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 Centro ricerche per le energie non convenzionali, Istituto ENI Donegani, ENI S.p.A., via G. Fauser 4, 28100 Novara, Italy b Istituto per la Sintesi Organica e la Fotoreattivit a (CNR-ISOF), Consiglio Nazionale delle Ricerche, via P. Gobetti 101, 40129 Bologna, Italy article info Article history: Received 10 November 2011 Received in revised form 21 December 2011 Accepted 23 December 2011 Available online 25 January 2012 Keywords: Polymer solar cells PEDOT electrode Graphene electrode Carbon nanotubes electrode ITO-free electrode Metal-free electrode abstract 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. Contents 1. Introduction ....................................................................................................... 97 2. 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 3. Carbon materials .................................................................................................. 106 3.1. Carbon nanotubes ........................................................................................... 106 3.2. Graphene .................................................................................................. 108 3.3. Diamonds .................................................................................................. 110 4. Concluding remarks ................................................................................................ 110 References ....................................................................................................... 111 1. Introduction Thin-films solar cells based on polymeric photoactive materi- als represent a promising technology to afford low-cost, readily available energy. In the last years a large number of academic 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 Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.12.022 n Corresponding author. Tel.: þ390321447001. E-mail address: [email protected] (C. Carbonera). Solar Energy Materials & Solar Cells 100 (2012) 97–114
Transcript
Solar Energy Materials & Solar Cells 100 (2012) 97–114
Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells
0927-02
doi:10.1
n Corr
E-m
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 Centro ricerche per le energie non convenzionali, Istituto ENI Donegani, ENI S.p.A., via G. Fauser 4, 28100 Novara, Italyb Istituto per la Sintesi Organica e la Fotoreattivit�a (CNR-ISOF), Consiglio Nazionale delle Ricerche, via P. Gobetti 101, 40129 Bologna, Italy
a r t i c l e i n f o
Article history:
Received 10 November 2011
Received in revised form
21 December 2011
Accepted 23 December 2011Available online 25 January 2012
Thin-films solar cells based on polymeric photoactive materi-als represent a promising technology to afford low-cost, readilyavailable energy. In the last years a large number of academic
ll rights reserved.
bonera).
groups and industrial companies have started research programsaiming to achieve efficient, durable and cheap solar cells that canenter the market of photovoltaics [1].
The optimization of polymer solar cells (PSCs) encompassesthe development of new approaches in the design of both activematerials and device architectures [2–7]. A relevant part of theliterature on PSCs concerns the active components of the cell,mainly the electron-donor [8–10] and, to a minor extent, also theelectron-acceptor [6,7]. The aim is the preparation of materials
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–11498
with broad absorption, ideal energy levels, high charge carriermobility, controlled morphology, high stability.
The device architecture of PSCs usually comprises buffermaterials [11,12] interposed between the active layer and oneor both the electrodes. They are used to improve the electrodeselectivity, to tune the electrode work function, to render moreohmic the active layer/electrodes interfaces [13], to act as opticalspacers.
The device electrodes assure the collection of photogeneratedcharge carriers and the difference of their work function providesthe driving-force for carriers migration by generating a built-inpotential. 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 theanode. Tin-doped indium oxide (ITO) is commonly used as thetransparent electrode, acting as the anode in conventional solarcells [14] or as the electron-collecting electrode in inverted cells[3], though very thin metal layers (o20 nm) have been alsoproposed [15–17].
The literature related to the variety of materials for electrodefabrication is by far inferior, compared to the other components ofPSCs, though electrode deposition is considered a critical andlimiting step in roll-to-roll (R2R) processes [18–29], which canopen the way for flexible, light-weighing, cheap polymer solarcells, potentially very competitive on the market. Indeed, accord-ing to a recently published study [30], flexible PSCs on plasticsupport would have both a substantially lower environmentalimpact and a significantly reduced cost, compared to rigid panelson glass substrate. In R2R fabrication, each layer of the devicestructure is printed or deposited in form of inks. Worth to benoted, in order to limit the environmental impact of the manu-facturing process of flexible solar modules toxic solvents must notbe used for the ink formulations, and substituted with environ-mentally and health friendly compounds (water or alcohols)[31–32]. In this context, ITO and metals do not lend themselvesto printing processes readily.
ITO is commonly manufactured with a high throughput bysputtering or e-beam evaporation, in a high-vacuum, high-tem-perature, energy-costly process, and with significant differencesof its properties from batch to batch [33–35]. Commercial ITO/PET(PET is poly(ethylene terephthalate) or ITO/PEN (PEN ispoly(ethylene 2,6-naphthalenedirboxylate) rolls produced off-lineare used as starting substrates in R2R fabrication of PSCs and a
Embodied Energy Input Materials
Nitrogen48.19%
ITO/glass 50.40%
Ca0.02%
P3HT0.07%
PCBM0.44%
PEDOT:PSS0.61%
Al0.06%
Toluene0.21%
Others1.41%
C
Ac
Fig. 1. Material energy content and process energetic cost for the manufacturin
lithographic patterning step is required to obtain the desiredgeometries. Alternatively, ITO could be sputtered as the last layer,but damaging the underlying organic layer [36–38] (buffermaterials may mitigate this problem), or it can be deposited in-line as a paste or ink, or through sol–gel techniques, but withinferior electric and optical properties [39–42], so these alter-natives seem not to be viable. In any case, one of the main faultsof ITO on a flexible support consists in the unsatisfactorymechanical properties [43–47], because it tends to crack and/orto delaminate, especially after repeated bending cycles. Anotherissue is that the atomic elements, in particular indium, tend tomigrate into the active and/or the buffer layer [48], promotingtheir degradation [49,50]. Finally, indium is an expensive andscarcely available metal [14,30,51], and, given that alternativetransparent oxides show worse properties [14], effective ITOsubstitutes for PSCs would be highly desirable [52].
