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Large area ITO-free organic solar cells on steel substrate
Organic Electronics 13 (2012) 3310–3314
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Large area ITO-free organic solar cells on steel substrate Yulia Galagan ⇑, Date J.D. Moet, Dorothee C. Hermes, Paul W.M. Blom, Ronn Andriessen Holst Centre, PO Box 8550, 5605 KN Eindhoven, The Netherlands
a r t i c l e
i n f o
Article history: Received 17 August 2012 Received in revised form 28 September 2012 Accepted 28 September 2012 Available online 26 October 2012 Keywords: Stainless steel Metal substrate Organic solar cells ITO-free Transparent cathode Optical modeling
a b s t r a c t Large area ITO-free organic solar cells on steel substrates are demonstrated. The stainless steel was used as a substrate with excellent barrier properties and as a bottom electrode with very low sheet resistance. The low sheet resistance of the steel electrode allows manufacturing of large area devices with good performance. The individual cells with the size of 5 10 cm2 and efficiency of 1.3% were manufactured. The OPV cells with steel electrode demonstrate fill factor in the range of 58–69%, as measured under illumination through a mask with an aperture of 1.0 1.0 cm2. A thermally evaporated semitransparent top cathode, contained ZnS and thin metal layers, was used to complete the device stack. In order to quantitate effect of the cathode transmittance, the optical simulations using the transfer-matrix formalism has been performed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The growing interest in organic photovoltaics (OPV) is caused by its promise of low cost energy conversion. In order to have an impact on the power generation market on the longer term, organic photovoltaics should combine high power conversion efficiency with low production costs and long term stability. Although the OPV technology looks very promising, there are still several issues that must be solved before this technology will occupy a large segment of the energy market. The potential low cost is based on the usage of low cost materials and substrates, and on the high production speeds that can be reached by using roll-to-roll printing and coating techniques [1–8]. The presence of a transparent conductive electrode such as indium–tin oxide (ITO) limits the reliability [9,10] and significantly increases the cost [11–13] of organic photovoltaic devices as it is brittle and expensive. Moreover, the relatively high sheet resistance of ITO electrodes on flexible substrates limits the maximum width of a single cell [14]. Moreover, recently, it was shown than ⇑ Corresponding author. Tel.: +31 404020447; fax: +31 404020699. E-mail address: [email protected] (Y. Galagan). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.09.039
in organic solar cells the effect of high sheet resistance of ITO electrodes on Isc and FF is much stronger that classical series resistance effect: it reduces voltage-dependent photocurrent [15]. The substrates [16,17] for low cost OPV device manufacturing must satisfy numerous requirements, such as flexibility, optical quality or transparency, substrate smoothness in the nanometer range, the ability to support processing at moderately high temperatures, good dimensional stability, good resistance to any chemicals used during processing and low water absorption. Polymer foil substrates are highly flexible, can be inexpensive and are compatible with roll-to-roll processing. Transparent plastic substrates have the advantage of being compatible with any organic solar cell architecture. The most common plastic substrates used for fabrication of OPV devices are polyethylene terephthalate (PET), polyethylene naphthalate (PEN) [10], polycarbonate (PC), polyethersulfone (PES) and polyimide (PI) [18]. Polyimide (e.g., KaptonÒ from DuPont) absorbs in the visible region, which makes it less suitable for some device architectures. However, the advantage of this plastic substrate is its compatibility with high process temperatures over 350 °C, while other plastic substrates
Y. Galagan et al. / Organic Electronics 13 (2012) 3310–3314
have limited process temperature capabilities. Another disadvantage of plastic substrates is the lack of dimensional stability during processing at elevated temperatures. There are no polymeric substrates which meet the extremely demanding requirements for low moisture and water permeability for OPV applications. The typical water and oxygen permeation rates of flexible plastic substrates are 1–10 g/m2/day and 1–10 cm3/m2/day [19], respectively, whereas typically required for organic electronic devices are 10 3–10 6 g/m2/day and 10 3– 10 5 cm3/m2/day [20–23]. Therefore, plastic substrates require additional barrier layer coatings. Barrier coatings can reduce permeability of water and oxygen, but simultaneously increase the cost of solar cell manufacturing significantly. Thin metal foil substrates [16,24] are particularly attractive for flexible OPV devices, because they provide an excellent barrier to water and oxygen, which are the most critical components affecting the lifetime of organic solar cells. Additionally, stainless steel foil has superior chemical resistance to most of the chemicals used during OPV device fabrication. The smoothness of the metal substrates can be increased by polishing or by applying additional planarization layers. The conductivity of metal substrates can be considered both as an advantage and disadvantage. Sometimes the metal substrate can serve as back contact, but very often the conductive properties of the metal substrate are not used. The conducting substrates are then completely insulated from the actual OPV device by an additional, insulating planarization layer. Stainless steel substrates are dimensionally stable and have excellent barrier properties against water and oxygen. Generally, stainless steel substrates are more durable then plastic substrates. In this work we report on a flexible large area organic solar cell on a flexible metal foil substrate. The stainless steel, with mean roughness value (Ra) is 60 nm, and rootmean-square roughness (Rq) amounts to 76 nm, was used as a substrate with excellent barrier properties and as a bottom electrode with very low sheet resistance (less than 0.5 X/sq). The low sheet resistance of the steel electrode allows manufacturing of large area devices with good performance. A semitransparent top cathode was applied to complete the device stack.
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Fig. 1. (a) Picture of a finished OPV device on steel foil. (b) Schematic layout of the device structure, showing the resist (ISO) structure, the PEDOT:PSS and P3HT:PCBM layers, as well as the semitransparent top electrode.
