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Large-area blade-coated organic solar cells processed from halogen-free solvent
Organic Electronics 75 (2019) 105376
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Large-area blade-coated organic solar cells processed from halogen-free solvent
T
Szu-Han Chena, Chung‐Hung Liaob, Chih‐Yu Changc,h,∗, Kuan‐Min Huangd, Jen‐Yueh Chene, Chao‐Hsuan Chend, Hsin-Fei Menga,∗∗, Hsiao-Wen Zand, Sheng-Fu Horngb, Yen-Chung Linf, Min-Hsin Yehg a
Institute of Physics, National Chiao Tung University, Hsinchu, 300, Taiwan Institute of Electronic Engineering, National Tsing Hua University, Hsinchu, 300, Taiwan c Graduate Institute of Nanomedicine and Medical Engineering, Taipei Medical University, Taipei, 110, Taiwan d Department of Photonics and Institute of Electro‐Optical Engineering, National Chiao Tung University, Hsinchu, 300, Taiwan e Electrophysics, National Chiao Tung University, Hsinchu, 300, Taiwan f Department of Internal Medicine, Taipei Medical University, Taipei, 110, Taiwan g Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan h Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Organic solar cells Blade-coating Halogen-free solvent Large area Air-processing
High performance organic solar cells (OSCs) are generally prepared by halogenated solvents, which are detrimental to the environment and human health. The replacement of hazardous halogenated solvents is a priority in the commercialization of solution-processed OSCs. In this study, poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl) benzo[1,2‐b;4,5‐b’]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6-diyl)] (PBDTTT‐EFT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM)-based device processed with binary halogen-free solvents (toluene and N-methyl-2-pyrrolidone) is fabricated by blade-coating method. Through the combination of morphological manipulation of active layer and interfacial engineering, the resulting OSCs deliver a promising power conversion efficiency (PCE) up to 11.09%. Importantly, large-area (216 cm2) blade-coated bulk-heterojunction OSCs is also demonstrated, and the resulting devices deliver a high PCE up to 5.03%, which is comparable to those of the devices processed with halogenated solvents (PCE = 5.20%). Additionally, the devices processed with halogen-free solvents also exhibit superior stability, maintaining 88% of its initial efficiency after 3960 h of continuous operation. More encouragingly, halogen-free solvents-based manufacturing process can also be performed under ambient conditions, and a moderate PCE of 3.67% is attained for large-area (216 cm2) OSCs. To the best of our knowledge, 3.67% represents the highest efficiency ever reported for air-processed large-area OSCs. This work paves the way towards the realization of environmentally-friendly large-area OSCs with high performance and long-term stability.
1. Introduction
devices (less than 1 cm2) fabricated by spin coating [12,13]; (iii) the devices typically suffer from rapid degradation under the ambient conditions [14–17]. Although high-efficiency small-area OSCs (active area < 0.042 cm2) processed from halogen-free solvents have been demonstrated [18], the realization of large-scale devices with high performance and long-term stability remains highly challenging. In contrast to spin-coating procedure, which inevitably causes waste of materials and is incompatible with high-volume production, doctorblade coating provides several unique advantages such as small amount of material waste, large-area uniformity, and high compatibility with
Organic solar cells (OSCs) have attracted valuable importance in the areas of renewable energy because of their possibility of fabricating large area, cost-effective, flexible, light-weight devices [1–6]. Despite significant efforts towards improving power conversion efficiency (PCE), three key issues still impede the commercialization of OSCs: (i) state-of-the-art devices are generally prepared using halogen-containing solvents, which are toxic and can be harmful to the environment [7–11]; (ii) high efficiency devices are still constrained to lab-scale
∗
Corresponding author. Corresponding author. Graduate Institute of Nanomedicine and Medical Engineering, Taipei Medical University, Taipei, 110, Taiwan. E-mail addresses: [email protected], [email protected] (C. Chang), [email protected] (H.-F. Meng).
∗∗
https://doi.org/10.1016/j.orgel.2019.105376 Received 16 May 2019; Received in revised form 5 July 2019; Accepted 24 July 2019 Available online 24 July 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
Organic Electronics 75 (2019) 105376
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Fig. 1. (a) The device configuration and the chemical structures of active materials used in this work. (b) Schematic illustrating the blade-coating processes. Table 1 Summary of the photovoltaic properties of PBDTTT-EFT-based small-area devices. The values in parenthesis are for the best performing devices. Device
Solvent
Interfacial layer
Voc [volt]
Jsc [mA cm−2]
FF [%]
PCE [%]
Aa Bb Cb
CB, DIO Toluene, NMP Toluene, NMP
LiF LiF ZrAcac
0.82 ± 0.00 (0.82) 0.85 ± 0.00 (0.85) 0.82 ± 0.02 (0.83)
19.02 ± 0.32 (19.44) 17.51 ± 0.13 (17.39) 17.58 ± 0.28 (17.89)
0.61 ± 0.01 (0.63) 0.63 ± 0.01 (0.67) 0.71 ± 0.04 (0.74)
9.45 ± 0.18 (9.75) 9.26 ± 0.11 (9.39) 10.91 ± 0.14 (11.09)
a b
Average and standard deviation values were obtained based on 30 devices. Average and standard deviation values were obtained based on 50 devices.