Concerning low work function metal electrodes, vacuum eva-poration can be integrated in R2R, although not in a trivial way,and damage of the organic layer is often observed [53,54]. Metal-based inks [55,56] or pastes can be used, but their effectivenessneeds further improvements. Krebs et al. showed also that silverinks based on organic components destroy the morphology of theunderlying layers [57]; by using metallic grids, the area coveredby the ink is lower, and the damage is limited. In addition, inksmade of low work function metals are still difficult to be usedroutinely [58], other than being easily oxidable.
On the whole, electrodes are expensive components of organicphotovoltaics, besides being hard to be included in continuousmanufacturing processes. They have been evaluated [59,60] to beresponsible for more than 50% of the materials energy contentand almost 45% of the energetic cost associated to the lab-scaleprocess of fabrication, causing the increase of the energy paybacktime of the final device beyond competitive values (Fig. 1).
A similar analysis was later performed on a preindustrial-scaleR2R process [61], which confirms and reinforces the previousconclusions: ITO on PET is responsible for more than 85% of thematerials energy content while the evaporation/patterning stepsaccount for about 50% of the direct process energy involved in thefabrication of an organic solar module [61]. Other independentstudies based on different assumption lead qualitatively to thesame conclusions, although a lower contribution of ITO is esti-mated [62]. Recently, the tremendous potential to reduce theenergy consumption during PSC manufacturing by all-solution
Direct Process Energy
ITO cleaning9.32%
N2 atmosphere maintenance
38.57%
Al evaporation17.30%
a evaporation16.47%
tive layer spin-coating0.25%
PEDOT:PSS spin-coating4.95%
Annealing13.13%
g of polymer solar cells on laboratory scale. Data extracted from Ref. [59].
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processes has been demonstrated [63], and an Energy PaybackTime as low as one day can be attained under particularconditions.
In this paper, the literature on alternative electrode materialsfor polymer solar cells is reviewed, while extensive environmen-tal and economic assessments can be found in recent publications[64,65]. In perspective, such alternative materials can be appliedin all-solution operations that should lead to less expensive andless energy demanding manufacturing processes.
The databases used to select the relevant papers were CAS (viaSTN International-FIZ Karlsruhe), Science Direct and the data-bases of ISI Web of KnowledgeSM. The search terms were ‘‘poly-mer solar cells’’, ‘‘organic solar cells’’, ‘‘organic photovoltaics’’,‘‘polymer photovoltaics’’, ‘‘ITO free’’, ‘‘metal free’’, ‘‘indium free’’,‘‘electrode’’, ‘‘anode’’ and ‘‘cathode’’. In addition, the referencescited by the found documents were taken into account, and anadditional search on ‘‘cited reference search’’ in the ISI systemwas carried out. Low-molecular weight and hybrid solar cellshave not been considered. The relevant papers, grouped bypublication year and subject, are summarized in Table 1. Thenumber of published paper increased in the last ten years, parallelto the rising interest for low-cost production of PSCs. However onlya few classes of effective materials have been developed, namelypoly(3,4-ethylenedioxythiophene) derivatives, poly(aniline)s, carbonnanotubes and graphene. These families will be discussed in detailin the following paragraphs.
2. Polymer electrodes
Due to its high transparency in the visible light spectrum, easyaqueous solution processing, and application for flexible devices,
Table 1Reference literature considered in the present review, classified by year and main sub
poly(3,4-ethylenedioxitiophene):poly(styrenesulfonate) (PEDOT:PSS)is definitely the predominant conducting polymer investigated asalternative low-cost electrode for ITO-free and metal-free solarcells, and in general for organic electronics. Indeed, the simulta-neous attainment of high conductivity and high transparency isnot an easy task, so it is not surprising that just a few examples ofpolymer electrodes different from PEDOT:PSS have been reportedfor solar cells application (Fig. 2). In the present section, waterdispersion PEDOT:PSS used as electrode in PSCs is largelyreviewed in the first two paragraphs, in situ prepared PEDOT isaccounted for in the third paragraph, while the last one is devotedto alternative polymeric materials.
2.1. PEDOT:PSS
PEDOT:PSS was developed and originally commercialized byBayer AG under the trade name of Baytrons, then by HC StarkGmbH and currently by Heraeus Holding GmbH under thetradename of CleviosTM. An exhaustive description of its synth-esis, modifications, properties and applications is reported inseveral reviews [66,67]. Agfa Gevaert N.V. is also commercializingPEDOT:PSS grades with the trade name OrgaconTM. Many for-mulations with different viscosity (suspensions in water or mixedsolvents, inks, pastes), conductivity and transparency are nowa-days available (Table 2).
Since the late 1990s, PEDOT:PSS has been the most widelyused 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) itis effective in transporting holes to the anode and in blockingelectrons; (iii) its high work function (usually reported between
ject.
Carbon materials
mers Carbon nanotubes Graphene Diamonds
[144] – –
– – –
– – –
– – –
– – –
– – –
[150] – –
[151–153,159] – –
[138,154] – –
[163] [170] –
[137,156,162] [171,172,177] –
[157,160] [173,176] [143]
[155] [174,175] –
OT:PSS, PHMEDT and PANI.
