laborato-
ries.148 Semitransparent monolithic modules used Pt-loaded
Sn:In2O3 (ITO) nanoparticles. In the original work by Kay,
module area efficiency was 5.3%, which was 94% of the
active area efficiency.147 However, this design was not stable
because no barrier was used between adjacent cells and elec-
trolyte composition could change from cell to cell. Unfortu-
nately, adding barriers between cells reduces the fraction of
active area in the module. Pettersson developed an encapsu-
lation method to separate cells of monolithic modules that
was stable at low light intensity but degraded quickly at 1
sun conditions.149
Pettersson also reported on monolithic current-collecting
modules which use one current collecting strip between each
pair of cells instead of series connected modules.139 These
devices had module area efficiencies above 4% at 1000 W/m2,
and accelerated aging showed similar stability to single
cells—negligible loss under light soaking but nearly 40% loss
during storage at 80 �C for 1000 h. The current-collecting
monolithic module was first introduced by Hinsch et al.,although performance and stability of the module were not
reported.150 More research and development is needed to
determine the most efficient, stable, and manufacturable mod-
ule designs.
B. Processing methods
DSSC materials and module architectures must be amena-
ble to low-cost, high-throughput processing, in addition to
being efficient and stable. Details of manufacturing lines are
not widely available because of the obvious commercial im-
portance to their owners. Manufacturing processes will also
vary significantly depending on whether DSSC modules are
rigid or flexible, the module architecture, and the substrate
material. Materials and manufacturing steps must be well-
matched. Dye molecules are sensitive to temperature above
�100 �C, so any high temperature processing should be
completed before dyeing. It is possible to seal cells and leave
small fill holes for subsequent, dyeing, rinsing, and electro-
lyte filling. With this procedure, the design of the filling sys-
tem and the module is critical. Sastrawan et al. at Fraunhofer
ISE have designed meander-type parallel current collecting
modules with partially interdigitated current collectors,
Fig. 1(a), that minimizes the number of cells that need to be
independently filled.151 Further details regarding processing
advantages of different module designs were described by
Tulloch.152
In order to compete with other current PV technologies
and reach future targets in terms of cost, manufacturing line
speeds of 2 to >20 m/min are likely to be necessary.33 Proc-
essing in the research lab for small cells is commonly per-
formed by hand and without regard for time; however,
suitable automated and high-speed protocols must be
FIG 10. (Color online) Schematic cross sections of four types of DSSC module designs: (a) parallel-connected, (b) Z-interconnected, (c) W-interconnected,
(d) series-connected monolithic. Not drawn to scale. Current is primarily conducted into the page in (a) and horizontally in (b)–(d). Adapted from Ref. 33.
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developed for manufacturing. For example, TiO2 nanopar-
ticle films are commonly sintered at 450 �C for 30 min and
dyeing is routinely done overnight. These procedures would
require exceedingly long process lines to accomplish. Alter-
natives materials and processes such as fast-curing TiO233
and dyeing procedures requiring only minutes153 have
already been developed.
One of the advantages of DSSCs compared to crystalline
silicon and thin films is low-cost, low-energy processing.
Energy payback periods of less than one year are expected.31
No cleanroom, vacuum processing, or high temperatures
(above 450 �C) are required. No new technologies need to be
developed for DSSC manufacturing. High throughput proc-
esses can be borrowed from other industries including thin
film PV, printing, and laminating. For example, TiO2 layers
are typically deposited by screen printing and cured in an in-
line oven. Availability of standard equipment and processing
will enable fast development of new manufacturing lines for
DSSC modules. Equipment for small scale automation of
many of the DSSC fabrication processes is already offered
by Dyesol.
