DOI: 10.1002/asia.201501400
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Donor–Acceptor Systems
Enhancement of Open-Circuit Voltage by Using the 58-p Silylmethyl Fullerenes in Small-Molecule Organic Solar Cells Il Jeon+,[a] Cl¦ment Delacou+,[b] Takafumi Nakagawa,[a] and Yutaka Matsuo*[a] Abstract: The application of 58-p-1,4-bis(silylmethyl)[60]fullerenes, C60(CH2SiMe2Ph)(CH2SiMe2Ar) (Ar = Ph and 2-methoxylphenyl for SIMEF-1 and SIMEF-2, respectively), in small-molecule organic solar cells with a diketopyrrolopyrrole donor (3,6-bis[5-(benzofuran-2-yl)thiophen-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)2)) is demonstrated. With the 58-p-silylmethyl fullerene acceptor, SIMEF1, the devices showed the highest efficiency of 4.57 % with an average of 4.10 %. They manifested an improved open-
Introduction The development of organic solar cells (OSCs) has become increasingly important for their low-cost and eco-friendly nature. They are regarded as next-generation renewable energy sources with power conversion efficiencies (PCEs) reaching 10 % for a nontandem device.[1–4] For OSCs, the near-infrared absorption is considered to be a vital property that leads to a high PCE. To this end, copolymers that possess alternate electron-deficient and -rich units have readily been used as electron-donor species.[5–11] However in recent years, research into small-molecule OSCs (SMOSCs) has progressed greatly and their efficiencies are on par with their polymer-based counterparts.[12, 13] The improvement of SMOSCs mainly comes from the design of lowmolecular-weight organic molecules that give higher absorption coefficients, fast charge transport, and enhanced miscibility with the fullerene acceptors.[14–16] Among small-molecules,[17–24] Nguyen et al.[24] investigated diketopyrrolopyrrole (DPP), which initially showed a high field-effect mobility owing to good p–p stacking interactions, involving fused aromatic [a] I. Jeon,+ Dr. T. Nakagawa, Prof. Y. Matsuo Department of Chemistry, School of Science The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033 (Japan) E-mail: [email protected]
[b] C. Delacou+ Department of Mechanical Engineering, School of Engineering The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656 (Japan) [+] These authors contributed equally to this work. The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/asia.201501400. This manuscript is part of a special issue on energy conversion and storage. Click here to see the Table of Contents of the special issue. Chem. Asian J. 2016, 11, 1268 – 1272
circuit voltage (1.03 V) owing to the high-lying LUMO level of SIMEF-1, while maintaining a high short-circuit density (9.91 mA cm¢2) through controlling the crystallinity of DPP by thermal treatment. On the other hand, despite even higher open-circuit voltage (1.05 V), SIMEF-2-based devices showed lower performances of 3.53 %, owing to a low shortcircuit current density (8.33 mA cm¢2) and fill factor (0.40) arising from the asymmetric structure, which results in a lower mobility and immiscibility.
rings in a planar conjugated polymer. The DPP core in smallmolecules for OSCs also exhibited excellent results, for instance, a PCE of greater than 4 % was achieved in combination with phenyl-C71-butyric acid methyl ester (PC71BM).[25–27] However, there are only a few other examples of fullerene acceptors that show good performance.[28–30] It was previously reported that higher efficiencies could be obtained by using 1,4-bis(dimethylphenylsilylmethyl)[60]fullerene (SIMEF-1) acceptors with a tetrabenzoporphyrin donor owing to the high-lying LUMO level of SIMEF-1 and outstanding packing; these generate a high open-circuit voltage (VOC) and improve the short-circuit current (JSC), respectively.[31, 32] SIMEF-1 has a columnar fullerene core array, which brings about high electron mobility and undergoes thermal crystallization for good phase separation with electron-donating materials.[33] Dimethyl(ortho-anisyl)silylmethyl(dimethylphenylsilylmethyl)[60]fullerene (SIMEF-2) is another silylmethyl derivative that was found to give an even higher VOC than that of SIMEF1 without a decrease in JSC in an inverted OSC, in which poly(3-hexylthiophene) (P3HT) was used as a donor.[34] SIMEF-2 has the advantage of having a methoxy group (compared with SIMEF-1), which makes the purification easier. SIMEF-2 was reported to have a LUMO level of ¢3.72 eV, which was higher than that of SIMEF-1 (¢3.74 eV). However, the electron mobility of SIMEF-2 was measured to be 3 Õ 10¢3 cm2 V¢1 s¢1 from a space-charge limited current (SCLC) model; this value was lower than that of SIMEF-1 (8 Õ 10¢3 cm2 V¢1 s¢1).[31, 35] Herein, we fabricated SMOSCs by using SIMEF derivatives and a DPP-based small-molecule electron donor. SIMEF derivatives with small-molecules in a bulk heterojunction have not been reported so far, except for tetrabenzoporphyrin-based OSCs. The DPP:SIMEF-1 solar cell devices gave the highest recorded PCE of 4.57 % with an average PCE of 4.10 %, owing to
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Full Paper an increase in VOC ; these values are comparable to the highest PCE of 4.38 % and average PCE of 3.96 % recorded for DPP:PC61BM control devices. In the case of DPP:SIMEF-2-based devices, the PCE was 3.53 %. Despite a high VOC, difficulty in controlling the growth of DPP crystallinity led to JSC values that were lower than those of devices with SIMEF-1. In addition, the intrinsically low mobility of SIMEF-2 and immiscibility with DPP led to a low fill factor (FF).
