Technology development for roll-to-roll production of organic photovoltaics
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Chemical Engineering and Processing xxx (2010) xxx–xxx
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Chemical Engineering and Processing:
Process
Intensi?cation
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c e
p
Technology development for roll-to-roll production of organic photovoltaics
Yulia Galagan a ,?,Ike G.de Vries a ,Arjan https://www.sodocs.net/doc/269250557.html,ngen a ,Ronn Andriessen a ,Wiljan J.H.Verhees b ,Sjoerd C.Veenstra b ,Jan M.Kroon b
a Holst Centre,High Tech Campus 31,P.O.Box 8550,5605KN Eindhoven,The Netherlands
b
Energy Research Centre of the Netherlands (ECN),P.O.Box 1,1755ZG Petten,The Netherlands
a r t i c l e i n f o Article history:
Received 27March 2010
Received in revised form 23June 2010Accepted 21July 2010Available online xxx Keywords:
Organic solar cell Photovoltaics Roll-to-Roll Coating Slot die
OPV module
a b s t r a c t
In order to reach the objective of low-cost,large area organic photovoltaic systems,we build up a knowl-edge base concerning the in?uence of process conditions on the performance of polymer solar cells.A large area solar cell module,with roll-to-roll coated PEDOT:PSS and photoactive layers (based on P3HT:[C60]PCBM blend)on a ?exible substrate,has been demonstrated.Both the PEDOT:PSS and pho-toactive layer were deposited by slot die coating.A non-chlorinated solvent was used for the deposition of the photoactive blend.The ?exible solar cell module illustrated a power conversion ef?ciency of 0.7%under AM 1.5conditions.Methods to further improve the technological process are proposed.
? 2010 Elsevier B.V. All rights reserved.
1.Introduction
The main objectives in the ?eld of organic photovoltaics (OPV)are achieving high ef?ciency,long term stability and low-cost.Low-cost can be achieved by combining a low bill of materi-als with fast roll-to-roll (R2R)manufacturing techniques.Indeed compared with Si-based solar cells,organic solar cells should be less expensive and easier in manufacturing,due to lower energy consumption,free from vacuum process,possibility of direct pat-terning during coating.However,in comparison to evaporation processes a solution process shows quite more dif?culties in terms of wetting/dewetting and the control of self-assembly pro-cesses.The most commonly used deposition technique for the manufacturing of lab scale devices is spin coating,which is not roll-to-roll compatible.However,spin coating is still widely used for studying and understanding the fundamental principles of OPV.Roll-to-roll compatible methods,especially blade and wire bar coatings,are employed as intermediate steps towards real roll-to-roll processing [1,2].Roll-to-roll coating and printing of organic semiconductors have become a focus of many companies and research groups.Applications based on organic semiconductors,such as organic light emitting diodes and organic photovoltaic devices have strict requirement to the layer properties,such as
?Corresponding author.Tel.:+31404020447;fax:+31404020699.E-mail address:yulia.galagan@tno.nl (Y.Galagan).thickness,uniformity and over layer accuracy.Many different print-ing and coating techniques are applied and developed for the deposition of thin layers of organic semiconductors [3].For exam-ple,jet printing is a well-studied method for the deposition of PEDOT:PSS [4,5],and polymer–fullerene blends [6–8].The advan-tage of this method is the possibility to print patterned layers in one printing step.The challenge is to ?nd appropriate solvent sys-tems for polymer–fullerene blends,which will provide appropriate spreading and wetting of the ink on the substrate and homoge-neous drying with required morphology of the active layer.Gravure printing,which is widely used for the printing of organic transistors [9,10],has also been applied for the deposition of OPV layers [11].The main dif?culty in gravure printing is viscosity of the ink,which is for OPV blend is slightly lower than required for gravure printing.There are a number of publications on spray coating of photovoltaic inks [4,12–15].This deposition method is very ef?cient especially for low viscosity solutions which complicate the use of other depo-sition techniques.Pad printing is a rather unconventional method which has been employed in a roll-to-roll process for the fabrica-tion of OPV [16].Screen printing has been applied for the deposition of photoactive layers based on MDMO-PPV:C60-PCBM [17]and MEH-PPV:C60-PCBM [18,19].As well as a complete process for production of ?exible large area polymer solar cells entirely using screen printing has been demonstrated [20].There are many pub-lications where slot die coating was chosen for the deposition of several layers,including active layer,in polymer based solar cells [21–26].As one of the coating techniques,slot die deposition can provide very thin,uniform,non-patterned layers.One-dimensional
0255-2701/$–see front matter ? 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.cep.2010.07.012
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Table1
Solvent properties.