Table 2Properties 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 – 0.4 – – PEDOT:PSS 1:2.5 w/w HTLb
P AI 4083a – 2–20�10�4 5.0–5.2 5–15 PEDOT:PSS 1:6 w/w HTLb
P CH 8000 – 3–10�10�6 – 9–20 PEDOT:PSS 1:20 w/w HTL b
PH 500 170c 500d 4.8–5.0 8–25 – Electrode
PH 510 – – – 20–100 – Electrode
PH 750 130c 750d 4.8–5.0 30 – Electrode
PH 1000 100c 1000d 4.8–5.0 15–50 – Electrode
P HC V4 – 400d – 100–350 High boiling point solvent Electrode
F CPP105 DM o7000c – – 30–60 High adesive formulation (includes a binder) Electrode
S V3 700c – – 3000–6000 Screen-printing formulation –
a Alternatives for ‘‘P AI’’ can also be found in the literature, such as ‘‘P VP AI’’; ‘‘PVP’’; ‘‘P Al’’; etc.b HTL: hole transport layer.c Value for a thickness of 100 nm or 90% of transmittance.d Value for PEDOT:PSSþDMSO (pure PH 500: 1 S/cm).e Value for P77 (thr/cm) for EL-P 3040 and P77/55 for EL-P 5010.
Table 3Additives reported in the literature for PEDOT:PSS modification: added to the PEDOT:PSS suspension (ADDED) or spin-coated onto
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4.8 and 5.2 eV) allows the formation of an ohmic contact withmost common donor polymers; (iv) it is stable in ambientconditions. In addition, the importance of PEDOT:PSS layer inthe planarization of ITO superficial spikes has been often under-lined [68–70].
Before the development of high-conductivity PEDOT:PSS (around2008), low-conductivity grades of PEDOT:PSS were the subject of adozen papers on ITO-free PSCs with polymeric anodes (Table 1). Toour best knowledge, the first attempt of using PEDOT:PSS for ITO-free polymer solar cells was reported in 1999 [71]. After that, anumber of groups have devoted their research activity to improvethe electrical properties of commercially available PEDOT:PSS for-mulations (mainly, by using suitable additives, Table 3), or to opennew roads by using ‘‘in situ’’ polymerization approaches [72,73]. The
main goal was, and still is, to obtain a good compromise betweengood conductivity and transparency, along with a high workfunction and good processability.
2.1.1. Low conductivity PEDOT:PSS
Because of the low conductivity of buffer layer grade PED-OT:PSS, the related ITO-free solar cells show an extremely highsheet resistance (typically of 104–105 O/sq) [74], leading to verypoor device performance [71,74,75]. However, the conductivity ofPEDOT:PSS can be increased of orders of magnitude by dopingwith appropriate additives, usually followed by thermal treat-ments [74,76,77].
MEH-PPV:PCBM (MEH-PPV is poly[2-methoxy-5-(20-ethylhexy-loxy)-p-phenylene vinylene] and PCBM is [6,6]-phenyl-C61-butyric
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114 101
acid methylester) solar cells including sorbitol-doped Baytron P asanode, with a sheet resistance of the order of 103 O/sq and atransmittance of over 80% in the 350–600 nm range, reached apower conversion efficiency (PCE) of 0.36%, compared to 0.46%obtained for similar devices made with an ITO/untreated-PEDOT:PSSanode. Addition of meso-erythritol to Baytron P AI 4071 enhancesthe conductivity from 0.4 S/cm to 155 S/cm [77,78] andMEH-PPV:PCBM solar cells fabricated with this modified PEDOT:PSSas the anode exhibited a PCE of 1.5%, close in performanceto a reference cell with conventional ITO/untreated-PEDOT:PSSanode [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].
TEM inages
PristinePEDOT:PSS
PEDOT:PSS+ methoxyethanol
PEDOT:PSS+ ethylene glycol
Fig. 4. TEM images (left) and schematic tridimensional morphologies (right) of Baytr
ethylene glycol (bottom). Adapted from Ref. [80]. Reproduced by permission of Royal
number of oxygenated solvents to enhance the conductivity ofBaytron P AI 4071: ethanol, methoxyethanol, dimethoxyethaneand ethyleneglycol [80]. These compounds were not mixed to thePEDOT: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 werethe most efficient (PCE¼3.39% for 1 mm2 device area), althoughthe reference ITO/untreated-PEDOT:PSS cell exhibited a higherefficiency (3.80%) on an even larger area (4 mm2). The transmit-tance of the anode at 550 nm was about 93%, practically indepen-dent on the modifying agent. In all cases some differences in themorphology of the PEDOT:PSS layer were observed: the additiveinduced particles coalescence, hence a greater phases segregationleading to longer conduction paths (Fig. 4).
The conductivity of a buffer layer grade PEDOT:PSS (unspecifiedgrade; probably P AI 4071, according to the reported characteristics)can be increased from 0.2 S/cm up to almost 100 S/cm after atreatment (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) orup to 190 S/cm after a treatment with copper chloride or bromide[75]. The enhanced conductivity has been explained in terms ofconformational changes of PEDOT chains, caused by the screeningof coulombic attractions between PEDOT and PSS. The photovol-taic response of P3HT:PCBM solar cells with treated PEDOT:PSSanodes was better than that with untreated ones, but somewhaterratic if compared with the conductivity values (Table 1). Thiswas attributed to the different roughness of the treated PEDOT:PSSanodes. However, overall the zwitterion molecules gave solar cellswith higher photovoltaic parameters than copper salts.
Similar enhancements of conductivity were achieved by treat-ing Baytron P with solvent/water mixtures (solvent ¼ methanol,ethanol, isopropanol, acetonitrile, acetone, tetrahydrofurane) athigh temperatures [81]. The improvement of conductivity wasfound to be dependent on the mixture composition, the dielectric
3D morphology
TOP view SIDE view
on P AI 4071 films pristine (top) and modified with methoxyethanol (middle) or
Society of Chemistry.
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constant of the organic solvent and the temperature of the solventtreatment. P3HT:PCBM solar cells including ethanol/water andacetonitrile/water treated PEDOT:PSS at 140 1C exhibited efficien-cies approaching 3% (Table 4). The reason for the observedbehaviour was attributed to the preferential solvation of hydro-phylic PSS by water, and hydrophobic PEDOT by the organicsolvent. In turn, this effect would cause a phase segregation of thetwo polymers and a coil-to-linear conformational transition inPEDOT, similar to the previously reported treatments with zwit-terions [75].
