Solar Energy Materials & Solar Cells 100 (2012) 97–114
Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
Review
Polymer- and carbon-based electrodes for polymer solar cells:
Toward low-cost, continuous fabrication over large area
Riccardo Po a, Chiara Carbonera a,n, Andrea Bernardi a, Francesca Tinti b, Nadia Camaioni b
a
b
Centro ricerche per le energie non convenzionali, Istituto ENI Donegani, ENI S.p.A., via G. Fauser 4, 28100 Novara, Italy
Istituto per la Sintesi Organica e la Fotoreattivita (CNR-ISOF), Consiglio Nazionale delle Ricerche, via P. Gobetti 101, 40129 Bologna, Italy
a r t i c l e i n f o
abstract
Article history:
Received 10 November 2011
Received in revised form
21 December 2011
Accepted 23 December 2011
Available online 25 January 2012
The growing interest in organic photovoltaics and the potential for a future mass production urges to
find alternatives to the presently employed materials that are well performing but not convenient from
the point of view of large area fabrication. Electrodes based on non abundant elements, or that
constitute an issue for devices (i) long term stability, (ii) mechanical robustness and (iii) continuous
fabrication process, shall be possibly soon replaced by earth abundant, easy processable and
sustainable materials. Many groups have recently started to devote their research work on materials
not containing metals or metal oxides, and the time has come to summarise the progress that has been
reached so far.
& 2012 Elsevier B.V. All rights reserved.
Keywords:
Polymer solar cells
PEDOT electrode
Graphene electrode
Carbon nanotubes electrode
ITO-free electrode
Metal-free electrode
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Polymer electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
2.1.
PEDOT:PSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
2.1.1.
Low conductivity PEDOT:PSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
2.1.2.
High conductivity PEDOT:PSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
2.2.
In-situ prepared PEDOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
2.3.
Other polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Carbon materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
3.1.
Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
3.2.
Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
3.3.
Diamonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
1. Introduction
Thin-films solar cells based on polymeric photoactive materials represent a promising technology to afford low-cost, readily
available energy. In the last years a large number of academic
n
Corresponding author. Tel.: þ390321447001.
E-mail address: [email protected] (C. Carbonera).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2011.12.022
groups and industrial companies have started research programs
aiming to achieve efficient, durable and cheap solar cells that can
enter the market of photovoltaics [1].
The optimization of polymer solar cells (PSCs) encompasses
the development of new approaches in the design of both active
materials and device architectures [2–7]. A relevant part of the
literature on PSCs concerns the active components of the cell,
mainly the electron-donor [8–10] and, to a minor extent, also the
electron-acceptor [6,7]. The aim is the preparation of materials
98
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
with broad absorption, ideal energy levels, high charge carrier
mobility, controlled morphology, high stability.
The device architecture of PSCs usually comprises buffer
materials [11,12] interposed between the active layer and one
or both the electrodes. They are used to improve the electrode
selectivity, to tune the electrode work function, to render more
ohmic the active layer/electrodes interfaces [13], to act as optical
spacers.
The device electrodes assure the collection of photogenerated
charge carriers and the difference of their work function provides
the driving-force for carriers migration by generating a built-in
potential. The cathode is usually a low work function metal
(aluminium, calcium, barium, silver, etc.) or a metal pair (Ca/Al,
Mg/Al, etc.), whereas a high work function material is used for the
anode. Tin-doped indium oxide (ITO) is commonly used as the
transparent electrode, acting as the anode in conventional solar
cells [14] or as the electron-collecting electrode in inverted cells
[3], though very thin metal layers ( o20 nm) have been also
proposed [15–17].
The literature related to the variety of materials for electrode
fabrication is by far inferior, compared to the other components of
PSCs, though electrode deposition is considered a critical and
limiting step in roll-to-roll (R2R) processes [18–29], which can
open the way for flexible, light-weighing, cheap polymer solar
cells, potentially very competitive on the market. Indeed, according to a recently published study [30], flexible PSCs on plastic
support would have both a substantially lower environmental
impact and a significantly reduced cost, compared to rigid panels
on glass substrate. In R2R fabrication, each layer of the device
structure is printed or deposited in form of inks. Worth to be
noted, in order to limit the environmental impact of the manufacturing process of flexible solar modules toxic solvents must not
be used for the ink formulations, and substituted with environmentally and health friendly compounds (water or alcohols)
[31–32]. In this context, ITO and metals do not lend themselves
to printing processes readily.
ITO is commonly manufactured with a high throughput by
sputtering or e-beam evaporation, in a high-vacuum, high-temperature, energy-costly process, and with significant differences
of its properties from batch to batch [33–35]. Commercial ITO/PET
(PET is poly(ethylene terephthalate) or ITO/PEN (PEN is
poly(ethylene 2,6-naphthalenedirboxylate) rolls produced off-line
are used as starting substrates in R2R fabrication of PSCs and a
lithographic patterning step is required to obtain the desired
geometries. Alternatively, ITO could be sputtered as the last layer,
but damaging the underlying organic layer [36–38] (buffer
materials may mitigate this problem), or it can be deposited inline as a paste or ink, or through sol–gel techniques, but with
inferior electric and optical properties [39–42], so these alternatives seem not to be viable. In any case, one of the main faults
of ITO on a flexible support consists in the unsatisfactory
mechanical properties [43–47], because it tends to crack and/or
to delaminate, especially after repeated bending cycles. Another
issue is that the atomic elements, in particular indium, tend to
migrate into the active and/or the buffer layer [48], promoting
their degradation [49,50]. Finally, indium is an expensive and
scarcely available metal [14,30,51], and, given that alternative
transparent oxides show worse properties [14], effective ITO
substitutes for PSCs would be highly desirable [52].
Concerning low work function metal electrodes, vacuum evaporation can be integrated in R2R, although not in a trivial way,
and damage of the organic layer is often observed [53,54]. Metalbased inks [55,56] or pastes can be used, but their effectiveness
needs further improvements. Krebs et al. showed also that silver
inks based on organic components destroy the morphology of the
underlying layers [57]; by using metallic grids, the area covered
by the ink is lower, and the damage is limited. In addition, inks
made of low work function metals are still difficult to be used
routinely [58], other than being easily oxidable.
On the whole, electrodes are expensive components of organic
photovoltaics, besides being hard to be included in continuous
manufacturing processes. They have been evaluated [59,60] to be
responsible for more than 50% of the materials energy content
and almost 45% of the energetic cost associated to the lab-scale
process of fabrication, causing the increase of the energy payback
time of the final device beyond competitive values (Fig. 1).
A similar analysis was later performed on a preindustrial-scale
R2R process [61], which confirms and reinforces the previous
conclusions: ITO on PET is responsible for more than 85% of the
materials energy content while the evaporation/patterning steps
account for about 50% of the direct process energy involved in the
fabrication of an organic solar module [61]. Other independent
studies based on different assumption lead qualitatively to the
same conclusions, although a lower contribution of ITO is estimated [62]. Recently, the tremendous potential to reduce the
energy consumption during PSC manufacturing by all-solution
Direct Process Energy
Embodied Energy Input Materials
Al evaporation
17.30%
Nitrogen
48.19%
N2 atmosphere
maintenance
38.57%
PEDOT:PSS
0.61%
PCBM
0.44%
Others
1.41%
Ca evaporation
16.47%
P3HT
0.07%
Ca Active layer spincoating
0.02%
0.25%
ITO/glass
50.40%
Al
0.06%
Toluene
0.21%
ITO cleaning
9.32%
Annealing
13.13%
PEDOT:PSS spincoating
4.95%
Fig. 1. Material energy content and process energetic cost for the manufacturing of polymer solar cells on laboratory scale. Data extracted from Ref. [59].
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
processes has been demonstrated [63], and an Energy Payback
Time as low as one day can be attained under particular
conditions.
In this paper, the literature on alternative electrode materials
for polymer solar cells is reviewed, while extensive environmental and economic assessments can be found in recent publications
[64,65]. In perspective, such alternative materials can be applied
in all-solution operations that should lead to less expensive and
less energy demanding manufacturing processes.
