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Semiconductor nanostructure-based photovoltaic solar cells

Genqiang Zhang,a Scott Finefrock,a Daxin Liang,ab Gautam G. Yadav,a Haoran Yang,a Haiyu Fanga

and Yue Wu*a

Received 10th February 2011, Accepted 10th March 2011

DOI: 10.1039/c1nr10152h

Substantial efforts have been devoted to design, synthesize, and integrate various semiconductor

nanostructures for photovoltaic (PV) solar cells. In this article, we will review the recent progress in this

exciting area and cover the material chemistry and physics related to all-inorganic nanostructure solar

cells, hybrid inorganic nanostructure-conductive polymer composite solar cells, and dye-sensitized

solar cells.

The worldwide demand for energy is causing a dramatic envi-

ronmental impact on the global climate due to the combustion of

fossil fuels. In order to improve the sustainability of our society,

photovoltaic solar cells, as a significant low-carbon energy

source, have received tremendous attention recently. For over

the past two decades, crystalline silicon based single junction

solar cells, also known as first generation solar cells, have

dominated the photovoltaic (PV) market with an efficiency of

aSchool of Chemical Engineering, Purdue University, West Lafayette, IN,47907, USA. E-mail: yuewu@purdue.edu; Fax: +1 765-494-0805; Tel: +1765-494-6028bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry,Jilin University, 2699 Qianjin Street, Changchun, 130012, PR China

Genqiang Zhang

Genqiang Zhang received BS

degree in Materials Chemistry

in 2004 and PhD degree in

Condensed Matter Physics in

2009, both from University of

Science and Technology of

China supervised by Prof.

Xiaoguang Li. After that he

became a research associate in

Chemical Engineering of Purdue

University with Prof. Yue Wu.

He is interested in the solution

phase synthesis, property study

and device applications of

nanostructured thermoelectric

materials and solar cells.

2430 | Nanoscale, 2011, 3, 2430–2443

�25%,1 quite close to the theoretical limit value of �31%.2

Recently, thin film solar cells, also known as second generation

solar cells, utilizing a much thinner absorbing layer have been

considered as a possible solution to lower the solar cell cost

compared with established silicon technology. Until now, the

maximum efficiencies for second generation solar cells are

�19.9% for the Cu(In, Ga)Se2 (CIGS) system, �16.5% for the

CdTe system,�15% for amorphous Si film.3,4 The reasons for the

lower efficiency in thin film solar cells are mainly the absorption

of less of the solar spectrum due to the thinner cell and higher

density of non-radiative recombination centers due to the poly-

crystalline or amorphous features.5 Among the various types of

solar cells, semiconductor nanostructure-based PV devices are

promising candidates for the third generation solar cells because

Yue Wu

Prof. Yue Wu received his

bachelor degree in chemistry

with best thesis award from

University of Science and Tech-

nology of China in 2001. Then,

he went to Harvard University

and studied for his doctoral

degree under the supervision of

Prof. Charles M. Lieber. He

received his doctoral degree in

June, 2006 and was awarded the

Miller Fellowship from Univer-

sity of California at Berkeley.

At Berkeley, he worked with

Prof. A. Paul Alivisatos for

three years and joined School of

Chemical Engineering at Purdue University as an assistant

professor in August, 2009. Prof. Wu’s research interest is focusing

on the broadly defined nanostructured materials for their potential

applications in nanoscale devices and renewable energy-related

field.

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 Well-defined CdSe nanocrystals with tunable aspect ratios, the

scale bar in (C) is for (A)–(C). Further details are described in ref. 14.

Reprinted with permission from American Association for the

Advancement of Science.

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they have several advantages over the traditional single crystal or

thin film solar cells including low temperature processing, scal-

ability, and low cost manufacturing. These benefits could be

capitalized upon if advanced synthesis methods and device

fabrication techniques are used to create devices with efficiency

comparable to single crystal and/or thin film solar cells. Many

semiconductor nanostructures have been demonstrated for

applications in photovoltaic devices, for example, cadmium

chalcogenides (CdTe, CdSe, CdS), lead chalcogenides (PbTe,

PbSe, PbS), I–III–VI2 material systems (CuInSe2 based semi-

conductors), zinc chalcogenides (ZeTe, ZnO), I2–VI semi-

conductors (Cu2O, Cu2S), iron pyrite (FeS2), etc.6–10 In this

review article, we give a comprehensive introduction to the

representative synthesis process, size and shape control, self-

assembly behavior and electrical characterization of those

promising nanostructures that have been built into PV devices in

the previous literatures.

1. The synthesis of semiconductor nanostructures forphotovoltaic devices

1.1. Nanocrystal synthesis

1.1.1. Thermal decomposition process. Thermal decomposi-

tion synthesis, which has been broadly applied to synthesize

semiconductor nanocrystals, generally involves reactions in an

organic solvent under air-free conditions using a Schlenk line. In

this method, the organometallic compounds or metal-surfactant

complexes usually act as the precursors and a subsequent hot

injection of precursor solution into organic solvent with

a surfactant is necessary to obtain high quality nanocrystals of

various materials, including semiconductor nanostructures for

photovoltaic devices. The thermal decomposition synthesis of

semiconductor nanocrystals was pioneered by Bawendi and

co-workers,11 who used dimethylcadmium (Me2Cd) as the

Cd source, bis(trimethylsilyl) [(TMS)2E] (E ¼ Te, Se, S) or

tri-n-octylphosphine tellurium (TOPTe) and TOPSe as the

chalcogen source and mixed phosphine/phosphine oxide solution

as the coordinating solvent. The rapid injection of precursor

solution into the hot solvent led to a short burst of homogeneous

nucleation and a subsequent slow growth process, which could

possibly be due to the sudden temperature drop associated with

the introduction of room temperature reagents, generated

monodisperse CdE nanocrystals. The obtained nanocrystals

could be well dispersed in organic solvents and fine tuning of

their diameters resulted in a notable quantum confinement effect.

This work provided a new rationale for the synthesis of high

quality semiconductor nanocrystals. Subsequently, Peng et al.

successfully obtained high quality CdE nanocrystals using the

more convenient and less harmful CdO as a precursor.12,13 Up to

date, various monodisperse nanocrystals have been synthesized

through the hot injection process based on the thermal decom-

position reaction. For example, shape controlled CdSe nano-

crystals with different aspect ratios have been achieved, which

greatly benefit the application of CdSe in PV devices.14–17 Fig. 1

gives the typical transmission electron microscopy (TEM) images

for CdSe nanocrystals with aspect ratios ranging from 1 to 10,

which were achieved by adjusting the initial monomer

This journal is ª The Royal Society of Chemistry 2011

concentrations.12 Through similar procedures, shape controlled

CdTe nanocrystals have also been synthesized.11,18–22

As a family of promising candidates for thin film PV appli-

cations, the synthesis of the copper indium diselenide (CIS) based

materials has long attracted intense interest.23–30 However, the

products synthesized through previously reported routes have

limitations in dispersion, impurities, difficult compositional and

structural control, etc. Fortunately, great progress has been

reached in some recent literatures through the thermal decom-

position routes.31–37 For example, the Hillhouse group developed

an effective solution synthesis of phase selective CuInSe2 nano-

crytals,31 in which the order of Se precursor injection into

oleylamine solution was varied in order to induce different

crystal structures in the final products. The results indicated that

sphalerite nanocrystals were obtained if Se precursor was added

into the solution of CuCl and InCl3 in oleylamine at 285 �C due

to abrupt supersaturation of the reaction mixture and a subse-

quent rapid nucleation and growth process, while pure chalco-

pyrite nanocrystals were obtained when the Se precursor was

injected along with other precursors at 130 �C, followed by

a higher temperature growth process. Fig. 2A and B show typical

TEM and high-resolution transmission electron microscopy

(HRTEM) images of the as-synthesized chalcopyrite CuInSe2nanocrystals, indicating the good crystallinity of the products. In

addition, CuInSe2 nanorings were formed (Fig. 2C and D)

through a secondary ligand during the nanocrystal growth by

dissolving the precursors in TOP and hot injection into oleyl-

amine. This method has also been extended to other CIS based

ternary and quaternary nanocrystals.32–34,36 In addition, Korgel

and co-workers reported the synthesis of monodisperse, solu-

tion-stable chalcopyrite CuInSe2 nanocrystals with trigonal

pyramidal morphology, as shown in Fig. 2E and F. The synthetic

procedure, which was similar to previous literatures, required the

following procedures: InCl3 and CuCl were first combined in

oleylamine under air-free conditions and heated to 130 �C, fol-lowed by the addition of selenourea at 100 �C. Then, the reactionmixture was heated up to 240 �C for nanocrystal growth. The

critical modification in this method was the use of selenourea as

the Se source, which decided the final shape of the products. The

trigonal pyramidal nanocrystals exhibited a well defined

arrangement on the TEM grid (Fig. 2E) and single crystal nature

according to microstructure analysis (Fig. 2F).

As a candidate material that contains both earth abundant

elements and environmentally friendly, copper(I) sulfide has been

well studied in PV devices since the 1960s.38–43 Cu2S nano-

structures with various morphologies, including nanoparticles,

nanodisks, nanorods/wires and hexagonal platelets have been

Nanoscale, 2011, 3, 2430–2443 | 2431

Fig. 2 Shape controlled synthesis of CuInSe2 nanocrystals: (A, C and E)

TEM images; (B, D and F) corresponding HRTEM analysis. Further

details are described in ref. 31 and 37. Reprinted with permission from

American Chemical Society.

