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Solution-processed silver opaque electrode for organic solar devices
Solar Energy Materials & Solar Cells 126 (2014) 192–196
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Solution-processed silver opaque electrode for organic solar devices Rafael Betancur n, Franklin Jaramillo n Centro de investigación, innovación y desarrollo de materiales-CIDEMAT, Universidad de Antioquia UdeA, Calle 70 No 52-21, Medellín, Colombia
art ic l e i nf o
a b s t r a c t
Article history: Received 20 January 2014 Received in revised form 4 March 2014 Accepted 13 March 2014
A way to upscale the manufacture of organic solar cells is through roll-to-roll processing in which is required to develop fully solution-processed devices. However, deposition via solution of opaque electrodes as final layer on top of the device stack has shown to still be challenging. In this work we present a simple method to develop such electrode based on silver plate-shaped micro-particles. We demonstrate that this electrode do not affect the underlying device structure while providing an optimal performance in terms of low sheet resistance and electrical stability. The final fabricated devices achieved up to 2.2% efficiency and very low device series resistance values. & 2014 Elsevier B.V. All rights reserved.
Keywords: Organic solar cell Solution-processed devices Solution-processed electrode
1. Introduction Over the past 50 years an important industrial effort has been done to develop electronic applications. This development has been mainly supported in the microelectronics technology growth and therefore guided towards decreasing more and more the size of the different components. However, recent applications such as photovoltaic devices and flat panel displays have open a new electronic branch known as macroelectronics [1] where the most critical factor is not achieving small sizes but large-area electronics implementations. Of course, this different orientation holds several manufacture challenges including finding inexpensive and scalable deposition methods and more suitable materials. In the case of the organic solar cells, a potential alternative is developing solutionprocessed devices with high mechanical flexibility to make them compatible with roll-to-roll manufacturing and enabling the production of cost-effective large-area photovoltaic panels [2]. A major challenge in the development of fully solutionprocessed devices has been depositing electrodes via solution as final layer of the photovoltaic device multilayer stack [3]. To such end, several materials have been tested including PEDOT, graphene and silver. PEDOT can be easily deposited by spin or spray coating [4–9]. Reported efficiencies reached 2.5% [4,5] and the final device area delimitation requires lift-off steps [5,8]. Graphene has also been used as electrode via a lamination process [10,11]. A maximum efficiency of 3.0% has been reported [10] and the main challenges are related to the processing and deposition of the graphene sheets. Finally, silver has allowed to reach the best
results up to date. Concretely, silver nanowires [3,12–15], deposited by spray coating [3,12], spin coating [13] or lamination [14,15], enabled efficiencies as high as 5% in a PBDTTPD:PC70BM device [12]. A simpler and cheaper way to develop a silver electrode, avoiding the need of synthetizing nanowires, is by directly implementing commercial silver formulations [2,16–20]. This kind of silver electrode enables the application of industrial-oriented deposition methods such as printing – screen, flexographic or inkjet [2,16,17,19,20] – or doctor blade [18]. From these, screen printing has demonstrated to be the most robust method [20]. Additionally, the application of printing technology in the back electrode, and the rest of the device layers, has allowed to reach the highest installation rate, in installed watts per minute, among all the PV technologies [19]. Up to date, the best device efficiency reported using silver formulations has been 2.2% [2]. However, in particular for P3HT:PCBM-based devices, this method to deposit the silver electrode, seems to be affecting either the fill factor (values below 40%) [2] or the open circuit voltage (values below 540 mV) [16,17,21,22] of the final devices. In this work we explore the deposition of solution-processed silver electrodes by spincoating. By properly selecting the deposition conditions, we achieved an opaque electrode without affecting the underlying structure and consequently preserving a relative high FF and Voc. This strategy resulted in a simple, cheap and reproducible method to fabricate solar cells up to 2.2% efficient.
2. Materials and methods 2.1. Materials preparation
n
Corresponding authors. Tel.: þ 574 2196680. E-mail addresses: [email protected] (R. Betancur), [email protected] (F. Jaramillo). http://dx.doi.org/10.1016/j.solmat.2014.03.037 0927-0248/& 2014 Elsevier B.V. All rights reserved.