(P3HT) (purchased from Plextronics, Plexcore OS 2100) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (99%, purchased from Solenne BV) were dissolved in 1,2dichlorobenzene with a mixing ratio of 1:1 by weight, with total solid content of 4%. The solution was stirred for 3 h at 80 °C. The photoactive layer was obtained by spin coating at 1000 rpm for 30 s, resulting in a dry layer thickness of 220 nm. The semi-transparent cathode [25] (LiF/Al/Ag/ ZnS) was thermally evaporated in a vacuum chamber (pressure of 10 7 Torr) through a shadow mask with deposition rate of 0.2 nm/s. Finally, a thin film transparent barrier [26] was deposited on top of the completed devices. Then, the stainless steel substrate with finished devices was delaminated from the glass carrier. Current–voltage curves were measured using a xenonlamp-based solar simulator (Oriel (LS0104) 150 W). The devices were illuminated through an illumination mask with an aperture of 1.0 1.0 cm2.
3. Results and discussion 2. Experimental The flexible metal foil substrates used for fabrication of organic solar cells were 50 lm thick stainless steel foil of type 1.4301 from Hasberg Schneider GmbH. The 15.2 15.5 cm2 substrates were temporarily laminated to a glass carrier, which is required for spin coating of the subsequent layers. Negative photoresist was spin coated on top of the steel substrates with a layer thickness of 1 lm. The photoresist layer was structured via photolithography in such a way, that each substrate contained two devices with an active area of 5 10 cm2. A schematic illustration of the resist structure is shown in Fig. 1. PEDOT:PSS (Clevios P VP AI 4083, H.C. Starck) was spin coated, forming a 100 nm thick layer. Poly(3-hexylthiophene)
Thin film OPV devices are very sensitive to surface roughness. High roughness structures over short distances must be avoided. Substrate smoothness in the nanometer range is required to provide a surface that will promote high-quality deposition of subsequent layers and prevent the penetration of potential substrate spikes or irregularities into the organic layers. However, an intermediate roughness over long distance is acceptable. The roughness of the stainless steel substrates used in this study is shown in Fig. 2. As can be observed from the image, recorded with white light interferometry, the roughness of the film is in the range of 60–70 nm and there are no large thickness variations on the submicron scale. The deposition of the organic layers onto these substrates did not demonstrate any
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Fig. 2. 3D height view of the steel foil surface obtained by white light interferometry at a magnification of 50. The arithmetic mean roughness value (Ra) is 60 nm, the root-mean-square roughness (Rq) amounts to 76 nm on the scanned area of 123 94 lm.
Fig. 4. Upper panel: the maximum current density (Jmax) that can be extracted from ITO-based P3HT:PCBM solar cells (black line) and devices on steel foil (red line) for varying photoactive layer thickness (L), as determined by optical modeling. Bottom panel: the ratio of Jmax for steel devices to Jmax of ITO-based devices. At L = 220 nm, the ratio amounts to 0.6. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Current–voltage (I–V) characteristics of a 5 10 cm2 solar cell on steel foil, illuminated through a 1 1 cm2 mask aperture at the edge (open black symbols) and in the middle (open red symbols) of the device. The solid symbols mark the corresponding dark current. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
difficulties. As shown in Fig. 3, the finished devices exhibit low leakage currents, indicating, that local shunts do not play a role. The active device area of the solar cells on steel foil was 50 cm2. Since the solar simulator used in this study cannot provide uniform illumination over such an area, the performance of the devices was measured with a 1 1 cm2 illumination mask in different places of the devices. Fig. 3 illustrates typical I–V curves of devices. The illuminated area is selected either close to the contacts or in the middle of the device, which is the most distanced point from the contacts. The results illustrate a slight decrease in fill factor (FF) from 69% to 58% due to resistive losses. It is obvious that the steel electrode does not limit the fill factor as it has a very low sheet resistance. Rather, the fill factor is determined by the sheet resistance of the transparent electrode [25], which has previously been determined to be 5 X/sq.
The short-circuit current density (Jsc) amounts to 4.2 mA/cm2. We attribute this rather low value to the limited transmittance of the top electrode and thin film encapsulation. In order to quantitate this effect, we performed optical simulations using the transfer-matrix formalism [27]. By calculating the absorption in the active layer, taking into account interfacial reflections and interference effects, and assuming unity internal quantum efficiency, one can calculate the maximum current density (Jmax) that can be extracted from the solar cell under illumination with white light at an intensity of 1 sun [28]. In the upper panel of Fig. 4, Jmax is plotted versus the P3HT:PCBM layer thickness for a solar cell on steel foil, as used in the present study, as well as for a regular device on an ITO-covered glass substrate. The structure of the regular ITO-based device was glass (0.7 mm)/ITO (70 nm)/PEDOT:PSS (100 nm)/P3HT:PCBM/LiF (1 nm)/Al (100 nm). Both in experiment and in the optical simulations, the ITObased device is illuminated through the glass substrate, whereas the steel device is illuminated through the semitransparent barrier and top electrode. The calculated ratio of Jmax of the steel device and Jmax of the ITO device is plotted in the bottom panel of Fig. 4. It can be seen that this ratio amounts to 0.6 at a photoactive layer thickness of 220 nm. This number is in excellent agreement with experimental results on ITO-based solar cells, which showed a Jsc of 7.3 mA/cm2 under similar conditions. Another feature of the J–V curves in Fig. 3 is the rather low open-circuit voltage. As mentioned above, the steel devices were illuminated through a shadow mask with an aperture area (Amask) of 1 cm2. Compared to the cell area (Acell) of 50 cm2, this amounts to an illuminated region of
Y. Galagan et al. / Organic Electronics 13 (2012) 3310–3314
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ITO-based devices. The main conclusion of this study is that for innovative approach of manufacturing organic solar cells on steel substrate, highly transparent and high conductive top electrode is required. Increasing of Jsc in steel-based devices with existing electrode is possible by varying the thickness of the layers in the device stack.