Results and Discussion Device Performance and Comparison
Table 1. Photovoltaic parameters of the devices measured under standard one-sun conditions (AM 1.5G, 100 mW cm¢2).
Entry 1 2 3 4 5 6 7 8 9 10
Acceptor
Spin ratio [rpm]
4500 SIMEF-1
SIMEF-2 PC61BM PCBNB[c]
1500 3000 6000 4500 4500 3000
Annealing
VOC [V]
JSC [mA cm¢2]
FF
PCE [%] (average)
SVA[a] TA 90 8C[b] TA 110 8C[b] TA 130 8C[b]
1.05 1.03 1.04 1.03 1.02 1.02 1.03 1.05 0.92 0.94
6.67 9.91 7.01 6.14 7.28 9.29 9.98 8.33 10.75 9.67
0.44 0.45 0.46 0.45 0.42 0.44 0.44 0.40 0.44 0.48
3.08 4.57 [4.10 0.41] 3.40 2.88 3.11 4.15 4.48 3.53 [3.29 0.21] 4.38 [3.96 0.25] 4.42
TA 90 8C[b] TA 90 8C[b] TA 100 8C[b] TA 100 8C[b]
[a] SVA for devices placed inside a saturated CH2Cl2 vapor container for 2 min. [b] TA
SMOSC fabrication was carried out by using the bulk at each temperature for 10 min in a nitrogen atmosphere.[c] PCBNB = [6,6]-phenyl-C61butyric acid n-butyl ester. heterojunction of DPP and SIMEF derivatives. Figure 1 shows the molecular structures and energy levels of DPP and SIMEF derivatives. The active layers extraction. We studied the DPP:SIMEF-1-based devices accordwere prepared from a 20 mg mL¢1 solution with a donor to acing to their active-layer thickness (Table 1, entries 1, 5–7). A ceptor ratio of 3:2 in CHCl3, which was a typical solvent for spinning speed of 4500 rpm with a thickness of 90 nm gave DPP.[27, 36] Postannealing treatment is another vital factor in the best performance. Thus, the rest of the devices were fabricated under these conditions. With the optimized spinning speed of 4500 rpm and annealing temperature of 90 8C, the average PCE of DPP:SIMEF-1 was 4.10 % (Table 1, entry 2). Compared with the PCE of the DPP:PCBM reference device (3.96 %; Table 1, entry 9), the DPP:SIMEF-1 device performed equally well. This was due to the higher VOC value, arising from the high-lying LUMO energy level, as expected (Figure 1). However, DPP:SIMEF-1 devices showed marginally lower JSC values, which implied that their charge extraction in DPP:SIMEF was not as good as that in DPP:PCBM. Based on previous studies of polymer:fullerene solar cells,[40, 41] unfavorable packing between SIMEF and DPP is speculated to be the cause of low JSC value. Relative to the DPP:PCBNB-based devices, the DPP:SIMEF-1-based device performed better only as a result of a higher VOC value (Table 1, Figure 1. Structures and energy level diagrams of DPP, PC61BM, SIMEF-1, and entries 1 and 10). SIMEF-2. According to the J–V curves in Figure 2 and the photovoltaic parameters in Table 1, entry 8, SIMEF-2-based devices also showed an enhancement in VOC with respect to that of the PCBM-based reference devices. This is again attributed to the SMOSC fabrication because it induces the crystallinity of both high-lying LUMO energy level. The VOC value of the DPP:SIMEFdonor and acceptor species; this greatly affects photovoltaic 2 devices was 0.02 V higher than those of the DPP:SIMEF-1 deperformance. Two established postannealing methods, solvent vices. This is because SIMEF-2 possesses an even higher LUMO vapor annealing (SVA) and thermal annealing (TA), were apenergy (¢3.72 eV) than that of SIMEF-1 (¢3.74 eV). However, plied to the DPP:SIMEF-based devices.[37–39] TA produced SIMEF-2-based devices exhibited a lower PCE (3.53 %) than that a higher efficiency and was more reproducible (Table 1, enof the SIMEF-1-based devices (4.57 %). The is because the tries 1 and 2). Therefore, the effect of temperature on TA was SIMEF-1-based devices gave lower values of both JSC and FF. studied. Annealing temperatures of 90, 110, and 130 8C were SIMEF-based devices, in general, showed a concomitant reductested, and 90 8C gave the highest efficiency (Table 1, ention in JSC, but the decrease in SIMEF-2-based devices was tries 2–4). The VOC and FF values did not