DOI: 10.1002/ente.201300057

Freely available OPV—The fast way to progressFrederik C. Krebs,*[a] Markus Hçsel,[a] Michael Corazza,[a] B�renger Roth,[a] Morten V. Madsen,[a]

Suren A. Gevorgyan,[a] Roar R. Søndergaard,[a] Dieter Karg,[b] and Mikkel Jørgensen[a]

Abundance, fast manufacture, and low cost are what ideallyepitomize organic and polymer photovoltaics. However, theyhave remained esoteric (in physical form) almost since theirinception and though they have been extensively studiedthey cannot be said to be generally available to the publicwith the exception of a few samples. It is obvious that toqualify as a technology, polymer photovoltaics have to begenerally available in significant quantities. We recently re-ported a fast, efficient combined printing and coatingmethod[1] that enabled roll-to-roll processing of the polymersolar cell stack directly onto almost any flexible material,which ideally comprises a thin flexible barrier substrate.

Herein we describe the fabrication of 20 928 small modules(10.0 �14.2 cm2) directly on barrier foil by employing a newlydesigned front electrode grid. This type of encapsulation re-sults from efficient edge sealing by laser-cutting of the finalmodules. These “freeOPV” modules are, as the name sug-gests, made freely available to anyone who registers on ourscientific website.[2] The general idea behind the establish-ment of such a program is that the power of analysis is close-ly linked to the amount of available data and we thus encour-age feedback from any technical or scientific study regardlessof its nature. The website will furthermore function as a plat-form through which new materials can be evaluated in thecontext of this new module.

The indium-tin oxide (ITO)-free solar cell modules wereprepared by using previously described procedures,[1] al-though this current work was performed at higher speedsand with a module design specific to this purpose. A few dis-tinct advances and differences are described in the followingparagraphs. An illustration of the complete solar-cell stack isshown in Figure 1.

The front silver grid of the solar cell is processed by flexo-printing at high speed (20 mmin�1). We have previously re-ported the use of a hexagonal front grid made of silver incombination with highly conductive poly(3,4-ethylenedioxy-

thiophene) poly(styrenesulfonate) (PEDOT:PSS) as a trans-parent electrode, but more-detailed studies have shown thatthe presence of small electrical shorts in the solar cell ismuch more pronounced in the areas where the silver frontelectrode grid and the silver back electrode grid overlap. Ide-ally a design should be developed for which no overlapoccurs, but considering the current control of registration(horizontally and in the web direction) and the unpredictablethermal shrinkage and stretching (in the cross-web and in theweb direction respectively) of the foil during the process, it isnot currently possible to handle such precision at sufficientlyhigh speeds. As an alternative approach, a comb structurewith slants of �58 for the respective grids has been chosen.In such a design there is only one region of overlap (ora maximum of two) thus minimizing the number of likely

Figure 1. Outline of the multilayer structure of the general structure of thefreeOPV sample (top) and an illustration of the a laser-cut freeOPV from thefinal roll-to-roll processed foil.

[a] Prof. F. C. Krebs, M. Hçsel, M. Corazza, B. Roth, Dr. M. V. Madsen,Dr. S. A. Gevorgyan, Dr. R. R. Søndergaard, Dr. M. JørgensenDepartment of Energy Conversion and StorageTechnical University of DenmarkFrederiksborgvej 399, DK-4000 Roskilde (Denmark)E-mail: frkr@dtu.dk

[b] Dr. D. KargDCG Systems GmbH InstitutionAm Weichselgarten 7, 91058 Erlangen (Germany)

� 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.This is an open access article under the terms of the Creative CommonsAttribution Non-Commercial License, which permits use, distribution andreproduction in any medium, provided the original work is properly citedand is not used for commercial purposes.

378 � 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Energy Technol. 2013, 1, 378 – 381

electrical shorts in the structure. Figure 2 a and b shows light-beam-induced current (LBIC) and dark lock-in thermogra-phy (DLIT) images of a module for which three of the eightcells are heavily shunted. In the thermographic image theshunted silver comb lines light up due to dissipation of heatat the short circuit located there. Careful analysis of theDLIT image using DLIT microscopy reveals just one pointof contact. This technique (Figure 2 c) enables extremelyhigh resolution in the thermal image. The grid line is 100 mmwide and the pixel size in the infrared image is 3 mm. Wefound this new IR microscopy technique to be of exceptionalvalue for the development of the grid electrode.