References
Fig. 5. Change in open-circuit voltage (DVoc) for an ITO-based device measured with an illumination mask (solid symbols) or using neutral density filters to tune the light intensity (open symbols). The bottom and top horizontal axes span the ratio of the area of the mask aperture to the cell area (Amask/Acell) and the relative light intensity (I/I0), respectively.
2%. The simple explanation of the Voc reduction under a localized illumination is that the entire cell can be viewed as the parallel connection of illuminated and dark subcells. In real cells (even in inorganic PV), effects of non-uniform illumination depend on the light intensity and the cell series resistance [29–31]. This effect is shown in Fig. 5, which shows the decrease in Voc for an ITO-based device that is characterized using either masked illumination with different Amask/Acell ratios or using neutral density filters to tune the light intensity. The measurements clearly indicate a similar effect of masking and filtering. Furthermore, the drop in Voc at an effective intensity of 2% is about 0.11 V, which agrees well with lowered Voc of the steel device compared to typical ITO-based devices exhibiting Voc = 0.56 V.
4. Conclusions Large area ITO-free organic solar cells on steel substrates are demonstrated. The stainless steel was used as a substrate with excellent barrier properties and as a bottom electrode with very low sheet resistance, which allows to obtain large area OPV devices with fill factor of 58–69%, as measured under illumination through a mask with an aperture of 1.0 1.0 cm2. Semitransparent cathode which has sheet resistance of 5 X/sq is a limiting factor in the maximum cell geometry. Calculation of maximum cell size and its dependence from sheet resistance of the electrodes has been calculated and reported separately [32]. The main factor limiting short circuit current density in the reported devices is transparency of the top electrode. In order to quantitate effect of the semitransparent cathode transmittance, the optical simulations using the transfer-matrix formalism has been performed. Optical modeling has explained short circuit current in the steel devices, which also was compared with Jsc of standard bottom illuminated
[1] Y. Galagan, I.G. de Vries, A.P. Langen, R. Andriessen, W.J.H. Verhees, S.C. Veenstra, J.M. Kroon, Technology development for roll-to-roll production of organic photovoltaics, Chem. Eng. Process. 50 (2011) 454–461. [2] F.C. Krebs, J. Fyenbo, D.M. Tanenbaum, S.A. Gevorgyan, R. Andriessen, B. Van Remoortere, Y. Galagan, M. Jørgensen, The OE-A OPV demonstrator anno domini 2011, Energy Environ. Sci. 4 (10) (2011) 4116–4123. [3] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. [4] T.R. Andersen, T.T. Larsen-Olsen, B. Andreasen, A.P.L. Böttiger, J.E. Carlé, M. Helgesen, E. Bundgaard, K. Norrman, J.W. Andreasen, M. Jørgensen, F.C. Krebs, Aqueous processing of low-band-gap polymer solar cells using roll-to-roll methods, ACS Nano. 5 (2011) 4188– 4196. [5] S.A. Gevorgyan, A.J. Medford, E. Bundgaard, S.B. Sapkota, H.F. Schleiermacher, B. Zimmermann, U. Würfel, A. Chafiq, M. LiraCantu, T. Swonke, M. Wagner, C.J. Brabec, O. Haillant, E. Voroshazi, T. Aernouts, R. Steim, J.A. Hauch, A. Elschner, M. Pannone, M. Xiao, A. Langzettel, D. Laird, M.T. Lloyd, T. Rath, E. Maier, G. Trimmel, M. Hermenau, T. Menke, K. Leo, R. Rösch, M. Seeland, H. Hoppe, T.J. Nagle, K.B. Burke, C.J. Fell, D. Vak, T.B. Singh, S.E. Watkins, Y. Galagan, A. Manor, E.A. Katz, T. Kim, K. Kim, P.M. Sommeling, W.J.H. Verhees, S.C. Veenstra, M. Riede, M. Greyson Christoforo, T. Currier, V. Shrotriya, G. Schwartz, F.C. Krebs, An inter-laboratory stability study of roll-to-roll coated flexible polymer solar modules, Sol. Energy Mater. Sol. C. 95 (2011) 1398–1416. [6] S.R. Dupont, M. Oliver, F.C. Krebs, R.H. Dauskardt, Interlayer adhesion in roll-to-roll processed flexible inverted polymer solar cells, Sol. Energy Mater. Sol. C. 97 (2012) 171–175. [7] T.T. Larsen-Olsen, B. Andreasen, T.R. Andersen, A.P.L. Böttiger, E. Bundgaard, K. Norrman, J.W. Andreasen, M. Jørgensen, F.C. Krebs, Simultaneous multilayer formation of the polymer solar cell stack using roll-to-roll double slot-die coating from water, Sol. Energy Mater. Sol. C. 97 (2012) 22–27. [8] R. Rösch, F.C. Krebs, D.M. Tanenbaum, H. Hoppe, Quality control of roll-to-roll processed polymer solar modules by complementary imaging methods, Sol. Energy Mater. Sol. C. 97 (2012) 176–180. [9] Y. Galagan, J.E. Rubingh, R. Andriessen, C.C. Fan, P. Blom, S.C. Veenstra, J.M. Kroon, ITO-free flexible organic solar cells with printed current collecting grids, Sol. Energy Mater. Sol. C. 95 (2011) 1339–1343. [10] F.C. Krebs, All solution roll-to-roll processed polymer solar cells free from indium–tin-oxide and vacuum coating steps, Org. Electron. 10 (2009) 761–768. [11] N. Espinosa, R. García-Valverde, A. Urbina, F.C. Krebs, A life cycle analysis of polymer solar cell modules prepared using roll-to-roll methods under ambient conditions, Sol. Energy Mater. Sol. C. 95 (2011) 1293–1302. [12] N. Espinosa, R. García-Valverde, F.C. Krebs, Life-cycle analysis of product integrated polymer solar cells, Energy Environ. Sci. 4 (2011) 1547–1557. [13] N. Espinosa, R. García-Valverde, A. Urbina, F. Lenzmann, M. Manceau, D. Angmo, F.C. Krebs, Life cycle assessment of ITO-free flexible polymer solar cells prepared by roll-to-roll coating and printing, Sol. Energy Mater. Sol. C. 97 (2012) 3–13. [14] C. Lungenschmied, G. Dennler, H. Neugebauer, S.N. Sariciftci, M. Glatthaar, T. Meyer, A. Meyer, Flexible, long-lived, large-area, organic solar cells, Sol. Energy Mater. Sol. C. 91 (2007) 379–384. [15] A. Manor, E.A. Katz, T. Tromholt, B. Hirsch, F.C. Krebs, Origin of size effect on efficiency of organic photovoltaics, J. App. Phys. 109 (2011) 074508. [16] J. Jensen, M. Mikkelsen, F.C. Krebs, Flexible substrates as basis for photocatalytic reduction of carbon dioxide, Sol. Energy Mater. Sol. C. 95 (2011) 2949–2958.
