FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

HView Article Online / Journal Homepage / Table of Contents for this issue

Hybrid nanocomposite materials with organic and inorganiccomponents for opto-electronic devices

Elisabeth Holder,*a Nir Tesslerb and Andrey L. Rogachc

Received 7th August 2007, Accepted 15th November 2007

First published as an Advance Article on the web 10th January 2008

DOI: 10.1039/b712176h

Hybrid structures consisting of semiconducting organic polymers and strongly luminescent

semiconductor nanocrystals offer favorable perspectives through highly saturated, tunable emission

and tunable absorption, in combination with an easy processability from solution and low materials

cost, which is of importance for several high-tech applications such as hybrid organic–inorganic

light-emitting diodes and solar cells. This Feature Article describes selected examples of the design and

synthesis of polymer compounds and inorganic semiconductor nanocrystals, followed by operation

principles and material choices for devices that are based on combinations of polymers with

semiconductor nanocrystals.

Elisabeth Holder

Elisabeth Holder studied

chemistry at the Eberhard-Karls

Universitat in Tubingen,

Germany and received her

Ph.D. in 2001 in the laboratory

of Prof. E. Lindner. Thereafter,

she had the benefit of a postdoc-

toral stay in Baltimore, USA at

the University of Maryland in

the Center for Fluorescence

Spectroscopy with Prof. J. R.

Lakowicz. Subsequently, she

pursued a postdoctoral stay at

the Technischen Universiteit

Eindhoven in the Netherlands in

the laboratory of Prof. U. S. Schubert. Since April 2006 she has

led the ‘‘Functional Polymers Group’’ at the University of Wupper-

tal in Germany and there she belongs also to the directorial board of

the Institute of Polymer Technology. From 2006–2007 she was

a visiting scientist at the Technischen Universiteit Eindhoven in

the Netherlands. Her research interests are in the synthesis and

characterization of new functional and macromolecular materials

involving metal–ligand complexes, organic–inorganic hybrid mate-

rials and composites for applications in light-emitting devices, solar

cells and sensors, with a focus on materials development for

engineering energy and electron transfer processes.

aFunctional Polymers Group and Institute of Polymer Technology,University of Wuppertal, Gaußstraße 20, D-42097 Wuppertal, Germany.E-mail: holder@uni-wuppertal.de; Fax: 0049-202-439-3880bOrganic Materials & Devices Laboratory, Nanoelectronic Center,Department of Electrical Engineering, Technion - Israel Institute ofTechnology, Haifa 32000, IsraelcPhotonics and Optoelectronics Group, Department of Physics and Centerfor Nanoscience (CeNS), Ludwig-Maximilians-Universitat Munchen,Amalienstr. 54, D-80799 Munich, Germany

1064 | J. Mater. Chem., 2008, 18, 1064–1078

Introduction

The motivation to control interfacial interactions on the

nanoscale leads to a new class of materials known as nanocom-

posites.1–4 The idea is to co-assemble organic and inorganic pre-

cursors into a nanocomposite material with molecular level

control over interfaces, structure, and morphology.5,6 The

designed structures may exhibit properties and functions

unattainable in the individual components. Harnessing the

advantages of organic–inorganic nanocomposite materials

Nir Tessler

Nir Tessler received his B.Sc.,

M.Sc., and Ph.D. from the Elec-

trical Engineering department at

the Technion Israel Institute of

Technology, Haifa, Israel. In

1995 he joined, as a postdoctoral

fellow, Prof. Sir Richard Friend’s

group at the Cavendish Labora-

tory, Cambridge, UK and in

1996 he demonstrated the world’s

first optically pumped conjugated

polymer laser. In 1999 he left his

advanced EPSRC fellow position

at the Cavendish to head the

Organic Materials & Devices Laboratory at the Electrical Engineer-

ing Department, Technion, Haifa, Israel. He is a member of the

Microelectronic and Nanoelectronic Centers as well as the Russel

Berry Nanotechnology Institute at the Technion. His research inter-

ests are in the field of conducting, semiconducting and light emitting

organic materials. This includes photophysical processes (including

laser action and microcavities), transport of charges and of electron–

hole pairs (executions), processing of devices and circuits (diodes,

transistors, detectors), and also the combination between synthetic

and natural (biological) polymers as a tool to extend the understand-

ing and scope of each separate system. He is furthermore interested

in new device structures and processing procedures.

This journal is ª The Royal Society of Chemistry 2008

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

requires fine-tuning of the sizes, topologies, and spatial assembly

of individual domains and their interfaces. Furthermore, in order

to exploit the full potential of the technological applications of

the nanomaterials, it is very important to endow them with

good processability,7–20 which has ultimately guided research

towards using polymers as one of the components resulting in

a special class of hybrid materials: polymer nanocomposites.

Depending upon the nature of association between the

inorganic and polymeric components, nanocomposites can be

classified into two categories: one in which the inorganic material

(nanocrystals, colloids, polyoxometalates) is embedded in a

polymeric matrix,21 and the other where the organic polymer is

confined into an inorganic template.22 In the first case,

inorganic-in-organic composites, the good processability and

low density of the polymer component, in combination with

the high mechanical durability and well-defined optoelectronic

properties of the inorganic component, are harnessed. This

allows the combinations of the advantages of the rapidly

developing field in opto-electronic technology23 with attractive

properties of semiconductor nanocrystals (NCs). Optical proper-

ties of this class of lumophores are determined by the quantum

confinement effect, so that their emission color and the electron

affinity can be finely controlled, not only by the material choice,

but also by size within a single synthetic route. State of the art

syntheses, which can be carried out either in organic solvents

or in water, provide a broad palette of II–VI, III–V and IV–VI

NCs with variable size and a narrow size distribution leading

to narrow emission spectra—25–35 nm full width at half maxi-

mum in solution—tuneable from the UV to the near-infrared

spectral region.24,25 Proper surface passivation leads to improved

chemical stability and high photoluminescence (PL) quantum

yields of >50% for so-called core–shell NCs like CdSe/ZnS26 or

CdSe/CdS,27 where the large bandgap semiconductors (ZnS or

CdS) epitaxially overgrow the core material (CdSe) and the

band edges of the core material lie inside the bandgap of the

outer material (type I structures). Variable surface chemistry of

NCs allows for the ease of their processability from different

solvents and for their incorporation into different organic matri-

ces. The basic idea behind NC based organic light-emitting

diodes (OLEDs) is to achieve full color tuneability in a single

host material. Organic semiconductors such as conjugated

Andrey L: Rogach

Andrey Rogach received his PhD in p

in 1995 from the Belarusian State Un

at the Institute of Physical Chemistr

he also worked in 1997–2002 as a

a staff scientist. Since 2002 he has b

turing Labs at the Photonics & Optoe

Universitat (LMU) in Munich, Germ

a Principal Investigator of the Exce

was holder of the Walton Award of t

in 2005–2006. His research interest

materials, with an emphasis on the s

of colloidal semiconductor nanocryst

nanocrystal structures, energy transf

voltaics, and the use of water-soluble

systems.

This journal is ª The Royal Society of Chemistry 2008

polymers represent large polymer chains that are soft and flexi-

ble. On the other hand, semiconductor NCs as inorganic species

are robust. Inorganic semiconductors are well known to change

their emission color over a very wide spectral range, while in

organic semiconductors this effect is less pronounced and often

goes in hand with changed electrical properties. Although

organic molecules span the entire visible spectrum in terms of

emission wavelength, a change of material required to tune the

color of emission can result in a dramatic modification of the

charge transport properties and thus of the device characteri-

stics. A key goal of research into hybrid NC-based OLEDs is

therefore a separate optimization of charge transport and emis-

sion properties, which can be achieved in certain hybrid devices.6

Whereas organic semiconductors are typically hole transporting

materials, NC solids generally display strongly n-type beha-

vior,28 i.e. in most cases preferentially conduct electrons, which

make the two classes of materials a natural choice of partners.

Therefore, combination of both components within nanocompo-

sites enhances the range of potential optoelectronic applications.

Photodiodes based on organic polymers rely on the presence

of a second component facilitating charge separation, with

fullerenes being a typical example.29 An alternative way to

achieve efficient charge separation is to combine polymers with

inorganic semiconductor NCs (i.e. based on cadmium chalcoge-

nides or on lead chalcogenides). Size- and material-dependent

energy levels of a semiconductor NC allow for an elegant

solution in adjustment of their electron affinity, favoring charge

separation at the interface with polymers. The charge transfer

rate can be remarkably fast in the case of polymers chemically

bound to NCs, allowing a high percentage of excitons generated

in polymers and/or NCs to dissociate at the interface. Chemical

bonding of the NCs and polymers naturally overcomes problems

related to purely defined morphological issues of hybrid photo-

diodes. Furthermore, quantum confinement leads to an enhance-

ment of the absorption coefficients of NCs compared to the

respective bulk materials, making them efficient absorbers even

in relatively thin devices.30 The performance in such devices is

strongly dependent on the local and long-range film morpho-

logies. Such molecular and mesoscale structural features can in

many cases be modified by incorporation into a suitable polymer

host matrix to form a composite material.

hysical chemistry on synthesis and properties of silver nanoparticles

iversity in Minsk. From 1995 to 1996 he was a postdoctoral fellow

y, University of Hamburg (Germany) with Prof. H. Weller, where

visiting scientist, Alexander-von-Humboldt Fellow, and finally as

een a senior staff scientist heading the Chemistry and Microstruc-

lectronics group of Prof. J. Feldmann at the Ludwig-Maximilians-

any. He is a member of the Center for NanoScience (CeNS) and

llence Cluster ‘‘Nanosystems Initiative Munich’’ at the LMU. He

he Science Foundation Ireland at Trinity College Dublin (Ireland)

s are in the areas of chemistry and physics of nanometer-sized

ynthesis, supramolecular chemistry, spectroscopy and applications

als. His recent research activities address the formation of hybrid

er in composite nanocrystal-based systems, nanocrystals for photo-

semiconductor nanocrystals as luminescent markers in biological

J. Mater. Chem., 2008, 18, 1064–1078 | 1065

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

In this Feature Article, we describe examples of general

principles of device operation, followed by selected examples

on the design and synthesis of conjugated polymers and organic

compounds and inorganic semiconductor NCs, and material

choices for devices based on combinations of polymers with

NCs.

Organic LEDs

OLEDs based on charge-transporting and light-emitting organic

molecules or polymers31–33 have been gaining much interest in

both academic and applied research. In organic optoelectronic

devices there is much room to tailor both the devices and the

materials designs.

The first conjugated polymer LEDs appeared in their simplest

form and consisted of �100 nm of a PPV polymer film sand-

wiched between two electrodes: indium tin oxide, ITO (for hole

injection and transparency), and a lower work function metal

(for electron injection)34,35 (Fig. 1).

