ISSN 1754-5692

Energy&Environmental Science

COVER ARTICLEDrain et al.Commercially viable porphyrinoid dyes for solar cells

REVIEWHofmann and SchellnhuberOcean acidifi cation: a millennial challenge 1754-5692(2010)3:12;1-G

www.rsc.org/ees Volume 3 | Number 12 | December 2010 | Pages 1813–2020

Vo

lum

e 3

|

Nu

mb

er 1

2

| 2

01

0

Energy &

Enviro

nm

ental S

cience

Pa

ge

s 1

81

3–

20

20

www.rsc.org/publishingRegistered Charity Number 207890

Dank u wel kiitos takk fyrir

aitäh děkuji D’akujem БлагодаряСпасибо Thank you Tak

grazie Takk Tack 唔該 Danke

Merci gracias Ευχαριστω

どうもありがとうございます。

As a result of your commitment and support, RSC journals have a reputation for

the highest quality content. Your expertise as a referee is invaluable – thank you.

To our referees:

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online / Journal Homepage / Table of Contents for this issue

PERSPECTIVE www.rsc.org/ees | Energy & Environmental Science

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

Commercially viable porphyrinoid dyes for solar cells

Ivana Radivojevic,†a Alessandro Varotto,†a Christopher Farleya and Charles Michael Drain*ab

Received 29th March 2010, Accepted 15th June 2010

DOI: 10.1039/c0ee00009d

Multifunctional molecules bearing different dyes, such as donor–acceptor systems, synthesized by

covalent chemistry have provided a wealth of information on the fundamental nature of electron and

energy transfer in organic systems and there is a growing literature on the materials properties of dyes

on surfaces. However, in the vast majority of cases the synthetic costs of producing these covalently

bound systems prohibit them from deployment in commercially viable devices. Thus, to achieve both

the needed multifunctionality and to bring the synthetic costs in line with potential commercialization,

supramolecular approaches to the formation of photonic materials can be exploited. This perspective

focuses on porphyrinoids as exemplary dyes, but the concepts and design principles extend to other

chromophores.

Once human beings realize something can be done, they’re not

satisfied until they’ve done it. Frank Herbert.

Introduction

One of the fundamental scientific issues in the chemical sciences

that can ‘‘enable new opportunities to meet societal needs’’ is

‘‘How far can we push self-assembly?’’1,2 Self-assembly can play

pivotal roles in addressing some of the great challenges in science

today such as the development of cost-effective solar energy

harvesting systems and devices with lower power consumption in

order to meet the ever increasing energy requirements of the

world in an environmentally acceptable way.3 The continued

development of concepts in supramolecular chemistry has other

important applications, e.g. in sensors, smart materials, and

medicine. The overarching theme of this perspective is to provide

a roadmap to develop self-assembly and self-organization

approaches to create functional, photonic materials and proto-

typical solar energy harvesting devices containing the various

porphyrinoids. Other highly stable dyes and polymers are dis-

cussed elsewhere, vide infra. As concluded by Kalowekamo and

Baker, the manufacturing costs for organic and hybrid devices

can be quite competitive compared to Si-based standards.4 This

aHunter College and Graduate Center of the City University of New York,695 Park Avenue, New York, New York, 10065, USA. E-mail: cdrain@hunter.cuny.edubThe Rockefeller University, 1230 York Avenue, New York, New York,10065, USA

† IR and AV contributed equally to this manuscript.

Broader context

Organic and hybrid organic/inorganic photovoltaic devices have gre

viable because of the reduced manufacturing costs, reduced depend

mental impact because of reduced energy to make the devices and m

needed for deployment of these devices, the dyes used in solar energ

several classes of dyes that may be suitable, porphyrinoids, such as p

very attractive because of their stability and wide range of photoni

This journal is ª The Royal Society of Chemistry 2010

perspective aims to enthuse development of new concepts and

designs of hierarchical dye materials for applications such as

solar energy conversion and photonic devices using porphyr-

inoids (Fig. 1–4) as exemplary materials.

Concepts

Self-assembly

The near exponential growth in the number of publications on

self-assembled porphyrinic systems in the last two decades5–17 was

propelled by: (a) the potential to make functional materials from

Fig. 1 Diverse dyes, from left to right: C60, free base 5,10,15,20-tetra-

phenylporphyrin; 5,10,15,20-tetraphenylporphyrinato Fe(III)Cl;

5,10,15,20-tetra-(4-carboxyphenyl)porphyrin; tetra-(tert-butyl)phthalo-

cyaninato Zn(II); hexadecylfluorophthalocyaninato Zn(II); monoamino-

tri-(tert-butyl)phthalocyaninato Zn(II); mono(thiopentyl)pentadecyl-flu-

orophthalocyaninato Zn(II); Pc 735; Pc787. See Fig. 2–4.

at potential in a variety of applications and can be economically

ence on rare metals, versatility of substrates, and less environ-

inimized use of toxic metals. Because of the very large quantities

y absorption must also be commercially viable. While there are

orphyrins and phthalocyanines, and the metallo derivatives, are

c properties.

Energy Environ. Sci., 2010, 3, 1897–1909 | 1897

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

tectons that are significantly easier to synthesize compared to

covalently linked arrays, and (b) the large number of potential

applications of porphyrinoids including: photonics, sensors,

catalysts, electronics, and components of solar energy utilization

devices. Self-assembly11 results in discrete supramolecular

Fig. 2 Structures of dye molecules.Ivana Radivojevic

Ivana Radivojevic finished her

PhD at Hunter College of The

City University New York in

2010. She received her Diploma

in Chemical Engineering from

Faculty of Technology and

Metallurgy, University of Bel-

grade, Serbia, in 2003.

Currently, she is working in the

Nanophotonics and Material

Chemistry laboratory of Prof.

Charles Michael Drain. Her

research is focused on charac-

terization of self-organized

functional nanostructured materials on surfaces for photonic

applications.

Christopher Farley

Christopher Farley received his

Bachelor’s in physics from Fordham

University in 2004. After several

years away from science, he recently

entered The Graduate Center at The

City University of New York to

pursue a PhD in Chemistry with

a concentration in Nanotechnology

and Materials Science. He is

currently working in Prof. Charles

Michael Drain’s laboratory at

Hunter College, CUNY, investi-

gating the photophysics of potential

nanoscale materials for photonic

devices.

Alessandro Varotto

Alessandro Varotto is a post-

doctoral Scholar at the Univer-

sity of California, Santa

Barbara with the Mitsubishi

Chemical-Center for Advanced

Materials, in Prof. Fred Wudl’s

group. He received his ‘Laurea’

degree in Chemistry from

University of Padova, Italy, in

Prof. Tommaso Carofiglio lab

and his PhD in Chemistry from

The City University of New

York in 2009, under the super-

vision of Prof. Charles Michael

Drain at Hunter College. His

doctoral thesis focused on self-

organized organic dyes on

surfaces for photonic devices.

1898 | Energy Environ. Sci., 2010, 3, 1897–1909

systems that are usually topologically closed because the

component molecules are carefully designed with complementary

recognition groups and geometries to maximize specific inter-

molecular interactions. This strategy allows the predictable

formation of nanoarchitectures such as squares and rosettes with

a degree of predictability in their supramolecular properties such

as energy or electron transfer and luminescence. The bottom-up

design and organization of molecules into materials are

aided conceptually by considering four levels of

structure: molecular (primary), supramolecular (secondary), the

organization of supramolecular systems into solid state materials

such as crystals (tertiary), and materials in devices

(quaternary).11

Self-organization

Self-organization generally relies on both specific and non-

specific intermolecular interactions to yield non-discrete

Charles Michael Drain

Charles Michael Drain studied

art at the University of Missouri

at St Louis, and took a chem-

istry course to understand the

chemistry of lithography only to

find that he liked chemistry, and

that the mechanism was largely

unknown. He earned his PhD at

Tufts University with Prof.

Barry B. Cordon in 1989. His

studies on the supramolecular

chemistry of porphyrins began

with postdoctoral work with

Prof. David Mauzerall at

Rockefeller University, followed

by two years with Prof. Jean-Marie Lehn in Strasbourg and Prof.

Dewey Holten at Washington University. He joined the faculty at

Hunter College in 1996.

This journal is ª The Royal Society of Chemistry 2010

Fig. 3 Facile modification of a core porphyrin platform, TPPF20 enables

investigation of new molecular design concepts. For example, R ¼(CH2)11CH3, or CH2CH2(CF2)9CF3. Using a core Por platform affords

chemically compatible systems.

Fig. 4 Facile modification of a core phthalocyanine platform, PcF16

enables investigation of new molecular design concepts. For example,

Pc735 has eight S(CH2)11CH3 mostly on the outside positions and Pc787

substitutes all of the F for the thioalkane. SCH2CH2(CF2)9CF3 can also

be used to form highly fluorous derivatives. Using a core Pc platform

affords chemically compatible systems.

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

systems that are less ordered than those resulting from self--

assembly—e.g. liquid crystals, mesogenic materials, and mono-

layers on surfaces.13,18,19 As an alternative strategy to the

formation of hierarchical functional materials, self-organization

offers several advantages and some drawbacks compared to self-

assembly. There is usually a defect threshold below which self-

organized systems maintain their function. However, a defect in

a self-assembled system results in a new supramolecular material

that may have diminished or undesired properties. The syntheses

of the molecular components of self-organized systems can be

easier than those designed for self-assembly because non-specific

intermolecular interactions oftentimes allow simpler molecular

structures to be used (less information has to be programmed

into the molecule).

An important advantage afforded by self-assembled/organized

materials with potential commercial applications is the high yield

syntheses of molecular components.20–26 Additionally, materials

assembled by specific intermolecular interactions such as metal

ion coordination, complementary hydrogen bonding, and certain

electrostatic constructs are generally more robust than those

organized by dispersion forces and hydrophobic/hydrophilic

properties.24 The disadvantages of self-assembled/organized

systems largely stem from the complex equilibriums inherent to

supramolecular entities (thermodynamic products) that make

both characterization and material/device stability keystone

issues in real-world applications. On the other hand, these

equilibriums also can be exploited to anneal the material to

improving device performance and yield, and afford a mecha-

nism of self-repair.18,27

This journal is ª The Royal Society of Chemistry 2010

Surfaces

Applications of functional materials require interactions with

surfaces in devices; however, the structure and function of self-

assembled/organized materials also depend on interactions with

these surfaces. Surface interactions play a complex role in the

hierarchical organization of supramolecular systems, and this

must be understood as part of the design process.13,14,28,29 Por-

phyrinoids can be organized on a variety of surfaces as chemically

bound monolayers (SAMs)28,30–40 or by adsorption.41–43 The

surface deposition of discrete arrays, such as square tetramers pre-

assembled by metal ion coordination or H-bonds, is much more

difficult.13,28 This difficulty is due to the intertwined factors

affecting the equilibriums of self-assembled systems as they are

applied to surfaces: e.g. solvent evaporation, fluid dynamics,

concentration changes, and surface energetics. Consequently, self-

assembled arrays can: (a) fall apart as the components separate

into different domains, (b) aggregate, or (c) reorganize into

different structures. A detailed understanding of molecular–

surface and supramolecular–surface interactions can be exploited

both as a design element for the formation of hierarchical

photonic materials and as a means to photonically couple nano-

materials to the macroscopic world. Discussion of the reliable and

predictable control of the structure of supramolecular materials at

interfaces is an essential aspect of this work.