The most efficient and stable modules to date have been
produced using two glass substrates in a sandwich configura-
tion. However, glass is a very costly component in the mod-
ule, especially when coated with transparent conducting
oxide. Rigid glass is useful for some applications such as
building integrated PV, but flexibility and light weight are
desired for other applications including portable charging
stations. In that case, roll to roll processing of DSSC modules
on metal foils or polymer substrates is desirable. Great pro-
gress has already been made in this area. Ikegami et al., with
Peccell Technologies, have applied the Z-interconnected
module architecture on plastic [polyethylene naphthalate
(PEN)] substrates.154 Modules 30� 30 cm2 showed efficien-
cies of 3.7% after a brief aging period, and maintained at
least half that efficiency through 880 h of illumination at
1000 W/m2. Cells also showed good stability upon continu-
ous heating at 55 �C and 95% RH for over 200 h, as well as
20 temperature cycles from �10 to 50 �C.
The first commercial DSSC products were produced by
G24 Innovations in 2009. G24i is operating a 2 MW pilot
line (4� 106 units per year of 0.5 W modules), with 10 MW
production line under construction.141 Their design com-
prises flexible Z-interconnected sandwich modules with
working electrode on titanium foil and counterelectrode on
PEN or polyethylene terephthalate (PET). Modules are man-
ufactured on roll to roll processing equipment for indoor or
outdoor applications to charge portable electronics. The
20� 15 cm2 modules designed for indoor use are only
1.2 mm thick and weigh only 17 g, while 20� 14 cm2 out-
door modules are 1.8 mm thick and weigh 50 g due to addi-
tional encapsulation features. Typical power output at 1 sun
illumination is 550 mW at 5 V, giving module area effi-
ciency of �2.2%.155 Products come with 1 year warranty.
Outdoor testing shows negligible change in efficiency of a
1.6% module over 8 months in Wales.141
Many different combinations of materials, module designs,
and processing methods are currently being pursued. It is too
early to tell which is the best strategy, and there are likely
multiple potentially successful approaches depending on the
desired application. A balance must be struck between effi-
ciency, stability, and cost. Hagfeldt et al. have suggested that
the winning strategies may be determined by the most func-
tional encapsulation process.41 For now, key groundwork is
being laid in the form of many demonstrations and a few pilot
plants and high-volume production facilities. These steps will
initiate a chain of suppliers of materials and processing equip-
ment for the DSSC industry that will bring down module costs
and lead to more investment in this promising technology.
C. Materials costs and availability
Global DSSC production is predicted to exceed 100 MW
in the year 2012, increasing dramatically from 5 MW in
2009.141 The 2012 prediction is 0.1–1 % of the global PV
market, and market share should continue to climb as com-
panies move from pilot plants to manufacturing facilities
and economies of scale begin to aid in cost reduction. It is
important at this point to assess the cost and availability of
key raw materials for DSSCs in order to calculate likely sce-
narios for module costs and scale of manufacturing. Many
assumptions contribute to forecasting materials availability
and costs, and predictions can vary substantially. The most
detailed analysis that is publicly available has been per-
formed by Dyesol.33 Numerical data presented here is
directly from or derived from that reference unless otherwise
noted.
Availability of materials used in conventional high-
efficiency DSSCs would enable economical production of
hundreds of gigawatts of DSSC panels, with terawatt pro-
duction possible for slight modifications of materials such as
ruthenium-free dyes and platinum-free electrocatalysts. Such
modifications currently produce lower efficiencies than con-
ventional cells, but research is ongoing to improve perform-
ance. In terms of both cost and availability, ruthenium,
platinum, and silver are quite precious but are used in very
small quantities in DSSCs. For example, only 0.1 g/m2 Ru
and 0.02 g/m2 Pt are required for DSSCs. Identified resour-
ces of Ru exceed 11 000 tons and annual global consumption
has not surpassed 50 tons prior to 2008. Utilizing 10% of
known Ru reserves in 7% efficient DSSC panels would ena-
ble production of 400 GWp. 400 GWp DSSC production
would require only 0.3% of the world’s known Pt reserves.
Ag in interconnects and Sn in TCO films are required in
higher volume than Pt and Ru, and more severe competition
for these metals could become problematic in coming deca-
des.156 No limitations are expected from availability of other
DSSC materials.