The optimized structure was ITO/ZnO/P3HT:PCBM/PEDOT/Ag. 25 mm 25 mm ITO substrates (PG&O) 10 Ω/sq were etched,
R. Betancur, F. Jaramillo / Solar Energy Materials & Solar Cells 126 (2014) 192–196
using a mixture of HCl:H2O (1:1) and zinc powder, to form two 3 mm stripes. A ZnO precursor [23] was prepared by mixing zinc acetate dihydrate (100 mg, Sigma Aldrich), ethanol amine (28 μL, Sigma Aldrich) and 2-methoxy ethanol (1 mL, Sigma Aldrich) and stirred overnight. The active layer was made blending P3HT 4002E (20 mg, Rieke) and PCBM (20 mg, Sigma Aldrich) dissolved in dichlorobenzene (1 mL, Sigma Aldrich) and stirred overnight at 60 1C inside a glovebox. The PEDOT CPP 105-D (Clevios) was used as received. Finally, commercial silver paint from SPI suppliess was dispersed in toluene to an approximate concentration of 20 wt%. The final suspension was stirred constantly. 2.2. Device fabrication The patterned ITO substrates were cleaned with soap and later placed in an ultrasonic bath with organic solvents followed by a thermal drying (10 min 120 1C) and a UV-ozone treatment (10 min). The rest of the layers were deposited by spincoating starting with the ZnO precursor on the ITO at 6000 rpm. This layer was thermally annealed in air during 10 min at 200 1C and its thickness was about 40 nm. Afterwards, the devices were placed inside a glovebox to complete the remaining layers. The active layer was deposited at 1000 rpm followed by a slow drying process of about 15 min reaching a thickness of 150 nm. The PEDOT layer was deposited at 1000 rpm and the partial device was thermally annealed during 10 min at 130 1C. The thickness of this layer was around 200 nm as determined by SEM. Finally, the silver layers were deposited at different speeds ranging from 300 to 1500 rpm and also included a step at 4000 rpm to evaporate the remaining solvents. Their final thickness ranged from 1.6 to 4.2 μm. As mentioned previously, the silver suspension was stirred constantly to prevent precipitation of the micro-particles. Finally, the continuous silver electrode was scratched in order to define the area of eight individual cells. Such area was accurately determined by image analysis employing ImageJs [27] and was 8.3 mm2 in average. 2.3. Optical characterization Raman spectra (LabRAM HR), equipped with an optical microscope Olympus BX41, was used to verify the integrity of the PEDOT layer after the interaction with toluene which was used as solvent for the silver. UV–vis spectrophotometry (Cary 300 Agilent) was performed to estimate the thickness of the ITO, ZnO and active layers. It also enabled to calculate the total device luminosity. Images of the silver electrodes were acquired with a Nikon eclipse E200 optical microscope at 200 . Image processing was
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implemented using ImageJs [27] enabling to determine a uniformity factor as the percentage of covered area, excluding surface pinholes and defects, compared to the total image area. Finally, scanning electron microsopy (JEOL JSM-6490) with integrated energy dispersive X-ray spectrometer pictures were taken to check the silver layer morphology, thickness and the device stack distribution after a cryogenic cut. 2.4. Electrical characterization A Keithley 4200-SCS equipped with a four-probe head was used to determine the sheet resistance of the electrodes. The average and standard deviation values came from two sets of samples where each sample was measured 20 times. On the other hand, the same Keithley 4200-SCS was used in combination with an Oriel sol3A sun simulator, calibrated to standard conditions AM1.5G (1000 W/m2) using an oriel 91150V reference cell and meter, to determine the photovoltaic parameters of the different devices. Each sample contained 8 different cells which enabled the calculation of average values and standard deviations. The whole experiment was repeated twice with consistent results.
3. Results and discussion In order to form a silver layer via solution, it was necessary to find an adequate solvent. The original solvent of the silver paint, according to the supplier, was based in a combination of ethyl acetate, N-butil acetate, acetone and toluene which, when applied directly, affected the PEDOT layer and consequently the device performance. Several solvents – including acetone, ethyl acetate, dichloromethane, acetylacetone and isopropyl alcohol – were tested to dissolve the silver. The most important challenge was preventing phase segregation in the solution which was closely related to obtaining an electrically connected silver micro-particles layer. At the end the before mentioned requirement was fulfilled by toluene. The second requirement for the silver solvent was leaving unaffected the underlying device stack which otherwise would disturb the overall device performance. The silver layer was in direct contact with the PEDOT layer which consequently act as a protective layer. When using an 85 nm PEDOT layer, the active layer was not enough protected and its dissolution was apparent. This drawback was solved by increasing the PEDOT thickness up to 200 nm. This thickness is remarkably low compared to equivalent reported PEDOT films in printing setups where it can reach up to 800 nm [28]. The Raman spectra obtained for such 200 nm PEDOT layer (Fig. 1a) – with and without thermal annealing, before and
Fig. 1. Effect of the toluene on the PEDOT layer. (a) Raman and (b) UV–vis spectra including the resulting luminosity.