change with annealmore severe. A decrease in JSC means that the interface being temperature, but JSC was affected greatly. This is due to retween donor and acceptor domains is reduced, which is affectduction in donor to acceptor interfaces, which are directly reed by the increase in crystallinity of the domains. Nevertheless, lated to the amount of charge extracted. Small-molecules, an increase crystallinity induces an increase in FF in general. such as DPP, are known to have a relatively low mobility, Thus, we suspect the low mobility and immiscibility of SIMEF-2 which means that the active layer thickness is critical to charge could be the reasons for low JSC and FF values. Chem. Asian J. 2016, 11, 1268 – 1272
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Figure 2. J–V curves of DPP:PC61BM (black), DPP:SIMEF-1 (red), and DPP:SIMEF-2 (blue) OSCs under light (solid) and dark (dashed) conditions.
Optical Property and Morphology Analysis Figure 3 shows the UV/Vis absorption spectra of DPP:SIMEF and DPP:PCBM films before and after TA. Redshifts in the absorption spectra appeared in all of the films after annealing. This is a typical characteristic of DPP films prepared in CHCl3 that can be ascribed to J aggregation and growth of DPP crystallites.[36]
Figure 4. Thin-film XRD spectra of DPP:PC61BM, DPP:SIMEF-1, and DPP:SIMEF-2 thin films.
asymmetry. This makes SIMEF-2 pack more loosely with each other, which encourages DPP crystallization. Because an increase in the DPP domain will decrease the interface between donor and acceptor, this explains why the SIMEF-2-based devices showed lower JSC values than those of the SIMEF-1-based devices. AFM was carried out to investigate the effect of thermal treatment on morphological domains of DPP:SIMEF films (Figure 5). AFM images of SIMEF-based photoactive thin films show bright DPP donor aggregations with increasing TA temperatures. When considering that the crystallization temperature of SIMEF is 150 8C, it is clear that the observed crystals are DPP.[31] From the images in Figure 5 b and f, we can see that 90 8C annealing produced the best morphology with the highest miscibility for both SIMEF-1 and SIMEF-2 blend films. This agrees with the best PCE obtained from 90 8C annealing (Table 1, entry 2). The fact that SIMEF-2 blends showed much stronger aggregation of DPP corroborates the XRD characterization shown in Figure 4, as well as our explanation for the low JSC value for the SIMEF-2-based devices.[44]
Conclusion We have applied two types of 58-p SIMEF derivatives, namely, SIMEF-1 and SIMEF-2, each with a small-molecule donor: DPP. Figure 3. UV/Vis absorption spectra of DPP:PC61BM (black), DPP:SIMEF1 (red), and DPP:SIMEF-2 (blue) thin films before (dashed) and after (solid) TA at 90 8C for 10 min.
Aggregation characteristics of the films are related to crystallization during the annealing process and can be observed by using XRD.[27] The height of the spectra taken from the active layers prepared on glass substrates under the same conditions as those used for the optimized devices indicate the crystallinity of DPP (Figure 4). It means that photoactive films composed of DPP and SIMEF-2 have similar crystallinity to those of DPP and PCBM. By comparing SIMEF-1 with SIMEF-2, we can see that DPP crystalizes better with SIMEF-2. It is known that the structures of acceptor species affect the crystallinity of DPP greatly.[42, 43] SIMEF-2 has a methoxy group that brings about Chem. Asian J. 2016, 11, 1268 – 1272
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Figure 5. AFM images and root-mean-squared roughness values of bulk heterojunction thin films of DPP:SIMEF-1 and DPP:SIMEF-2 prepared from solutions in chloroform for nonannealed samples (a and e) and samples annealed at 90 (b and f), 110 (c and g), and 130 8C (d and h).