2.1.2. High conductivity PEDOT:PSS
The first – and, for a long time, only – comprehensive report ona high conductive PEDOT:PSS (Orgacon EL-P 3040) for ITO sub-stitution dates back to 2004 [82]. However, ITO-free solar cellsshow poor performance also for high conductivity formulations ofPEDOT:PSS [82–86], unless the electrical properties of the poly-mer 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 con-ductivity up to 470 S/cm [85], that is just one order of magnitudelower than that of ITO on glass of about one third of ITO on plasticsubstrates. P3HT:PCBM solar cells with DMSO-modified CleviosPH500 showed excellent performance, both on glass substrate(PCE 3.27% compared with 3.66% measured for reference deviceson glass/ITO) and PET substrate (PCE of 2.8% against 2.9% for solarcells on PET/ITO). In addition, ITO-free cells on flexible substraterevealed a much better mechanical resistance and, differentlyfrom ITO-based cells, did not reduce significantly their efficiencyeven after 300 bending cycles (Fig. 5). Efficient fully spray-coatedITO-free P3HT:PCBM solar cells, including DMSO-modified CleviosPH500 anodes, have been also demonstrated [87,88].
Similar results have been obtained with Clevios PH510 mod-ified with DMSO [86]. This PEDOT:PSS formulation differs fromClevios PH 500 for a higher solid content and a consequent betterprocessability. The dramatic increase of conductivity observed forfilms of DMSO-modified high conductivity PEDOT:PSS (aboutthree orders of magnitudes for the addition of 7% DMSO forClevios PH510) was explained in terms of nanomorphologyevolution and increased uniformity of the distribution ofPEDOT-rich regions trough the PEDOT:PSS film. PEDOT:PSSusually arranges in grains with hydrophobic and highly conduc-tive PEDOT-rich core and a hydrophilic insulating PSS-rich shellacting as a passive barrier for charge transport. The size of thegrains rose up as a consequence of the addition of DMSO (Fig. 6),leading to an increased film roughness. This causes a reduction ofthe contact surface between grains leading to a superior chargetransport within the layer. At the same time, the addition ofDMSO resulted in a reduction of the PSS content at the surface ofPEDOT grains (as revealed by XPS measurement), weakening thebarrier effect with a consequent improvement of conductivity.ITO-free P3HT:PCBM solar cells including Clevios PH 510 polymeranodes modified with DMSO exhibited a PCE of 3.48% for a devicearea of a few mm2 [86], decreasing as the area was increased [89],as expected. A thorough study of the mechanical properties ofgravure printed layers of Clevios PH510 with 7% DMSO has beenrecently published [90]. Very low changes in resistivity (less than1%) have been registered even after 2000 bending cycles, both forstretching and for bending of the film at different angles.
To further improve the electrical properties of PEDOT:PSSanodes, mixed additives [91–93] have been proposed, also com-bined with a bilayer approach [94]. Conductivities of 200–320 S/cmhave been reported for Clevios PH 500 additivated by 5–10% ofDMSO and 5–10% of isopropanol or 2% of a surfactant [95], whilethe addition of 5% DMSOþ13% isopropanol to Clevios PH750anodes resulted in a conductivity of about 590 S/cm [90]. In the
latter case, the related ITO-free P3HT:bisPCBM solar cells exhib-ited an even higher PCE than that of the ITO-based referencedevice (3.5% vs. 3.3%). 1% of a surfactant was added to a conductiveink made of PH 500þ5% DMSO and patterned anodes was madewith PDMS masking and brush painting technique. A sheetresistance of 350 O/sq was reported [96].
High conductivity PEDOT:PSS has been also proposed toreplace the top metal contact of inverted polymer solar cells[97–100], thus realizing semitransparent devices for smart win-dows application. To this end, appropriate protocols are requiredto improve the wettability of the hydrophobic active layer by thePEDOT:PSS aqueous dispersion, such as a poly(allylamine hydro-chloride):dextran nanogel interlayer [98]. Inverted semitranspar-ent P3HT:PCBM solar cells with a top anode based on Clevios PH500 exhibiting power conversion efficiency of about 2% have beenreported [97,98], while in the case of a mixture of Clevios PH1000and Clevios F CPP 105 DM, containing DMSO and showing aconductivity of 400 S/cm, a PCE of 2.4% was obtained [99].Recently, an efficiency of 2.7% has been reported for invertedP3HT:PCBM solar cells with a top semitransparent contact, spray-coated from a dispersion of Clevios PH500 with 5% DMSO [100].
If combined with appropriate interfacial layers [11], PED-OT:PSS can also serve as polymer cathode [101,102], openingthe way to full polymer electrode solar cells [102,103].P3HT:PCBM inverted solar cell with a top Ag anode and atransparent cathode made of PEDOT:PSS (Clevios PH500 withthe addition of 5% DMSO), an interfacial layer of ZnO nanoparti-cles and a C60 self-assembled monolayer (Fig. 7) showed inter-esting efficiencies on both glass and plastic substrate (3.08% and2.99%, respectively) [102]. However, when the top Ag anode wasreplaced by Clevios PH500þ5% DMSO (Fig. 7), the efficiencydropped to 0.47%.
Much better results were achieved for ITO-free and metal-freeinverted P3HT:PCBM solar cells on glass substrate by using aPEDOT:PSS formulation (Clevios PH1000) with a higher conductiv-ity (680 S/cm) for both anode and cathode electrodes [103]. Again, abuffer layer grade PEDOT:PSS (Clevios CPP 105 D) was interposedbetween the active layer and the top anode, while a ZnO interfaciallayer was used to realize the electron-selective bottom cathode. Theresulting all-polymer solar cells showed an optical transmittance of10 to 55% in the range from 400 to 800 nm and exhibited anaverage power conversion efficiency of 1.8%.