The databases used to select the relevant papers were CAS (via
STN International-FIZ Karlsruhe), Science Direct and the databases of ISI Web of KnowledgeSM. The search terms were ‘‘polymer solar cells’’, ‘‘organic solar cells’’, ‘‘organic photovoltaics’’,
‘‘polymer photovoltaics’’, ‘‘ITO free’’, ‘‘metal free’’, ‘‘indium free’’,
‘‘electrode’’, ‘‘anode’’ and ‘‘cathode’’. In addition, the references
cited by the found documents were taken into account, and an
additional search on ‘‘cited reference search’’ in the ISI system
was carried out. Low-molecular weight and hybrid solar cells
have not been considered. The relevant papers, grouped by
publication year and subject, are summarized in Table 1. The
number of published paper increased in the last ten years, parallel
to the rising interest for low-cost production of PSCs. However only
a few classes of effective materials have been developed, namely
poly(3,4-ethylenedioxythiophene) derivatives, poly(aniline)s, carbon
nanotubes and graphene. These families will be discussed in detail
in the following paragraphs.
2. Polymer electrodes
Due to its high transparency in the visible light spectrum, easy
aqueous solution processing, and application for flexible devices,
99
poly(3,4-ethylenedioxitiophene):poly(styrenesulfonate) (PEDOT:PSS)
is definitely the predominant conducting polymer investigated as
alternative low-cost electrode for ITO-free and metal-free solar
cells, and in general for organic electronics. Indeed, the simultaneous attainment of high conductivity and high transparency is
not an easy task, so it is not surprising that just a few examples of
polymer electrodes different from PEDOT:PSS have been reported
for solar cells application (Fig. 2). In the present section, water
dispersion PEDOT:PSS used as electrode in PSCs is largely
reviewed in the first two paragraphs, in situ prepared PEDOT is
accounted for in the third paragraph, while the last one is devoted
to alternative polymeric materials.
2.1. PEDOT:PSS
PEDOT:PSS was developed and originally commercialized by
Bayer AG under the trade name of Baytrons, then by HC Stark
GmbH and currently by Heraeus Holding GmbH under the
tradename of CleviosTM. An exhaustive description of its synthesis, modifications, properties and applications is reported in
several reviews [66,67]. Agfa Gevaert N.V. is also commercializing
PEDOT:PSS grades with the trade name OrgaconTM. Many formulations with different viscosity (suspensions in water or mixed
solvents, inks, pastes), conductivity and transparency are nowadays available (Table 2).
Since the late 1990s, PEDOT:PSS has been the most widely
used anode buffer layer in polymer solar cells [11]. Indeed,
PEDOT:PSS possesses a combination of favourable properties:
(i) it has a good optical transparency in the visible range; (ii) it
is effective in transporting holes to the anode and in blocking
electrons; (iii) its high work function (usually reported between
Table 1
Reference literature considered in the present review, classified by year and main subject.
Year
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
Polymeric materials
Carbon materials
PEDOT:PSS
Other polymers
Carbon nanotubes
Graphene
Diamonds
[71]
–
–
[74]
–
[76,78,82]
[77,114]
[83,127,128,130]
[79,115]
[80,84,85,94,97,106,125]
[86,90,91,102,106,107,108,116,120,123,131]
[75,87,98,101,103,104,109,110,118,121,122,126]
[81,88–90,93,96,99,111,112,113,115,119,129]
–
–
–
–
–
–
–
–
–
–
[131,132]
–
[133]
[144]
–
–
–
–
–
[150]
[151–153,159]
[138,154]
[163]
[137,156,162]
[157,160]
[155]
–
–
–
–
–
–
–
–
–
[170]
[171,172,177]
[173,176]
[174,175]
–
–
–
–
–
–
–
–
–
–
–
[143]
–
Fig. 2. Chemical structures of PEDOT:PSS, PHMEDT and PANI.
100
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
Table 2
Properties of selected commercial grades of PEDOT:PSS.
Grade
Sheet resistance
(O/sq)
Conductivity
(S/cm)
Work function
(eV)
Viscosity
(mPa s)
Notes
Suggested
application
Baytron/Clevios (Bayer/HC Stark/Heraeus)
P AI 4071a
–
P AI 4083a
–
P CH 8000
–
PH 500
170c
PH 510
–
PH 750
130c
PH 1000
100c
P HC V4
–
F CPP105 DM
o 7000c
S V3
700c
0.4
2–20 10 4
3–10 10 6
500d
–
750d
1000d
400d
–
–
–
5.0–5.2
–
4.8–5.0
–
4.8–5.0
4.8–5.0
–
–
–
–
5–15
9–20
8–25
20–100
30
15–50
100–350
30–60
3000–6000
PEDOT:PSS 1:2.5 w/w
PEDOT:PSS 1:6 w/w
PEDOT:PSS 1:20 w/w
–
–
–
–
High boiling point solvent
High adesive formulation (includes a binder)
Screen-printing formulation
HTLb
HTLb
HTL b
Electrode
Electrode
Electrode
Electrode
Electrode
Electrode
–
Orgacon (Agfa)
EL-P 3040
EL-P 5010
–
–
–
–
48000
120000
Screen-printing formulation
Screen-printing formulation
Electrode
–
1000e
200e
a
Alternatives for ‘‘P AI’’ can also be found in the literature, such as ‘‘P VP AI’’; ‘‘PVP’’; ‘‘P Al’’; etc.
HTL: hole transport layer.
c
Value for a thickness of 100 nm or 90% of transmittance.
d
Value for PEDOT:PSS þDMSO (pure PH 500: 1 S/cm).
e
Value for P77 (thr/cm) for EL-P 3040 and P77/55 for EL-P 5010.
b
Table 3
Additives reported in the literature for PEDOT:PSS modification: added to the PEDOT:PSS suspension (ADDED) or spin-coated onto
the PEDOT:PSS layer (SPIN-COATED).
Additive
ADDED
Spin-coated
Methanol
Ethanol
Isopropanol
Methoxyethanol
Dimethoxyehane
Ethylene glycol
Diethylene glycolþ 3-glycidyloxypropyltrimethoxysilane þ surfactant
Glycerine
Sorbitol
Erhythritol
Acetonitrile
Tetrahydrofurane
Acetone
2-{1-[(dimethylamino)carbonyl]-4-pyridiniumyl}ethanesulfonate
3-[dodecyl(dimethyl)ammonio]-1-propanesulfonate
3-[dimethyl(nonyl)ammonio]propyl sulfate
N-methylpyrrolidone
N,N-dimethylformamide
Dimethylsulfoxide (DMSO)
DMSOþ surfactant
DMSOþ isopropanol
DMSOþ isopropanolþ surfactant
DMSOþ sorbitol
DMSOþ diethyleneglycol
DMSOþ diethyleneglycol þsorbitol þsurfactant
–
–
–
–
–
[93,104]
[79]
[74,76]
[74,93,127]
[77,78]
–
–
–
–
–
–
[93]
[93]
[85–87,89,90,93,99,101–103,116]
[96,120–122]
[88,90,100]
[95]
[93,97]
[94]
[94]
[81]
[80,81]
[81]
[80,116]
[80]
[80]
–
–
–
–
[81]
[81]
[81]
[75]
[75]
[75]
–
–
–
–
–
–
–
–
–
4.8 and 5.2 eV) allows the formation of an ohmic contact with
most common donor polymers; (iv) it is stable in ambient
conditions. In addition, the importance of PEDOT:PSS layer in
the planarization of ITO superficial spikes has been often underlined [68–70].
Before the development of high-conductivity PEDOT:PSS (around
2008), low-conductivity grades of PEDOT:PSS were the subject of a
dozen papers on ITO-free PSCs with polymeric anodes (Table 1). To
our best knowledge, the first attempt of using PEDOT:PSS for ITOfree polymer solar cells was reported in 1999 [71]. After that, a
number of groups have devoted their research activity to improve
the electrical properties of commercially available PEDOT:PSS formulations (mainly, by using suitable additives, Table 3), or to open
new roads by using ‘‘in situ’’ polymerization approaches [72,73]. The
main goal was, and still is, to obtain a good compromise between
good conductivity and transparency, along with a high work
function and good processability.
2.1.1. Low conductivity PEDOT:PSS
Because of the low conductivity of buffer layer grade PEDOT:PSS, the related ITO-free solar cells show an extremely high
sheet resistance (typically of 104–105 O/sq) [74], leading to very
poor device performance [71,74,75]. However, the conductivity of
PEDOT:PSS can be increased of orders of magnitude by doping
with appropriate additives, usually followed by thermal treatments [74,76,77].