Porphyrinoids

The remarkable stability, diverse photophysical and chemical

properties make porphyrinoids (Fig. 2) exemplary molecules to

construct hierarchical functional materials.9,44–49 Additionally,

the functional properties of porphyrinoids can be systematically

varied both by metalation with nearly every metal in the periodic

table and by substituents on the macrocycle, e.g.: excited state

lifetimes,50,51 redox potentials, catalytic activities,52,53 magne-

tism,12,54–60 optical cross-sections, and molecular dynamics. The

functionality of these materials is also profoundly affected by the

structural or architectural organization of the chromophores, as

well as environmental factors such as solvent, matrix, and

surface.12,16,61–68 The structural rigidity and topological diversity

of the porphyrins, phthalocyanines, and the metalo complexes

make these dyes well suited for the engineering of supramolec-

ular materials.

Materials composed of porphyrinoids can be very robust to

real-world conditions such as elevated temperatures, the presence

of dioxygen and/or water.10,38,69–71 Porphyrinoids are versatile

tectons that afford a rich diversity in self-assembled nano-archi-

tectures because of their rigid molecular topology.13,65,72–89 A few

reports describe the photonic properties of self-assembled

(discrete) porphyrin (Por) and phthalocyanine (Pc) mate-

rials.61,72,90–94 Since the photonic and chemical properties of Pc are

complementary to the Por (e.g. the UV-visible spectra, lumines-

cence, stability, metal ion binding are different), supramolecular

materials composed of combinations of chromophores will enable

devices that are otherwise unobtainable using only one dye. As

expected, additional fused benzenes on the PC core, e.g. to yield

naphthocyanine, shift the absorption spectra to the red, but the

additional fused benzene rings make the ligands more vulnerable

to oxidative degradation.95

Energy Environ. Sci., 2010, 3, 1897–1909 | 1899

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

Simple Pc derivatives are commodity chemicals used as dyes,

toners, and photonics. Simple tetraaryl Por can be made in

greater than 60% yield and can have similar applications.96 In

both cases, less symmetric porphyrinoid dyes requiring complex

multistep synthetic procedures resulting in low yields based on

the initial reagents and/or extensive chromatography are not

considered in this perspective. For both Pc and Por, mixtures of

positional isomers or even mixtures of compounds may be used

as the photo-active material.97 Core Por and Pc platforms that

can serve as test-bed for new molecular design concepts have

been reported (Fig. 3 and 4).98–100

Fig. 5 Sequentially dipping a substrate into separate solutions of

a cationic tetra-N-methylpyridinium porphyrin (top left) and an anionic

polyoxometalate (top right) allows formation of thin films with highly

controlled optical densities on a variety of substrates (bottom). These

films are stable even to sonicating in water (W), toluene (T) and 100 mM

NaCl. This is an alternative to spin coating and vacuum deposition

methods.

Dyes in solar cells

The advantages, disadvantages, and architectural considerations

of organic and dye-based solar cell devices are well reviewed.101–110

These devices generally fall into two groups: (a) dye-sensitized

solar cells wherein a dye resides on an inorganic semiconductor

surface such as TiO2, SnO2, and ZnO;111 (b) organic solar cells

that use conducting or semiconducting polymers. The main

concerns with dye systems are that of charge separation and

transport versus recombination kinetics. Recombination kinetics

should be much slower than electron injection/collection. These

depend on the component dyes, the substrate and the presence of

intervening electron acceptors in the organic devices. Since

a good solar cell should have a 20+ year lifetime, the dye will

need to undergo more than 108 turnover cycles without signifi-

cant decomposition.112

The goal of this perspective is to describe porphyrinoids that

can be readily synthesized in large scales, and the chelates of

earth-abundant metal ions. We focus herein on the dye systems

that can be incorporated into devices that are designed for

photovoltaics, photocatalysts for water splitting, or to derive the

overpotential for water splitting catalysts.

Matching the excited state energies to the device design, as in

substrate band gaps, is critical to performance. Charge injection

or separation from the singlet state yields the most potential

energy, but the triplet state may be sufficient in some device

designs. Cyclic voltammetry and optical spectra allow the ener-

gies of the HOMO and LUMO to be evaluated.113,114 Also, the

photophysical properties of the dye need to be determined,49 e.g.:

singlet state lifetimes, yield of inter-system-crossing to the triplet

state, and triplet state lifetimes.

Surface deposition

There are many means to deposit porphyrinoids onto surfaces

ranging from chemical vapor deposition at elevated temperatures

and high vacuum, to solution casting. The latter approach can be

accomplished by spin casting, aerosol spraying, dipping, layer-by-

layer methods, and electrodeposition. Each fabrication method

has associated energy consumption and results in different surface

morphologies. With judicious choice of substituents, it is possible

to use self-organization via solution deposition to achieve highly

ordered structures similar to those observed in materials made by

chemical vapor deposition (CVD). Layer-by-layer methods for

Por115 and Pc116 are more recent and afford opportunities to

incorporate other active materials within the active layer in

a sequential manner (Fig. 5). There is also considerable work in

1900 | Energy Environ. Sci., 2010, 3, 1897–1909

examining the electrochemical28,30–40 and transport117 properties of

Por and Pc covalently bound to surfaces, but for the most part

these dyes are not commercially viable on the scales needed to

deploy them for common usage. Though there are many papers

describing porphyrins on surfaces, we concentrate here on active

devices with simple dyes.

As opposed to covalently linked donor–bridge–acceptor and

related molecules,118 axial coordination of acceptors to metal-

loporphyrinoids can be a simpler means to constructing dye

systems where the initial charge separation remains efficient, e.g.

with SiPc-C60,119 while significantly reducing the synthetic

efforts. These donor–acceptor systems facilitate charge disloca-

tion in active layers of several types of photovoltaic devices.

These constructs have been incorporated into a variety of active

layer architectures, vide infra.

Phthalocyanines

Many Pc are commercially used in displays, optical recording

media, dyes, and various inks because of their tunable photonic

properties. Pc are robust, and the product of high-yield reactions

of commodity chemicals.

Unsubstituted Pc in solution display a strong and narrow

absorption peak in the red (3 > 105 M�1 cm�1); the wavelength

and intensity vary according to the specific metal ion chelated.

This absorption is the result of a single transition from the

ground state (HOMO) to the excited state (LUMO) of the

molecular orbitals. Once Pc are deposited on a surface as

a film, they tend to aggregate predominantly by p–p interac-

tions. Not only does this interaction broaden the absorption

peak, but it can also give rise to additional absorption peaks

because of a splitting in the energy of the LUMO. This is

a result of the formation of face-to-face H-aggregates, and/or

slipped cofacial aggregates, and/or head-to-tail J aggregates

This journal is ª The Royal Society of Chemistry 2010

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

depending on the molecular structure.120 Because the substitu-

ents on Pc are appended directly to the chromophore, the

electronic properties of the substituents play a pivotal role in

the HOMO–LUMO energy gap;97,121,122 therefore, affecting the

ground electronic spectra and excited state luminescence

properties.

Changing the metal ion center can also vary the location of

the Q-band.122 Metals with large ionic radii, such as lead, tend

to bend the normally flat core of a Pc macrocycle thereby

decreasing the symmetry of the molecule and lifting the

degeneracy of the molecular orbitals, which in turn affect the

opto-electronic properties.123 Therefore, careful consideration

of these properties is necessary for incorporation of Pc dyes

into solar cells.

Unsubstituted Pc are not soluble enough to form concen-

trated solutions for the preparation of films by spin-coating or

drop casting; therefore, most devices with the Pc core are

fabricated by thermal evaporation under vacuum.124 CuPc and

ZnPc are widely investigated as donor materials in the prepa-

ration of thermally evaporated solar cells, both in bulk hetero-

junction106 (BHJ) and layered architectures.125 Other

metallophthalocyanines, such as Ni, Si and Fe, are also widely

used. Synthesis of Pc with exocyclic organic groups to increase

solubility in either organic or aqueous solvents, and to form

liquid crystalline materials is well established and commercially

used in some displays.120,122 Modification of the Pc macrocycle

by high yield click-type reactions97 has more recently afforded

derivatives that were inaccessible because of labile functional

groups that decompose in the Pc-forming reaction.126 Pc are

excellent candidate dyes, but generally cover the red side of the

solar energy spectrum, thus necessitating the use of other dyes

such as porphyrins.

Porphyrins

In photosynthesis, nature uses carotinoids and Por with various

modifications to the macrocycle to make the chlorophylls to

cover the solar energy spectrum,127 but neither are robust enough

to incorporate into devices with 20 year lifetimes using current

device designs. The parent Por macrocycle, however, is robust

enough so these are good candidates for light harvesting since

they exhibit strong absorption in visible region, 400–700 nm. The

maximum absorption of Por is a B (Soret) band at 390–430 nm

with molar extinction coefficient (3) on the order of 105 M�1 cm�1

and several Q-bands between 500 and 650 nm with 3 10–20 fold

less.128 The Por fluorescence quantum yields are low (<15%) and

have 1–15 ns lifetimes, but the triplet quantum yields are corre-

spondingly high. Both pyrrole substituted and tetraaryl-

porphyrins can be synthesized in high yields, e.g. 25–67%

reported by Sharghi et al. for the latter.129 For the present

discussion only symmetric compounds are considered because

the synthesis uses pyrrole and benzaldehydes in the presence of

a catalyst (CF3SO2Cl), but no organic oxidants. The purification

is facile, making tetraarylporphyrins attractive for industrial

production.

