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Efficiency enhancement of organic photovoltaic modules prepared via wash processing
Solar Energy Materials & Solar Cells 143 (2015) 58–62
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Efficiency enhancement of organic photovoltaic modules prepared via wash processing Young Min Lee a,b, Jang Whan Lee a,b, Dong Kwon Choi b,n, Jae-Woong Yu a,n a Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, 1732 Deogyeong-daro, Giheung-gu, Yongin, Gyeonggi 446-701, Republic of Korea b Sung An Machinery Co., Ltd., 33, Madogongdan, Mado-myeon, Hwaseong City, Gyeonggi 445-861, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 28 April 2015 Received in revised form 15 June 2015 Accepted 16 June 2015
To enhance OPV performance, a novel module-fabrication process was developed. Chlorobenzene and acetone were employed to wash away unnecessary residual high-resistance materials after full modules were fabricated. Monolithic interconnecting electrode lines between the anode and the cathode were formed using a dispenser after wash process. The wash process allowed for minimization of interconnection resistance and enhancement of power conversion efficiency. Module efficiency changes were also studied by varying the coating conditions of PEIE. The power conversion efficiency increased as a result of the wash-off process compared with modules fabricated via conventional processes. The power conversion efficiency enhancement was attributed to increases in open circuit voltage and fill factor. Applying this wash-off process to larger-area OPV modules is expected to be effective for further enhancing power conversion efficiency. & 2015 Elsevier B.V. All rights reserved.
Keywords: Organic photovoltaic Module wash-off process
1. Introduction Due to the demand for sources of inexpensive renewable energy, organic photovoltaics (OPVs) have attracted a large amount of research and development focus in the last two decades [1–7]. Before OPVs can replace conventional power generation methods, it is important for the technology to be price competitive. This can be achieved through the utilization of mass production fabrication techniques such as roll-to-roll processing. Rollto-roll processing technology, with its high production throughput, is recognized as a method for low-cost mass production of large-area flexible organic electronics [8–18]. Since the first roll-to-roll-processed OPV module was reported by Krebs, many developmental studies have been conducted [15]. For use in everyday power applications, an OPV module structure must have connections between sub cells. Series connections are the most commonly employed method due to their relative ease of processing. Furthermore, according to OPV module shading tests, 10% shade exposure can shut down an entire module [19]. Therefore, optimized module fabrication technologies are critical to yield the maximum power output for a given module area. For example, we previously demonstrated that longitudinal partitioning of the OPV module leads to enhancement in efficiency n
Corresponding authors. E-mail addresses: [email protected] (D.K. Choi), [email protected] (J.-W. Yu).
http://dx.doi.org/10.1016/j.solmat.2015.06.030 0927-0248/& 2015 Elsevier B.V. All rights reserved.
[20]. Many studies have assessed the effects of module geometry on power conversion performance. In this study, we investigated the influence of OPV module fabrication methods on module performance. In particular, we studied the ability to form ohmic contacts as a result of washing away unnecessary active and transporting layers and correlated this ability with enhanced module performance.
2. Materials and methods 2.1. Module fabrication In this study, 100 mm 100 mm2 ITO glass substrates were etched via photolithographic processes to yield seven sub cells with 8-mm-wide ITO stripes. Prepared substrates were cleaned in series with acetone, isopropyl alcohol, and de-ionized water in an ultrasonic bath for 10 min. Subsequently, the substrates were dried under nitrogen gas in a 120 °C convection oven. Atmospheric pressure plasma (Power: 8 kV, N2 gas: 200 l/m) was used to increase the wettability of the substrates. An electron extraction buffer layer was formed using 0.4 wt% polyethylenimine, ethoxylated (PEIE, purchased from Sigma-Aldrich, MW ¼75,000) stock solution (dissolved in 2-methoxyethanol) in conjunction with a slot die coater (coating speed was 10–20 mm/s, shim plate thickness was 30 μm), followed by drying at 110 °C for 10 min in a convection oven. The resulting PEIE layer thickness was 5–10 nm
Y.M. Lee et al. / Solar Energy Materials & Solar Cells 143 (2015) 58–62
study, inverted OPV modules were prepared. To simulate a roll-toroll processing environment, all fabricated device layers were formed via slot die-coating or screen printing. Fig. 1 reveals the
50
2
Cell 1 (6.6 cm ) 2
40 Current (mA)
as measured with a thickness profiler (TENCORs, P-10 α-step). Next, 4.6 wt% P3HT (purchased from Rieke Metals, EE grade) and PCBM (purchased from Nano-C) were blended and dissolved in anhydrous chlorobenzene at a 1:0.6 weight ratio. The dissolved polymer blend solution was slot die-coated onto ITO glass/PEIE at a speed of 15 mm/s. The resultant active layer was 230 nm thick and was dried at 110 °C for 12 min. The hole extraction layer was generated by screen printing a PEDOT:PSS (Agfa EL-P5010) coating layer and was dried at 110 °C for 10 min. A silver paste (Toyo UV) coating was also applied via screen printing (width of silver electrode was 6 mm) and was sintered using heat and UV light (110 °C for 10 min and 1 min at 2400 mJ). Following fabrication of the OPV module, unnecessary buffer and active layers were removed by sequential washing with chlorobenzene and acetone solutions. Monolithic interconnecting electrode lines (formed with Elcoat p-100 whose width was 2 mm) were subsequently formed using a dispenser.
