hybrid solar cells: basic principles and the role of ligands
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Hybrid solar cells: basic principles and the role of ligands
Adam J. Moule,* Lilian Chang, Chandru Thambidurai, Ruxandra Vidu and Pieter Stroeve
Received 27th September 2011, Accepted 10th November 2011
DOI: 10.1039/c1jm14829j
For the last decade, researchers have attempted to construct photovoltaic (PV) devices using a mixture
of inorganic nanoparticles and conjugated polymers. The goal is to construct layers that use the best
properties of each material e.g., flexibility from the polymer and high charge mobility from the
nanoparticles or blue absorbance from the polymer complementing red absorbance from the
nanoparticles. This critical review discusses the main obstacles to efficient hybrid organic/inorganic PV
device design in terms of contributions to the external and internal quantum efficiencies. We discuss in
particular the role that ligands on the nanoparticles play for mutual solubility and electronic processes
at the nanoscale. After a decade of work to control the separation distance between unlike domains and
the connectivity between like domains at the nanoscale, hybrid PV device layers are gaining in
efficiency, but the goal of using the best properties of two mixed ma
photovoltaics. There are other types of PVs such as single and
ve also captured a significant
f their high power conversion
n 2004, Lilian Chang graduated
rom the Science University of
alaysia with a BS in Chemical
ngineering. She completed her
S in Chemical Engineering at
an Jose State University, CA
2008. For her MS disserta-
Dynamic Article LinksC
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efficiency (PCE). Hybrid solar cells have a low PCE of about
3%,4 but they are a potentially strong candidate in the PVmarket
because of high optical absorption, low production cost, easy
fabrication and major pay back.5This device type is called hybrid
because the active layer consists of a blend of electron donor
polymer and electron acceptor inorganic nanostructures
(CdSe,4,615 CdTe,13,16 CdS,17,18 CuInS2,19 PbSe,2022 PbS,23,24
TiO2,2529 ZnO3039). These inorganic semiconductor nano-
structures have different structures: nanorods, nanoparticles
(NPs), branched structures (e.g., hyperbranched and tetrapods),
etc. They have been widely studied in the literature for better
optical absorption and improved pathways for electron transport
in the bulk-heterojunction (BHJ) architecture.40,41 The BHJ
concept employs a mixed electron donor and electron acceptor
active layer with charge-separating heterojunctions dispersed
throughout the layer. The ideal BHJ morphology contains
domains that are smaller than the typical exciton dissociation
length.
In this review paper, we focus in detail on hybrid BHJ solar
cells, especially on its basic principles, the synthesis of inorganic
NP, and the effect of ligands. Several reviews on a broader
subject of organic solar cells have been already published
elsewhere.4247
Mechanism of operation of hybrid solar cells
The mechanism of operation for inorganic/organic hybrid solar
cells is quite complex. The mechanism can be broken up into
a number of steps, as shown in Fig. 1, for which the efficiency (h)
is represented by a number ranging from 0 to 1. Successful
operation of an organic PV (OPV) device requires that most or
all of the steps have h close to 1. The overall efficiency to convert
incident photons to current, known as the external quantum
efficiency (EQE), can be written as
Chandru Thambidurai
Chandru Thambidurai received
his BTech degree in Chemical
and Electrochemical Engi-
neering from Central Electro-
chemical Research Institute
(CECRI), India in 2004. He
then received his PhD in Chem-
istry from the University of
Georgia in 2009 under the guid-
ance of Prof. John L. Stickney.
During his PhD, he focused on
Electrochemical Atomic Layer
Deposition for various metal and
semiconductor thin films. He
then carried out his post-
doctoral research at the Univer-
sity of California, Davis under the supervision of Prof. Pieter
Stroeve. He is currently a process development engineer at Copper
Interconnect Group, Applied Materials. His research interest
includes electrochemistry, electroplating and solar cells.
Dr Ruxandra Vidu is the Asso-
ciate Director of the California
Solar Energy Collaborative at
UC Davis. She is a published
research scientist and university
lecturer with 20 years of expe-
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View OnlineRuxandra Vidu
rience in Chemical Engineering
and Materials Science, with
extensive expertise in thin-films
and nanostructure fabrication
and characterization for solar
materials. Dr Vidus research
emphasizes the use of nano-
science and nanotechnology in
the development of new mate-
rials and processes. Her areas of
interest include electrochemistry, surface modification and char-
acterization, electrochemical alloying and doping, ultra-thin film
systems and template synthesis of nano-structured materials for
thermoelectric, battery technology, fuel cells and solar energy
related applications.2352 | J. Mater. Chem., 2012, 22, 23512368EQE(l, V) hA(l) hdiff hdiss(V) htr(V) hcc(V) (1)
where l is the wavelength of the incident light andV is the voltage
across the cell. hA(l) is the photon absorption yield or fraction of
incident photons absorbed in the active layer as a function of l as
illustrated in Fig. 1(a). Factors such as absorption spectra of the
hybrid organic/inorganic layer, the active layer thickness, and
device architecture determine the photon absorption yield.48,49 In
general, increased absorbance or increased thickness of the active
layer also increases hA(l).
hdiff (Fig. 1(b)) is the exciton diffusion yield, which is the ratio
of excitons that diffuse to a heterojunction without recombina-
tion to the total number of generated excitons. This efficiency can
be increased by either increasing the exciton diffusion length or
reducing the average donor/acceptor distance within the active
layer.50
hdiss(V), as shown in Fig. 1(c), is the exciton dissociation yield,
which is the ratio of the number excitons that dissociate to free
charges at a donor/acceptor interface to the total number of
excitons at donor/acceptor interfaces. Dissociation is dependent
Pieter Stroeve
Pieter Stroeve is a Distinguished
Professor of Chemical Engi-
neering and Materials Science
at the University of California,
Davis. He was born in The
Netherlands and immigrated to
the USA with his family when he
was 14 years old. Professor
Stroeve conducts fundamental
research in colloid and surface
science, self-assembled mono-
layers, LangmuirBlodgett
films, supramolecular structures
on surfaces, supported lipid
bilayers, mass transport in
colloids and tissues, nanotech-
nology, bionanotechnology, electrochemistry, solar energy and
nanoporous membrane separations.This journal is The Royal Society of Chemistry 2012
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View Onlineon an internal electric field generated by differences between the
lowest unoccupied molecular orbital (LUMO) of the donor and
the electron affinity of the acceptor. If dissociated charges remain
weakly bound at the interface they are referred to as charge
transfer (CT) states.5153 If mobile dissociated charges are
generated, they are referred to as charge separated (CS) states.54
hdiss measures both the direct formation of CS and CT states that
separate to CS states. The energy required to dissociate the
electronhole pair in the interface for organic solar cell is higher
than its binding energy, 400 meV, when compared to a few meV
in inorganic semiconductors. This is because of enhanced
Fig. 1 Illustration of the consecutive steps, from (a) to (e), for photocu
absorption of a photon. Excitons can be generated in either the donor (
orbital, HOMO highest occupied molecular orbital, CB conductioncoulombic attraction between holes and electrons due to the low
electric permittivity and localized electron and hole wave func-
tions in organic semiconductors. The dissociation efficiency can
be increased by increasing the potential difference between the
donor LUMO and acceptor electron affinity. This can occur due
to a high total electric field across the device or by designing
morphological features that increase the distance between the
electron and hole. Field enhancement increases the efficiency of
the CT/CS process, but since excitons are uncharged particles,
the exciton separation probability is not field dependent. In many
studies hdiss and hdiff are treated as one term since measurement
of the formation of polaron states using transient absorption is
typically used to determine dissociation efficiency.55
htr(V), shown in Fig. 1(d), is the charge transport yield, which
is the ratio of the number of free charge carriers transported to
the collecting electrode to the number of excitons dissociated at
the heterojunction. Structural defects and impurities in the active
layer can cause the charge carriers to become trapped and to
recombine, which reduces the transport efficiency. htr can be
increased by increasing the domain size and crystallinity of both
the donor and acceptor components.56 htr is also increased by
reducing energetic barriers between like domains in the active
layer. Finally, htr is increased by applying a reverse bias on the
device.