Even in these single-layer and single-material LEDs one can

consider all the basic processes which have to be optimized in

any LED. The processes leading to electroluminescence (EL) in

OLEDs, when a positive bias is applied, can be divided as

follows: charge injection and charge transport followed by

charge recombination to form excitons (singlets and triplets)

and subsequent radiative recombination of some of the singlet

excitons leading to light emission. The enormous improvement

of the performance of OLEDs over the last two decades has

been achieved by the optimization of the introduced processes

leading to light emission. The hole injection can be improved

by introduction of conducting polymers (such as PEDOT)36 as

well as by engineering the interface using monolayers that

introduce a graded bandgap or electric dipole.37,38 The electron

injection and the balancing of electron and hole injection was

largely done through systematic modifications of the polymer

structure and the resulting electronic properties.39,40 The exciton

recombination and the maximum quantum efficiency seemed to

be elusive for some time as it was believed that the maximum EL

efficiency was limited to 25% due to the formation of non-

emissive triplets. However, in recent years the use of triplet

emitters41,42 paved the way to significantly higher efficiencies by

placing the theoretical limit at 100%.14,43 At the same time it

Fig. 1 Schematic energy band representation of an ITO/PEDOT : PSS/

PPV/Ca LED, showing the basic processes leading to electroluminescence.

1066 | J. Mater. Chem., 2008, 18, 1064–1078

also became apparent that the 25 : 75 singlet to triplet ratio

is not so general and ratios closer to 50 : 50 were reported for

polymers.44,45

This field has been rapidly progressing23,46 so that the currently

available OLEDs for color application are highly advanced in

terms of lifetime (100 000 h), operation voltage (below 5 V)

and efficiency (above 20 lm W�1), and most of these record

performances are being reported by commercial companies.47–49

Organic photovoltaics

The constraints in the use of organic semiconductors in photo-

voltaic applications are very much determined by the fact that

the excited states produced by photon absorption are usually

excitons that have relatively high binding energies and do not

dissociate to give electrons and holes.32 Exciton ionization in

the bulk is therefore not a promising method to follow. However,

it is known that exciton dissociation is efficient at interfaces

formed between materials with different electron affinities and

ionization potentials, where the electron is accepted by the mate-

rial with larger electron affinity and the hole by the material with

lower ionization potential.50 The built-in potential in a single-

layer organic photodiode, which results from the difference in

work function of the cathode and anode, is usually insufficient

to induce efficient charge separation. Therefore, in order to

obtain free charges in a conjugated polymer, a more effective

exciton dissociation mechanism is required and is typically

manifested in the form of a junction. The simplest structure is

that of a bilayer heterojunction, formed by a layer of a hole-

accepting material and a second layer of an electron-accepting

material. It has been shown that if the energy discontinuity at

the junction is sufficiently large exciton dissociation (charge

generation) is efficient.51,52 The intensively studied bilayer device

is fabricated with a layer of conjugated polymer and a layer of

buckminsterfullerene, C60, where the achieved external mono-

chromatic quantum efficiency values are about 9%.53,54

Fig. 2 schematically illustrates how exciton dissociation and

charge collection can be improved by using a polymer blend as

the active layer in photodiodes. In this way, when an exciton is

created in the film after light absorption it is easier for it to

find an interface where dissociation may occur. The free charges

obtained are afterwards transferred to the appropriate phase of

electron acceptor or hole acceptor. Subsequently, they are driven

by the electrical field to the electrodes. Very recently, such

devices have been reported to top 5% power conversion

efficiency.55–57

Synthesis of conjugated polymers

Since the discovery of electroluminescence from conjugated

polymers by Burroughes, Friend, Holmes and coworkers,58 there

has been considerable industrial and academic interest in semi-

conducting polymers as the active material for EL applications.

There is a passionate pursuit of the design and development of

materials that meet application criteria such as desired emission

colors with high emission intensity and efficiency, and good

environmental stability. In particular the control of the emission

color, of charge-accepting and -transporting properties, which

are important for optimizing device efficiency, and of electrical

This journal is ª The Royal Society of Chemistry 2008

Fig. 3 Some main representatives of poly(arylene vinylene)s: poly

(phenylene vinylene) (PPV), methyl ethylhexyl poly(phenylene vinylene)

(MEH-PPV), dihexyl cyano poly(phenylene vinylene) (CN-PPV).

Fig. 4 Main structure of poly(arylene ethynylene): poly(phenylene

ethynylene). R represents some oxyalkyl chains.

Fig. 2 Cross section of a hetero-junction photovoltaic device with a

conjugated polymer for hole transport and C60 fullerene for electron

transport (redrawn according to ref. 17). Phase separation increases the

number of active interfaces in the device, improving exciton dissociation.

Optimized phase separation may also improve charge transport in the

device. Holes are collected at the anode and electrons are collected at

the cathode.

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

and optical stability, which are vital factors in determining device

lifetimes, must be addressed. In light absorbing applications

there is a stronger focus on a broad spectral response to the

absorbed light, the charge transport and the morphological

behavior of the conjugated polymers such as phase separation

and charge transport within the respective domains on a meso-

scopic level (nm–mm). Some of the most common methods to

address the above-mentioned issues will be briefly discussed in

the following sections.

Methods of synthesis

A large variety of homo-,59–62 co-,59,60,62–65 block co-,66–71

dendritic72–83 and end-group functionalized semiconducting

polymers84–87 has been synthesized, mainly stimulated by the

developments in the late 1980s and early 1990s. This becomes

obvious when looking at the enormous amount of scientific pub-

lications coming close to 15 000 and in the numerous review

articles dealing with these classes of polymers.59,60,62,88–108 Most

of the articles describing synthetic procedures deal with deriva-

tives of semiconducting polymers of the poly(arylene vinylene)

(PAV),58 poly(arylene ethynylene) (PAE),109 and polyarylene

(PA)40,60,62,94 families. Large numbers of derivatives have been

synthesized to optimize absorbing and emitting behavior, charge

accepting or transporting properties as well as the long-range

assembly phenomena.

Main synthetic procedures for poly(arylene vinylene)s

Widely used in the poly(arylene vinylene) (PAV) (for some

representatives see Fig. 3) synthesis are the quinodimethane

polymerization reactions such as the Wessling–Zimmerman

route,110 and modifications like the Vanderzande route,111 the

Gilch route112 or the Horhold route.113 PAVs are also available

via polycondensation reactions such as the Witting–Horner

reaction.114 Furthermore, there are transition-metal-mediated

reaction methods to obtain PAVs. Prominent is the Heck cou-

pling reaction,115 and the Stille coupling reaction.116 Metathesis

This journal is ª The Royal Society of Chemistry 2008

polymerization procedures also lead to PAVs. Famous are the

Csanyi route117 and the ring-opening metathesis polymerization

(ROMP) reaction118 by Grubbs.

The structure–property relationship in PAVs regarding the

emission color is mainly affected by two structural elements:

the effects of the substituents and the degree of conjugation

along the backbone.

In EL devices balanced charge injection is a fundamental

prerequisite for high EL quantum efficiencies. PAVs represent

the most promising class of polymers with respect to the fabrica-

tion of such EL devices and consequently the optimization of the

structure–property relationship is mandatory. Hole and electron

transporting moieties can be connected to the polymer backbone

inducing improved charge transport. On the other hand, the

presence of meta-linkages or silicon atoms119 effectively reduces

the p-conjugation length of the polymer main chain. As a direct

result the emission wavelength is shifted to the blue region.97

Main synthetic procedures for poly(arylene ethynylene)s

The poly(arylene ethynylene) (PAE) (for a representative struc-

ture see Fig. 4) synthesis15,109,120,121 has been mainly developed

based on procedures of the so-called Hagihara–Sonogashira

coupling122 and by a metathesis procedure developed by

Bunz.109 The structure–property relationship of PAEs is given

by their more rigid backbone than that of PAVs, so that they

show a greater tendency to strongly aggregate which leads to

a strong red-shift of their emission in the solid state.

J. Mater. Chem., 2008, 18, 1064–1078 | 1067

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

Due to their rigid nature the PAEs are applied in nanotech-

nology as ‘‘molecular wires’’ or as ‘‘molecular alligator clips’’

in between two gold electrodes. Another use of these polymers

resulting from their structure–property relationship is in sensing

applications104 basically due to their high PL quantum efficien-

cies that can be efficiently quenched.97

Main synthetic procedures for poly(arylene)s

Polyarylenes (PAs) (for some structures see Fig. 5) are accessible

via the oxidative coupling of arenes,123 but also via the reductive

coupling of arenes which is namely the coupling according

to Yamamoto.124 Transition-metal-mediated cross-coupling

reactions are also paving the way to PAs: namely the Suzuki

polycondensation,125 McCullough126 and Rieke routes,127 as well

as the Grignard metathesis (GRIM) method.128 The structure–

property relationship of PAs is largely structure dependent. The

nature of the arene units strongly affects the emission. Therefore

a facile copolymer design allows tuning of the emission color of

PA polymers. Furthermore, the planarity of the PA system does

affect the emission color additionally: the more planar the system

is, the better the conjugation and thus the smaller the bandgaps.

Substituents also affect the color as steric interactions can produce

out-of-plane twisting of the polymer backbone leading to blue-

shifted emission. Since aggregation can lead to red-shifts in

emission, or enhance non-radiative pathways, attaching bulky

side groups to increase interchain separation can be used to

control the emission color and to enhance the PL efficiency. The

charge accepting and transport properties can be influenced by

introducing suitable units in the main chain, or in the side chain.

The regular alternation of electron-rich and electron-deficient

repeat units induces a high planarity of the conjugated backbone

with intramolecular charge transfer, resulting in low bandgaps.97

meta-Linkages129 or the presence of silicon atoms,119,130 amongst

others,131–133 are known to effectively reduce the p-conjugation

length of the polymer main chain, also for PAs. As a direct result

the emission wavelength is shifted to the blue region97 making

them attractive matrices for triplet emitters14 or semiconductor

NCs.61 In particular polyfluorenes40,62,94 became very prominent

as host polymers in light-emitting diodes, because of their wide

bandgap and good conducting properties. Through their

Fig. 5 Some representative structures of poly(arylene)s. Shown are

poly(phenylene) (PP), poly(3-hexyl thiophene) (P3HT), poly(fluorene)

(PF) and poly(carbazole) (PC), where R represents an alkyl chain.

1068 | J. Mater. Chem., 2008, 18, 1064–1078

modification with hole- or electron-conducting moieties, efficient

charge transport materials hosting other fluorophores can be

designed.14,43,61,134 Introducing a silicon atom in the 9-position130

allows them to be made stable against oxidation in the 9-position

that causes an undesired green emission band.130 For photo-

diodes and sensor applications on the other hand, poly(thio-

phene)s emerged to become a very important class of polymers.60

PAs are basically applied in all fields of polymer (opto)electronic

research.