59
Cell 2 (6.6 cm ) 2
2Cell array (13.2 cm ) 30 20 10 0 0
200
400
2.2. Characterization A Class AAA Xe solar simulator (McScience K3600) was used as a light source, and all measurements were performed under a 1 sun condition (100 mW/cm2). Measurements were not corrected for reflection losses or absorptions from the ITO electrode. The I–V performance was measured with a Keithley 2400 source measurement unit. The cross-sectional analysis was performed by TESCAN Lyra3 dual beam FIB analyzer (Resolution in high vacuum is 1.2 nm at 30 kV).
1000
1200
2
Cell 1 (6.6 cm ) 2
Cell 2 (6.6 cm )
40 Current (mA)
2
2Cell array (13.2 cm ) 30 20 10
Conventional OPV modules are formed through series connections of several sub cells to yield the maximum power output for a given module area. We investigated the influence of ohmic contacts, formed by washing away unnecessary active and transporting layers, on the performance of the OPV module. In this
0 0
[HTL layer pattern printing]
200
400
600 Bias (V)
800
1000
1200
Fig. 2. I–V characteristics of sub cells and two cell array; (a) conventional process, (b) wash-off process.
[ETL layer pattern coating]
[Preparation of substrate]
[Ag electrode printing]
800
50
3. Results and discussion
[Preparation of substrate]
600 Bias (V)
[Active layer pattern coating]
[Ag pattern printing]
[ETL layer coating]
[Active layer coating]
[Remove ETL / Active layer]
[HTL layer printing]
[Inter connection monolithic Electrode line]
Fig. 1. OPV module fabrication process; (a) existing standard fabrication process, (b) low TCO-metal interfacial contact resistance process (wash-off process).
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Y.M. Lee et al. / Solar Energy Materials & Solar Cells 143 (2015) 58–62
Table 1 Performance of OPV cells and modules. Conventional process
Voc (V) Isc (mA) Jsc (mA/cm2) Fill factor Efficiency (%)
Wash-off process
Cell average
Standard deviation
Array average
Cell average
Standard deviation
Array average
0.581 32.63 4.907 0.349 0.995
7 0.000829 7 0.38034 7 0.057412 7 0.017385 7 0.039026
0.871 28.27 2.142 0.368 0.688
0.556 40.05 6.023 0.329 1.102
7 0.002947 7 0.71581 7 0.107574 7 0.008814 7 0.016887
1.001 36.77 2.785 0.34 0.948
Fig. 3. The cross-sectional analysis images of the OPV module prepared via the conventional process (a) and the wash processing method (b).
differences between conventional fabrication processes and the new solvent wash process highlighted in this study. This wash process removes organic semiconducting (i.e. high resistance) materials and minimizes the interfacial resistance between the transparent conducting oxide (TCO) and metal electrode. Fig. 2(a) and (b) shows the I–V characteristics of modules prepared via conventional processes (a) and the new wash process (b). Each sub cell had a measured dimension of 11 cm 0.6 cm (6.6 cm2 area), and modules were prepared with two cells connected in series. As shown in Fig. 2(a), the open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the conventional module (two-sub cell array) at AM 1.5 100 mW/cm2 condition were 0.871 V, 2.142 mA/cm2, 0.368, and 0.688 %, respectively. As shown in Fig. 2(b), the open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the washed module (two-sub cell array) at AM 1.5 100 mW/cm2 condition were 1.001 V, 2.785 mA/cm2, 0.340, and 0.948 %, respectively. Individual sub cell and module data are summarized in Table 1. During the module fabrication process, the open circuit voltage was increased as two cells were connected in series. Open circuit voltage enhancement in large-area printed devices is not easily achieved compared to the results obtained with small laboratory-scale devices. With conventional fabrication processing methods, the open circuit voltage of a two-cell array increased by 1.5-fold compared to the single-cell value (from 0.581 V to 0.871 V). The open circuit voltage after wash processing was doubled when using a two-cell array (from 0.556 V to 1.001 V). The short circuit current density of the two-cell array also increased following wash processing compared to the value obtained with conventional processing, as summarized in Table 1. The fill factor was also reduced when a two-cell array was fabricated using conventional fabrication processing methods. However, the fill factor of a two-cell array prepared via wash processing was increased. The combined effect of these changes resulted in significantly increase in power conversion efficiency following wash processing compared with that of the conventional process (0.688 for conventional and 0.948 for wash processing).