This journal is The Royal Society of Chemistry 2012hcc(V) is the charge collection yield, as illustrated in Fig. 1(e),
which is the ratio of the number of charge carriers collected
externally to the number of the charge carriers transported to the
electrode. With Ohmic electrodes, hcc(V) is typically 1.
The power conversion efficiency (PCE) is defined as the ratio
of the electrical power output to the absorbed sunlight energy
PCE VocJscFFPin
(2)
where Voc is the open circuit voltage, FF is the filling factor, Jsc is
the short circuit current density, and Pin is the incident optical
t generation in inorganic/organic hybrid solar cells, beginning from the
mer) or acceptor (NP) phase. (LUMO lowest unoccupied moleculard, VB valence band).power on the device assuming an AM1.5G solar spectrum.
Because the Jsc is maximized by increasing the EQE across the
absorbance spectrum of the active layer, eqn (1) and (2) are
explicitly related. The PCE takes into account not only the
quantum efficiency at zero bias, but also the energetic efficiency
of converting a spectrum of photon energies to photocurrent at
all potentials. In this article we will discuss how morphology and
fabrication of hybrid organic/inorganic devices affect the various
efficiency terms in eqn (1) and ultimately the PCE. For the scope
of this article we will focus primarily on the role that ligands on
the surface of the inorganic phase play on device efficiency in
a BHJ configuration.
State-of-the-art in hybrid organic/inorganic PV devices
Intensified research on hybrid organic/inorganic PV devices
began with the publication of a Science article by Huynh et al.
that demonstrated a PCE of over 2% using a mixture of CdSe
nanoparticles (NPs) and P3HT as the active layer in a BHJ
architecture.57 Since then, a number of articles and review articles
have been published that attempted to increase the PCE by
designing improved morphology for the active layer.11,14,58,59
Fig. 2 shows an energy level diagram that summarizes the
operation of a CdSe-NP/P3HT PV device. Excitation occurs at
J. Mater. Chem., 2012, 22, 23512368 | 2353
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either the NP or polymer phase. The resulting excitons then
diffuse to an NP/polymer interface for dissociation into free
charges. Indium tin oxide (ITO) is the most widely used trans-
parent conducting oxide (TCO) in low-temperature deposition
processes because it has relatively high optical transmission for
a given specific sheet resistance. In this configuration, ITO serves
as the hole-collecting electrode while electrons are extracted at
the aluminium (Al) back electrode. PEDOT:PSS is a transparent,
conductive polymer which functions as a hole transport layer
(HTL). A flow diagram of the processes involved in the fabri-
cation of a typical hybrid BHJ device is shown in Fig. 3. The
versatility offered by a solution-processable route makes it an
attractive technology for producing low-cost printable PV
devices. The major advantages and disadvantages of this device
type are:
(a) Advantageboth electron donor (P3HT) and acceptor
(CdSe) components contribute to light absorption, which
increases hA.
(b) Advantagethe identity of the inorganic component can
be altered to increase Voc.
(c) Advantagethe shape of the NPs can be altered to increase
the surface area, which can increase hdiff, hdiss, and htr.(d) Advantagefabrication is simple with both components
sc
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assembled.
(e) Disadvantagethe phase separation in self-assembled
morphologies is usually too large.
(f) Disadvantagethe solubility of NPs is limited, even with
larger ligands.
(g) Disadvantagethe ligands that provide solubility for the
NPs are usually insulating, which reduces hdiss by increasing the
D/A distance, reduces htr by creating insulating barriers between
electron conducting domains, and reduces hcc by creating insu-
lating barriers between the NPs and the electron collecting
electrode. The presence of this last barrier is often observed as an
S-kink in the IV characteristics.60
Fig. 2 Energy diagram and schematic of a CdSe/P3HT device.
Reprinted with permission from reference 11. Copyright 2009 Elsevier
Science.2354 | J. Mater. Chem., 2012, 22, 23512368tion, smaller particles dissolve to support the growth of larger
ones which leads to a spherical shape. The other crucial factor
that influences shape control is the ratio of surfactants to
injection volume.
Effect of shape on polymer/nanocrystal photovoltaics
Shape control of the NPs in hybrid BHJ devices has proven to be
critical in improving efficiency.75 It was found that EQE was
much lower in films with spherical particles than with elongated
particles.6 The presumed reasons for the higher EQE with elon-
gated particles are:
(a) Increased electron mobility with larger domain sizes for
elongated particles (increased htr)
(b) Reduced geminate recombination of electrons and holes
(increased hdiss)
(c) Reduced number of junctions between particles (increased
htr).with differing band gaps and with differing lamp spectra. As
described in the Introduction, EQE can be broken into at least
five consecutive steps and the product of all defines the external
quantum efficiency.59,66
Light-absorbing nanostructures
One of the most popular methods for the preparation of inor-
ganic crystals is chemical synthesis. The formation of various
nanocrystals such as CdSe,4,12,57,67 PbSe,68 PbS,69 CuS,70
CuInSe219 using solution phase synthesis has been reported for
use in BHJ solar cells. The main advantage of solution phase
over gas phase71 and solid state synthesis is the ability to isolate
the materials, which facilitates incorporation of materials into
the BHJ layer in post-synthetic steps. Semiconductor nano-
crystals can be prepared by thermal decomposition of hot
organometallic precursors in a hot mixture of trioctylphosphine
oxide (TOPO), tributyl- or trioctylphosphine (TOP) or hex-
ylphosphonic acid (HPA) and small amounts of various phos-
phonic acids as strong ligands.7274 The growth kinetics and
shape of the nanocrystals are mainly determined by the Gibbs
Thompson law, i.e., the solubility of crystals increases as the
size of the crystals decreases. It was demonstrated that the
monomer concentration controls the formation of CdSe nano-
crystals with various shapes: rod, arrow, teardrop, and tetrapod
(Fig. 4). That is, at higher monomer concentration, smaller
particles grow faster than larger particles which can lead to
anisotropic shapes.72,75 On the other hand, at lower concentra-A second well-studied device type involves the creation of
a porous or high surface area contiguous inorganic phase that is
filled with an organic hole conductor. This device architecture
has been discussed elsewhere.6165
Although the general operation of hybrid PV is simple, their
mechanism of converting light into energy is complicated. There
are many factors that affect device performance which may
explain why the maximum efficiency attained is only 3%.
