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Cite this: DOI: 10.1039/c2jm33838f

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Metal oxides for interface engineering inpolymer solar cellsSong Chen, Jesse R. Manders, Sai-Wing Tsang and Franky So*

DOI: 10.1039/c2jm33838f

Significant progress in power conversion efficiencies and stabilities of polymer solar cells hasbeen achieved using semiconducting metal oxides as charge extraction interlayers. Bothn- and p-type transition metal oxides with good transparency in the visible as well as infraredregion make good Ohmic contacts to both donors and acceptors in polymer bulkheterojunction solar cells. Their compatibility with roll-to-roll processing makes them veryattractive for low cost manufacturing of polymer solar cells. In this review, we will present therecent results on synthesis and characterization of these metal oxides along with the deviceperformance of the solar cells using these metal oxides as interlayers.

1. Introduction

Polymer solar cells (PSCs) have drawn a

lot of research interest in the past decade

because of their potential to be low-cost

light harvesting devices.1 The race for

power conversion efficiency (PCE) of

PSCs has been driven by the development

of photoactive materials2,3 and device

architectures.4–7 In particular, a large

number of donor–acceptor (D–A) poly-

mers3,8–10 have been synthesized by

controlling the highest occupied molec-

ular orbital (HOMO) and lowest unoc-

cupied molecular orbital (LUMO) levels

of the donor and the acceptor units of the

resulting conjugated polymers. This

bandgap engineering enables the realiza-

tion of many low bandgap polymers with

deep-lying HOMO energies, resulting in

enhancements in short circuit currents

(Jsc) as well as open circuit voltages (Voc),

which are two key parameters for photo-

voltaic cells. Using these photo-active

materials, PCEs of 7–9% (ref. 9 and

11–16) have already been demonstrated

with the incorporation of charge extrac-

tion interlayer materials, among which

transition metal oxides are very attractive

because of their ability to efficiently

Department of Materials Science andEngineering, University of Florida, 32611-6400,USA. E-mail: fso@mse.ufl.edu

This journal is ª The Royal Society of Chemistry

extract charge carriers and their compat-

ibility with high volume roll-to-roll (R2R)

processing.17,18 While solution processed

electron extraction materials such as zinc

oxide have been demonstrated in both

lab-scale devices and scaled-up modules,

further development of high work func-

tion solution processed metal oxides for

hole extraction layers is still needed.

Most PSCs are made with bulk heter-

ojunctions (BHJs)19 wherein an electron

donating polymer and an electron ac-

cepting fullerene derivative form nano-

scaled interpenetrating networks allowing

efficient exciton dissociation and carrier

transport. Unlike inorganic solar cells

where Ohmic contacts can be made by

surface doping, PCSs require alternative

strategies for the interface engineering.

Specifically, poor Ohmic contacts with

transparent conducting oxides (TCO) or

metallic electrodes are due to the

mismatch of work function,20,21 the pres-

ence of interfacial dipoles22–24 as well as

high densities of interfacial trap states.25,26

To achieve good Ohmic contacts, various

charge extracting interlayers have been

used between the BHJ layer and the elec-

trodes. Among the electrode interlayer

materials used in high performance PSCs,

transition metal oxides are promising

because of their better environmental

stability, higher optical transparency and

2012

easier synthesis routes than alkali metal

compounds,27–30 aqueous conducting

polymers31–34 and conjugated

polyelectrolytes.14,35

The PCE of PSCs is a product of Voc,

Jsc and fill factor (FF). The Voc is deter-

mined by recombination as well as the

energy level alignment between the pho-

toactive polymer donor and the fullerene

acceptor, Jsc is determined by the light

harvesting and the charge separation

efficiency under large extraction fields,

and FF is determined by the device series

resistance, the dark current and the

charge recombination/extraction rate

under low internal fields. The n- and

p-type semiconductivities in metal oxides

are, in general, due to the intrinsic point

defects such as atomic vacancies present

in the oxides. Fig. 1 shows the HOMO as

well as LUMO energy levels of some of

the state-of-the-art photovoltaic poly-

mers, electron accepting fullerene deriva-

tives and the electron affinities and

ionization energies of high/low work

function metal oxides. These metal oxides

form Ohmic contacts to photovoltaic

polymers and fullerenes through favor-

able vacuum level shift, energy level

bending and Fermi level pinning at the

polymer–electrode interfaces. In addition,

the use of oxide interlayers circumvents

the direct contact between a photoactive

J. Mater. Chem.

Fig. 1 The energy level diagram of state-of-art photovoltaic polymers, electron accepting fullerene

derivatives and transition metal oxides.

Fig. 2 Conventional (top) and inverted

(bottom) polymer solar cells. The usage of low

work function (WF) oxide in a conventional

cell is optional.

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polymer and electrodes where high

densities of carrier traps or unfavorable

interface dipoles hinder efficient charge

collection. Equally important is the use of

metal oxides for Ohmic contacts to

maximize theVoc because the reduction of

built-in potential leads to an increase in

dark current as well as carrier

recombination.

In this paper, we will review the

synthesis and characterization of some of

the vacuum-deposited and solution pro-

cessed metal oxides including zinc oxide,

titanium oxide, molybdenum oxide,

tungsten oxide, vanadium oxide and

nickel oxide for interlayers in PSCs. We

will also describe how their properties

affect the performance of some of the

state-of-the-art PSCs.

2. Metal oxides as cathodeinterlayers

Depending on the solar cell device

geometry, as shown in Fig. 2, the cathode

interlayer is either coupled with the top

electrode, usually a low work function

metal, or with the bottom TCO electrode.