To further study the effects of wash processing on OPV modules, a large-area module was fabricated using a 10 cm 10 cm ITO substrate. In this study, PEIE-coated ITO was used as the lowwork function electrode (cathode), as reported in other literature [21]. Since the performance of the ITO work function is highly dependent on the thickness of the PEIE coating, it is very important to optimize the PEIE coating conditions. A full-area coating was prepared due to the difficulty in printing a very thin PEIEpatterned structure via slot die coating. Different treatment conditions were prepared to determine the optimal PEIE coating thickness. The ITO substrate was patterned via photolithography, and a 4.6 wt% P3HT:PCBM blend with a 1:0.6 weight ratio was coated on top of the PEIE layer via slot die coating. A PEDOT:PSS coating was placed on top of the active layer via screen printing. Lastly, a silver electrode was deposited via screen printing. After the module was fully fabricated, unnecessary layers (PEIE, active, and PEDOT:PSS layers) were removed by sequential chlorobenzene and acetone washes. Fig. 3 shows the cross-sectional analysis of the OPV module prepared via the conventional process (a) and the wash processing method (b). According to FIB analysis, the thickness of active layer was about 230 nm and the thickness of PEDOT: PSS was about 3.6 μm. Thickness of the charge collecting silver electrode (TOYO UV Ag) and the interconnection electrode was 25 and 15 μm, respectively. As shown, modules prepared via either process showed very similar cross-sectional planes. The chargecollecting silver electrode protected the underlying organic materials (PEIE, active, and PEDOT:PSS layers) from being washed away by solvent. Following these processes, the cathode and anode were directly interconnected via monolithic electrode lines using a dispenser. According to SEM–EDS analysis, the silver charge collection layer (formed with Toyo UV paste whose thickness was 16 μm) showed many gaps between the silver grains. The interconnection electrode, formed with Elcoat p-100 (manufactured by CANS), was produced with a thickness of 25 μm. The interconnection electrode layer showed more meticulous structures. The conductivity of the charge collection electrode was 0.00438 Ω/□, while that of the interconnection electrode was 0.007 Ω/□. Therefore, it can be concluded that metallic
Y.M. Lee et al. / Solar Energy Materials & Solar Cells 143 (2015) 58–62
0.07 ml 0.093 ml 0.14 ml
Current (mA)
40 30 20 10 0 0
1000
2000
3000
4000
5000
Bias (mV) Fig. 4. The I–V behavior of an OPV module consisting of seven sub cells with different PEIE thickness.
Table 2 Summary of OPV modules performance consisting of seven sub cells with different PEIE thickness. (Micro gear pump flow rate was 20 rpm). PEIE coating condition Coating rate (mm/s) Flow rate (ml) Voc (V) Isc (mA) Jsc (mA/cm2) Fill factor Efficiency (%)
20 0.07 3.862 70.011 34.8517 0.102 0.996 70.003 0.4797 0.008 1.843 7 0.041
15 0.093 3.744 70.044 27.998 7 0.127 0.8007 0.004 0.4337 0.004 1.295 7 0.031
10 0.14 3.6437 0.059 22.9917 0.601 0.6577 0.017 0.434 70.001 1.039 7 0.014
40
Current (mA)
30
20 module #1 module #2 module #3 module #4 module #5
10
0 0
1000
2000 Bias (mV)
3000
4000
Fig. 5. The I–V behavior of an OPV module prepared using the conventional process.