As clearly emerges, the most critical parameter of PEDOT:PSSelectrodes is still the low conductivity, though in last years it israpidly approaching that of ITO (Fig. 8). This results in high sheetresistance of the polymer electrode, leading to solar cells withhigh series resistance and poor ability of current extraction. Thethickness of the PEDOT:PSS could be increased to reduce its sheetresistance, but to the detriment of its transparency, in the case ofthe illuminated electrode. A lower sheet resistance results in animproved fill factor, but the less efficient solar light harvestingcould negatively reflect on the short circuit current (Jsc), asillustrated in the case of Fig. 9, though the parallel reduction ofthe device series resistance has a beneficial effect also on Jsc.
The strong correlation between cell performance and seriesresistance has been clearly shown by Kim et al. [104] for ITO-freeP3HT:PCBM solar cells made with a Clevios PH 500 anodemodified with ethylene glycol (conductivity of the order of102 S/cm) and it is illustrated in Fig. 10. In that case the seriesresistance of the cell was varied by changing the device geometryand not the thickness of the PEDOT:PSS layer, thus its sheetresistance or transparency.
To compensate the high sheet resistance of PEDOT:PSS, extre-mely critical in large area applications, metal grids can beintegrated with the polymer electrode. For this purpose, silver isthe most frequently used material [79,82,105–113], but gold
Table 4Non 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.
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.
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114104
[105,114–117] and copper grids [105,118] have been alsoreported. Either evaporation [106,111,114–117] or printing tech-niques [79,82,105,107–113] are used for the grid deposition.Recently, Kylberg et al. reported an original approach consistingin the fabrication of composite woven mesh electrodes consisting
of molibdenum nanowires and polymer fibres embedded in aOrgacon EL-P3040 matrix [119].
Before concluding this session, it is worth mentioning thequite recent use of PEDOT:PSS – alone or combined with silver –on textile substrates, to give ‘‘fabric electrodes’’ [120–123].
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 PH500þ5% 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].
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114 105
Working devices have been demonstrated, that are compatiblewith continuous fabrication processes.
2.2. In-situ prepared PEDOT
The use of commercial PEDOT formulations for metal-freeanode depositions is more straightforward, however in situ
synthesised PEDOT may offer additional advantages in terms ofgreater flexibility and properties tunability. Several techniquesare known to polymerize the EDOT monomer directly on thetarget substrate, all requiring three main ingredients: the mono-mer, 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 and81% transparency) from EDOT and Fe(OTs)3 plus imidazole tocontrol oxidation [124,125]. To enhance the adhesion betweenPEDOT and glass substrates, the addition of silicate in the in-situpolymerization has been suggested, leading to PEDOT:SiOx hybridanodes [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 orsilk screen printing on the substrate a solution containing anoxidising agent plus a basic inhibitor. The substrate is thentransferred to a polymerization chamber containing an EDOTreservoir creating a monomer vapour phase that reacts on thesubstrate thanks to the presence of the oxidant. In 2006, almostcontemporarily, Winther-Jensen et al. [128] and Admassie et al.[127] published the results of their investigations on the possibi-lity 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 cellsmade of VPP:PEDOT showing high conductivity (775 S/cm) andhigh transparency (84%) have been reported with an efficiencycomparable to that measured on similar devices made with aPEDOT:PSS:sorbitol anode [127]. Vapour phase oxidative poly-merization of PEDOT:OTs made using Fe(OTs)3 as an oxidant forEDOT has been very recently suggested [129] and conductivitiesof the order of 650 S/cm have been reported, with a very strongadhesion of PEDOT:OTs on the glass substrate. VPP:PEDOT hasbeen also proposed as the electron-collecting electrode in poly-mer solar cells [130], but with very poor performance of therelated devices.
2.3. Other polymers
A polymer anode made of in-situ polymerized 3,4-(1-hydro-xymethyl)ethylenedioxythiophene (Fig. 2) has been proposed asan alternative to the more ‘‘conventional’’ PEDOT [131]. Thehydroxymethyl groups increase the interaction and the adhesionto the glass substrate, as revealed by the AFM images showing abetter uniformity of the films, compared to the common Clevios PAI 4083. A 45 nm thick layer of poly[3,4-(1-hydroxymethyl)ethylenedioxythiophene]:toluenesulfonate (PHMEDT:TS) exh-ibited a transmittance of 87% at 510 nm and a conductivity of
PEDOT:PSSglass
ITO
P3HT:PCBMLiF/Al
shadowmask
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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].
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114106
700 S/cm. When used as anode in ITO-free devices, P3HT:PCBMsolar cells with an efficiency of 0.61% were obtained.
Polyaniline (Fig. 2) is the only reported polymer radicallydifferent from PEDOT, though its low transmittance below480 nm (o40%) [132] could represent a limit when used astransparent electrode. Through a thickness-controlled drop-castingmethod [133], camphorsulfonic acid doped polyaniline (PANI:CSA)electrodes exhibiting a conductivity of about 600 S/cm, a highoptical transmittance of about 85% at 550 nm and goodperformance preservation after 50 bending cycles have beendemonstrated. ITO-free P3HT:PCBM solar cells including PANI:CSA as anode and made onto a flexible substrate exhibited a PCEof 2% [133].
Although only a few alternatives to PEDOT:PSS have beenproposed till now, the research on conductive polymers exhibit-ing a proper properties balance is in progress [134] and validoptions could be available soon.