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Full Paper We obtained the highest PCE values of 4.57 and 3.53 %, respectively. They both possessed a high VOC of around 1 eV owing to the high-lying LUMO. However, they exhibited a reduction in JSC owing to unfavorable packing within the photoactive layer. Overall, compared with the reference DPP:PC61BM devices, which gave the highest PCE of 4.38 % and an average PCE of 3.96 %, DPP:SIMEF-1 devices showed comparable performances, whereas DPP:SIMEF-2 devices showed lower performances. The SIMEF-2 devices had exceptionally low JSC and FF values on account of high DPP crystallinity, intrinsic low mobility, and poor miscibility with DPP molecules, as corroborated by XRD data (Figure 4) and AFM images (Figure 5). In summary, these findings provide vital insights into the interactions of SIMEF fullerene derivatives with DPP small-molecules and their solar cell applications; the correlation of JSC reduction and VOC enhancement has been stressed herein. We anticipate that this work will contribute to research into SMOSCs.
Device Characterization Current–voltage (J–V) characteristics were measured by using a software-controlled source meter (Keithley 2400) under dark conditions and one sun AM 1.5G simulated sunlight irradiation (100 mW cm¢2) by using a solar simulator (EMS-35AAA, Ushio Spax Inc.), which was calibrated by using a silicon diode (BS-520BK, Bunkokeiki).
Film Characterization UV/Vis absorption spectra were measured on a JASCO V-670 spectrometer (Nihon bunko). AFM images were recorded by using a Bruker Multimode atomic force microscope operating in tapping mode (Si probes, nominal frequency 70 kHz). Out-of-plane XRD was carried out on a Rigaku Smartlab diffractometer with CuKa radiation operating at a power of 9 kW (45 kV, 200 mA). The diffraction pattern of each sample was recorded between 2q = 2 and 148 at 0.58 increments, the durations of which were 3 s.
Acknowledgements Experimental Section Material and Methods 3,6-Bis[5-(benzofuran-2-yl)thiophen-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)2) was purchased from Lumtec and further purified by column chromatography on silica gel (WakogelÒ 60N) by using chloroform as the eluent. SIMEF derivatives were synthesized and purified by following our previously reported methods.[31, 35] SIMEF-2 was recrystallized from toluene/MeOH and/ or chlorobenzene/MeOH.
Device Fabrication A device was fabricated with the following architecture: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS)/DPP(TBFu)2 :fullerene derivative/LiF/Al. First, patterned ITO substrates (155 nm, 9 W/sq.) were sonicated in acetone for 20 min followed by two additional 20 min sonication cycles in isopropanol. Next, the substrates were dried under a stream of nitrogen and then subjected to 30 min UV/O3 treatment. PEDOT:PSS (Clevios AI4083) was spin-coated onto the clean ITO substrates at a rate of 3000 rpm for 45 s. Drying of the PEDOT:PSS films was first achieved in air at 120 8C for 10 min and then in a nitrogen-filled glove box at 130 8C for an additional 5 min. Active layers were then deposited by spin-coating at a rate of 3000 rpm for 60 s. The optimized donor/acceptor ratio (w/w) for DPP(TBFu)2/PC61BM and DPP(TBFu)2/SIMEF was 3:2 in a total concentration of 20 mg mL¢1 in CHCl3. The active-layer thickness was approximately 90 nm, as measured by using a step profiler. For the active layers after spincoating, either TA or SVA was applied. TA was performed by placing the samples on a hot plate at a temperature of 90 8C under nitrogen. SVA was performed by placing one substrate at a time in a sealed vessel saturated with CH2Cl2 for 2 min. Following the annealing process, the substrates were placed in an evaporator chamber, in which a 0.8 nm layer of LiF was first deposited followed by a 100 nm thick layer of Al. The pressure of the evaporation chamber never exceeded 5 Õ 10¢4 Pa during deposition. Devices were sealed in a nitrogen-rich environment by using a UV-curable epoxy before measuring the photovoltaic characteristics. Chem. Asian J. 2016, 11, 1268 – 1272
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This work was supported by a Grants-in-Aid for Scientific Research (15H02219). Part of this work was supported by the Strategic Promotion of Innovative Research and Development, Japan Science and Technology Agency (JST). I.J. appreciates the Japan Student Services Organization and the Japan Society for the Promotion of Science. Keywords: donor–acceptor systems · fullerenes · photochemistry · solar cells
electrochemistry
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Donor–Acceptor Systems
Enhancement of Open-Circuit Voltage by Using the 58-p Silylmethyl Fullerenes in Small-Molecule Organic Solar Cells Il Jeon+,[a] Cl¦ment Delacou+,[b] Takafumi Nakagawa,[a] and Yutaka Matsuo*[a] Abstract: The application of 58-p-1,4-bis(silylmethyl)[60]fullerenes, C60(CH2SiMe2Ph)(CH2SiMe2Ar) (Ar = Ph and 2-methoxylphenyl for SIMEF-1 and SIMEF-2, respectively), in small-molecule organic solar cells with a diketopyrrolopyrrole donor (3,6-bis[5-(benzofuran-2-yl)thiophen-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)2)) is demonstrated. With the 58-p-silylmethyl fullerene acceptor, SIMEF1, the devices showed the highest efficiency of 4.57 % with an average of 4.10 %. They manifested an improved open-