Thus, the use of Por to cover the blue end of the solar spectrum

and Pc to cover the red end, and both have good optical densities

in the middle, affords a range of dyes to incorporate into various

devices.

This journal is ª The Royal Society of Chemistry 2010

Devices with phthalocyanines

BHJ versus bilayer devices

Generally, bilayer devices are limited by the short diffusion

length of the exciton that forms in the donor (D) layer, which

needs to diffuse to the acceptor (A) layer before recombining.

Thus, recombination, rather than charge transport properties, is

a main limiting factor to device efficiency for this geometry.124

Additionally, the series resistance that develops between the D

and A layers contributes substantially to lower the performance.

Forrest et al. reported one of the most efficient bilayer devices

using CuPc/C60 that had a very low series resistance and a 4.2%

power conversion efficiency.130

BHJ architectures, in which donor and acceptor molecules are

blended or co-deposited within the same layer, offer a more

intimate contact and favor the charge separation process. On the

other hand this geometry suffers from poor crystalline order, low

carrier mobility, and increased charge-trap densities.124 Room

for improvement resides on controlling the film morphology to

form interpenetrating fingers of D and A. Such systems can

improve both charge transfer and charge separation. Leo and co-

workers demonstrated an improved efficiency in ZnPc/C60 by

interface modification at the nanoscale.131 The improved effi-

ciency was achieved by co-deposition of sequential multi-layers

of D and A and blended D/A, resulting in enhanced exciton

dissociation and photocurrent extraction. The improved

performance was explained by the formation of an inter-

penetrating network of donors and acceptors, thereby increasing

the photocurrent and suppressing charge recombination. In this

and another work,132 the ratio of co-deposited Pc and C60

appears to affect the Voc of the device. This is in contrast with the

theory that the Voc is defined by the difference in energy of the

donor HOMO and LUMO of the acceptor. However, a more

recent theoretical treatment of experimental data about the

origin of the Voc was suggested,105 wherein the Voc can be influ-

enced by the shape of the orbitals of the D. The interface between

the generally flat D and A molecules is also important in that

molecules bearing bulky groups (such orthogonal phenyls) that

prevent strong intermolecular interactions, display higher Voc.105

The greater Voc was explained by a steric effect that reduces

charge recombination.

Pc have also been used as an additive to improve polymer/C60

BHJ solar cells. Polymer-containing photovoltaics have already

surpassed 5–6% power conversion efficiency.133 The higher

performance is mainly due to a better degree of phase segregation

between D (polymer) and A (usually [6,6]-phenyl-C61-butyric

acid methyl ester, PCBM). Once the film is annealed, it tends to

form optimally sized domains108 that facilitate charge separation

and transport to the electrodes. Soluble small molecules may lack

the ability to form similar domains. For example, a SiPc

improved the efficiency of a polymer solar cell when blended in

the film largely because the Jsc increased considerably.119 The

authors explained this increase with two mechanisms: a direct

contribution from the dye resulting from the extra absorption in

the red, and an indirect contribution because the dye molecules

promote charge separation from the polymer excitons at the

interface, due to the proper alignment of the orbitals, even if they

do not absorb photons directly.

Energy Environ. Sci., 2010, 3, 1897–1909 | 1901

Fig. 6 The band gap of chemically compatible dyes on a core Pc plat-

form can be systematically tuned by a balance of electron withdrawing F

and electron donating SR groups. The dark purple areas are where two or

more overlap.

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

Several groups have reported that Pc can be used to increase

the Voc of a device. As shown by Murata and co-workers, when

a thin layer of a Cu or Zn Pc was deposited between a pentacene

donor and C60 acceptor the Voc increased as a result of the

HOMO of the Pc.134 The nearly isoenergetic HOMO of the Cu

and Zn Pc is higher in energy than that of pentacene. ZnPc was

reported to increase both Voc and Jsc. The longer excited state

lifetime of ZnPc (3.3 ns) compared to the CuPc (6 ps) enhances

the efficiency of charge separation at the interface.

Tandem solar cells

One of the most direct ways to increase the efficiency of an

organic dye based solar cell is to increase the spectral coverage of

the light-harvesting layer. This can be achieved simply by using

a tandem architecture, in which layers of two (or more) D of

different optical band gaps are superimposed. This type of

architecture has been used in inorganic solar cells to minimize the

loss caused by thermalization of charge carriers.135 In organic

solar cells, this concept is adopted either to increase the Voc by

stacking heterojunctions of the same material, or to increase the

photon harvesting by combining materials having complementary

absorption spectra. Organic tandem solar cells have been recently

reviewed.125 In 2002, Forrest and Yakimov utilized CuPc to

fabricate a series of stacked heterojunctions (with 2, 3, 4 and 5

layers).136 The Voc in a double heterojunction was given by the sum

of the Voc of the single cells, but surprisingly the overall efficiency

was more than double that of the same single heterojunction. On

the other hand, three or more stacked junctions lowered the

overall efficiency, likely due to a reduction in light absorption.

Sariciftci and co-workers combined poly(3-hexylthiophene)

(PH3T) and ZnPc in a double stacked heterojunction to increase

the spectral coverage.135,137,138 This cell design delivered a Voc equal

to the sum of the Voc delivered by the single cells, whereas Jsc was

dominated by the smallest of the two Jsc (perhaps because the

layer thickness was not optimized). Implementing the powerful

concept of a tandem solar cell can be challenging because of the

non-trivial device design and fabrication issues. Indeed, the

stacked heterojunctions must be separated by an intermediate

layer to separate and connect the front and back cell.139

To circumvent these fabrication issues, we investigated the

idea of blending three Pc derivatives with different optical gaps

(Fig. 6) in the same layer to increase the photon absorption.97 In

this case the Voc and FF were equal (or similar) to that of the

devices built with one derivative, whereas the Jsc was higher than

1902 | Energy Environ. Sci., 2010, 3, 1897–1909

the sum of devices with the three individual dyes. Although the

overall efficiency of the blended dye devices was low, due to both

the all-organic device architecture and poor charge transport

properties of the soluble Pc derivatives, this work demonstrates

that simply blending the dyes may be advantageous compared to

layered structures. The blended dye concept can be applied to

other systems as well.

Perhalogenated phthalocyanines

Due to the high electronegativity of fluorines and chlorines,

molecular orbitals in perhalogenated Pc are strongly stabilized

(Pauli electronegativity140 for Cl ¼ 3.16 and for F ¼ 3.98). The

electron-withdrawing halogens tend to lower the LUMO which

allows for efficient electron injection and also makes Pc more stable

to ambient oxidation.141 Because of these properties, CuPcF16 was

employed in a solar cell as an n-type electron-transporting layer.142

In this work, CuPcF16 was used as a connecting layer in a tandem

architecture. The all-organic metalloPc connecting layers have

a better transparency and lower sublimation temperature than

inorganic semiconductors. A tunneling mechanism was used to

explain the photogenerated carrier combination. The use of all-

organic layers is also beneficial in reducing the resistance between

the stacked layers. For this purpose, other electron-withdrawing

groups such as CN and pyridyl groups can be used, and Zn, Fe, and

Co metalloPc were studied.

Fluoro- and chloro-Pc can also be adopted as a convenient

platform for the preparation of other derivatives by substitution

of the halogens with nucleophiles, such as thiols, amines, and

alcohols.97,98,143 The substitution with electron-donating groups

on the periphery of the macrocycle decreases the optical gap of

the material. Also, since the substituent effect is proportional to

the number of substituents, it is possible to tune the optical

properties accordingly, e.g. from �670 nm for PcF16 to �790 nm

for Pc(SR)16 (Fig. 6). Although many of these Pc derivatives are

not commercially available at present, the synthesis and purifi-

cation are facile, so have potential for commercialization.

Future directions

A variety of commercially available or commercially viable Pc

can be used in the preparation of solar cells. Unsubstituted Pc

macrocyles and their metalo complexes are non-soluble; there-

fore they must be evaporated under vacuum to form high-quality

layers. Future studies should address the means to form highly

organized films of soluble Pc to enable solution processing for the

fabrication of the devices. In addition to directing the organi-

zation of the nanofilms, which consequently can affect the charge

transport properties, the exocyclic motif should bring an added

functionality to the materials, e.g. facilitate charge dislocation,

film stability, and film quality. Other earth abundant metals

should be considered since they have a profound effect on the

physical properties of the dyes. The morphological features of

the domains at the nanoscale should be correlated with perfor-

mance of the solar cell and the factors that dictate efficiency.

Dyes for DSSC

Dye-sensitized solar cells (DSSC) have been extensively studied

since they have great potential to become one of the cheapest

This journal is ª The Royal Society of Chemistry 2010

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

photovoltaic devices.104,144,145 In particular, they are attractive

because of the simple design and fabrication methods, reduced

material and manufacturing costs compared to commercial Si

based solar cells, and scaled-up large demonstration installa-

tions. Since the initial development of DSSC in the late 1990s in

the Gr€atzel laboratory, a substantial amount of research has

been devoted to improve individual components and optimize

the performance of DSSC, but the greatest conversion efficiency

is �11%.

The various components in DSSC determine the performance

of the dye. In DSSC a wide-band gap semiconductor such as

TiO2, ZnO, SnO2 or other metal oxide is sensitized with a dye

molecule that absorbs solar light. Nanocrystalline/nano-

structured materials are of greatest interest at present. To date,

the best hole transporting material (HTM) in these systems is

a liquid electrolyte containing I�/I3�. When visible light is

absorbed by the dye, an electron from either the singlet or triplet

excited state is injected into the conduction band of TiO2 with

charge injection rates ranging from 100s of fs to tens of ps. The

oxidized dye is regenerated by the redox mediator. The negative

charge is collected on the photoanode such as indium-tin-oxide

(ITO). The main issues with deployment of DSSC include: long

term instability, the electrolyte, photobleaching, and the best

dyes so far contain ruthenium. Manufacturing scalability can be

an issue, e.g. the efficiency rapidly drops from 10.4% for a 1 cm2

cell to 6.3% for 26.5 cm2 cell.112 Quasi-solid and solid HTM to

replace the I�/I3� typically have efficiencies of ca. 5%.146

Optimal properties of the sensitizer include: (1) strong absor-

bance over most of the solar spectrum, including the near IR; (2)

the dye should be strongly grafted to the semiconductor surface;

(3) electron injection to conduction band of the metal oxide

should be of high quantum efficiency; (4) the dye should be easily

regenerated by the redox couple; (5) the dye should be stable in

operation for over 20 years. The best performing dyes so far are

ruthenium and osmium based polypyridyl complexes in terms of

highest conversion efficiency and long term stability.112 The main

drawback of these dyes is the high costs of the metals, which

would significantly increase if they were to be deployed world-

wide. Other dyes have been investigated in an attempt to find

cheaper alternatives. In addition to the properties outlined

above, Por chromophores have attracted much attention since

these molecules are key to photosynthesis. To date, synthetic

organic dyes and natural organic dyes make lower efficiency

DSSC and other devices, but only a few potential commercially

viable derivates have been investigated.