Conjugated polymers used in light-emitting devices

The use of semiconducting, conjugated polymers has a long

tradition in light-emitting diodes.16,58,135 All UV-Vis colors could

be achieved.60,65,97 Furthermore, low bandgap polymers64 have

been designed for emission in the NIR region.90,136 To achieve

such low bandgap semiconducting polymers,90,92,137–140 potential

excited state electron donor and potential excited state electron

acceptor units are copolymerized (for the excited state electron

donor–excited state electron acceptor principle in order to

achieve low bandgap polymers see Fig. 6).

Electron and hole transporting functionalities have been intro-

duced into semiconducting polymers as well as emissive units

tuning the respective emission color. Recent development to

enhance the device efficiency involves the use of triplet emitters

for enhanced efficiency (theoretically 100%, 25% singlet and

75% triplet excitons that combine for the internal light emission

efficiency).43 Such systems usually consist of a fluorescent donor

and a phosphorescent acceptor. Possible triplet emitters making

all emission colors possible are mostly metal complexes of

platinum141 and iridium.14,43,142 Such phosphorescent small-

molecule OLEDs reveal external efficiencies of about 22%,14,43

while internal efficiencies of organic only polymer LEDs are

about 15–20%.143

Conjugated polymers used in photovoltaic devices

In bulk heterojunction solar cells102,144–146 traditionally two types

of conjugated polymers102 are widely used, namely derivatives of

Fig. 6 Principle of lowering the optical bandgap in low bandgap

polymer composites. Absorption takes place from the donor’s highest

occupied molecule orbital (HOMO) to the lowest unoccupied molecule

orbital (LUMO) of the excited state electron acceptor moiety. Typical

excited state electron donor moieties are thienyl derivatives and typical

excited state electron acceptor moieties are quinoxalines, benzothiadia-

zoles or thienylpyrazines.

This journal is ª The Royal Society of Chemistry 2008

Fig. 7 (a) High-resolution transmission electron microscopy image of

a CdSe NC demonstrating the quality of a single crystal semiconductor

core. (b) Schematic representation of a NC consisting of a semiconductor

core coated by trioctylphosphine oxide (TOPO) molecules (c) which is

one of the organic molecules typically used as a ligand. (d) Schematic rep-

resentation of a water-soluble CdTe NC coated by short-chain mercapto-

acid molecules, another typical organic ligand.

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

poly(phenylene vinylene) PPV147–150 and polythiophenes. With

poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenyleneviny-

lene] (MDMO PPV) and [6,6]-phenyl-C61-butyric acid methyl

ester (C60-PCBM) or [6,6]-phenyl-C71-butyric acid methyl ester

(C70-PCBM) efficiencies of around 3% could be achieved.151

With poly(3-hexyl-thiophene) (P3HT)152 as hole transporter

and C60-PCBM as electron transporter 5% efficiency has been

reported for optimized cells.153 Many other conjugated polymers

have been tested, but none of them could thus far (used solely)

top the 5% of P3HT, a regio-regular polythiophene with good

stacking behavior. To date, many efforts have been focused on

the design of conjugated polymers utilizing the donor–acceptor

approach90,137 (Fig. 6) in order to achieve low bandgap polymers

for tuning the absorption over the 300–1500 nm spectral range;

2.4% efficiency has been reported combining a low bandgap

polymer with PCBM,154 and it is currently an active matter of

research to reach the targeted 6–8% efficiency of this kind of

materials being announced.155 By an elegant blending approach

utilizing alkanedithiols, introduced by Heeger and coworkers,55

the efficiency of such solar cells using low bandgap polymers

can be dramatically increased to 5.5% through altering the film

morphology. The Heeger team furthermore showed an improve-

ment by changing the solar cell architecture to a tandem cell

achieving 6% efficiency.56 To the best of our knowledge, this is

the highest efficiency to date reported for organic polymer

photovoltaics. These recent and very promising developments

give realistic hope that the targeted efficiencies will be achieved

by further tailoring the low bandgap polymers and by optimizing

the morphologies of the polymer solar cells.

Fig. 8 Schematic representation of a core–shell NC and the band struc-

tures for typical type I core–shell materials CdSe/ZnS and CdSe/CdS

(reproduced with permission from ref. 24, Copyright 2004 Wiley).

Inorganic semiconductor nanocrystals

The availability of reliable colloidal syntheses leading to semi-

conductor NCs being uniform in composition, size, shape,

and surface chemistry is crucial both for the study of their size-

dependent properties and for further use in different applica-

tions. Briefly describing the state of the art in the semiconductor

NC area we outline the challenges as well as the flexibility in

defining the NC structures (organic passivation, core-only

NCs, type I and type II core–shell NCs) and hence their proper-

ties (tuneable energy levels and emission spectral range). A

typical semiconductor NC, which can also be thought of as a

colloidal quantum dot, consists of an inorganic core, which is

comparable to or smaller in size than the Bohr exciton diameter

of the corresponding bulk material, surrounded (‘‘passivated’’)

by an organic shell of ligands (Fig. 7). The choice of a suitable

capping ligand (‘‘stabilizer’’) can be considered as a key point

in an advanced colloidal synthesis of semiconductor NCs. Stabi-

lizers regulate the growth rate and the size of NCs, prevent them

(to a greater or lesser extent) from (photo-)oxidation and provide

a dielectric barrier at the surface thus eliminating (partially)

surface traps. Importantly, stabilizers, being organic molecules

with free functional groups, make NCs stable as colloidal disper-

sions in different solvents and allow their further handling

(attachment to different surfaces, covalent coupling with each

other and with other molecules, etc.). A very convenient and

generally applicable liquid–solid–solution (LSS) synthesis

approach was recently introduced allowing the synthesis of

This journal is ª The Royal Society of Chemistry 2008

numerous NCs, including TiO2, CuO, ZrO2, SnO2, CdS, Ag2S,

ZnS, PbS, MnS, ZnSe and CdSe.156

Nanocrystals for applications at visible wavelengths

The synthesis of CdSe NCs by the reaction of dimethylcadmium

with trioctylphosphine selenide in a trioctylphosphine oxide–

trioctylphosphine (TOPO–TOP)157 or TOPO–TOP–hexadecyl-

amine (HDA)158 mixtures of coordinating solvents at 250–300�C allows the preparation of NCs with sizes from �1.7 to

15 nm soluble in a variety of organic solvents like toluene,

n-hexane, chloroform, etc.

Type I core–shell nanocrystals

A large step towards the preparation of robust highly lumines-

cent NCs was the passivation of their surface with an inorganic

shell of a semiconductor with wider bandgap (usually ZnS or

CdS)26,27 in analogy with the well developed techniques for the

growth of type I quantum wells (Fig. 8). The lattice mismatch

between the CdSe core and the ZnS or CdS shell is small enough

to allow epitaxial growth, if the shell thickness is low.159 In type I

core–shell NCs the large bandgap semiconductor forms a closed

outer shell and the band edges of the core material lie inside the

bandgap of the outer material. The outer inorganic shell

J. Mater. Chem., 2008, 18, 1064–1078 | 1069

Fig. 9 Size-dependent change of the emission color of colloidal

solutions of CdSe/ZnS core–shell nanocrystals. The particles with the

smallest (�1.7 nm) CdSe core emit blue, the particles with the largest

(�5 nm) core emit red (reproduced with permission from ref. 24, Copy-

right 2004 Wiley).

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

efficiently passivates core surface states and considerably

improves the chemical stability and photostability of the NCs,

resulting in high PL efficiency. Using CdSe core NCs of different

sizes for growing the core–shell particles, a series of colloidal

solutions emitting from blue to red with the emission band as

narrow as 25–35 nm (FWHM) and room temperature PL quan-

tum yield as high as 50–80% can be synthesized (Fig. 9).

Type II core–shell nanocrystals

A type II colloidal heterostructure consists of semiconductor

materials where both the valence and conduction bands in the

core are lower (or higher) than in the shell. As a result, one

charge carrier is mostly confined to the core, and the other to

the shell (Fig. 10). For CdTe/CdSe NCs, the hole wave function

is predominantly located in CdTe core, and the electron wave

Fig. 10 Schematic representation of a cross section of CdTe/CdSe NC

(left) and CdSe/ZnTe NC (right), with corresponding potential diagrams.

A 20 A core radius and a 4 A shell thickness were used in the modeling

(redrawn according to ref. 160).

1070 | J. Mater. Chem., 2008, 18, 1064–1078

function in the CdSe shell; for CdSe/ZnTe NCs the electron–

hole localization is vice versa.160 Emission from type II CdTe/

CdSe, or CdSe/ZnTe NCs originates from the radiative recombi-

nation of the electron–hole pairs across the core–shell interface,

and the emission energy is determined by the corresponding

band offset. As a result, typeII core–shell NCs emit at lower

energies than corresponding core-only materials, covering the

spectral range from 700 to 1000 nm depending on core size

and shell thickness.160

Nanocrystals for near-IR applications

In contrast to the cadmium chalcogenide NCs absorbing and

emitting in the visible spectral range, the fabrication of near-

IR nanocrystalline materials has only relatively recently been

explored.156,161–174 The use of IR emitting colloidal NCs for

telecommunications has been the subject of several reviews.175,176

In what follows, we provide some examples of IV–VI and III–V

IR active NCs; the interested reader is referred to a recent review

for details.177

PbS and PbSe nanocrystals

PbS178,179 and PbSe180 NCs are of great potential interest due to

their bulk unique properties. Bulk PbS and PbSe materials have

a cubic (rock salt) crystal structure, a narrow direct bandgap

(0.28–0.41 eV at 300 K) and a large exciton Bohr radius (for

PbSe 46 nm, about eight times larger than that of CdSe).167 As

a result, size quantization effects are more pronounced in PbSe

than in cadmium chalcogenides, and the value of the bandgap

energy makes the PbSe nanocrystals interesting as emitters in

the near-IR region.