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after depositing on it toluene – displayed the typical PEDOT band between 1400 and 1500 cm 1 corresponding to the Cα ¼ Cβ stretching vibrations modes [29,30] and the rest of the observed bands are typical of key modes [30] confirming that the PEDOT layer was not chemically modified by the toluene. On the other hand, toluene did not thin down the PEDOT layer as confirmed by the UV–vis spectra in Fig. 1b. All spectra look the same and indeed in all cases the luminosity, which corresponds to the brightness as perceived by a human observer, stayed close to 85%. Depending on the deposition conditions, the silver layer quality and its performance changed. As mentioned previously, in order to prevent precipitation, the silver suspension was stirred constantly. However, after placing the suspension on top of the PEDOT, a rapid precipitation appeared. Different spincoating speeds – ranging from 300 to 1500 rpm – were tested for depositing the silver layers. Each sample was labelled according to its deposition speed and Fig. 2a presents their corresponding micrographs. The different spincoating conditions induced notorious changes in the quality of the layers. Interestingly, the rheology of the suspension determined that higher rotation speeds implied thicker and less uniform layers as shown in Fig. 2b. Additionally, the sheet resistance presented in Fig. 2c linearly depended on the uniformity of the layers, where the best performance was found to be 2.3 71.6 Ω/sq for the Ag-300 layer with 97.7% uniformity. In terms of opacity, all electrodes performed satisfactorily showing a luminosity below 3% (Fig. 2c). On the other hand, the cyclingdurability of the silver electrodes was evaluated. As seen in Fig. 2d, 100 I–V cycles were used to corroborate the stability of the electrical resistance of the electrodes and their low leakage
current. The lowest resistance was found for the samples Ag-300 and Ag-500 which present the highest uniformity. Correspondingly, the highest resistance was found for the sample with the lowest uniformity. Variations in the resistance can be attributed to the presence of pinholes as evidenced in Fig. 2a. Formation of such pinholes are related to the deposition speed but also to the precipitation of the silver micro-particles in the first stage of the spincoating procedure. A further analysis of the silver electrode revealed that the detailed morphology is based on plate-shaped micro-particles. Such micro-particles were sized around 5 μm according to SEM imaging (Fig. 3a). As shown in Fig. 3b, the electrode deposition does not affect physically the underlying device stack and the aggregation of the micro-particles determines a total electrode thickness below 3 μm for the best performing silver layer (Ag300). Compared to the reported 5–6 μm thick silver electrodes deposited by printing techniques [24–26], our resulting silver electrode is thinner which can be mainly attributed to the use of a low solid content in the silver formulation (20 wt% vs 80–86 wt% reported by other authors [2]). This low concentration of the silver suspension stimulates the toluene evaporation enabling an opaque electrode mainly formed by silver. In fact, other elements, such as the solvent and compounds present in the original silver paint, just have marginal occurrence in the final electrode as validated by the energy dispersive X-ray spectrometry presented in Fig. 3c. The different opaque solution-processed electrodes were implemented in photovoltaic devices achieving a maximum efficiency of 2.2% (Fig. 4). Their characterization parameters are presented in Table 1. Jsc and Rs resulted to be very similar in all devices. It is
Fig. 2. Optical and electrical characterization of the solution-deposited opaque electrodes. (a) Each sample was labelled according to its deposition speed. Micrographs evidence a strong dependence of the layer quality on the spincoating conditions which is reflected in (b) their uniformity and thickness. (c) The sheet-resistance (black squares) and luminosity (red circles) of the electrodes showed a linear relationship with their uniformity and finally (d) their electrical stability is demonstrated after a stable I–V behavior during 100 voltage swept cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. SEM and EDS analysis of the devices implementing the solution-processed opaque silver electrode. (a) Superficial analysis of the electrode showed a morphology based on plate-shaped micro-particles in the range of 5 μm. (b) In the optimal device, the silver layer thickness ranged from 1.6 to 2.9 μm thick and did not affect the underlying layer stack. (c) Energy dispersive X-ray spectrometry demonstrated that the opaque electrode was mostly silver.
Table 1 Electrical characterization parameters for the devices implementing the solutionbased silver opaque electrode at the different deposition conditions (best results in parenthesis). Samplea Jsc (mA/cm2) Voc (mV) FF (%) η (%) Ag-300 Ag-500 Ag-1000 Ag-1500 a
Fig. 4. Photovoltaic performance of the best devices incorporating the solutionprocessed opaque electrode.
worth pointing out that the Rs was remarkably low achieving up to 15 Ω.cm2 which confirms the ohmic contact of the silver electrode with the PEDOT layer. The differences between devices arouse after the strong dependence of the Voc with the parameters of the silver deposition. In fact, the devices corresponding to the silver with the higher uniformity (Ag-300 and Ag-500) exhibited a Voc over 600 mV, which is around the maximum voltage achievable with the bulk hetero-junction P3HT:PCBM, while the Voc for the other devices was lower. A clear tendency can be observed from Table 1. Finally, the Ag-300 device outperformed the other devices when considering the shunt resistance. Its better Rsh was consequently reflected in a higher FF and a better electrical rectification compared
6.3 7 0.3 6.6 7 0.5 6.0 7 0.5 6.5 7 0.3
6107 3 6167 7 581 7 13 569 7 29
557 1 427 2 397 3 447 4
2.17 0.1 (2.2) 1.7 7 0.3 (2.0) 1.4 7 0.2 (1.6) 1.6 7 0.2 (1.9)
Rs (Ω.cm2) Rsh (Ω.cm2) 157 1 397 14 217 2 167 4
8117 389 3337 85 239 7 46 3137 171
The labelling numbers represent the deposition speed of the films in rpm.
to the other devices as shown in Fig. 4. All these features made the Ag-300 electrode based device the best one in terms of power conversion efficiency, accounting (2.170.1)%, and stability as reflected in the low variations of its photovoltaic parameters (Table 1).
