Additive effect for organic solar cell fabrication by multi layer inking and stamping

Journal of Science: Advanced Materials and Devices (2018) 221e225 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Additive effect for organic solar cell fabrication by multi-layer inking and stamping Sheng Bi a, b, Zhongliang Ouyang c, Qinglei Guo d, Chengming Jiang a, b, * a Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, PR China b Institute of Photoelectric Nanoscience and Nanotechnology, Dalian University of Technology, Dalian 116024, PR China c Department of Electrical and Computer Engineering, Center for Materials for Information Technology, The University of Alabama, Box# 870209, Tuscaloosa, AL 35487, USA d Department of Material Science and Engineering, Frederick Seitz Material Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA a r t i c l e i n f o a b s t r a c t Article history: Received 29 January 2018 Received in revised form 14 March 2018 Accepted April 2018 Available online 11 April 2018 Large-scale printing fabrication of organic solar cells (OSCs) has attracted much attention in recent decades due to its efficient industrial application Additive in the organic layer is one of the crucial factors that promote both quality of transferred pattern and the power conversion efficiency of the solar cell Here, an organic material, 3-Glycidyloxypropyl trimethoxysilane (GLYMO), as an additive was used in cost-efficient multi-layer inking and stamping processes to fabricate OSCs Polydimethylsiloxane (PDMS) was used as a transfer carrier that carries patterns from silicon mold to indium tin oxide (ITO) glass or polyethylene terephthalate (PET) to fabricate rigid or flexible organic solar cell devices By investigating the effects of chemical additives on OSCs performance in a regular procedure, the amount of additive was found which provides the best power conversion efficiency of 1.71% Further refining the multi-layer inking and stamping process by using the amount of additive found in previous experiments, highresolution transferred patterns with maximum efficiency were produced The overall OSCs efficiency and high yield pattern transfers indicate high potential for future printing processing and will thus reduce OSCs production costs © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Organic solar cells Pattern transfer Additives Flexible substrate Power conversion efficiency Introduction Renewable and low-cost energy sources have gained increased attention as the global supply of fossil fuels decreases and the modern energy crisis intensifies Since the annual solar radiation from the sun produces significantly more energy than that consumed by the entire world's population in a year, much research has been invested into photovoltaic cells to harvest the energy of the Sun [1e6] Organic solar cells (OSCs) serve as a more viable possibility in the future that is both cost and energy efficient to replace conventional energy sources [7e9] However, the spincoating method widely used in laboratory is difficult as well as * Corresponding author Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, PR China E-mail address: jiangcm@dlut.edu.cn (C Jiang) Peer review under responsibility of Vietnam National University, Hanoi relatively expensive for the fabrication of large area devices Furthermore, spin coating technique is unable to fabricate thin films on flexible substrates with the same uniformity as on rigid ITO glass substrates Recently developed inexpensive high yield pattern transfer techniques have been used to overcome the incompatibility of certain organic electronics on both rigid and flexible substrates [10e18] The inking and stamping pattern transfer method, which uses cost-efficient PDMS elastomer stamps, has been applied to successfully transfer conducting polymer PEDOT:PSS to make organic thin film transistors (OTFT) [19] Multi-layer inking and stamping of metals and polymers in a single step has also been developed to fabricate polymer light-emitting diodes (PLED) on both ITO and flexible substrates [20e23] Direct multilayer pattern transfer is noted to preserve the functionality of the patterned polymer layers in organic devices and still maintain high-resolution transferred patterns [19,24e26] A high yield multi-layer pattern transfer depends on the relative adhesion strengths