Solvent Vapor pressure
(mm Hg)Boiling
point(?C)
Surface tension
(dynes cm?1)
Viscosity
(mPa s)
Chlorobenzene11.80(25?C)13233.00.80(20?C)
o-Xylene 5.10(20?C)14428.70.76(25?C) patterning is possible by coating stripes which can be used for making OPV modules.
The focus of our research is the development of low-cost,?ex-ible,polymer based photovoltaic devices using roll-to-roll coating and printing technologies.In order to reach the objective of low-cost,large area OPV,we build up a knowledge base concerning the in?uence of process conditions on the performance of the poly-mer solar cells.First of all,the inks with the required properties should be formulated and the deposition method appropriate for this ink should be chosen.The in?uence of the deposition method and parameters on the layer properties has to be determined.The in?uence of drying conditions on the layer properties,such as mor-phology and uniformity,has to be analyzed.Depending on the deposition method,it is important to determine how the pattern-ing of the layers will be applied.The design and the cell architecture should be optimized.It is important to determine the correlation between all these parameters and the device performance,?nd a balance and transfer this knowledge to a roll-to-roll process.
2.Effect of solvent
In this study the P3HT:[C60]PCBM blend has been used as the photoactive layer.The most optimal solvents to reach high ef?-ciencies for this mixture are o-dichlorobenzene and clorobenzene. However,chlorinated solvents cannot be used in the mass-production because of environmental reasons.That is why,the ?rst topic for the technology development is searching the alter-native solvents which will provide appropriate morphology and hence,high ef?ciencies of the solar cell devices.The choice of solvent for OPV blend is one of the major factors which have in?u-ence on the performance of the solar cell.Indeed,the solvent is responsible for the good wettability of the photoactive blend on the previous layer,drying,and phase separation in the photoac-tive layer.To provide effective donor–acceptor charge transfer and transport in bulk heterojunction solar cell,the photoactive layer has to demonstrate right morphology,which means appropriate domain size,crystallinity and vertical distribution of both com-ponents.The choice of solvent,drying condition and annealing temperature are the most critical factors for the morphology.The effect of morphology can be described by the formation of effective network between donor and acceptor which effective for charge transport.The phase separation in P3HT/PCBM system has been well studied and is understood in several solvent systems[27–31]. The properties of the solvents,such as boiling point,vapor pres-sure,solubility and surface tension have the considerable impact on the morphology of the photoactive layer.There several pub-lications where the alternative environmentally friendly solvents have been used for deposition photoactive layer[6,12,32].Toluene solution demonstrated not preferable morphology for P3HT/PCBM blend[33].Xylene is also well-studied solvent for this system and the solubility of both P3HT and PCBM in o-xylene is better in com-parison with toluene.Moreover,the boiling point and viscosity of o-xylene are very close to the values of clorobenzene(Table1). For
all these reasons we chose o-xylene as a solvent for this study.
Bulk heterojunction solar cells have been prepared by spin coating of PEDOT and OPV layers on ITO coated glass substrate.
A LiF/Al top electrode was applied by vacuum evaporation.The Fig.1.Viscosity of the P3HT:[C60]PCBM(1:1)mixture in chlorobenzene and o-xylene as function of P3HT concentration.