MEH-PPV:PCBM (MEH-PPV is poly[2-methoxy-5-(20 -ethylhexyloxy)-p-phenylene vinylene] and PCBM is [6,6]-phenyl-C61-butyric
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
acid methylester) solar cells including sorbitol-doped Baytron P as
anode, with a sheet resistance of the order of 103 O/sq and a
transmittance of over 80% in the 350–600 nm range, reached a
power conversion efficiency (PCE) of 0.36%, compared to 0.46%
obtained for similar devices made with an ITO/untreated-PEDOT:PSS
anode. Addition of meso-erythritol to Baytron P AI 4071 enhances
the conductivity from 0.4 S/cm to 155 S/cm [77,78] and
MEH-PPV:PCBM solar cells fabricated with this modified PEDOT:PSS
as the anode exhibited a PCE of 1.5%, close in performance
to a reference cell with conventional ITO/untreated-PEDOT:PSS
anode [77].
Other different oxydrylated compounds, like diethyleneglycol
[79], have been experimented as additives. Hsiao et al. used a
Fig. 3. Power conversion efficiencies of P3HT:PCBM solar cells with anodes made
of Baytron P AI 4071 modified with different additives vs. sheet resistance. Data
from Ref. [80].
101
number of oxygenated solvents to enhance the conductivity of
Baytron P AI 4071: ethanol, methoxyethanol, dimethoxyethane
and ethyleneglycol [80]. These compounds were not mixed to the
PEDOT:PSS suspension, but spin coated over the PEDOT:PSS layer.
Ethylene glycol was found to be the most effective surface modifier
(Fig. 3). Accordingly, P3HT:PCBM (P3HT is poly(3-hexylthiophene))
solar cells with a ethylene glycol-modified Baytron P AI 4071 were
the most efficient (PCE¼3.39% for 1 mm2 device area), although
the reference ITO/untreated-PEDOT:PSS cell exhibited a higher
efficiency (3.80%) on an even larger area (4 mm2). The transmittance of the anode at 550 nm was about 93%, practically independent on the modifying agent. In all cases some differences in the
morphology of the PEDOT:PSS layer were observed: the additive
induced particles coalescence, hence a greater phases segregation
leading to longer conduction paths (Fig. 4).
The conductivity of a buffer layer grade PEDOT:PSS (unspecified
grade; probably P AI 4071, according to the reported characteristics)
can be increased from 0.2 S/cm up to almost 100 S/cm after a
treatment (drop casting and drying) with zwitterion molecules
(2-{1-[(dimethylamino)carbonyl]-4-pyridiniumyl}ethanesulfonate,
DMCSP; 3-[dodecyl(dimethyl)ammonio]-1-propanesulfonate,
DDMAP; 3-[dimethyl(nonyl)ammonio]propyl sulfate, DNSPN) or
up to 190 S/cm after a treatment with copper chloride or bromide
[75]. The enhanced conductivity has been explained in terms of
conformational changes of PEDOT chains, caused by the screening
of coulombic attractions between PEDOT and PSS. The photovoltaic response of P3HT:PCBM solar cells with treated PEDOT:PSS
anodes was better than that with untreated ones, but somewhat
erratic if compared with the conductivity values (Table 1). This
was attributed to the different roughness of the treated PEDOT:PSS
anodes. However, overall the zwitterion molecules gave solar cells
with higher photovoltaic parameters than copper salts.
Similar enhancements of conductivity were achieved by treating Baytron P with solvent/water mixtures (solvent ¼ methanol,
ethanol, isopropanol, acetonitrile, acetone, tetrahydrofurane) at
high temperatures [81]. The improvement of conductivity was
found to be dependent on the mixture composition, the dielectric
3D morphology
TEM inages
TOP view
SIDE view
Pristine
PEDOT:PSS
PEDOT:PSS
+ methoxyethanol
PEDOT:PSS
+ ethylene glycol
Fig. 4. TEM images (left) and schematic tridimensional morphologies (right) of Baytron P AI 4071 films pristine (top) and modified with methoxyethanol (middle) or
ethylene glycol (bottom). Adapted from Ref. [80]. Reproduced by permission of Royal Society of Chemistry.
102
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
constant of the organic solvent and the temperature of the solvent
treatment. P3HT:PCBM solar cells including ethanol/water and
acetonitrile/water treated PEDOT:PSS at 140 1C exhibited efficiencies approaching 3% (Table 4). The reason for the observed
behaviour was attributed to the preferential solvation of hydrophylic PSS by water, and hydrophobic PEDOT by the organic
solvent. In turn, this effect would cause a phase segregation of the
two polymers and a coil-to-linear conformational transition in
PEDOT, similar to the previously reported treatments with zwitterions [75].
2.1.2. High conductivity PEDOT:PSS
The first – and, for a long time, only – comprehensive report on
a high conductive PEDOT:PSS (Orgacon EL-P 3040) for ITO substitution dates back to 2004 [82]. However, ITO-free solar cells
show poor performance also for high conductivity formulations of
PEDOT:PSS [82–86], unless the electrical properties of the polymer anode are enhanced with appropriate additives.
The addition of 5% of a high boiling point polar solvent
(dimethylsulfoxide, DMSO) to Clevios PH 500 increases its conductivity up to 470 S/cm [85], that is just one order of magnitude
lower than that of ITO on glass of about one third of ITO on plastic
substrates. P3HT:PCBM solar cells with DMSO-modified Clevios
PH500 showed excellent performance, both on glass substrate
(PCE 3.27% compared with 3.66% measured for reference devices
on glass/ITO) and PET substrate (PCE of 2.8% against 2.9% for solar
cells on PET/ITO). In addition, ITO-free cells on flexible substrate
revealed a much better mechanical resistance and, differently
from ITO-based cells, did not reduce significantly their efficiency
even after 300 bending cycles (Fig. 5). Efficient fully spray-coated
ITO-free P3HT:PCBM solar cells, including DMSO-modified Clevios
PH500 anodes, have been also demonstrated [87,88].
Similar results have been obtained with Clevios PH510 modified with DMSO [86]. This PEDOT:PSS formulation differs from
Clevios PH 500 for a higher solid content and a consequent better
processability. The dramatic increase of conductivity observed for
films of DMSO-modified high conductivity PEDOT:PSS (about
three orders of magnitudes for the addition of 7% DMSO for
Clevios PH510) was explained in terms of nanomorphology
evolution and increased uniformity of the distribution of
PEDOT-rich regions trough the PEDOT:PSS film. PEDOT:PSS
usually arranges in grains with hydrophobic and highly conductive PEDOT-rich core and a hydrophilic insulating PSS-rich shell
acting as a passive barrier for charge transport. The size of the
grains rose up as a consequence of the addition of DMSO (Fig. 6),
leading to an increased film roughness. This causes a reduction of
the contact surface between grains leading to a superior charge
transport within the layer. At the same time, the addition of
DMSO resulted in a reduction of the PSS content at the surface of
PEDOT grains (as revealed by XPS measurement), weakening the
barrier effect with a consequent improvement of conductivity.
ITO-free P3HT:PCBM solar cells including Clevios PH 510 polymer
anodes modified with DMSO exhibited a PCE of 3.48% for a device
area of a few mm2 [86], decreasing as the area was increased [89],
as expected. A thorough study of the mechanical properties of
gravure printed layers of Clevios PH510 with 7% DMSO has been
recently published [90]. Very low changes in resistivity (less than
1%) have been registered even after 2000 bending cycles, both for
stretching and for bending of the film at different angles.
To further improve the electrical properties of PEDOT:PSS
anodes, mixed additives [91–93] have been proposed, also combined with a bilayer approach [94]. Conductivities of 200–320 S/cm
have been reported for Clevios PH 500 additivated by 5–10% of
DMSO and 5–10% of isopropanol or 2% of a surfactant [95], while
the addition of 5% DMSOþ13% isopropanol to Clevios PH750
anodes resulted in a conductivity of about 590 S/cm [90]. In the
latter case, the related ITO-free P3HT:bisPCBM solar cells exhibited an even higher PCE than that of the ITO-based reference
device (3.5% vs. 3.3%). 1% of a surfactant was added to a conductive
ink made of PH 500þ5% DMSO and patterned anodes was made
with PDMS masking and brush painting technique. A sheet
resistance of 350 O/sq was reported [96].
High conductivity PEDOT:PSS has been also proposed to
replace the top metal contact of inverted polymer solar cells
[97–100], thus realizing semitransparent devices for smart windows application. To this end, appropriate protocols are required
to improve the wettability of the hydrophobic active layer by the
PEDOT:PSS aqueous dispersion, such as a poly(allylamine hydrochloride):dextran nanogel interlayer [98]. Inverted semitransparent P3HT:PCBM solar cells with a top anode based on Clevios PH
500 exhibiting power conversion efficiency of about 2% have been
reported [97,98], while in the case of a mixture of Clevios PH1000
and Clevios F CPP 105 DM, containing DMSO and showing a
conductivity of 400 S/cm, a PCE of 2.4% was obtained [99].