conductivity of the two electrodes is not the reason for the power conversion efficiency enhancement. Fig. 4 shows the I–V behavior of an OPV module consisting of seven sub cells (individual sub cell dimensions: 0.5 cm 10 cm), and the results are summarized in Table 2. PEIE thickness was measured using a surface profiler and was 5 and 10 nm for coating rates of 20, 15, and 10 mm/s with a 20 rpm micro gear pump rate, which the total flow rate was about 0.07, 0.093 and 0.14 ml, respectively. The best power conversion efficiency was obtained with a 20 mm/s coating rate. The open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the seven-cell module at AM 1.5 100 mW/cm2 condition were 3.862 70.011 V, 0.996 70.003 mA/cm2, 0.4797 0.008, and 1.8437 0.041%, respectively. As summarized in Table 2, thinnest PEIE layer yielded the best open circuit voltage, the short circuit current, fill factor and power conversion efficiency. In determining the superiority of this fabrication process, a module having comparable size and characteristics was prepared via
61
conventional processing (i.e., fabricated using the same procedures minus the solvent wash process). Fig. 5 shows the I–V behavior of an OPV module prepared using the conventional process. The open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the seven-cell module at AM 1.5 100 mW/cm2 condition were 3.42670.067 V, 1.03170.019 mA/cm2, 0.38770.792, and 1.36770.030%, respectively. The substrate size of the module was 10 10 cm2, and the effective cell area was 35 cm2. The power conversion efficiency enhancement resulting from the washing process led to significant increase compared to the value obtained from conventional processing (from 1.367% to 1.843%). This efficiency increase is primarily attributed to increases in open circuit voltage and fill factor. The washing process dissolves many insulating (PEIE, active, and PEDOT:PSS layers, i.e., high-resistance materials) components between the anode and cathode. It also allows for direct contact between two electrodes via a monolithic metal line. The results demonstrate that the short circuit current density was relatively unchanged between the conventional and wash processing methods. This suggests that the current production of an effective cell area was likewise relatively unchanged. Due to a series connection efficiency enhancement achieved through use of a metal connecting line between the anode and cathode, the electric power conversion efficiency was improved. If wash processing is applicable for large-area modules such as long roll-to-roll-printed OPV modules, the resultant power conversion efficiency enhancement may be profound.
4. Conclusions A novel roll-to-roll applicable OPV module fabrication process was presented in this study. The influence of ohmic contact achieved via washing away highly resistant materials (active and transporting layers) was investigated. Common organic solvents such as chlorobenzene and acetone were used to dissolve unnecessary buffer and active layers. Monolithic interconnecting electrode lines were formed using a dispenser. Module efficiency reducing factors were removed by minimizing interconnection resistance. PEIE coating conditions were varied to study their effects on module efficiency. Modules consisted of seven sub cells connected in series on a 10 10 cm2 substrate (individual sub cell dimensions: 0.5 10 cm2), such that the effective cell area was 35 cm2. The effective power conversion efficiency as a result of wash processing was increased significantly compared to that of modules fabricated using the conventional process (efficiency improved from 1.367% to 1.843% for wash processing). Efficiency enhancement was attributed to increases in open circuit voltage and fill factor. Since power conversion enhancement was obtained for a small area (35 cm2), if the wash process can be applied to larger-area modules, the efficacy of power conversion efficiency enhancement may be greatly increased.
Acknowledgments This research was supported by Industrial Core Technology Development Program from Ministry of Trade, Industry & Energy (Grant no. 10035648), South Korea.
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[3] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Efficient photodiodes from interpenetrating polymer networks, Nature 376 (1995) 498–500. [4] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Adv. Funct. Mater. 11 (2001) 15–26. [5] K.M. Coakley, M.D. McGehee, Conjugated polymer photovoltaic cells, Chem. Mater. 16 (2004) 4533–4542. [6] W.L. Ma, C.Y. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15 (2005) 1617–1622. [7] M. Campoy-Quiles, T. Ferenczi, T. Agostinelli, P.G. Etchegoin, Y. Kim, T.D. Anthopoulos, P.N. Stavrinou, D.D.C. Bradley, J. Nelson, Morphology evolution via self-organization and lateral and vertical diffusion in polymer: fullerene solar cell blends, Nat. Mater. 7 (2008) 158–164. [8] 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. Cells 95 (2011) 1293–1302. [9] B. Azzopardi, C.J.M. Emmott, A. Urbina, F.C. Krebs, J. Mutale, J. Nelson, Economic assessment of solar electricity production from organic-based photovoltaic modules in a domestic environment, Energy Environ. Sci. 4 (2011) 3741–3753. [10] F.C. Krebs, S.A. Gevorgyan, J. Alstrup, A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies, J. Mater. Chem. 19 (2009) 5442–5451. [11] F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394–412. [12] F.C. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T.D. Nielsen, J. Fyenbo, K. Larsen, J. Kristensen, 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.