Introduction The development of organic solar cells (OSCs) has become increasingly important for their low-cost and eco-friendly nature. They are regarded as next-generation renewable energy sources with power conversion efficiencies (PCEs) reaching 10 % for a nontandem device.[1–4] For OSCs, the near-infrared absorption is considered to be a vital property that leads to a high PCE. To this end, copolymers that possess alternate electron-deficient and -rich units have readily been used as electron-donor species.[5–11] However in recent years, research into small-molecule OSCs (SMOSCs) has progressed greatly and their efficiencies are on par with their polymer-based counterparts.[12, 13] The improvement of SMOSCs mainly comes from the design of lowmolecular-weight organic molecules that give higher absorption coefficients, fast charge transport, and enhanced miscibility with the fullerene acceptors.[14–16] Among small-molecules,[17–24] Nguyen et al.[24] investigated diketopyrrolopyrrole (DPP), which initially showed a high field-effect mobility owing to good p–p stacking interactions, involving fused aromatic [a] I. Jeon,+ Dr. T. Nakagawa, Prof. Y. Matsuo Department of Chemistry, School of Science The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033 (Japan) E-mail: [email protected]
[b] C. Delacou+ Department of Mechanical Engineering, School of Engineering The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656 (Japan) [+] These authors contributed equally to this work. The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/asia.201501400. This manuscript is part of a special issue on energy conversion and storage. Click here to see the Table of Contents of the special issue. Chem. Asian J. 2016, 11, 1268 – 1272
circuit voltage (1.03 V) owing to the high-lying LUMO level of SIMEF-1, while maintaining a high short-circuit density (9.91 mA cm¢2) through controlling the crystallinity of DPP by thermal treatment. On the other hand, despite even higher open-circuit voltage (1.05 V), SIMEF-2-based devices showed lower performances of 3.53 %, owing to a low shortcircuit current density (8.33 mA cm¢2) and fill factor (0.40) arising from the asymmetric structure, which results in a lower mobility and immiscibility.
rings in a planar conjugated polymer. The DPP core in smallmolecules for OSCs also exhibited excellent results, for instance, a PCE of greater than 4 % was achieved in combination with phenyl-C71-butyric acid methyl ester (PC71BM).[25–27] However, there are only a few other examples of fullerene acceptors that show good performance.[28–30] It was previously reported that higher efficiencies could be obtained by using 1,4-bis(dimethylphenylsilylmethyl)[60]fullerene (SIMEF-1) acceptors with a tetrabenzoporphyrin donor owing to the high-lying LUMO level of SIMEF-1 and outstanding packing; these generate a high open-circuit voltage (VOC) and improve the short-circuit current (JSC), respectively.[31, 32] SIMEF-1 has a columnar fullerene core array, which brings about high electron mobility and undergoes thermal crystallization for good phase separation with electron-donating materials.[33] Dimethyl(ortho-anisyl)silylmethyl(dimethylphenylsilylmethyl)[60]fullerene (SIMEF-2) is another silylmethyl derivative that was found to give an even higher VOC than that of SIMEF1 without a decrease in JSC in an inverted OSC, in which poly(3-hexylthiophene) (P3HT) was used as a donor.[34] SIMEF-2 has the advantage of having a methoxy group (compared with SIMEF-1), which makes the purification easier. SIMEF-2 was reported to have a LUMO level of ¢3.72 eV, which was higher than that of SIMEF-1 (¢3.74 eV). However, the electron mobility of SIMEF-2 was measured to be 3 Õ 10¢3 cm2 V¢1 s¢1 from a space-charge limited current (SCLC) model; this value was lower than that of SIMEF-1 (8 Õ 10¢3 cm2 V¢1 s¢1).[31, 35] Herein, we fabricated SMOSCs by using SIMEF derivatives and a DPP-based small-molecule electron donor. SIMEF derivatives with small-molecules in a bulk heterojunction have not been reported so far, except for tetrabenzoporphyrin-based OSCs. The DPP:SIMEF-1 solar cell devices gave the highest recorded PCE of 4.57 % with an average PCE of 4.10 %, owing to
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Full Paper an increase in VOC ; these values are comparable to the highest PCE of 4.38 % and average PCE of 3.96 % recorded for DPP:PC61BM control devices. In the case of DPP:SIMEF-2-based devices, the PCE was 3.53 %. Despite a high VOC, difficulty in controlling the growth of DPP crystallinity led to JSC values that were lower than those of devices with SIMEF-1. In addition, the intrinsically low mobility of SIMEF-2 and immiscibility with DPP led to a low fill factor (FF).