In PSCs, three approaches are used to

make Ohmic contacts to the electron

accepting fullerene derivatives in the BHJ

layer. The first approach is to use alkali

metals or related compounds such as

cesium carbonate (Cs2CO3)7,36 and

lithium fluoride (LiF) to make Ohmic

contacts to fullerenes due to their low

work functions (<3.0 eV). The disadvan-

tage of this approach is, however, that

J. Mater. Chem.

alkali metal compounds are prone to

oxidation and lead to rapid device

degradation. The second approach is to

use alcohol/water-soluble conjugated

polyelectrolytes14,35,37 for cathode inter-

layer materials. Due to the intramolecular

dipole moment and their ability to form

self-assembled monolayers, these conju-

gated polyelectrolytes can induce an

interface dipole pointing from the

cathode to the BHJ layer in the conven-

tional device geometry, thus increasing

the built-in potential of the device. Using

this approach, high PCE values (8.3%)

have been achieved in devices using

polyelectrolytes for the top cathode

This journ

interlayer.14 The third approach is to use

transition metal oxides such as ZnO and

TiO2 with work functions matching the

LUMO levels of fullerenes. Due to their

chemical resistance to oxygen and mois-

ture, optical transparency and facile

solution processability, these n-type

semiconducting oxides effectively replace

low work function metals for cathode

contacts, resulting in high efficiency

devices demonstrated in both conven-

tional and inverted device geometries.

2.1 Zinc oxide (ZnO)

ZnO is currently the most commonly used

electron transporting layer (ETL) mate-

rial in PSCs. Its work function13,38

provides a good Fermi level match to the

Fermi levels of TCOs as well as the

LUMO energy of [6,6]-phenyl-C61 (or

C71) butyric acid methyl ester (PC61BM

or PC71BM). Because of the large

bandgap energy, the ionization potential

of ZnO is large enough to let it act as a

hole blocker and thus increases the shunt

resistance of a BHJ cell. Optically, ZnO

absorbs in the UV part of the spectrum

and hence it serves as a low-pass filter for

PC71BM and photoactive polymers used

for solar cells. Using zinc salts as precur-

sors, solution processable ZnO can be

formed through sol–gel,39 solvothermal40

or a hydrothermal process,41 as a result,

various nanostructures of ZnO such as

colloidal nanoparticles and sol–gel ZnO

films have been used as ETLs in PSCs.

2.1.1 ZnO colloidal nanoparticles

(NPs). The use of ZnO NPs enables the

material to be utilized in its crystalline

phase while its thin film deposition is

compatible with solution processing. ZnO

NPs have been used for R2R processing

of large area solar modules.18 For lab-

scale device fabrication, ZnO NPs are

usually deposited by spin-coating as a

bottom or top interlayer depending on the

device geometry.

ZnO NPs are readily synthesized from

zinc acetate dihydrate in alcoholic

solvents with a strong base such as

sodium hydroxide (NaOH),42 potassium

hydroxide (KOH)41,43,44 or tetramethyl-

ammonium hydroxide (TMAH).13,45 The

resulting wurtzite-type ZnO NPs have

diameters of 3–6 nm, bandgap energies of

3.3–3.6 eV, and they form transparent

colloids in polar or non-polar

al is ª The Royal Society of Chemistry 2012

Fig. 3 J–V characteristics under AM 1.5 G

illumination (top) and external quantum effi-

ciencies spectra (bottom) of the ITO/ZnO NP/

PDTG-TPD:PC71BM/MoO3/Ag device.

Reprinted with the permission of ref. 13.

Copyright 2012 Wiley-VCH.

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solvents.13,41–45 To fabricate PSCs with an

inverted device geometry, i.e., devices

with a bottom cathode, ZnO NP colloids

are spin-coated onto TCO substrates as

the bottom electrode interlayer. Using

such an approach, inverted poly(3-hex-

ylthiophene) (P3HT) cells with a PCE of

4% have been demonstrated.46,47 More

recently, using low bandgap polymers

such as dithienosilole-thienopyrrole-4,6-

dione (PDTS-TPD) and dithienogermole-

thienopyrrole-4,6-dione (PDTG-TPD),

PCEs of 6.6% and 7.3% have also been

reported.9,44

Because of the defects present in

colloidal ZnO NPs, devices require UV

light exposure to enhance the conduc-

tivity of ZnO NPs. Such photo-conduc-

tivity enhancement occurs within a few

seconds of the UV exposure and the

phenomena can be explained by photo-

doping and defect filling. However, this

photo-doping effect only lasts for a short

time and such a post UV-soaking alone is

not sufficient for optimum device perfor-

mance. While the defect states give rise to

the n-type semiconductivity of ZnO, these

defects also lead to a recombination

pathway for dissociated carriers in the

adjacent BHJ layer. In addition, due to

the small crystal diameter, ZnO NPs are

known to have up to 30% of the atoms

exposed to the particle surface forming

dangling bonds which contribute to a

high density of gap states. These gap state

defects also give rise to the color center

emission in the visible spectrum when

ZnO NPs are photoexcited. Recently, it

has been found that UV-ozone (UVO)

treatment was an effective approach to

passivate these defect states as evident

from the quenching of the color center

while maintaining the work function of

ZnO.13WithUVO-treated ZnONPs as an

ETL, PDTG-TPD:PC71BM cells show a

significant enhancement of short circuit

current due to reduced interface recom-

bination, resulting in a PCE as high as

8.1% which is among the highest reported

values of a single junction inverted PSC.

These results are illustrated in Fig. 3.

However, this UVO treatment only works

with devices having a photoactive poly-

mer with a deep HOMO energy—usually

benchmarked by an oxidization threshold

energy of 5.27 eV.48 Alternatively, for

polymers with shallow HOMO energies,

such as P3HT, it has been reported that

using a self-assembled C60 layer can block

This journal is ª The Royal Society of Chemistry

the direct contact between the ZnO NPs

ETL and the absorber, thus enhancing the

efficiency to 4.9%.49

In addition to its use as an ETL in

single junction cells, ZnO is also used to

form the interconnecting unit in tandem

PSCs. Such an interconnecting unit,

essentially a tunneling p–n junction

sandwiched by two BHJ cell stacks, forms

a hole–electron recombination zone

across which the Fermi levels of the hole

transporting layer and the electron

transporting layer are aligned to minimize

the Voc loss in a tandem cell. To achieve

good performance in tandem cells, the p–

n junction materials should be heavily

doped in order to reach the energy equi-

librium. To date, the most efficient

tandem cell reported utilizes ZnO NPs as

the bottom cathode interlayer, P3HT:

indene-C60 bisadduct (ICBA)50 as the

front cell, ZnO NPs/poly(3,4-ethyl-

enedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS) as the interconnecting unit

and a low band gap BHJ stack as the rear

cell. The resulting devices have a PCE of

8.6% in which the Voc value is very close

to the sum of the two single junction cells,

indicating a negligible photovoltage loss

across the interconnecting unit.16 Using a

very similar device architecture, a new

2012

record efficiency of 10.6% was mentioned

in a recent progress report.51

Compared with low work function

metals and polyelectrolytes, the electronic

properties of ZnO are insensitive to the

moisture and oxygen in the ambient; as a

result, the device lifetime is enhanced

using ZnO as a cathode interlayer.