Therefore, a better understanding of the charge separation
mechanism in hybrid PV devices becomes very important. The
focus of discussion in this review is mainly on EQE since it offers
a more universal comparison across devices made in various
laboratories. It makes little sense to compare J between devicesThis journal is The Royal Society of Chemistry 2012
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View OnlineIn several studies of NP polymer mixtures, it was found that
the number density of NP acceptors needed to effectively quench
the exciton emission from polymers is much lower than the
percolation threshold for electron transport though the NPs.26,76
In fact once mixtures of NPs and polymers were prepared into
optimized devices, the NP volume ratio is always quite
high.19,31,32,34,7779 Finally, it was found that increased particle
Fig. 3 Flow diagram of the processes in thelength leads to a higher EQE in PV devices made up of a mixture
of CdSe NPs and P3HT.40 The combination of these results
showed that the device PCE was limited most by a poor
connection between NPs across the device since they demon-
strate efficient charge separation (increased hdiff and hdiss) but
poor charge transport (decreased htr).80
One method to improve charge transport and to cause elon-
gated NPs to align in a vertical configuration is to synthesize
branched particles8 and tetrapods.13,67 Tetrapod-shaped nano-
structures with core and arms of different materials are synthe-
sized with the seeded growth method.81,82 The tetrapod shape
gives direct pathways for electrons to be transported across the
active layer to a carrier-selective electrode. The tetrapods align
with three arms touching the cathode material and the fourth
arm pointed upwards through the active layer thickness. The
Fig. 4 CdSe nanocrystals with 7 7 nm (a), 7 30 nm (b), 7 60 nm dimreference 57. Copyright 2002 American Association for the Advancement of
This journal is The Royal Society of Chemistry 2012core seed is the nucleation of cubic spheralites72,74 and with the
injection of precursors and hot mixture of surfactants, it extends
along the hexagonal phase, where the wurtzite starts to dominate
four out of eight (111) facets, forming the tetrapod structure
(Fig. 5). These structures are developed with different core and
arm materials which is an improvement over single materials83
because it permits us to finely tune the material properties. The
brication of a typical hybrid BHJ solar cell.alkyl chain phosphonic acids selectively stabilize the non-polar
lateral facets of CdSe and CdTe wurzite structures. The materials
used for core and arm are listed in Fig. 5(a).
PV devices prepared from a mixture of polymer and branched
inorganic NPs showed a clear increase in the EQE when
compared to devices prepared with inorganic rods.13 The three
dimensional shape is much more effective at transporting charges
across the BHJ layer. Gur et al. showed that although hyper-
branched CdSe improves the percolation network (increased htr),
the donor polymer is unable to penetrate the branches (decreased
hdiss),8 which emphasizes the need for optimization of the blend
morphology, and not just the morphology of either the acceptor
or the donor. In some cases with the tetrapods, it was necessary
to prepare a hole-transport only layer between the anode and the
BHJ layer as the tetrapods are large enough to short-circuit the
ensions (c) and its respective EQE (d). Reprinted with permission from
Science (AAAS).
J. Mater. Chem., 2012, 22, 23512368 | 2355
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View OnlineFig. 5 Sketch represents the formation of tetrapod shaped nanocrystals
from the seeded growth approach. (a) Reprinted with permission from
reference 81. Copyright 2009 American Chemical Society. (b) Repro-
duced with permission from Macmillan Pulishers Ltd., reference 83,
copyright 2003.device between the two electrodes without the hole only layer to
force rectification.67 Ultimately, the solubility of tetrapods in
conjugated polymers is quite low, leading to phase separation on
a length scale larger than the exciton diffusion length. For this
reason, the EQE of solution cast mixtures of NPs and polymers
generally remains less than 50%.34,67 In 2010, however, a hybrid
solar cell employing the use of pyridine-capped CdSe tetrapods
with a low band gap polymer, PCPDTBT, achieved a PCE of
3.19% with a maximum EQE of 55% in the range of 630720
nm.4 The use of a low band gap polymer allows efficient har-
vesting of the red and near-IR regions of the solar spectrum,
increasing hA, while tetrapod-CdSe increases htr. One study
showed vertical phase-separation in an OC1C10-PPV/tetrapod-
CdSe blend with the tetrapod-rich layer near the cathode and the
polymer-rich layer near the anode, hence increasing hcc.15
Ternary tetrapod nanocrystals, such as CdSexTe1x, have alsobeen explored,13 providing another parameter besides nano-
crystal size to tune the energy levels of the acceptor by changing
the Se or Te content.
Effect of ligands on polymer/nanocrystal photovoltaics
Inorganic semiconductor NPs are typically in the range of 1100
nm. The tunability of the semiconductors optical band gap by
controlling the nanostructure diameter is valuable for nanoscale
photonic, light emitting diode (LED) and photovol-
taics.7,10,57,8498 Changing the diameter of the inorganic nano-
structures modifies the semiconductor absorption range and also
the effective ionization energy and electron affinity of the
2356 | J. Mater. Chem., 2012, 22, 23512368particles. This is made possible by precise tuning of the energy of
discrete electronic states and optical transitions99101 using the
quantum confinement effect. Brabec et al. has shown a direct
correlation of Voc to the electron affinity of a series of fullerene
derivatives in polymer/fullerene BHJ solar cells.102 According to
this study, the maximum achievable Voc for a polymer/fullerene
BHJ solar cell can be approximated based on the energy differ-
ence between the HOMO of the donor and the LUMO of the
acceptor.102 Band gap tunability allows for optimization of the
relative LUMO/VB gap between the polymer and NP and can be
used to increase the Voc of a device. The disadvantage is that an
NP sample must have a narrow size range or larger particles will
act as charge traps due to a locally lower band gap. Fig. 6 shows
how the absorbance of CdSe103 and PbSe69 can be changed by
altering the NP diameter.
NPs are typically formed in solution and their size is deter-
mined by controlling the ratio of inorganic precursors to ligands
and the temperature of formation.72,75 Capping ligands cover the
surface of the nanoparticle and control the solubility of nano-
crystals in solution. Ideally, ligands should possess head groups
that have high affinity to nanocrystals and end groups that
provide for solubility. In an NP/polymer solution, ligands are
used to solvate the NP in a good solvent for the polymer and also
to provide solubility in the polymer matrix. Photoexcited NPs
will form an exciton, with the hole on trap sites at the NP surface,
or a charge separated complex is formed that removes the hole
completely from the NP.105
Besides the solubility of NP in solution, the choice of ligands
strongly affects the degree of phase separation and ultimately the
film morphology of NP/polymer solution cast films. The poly-
merNP morphology strongly influences hdiff, hdiss, htr, and hcc:
hdiff, since the ligand group determines the distance andmedium through which charge separation occurs;
hdiss, since the ligand can change the donor and acceptordistance and tunneling is exponentially dependent on distance;
htr, since the ligand size will determine the probability ofcharge hopping between particles; and
hcc, since the ligand size will determine the probability ofcharge hopping between a particle and the electrode.
To a much smaller extent, hA is also affected by the
morphology. The absorbance spectrum in the polymer depends
on the degree of stacking and the polymer domain size. Addition
of NPs can reduce the degree of order in the polymer, changing
the shape of the absorbance spectrum and hence hA. We discuss
here how the ligand type affects the various efficiencies, sepa-
rating morphological effects from other effects due directly to the
structure of ligand.
The list of frequently used organic capping ligands includes
alkyl thiols,106109 amines,110 phosphines, phosphine oxides,111
and carboxylic acids (Fig. 7). Ligands have a big impact on the
electrical performance of the NPs. Ligands that act as electrical
insulators with large band gaps can severely reduce device effi-
ciency by impeding charge transport between nanocrystals
(reduced htr) and reducing charge separation at semiconducting
polymerNP interfaces (reduced hdiss). Some examples of insu-
lating ligands are long alkyl chain ligands (commonly oleic acid
(OA) and trioctylphosphine oxide (TOPO)), which are typically
used during NP synthesis to provide controlled growth condi-
tions with dispersed nanocrystals. Poor device performance isThis journal is The Royal Society of Chemistry 2012
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seen in TOPO-capped CuInSe2 NPs blended with P3HT in a BHJ
architecture.19 The devices suffer from low Jsc due to the presence
of the insulating ligand, although theVoc and FF delivered by the
devices were adequate. As further evidence, the IV results also
Fig. 6 Absorption spectra of (a) CdSe semiconductor nanocrystals ranging
Copyright 2010 American Chemical Society. (b) PbSe quantum dots spanning
red-shifted), showing quantum confinement and size tenability. Copyright 200
from reference 69.