Recently, an efficiency of 2.7% has been reported for inverted
P3HT:PCBM solar cells with a top semitransparent contact, spraycoated from a dispersion of Clevios PH500 with 5% DMSO [100].
If combined with appropriate interfacial layers [11], PEDOT:PSS can also serve as polymer cathode [101,102], opening
the way to full polymer electrode solar cells [102,103].
P3HT:PCBM inverted solar cell with a top Ag anode and a
transparent cathode made of PEDOT:PSS (Clevios PH500 with
the addition of 5% DMSO), an interfacial layer of ZnO nanoparticles and a C60 self-assembled monolayer (Fig. 7) showed interesting efficiencies on both glass and plastic substrate (3.08% and
2.99%, respectively) [102]. However, when the top Ag anode was
replaced by Clevios PH500þ5% DMSO (Fig. 7), the efficiency
dropped to 0.47%.
Much better results were achieved for ITO-free and metal-free
inverted P3HT:PCBM solar cells on glass substrate by using a
PEDOT:PSS formulation (Clevios PH1000) with a higher conductivity (680 S/cm) for both anode and cathode electrodes [103]. Again, a
buffer layer grade PEDOT:PSS (Clevios CPP 105 D) was interposed
between the active layer and the top anode, while a ZnO interfacial
layer was used to realize the electron-selective bottom cathode. The
resulting all-polymer solar cells showed an optical transmittance of
10 to 55% in the range from 400 to 800 nm and exhibited an
average power conversion efficiency of 1.8%.
As clearly emerges, the most critical parameter of PEDOT:PSS
electrodes is still the low conductivity, though in last years it is
rapidly approaching that of ITO (Fig. 8). This results in high sheet
resistance of the polymer electrode, leading to solar cells with
high series resistance and poor ability of current extraction. The
thickness of the PEDOT:PSS could be increased to reduce its sheet
resistance, but to the detriment of its transparency, in the case of
the illuminated electrode. A lower sheet resistance results in an
improved fill factor, but the less efficient solar light harvesting
could negatively reflect on the short circuit current (Jsc), as
illustrated in the case of Fig. 9, though the parallel reduction of
the device series resistance has a beneficial effect also on Jsc.
The strong correlation between cell performance and series
resistance has been clearly shown by Kim et al. [104] for ITO-free
P3HT:PCBM solar cells made with a Clevios PH 500 anode
modified with ethylene glycol (conductivity of the order of
102 S/cm) and it is illustrated in Fig. 10. In that case the series
resistance of the cell was varied by changing the device geometry
and not the thickness of the PEDOT:PSS layer, thus its sheet
resistance or transparency.
To compensate the high sheet resistance of PEDOT:PSS, extremely critical in large area applications, metal grids can be
integrated with the polymer electrode. For this purpose, silver is
the most frequently used material [79,82,105–113], but gold
Table 4
Non exhaustive survey of P3HT:PCBM solar cells including PEDOT:PSS as the anode, having the structure Glass/PEDOT:PSS/P3HT:PCBM/cathode (with the cathode made of a low work-function metal or a buffer layer/metal).
The photovoltaic parameters are compared to those measured for reference devices (values in round and square brackets) when available.
Cell architecture (reference electrode)
PEDOT:PSS anode properties
2
Photovoltaic parameters @100 mW/cm2
Area (mm ) Thick. (nm) R& (O/sq) T (%)
Jsc (mA cm
Glass/Baytron PH500þ 5%DMSO (Glass/ITO/Baytron P AI 4083)
PET/Baytron PH500 þ 5%DMSO (PET/ITO/Baytron P AI 4083)
Glass/Clevios PH510 þ 7%DMSO (Glass/ITO/Baytron P AI 4083)
Glass/Baytron PH500þ 5%DMSO (spray) (Glass/ITO/Clevios PH500 þ 5%DMSO) (spin)
PET/PH510 þ 7%DMSO
Glass/Baytron P AI 4071/ethylene glycol
Glass/Baytron P AI 4071/ethylene glycol
Glass/Baytron P AI 4071/methoxyethanol
Glass/Clevios PH500 þ 5%DMSO þ5%isopropanol
Glass/Clevios PH750 þ5%DMSO þ 13%isopropanol (Glass/ITO/PEDOT:PSS (10 3 S/cm))
Glass/Baytron PH500þ 5% ethylene glycol
Glass/Clevios PH500 þ 10% sorbitol (Glasss/ITO/Clevios PH500)
Glass/Clevios PH500 þ 5% DMSO (Glasss/ITO/Clevios PH500)
Glass/Clevios PH500 þ 5% DMSO þ 5% sorbitol (Glasss/ITO/Clevios PH500)
Glass/Clevios PH500 þ 5% ethylene glycol (Glasss/ITO/
Clevios PH500)
Glass/Clevios PH500 þ 5% N-methylpyrrolidone (Glasss/ITO/
Clevios PH500)
Glass/Clevios PH500 þ 5% N,N-dimethylformamide (Glasss/ITO/
Clevios PH500)
Glass/Clevios PH500:DMSO:DEG:sorbitol:surfynol/Clevios PH500:DMSO:DEG
(Glass/ITO/Baytron P AI 4083)
Glass/Clevios P:CuCl2
Glass/Clevios P:CuBr2
Glass/Clevios P:DMCSP
Glass/Clevios P:DDMAP
Glass/Clevios P:DNSPN
Glass/Clevios P þ ethanol/water
Glass/Clevios P þ acetonitrile/water
Glass/PEDOT:OTs:SiOx
4.34
4.34
4.66
4.66
–
1
2
1
20
20
9
4
4
4
4
100
100
300
–
100
50
50
50
240
168
270
200
200
200
200
213
213
63
–
359
–
–
–
132
101
–
–
–
–
–
490 (Vis.)
490 (Vis.)
77 (550 nm)
–
89 (550 nm)
92–94 (550 nm)
92–94 (550 nm)
92–94 (550 nm)
84 (500 nm)
95 (500 nm)
70–80 (400–650 nm)
–
–
–
490 (vis.)
4
200
–
4
200
14
11
11
11
11
11
11
11
8-10
a
)
Ref.
Voc (V)
FF
9.73 (8.42)
9.16 (8.24)
8.53 (9.79)
6.62 (8.06)
9.10
8.99
6.75
9.50
–
–
6.88
7.10 (10.6)
9.6 (10.6)
11.3 (10.6)
10.5 (10.6)
0.62
0.61
0.57
0.61
0.50
0.59
0.57
0.58
–
–
0.53
0.63
0.60
0.57
0.59
0.54
0.50
0.68
0.54
0.45
0.64
0.45
0.57
–
–
0.65
0.40
0.61
0.56
0.61
–
8.20 (10.6)
–
–
2.70 (10.6)
110
–
78 (300–750 nm)
8.50 ( 9.00) 0.58 ( 0.59) 0.52 ( 0.63) 2.60 ( 3.40) [94]
130
130
130
130
130
130
130
95
455
409
950
1570
836
1054
974
120
–
–
–
–
–
–
–
80 (510 nm)
2.85
6.62
7.00
8.51
8.25
8.99
7.84
4.89
(0.62)
(0.62)
(0.62)
(0.62)
3.27 (3.66)
2.80 (2.90)
3.29 (3.89)
2.17 (2.86)
2.00
3.39
1.72
3.13
2.20a
3.50 (3.30)
2.37
1.80 (3.88)
3.51 (3.88)
3.64 (3.88)
3.77 (3.88)
[85]
[85]
[86]
[87]
[90]
[80]
[80]
[80]
[91]
[90]
[104]
[93]
[93]
[93]
[93]
0.57 (0.59)
0.43 (0.62)
2.01 (3.88)
[93]
0.51 (0.59)
0.28 (0.62)
0.37 (3.88)
[93]
0.52
0.52
0.60
0.57
0.55
0.61
0.63
0.54
(0.64)
(0.63)
(0.58)
(0.59)
PCE (%)
(0.59)
(0.59)
(0.59)
(0.59)
0.29
0.31
0.30
0.43
0.40
0.52
0.51
0.40
(0.68)
(0.56)
(0.68)
(0.60)
0.43
1.08
1.24
2.08
1.79
2.87
2.51
1.05
[75]
[75]
[75]
[75]
[75]
[75]
[75]
[126]
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
Anode
2
Measured @85 mW/cm2.