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[17] Y. Galagan, R. Andriessen, Organic photovoltaics: technologies and manufacturing, in: V. Fthenakis (Ed.), Third Generation Photovoltaics, InTech, 2012, pp. 61–90. [18] M. Manceau, D. Angmo, M. Jørgensen, F.C. Krebs, ITO-free flexible polymer solar cells: from small model devices to roll-to-roll processed large modules, Org. Electron. 12 (2011) 566–574. [19] J.S. Lewis, M.S. Weaver, Thin-film permeation-barrier technology for flexible organic light-emitting devices, IEEE J. Sel. Topics Quantum Electron. 10 (2004) 45–57. [20] S. Cros, R. De Bettignies, S. Berson, S. Bailly, P. Maisse, N. Lemaitre, S. Guillerez, Definition of encapsulation barrier requirements: a method applied to organic solar cells, Sol. Energy Mater. Sol. C. 95 (SUPPL. 1) (2011) S65–S69. [21] J.A. Hauch, P. Schilinsky, S.A. Choulis, S. Rajoelson, C.J. Brabec, The impact of water vapor transmission rate on the lifetime of flexible polymer solar cells, Appl. Phys. Lett. 93 (10) (2008). [22] M. Hermenau, S. Schubert, H. Klumbies, J. Fahlteich, L. MüllerMeskamp, K. Leo, M. Riede, The effect of barrier performance on the lifetime of small-molecule organic solar cells, Sol. Energy Mater. Sol. C. 97 (2012) 102–108. [23] F.C. Krebs, Air stable polymer photovoltaics based on a process free from vacuum steps and fullerenes, Sol. Energy Mater. Sol. C. 92 (7) (2008) 715–726. [24] T. Chuang, M. Troccoli, P. Kuo, A. Jamshidi-Roudbari, M.K. Hatalis, I. Biaggio, A.T. Voutsas, Top-emitting 230 dots/in. active-matrix polymer light-emitting diode displays on flexible metal foil substrates, Appl. Phys. Lett. 90 (2007) 151114.
[25] Y. Galagan, M.G. Debije, P.W.M. Blom, Semitransparent organic solar cells with organic wavelength dependent reflectors, Appl. Phys. Lett. 98 (2011) 043302. [26] F. van Assche, H. Rooms, E. Young, J. Michels, T. van Mol, G. Rietjens, P. van de Weijer, P. Bouten, Thin-film barrier on foil for organic LED lamps, Presented at AIMCAL fall technical conference, Myrtle Beach, South Carolina, 2008. [27] L.A.A. Pettersson, L.S. Roman, O. Inganäs, Modeling photocurrent action spectra of photovoltaic devices based on organic thin films, J. Appl. Phys. 86 (1999) 487–496. [28] J.D. Kotlarski, P.W.M. Blom, L.J.A. Koster, M. Lenes, L.H. Slooff, Combined optical and electrical modeling of polymer: Fullerene bulk heterojunction solar cells, J. Appl. Phys. 103 (2008) 084502. [29] L.J.A. Koster, V.D. Mihailetchi, R. Ramaker, P.W.M. Blom, Light intensity dependence of open-circuit voltage of polymer: fullerene solar cells, Appl. Phys. Lett. 86 (2005) 123509. [30] A. Manor, E.A. Katz, R. Andriessen, Y. Galagan, Study of organic photovoltaics by localized concentrated sunlight: towards optimization of charge collection in large-area solar cells, Appl. Phys. Lett. 99 (2011) 173305. [31] E.A. Katz, J.M. Gordon, W. Tassew, D. Feuermann, Photovoltaic characterization of concentrator solar cells by localized irradiation, J. Appl. Phys. 100 (2006) 044514. [32] Y. Galagan, B. Zimmermann, B., E.W.C. Coenen, M. Jørgensen, D.M. Tanenbaum, F.C. Krebs, H. Gorter, S. Sabik, L.H. Slooff, S.C. Veenstra, J.M. Kroon, R. Andriessen, Current collecting grids for ITO-free solar cells, Adv. Energy Mater. 2 (2012) 103–110.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Large area ITO-free organic solar cells on steel substrate Yulia Galagan ⇑, Date J.D. Moet, Dorothee C. Hermes, Paul W.M. Blom, Ronn Andriessen Holst Centre, PO Box 8550, 5605 KN Eindhoven, The Netherlands
a r t i c l e
i n f o
Article history: Received 17 August 2012 Received in revised form 28 September 2012 Accepted 28 September 2012 Available online 26 October 2012 Keywords: Stainless steel Metal substrate Organic solar cells ITO-free Transparent cathode Optical modeling
a b s t r a c t Large area ITO-free organic solar cells on steel substrates are demonstrated. The stainless steel was used as a substrate with excellent barrier properties and as a bottom electrode with very low sheet resistance. The low sheet resistance of the steel electrode allows manufacturing of large area devices with good performance. The individual cells with the size of 5 10 cm2 and efficiency of 1.3% were manufactured. The OPV cells with steel electrode demonstrate fill factor in the range of 58–69%, as measured under illumination through a mask with an aperture of 1.0 1.0 cm2. A thermally evaporated semitransparent top cathode, contained ZnS and thin metal layers, was used to complete the device stack. In order to quantitate effect of the cathode transmittance, the optical simulations using the transfer-matrix formalism has been performed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The growing interest in organic photovoltaics (OPV) is caused by its promise of low cost energy conversion. In order to have an impact on the power generation market on the longer term, organic photovoltaics should combine high power conversion efficiency