Fig. 7 A new mode of binding Pc and Por to oxide surfaces uses Zr(IV)

and Hf(IV), which significantly protrude out of the macrocycle allowing

the oxophylic metal ion to simultaneously bind to the surface. This is

indicated by the strong binding of the group IV metal ion to both the

porphyrin and the defect site of a polyoxometalate shown in the crystal

structure of a Por–Hf–polyoxometalate complex (top). A scheme of the

possible binding of the Hf or Zr ions to both a Por and a TiO2 (bottom).

DSSC with porphyrins

The better Por sensitizers have the dye core serving as a har-

vesting system, a p-conjugated bridge, and an anchoring group

that binds to the oxide surface.49 The anchoring groups include

carboxylates, phosphonates, and sulfonates.147–149 The structure

and position of the anchoring group significantly impact solar

cell operation. One of the essential factors for high performance

of DSSC is good communication between LUMO orbital of the

dye and 3d orbital of Ti.150 Though PO32� groups significantly

strengthen binding to metal oxide surface and can increase the

stability of the cell, the overall efficiency of DSSC is lower, for

reasons that are not yet completely understood. One speculation

This journal is ª The Royal Society of Chemistry 2010

is that the electronic coupling between the dye and semi-

conductor is poor. This behavior is characteristic also for Ru

based polypyridyl complexes.151

For example, the position of a carboxylic acid group (para or

meta) on tetraarylporphyrins dictates the orientation and type of

binding to the oxide surface. Galoppini and co-workers studied

photophysical and electrochemical properties of Zn Por mole-

cules with carboxylic derivatives as binding groups to TiO2 and

ZnO surfaces. When the binding moieties are all on the meta

position, presumably resulting in binding planar to the surface

and preventing aggregation of chromophores, an improved

electron injection into TiO2 is observed because the dyes are

closer to the surface.149,152 Similarly, Campbell et al., and Odobel

et al., have demonstrated the importance of controlling the

orientation and distance from the dye to the surface.101,150

An alternative mode of binding the dyes to oxide surfaces uses

oxophilic Hf(IV) and Zr(IV) metal ions that protrude from the Por

macrocycle core (Fig. 7).153,154 Since the chromophore orbitals

are strongly coupled with the metal ion orbitals, and the metal

ion is simultaneously bound to the oxide surface, charge injection

is facilitated via both the metal ion and the proximity of the dye

to the surface.

In single dye systems, for the maximum conversion efficiency

the dye should absorb part of the NIR spectrum from 900–1000

nm. But the best performing dyes generally absorb only to �700

nm.112 To extend the spectral coverage of solar energy to longer

Energy Environ. Sci., 2010, 3, 1897–1909 | 1903

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

wavelengths, the HOMO–LUMO energies of the molecules can

be adjusted by modifications of the porphyrin macrocycle. While

the LUMO has to stay sufficiently above the conduction band of

TiO2 to maintain fast electron injection, the HOMO has to be

below the redox potential of the redox mediator to assure

reformation of the neutral dye and retain dye stability.155 Elon-

gation of the p-system of the aromatic macrocycle shifts the

absorption spectra towards the red with some peak broadening

by decreasing the HOMO–LUMO gap. Interesting examples

include the Zn b,b0-quinoxalino porphyrins bearing one or two

carboxyl binding groups.156 Both porphyrins have broader Soret

bands, and increase short circuit current in devices compared to

a control molecule without carboxyquinoxalino groups. The

molecule substituted with only one carboxyl group results in

devices with a 5.2% power conversion efficiency, yet devices

containing the Por with two carboxyl groups have 4.0% effi-

ciency. Lee et al. showed that Soret band of a porphyrin can be

red shifted by as much as 60 nm,157 but the efficiency using these

sensitizers does not go over 1% due to strong aggregation

between molecules, and orientation of the anchoring groups.

A so-called ‘‘push–pull’’ Por with an electron donating diary-

lamino group on the meso position opposite an anchoring

COOH group has improved charge separation properties and

extended absorption to longer wavelengths.158 This results in

larger Jsc and a 6.0% efficiency. Varying both the alkanes and the

bridge can lead to devices with�7% efficiencies.159 Recombination

processes in DSSC tend to decrease Voc and Jsc significantly,

thereby reducing overall conversion efficiencies. Long alkyl chains

on the Por macrocycle can help protect free sites on the TiO2

surface. Forneli and co-workers found that recombination

between the injected electron and oxidized electrolyte was reduced

when free base meso tetraphenylporphyrins appended with

hydrocarbon chains are deposited on TiO2.160 The alkanes had

little effect on recombination with the oxidized form of the dye.

Often, pre-treating the TiO2 surface with acids such as cholic

or decylphosphonic helps: (a) reduce the aggregation of

porphyrins, (b) reduce desorption from the surface, (c) reduce the

recombination by blocking free TiO2 sites. This strategy,

however, significantly lowers the dye surface coverage. A newer

approach that treats the dye-loaded electrode with phosphinic

acid moves the Fermi level of TiO2 to a more positive value

thereby increasing Jsc, and slowing down charge recombination

processes.161 A series of green Zn Por gave the best result in terms

of conversion efficiencies ranging from 5.2%–7.0%. Gr€atzel and

co-workers synthesized b substituted Zn Por with aryl groups as

electron donors and malonic acid as an acceptor group. The best

performing Por resulted in an efficiency of 7.1% using a liquid

electrolyte cell, and an efficiency of 3.6% in solid cell with

a specialized hole transporter.162,163

DSSC with phthalocyanines

Pc are complementary to the Por for longer wavelength sensiti-

zation since they have strong Q bands in the region of 500–600

nm and Soret bands of smaller intensity around 300 nm.17 The

large extinction coefficients (3 ¼ 105 M�1 cm�1) are sufficient to

reduce the thickness of dye sensitized films relative to many other

dye systems. The core Pc chromophore is very stable, thus has

good potential for applications in DSSC. Pc can be synthesized in

1904 | Energy Environ. Sci., 2010, 3, 1897–1909

high yeilds98,120 and are currently used as dyes and colorants in

the textile industry, as inks, and for recordable CDs. Simple Pc

tend to aggregate on TiO2 surfaces to a greater extent than Por,

which is observed as a broadening of absorption spectra

compared to the solution phase. This aggregation generally

results in the deactivation of the excited state, thereby reducing

electron injection.

There are numerous initiatives to expand Pc absorption to the

near-IR region and improve solubility in common organic

solvents, e.g. the significant research by Torres, Durant and co-

workers.122,144 While Ru–Pc have been used in DSSC systems,164–166

other metal ions inserted into Pc core, e.g. Ti, Zn, Fe will make the

dyes more economically feasible.167,168 Pc are most commonly

substituted with carboxylic or sulfonic groups for better attach-

ment to oxide surface, but unfortunately, these tend to have poor

solubility and do not perform well.169 Since the efficiency of DSSC

with Pc is typically less than 3%, Pc are more generally used in the

fabrication of thin-film devices where they can be thermally

evaporated onto conducting or semiconducting surfaces rather

than processed from the solution phase.

One of the best performing ZnPc was reported by Gr€atzel and

Nazeeruddin and has an extended p-conjugated system and is

unsymmetric.169,170 To prevent aggregation and improve solu-

bility, three bulky tert-butyl groups were appended to the mac-

rocycle, and two carboxylic groups immobilize Pc on TiO2. This

molecular design gives 2.35% efficiency. Similarly, a ZnPc with

a COOH linker directly attached to the macrocycle slightly

improves cell efficiency to 3.52%.169,171

Adding coadsorbents such as chenodeoxycholic acid on TiO2

can have multiple effects on the performance of solar cells.

Generally, the TiO2 band edge shifts to negative potentials,

consequently increasing Voc in the cell. Chenodeoxycholic acid

tends to suppress recombination and prevents dye aggregation

on the surface, but decreases dye coverage, which results in lower

observed IPCE values and lower photocurrents.149,172

Applications of Pc in DSSC are also limited due to the rela-

tively low energy of the LUMO orbital versus the conduction

band of TiO2. Imahori et al. investigated the differences between

free base and ZnPc substituted with eight phenyl groups, six of

them with tert-butyl and two with carboxyl, for binding the

semiconductor surface. Interestingly, the free base Pc did not

display a photocurrent response, presumably because of the low

energy LUMO orbital. Incorporation of Zn(II) into the Pc

increases the LUMO level, driving more favorable electron

injection. The overall conversion efficiency of the DSSC was as

low as 0.57% with or without coadsorbent indicating suppressed

aggregation and self-quenching of the excited state.173 These

studies demonstrate that a good understanding of the photo-

chemical and photophysical properties can aid in the design of

improved Pc.

DSSC with other organic dyes

Beside metalloporphyrinoids, a significant research effort has

focused on designing metal free organic dyes for solar cell devices

that have large extinction coefficients (3 ¼ 105 cm�1 M�1) and are

easy to synthesize. These two characteristics make them

commercially approachable from the cost point of view.

However, DSSC with organic dyes are often not very stable due

This journal is ª The Royal Society of Chemistry 2010

Fig. 8 Fluorous alkanes on a Por induce the formation of thin films with

C60 (1 : 1) because of F–p interactions, whereas the hydrocarbon

analogues do not. This strategy allows formation of robust films and

obviates making derivatives of the fullerene. See Fig. 3 for synthesis.