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synthesized by different chemical routes. Using the thermal

decomposition method, shape and size controlled Cu2S nano-

crystals have also been achieved. For example, Cu2S nano-

crystals, nanorods and nanodisks with chain-like arrangement

were obtained by the Korgel group, as shown in Fig. 3.44–46 In

addition, size tunable Cu2S spherical nanocrystals have been

achieved by simply adjusting the reaction time.47

1.1.2. Nonaqueous sol–gel process. The conventional sol–gel

process has been widely adopted to synthesize oxide particles by

hydrolysis and condensation of precursors in acidic or basic

aqueous medium.48 Generally, the products obtained through

this method have limitations in size and shape control, broad size

distribution, etc., due to the polarity of H2O that is used as

Fig. 3 Typical TEM images of shape controlled synthesis of Cu2S

nanocrystals. Further details are described in ref. 46. Reprinted with

permission from American Chemical Society.

2432 | Nanoscale, 2011, 3, 2430–2443

a medium.49 The nonaqueous sol–gel process, which involves

only metal oxide precursors and organic solvents, has been well

developed for the synthesis of high quality metal oxide nano-

structures.50,51 For semiconductor oxide nanostructures that

could be used as solar cell building blocks, TiO2 spherical and

rod-shaped nanocrystals have been obtained using several

alternative routes with different organic solvents, precursors

under different growth mechanisms including aminolysis and

reactions with alcohols.52–58 In addition, various ZnO nano-

structures including spheres, hexagons and rod/wire-shaped

nanocrystals with controlled aspect ratios can also be obtained

through this method by adjustment of experimental parameters

such as precursor, solvent, reaction time, temperature, etc.59–67

1.1.3. Other available methods. Although the dominant

routes for the synthesis of high quality semiconductor nano-

crystals are the thermal decomposition based on hot injections

and the nonhydrolytic sol–gel process, there are a variety of

alternative routes including the hydro/solvothermal process,

polyol synthesis and the aqueous solution method, etc. Through

these methods, various semiconductor nanostructures that are

applicable in photovoltaic devices have been obtained. For

example, a general hydrothermal based strategy has been

developed to effectively synthesize the nanocrystals of CdSe, PbS

and CdS.68 Shape controlled PbTe nanostructures and PbSe

hollow spheres were obtained through an ethylene glycol based

polyol process with different experimental procedures.69,70

Furthermore, aqueous phase synthesis has been recently devel-

oped to obtain various semiconductor nanocrystals such as

CdTe, CdSe, CdS and their heterostuctures.71–75

1.2. One-dimensional nanostructure synthesis

One-dimensional (1D) nanowires and nanotubes represent

another group of building blocks for nanostructured photovol-

taic solar cells. Compared to the nanocrystal-based approach,

one significant advantage of these 1D nanostructures possess is

the more continuous electrical transport. Many demonstrations

of nanowire or nanotube based PV devices have been explored as

promising ways to improve the energy conversion efficiency.

Generally, there are two types of design approaches: one is based

on nanowire/nanotube arrays for large scale application and the

other is the single nanowire devices that have been considered as

a possible high efficiency, robust and integrated nanoscale power

source.76 Previous literatures have demonstrated that Si, GaN,

and ZnO nanowires and TiO2 nanotubes could be successfully

applied as building blocks in PV devices. In this section, we

introduce the current synthesis methods for these 1D nano-

structures and heterostructures.

1.2.1. Vapor-phase synthesis of nanowires. The vapor phase

deposition process is the dominant route for synthesizing Si and

GaN nanowires as well as their axially or radially modulated

nanowire heterostructures. Most of this nanowire growth is

based on the vapor–liquid–solid (VLS) mechanism and the

diameter of the nanowires is defined by the size of the liquid alloy

droplets formed by gas precursors and catalyst particles that

create a surface energy difference to favor the 1D growth.77–81

For example, the Lieber group obtained molecular-scale Si

This journal is ª The Royal Society of Chemistry 2011

Fig. 4 HRTEM images of Si nanowires with different diameters. The

scale bars are 5 nm. Further details are described in ref. 80. Reprinted

with permission from American Chemical Society. Fig. 5 (A) Schematic illustration and (B) SEM images in back-scattered

electron mode of p–i–n Si coaxial nanowires. The scale bars are 1 mm

(top) and 200 nm (bottom), respectively. Further details are described in

ref. 82. Reprinted with permission from Nature Publishing Group.

Fig. 6 (A) Schematic illustration of cross-sectional view for the GaN

based radial heterostructure nanowires. (B) Bright-field TEM cross-

sectional image, (C) normalized EDS line profiles and (D) dark-field

scanning TEM image of a GaN/InxGa1-xN/GaN/AlGaN CMS nano-

wire. Scale bars are 100 nm in (B) and 50 nm in (D). Further details are

described in ref. 84. Reprinted with permission from American Chemical

Society.

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nanowires with controlled diameters using silane in hydrogen as

reactant and various sizes of gold nanoclusters as catalyst.80

Fig. 4a–c give a typical HRTEM analysis of Si nanowires with

diameters of 13.2, 5.7 and 3.5 nm, respectively. One can see that

the nanowires are single crystals with no obvious amorphous

oxide layers.

In order to build Si-based solar cells, one needs to fabricate

a p-type/n-type (p–n) or p-type/intrinsic/n-type (p–i–n) junction

structure in axial or radial geometry. The previous success in the

area of controlled synthesis of Si nanowires has greatly benefited

the work on special architecture fabrication. In the growth of

coaxial (radial) nanowire heterostructures, the Lieber group

designed and successfully synthesized coaxial Si nanowires in

a p–i–n geometry, as shown in Fig. 5A and B.82 They first

fabricated p-type Si nanowires according to the previous litera-

tures, followed by the deposition of an intrinsic Si shell at

a higher temperature and lower pressure compared with p-type Si

core growth. It should be noted that phosphine was used as

a reactant in order to achieve n-type doping of the outer shell to

create a p–i–n junction. Recently, wide band gap III–V alloy

based coaxial nanowires have also been indicated as promising

candidates for single nanowire PV devices with large open circuit

voltage.83 Their synthesis has been achieved in previous litera-

tures by the Lieber group as shown in Fig. 6,84–86 in which both

the element line scanning result and dark-field TEM image

indicate the formation of GaN/InxGa1-xN/GaN/AlGaN multi-

shell structure.84

Besides the radial nanowire heterostructures, axial p–i–n Si

nanowire heterostructures have also been obtained by simply

switching dopant precursor gases at different stages of the

growth. Fig. 7 shows a typical Scanning Electron Microscopy

(SEM) image of an axially modulated p–i–n Si nanowire.87 In

addition, the SEM image of the nanowire after selective wet

etching, as shown in Fig. 7C, provides clear evidence to verify the

formation of an axial p–i–n nanostructure.87

1.2.2. Solution phase method. Recently, the solution phase

method has also been applied for synthesis of aligned Si nano-

wires which can be used in PV devices after further modifications.

Fig. 8 gives a typical SEM image and HRTEM characterization

of large scale Si nanowire arrays on silicon wafers that were

synthesized through a facile chemical etching technique based on

galvanic displacement reactions.88–91 In a typical synthesis, the

cleaned silicon wafers were immersed into an aqueous HF/silver

nitrate solution in a sealed vessel and treated at 50 �C for a set

amount of time.88 The synthetic conditions and procedures are

simple compared with those of vapor phase deposition and the

products appear as aligned nanowire arrays with a single

This journal is ª The Royal Society of Chemistry 2011

crystalline nature, as shown in Fig. 8. However, the obtained Si

nanowires need post-treatments, such as the removal of the Ag

and oxide layers generated during the synthesis and deposition of

n-type shell materials to form the p–n junction geometry for

application in photovoltaic devices.92

ZnO nanowire arrays can also be obtained through a solution

phase synthesis on an ITO substrate. In order to obtain aligned

ZnO nanowires, the substrate was first decorated with a ZnO

layer and then immersed in an aqueous solution containing

appropriated zinc nitrate hexahydrate, hexamethylenetetramine

and polyethyleneimine for about 30–60 min at 90 �C.93,94 The

products can be built into nanowire array photovoltaic devices

after decoration by Cu2O or TiO2 particles or shell layers.94,95

1.3. TiO2 nanotube synthesis

The outstanding performance of TiO2 nanoparticle dye-sensi-

tized solar cells has attracted great attention to the synthesis of

TiO2 nanotubes and nanotube arrays and a variety of effective

methods have been developed, such as porous alumina template

assisted deposition,96–99 organic precursor based sol–gel

process,100,101 hydrothermal reaction,102–104 etc. Among these

synthetic routes, the anodization of titanium process which was

first reported by Grimes and co-workers in 2001,105 has proven to

be the most successful strategy and was greatly modified to

Nanoscale, 2011, 3, 2430–2443 | 2433

Fig. 7 (A) Schematic illustration and (B) SEM image of axial p–i–n Si

nanowire; (C) SEM images of a p–i–n axial NW after selective wet

etching, the scale bars are 1 mm. Further details are described in ref. 87.

Reprinted with permission from American Chemical Society.

Fig. 9 (A and B) Typical SEM images of TiO2 nanotube arrays; (C)

digital images of transparent TiO2 nanotube array films with different

thicknesses; (D) TEM image of a single nanotube and the corresponding

SAED pattern. Further details are described in ref. 114. Reprinted with

permission from Nature Publishing Group.

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obtain high quality TiO2 nanotube arrays with remarkable

properties enabling their application in PV devices.106–114 The

growth of vertically aligned TiO2 nanotube arrays can be ach-

ieved on a fluorine doped tin oxide coated glass substrate,114 and

the length of the nanotube could reach 20 micrometres and the

pore diameter was about 95 nm with a wall thickness of about 10

nm, as shown in Fig. 9A and B. The images shown in Fig. 9C

indicate that the TiO2 nanotube array films are transparent,

which will allow photons to interact with the dye molecules more

efficiently. In addition, the selected area electron diffraction

(SAED) pattern proved that the products were single crystalline

with an anatase phase.