Fig. 3. Degradation profiles of the devices as a function of operation time in ambient conditions. Device A was processed with CB/DIO (interlayer: LiF). Device B was processed with toluene/NMP (interlayer: LiF). Device C was processed with toluene/NMP (interlayer: ZrAcac).
Fig. 2. J-V curves of the PBDTTT-EFT:PC71BM-based small-area OSCs processed with different solvents. Device A was processed with CB/DIO (interlayer: LiF). Device B was processed with toluene/NMP (interlayer: LiF). Device C was processed with toluene/NMP (interlayer: ZrAcac).
the potential of using o-methylanisole as processing solvent in bladecoated OSCs, leading to a PCE up to 8.4% [25]. In this study, blade coating with the halogen-free solvent, involving toluene and N-methyl-2-pyrrolidone (NMP), is applied to bulk-heterojunction OSCs employing poly[4,8-(bis(5-(2-ethylhexyl)thiophen-2-yl) benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PBDTTT-EFT, also known as PTB7-Th) blended with [,]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layer. We select toluene/NMP as the
roll-to-roll processs [19]. These characteristics make doctor-blade coating ideally suitable for large-area OSCs manufacturing [6,20–22]. For example, we have demonstrated that blade-coated OSCs with active area of 216 cm2 exhibit both high PCE (up to 5.6%) and good stability [23]. Heremans et al. investigate the essential factors (e.g. toxicity of solvents, wettability of solvents on substrates, drying time) for bladecoated OSCs, leading to new halogen-free solvent pathways for the scalable deposition of OSCs’ materials [24]. Recently, Ye et al. present 2
Organic Electronics 75 (2019) 105376
S.-H. Chen, et al.
Fig. 4. The high resolution AFM images of PBDTTT-EFT/PC71BM layer: (a) processed with chlorobenzene/1,8-diiodooctane, (b) processed with toluene/NMP, (c) overcoated with LiF, (d) overcoated with ZrAcac.
Fig. 5. (a) Schematic illustration of the cross-sectional structure of the large‐area device. (b) The side view of the schematic diagrams for large‐area device. (c) A photography of our large-area device.
the devices reaches 11.09%. In addition, the device processed with halogen-free solvents also exhibit superior stability, maintaining 88% of its initial efficiency after 3960 h of continuous operation. More importantly, halogen-free solvents-based manufacturing process can also be performed under ambient conditions, and an impressive PCE of
halogen-free processing solvent because it exhibits reasonably good solubility for conjugated polymers/fullerenes and has been studied as a promising strategy for the development of high-performance OSCs [26,27]. Through the combination of morphological manipulation of active layer and interfacial engineering, the highest achievable PCE of 3
Organic Electronics 75 (2019) 105376
S.-H. Chen, et al.
removal, and use a rubber to remove the unnecessary areas in the gaps between two cells. As for the interfacial layer material zirconium acetylacetonate (ZrAcac), a concentration of 0.8 mg/mL is dissolved in isopropanol. The interlayer is blade coated at 80 °C, and hot air is applied from above. For the deposition of interfacial layer LiF, the thickness is 0.8 nm. Then, the device is completed with the cathode by using thermal evaporation in a vacuum chamber with a base pressure below 3.0 × 10−6 Torr.
3. Results and discussions The device structure used in this study was glass substrate/indium–tin-oxide (ITO)/poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS)/PBDTTT-EFT:PC71BM/interlayer/Al (see Fig. 1a). The chemical structures of active materials and the solvents examined herein were also shown in Fig. 1a. Two solvent systems for the preparation of bulk-heterojunction (BHJ) active layer, involving halogen-free solvent (toluene/NMP additive) and commonly used halogen-containing solvent (chlorobenzene/1,8-diiodooctane additive), were examined. It should be noted that all the organic layers, including PEDOT:PSS hole-transport layer, bulk heterojunction active layer, and cathode interlayer, were deposited by scalable blade-coating approach. The schematic illustrating the blade-coating processes was shown in Fig. 1b. The wet film was rapidly dried by the hot wind and the hot plate. The film thickness variation within 5 nm could be achieved. The device characteristics of small-area devices (active area = 0.04 cm2) processed with different solvents were summarized in Table 1 and Fig. 2. For the devices processed with chlorobenzene/1,8diiodooctane (device A), a high PCE of 9.75% was obtained, with an open-circuit voltage (Voc) of 0.82 V, a short-circuit current (Jsc) of 19.44 mA cm−2 and a fill factor (FF) of 0.63 (Table 1 and Fig. 2). Encouragingly, for the device processed with toluene/NMP (device B), a comparable PCE of 9.39% was observed. This represents that the halogen-free solvent toluene/NMP can be successfully used as the alternative to commonly used chlorobenzene/1,8-diiodooctane solvent for high-efficiency OSCs. In order to understand the morphology of PBDTTT-EFT:PC71BM processed with different solvents, atomic force microscopy (AFM) analysis was performed. As shown in Fig. 4a–b, the root mean square (rms) surface roughness were 1.79 and 2.00 nm for the films processed with chlorobenzene/1,8-diiodooctane and toluene/NMP, respectively. Such surface low roughness implies that little or no phase separation exist for both solvent systems. To gain a deeper insight into the morphology of active layer, transmission electron microscopy (TEM) analysis was performed. As shown in Fig. S2 (supporting information), finely mixed polymer/fullerene morphologies were observed, which would be beneficial to the overall device performance [28]. It should be noted that the spin-coated films processed with toluene/NMP was much rougher (rms roughness ~4.53 nm; see supporting information Fig. S1) than that of blade-coated film, which suggests that blade coating may be beneficial for halogen-free system. Stability is usually a fundamental problem for many organic devices. OSCs may have morphological changes in the active layer heterojunction or in the interface with the electrode upon long-term operation [29], and studies indicate that the stability of the solar cell is
Fig. 6. J-V curves of the PBDTTT-EFT:PC71BM-based large-area OSCs processed with different solvents. Device D was processed with CB/DIO (interlayer: LiF). Device E was processed with toluene/NMP (interlayer: LiF). Device F was processed with toluene/NMP (interlayer: ZrAcac).