3. Carbon materials
Carbon nanotubes are regarded as promising candidates forthe replacement of transparent conducting oxides in the emer-ging field of plastic electronics, due to their transparency in thinfilms, high electrical conductivity, excellent mechanical proper-ties and inherent flexibility, and the potential for roll-to-rollprocessing [135]. Single-wall (SWNTs) and multi-wall carbonnanotubes (MWNTs) [136], as well as few-wall carbon nanotubes(FWNTs), 2–5 walls [137], have been investigated as transparentanodes in polymer solar cells. The aim is to replace the expensive,stiff and brittle ITO substrate, though improved solar cellshave been demonstrated, compared to ITO-only devices, whencarbon nanotubes have been combined with an underlying ITOelectrode [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 transparentelectrode material [141,142]. Nevertheless the performance ofITO-free polymer solar cells including graphene electrodes arestill rather poor and more work is required in order to makegraphene a promising replacement of transparent conductiveoxides.
Even p-doped nanocrystalline diamonds have been recentlyproposed as transparent anodes for ITO-free polymer solarcells [143].
3.1. Carbon nanotubes
Carbon nanotubes were first proposed by Ago et al. to replacethe ITO electrode in polymer photovoltaic devices [144]. Multi-walled carbon nanotubes were used as hole-collecting electrode insolar cells made of a composite of MWNTs and poly(p-phenylenevinylene) (PPV). The catalytically synthesized MWNTs [145] wereoxidized in acid solution [146] and the nanotube water dispersionwas spin-coated onto glass substrates to a thickness rangingbetween 20 and 300 nm. The polymer layer was obtained by firstspin-coating the PPV precursor onto the MNWT films, followed bythermal conversion [147]. The photovoltaic properties of MWNT/PPV/Al devices were compared to those of ITO/PPV/Al referencedevices by illuminating through the semitransparent aluminiumcathode. An external quantum efficiency (EQE) about twice thatof the reference cells was observed with MWNT hole-collector.The enhanced EQE was attributed to the complex interpenetratingnetwork of the polymer chains with the underlying MWNTrough layer and to the stronger built-in electric field due tothe higher work function of MWNTs (5.1 eV) [148] with respectto ITO.
Following the work of Wu et al. [149], showing that filmsmade of high purity single-wall carbon nanotubes could representa promising alternative to ITO, solar cells made of P3HT:PCBM asactive layer were reported, with an improved power conversionefficiency when a SWNT layer was substituted to the common ITOelectrode [150], but for very thick drop cast active layers of800 nm. The best photovoltaic properties were obtained withSWNT 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, gavea PCE of 0.99%, compared to 0.69% exhibited by a reference cellfabricated onto glass/ITO electrode. The photovoltaic parametersmainly enhanced in the SWNT-based cell were the short-circuitcurrent and the open-circuit voltage (Table 5), the fill factor beingsimilar (0.30 and 0.32, for the SWNT and the ITO-based cell,respectively). The enhanced performance of the SWNT cell, thoughthe poor optical transmission of the bottom hole-collector, wasexplained with the presence of voids in the SWNT layer, throughwhich the overlying PEDOT:PSS/P3HT:PCBM infiltrated andreached the surface of the glass substrate. So, despite the lowtransparency of the SWNT layer in comparison to ITO, the presenceof those voids provided sufficient exposure of the active layer toillumination.
Table 5Non 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.
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.
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114 107
A higher efficiency of 1.5% was reported by van de Lagemaatet al. for P3HT:PCBM solar cells deposited onto high puritybundles of SWNTs, with a sheet resistance of 50 O/sq and anoptical transmission of 70% at 650 nm [151]. Arc-producedSWNTs were purified and dispersed in water and alcohol. Theresultant ink was spray-coated onto glass substrates. The drop-casted P3HT:PCBM active layer was rather thick (0.5–1.0 mm) alsoin that case. The related solar cells gave Voc¼0.56 V,Jsc¼9.2 mA cm�2, and FF¼0.29, resulting in an efficiency of1.5%, lower than that exhibited (2.0%) by a reference cell madeonto an ITO electrode. The poor FF value indicated that theperformance 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 spin-coated active layers was only possible onto smooth SWNT films(with a root-mean-square, rms, surface roughness less than10 nm over a surface of 25 mm2) [152]. A transfer-printingmethod [153] was used to produce SWNT electrodes on flexiblepoly(ethylene terephthalate) (PET) substrates. Briefly, arc-dis-charge produced SWNTs were dissolved in solution with surfac-tants and sonicated. The solution was vacuum filtered over aporous alumina membrane and after drying the SWNT film waslifted off with a poly(dimethylsiloxane) (PDMS) stamp andtransferred to the PET substrate by printing. 30-nm-thick filmsshowed an optical transmission of 85% in the visible range with asheet resistance of 200 O/sq. The efficiency exhibited by the solarcells made on PET/SWNT substrate approached that of glass/ITObased devices (2.5% vs 3%). The reduced PCE in the SWNT deviceswas mainly due to the reduced FF compared to ITO-based cells(0.51 vs 0.61, respectively), attributed to the relatively high sheetresistance of the nanotube electrode.
High sheet resistance of electrode limits the device perfor-mance. It can be reduced by increasing the nanotube layerthickness, but at the expense of a less favourable optical trans-mission. So, the optimum balance between low R& and hightransparency is required. As an example, the usual trade-offbetween these quantities is illustrated in Fig. 11 for SWNTelectrodes on glass substrates and deposited following themethod reported by Wu et al. [149]. The best photovoltaicproperties 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%, approachingthat exhibited by similar ITO-based devices. An effective approach
to achieve SWNT electrodes with very high transparency and lowsheet resistance is to increase the fraction of metallic SWNTsquite a lot, with a dramatic improvement of the photovoltaicperformance of the related devices [155].