Without any device encapsulation, in-

verted P3HT:PC71BM cells with ZnO

NPs as the bottom cathode interlayer and

evaporated molybdenum oxide as the top

anode interlayer experienced less than

20% reduction in PCE after being exposed

to ambient for 40 days.46 Similarly,

inverted PDTS-TPD:PC71BM cells

retained 85% of their initial PCE (6.6%)

after storage in air over a period of a

month.44 Similar trends have also been

observed in polymer–CdSe hybrid solar

cells39 and quantum dot light emitting

diodes (QLED),45 indicating the use of

ZnO NP ETL is promising to enhance

lifetimes of solution processable elec-

tronic devices.

2.1.2 Sol–gel ZnO (films). In addition

to nanoparticles, ZnO thin films can also

be formed through sol–gel reaction

between zinc salts and hydroxyl groups

from solvent and ambient water. The so-

called ‘‘precursor-based’’52 or ‘‘sol–gel

derived’’53 ZnO refers to the preparation

of ZnO films by annealing zinc acetate

dihydrate solution (in ethanol) at 200 �C.Although this process requires amine

additives and high temperature annealing

to complete the hydrolysis reaction, it is

an effective way to prepare ZnO films as

the bottom cathode interlayers without

any additional material synthesis steps.

Using spray pyrolysis54 and inkjet

printing, sol–gel ZnO films can be pro-

cessed over a large scale with good

thickness control. For the lab-scale

device fabrication, the first inverted PSC

with a PCE of over 6% was made with

sol–gel ZnO and poly[[9-(1-octylnonyl)-

9H-carbazole-2,7-diyl]-2,5-thiophenediyl-

2,1,3-benzothiadiazole-4,7-diyl-2,5-thio-

phenediyl] (PCDTBT):PC71BM;53 the

PCE was close to the value obtained for a

conventional device with a top cathode

layer.55

In addition to pristine sol–gel ZnO

films, sol–gel ZnO polymer composites

have also been used for inverted PSCs.

For example, ZnO–polyvinylpyrrolidone

(PVP) composites have been made and

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PVP has been found to cap the ZnO

clusters in the composite film to improve

the film uniformity.56However, due to the

smaller surface energy of PVP, the top

surface of ZnO films tend to be PVP-rich

which forms an extraction barrier when

these composite films are used in inverted

cells as a bottom cathode interlayer.

Upon the removal of extra PVP by UVO

treatment, inverted cells of PDTG-

TPD:PC71BM also show improved elec-

trical coupling at the ETL–BHJ interface

resulting in a PCE exceeding 8% with

significantly improved values in FF and

Jsc.15 The surface PVP removal was

confirmed by the significant work func-

tion change of ZnO–PVP,13 the composi-

tional change in the X-ray photoelectron

spectra and the morphological change in

the atomic force microscopy (AFM)

phase images.15

In summary, today, ZnO is widely used

as an electron extraction layer for PSCs in

both research and manufacturing and it is

promising for both small area coating and

large area printing.

2.2 Titanium dioxide (TiO2)

TiO2 is a wide gap semiconductor with its

conduction band minima composed of

the Ti 3d band and its valence maxima

composed primarily of the O 2p states. As

a result of oxygen deficiency and the

occupied defect states, TiOx (x < 2) is an

n-type semiconductor. However, as a

typical electron transporting material in

dye sensitized solar cells (DSSC),57,58 the

traditional preparation methods of

nanocrystalline TiOx are not compatible

with processing of PSCs. For example, a

high temperature (450 �C) is required to

obtain crystalline TiOx and this high

temperature processing cannot be used

for plastic substrate processing. More-

over, mesoporous TiOx films formed

through TiCl4 treatment59 have a large

roughness and cannot be used as an

electrode interlayer. Thus, to date, most

TiOx films used for PSCs are in amor-

phous phase made by a sol–gel process.

The solution precursor is typically

prepared using titanium isopropoxide

along with solvent additives. After spin-

coating, TiOx films are formed upon

hydrolysis at �150 �C. Such a method is

practically similar to the synthesis sol–gel

ZnO presented above. According to

X-ray photoelectron spectroscopy results,

J. Mater. Chem.

solution processed TiOx was found to be

oxygen deficient with x ¼ 1.34.60

PSCs fabricated with sol–gel TiOx have

been reported in both conventional and

inverted cells. In conventional cells, the

incorporation of TiOx as an ETL shows

enhanced Jsc and FFwhen compared with

devices using aluminum electrodes. Using

PCDTBT:PC71BM as the absorber, the

optimum devices have a PCE of 6.1%.55

The enhancement of PCE was attributed

to the improved electrical coupling with

PC71BM and the enhanced light harvest-

ing due to optical interference with the

TiOx film as an optical spacer.60 With a

BHJ layer as thin as 60–80 nm, this is

usually a good strategy for polymers with

low hole mobilities and high absorption

coefficient, the optical field distribution in

the absorber is sensitive to its distance to

the back electrodes, making the J–V

characteristics highly dependent on the

spacer thickness.61 When TiOx is used as

the bottom electrode interlayer and PE-

DOT:PSS/gold as the top electrode, the

PCE of the resulting inverted

P3HT:PC71BM cells is about 3%.62,63

Similar to ZnO, TiOx has also been used

as the n-type material for the interconnect-

ing charge recombination layer for solution

processed tandemPSCs.With P3HT:ICBA

and poly(4,4-dioctyldithieno(3,2-b:20,30-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-

4,7-diyl) (PDTS-BTD):PC71BM as the two

absorbing layers, a PCE of 7% (ref. 64) was

reported.