with low production costs and long term stability. Although the OPV technology looks very promising, there are still several issues that must be solved before this technology will occupy a large segment of the energy market. The potential low cost is based on the usage of low cost materials and substrates, and on the high production speeds that can be reached by using roll-to-roll printing and coating techniques [1–8]. The presence of a transparent conductive electrode such as indium–tin oxide (ITO) limits the reliability [9,10] and significantly increases the cost [11–13] of organic photovoltaic devices as it is brittle and expensive. Moreover, the relatively high sheet resistance of ITO electrodes on flexible substrates limits the maximum width of a single cell [14]. Moreover, recently, it was shown than ⇑ Corresponding author. Tel.: +31 404020447; fax: +31 404020699. E-mail address: [email protected] (Y. Galagan). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.09.039
in organic solar cells the effect of high sheet resistance of ITO electrodes on Isc and FF is much stronger that classical series resistance effect: it reduces voltage-dependent photocurrent [15]. The substrates [16,17] for low cost OPV device manufacturing must satisfy numerous requirements, such as flexibility, optical quality or transparency, substrate smoothness in the nanometer range, the ability to support processing at moderately high temperatures, good dimensional stability, good resistance to any chemicals used during processing and low water absorption. Polymer foil substrates are highly flexible, can be inexpensive and are compatible with roll-to-roll processing. Transparent plastic substrates have the advantage of being compatible with any organic solar cell architecture. The most common plastic substrates used for fabrication of OPV devices are polyethylene terephthalate (PET), polyethylene naphthalate (PEN) [10], polycarbonate (PC), polyethersulfone (PES) and polyimide (PI) [18]. Polyimide (e.g., KaptonÒ from DuPont) absorbs in the visible region, which makes it less suitable for some device architectures. However, the advantage of this plastic substrate is its compatibility with high process temperatures over 350 °C, while other plastic substrates
Y. Galagan et al. / Organic Electronics 13 (2012) 3310–3314
have limited process temperature capabilities. Another disadvantage of plastic substrates is the lack of dimensional stability during processing at elevated temperatures. There are no polymeric substrates which meet the extremely demanding requirements for low moisture and water permeability for OPV applications. The typical water and oxygen permeation rates of flexible plastic substrates are 1–10 g/m2/day and 1–10 cm3/m2/day [19], respectively, whereas typically required for organic electronic devices are 10 3–10 6 g/m2/day and 10 3– 10 5 cm3/m2/day [20–23]. Therefore, plastic substrates require additional barrier layer coatings. Barrier coatings can reduce permeability of water and oxygen, but simultaneously increase the cost of solar cell manufacturing significantly. Thin metal foil substrates [16,24] are particularly attractive for flexible OPV devices, because they provide an excellent barrier to water and oxygen, which are the most critical components affecting the lifetime of organic solar cells. Additionally, stainless steel foil has superior chemical resistance to most of the chemicals used during OPV device fabrication. The smoothness of the metal substrates can be increased by polishing or by applying additional planarization layers. The conductivity of metal substrates can be considered both as an advantage and disadvantage. Sometimes the metal substrate can serve as back contact, but very often the conductive properties of the metal substrate are not used. The conducting substrates are then completely insulated from the actual OPV device by an additional, insulating planarization layer. Stainless steel substrates are dimensionally stable and have excellent barrier properties against water and oxygen. Generally, stainless steel substrates are more durable then plastic substrates. In this work we report on a flexible large area organic solar cell on a flexible metal foil substrate. The stainless steel, with mean roughness value (Ra) is 60 nm, and rootmean-square roughness (Rq) amounts to 76 nm, was used as a substrate with excellent barrier properties and as a bottom electrode with very low sheet resistance (less than 0.5 X/sq). The low sheet resistance of the steel electrode allows manufacturing of large area devices with good performance. A semitransparent top cathode was applied to complete the device stack.
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Fig. 1. (a) Picture of a finished OPV device on steel foil. (b) Schematic layout of the device structure, showing the resist (ISO) structure, the PEDOT:PSS and P3HT:PCBM layers, as well as the semitransparent top electrode.