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increased loading of TOPO-capped CuInSe2. Researchers haveFig. 7 Ligands commonly used to cap NPs in a hybrid solar cell.
This journal is The Royal Society of Chemistry 2012also looked into replacing insulating ligands on NPs with ligands
that favor electron transport or are less insulating. Wang et al.
found that when hexadecylamine (HDA) ligands on CdS nano-
rods were replaced with pyridine, the Jsc of MEH-PPV/CdS solar
cells improved.18 This is likely because of increased hdiss and htrdue to more intimate contact with the donor polymer and
between nanorods.
This realization spurred research into nano-crystal surfaces
that are free of surfactant, which can be obtained using weak
binding ligands that can be removed through solvent evapora-
tion either by thermal treatment (because of its high volatile
from 1.8 nm to 20 nm. Reprinted with permission from reference 104.
a range of tunable sizes from smallest (most blue-shifted) to largest (most
3 Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permissioncharacter),9 grafting,112114 or aging.105 Complete removal of the
surfactant ligands would provide direct contact between the
nanocrystals and the polymer.59
Besides ligands listed in Fig. 7, novel ligands that have been
explored are conjugated organic molecules such as oligo (phe-
nylene vinylene), shown in Fig. 8, which allows charge transport
while preventing NP aggregation. In the same vein, other inno-
vations include the modification of conjugated polymer (donor
component) to have end groups that interact with the NPs and
effectively disperse the NPs while maintaining intimate contact
between donor and acceptor10 or using the donor polymer itself
as a ligand on the NPs.23 In the former, a PCE of 1.5% was
obtained for P3HT/CdSe solar cells using P3HT modified with
amino acid end groups.10 Amino acid-terminated P3HT partially
replaces the pyridine ligand on the CdSe nanorods, enhancing
miscibility of the two components and increasing hdiss. In the
latter, the lack of a percolation pathway for electrons was blamed
for low Jsc since the NPs were surrounded by the donor polymer,
decreasing htr.23
As previously discussed, the ligand group dictates the NP
solvation properties and their miscibility in a particular medium.
When long chain alkyl ligands are injected into a hot reaction of
metalorganic precursors, homogeneous nucleation and
a narrow size distribution of hydrophobic nanocrystals115 occur,
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View Onlinealong with the formation of surface defects due to dangling
bonds that are believed to be the origin of charge carrier trap
sites. Traps density can be reduced by using the aforementioned
weak binding organic ligands (commonly pyridine106,115,116 and
butylamine11) which can be introduced via surface ligand
exchange reactions. Alternatively, surface traps can be passiv-
ated using core/shell NP systems by growing additional layers of
higher band gap shell material over the core nanocrystals (CdSe/
ZnS, CdSe/CdS).108,110,117121 The large band gap shell passivates
the cationic and anionic dangling bonds on the nanocrystals.
Since the focus of this review is on the effects of organic ligands
on nanocrystals with regards to hybrid PV devices, the effect of
inorganic surface modification is beyond the scope of this review
and will not be addressed.
Aldakov et al. and Olson et al. studied the effect of various
ligands on CdSe NPs on the morphology and IV characteristics
of P3HT/CdSe11,58 and MDMO-PPV/CdSe58 BHJ PV devices.
The goal was to mimic the improved performance and electron
transport achieved using branched CdSe particles by forming
well ordered CdSe NP domains. Both groups noted an intimate
correlation between the choice of ligands, the degree of phase
separation, and device performance. Parameters important for
affecting the morphology between P3HT and CdSe are the
Fig. 8 Schematic diagram of 4 nm CdSe dots covered with a monolayer
of ligand (oligo (phenylene vinylene)) attached to the particle surface.
Reprinted with permission from reference 122. Copyright 2007 Springer.nanocrystal/polymer ratio, processing time, and ligand exchange
efficiency. CdSe particles were synthesized using TOPO ligands
that were later replaced by stirring the nanocrystals in an excess
of a different capping ligand under reflux for 2448 h. Both
groups agreed that nanorod-based devices display better
PCE due to better charge transport, and that a higher NP con-
tent within the active layer increases PCE likely due to the
formation of a percolation pathway for electrons to the cathode
(increased htr).
Olson et al.11 found that butyl amine-capped CdSe particles
with a w/w mixing ratio of 12 : 1 (CdSe : P3HT) and post heated
at 110 C produced the device with the highest PCE. Theirdevices, displayed in Fig. 9, show a large range in PCE that is
matched by widely varying domain sizes. The smallest domains
result in the highest efficiency. This correlation shows that by far
the largest effect of ligands on the CdSe particles is the control of
NP solubility and the formation of domains with a size scale
approaching the exciton diffusion length. This means that hdiff is
2358 | J. Mater. Chem., 2012, 22, 23512368the efficiency parameter being adjusted. Reduction of the domain
size and thereby the diffusion distance increases hdiff. Further
decreases in the domain size of P3HT/CdSe could be gained by
alteration of the solvent. When a solvent that is equally good/
poor for the polymer and NPs is used, the domain size is
reduced.9 For the mixture of P3HT/CdSe, it was found that
a mixture of pyridine and chloroform, both non-ideal solvents
for the polymer reduced the polymer solubility to the level of the
CdSe particles. The solubility of the CdSe particles can be
improved by adding excess ligand to the solvent, which in this
case was pyridine. A strong enhancement of the photocurrent
was achieved after heating, due to removal of both interfacial
and excess pyridine, which acts as a non-radiative recombination
site for excitons in the polymer.
Another method for controlling the morphology of polymer/
NP is the use of thermally cleavable solubilizing ligands. Seo
et al. replaced the typical TOPO ligand on CdSe nanocrystals
with tert-butyl N-(2-mercaptoethyl) carbamate ligands123
(Fig. 10). Again CdSe particles with the new capping group were
prepared using a solvent exchange reaction. Then the mixture of
end-capped particles and polymer was coated onto the substrate.
Before heating, there was little electrical contact between CdSe
particles and no photocurrent could be measured. After heating
to above 200 C, the ligand was thermally cleaved with isobuteneand carbon dioxide evaporating from the film. The remaining
2-mercaptoethylamine ligands were considerably smaller than
the original ligand and the electrical characteristics of the device
improved considerably due to improved contact between CdSe
particles and also between CdSe and the polymer. They show an
increase in PCE from 0.21% to 0.44% by increasing the heat-
treatment temperature from 150 C to 250 C, largely due toincrease in Jsc and FF. Practically, this concept is less successful
than replacement of TOPO with pyridine because the thermal
cleaving of the carbamate ligands occurred at a higher temper-
ature compared to the temperature required for the removal of
pyridine. The choice of treatment temperature is critical because
the Tg of P3HT is well below 200C.124126 Hence even if the
carbamate ligand greatly reduces phase separation, the high
heat-treatment temperature is likely to reintroduce large scale
phase separation due to the high mobility of P3HT, which is why
PCE is still low in their best devices.