among the https://doi.org/10.1016/j.jsamd.2018.04.004 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 222 S Bi et al / Journal of Science: Advanced Materials and Devices (2018) 221e225 layers of thin-film, the PDMS stamp and the substrate For an entire stack of thin-films to be transferred, the adhesion between the organic layer and the substrate must be the strongest of all interlayer attractions and the adhesion between the PDMS and the stamp must be the weakest Therefore, additive is essential to the multi-layer pattern transfer The process of the multi-layer inking and stamping still needs to be optimized In this study, we utilize a chemical additive in the multi-layer inking and stamping technique to successfully fabricate OSCs on both ITO glass and PET flexible substrates We established a reliable procedure to investigate the effects of the additives on the pattern transfer and the overall performance of solar cells, and eventually to fabricate high-resolution multi-layer inked and stamped OSCs Scanning Electron Microscope (SEM) was used to demonstrate the quality of the transferred patterns as well as the separated crosssection layers of the transferred patterns Atomic Force Microscopy (AFM) images document the recessions found between the transferred patterns A currentevoltage (IeV) curve was measured and the energy-conversion efficiency was calculated An optical image of successful OSCs fabrication on PET substrate was also taken In the experiment, the spin-coating method was used as an example to deposit organic films onto the PDMS mold Other methods such as dip-coating, for instance, might also work to complete the PDMS mold fabrication The pattern transfer technique is an efficient way of making sub-micro patterns instead of using photolithography, metal deposition, developing, lift-off, etc It was found a lot more useful that the soft PDMS mold is appropriately applicable on flexible substrates We anticipate that our method can improve the development of the devices and promote industrial production of OSCs Experimental Poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) were purchased from Solarmer and Nano-C respectively and just used without any further treatment The ITO glass was cleaned in detergent, de-ionized water, acetone and isopropyl alcohol in sequence, and treated with oxygen plasma at 30 W for to increase its surface energy [27,28] A silicon wafer PDMS master mold, initially etched by photolithography and reactive ion etching, was put pattern-side up into a petri dish Sylgard 184 silicone elastomer base mixed with a curing agent at a weight ratio of 8:1 was poured into the petri dish and put in a vacuum oven overnight at room temperature to remove the excess bubbles and was then heated to 100  C for 1.5 h to completely solidify the PDMS solution A 30 nm thick layer of gold was sputtered onto the PDMS stamp, followed by a 50 nm layer of aluminum deposited by thermal evaporation at a rate of Å/s P3HT and PCBM (1:1 wt, concentration of 25 mg/mL in chlorobenzene) was spincoated onto the PDMS with a spin-speed of 900 rpm for 45 s The PDMS was then treated with oxygen plasma at 30 W for 10 s followed by spin-coating PEDOT:PSS (purchased from HC Stark) onto the PDMS at 5000 rpm Various amounts (2.5 ml, ml, 10 ml, 20 ml, 100 ml) of 3-Glycidyloxypropyl trimethoxysilane (GLYMO) (chemical structure shown in Fig 1(b)) were added to ml of PEDOT:PSS solution and left at room temperature overnight before use to Fig The chemical structures of (a) P3HT (b) 3-Glycidyloxypropyl trimethoxysilane (GLYMO) increase the adhesive properties of the solution The “inked” PDMS is then immediately stamped onto the pre-cleaned ITO glass on a hot plate at 80  C for and then slowly peeled off The entire process is illustrated in Fig All fabrication procedures were undertaken in nitrogen filled glove box To accurately test the effect of the GLYMO additive to the performance of the solar cell, spin-coated solar cells on rigid ITO substrate were fabricated The regular structure of the P3HT/PCBM system was used The ITO glass substrate was first cleaned following the procedure mentioned above A 40-nm-thick PEDOT:PSS anode buffer layer with various amounts of GLYMO was spin-coated on top of the precleaned ITO substrate The P3HTPCBM solution was then deposited by spin coating at a speed of 900 