(P3HT) (purchased from Plextronics, Plexcore OS 2100) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (99%, purchased from Solenne BV) were dissolved in 1,2dichlorobenzene with a mixing ratio of 1:1 by weight, with total solid content of 4%. The solution was stirred for 3 h at 80 °C. The photoactive layer was obtained by spin coating at 1000 rpm for 30 s, resulting in a dry layer thickness of 220 nm. The semi-transparent cathode [25] (LiF/Al/Ag/ ZnS) was thermally evaporated in a vacuum chamber (pressure of 10 7 Torr) through a shadow mask with deposition rate of 0.2 nm/s. Finally, a thin film transparent barrier [26] was deposited on top of the completed devices. Then, the stainless steel substrate with finished devices was delaminated from the glass carrier. Current–voltage curves were measured using a xenonlamp-based solar simulator (Oriel (LS0104) 150 W). The devices were illuminated through an illumination mask with an aperture of 1.0 1.0 cm2.
3. Results and discussion 2. Experimental The flexible metal foil substrates used for fabrication of organic solar cells were 50 lm thick stainless steel foil of type 1.4301 from Hasberg Schneider GmbH. The 15.2 15.5 cm2 substrates were temporarily laminated to a glass carrier, which is required for spin coating of the subsequent layers. Negative photoresist was spin coated on top of the steel substrates with a layer thickness of 1 lm. The photoresist layer was structured via photolithography in such a way, that each substrate contained two devices with an active area of 5 10 cm2. A schematic illustration of the resist structure is shown in Fig. 1. PEDOT:PSS (Clevios P VP AI 4083, H.C. Starck) was spin coated, forming a 100 nm thick layer. Poly(3-hexylthiophene)
Thin film OPV devices are very sensitive to surface roughness. High roughness structures over short distances must be avoided. Substrate smoothness in the nanometer range is required to provide a surface that will promote high-quality deposition of subsequent layers and prevent the penetration of potential substrate spikes or irregularities into the organic layers. However, an intermediate roughness over long distance is acceptable. The roughness of the stainless steel substrates used in this study is shown in Fig. 2. As can be observed from the image, recorded with white light interferometry, the roughness of the film is in the range of 60–70 nm and there are no large thickness variations on the submicron scale. The deposition of the organic layers onto these substrates did not demonstrate any
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Fig. 2. 3D height view of the steel foil surface obtained by white light interferometry at a magnification of 50. The arithmetic mean roughness value (Ra) is 60 nm, the root-mean-square roughness (Rq) amounts to 76 nm on the scanned area of 123 94 lm.
Fig. 4. Upper panel: the maximum current density (Jmax) that can be extracted from ITO-based P3HT:PCBM solar cells (black line) and devices on steel foil (red line) for varying photoactive layer thickness (L), as determined by optical modeling. Bottom panel: the ratio of Jmax for steel devices to Jmax of ITO-based devices. At L = 220 nm, the ratio amounts to 0.6. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Current–voltage (I–V) characteristics of a 5 10 cm2 solar cell on steel foil, illuminated through a 1 1 cm2 mask aperture at the edge (open black symbols) and in the middle (open red symbols) of the device. The solid symbols mark the corresponding dark current. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
difficulties. As shown in Fig. 3, the finished devices exhibit low leakage currents, indicating, that local shunts do not play a role. The active device area of the solar cells on steel foil was 50 cm2. Since the solar simulator used in this study cannot provide uniform illumination over such an area, the performance of the devices was measured with a 1 1 cm2 illumination mask in different places of the devices. Fig. 3 illustrates typical I–V curves of devices. The illuminated area is selected either close to the contacts or in the middle of the device, which is the most distanced point from the contacts. The results illustrate a slight decrease in fill factor (FF) from 69% to 58% due to resistive losses. It is obvious that the steel electrode does not limit the fill factor as it has a very low sheet resistance. Rather, the fill factor is determined by the sheet resistance of the transparent electrode [25], which has previously been determined to be 5 X/sq.
The short-circuit current density (Jsc) amounts to 4.2 mA/cm2. We attribute this rather low value to the limited transmittance of the top electrode and thin film encapsulation. In order to quantitate this effect, we performed optical simulations using the transfer-matrix formalism [27]. By calculating the absorption in the active layer, taking into account interfacial reflections and interference effects, and assuming unity internal quantum efficiency, one can calculate the maximum current density (Jmax) that can be extracted from the solar cell under illumination with white light at an intensity of 1 sun [28]. In the upper panel of Fig. 4, Jmax is plotted versus the P3HT:PCBM layer thickness for a solar cell on steel foil, as used in the present study, as well as for a regular device on an ITO-covered glass substrate. The structure of the regular ITO-based device was glass (0.7 mm)/ITO (70 nm)/PEDOT:PSS (100 nm)/P3HT:PCBM/LiF (1 nm)/Al (100 nm). Both in experiment and in the optical simulations, the ITObased device is illuminated through the glass substrate, whereas the steel device is illuminated through the semitransparent barrier and top electrode. The calculated ratio of Jmax of the steel device and Jmax of the ITO device is plotted in the bottom panel of Fig. 4. It can be seen that this ratio amounts to 0.6 at a photoactive layer thickness of 220 nm. This number is in excellent agreement with experimental results on ITO-based solar cells, which showed a Jsc of 7.3 mA/cm2 under similar conditions. Another feature of the J–V curves in Fig. 3 is the rather low open-circuit voltage. As mentioned above, the steel devices were illuminated through a shadow mask with an aperture area (Amask) of 1 cm2. Compared to the cell area (Acell) of 50 cm2, this amounts to an illuminated region of
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ITO-based devices. The main conclusion of this study is that for innovative approach of manufacturing organic solar cells on steel substrate, highly transparent and high conductive top electrode is required. Increasing of Jsc in steel-based devices with existing electrode is possible by varying the thickness of the layers in the device stack.