Alternatively, the capping ligand can be removed completely
by using ligands with a low boiling point and weak attachment to
the NP.9 The main expected advantage in removing the capping
ligand completely is that the NPs will come into closer physical
contact, reducing the barrier for charge transport between
particles and thereby increasing htr. hdiss should also increase due
to better contact with the donor. Fig. 11 shows a cartoon of how
solvent removal reduces the distance between NPs. As a case in
point, an increase in EQE maximum to 59% was obtained for
P3HT/CdSe solar cells after heat treatment at 120 C to removeinterfacial pyridine.9
As a testament of the importance of morphology control in
polymer/nanoparticle hybrid devices, a group demonstrated that
P3HT/CdSe solar cells can achieve a maximum EQE of 70% by
using a high boiling point solvent, 1,2,4-trichlorobenzene (TClB)
for P3HT, and pyridine-capped CdSe nanorods.12 TClB allows
P3HT to self-organize into ordered structures in the presence of
pyridine and CdSe nanorods. The active layer comprises P3HTThis journal is The Royal Society of Chemistry 2012
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View Onlinenanofibers and CdSe nanorods that provide percolation path-
ways for both electron and holes, enhancing htr. Potentially,
some combination of increasing electron transport, through
perhaps the tetrapod structure, and increasing hole transport
through improved ordering of the polymer domain can propel us
forward in the race for record efficiencies.
A chemical method for removing ligand from CdSe particles
post-synthesis for use in PV devices was demonstrated by Zhou
et al. This group reports the highest PCE of 2% for CdSe/P3HT
PV devices.14 The CdSe particles were prepared using a HDA
ligand and were then treated in hexanoic acid to remove the
HDA. The resulting ligand salt was removed using centrifuga-
tion. Ligand-free CdSe particles were then dispersed into
a dichlorobenzene solution with P3HT and spin-coated. Fig. 12
(a) shows a TEM image of HDA-coated CdSe particles and
Fig. 9 AFM images and JV characteristics of CdSe nanocrystal/P3HT
pyridine, (d) oleic acid and (e) tributylamine. Reprinted with permissionFig. 12(b) shows an image of CdSe after the HDA was removed.
Clearly the distance between particles is greatly reduced which
should increase electron mobility and htr. Intriguing elements of
the article include a statement that NMR shows that the ligands
remained on the CdSe particles after HDA treatment and
another observation that CdSe particles remain well dispersed
after removal of HDA. On the other hand, the excellent device
results suggest that the HDA treatment did decrease the
Fig. 10 (a) Absorption and PL spectra for tBOC covered CdSe nanocrystals
thermal deprotection processing. Reprinted with permission from reference 1
This journal is The Royal Society of Chemistry 2012interparticle distance and improves PCE. The authors proposed
that the insulating ligand sphere around the CdSe particles is
reduced using hexanoic acid, hence improving charge transfer
between P3HT and CdSe particles (increased hdiss), and electron
transport between CdSe particles (increased htr), as illustrated in
Fig. 12(c). More work to understand this chemical mechanism is
necessary.
Several groups have attempted to use PbS and PbSe nano-
crystals as electron acceptors in polymer/nanocrystal PV
devices20,21,23,24,127131 because these materials have low band gaps
of 0.37 and 0.26 eV, respectively, in the bulk. Although PbS and
PbSe absorb across the full near infrared radiation spectrum, PV
devices fabricated from mixtures of PbS and PbSe nanocrystals
with polymers result in lower PCE than polymer/CdSe devices.
The ultra low band gap materials suffer from three problems:
ms using different capping ligands: (a) butyl amine, (b) stearic acid, (c)
reference 11. Copyright 2009 Elsevier Science.first, the Voc of all devices is below 0.4 eV; second, the nano-
particles have a much lower absorbance than P3HT so very little
of the incident light from 650 nm to 2000 nm is actually absor-
bed; and finally, the nanoparticles suffer from the same
connectivity problems as CdSe particles due to the presence of
ligands.
Some groups use a chemical pre-treatment step on PbS
nanoparticles to remove the ligand group.21,131 As with the CdSe
and TEM images of CdSe nanocrystals. (b) Schematic illustration of the
23. Copyright 2009 The American Institute of Physics.
J. Mater. Chem., 2012, 22, 23512368 | 2359
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have band gaps in the UV and so do not contribute significantly
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View OnlineFig. 11 Removal of interfacial pyridine between nanorods (top) and
excess pyridine from P3HT (bottom) after heating. Adapted from refer-
ence 9. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA.particles, removal of the ligand group led to better packing of the
PbS particles and a smoother surface topology. However, the
resulting devices still had a PCE of less than 0.1%.21,131 A
surfactant-free PbS synthesis approach has also been intro-
duced, affording a PCE of 0.7% in MEH-PPV/PbS solar cells.24
Poor device performance was reported for oleate-capped PbSe
particles employed in a P3HT/PbSe hybrid solar cell, most likely
because of the large ligands surrounding the PbSe particles.22
Hanrath et al. performed a study of PbSe nanocrystals that were
synthesized with oleate ligands and subsequently replaced with
pyridine (as discussed for CdSe above). They showed that even
after ligand replacement, larger PbSe particles tended to main-
tain individual nanocrystal character, while smaller particles
bound together to form networks of PbSe (Fig. 13).132 They also
performed photoluminescence quenching experiments with both
ligands and both particle sizes in mixed nanoparticle/P3HT
layers. These experiments led to the conclusion that
diss
Fig. 12 TEM of (a) well-dispersed CdSe quantum dots due to the
presence of HDA ligand sphere (b) aggregation of CdSe NPs after the
acid treatment leads to efficient percolation pathways. (c) Schematic
illustration of a proposed reduced ligand sphere model. Reprinted with
permission from reference 14. Copyright 2010 The American Institute of
Physics.
2360 | J. Mater. Chem., 2012, 22, 23512368between donor and acceptor sites. Also the lack of ligands allows
formation of pathways for electron transport, increasing htr.
Hybrid devices formed from the polymer, MDMO-PPV, with
ZnO particles were fabricated to form PV devices with a PCE of
1.6%.31 This PCE compares well with that of the MDMO-PPV/to the generation of photocurrent. It was found that charge
transfer to ZnO is more efficient than to TiO2 particles/rods135
and that the small ZnO particles build a more percolating
network of electron acceptor material than TiO2. Problems with
charge transport between ZnO particles were considerably
smaller than with CdSe particles because of minor or non-exis-
tent ligand groups.
Besides conventional chemical synthetic methods, a solgel
processing route for the fabrication of ZnO and TiO2 NPs has
been widely explored. This chemical solution method typically
uses alkoxide precursors that undergo primary reactions like
hydrolysis and condensations with the addition of reactants and
solvents. The interaction of precursors and reagents highly
depends on the chemical reactivity of the compounds and reflux
temperature. The other factors that determine the particle size
are initial concentration of the precursor and the reaction time.136
The solgel process for the synthesis of TiO2 NPs is typically
based on the hydrolysis of titanium precursors under acidic
conditions which promotes the formation of the TiOTi
bond.137 Since metal alkoxides easily hydrolyse in ambient
atmosphere, and are thus expensive, one way to synthesize ZnO
NPs is using the method of Pacholski et al.,138 which involves the
hydrolysis and condensation of a simple salt, zinc acetate dihy-
drate, by potassium hydroxide in methanol.