1.4. Nanocrystal assembly

The controlled assembly of the as-synthesized nanocrystals or

nanorods will be the prerequisite for their applications in various

nanodevices. There are several available techniques to assemble

nanostructures, including self-assembly by slow evaporation of

solvents,115–118 Langmuir–Blodgettry (LB),119 layer-by-layer

(LBL) deposition,120,121 etc. Bawendi and co-workers first carried

out the self-assembly of spherical CdSe nanocrystals in octane

and octanol to create a superlattice structure through selective

solvent evaporation under reduced pressure.115 There have been

many other reports on the study of self-assembly behavior of

semiconductor nanostructures.122–124 For example, Fig. 10A

gives the SEM image of self-assembled 3-D colloidal crystals of

Cu2S nanodisks and Fig. 10B gives the TEM image of self-

assembled 2-D layers of CdSe/CdS nanorods.46,123 In general,

self-assembly behavior requires high quality of nanocrystals or

nanorods, such as monodispersity, regular shape (spherical,

cubic, or regular rods), and small diameters. The LB technique

has been demonstrated as a low cost, high-throughput and easily

integrated method for controlled assembly of various

Fig. 8 (A) Typical SEM image of the aligned Si nanowire arrays and (B)

corresponding HRTEM analysis and ED pattern (the inset). Further

details are described in ref. 91. Reprinted with permission from Wiley.

2434 | Nanoscale, 2011, 3, 2430–2443

nanocrystals and nanowires. The assembly of nanocrystals

through this method is usually achieved in a water-filled trough

equipped with a mobile barrier and a pressure sensor. The

organic solution containing dispersed nanoparticles is spread

dropwise onto the water surface and self-organization is formed

at the high surface pressures when the nanocrystal solution was

pushed to form a close packed monolayer on the water surface.119

The LBL deposition method, which involves the repeated process

of dip-coating a thin layer of nanocrystals followed by chemical

treatment after each new layer, has been broadly applied to

fabricate nanostructured film based PV devices. Fig. 11 gives an

example of the morphology of PbSe nanocrystal film prepared by

LBL dip-coating process.

Fig. 10 (A) SEM image of self-assembled three dimensional colloidal

crystals of Cu2S nanodisks; (B) TEM image of self-assembled two

dimensional layers of CdSe/CdS nanorods, the scale bar is 50 nm. Further

details are described in ref. 46 and 123. Reprinted with permission from

American Chemical Society.

This journal is ª The Royal Society of Chemistry 2011

Fig. 11 SEM images of PbSe NC films prepared by layer-by-layer dip-

coating onto ITO substrates. (a) Plan view. The scale bar is 50 nm.

(b) Cross-section. The scale bars are 100 nm. Further details are described

in ref. 121. Reprinted with permission from American Chemical Society.

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2. Semiconductor nanostructure based solar cells

2.1. All inorganic nanocrystal based solar cells

Several semiconductor nanocrystals have been utilized as

building blocks for all inorganic nanocrystal solar cells. Gener-

ally, to fabricate a device, semiconductor nanocrystals dispersed

in a solvent were deposited on an appropriate substrate, such as

indium tin oxide (ITO) glass or a plastic substrate by spin-

casting, dip-coating, or other techniques to form a homogenous

nanocrystal film with high density. Various semiconductor

nanocrystals have been applied in the fabrication of photovoltaic

devices, including CdSe, CdS, PbSe, PbS, Cu2S, Cu(In, Ga)Se2,

Cu2ZnSnS4, etc.125–127 For example, the Alivisatos group built

PV devices based on a CdSe/CdTe bilayer film by spin casting

from CdSe and CdTe nanorod colloidal solutions.128 It was

proposed that the photovoltaic conversion of their device was

based on a donor–acceptor mechanism according to the energy

diagram of CdSe and CdTe shown in Fig. 12A, in which

photoexcitations experienced an energetic driving force for the

charge transfer process and the exciton will separate into elec-

trons and holes at the interface. A typical SEM image showed

Fig. 12 (A) An energy diagram of valence and conduction band levels

for CdTe and CdSe. (B) Typical SEM image of the nanocrystal film

through spin-casting process, the scale bar is 1 mm. (C) I–V characteristics

for the CdSe/CdTe nanorods based device in the dark (dotted) and under

simulated one-sun AM1.5G illumination (solid). Further details are

described in ref. 128. Reprinted with permission from American Asso-

ciation for the Advancement of Science.

This journal is ª The Royal Society of Chemistry 2011

that the formed film was homogenous and pinhole-free

(Fig. 12B). The I–V curve in the dark and in the light indicates

that the device showed strong photoresponse and diode rectifi-

cation behavior (Fig. 12C). The cell also exhibited a notable

photovoltaic effect: the short circuit current (Isc) and the open-

circuit voltage (Voc) were as high as 0.58 mA cm�2 and 0.41 V,

respectively. The post-annealing process and the top contact

electrode greatly influence the device properties and an AM1.5

power conversion efficiency of 2.9% has been achieved by

choosing a Ca 20 nm/Al 80 nm layer as the electrode.

Schottky junction based solar cells, which separate the pho-

togenerated carrier due to the energy barrier at the metal–semi-

conductor interface, have also been well studied recently in all

inorganic nanocrystal PV devices, especially on those based on

lead chalcogenides.33,129–134 For example, Nozik and co-workers

fabricated PbSe nanocrystal solar cells through a LBL technique

on ITO glass substrate as shown in Fig. 13A.120 In the fabrication

process, small molecules, such as 1,2-ethanedithiol (EDT), are

used to replace the long alkyl chain surfactants on the PbSe

nanocrystal surface to improve the charge transport, while the

electrons and holes were separated by a Schottky barrier at the

interface between the evaporated metal electrode and nano-

crystal film (Fig. 13B). Similar devices have also been fabricated

using PbS nanocrystal films.135 The external quantum efficiency

(EQE) of these devices resembles the absorption spectra in the

visible and near-infrared spectral regions and typically peak at

55–65% below 800 nm, as shown in Fig. 13C. In addition, the

results indicated that the Voc of the device strongly depended on

Fig. 13 (A) Cross-sectional SEM image of the PbSe nanocrystal based

device with a structure of ITO/NC film/metal contact, the scale bar is 100

nm. (B) Proposed equilibrium band diagram. (C) External quantum

efficiency (EQE) of a device with a 140 nm thick film. (D) The relation-

ship between Voc and nanocrystal particle sizes. (E) The dependence of

Voc with metal work function.113 Further details are described in ref. 120.

Reprinted with permission from American Chemical Society.

Nanoscale, 2011, 3, 2430–2443 | 2435

Fig. 15 (A) SEM images of the thermal treated CuInSe2 nanocrystal film

on Mo coated soda lime glass. (B) The I–V curve of the CuInSe2 based

photovoltaic device.24 Further details are described in ref. 31. Reprinted

with permission from American Chemical Society.

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the nanocrystal particle sizes and metal work functions; their

relationships are demonstrated in Fig. 13D and E.

Copper(I) sulfide with an indirect gap of 1.2 eV has been

broadly studied in thin film PV devices when combined with

CdS.38–42 The Alivisatos group has developed the PV devices

based on high quality Cu2S nanocrystals with an average size of

about 5.4 nm (Fig. 14A). The absorption spectrum shown in

Fig. 14B indicated that the Cu2S nanocrystals exhibited a wide

absorption up to about 1000 nm and the photoluminescence (PL)

characterization showed a single peak at 1.32 eV. Cu2S–CdS

bilayer photovoltaic devices have been fabricated by spin-casting

a 300 nm Cu2S layer and a 100 nm CdS layer on ITO glass

substrates. A typical I–V curve of the device (Fig. 14C) indicated

that theVoc and Isc of the solar cell can reach up to 0.6 V and 5.63

mA cm�2, respectively, corresponding to a power conversion

efficiency of 1.6% with a fill factor of 40%. Another important

feature was that the device can also be built on a flexible plastic

substrate, which only showed a slight decrease of efficiency

(�8%) even being bent to a curvature of 105 �C (Fig. 14D). This

feature may greatly expand the application of nanocrystals based

solar cells for flexible power generation in mobile devices.

I–III–VI2 compounds have been demonstrated as promising

candidates for high efficiency PV devices in thin film solar cells.136

Therefore, the I–III–VI2 and related nanocrystal based PV

devices have also attracted a lot of attention.31–34,36 For example,

the Hillhouse group recently fabricated PV devices based on

thermally sintered CuInSe2 nanocrystal films using CuInSe2nanocrystal ink through drop-casting (Fig. 15A), followed by

chemical bath deposition of a CdS layer and RF sputtering of 50

nm intrinsic ZnO and 300 nm ITO layers. The devices had

a power conversion efficiency and fill factor of 2.8% and 39%

respectively under standard AM1.5 illumination; the corre-

sponding I–V curve is shown in Fig. 15B. As a further develop-

ment in this group of PV devices, researchers have been trying to

Fig. 14 (A) TEM, high resolution TEM (top inset) image and (B) the

typical UV-vis absorption spectrum of the as-synthesized of Cu2S

nanocrystals. (C) I–V curve of the Cu2S–CdS nanocrystal based photo-

voltaic devices under zero (black) and standard illumination (red). (D)

The I–V curve of the Cu2S–CdS nanocrystal based photovoltaic device

on the plastic substrate under zero (black) and standard (red) illumina-

tion with a curvature of 105�, and I–V curve after released to flat (green).

Further details are described in ref. 133. Reprinted with permission from

American Chemical Society.