rpm for 40 s on the top of the PEDOT:PSS layer Then, the entire device was put into the vacuum oven and annealed at 140  C for 20 An 80 nm thick Al layer was subsequently thermally evaporated on it at the vacuum pressure of  10À6 torr The current-voltage (IeV) characterization of the polymer photovoltaic cells was conducted using a computer-controlled measurement unit (B1500A semiconductor parameter analyzer) from Agilent Technologies under ambient condition with illumination of the AM1.5G, at 100 mW/cm2 The open circuit voltage (Voc) and the short circuit current (Isc) were measured The fill factor (FF, that is the available power at the maximum power point divided by the open circuit voltage and the short circuit current) and the power conversion efficiency (PCE) were determined The GLYMO acts as a plasticizer, which increases the chain mobility of the polymers, resulting in a lower processing temperature and pressure [29] GLYMO is able to prevent the spin-coated PEDOT:PSS thin film from completely dry out immediately Also, it helps with sticking the layers on the mold to the substrate Moreover, adding glycerol can also enhance the conductivity of PEDOT:PSS [30] Results and discussion In order to test the effect of GLYMO on the OSCs efficiency, a set of control experiments were performed on spin-coated solar cells with different amounts of GLYMO (0.0 ml, 2.5 ml, 5.0 ml, 10.0 ml, 20.0 ml, 100.0 ml) added to ml of PEDOT:PSS Current-voltage characterizations are displayed in Fig 3(a) and derived parameters in Table When the amount of GLYMO increases from 2.5 ml to ml, Voc remains relatively constant, while the Jsc greatly increases from 6.84 mA/cm2 to 7.03 mA/cm2 The FF raises from 38.58% to 40.24%, as shown in Table 1, indicating that the highest occupied molecular orbit and lowest unoccupied molecular orbit remain the same, while the resistance inside the devices decreases However, when the GLYMO concentration further increases, a significant change occurrs in the devices, dramatically decreasing their short circuit current When 100 ml of GLYMO was added, the OSCs short circuit current decreases to a negligible amount, as illustrated in Fig 4(b) It was found that 5.0 ml of GLYMO was the ideal amount required per ml of PEDOT:PSS to achieve both the highest OSCs efficiency and a high yield pattern transfer Higher or lower concentrations of GLYMO would both significantly reduce the OSCs efficiency A small amount of GLYMO additive in the PEDOT:PSS solution is able to enhance the conductivity of PEDOT:PSS However, when further increase the amount of GLYMO, the mismatch of the energy levels between the GLYMO and the P3HT/PCBM will result in a charge transport block, leading to an increase of the recombination efficiency and a decrease of charge generation efficiency, which causes a poor performance of the solar cell Fig 4(a) illustrate that the high yield multi-layer pattern transfer was successfully performed on the ITO glass substrate Each rectangular pattern represents a separate OSCs device The relatively S Bi et al / Journal of Science: Advanced Materials and Devices (2018) 221e225 223 Fig Schematic of the transfer procedure Deposition of films onto PDMS stamp; Oxygen plasma treatment on ITO glass; Press PDMS onto plasma treated ITO glass; Slowly peel off the PDMS from the ITO glass Fig (a) IeV characterizations of spin-coated P3HT/PCBM OSCs devices with 0.0 ml, 2.5 ml, 5.0 ml, 10.0 ml, 20.0 ml and 100 ml of GLYMO added to ml of PEDOT:PSS solution (b) Comparison of OSCs efficiency vs the amount of GLYMO added to ml of PEDOT:PSS solution Table Voc (V), Jsc (mA/cm2), FF(%), and efficiency values of spin-coated solar cells with various amounts of GLYMO added 0.0 ml 2.5 ml 5.0 ml 10.0 ml 20.0 ml 100 ml Voc (V) Jsc (mA/cm2) FF (%) Efficiency (%) 0.66 0.60 0.61 0.58 0.58 0.56 5.93 6.84 7.03 5.69 3.92 2.77E-3 43.67 38.58 40.24 30.95 20.73 14.18 1.66 ± 1.52 ± 1.67 ± 1.13 ± 0.30 ± 2.2E-4 0.081 0.065 0.089 0.173 0.097 ± 1E-4 smooth surface and the high yield of the transferred patterns with the minimal deformities, such as cracks or buckles, signify an optimal metal deposition, an appropriate additive use (the sufficient amount of GLYMO added to the PEDOT:PSS solution), and the careful handling of the PDMS stamp during the spin-coating process Pattern transfer on flexible PET substrate is another step that