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Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing Jason 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 for improving 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

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Page 1: Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

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

Page 2: Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

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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Page 3: Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

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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Page 4: Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

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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Page 5: Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

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

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

Page 6: Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

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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Page 7: Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

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

020801-7 Jason B. Baxter: Commercialization of dye sensitized solar cells 020801-7

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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

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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

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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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2010).4C. A. Wolden, J. Kurtin, J. B. Baxter, I. Repins, S. E. Shaheen, J. T. Tor-

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