3.07% is attained. These findings offer a rational guideline for developing environmentally friendly large-area OSCs with high performance and long-term stability. 2. Experimental Organic solar cells have a structure of pre-patterned indium–tinoxide (ITO) glass/PEDOT:PSS/BHJ layer/interfacial layer/Al. The ITO coated glass substrate was first cleaned with acetone in an ultrasonic bath for 20 min and rinsed three times with deionized water to ensure that the glass was clean and then cleaned with a UV ozone cleaner for 20 min. For small-area devices, PEDOT:PSS layer was spin coated at 2500 rpm on a pre-cleaned ITO substrate for 40s and baked at 150 °C for 15 min. The active layer material of a bulk heterojunction organic solar cell used PBDTTT-EFT as the donor and PC71BM as the acceptor. Blade coating is used for the deposition of the active layer as it can achieve high material usage and is scalable to large-area OSC. PBDTTT-EFT and PC71BM powders are dissolved in chlorobenzene and toluene at 60 °C for 1 day. The solution concentrations of 5 mg:7.5 mg:500 μL were prepared. 1,8-diiodooctane (DIO, 3% v/v) and N-methyl-2-pyrrolidone (NMP, 3% v/v) are respectively added to the solution 2 h before deposition. The temperature of the blade substrate was set at 80 °C. During the blade coating process, hot air is simultaneously applied from the top using a blower with a diffusing showerhead to enhance the drying speed and uniformity. After coating, all PBDTTT-EFT:PC71BM layers were annealed at 100 °C for 10 min in nitrogen. For large-area devices, PEDOT:PSS is first diluted by Ethanol in a 1:2 vol ratio before blade coating. The blade coating method of the active layer is similar to that of small-area devices, but the uniformity is significantly improved by fine tuning the injection and the blade motion. The solution concentrations were prepared by 10 mg:15 mg:1000 μL with the addition of 3% v/v NMP. After blade coating, the films are baked at 150 °C for 15 min. A metal mask is used to define the effective area. We pattern the active layer by mechanical
Table 2 Summary of the photovoltaic properties of large-area devices with active area of 216 cm2. The values in parenthesis are for the best performing devices. Device a
D Ea Fb a b
Solvent
Interlayer
Voc [volt]
Jsc [mA cm−2]
FF [%]
PCE [%]
CB, DIO Toluene, NMP Toluene, NMP
LiF LiF ZrAcac
12.14 ± 0.36 (12.21) 12.52 ± 0.39 (12.99) 11.78 ± 0.13 (11.87)
0.81 ± 0.09 (0.83) 0.76 ± 0.02 (0.78) 0.81 ± 0.02 (0.85)
0.51 ± 0.02 (0.54) 0.47 ± 0.01 (0.49) 0.48 ± 0.07 (0.49)
5.16 ± 0.03 (5.20) 5.00 ± 0.01 (5.03) 5.12 ± 0.07 (5.27)
Average and standard deviation values were obtained based on 6 devices. Average and standard deviation values were obtained based on 5 devices. 4
Organic Electronics 75 (2019) 105376
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Table 3 Photovoltaic parameters of large-area air-processed OSCs (active area = 216 cm2) processed from halogen-free solvents. The values in parenthesis are for the best performing devices. Device
Active layer processing
Voc [volt]
Jsc [mA cm−2]
FF [%]
PCE [%]
Ga Ha
Annealed 100 °C Non-annealing
9.53 ± 1.42 (11.24) 12.01 ± 0.02 (12.31)
0.73 ± 0.09 (0.84) 0.57 ± 0.03 (0.58)
0.42 ± 0.01 (0.39) 0.40 ± 0.02 (0.39)
3.21 ± 0.43 (3.67) 2.71 ± 0.27 (2.83)
a
Average and standard deviation values were obtained based on 5 devices.