OPV blend consists of a mixture of P3HT(purchased from Merck, Mw=20,050g/mol)and PCBM(purchased from Solene)in the ratio 1:1.The photoactive layer was spin coated from chlorobrnzene and o-xylene solutions.The concentration of the polymer was 15mg/ml.
The OPV ink was processed in ambient conditions.The solutions were stirred for14h at70?C and then the solutions were cooled down to room temperature and processed within30min.The cool-ing down of the polymeric solution typically leads to increasing of the interaction between polymer coils,resulting in aggregation and in some cases gelation due to physical cross-linking between the polymer chains.The time of gelation strongly depends on the molecular weight of the polymer[34]and the solubility of the polymer in a certain solvent.The gelation of the P3HT:[C60]PCBM mixture,indicated by an increase in the viscosity of the mixture is shown in Fig.1.The viscosity was measured after30min when solu-tion was cooled down.o-Xylene solution of P3HT/PCBM mixture shows up a high tendency for gelation when the concentration of the solution increases.P3HT in o-xylene solution illustrates rather fast gel formation that causes particles,?lms defects and the reduc-tion of solution’s shelf lifetime[34,35].
AFM topographic images of P3HT/PCBM layer deposited from o-xylene demonstrates higher roughness in comparison with chlorobenzene as-casted layer,as shown in Fig.2(a and c).Higher roughness of o-xylene deposited layer corresponds with the polymer aggregates which were formed in the solution.UV–vis absorption spectrum of as-casted P3HT/PSBM layers deposited from o-xylene demonstrates blue-shift in comparison with the same layer deposited from chlorobenzene(Fig.3).The shift of?–?* transition absorption peaks to higher energy indicates an increas-ing density of conformational defects,equivalent to non-plarnarity, and cause loss in conjugation[36,37].Red-shift of the absorption maximum in chlorobenzene deposited layer points on better crys-tallinity of P3HT.
Thermal annealing at110?C during10min increases crys-tallinity of both chlorobenzene and o-xylene deposited photoactive layers,which is indicated on UV–vis spectra(Fig.3).Topology of the chlorobenzene deposited layer after annealing illustrates increas-ing of roughness that typically also corresponds to increasing of crystallinity.However,the roughness of o-xylene deposited layer decreases after annealing that can indicate the destroying of amor-phous aggregates.
The solar cells,made from both chlorobenzene and o-xylene, illustrate increasing of the performance after thermal annealing (Table2).The thickness of photoactive layer has been optimized to200nm.J–V curves of the devices after thermal annealing are given in Fig.4.Lower current in the device produced from o-xylene can be explained by difference in the morphology of photoactive
Y.Galagan et al./Chemical Engineering and Processing xxx (2010) xxx–xxx
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Fig.2.AFM images of OPV layer deposited from (a)o-xylene without annealing,(b)o-xylene annealed during 10min at 110?C,(c)chlorobenzene without annealing and d)chlorobenzene annealed during 10min at 110?
C.
Fig.3.UV–vis spectra of P3HT/PCBM layers deposited from chlorobenzene and o-xylene before and after thermal annealing.The thicknesses of the layers are 200nm.
layer,Morphology difference mainly cause due to differences in solvents properties such as boiling point,vapor pressure,viscosity and surface tension and polarity.Another important parameter is
Table 2
The characteristics of photovoltaic devices,produced from different solvents,with and without thermal annealing.
Device (solvent,annealing)J sc (mA/cm 2)V oc (V)FF áChlorobenzene,non-annealed 3.640.5720.2960.62Chlorobenzene,annealed 7.750.5460.572 2.42o-Xylene,non-annealed 4.710.5070.3320.79o-Xylene,annealed
7.05
0.535
0.581
2.19
Fig.4.J –V curves of OPV devices,deposited from chlorobenzene and o-xylene solu-tions.The active area of the devices is 0.09cm 2.
solubility in the solvent that can bring to the formation of the aggre-gates in the solution.Aggregation and gelation have big in?uence on the morphology and device performance.3.Scaling up
Scaling up of the active area of the solar cell devices typically leads to ef?ciency losses [38–40].The trend of the losses is strongly depends on the sheet resistance of the electrode.Thus,scaling up of the active area from 0.09cm 2to 6cm 2summarized in about
4
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Table 3
The characteristics of photovoltaic devices with different active areas.Devices pro-duced from chlorobenzene solution.Solar cell parameters are corresponding to Fig.5.