There have also been reports that PSCs

with a TiOx ETL show a strong depen-

dence on UV illumination65–67 acquired

from the A.M. 1.5G light exposure.

Showing poor initial device characteris-

tics before UV illumination, these devices

not only require prolonged UV exposure

to obtain optimum J–V characteristics

but continuous UV illumination is also

necessary to keep the TiOx layer ‘‘acti-

vated’’ as some study has revealed that

both Jsc and FF drop rapidly when the

UV portion of the AM 1.5 G spectrum is

filtered.63,68 Such a phenomenon, along

with the work function change67 uponUV

illumination, is in contrast with what has

been observed in ZnO devices, and cannot

be simply interpreted as a result of UV

induced photo-doping. Schmidt et al.

tracked the change in FFs of P3HT cells

after UV exposure under different

ambient conditions. The results indicated

that oxygen is chemisorbed at the TiOx

This journ

surface, leading to band bending in the

TiOx and suppressing electron extrac-

tion.67 Unless improvement is made on

the UV sensitivity of these TiOx based

devices, their use for PSCs will be limited.

3. Metal oxides as anodeinterlayers

For anode interlayers in PSCs, their work

functions need to large enough so that the

Fermi levels of the interlayers match the

HOMO levels of photovoltaic polymers.

UVO treated TCO have a work function

below 5 eV (ref. 69 and 70) and this low

work function does not match the

HOMO energy of many photovoltaic

polymers; therefore, good Ohmic

contacts cannot be made without an

anode interlayer in conventional cells.

For inverted devices, while a high work

function metal may be used for the top

anode, good Ohmic contacts cannot be

made to conjugated polymers with a high

HOMO energy because of the electron

transfer from the metal to the organic

absorber, thus creating an interface dipole

and reducing the device built-in potential.

This reduction of built-in potential of the

BHJ diode not only increases the series

resistance under forward bias but also

decreases the extraction field at short

circuit condition. Therefore, in order to

achieve high Jscs, Vocs and FFs in PSCs,

anode interlayer materials with high work

functions are needed. To date, despite the

problem of device lifetime, PEDOT:PSS

is still the anode interlayer material used

in most PSCs. Recently, several vacuum-

deposited transition metal oxides with

high work functions, good stability and

high optical transparency have gained

significant research interest and PSCs

incorporated with these metal oxides have

demonstrated good device performance.

Depending on the different mechanisms

of hole extraction, both n-type and p-type

semiconducting oxides are used for anode

interlayers. Most transition metal oxides

such as molybdenum oxide, tungsten

oxide, and vanadium oxide are n-type and

they are commonly used for anode Ohmic

contacts for both OPV and OLED

devices. These oxides have work functions

exceeding 6 eV and form strong acceptors

with organic materials. When they are

used as anode interlayers, Fermi level

pinning occurs and increases the built-in

potential. Under an extraction field,

al is ª The Royal Society of Chemistry 2012

Fig. 5 Plot of HOMO offset (DEH) versus the

difference between the substrate work function

and organic ionization energy (f-IEorg) for

a-NPD on NiO and MoO3, where the electron

chemical potential of oxides was tuned by

inducing oxygen vacancy defects. Reprinted

with the permission of ref. 81. Copyright 2012

Mcmillan Publishers Limited.

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electrons are injected through the

conduction band of the oxide and re-

combine with the photo-generated holes

at the oxide–organic interface. The

process is equivalent to ‘‘hole extraction’’.

Nickel oxide is a transition metal oxide

known to be p-type. The large work

function in NiO allows a good match

between the valence band and the HOMO

level of most photoactive polymers. In

early works, these high work function

n-type and p-type semiconducting oxides

are vacuum deposited. Recent works also

showed these oxides are solution

processable. In the following section, we

will review examples of both n-type and

p-type oxides which have been used as

anode interlayer materials.

Fig. 4 Energy level alignment at the MoO3–

organics interface. Reprinted with the permis-

sion of ref. 72. Copyright 2009 American

Institute of Physics.

3.1 Molybdenum oxide (MoO3)

3.1.1 Thermally evaporated MoO3

(e-MoO3). The use of MoO3 started with

the exploration of efficient hole injection

materials used in OLEDs where the hole

transporting layer has a deep HOMO

level.71 Because of its low melting

temperature (795 �C), molybdenum oxide

can be deposited by thermal evaporation

which allows accurate thickness control at

a nanometer scale. MoO3 is an excellent

hole injection material and was once

misidentified as a p-type semiconductor

until the n-type characteristics were

directly determined by ultraviolet photo-

electron spectroscopy (UPS).72 Similar to

TiOx, the 4d band of molybdenum is

unoccupied and contributes to the

conduction band of the oxide. Evident by

results of XPS and UPS measurements,

oxygen deficiency in e-MoO3 gives rise to

the defect band which lifts the Fermi level

closer to the conduction band.73,74 Like

other n-type oxides, e-MoO3 forms

Ohmic contacts with many organic hole

transporting materials. Based on the

results of in situ UPS results, upon depo-

sition of MoO3 onto ITO substrates, the

interface experiences a significant vacuum

level shift of 2 eV which aligns the Fermi

levels of MoO3 and ITO. The work

function of MoO3 is 6.7 � 0.2 eV and the

Fermi level lies at an energy of 0.5 �0.2 eV below the conduction band edge

and 2.7 � 0.2 eV above the valence band

edge.72,74–77 Further study reveals that the

work function of MoO3 exhibits a strong

dependence on the stoichiometry and is

highly sensitive to surface contamination.

This journal is ª The Royal Society of Chemistry

Upon oxygen or air exposure, the work

function decreases to 5.3–5.7 eV (ref.