large-area devices processed with toluene/NMP, and the photovoltaic parameters were summarized in Table 2. The resulting devices exhibited a PCE of 5.03%, with an Voc of 12.99 V, a Jsc of 0.78 mA cm−2, and a FF of 0.49 (Table 2 and Fig. 6). It should be emphasized again that the device performance based on toluene/NMP (device E) was nearly comparable to that of the devices based on chlorobenzene/1,8diiodooctane (device D), as shown in Table 2 and Fig. 6. These results clearly indicate that the newly-developed halogen-free solvent system is applicable for large-area OSCs. Similar to those observed for small larea devices, the PCE could be further boosted to 5.27% by employing ZrAcac as the cathode interlayer (device F) (Table 2 and Fig. 6). Impressively, halogen-free solvents-based manufacturing process can also be performed under ambient conditions. So far, only very few studies have attempted to demonstrate air-processed OSCs based on halogen-free solvents [18,35]. The device structure of air-processed devices was glass substrate/ITO/PEDOT:PSS/PBDTTT-EFT:PC71BM/ ZrAcac/Al, whose active area was 214 cm2. Our results indicated that an appropriate thermal annealing treatment seems to be essential for high performance. As shown in Table 3 and Fig. 7, after thermal annealing at 100 °C for 10 min, the PCE of air-processed devices with active area of 216 cm2 reached 3.67% (device G), which is superior to the devices without thermal annealing (PCE = 2.83%; device H). This might arise from the fact that optimal morphologies are usually obtained upon thermal annealing [2]. It should be noted that the highest achievable efficiency of air-processed devices (device G) was still inferior to those of the inert atmosphere-processed devices (device F), suggesting that ambient air inevitably causes adverse effects on the optoelectronic properties of BHJ layer. Further investigations are our on-going efforts. It should be noted that this is the first demonstration of large-area air-processed OSCs based on halogen-free solvent. To the best of our knowledge, 3.67% PCE represents the highest efficiency ever reported for air-processed OSCs, regardless of processing solvent and active area, as shown in Supporting Information Table S1. The advantages of preparing devices in air is not only to speed up the process, but also to the large-area preparation of organic solar cells.
Fig. 7. J-V curves of air-processed OSCs (active area = 216 cm2) processed from halogen-free solvents. The active layer of device G was annealed at 100 °C for 10 min, while the active layer of device H was non-annealed.
improved in various ways, such as the use of an appropriate interlayer [30]. Lithium fluoride (LiF) has been widely used as the cathode interlayer for OSCs. However, LiF still presents some inadequacies, such as poor surface coverage, high susceptibility to atmospheric moisture, and insufficient thermal stability [31–33]. We thus examine the effectiveness of using zirconium acetylacetonate (ZrAcac; see Fig. 1a for the chemical structure) as the interlayer for halogen solvents-free devices. It should be mentioned again that ZrAcac layer was deposited by scalable blade-coating approach. Very encouragingly, the devices with ZrAcac achieved superior performance to those with LiF, with a PCE up to 11.09% (Table 1 and Fig. 2). The PCE improvement mainly came from relatively high FF (0.74; see Table 1 and Fig. 2), suggests that ZrAcac interlayer can afford superior charge selectivity at the cathode interface. The device stability was also examined under continuous operation (with 100 mW cm−2 AM1.5 light illumination) in ambient air, and the results were shown in Fig. 3. The devices processed with toluene/NMP (device B) exhibited superior stability than the devices processed with chlorobenzene/1,8-diiodooctane (device A), maintaining 50% of its initial efficiency after 5200 h of continuous operation. These results are consistent with previous findings and can be attributed to that fact that the presence of 1,8-diiodooctane can cause serious light instability of the active layer [34]. More importantly, the device stability was found to be further improved by replacing LiF with ZrAcac interlayer (device C), maintaining 88% of its initial efficiency after 3960 h of continuous operation. Having determined that high-performance small-area devices can be achieved by the combination of morphological manipulation of active layer and interfacial engineering, large-area devices with total active area of 216 cm2 were also fabricated by blade coating. The design and the schematic diagrams of the large‐area OSC are shown in Fig. 5. For the modules, the cells were connected in 16 series on 280 mm × 200 mm indium in oxide (ITO) glass. Each stripe has a width of 12 mm and a length of 175 mm, and 2 mm of the gap. The way to form a series connection is to connect the cathode of the cell on the left to the ITO of the cell on the right. Fig. 6 (a) shows the measured current J–V characteristics of the
4. Conclusion In summary, high-efficiency OSC based on PBDTTT-EFT was successfully fabricated by using blade coating in halogen-free toluene solution. This achievement may be able to replace the toxic solvents in the preparation of OSC to achieve environmental protection. The highest achievable PCE reaches 11.09%. The large-area solar cells with active area of 216 cm2 also deliver a promising photoelectric conversion efficiency of 5.03%. Remarkably, a PCE of 3.67% can be achieved by airprocessed halogen-free blade-coated OSCs. To the best of our knowledge, 3.67% represents the highest efficiency ever reported for airprocessed OSCs. More importantly, the resulting devices also exhibit good stability under continuous operation in ambient air. These results show that it is achievable to use halogen-free solvents to replace toxic solvents in the development of ambient-processed high-efficiency OSCs. This work offers a rational guideline for developing environmentally friendly large-area OSCs with high performance and long-term stability.