Active area,cm 2(cm ×cm)J sc (mA/cm 2)V oc (V)FF á0.09(0.3×0.3)7.750.5460.572 2.426(2×3)
7.46
0.530
0.478
1.89
Fig.5.J –V curves of OPV devices,deposited from chlorobenzene solution with dif-ferent sizes of active area:0.09cm 2and 6cm 2.
20%of ef?ciency losses for the ITO-based devices on glass substrate (Table 3).The JV curves of the devices are shown in Fig.5.
The ef?ciency of the devices demonstrates the rapid decay upon increasing the width of the solar cell [39].There has been illus-trated that only the width of the electrode is relevant for the device ef?ciency.To keep the resistive losses in the electrode as small as possible,the width has to be narrow with the contacts taken on the long side.Based on the experimental [38–40]and theoretical modeling [39],it was illustrated that the minimum loses in the ef?-ciency of the solar cells are expected when the width of the cell is not higher than 1cm.The effective coverage of the large surface area is possible with interconnection of several devices.4.Substrate (glass versus foil)
The ef?ciency of the solar cell is a function of electrode dimen-sion and its sheet resistance.Typically,the sheet resistance of ITO/glass substrate is 10–15 / ,while the sheet resistance of ITO on PET substrate is around 60 / .A rapid decay of the ef?ciency was shown upon increasing the width of the solar cell [39].More-over,a much faster decay was observed in case of a foil substrate with high sheet resistance.Thus,the ef?ciency of the solar cells on plastic substrate drops down almost two times in comparison with the sell on glass substrate.The series resistance induced by the sheet resistance of electrode can dramatically reduce charge col-lection in the device.The J –V curves of the devices with the active area of 2×2cm 2on glass and foil substrate are shown in Fig.6,the characteristics of the devices are summarized in Table 4.
Another factor which has an in?uence on the devices per-formance is the transparency of the substrate.In general the
Table 4
The characteristics of photovoltaic devices on glass and foil substrates.Devices produced from chlorobenzene
solution.Solar cell parameters are corresponding to Fig.6.Substrate J sc (mA/cm 2)V oc (V)FF áGlass/ITO 7.510.52652.32 2.07Foil/ITO
6.59
0.489
29.60
0.95
Fig.6.J –V curves of OPV devices with active area of 4cm 2,prepared on glass and foil substrate.The sheet resistance of ITO on glass is 13 / ,and on foil is 60 / .
transparency of the PEN/PET foil is slightly lower in comparison with high quality glass.Moreover,the ?exible devices reported in this study have been encapsulated using thin ?lm encapsulating technology of Holst Centre [41].The transmittance of PEN foil with thin ?lm barrier versus transmittance of glass is given in Fig.7.The difference in transparency can explain the difference in J sc for glass-based and ?exible devices.While,the difference in ?ll factors for both types of the devices is related to the sheet resistance of ITO.A solution to overcome the problem with sheet resistance of the ITO is to limit the path of the charge carriers in this electrode.In the other words,to design individual cells in such a way,that has large width but short length.5.Module design
Standard single organic solar cell has an open-circuit voltage below 1V.In solar cells based on P3HT/PCBM blend this value is typ-ical in around 0.5V.The voltage at maximum power point is lower,even at high illumination level.The value of current is strongly depends on light intensity and the size of active area of the device.Scaling up of the active area can increase the actual output cur-rent,but the voltage delivered by photovoltaic device will remain unchanged.For electrical powering of electronic
tools and devices is very often required higher voltage.It can be achieved by inter-connection of number of single cells in series for delivering higher voltage.The interconnection can be performed by external wiring,
Fig.7.The transmission of the PEN/barrier and glass substrates with pre-coated ITO and PEDOT:PSS layers and absorption spectrum of P3HT/PCBM layer.