2436 | Nanoscale, 2011, 3, 2430–2443

find more cost-effective materials that possess efficiencies similar

to those of I–III–VI2 compounds. One of the key challenges is to

replace indium, which is the most expensive and least abundant

element in the formula. Cu2ZnSnS4 with a band gap and an

absorption coefficient both similar to those of Cu(In, Ga)Se2may fit the demands of high-efficiency and low cost, and the

fabrication of Cu2ZnSnS4 nanostructure-based PV devices has

been explored by the Hillhouse group recently as well.33

2.2. Polymer–nanocrystal hybrid solar cells

Hybrid solar cells are usually made from conjugated polymers

and inorganic semiconductor nanocrystal composites, which

possess the beneficial features of both materials and hold great

potential for large-scale cost-effective manufacturing. Typically,

the organic semiconducting polymers in hybrid devices are the

electron donors and hole-transport medium while the inorganic

nanocrystals usually act as the electron acceptors and electron-

transport medium. In previous literatures, various hybrid solar

cells have been studied and much progress has been achieved,

including devices that utilized polymer in conjunction with CdSe

nanocrystals14,137 and hyperbranched nanostructures,138 CdS

nanowires,139,140 PbSe, PbS nanoparticles,141–145 CuInS2, Si

nanocrystals146 and ZnO and TiO2 nanostructures.147,148 The

Alivisatos group demonstrated the CdSe nanocrystal/poly(3-

hexylthiophene) (P3HT) hybrid solar cells and systematically

studied the influence of nanocrystal diameter and length on

device efficiency.14 The device fabrication involved spin-casting

a solution of 90 wt% CdSe nanorods in P3HT onto an ITO glass

substrate coated with polystyrene sulfonic acid doped poly-

(3,4-ethylene dioxythiophene) (PEDOT:PSS) and aluminium as

the top contact. Fig. 16A lists the different EQEs of the devices

based on CdSe nanocrystals with different aspect ratios. It is very

clear that the EQE has notably enhanced by a factor of

approximately 3 when the aspect ratio of nanocrystals increased

from 1 to 10 and reached up to 55% in a device made from 7 nm

by 60 nm nanorods under 0.1 mW cm�2 illumination at 485 nm.14

The diameter of the nanocrystals also plays an important role on

the device efficiency as shown in Fig. 16B, in which the absorp-

tion edge of CdSe nanocrystal films was effectively tuned from

650 nm to 720 nmwhen the diameter increased from 3 to 7 nm. In

this device, a high power conversion efficiency of 6.9% has been

obtained under a 0.1 mW cm�2 illumination at 515 nm using the

blend containing 7 nm by 60 nm CdSe nanorods. In addition to

simple nanocrystals, the Alivisatos group also built hybrid solar

This journal is ª The Royal Society of Chemistry 2011

Fig. 16 External quantum efficiency of solar cell based 7 nm diameter

CdSe nanorods with lengths of 7, 30 and 60 nm respectively. (B)

Photocurrent spectra for two devices with 60 nm long nanorods with

diameters 7 and 3 nm.7 Further details are described in ref. 14. Reprinted

with permission from American Association for the Advancement of

Science.

Fig. 18 (A) External quantum efficiency as a function of wavelength for

all four devices based on P3OT layer with different thicknesses (Devices

A, B, C, D: 160, 115, 80 and 0 nm, respectively) and 160 nm PbS nano-

crystal layer. The inset shows the absorption spectrum of P3OT. (B)

Measured external quantum efficiency and calculated internal quantum

efficiency at the first 1260 nm and the onset of the third exciton peaks (720

nm).137 Further details are described in ref. 144. Reprinted with permis-

sion from American Institute of Physics.

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cells based on CdSe hyperbranched nanostructures with the

purpose of solving the unpredictable and irreproducible

morphology in the nanocomposite blend.138 The hyperbranched

CdSe nanostructures spontaneously formed a 3-D array on the

substrates and an improved power conversion efficiency of 2.18%

under a AM1.5 illumination has been demonstrated.138

Infrared nanocrystal based hybrid solar cells have also been

broadly characterized recently in order to generate photocurrent

using lower energy photons. McDonald et al. recently investi-

gated the infrared photovoltaic properties of a nanocomposite

where PbS nanocrystals were incorporated into the polymer

matrix of poly[2-methoxy-5-(2-ethylhexyloxy-pphenyleneviny-

lene)] (MEH-PPV).141 The obtained nanocomposites were then

deposited on a poly(p-phenylenevinylene) (PPV) coated ITO

glass substrate to form a sandwich structure device that showed

a photovoltaic effect with an internal quantum efficiency of

�0.0006% and an external quantum efficiency of �0.0008%, as

shown in Fig. 17. However, the efficiency under a �5 bias can be

enhanced to about 3%, which is about 500 times higher than

when under zero bias.141 They subsequently fabricated an

improved PbS nanocrystal based hybrid solar cell through a poly

(3-octylthiophene) (P3OT)-nanocrystal bi-layer structure on

a PEDOT:PSS coated ITO glass substrate.144 Fig. 18A gives the

curves of EQE as functions of wavelength for four devices with

different P3OT layer thicknesses (160, 115, 80, 0 nm) together

with a 160 nm PbS nanocrystal layer. The EQE was clearly

dependent on the P3OT layer thickness and reached to

Fig. 17 (A) Dark current and photocurrent curves near zero bias, the

inset is the proposed simplified band diagram depicting the relative

energy alignment after the magnesium electrode deposition. (B) Short-

circuit current (circles) and corresponding internal quantum efficiency

(squares) as a function of incident power. Inset: stability of the short-

circuit current as a function of time for a sample with much lower

photovoltaic response than in (A). Further details are described in ref.

141. Reprinted with permission from Nature Publishing Group.

This journal is ª The Royal Society of Chemistry 2011

a maximum value of about 1.8% at the wavelength of 720 nm,

with a calculated internal quantum efficiency of 11.3% when the

P3OT layer thickness was 115 nm (Fig. 18B). In addition to PbS

nanocrystals, PbSe nanocrystals have also been used in hybrid

solar cells.143 Xu and co-workers built hybrid solar cells

employing 6 nm PbSe nanocrystals blended with P3HT and

obtained an internal quantum efficiency of 21% and power

conversion efficiency of 0.14% under AM1.5 illumination.143

Bulk silicon has long been used in first generation solar cells.

Recently, silicon nanocrystal based hybrid solar cells have also

been investigated.146 The Kortshagen group built PV devices by

spin-coating Si nanocrystal/P3HT blended film with thickness of

about 100 nm onto ITO coated glass substrate with aluminium as

the top contact. The I–V curve of the hybrid solar cell in Fig. 19A

shows a power conversion efficiency of 1.15% and a fill factor of

about 46%. The Isc density and Voc of the device reach 3.3 mA

cm�2 and 0.75 eV, respectively. The data clearly showed that the

hybrid solar cell possesses enhanced properties compared with

those of the P3HT solar cell, as shown in the inset of Fig. 19A. In

addition, the incident photon-to-current conversion efficiency

(IPCE) of the device reaches as high as 26% near the wavelength

of 500 nm in Fig. 19B, but most of the collected current was

contributed by the P3HT absorption while the Si nanocrystal

absorption only boosted the IPCE above the wavelength of

500 nm.

Besides the traditional group V and compound semi-

conductors, nanostructured semiconducting oxides have also

been proposed as promising candidates for hybrid solar cells.

Until now, most of the researches have been focusing on ZnO

and TiO2 nanostructure based hybrid solar cells.149–156 Among

these nanostructures, the composites containing nanorod/wire

arrays have been considered a more interesting architecture for

Nanoscale, 2011, 3, 2430–2443 | 2437

Fig. 19 (A) I–V curves of a 35 wt% 3–5 nm Si nanocrystals/P3HT hybrid

solar cell and P3HT-only solar cell (the inset) under 100 mW cm�2 AM1.5

illumination. (B) Incident photon-to-current efficiency spectrum of a 35

wt% 3–5 nm Si NCs/P3HT solar cell. Further details are described in ref.

146. Reprinted with permission from American Chemical Society.

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solar cells due to the optimal effect of light trapping and more

continuous electrical transport.147 For example, NREL

researchers fabricated P3HT/ZnO rod based photovoltaic

devices and obtained a Isc of 2.2 mA cm�2, a Voc of about 0.44 V

and a fill factor of 56% with a power conversion efficiency of

0.53%.157 The Ravirajan and Peiro group built P3HT and

hydrothermally synthesized ZnO nanorod arrays based PV

devices and a power conversion efficiency of 0.2% was achieved

in their reports.158,159 Recently, the Yang group studied ZnO/

TiO2 core/shell nanorod array/P3HT based hybrid solar cells.147

In their study, they first fabricated ZnO nanorod arrays through

a two-step solution method on an ITO glass substrate followed

by the deposition of a TiO2 shell using an atomic layer deposition

(ALD) system.160 Fig. 20A gives the typical SEM image of the

ZnO/TiO2 core/shell nanorod array filled with P3HT with a TiO2

shell thickness of 7 nm and a total film thickness of about 600

Fig. 20 (A) Cross-sectional SEM image of a ZnO–TiO2 array grown on

silicon and then infiltrated with P3HT and annealed at 220 �C for 1 h.

(B) I–V curves of a ZnO–TiO2 nanorod/P3HT hybrid solar cells with

a 7 nm shell (circles) and a P3HT/TiO2 bilayer cell (triangles) in the dark

(open symbols) and under 100 mW cm�2 AM1.5 simulated illumination

(closed symbols). The inset is the external quantum efficiency of the

nanorod cell. Further details are described in ref. 147. Reprinted with

permission from American Chemical Society.