76–78) which is still sufficient to yield

good Ohmic contacts with organic hole

transporting materials.76 Further reduc-

tion of the suboxide gives rise to growth of

gap states that will finally reach the Fermi

energy, resulting in the metallic behavior

of MoO2.79

In order to understand how a p-type

contact is made with an electron accepting

material, UPS experiments were con-

ducted to study the energy level evolution

of organic materials deposited onto the

MoO3 surface. Here, we use a small

molecule hole transporting material—

aluminum phthalocyanine chloride

(AlPcCl) as an example. As shown in

Fig. 4, the HOMO level of AlPcCl shifts

by 1.6 eV when a few nanometers of the

material are deposited on top of the ITO

surface, resulting in a strong band

bending of AlPcCl within the first few

nanometers at the interface.72 Similar

band bending is also directly confirmed by

studying the N,N0-diphenyl-N,N0-bis(1-naphthyl)-1,10-biphenyl-4,40-diamine (a-

NPD)–MoO3 interface.74,80 Such band

bending indicates there is a strong elec-

tron transfer from the HOMO levels of

organic materials to the 4d band of

molybdenum, leading to Fermi level

pinning at the oxide interface. Such a

phenomenon can be explained by the

thermodynamics that, for either n-type or

p-type oxides, given the metal oxide work

functions are larger than the HOMO

levels of organic materials, the HOMO

offset, which is defined as the difference

between the HOMO levels and Fermi

energy after equilibrium, will approach a

fixed minimum, resulting in Fermi level

pinning as shown in Fig. 5.81 In addition,

due to such a high work function and a

strong electron accepting capability,

2012

MoO3 is an efficient p-type dopant in

organic hole transporting materials.75

For PSCs with a high PCE, the

optimum HOMO level of the photovol-

taic polymers generally resides between

�5.2 eV and �5.6 eV considering the

trade-off relationship between Voc and

optical absorption band.82 With these

HOMO energies, good Ohmic contacts to

organic materials can readily be made

with e-MoO3 as an interlayer and the

device data of the resulting PSCs are

summarized in Table 1. Furthermore, our

recent work also showed that compared

with the conventional bottom MoO3

p-contact, enhanced Ohmic contacts can

be made when MoO3 and silver are

sequentially evaporated onto organic

materials as a top p-contact.83 Using such

a top p-contact, inverted solar cells are

now one of the prototypical structures for

the lab-scale device demonstration.

Inverted P3HT:PC71BM cells with PCEs

of 4.5–5% (ref. 47 and 49) have been

reported and these values are consistently

higher than those for conventional

devices using PEDOT:PSS as the bottom

electrode interlayer. Similarly, PDTS-

TPD:PC71BM cells with a PCE reaching

7.8% (ref. 13) have been demonstrated

and these results are slightly higher than

the 7.3% PCE reported using PE-

DOT:PSS as a bottom p-contact.12 In

addition, PDTG-TPD:PC71BM cells were

reported showing a PCE exceeding 8%

with the inverted device structures.13,15

Using MoO3 to replace PEDOT:PSS for

the bottom anode contact, the resulting

devices show comparable initial

J. Mater. Chem.

Table 1 A summary of high efficiency polymer solar cells using transition metal oxide interlayers

Polymer Anode interlayer Cathode interlayer Jsc (mA cm�2) Voc (V) FF PCE (%)

P3HT49 e-MoO3 ZnO NP 12.6 0.63 0.62 4.9P3HT112 NiOx (PLD) LiF 11.3 0.64 0.69 5.2PCDTBT55 PEDOT:PSS s-TiOx 10.6 0.88 0.66 6.1PCDTBT53 e-MoO3 sol–gel ZnO 10.4 0.88 0.69 6.3PCDTBT111 s-NiOx Ca 11.5 0.88 0.65 6.7PDTS-TPD44 e-MoO3 ZnO NP 11.3 0.89 0.67 6.7PCDTBT84 e-MoO3 s-TiOx 11.9 0.91 0.66 7.2PDTS-TPD13 e-MoO3 ZnO NP 13.1 0.90 0.66 7.8PDTG-TPD15 e-MoO3 sol–gel ZnO–PVP 14.0 0.86 0.67 8.1PDTG-TPD13 e-MoO3 ZnO NP 14.1 0.86 0.67 8.1

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performance with much enhanced

stability84 (Fig. 6), indicating high work

function n-type metal oxides can replace

PEDOT:PSS in both conventional and

inverted devices. In recent years, MoO3

has been widely used as a hole injection

layer in organic electronic devices to

replace PEDOT:PSS.

3.1.2 Solution processed MoO3

(s-MoOx). For R2R processing, it is

desirable to deposit MoO3 by a solution

process instead of vacuum deposition. To

use s-MoOx for PSCs, the material should

have a high work function as well as low

surface roughness. In the following

section, we will review several represen-

tative routes to prepare s-MoOx and their

use in P3HT:PCBM cells.

Liu et al.85 presented a facile way to

make s-MoOx thin films, which represents

the first attempt for PSC applications.

The aqueous molybdenum containing a

precursor, ammonium molybdate

[(NH4)6Mo7O24], was dissolved in a

Fig. 6 Normalized PCEs as a function of

storage time for PCDTBT:PC71BM cells

fabricated with PEDOT:PSS and MoO3 in air

under ambient conditions (no encapsulation).

The devices with MoO3 had an initial PCE of

6.4%. Reprinted with the permission of ref. 84.

Copyright 2011 Wiley-VCH.