103
104
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
Fig. 5. Changes in J–V curves of flexible P3HT:PCBM solar cells during repeated bending: (a) ITO anode (conventional Organic Solar Cell, OSC); (b) Clevios PH500 anode
(ITO-Free Organic Solar Cell, IFOSC). (c) Changes in device efficiency vs. bending cycle. From Ref. [85] by permission of Wiley-VCH.
Fig. 6. AFM topographies (a)–(d) and phase images (e)–(h) of Clevios PH 510 films containing different amounts of dimethylsulfoxide. From Ref. [86], reproduced by
permission of Royal Society of Chemistry.
[105,114–117] and copper grids [105,118] have been also
reported. Either evaporation [106,111,114–117] or printing techniques [79,82,105,107–113] are used for the grid deposition.
Recently, Kylberg et al. reported an original approach consisting
in the fabrication of composite woven mesh electrodes consisting
of molibdenum nanowires and polymer fibres embedded in a
Orgacon EL-P3040 matrix [119].
Before concluding this session, it is worth mentioning the
quite recent use of PEDOT:PSS – alone or combined with silver –
on textile substrates, to give ‘‘fabric electrodes’’ [120–123].
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
105
Fig. 7. Inverted solar cells with a PEDOT:PSS cathode and a top Ag (a) or a PEDOT:PSS anode. The PEDOT:PSS layer (CLEVIOS P VP AI 4083) over the P3HT:PCBM active layer
acts as a buffer layer. From Ref. [102] by permission of Elsevier B.V.
Fig. 8. Record conductivity over the years of PEDOT:PSS anodes used in ITO-free
polymer solar cells.
Fig. 9. Variation of Jsc, FF and PCE (the open-circuit voltage does not show
meaningful variations) of inverted P3HT:PCBM solar cells with the sheet resistance
of the illuminated PEDOT:PSS cathode (Clevios PH500þ 5% DMSO, thickness ranging
between 40 and 220 nm). The values of the photovoltaic parameters are expressed
as the percentage of those measured on a reference cell with the same structure but
with ITO replacing the polymer cathode. The percentages given in the graph is the
transmittance at 510 nm of the PEDOT:PSS cathode. Data from Ref. [102].
Working devices have been demonstrated, that are compatible
with continuous fabrication processes.
2.2. In-situ prepared PEDOT
The use of commercial PEDOT formulations for metal-free
anode depositions is more straightforward, however in situ
synthesised PEDOT may offer additional advantages in terms of
greater flexibility and properties tunability. Several techniques
are known to polymerize the EDOT monomer directly on the
target substrate, all requiring three main ingredients: the monomer, an oxidant (usually Fe(OTs)3 or FeCl3), and a weak base (e.g.,
pyridine or imidazole) as oxidation regulator (inhibitor).
Chemical oxidative polymerization has been used to prepare,
for example, PEDOT films (conductivity as high as 750 S/cm and
81% transparency) from EDOT and Fe(OTs)3 plus imidazole to
control oxidation [124,125]. To enhance the adhesion between
PEDOT and glass substrates, the addition of silicate in the in-situ
polymerization has been suggested, leading to PEDOT:SiOx hybrid
anodes [126].
Another approach consists in vapour phase polymerization
(VPP) (see references in [127]), that provides high conductivity for
‘‘in situ’’ prepared PEDOT. This method consists in spin coating or
silk screen printing on the substrate a solution containing an
oxidising agent plus a basic inhibitor. The substrate is then
transferred to a polymerization chamber containing an EDOT
reservoir creating a monomer vapour phase that reacts on the
substrate thanks to the presence of the oxidant. In 2006, almost
contemporarily, Winther-Jensen et al. [128] and Admassie et al.
[127] published the results of their investigations on the possibility of employing vapour phase polymerised PEDOT (VPP-PEDOT),
envisaging the possibility of scaling up the process to large areas.
In both cases pyridine was used as inhibitor. ITO-free solar cells
made of VPP:PEDOT showing high conductivity (775 S/cm) and
high transparency (84%) have been reported with an efficiency
comparable to that measured on similar devices made with a
PEDOT:PSS:sorbitol anode [127]. Vapour phase oxidative polymerization of PEDOT:OTs made using Fe(OTs)3 as an oxidant for
EDOT has been very recently suggested [129] and conductivities
of the order of 650 S/cm have been reported, with a very strong
adhesion of PEDOT:OTs on the glass substrate. VPP:PEDOT has
been also proposed as the electron-collecting electrode in polymer solar cells [130], but with very poor performance of the
related devices.
2.3. Other polymers
A polymer anode made of in-situ polymerized 3,4-(1-hydroxymethyl)ethylenedioxythiophene (Fig. 2) has been proposed as
an alternative to the more ‘‘conventional’’ PEDOT [131]. The
hydroxymethyl groups increase the interaction and the adhesion
to the glass substrate, as revealed by the AFM images showing a
better uniformity of the films, compared to the common Clevios P
AI 4083. A 45 nm thick layer of poly[3,4-(1-hydroxymethyl)
ethylenedioxythiophene]:toluenesulfonate (PHMEDT:TS) exhibited a transmittance of 87% at 510 nm and a conductivity of
LiF/Al
P3HT:PCBM
ITO
PEDOT:PSS
glass
shadow
mask
8
3.5
7
3.0
Sheet resistance (Ω/sq)
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
Jsc (mA cm-2)
106
d
6
2.5
5
10
15
20
d (mm)
Fig. 10. Series resistance and short-circuit density current of P3HT:PCBM solar cells with ethylene glycol-modified Clevios PH 500 anode as a function of the
anode–cathode distance. Data from Ref. [104].
700 S/cm. When used as anode in ITO-free devices, P3HT:PCBM
solar cells with an efficiency of 0.61% were obtained.
Polyaniline (Fig. 2) is the only reported polymer radically
different from PEDOT, though its low transmittance below
480 nm (o40%) [132] could represent a limit when used as
transparent electrode. Through a thickness-controlled drop-casting
method [133], camphorsulfonic acid doped polyaniline (PANI:CSA)
electrodes exhibiting a conductivity of about 600 S/cm, a high
optical transmittance of about 85% at 550 nm and good
performance preservation after 50 bending cycles have been
demonstrated. ITO-free P3HT:PCBM solar cells including PANI:
CSA as anode and made onto a flexible substrate exhibited a PCE
of 2% [133].
Although only a few alternatives to PEDOT:PSS have been
proposed till now, the research on conductive polymers exhibiting a proper properties balance is in progress [134] and valid
options could be available soon.
3. Carbon materials
Carbon nanotubes are regarded as promising candidates for
the replacement of transparent conducting oxides in the emerging field of plastic electronics, due to their transparency in thin
films, high electrical conductivity, excellent mechanical properties and inherent flexibility, and the potential for roll-to-roll
processing [135]. Single-wall (SWNTs) and multi-wall carbon
nanotubes (MWNTs) [136], as well as few-wall carbon nanotubes
(FWNTs), 2–5 walls [137], have been investigated as transparent
anodes in polymer solar cells. The aim is to replace the expensive,
stiff and brittle ITO substrate, though improved solar cells
have been demonstrated, compared to ITO-only devices, when
carbon nanotubes have been combined with an underlying ITO
electrode [138].
Also graphene [139], the rising star of material science,
exhibits remarkable mechanical and electronic properties [140]
and is recently attracting much attention as novel transparent
electrode material [141,142]. Nevertheless the performance of
ITO-free polymer solar cells including graphene electrodes are
still rather poor and more work is required in order to make
graphene a promising replacement of transparent conductive
oxides.
Even p-doped nanocrystalline diamonds have been recently
proposed as transparent anodes for ITO-free polymer solar
cells [143].
3.1. Carbon nanotubes
Carbon nanotubes were first proposed by Ago et al. to replace
the ITO electrode in polymer photovoltaic devices [144]. Multiwalled carbon nanotubes were used as hole-collecting electrode in
solar cells made of a composite of MWNTs and poly(p-phenylene
vinylene) (PPV). The catalytically synthesized MWNTs [145] were
oxidized in acid solution [146] and the nanotube water dispersion
was spin-coated onto glass substrates to a thickness ranging
between 20 and 300 nm. The polymer layer was obtained by first
spin-coating the PPV precursor onto the MNWT films, followed by
thermal conversion [147]. The photovoltaic properties of MWNT/
PPV/Al devices were compared to those of ITO/PPV/Al reference
devices by illuminating through the semitransparent aluminium
cathode. An external quantum efficiency (EQE) about twice that
of the reference cells was observed with MWNT hole-collector.