4. Conclusions Finding an appropriate solvent allowed to deposit an opaque silver layer without affecting its own electrical connection between micro-particles and preventing modifying physically or chemically the underlying stacked device. At the end, an electrode below 3 μm thick, with low sheet resistance, electrically stable and formed by plate-shaped micro-particles with few pinholes was achieved. Such solution-processed silver opaque electrode allowed
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reaching a 2.2% efficient solar device with a very low series resistance demonstrating its compatibility with the whole device stack. The solution-processed opaque electrode presented in this work constitutes a simple and reproducible way to develop organic solar cells. A significant reduction in the processing time was achieved after avoiding the inclusion of vacuum steps. Recognizing that further work is necessary in the implementation of industrially-oriented deposition methods, the ideas presented in this work are addressed towards the solution processing of organic solar cells which is the most probable way to upscale the photovoltaic organic technology [2] and can also directly contribute to the field of large-area electronics. Acknowledgments We would like to thank to the program “Estrategia de Sostenibilidad 2013-2014 de la Universidad de Antioquia”. We also want to thank to Empresas Públicas de Medellín-EPM for funding the project consecutive number 151530. The support in the RAMAN and SEM measurements by Harol Torres and Dayana Meza is fully appreciated. References [1] D. Mitzi, ed., Solution processing of inorganic materials, Wiley, 2009. [2] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. [3] F. Guo, X. Zhu, K. Forberich, J. Krantz, T. Stubhan, M. Salinas, et al., ITO-free and fully solution-processed semitransparent organic solar cells with high fill factors, Adv. Energy Mater 3 (2013) 1062–1067. [4] H.P. Kim, H.J. Lee, A.R. Bin Mohd Yusoff, J. Jang, Semi-transparent organic inverted photovoltaic cells with solution processed top electrode, Sol. Energy Mater. Sol. Cells 108 (2013) 38–43. [5] S.K. Hau, H.-L. Yip, J. Zou, A.K.-Y. Jen, Indium tin oxide-free semi-transparent inverted polymer solar cells using conducting polymer as both bottom and top electrodes, Org. Electron. 10 (2009) 1401–1407. [6] A. Colsmann, M. Reinhard, T.-H. Kwon, C. Kayser, F. Nickel, J. Czolk, et al., Inverted semi-transparent organic solar cells with spray coated, surfactant free polymer top-electrodes, Sol. Energy Mater. Sol. Cells 98 (2012) 118–123. [7] J. Czolk, A. Puetz, D. Kutsarov, M. Reinhard, U. Lemmer, A. Colsmann, Inverted semi-transparent polymer solar cells with transparency color rendering indices approaching 100, Adv. Energy Mater 100 (2012) (n/a–n/a). [8] Y. Zhou, H. Cheun, S. Choi, W.J. Potscavage, C. Fuentes-Hernandez, B. Kippelen, Indium tin oxide-free and metal-free semitransparent organic solar cells, Appl. Phys. Lett. 97 (2010) 153304. [9] Y.-F. Lim, S. Lee, D.J. Herman, M.T. Lloyd, J.E. Anthony, G.G. Malliaras, Spraydeposited poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) top electrode for organic solar cells, Appl. Phys. Lett. 93 (2008) 193301. [10] Z. Liu, J. Li, Z.-H. Sun, G. Tai, S.-P. Lau, F. Yan, The application of highly doped single-layer graphene as the top electrodes of semitransparent organic solar cells, ACS Nano 6 (2012) 810–818. [11] Y.-Y. Lee, K.-H. Tu, C.-C. Yu, S.-S. Li, J.-Y. Hwang, C.-C. Lin, et al., Top laminated graphene electrode in a semitransparent polymer solar cell by simultaneous thermal annealing/releasing method, ACS Nano 5 (2011) 6564–6570.
[12] Z.M. Beiley, M.G. Christoforo, P. Gratia, A.R. Bowring, P. Eberspacher, G. Y. Margulis, et al., Semi-transparent polymer solar cells with excellent subbandgap transmission for third generation photovoltaics, Adv. Mater. 25 (2013) 7020–7026. [13] C. Chen, L. Dou, R. Zhu, C. Chung, T. Song, Y.B. Zheng, et al., Visibly transparent polymer solar cells produced by solution processing, J. Am. Chem. Soc. 6 (2012) 7185–7190. [14] B.E. Hardin, W. Gaynor, I.-K. Ding, S.-B. Rim, P. Peumans, M.D. McGehee, Laminating solution-processed silver nanowire mesh electrodes onto solidstate dye-sensitized solar cells, Org. Electron. 12 (2011) 875–879. [15] J.-Y. Lee, S.T. Connor, Y. Cui, P. Peumans, Semitransparent organic photovoltaic cells with laminated top electrode, Nano Lett. 10 (2010) 1276–1279. [16] J.J. van Franeker, W.P. Voorthuijzen, H. Gorter, K.H. Hendriks, R.a.J. Janssen, A. Hadipour, et al., All-solution-processed organic solar cells with conventional architecture, Sol. Energy Mater. Sol. Cells 117 (2013) 267–272. [17] D. Angmo, M. Hösel, F.C. Krebs, All solution processing of ITO-free organic solar cell modules directly on barrier foil, Sol. Energy Mater. Sol. Cells 107 (2012) 329–336. [18] O. Hagemann, M. Bjerring, N.C. Nielsen, F.C. Krebs, All solution processed tandem polymer solar cells based on thermocleavable materials, Sol. Energy Mater. Sol. Cells 92 (2008) 1327–1335. [19] F.C. Krebs, N. Espinosa, M. Hösel, R.R. Søndergaard, M. Jørgensen, 25th Anniversary article: rise to power—OPV-based solar parks, Adv. Mater. 26 (2014) 29–39. [20] M. Hösel, R.R. Søndergaard, D. Angmo, F.C. Krebs, Comparison of fast roll-toroll flexographic, inkjet, flatbed, and rotary screen printing of metal back electrodes for polymer solar cells, Adv. Eng. Mater. (2013) (n/a–n/a). [21] K. Norrman, M. V Madsen, S. a Gevorgyan, F.C. Krebs, Degradation patterns in water and oxygen of an inverted polymer solar cell, J. Am. Chem. Soc. 132 (2010) 16883–16892. [22] T.R. Andersen, H.F. Dam, B. Andreasen, M. Hösel, M.V. Madsen, S.a. Gevorgyan, et al., A rational method for developing and testing stable flexible indium- and vacuum-free multilayer tandem polymer solar cells comprising up to twelve roll processed layers, Sol. Energy Mater. Sol. Cells (2013) 1–9. [23] Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Inverted polymer solar cells integrated with a low-temperature-annealed sol–gel-derived ZnO Film as an electron transport layer, Adv. Mater. 