J. Mater. Chem.

mixture of hydrochloric acid and de-

ionized water before spin-casting. After

annealing the films at 160 �C in nitrogen,

the resulting sol–gel based s-MoOx films

show a very similar X-ray diffraction

(XRD) pattern compared with e-MoO3

films. The reported P3HT:PCBM cells

showed a PCE of 3.1% which is close to

the PEDOT:PSS device fabricated under

the same condition. Despite the low

temperature process and acceptable PCE

values, aggregation of s-MoOx resulted in

poor uniformity and thus limits its use for

large scale processing. Shortly afterwards,

Meyer et al.78 reported the UPS

measurements and hole injection study of

s-MoOx films made by MoO3 nano-

crystals. In order to realize homogeneous

films, a block copolymer was used to

facilitate the nanocrystal dispersion in

xylene. After spin-coating, oxygen plasma

was carried out to remove the dispersing

agent and also to tune the work function

of s-MoOx. With thermal annealing in

nitrogen ambient at 100 �C, s-MoOx

showed a work function of 6.0 eV which

was close to the air exposed e-MoO3 film

(5.7 eV). Using these MoO3 nanocrystals,

Stubhan et al.86 fabricated P3HT:PCBM

cells with s-MoOx interlayers with PCEs

of 2.5%, which is lower than the results

obtained for typical P3HT cells. The PCE

was probably limited by the rough surface

of s-MoOx due to the large nanocrystal

size of �15 nm. Hammond et al.87 also

reported that oxygen plasma treatment

on s-MoOx films, made from the molyb-

denum tricarbonyl trispropionitrile

[Mo(CO)3(EtCN)3] precursor, increased

the work function of s-MoOx to 5.4 eV,

resulting in P3HT cells with PCEs of

3.5%. Others also recently reported

similar device performance without using

oxygen plasma treatment. Girotto et al.88

This journ

prepared precursors by dissolving MoO3

powder into H2O2. However, to form

s-MoOx films to be used for PSCs, the

process requires a high annealing

temperature (�300 �C). Zilberberg et al.89

presented a precursor based on bis(2,4-

pentanedionato) molybdenum(VI)

dioxide and isopropanol. After annealing

in nitrogen at 150 �C, a smooth (rough-

ness r.m.s. <3 nm) s-MoOx film forms

with a work function of 5.3 eV without

additional treatments.

s-MoOx is a good candidate to replace

PEDOT:PSS for PSCs with greatly

improved device stability.88,89 However,

the work function values of s-MoOx films

tend to be lower than that of e-MoO3,

thus affecting the quality of the resulting

Ohmic contacts. In the above mentioned

work, P3HT:PCBM cells with s-MoOx

generally show a Voc value of 0.55 V

which is 50 mV lower than that for the

same devices made with e-MoO3. An

exploration of better aqueous precursors

as well as processing conditions is needed.

In spite of progress made, the uniformity

issue of ultra-thin s-MoOx films has to be

resolved to enable their use for large scale

manufacturing.

3.2 Tungsten oxide (WO3)

Another n-type metal oxide with a high

work function is WO3; similar to MoO3,

its electronic structure highly depends on

the stoichiometry, the crystalline struc-

ture and thus the deposition conditions.

Back in the 1980s, the valence band

structure and the Fermi energy of ther-

mally evaporated WO3 (e-WO3) were

well studied by UPS.90 Evaporated films

of amorphous WO3 are generally defi-

cient in oxygen which gives rise to the

gap states and n-type semiconductivity.

Further depletion of oxygen is observed

when WO3 films are annealed at high

temperatures in vacuum and eventually

converted to metallic WO2. During the

annealing process, the occupancy of gap

states at the valence band edge grows

significantly and leads to a reduction of

work function from 6.05 eV to 5.5 eV.

The work function is also sensitive to

oxygen exposure which reduces the work

function further to 4.7 eV. The optical

band gap of these evaporated films has

been determined to be 3.25–3.41 eV and

was found to decrease by 0.5 eV upon

annealing in air.

al is ª The Royal Society of Chemistry 2012

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Inverted P3HT:PCBM cells using

e-WO3 as a top anode interlayer and

s-TiOx as a bottom cathode interlayer

have been reported by Tao et al.91With an

optimum e-WO3 film thickness, the

devices display a Voc of 0.6 V and a FF

over 0.60, revealing the high work func-

tion that was reported earlier.80,90 Similar

device performance was also demon-

strated using conventional devices

wherein e-WO3 films were used as the

bottom anode interlayer with low surface

roughness (r.m.s. ¼ 0.88 nm).92 The

tungsten suboxide deposited by thermal

evaporation and subsequent hydrogen

reduction shows metallic conductivity

with a large work function and makes

good Ohmic contact to materials with

deep HOMO energies such as poly[(9,9-

dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-

(2,10,3)-thiadiazole)] (F8BT) (5.9 eV).

Recently, solar cells with good perfor-

mance using solution processed WO3

have also been reported. Using tungsten

ethoxide [W(OC2H5)6] solution (in

ethanol) as the precursor, spin-coated

films show a larger work function than

PEDOT:PSS, which leads to a 3.4% PCE

(Jsc ¼ 8.63 mA cm�2; Voc ¼ 0.62 V; FF ¼0.63) for a P3HT:PCBM cell made in a

conventional structure. The devices show

a much enhanced lifetime by maintaining

90% of the initial value after being

exposed to air for nearly 200 hours

without any encapsulation.93 These

promising results make s-WO3 a better

choice than other solution processed

high-work-function oxides for large-scale

coating in the R2R process.

3.3 Vanadium oxide (V2O5)

The third n-type semiconducting oxide

used as the anode interlayer is V2O5. Its

band gap determined by UPS and IPES is

2.8 eV,94 revealing that its absorption

band partially covers the absorption band

of PC71BM. Similar to MoO3 and WO3,

the band structure of e-V2O5 is highly

sensitive to the ambient environment.