Lead chalcogenide semiconductor NCs were first generated

inside polymer and oxide glass hosts over a decade ago.180,181 A

more versatile liquid-phase synthesis for PbSe colloidal NCs

was first reported by Murray et al. in 2001.161 In the following

year, the groups of Guyot-Sionnest164 and Krauss170 reported

simultaneously similar methods to prepare PbSe NCs. Very

small PbSe NCs (2–3 nm in diameter)162 as well as larger PbSe

quantum wires, rods, and cubes163 have also been successfully

synthesized. High quality PbSe NCs were synthesized from

lead oleate and trioctylphosphine selenide in the diphenyl

ether–TOP medium at 90–220 �C.161 The isolated monodisperse

PbSe NC fractions sized from �3 nm to 15 nm exhibit a

pronounced quantum size effect resulting in a shift of the first

absorption maximum from �1200 nm for 3 nm particles to

2200 nm for 9 nm NCs. The best reported values for PL

efficiency of PbSe nanocrystals were as high as �60%.182 The

passivation of the PbSe NC surface via the epitaxial growth of

a PbS shell resulted in an improvement of both PL efficiency

and NC stability.162

InP and InAs nanocrystals

A first step towards controllable synthesis of III–V NCs was

made by Micic et al. who adapted the Well’s dehalosylilation

reaction for the synthesis of nanocrystalline InP colloids.183

Alivisatos et al. have extended this approach to InAs.184 The

luminescence properties of as-prepared InP and InAs NCs are

rather poor as compared to those of II–VI compounds. The

This journal is ª The Royal Society of Chemistry 2008

Fig. 11 Structures of the non-interacting polymers used in blend systems for hybrid devices. Shown are the structures of MEH-PPV, MEH-CN-PPV,

P3HT, PF, F8BT, CN-PPP, PF-co-PPV, yellow-PPV and MeLPPP.

Fig. 12 Structures of PF-type polymers used in blend systems. Shown

are the structures of CP1 and CP2. CP1 represents a polycationic

semiconducting polymer, while CP2 is an end-capped PF.

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

growth of an epitaxial shell of a higher bandgap II–VI or III–V

semiconductor on the core InAs NCs184 or photochemical etch-

ing of InP NCs185 increases their PL efficiency.

Semiconductor nanoparticles synthesized in aqueous medium

Various II–VI NCs have been prepared in aqueous solutions

using different thiols as stabilizing agents: CdS,186 CdSe,187

CdTe,188 CdxHg1�xTe,189 and HgTe.190 As in the non-aqueous

syntheses mentioned above, the entire spectral range from 500

nm to 2000 nm is available by varying the size and composition

of the nanoparticles. In particular, the luminescence spectra of

HgTe NCs of different sizes cover the spectral region between

1000 and 1800 nm. The luminescence quantum yields of CdTe,

CdxHg1�xTe and HgTe NCs as synthesized in water routinely

reach values of up to 40–50%.173,188

Design of conjugated polymers used inorganic–inorganic devices

The charge transport properties of NC-only films are poor,191

and it is difficult to achieve an electrical contact to single NCs

because of their small size. Taking these aspects together, these

particularities of NCs make them attractive materials for fabri-

cation of hybrid semiconductor NC/organic devices as described

below. The organic/inorganic compatibility depends on the

This journal is ª The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 1064–1078 | 1071

Fig. 13 Structures of end-group functionalized oligomers: P3HT,86 OP-

PE(SH2)217 and PPV-SH87 as well as OPPV-PO.211 In the composites the

endgroups are connected to NCs.

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

versatile chemistry of the host polymer. Hence, the polymers are

designed to attain both the optoelectronic properties desired for

the specific application in mind, and chemical compatibility

providing organic/inorganic homogeneous mixtures.

Fig. 14 Synthesis of polymer-modified

1072 | J. Mater. Chem., 2008, 18, 1064–1078

In analogy to the already very successful donor/acceptor

approach for phosphorescent systems,14 wide bandgap semi-

conducting polymers are used as donor hosts for semiconductor

NCs.61 In such blend systems, efficient energy transfer from host

(polymer) to guest (NCs) takes place when the donor and acceptor

moieties are in close spatial proximity and have a sufficient spec-

tral overlap.181,192,193 Energy and charge transfer from the polymer

to the NCs requires an energy band offset at the organic/inorganic

interface. To date, conjugated polymer hosts used in hybrid device

technology include mainly the commercially available p-type wide

bandgap polymers. MEH-PPV is the most common polymer

used as host in hybrid light-absorbing or -emitting devices.194–198

Some authors reported yellow-PPV,198 CN-PPV199,200 or methyl-

substituted ladder-type poly(para-phenylene) (MeLPPP)201 as

hosts (Fig. 11). Polyfluorene (PF) (n-type polymer),61 and several

PF copolymers such as PF-co-PPV202 and 9,9-dialkylfluorene-

benzothiadiazole (FxBT; x ¼ 6 or 8)203,204 were also recently

used as polymer matrices in hybrid LEDs (Fig. 11).

Hybrid solar cells comprise mainly MEH-PPV195,197,205,206 or

p-type semiconducting polymers of the polythiophene family

such as P3HT205,207,208 or poly(3-octyl-thiophene) (P3OT).209 A

recent example of a red PF is also available.210

Mixing NCs into commercially available cationic PF-based

polyelectrolytes (Fig. 12) resulted in improved NC stability,

and reduced polymer interchain interactions.204 These effects

are both desirable in hybrid LEDs as they enhance the device

stability and device efficiency by reducing non-radiative losses.

A competitive approach to suppress polymer host interchain

interaction is the introduction of linear84,85,211,212 and side-func-

tional,87,213 or dendritic capping ligands.214–216 Blending the

NCs with polymers was also shown to result in enhanced energy

transfer from the polymer to the NCs.214 In Fig. 13, some of the

end-group functionalized oligomers are shown.86,217

In addition to the already shown polymer blends and surface

functional oligomers, functionalized semiconducting polymers

have also been designed in order to specifically modify the

NCs.85 A direct method is shown in Fig. 14.

Another approach for polymers uses the side chains, intro-

duced in thienyl monomers, in order to design materials that

can be grafted onto the surface of CdTe NCs (Fig. 15).218

Therefore 2,5-dibromo-3-[(tetrahydro-4H-thiopyran-4-ylidene)

methyl]thiophene has been prepared and was subsequently

copolymerized in a nickel-catalyzed C–C coupling procedure.

NCs (redrawn according to ref. 85).

This journal is ª The Royal Society of Chemistry 2008

Fig. 16 NC modified with thiophene dendrons (redrawn according to

ref. 215).

Fig. 15 Polythiophene-modified CdTe NC(redrawn according toref. 218).

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

The surface active side chains of the polymer were utilized in

order to trap the NCs via a ligand-exchange reaction. Further-

more, thiophene dendrimers synthesized in a convergent

approach were used in order to modify the NC surface with

polymeric organic materials as shown in Fig. 16.215,216

Visible LEDs based on nanocrystal–organiccompound composites

The first paper on hybrid NC/polymer LEDs from the Berkeley

group appeared in 1994219 and reported on a bi-layer device

comprising a thin (few tens of nm) layer of CdSe NCs deposited

on a conducting support with the help of a bifunctional linker

hexane dithiol, and a 100 nm thick layer of a soluble PPV deriva-

tive. The structure was sandwiched between an ITO-coated glass

and an Mg/Ag electrode, which functioned as anode and

cathode, respectively. The device with a hole transporting PPV

layer close to the ITO, with electrons injected into a layer of

NCs and holes injected into a layer of polymer (forward bias),

exhibited an emission characteristic of the CdSe NCs at an

operating voltage of only 4 V, with an EL band tunable from

yellow to red by changing the NC size.

A subsequent paper from two MIT groups220 reported on a

single-layer device where CdSe NCs were homogeneously

distributed within a polymer layer (70–120 nm thick) of poly-

vinylcarbazole as a hole conducting component, which addi-

tionally contained an oxadiazole derivative (butyl-PBD) as an

electron transporting molecular species. The resulting volume

fraction of CdSe NCs in a film sandwiched between ITO and

Al electrodes was 5–10%, below the percolation threshold for

charge transport to occur between the NCs. The PL and EL

spectra of the devices were reasonably narrow (<40 nm

FWHM) and nearly identical, and were tunable from 530 to

650 nm by varying the NC size. Subsequent papers reported

on bi-layer devices based on core–shell NCs221,222 and showed

This journal is ª The Royal Society of Chemistry 2008

sufficient improvements over the core-only CdSe-based devices,

namely a factor of 20 increase in efficiency and a factor of 100

in lifetime.

Tri-layer hybrid NC/organic molecule LEDs with a single

monolayer of CdSe/ZnS NCs sandwiched between two organic

thin films have been introduced recently by two MIT groups.223

In these devices the external quantum efficiency exceeded 0.4%

for a broad range of luminance. At 125 mA cm�2, the brightness

of the three layer device was 2 000 cd m�2 (i.e. 1.6 cd A�1), which

constitutes a 25-fold improvement over the best previously

reported visible NC-based LEDs.221

The first hybrid NC-based LED made by layer-by-layer

assembly was reported in ref. 224. It was built up from 20 alter-

nating double layers of a precursor of PPV (pre-PPV) and CdSe

NCs capped by thiolactic acid, followed by thermal conversion

of pre-PPV to PPV. The device emitted white light originating

mainly from recombination through NC trap sites, with

a turn-on voltage of 3.5–5 V and an external quantum

efficiency of 0.0015%.

EL of different colors (from green to red) was obtained from

layer-by-layer assembled LEDs based on thioglycolic acid cap-

ped CdTe NCs of different sizes and poly(diallyldimethylammo-

nium chloride) (PDDA).225 In spite of the use of an insulating

polymer, external quantum efficiencies of 0.1% were achieved.

Light emission was observed at current densities of 10 mA

cm�2 and at exceptionally low onset voltages of 2.5–3.5 V (i.e.

only just above the bandgap of the CdTe NCs).

Near-infrared LEDs based on nanocrystal–polymercomposites

Because of the limited choice of polymers and organic dyes emit-

ting in the range of telecommunications windows (1.3 and 1.55

micron), the use of the near-infrared emitting NCs in hybrid

organic–inorganic LEDs provides a particularly attractive alter-

native. The first paper on such LEDs appeared in 2002 and

reported on single-layer blend devices.203 The devices showed

broad emission in the range of 1.3 microns, which was attributed

to the inhomogeneously broadened spectrum of the ensemble of

differently sized NCs, with a rather high turn-on voltage of 15 V

and a remarkably high external quantum efficiency reaching

0.5% at high operating voltages.203,226

In subsequent reports, EL spectra of single-layer devices based

on mixtures of PbS NCs with MEH-PPV or poly(2-(6-cyano-60-

methylheptyloxy)-1,4-phenylene) (CN-PPP) were shown to be

potentially tunable across the range of 1000 to 1600 nm.181,227,228

Internal quantum efficiencies of 1.9% could be achieved in such

diodes with MEH-PPV and PbS nanocrystals.196 Other reports

on HgTe201 or PbSe198 based LEDs showed relatively poor EL

efficiency probably due to non matched properties of the NCs

and the organic molecules/polymers hosting them.