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Organic Electronics 75 (2019) 105376
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Acknowledgements
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Large-area blade-coated organic solar cells processed from halogen-free solvent
T
Szu-Han Chena, Chung‐Hung Liaob, Chih‐Yu Changc,h,∗, Kuan‐Min Huangd, Jen‐Yueh Chene, Chao‐Hsuan Chend, Hsin-Fei Menga,∗∗, Hsiao-Wen Zand, Sheng-Fu Horngb, Yen-Chung Linf, Min-Hsin Yehg a
Institute of Physics, National Chiao Tung University, Hsinchu, 300, Taiwan Institute of Electronic Engineering, National Tsing Hua University, Hsinchu, 300, Taiwan c Graduate Institute of Nanomedicine and Medical Engineering, Taipei Medical University, Taipei, 110, Taiwan d Department of Photonics and Institute of Electro‐Optical Engineering, National Chiao Tung University, Hsinchu, 300, Taiwan e Electrophysics, National Chiao Tung University, Hsinchu, 300, Taiwan f Department of Internal Medicine, Taipei Medical University, Taipei, 110, Taiwan g Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan h Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Organic solar cells Blade-coating Halogen-free solvent Large area Air-processing
High performance organic solar cells (OSCs) are generally prepared by halogenated solvents, which are detrimental to the environment and human health. The replacement of hazardous halogenated solvents is a priority in the commercialization of solution-processed OSCs. In this study, poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl) benzo[1,2‐b;4,5‐b’]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6-diyl)] (PBDTTT‐EFT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM)-based device processed with binary halogen-free solvents (toluene and N-methyl-2-pyrrolidone) is fabricated by blade-coating method. Through the combination of morphological manipulation of active layer and interfacial engineering, the resulting OSCs deliver a promising power conversion efficiency (PCE) up to 11.09%. Importantly, large-area (216 cm2) blade-coated bulk-heterojunction OSCs is also demonstrated, and the resulting devices deliver a high PCE up to 5.03%, which is comparable to those of the devices processed with halogenated solvents (PCE = 5.20%). Additionally, the devices processed with halogen-free solvents also exhibit superior stability, maintaining 88% of its initial efficiency after 3960 h of continuous operation. More encouragingly, halogen-free solvents-based manufacturing process can also be performed under ambient conditions, and a moderate PCE of 3.67% is attained for large-area (216 cm2) OSCs. To the best of our knowledge, 3.67% represents the highest efficiency ever reported for air-processed large-area OSCs. This work paves the way towards the realization of environmentally-friendly large-area OSCs with high performance and long-term stability.
1. Introduction
devices (less than 1 cm2) fabricated by spin coating [12,13]; (iii) the devices typically suffer from rapid degradation under the ambient conditions [14–17]. Although high-efficiency small-area OSCs (active area < 0.042 cm2) processed from halogen-free solvents have been demonstrated [18], the realization of large-scale devices with high performance and long-term stability remains highly challenging. In contrast to spin-coating procedure, which inevitably causes waste of materials and is incompatible with high-volume production, doctorblade coating provides several unique advantages such as small amount of material waste, large-area uniformity, and high compatibility with
Organic solar cells (OSCs) have attracted valuable importance in the areas of renewable energy because of their possibility of fabricating large area, cost-effective, flexible, light-weight devices [1–6]. Despite significant efforts towards improving power conversion efficiency (PCE), three key issues still impede the commercialization of OSCs: (i) state-of-the-art devices are generally prepared using halogen-containing solvents, which are toxic and can be harmful to the environment [7–11]; (ii) high efficiency devices are still constrained to lab-scale
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Corresponding author. Corresponding author. Graduate Institute of Nanomedicine and Medical Engineering, Taipei Medical University, Taipei, 110, Taiwan. E-mail addresses: [email protected], [email protected] (C. Chang), [email protected] (H.-F. Meng).
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https://doi.org/10.1016/j.orgel.2019.105376 Received 16 May 2019; Received in revised form 5 July 2019; Accepted 24 July 2019 Available online 24 July 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) The device configuration and the chemical structures of active materials used in this work. (b) Schematic illustrating the blade-coating processes. Table 1 Summary of the photovoltaic properties of PBDTTT-EFT-based small-area devices. The values in parenthesis are for the best performing devices. Device
Solvent
Interfacial layer
Voc [volt]
Jsc [mA cm−2]
FF [%]
PCE [%]
Aa Bb Cb
CB, DIO Toluene, NMP Toluene, NMP
LiF LiF ZrAcac
0.82 ± 0.00 (0.82) 0.85 ± 0.00 (0.85) 0.82 ± 0.02 (0.83)
19.02 ± 0.32 (19.44) 17.51 ± 0.13 (17.39) 17.58 ± 0.28 (17.89)
0.61 ± 0.01 (0.63) 0.63 ± 0.01 (0.67) 0.71 ± 0.04 (0.74)
9.45 ± 0.18 (9.75) 9.26 ± 0.11 (9.39) 10.91 ± 0.14 (11.09)
a b
Average and standard deviation values were obtained based on 30 devices. Average and standard deviation values were obtained based on 50 devices.