[13] F.C. Krebs, Fabrication and processing of polymer solar cells: polymer solar cell modules prepared using roll-to-roll methods: knife-over-edge coating, slotdie coating and screen printing, Sol. Energy Mater. Sol. Cells 93 (2009) 465–475. [14] F.C. Krebs, Roll-to-roll fabrication of monolithic large-area polymer solar cells free from indium–tin-oxide, Sol. Energy Mater. Sol. Cells 93 (2009) 1636–1641. [15] 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. [16] F.C. Krebs, T. Tromholt, M. Jørgensen, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale 2 (2010) 873–886. [17] 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. [18] C. Koidis, S. Logothetidis, S. Kassavetis, C. Kapnopoulos, P.G. Karagiannidis, D. Georgiou, A. Laskarakis, Effect of process parameters on the morphology and nanostructure of roll-to-roll printed P3HT:PCBM thin films for organic photovoltaics, Sol. Energy Mater. Sol. Cells 93 (2009) 36–46. [19] R. Steim, P. Schilinsky, S.A. Choulis, C.J. Brabec, Flexible polymer photovoltaic modules with incorporated organic bypass diodes to address module shading effects, Sol. Energy Mater. Sol. Cells 93 (2009) 1963–1967. [20] N.S. Kang, B.-K. Ju, J.-W. Yu, Module structure for an organic photovoltaic device, Sol. Energy Mater. Sol. Cells 116 (2013) 219–223. [21] N.Y. Zhou, C. Fuentes-Hernandez, J. Shim, J. Meyer, A.J. Giordano, H. Li, P. Winget, T. Papadopoulos, H. Cheun, J. Kim, M. Fenoll, A. Dindar, W. Haske, E. Najafabadi, T.M. Khan, H. Sojoudi, S. Barlow, Sl Graham, J.-L. Brédas, S.R. Marder, An Kahn, B. Kippelen, A universal method to produce low-work function electrodes for organic electronics, Science 336 (2012) 327–332.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Efficiency enhancement of organic photovoltaic modules prepared via wash processing Young Min Lee a,b, Jang Whan Lee a,b, Dong Kwon Choi b,n, Jae-Woong Yu a,n a Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, 1732 Deogyeong-daro, Giheung-gu, Yongin, Gyeonggi 446-701, Republic of Korea b Sung An Machinery Co., Ltd., 33, Madogongdan, Mado-myeon, Hwaseong City, Gyeonggi 445-861, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 28 April 2015 Received in revised form 15 June 2015 Accepted 16 June 2015
To enhance OPV performance, a novel module-fabrication process was developed. Chlorobenzene and acetone were employed to wash away unnecessary residual high-resistance materials after full modules were fabricated. Monolithic interconnecting electrode lines between the anode and the cathode were formed using a dispenser after wash process. The wash process allowed for minimization of interconnection resistance and enhancement of power conversion efficiency. Module efficiency changes were also studied by varying the coating conditions of PEIE. The power conversion efficiency increased as a result of the wash-off process compared with modules fabricated via conventional processes. The power conversion efficiency enhancement was attributed to increases in open circuit voltage and fill factor. Applying this wash-off process to larger-area OPV modules is expected to be effective for further enhancing power conversion efficiency. & 2015 Elsevier B.V. All rights reserved.
Keywords: Organic photovoltaic Module wash-off process
1. Introduction Due to the demand for sources of inexpensive renewable energy, organic photovoltaics (OPVs) have attracted a large amount of research and development focus in the last two decades [1–7]. Before OPVs can replace conventional power generation methods, it is important for the technology to be price competitive. This can be achieved through the utilization of mass production fabrication techniques such as roll-to-roll processing. Rollto-roll processing technology, with its high production throughput, is recognized as a method for low-cost mass production of large-area flexible organic electronics [8–18]. Since the first roll-to-roll-processed OPV module was reported by Krebs, many developmental studies have been conducted [15]. For use in everyday power applications, an OPV module structure must have connections between sub cells. Series connections are the most commonly employed method due to their relative ease of processing. Furthermore, according to OPV module shading tests, 10% shade exposure can shut down an entire module [19]. Therefore, optimized module fabrication technologies are critical to yield the maximum power output for a given module area. For example, we previously demonstrated that longitudinal partitioning of the OPV module leads to enhancement in efficiency n
Corresponding authors. E-mail addresses: [email protected] (D.K. Choi), [email protected] (J.-W. Yu).
http://dx.doi.org/10.1016/j.solmat.2015.06.030 0927-0248/& 2015 Elsevier B.V. All rights reserved.
[20]. Many studies have assessed the effects of module geometry on power conversion performance. In this study, we investigated the influence of OPV module fabrication methods on module performance. In particular, we studied the ability to form ohmic contacts as a result of washing away unnecessary active and transporting layers and correlated this ability with enhanced module performance.