2438 | Nanoscale, 2011, 3, 2430–2443

nm. The I–V curves in Fig. 20B showed the Isc and Voc of the

device to be 0.77 mA cm�2 and 0.51 V, with a fill factor of about

51% and a power conversion efficiency of about 0.23%. The

external quantum efficiency shown in the inset of Fig. 20B is

about 8.5% at the P3HT absorption peak of 495 nm.147

2.3. TiO2 nanotube dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) have been considered as the

most close-to-commercialization nanostructured PV devices due

to the advantages of high performance, shorter energy pay-back

time, lower sensitivity to light angle of incidence, insensitivity to

temperature changes, easy fabrication on rigid and flexible

substrate, etc.161 The efficiency of DSCs has been enhanced to

11.18% after the first report of a power conversion efficiency of

about 7.12%.162,163 In earlier literatures, the DSCs were fabri-

cated using a high-surface-area titania anatase nanoparticle film

on the transparent conducting oxide (TCO) glass as the photo-

anode, a monolayer of a light-absorbing chromophore, a plat-

inum coated-TCO glass counter-electrode, and an iodide

electrolyte. Recently, TiO2 nanotube array based DSCs have

received increasing interest because this kind of nanoarchitecture

has many features which could enhance efficiency. For example

the highly ordered structure could improve photogenerated

charge carrier lifetimes, the adjustable geometrical feature could

allow devices to achieve specific light absorption and propaga-

tion characteristics, and the oriented nature makes them attrac-

tive electron percolation pathways for vertical charge transfer

between interfaces, and so on.164 It has been reported that the

TiO2 nanotube array based solar cells possess higher charge-

collection and light-harvesting efficiencies due to the enhanced

light-scattering properties of nanotubes.165 In addition, the

Fig. 21 (A) Schematic illustration of TiO2 nanotube array based DSCs.

(B) I–V curves for the TiO2 nanotube array based device in the dark

(black squares) and under 1.5 AM illumination (red circles). Further

details are described in ref. 113. Reprinted with permission from Amer-

ican Chemical Society.

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electron diffusion length of the TiO2 nanotube could be at least

three times better than in previous nanoparticle film based

DSCs.166 All these features have undoubtedly stimulated the

research on TiO2 nanotube array based DSCs and significant

progress has been achieved in recent studies. For example, the

Durstock group fabricated PV devices using sol–gel template

synthesized TiO2 nanotube arrays as the working electrode

according to the architecture shown in Fig. 21A.113 Fig. 21B gives

the performance of the devices using TiO2 nanotubes with an

average outer diameter, length and wall thickness of 295 nm, 15.3

mm and 42 nm, respectively. As can be seen, the device exhibited

an Voc of about 0.61 V and a Isc density of 8.26 mA cm�2 with

a fill factor as high as 70%. The overall power conversion effi-

ciency of the device was calculated as about 3.5%. In addition,

their results indicate that the IPCE of the device exhibits a peak

of about 20% at a wavelength of 520 nm. The Grimes group has

fabricated several devices based on electrochemically synthesized

TiO2 nanotube arrays.106,109,110,114 In order to obtain high quality

TiO2 nanotube arrays, titanium films were first deposited on

glass substrates using magnetron sputtering, followed by an

electrochemical process involving electrolytes consisting of 2–4

vol% dimethylsulfoxide (DMSO) and 0–4 vol% water under

different voltages for controlled pore diameters and wall thick-

nesses of the resulting nanotubes.114 Fig. 22 shows the perfor-

mance of their devices fabricated from as synthesized TiO2

nanotube arrays of various lengths. Fig. 22A shows the power

conversion efficiency and fill factor of the devices are notably

dependent on the length of the TiO2 nanotubes. Among these

devices, the DSC built with 17.6 mm long nanotubes fabricated

with 20 mm thick titanium films possesses a power conversion of

6.9% with a Voc of 0.73 V and a Isc of 15.8 mA cm�2 and a fill

Fig. 22 Performance of the transparent nanotube array film basedDSCs

with different nanotube lengths: (A) I–V curves; (B) IPCE as a function of

wavelength.107 Further details are described in ref. 114. Reprinted with

permission from Nature Publishing Group.

This journal is ª The Royal Society of Chemistry 2011

factor of 59%. The power conversion efficiency of TiO2 nanotube

based PV devices is still lower than that of nanoparticle film

based devices, partially due to the roughness of the nanotubes.

However, it should be noted that the dye used in these devices is

the commercially available N719 without any purification while

high purity dyes were needed in nanoparticle film devices in order

to achieve high efficiency. Also, there was no scattering layer here

that is a precondition in nanoparticle film DSCs.

Furthermore, DSCs capitalizing on F€orster resonance energy

transfer (FRET) have been synthesized to increase the quantum

yields for red/IR photons which account for 33% of the solar

energy received on earth’s surface.167,168 For example, the Grimes

group saw a four-fold increase in quantum yield in the red/NIR

region using a TiO2 nanowire array surrounded by N719 and

a FRET donor dye, zinc 2,9,16,23-tetra-tert-butyl-29H,31H-

phthalocyanine (ZnPc-TBB). Using this concept, the Grimes

group built a DSC in which a TiO2 nanotube array transported

electrons, intercalated spiro-OMeTAD transported holes, 5-ca-

rboxyl-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-indol-2-yli-

dene)-methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene] methyl]-

3,3-trimethyl-1-octyl-3H-indolium (SQ-1) organic dye absorbed

red/NIR photons and acted as a FRET acceptor, and 4-(dicya-

nomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran

absorbed visible light and acted as a FRET donor. The device

efficiency was 1.64%.169 However, this did not surpass their

earlier work on a DSC consisting of a TiO2 array intercalated

with the p-type conducting polymer P3HT and sensitized by SQ-

1, which achieved an efficiency of 3.2%.167 In this DSC, FRET

has been hypothesized to occur from the P3HT to SQ-1, but this

has not been confirmed.

2.4. Single nanowire solar cells

Single nanowire based photovoltaic devices have been proposed

as high-efficiency, robust, integrated nanoscale power sources.

There are several advantages of single nanowire solar cells, which

include: (1) the bottom-up synthetic techniques allow for the

rational control of nanomaterial parameters which may influence

the efficiencies of the devices; (2) single nanowire devices could be

applied as building blocks for conventional and nanoscale elec-

tronics power sources for low-power applications. Two kinds of

single nanowire solar cells have been developed, which consist of

p–i–n dopant modulation in axial and radial geometries. For

p–i–n axial nanowire solar cells, the electron–hole pairs are

usually generated through the absorption of photons with ener-

gies equal to or greater than the band gap of nanowire. The

carrier generation and separation are most efficient within the

depletion region due to the built-in field across the p–i–n

junction.87 In the axial p–i–n nanowire, the role of the p-type and

n-type region is to provide contact to the junction embedded

within the nanowire and their lengths can therefore be made

quite short which may enhance the integration of single nanowire

solar cells. The Lieber group recently built single nanowire PV

devices using p–i–n silicon axial nanowires obtained by VLS

growth.87 Fig. 23A gives the room temperature I–Vmeasurement

of devices based on p–i–n silicon axial nanowires with different

i-region thicknesses under dark conditions. It clearly indicated

that the devices exhibited well-defined rectification behavior with

a current onset in forward bias at about 0.6 V. The effect of

Nanoscale, 2011, 3, 2430–2443 | 2439

Fig. 23 (A) Dark I–V characteristics of p–i–n silicon single nanowire

with different i-region lengths (red, green, and black curves correspond to

i-segment lengths of 0, 2, and 4 mm, respectively). (The inset is the SEM

image of single nanowire device with i-length of 2 mm; the scale bar is

4 mm.) (B) Light I–V curves for the i-length of 0, 2, and 4 mm devices in

panel (A); the illumination intensity was 100 mW cm�2, AM1.5G.

Further details are described in ref. 87. Reprinted with permission from

American Chemical Society.

Fig. 24 Typical SEM images of the radial modulated single nanowire

photovoltaic device, the scale bars are 100 nm (left), 200 nm (middle) and

1.5 mm (right). Further details are described in ref. 82. Reprinted with

permission from Nature Publishing Group.

Fig. 25 (A–C) characterization of the p–i–n coaxial silicon nanowire

photovoltaic devices: (A) I–V curves under dark and light; (B) Light I–V

curves for two different n-shell contact locations, the inset is the optical

microscopy image of the device, scale bar: 5 mm. (C) Isc and Jsc of the

devices as a function of device lengths; (D) light I–V curves of two silicon

coaxial nanowire devices individually and connected in series and in

parallel. Further details are described in ref. 82. Reprinted with permis-

sion from Nature Publishing Group.

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i-region thickness on photovoltaic properties of the devices is

shown in Fig. 23B, which indicates that the Voc and Isc for the

devices with an i-region of 0, 2 and 4 mm were 0.12 V and 3.5 pA,

0.24 V and 14 pA, and 0.29 V and 31.1 pA, respectively, and

clearly illustrates that a systematic improvement in both Voc and

Isc was achieved along with the increasing i-region lengths. The

fill factor for the 4 mm i-region device is about 51% and could

yield a maximum power output per nanowire of 4.6 pW The

power conversion efficiency was estimated to be about 0.5%

based on a device with projected active area of 4 mm i-region

under AM1.5 illumination, which is close to the value reported

for single Si nanowire with metal–semiconductor junctions.170

Compared to the p–i–n axially modulated single nanowire

cells, the p–i–n radially modulated nanowire photovoltaic

devices may possess different benefits. For example, in radially

modulated single nanowire devices, the carrier separation could

be reached in the radial direction versus the longer axial direction

and the carrier collection distance should be much smaller or

comparable to the minority carrier diffusion length, so the

photogenerated carriers should reach the p–i–n junction with

higher efficiency. The Lieber group exploited effective routes for

the fabrication of radially modulated single nanowire solar

cells.82 In order to obtain the devices, the core/shell silicon

nanowires were selectively etched using KOH solution to expose

the p-core in a lithographically defined region, followed by the

deposition of metal contacts on the p-core and the n-shell.