Results and Discussion Device Performance and Comparison
Table 1. Photovoltaic parameters of the devices measured under standard one-sun conditions (AM 1.5G, 100 mW cm¢2).
Entry 1 2 3 4 5 6 7 8 9 10
Acceptor
Spin ratio [rpm]
4500 SIMEF-1
SIMEF-2 PC61BM PCBNB[c]
1500 3000 6000 4500 4500 3000
Annealing
VOC [V]
JSC [mA cm¢2]
FF
PCE [%] (average)
SVA[a] TA 90 8C[b] TA 110 8C[b] TA 130 8C[b]
1.05 1.03 1.04 1.03 1.02 1.02 1.03 1.05 0.92 0.94
6.67 9.91 7.01 6.14 7.28 9.29 9.98 8.33 10.75 9.67
0.44 0.45 0.46 0.45 0.42 0.44 0.44 0.40 0.44 0.48
3.08 4.57 [4.10 0.41] 3.40 2.88 3.11 4.15 4.48 3.53 [3.29 0.21] 4.38 [3.96 0.25] 4.42
TA 90 8C[b] TA 90 8C[b] TA 100 8C[b] TA 100 8C[b]
[a] SVA for devices placed inside a saturated CH2Cl2 vapor container for 2 min. [b] TA
SMOSC fabrication was carried out by using the bulk at each temperature for 10 min in a nitrogen atmosphere.[c] PCBNB = [6,6]-phenyl-C61butyric acid n-butyl ester. heterojunction of DPP and SIMEF derivatives. Figure 1 shows the molecular structures and energy levels of DPP and SIMEF derivatives. The active layers extraction. We studied the DPP:SIMEF-1-based devices accordwere prepared from a 20 mg mL¢1 solution with a donor to acing to their active-layer thickness (Table 1, entries 1, 5–7). A ceptor ratio of 3:2 in CHCl3, which was a typical solvent for spinning speed of 4500 rpm with a thickness of 90 nm gave DPP.[27, 36] Postannealing treatment is another vital factor in the best performance. Thus, the rest of the devices were fabricated under these conditions. With the optimized spinning speed of 4500 rpm and annealing temperature of 90 8C, the average PCE of DPP:SIMEF-1 was 4.10 % (Table 1, entry 2). Compared with the PCE of the DPP:PCBM reference device (3.96 %; Table 1, entry 9), the DPP:SIMEF-1 device performed equally well. This was due to the higher VOC value, arising from the high-lying LUMO energy level, as expected (Figure 1). However, DPP:SIMEF-1 devices showed marginally lower JSC values, which implied that their charge extraction in DPP:SIMEF was not as good as that in DPP:PCBM. Based on previous studies of polymer:fullerene solar cells,[40, 41] unfavorable packing between SIMEF and DPP is speculated to be the cause of low JSC value. Relative to the DPP:PCBNB-based devices, the DPP:SIMEF-1-based device performed better only as a result of a higher VOC value (Table 1, Figure 1. Structures and energy level diagrams of DPP, PC61BM, SIMEF-1, and entries 1 and 10). SIMEF-2. According to the J–V curves in Figure 2 and the photovoltaic parameters in Table 1, entry 8, SIMEF-2-based devices also showed an enhancement in VOC with respect to that of the PCBM-based reference devices. This is again attributed to the SMOSC fabrication because it induces the crystallinity of both high-lying LUMO energy level. The VOC value of the DPP:SIMEFdonor and acceptor species; this greatly affects photovoltaic 2 devices was 0.02 V higher than those of the DPP:SIMEF-1 deperformance. Two established postannealing methods, solvent vices. This is because SIMEF-2 possesses an even higher LUMO vapor annealing (SVA) and thermal annealing (TA), were apenergy (¢3.72 eV) than that of SIMEF-1 (¢3.74 eV). However, plied to the DPP:SIMEF-based devices.[37–39] TA produced SIMEF-2-based devices exhibited a lower PCE (3.53 %) than that a higher efficiency and was more reproducible (Table 1, enof the SIMEF-1-based devices (4.57 %). The is because the tries 1 and 2). Therefore, the effect of temperature on TA was SIMEF-1-based devices gave lower values of both JSC and FF. studied. Annealing temperatures of 90, 110, and 130 8C were SIMEF-based devices, in general, showed a concomitant reductested, and 90 8C gave the highest efficiency (Table 1, ention in JSC, but the decrease in SIMEF-2-based devices was tries 2–4). The VOC and FF values did not