Nanocrystal-based photovoltaic devices

Semiconductor NCs have recently been incorporated into conju-

gated polymers to create hybrid photovoltaic cells, aimed at

achieving ease of processing, low cost, substrate flexibility, large

area coverage and, from a longer term perspective, enhanced

efficiency. A particular noteworthy example is the hybrid NC/

J. Mater. Chem., 2008, 18, 1064–1078 | 1073

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

polymer photovoltaic device consisting of a blend of CdSe

nanorods (7 nm � 60 nm) with P3HT, which showed a solar

conversion efficiency of 1.7% (under air mass (AM) 1.5 global

solar conditions).30 The superior efficiency of this device over

net-polymer-based ones arises from the fast charge separation

at the NC/polymer interface and the enhanced electron transport

in the structure due to the relatively high intrinsic carrier mobili-

ties in inorganic semiconductor NCs. The performance of such

cells assembled from solution can attain external quantum

efficiencies of over 54% and a monochromatic power conversion

efficiency of 6.9% (under 0.1 mW per square centimeter illumina-

tion at 515 nm) in optimized devices tuning the composite mor-

phology by introducing large polymer/NC interfaces upon

thermal treatment.229 End-functionalized polymers in the hybrid

solar cells86 provided the power conversion efficiency (AM 1.5)

of 1.5%, being attached to CdSe nanorods of about 7 nm in

diameter and 30 nm in length.86

The first proof-of-principle photovoltaic device with dendritic

structures built from thienyl moieties79,82,83,215,216 appeared and

showed promise due to the favorable assembly behavior of the

thienyl dendrons used.215 Simple, not yet optimized devices

resulted in power conversion efficiencies of 0.29%. The impor-

tance of the morphology optimization in such devices in terms

of creating large interfaces becomes furthermore apparent

when looking at multi-armed CdS nanorods in MEH-PPV.230

After thermal treatment the reorganization of the inorganic

NC/polymer interface leads to an increase of exciton dissociation

efficiency in the system, providing photovoltaic devices with

power conversion efficiencies of up to 1.17% under AM 1.5

illumination (100 mW cm�2).

The use of metal–ligand complexes collecting the triplet

excitons from the conjugated polymers was also found to

enhance efficiencies in composite structures for solar cell applica-

tions.231 The system investigated was P3HT/CdSe doped with

10% of Ir(mppy)3. In such devices the short-circuit current was

found to increase by 100% revealing a much increased popula-

tion of triplet excitons.

A major drawback of the above mentioned devices utilizing

cadmium chalcogenides is that both the NCs and the introduced

polymers absorb light in the visible part of the solar spectrum

(below 800 nm), leaving a considerable amount of near infrared

solar energy (800–2800 nm) unused.

To generate photocurrent from low energy photons, narrow

bandgap NCs have been used in NC/polymer composites to

extend the photovoltaic response into the infrared.195 In this

study, PbS NCs were incorporated in the polymer matrix of

MEH-PPV, and NIR activity was demonstrated. Subsequently,

a single and double layer structure based on PbS or PbSe NCs

has been described and demonstrated spectral coverage up to

�1600 nm and power efficiencies up to 0.14%.232 The reason

for the relatively low efficiency at NIR wavelengths is directly

related to the low absorption cross section of the NCs that

becomes comparable to that of conjugated polymers only at

about double or triple the energy of the NC bandgap.198 Further-

more, it has to be mentioned that the mobility of holes and

electrons is equal in PbS NCs, which is different to typical elec-

tron acceptors such as fullerene or CdSe.233,234 Charge transport

in semiconducting polymer/PbS composite structures is still

a matter of active research.234 On the other hand, simply by

1074 | J. Mater. Chem., 2008, 18, 1064–1078

changing the hole conductor from MEH-PPV to P3OT sufficient

improvement in simple bilayer devices exceeding the external

quantum efficiency of 1% has been achieved.209 This may be

determined by rapid extraction of holes at the interface from

the NCs into the surrounding polymer, which is due to the favor-

able alignment of the heterostructure band offsets between the

polymer P3OT and the quantum-confined states of PbS

NCs.209 Introducing octylamine ligands to the PbS NCs

combined with the MEH-PPV hole transporting polymer may

be another reason for the drastic improvement of these devices.

PbS NCs used were either oleic acid or octylamine capped. The

octylamine-capped NCs allowed over two orders of magnitude

more photocurrent under �1 V bias; they also showed an infra-

red photovoltaic response, while devices using oleic acid-capped

NCs did not. Further improvement in the photovoltaic perfor-

mance of devices based on octylamine-capped PbS NCs occurred

upon thermal annealing: This simple procedure led to a 200-fold

increase in short circuit current, a 600-fold increase in maximum

power output, and an order of magnitude faster response time.

These already very promising developments in hybrid photo-

voltaic devices become apparent in many patent applications

filed recently.235–238 Novel approaches can further improve effi-

ciencies by e.g. taking advantage of multiphoton excitation239,240

in composite NC-based systems.

Conclusions

Hybrid structures consisting of semiconducting polymers and

strongly luminescent semiconductor NCs offer favorable

perspectives through highly saturated tunable emission as well

as tunable absorption features, which are of importance for

several high-tech applications such as hybrid organic–inorganic

light-emitting diodes and solar cells. The combination of organic

and inorganic components in hybrid materials offers advantages

that are not accessible with solely one of the materials classes.

Hybrid structures created by combination of inorganic and

organic components merge advantageous optical and conduct-

ing properties with robustness and flexibility. The field of

research, highlighted with selected examples in this publication,

will rapidly develop in the near future, ideally leading to a new

generation of organic–inorganic LEDs and photovoltaic devices.

Acknowledgements

EH and ALR gratefully acknowledge financial support from the

Deutsche Forschungsgemeinschaft (DFG). EH acknowledges

Prof. Ullrich Scherf supporting the Functional Polymers Group.

ALR gratefully acknowledges financial support of the German

Excellence Initiative of the DFG via the ‘‘Nanosystems Initiative

Munich (NIM)’’.

References

1 A. C. Balazs, T. Emrick and T. P. Russell, Science, 2006, 314, 1107–1110.

2 C. Sanchez, B. Julian, P. Belleville and M. Popall, J. Mater. Chem.,2005, 15, 3559–3592.

3 C. Sanchez, B. Lebeau, F. Chaput and J. P. Boilot, Adv. Mater.,2003, 15, 1969–1994.

4 E. Lancelle-Beltran, P. Prene, C. Boscher, P. Belleville, P. Buvat andC. Sanchez, Adv. Mater., 2006, 18, 2579–2582.

This journal is ª The Royal Society of Chemistry 2008

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

5 G. A. Ozin, Adv. Mater., 1992, 4, 612–649.6 M. J. MacLachlan, I. Manners and G. A. Ozin, Adv. Mater., 2000,12, 675–681.

7 S.-C. Chang, J. Bharathan, Y. Yang, R. Helgeson, F. Wudl,M. B. Ramey and J. R. Reynolds, Appl. Phys. Lett., 1998, 73,2561–2563.

8 H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda,M. Inbasekaran, W. Wu and E. P. Woo, Science, 2000, 290, 2123–2126.

9 T. Kawase, T. Shimoda, C. Newsome, H. Sirringhaus andR. H. Friend, Thin Solid Films, 2003, 438–439, 279–287.

10 Y. Xia and R. H. Friend, Macromolecules, 2005, 38, 6466–6471.11 J. F. Dijksman, P. C. Duineveld, M. J. J. Hack, A. Pierik, J. Rensen,

J. E. Rubingh, I. Schram and M. M. Vernhout, J. Mater. Chem.,2007, 17, 511–522.

12 B.-J. de Gans, P. C. Duineveld and U. S. Schubert, Adv. Mater.,2004, 16, 203–213.

13 S.-C. Chang, J. Liu, J. Bharathan, Y. Yang, J. Onohara and J. Kido,Adv. Mater., 1999, 11, 734–737.

14 E. Holder, B. M. W. Langeveld and U. S. Schubert, Adv. Mater.,2005, 17, 1109–1121.

15 E. Tekin, H. Wijlaars, E. Holder, D. A. M. Egbe and U. S. Schubert,J. Mater. Chem., 2006, 16, 4294–4298.

16 E. Tekin, E. Holder, D. Kozodaev and U. S. Schubert, Adv. Funct.Mater., 2007, 17, 277–284.

17 V. Marin, E. Holder, M. M. Wienk, E. Tekin, D. Kozodaev andU. S. Schubert, Macromol. Rapid Commun., 2005, 26, 319–324.

18 E. Tekin, E. Holder, V. Marin, B.-J. De Gans and U. S. Schubert,Macromol. Rapid Commun., 2005, 26, 293–297.

19 V. Marin, E. Holder, R. Hoogenboom, E. Tekin and U. S. Schubert,Dalton Trans., 2006, 1636–1644.

20 E. Tekin, P. J. Smith, S. Hoeppener, A. M. J. van den Berg,A. S. Susha, A. L. Rogach, J. Feldmann and U. S. Schubert,Adv. Funct. Mater., 2007, 17, 23–28.

21 R. Gangopadhyay and A. De, Chem. Mater., 2000, 12, 2064–2064.22 C. O. Oriakhi, J. Chem. Educ., 2000, 77, 1138–1146.23 S. R. Forrest, Nature, 2004, 428, 911–918.24 A. L. Rogach, D. V. Talapin and H. Weller, in Colloids and Colloid

Assemblies, ed. F. Caruso, Wiley-VCH, Weinheim, 2004, pp.52–95.

25 A. L. Rogach, A. Eychmuller, S. G. Hickey and S. V. Kershaw,Small, 2007, 3, 536–557.

26 B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine,H. Mattoussi, R. Ober, K. F. Jensen and M. G. Bawendi, J. Phys.Chem. B, 1997, 101, 9463–9475.

27 X. Peng, M. C. Schlamp, A. V. Kadavanich and A. P. Alivisatos,J. Am. Chem. Soc., 1997, 119, 7019–7029.

28 D. Yu, C. Wang and P. Guyot-Sionnest, Science, 2003, 300, 1277–1280.

29 N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science,1992, 258, 1474–1476.

30 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295,2425–2427.

31 A. J. Heeger, Rev. Mod. Phys., 2001, 73, 681–700.32 R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes,

R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dossantos,J. L. Bredas, M. Logdlund and W. R. Salaneck, Nature, 1999,397, 121–128.

33 S. R. Forrest, IEEE J. Sel. Top. Quantum Electron., 2000, 6, 1072–1083.

34 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature,1990, 347, 539–541.

35 D. Braun and A. J. Heeger, Appl. Phys. Lett., 1991, 58,1982–1984.

36 S. A. Carter, M. Angelopoulos, S. Karg, P. J. Brock, J. C. Scott,J. C. Carter, I. Grizzi, S. K. Heeks, D. J. Lacey, S. G. Latham,P. G. May, O. R. Delospanos, K. Pichler, C. R. Towns andH. F. Wittmann, Appl. Phys. Lett., 1997, 70, 2067–2069.

37 L. Zuppiroli, L. Si-Ahmed, K. Kamaras, F. Nuesch, M. N. Bussac,D. Ades, A. Siove, E. Moons and M. Gratzel, Eur. Phys. J. B, 1999,11, 505–512.