Fig. 3. Degradation profiles of the devices as a function of operation time in ambient conditions. Device A was processed with CB/DIO (interlayer: LiF). Device B was processed with toluene/NMP (interlayer: LiF). Device C was processed with toluene/NMP (interlayer: ZrAcac).
Fig. 2. J-V curves of the PBDTTT-EFT:PC71BM-based small-area OSCs processed with different solvents. Device A was processed with CB/DIO (interlayer: LiF). Device B was processed with toluene/NMP (interlayer: LiF). Device C was processed with toluene/NMP (interlayer: ZrAcac).
the potential of using o-methylanisole as processing solvent in bladecoated OSCs, leading to a PCE up to 8.4% [25]. In this study, blade coating with the halogen-free solvent, involving toluene and N-methyl-2-pyrrolidone (NMP), is applied to bulk-heterojunction OSCs employing poly[4,8-(bis(5-(2-ethylhexyl)thiophen-2-yl) benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PBDTTT-EFT, also known as PTB7-Th) blended with [,]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layer. We select toluene/NMP as the
roll-to-roll processs [19]. These characteristics make doctor-blade coating ideally suitable for large-area OSCs manufacturing [6,20–22]. For example, we have demonstrated that blade-coated OSCs with active area of 216 cm2 exhibit both high PCE (up to 5.6%) and good stability [23]. Heremans et al. investigate the essential factors (e.g. toxicity of solvents, wettability of solvents on substrates, drying time) for bladecoated OSCs, leading to new halogen-free solvent pathways for the scalable deposition of OSCs’ materials [24]. Recently, Ye et al. present 2
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Fig. 4. The high resolution AFM images of PBDTTT-EFT/PC71BM layer: (a) processed with chlorobenzene/1,8-diiodooctane, (b) processed with toluene/NMP, (c) overcoated with LiF, (d) overcoated with ZrAcac.
Fig. 5. (a) Schematic illustration of the cross-sectional structure of the large‐area device. (b) The side view of the schematic diagrams for large‐area device. (c) A photography of our large-area device.
the devices reaches 11.09%. In addition, the device processed with halogen-free solvents also exhibit superior stability, maintaining 88% of its initial efficiency after 3960 h of continuous operation. More importantly, halogen-free solvents-based manufacturing process can also be performed under ambient conditions, and an impressive PCE of
halogen-free processing solvent because it exhibits reasonably good solubility for conjugated polymers/fullerenes and has been studied as a promising strategy for the development of high-performance OSCs [26,27]. Through the combination of morphological manipulation of active layer and interfacial engineering, the highest achievable PCE of 3
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removal, and use a rubber to remove the unnecessary areas in the gaps between two cells. As for the interfacial layer material zirconium acetylacetonate (ZrAcac), a concentration of 0.8 mg/mL is dissolved in isopropanol. The interlayer is blade coated at 80 °C, and hot air is applied from above. For the deposition of interfacial layer LiF, the thickness is 0.8 nm. Then, the device is completed with the cathode by using thermal evaporation in a vacuum chamber with a base pressure below 3.0 × 10−6 Torr.
3. Results and discussions The device structure used in this study was glass substrate/indium–tin-oxide (ITO)/poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS)/PBDTTT-EFT:PC71BM/interlayer/Al (see Fig. 1a). The chemical structures of active materials and the solvents examined herein were also shown in Fig. 1a. Two solvent systems for the preparation of bulk-heterojunction (BHJ) active layer, involving halogen-free solvent (toluene/NMP additive) and commonly used halogen-containing solvent (chlorobenzene/1,8-diiodooctane additive), were examined. It should be noted that all the organic layers, including PEDOT:PSS hole-transport layer, bulk heterojunction active layer, and cathode interlayer, were deposited by scalable blade-coating approach. The schematic illustrating the blade-coating processes was shown in Fig. 1b. The wet film was rapidly dried by the hot wind and the hot plate. The film thickness variation within 5 nm could be achieved. The device characteristics of small-area devices (active area = 0.04 cm2) processed with different solvents were summarized in Table 1 and Fig. 2. For the devices processed with chlorobenzene/1,8diiodooctane (device A), a high PCE of 9.75% was obtained, with an open-circuit voltage (Voc) of 0.82 V, a short-circuit current (Jsc) of 19.44 mA cm−2 and a fill factor (FF) of 0.63 (Table 1 and Fig. 2). Encouragingly, for the device processed with toluene/NMP (device B), a comparable PCE of 9.39% was observed. This represents that the halogen-free solvent toluene/NMP can be successfully used as the alternative to commonly used chlorobenzene/1,8-diiodooctane solvent for high-efficiency OSCs. In order to understand the morphology of PBDTTT-EFT:PC71BM processed with different solvents, atomic force microscopy (AFM) analysis was performed. As shown in Fig. 4a–b, the root mean square (rms) surface roughness were 1.79 and 2.00 nm for the films processed with chlorobenzene/1,8-diiodooctane and toluene/NMP, respectively. Such surface low roughness implies that little or no phase separation exist for both solvent systems. To gain a deeper insight into the morphology of active layer, transmission electron microscopy (TEM) analysis was performed. As shown in Fig. S2 (supporting information), finely mixed polymer/fullerene morphologies were observed, which would be beneficial to the overall device performance [28]. It should be noted that the spin-coated films processed with toluene/NMP was much rougher (rms roughness ~4.53 nm; see supporting information Fig. S1) than that of blade-coated film, which suggests that blade coating may be beneficial for halogen-free system. Stability is usually a fundamental problem for many organic devices. OSCs may have morphological changes in the active layer heterojunction or in the interface with the electrode upon long-term operation [29], and studies indicate that the stability of the solar cell is
Fig. 6. J-V curves of the PBDTTT-EFT:PC71BM-based large-area OSCs processed with different solvents. Device D was processed with CB/DIO (interlayer: LiF). Device E was processed with toluene/NMP (interlayer: LiF). Device F was processed with toluene/NMP (interlayer: ZrAcac).