The enhanced EQE was attributed to the complex interpenetrating
network of the polymer chains with the underlying MWNT
rough layer and to the stronger built-in electric field due to
the higher work function of MWNTs (5.1 eV) [148] with respect
to ITO.
Following the work of Wu et al. [149], showing that films
made of high purity single-wall carbon nanotubes could represent
a promising alternative to ITO, solar cells made of P3HT:PCBM as
active layer were reported, with an improved power conversion
efficiency when a SWNT layer was substituted to the common ITO
electrode [150], but for very thick drop cast active layers of
800 nm. The best photovoltaic properties were obtained with
SWNT layers showing the lowest sheet resistance (R& ¼282 O/sq),
among the investigated ones prepared with different thickness,
though their reduced optical transmission. The related solar cell,
with the structure glass/SWNT/PEDOT:PSS/P3HT:PCBM/Ga:In, gave
a PCE of 0.99%, compared to 0.69% exhibited by a reference cell
fabricated onto glass/ITO electrode. The photovoltaic parameters
mainly enhanced in the SWNT-based cell were the short-circuit
current and the open-circuit voltage (Table 5), the fill factor being
similar (0.30 and 0.32, for the SWNT and the ITO-based cell,
respectively). The enhanced performance of the SWNT cell, though
the poor optical transmission of the bottom hole-collector, was
explained with the presence of voids in the SWNT layer, through
which the overlying PEDOT:PSS/P3HT:PCBM infiltrated and
reached the surface of the glass substrate. So, despite the low
transparency of the SWNT layer in comparison to ITO, the presence
of those voids provided sufficient exposure of the active layer to
illumination.
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
107
Table 5
Non exhaustive survey of polymer solar cells including carbon nanotubes as the bottom transparent anode. The photovoltaic parameters are compared to those measured
for a reference cell (values in parenthesis), when available, fabricated in the same conditions but onto a glass/ITO substrate.
Cell architecture
glass/SWNT/PEDOT:PSS/P3HT:PCBM/Ga:In
glass/SWNT/PEDOT:PSS/P3HT:PCBM/Al
PET/SWNT/PEDOT:PSS/P3HT:PCBM/Al
glass/SWNT/PEDOT:PSS/P3HT:PCBM/Al
PET/SWNT/ZnO-nw/P3HT/Au
glass/FWNT/P3HT:PCBM/Al
glass/SWNT/PEDOT:PSS/P3HT:PCBM/Ca/Al
glass/SWNT(DCE)/PEDOT:PSS/P3HT:PCBM/LiF/Al
glass/SWNT(H2O:SDS)/PEDOT:PSS/P3HT:PCBM/LiF/Al
glass/SWNT(H2O:SDBS)/PEDOT:PSS/P3HT:PCBM/LiF/
Al
glass/SWNT/PEDOT:PSS/P3HT:PCBM/Ca/Al
glass/SWNT/P3HT:PCBM/Ca/Al
glass/SWNT/PEDOT:PSS/P3HT:PCBM/LiF/Al
Area (mm2) Nanotube electrode properties
Photovoltaic parameters @ 100 mW cm 2
Ref.
Thick.
(nm)
R&
(O/sq)
T (%)
Jsc
(mA cm 2)
Voc (V)
FF
PCE (%)
7
10
4
–
–
–
10
–
–
–
300
–
30
80
–
–
40
24
26
28
282
50
200
362
250
86
60
128
57
68
45 (@ 650 nm)
70 (@ 650 nm)
85 (@ 500 nm)
64 (@ 520 nm)
65
70 (@ 550 nm)
65–70 (@ 550 nm)
90 (@ 550 nm)
65 (@ 550 nm)
70 (@ 550 nm)
6.65 (5.00)
9.24
7.80 (8.00)
4.30 (4.90)
–
4.46 (4.87)
11.50 (12.70)
9.90 (8.40)
7.30 (8.40)
6.70 (8.40)
0.50 (0.43)
0.56
0.61 (0.61)
0.58 (0.60)
–
0.36 (0.38)
0.58 (0.58)
0.55 (0.55)
0.59 (0.55)
0.55 (0.55)
0.30 (0.32)
0.29
0.52 (0.61)
0.48 (0.46)
–
0.38 (0.37)
0.48 (0.49)
0.43 (0.50)
0.46 (0.50)
0.31 (0.50)
0.99 (0.69)
1.50 (2.00)
2.50 (3.00)
1.20 (1.35)
0.60
0.61 (0.68)
3.10 (3.60)
2.30 (2.30)
2.20 (2.30)
1.20 (2.30)
4
4
4
–
–
–
56
24
–
70 (@ 500 nm)
50 (@ 500 nm)
92 (@ 550 nm)
13.78 (10.91) 0.57 (0.59) 0.53 (0.55) 4.13 (3.51) [160]
11.39 (7.42) 0.54 (0.49) 0.55 (0.41) 3.37 (1.44) [160]
10.52 (8.88) 0.56 (0.57) 0.35 (0.62) 2.05 (3.15) [155]
A higher efficiency of 1.5% was reported by van de Lagemaat
et al. for P3HT:PCBM solar cells deposited onto high purity
bundles of SWNTs, with a sheet resistance of 50 O/sq and an
optical transmission of 70% at 650 nm [151]. Arc-produced
SWNTs were purified and dispersed in water and alcohol. The
resultant ink was spray-coated onto glass substrates. The dropcasted P3HT:PCBM active layer was rather thick (0.5–1.0 mm) also
in that case. The related solar cells gave Voc ¼0.56 V,
Jsc ¼9.2 mA cm 2, and FF¼ 0.29, resulting in an efficiency of
1.5%, lower than that exhibited (2.0%) by a reference cell made
onto an ITO electrode. The poor FF value indicated that the
performance was limited by the device series resistance (Rs),
orders of magnitudes higher compared to ITO-based devices.
The preparation of P3HT:PCBM solar cells with thin spincoated active layers was only possible onto smooth SWNT films
(with a root-mean-square, rms, surface roughness less than
10 nm over a surface of 25 mm2) [152]. A transfer-printing
method [153] was used to produce SWNT electrodes on flexible
poly(ethylene terephthalate) (PET) substrates. Briefly, arc-discharge produced SWNTs were dissolved in solution with surfactants and sonicated. The solution was vacuum filtered over a
porous alumina membrane and after drying the SWNT film was
lifted off with a poly(dimethylsiloxane) (PDMS) stamp and
transferred to the PET substrate by printing. 30-nm-thick films
showed an optical transmission of 85% in the visible range with a
sheet resistance of 200 O/sq. The efficiency exhibited by the solar
cells made on PET/SWNT substrate approached that of glass/ITO
based devices (2.5% vs 3%). The reduced PCE in the SWNT devices
was mainly due to the reduced FF compared to ITO-based cells
(0.51 vs 0.61, respectively), attributed to the relatively high sheet
resistance of the nanotube electrode.
High sheet resistance of electrode limits the device performance. It can be reduced by increasing the nanotube layer
thickness, but at the expense of a less favourable optical transmission. So, the optimum balance between low R& and high
transparency is required. As an example, the usual trade-off
between these quantities is illustrated in Fig. 11 for SWNT
electrodes on glass substrates and deposited following the
method reported by Wu et al. [149]. The best photovoltaic
properties
were
obtained
for
80-nm-thick
electrodes
(R& ¼362 O/sq, optical transmission of 64% at 520 nm) [154].
P3HT:PCBM solar cells gave an efficiency of 1.2%, approaching
that exhibited by similar ITO-based devices. An effective approach
[150]
[151]
[152]
[154]
[163]
[137]
[156]
[157]
[157]
[157]
Fig. 11. Correlation between SWNT film transparency (represented by the
transmittance at 520 nm) and sheet resistance by varying the film thickness.
From Ref. [154] by permission of Elsevier B.V.
to achieve SWNT electrodes with very high transparency and low
sheet resistance is to increase the fraction of metallic SWNTs
quite a lot, with a dramatic improvement of the photovoltaic
performance of the related devices [155].