Application in hybrid devices
Since the band gap of these transparent semiconducting nano-
particles lies in the UV, these particles make very little contri-
bution to light absorption. The hA as a function of layer
thickness for a layer with these particles is reduced because only
the polymer contributes to light absorption. On the other hand,
as a balance to this negative attribute, TiO2 and ZnO nano-
particles are synthesized using a hydrolysis and condensation
approach and thus, they do not need ligands for stabilization.138
Unlike CdSe nanocrystals, these oxides have direct contact with
the polymer, which increases h due to reduced distancephotoexcitation of PbSe results in energy transfer to P3HTnot
charge transfer. This conclusion is derived from the fact that
both size and ligand type did not change the photoluminescence
yield.
Transparent nanostructures
Other types of inorganic semiconductors used in the hybrid
nanoparticle/polymer BHJs are metal oxides such as ZnO
NPs,3136 ZnO nanorods,34 TiO2 NPs,29,31,32,34,78,133 and TiO2
nanorods.25,28 These materials have the advantages of ultrafast
photoinduced charge transfer occurring between the conjugated
polymers and the metal oxide structures and their high electron
mobility (e.g., ZnO, 100 cm2 V1 s1 (ref. 134)). Considerable
research was invested in mixing these metal oxides with conju-
gated polymers in the BHJ architecture. Both of these oxide NPsThis journal is The Royal Society of Chemistry 2012
-
fullerene devices. The same group also made P3HT/ZnO PV
devices but obtained a lower PCE of 0.9%, although P3HT
absorbs a much larger portion of the solar spectrum.33 The
explanation for this reduced efficiency is that the P3HT and ZnO
formed a much larger domain structure. This large domain
structure separates the donor and acceptor components too
much, resulting in a large drop in the exciton diffusion efficiency
(hdiff). An electrical modeling analysis of the ZnO nanoparticle
based devices showed that the electron mobility is sufficient to
obtain a higher device efficiency than was measured, but that
unlike the fullerene, ZnO nanoparticles do not cause the
MDMO-PPV coils to uncoil, resulting in reduced hole mobility.35
It was suggested in this article that the combination of ZnO
nanoparticles with another polymer may yield significantly better
results if the morphology is improved.35 In another study,
n-propylamine was added as a ligand to improve solubility of
Fig. 13 TEM image of 4 nm PbSe nanocrystals before (A) and after (B) ligand displacement with pyridine. The insets on the upper right show high
resolution images of isolated and aggregated nanocrystals respectively. The insets on the lower right provide schematic representations of the nano-
crystal and the surrounding zone shielded by the surface ligand. Reprinted with permission from reference 132. Copyright 2009 American Chemical
Society.
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for (c) ZnO:P3HT BHJ layer and (d) ZnO:P3HT-E BHJ layer. Reprinted with
This journal is The Royal Society of Chemistry 2012BHJ layer. (Bottom) Reconstructed volumes from electron tomography
permission from reference 38. Copyright 2011Wiley-VCH Verlag Gmbh.
J. Mater. Chem., 2012, 22, 23512368 | 2361
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View OnlineZnO-nanorods.34 However, the presence of the ligands caused
a decrease in Jsc, likely due to decrease in htr.
An interesting application of ZnO NP hybrid solar cells is
demonstrated by Krebs et al. where the ZnO NP is blended
with poly-(3-(2-methylhexan-2-yl)-oxy-carbonyldithiophene)
(P3MHOCT) in an inverted architecture, i.e., electron is collected
at the ITO while holes are collected at the back metal electrode.
P3MHOCT thermocleaves to P3CT at 210 C which is insoluble,affording a PCE of 0.18%. The inverted architecture avoids the
use of a reactive metal electrode (stable hcc over time) and leaves
ZnO NPs dispersed in an insoluble matrix, providing a stable
device.139
Hybrid devices employing TiO2 NP generally show PCEs that
are lower than when ZnO NP is used as the electron acceptor.
Kwong et al. studied P3HT/TiO2NP hybrid devices using
various casting solvents and determined that devices cast from
xylene afforded the highest PCE mainly due to higher Jsc, which
likely means a more efficient percolated pathway for charge
transport (increased htr), although interestingly, AFM reveals
almost similar domain sizes between films cast from chloroben-
zene and xylene.29 Ligand effects in TiO2 hybrid devices were also
examined.25 Interestingly, PCE improved when thiophenol was
added as a ligand, likely due to low solubility of pristine TiO2nanorods in toluene. The low FF probably indicates low hdissbecause the presence of ligands prevents intimate contact with
MEH-PPV. A PCE of 0.83% was achieved in a P3HT/pyridine-
capped TiO2nanorod system, where the active layer was cast
from trichlorobenzene, which resulted in more ordered P3HT
domains (increased htr).28 A layer of pure TiO2 nanorod was
deposited on top of the active layer which improves hcc.
As an alternative to the method that involves first preparing
the metal oxide NPs then mixing it with the donor material, solar
cells can also be fabricated via the precursor route, where zinc
and titanium precursor is mixed with the donor material before
undergoing hydrolysis to form ZnO30,3739 and TiO2.26,140,141 In
order to attain compatibility between the hydrophobic donor
and the precursor solution, a less polar solvent such as tetrahy-
drofuran (THF) can be used to replace the polar solgel solvents.
Fabrication of hybrid solar cells via the precursor route is feasible
given that the devices offer a PCE of 1.1%30 with MDMO-PPV
which is only slightly less than when ZnO NPs were used with
MDMO-PPV (PCE 1.6%). The precursor route has also beenexplored for P3HT:ZnO37 and MDMO-PPV:TiO2
26 hybrid BHJ
solar cells. The P3HT:ZnO-precursor BHJ devices37 outperform
P3HT:nc-ZnO devices.31 Data by Moet et al. suggested partial
degradation of dialkoxy-substituted PPV polymer when mixed
with precursor ZnO, resulting in low device performance.37 Since
then, precursor-ZnO:P3HT devices have achieved a PCE of 2%
by simply making the active layer thicker.39 The thicker ZnO:
P3HT BHJ layer resulted in finer phase-separated domains,39
increasing hdiff, although the device may have to endure lower
htr, since the path length for charge carrier to travel for collection
at the electrode is longer. Conversely, thinner devices suffer from
low ZnO content and extensive phase separation, as shown in
Fig. 14(a) and (c), lowering hcc, hdiff and htr. The same research
group introduced an ester-functionalized side chain of P3HT
(P3HT-E) to mix more intimately with ZnO (finer phase-sepa-
ration),38 resulting in an increase in Jsc (increase in hdiff). Limi-
tations to the ZnO:P3HT-E BHJ devices include a lower degree2362 | J. Mater. Chem., 2012, 22, 23512368networks rather than particles, which should greatly increase htr,
but the materials also vertically segregate, which reduces hdiss.
This is the first report of an in situ formation of semiconductor
nanoparticles, so very little has been done to optimize the
fabrication process or morphology. However this route to
a hybrid PV layer formation shows great promise to increase
EQE and PCE beyond the current records.
Conclusions and outlook
This article discusses the effects that the fabrication method and
the presence of ligands can have on inorganic nanostructures
used for hybrid inorganic/polymer solar cells. Even small
changes in the synthesis or fabrication method can have colossal
impacts on the resulting layer morphology and device efficiency.
This article addresses the likely impact that various synthesis and
fabrication choices have on the quantum efficiencies of light
absorption, exciton migration, charge dissociation, chargemobility (low htr).
The BHJ concept contends with disordered nanostructures in
exchange for the ease of all-solution fabrication using donor and
acceptor mixtures. Other ordered configurations136142145 have
been explored to improve upon the morphology of inorganic-
oxide NPs and polymers in the active layer but will not be
examined in this paper.