change with annealmore severe. A decrease in JSC means that the interface being temperature, but JSC was affected greatly. This is due to retween donor and acceptor domains is reduced, which is affectduction in donor to acceptor interfaces, which are directly reed by the increase in crystallinity of the domains. Nevertheless, lated to the amount of charge extracted. Small-molecules, an increase crystallinity induces an increase in FF in general. such as DPP, are known to have a relatively low mobility, Thus, we suspect the low mobility and immiscibility of SIMEF-2 which means that the active layer thickness is critical to charge could be the reasons for low JSC and FF values. Chem. Asian J. 2016, 11, 1268 – 1272
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Figure 2. J–V curves of DPP:PC61BM (black), DPP:SIMEF-1 (red), and DPP:SIMEF-2 (blue) OSCs under light (solid) and dark (dashed) conditions.
Optical Property and Morphology Analysis Figure 3 shows the UV/Vis absorption spectra of DPP:SIMEF and DPP:PCBM films before and after TA. Redshifts in the absorption spectra appeared in all of the films after annealing. This is a typical characteristic of DPP films prepared in CHCl3 that can be ascribed to J aggregation and growth of DPP crystallites.[36]
Figure 4. Thin-film XRD spectra of DPP:PC61BM, DPP:SIMEF-1, and DPP:SIMEF-2 thin films.
asymmetry. This makes SIMEF-2 pack more loosely with each other, which encourages DPP crystallization. Because an increase in the DPP domain will decrease the interface between donor and acceptor, this explains why the SIMEF-2-based devices showed lower JSC values than those of the SIMEF-1-based devices. AFM was carried out to investigate the effect of thermal treatment on morphological domains of DPP:SIMEF films (Figure 5). AFM images of SIMEF-based photoactive thin films show bright DPP donor aggregations with increasing TA temperatures. When considering that the crystallization temperature of SIMEF is 150 8C, it is clear that the observed crystals are DPP.[31] From the images in Figure 5 b and f, we can see that 90 8C annealing produced the best morphology with the highest miscibility for both SIMEF-1 and SIMEF-2 blend films. This agrees with the best PCE obtained from 90 8C annealing (Table 1, entry 2). The fact that SIMEF-2 blends showed much stronger aggregation of DPP corroborates the XRD characterization shown in Figure 4, as well as our explanation for the low JSC value for the SIMEF-2-based devices.[44]
Conclusion We have applied two types of 58-p SIMEF derivatives, namely, SIMEF-1 and SIMEF-2, each with a small-molecule donor: DPP. Figure 3. UV/Vis absorption spectra of DPP:PC61BM (black), DPP:SIMEF1 (red), and DPP:SIMEF-2 (blue) thin films before (dashed) and after (solid) TA at 90 8C for 10 min.
Aggregation characteristics of the films are related to crystallization during the annealing process and can be observed by using XRD.[27] The height of the spectra taken from the active layers prepared on glass substrates under the same conditions as those used for the optimized devices indicate the crystallinity of DPP (Figure 4). It means that photoactive films composed of DPP and SIMEF-2 have similar crystallinity to those of DPP and PCBM. By comparing SIMEF-1 with SIMEF-2, we can see that DPP crystalizes better with SIMEF-2. It is known that the structures of acceptor species affect the crystallinity of DPP greatly.[42, 43] SIMEF-2 has a methoxy group that brings about Chem. Asian J. 2016, 11, 1268 – 1272
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Figure 5. AFM images and root-mean-squared roughness values of bulk heterojunction thin films of DPP:SIMEF-1 and DPP:SIMEF-2 prepared from solutions in chloroform for nonannealed samples (a and e) and samples annealed at 90 (b and f), 110 (c and g), and 130 8C (d and h).