23 (2011) 1679–1683. [24] S.R. Dupont, M. Oliver, F.C. Krebs, R.H. Dauskardt, Interlayer adhesion in rollto-roll processed flexible inverted polymer solar cells, Sol. Energy Mater. Sol. Cells 97 (2012) 171–175. [25] D.M. Tanenbaum, M. Hermenau, E. Voroshazi, M.T. Lloyd, Y. Galagan, B. Zimmermann, et al., The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices, RSC Adv 2 (2012) 882–893. [26] F.C. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T.D. Nielsen, et al., A complete process for production of flexible large area polymer solar cells entirely using screen printing—first public demonstration, Sol. Energy Mater. Sol. Cells 93 (2009) 422–441. [27] M.D. Abràmoff, P.J. Magalhães, J.R. Sunanda, Image processing with ImageJ, Biophotonics Int 11 (2004) 36–42. [28] D. Angmo, J. Sweelssen, R. Andriessen, Y. Galagan, F.C. Krebs, Inkjet printing of back electrodes for inverted polymer solar cells, Adv. Energy Mater 3 (2013) 1230–1237. [29] J. Ouyang, Q. Xu, C.-W. Chu, Y. Yang, G. Li, J. Shinar, On the mechanism of conductivity enhancement in poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) film through solvent treatment, Polymer (Guildford) 45 (2004) 8443–8450. [30] A.a. Farah, S.a. Rutledge, A. Schaarschmidt, R. Lai, J.P. Freedman, A.S. Helmy, Conductivity enhancement of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) films post-spincasting, J. Appl. Phys. 112 (2012) 113709.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Solution-processed silver opaque electrode for organic solar devices Rafael Betancur n, Franklin Jaramillo n Centro de investigación, innovación y desarrollo de materiales-CIDEMAT, Universidad de Antioquia UdeA, Calle 70 No 52-21, Medellín, Colombia
art ic l e i nf o
a b s t r a c t
Article history: Received 20 January 2014 Received in revised form 4 March 2014 Accepted 13 March 2014
A way to upscale the manufacture of organic solar cells is through roll-to-roll processing in which is required to develop fully solution-processed devices. However, deposition via solution of opaque electrodes as final layer on top of the device stack has shown to still be challenging. In this work we present a simple method to develop such electrode based on silver plate-shaped micro-particles. We demonstrate that this electrode do not affect the underlying device structure while providing an optimal performance in terms of low sheet resistance and electrical stability. The final fabricated devices achieved up to 2.2% efficiency and very low device series resistance values. & 2014 Elsevier B.V. All rights reserved.
Keywords: Organic solar cell Solution-processed devices Solution-processed electrode
1. Introduction Over the past 50 years an important industrial effort has been done to develop electronic applications. This development has been mainly supported in the microelectronics technology growth and therefore guided towards decreasing more and more the size of the different components. However, recent applications such as photovoltaic devices and flat panel displays have open a new electronic branch known as macroelectronics [1] where the most critical factor is not achieving small sizes but large-area electronics implementations. Of course, this different orientation holds several manufacture challenges including finding inexpensive and scalable deposition methods and more suitable materials. In the case of the organic solar cells, a potential alternative is developing solutionprocessed devices with high mechanical flexibility to make them compatible with roll-to-roll manufacturing and enabling the production of cost-effective large-area photovoltaic panels [2]. A major challenge in the development of fully solutionprocessed devices has been depositing electrodes via solution as final layer of the photovoltaic device multilayer stack [3]. To such end, several materials have been tested including PEDOT, graphene and silver. PEDOT can be easily deposited by spin or spray coating [4–9]. Reported efficiencies reached 2.5% [4,5] and the final device area delimitation requires lift-off steps [5,8]. Graphene has also been used as electrode via a lamination process [10,11]. A maximum efficiency of 3.0% has been reported [10] and the main challenges are related to the processing and deposition of the graphene sheets. Finally, silver has allowed to reach the best
results up to date. Concretely, silver nanowires [3,12–15], deposited by spray coating [3,12], spin coating [13] or lamination [14,15], enabled efficiencies as high as 5% in a PBDTTPD:PC70BM device [12]. A simpler and cheaper way to develop a silver electrode, avoiding the need of synthetizing nanowires, is by directly implementing commercial silver formulations [2,16–20]. This kind of silver electrode enables the application of industrial-oriented deposition methods such as printing – screen, flexographic or inkjet [2,16,17,19,20] – or doctor blade [18]. From these, screen printing has demonstrated to be the most robust method [20]. Additionally, the application of printing technology in the back electrode, and the rest of the device layers, has allowed to reach the highest installation rate, in installed watts per minute, among all the PV technologies [19]. Up to date, the best device efficiency reported using silver formulations has been 2.2% [2]. However, in particular for P3HT:PCBM-based devices, this method to deposit the silver electrode, seems to be affecting either the fill factor (values below 40%) [2] or the open circuit voltage (values below 540 mV) [16,17,21,22] of the final devices. In this work we explore the deposition of solution-processed silver electrodes by spincoating. By properly selecting the deposition conditions, we achieved an opaque electrode without affecting the underlying structure and consequently preserving a relative high FF and Voc. This strategy resulted in a simple, cheap and reproducible method to fabricate solar cells up to 2.2% efficient.