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

to desorption from the oxide surface and photobleaching under

long illumination times and high temperatures. Though many

organic dyes do not cover a broad range of solar spectrum,

greater absorptivities enable production of thinner TiO2 films,

and may result in higher quantum efficiencies and reduced

costs.68,109 A generalized structure of organic dyes can be repre-

sented as donor/p-conjugated bridge/acceptor, thus in addition

to the chromophore, the bridge and acceptor are crucial

parameters. Conjugated bridges, e.g. thiophene, can improve

charge transfer characteristics. Liquid DSSC with organic dyes

can be around 9% efficient using an indoline dye by optimizing

semiconducting surface to prevent aggregation.109,174–177 An ionic

liquid device led to 7% efficiency, while a solid state solar cell

reached 4%.109 Other examples of organic sensitizers include

hemicyanine (6.3%), cyanine (4.8%) and squaraine (4.5%) effi-

ciency.109,178 Hara et al. reported that coumarin dye-based

devices can have 7.7% efficiencies.179,180

Future directions

The metal ion in Por and Pc can concomitantly affect (1) the

excited state photophysics, fluorescence and phosphorescence

quantum yields and lifetimes; (2) the electrochemical potentials;

(3) the packing of the chromophores in films; and (4) can be used

to form supramolecular materials. Yet, only a handful of metal

ions have been investigated. Compared to most Ru based

complexes, Por and Pc absorb 10 times more light, but since these

have narrower absorption bands, combinations of Por and Pc

will be needed to cover the solar spectrum. However, these

combinations will allow much thinner active layers, e.g. poten-

tially reducing the typically 10 mm thick semiconducting layer of

dye-coated TiO2 nanoparticles to 1 mm in DSSC, thereby

decreasing losses during charge transport through this layer and

reducing costs.155

Fig. 9 Porphyrinoid dyes that are potentially commercially viable are

made from simple starting materials without separation of isomers,

atropisomers, or other dye compounds if formed (e.g. when two or more

aldehydes or pyrroles are used in Por synthesis, or phthalonitriles in Pc

synthesis). The sphere represents an earth-abundant metal ion (generally

+2 to +4 oxidation state). (A) Halogenated Pc, (B) some Pc derivatives,

(C) octaethylporphyrin, (D) the meso aryl groups are orthogonal to the

macrocycle in tetraaryporphyrin derivatives.

Cosensitization

Cosensitization with two or more dyes (see above) is a new

approach to enhancing light harvesting. Since they compete for

free sites on TiO2 surface, a limited number of dyes can be loaded

per unit area. Organic molecules with high molar extinction

coefficients mean that fewer dye molecules are required for

sensitizing the nanocrystalline films because of the increased

optical absorption spectra of the dye monolayer. Zhang et al.

used a combination of three different organic dyes (merocyanine,

hemicyanine, and a squarylium cyanine dye) to construct devices

with overall higher efficiencies than each individual dye. The

three dyes covered wavelengths from 380 nm to about 700 nm,

and a 6.5% efficiency was reported.181 Gr€atzel et al. showed that

a combination of red and blue dyes with an ionic liquid elec-

trolyte increases the photocurrent and results in 6.4% efficient

devices.182,183 Using ‘‘molecular cocktails’’ of three organic dyes

covering 400–700 nm results in cells that are 7.74% efficient,

again more than the performance of each individual dye.171

Cosensitization for the BHJ cell design need not be limited to

Pc, vide supra. Por, Pc, and semiconducting polymers in appro-

priately designed cells, may afford greater spectral coverage with

minimized film thicknesses and greater charge dislocation and

transport. It remains to be seen if the simple mixtures of different

This journal is ª The Royal Society of Chemistry 2010

Pc dyes in an active layer are competitive with the dyes layered

one at a time.

The addition of C60 derivatives to BHJ devices to enhance the

initial charge separation is widely reported. These derivatives

often have ligands to bind to the metalo dyes, but the optimum

system would use underivatized fullerenes since these are better

electron acceptors. Other motifs on the dyes can be used to

enhance intermolecular interactions with C60 as a means to

design self-organized thin films of Por and Pc with the fullerene.

Energy Environ. Sci., 2010, 3, 1897–1909 | 1905

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

Appending fluorous alkanes and exploiting both p–p and F–p

interactions is a recent example (Fig. 8), and the fluorous alkanes

may enhance the stability of the films.

Conclusions

Even though the maximum efficiency of BHJ and DSSC remains

low compared to commercial Si based solar cells (maximally

�20%), significant understanding of the design criteria and

mechanisms has allowed the fabrication and testing of new

device designs and new molecular designs. Though a great

variety of new dyes and metallodyes have been designed, in terms

of commercialization most are likely too costly or too fragile to

deploy. Dyes such as simple Por and Pc derivatives (e.g. Fig. 9),

with very high molar extinction coefficients, can compete

strongly with the best performing Ru bipyridyl complexes

because they have the advantage of low material costs, simple

synthesis, and the possibility to allow fabrication of thinner

semiconductor films. Thinner devices make them more favorable

for large scale applications because of reduced materials usage.

However, increasing efficiency with commercially viable dye

remains a keystone issue.

Self-assembly and self-organization may allow formation of

active layers with several spectrally complementary chromo-

phores in pre-specified geometries to more effectively harvest

solar energy and convert it into electrochemical potential energy.

Robust but reversible intermolecular interactions, such as metal

ion coordination,10,17,67,72,73 can realize complex supramolecular

architectures by self-assembly such that the chromophores are in

a specified order so that the direction of electron or energy

transfer is correspondingly predictable, albeit this does not

assure high quantum yields. Conversely, self-organized nano-

particles (NP aggregates) of dyes such as Por16,70,184 and Pc185,186

with much less structural order, or colloidal crystals187 can

display significantly enhanced or modulated photonic properties

because of quantum mechanical effects at this scale.185,188 A

considerable advantage of the organic NP systems is that the

dyes can be simple commercial molecules and the exocyclic motif

used to enhance materials and photonic properties rather than

potentiate intermolecular interactions. Thus, in conjunction with

advances in device design, the deployment of commercially

viable dyes in the active layers, including chelates of earth-

abundant metal ion, will require supramolecular design concepts.

Other hierarchical nanomaterials are a rapidly emerging as

new active components for solar energy harvesting and utiliza-

tion. In addition to the functional groups mentioned above,

several polymeric materials composed of symmetric meso aryl Por

have shown promise in several types of solar energy applications.

Oxidative electropolymerization of tetraaminophenyl Por forms

a covalently linked polymer that is analogous to the polyaniline,

and these polymeric films are useful materials for solar energy

harvesting.189 Ionic free base- and metallo- porphyrins (especially

the tetrasulfoxyphenyl, tetracarboxyphenyl, and tetrapyridinium

derivatives) have long been known to form a plethora of nano-

structured materials depending on pH, ionic strength, mixing, and

deposition. Thus fractal patterns, nanotubes, nanorods, and other

morphologies have been shown to have useful photonic properties

that can translate into light harvesting materials.190,191 These can

also be organized onto nanotubular supports such as carbon

1906 | Energy Environ. Sci., 2010, 3, 1897–1909

nanotubes.192-194 Mixing anionic and cationic solutions of these

porphyrins afford a mode of self-organization that forms complex

but robust nanostructures, for example the nanomaterials

composed of pyridinium and sulfate Por reported by Shellnut and

coworkers.195,196 Self-organized films of tetrapyridyl Por organized

by Pd(II) coordination, and new dyes have demonstrated

a capacity for solar energy harvesting.197 Many of these structure

are co-formed with fullerene nanotubes. Other mixed dye systems

with Pc are reported to enhance solar energy harvesting.198,199 In

addition to C60 and carbon nanotubes, pereleydiimides (PDI) are

commercially viable electron acceptors in heterojunction devices

and porphyrin-PDI constructs have been studied by Waselewski

and coworkers.200-204 There should be a way to take advantage of

Pc aggregation to form analogous nanostructures as those

reported in the extensive literature on Por. Metal organic frame-

work solids of symmetrically substituted metalloporphyrins,

especially the cobalt complex, have the potential to adsorb and

reduce CO2 in photodriven reactions.205 From our perspective,

organic components of solar cells have a bright future.

Acknowledgements

This work was supported by the National Science Foundation

(NSF, CHE-0847997) and a collaborative grant (CHE-0848786)

to Professor James D. Batteas of Texas A&M University and to

C.M.D. Hunter College science infrastructure is supported by

the NSF, the National Institutes of Health (including RCMI,

G12-RR-03037), and CUNY. The authors wish to thank Angela

Melillo and Armond Pietrocarlo for helping with manuscript

preparation.

References

1 R. F. Service, Science, 2005, 309, 95.2 C. P. Casey, Chem. Int., 2005, 27, 8–11.3 R. F. Service, Science, 2005, 309, 548–551.4 J. Kalowekamo and E. Baker, Sol. Energy, 2009, 83, 1224–1231.5 C. M. Drain, B. Christensen and D. C. Mauzerall, Proc. Natl. Acad.

Sci. U. S. A., 1989, 86, 6959–6962.6 C. M. Drain and D. Mauzerall, Bioelectrochem. Bioenerg., 1990, 24,

263–268.7 C. M. Drain and D. C. Mauzerall, Biophys. J., 1992, 63, 1556–1563.8 C. M. Drain and J.-M. Lehn, Chem. Commun., 1994, 2313–2315

(correction 1995, 2503).9 C. M. Drain, K. C. Russel and J.-M. Lehn, Chem. Commun., 1996,

337–338.10 C. M. Drain, F. Nifiatis, A. Vasenko and J. D. Batteas, Angew.

Chem., Int. Ed., 1998, 37, 2344–2347.11 C. M. Drain, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5178–5182.12 C. M. Drain, J. D. Batteas, G. W. Flynn, T. Milic, N. Chi,

D. G. Yablon and H. Sommers, Proc. Natl. Acad. Sci. U. S. A.,2002, 99, 6498–6502.

13 T. N. Milic, N. Chi, D. G. Yablon, G. W. Flynn, J. D. Batteas andC. M. Drain, Angew. Chem., Int. Ed., 2002, 41, 2117–2119.

14 C. M. Drain, G. Smeareanu, J. Batteas and S. Patel, in DekkerEncyclopedia of Nanoscience and Nanotechnology, ed. J. A.Schwartz, C. I. Contescu and K. Putyera, Marcel Dekker, Inc.,New York, 2004, vol. 5, pp. 3481–3502.