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Fig.8.Schematic illustration of the organic solar cell module.
which is commonly used in the wafer-based Si solar cells.However,application coating and printing techniques for manufacturing of solar cells gives possibility of direct patterning of the layers.The patterning printing opens new possibilities of manufacturing solar sell modules with internal interconnection.The printing of com-plete module provides signi?cant decreasing of the manufacturing cost and increasing of stability of the solar cell module.
Organic solar cell module reported in this study contained three cells connected in series.The width of single cell was 1cm and the length was 7cm.The schematic illustration of the organic solar cell module is given in Fig.8.
6.Deposition technique for roll-to-roll manufacturing Benchmarking of deposition techniques for roll-to-roll manu-facturing is a complicated process.For the wet chemical deposition of organic semiconductors a large number of deposition techniques can be used.The choice is mainly based on the following factors:?stability of printing/coating step for the desired materials (vis-cosity of the ink),
?the capacity of the technique to print/coat the desired feature sizes (lateral resolution,thickness and uniformity),?throughput of the process.
Each layer in OPV device has its own requirements for the deposition technique.The choice of the technique should be done individually for the each layer.The ?rst and the most limiting requirement of the deposition technique is viscosity of the ink.There is a large range of viscosities that can be used for the dif-ferent printing processes.The viscosity of the ink required for the different deposition techniques are given in Table 5.
The viscosity of the ink is one of the critical factors for the choice of the deposition technology.The P3HT:[C60]PCBM blend has a vis-cosity in the range of 1–5mPa s,which is related to the low viscosity of the solvent itself,and to the low solubility of the components in the solvents.For the deposition of PEDOT:PSS and OPV layers,both solutions have rather low viscosity,and therefore we have used slot die R2R coating process.At slot die coating,the coating ink is squeezed out by gravity or under pressure through a slot onto the substrate.The picture of the slot die printing unit used in this experiment is shown in Fig.9.
The patterning of the layer can be done in a separate process step after the coating or during the coating process.As one of the coat-ing techniques,slot die can provide very uniform non-patterned layers.Post-patterning of the layer can be applied after the coat-ing,for example by laser ablation.Pre-patterning can be applied
Table 5
Viscosity requirements for the different deposition techniques [42–44].
Deposition technique Viscosity (mPa s)Jet printing 1–40
Slot die coating 1–10,000Gravure printing 15–200Flexo printing 15–500Screen printing
50–50,000
Fig.9.Slot die printing unit.
during the coating process,by patch/intermittent coating or self-assembled coating.In this study we report monolithic roll-to-roll coating of PEDOT:PSS and OPV photoactive layers.The patterning was performed as an additional step by solvent etching.7.Roll-to-roll line
The roll-to-roll coating line used in this study is equipped with a slot die module and a drying unit (three sections of 1.5m each).The web width is 30cm,the width of the slot die is 24cm.The speed of the line is in the range of 5–30m/min.In Fig.10,the roll-to-roll line and clean room environment used in this study are shown.The surface treatments which are sometimes required for improving the wettability were applied inline by a Corona unit.8.OPV module with roll-to-roll coated PEDOT:PSS and photoactive layers
The PEDOT:PSS (Orgacon TM )formulation for slot die coating was delivered by Agfa-Gevaert.A thin layer of PEDOT:PSS was deposited with a speed of 10m/min and dried with the same speed at 110?C.The thickness of the dry layers was about 110nm and illustrates high https://www.sodocs.net/doc/269250557.html,yer thickness variation was within ±2%.The thickness and uniformity of the layer was checked by ellipsometry.The result of measurements is shown in Fig.
11.