An extremely important factor in the operation of organicsolar cells and modules is to ensure that the cells are proper-ly encapsulated, while retaining the essential access to theelectrodes. The previous method was to protect the organicsolar cells by applying a barrier foil containing a pressure-sensitive adhesive over the active area while leaving the elec-trodes exposed for external access.[3] However, such sealingis very sensitive to the slow diffusion from the edges of theseal (a distance of a few millimeters). As an alternative, wepresent a method for encapsulating the solar cells by usinga UV-curable adhesive (DELO Katiobond LP655). Access tothe electrodes is subsequently achieved by piercing the fin-ished solar cells through the area where a thick conductor isprinted (we have employed both carbon and silver) witha metal push button (Figure 3).

The adhesive was applied to the encapsulation barrier foilby flexoprinting (30 cm3 m�2 anilox cylinder) and this foil wasfed into a nip together with the solar cells where the com-bined foil was subsequently exposed to UV light from anarray of twelve lamps. The lamination process was performed

at a web speed of 2 m min�1. The area containing the extra-thick silver layer (“Ag connector” in Figure 1) was used tomake electrical contact after lamination by piercing a nickel-free metal connector through the foil.

The finished solar cells were finally cut into individualunits by laser cutting using a 90 W roll-to-roll CO2 laser witha laser speed of 4.5 mmin�1. Besides the obvious issue ofspeed an additional advantage of laser cutting is that it mini-mizes the mechanical stress at the edges of the solar cell thatwould certainly be present if the cells were cut by conven-tional means using a knife. It is furthermore reasonable toassume that melting of the substrate at the edges actuallyseals the multilaminate further and avoids introducing a de-lamination defect/fracture that has a tendency to propagate.Figure 4 shows a photo of the laser-cutting process (a movieshowing the process can be found in the Supporting Informa-tion).

Despite the high speed of production the precision and ac-curacy in each step of coating, printing, encapsulating, andcutting provides high consistency in the device performancewith a very low percentage of defective or malfunctioningdevices. Figure 5 a shows the distribution of the photovoltaicparameters for 80 samples randomly chosen from the roll.The I–V curve of a typical sample is shown in Figure 5 b.

The results of short-term stability measurements in accord-ance with the International Summit on OPV Stability (ISOS-L and ISOS-D)[4] suggest that this new generation of samples

Figure 3. Illustration of the fully encapsulated solar cell. The black and whitearrows show the diffusive pathway from the edges to the solar cell.

Figure 4. Photograph of the laser cutting process.

Figure 2. LBIC image (A) and DLIT image (B) of a freeOPV sample moduleshowing severe shunting in 3 of the 8 serially connected cells. In frame C isa superimposed photograph and an IR image. The heat spot occurs exactlyat the overlap of the two silver grids (one of the silver lines is blocked by thesolar cell stack in the picture). The scale bars in A and B are 10 mm and in Cit measures 100 mm.

Energy Technol. 2013, 1, 378 – 381 � 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.entechnol.de 379

is approximately as stable as its roll-to-roll processed prede-cessors, which have been shown to exceed 10 000 h lifetimeunder outdoor exposure conditions.[5] As a result of theaforementioned full encapsulation, the cells also exhibit anextremely long shelf life that allows for the samples to beshipped across long distances without degradation of the per-formance. However, due to their highly flexible nature thecells can be sensitive towards constant handling, excessiveflexing, mechanical stresses, and heating, which may intro-duce flaws in the encapsulation and deteriorate the stability.Thus, the lifetime of such a sample is linked to its use, appli-cation, and handling during shipment. For the first experi-ment announced on the website, we aim to establish how themodules are affected by shipment without packaging (i.e., bysending them as a postcard). Initial results are promising andthe interested reader can still participate in this study.