2. Materials and methods 2.1. Module fabrication In this study, 100 mm 100 mm2 ITO glass substrates were etched via photolithographic processes to yield seven sub cells with 8-mm-wide ITO stripes. Prepared substrates were cleaned in series with acetone, isopropyl alcohol, and de-ionized water in an ultrasonic bath for 10 min. Subsequently, the substrates were dried under nitrogen gas in a 120 °C convection oven. Atmospheric pressure plasma (Power: 8 kV, N2 gas: 200 l/m) was used to increase the wettability of the substrates. An electron extraction buffer layer was formed using 0.4 wt% polyethylenimine, ethoxylated (PEIE, purchased from Sigma-Aldrich, MW ¼75,000) stock solution (dissolved in 2-methoxyethanol) in conjunction with a slot die coater (coating speed was 10–20 mm/s, shim plate thickness was 30 μm), followed by drying at 110 °C for 10 min in a convection oven. The resulting PEIE layer thickness was 5–10 nm
Y.M. Lee et al. / Solar Energy Materials & Solar Cells 143 (2015) 58–62
study, inverted OPV modules were prepared. To simulate a roll-toroll processing environment, all fabricated device layers were formed via slot die-coating or screen printing. Fig. 1 reveals the
50
2
Cell 1 (6.6 cm ) 2
40 Current (mA)
as measured with a thickness profiler (TENCORs, P-10 α-step). Next, 4.6 wt% P3HT (purchased from Rieke Metals, EE grade) and PCBM (purchased from Nano-C) were blended and dissolved in anhydrous chlorobenzene at a 1:0.6 weight ratio. The dissolved polymer blend solution was slot die-coated onto ITO glass/PEIE at a speed of 15 mm/s. The resultant active layer was 230 nm thick and was dried at 110 °C for 12 min. The hole extraction layer was generated by screen printing a PEDOT:PSS (Agfa EL-P5010) coating layer and was dried at 110 °C for 10 min. A silver paste (Toyo UV) coating was also applied via screen printing (width of silver electrode was 6 mm) and was sintered using heat and UV light (110 °C for 10 min and 1 min at 2400 mJ). Following fabrication of the OPV module, unnecessary buffer and active layers were removed by sequential washing with chlorobenzene and acetone solutions. Monolithic interconnecting electrode lines (formed with Elcoat p-100 whose width was 2 mm) were subsequently formed using a dispenser.
59
Cell 2 (6.6 cm ) 2
2Cell array (13.2 cm ) 30 20 10 0 0
200
400
2.2. Characterization A Class AAA Xe solar simulator (McScience K3600) was used as a light source, and all measurements were performed under a 1 sun condition (100 mW/cm2). Measurements were not corrected for reflection losses or absorptions from the ITO electrode. The I–V performance was measured with a Keithley 2400 source measurement unit. The cross-sectional analysis was performed by TESCAN Lyra3 dual beam FIB analyzer (Resolution in high vacuum is 1.2 nm at 30 kV).
1000
1200
2
Cell 1 (6.6 cm ) 2
Cell 2 (6.6 cm )
40 Current (mA)
2
2Cell array (13.2 cm ) 30 20 10
Conventional OPV modules are formed through series connections of several sub cells to yield the maximum power output for a given module area. We investigated the influence of ohmic contacts, formed by washing away unnecessary active and transporting layers, on the performance of the OPV module. In this
0 0
[HTL layer pattern printing]
200
400
600 Bias (V)
800
1000
1200
Fig. 2. I–V characteristics of sub cells and two cell array; (a) conventional process, (b) wash-off process.
[ETL layer pattern coating]
[Preparation of substrate]
[Ag electrode printing]
800
50
3. Results and discussion
[Preparation of substrate]
600 Bias (V)
[Active layer pattern coating]
[Ag pattern printing]
[ETL layer coating]
[Active layer coating]
[Remove ETL / Active layer]
[HTL layer printing]
[Inter connection monolithic Electrode line]
Fig. 1. OPV module fabrication process; (a) existing standard fabrication process, (b) low TCO-metal interfacial contact resistance process (wash-off process).