15 C. M. Drain, G. Bazzan, T. Milic, M. Vinodu and J. C. Goeltz, Isr. J.Chem., 2005, 45, 255–269.

16 C. M. Drain, G. Smeureanu, S. Patel, X. Gong, J. Garno andJ. Arijeloye, New J. Chem., 2006, 30, 1834–1843.

17 C. M. Drain, A. Varotto and I. Radivojevic, Chem. Rev., 2009, 109,1630–1658.

18 J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,Wiley VCH, Weinheim, 1995.

This journal is ª The Royal Society of Chemistry 2010

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

19 J.-M. Lehn, Science, 2002, 295, 2400–2403.20 A. P. de Silva, S. Uchiyama, T. P. Vance and B. Wannalerse, Coord.

Chem. Rev., 2007, 251, 1623–1632.21 S. J. Loeb, Chem. Soc. Rev., 2007, 36, 226–235.22 F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and

A. P. H. J. Schenning, Chem. Rev., 2005, 105, 1491–1546.23 G. S. Papaefstathiou and L. R. MacGillivray, Coord. Chem. Rev.,

2003, 246, 169–184.24 A. P. Alivisatos, P. F. Barbara, A. W. Castleman, J. Chang,

D. A. Dixon, M. L. Klein, G. L. McLendon, J. S. Miller,M. A. Ratner, P. J. Rossky, S. I. Stupp and M. E. Thompson,Adv. Mater., 1998, 10, 1297–1336.

25 L. Basabe-Desmonts, D. N. Reinhoudt and M. Crego-Calama,Chem. Soc. Rev., 2007, 36, 993–1017.

26 C. M. Drain, I. Goldberg, I. Sylvain and A. Falber, Top. Curr.Chem., 2005, 245, 55–88.

27 M. Schmittel, V. Kalsani and J. W. Bats, Inorg. Chem., 2005, 44,4115–4117.

28 T. Milic, J. C. Garno, J. D. Batteas, G. Smeureanu and C. M. Drain,Langmuir, 2004, 20, 3974–3983.

29 C. M. Drain, T. Milic, J. C. Garno, G. Smeureanu and J. D. Batteas,Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), 2004, 45, 346–347.

30 P. Thamyongkit, L. Yu, K. Padmaja, J. Jiao, D. F. Bocian andJ. S. Lindsey, J. Org. Chem., 2006, 71, 1156–1171.

31 K. Padmaja, W. J. Youngblood, L. Wei, D. F. Bocian andJ. S. Lindsey, Inorg. Chem., 2006, 45, 5479–5492.

32 K. Padmaja, L. Wei, J. S. Lindsey and D. F. Bocian, J. Org. Chem.,2005, 70, 7972–7978.

33 L. Wei, D. Syomin, R. S. Loewe, J. S. Lindsey, F. Zaera andD. F. Bocian, J. Phys. Chem. B, 2005, 109, 6323–6330.

34 L. Wei, H. Tiznado, G. Liu, K. Padmaja, J. S. Lindsey, F. Zaera andD. F. Bocian, J. Phys. Chem. B, 2005, 109, 23963–23971.

35 J. S. Lindsey, Acc. Chem. Res., 2010, 43, 300–311.36 A. A. Yasseri, D. Syomin, R. S. Loewe, J. S. Lindsey, F. Zaera and

D. F. Bocian, J. Am. Chem. Soc., 2004, 126, 15603–15612.37 Q. Li, S. Surthi, G. Mathur, S. Gowda, Q. Zhao, T. A. Sorenson,

R. C. Tenent, K. Muthukumaran, J. S. Lindsey and V. Misra,Appl. Phys. Lett., 2004, 85, 1829–1831.

38 K. Muthukumaran, R. S. Loewe, A. Ambroise, S.-I. Tamaru, Q. Li,G. Mathur, D. F. Bocian, V. Misra and J. S. Lindsey, J. Org. Chem.,2004, 69, 1444–1452.

39 Z. Liu, A. A. Yasseri, R. S. Loewe, A. B. Lysenko, V. L. Malinovskii,Q. Zhao, S. Surthi, Q. Li, V. Misra, J. S. Lindsey and D. F. Bocian,J. Org. Chem., 2004, 69, 5568–5577.

40 A. Balakumar, A. B. Lysenko, C. Carcel, V. L. Malinovskii,D. T. Gryko, K. H. Schweikart, R. S. Loewe, A. A. Yasseri,Z. Liu, D. F. Bocian and J. S. Lindsey, J. Org. Chem., 2004, 69,1435–1443.

41 L. Scudiero, K. W. Hipps and D. E. Barlow, J. Phys. Chem. B, 2003,107, 2903–2909.

42 D. E. Barlow, L. Scudiero and K. W. Hipps, Langmuir, 2004, 20,4413–4421.

43 A. Ogunrinde, K. W. Hipps and L. Scudiero, Langmuir, 2006, 22,5697–5701.

44 K. E. Splan and J. T. Hupp, Langmuir, 2004, 20, 10560–10566.45 A. Prodi, C. Chiorboli, F. Scandola, E. Iengo, E. Alessio,

R. Dobrawa and F. W€urthner, J. Am. Chem. Soc., 2005, 127,1454–1462.

46 K. Ogawa and Y. Kobuke, J. Photochem. Photobiol., C, 2006, 7, 1–16.47 R. F. Kelley, S. Won Suk, B. Rybtchinski, L. E. Sinks and

M. R. Wasielewski, J. Am. Chem. Soc., 2007, 129, 3173–3181.48 E. Alessio, Non-covalent Multi-Porphyrin Assemblies: Synthesis and

Properties, Springer Verlag, Berlin, 2007.49 X.-F. Wang and H. Tamiaki, Energy Environ. Sci., 2010, 3, 94–106.50 C. M. Drain, S. Gentemann, J. A. Roberts, N. Y. Nelson,

C. J. Medforth, S. Jia, M. C. Simpson, K. M. Smith, J. Fajer,J. A. Shelnutt and D. Holten, J. Am. Chem. Soc., 1998, 120, 3781–3791.

51 L. Yu, K. Muthukumaran, I. V. Sazanovich, C. Kirmaier, E. Hindin,J. R. Diers, P. D. Boyle, D. F. Bocian, D. Holten and J. S. Lindsey,Inorg. Chem., 2003, 42, 6629–6647.

52 R. Schl€ogl and S. B. Abd Hamid, Angew. Chem., Int. Ed., 2004, 43,1628–1637.

53 D. Mansuy, Coord. Chem. Rev., 1993, 125, 129–141.54 F. Lerouge, G. Cerveau, R. J. P. Corriu, C. Stern and R. Guilard,

Chem. Commun., 2007, 1553–1555.

This journal is ª The Royal Society of Chemistry 2010

55 V. Laget, C. Hornick, P. Rabu, M. Drillon and R. Ziessel, Coord.Chem. Rev., 1998, 178–180, 1533–1553.

56 N. Ishikawa, M. Sugita, T. Ishikawa, S.-y. Koshihara and Y. Kaizu,J. Phys. Chem. B, 2004, 108, 11265–11271.

57 M. Zhao, C. Zhong, C. Stern, A. G. M. Barrett and B. M. Hoffman,J. Am. Chem. Soc., 2005, 127, 9769–9775.

58 M. Zhao, C. Zhong, C. Stern, A. G. M. Barrett and B. M. Hoffman,Inorg. Chem., 2004, 43, 3377–3385.

59 J. S. Miller, Polyhedron, 2001, 20, 1723–1725.60 N. Ishikawa, M. Sugita, T. Ishikawa, S.-y. Koshihara and Y. Kaizu,

J. Am. Chem. Soc., 2003, 125, 8694–8695.61 K. F. Cheng, N. A. Thai, K. Grohmann, L. C. Teague and

C. M. Drain, Inorg. Chem., 2006, 45, 6928–6932.62 M. Koepf, A. Trabolsi, M. Elhabiri, J. A. Wytko, D. Paul,

A. M. Albrecht-Gary and J. Weiss, Org. Lett., 2005, 7, 1279–1282.63 H. Imahori, J. Phys. Chem. B, 2004, 108, 6130–6143.64 M. S. Choi, T. Yamazaki, I. Yamazaki and T. Aida, Angew. Chem.

Int. Ed., 2004, 43, 150–158.65 H. E. Toma, J. Braz. Chem. Soc., 2003, 14, 845–869.66 H. Imahori, Y. Mori and Y. Matano, J. Photochem. Photobiol., C,

2003, 4, 51–83.67 (a) C. M. Drain, J. T. Hupp, K. S. Suslick, M. R. Wasielewski and

X. Chen, J. Porphyrins phthal., 2002, 6, 243–256; (b) K. Ariga,J. P. Hill, Y. Wakayama, M. Akada, E. Barrena and D. G. deOteyza, J. Porphyrins phthal., 2009, 13, 23–34.

68 F.-T. Kong, S.-Y. Dai and K.-J. Wang, Adv. OptoElectron., 2007, 1–13.

69 Z. Liu, A. A. Yasseri, J. S. Lindsey and D. F. Bocian, Science, 2003,302, 1543–1545.

70 X. Gong, T. Milic, C. Xu, J. D. Batteas and C. M. Drain, J. Am.Chem. Soc., 2002, 124, 14290–14291.

71 P. Pasetto, X. Chen, C. M. Drain and R. W. Franck, Chem.Commun., 2001, 81–82 (erratum 507).

72 K. F. Cheng, N. A. Thai, L. C. Teague, K. Grohmann andC. M. Drain, Chem. Commun., 2005, 4678–4680.

73 S. J. Lee and J. T. Hupp, Coord. Chem. Rev., 2006, 250, 1710–1723.74 J. T. Hupp, Struct. Bonding, 2006, 121, 145–165.75 M. H. Keefe, K. D. Benkstein and J. T. Hupp, Coord. Chem. Rev.,

2000, 205, 201–228.76 V. F. Slagt, P. W. N. M. van Leeuwen and J. N. H. Reek, Dalton

Trans., 2007, 2302–2310.77 K. Tashiro and T. Aida, Chem. Soc. Rev., 2007, 36, 189–197.78 M. Schmittel, R. S. K. Kishore and J. W. Bats, Org. Biomol. Chem.,

2007, 5, 78–86.79 A. Satake, O. Shoji and Y. Kobuke, J. Organomet. Chem., 2007, 692,

635–644.80 S. George and I. Goldberg, Cryst. Growth Des., 2006, 6, 755–762.81 F. Nishiyama, T. Yokoyama, T. Kamikado, S. Yokoyama and

S. Mashiko, Appl. Phys. Lett., 2006, 88, 253113.82 A. Hosseini, M. C. Hodgson, F. S. Tham, C. A. Reed and

P. D. W. Boyd, Cryst. Growth Des., 2006, 6, 397–403.83 (a) J. A. A. W. Elemans, V. F. Slagt, A. E. Rowan and R. J. M. Nolte,

Isr. J. Chem., 2005, 45, 271–279; (b) R. v. Hameren, P. Sch€on,A. M. v. Buul, J. Hoogboom, S. V. Lazarenko, J. W. Gerritsen,H. Engelkamp, P. C. M. Christianen, H. A. Heus, J. C. Maan,T. Rasing, S. Speller, A. E. Rowan, J. A. A. W. Elemans andR. J. M. Nolte, Science, 2006, 314, 1433–1436; (c) J. Hoogboom,P. M. L. Garcia, M. B. J. Otten, J. A. A. W. Elemans, J. Sly,S. V. Lazarenko, T. Rasing, A. E. Rowan and R. J. M. Nolte, J.Am. Chem. Soc., 2005, 127, 11047–11052.