Ultrasmooth, high-quality, and highly uniform SWNT electrodes have been reported by Tenent et al. [156], produced by
ultrasonic spraying. The SWNTs were sprayed from aqueous
dispersions containing sodium dodecyl sulphate (SDS) or sodium
carboxymethyl cellulose (CMC) as surfactants. After the deposition, a treatment with nitric acid removed surfactants and doped
the nanotubes in a single step. It was demonstrated that more
uniform SWNT films can be obtained with CMC surfactant,
compared to SDS. In addition, modest sonication was required
to achieve high quality SWNT dispersion in the case of CMC,
resulting in longer nanotubes than those found after the more
robust sonication treatment necessary for the SDS dispersions. An
rms roughness of 3 nm over a 10 mm 10 mm area was measured for the CDC-sprayed films, with an excellent uniformity
over large areas (6 in. 6 in.), as shown in Fig. 12. Due to the high
quality of the SWNT ultrasonic CMC-sprayed electrodes,
108
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
Fig. 12. Optical microscopy images of SWNT films sprayed from a SDS dispersion (a) or a CMC dispersion (b), before treatment with nitric acid. (c) Photograph of a 6 6 in.
glass substrate sprayed from a CMC dispersion. (d) UV–vis-NIR spectra for a CMC-sprayed SWNT film onto glass substrate. (e) IR spectra for a CMC-sprayed SWNT film onto
a silicon wafer. From Ref. [156], copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
combined with a high transparency ( 65–70% at 550 nm) and
low sheet resistance (60 O/sq), highly efficient P3HT:PCBM solar
cells were demonstrated (PCE 3.1%), approaching the performance
of similar ITO-based cells (PCE 3.6%).
SWNT films obtained with CMC surfactant are believed to be
less mechanically robust than those derived with common surfactants [157], such as SDS or sodium dodecyl benzene sulphonate (SDBS). The effect of SDS and SDBS surfactants on the
properties of pressure-driven spray-deposited SWNT films has
been recently reported and compared to surfactant-free, highpurity SWNTs spin-coated from dichloroethane (DCE) [157]. The
combination of thermal and nitric acid treatments efficiently
removed the surfactants, and mechanically robust spray-coated
films were obtained, with no tendency to delaminate. Spraycoated films with comparable R& and transparency were obtained
with SDS and SDBS surfactants, though with a higher roughness in
the latter case. P3HT:PCBM solar cells including a bottom SWNT
anode electrodes sprayed with SDS or SDBS surfactants gave a PCE
of 2.2% and 1.2%, respectively, comparable to that exhibited by
reference devices using DCE-coated SWNT or ITO as anode
(PCE¼2.3%). The modest performance of the devices made with
SDBS–SWNTs, was attributed to the higher electrode roughness,
resulting in a poor contact with the overlying PEDOT:PSS layer.
In ITO-free solar cells, carbon nanotube electrodes are usually
combined with PEDOT:PSS and reduced photovoltaic performance
have been reported for solar cells without the buffer layer
[150–152,158]. In most cases PEDOT:PSS is deposited onto the
surface of the nanotube film, though PEDOT:PSS doped with
SWNTs has also been considered by spin-coating onto glass
substrate a mixed PEDOT:PSS-SWNTs aqueous dispersion [159].
The relatively high surface roughness of carbon nanotube films
can be highly disadvantageous for devices and the overlying
PEDOT:PSS layer reduces the roughness of the nanotube deposition, thus decreasing device shorting probability; it also improves
the lateral conductivity of nanotube films by filling their porosity
[139], resulting in a reduced series resistance of the electrode
[152]. However, it has been recently shown that high-quality and
very smooth SWNT electrodes, deposited by ultrasonic spray
method, can efficiently replace both ITO and PEDOT:PSS [160].
ITO-free and PEDOT-free P3HT:PCBM solar cells exhibited a PCE of
3.37%, compared to 3.51% and 4.13% of ITO/PEDOT:PSS and
SWNT/PEDOT:PSS solar cells, respectively.
Carbon nanotube electrodes are mainly proposed as holecollectors, to replace the bottom ITO electrode in polymer solar
cells, however inverted solar cells [161] with a top anode made of
a SWNT layer sandwiched between two PEDOT:PSS layer has
been reported by Tanaka et al. [162]. SWNTs have been also used
as the bottom electron-collecting electrode in inverted solar cells
with an active layer made of P3HT and ZnO nanowires as
electron-acceptors [163].
As expected, a much improved stability in bending tests of
solar cells incorporating a PET/nanotube electrode is reported
[152], compared to those fabricate onto PET/ITO substrates.
3.2. Graphene
Transparent graphene films were obtained through a bottomup chemical approach by Wang et al. [164]. An hexadodecylsubstituted superphenalene [165] was spin-coated from chloroform solution, followed by heat treatments up to 1100 1C. By
varying the solution concentration, graphene films of different
thickness and transparency were achieved, resulting from the
thermal fusion of the superphenalene molecules. Transmittances
between 66 and 90% (at 500 nm) and a sheet resistance of the
order of kO/sq were achieved for very smooth films with thickness ranging between 30 and 4 nm. Graphene films were used as
hole collecting electrodes in ITO-free P3HT:PCBM solar cells,
showing a PCE of 0.29%, compared to 1.17% of reference ITObased devices (Table 6). The relatively low values of Jsc and FF of
the graphene-based device, compared to the reference device,
were due to the high R& of graphene anodes.
Graphene is usually obtained from the reduction of graphene
oxide (GO) [166,167]. Thin films of GO can be prepared from
aqueous dispersions obtained by the exfoliation of graphite oxide
using the modified Hummers method [168]. Uniform layers of GO
were deposited with vacuum filtration technique [169] through a
mixed cellulose ester membrane, followed by a transfer process
onto a substrate [170]. The insulating GO layers were reduced
with a combined hydrazine treatment/thermal annealing (200 1C)
process. Solar cells incorporating the reduced and Cl-doped GO
films as transparent (64% of transmittance at 550 nm) hole
collectors exhibited very poor performance (PCE of 0.1%), again
mainly limited by the high sheet resistance (40 kO/sq) of the
graphene electrode.
The poor dispersibility of reduced graphene in aqueous
dispersion can be dramatically improved by using aromatic
molecules with nanographene units (Fig. 3), such as pyrene1-sulfonic acid sodium salt (PyS) or the diasodium salt of
3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonic acid
(PDI), as dispersants [171]. Stable and precipitate-free aqueous
[164]
[170]
[171]
[171]
[172]
[172]
[172]
[173]
[174]
[176]
[177]
(0.61)
(0.61)
(0.61)
(0.37)
(0.68)
(0.49)
0.29 (1.17)
0.10
0.78
1.12
0.21 (3.10)
0.74 (3.10)
1.71 (3.10)
0.68 (1.21)
1.17 (3.43)
0.13 (3.59)
0.85
0.25
–
0.26
0.31
0.27
0.24
0.51
0.26
0.33
0.25
0.42
0.38 (0.41)
–
0.50
0.50
0.32 (0.56)
0.55 (0.56)
0.55 (0.56)
0.54 (0.51)
0.52 (0.59)
0.46 (0.60)
0.58
(1.00)
0.36
–
5.40
8.20
2.39
5.56
6.05
4.82
6.91
1.18
3.47
85 (@ 500 nm)
64 (@ 550 nm)
70
68
91–72 (@ 550 nm)
91–72 (@ 550 nm)
91–72 (@ 550 nm)
81 (@ 550 nm)
84 (400–600 nm)
69 (@ 550 nm)
87
18
40
2
1
0.21-1.35
0.21-1.35
0.21-1.35
1.65
0.374
17.9
0.6
–
–
8
8
–
–
–
–
4.66
–
–
quartz/graphene//P3HT:PCBM/Ag
quartz/graphene/PEDOT:PSS/P3HT:PCBM/Al
quartz/graphene/PEDOT:PSS/P3HT:PCBM/ZnO/Al
quartz/graphene-PyS/PEDOT:PSS/P3HT:PCBM/ZnO/Al
glass/graphene/PEDOT:PSS/P3HT:PCBM/LiF/Al
glass/graphene-UV/PEDOT:PSS/P3HT:PCBM/LiF/Al
glass/graphene-PBASE/PEDOT:PSS/P3HT:PCBM/LiF/Al
quartz/graphene/PEDOT:PSS/P3HT:PCBM/Al
glass/graphene/PEDOT:PSS/P3HT:PCBM/Ca:Al
quartz/graphene/PEDOT:PSS/P3HT:PCBM/LiF/Al
glass/graphene-SWNT/PEDOT:PSS/P3HT:PCBM/Ca:Al
–
14
10
10
6-30
6-30
6-30
17
–
25
5
Jsc (mA cm
T (%)
R& (kO/sq)
Thick. (nm)
(9.03)
(9.03)
(9.03)
(6.49)
(8.51)
(12.32)
)
Voc (V)
FF
(0.48)
PCE (%)
Ref.