Based on the results of in situ UPS

measurements, e-V2O5 films deposited

under ultra-high-vacuum conditions

(<10�10 Torr) reaches a work function

value as large as 7.0 eV, and thus provides

an excellent Ohmic contact to organic

materials with large HOMO energies.94

P3HT:PCBM cells using e-V2O5 (�10�6

Torr) as an anode interlayer show an

This journal is ª The Royal Society of Chemistry

optimum PCE of �3%, which is close to

similar devices with aMoO3 interlayer.7,95

Uponair exposure, thework functionof e-

V2O5 drops significantly to 5.3 eV along

with a significant reduction of electron

affinity and growth of gap states.95

Therefore, compared with the V2O5

deposited in high vacuum, such contami-

nated e-V2O5 is just lightly n-doped and

thus less likely to induce efficient charge

transfer, resulting in a larger contact

barrier with organic materials. As is

evident byXPS spectra, the band structure

change is attributed to the adsorbates that

partially reduce V5+ at the top surface.95

Vanadium(V) oxitriisopropoxide is a

common precursor to synthesize sol–gel

V2O5 at room temperature. Due to air

exposure during the hydrolysis reaction,

the work function, ionization potential

and electron affinity of this solution pro-

cessed V2O5 (s-V2O5) are very close to

those of the contaminated e-V2O5. The

optical gap of sol–gel V2O5 films is 2.3 eV,

indicating the change of its electronic

structure from the bulk to the top

surface.96 Compared with e-V2O5 without

any air exposure, P3HT devices with

s-V2O5 show a reduction ofVoc due to the

poor energy level alignment at the anode

interface. The resulting energy alignment

does not induce a hole extraction barrier;

thus, the reduction of Jsc can only be

explained by the decrease of an extraction

field. Compared with MoO3, less work

has been done on V2O5. For a better

understanding of the charge extraction

mechanism at the s-V2O5–polymer inter-

face, further studies of the interface elec-

tronic structures are still needed.

3.4 Nickel oxide (NiO)

Among the metal oxides presented in this

review, nickel oxide (NiO) is the only

p-type semiconducting oxide. In contrast

to all the above n-type oxides where their

semiconductivity originates from the

negative charge compensation at defect

sites of oxygen vacancies and/or cation

interstitials, the p-type characteristics of

NiO stems from positive charge compen-

sation at the thermodynamically favored

Ni2+ vacancies97–99 and is related to its

complex band structure100 wherein both

the valence band maximum and conduc-

tion band minimum are believed to

contain contributions from the Ni 3d

states.101–103

2012

The work function of NiO strongly

depends on its surface chemistry, crystal

orientation and the thin film deposition

conditions. For NiO formed by in situ

oxidization of sputtered nickel films, the

work function is as large as 6.7 eV.104

NiO films can also be formed by UVO

treatment or oxygen plasma treatment of

metallic nickel or solution processed

nickel containing precursors. The work

function of ex situ oxidized NiO films is

5.0–5.6 eV,104–106 close to those of in situ

prepared samples with subsequent air

exposure.104 Such a work function

change is summarized in Table 2 along

with the high work function oxides pre-

sented above. It is well documented that

the NiO surface readily adsorbs surface

contaminants such as nitric oxide,

various carbonaceous species and

hydroxyl species107–109 within minutes of

air exposure and results in a work func-

tion reduction. Interestingly, a recent

study by Ratcliff et al. on solution-pro-

cessed NiO concluded that post-deposi-

tion O2 plasma treatment increased the

work function from 4.7 eV to 5.3 eV by

creating a dipolar nickel oxyhydroxide

(NiOOH) on the NiO surface.101

However, in earlier work by Ratcliff’s

collaborators,105 it was shown that the

work function quickly decreased after O2

plasma treatment even when the NiO

film was stored in a N2 environment. It is

possible that in these studies, the O2

plasma treatment not only induced a

favorable NiOOH surface dipole but also

removed the surface contaminants, re-

sulting in NiO with a larger work func-

tion and making more surface lattice

sites available for hydroxylation. Upon

exposure to the nitrogen environment

after the O2 plasma treatment, it is

probable that contaminants were ad-

sorbed by NiO, which reduced the work

function again. Despite these changes,

the work function of NiO is sufficient for

the energy alignment with the HOMO

levels of many photovoltaic polymers.

Moreover, due to the small electron

affinity (�2.1 eV) and wide band gap,100

NiO acts as an effective electron blocker

by introducing a large electron trans-

porting barrier at the photovoltaic

polymer interface.110,111 As a result of the

electronic structure of NiO thin films,

high efficiency solar cells have been

fabricated and the results will be pre-

sented in the next section.

J. Mater. Chem.

Table 2 The reported work function values of high-work-function transition metal oxides that are prepared under different conditions. The results wereobtained using UPS or Kelvin Probe

Work function (eV) MoO3 WO3 V2O5 NiO

Vacuum deposited 6.5–6.9 (ref. 72, 74–77) 6.1–6.6 (ref. 80 and 90) 6.8–7.0 (ref. 94 and 123) 6.7 (ref. 104)Air exposed 5.3–5.7 (ref. 76–78) 4.7 (ref. 90) 5.3 (ref. 95) 5.5 (ref. 104)Solution processed 5.3–6.0 (ref. 78, 87 and 89) 4.8 (ref. 93) 5.3 (ref. 96) 5.3–5.4 (ref. 101, 105 and 110)

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3.4.1 Vacuum deposited NiO. In

earlier studies on PV cells with NiO

interlayers, NiO films were deposited in

vacuum where the O2 partial pressure and

moisture level during film growth can be

accurately controlled. Irwin et al.112 first

demonstrated NiO hole extracting/elec-

tron blocking layers in conventional

P3HT:PCBM cells by depositing NiO via

pulsed laser deposition (PLD) on top of

ITO. They observed an increase in Jsc,Voc

and FF when compared to devices with

no hole extraction/electron blocking

layers and devices with PEDOT:PSS as

the hole extraction layer. The optimized

solar cells incorporated a 10 nm thickNiO

layer to minimize series resistance and

optical absorption by NiO and achieved a

PCE greater than 5%. Encapsulated

devices with NiO showed good stability

up to 60 days of continuous operation,

while the stability curve trends for much

longer times. Conductive AFM measure-

ments from the same group revealed a

reduction in conductivity ‘‘hot and cold

spots’’ when NiO is deposited on ITO,

suggesting a homogeneous electrical

surface which reduced leakage and

recombination at the NiO–organics

interface.113 UPS measurements revealed

a large work function and ionization

potential which are sufficient for align-

ment with the HOMO energies of many

state-of-the-art photovoltaic polymers.