3.07% is attained. These findings offer a rational guideline for developing environmentally friendly large-area OSCs with high performance and long-term stability. 2. Experimental Organic solar cells have a structure of pre-patterned indium–tinoxide (ITO) glass/PEDOT:PSS/BHJ layer/interfacial layer/Al. The ITO coated glass substrate was first cleaned with acetone in an ultrasonic bath for 20 min and rinsed three times with deionized water to ensure that the glass was clean and then cleaned with a UV ozone cleaner for 20 min. For small-area devices, PEDOT:PSS layer was spin coated at 2500 rpm on a pre-cleaned ITO substrate for 40s and baked at 150 °C for 15 min. The active layer material of a bulk heterojunction organic solar cell used PBDTTT-EFT as the donor and PC71BM as the acceptor. Blade coating is used for the deposition of the active layer as it can achieve high material usage and is scalable to large-area OSC. PBDTTT-EFT and PC71BM powders are dissolved in chlorobenzene and toluene at 60 °C for 1 day. The solution concentrations of 5 mg:7.5 mg:500 μL were prepared. 1,8-diiodooctane (DIO, 3% v/v) and N-methyl-2-pyrrolidone (NMP, 3% v/v) are respectively added to the solution 2 h before deposition. The temperature of the blade substrate was set at 80 °C. During the blade coating process, hot air is simultaneously applied from the top using a blower with a diffusing showerhead to enhance the drying speed and uniformity. After coating, all PBDTTT-EFT:PC71BM layers were annealed at 100 °C for 10 min in nitrogen. For large-area devices, PEDOT:PSS is first diluted by Ethanol in a 1:2 vol ratio before blade coating. The blade coating method of the active layer is similar to that of small-area devices, but the uniformity is significantly improved by fine tuning the injection and the blade motion. The solution concentrations were prepared by 10 mg:15 mg:1000 μL with the addition of 3% v/v NMP. After blade coating, the films are baked at 150 °C for 15 min. A metal mask is used to define the effective area. We pattern the active layer by mechanical
Table 2 Summary of the photovoltaic properties of large-area devices with active area of 216 cm2. The values in parenthesis are for the best performing devices. Device a
D Ea Fb a b
Solvent
Interlayer
Voc [volt]
Jsc [mA cm−2]
FF [%]
PCE [%]
CB, DIO Toluene, NMP Toluene, NMP
LiF LiF ZrAcac
12.14 ± 0.36 (12.21) 12.52 ± 0.39 (12.99) 11.78 ± 0.13 (11.87)
0.81 ± 0.09 (0.83) 0.76 ± 0.02 (0.78) 0.81 ± 0.02 (0.85)
0.51 ± 0.02 (0.54) 0.47 ± 0.01 (0.49) 0.48 ± 0.07 (0.49)
5.16 ± 0.03 (5.20) 5.00 ± 0.01 (5.03) 5.12 ± 0.07 (5.27)
Average and standard deviation values were obtained based on 6 devices. Average and standard deviation values were obtained based on 5 devices. 4
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Table 3 Photovoltaic parameters of large-area air-processed OSCs (active area = 216 cm2) processed from halogen-free solvents. The values in parenthesis are for the best performing devices. Device
Active layer processing
Voc [volt]
Jsc [mA cm−2]
FF [%]
PCE [%]
Ga Ha
Annealed 100 °C Non-annealing
9.53 ± 1.42 (11.24) 12.01 ± 0.02 (12.31)
0.73 ± 0.09 (0.84) 0.57 ± 0.03 (0.58)
0.42 ± 0.01 (0.39) 0.40 ± 0.02 (0.39)
3.21 ± 0.43 (3.67) 2.71 ± 0.27 (2.83)
a
Average and standard deviation values were obtained based on 5 devices.