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Full Paper We obtained the highest PCE values of 4.57 and 3.53 %, respectively. They both possessed a high VOC of around 1 eV owing to the high-lying LUMO. However, they exhibited a reduction in JSC owing to unfavorable packing within the photoactive layer. Overall, compared with the reference DPP:PC61BM devices, which gave the highest PCE of 4.38 % and an average PCE of 3.96 %, DPP:SIMEF-1 devices showed comparable performances, whereas DPP:SIMEF-2 devices showed lower performances. The SIMEF-2 devices had exceptionally low JSC and FF values on account of high DPP crystallinity, intrinsic low mobility, and poor miscibility with DPP molecules, as corroborated by XRD data (Figure 4) and AFM images (Figure 5). In summary, these findings provide vital insights into the interactions of SIMEF fullerene derivatives with DPP small-molecules and their solar cell applications; the correlation of JSC reduction and VOC enhancement has been stressed herein. We anticipate that this work will contribute to research into SMOSCs.
Device Characterization Current–voltage (J–V) characteristics were measured by using a software-controlled source meter (Keithley 2400) under dark conditions and one sun AM 1.5G simulated sunlight irradiation (100 mW cm¢2) by using a solar simulator (EMS-35AAA, Ushio Spax Inc.), which was calibrated by using a silicon diode (BS-520BK, Bunkokeiki).
Film Characterization UV/Vis absorption spectra were measured on a JASCO V-670 spectrometer (Nihon bunko). AFM images were recorded by using a Bruker Multimode atomic force microscope operating in tapping mode (Si probes, nominal frequency 70 kHz). Out-of-plane XRD was carried out on a Rigaku Smartlab diffractometer with CuKa radiation operating at a power of 9 kW (45 kV, 200 mA). The diffraction pattern of each sample was recorded between 2q = 2 and 148 at 0.58 increments, the durations of which were 3 s.
Acknowledgements Experimental Section Material and Methods 3,6-Bis[5-(benzofuran-2-yl)thiophen-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)2) was purchased from Lumtec and further purified by column chromatography on silica gel (WakogelÒ 60N) by using chloroform as the eluent. SIMEF derivatives were synthesized and purified by following our previously reported methods.[31, 35] SIMEF-2 was recrystallized from toluene/MeOH and/ or chlorobenzene/MeOH.
Device Fabrication A device was fabricated with the following architecture: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS)/DPP(TBFu)2 :fullerene derivative/LiF/Al. First, patterned ITO substrates (155 nm, 9 W/sq.) were sonicated in acetone for 20 min followed by two additional 20 min sonication cycles in isopropanol. Next, the substrates were dried under a stream of nitrogen and then subjected to 30 min UV/O3 treatment. PEDOT:PSS (Clevios AI4083) was spin-coated onto the clean ITO substrates at a rate of 3000 rpm for 45 s. Drying of the PEDOT:PSS films was first achieved in air at 120 8C for 10 min and then in a nitrogen-filled glove box at 130 8C for an additional 5 min. Active layers were then deposited by spin-coating at a rate of 3000 rpm for 60 s. The optimized donor/acceptor ratio (w/w) for DPP(TBFu)2/PC61BM and DPP(TBFu)2/SIMEF was 3:2 in a total concentration of 20 mg mL¢1 in CHCl3. The active-layer thickness was approximately 90 nm, as measured by using a step profiler. For the active layers after spincoating, either TA or SVA was applied. TA was performed by placing the samples on a hot plate at a temperature of 90 8C under nitrogen. SVA was performed by placing one substrate at a time in a sealed vessel saturated with CH2Cl2 for 2 min. Following the annealing process, the substrates were placed in an evaporator chamber, in which a 0.8 nm layer of LiF was first deposited followed by a 100 nm thick layer of Al. The pressure of the evaporation chamber never exceeded 5 Õ 10¢4 Pa during deposition. Devices were sealed in a nitrogen-rich environment by using a UV-curable epoxy before measuring the photovoltaic characteristics. Chem. Asian J. 2016, 11, 1268 – 1272
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This work was supported by a Grants-in-Aid for Scientific Research (15H02219). Part of this work was supported by the Strategic Promotion of Innovative Research and Development, Japan Science and Technology Agency (JST). I.J. appreciates the Japan Student Services Organization and the Japan Society for the Promotion of Science. Keywords: donor–acceptor systems · fullerenes · photochemistry · solar cells
electrochemistry
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