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Y.M. Lee et al. / Solar Energy Materials & Solar Cells 143 (2015) 58–62
Table 1 Performance of OPV cells and modules. Conventional process
Voc (V) Isc (mA) Jsc (mA/cm2) Fill factor Efficiency (%)
Wash-off process
Cell average
Standard deviation
Array average
Cell average
Standard deviation
Array average
0.581 32.63 4.907 0.349 0.995
7 0.000829 7 0.38034 7 0.057412 7 0.017385 7 0.039026
0.871 28.27 2.142 0.368 0.688
0.556 40.05 6.023 0.329 1.102
7 0.002947 7 0.71581 7 0.107574 7 0.008814 7 0.016887
1.001 36.77 2.785 0.34 0.948
Fig. 3. The cross-sectional analysis images of the OPV module prepared via the conventional process (a) and the wash processing method (b).
differences between conventional fabrication processes and the new solvent wash process highlighted in this study. This wash process removes organic semiconducting (i.e. high resistance) materials and minimizes the interfacial resistance between the transparent conducting oxide (TCO) and metal electrode. Fig. 2(a) and (b) shows the I–V characteristics of modules prepared via conventional processes (a) and the new wash process (b). Each sub cell had a measured dimension of 11 cm 0.6 cm (6.6 cm2 area), and modules were prepared with two cells connected in series. As shown in Fig. 2(a), the open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the conventional module (two-sub cell array) at AM 1.5 100 mW/cm2 condition were 0.871 V, 2.142 mA/cm2, 0.368, and 0.688 %, respectively. As shown in Fig. 2(b), the open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the washed module (two-sub cell array) at AM 1.5 100 mW/cm2 condition were 1.001 V, 2.785 mA/cm2, 0.340, and 0.948 %, respectively. Individual sub cell and module data are summarized in Table 1. During the module fabrication process, the open circuit voltage was increased as two cells were connected in series. Open circuit voltage enhancement in large-area printed devices is not easily achieved compared to the results obtained with small laboratory-scale devices. With conventional fabrication processing methods, the open circuit voltage of a two-cell array increased by 1.5-fold compared to the single-cell value (from 0.581 V to 0.871 V). The open circuit voltage after wash processing was doubled when using a two-cell array (from 0.556 V to 1.001 V). The short circuit current density of the two-cell array also increased following wash processing compared to the value obtained with conventional processing, as summarized in Table 1. The fill factor was also reduced when a two-cell array was fabricated using conventional fabrication processing methods. However, the fill factor of a two-cell array prepared via wash processing was increased. The combined effect of these changes resulted in significantly increase in power conversion efficiency following wash processing compared with that of the conventional process (0.688 for conventional and 0.948 for wash processing).
To further study the effects of wash processing on OPV modules, a large-area module was fabricated using a 10 cm 10 cm ITO substrate. In this study, PEIE-coated ITO was used as the lowwork function electrode (cathode), as reported in other literature [21]. Since the performance of the ITO work function is highly dependent on the thickness of the PEIE coating, it is very important to optimize the PEIE coating conditions. A full-area coating was prepared due to the difficulty in printing a very thin PEIEpatterned structure via slot die coating. Different treatment conditions were prepared to determine the optimal PEIE coating thickness. The ITO substrate was patterned via photolithography, and a 4.6 wt% P3HT:PCBM blend with a 1:0.6 weight ratio was coated on top of the PEIE layer via slot die coating. A PEDOT:PSS coating was placed on top of the active layer via screen printing. Lastly, a silver electrode was deposited via screen printing. After the module was fully fabricated, unnecessary layers (PEIE, active, and PEDOT:PSS layers) were removed by sequential chlorobenzene and acetone washes. Fig. 3 shows the cross-sectional analysis of the OPV module prepared via the conventional process (a) and the wash processing method (b). According to FIB analysis, the thickness of active layer was about 230 nm and the thickness of PEDOT: PSS was about 3.6 μm. Thickness of the charge collecting silver electrode (TOYO UV Ag) and the interconnection electrode was 25 and 15 μm, respectively. As shown, modules prepared via either process showed very similar cross-sectional planes. The chargecollecting silver electrode protected the underlying organic materials (PEIE, active, and PEDOT:PSS layers) from being washed away by solvent. Following these processes, the cathode and anode were directly interconnected via monolithic electrode lines using a dispenser. According to SEM–EDS analysis, the silver charge collection layer (formed with Toyo UV paste whose thickness was 16 μm) showed many gaps between the silver grains. The interconnection electrode, formed with Elcoat p-100 (manufactured by CANS), was produced with a thickness of 25 μm. The interconnection electrode layer showed more meticulous structures. The conductivity of the charge collection electrode was 0.00438 Ω/□, while that of the interconnection electrode was 0.007 Ω/□. Therefore, it can be concluded that metallic
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0.07 ml 0.093 ml 0.14 ml
Current (mA)
40 30 20 10 0 0
1000
2000
3000
4000
5000
Bias (mV) Fig. 4. The I–V behavior of an OPV module consisting of seven sub cells with different PEIE thickness.