2
Photovoltaic parameters @ 100 mW cm 2
Graphene electrode properties
Area (mm2)
Cell architecture
Table 6
Non-exhaustive survey of polymer solar cells including graphene as the bottom transparent anode. The photovoltaic parameters are compared to those measured for a reference cell (values in parenthesis), when available,
fabricated in the same conditions but onto a glass/ITO substrate.
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
109
dispersions have been obtained with PyS (graphene–PyS) and PDI
(graphene–PDI) dispersants, compared to the highly aggregated
dispersion without any dispersant (Fig. 13).
Upon a thermal annealing at high temperature (1000 1C), the
conductivity of spray-coated films of reduced graphene,
graphene–PyS and graphene–PDI was greatly enhanced, reaching
values of the order of 103 S cm 1 for the samples deposited with
the dispersants, more than twice higher than that exhibited by
the annealed pristine graphene. The improved electrical properties of graphene films containing the nanographene units,
reflected in the improved performance of the related P3HT:PCBM
solar cells, compared to reference devices including pristine
graphene in the anode. In the case of graphene–PyS, a power
conversion efficiency of 1.12% was achieved, while the reference
cell, with lower Jsc and FF, showed a PCE of 0.78% (Table 6).
Graphene transparent anodes for polymer solar cells have
been also synthesised by chemical vapour deposition (CVD)
[172–175]. Highly crystalline and nearly defect-free graphene
films grown by CVD onto Ni-coated SiO2/Si wafer were transferred to glass substrate through a PDMS-based stamping process
[172]. The graphene layers were used in ITO-free solar cells
having the structure glass/graphene/PEDOT:PSS/P3HT:PCBM/LiF/
Al. The treatment of graphene with UV/ozone or with a pyrene
derivative (PBASE) improved the graphene wettability, resulting
in a better uniformity of the overlying PEDOT:PSS layer. The
device PCE, starting from 0.21% for pristine graphene anode,
increased to 0.74% and 1.71% for UV-treated graphene (graphene-UV) and PBASE-modified graphene (graphene-PBASE),
respectively. The poor fill factor of cells made with graphene–
UV (0.24) compared to graphene–PBASE anode (0.51), indicated a
higher Rs attributed to a deterioration of graphene electrical
properties upon UV/ozone irradiation. Kalita et al. [173] reported
transparent graphene films from CVD-deposited camphor
(C10H16O), followed by pyrolysis at 900 1C. Graphene electrodes
with a transmittance of 81% at 550 nm and R& of 1.645 kO/sq
were incorporated in ITO-free P3HT:PCBM solar cells, exhibiting a
PCE of 0.68%.
Choi et al. [174] reported ITO-free solar cells with a multilayer
(4 layers) graphene hole-collecting electrode, again prepared
using the CVD method and a transfer process onto glass substrate.
The multilayered graphene showed a sheet resistance of 374 O/sq
and a transparency of 84.2%. The related P3HT:PCBM solar cells,
also including a PEDOT:PSS buffer layer, exhibited a PCE of 1.17%,
compared to 3.43% of the reference ITO-based cell.
Recently, inverted semitransparent solar cells with a top
graphene anode have been demonstrated [175]. Also in this case,
the graphene electrode has first been obtained with CVD and then
transferred through a lamination process. Solar cells with the
structure ITO/ZnO/P3HT:PCBM/GO/graphene (graphene oxide
acts as the hole transporting layer) exhibited an efficiency of
2.5% for a thickness of 8 nm of the graphene anode (8 layers), not
so far from the PCE of 3.3% calculated for the reference ITO/ZnO/
P3HT:PCBM/GO/Ag cell.
Very poor performance (PCE 0.13%) was exhibited by
P3HT:PCBM solar cells including graphene hole collectors, prepared using the modified Hummers method from flake graphite,
spin-coated from an aqueous dispersion and reduced through
exposure to hydrazine vapour [176]. A further heat treatment at
700 1C was needed to decrease the sheet resistance of 25-nmthick reduced graphene film from 1010 to 104 O/sq.
Graphene–SWNT hybrid hole collectors have been also proposed for ITO-free polymer solar cells, to take advantage of the
extended conjugated network in which nanotubes can act as
conducting wires connecting graphene sheets [177]. The hybrid
graphene–SWNT layers were obtained through a solution-based
method, without surfactants or high-temperature processes.
110
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
Fig. 13. Molecular schemes and images of aqueous dispersions of: (a) reduced graphene; (b) graphene–PDI (after centrifugation); (c) graphene–PyS (after centrifugation).
From Ref. [171] by permission of Wiley-VCH.
Chemically converted graphene and SWNT were dissolved in pure
hydrazine and spin-coated onto PET or glass substrates.
P3HT:PCBM solar cells including the hybrid hole collectors
(transmittance of 87% and R& of 600 O/sq) displayed a PCE
of 0.85%.
3.3. Diamonds
Films made of oxygenated nanocrystalline diamonds (NCD)
have been recently proposed as efficient anode electrodes for
photovoltaic devices [143]. Transparent nanocrystalline diamond
films, exhibiting a transparency of around 70% in the visible
range, were prepared by the plasma-enhanced vapour deposition
[178] and were p-doped by introducing boron into the gas feeds.
The properties of NCD layers with different surface terminations
(H, OH, O) were investigated and compared with those of ITO. The
electrical properties were not meaningfully affected by the
termination. For example, the sheet resistance was around
300 O/sq for all the NCD samples. Differently from the electrical
properties, the electrode work function was found to be significantly increased by the surface termination, starting from 4.1 eV
for H-NCD, increasing to 5.0 eV upon photochemical hydroxylation of H-NCD to OH-NCD, and reaching 5.3 eV for oxygenated
diamonds.
NCD films were not incorporated in polymer solar cells,
however photoelectrochemical measurements were performed
on P3HT-coated NCD electrodes and higher photocurrents were
observed for higher work function of the electrode (O-NCD 4OHNCD4H-NCD). The reference P3HT-coated ITO sample exhibited
the lowest photocurrent under the same experimental conditions.
4. Concluding remarks
Indium-tin-oxide, as wells as metal electrodes, are expensive
components in the technology of polymer solar cells and and pose
several technological issues when their deposition must be
integrated in fully roll-to-roll production process. The competitiveness of this emerging photovoltaic technology will also
strongly depend on the availability of alternative and earthabundant electrode materials exhibiting good electrical and
optical properties, long-term stability, inherent flexibility, easy
processability in a continuous production process.
It is not a trivial task to find valid alternatives that meet all the
necessary requirements and, on the whole, we record a modest
effort in this field. Moreover, from the literature, we observe that,
differently from anodes, traditional cathodes prove to be more
difficult to be replaced by new materials, because the tuning of
the work function is still a challenging issue. Up to now, just a few
classes of materials have been taken into account as potential
electrodes in ITO-free and/or metal-free polymer solar cells,
mainly PEDOT:PSS, carbon nanotubes, and, more recently, graphene. They can act both as hole-collectors and electron-collectors, if combined with appropriate buffer layers.
Small-area ITO-free solar cells, including high-conductivity
PEDOT:PSS or carbon nanotube electrodes, with performance
approaching that of ITO-based devices have been reported.
Recently, solution-processed, ultra-smooth, and high-quality
(high purity and almost defect-free) carbon nanotube films have
been demonstrated [152], envisaging encouraging perspectives as
flexible and valid substitutes of the brittle indium-tin-oxide.
Though the conductivity of PEDOT:PSS and carbon nanotube
electrodes is rapidly improving, their low sheet resistance is still
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114
an issue, particularly critical for transparent electrodes, for which
high transparency is also required, and for large-area solar cells
[89,176]. So, the significant improvement of electrode conductivity is a necessary prerequisite for practical applications. In
particular, according to Servaites et al. [179] this factor becomes
critical for devices with area 41 cm2, that is, practically speaking,
for solar cells of commercial interest.
Concerning graphene electrodes [180], the sheet resistance is
still very high, limiting the performance of the related solar cells,
and a scalable and controllable method for the preparation of
high-quality graphene layers is still lacking. However graphene,
the rising star of material science, has the great advantage of
‘‘learning’’ from nanotubes, so a rapid progress could be expected.
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