84 J. Elemans, A. E. Rowan and R. J. M. Nolte, J. Mater. Chem., 2003,11, 2661–2670.

85 D. S. Lawrence, T. Jiang and M. Levett, Chem. Rev., 1995, 95, 2229–2260.

86 D. Philp and J. F. Stoddart, Angew. Chem., Int. Ed., 1996, 35, 1154–1196.

87 P. R. Carlier, Angew. Chem., Int. Ed., 2004, 43, 2602–2605.88 K. Kameyama, A. Satake and Y. Kobuke, Tetrahedron Lett., 2004,

45, 7617–7620.89 M. de Napoli, S. Nardis, R. Paolesse, M. G. H. Vicente, R. Lauceri

and R. Purrello, J. Am. Chem. Soc., 2004, 126,5934–5935.

90 M. E. El-Khouly, O. Ito, P. M. Smith and F. D’Souza, J. Photochem.Photobiol., C, 2004, 5, 79–104.

91 F. D’Souza and O. Ito, Coord. Chem. Rev., 2005, 249, 1410–1422.

Energy Environ. Sci., 2010, 3, 1897–1909 | 1907

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

92 J. A. A. W. Elemans, R. v. Hameren, R. J. M. Nolte andA. E. Rowan, Adv. Mater., 2006, 18, 1251–1266.

93 S. Karan, D. Basak and B. Mallik, Chem. Phys. Lett., 2007, 434,265–270.

94 M. Wang, Y.-L. Yang, K. Deng and C. Wang, Chem. Phys. Lett.,2007, 439, 76–80.

95 N. Kobayashi, S.-i. Nakajima, H. Ogata and T. Fukuda, Chem.–Eur. J., 2004, 10, 6294–6312.

96 K. Kadish, K. M. Smith and R. Guiard, The Porphyrin Handbook,Academic Press, New York, 2000–2003.

97 A. Varotto, C.-Y. Nam, I. Radivojevic, J. P. C. Tom�e,J. A. S. Cavaleiro, C. T. Black and C. M. Drain, J. Am. Chem.Soc., 2010, 132, 2552–2554.

98 C. C. Leznoff and J. L. Sosa-Sanchez, Chem. Commun., 2004, 338–339.

99 D. Samaroo, C. E. Soll, L. J. Todaro and C. M. Drain, Org. Lett.,2006, 8, 4985–4988.

100 D. Samaroo, M. Vinodu, X. Chen and C. M. Drain, J. Comb. Chem.,2007, 9, 998–1011.

101 W. M. Campbell, A. K. Burrell, D. L. Officer and K. W. Jolley,Coord. Chem. Rev., 2004, 248, 1363–1379.

102 F. Yang and S. R. Forrest, ACS Nano, 2008, 2, 1022–1032.103 P. Heremans, D. Cheyns and B. P. Rand, Acc. Chem. Res., 2009, 42,

1740–1747.104 M. Gr€atzel, Acc. Chem. Res., 2009, 42, 1788–1798.105 M. D. Perez, C. Borek, S. R. Forrest and M. E. Thompson, J. Am.

Chem. Soc., 2009, 131, 9281–9286.106 J. Roncali, Acc. Chem. Res., 2009, 42, 1719–1730.107 W. J. Potscavage, A. Sharma and B. Kippelen, Acc. Chem. Res.,

2009, 42, 1758–1767.108 J. Peet, A. J. Heeger and G. C. Bazan, Acc. Chem. Res., 2009, 42,

1700–1708.109 A. Mishra, M. K. R. Fischer and P. B€auerle, Angew. Chem., Int. Ed.,

2009, 48, 2474–2499.110 B. Kippelen and J.-L. Br�edas, Energy Environ. Sci., 2009, 2, 251–

261.111 K. Yu and J. Chen, Nanoscale Res. Lett., 2009, 1–10.112 M. Gr€atzel, J. Photochem. Photobiol., C, 2003, 4, 145–153.113 J. Heinz, Angew. Chem., Int. Ed., 1984, 23, 831–847.114 S. Kim, J. K. Lee, S. O. Kang, J. Ko, J.-H. Yum, S. Fantacci, F. De

Angelis, D. Di Censo, M. K. Nazeeruddin and M. Gr€atzel, J. Am.Chem. Soc., 2006, 128, 16701–16707.

115 G. Bazzan, W. Smith, L. Francesconi and C. M. Drain, Langmuir,2007, 24, 3244–3249.

116 W. J. Doherty, R. Friedlein and W. R. Salaneck, J. Phys. Chem. C,2007, 111, 2724–2729.

117 Y.-H. Chan, A. E. Schuckman, L. M. P�erez, M. Vinodu,C. M. Drain and J. D. Batteas, J. Phys. Chem. C, 2008, 112, 6110–6118.

118 B. Albinsson and J. M�artensson, J. Photochem. Photobiol., C, 2008,9, 138–155.

119 S. Honda, T. Nogami, H. Ohkita, H. Benten and S. Ito, ACS Appl.Mater. Interfaces, 2009, 1, 804–810.

120 C. C. Leznoff and A. B. P. Lever, Phthalocyanines Properties andApplications, Wiley VCH, New York, 1993.

121 N. Kobayashi, H. Ogata, N. Nonaka and E. A. Luk’yanets, Chem.–Eur. J., 2003, 9, 5123–5134.

122 Y. Rio, M. S. Rodr�ıguez-Morgade and T. Torres, Org. Biomol.Chem., 2008, 6, 1877–1894.

123 D. R. Tackley, G. Dent and W. E. Smith, Phys. Chem. Chem. Phys.,2001, 3, 1419–1426.

124 P. Peumans, A. Yakimov and S. R. Forrest, J. Appl. Phys., 2003, 93,3693–3723.

125 T. Ameri, G. Dennler, C. Lungenschmied and C. Brabec, EnergyEnviron. Sci., 2009, 2, 347–363.

126 X. Chen, J. Thomas, P. Gangopadhyay, R. A. Norwood,N. Peyghambarian and D. V. McGrath, J. Am. Chem. Soc., 2009,131, 13840–13843.

127 G. Calogero, G. Di Marco, S. Caramori, S. Cazzanti, R. Argazzi andC. A. Bignozzi, Energy Environ. Sci., 2009, 2, 1162–1172.

128 M. Gouterman, in The Porphyrins, ed. D. Dolphin, Academic Press,New York, 1978, pp. 1–153.

129 H. Sharghi and A. H. Nejad, Tetrahedron, 2004, 60, 1863–1868.130 J. Xue, S. Uchida, B. P. Rand and S. R. Forrest, Appl. Phys. Lett.,

2004, 84, 3013–3015.

1908 | Energy Environ. Sci., 2010, 3, 1897–1909

131 Z. R. Hong, B. Maennig, R. Lessmann, M. Pfeiffer, K. Leo andP. Simon, Appl. Phys. Lett., 2007, 90, 203505.

132 P. Sullivan, S. Heutz, S. M. Schultes and T. S. Jones, Appl. Phys.Lett., 2004, 84, 1210–1212.

133 J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Danteand A. J. Heeger, Science, 2007, 317, 222–225.

134 (a) Y. Kinoshita, T. Hasobe and H. Murata, Appl. Phys. Lett., 2007,91, 083518; (b) T. Hasobe, Phys. Chem. Chem. Phys., 2010, 12, 44–57.

135 G. Dennler, H.-J. Prall, R. Koeppe, M. Egginger, R. Autengruberand N. S. Sariciftci, Appl. Phys. Lett., 2006, 89, 073502–073503.

136 A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 2002, 80, 1667–1669.

137 H. Hoppe and N. Sariciftci, in Photoresponsive Polymers II,Springer-Verlag, Berlin, 2008, pp. 1–86.

138 M. Egginger, R. Koeppe, F. Meghdadi, P. A. Troshin,R. N. Lyubovskaya, D. Meissner and N. S. Sariciftci, Proc. SPIE-Int. Soc. Opt. Eng., 2006, 6192, 61921Y-8.

139 X. Gong, M. Tong, Y. Xia, W. Cai, J. S. Moon, Y. Cao, G. Yu,C.-L. Shieh, B. Nilsson and A. J. Heeger, Science, 2009, 325,1665–1667.

140 A. L. Allred, J. Inorg. Nucl. Chem., 1961, 17, 215–221.141 Q. Tang, H. Li, Y. Liu and W. Hu, J. Am. Chem. Soc., 2006, 128,

14634–14639.142 B. Yu, F. Zhu, H. Wang, G. Li and D. Yan, J. Appl. Phys., 2008,

104, 114503–114505.143 C. C. Leznoff, A. Hiebert and S. Ok, J. Porphyrins phthalocyanines,

2007, 11, 537–546.144 B. C. O’Regan and J. R. Durrant, Acc. Chem. Res., 2009, 42, 1799–

1808.145 L. M. Goncalves, V. D. Z. Bermudez, H. A. Ribeiro and

A. M. Mendes, Energy Environ. Sci., 2008, 655–667.146 L. Schmidt-Mende, U. Bach, R. Humphry-Baker, T. Horiuchi,

H. Miura, S. Ito, S. Uchida and M. Gr€atzel, Adv. Mater., 2005,17, 813–815.

147 S. Cherian and C. C. Wamser, J. Phys. Chem. B, 2000, 104,3624–3629.

148 J. Jasieniak, M. Johnston and E. R. Waclawik, J. Phys. Chem. B,2004, 108, 12962–12971.

149 J. Rochford and E. Galoppini, Langmuir, 2008, 24, 5366–5374.150 F. Odobel, E. Blart, M. Lagr�ee, M. Villieras, H. Boujtita,

N. E. Murr, S. Caramori and C. A. Bignozzi, J. Mater. Chem.,2003, 13, 502–510.

151 C. She, J. Guo, S. Irle, K. Morokuma, D. L. Mohler, H. Zabri,F. Odobel, K.-T. Youm, F. Liu, J. T. Hupp and T. Lian, J. Phys.Chem. A, 2007, 111, 6832–6842.