References
Fig. 5. Change in open-circuit voltage (DVoc) for an ITO-based device measured with an illumination mask (solid symbols) or using neutral density filters to tune the light intensity (open symbols). The bottom and top horizontal axes span the ratio of the area of the mask aperture to the cell area (Amask/Acell) and the relative light intensity (I/I0), respectively.
2%. The simple explanation of the Voc reduction under a localized illumination is that the entire cell can be viewed as the parallel connection of illuminated and dark subcells. In real cells (even in inorganic PV), effects of non-uniform illumination depend on the light intensity and the cell series resistance [29–31]. This effect is shown in Fig. 5, which shows the decrease in Voc for an ITO-based device that is characterized using either masked illumination with different Amask/Acell ratios or using neutral density filters to tune the light intensity. The measurements clearly indicate a similar effect of masking and filtering. Furthermore, the drop in Voc at an effective intensity of 2% is about 0.11 V, which agrees well with lowered Voc of the steel device compared to typical ITO-based devices exhibiting Voc = 0.56 V.
4. Conclusions Large area ITO-free organic solar cells on steel substrates are demonstrated. The stainless steel was used as a substrate with excellent barrier properties and as a bottom electrode with very low sheet resistance, which allows to obtain large area OPV devices with fill factor of 58–69%, as measured under illumination through a mask with an aperture of 1.0 1.0 cm2. Semitransparent cathode which has sheet resistance of 5 X/sq is a limiting factor in the maximum cell geometry. Calculation of maximum cell size and its dependence from sheet resistance of the electrodes has been calculated and reported separately [32]. The main factor limiting short circuit current density in the reported devices is transparency of the top electrode. In order to quantitate effect of the semitransparent cathode transmittance, the optical simulations using the transfer-matrix formalism has been performed. Optical modeling has explained short circuit current in the steel devices, which also was compared with Jsc of standard bottom illuminated
[1] Y. Galagan, I.G. de Vries, A.P. Langen, R. Andriessen, W.J.H. Verhees, S.C. Veenstra, J.M. Kroon, Technology development for roll-to-roll production of organic photovoltaics, Chem. Eng. Process. 50 (2011) 454–461. [2] F.C. Krebs, J. Fyenbo, D.M. Tanenbaum, S.A. Gevorgyan, R. Andriessen, B. Van Remoortere, Y. Galagan, M. Jørgensen, The OE-A OPV demonstrator anno domini 2011, Energy Environ. Sci. 4 (10) (2011) 4116–4123. [3] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. [4] T.R. Andersen, T.T. Larsen-Olsen, B. Andreasen, A.P.L. Böttiger, J.E. Carlé, M. Helgesen, E. Bundgaard, K. Norrman, J.W. Andreasen, M. Jørgensen, F.C. Krebs, Aqueous processing of low-band-gap polymer solar cells using roll-to-roll methods, ACS Nano. 5 (2011) 4188– 4196. [5] S.A. Gevorgyan, A.J. Medford, E. Bundgaard, S.B. Sapkota, H.F. Schleiermacher, B. Zimmermann, U. Würfel, A. Chafiq, M. LiraCantu, T. Swonke, M. Wagner, C.J. Brabec, O. Haillant, E. Voroshazi, T. Aernouts, R. Steim, J.A. Hauch, A. Elschner, M. Pannone, M. Xiao, A. Langzettel, D. Laird, M.T. Lloyd, T. Rath, E. Maier, G. Trimmel, M. Hermenau, T. Menke, K. Leo, R. Rösch, M. Seeland, H. Hoppe, T.J. Nagle, K.B. Burke, C.J. Fell, D. Vak, T.B. Singh, S.E. Watkins, Y. Galagan, A. Manor, E.A. Katz, T. Kim, K. Kim, P.M. Sommeling, W.J.H. Verhees, S.C. Veenstra, M. Riede, M. Greyson Christoforo, T. Currier, V. Shrotriya, G. Schwartz, F.C. Krebs, An inter-laboratory stability study of roll-to-roll coated flexible polymer solar modules, Sol. Energy Mater. Sol. C. 95 (2011) 1398–1416. [6] S.R. Dupont, M. Oliver, F.C. Krebs, R.H. Dauskardt, Interlayer adhesion in roll-to-roll processed flexible inverted polymer solar cells, Sol. Energy Mater. Sol. C. 97 (2012) 171–175. [7] T.T. Larsen-Olsen, B. Andreasen, T.R. Andersen, A.P.L. Böttiger, E. Bundgaard, K. Norrman, J.W. Andreasen, M. Jørgensen, F.C. Krebs, Simultaneous multilayer formation of the polymer solar cell stack using roll-to-roll double slot-die coating from water, Sol. Energy Mater. Sol. C. 97 (2012) 22–27. [8] R. Rösch, F.C. Krebs, D.M. Tanenbaum, H. Hoppe, Quality control of roll-to-roll processed polymer solar modules by complementary imaging methods, Sol. Energy Mater. Sol. C. 97 (2012) 176–180. [9] Y. Galagan, J.E. Rubingh, R. Andriessen, C.C. Fan, P. Blom, S.C. Veenstra, J.M. Kroon, ITO-free flexible organic solar cells with printed current collecting grids, Sol. Energy Mater. Sol. C. 95 (2011) 1339–1343. [10] F.C. Krebs, All solution roll-to-roll processed polymer solar cells free from indium–tin-oxide and vacuum coating steps, Org. Electron. 10 (2009) 761–768. [11] N. Espinosa, R. García-Valverde, A. Urbina, F.C. Krebs, A life cycle analysis of polymer solar cell modules prepared using roll-to-roll methods under ambient conditions, Sol. Energy Mater. Sol. C. 95 (2011) 1293–1302. [12] N. Espinosa, R. García-Valverde, F.C. Krebs, Life-cycle analysis of product integrated polymer solar cells, Energy Environ. Sci. 4 (2011) 1547–1557. [13] N. Espinosa, R. García-Valverde, A. Urbina, F. Lenzmann, M. Manceau, D. Angmo, F.C. Krebs, Life cycle assessment of ITO-free flexible polymer solar cells prepared by roll-to-roll coating and printing, Sol. Energy Mater. Sol. C. 97 (2012) 3–13. [14] C. Lungenschmied, G. Dennler, H. Neugebauer, S.N. Sariciftci, M. Glatthaar, T. Meyer, A. Meyer, Flexible, long-lived, large-area, organic solar cells, Sol. Energy Mater. Sol. C. 91 (2007) 379–384. [15] A. Manor, E.A. Katz, T. Tromholt, B. Hirsch, F.C. Krebs, Origin of size effect on efficiency of organic photovoltaics, J. App. Phys. 109 (2011) 074508. [16] J. Jensen, M. Mikkelsen, F.C. Krebs, Flexible substrates as basis for photocatalytic reduction of carbon dioxide, Sol. Energy Mater. Sol. C. 95 (2011) 2949–2958.