Figure 11 shows the distribution of present (2009) com-
ponent costs in US$/m2, assuming a single glass substrate
and production on the order of 100 000 m2. Data is taken
from the middle of the ranges given by Desilvestro et al.33
The total component cost is $55 per m2, or $0.78/Wp, assum-
ing a module area efficiency of 7%. The European NANO-
MAX consortium estimated additional manufacturing costs
to be 20–40 % of the materials costs.157 Adding balance of
020801-15 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-15
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systems and other manufacturing costs to assemble compo-
nents into modules, installed costs of $2–4/Wp seem reason-
able at present. This cost is in the same ballpark as c-Si and
thin film PV, and economies of scale and manufacturing
learning curves can be expected to decrease this price signifi-
cantly at higher production volumes. However, lower DSSC
efficiencies would require significantly larger areas to pro-
duce the same amount of power as c-Si or CdTe. Therefore,
improving cell and module efficiency, as well as lifetime, is
also essential to reducing cost and competing with other PV
technologies.
The most costly components in DSSCs are the dye, TiO2,
and glass/TCO substrate. In the case of the dye and TiO2, the
raw materials themselves are not very costly compared to
the processing and synthesis steps. Ru is only �10% of the
dye cost. The rest of the dye molecule is inexpensive organ-
ics, but the synthesis and attachment of ligands and subse-
quent purification are quite expensive. Manufacturing dye at
kilogram compared to gram volumes reduces price from
$700/g to $70/g, and smaller but still significant reduction in
processing costs can be expected for production of thousands
of kilograms.
TiO2 and Ti raw material is abundant and cheap, but
hydrothermal synthesis of TiO2 colloids is relatively expen-
sive. Hydrothermally prepared colloidal TiO2 currently costs
�$500/kg in ton quantities, about 30–50 % more than target
production costs. Other preparation methods such as flame
pyrolysis are less expensive, but control over particle size
and shape is sacrificed and efficiencies are generally lower.
TCO-coated glass is also expensive, and the cost of glass
per module area doubles for sandwich cells compared to
monolithic modules. It is expected that economies of scale
will be sufficient to lower the cost of glass to acceptable lev-
els when produced at high volumes. No technological break-
through is needed. Additionally, partnerships between glass
companies and DSSC manufacturers may further reduce the
cost of integrating the TCO/glass into DSSC modules. For
example, Dyetec Solar is a new joint venture between Dye-
sol and Pilkington Glass that is focused on glass-based
DSSCs for building-integrated PV applications. Alterna-
tively, different transparent conducting substrates based on
carbon nanotubes, graphene, or conductive polymers may
eventually offer acceptable performance at lower cost. Other
alternatives include metal foils such as Ti or steel with trans-
parent polymer encapsulation. High quality barrier layers are
still quite expensive, and there is some tradeoff between cost
and expected lifetime.
Pt is expensive but is used in extremely small quantities.
Cost of electrolytes based on organic solvents is not problem-
atic, although ionic liquids used to improve stability are still
more expensive than desired. Ag for contacts and interconnects
imparts a significant cost. Al and Cu are cheaper alternatives
for low power modules, but resistive losses could become sig-
nificant for large panels. Encapsulation has not been directly
accounted for here and could be significant, particularly if high
performance barrier laminates are needed to replace glass.
A number of directions in research and development are
expected to have significant impact on module costs.
Research into dyes with higher extinction coefficients will not
only improve efficiency, but also reduce cost by requiring less
dye and TiO2 per module area. Metal-free organic dyes would
eliminate the need for costly and rare ruthenium. Reducing
processing costs and improving yield in dye production would
significantly reduce the overall module costs. Alternatives to
hydrothermal synthesis for very high volume production of
TiO2 colloids that maintain good performance would be
highly beneficial. These research directions mainly impact the
cost of supplied materials. Improvements in process integra-
tion and module design will also yield significant benefits.