Growth of semiconducting networksa promisingapproach
The focus of this review has been on how various fabrication
steps lead to increases in quantum efficiency in hybrid inorganic/
organic devices. A summary of the performances of some notable
hybrid BHJ devices is presented in Table 1. As shown in Table 1,
a combination of the presence of ligands, poor connection
between semiconducting domains, and/or overly large phase
separation leads to reduced hdiss or htr in most of the device types.
Great creativity has been used to reduce these problems, but the
fact remains that device efficiency in hybrid devices is limited by
ineffective self-assembly of the polymer and semiconductor
domains and by poor electrical contact between NPs due to low
temperature sintering.
Last year a new approach to morphology control in semi-
conductor/polymer devices was introduced. Leventis et al.
introduced a method to deposit precursors for CdS in solution
with P3HT.146 As summarized in Fig. 15, the precursor Cd
(S2COEt)2 and excess pyridine reacts with mild heating (50150C) to form CdS and the side products, COS, C2H4, and H2S,which are all gasses. The resulting device is composed of CdS and
P3HT domains with no ligands. The CdS forms extendedof crystallinity in P3HT-E, due to better dispersion of ZnO in the
polymer matrix, and reduction in ZnO connectivity. Both effects
lower htr. This highlights the delicate balance between connec-
tivity (affecting htr) and consideration for the typical exciton
diffusion length (affecting hdiff). The former requires larger
domains, and the latter requires smaller domains. BHJ devices
fabricated from the TiO2 precursor do not perform as well as
from the ZnO precursor. This is likely due to the conversion of
the TiO2 precursor (titanium(IV)isopropoxide is commonly used)
to amorphous TiO2 at room temperature, which reduces chargeThis journal is The Royal Society of Chemistry 2012
-
devices by increa
nique also promi
mixed morpholog
solvent selection.
P3HT poly(3-hexylthiophene)
two
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View Onlinetransport, and charge collection, respectively. The internal
quantum efficiency is limited by a large exciton diffusion
distance, by poor charge separation at heterojunction interfaces
caused by the presence of ligands, and by poor charge transport
due to non-continuous domains and the presence of insulating
ligands between the nanoparticles.
In the area of synthesis, a lot of research attention has been
focused on the creation of nanoparticles with a higher aspect
ratio such as rods or branched structures so that pathways for
electrons are created through the mixed layer. Low band gap
nanoparticles such as PbS, PbSe, and CuInS2 are particularly
desirable because they have the capability to greatly enhance the
absorption range of the active PV layer. However, thus far, CdSe
and ZnO nanoparticles with a band gap of 1.9 eV or above have
generated devices with the highest efficiency. More work needs to
be focused on synthesis of low band gap nanoparticles for use in
hybrid PV devices and more understanding of the photophysics
to these particles is also necessary. In many cases, even a simple
study such as understanding the electronic effect of particle size
has not been performed, but must done be in order to advance
this field.
In order to attain high efficiency, there is a significant need
for better understanding of the role that ligands play in charge
transport and charge transfer across heterojunction interfaces.
These ligands can enhance the dispersion of semiconductor
nanoparticles in solution and in the polymer to create
morphological features of the correct dimension, but at the
same time can also create electronic problems. A number of
groups have experimented with removing or replacing ligands
before or after deposition of the mixed nanoparticle/polymer
solution. In general PV devices that were fabricated with smaller
Fig. 15 Chemical precursors for CdS react with mild heating to form a ne
device that shows an initial PCE of 1.5% under low light conditions to 0.7
2010 American Chemical Society.or no ligands showed higher power conversion efficiency. But
the fundamental effect of ligands is still not well understood.
Studies focused on the use of conjugated or polar ligands
designed to actively participate in the device electronics have
not yet been explored. It is well known from studies of dye-
sensitized solar cells that the organic dye passivates the inor-
ganic surface and helps to control both charge transport and
recombination rate. A similar level of understanding on the
effect of ligands for solution-processed nanoparticles in photo-
voltaics is lacking.
Finally, we feature a new technique to grow networks of
semiconducting domains using in situ chemistry rather than
simple mixing of materials. This new technique shows promise to
increase the quantum efficiency of hybrid inorganic/organic PV
This journal is The Royal Society of Chemistry 2012P3HT-E ester-functionalized side chain P3HT
derivative
OC1C10-PPV poly[2-methoxy-5-(30,70-dimethyloctyloxy)-p-phenylenevinylene]
PCPDTBT poly[2,6-(4,4-bis-(2-ethyhexyl)-4H-cyclopenta
[2,1-b; 3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]
APFO-3 poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole))nr
TClBMEH-PPV
TOPO
TOP
HDA
MPP
Acknowledgem
This work was
Collaborative (C
ifornia Energy Co
through NSF-CBnanorod
1,2,4-trichlorobenzenehp hyperbranchedtp tetrapodsqd quantum dotsnc nanocrystalsVB valence bandCB conduction bandLUMO lowest unoccupied molecular orbitalHOMO highest occupied molecular orbitalAbbreviationsexplored.New ideas such as this should be aggressivelyture), heating meses to allow a large degree of control of the
y by changing the reaction speed (tempera-
thod, precursor, and polymer morphology viaextended semiconsing transport efficiency via the formation of
ducting domains without ligands. This tech-rk of CdS with P3H
nder one sun. ReprT in the pores. The resulting mixed layer forms a PV
inted with permission from reference 146. Copyrightpoly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-
phenylene vinylene)
trioctylphosphine oxide
trioctylphosphine
hexadecylamine
maximum power point
ents
supported by the California Solar Energy
SEC) through a contract funded by the Cal-
mmission (CEC). Lilian Chang was supported
ET award number 0933435.
J. Mater. Chem., 2012, 22, 23512368 | 2363
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Table 1 Summary of notable hybrid BHJ PV devices
Donor AcceptorLoadinga
(D : A) Ligand Voc/VJsc/mAcm2 FF
PCE(%)
EQEmax (%) Affected h Ref.
P3HT CuInSe2 1 : 6 TOPO 1.0 0.3 0.5 Low Jsc likely linked to low htr because ofthe presence of large insulating ligandmolecules.
19
P3HT nc-CdSe 87 wt%CdSe
HDA 0.6323 5.8 0.56 2 Hexanoic acid wash reduced insulatingligand sphere around QD, both htr andhdiss are expected to increase. Device datawithout acid wash not provided.
14
P3HT nc-CdSe 1 : 12 Butylamine 0.55 6.9 0.47 1.8 Balance between NP aggregation andphase separation required through choiceof ligand and annealing conditions,increasing htr and hdiss.
11
P3HT nr-CdSe 90 wt%CdSe
Pyridine 0.4 0.018b 0.5 59 Removal of interfacial pyridine,enhancing htr and hdiss.
9
P3HT nr-CdSe 90 wt%CdSe
Pyridine 0.62 8.79 0.5 2.9 70 Use of high bp solvent allows self-ordering of P3HT in the presence ofpyridine and nr-CdSe, increasing htr.
12
P3HT-amino acid nr-CdSe 3 : 2 Pyridine 1.5b P3HT-amino acid partially replacespyridine surfactant, enhancing miscibilitywith NCs, hence increasing hdiss.
10
PCPDTBT tp-CdSe 1 : 9 Pyridine 0.678 10.1 0.51 3.19 55 Use of low band gap polymer increaseshA, while tetrapod CdSe increases htr.