2. Materials and methods 2.1. Materials preparation
n
Corresponding authors. Tel.: þ 574 2196680. E-mail addresses: [email protected] (R. Betancur), [email protected] (F. Jaramillo). http://dx.doi.org/10.1016/j.solmat.2014.03.037 0927-0248/& 2014 Elsevier B.V. All rights reserved.
The optimized structure was ITO/ZnO/P3HT:PCBM/PEDOT/Ag. 25 mm 25 mm ITO substrates (PG&O) 10 Ω/sq were etched,
R. Betancur, F. Jaramillo / Solar Energy Materials & Solar Cells 126 (2014) 192–196
using a mixture of HCl:H2O (1:1) and zinc powder, to form two 3 mm stripes. A ZnO precursor [23] was prepared by mixing zinc acetate dihydrate (100 mg, Sigma Aldrich), ethanol amine (28 μL, Sigma Aldrich) and 2-methoxy ethanol (1 mL, Sigma Aldrich) and stirred overnight. The active layer was made blending P3HT 4002E (20 mg, Rieke) and PCBM (20 mg, Sigma Aldrich) dissolved in dichlorobenzene (1 mL, Sigma Aldrich) and stirred overnight at 60 1C inside a glovebox. The PEDOT CPP 105-D (Clevios) was used as received. Finally, commercial silver paint from SPI suppliess was dispersed in toluene to an approximate concentration of 20 wt%. The final suspension was stirred constantly. 2.2. Device fabrication The patterned ITO substrates were cleaned with soap and later placed in an ultrasonic bath with organic solvents followed by a thermal drying (10 min 120 1C) and a UV-ozone treatment (10 min). The rest of the layers were deposited by spincoating starting with the ZnO precursor on the ITO at 6000 rpm. This layer was thermally annealed in air during 10 min at 200 1C and its thickness was about 40 nm. Afterwards, the devices were placed inside a glovebox to complete the remaining layers. The active layer was deposited at 1000 rpm followed by a slow drying process of about 15 min reaching a thickness of 150 nm. The PEDOT layer was deposited at 1000 rpm and the partial device was thermally annealed during 10 min at 130 1C. The thickness of this layer was around 200 nm as determined by SEM. Finally, the silver layers were deposited at different speeds ranging from 300 to 1500 rpm and also included a step at 4000 rpm to evaporate the remaining solvents. Their final thickness ranged from 1.6 to 4.2 μm. As mentioned previously, the silver suspension was stirred constantly to prevent precipitation of the micro-particles. Finally, the continuous silver electrode was scratched in order to define the area of eight individual cells. Such area was accurately determined by image analysis employing ImageJs [27] and was 8.3 mm2 in average. 2.3. Optical characterization Raman spectra (LabRAM HR), equipped with an optical microscope Olympus BX41, was used to verify the integrity of the PEDOT layer after the interaction with toluene which was used as solvent for the silver. UV–vis spectrophotometry (Cary 300 Agilent) was performed to estimate the thickness of the ITO, ZnO and active layers. It also enabled to calculate the total device luminosity. Images of the silver electrodes were acquired with a Nikon eclipse E200 optical microscope at 200 . Image processing was
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implemented using ImageJs [27] enabling to determine a uniformity factor as the percentage of covered area, excluding surface pinholes and defects, compared to the total image area. Finally, scanning electron microsopy (JEOL JSM-6490) with integrated energy dispersive X-ray spectrometer pictures were taken to check the silver layer morphology, thickness and the device stack distribution after a cryogenic cut. 2.4. Electrical characterization A Keithley 4200-SCS equipped with a four-probe head was used to determine the sheet resistance of the electrodes. The average and standard deviation values came from two sets of samples where each sample was measured 20 times. On the other hand, the same Keithley 4200-SCS was used in combination with an Oriel sol3A sun simulator, calibrated to standard conditions AM1.5G (1000 W/m2) using an oriel 91150V reference cell and meter, to determine the photovoltaic parameters of the different devices. Each sample contained 8 different cells which enabled the calculation of average values and standard deviations. The whole experiment was repeated twice with consistent results.