As mentioned above, the aim of distributing freeOPVsamples is to generate a platform from which the roll-to-rollprocessing technology can be evaluated. Such evaluation ismost efficiently performed with the technology freely avail-able for everyone. Organic solar cell research has been con-ducted for more than 25 years now with the vision of mass-produced flexible roll-to-roll processed solar cells, but onlya few researchers have actually had a flexible organic solarcell in their hands. It is our belief that the research progress

is best evaluated by using comparisons among results withthe same processing origin and this platform is intended toprovide such an origin. For the same reasons the transparentITO-free substrate Flextrode[2] is also freely available.

All freeOPV samples are equipped with a 2D barcodegiving them a unique ID and full tracability. It will thus bepossible to retrieve and reference all information on theprocessing and handling of a given cell and the platform willallow the receiver to give feedback on his/her specific cellthrough the website.[2] The purpose of this is to developa methodology for processing, testing, and distributing anenormous amount of solar cells with minimal influence froma human operator while maintaining full traceability. Thesoftware we developed for this purpose can be experiencedon the website. The 2D barcode can be scanned with anymodern mobile device; for those without access to sucha device there is also a code that can be typed in for extrac-tion of the information. Again the purpose of the effort is toorganize a fully automated platform for handling everyaspect of the module along the value chain (preparation, dis-tribution, service, and decommission).

The platform is furthermore thought of as a possiblevector for testing of new materials in a roll-to-roll context.Very few research groups have access to roll-to-roll process-ing equipment for testing of their new materials and the plat-form will provide a means to do so. The module structure de-scribed here is sufficiently refined to enable development ofnew active materials and interface layers for this structure;pending success new results can be rapidly integrated infuture freeOPV samples, again in a way that everyone cantest and see for themselves. We have thus chosen an initialstandard with this first freeOPV sample and a direct compar-ison can be performed by substituting just one or more ofthe components.

We have devised an efficient method to prepare small,flexible, ITO-free polymer solar-cell modules directly on bar-rier foil. The method is in principle generic and though wehave exemplified the modules here with P3HT:PCBM as thecommonly known active material, this methodology can alsoserve as a generic platform for the development of new andmore-effective materials combinations, both with respect toperformance and stability. The modules are true to the art inthe sense that they are flexible, prepared using fast printingand coating methods, and of such a low cost that they can bemade freely available to the public through a website. In factthe postage of the solar cell is significantly more expensivethan the solar cell itself. All conceivable scientific or techni-cal studies are encouraged and welcomed regardless of theirnature.

Acknowledgements

This work was supported by the Danish Ministry of Science,Innovation and Higher Education through the EliteForsk ini-tiative, through the 2011 Grundfos Award. Partial support wasalso obtained from and the EU-Indian framework of the

Figure 5. Distribution of the device performance for 80 modules (A) and theI-V curve of a typical module (B).

380 www.entechnol.de � 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Energy Technol. 2013, 1, 378 – 381

“Largecells” project as part of the European Commission�sSeventh Framework Programme (FP7/2007-2013, grant no.261936) and the Framework 7 ICT 2009 collaborative projectROTROT (grant no. 288565) and FP7-NMP-2011-LARGE-5collaborative project Clean4Yield (grant no. 281027) and theEurotech Universities Alliance project “Interface science forphotovoltaics (ISPV)”

Keywords: laser cutting · ITO-free · photovoltaics ·polymers · roll-to-roll processing

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[2] The Flextrode substrate and the freeOPV solar cell modules can beobtained free of charge by submitting a postal address and registeringat the www.plasticphotovoltaics.org website.

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[5] S. A. Gevorgyan, M. V. Madsen, H. F. Dam, M. Jørgensen, C. J. Fell,K. F. Anderson, B. C. Duck, A. Mescheloff, E. A. Katz, A. Elschner,R. Rçsch, H. Hoppe, M. Hermenau, M. Riede, F. C. Krebs, Sol.Energy Mater. Sol. Cells 2013, 116, 187 –196.

Received: May 24, 2013Published online on July 2, 2013

Energy Technol. 2013, 1, 378 – 381 � 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.entechnol.de 381