4
OC1C10-PPV tp-CdSe 1 : 6 Pyridine 0.76 6.42 0.44 2.4 52 Vertical phase separation with tetrapod-rich layer near cathode and polymer-richlayer near anode, increasing hcc.
15
P3HT hb-CdSe 78 vol%CdSe
Pyridinec 0.6 3.5b 0.55b 1.1b 23b Hyperbranched CdSe improves thepercolation network, increasing htr.Polymer unable to penetrate branches,decreasing hdiss.
8
MEH-PPV tp-CdSe 1 : 9 Pyridine 0.69 2.86 0.46 1.13 47 Tetrapod CdSe increases htr. 13tp-CdTe 1 : 9 Pyridine 0.33 0.024 0.33 0.003 too low Energy transfer of excitons favored, while
charge transfer from polymer to CdTe isforbidden, hence very low hdiss.
tp-CdSe0.78Te0.22
1 : 9 Pyridine 0.69 1.57 0.36 0.49 48b Reduced HOMO and LUMO energydifference with MEH-PPV compared toCdSe, decreasing hdiss. Light absorptionmainly in the ternary NP (EQE max at400nm), reducing hA.
MEH-PPV nc-CdTe 1 : 3 Pyridine 0.37 0.49 0.27 0.052 Islands of nc-CdTe observed indicatingthe inefficient percolation network, i.e.,low htr.
16
MEH-PPV nr-CdS 1 : 6 Pyridine 0.85 2.28 0.462 0.89 Replacement of HDA with pyridineincreases Jsc, likely due to more intimatecontact with polymer, increasing hdiss.Annealing removes interfacial pyridine,increasing htr.
18
P3HT nc-CdS 1 : 4.7 None 0.611 3.54 0.333 0.72 36.5 In situ growth of the nc-CdS networkincreases htr but CdS and P3HT verticallysegregate, reducing hdiss.
146
MEH-PPV PbS 5060 wt% PbS
Surfactant-free
1.0 0.13 0.28 0.7 21 Surfactant-free synthesis improves PbScontact between polymer and with oneanother, increasing both hdiss and htr.
24
MDMO-PPV PbS 1 : 2 MDMO-PPV
0.78 0.0025 0.56 0.0013 Low Jsc likely indicates lack ofpercolation pathway for electrons sincePbS NPs are surrounded by MDMO-PPV, decreasing htr. High seriesresistance was reported.
23
P3HT PbS 1 : 1 Acetic acid 0.35 0.08 0.36 0.01 Exchanged to a smaller ligand improvesJsc, i.e., increase in htr.
21
P3HT PbSe 80 wt%PbSe
Oleate 0.28 0.9 0.39 0.1 PCE increased to 0.19% when the P3HTlayer is introduced at anode (removeshunt contact between PbSe and anode),increasing hcc. Overall poor deviceperformance likely due to large ligandssurrounding PbSedecreased htr andhdiss.
22
MDMO-PPV nc-ZnO 67 wt%ZnO
None 0.814 2.4 0.59 1.6 40 Non-existent ligand ensures intimatemixing between polymer and ZnO,increasing hdiss. Also intimate contactbetween ZnO NPs increases htr.
31
2364 | J. Mater. Chem., 2012, 22, 23512368 This journal is The Royal Society of Chemistry 2012
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Table 1 (Contd. )
Donor AcceptorLoadinga
(D : A) Ligand Voc/VJsc/mAcm2 FF
PCE(%)
EQEmax (%) Affected h Ref.
P3HT nc-ZnO 26 vol%ZnO
None 0.69 2.19 0.55 0.92 27 P3HT:ZnO forms larger domainscompared to MDMO-PPV:ZnO,decreasing hdiff.
33
P3MHOCT(thermocleaves toP3CT)
np-ZnO 1 : 2 None 0.516 1.00 0.35 0.18 20 Inverted structure with a layer of ZnO onITO as ETL, avoiding the use of reactivemetal electrode, stable hcc overtime. ZnONPs dispersed in an insoluble rigidpolymer matrix, giving stablemorphology, hdiff and hdiss should notchange.
36
MDMO-PPV nr-ZnO 26 vol% 4 vol% n-propylamine
0.52 1.1 0.62 MPP lower than without ligand. Ligandadded to improve solubility of nr-ZnObut the presence of ligands likely causedthe decrease in Jsc probably due todecrease in htr. Voc also decreasesprobably because of decrease in hcc.
34
nc-ZnO 26 vol% 2 vol% n-propylamine
0.60.7
1.51.6 0.59 The presence of surfactant caused a bigdecrease in Jsc while Voc slightlydecreased and FF remained unchanged,indicating a drop in htr.
MDMO-PPV ZnOprecursor
15 vol%ZnO
None 1.14 2.0 0.42 1.1 27 Almost comparable to nc-ZnO devices,except for lower FF. The highly reactiveZnO precursor degrades MDMO-PPV,which likely reduces the FF, decreasinghdiss. No explanation offered on higherVoc compared to devices using nc-ZnO.
30
P3HT ZnOprecursor
15 vol%ZnO
None 0.83 3.5 0.50 1.4 26 P3HT more stable to diethylzinc thanMDMO-PPV. Homogeneous blendincreases hdiff and hdiss.
37
P3HT ZnOprecursor
20 vol%ZnO
None 0.75 5.2 0.52 2.0 44 Thin P3HT:ZnO devices suffer lowperformance due to electrode-quenchingeffect, low ZnO content and extensivephase separation, leading to low hcc, hdissand htr. Thicker P3HT:ZnO layersincreases Jsc because of finer phase-separated domains but likely suffer fromtraps in the longer charge carrier pathway(low htr).
39
P3HT-E ZnOprecursor
17 vol%ZnO
None 1.02 2.1 0.40 0.83 25 ZnO disperses better in P3HT-E thanP3HT. P3HT-E gives higher Voc due tolarger optical band gap and higher Jsc dueto more intimate mixing (higher hdiss andhdiff). Jsc of P3HT-E devices decrease withincreasing active layer thicknessindicating limitations in hcc. Intimatemixing also means a lower degree ofcrystallinity for P3HT-E (decreased htr).
38
P3HT NPTiO2 60 vol%TiO2
None 0.44 2.759 0.356 0.424 15b Devices spin-coated from xylene gave thebest performance mainly due to higher Jscwhich likely means a more efficientpercolated pathway (higher htr). AFMreveal almost similar domain sizesbetween ClB and xylene films.
29
PPV NPTiO2 40 wt%TiO2
None 0.46 0.1397 0.26 0.018 Lower degree of TiO2 crystallizationlikely decreases htr and hdiss.
27
MEH-PPV nr-TiO2 70 wt%TiO2
Thiophenol 0.70 0.456 0.34 0.157 Devices using thiophenol ligand performbetter than pristine TiO2 due to lowsolubility of the latter in toluene. Low FFlikely indicates low hdiss due to thepresence of ligand.
25
P3HT nr-TiO2 53 wt%TiO2
Pyridine 0.52 2.97 0.54 0.83 28 Layer of pure nr-TiO2 on top of activelayer improves hcc. Devices cast fromTClB perform better due to more orderedP3HT domains even in the presence of nr-TiO2 and increase in htr.
28
MDMO-PPV TiO2precursor
20 vol%TiO2
None 0.52 0.6 0.42 11 Poor conversion of precursor into TiO2resulting in lower htr and lower hdiss.
26
a Unless otherwise stated. b Values estimated from graph. c Presumed based on authors other papers.
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