Ultrasmooth, high-quality, and highly uniform SWNT electro-des have been reported by Tenent et al. [156], produced byultrasonic spraying. The SWNTs were sprayed from aqueousdispersions containing sodium dodecyl sulphate (SDS) or sodiumcarboxymethyl cellulose (CMC) as surfactants. After the deposi-tion, a treatment with nitric acid removed surfactants and dopedthe nanotubes in a single step. It was demonstrated that moreuniform SWNT films can be obtained with CMC surfactant,compared to SDS. In addition, modest sonication was requiredto achieve high quality SWNT dispersion in the case of CMC,resulting in longer nanotubes than those found after the morerobust sonication treatment necessary for the SDS dispersions. Anrms roughness of �3 nm over a 10 mm�10 mm area was mea-sured for the CDC-sprayed films, with an excellent uniformityover large areas (6 in.�6 in.), as shown in Fig. 12. Due to the highquality of the SWNT ultrasonic CMC-sprayed electrodes,
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.
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114108
combined with a high transparency (�65–70% at 550 nm) andlow sheet resistance (60 O/sq), highly efficient P3HT:PCBM solarcells were demonstrated (PCE 3.1%), approaching the performanceof similar ITO-based cells (PCE 3.6%).
SWNT films obtained with CMC surfactant are believed to beless mechanically robust than those derived with common sur-factants [157], such as SDS or sodium dodecyl benzene sulpho-nate (SDBS). The effect of SDS and SDBS surfactants on theproperties of pressure-driven spray-deposited SWNT films hasbeen recently reported and compared to surfactant-free, high-purity SWNTs spin-coated from dichloroethane (DCE) [157]. Thecombination of thermal and nitric acid treatments efficientlyremoved the surfactants, and mechanically robust spray-coatedfilms were obtained, with no tendency to delaminate. Spray-coated films with comparable R& and transparency were obtainedwith SDS and SDBS surfactants, though with a higher roughness inthe latter case. P3HT:PCBM solar cells including a bottom SWNTanode electrodes sprayed with SDS or SDBS surfactants gave a PCEof 2.2% and 1.2%, respectively, comparable to that exhibited byreference devices using DCE-coated SWNT or ITO as anode(PCE¼2.3%). The modest performance of the devices made withSDBS–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 usuallycombined with PEDOT:PSS and reduced photovoltaic performancehave been reported for solar cells without the buffer layer[150–152,158]. In most cases PEDOT:PSS is deposited onto thesurface of the nanotube film, though PEDOT:PSS doped withSWNTs has also been considered by spin-coating onto glasssubstrate a mixed PEDOT:PSS-SWNTs aqueous dispersion [159].The relatively high surface roughness of carbon nanotube filmscan be highly disadvantageous for devices and the overlyingPEDOT:PSS layer reduces the roughness of the nanotube deposi-tion, thus decreasing device shorting probability; it also improvesthe 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 andvery smooth SWNT electrodes, deposited by ultrasonic spraymethod, can efficiently replace both ITO and PEDOT:PSS [160].ITO-free and PEDOT-free P3HT:PCBM solar cells exhibited a PCE of3.37%, compared to 3.51% and 4.13% of ITO/PEDOT:PSS andSWNT/PEDOT:PSS solar cells, respectively.
Carbon nanotube electrodes are mainly proposed as hole-collectors, to replace the bottom ITO electrode in polymer solarcells, however inverted solar cells [161] with a top anode made of
a SWNT layer sandwiched between two PEDOT:PSS layer hasbeen reported by Tanaka et al. [162]. SWNTs have been also usedas the bottom electron-collecting electrode in inverted solar cellswith an active layer made of P3HT and ZnO nanowires aselectron-acceptors [163].
As expected, a much improved stability in bending tests ofsolar 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 bottom-up chemical approach by Wang et al. [164]. An hexadodecyl-substituted superphenalene [165] was spin-coated from chloro-form solution, followed by heat treatments up to 1100 1C. Byvarying the solution concentration, graphene films of differentthickness and transparency were achieved, resulting from thethermal fusion of the superphenalene molecules. Transmittancesbetween 66 and 90% (at 500 nm) and a sheet resistance of theorder of kO/sq were achieved for very smooth films with thick-ness ranging between 30 and 4 nm. Graphene films were used ashole collecting electrodes in ITO-free P3HT:PCBM solar cells,showing a PCE of 0.29%, compared to 1.17% of reference ITO-based devices (Table 6). The relatively low values of Jsc and FF ofthe 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 grapheneoxide (GO) [166,167]. Thin films of GO can be prepared fromaqueous dispersions obtained by the exfoliation of graphite oxideusing the modified Hummers method [168]. Uniform layers of GOwere deposited with vacuum filtration technique [169] through amixed cellulose ester membrane, followed by a transfer processonto a substrate [170]. The insulating GO layers were reducedwith a combined hydrazine treatment/thermal annealing (200 1C)process. Solar cells incorporating the reduced and Cl-doped GOfilms as transparent (64% of transmittance at 550 nm) holecollectors exhibited very poor performance (PCE of 0.1%), againmainly limited by the high sheet resistance (40 kO/sq) of thegraphene electrode.
The poor dispersibility of reduced graphene in aqueousdispersion can be dramatically improved by using aromaticmolecules with nanographene units (Fig. 3), such as pyrene-1-sulfonic acid sodium salt (PyS) or the diasodium salt of3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonic acid(PDI), as dispersants [171]. Stable and precipitate-free aqueous
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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 aggregateddispersion without any dispersant (Fig. 13).
Upon a thermal annealing at high temperature (1000 1C), theconductivity of spray-coated films of reduced graphene,graphene–PyS and graphene–PDI was greatly enhanced, reachingvalues of the order of 103 S cm�1 for the samples deposited withthe dispersants, more than twice higher than that exhibited bythe annealed pristine graphene. The improved electrical proper-ties of graphene films containing the nanographene units,reflected in the improved performance of the related P3HT:PCBMsolar cells, compared to reference devices including pristinegraphene in the anode. In the case of graphene–PyS, a powerconversion efficiency of 1.12% was achieved, while the referencecell, with lower Jsc and FF, showed a PCE of 0.78% (Table 6).