Typical SEM images of the architectures can be seen in Fig. 24.

The performance of the p–i–n coaxial silicon nanowire devices

under AM1.5G illumination is shown in Fig. 25A–C. The

representative I–V curves of the best device shown in Fig. 25A

indicate a Voc of 0.26 V and an Isc of 0.503 nA with a fill factor of

55.0%. In addition, the power output for the coaxial silicon single

nanowire device reaches 72 pW with excellent stability. The I–V

results recorded using contacts to different positions from the

p-core contact (Fig. 25B) exhibited the same photovoltaic

response, thus indicating that the n-shell is equipotential with

radial carrier separation occurring uniformly along the entire

length of the core/shell silicon nanowire device. The Lieber group

also studied the effect of nanowire length on device performance,

as shown in Fig. 25C. As can be seen, the Isc of the devices

2440 | Nanoscale, 2011, 3, 2430–2443

exhibits a linearly increasing behavior with increasing nanowire

lengths while Voc is essentially independent of the nanowire

length. The linear behavior of the Isc with nanowire lengths

suggests that the photogenerated carriers are collected uniformly

along the length of these radial nanostructures, and that scat-

tering of light by the metal contacts does not have a major

contribution on the observed photocurrent. The overall power

conversion efficiency reaches 3.4%, which is much higher

compared with that of axially modulated silicon nanowire solar

cells. In addition, they studied the performance of the inter-

connected devices in series and in parallel, as shown in Fig. 25D.

Notably, the Voc and Isc of the interconnected devices are

approximately the sum of the two, which may provide an effec-

tive route to enhanced power output for future scalable

applications.

Besides Si nanowire based devices, III–V related coaxial single

nanowire photovoltaic devices have also been successfully

fabricated and their performance was systematically studied by

Dong et al. The Voc and Isc of the device ranged from 1.0–2.0 V

and 0.39–0.059 mA cm�2 with varying composition of the

nanowire and the maximum power conversion efficiency reached

about 0.19%.83

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3. Conclusion

We have discussed the state of the art of various nanostructure

based solar cell devices, together with a brief introduction of

nanostructure synthesis and nanocrystal assembly methods. The

nanostructure based solar cells have achieved exciting progress

due to both the great advancement of nanostructure synthesis

methods and device fabrication techniques. In addition, there

have been many reports related to the optimization of device

performance by post-treatment. However, the most important

challenge regarding nanostructured solar cells is how to enhance

the conversion efficiency of the devices in order to reach the goal

of scalable applications and promising market foreground. It is

true that there are still difficulties in the development of nano-

structure based solar cells, particularly on how to take the full

advantage of the unique electrical and optical properties of the

nanostructures. Currently there are many studies on how to

improve the device efficiency by converting the nanostructured

film into microstructures using high temperature thermal

annealing or sintering. Such an approach, in our opinion, has no

obvious benefit because of the similar energy consumption

during device fabrication yet the PV devices fabricated in such

a way typically have much lower performance comparing to the

traditional thin film devices.

Acknowledgements

Yue Wu thanks the DuPont Young Faculty Award and the new

faculty start-up funding from Purdue University. Daxin Liang

thanks the fellowship support from the Chinese Scholar Council.

References

1 J. Zhao, A. Wang, M. Green and F. Ferrazza, Appl. Phys. Lett.,1998, 73, 1991.

2 W. Shockley and H. Queisser, J. Appl. Phys., 1961, 32, 510.3 X. Wu, J. C. Keane, R. G. Dhere, C. DeHart, D. S. Albin, A. Duda,T. A. Gessert, S. Asher, D. H. LKevi and P. Sheldon, Proceedings ofthe 17th European Photovoltaic Solar Energy Conference, 2001,p. 995.

4 U. Rau andH.W. Schock,Appl. Phys. A:Mater. Sci. Process., 1999,69, 131.

5 L. Tsakalakos, Mater. Sci. Eng., R, 2008, 62, 175.6 M. A. Green, K. Emery, D. L. King, S. Igari and W. Warta, Prog.Photovoltaics, 2001, 9, 287.

7 C. Wadia, A. P. Alivisatos and D. M. Kammen, Environ. Sci.Technol., 2009, 43, 2072.

8 H. W. Hillhouse and M. C. Beard, Curr. Opin. Colloid Interface Sci.,2009, 14, 245.

9 E. H. Sargent, Nat. Photonics, 2009, 3, 325.10 E. H. Sargent, Adv. Mater., 2008, 20, 3958.11 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc.,

1993, 115, 8706.12 Z. A. Peng and X. G. Peng, J. Am. Chem. Soc., 2001, 123, 183.13 X. G. Peng, L. Manna, W. D. Yang, J. Wickham, E. Scher,

A. Kadavanich and A. P. Alivisatos, Nature, 2000, 404, 59.14 W. Huynh, J. Dittmer and A. Alivisatos, Science, 2002, 295, 2425.15 L. Tsakalakos, Mater. Sci. Eng., R, 2008, 62, 175.16 S. Kumar and G. D. Scholes, Microchim. Acta, 2008, 160, 315.17 L.-s. Li, J. Hu, W. Yang and A. P. Alivisatos, Nano Lett., 2001, 1,

349.18 I. Gur, N. Fromer, M. Geier and A. Alivisatos, Science, 2005, 310,

462.19 L. Carbone, S. Kudera, E. Carlino, W. J. Parak, C. Giannini,

R. Cingolani and L. Manna, J. Am. Chem. Soc., 2006, 128, 748.20 W. W. Yu, Y. A. Wang and X. G. Peng, Chem. Mater., 2003, 15,

4300.

This journal is ª The Royal Society of Chemistry 2011

21 A. Fiore, R. Mastria, M. G. Lupo, G. Lanzani, C. Giannini,E. Carlino, G. Morello, M. De Giorgi, Y. Li, R. Cingolani andL. Manna, J. Am. Chem. Soc., 2009, 131, 2274.

22 L. Manna, D. J. Milliron, A. Meisel, E. C. Scher andA. P. Alivisatos, Nat. Mater., 2003, 2, 382.

23 B. Li, Y. Xie, J. X. Huang and Y. T. Qian, Adv. Mater., 1999, 11,1456.

24 Y. Jiang, Y. Wu, X. Mo, W. C. Yu, Y. Xie and Y. T. Qian, Inorg.Chem., 2000, 39, 2964.

25 Y. Jiang, Y. Qu, S. W. Yuan, B. Xie, S. Y. Zhang and Y. T. Qian, J.Mater. Res., 2001, 16, 2805.

26 Q. Li, Y. Ding, X. M. Liu and Y. T. Qian, Mater. Res. Bull., 2001,36, 2649.

27 J. P. Xiao, Y. Xie, R. Tang and Y. T. Qian, J. Solid State Chem.,2001, 161, 179.

28 J. P. Xiao, Y. Xie, Y. J. Xiong, R. Tang and Y. T. Qian, J. Mater.Chem., 2001, 11, 1417.

29 H. Grisaru, O. Palchik, A. Gedanken, V. Palchik, M. A. Slifkin andA. M. Weiss, Inorg. Chem., 2003, 42, 7148.

30 S.-H. Choi, E.-G. Kim and T. Hyeon, J. Am. Chem. Soc., 2006, 128,2520.

31 Q. Guo, S. J. Kim, M. Kar, W. N. Shafarman, R. W. Birkmire,E. A. Stach, R. Agrawal and H. W. Hillhouse, Nano Lett., 2008, 8,2982.

32 Q. Guo, G. M. Ford, H. W. Hillhouse and R. Agrawal, Nano Lett.,2009, 9, 3060.

33 Q. J. Guo, H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc.,2009, 131, 11672.

34 M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke,L. Dunn, A. Dodabalapur, P. F. Barbara and B. A. Korgel, J.Am. Chem. Soc., 2008, 130, 16770.

35 B. Koo, R. N. Patel and B. A. Korgel, Chem. Mater., 2009, 21,1962.

36 C. Steinhagen,M. G. Panthani, V. Akhavan, B. Goodfellow, B. Kooand B. A. Korgel, J. Am. Chem. Soc., 2009, 131, 12554.

37 B. Koo, R. N. Patel and B. A. Korgel, J. Am. Chem. Soc., 2009, 131,3134.

38 A. Rothwarf and A. M. Barnett, IEEE Trans. Electron Devices,1977, 24, 381.

39 K. W. B€oer, J. Cryst. Growth, 1982, 59, 111.40 W. D. Gill and R. H. Bube, J. Appl. Phys., 1970, 41, 3731.41 A. L. Fahrenbr and R. H. Bube, J. Appl. Phys., 1974, 45, 1264.42 F. Pfisterer, Thin Solid Films, 2003, 431, 470.43 A. Rothwarf, Sol. Cells, 1980, 2, 115.44 A. E. Saunders, A. Ghezelbash, D. M. Smilgies, M. B. Sigman and

B. A. Korgel, Nano Lett., 2006, 6, 2959.45 T. H. Larsen, M. Sigman, A. Ghezelbash, R. C. Doty and

B. A. Korgel, J. Am. Chem. Soc., 2003, 125, 5638.46 M. B. Sigman, A. Ghezelbash, T. Hanrath, A. E. Saunders, F. Lee

and B. A. Korgel, J. Am. Chem. Soc., 2003, 125, 16050.47 S. Li, H. Z.Wang,W.W. Xu, H. L. Si, X. J. Tao, S. Y. Lou, Z. L. Du

and L. S. Li, J. Colloid Interface Sci., 2009, 330, 483.48 C. J. Brinker and G. W. Scherer, AcademicPress, Boston, 1990.49 J. Park, J. Joo, S. G. Kwon, Y. Jang and T. Hyeon, Angew. Chem.,

Int. Ed., 2007, 46, 4630.50 N. Pinna and M. Niederberger, Angew. Chem., Int. Ed., 2008, 47,

5292.51 Y.-w. Jun, J.-s. Choi and J. Cheon, Angew. Chem., Int. Ed., 2006, 45,

3414.52 H. Kominami, J. Kato, Y. Takada, Y. Doushi, B. Ohtani,

S. Nishimoto, M. Inoue, T. Inui and Y. Kera, Catal. Lett., 1997,46, 235.

53 C. Wang, Z. Deng and Y. D. Li, Inorg. Chem., 2001, 40, 5210.54 M. Niederberger, M. H. Bartl and G. D. Stucky, Chem. Mater.,

2002, 14, 4364.55 J. Wang, J. Polleux, J. Lim and B. Dunn, J. Phys. Chem. C, 2007,

111, 14925.56 G. Garnweitner, M. Antonietti and M. Niederberger, Chem.