38 P. K. H. Ho, J. S. Kim, J. H. Burroughes, H. Becker, S. F. Y. Li,T. M. Brown, F. Cacialli and R. H. Friend, Nature, 2000, 404,481–484.

This journal is ª The Royal Society of Chemistry 2008

39 H. Becker, H. Spreitzer, W. Kreuder, E. Kluge, H. Schenk, I. Parkerand Y. Cao, Adv. Mater., 2000, 12, 42–48.

40 U. Scherf and E. J. W. List, Adv. Mater., 2002, 14, 477–487.41 M. A. Baldo, D. F. Obrien, Y. You, A. Shoustikov, S. Sibley,

M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151–154.42 V. Cleave, G. Yahioglu, P. LeBarny, R. H. Friend and N. Tessler,

Adv. Mater., 1999, 11, 285–288.43 A. Kohler, J. S. Wilson and R. H. Friend, Adv. Mater., 2002, 14,

701–707.44 D. Beljonne, Z. Shuai, J. Cornil, J. P. Calbert and J. L. Bredas,

J. Photochem. Photobiol., A, 2001, 144, 57–62.45 M. Wohlgenannt, X. M. Jiang, Z. V. Vardeny and R. A. J. Janssen,

Phys. Rev. Lett., 2002, 88, 197401.46 D. Tanaka, H. Sasabe, Y.-J. Li, S.-J. Su, T. Takeda and J. Kido,

Jpn. J. Appl. Phys., Part 2, 2007, 46, L10–L12.47 M. Leadbeater, N. Patel, B. Tierney, S. O’Connor, I. Grizzi and

C. Towns, Dig. Tech. Pap. Soc. Inf. Disp. Int. Symp., 2004, 35,162–163.

48 M. E. Thompson, S. Garon, R. Kwong, J. Brooks and M.-H. M. Lu,US Pat. Appl., US2003-405009, 2004.

49 M. Hack, M. S. Weaver, V. Adamovich, R. C. Kwong, M. H. Luand J. J. Brown, Proc. SPIE–Int. Soc. Opt. Eng., 2005, 5961,596102.

50 C. W. Tang, Appl. Phys. Lett., 1986, 48, 183–185.51 N. S. Sariciftci, D. Braun, C. Zhang, V. I. Srdanov, A. J. Heeger,

G. Stucky and F. Wudl, Appl. Phys. Lett., 1993, 62, 585–587.52 J. J. M. Halls, K. Pichler, R. H. Friend, S. C. Moratti and

A. B. Holmes, Appl. Phys. Lett., 1996, 68, 3120–3122.53 H. Spanggaard and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2004,

83, 125–146.54 C. J. Brabec and S. N. Sariciftci, Monatsh. Chem., 2001, 132, 421–

431.55 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger

and G. C. Bazan, Nat. Mater., 2007, 6, 497–500.56 J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante

and A. J. Heeger, Science, 2007, 317, 222–225.57 G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery and

Y. Yang, Nat. Mater., 2005, 4, 864–868.58 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,

K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature,1990, 347, 539–541.

59 P. Sigwalt, G. Wegner, J.-F. Morin, M. Leclerc, D. Ades andA. Siove, Macromol. Rapid Commun., 2005, 26, 761–778.

60 I. F. Perepichka, D. F. Perepichka, H. Meng and F. Wudl,Adv. Mater., 2005, 17, 2281–2305.

61 H. V. Demir, S. Nizamoglu, T. Ozel, E. Mutlugun, I. O. Huyal,E. Sari, E. Holder and N. Tian, New J. Phys., 2007, 9, 362.

62 M. Leclerc, J. Polym. Sci., Part A: Polym. Chem., 2001, 39, 2867–2873.

63 Y. Zhu, R. D. Champion and S. A. Jenekhe, Macromolecules, 2006,39, 8712–8719.

64 A. Tsami, X.-H. Yang, S. Adamczyk, T. Farrell, R. Heiderhoff,L. J. Balk, D. Neher and E. Holder, J. Polym. Sci., Part A: Polym.Chem., 2007, submitted.

65 A. Tsami, X.-H. Yang, F. Galbrecht, T. Farrell, H. Li,S. Adamczyk, R. Heiderhoff, L. J. Balk, D. Neher and E. Holder,J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 4773–4785.

66 H. J. Brouwer, A. Hilberer, V. V. Krasnikov, M. Werts, J. Wildemanand G. Hadziioannou, Synth. Met., 1997, 84, 881–882.

67 X. L. Chen and S. A. Jenekhe, Synth. Met., 1997, 85, 1431–1434.68 P. Leclere, V. Parente, J. L. Bredas, B. Francois and R. Lazzaroni,

Chem. Mater., 1998, 10, 4010–4014.69 C. Towns and P. Wallace, PCT Int. Appl., WO2002GB04752

20021022, 2003.70 U. Asawapirom, F. Bulut, T. Farrell, C. Gadermaier, S. Gamerith,

R. Guentner, T. Kietzke, S. Patil, T. Piok, R. Montenegro, B. Stiller,B. Tiersch, K. Landfester, E. J. W. List, D. Neher, C. S. Torres andU. Scherf, Macromol. Symp., 2004, 212, 83–91.

71 T. W. Buennagel, F. Galbrecht and U. Scherf, Macromolecules,2006, 39, 8870–8872.

72 Z. Xu, M. Kahr, K. L. Walker, C. L. Wilkins and J. S. Moore,J. Am. Chem. Soc., 1994, 116, 4537–4550.

73 Z. Xu and J. S. Moore, Angew. Chem., Int. Ed. Engl., 1993, 32,246–248.

74 Z. Zhu and J. S. Moore, J. Org. Chem., 2000, 65, 116–123.

J. Mater. Chem., 2008, 18, 1064–1078 | 1075

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

75 S. F. Swallen, Z.-Y. Shi, W. Tan, Z. Xu, J. S. Moore andR. Kopelman, J. Lumin., 1998, 76(77), 193–196.

76 R. E. Bauer, A. C. Grimsdale and K. Muellen, Top. Curr. Chem.,2005, 245, 253–286.

77 H. John, R. Bauer, P. Espindola, P. Sonar, J. Heinze andK. Muellen, Angew. Chem., Int. Ed., 2005, 44, 2447–2451.

78 M. Maus, R. De, M. Lor, T. Weil, S. Mitra, U.-M. Wiesler,A. Herrmann, J. Hofkens, T. Vosch, K. Muellen andF. C. De Schryver, J. Am. Chem. Soc., 2001, 123, 7668–7676.

79 C. Xia, X. Fan, J. Locklin and R. C. Advincula, Org. Lett., 2002, 4,2067–2070.

80 N. Kopidakis, W. J. Mitchell, J. van de Lagemaat, D. S. Ginley,G. Rumbles, S. E. Shaheen and W. L. Rance, Appl. Phys. Lett.,2006, 89, 103524.

81 W. J. Mitchell, N. Kopidakis, G. Rumbles, D. S. Ginley andS. E. Shaheen, J. Mater. Chem., 2005, 15, 4518–4528.

82 C. Xia, X. Fan, J. Locklin, R. C. Advincula, A. Gies andW. Nonidez, J. Am. Chem. Soc., 2004, 126, 8735–8743.

83 C.-Q. Ma, E. Mena-Osteritz, T. Debaerdemaeker, M. M. Wienk,R. A. Janssen and P. Baeuerle, Angew. Chem., Int. Ed., 2007, 46,1679–1683.

84 Q. Zhang, T. P. Russell and T. Emrick, Chem. Mater., 2007, 19,3712–3716.

85 H. Skaff, K. Sill and T. Emrick, J. Am. Chem. Soc., 2004, 126,11322–11325.

86 J. Liu, T. Tanaka, K. Sivula, A. P. Alivisatos and J. M. J. Frechet,J. Am. Chem. Soc., 2004, 126, 6550–6551.

87 O. P. Varnavski, M. Ranasinghe, X. Yan, C. A. Bauer, S.-J. Chung,J. W. Perry, S. R. Marder and T. Goodson, III, J. Am. Chem. Soc.,2006, 128, 10988–10989.

88 A. C. Grimsdale and K. Mullen, Adv. Polym. Sci., 2006, 199, 1–82.89 B. Liu and G. C. Bazan, Opt. Eng., 2005, 94, 179–266.90 J. Roncali, Chem. Rev., 1997, 97, 173–205.91 D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev., 2000,

100, 2537–2574.92 H. A. M. van Mullekom, J. A. J. M. Vekemans, E. E. Havinga and

E. W. Meijer, Mater. Sci. Eng., R: Rep., 2001, 32, 1–40.93 A. J. Heeger, J. Phys. Chem. B, 2001, 105, 8475–8491.94 D. Neher, Macromol. Rapid Commun., 2001, 22, 1365–1385.95 T. Yamamoto, Synlett, 2003, 425–450.96 E. Moons, J. Phys.: Condens. Matter, 2002, 14, 12235–12260.97 F. Babudri, G. M. Farinola and F. Naso, J. Mater. Chem., 2004, 14,

11–34.98 B. Liu and G. C. Bazan, Chem. Mater., 2004, 16, 4467–4476.99 A. Moliton and R. C. Hiorns, Polym. Int., 2004, 53, 1397–1412.

100 A. P. H. J. Schenning and E. W. Meijer, Chem. Commun., 2005,3245–3258.

101 M. J. Frampton and H. L. Anderson, Angew. Chem., Int. Ed., 2007,46, 1028–1064.

102 S. Guenes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007,107, 1324–1338.

103 F. Galbrecht, T. W. Buennagel, U. Scherf and T. Farrell, Macromol.Rapid Commun., 2007, 28, 387–394.

104 S. W. Thomas, III, G. D. Joly and T. M. Swager, Chem. Rev., 2007,107, 1339–1386.

105 B. J. Schwartz, Annu. Rev. Phys. Chem., 2003, 54, 141–172.106 J. V. Grazulevicius, P. Strohriegl, J. Pielichowski and

K. Pielichowski, Prog. Polym. Sci., 2003, 28, 1297–1353.107 U. Scherf, J. Mater. Chem., 1999, 9, 1853–1864.108 A. D. Schluter and Z. Bo, Handbook of Organopalladium

Chemistry for Organic Synthesis, ed. E. Negishi, Wiley-Interscience,New York, 2002, vol. 1, pp. 825–861.

109 U. H. F. Bunz, Chem. Rev., 2000, 100, 1605–1644.110 R. A. Wessling, J. Polym. Sci., Polym. Symp., 1985, 72, 55–66.111 F. Louwet, D. Vanderzande, J. Gelan and J. Mullens,

Macromolecules, 1995, 28, 1330–1331.112 H. G. Gilch and W. L. Wheelwright, J. Polym. Sci., Polym. Chem.