large-area devices processed with toluene/NMP, and the photovoltaic parameters were summarized in Table 2. The resulting devices exhibited a PCE of 5.03%, with an Voc of 12.99 V, a Jsc of 0.78 mA cm−2, and a FF of 0.49 (Table 2 and Fig. 6). It should be emphasized again that the device performance based on toluene/NMP (device E) was nearly comparable to that of the devices based on chlorobenzene/1,8diiodooctane (device D), as shown in Table 2 and Fig. 6. These results clearly indicate that the newly-developed halogen-free solvent system is applicable for large-area OSCs. Similar to those observed for small larea devices, the PCE could be further boosted to 5.27% by employing ZrAcac as the cathode interlayer (device F) (Table 2 and Fig. 6). Impressively, halogen-free solvents-based manufacturing process can also be performed under ambient conditions. So far, only very few studies have attempted to demonstrate air-processed OSCs based on halogen-free solvents [18,35]. The device structure of air-processed devices was glass substrate/ITO/PEDOT:PSS/PBDTTT-EFT:PC71BM/ ZrAcac/Al, whose active area was 214 cm2. Our results indicated that an appropriate thermal annealing treatment seems to be essential for high performance. As shown in Table 3 and Fig. 7, after thermal annealing at 100 °C for 10 min, the PCE of air-processed devices with active area of 216 cm2 reached 3.67% (device G), which is superior to the devices without thermal annealing (PCE = 2.83%; device H). This might arise from the fact that optimal morphologies are usually obtained upon thermal annealing [2]. It should be noted that the highest achievable efficiency of air-processed devices (device G) was still inferior to those of the inert atmosphere-processed devices (device F), suggesting that ambient air inevitably causes adverse effects on the optoelectronic properties of BHJ layer. Further investigations are our on-going efforts. It should be noted that this is the first demonstration of large-area air-processed OSCs based on halogen-free solvent. To the best of our knowledge, 3.67% PCE represents the highest efficiency ever reported for air-processed OSCs, regardless of processing solvent and active area, as shown in Supporting Information Table S1. The advantages of preparing devices in air is not only to speed up the process, but also to the large-area preparation of organic solar cells.
Fig. 7. J-V curves of air-processed OSCs (active area = 216 cm2) processed from halogen-free solvents. The active layer of device G was annealed at 100 °C for 10 min, while the active layer of device H was non-annealed.
improved in various ways, such as the use of an appropriate interlayer [30]. Lithium fluoride (LiF) has been widely used as the cathode interlayer for OSCs. However, LiF still presents some inadequacies, such as poor surface coverage, high susceptibility to atmospheric moisture, and insufficient thermal stability [31–33]. We thus examine the effectiveness of using zirconium acetylacetonate (ZrAcac; see Fig. 1a for the chemical structure) as the interlayer for halogen solvents-free devices. It should be mentioned again that ZrAcac layer was deposited by scalable blade-coating approach. Very encouragingly, the devices with ZrAcac achieved superior performance to those with LiF, with a PCE up to 11.09% (Table 1 and Fig. 2). The PCE improvement mainly came from relatively high FF (0.74; see Table 1 and Fig. 2), suggests that ZrAcac interlayer can afford superior charge selectivity at the cathode interface. The device stability was also examined under continuous operation (with 100 mW cm−2 AM1.5 light illumination) in ambient air, and the results were shown in Fig. 3. The devices processed with toluene/NMP (device B) exhibited superior stability than the devices processed with chlorobenzene/1,8-diiodooctane (device A), maintaining 50% of its initial efficiency after 5200 h of continuous operation. These results are consistent with previous findings and can be attributed to that fact that the presence of 1,8-diiodooctane can cause serious light instability of the active layer [34]. More importantly, the device stability was found to be further improved by replacing LiF with ZrAcac interlayer (device C), maintaining 88% of its initial efficiency after 3960 h of continuous operation. Having determined that high-performance small-area devices can be achieved by the combination of morphological manipulation of active layer and interfacial engineering, large-area devices with total active area of 216 cm2 were also fabricated by blade coating. The design and the schematic diagrams of the large‐area OSC are shown in Fig. 5. For the modules, the cells were connected in 16 series on 280 mm × 200 mm indium in oxide (ITO) glass. Each stripe has a width of 12 mm and a length of 175 mm, and 2 mm of the gap. The way to form a series connection is to connect the cathode of the cell on the left to the ITO of the cell on the right. Fig. 6 (a) shows the measured current J–V characteristics of the
4. Conclusion In summary, high-efficiency OSC based on PBDTTT-EFT was successfully fabricated by using blade coating in halogen-free toluene solution. This achievement may be able to replace the toxic solvents in the preparation of OSC to achieve environmental protection. The highest achievable PCE reaches 11.09%. The large-area solar cells with active area of 216 cm2 also deliver a promising photoelectric conversion efficiency of 5.03%. Remarkably, a PCE of 3.67% can be achieved by airprocessed halogen-free blade-coated OSCs. To the best of our knowledge, 3.67% represents the highest efficiency ever reported for airprocessed OSCs. More importantly, the resulting devices also exhibit good stability under continuous operation in ambient air. These results show that it is achievable to use halogen-free solvents to replace toxic solvents in the development of ambient-processed high-efficiency OSCs. This work offers a rational guideline for developing environmentally friendly large-area OSCs with high performance and long-term stability.
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Acknowledgements
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