The photoactive layer was coated from o-xylene solution.The concentration of the inks was 10mg/ml.The layer was coated and
Fig.10.Roll-to-roll line in Holst Centre,equipped with slot die printing unit.
6
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https://www.sodocs.net/doc/269250557.html,yer thickness of roll-to-roll slot die coated PEDOR:PSS layer,measured every 5cm in coating direction.
dried with a speed of 10m/min.The drying has been performed at 90?C during 30s.Fig.12illustrates the roll of the PET foil with slot die coated PEDOT:PSS and OPV layers.The patterning of PEDOT and OPV layers has been performed manually by solvent etching.The top electrode (1nm of LiF and 100nm of Al)has been evaporated through a shadow mask,which adds extra time in the devices man-ufacturing.Any additional post-treatments have not been applied.The solar cell modules have been characterized under AM 1.5con-dition.The device was placed in a nitrogen container to protect the device from water and oxygen,as shown in Fig.13,and the measurements were performed under a Class A solar simulator.
The characteristics of the modules were the following:V oc –1.54V (three times the values of the single cell);J sc –3.98mA/cm 2,FF –0.33,MPP –0.7mW/cm 2(on active area).The J –V curves of the roll-to-roll coated device are given in Fig.14.The value of the open-circuit voltage of single cell is close to the values reported for this kind of materials.The low value of the short circuit cur-rent can be explained as effect of several factors.First of all,the sheet resistance of ITO foil used in this study was 60 / ,while the typical sheet resistance of ITO/glass substrate is 10–15 / .Taking into account high sheet resistance and large size of the cell,it is summarized in the lower short circuit current.Secondly,the ITO coated PET foil typically characterized by presence of big amount of spikes,which create shunts in the OPV devices.Fig.15illustrates the AFM topology image of ITO coated glass,as a reference,and ITO coated PET foil used in this study for roll-to-roll tests.Cur-rent leakage due to shunts is indicated by the low ?ll factor and decreased value of short circuit current.Next to that,morphology of the layer has not been optimized.As we have shown above,o-xylene has effect of decreasing ef?ciency in comparison with OPV devices deposited from chlorobenzene.Moreover,thermal
anneal-
Fig.13.OPV module in nitrogen container for the performance
measurements.
Fig.14.J –V curves of roll-to-roll coated OPV device.
ing,one of the important step to obtain right morphology,has been modi?ed for roll-to-roll process.In our study thermal annealing was combined with drying and lasted only 30s at 90?C,with is not enough for the creation of appropriate morphology.The ther-mal annealing which can easily be applied on the roll-to-roll mode can signi?cantly improve morphology and increase performance of the solar cell devices.The low value of the currents is mainly due to non-optimal morphology,transmission losses due to thick PEDOT and maybe somewhat effected by series resistance losses.Finally,the layer thickness of photoactive layer was far from opti-mum layer thicknesses.All these parameters have certain in?uence on the device performance.Optimization of each parameter can signi?cantly improve quality of the
devices.
Fig.12.Roll of the PET foil with slot die coated PEDOT and photoactive layers.
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Fig.15.AFM topology image of(a)ITO coated glass and(b)ITO coated PET foil.
9.Conclusions
The technology development from lab scale to industrial manu-facturing is a very complicated process.The feedback from the?rst roll-to-roll experiments has been taken into account for the further improving of the process.The main factors which have in?uence on the ef?ciency of the device are:solvent,layer thickness,deposition technique,size and design,dry time,thermal annealing,substrate, including TCO roughness and sheet resistance.In this study we have used non-chlorinated solvents,to prepare?exible polymer solar cell modules on a roll-to-roll line.The module was characterized as prepared without additional post-treatment,which is usually applied to improve the morphology of the polymer/fullerene blend. One of the most important factors to obtain high ef?cient devices is the uniform drying of OPV layers and creation of the appropriate morphology.Further optimization of the processing and drying can signi?cantly improve the results.Moreover,to demonstrate com-pletely printed solar cell module,the technology for the roll-to-roll deposition of the electrodes has to be developed.
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