3314
Y. Galagan et al. / Organic Electronics 13 (2012) 3310–3314
[17] Y. Galagan, R. Andriessen, Organic photovoltaics: technologies and manufacturing, in: V. Fthenakis (Ed.), Third Generation Photovoltaics, InTech, 2012, pp. 61–90. [18] M. Manceau, D. Angmo, M. Jørgensen, F.C. Krebs, ITO-free flexible polymer solar cells: from small model devices to roll-to-roll processed large modules, Org. Electron. 12 (2011) 566–574. [19] J.S. Lewis, M.S. Weaver, Thin-film permeation-barrier technology for flexible organic light-emitting devices, IEEE J. Sel. Topics Quantum Electron. 10 (2004) 45–57. [20] S. Cros, R. De Bettignies, S. Berson, S. Bailly, P. Maisse, N. Lemaitre, S. Guillerez, Definition of encapsulation barrier requirements: a method applied to organic solar cells, Sol. Energy Mater. Sol. C. 95 (SUPPL. 1) (2011) S65–S69. [21] J.A. Hauch, P. Schilinsky, S.A. Choulis, S. Rajoelson, C.J. Brabec, The impact of water vapor transmission rate on the lifetime of flexible polymer solar cells, Appl. Phys. Lett. 93 (10) (2008). [22] M. Hermenau, S. Schubert, H. Klumbies, J. Fahlteich, L. MüllerMeskamp, K. Leo, M. Riede, The effect of barrier performance on the lifetime of small-molecule organic solar cells, Sol. Energy Mater. Sol. C. 97 (2012) 102–108. [23] F.C. Krebs, Air stable polymer photovoltaics based on a process free from vacuum steps and fullerenes, Sol. Energy Mater. Sol. C. 92 (7) (2008) 715–726. [24] T. Chuang, M. Troccoli, P. Kuo, A. Jamshidi-Roudbari, M.K. Hatalis, I. Biaggio, A.T. Voutsas, Top-emitting 230 dots/in. active-matrix polymer light-emitting diode displays on flexible metal foil substrates, Appl. Phys. Lett. 90 (2007) 151114.
[25] Y. Galagan, M.G. Debije, P.W.M. Blom, Semitransparent organic solar cells with organic wavelength dependent reflectors, Appl. Phys. Lett. 98 (2011) 043302. [26] F. van Assche, H. Rooms, E. Young, J. Michels, T. van Mol, G. Rietjens, P. van de Weijer, P. Bouten, Thin-film barrier on foil for organic LED lamps, Presented at AIMCAL fall technical conference, Myrtle Beach, South Carolina, 2008. [27] L.A.A. Pettersson, L.S. Roman, O. Inganäs, Modeling photocurrent action spectra of photovoltaic devices based on organic thin films, J. Appl. Phys. 86 (1999) 487–496. [28] J.D. Kotlarski, P.W.M. Blom, L.J.A. Koster, M. Lenes, L.H. Slooff, Combined optical and electrical modeling of polymer: Fullerene bulk heterojunction solar cells, J. Appl. Phys. 103 (2008) 084502. [29] L.J.A. Koster, V.D. Mihailetchi, R. Ramaker, P.W.M. Blom, Light intensity dependence of open-circuit voltage of polymer: fullerene solar cells, Appl. Phys. Lett. 86 (2005) 123509. [30] A. Manor, E.A. Katz, R. Andriessen, Y. Galagan, Study of organic photovoltaics by localized concentrated sunlight: towards optimization of charge collection in large-area solar cells, Appl. Phys. Lett. 99 (2011) 173305. [31] E.A. Katz, J.M. Gordon, W. Tassew, D. Feuermann, Photovoltaic characterization of concentrator solar cells by localized irradiation, J. Appl. Phys. 100 (2006) 044514. [32] Y. Galagan, B. Zimmermann, B., E.W.C. Coenen, M. Jørgensen, D.M. Tanenbaum, F.C. Krebs, H. Gorter, S. Sabik, L.H. Slooff, S.C. Veenstra, J.M. Kroon, R. Andriessen, Current collecting grids for ITO-free solar cells, Adv. Energy Mater. 2 (2012) 103–110.