Additionally, the Voc enhancement was

attributed to the reduction of electron

leakage current. Since then, several

groups have also showed enhanced device

performance by depositing NiO by sput-

tering,114–116 PLD117 and O2 plasma

treatment of evaporated nickel films.118

While NiO films can easily be fabricated

by a wide variety of vacuum based tech-

niques, solution processed NiO is needed

for R2R processing.

3.4.2 Solution-deposited NiO.

Unlike ZnO and TiO2, a direct route from

the metal salt to a colloidal nanocrystal-

line NiO at low temperatures and without

J. Mater. Chem.

aliphatic insulating ligands has not been

demonstrated. This seems to be due to the

fundamental difference in the chemical

behavior of nickel compared to that of

zinc or titanium. For example, during the

synthesis of ZnO nanoparticles under the

excess base condition, the precipitated

zinc hydroxide [Zn(OH)2] does not

terminate the reaction in alcoholic solvent

because it rapidly forms soluble interme-

diate tetrahydroxozincate ions (i.e.

[Zn(OH)4]2�) which then undergo water-

catalyzed dehydration to yield colloidal

nanocrystalline ZnO.119 In the case of

synthesizing colloidal TiO2, researchers

take advantage of the oxophilicity of

titanium, i.e. it induces hydrolysis of

ambient water or directly abstracts

oxygen from its surroundings.120 This

process has been shown to readily yield

nanocrystalline TiO2 from a TiCl4precursor without any bulky insulating

ligands. However, the synthesis of nickel

oxide from wet chemical techniques

cannot follow either of these routes.

Nickel hydroxide is insoluble in alcohol

and cannot accept excess OH� to form

soluble complex anions. Also, it has not

been shown that nickel salts possess the

same ability to induce direct hydrolysis or

oxygen abstraction in alcoholic solutions

to form NiO. Despite these limitations,

thus far, there have been several routes to

employing solution-processed NiO in

solar cells, including the ‘‘sol–gel’’ route in

the conventional devices,105,110,111 pep-

tized nickel hydroxide in a conventional

device structure,121 and a dispersion of

fine NiO powder into solvents for the top

electrode interlayer in inverted solar

cells.122 The sol–gel route is more popular

for solution-processed NiO films used in

organic solar cells. The first report of

solution-processed NiO in PSCs came

from Steirer et al.,105 where they reported

that nickel formate formed a complex

with ethylenediamine to compose the

nickel ink. NiO films were formed by spin-

casting followed by heat treatment (250–

300 �C) in air. O2 plasma treatment

This journ

increased the NiO work function, which

enabled solar cells to be made with iden-

tical PCEs to those of the control devices

with PEDOT:PSS. Further work from

Steirer, Ratcliff, and collaborators has

developed a more comprehensive under-

standing of the surface interaction of NiO

with photoactive layers based on P3HT

and PCDTBT.101,110,111 The O2 plasma

treatment on NiO not only increased its

work function, it also increased its band

gap energy and enhanced its electron

blocking capability. UPS and XPS results

were correlated with device performance

and showed that the dipolar NiOOH

species on the NiO surface was respon-

sible for the increase in work function as

well as the enhanced hole extraction and

electron blocking at the NiO–active layer

interface. As shown in Fig. 7, these effects

produced solar cells with higher values of

Jsc, Voc and FFs than the control devices

using PEDOT:PSS, and the devices ach-

ieved a final PCE of 6.7% using

PCDTBT:PC71BM as the photoactive

layer.110,111

As a very promising solution processed

anode-interlayer material, the high

temperature for annealing during the NiO

film processing may be the bottleneck for

its flexible electronics applications. A

better understanding of the chemical

processes during heat treatment will help

to realize low-temperature solution pro-

cessed NiO films.

4. Summary and outlook

In summary, we reviewed the synthesis

and the electronic properties of transition

metal oxides used for both anode as well

as cathode interlayers in PSCs along with

the performance data of the resulting

devices. Low work function oxides, such

as ZnO and TiO2, with their work func-

tion matching the LUMO levels of elec-

tron-accepting fullerene derivatives are

used as electron extraction interlayers.

High work function oxides such asMoO3,

WO3 V2O5, and NiO form good Ohmic

al is ª The Royal Society of Chemistry 2012

Fig. 7 Solar cell performance of BHJ devices utilizingNiOx or PEDOT:PSS as the anode interlayer.

(a) J–V characteristics under AM 1.5 G illumination. Control devices with PEDOT:PSS exhibited

5.7% PCE compared to 6.7% of those with NiOx. Inset: J–V curves comparing BHJ solar cells with

as deposited and oxygen-plasma treatedNiOx films. (b) Dark J–V characteristics and corresponding

fits. Solar cell performance is improved withNiOx via a reduced reverse saturation current and diode

ideality factor. Reprinted with the permission of ref. 111. Copyright 2011 Wiley-VCH.

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contacts to many state-of-the-art photo-

voltaic polymers and can be used as hole

extraction layers. All these metal oxides

can be deposited by solution processes in

the form of colloidal nanoparticles or

aqueous precursors and the results are

promising for R2R processing. Specifi-

cally, the PCEs of PSCs with solution

processed low work function oxides such

as ZnO and TiO2 as cathode interlayers

are comparable with devices using

vacuum deposited low work function

electrodes. Therefore, these printable

oxides are becoming prototypical in the

manufacturing of plastic solar cells.

However, to realize all-solution-pro-

cessed plastic cells with stable perfor-

mance, further understanding of oxygen

substoichiometry in the high-work-func-

tion oxides and optimization of solution

processed anode interlayers are required.

The promising results achieved in this

area enable us to believe that interface

engineering based on solution processed

oxides will accelerate the commercializa-

tion of low-cost polymer photovoltaic

cells.

Acknowledgements

We acknowledge the support of Depart-

ment of Energy Basic Energy Sciences

(Award no. DE-FG0207ER46464) for

our work on solution processed ZnO.

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