was achieved As shown in inset of Fig 4(a), successful pattern transfer with clear separated patterns was observed This achievement demonstrates a great promise for using the multilayer inking and stamping technique to fabricate large amounts of OSCs through printing method To prove that the stripes between each two rectangle patterns were also transferred, AFM was carried out and results are shown in Fig 4(b) A clear recession as observed indicates good separation between the patterns due to an optimal pressure applied during the stamping process and the maximum stress at the corners of the patterns The dark orange middle section with well-defined top and bottom edges represents a clear recession between the two transferred patterns This pattern separation indicates that only desired layers on the patterned parts of the PDMS stamp were transferred while the remaining layers are still attached to the original stamp and, thus, allowing a more accurate area for each OSCs device to be measured A clearly separation of each layer is the key to ensure that charge transports can be generated and the electron and hole pairs can be successfully separated and transported to the cathode and anode, respectively In order to reveal the separation of metal, P3HT/PCBM and PEDOT:PSS thin-films on the ITO glass after the transfer process, an SEM image of the cross-section of this structure was taken as it is shown in Fig 4(c) From the image, distinctive separated edges with sharp contrast are observed It has a an apparent effect on the charge carrier generation and transportation in OSC devices Currentevoltage measurement was performed on a pattern transferred device with 460 mm  1000 mm dimensions, prepared with a 5.0 ml:1 ml GLYMO to PEDOT:PSS ratio, and results are shown in Fig We achieved a Voc of 0.57 V, a Jsc of 1.7 mA/cm2, FF of 21.44% and an efficiency of  10À4% The Voc seems comparable to that of a spin-coated solar cell, but the Jsc is rather low The comparable Voc indicates a good pattern transfer and a functional light absorption layer The low current is likely caused by the oxidation of Al at the interface between the Al and P3HT/PCBM layers, which may have led to significant degradation of the OSCs device Another effect might come from the oxygen plasma treatment on P3HT/PCBM 224 S Bi et al / Journal of Science: Advanced Materials and Devices (2018) 221e225 Fig IeV characterization of the OSCs (Au/Al/P3HT/PCBM/PEDOT:PSS/ITO) patterned by the multi-layer inking and stamping distinct We achieved an overall OSCs efficiency of 2.1  10À4% We anticipate this work may ultimately support the development of the multi-layer inking and stamping pattern transfer technique to a more viable and beneficial option for large-scale OSCs fabrication Acknowledgments This project was financially supported by National Natural Science Foundation of China (NSFC, 51702035 and 51602056), and Dalian University of Technology, China, DUT16RC(3)051 References Fig (a) The SEM image of a high yield pattern transfer onto ITO glass substrate The inset is an optical photograph of a multi-layer OSCs successfully transferred onto a PET flexible substrate (b) The AFM image of the recession between two separated patterns after the pattern transfer process (c) The SEM cross-sectional image of the transferred layers, Au/Al, P3HT/PCBM, PEDOT:PSS, from top to bottom onto ITO glass layer since oxygen could react with the P3HT molecules which will change its conjugated property, resulting in some disadvantages to the charge carrier generation and transport Conclusion In summary, GLYMO was used in the multi-layer pattern transfer process to print OSCs With an amount of 5.0 ml of GLYMO in ml of PEDOT:PSS, we managed to perform both high-yield transferred patterns and to reach the maximum power conversion efficiency Multi-layer patterns were successfully transferred from PDMS stamp to both ITO glass and PET flexible substrates with the optimum GLYMO additive Each layer was clearly separated after the transfer, and recessions between the transferred patterns were [1] D Wohrle, D Meissner, Organic solar-cells, Adv Mater (3) (Mar 1991) 129e138 [2] J You, et al., A polymer tandem solar cell with 10.6% power 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anticipate this work may ultimately support the development of the multi- layer inking and