Ed., 1966, 4, 1337–1349.113 D. Raabe, H. H. Hoerhold and U. Scherf, Makromol. Chem., Rapid

Commun., 1986, 7, 613–619.114 K. L. Brandon, P. G. Bentley, D. D. C. Bradley and D. A. Dunmur,

Synth. Met., 1997, 91, 305–306.115 Y. Liu, P. M. Lahti and F. La, Polymer, 1998, 39, 5241–5244.116 M. J. Marsella, D. K. Fu and T. M. Swager, Adv. Mater., 1995, 7,

145–147.

1076 | J. Mater. Chem., 2008, 18, 1064–1078

117 E. Thorn-Csanyi and P. Kraxner, Macromol. Chem. Phys., 1997,198, 3827–3843.

118 S. Tasch, W. Graupner, G. Leising, L. Pu, M. W. Wagner andR. H. Grubbs, Adv. Mater., 1995, 7, 903–906.

119 D.-H. Hwang, H.-K. Shim and D.-S. Sakong, US Pat., 08/562,025,1998.

120 T. Yamamoto, M. Takagi, K. Kizu, T. Maruyama, K. Kubota,H. Kanbara, T. Kurihara and T. Kaino, J. Chem. Soc., Chem.Commun., 1993, 797–798.

121 D. A. M. Egbe and E. Klemm, Macromol. Chem. Phys., 1998, 199,2683–2688.

122 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett.,1975, 4467–4470.

123 M. Leclerc, F. Martinez Diaz and G. Wegner, Makromol. Chem.,Rapid Commun., 1989, 190, 3105–3116.

124 T. Yamamoto, W. Yamada, M. Takagi, K. Kizu, T. Maruyama,N. Ooba, S. Tomaru, T. Kurihara, T. Kaino and K. Kubota,Macromolecules, 1994, 27, 6620–6626.

125 A. D. Schluter, J. Polym. Sci., Part A: Polym. Chem., 2001, 39,1533–1556.

126 R. D. McCullough and R. D. Lowe, J. Chem. Soc., Chem. Commun.,1992, 70–72.

127 T. A. Chen and R. D. Rieke, J. Am. Chem. Soc., 1992, 114, 10087–10088.

128 R. S. Loewe, P. C. Ewbank, J. Liu, L. Zhai and R. D. McCullough,Macromolecules, 2001, 34, 4324–4333.

129 A. van Dijken, J. J. A. M. Bastiaansen, N. M. M. Kiggen,B. M. W. Langeveld, C. Rothe, A. Monkman, I. Bach, P. Stoesseland K. Brunner, J. Am. Chem. Soc., 2004, 126, 7718–7727.

130 K. L. Chan, S. E. Watkins, C. S. K. Mak, M. J. McKiernan,C. R. Towns, S. I. Pascu and A. B. Holmes, Chem. Commun.,2005, 5766–5768.

131 P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak andP. E. Burrows, Org. Lett., 2006, 8, 4211–4214.

132 P. E. Burrows, A. B. Padmaperuma, L. S. Sapochak, P. Djurovichand M. E. Thompson, Appl. Phys. Lett., 2006, 88, 183503.

133 A. B. Padmaperuma, L. S. Sapochak and P. E. Burrows, Chem.Mater., 2006, 18, 2389–2396.

134 A. A. M. K. G. L. F. E. Aharon, Adv. Funct. Mater., 2006, 16,980–986.

135 A. J. Heeger, Angew. Chem., Int. Ed., 2001, 40, 2591–2611.136 A. Ajayaghosh, Chem. Soc. Rev., 2003, 32, 181–191.137 C. J. Brabec, C. Winder, N. S. Sariciftci, J. C. Hummelen,

A. Dhanabalan, P. A. Van Hal and R. A. J. Janssen, Adv. Funct.Mater., 2002, 12, 709–712.

138 F. Zhang, E. Perzon, X. Wang, W. Mammo, M. R. Andersson andO. Inganaes, Adv. Funct. Mater., 2005, 15, 745–750.

139 D. Muehlbacher, H. Neugebauer, A. Cravino, N. S. Sariciftci,J. K. J. van Duren, A. Dhanabalan, P. A. van Hal,R. A. J. Janssen and J. C. Hummelen, Mol. Cryst. Liq. Cryst. Sci.Technol., Sect. A, 2002, 385, 85–92.

140 C. Winder, D. Muehlbacher, H. Neugebauer, N. Serdar Sariciftci,C. Brabec, R. A. J. Janssen and J. Kees Hummelen, Mol. Cryst.Liq. Cryst. Sci. Technol., Sect. A, 2002, 385, 93–100.

141 B. Ma, P. I. Djurovich and M. E. Thompson, Coord. Chem. Rev.,2005, 249, 1501–1510.

142 V. Marin, E. Holder, R. Hoogenboom and U. S. Schubert,Chem. Soc. Rev., 2007, 36, 618–635.

143 P. K. H. Ho, J.-S. Kim, J. H. Burroughes, H. Becker, F. Y. L. Sam,T. M. Brown, F. Cacialli and R. H. Friend, Nature, 2000,404, 481–484.

144 J. Roncali, Chem. Soc. Rev., 2005, 34, 483–495.145 P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster and

D. E. Markov, Adv. Mater., 2007, 19, 1551–1566.146 C. J. Brabec, Springer Ser. Mater. Sci., 2003, 60, 159–248.147 H. Hoppe, M. Niggemann, C. Winder, J. Kraut, R. Hiesgen,

A. Hinsch, D. Meissner and N. S. Sariciftci, Adv. Funct. Mater.,2004, 14, 1005–1011.

148 H. Hoppe, D. A. M. Egbe, D. Muehlbacher and N. S. Sariciftci,J. Mater. Chem., 2004, 14, 3462–3467.

149 J. G. Muller, J. M. Lupton, J. Feldmann, U. Lemmer,M. C. Scharber, N. S. Sariciftci, C. J. Brabec and U. Scherf, Phys.Rev. B: Condens. Matter Mater. Phys., 2005, 72, 195208.

150 J. K. J. van Duren, X. Yang, J. Loos, C. W. T. Bulle-Lieuwma,A. B. Sieval, J. C. Hummelen and R. A. J. Janssen, Adv. Funct.Mater., 2004, 14, 425–434.

This journal is ª The Royal Society of Chemistry 2008

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

151 M. M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol,J. C. Hummelen, P. A. van Hal and R. A. J. Janssen, Angew.Chem., Int. Ed., 2003, 42, 3371–3375.

152 P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele and C. J. Brabec,Chem. Mater., 2005, 17, 2175–2180.

153 M. Reyes-Reyes, K. Kim and D. L. Carroll, Appl. Phys. Lett., 2005,87, 083506.

154 R. S. Ashraf, M. Shahid, E. Klemm, M. Al-Ibrahim and S. Sensfuss,Macromol. Rapid Commun., 2006, 27, 1454–1459.

155 L. J. A. Koster, V. D. Mihailetchi and P. W. M. Blom, Appl. Phys.Lett., 2006, 88, 093511.

156 X. Wang, J. Zhuang, Q. Peng and Y. Li, Nature, 2005, 437, 121–124.157 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc.,

1993, 115, 8706–8715.158 D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase and

H. Weller, Nano Lett., 2001, 1, 207–211.159 A. V. Baranov, Y. P. Rakovich, J. F. Donegan, T. S. Perova,

R. A. Moore, D. V. Talapin, A. L. Rogach, Y. Masumoto andI. Nabiev, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 68,165306.

160 S. Kim, B. Fisher, H. J. Eisler and M. Bawendi, J. Am. Chem. Soc.,2003, 125, 11466–11467.

161 C. B. Murray, S. H. Sun, W. Gaschler, H. Doyle, T. A. Betley andC. R. Kagan, IBM J. Res. Dev., 2001, 45, 47–56.

162 A. Sashchiuk, L. Langof, R. Chaim and E. Lifshitz, J. Cryst.Growth, 2002, 240, 431–438.

163 E. Lifshitz, M. Bashouti, V. Kloper, A. Kigel, M. S. Eisen andS. Berger, Nano Lett., 2003, 3, 857–862.

164 B. L. Wehrenberg, C. J. Wang and P. Guyot-Sionnest, J. Phys.Chem. B, 2002, 106, 10634–10640.

165 D. Kuang, A. Xu, Y. Fang, H. Liu, C. Frommen and D. Fenske,Adv. Mater., 2003, 15, 1747–1750.

166 K.-S. Cho, D. V. Talapin, W. Gaschler and C. B. Murray, J. Am.Chem. Soc., 2005, 127, 7140–7147.

167 M. Brumer, A. Kigel, L. Amirav, A. Sashchiuk, O. Solomesch,N. Tessler and E. Lifshitz, Adv. Funct. Mater., 2005, 15,1111–1116.

168 A. J. Houtepen, R. Koole, D. Vanmaekelbergh, J. Meeldijk andS. G. Hickey, J. Am. Chem. Soc., 2006, 128, 6792–6793.

169 J. S. Steckel, B. K. H. Yen, D. C. Oertel and M. G. Bawendi, J. Am.Chem. Soc., 2006, 128, 13032–13033.

170 H. Du, C. Chen, R. Krishnan, T. D. Krauss, J. M. Harbold,F. W. Wise, M. G. Thomas and J. Silcox, Nano Lett., 2002, 2,1321–1324.

171 K. R. Choudhury, Y. Sahoo, S. Jang and P. N. Prasad, Adv. Funct.Mater., 2005, 15, 751–756.

172 S. V. Kershaw, M. Burt, M. Harrison, A. Rogach, H. Weller andA. Eychmuller, Appl. Phys. Lett., 1999, 75, 1694–1696.

173 A. L. Rogach, M. T. Harrison, S. V. Kershaw, A. Kornowski,M. G. Burt, A. Eychmuller and H. Weller, Phys. Status Solidi B,2001, 224, 153–158.

174 M. T. Harrison, S. V. Kershaw, A. L. Rogach, A. Kornowski,A. Eychmuller and H. Weller, Adv. Mater., 2000, 12, 123–125.

175 S. V. Kershaw, M. Harrison, A. L. Rogach and A. Kornowski,IEEE J. Sel. Top. Quantum Electron., 2000, 6, 534–543.

176 M. T. Harrison, S. V. Kershaw, M. G. Burt, A. L. Rogach,A. Kornowski, A. Eychmuller and H. Weller, Pure Appl. Chem.,2000, 72, 295–307.