Table 2 Summary of OPV modules performance consisting of seven sub cells with different PEIE thickness. (Micro gear pump flow rate was 20 rpm). PEIE coating condition Coating rate (mm/s) Flow rate (ml) Voc (V) Isc (mA) Jsc (mA/cm2) Fill factor Efficiency (%)
20 0.07 3.862 70.011 34.8517 0.102 0.996 70.003 0.4797 0.008 1.843 7 0.041
15 0.093 3.744 70.044 27.998 7 0.127 0.8007 0.004 0.4337 0.004 1.295 7 0.031
10 0.14 3.6437 0.059 22.9917 0.601 0.6577 0.017 0.434 70.001 1.039 7 0.014
40
Current (mA)
30
20 module #1 module #2 module #3 module #4 module #5
10
0 0
1000
2000 Bias (mV)
3000
4000
Fig. 5. The I–V behavior of an OPV module prepared using the conventional process.
conductivity of the two electrodes is not the reason for the power conversion efficiency enhancement. Fig. 4 shows the I–V behavior of an OPV module consisting of seven sub cells (individual sub cell dimensions: 0.5 cm 10 cm), and the results are summarized in Table 2. PEIE thickness was measured using a surface profiler and was 5 and 10 nm for coating rates of 20, 15, and 10 mm/s with a 20 rpm micro gear pump rate, which the total flow rate was about 0.07, 0.093 and 0.14 ml, respectively. The best power conversion efficiency was obtained with a 20 mm/s coating rate. The open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the seven-cell module at AM 1.5 100 mW/cm2 condition were 3.862 70.011 V, 0.996 70.003 mA/cm2, 0.4797 0.008, and 1.8437 0.041%, respectively. As summarized in Table 2, thinnest PEIE layer yielded the best open circuit voltage, the short circuit current, fill factor and power conversion efficiency. In determining the superiority of this fabrication process, a module having comparable size and characteristics was prepared via
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conventional processing (i.e., fabricated using the same procedures minus the solvent wash process). Fig. 5 shows the I–V behavior of an OPV module prepared using the conventional process. The open circuit voltage, short circuit current density, fill factor, and power conversion efficiency of the seven-cell module at AM 1.5 100 mW/cm2 condition were 3.42670.067 V, 1.03170.019 mA/cm2, 0.38770.792, and 1.36770.030%, respectively. The substrate size of the module was 10 10 cm2, and the effective cell area was 35 cm2. The power conversion efficiency enhancement resulting from the washing process led to significant increase compared to the value obtained from conventional processing (from 1.367% to 1.843%). This efficiency increase is primarily attributed to increases in open circuit voltage and fill factor. The washing process dissolves many insulating (PEIE, active, and PEDOT:PSS layers, i.e., high-resistance materials) components between the anode and cathode. It also allows for direct contact between two electrodes via a monolithic metal line. The results demonstrate that the short circuit current density was relatively unchanged between the conventional and wash processing methods. This suggests that the current production of an effective cell area was likewise relatively unchanged. Due to a series connection efficiency enhancement achieved through use of a metal connecting line between the anode and cathode, the electric power conversion efficiency was improved. If wash processing is applicable for large-area modules such as long roll-to-roll-printed OPV modules, the resultant power conversion efficiency enhancement may be profound.
4. Conclusions A novel roll-to-roll applicable OPV module fabrication process was presented in this study. The influence of ohmic contact achieved via washing away highly resistant materials (active and transporting layers) was investigated. Common organic solvents such as chlorobenzene and acetone were used to dissolve unnecessary buffer and active layers. Monolithic interconnecting electrode lines were formed using a dispenser. Module efficiency reducing factors were removed by minimizing interconnection resistance. PEIE coating conditions were varied to study their effects on module efficiency. Modules consisted of seven sub cells connected in series on a 10 10 cm2 substrate (individual sub cell dimensions: 0.5 10 cm2), such that the effective cell area was 35 cm2. The effective power conversion efficiency as a result of wash processing was increased significantly compared to that of modules fabricated using the conventional process (efficiency improved from 1.367% to 1.843% for wash processing). Efficiency enhancement was attributed to increases in open circuit voltage and fill factor. Since power conversion enhancement was obtained for a small area (35 cm2), if the wash process can be applied to larger-area modules, the efficacy of power conversion efficiency enhancement may be greatly increased.
Acknowledgments This research was supported by Industrial Core Technology Development Program from Ministry of Trade, Industry & Energy (Grant no. 10035648), South Korea.
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