Commun., 2005, 397.57 G. Li, L. Li, J. Boerio-Goates and B. F. Woodfield, J. Am. Chem.

Soc., 2005, 127, 8659.58 Z. Zhong, T.-P. Ang, J. Luo, H.-C. Gan and A. Gedanken, Chem.

Mater., 2005, 17, 6814.59 S. C. Goel, M. Y. Chiang, P. C. Gibbons and W. E. Buhro, Mater.

Res. Soc. Symp. Proc., 1992, 271, 3.

Nanoscale, 2011, 3, 2430–2443 | 2441

Publ

ishe

d on

28

Apr

il 20

11. D

ownl

oade

d by

Uni

vers

ity o

f C

alif

orni

a -

Sant

a B

arba

ra o

n 16

/08/

2015

01:

10:5

4.

View Article Online

60 D. Koziej, F. Fischer, N. Kranzlin, W. R. Caseri andM. Niederberger, ACS Appl. Mater. Interfaces, 2009, 1, 1097.

61 I. Bilecka, I. Djerdj and M. Niederberger, Chem. Commun., 2008,886.

62 N. Pinna, G. Garnweitner, M. Antonietti and M. Niederberger, J.Am. Chem. Soc., 2005, 127, 5608.

63 G. Clavel, M. G. Willinger, D. Zitoun and N. Pinna, Adv. Funct.Mater., 2007, 17, 3159.

64 M. M. Demir, R. Munoz-Espi, I. Lieberwirth and G. Wegner, J.Mater. Chem., 2006, 16, 2940.

65 S. K. N. Ayudhya, P. Tonto, O. Mekasuwandumrong,V. Pavarajarn and P. Praserthdam, Cryst. Growth Des., 2006, 6,2446.

66 L. S. Panchakarla, A. Govindaraj and C. N. R. Rao, J. Cluster Sci.,2007, 18, 660.

67 J. Buha, I. Djerdj and M. Niederberger, Cryst. Growth Des., 2007, 7,113.

68 X. Wang, J. Zhuang, Q. Peng and Y. D. Li, Nature, 2005, 437, 121.69 G. Q. Zhang, W. Wang, Q. X. Yu and X. G. Li, Chem. Mater., 2009,

21, 969.70 G. Zhang, X. Lu, W.Wang and X. Li, Chem. Mater., 2007, 19, 5207.71 L. Li, H. F. Qian and J. C. Ren, Chem. Commun., 2005, 528.72 Z. Y. Gu, L. Zou, Z. Fang, W. H. Zhu and X. H. Zhong,

Nanotechnology, 2008, 19, 135604.73 L. Zou, Z. Y. Gu, N. Zhang, Y. L. Zhang, Z. Fang, W. H. Zhu and

X. H. Zhong, J. Mater. Chem., 2008, 18, 2807.74 M. N. Kalasad, A. K. Rabinal and B. G. Mulimani, Langmuir, 2009,

25, 12729.75 W. C. Law, K. T. Yong, I. Roy, H. Ding, R. Hu, W. W. Zhao and

P. N. Prasad, Small, 2009, 5, 1302.76 B. Tian, T. J. Kempa andC.M. Lieber,Chem. Soc. Rev., 2009, 38, 16.77 M. Yazawa,M. Koguchi, A.Muto,M. Ozawa andK. Hiruma,Appl.

Phys. Lett., 1992, 61, 2051.78 J. Westwater, D. P. Gosain, S. Tomiya, S. Usui and H. Ruda, J. Vac.

Sci. Technol., B: Microelectron. Nanometer Struct.–Process., Meas.,Phenom., 1997, 15, 554.

79 A. M. Morales and C. M. Lieber, Science, 1998, 279, 208.80 Y.Wu, Y. Cui, L. Huynh, C. J. Barrelet, D. C. Bell and C.M. Lieber,

Nano Lett., 2004, 4, 433.81 G. Zheng, W. Lu, S. Jin and C. M. Lieber, Adv. Mater., 2004, 16,

1890.82 B. Z. Tian, X. L. Zheng, T. J. Kempa, Y. Fang, N. F. Yu, G. H. Yu,

J. L. Huang and C. M. Lieber, Nature, 2007, 449, 885.83 Y. J. Dong, B. Z. Tian, T. J. Kempa and C. M. Lieber, Nano Lett.,

2009, 9, 2183.84 F. Qian, S. Gradecak, Y. Li, C. Y. Wen and C. M. Lieber, Nano

Lett., 2005, 5, 2287.85 Y. Li, J. Xiang, F. Qian, S. Gradecak, Y. Wu, H. Yan, D. A. Blom

and C. M. Lieber, Nano Lett., 2006, 6, 1468.86 F. Qian and C. M. Lieber, 2007 Ieee Leos Annual Meeting

Conference Proceedings, 2007, vols 1 and 2, 831.87 T. J. Kempa, B. Z. Tian, D. R. Kim, J. S. Hu, X. L. Zheng and

C. M. Lieber, Nano Lett., 2008, 8, 3456.88 K. Q. Peng, Y. J. Yan, S. P. Gao and J. Zhu, Adv. Mater., 2002, 14,

1164.89 K. Q. Peng, J. J. Hu, Y. J. Yan, Y. Wu, H. Fang, Y. Xu, S. T. Lee

and J. Zhu, Adv. Funct. Mater., 2006, 16, 387.90 K. Q. Peng, Z. P. Huang and J. Zhu, Adv. Mater., 2004, 16, 73.91 K. Q. Peng, Y. Xu, Y. Wu, Y. J. Yan, S. T. Lee and J. Zhu, Small,

2005, 1, 1062.92 E. C. Garnett and P. D. Yang, J. Am. Chem. Soc., 2008, 130, 9224.93 L. Greene, M. Law, D. H. Tan, J. Goldberger and P. D. Yang,Nano

Lett., 2005, 5, 1231.94 B. D. Yuhas and P. D. Yang, J. Am. Chem. Soc., 2009, 131, 3756.95 L. E. Greene, M. Law, B. D. Yuhas and P. D. Yang, J. Phys. Chem.

C, 2007, 111, 18451.96 P. Hoyer, Langmuir, 1996, 12, 1411.97 B. B. Lakshmi, P. K. Dorhout and C. R. Martin, Chem. Mater.,

1997, 9, 9.98 H. Imai, Y. Takei, K. Shimizu, M. Matsuda and H. Hirashima, J.

Mater. Chem., 1999, 9, 9.99 A. Michailowski, D. AlMawlwai, G. S. Cheng and M. Moskovits,

Chem. Phys. Lett., 2001, 349, 1.100 J. H. Jung, H. Kobayashi, K. J. C. van Bommel, S. Shinkai and

T. Shimizu, Chem. Mater., 2002, 14, 1445.

2442 | Nanoscale, 2011, 3, 2430–2443

101 S. Kobayashi, N. Hamasaki, M. Suzuki, M. Kimura, H. Shirai andK. Hanabusa, J. Am. Chem. Soc., 2002, 124, 6550.

102 T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino and K. Niihara,Langmuir, 1998, 14, 3160.

103 Q. Chen, W. Z. Zhou, G. H. Du and L. H. Peng, Adv. Mater., 2002,14, 1208.

104 B. D. Yao, Y. F. Chan, X. Y. Zhang, W. F. Zhang, Z. Y. Yang andN. Wang, Appl. Phys. Lett., 2003, 82, 281.

105 D. Gong, C. A. Grimes, O. K. Varghese, W. Hu, R. S. Singh,Z. Chen and E. C. Dickey, J. Mater. Res., 2001, 16, 3331.

106 C. A. Grimes, J. Mater. Chem., 2007, 17, 1451.107 N. K. Allam, K. Shankar and C. A. Grimes, Adv. Mater., 2008, 20,

3942.108 W. Chanmanee, A. Watcharenwong, C. R. Chenthamarakshan,

P. Kajitvichyanukul, N. R. de Tacconi and K. Rajeshwar, J. Am.Chem. Soc., 2008, 130, 965.

109 X. J. Feng, K. Shankar, O. K. Varghese, M. Paulose, T. J. Latempaand C. A. Grimes, Nano Lett., 2008, 8, 3781.

110 K. Shankar, J. Bandara, M. Paulose, H. Wietasch, O. K. Varghese,G. K. Mor, T. J. LaTempa, M. Thelakkat and C. A. Grimes, NanoLett., 2008, 8, 1654.