152 J. Rochford, D. Chu, A. Hagfeldt and E. Galoppini, J. Am. Chem.Soc., 2007, 129, 4655–4665.

153 A. Falber, B. P. Burton-Pye, I. Radivojevic, L. Todaro, R. Saleh,L. C. Francesconi and C. M. Drain, Eur. J. Inorg. Chem., 2009,2459–2466.

154 A. Falber, L. Todaro, I. Goldberg, M. V. Favilla and C. M. Drain,Inorg. Chem., 2008, 47, 454–467.

155 N. Robertson, Angew. Chem., Int. Ed., 2006, 45, 2338–2345.156 S. Eu, S. Hayashi, T. Umeyama, Y. Matano, Y. Araki and

H. Imahori, J. Phys. Chem. C, 2008, 112, 4396–4405.157 M. W. Lee, D. L. Lee, W. N. Yen and C. Y. Yeh, J. Macromol. Sci.,

Part A: Pure Appl. Chem., 2009, 46, 730–737.158 C.-W. Lee, H.-P. Lu, C.-M. Lan, Y.-L. Huang, Y.-R. Liang,

W.-N. Yen, Y.-C. Liu, Y.-S. Lin, E. W.-G. Diau and C.-Y. Yeh,Chem.–Eur. J., 2009, 15, 1403–1412.

159 H.-P. Lu, C.-L. Mai, C.-Y. Tsia, S.-J. Hsu, C.-P. Hsieh, C.-L. Chiu,C.-Y. Yeh and E. W.-G. Diau, Phys. Chem. Chem. Phys., 2009, 11,10270–10274.

160 A. Forneli, M. Planells, M. A. Sarmentero, E. Martinez-Ferrero,B. C. O’Regan, P. Ballester and E. Palomares, J. Mater. Chem.,2008, 18, 1652–1658.

161 A. Allegrucci, N. A. Lewcenko, A. J. Mozer, L. Dennany,P. Wagner, D. L. Officer, K. Sunahara, S. Mori and L. Spiccia,Energy Environ. Sci., 2009, 2, 1069–1073.

162 W. M. Campbell, K. W. Jolley, P. Wagner, K. Wagner, P. J. Walsh,K. C. Gordon, L. Schmidt-Mende, M. K. Nazeeruddin, Q. Wang,M. Gr€atzel and D. L. Officer, J. Phys. Chem. C, 2007, 111, 11760–11762.

163 Q. Wang, W. M. Campbell, E. E. Bonfantani, K. W. Jolley,D. L. Officer, P. J. Walsh, K. Gordon, R. Humphry-Baker,

This journal is ª The Royal Society of Chemistry 2010

Dow

nloa

ded

by U

nive

rsity

of

Suss

ex o

n 19

Aug

ust 2

012

Publ

ishe

d on

04

Aug

ust 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C0E

E00

009D

View Online

M. K. Nazeeruddin and M. Gr€atzel, J. Phys. Chem. B, 2005, 109,15397–15409.

164 B. C. O’Regan, I. L�opez-Duarte, M. V. Mart�ınez-D�ıaz, A. Forneli,J. Albero, A. Morandeira, E. Palomares, T. Torres andJ. R. Durrant, J. Am. Chem. Soc., 2008, 130, 2906–2907.

165 M. K. Nazeeruddin, R. Humphry-Baker, M. Gr€atzel andB. A. Murrer, Chem. Commun., 1998, 719–720.

166 A. Morandeira, I. L�opez-Duarte, M. V. Mart�ınez-D�ıaz, B. O’Regan,C. Shuttle, N. A. Haji-Zainulabidin, T. Torres, E. Palomares andJ. R. Durrant, J. Am. Chem. Soc., 2007, 129, 9250–9251.

167 E. Palomares, M. V. Mart�ınez-D�ıaz, S. A. Haque, T. Torres andJ. R. Durrant, Chem. Commun., 2004, 2112–2113.

168 P. Balraju, M. Kumar, M. S. Roy and G. D. Sharma, Synth. Met.,2009, 159, 1325–1331.

169 N. Robertson, Angew. Chem., Int. Ed., 2008, 47, 1012–1014.170 L. Giribabu, C. V. Kumar, P. Y. Reddy, J.-H. Yum, M. Gr€atzel and

M. K. Nazeeruddin, J. Chem. Sci., 2009, 121, 75–82.171 J.-J. Cid, J.-H. Yum, S.-R. Jang, M. K. Nazeeruddin, E. Mart�ınez-

Ferrero, E. Palomares, J. Ko, M. Gr€atzel and T. Torres, Angew.Chem., Int. Ed., 2007, 46, 8358–8362.

172 J.-H. Yum, S.-r. Jang, R. Humphry-Baker, M. Gr€atzel, J.-J. Cid,T. Torres and M. K. Nazeeruddin, Langmuir, 2008, 24, 5636–5640.

173 S. Eu, T. Katoh, T. Umeyama, Y. Matano and H. Imahori, DaltonTrans., 2008, 5476–5483.

174 S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet,P. Comte, M. K. Nazeeruddin, P. P�echy, M. Takata, H. Miura,S. Uchida and M. Gr€atzel, Adv. Mater., 2006, 18((9)), 1202–1205.

175 R. Argazzi, C. A. Bignozzi, T. A. Heimer, F. N. Castellano andG. J. Meyer, J. Am. Chem. Soc., 1995, 117, 11815–11816.

176 X. D. Li, D. W. Zhang, Z. Sun, Y. W. Chen and S. M. Huang,Microelectron. J., 2009, 40, 108–114.

177 R. Katoh, A. Furube, S. Mori, M. Miyashita, K. Sunahara,N. Koumura and K. Hara, Energy Environ. Sci., 2009, 2, 542–546.

178 H. Tian, X. Yang, R. Chen, A. Hagfeldt and L. Sun, Energy Environ.Sci., 2009, 2, 674–677.

179 K. Hara, M. Kurashige, Y. Dan-oh, C. Kasada, A. Shinpo, S. Suga,K. Sayama and H. Arakawa, New J. Chem., 2003, 27, 783–785.

180 K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga and H. Arakawa,Chem. Commun., 2001, 569–570.

181 Y. Chen, Z. Zeng, C. Li, W. Wang, X. Wang and B. Zhang, New J.Chem., 2005, 29, 773–776.

182 D. Kuang, P. Walter, F. N€uesch, S. Kim, J. Ko, P. Comte,S. M. Zakeeruddin, M. K. Nazeeruddin and M. Gr€atzel,Langmuir, 2007, 23, 10906–10909.

183 J.Fang,L.Su,J.Wu,Y.ShenandZ.Lu, Chem.Phys. Lett., 1997,270, 145–151.184 G. Smeureanu, A. Aggarwal, C. E. Soll, J. Arijeloye, E. Malave and

C. M. Drain, Chem.–Eur. J., 2009, 15, 12133–12140.

This journal is ª The Royal Society of Chemistry 2010

185 C. Nitschke, S. M. O’Flaherty, M. Kr€oll, J. J. Doyle and W. J. Blau,Chem. Phys. Lett., 2004, 383, 555–560.

186 E. V. Keuren, A. Bone and C. Ma, Langmuir, 2008, 24, 6079–6084.

187 S. Berhanu, M. A. McLachlan, D. W. McComb and T. S. Jones,Proc. SPIE-Int. Soc. Opt. Eng., 2008, 7052, 70521H-10.

188 R. Rangel-Rojo, H. Matsuda, H. Kasai and H. Nakanishi, J. Opt.Soc. Am. B, 2000, 17, 1376–1382.

189 M. G. Walter and C. C. Wamser, J. Phys. Chem, 2010, 114, 7563–7574.

190 T. Hasobe, Phys. Chem. Chem. Phys., 2010, 12, 44–57.191 G. De Luca, A. Romeo and L. M. Scolaro, J. Phys. Chem. B, 2006,

110, 7309–7315.192 G. De Luca, A. Romeo, V. Villari, N. Micali, I. Foltran, E. Foresti,

I. G. Lesci, N. Roveri, T. Zuccheri and L. M. Scolaro, J. Am. Chem.Soc., 2009, 131, 6920–6921.

193 E. Maligaspe, A. S. D. Sandanayaka, T. Hasobe, O. Ito andF. D’Souza, J. Am. Chem. Soc., 2010, 132, 8158–8164.

194 A. S. D. Sandanayaka, T. Murakami and T. Hasobe, J. Phys. Chem.C., 2009, 133, 18369–18378.

195 K. E. Martin, Z. Wang, T. Busani, R. M. Garcia, Z. Chen, Y. Jiang,Y. Song, J. L. Jacobsen, T. T. Vu, N. E. Schore,B. S. Swartzentruber, C. J. Medforth and J. A. Shelnutt, J. Am.Chem. Soc., 2010, 132, 8194–8201.

196 Z. Wang, Z. Li, C. J. Medforth and J. A. Shelnutt, J. Am. Chem.Soc., 2007, 129, 2440–2441.

197 H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42,1809–1818.

198 G. Bottari, G. de la Torre, D. M. Guldi and T. s. Torres, Chem. Rev.,2010, DOI: 10.1021/cr900254z.

199 B. E. Hardin, J.-H. Yum, E. T. Hoke, Y. C. Jun, P. P�echy,T. s. Torres, M. L. Brongersma, M. K. Nazeeruddin,M. Gr€atzel and M. D. McGehee, Nano Lett., 2010, DOI:10.1021/nl1016688.

200 A. W. Hains, C. Ramanan, M. D. Irwin, J. Liu, M. R. Wasielewskiand T. J. Marks, ACS Applied Materials & Interfaces, 2009, 2, 175–185.

201 J. E. Bullock, M. T. Vagnini, C. Ramanan, D. T. Co, T. M. Wilson,J. W. Dicke, T. J. Marks and M. R. Wasielewski, J. Phys. Chem.,2010, 114, 1794–1802.

202 M. T. Colvin, E. M. Giacobbe, B. Cohen, T. Miura, A. M. Scott andM. R. Wasielewski, J. Phys. Chem., 2010, 114, 1741–1748.

203 T. Miura, R. Carmieli and M. R. Wasielewski, J. Phys. Chem., 2010,114, 5769–5778.

204 M. R. Wasielewski, Acc. Chem. Res., 2009, 42, 1910–1921.205 E.-Y. Choi, P. M. Barron, R. W. Novotny, H.-T. Son, C. Hu and

W. Choe, Inorg. Chem., 2008, 48, 426–428.

Energy Environ. Sci., 2010, 3, 1897–1909 | 1909