177 A. L. Rogach, A. Eychmueller, S. G. Hickey and S. V. Kershaw,Small, 2007, 3, 536–557.

178 M. Mukherjee, A. Datta, S. K. Pradhan and D. Chakravorty,J. Mater. Sci. Lett., 1996, 15, 654–657.

179 M. Nagel, S. G. Hickey, A. Froemsdorf, A. Kornowski andH. Weller, Z. Phys. Chem., 2007, 221, 427–437.

180 A. Lipovskii, E. Kolobkova, V. Petrikov, I. Kang, A. Olkhovets,T. Krauss, M. Thomas, J. Silcox, F. Wise, Q. Shen and S. Kycia,Appl. Phys. Lett., 1997, 71, 3406–3408.

181 L. Bakueva, S. Musikhin, M. A. Hines, T. W. F. Chang, M. Tzolov,G. D. Scholes and E. H. Sargent, Appl. Phys. Lett., 2003, 82, 2895–2897.

182 C. E. Finlayson, A. Amezcua, P. J. A. Sazio, P. S. Walker,M. C. Grossel, R. J. Curry, D. C. Smith and J. J. Baumberg,J. Mod. Opt., 2005, 52, 955–964.

183 O. I. Micic, C. J. Curtis, K. M. Jones, J. R. Sprague and A. J. Nozik,J. Phys. Chem. B, 1994, 98, 4966–4969.

This journal is ª The Royal Society of Chemistry 2008

184 A. A. Guzelian, U. Banin, A. V. Kadavanich, X. Peng andA. P. Alivisatos, Appl. Phys. Lett., 1996, 69, 1432–1434.

185 D. V. Talapin, N. Gaponik, H. Borchert, A. L. Rogach, M. Haaseand H. Weller, J. Phys. Chem. B, 2002, 106, 12659–12663.

186 T. Vossmeyer, L. Katsikas, M. Giersig, I. G. Popovic, K. Diesner,A. Chemseddine, A. Eychmuller and H. Weller, J. Phys. Chem. B,1994, 98, 7665–7673.

187 A. L. Rogach, A. Kornowski, M. Y. Gao, A. Eychmuller andH. Weller, J. Phys. Chem. B, 1999, 103, 3065–3069.

188 N. Gaponik, D. V. Talapin, A. L. Rogach, K. Hoppe,E. V. Shevchenko, A. Kornowski, A. Eychmuller and H. Weller,J. Phys. Chem. B, 2002, 106, 7177–7185.

189 M. T. Harrison, S. V. Kershaw, M. G. Burt, A. Eychmuller,H. Weller and A. L. Rogach, Mater. Sci. Eng., B, 2000, 69, 355–360.

190 A. Rogach, S. Kershaw, M. Burt, M. Harrison, A. Kornowski,A. Eychmuller and H. Weller, Adv. Mater., 1999, 11, 552–555.

191 C. A. Leatherdale, C. R. Kagan, N. Y. Morgan, S. A. Empedocles,M. A. Kastner and M. G. Bawendi, Phys. Rev. B, 2000, 62, 2669–2680.

192 J. H. Warner, A. R. Watt, E. Thomsen, N. Heckenberg, P. Meredithand H. Rubinsztein-Dunlop, J. Phys. Chem. B, 2005, 109, 9001–9005.

193 M. Anni, L. Manna, R. Cingolani, D. Valerini, A. Creti andM. Lomascolo, Appl. Phys. Lett., 2004, 85, 4169–4171.

194 N. C. Greenham, X. G. Peng and A. P. Alivisatos, Phys. Rev. B:Condens. Matter, 1996, 54, 17628–17637.

195 S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr,E. J. D. Klem, L. Levina and E. H. Sargent, Nat. Mater., 2005, 4,138–142.

196 G. Konstantatos, C. Huang, L. Levina, Z. Lu and E. H. Sargent,Adv. Funct. Mater., 2005, 15, 1865–1869.

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

198 O. Solomeshch, A. Kigel, A. Saschiuk, V. Medvedev, A. Aharoni,A. Razin, Y. Eichen, U. Banin, E. Lifshitz and N. Tessler, J. Appl.Phys., 2005, 98, 074310.

199 D. S. Ginger and N. C. Greenham, Synth. Met., 1999, 101, 425–428.200 D. S. Ginger and N. C. Greenham, Phys. Rev. B: Condens. Matter

Mater. Phys., 1999, 59, 10622–10629.201 D. S. Koktysh, N. Gaponik, M. Reufer, J. Crewett, U. Scherf,

A. Eychmuller, J. M. Lupton, A. L. Rogach and J. Feldmann,ChemPhysChem, 2004, 5, 1435–1438.

202 Y. Li, A. Rizzo, M. Mazzeo, L. Carbone, L. Manna, R. Cingolaniand G. Gigli, J. Appl. Phys., 2005, 97, 113501.

203 N. Tessler, V. Medvedev, M. Kazes, S. H. Kan and U. Banin,Science, 2002, 295, 1506–1508.

204 B. T. Nguyen, J. E. Gautrot, C. Ji, P.-L. Brunner, M. T. Nguyen andX. X. Zhu, Langmuir, 2006, 22, 4799–4803.

205 E. Arici, D. Meissner and N. S. Sariciftci, Encyclopedia ofNanoscience and Nanotechnology, ed. H. S. Nalwa, AmericanScientific Publishers, Valencia, CA, 2004, vol. 3, pp. 929–944.

206 N. C. Greenham, X. Peng and A. P. Alivisatos, Phys. Rev. B:Condens. Matter, 1996, 54, 17628–17637.

207 B. Sun and N. C. Greenham, Phys. Chem. Chem. Phys., 2006, 8,3557–3560.

208 W. U. Huynh, X. Peng and A. P. Alivisatos, Adv. Mater., 1999, 11,923–927.

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

210 P. Wang, A. Abrusci, H. M. P. Wong, M. Svensson,M. R. Andersson and N. C. Greenham, Nano Lett., 2006, 6, 1789–1793.

211 M. Y. Odoi, N. I. Hammer, K. Sill, T. Emrick and M. D. Barnes,J. Am. Chem. Soc., 2006, 128, 3506–3507.

212 D. J. Milliron, A. P. Alivisatos, C. Pitois, C. Edder andJ. M. J. Frechet, Adv. Mater., 2003, 15, 58–61.

213 C. Querner, A. Benedetto, R. Demadrille, P. Rannou and P. Reiss,Chem. Mater., 2006, 18, 4817–4826.

214 C.-H. Chou, H.-S. Wang, K.-H. Wei and J. Y. Huang, Adv. Funct.Mater., 2006, 16, 909–916.

215 R. C. Advincula, Dalton Trans., 2006, 2778–2784.216 J. Locklin, D. Patton, S. Deng, A. Baba, M. Millan and

R. C. Advincula, Chem. Mater., 2004, 16, 5187–5193.217 A. Javier, C. S. Yun, J. Sorena and G. F. Strouse, J. Phys. Chem. B,

2003, 107, 435–442.

J. Mater. Chem., 2008, 18, 1064–1078 | 1077

Dow

nloa

ded

by H

einr

ich

Hei

ne U

nive

rsity

of

Due

ssel

dorf

on

16 M

arch

201

3Pu

blis

hed

on 1

0 Ja

nuar

y 20

08 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B71

2176

H

View Article Online

218 R. van Beek, A. P. Zoombelt, L. W. Jenneskens, C. A. van Walree,C. D. M. Donegla, D. Veldman and R. A. J. Janssen, Chem.–Eur. J.,2006, 12, 8075–8083.

219 V. L. Colvin, M. C. Schlamp and A. P. Alivisatos, Nature, 1994, 370,354–357.

220 B. O. Dabbousi, M. G. Bawendi, O. Onitsuka and M. F. Rubner,Appl. Phys. Lett., 1995, 66, 1316–1318.

221 M. C. Schlamp, X. G. Peng and A. P. Alivisatos, J. Appl. Phys.,1997, 82, 5837–5842.

222 H. Mattoussi, L. H. Radzilowski, B. O. Dabbousi, E. L. Thomas,M. G. Bawendi and M. F. Rubner, J. Appl. Phys., 1998, 83, 7965–7974.

223 S. Coe, W. K. Woo, M. Bawendi and V. Bulovic, Nature, 2002, 420,800–803.

224 M. Y. Gao, B. Richter and S. Kirstein, Adv. Mater., 1997, 9, 802–805.

225 M. Y. Gao, C. Lesser, S. Kirstein, H. Mohwald, A. L. Rogach andH. Weller, J. Appl. Phys., 2000, 87, 2297–2302.

226 V. Medvedev, M. Kazes, S. Kan, U. Banin, Y. Talmon andN. Tessler, Synth. Met., 2003, 137, 1047–1048.

227 T.-W. F. Chang, S. Musikhin, L. Bakueva, L. Levina, M. A. Hines,P. W. Cyr and E. H. Sargent, Appl. Phys. Lett., 2004, 84,4295–4297.

228 E. H. Sargent, J. Mod. Opt., 2004, 51, 2797–2803.

1078 | J. Mater. Chem., 2008, 18, 1064–1078

229 W. U. Huynh, J. J. Dittmer, W. C. Libby, G. L. Whiting andA. P. Alivisatos, Adv. Funct. Mater., 2003, 13, 73–79.

230 L. Wang, Y. Liu, X. Jiang, D. Qin and Y. Cao, J. Phys. Chem. C,2007, 111, 9538–9542.

231 C.-M. Yang, C.-H. Wu, H.-H. Liao, K.-Y. Lai, H.-P. Cheng,S.-F. Horng, H.-F. Meng and J.-T. Shy, Appl. Phys. Lett., 2007,90, 133509.

232 D. H. Cui, J. Xu, T. Zhu, G. Paradee, S. Ashok and M. Gerhold,Appl. Phys. Lett., 2006, 88, 183111–183113.

233 A. Watt, T. Eichman, H. Rubinsztein-Dunlop and P. Meredith,Los Alamos Nat. Lab., Prepr. Arch., Condens. Matter, 2004, 1–7.

234 A. Watt, T. Eichmann, H. Rubinsztein-Dunlop and P. Meredith,Appl. Phys. Lett., 2005, 87, 253109.

235 H.-S. Chen, S.-R. Chung, G.-Y. Chang and S.-J. Tsai, US Pat.Appl., US2006-515031, 2007.

236 S. R. Forrest, PCT. Int. Appl., WO2006-US46946, 2007.237 L. Levina, E. Sargent, P. Cyr, S. Zhang and S. McDonald, Can. Pat.

Appl., CA2005-2519608, 2006.238 E. Sargent, G. Konstantatos, L. Levina, I. Howard, E. J. D. Klem

and J. Clifford, US Pat. Appl., US2006-510263, 2007.239 J. W. M. Chon, M. Gu, C. Bullen and P. Mulvaney, Appl. Phys.

Lett., 2004, 84, 4472–4474.240 B. Fisher, J. M. Caruge, D. Zehnder and M. Bawendi, Phys. Rev.

Lett., 2005, 94, 087403.

This journal is ª The Royal Society of Chemistry 2008