Graphene transparent anodes for polymer solar cells havebeen also synthesised by chemical vapour deposition (CVD)[172–175]. Highly crystalline and nearly defect-free graphenefilms grown by CVD onto Ni-coated SiO2/Si wafer were trans-ferred to glass substrate through a PDMS-based stamping process[172]. The graphene layers were used in ITO-free solar cellshaving the structure glass/graphene/PEDOT:PSS/P3HT:PCBM/LiF/Al. The treatment of graphene with UV/ozone or with a pyrenederivative (PBASE) improved the graphene wettability, resultingin a better uniformity of the overlying PEDOT:PSS layer. Thedevice PCE, starting from 0.21% for pristine graphene anode,increased to 0.74% and 1.71% for UV-treated graphene (gra-phene-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 ahigher Rs attributed to a deterioration of graphene electricalproperties upon UV/ozone irradiation. Kalita et al. [173] reportedtransparent graphene films from CVD-deposited camphor(C10H16O), followed by pyrolysis at 900 1C. Graphene electrodeswith a transmittance of 81% at 550 nm and R& of 1.645 kO/sqwere incorporated in ITO-free P3HT:PCBM solar cells, exhibiting aPCE of 0.68%.
Choi et al. [174] reported ITO-free solar cells with a multilayer(4 layers) graphene hole-collecting electrode, again preparedusing the CVD method and a transfer process onto glass substrate.The multilayered graphene showed a sheet resistance of 374 O/sqand 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 topgraphene anode have been demonstrated [175]. Also in this case,the graphene electrode has first been obtained with CVD and thentransferred through a lamination process. Solar cells with thestructure ITO/ZnO/P3HT:PCBM/GO/graphene (graphene oxideacts as the hole transporting layer) exhibited an efficiency of2.5% for a thickness of 8 nm of the graphene anode (8 layers), notso 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 byP3HT:PCBM solar cells including graphene hole collectors, pre-pared using the modified Hummers method from flake graphite,spin-coated from an aqueous dispersion and reduced throughexposure to hydrazine vapour [176]. A further heat treatment at700 1C was needed to decrease the sheet resistance of 25-nm-thick reduced graphene film from 1010 to 104 O/sq.
Graphene–SWNT hybrid hole collectors have been also pro-posed for ITO-free polymer solar cells, to take advantage of theextended conjugated network in which nanotubes can act asconducting wires connecting graphene sheets [177]. The hybridgraphene–SWNT layers were obtained through a solution-basedmethod, without surfactants or high-temperature processes.
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114110
Chemically converted graphene and SWNT were dissolved in purehydrazine 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 PCEof 0.85%.
3.3. Diamonds
Films made of oxygenated nanocrystalline diamonds (NCD)have been recently proposed as efficient anode electrodes forphotovoltaic devices [143]. Transparent nanocrystalline diamondfilms, exhibiting a transparency of around 70% in the visiblerange, 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. Theelectrical properties were not meaningfully affected by thetermination. For example, the sheet resistance was around300 O/sq for all the NCD samples. Differently from the electricalproperties, the electrode work function was found to be signifi-cantly increased by the surface termination, starting from 4.1 eVfor H-NCD, increasing to 5.0 eV upon photochemical hydroxyla-tion of H-NCD to OH-NCD, and reaching 5.3 eV for oxygenateddiamonds.
NCD films were not incorporated in polymer solar cells,however photoelectrochemical measurements were performedon P3HT-coated NCD electrodes and higher photocurrents wereobserved for higher work function of the electrode (O-NCD4OH-NCD4H-NCD). The reference P3HT-coated ITO sample exhibitedthe lowest photocurrent under the same experimental conditions.
4. Concluding remarks
Indium-tin-oxide, as wells as metal electrodes, are expensivecomponents in the technology of polymer solar cells and and poseseveral technological issues when their deposition must beintegrated in fully roll-to-roll production process. The competi-tiveness of this emerging photovoltaic technology will alsostrongly depend on the availability of alternative and earth-abundant electrode materials exhibiting good electrical andoptical properties, long-term stability, inherent flexibility, easyprocessability in a continuous production process.
It is not a trivial task to find valid alternatives that meet all thenecessary requirements and, on the whole, we record a modesteffort in this field. Moreover, from the literature, we observe that,differently from anodes, traditional cathodes prove to be moredifficult to be replaced by new materials, because the tuning ofthe work function is still a challenging issue. Up to now, just a fewclasses of materials have been taken into account as potentialelectrodes in ITO-free and/or metal-free polymer solar cells,mainly PEDOT:PSS, carbon nanotubes, and, more recently, gra-phene. They can act both as hole-collectors and electron-collec-tors, if combined with appropriate buffer layers.
Small-area ITO-free solar cells, including high-conductivityPEDOT:PSS or carbon nanotube electrodes, with performanceapproaching 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 havebeen demonstrated [152], envisaging encouraging perspectives asflexible and valid substitutes of the brittle indium-tin-oxide.
Though the conductivity of PEDOT:PSS and carbon nanotubeelectrodes is rapidly improving, their low sheet resistance is still
R. Po et al. / Solar Energy Materials & Solar Cells 100 (2012) 97–114 111
an issue, particularly critical for transparent electrodes, for whichhigh transparency is also required, and for large-area solar cells[89,176]. So, the significant improvement of electrode conductiv-ity is a necessary prerequisite for practical applications. Inparticular, according to Servaites et al. [179] this factor becomescritical for devices with area 41 cm2, that is, practically speaking,for solar cells of commercial interest.
Concerning graphene electrodes [180], the sheet resistance isstill very high, limiting the performance of the related solar cells,and a scalable and controllable method for the preparation ofhigh-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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