3. Results and discussion In order to form a silver layer via solution, it was necessary to find an adequate solvent. The original solvent of the silver paint, according to the supplier, was based in a combination of ethyl acetate, N-butil acetate, acetone and toluene which, when applied directly, affected the PEDOT layer and consequently the device performance. Several solvents – including acetone, ethyl acetate, dichloromethane, acetylacetone and isopropyl alcohol – were tested to dissolve the silver. The most important challenge was preventing phase segregation in the solution which was closely related to obtaining an electrically connected silver micro-particles layer. At the end the before mentioned requirement was fulfilled by toluene. The second requirement for the silver solvent was leaving unaffected the underlying device stack which otherwise would disturb the overall device performance. The silver layer was in direct contact with the PEDOT layer which consequently act as a protective layer. When using an 85 nm PEDOT layer, the active layer was not enough protected and its dissolution was apparent. This drawback was solved by increasing the PEDOT thickness up to 200 nm. This thickness is remarkably low compared to equivalent reported PEDOT films in printing setups where it can reach up to 800 nm [28]. The Raman spectra obtained for such 200 nm PEDOT layer (Fig. 1a) – with and without thermal annealing, before and
Fig. 1. Effect of the toluene on the PEDOT layer. (a) Raman and (b) UV–vis spectra including the resulting luminosity.
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after depositing on it toluene – displayed the typical PEDOT band between 1400 and 1500 cm 1 corresponding to the Cα ¼ Cβ stretching vibrations modes [29,30] and the rest of the observed bands are typical of key modes [30] confirming that the PEDOT layer was not chemically modified by the toluene. On the other hand, toluene did not thin down the PEDOT layer as confirmed by the UV–vis spectra in Fig. 1b. All spectra look the same and indeed in all cases the luminosity, which corresponds to the brightness as perceived by a human observer, stayed close to 85%. Depending on the deposition conditions, the silver layer quality and its performance changed. As mentioned previously, in order to prevent precipitation, the silver suspension was stirred constantly. However, after placing the suspension on top of the PEDOT, a rapid precipitation appeared. Different spincoating speeds – ranging from 300 to 1500 rpm – were tested for depositing the silver layers. Each sample was labelled according to its deposition speed and Fig. 2a presents their corresponding micrographs. The different spincoating conditions induced notorious changes in the quality of the layers. Interestingly, the rheology of the suspension determined that higher rotation speeds implied thicker and less uniform layers as shown in Fig. 2b. Additionally, the sheet resistance presented in Fig. 2c linearly depended on the uniformity of the layers, where the best performance was found to be 2.3 71.6 Ω/sq for the Ag-300 layer with 97.7% uniformity. In terms of opacity, all electrodes performed satisfactorily showing a luminosity below 3% (Fig. 2c). On the other hand, the cyclingdurability of the silver electrodes was evaluated. As seen in Fig. 2d, 100 I–V cycles were used to corroborate the stability of the electrical resistance of the electrodes and their low leakage
current. The lowest resistance was found for the samples Ag-300 and Ag-500 which present the highest uniformity. Correspondingly, the highest resistance was found for the sample with the lowest uniformity. Variations in the resistance can be attributed to the presence of pinholes as evidenced in Fig. 2a. Formation of such pinholes are related to the deposition speed but also to the precipitation of the silver micro-particles in the first stage of the spincoating procedure. A further analysis of the silver electrode revealed that the detailed morphology is based on plate-shaped micro-particles. Such micro-particles were sized around 5 μm according to SEM imaging (Fig. 3a). As shown in Fig. 3b, the electrode deposition does not affect physically the underlying device stack and the aggregation of the micro-particles determines a total electrode thickness below 3 μm for the best performing silver layer (Ag300). Compared to the reported 5–6 μm thick silver electrodes deposited by printing techniques [24–26], our resulting silver electrode is thinner which can be mainly attributed to the use of a low solid content in the silver formulation (20 wt% vs 80–86 wt% reported by other authors [2]). This low concentration of the silver suspension stimulates the toluene evaporation enabling an opaque electrode mainly formed by silver. In fact, other elements, such as the solvent and compounds present in the original silver paint, just have marginal occurrence in the final electrode as validated by the energy dispersive X-ray spectrometry presented in Fig. 3c. The different opaque solution-processed electrodes were implemented in photovoltaic devices achieving a maximum efficiency of 2.2% (Fig. 4). Their characterization parameters are presented in Table 1. Jsc and Rs resulted to be very similar in all devices. It is
Fig. 2. Optical and electrical characterization of the solution-deposited opaque electrodes. (a) Each sample was labelled according to its deposition speed. Micrographs evidence a strong dependence of the layer quality on the spincoating conditions which is reflected in (b) their uniformity and thickness. (c) The sheet-resistance (black squares) and luminosity (red circles) of the electrodes showed a linear relationship with their uniformity and finally (d) their electrical stability is demonstrated after a stable I–V behavior during 100 voltage swept cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. SEM and EDS analysis of the devices implementing the solution-processed opaque silver electrode. (a) Superficial analysis of the electrode showed a morphology based on plate-shaped micro-particles in the range of 5 μm. (b) In the optimal device, the silver layer thickness ranged from 1.6 to 2.9 μm thick and did not affect the underlying layer stack. (c) Energy dispersive X-ray spectrometry demonstrated that the opaque electrode was mostly silver.