111 Y. Yang, X. H. Wang and L. T. Li, J. Am. Ceram. Soc., 2008, 91,3086.

112 C. C. Chen, W. D. Jehng, L. L. Li and E. W. G. Diau, J.Electrochem. Soc., 2009, 156, C304.

113 T. S. Kang, A. P. Smith, B. E. Taylor and M. F. Durstock, NanoLett., 2009, 9, 601.

114 O. K. Varghese, M. Paulose and C. A. Grimes, Nat. Nanotechnol.,2009, 4, 592.

115 C. B. Murray, C. R. Kagan and M. G. Bawendi, Science, 1995, 270,1335.

116 C. P. Collier, T. Vossmeyer and J. R. Heath, Annu. Rev. Phys.Chem., 1998, 49, 371.

117 M. P. Pileni, J. Phys. Chem. B, 2001, 105, 3358.118 A. Wei, Chem. Commun., 2006, 1581.119 A. R. Tao, J. X. Huang and P. D. Yang, Acc. Chem. Res., 2008, 41,

1662.120 J. M. Luther, M. Law, M. C. Beard, Q. Song, M. O. Reese,

R. J. Ellingson and A. J. Nozik, Nano Lett., 2008, 8, 3488.121 J. Luther, M. Law, Q. Song, C. Perkins, M. Beard and A. Nozik,Acs

Nano, 2008, 2, 271.122 C. C. Kang, C. W. Lai, H. C. Peng, J. J. Shyue and P. T. Chou, Acs

Nano, 2008, 2, 750.123 L. Carbone, C. Nobile, M. De Giorgi, F. D. Sala, G. Morello,

P. Pompa, M. Hytch, E. Snoeck, A. Fiore, I. R. Franchini,M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera, R. Cingolani,R. Krahne and L. Manna, Nano Lett., 2007, 7, 2942.

124 S. Ahmed and K. M. Ryan, Nano Lett., 2007, 7, 2480.125 N. Zhao, T. P. Osedach, L. Y. Chang, S. M. Geyer, D. Wanger,

M. T. Binda, A. C. Arango, M. G. Bawendi and V. Bulovic, ACSNano, 2010, 4, 3743.

126 L. H. Zhang, Y. Jia, S. S. Wang, Z. Li, C. Y. Ji, J. Q. Wei,H. W. Zhu, K. L. Wang, D. H. Wu, E. Z. Shi, Y. Fang andA. Y. Cao, Nano Lett., 2010, 10, 3583.

127 J. M. Luther, J. B. Gao, M. T. Lloyd, O. E. Semonin, M. C. Beardand A. J. Nozik, Adv. Mater., 2010, 22, 3704.

128 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science,2005, 310, 462.

129 G. I. Koleilat, L. Levina, H. Shukla, S. H. Myrskog, S. Hinds,A.G.Pattantyus-AbrahamandE.H. Sargent,AcsNano, 2008,2, 833.

130 E. J. D. Klem, D. D. MacNeil, L. Levina and E. H. Sargent, Adv.Mater., 2008, 20, 3433.

131 I. E. Anderson, A. J. Breeze, J. D. Olson, L. Yang, Y. Sahoo andS. A. Carter, Appl. Phys. Lett., 2009, 94, 063101.

132 K. Johnston, A. Pattantyus-Abraham, J. Clifford, S. Myrskog,D. MacNeil, L. Levina and E. Sargent, Appl. Phys. Lett., 2008, 92,151115.

133 Y. Wu, C. Wadia, W. L. Ma, B. Sadtler and A. P. Alivisatos, NanoLett., 2008, 8, 2551.

134 W. Ma, J. M. Luther, H. M. Zheng, Y. Wu and A. P. Alivisatos,Nano Lett., 2009, 9, 1699.

135 J. P. Clifford, K. W. Johnston, L. Levina and E. H. Sargent, Appl.Phys. Lett., 2007, 91, 253117.

136 I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf,C. L. Perkins, B. To and R. Noufi,Prog. Photovoltaics, 2008, 16, 235.

This journal is ª The Royal Society of Chemistry 2011

Publ

ishe

d on

28

Apr

il 20

11. D

ownl

oade

d by

Uni

vers

ity o

f C

alif

orni

a -

Sant

a B

arba

ra o

n 16

/08/

2015

01:

10:5

4.

View Article Online

137 K. Palaniappan, J. W. Murphy, N. Khanam, J. Horvath,H. Alshareef, M. Quevedo-Lopez, M. C. Biewer, S. Y. Park,M. J. Kim, B. E. Gnade and M. C. Stefan, Macromolecules, 2009,42, 3845.

138 I. Gur, N. A. Fromer, C. P. Chen, A. G. Kanaras andA. P. Alivisatos, Nano Lett., 2007, 7, 409.

139 J. C. Lee, W. Lee, S. H. Han, T. G. Kim and Y. M. Sung,Electrochem. Commun., 2009, 11, 231.

140 H. C. Liao, S. Y. Chen and D. M. Liu, Macromolecules, 2009, 42,6558.

141 S. McDonald, G. Konstantatos, S. Zhang, P. Cyr, E. Klem,L. Levina and E. Sargent, Nat. Mater., 2005, 4, 138.

142 S. Gunes, K. P. Fritz, H. Neugebauer, N. S. Sariciftci, S. Kumar andG. D. Scholes, in European Conference on Hybrid Organic Solar Cells(ECHOS 06), Paris, France, 2006, p. 420.

143 D. Cui, J. Xu, T. Zhu, G. Paradee, S. Ashok and M. Gerhold, Appl.Phys. Lett., 2006, 88, 183111.

144 A. Maria, P. W. Cyr, E. J. D. Klern, L. Levina and E. H. Sargent,Appl. Phys. Lett., 2005, 87, 213112.

145 S. Zhang, P. W. Cyr, S. A. McDonald, G. Konstantatos andE. H. Sargent, Appl. Phys. Lett., 2005, 87, 233101.

146 C. Y. Liu, Z. C. Holman and U. R. Kortshagen, Nano Lett., 2009, 9,449.

147 L. E. Greene, M. Law, B. D. Yuhas and P. Yang, J. Phys. Chem. C,2007, 111, 18451.

148 R. Kniprath, J. P. Rabe, J. T. McLeskey, D. Y. Wang andS. Kirstein, Thin Solid Films, 2009, 518, 295.

149 C. Y. Kwong, W. C. H. Choy, A. B. Djurisic, P. C. Chui, W. Chengand W. K. Chan, Nanotechnology, 2004, 15, 1156.

150 T.W. Zeng, Y. Y. Lin, H.H. Lo, C.W. Chen, C. H. Chen, S. C. Liou,H. Y. Huang and W. F. Su, Nanotechnology, 2006, 17, 5387.

151 L. H. Slooff, M. M. Wienk and J. M. Kroon, Thin Solid Films, 2004,451–52, 634.

152 P. A. van Hal, M. M. Wienk, J. M. Kroon, W. J. H. Verhees,L. H. Slooff, W. J. H. van Gennip, P. Jonkheijm andR. A. J. Janssen, Adv. Mater., 2003, 15, 118.

This journal is ª The Royal Society of Chemistry 2011

153 A. C. Arango, L. R. Johnson, V. N. Bliznyuk, Z. Schlesinger,S. A. Carter and H. H. Horhold, Adv. Mater., 2000, 12, 1689.

154 L. J. A. Koster, W. J. van Strien, W. J. E. Beek and P. W. M. Blom,Adv. Funct. Mater., 2007, 17, 1297.

155 W. J. E. Beek, L. H. Slooff, M. M. Wienk, J. M. Kroon andR. A. J. Janssen, Adv. Funct. Mater., 2005, 15, 1703.

156 W. J. E. Beek, M. M. Wienk and R. A. J. Janssen, J. Mater. Chem.,2005, 15, 2985.

157 D. C. Olson, J. Piris, R. T. Collins, S. E. Shaheen and D. S. Ginley,Thin Solid Films, 2006, 496, 26.

158 P. Ravirajan, A. M. Peiro, M. K. Nazeeruddin, M. Graetzel,D. D. C. Bradley, J. R. Durrant and J. Nelson, J. Phys. Chem. B,2006, 110, 7635.

159 A. M. Peiro, P. Ravirajan, K. Govender, D. S. Boyle, P. O’Brien,D. D. C. Bradley, J. Nelson and J. R. Durrant, J. Mater. Chem.,2006, 16, 2088.

160 L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger,G. Somorjai and P. D. Yang, Nano Lett., 2005, 5, 1231.

161 M. Gratzel, Nature, 2001, 414, 338.162 B. O’Regan and M. A. Gratzel, Nature, 1991, 353, 737.163 M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni,

G. Viscardi, P. Liska, S. Ito, B. Takeru and M. G. Gratzel, J. Am.Chem. Soc., 2005, 127, 16835.

164 C. A. Grimes, J. Mater. Chem., 2007, 17, 1451.165 K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett.,

2007, 7, 69.166 J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki and

A. B. Walker, J. Am. Chem. Soc., 2008, 130, 13364.167 G. K. Mor, S. Kim, M. Paulose, O. K. Varghese, K. Shankar,

J. Basham and C. A. Grimes, Nano Lett., 2009, 9, 4250.168 G. K. Mor, J. Basham, M. Paulose, S. Kim, O. K. Varghese,

A. Vaish, S. Yoriya and C. A. Grimes, Nano Lett., 2010, 10, 2387.169 K. Shankar, X. Feng and C. A. Grimes, ACS Nano, 2009, 3, 788.170 M. D. Kelzenberg, D. B. Turner-Evans, B. M. Kayes, M. A. Filler,

M. C. Putnam, N. S. Lewis and H. A. Atwater, Nano Lett., 2008, 8,710.

Nanoscale, 2011, 3, 2430–2443 | 2443