VI. CONCLUSIONS AND OUTLOOK
Successful commercialization of any PV technology
requires a combination of high efficiency, long term stabil-
ity, and low cost. DSSCs are just reaching the point where
pilot plants and small manufacturing facilities have become
feasible. Certified DSSC cell efficiencies exceeding 11% and
module efficiencies of 9.9% have been reported.5 DSSCs
have shown stable performance for over 20 000 h of continu-
ous illumination, thermal cycling, and several years of out-
door testing.28,30,142 These stability tests indicate potential
outdoor lifetimes beyond 20 years. Material and manufactur-
ing costs will continue to decline as manufacturing volume
increases and proper supply chains develop. There are no
material limitations inhibiting production of hundreds of
gigawatts and even terawatts of DSSC capacity.33,156
The combination of efficiency, lifetime, and cost puts
DSSC in position to compete with other PV technologies.
Efficiencies are significantly lower than c-Si and CdTe, so its
most direct competitor at present is amorphous Si for low-
cost, low-power markets such as charging consumer electron-
ics. Many DSSC demonstration modules are now available,
and G24i introduced the first commercial products in 2009.
The great challenge now lies in finding materials, module
architectures, and manufacturing processes that provide
FIG. 11. (Color online) Chart showing predicted breakdown of material
costs in $/m2 for different DSSC components. Data taken from the middle
of the range suggested by Desilvestro et al.33 based on 100,000 m2 annual
production.
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optimal combinations of high efficiency, long term stability,
and low cost. Increasing efficiency will be a primary driver
that enables DSSC to compete in the higher power markets
with CdTe and perhaps c-Si. Optimizing energetic alignment
of cell components while maintaining appropriate kinetics
could lead to efficiencies above 15%. With the vast number
of possible combinations of dye, redox couple, and anode,
the right combination is likely to exist. The challenge is in
developing a detailed understanding of the chemistry and
physics of DSSCs so that rational approaches can identify
such materials combinations without excessively long peri-
ods of trial and error. Efficiencies of small area cells have
not increased significantly in the last decade, and new break-
throughs will likely require changing multiple cell materials
simultaneously. Donor-p-acceptor organic dyes with Co
polypyridyl redox couples show significant promise.
Good stability has been demonstrated through both accel-
erated aging experiments and outdoor testing. Many of the
key mechanisms of degradation and failure have been identi-
fied. However, no standard protocol exists for aging and sta-
bility tests. Additionally, it is still not possible to predict
lifetimes from stability tests with the necessary degree of
confidence for commercial products. More research and de-
velopment is needed in these areas to determine which accel-
erating aging tests provide the best correlation to DSSC
lifetime.
Most of the manufacturing processes for DSSC modules
are derived from other well-known printing, laminating, or
PV manufacturing processes. However, the design of mod-
ules and integration of processing steps will be critical to
achieving high throughput and low cost without sacrificing
performance. Procedures such as dyeing and electrolyte fill-
ing that are trivial yet time consuming at the cell level must
be done rapidly and efficiently when producing DSSC mod-
ules at commercial scales. Encapsulation is critical to DSSC
lifetime and must be carefully and cost-effectively integrated
into the manufacturing.
In only 20 years, great progress has already been made in
bringing DSSCs from the first report of 7% efficient cells to
recent reports of 12.3% cells, 9.9% modules, and commer-
cial products. Many challenges remain to improve efficiency
and lifetimes while reducing cost. These challenges must be
met in order to advance beyond niche applications, such as
powering consumer electronics, and enter new markets such
as building integrated PV and remote power production.
With a strong core of academic researchers and growing in-
terest from many industrial players, continued rapid
advancement of DSSC technology should be expected.
ACKNOWLEDGMENTS
This work was supported by NSF CAREER Award No.
CBET-0846464 and NSF Grant No. CMMI-1000111. The
author acknowledges Borirak Opasanont and Jennifer Bing
for assistance with figures.
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