Table 1 Electrical characterization parameters for the devices implementing the solutionbased silver opaque electrode at the different deposition conditions (best results in parenthesis). Samplea Jsc (mA/cm2) Voc (mV) FF (%) η (%) Ag-300 Ag-500 Ag-1000 Ag-1500 a
Fig. 4. Photovoltaic performance of the best devices incorporating the solutionprocessed opaque electrode.
worth pointing out that the Rs was remarkably low achieving up to 15 Ω.cm2 which confirms the ohmic contact of the silver electrode with the PEDOT layer. The differences between devices arouse after the strong dependence of the Voc with the parameters of the silver deposition. In fact, the devices corresponding to the silver with the higher uniformity (Ag-300 and Ag-500) exhibited a Voc over 600 mV, which is around the maximum voltage achievable with the bulk hetero-junction P3HT:PCBM, while the Voc for the other devices was lower. A clear tendency can be observed from Table 1. Finally, the Ag-300 device outperformed the other devices when considering the shunt resistance. Its better Rsh was consequently reflected in a higher FF and a better electrical rectification compared
6.3 7 0.3 6.6 7 0.5 6.0 7 0.5 6.5 7 0.3
6107 3 6167 7 581 7 13 569 7 29
557 1 427 2 397 3 447 4
2.17 0.1 (2.2) 1.7 7 0.3 (2.0) 1.4 7 0.2 (1.6) 1.6 7 0.2 (1.9)
Rs (Ω.cm2) Rsh (Ω.cm2) 157 1 397 14 217 2 167 4
8117 389 3337 85 239 7 46 3137 171
The labelling numbers represent the deposition speed of the films in rpm.
to the other devices as shown in Fig. 4. All these features made the Ag-300 electrode based device the best one in terms of power conversion efficiency, accounting (2.170.1)%, and stability as reflected in the low variations of its photovoltaic parameters (Table 1).
4. Conclusions Finding an appropriate solvent allowed to deposit an opaque silver layer without affecting its own electrical connection between micro-particles and preventing modifying physically or chemically the underlying stacked device. At the end, an electrode below 3 μm thick, with low sheet resistance, electrically stable and formed by plate-shaped micro-particles with few pinholes was achieved. Such solution-processed silver opaque electrode allowed
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reaching a 2.2% efficient solar device with a very low series resistance demonstrating its compatibility with the whole device stack. The solution-processed opaque electrode presented in this work constitutes a simple and reproducible way to develop organic solar cells. A significant reduction in the processing time was achieved after avoiding the inclusion of vacuum steps. Recognizing that further work is necessary in the implementation of industrially-oriented deposition methods, the ideas presented in this work are addressed towards the solution processing of organic solar cells which is the most probable way to upscale the photovoltaic organic technology [2] and can also directly contribute to the field of large-area electronics. Acknowledgments We would like to thank to the program “Estrategia de Sostenibilidad 2013-2014 de la Universidad de Antioquia”. We also want to thank to Empresas Públicas de Medellín-EPM for funding the project consecutive number 151530. The support in the RAMAN and SEM measurements by Harol Torres and Dayana Meza is fully appreciated. References [1] D. Mitzi, ed., Solution processing of inorganic materials, Wiley, 2009. [2] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. [3] F. Guo, X. Zhu, K. Forberich, J. Krantz, T. Stubhan, M. Salinas, et al., ITO-free and fully solution-processed semitransparent organic solar cells with high fill factors, Adv. Energy Mater 3 (2013) 1062–1067. [4] H.P. Kim, H.J. Lee, A.R. Bin Mohd Yusoff, J. Jang, Semi-transparent organic inverted photovoltaic cells with solution processed top electrode, Sol. Energy Mater. Sol. Cells 108 (2013) 38–43. [5] S.K. Hau, H.-L. Yip, J. Zou, A.K.-Y. Jen, Indium tin oxide-free semi-transparent inverted polymer solar cells using conducting polymer as both bottom and top electrodes, Org. Electron. 10 (2009) 1401–1407. [6] A. Colsmann, M. Reinhard, T.-H. Kwon, C. Kayser, F. Nickel, J. Czolk, et al., Inverted semi-transparent organic solar cells with spray coated, surfactant free polymer top-electrodes, Sol. Energy Mater. Sol. Cells 98 (2012) 118–123. [7] J. Czolk, A. Puetz, D. Kutsarov, M. Reinhard, U. Lemmer, A. Colsmann, Inverted semi-transparent polymer solar cells with transparency color rendering indices approaching 100, Adv. Energy Mater 100 (2012) (n/a–n/a). [8] Y. Zhou, H. Cheun, S. Choi, W.J. Potscavage, C. Fuentes-Hernandez, B. Kippelen, Indium tin oxide-free and metal-free semitransparent organic solar cells, Appl. Phys. Lett. 97 (2010) 153304. [9] Y.-F. Lim, S. Lee, D.J. Herman, M.T. Lloyd, J.E. Anthony, G.G. Malliaras, Spraydeposited poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) top electrode for organic solar cells, Appl. Phys. Lett. 93 (2008) 193301. [10] Z. Liu, J. Li, Z.-H. Sun, G. Tai, S.-P. Lau, F. Yan, The application of highly doped single-layer graphene as the top electrodes of semitransparent organic solar cells, ACS Nano 6 (2012) 810–818. [11] Y.-Y. Lee, K.-H. Tu, C.-C. Yu, S.-S. Li, J.-Y. Hwang, C.-C. Lin, et al., Top laminated graphene electrode in a semitransparent polymer solar cell by simultaneous thermal annealing/releasing method, ACS Nano 5 (2011) 6564–6570.
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