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

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

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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View Onlinespin-coated from a common solution and morphology is self-

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

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,

J. Mater. Chem., 2012, 22, 23512368 | 2357

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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

have band gaps in the UV and so do not contribute significantly

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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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View OnlineFig. 14 (Top) TEM images of (a) Zn:P3HT BHJ layer and (b) ZnO:P3HT-E

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

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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