Efficient Ternary Organic Photovoltaics Using Two-Conjugated

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C: Energy Conversion and Storage; Energy and Charge Transport

Efficient Ternary Organic Photovoltaics Using TwoConjugated Polymers and a Nonfullerene Acceptor with Complementary Absorption and Cascade Energy-Level Alignment Yang-Yen Yu, Tzung-Wei Tsai, and ChihPing Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08595 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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The Journal of Physical Chemistry

Efficient Ternary Organic Photovoltaics using Two-Conjugated Polymers and a Nonfullerene Acceptor with Complementary Absorption and Cascade Energy-Level Alignment Yang-Yen Yu1,2*, Tzung-Wei Tsai1 and Chih-Ping Chen1* 1

Department of Materials Engineering, Ming Chi University of Technology, New Taipei City

243, Taiwan 2

Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan City

33302, Taiwan E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment

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Abstract

The earliest research concerned a device based on a donor-capped fullerene and nonfullerene acceptor or two planar donors and a small molecule acceptor. However, the absorption range of the device in visible light is insufficiently wide; thus, its current density is not very high. The device based on two planar donors and a small molecule acceptor is more likely to form defects and a depletion region in the active layer, resulting in a lower fill factor value. Therefore, many studies have focused on using planar and highly crystalline materials as donors with a nonfullerene acceptor because the combination of planar and highly crystalline materials can form a superior surface morphology to achieve highly efficient OPVs. In this study, the amorphous-like polymer benzodithiophene-alt-fluorobenzotriazole copolymer (J51) and the highly crystalline polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)

benzo

[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) were incorporated as two donors in the active layer, with the nonfullerene material 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6b′]dithiophene (ITIC) as an acceptor. Because of the complementary light harvesting and cascade energy level of these materials, we observed improvements in light harvesting, carrier transport, and thereby device performance. We obtained the highest efficiency of 9.06% for ternary OPVs, which outperformed the original J51:ITIC and PBDB-T:ITIC binary systems.

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Introduction

Organic photovoltaics (OPVs) have been extensively studied for the past 20 years because of the following advantages: use of the wet solution process for their synthesis, low cost, light weight, transparency, and flexibility.1-5 A fullerene acceptor, such as [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), has a high electron mobility6 and is thus frequently used as an active layer acceptor; the efficiency of device is as high as 10%.7-8 Compared with a fullerene acceptor, synthesizing and purifying nonfullerene materials and adjusting their material energy levels is easier. In addition, nonfullerene materials have a wide controllable absorption range for light harvesting.7, 9-12 Therefore, nonfullerene OPVs have begun to attract researchers’ attention.13 14Because binary OPVs might limit the light absorption range and intensity, a third material was introduced into the binary system to broaden the range of light absorption which can effectively increase the rates of charge dissociation and transportation as well as device stability .15-18 The ternary systems of OPVs can consist of multiple donor components19-23 or multiple acceptor components.24-27 Typically, the morphology of a ternary blend is determined by the miscibility of blend compositions and varies according to the system. The addition of a third component can effectively improve the performance of the original binary system because of improvements in the molecular arrangement, solution compatibility, visible spectrum absorption range, and blend film morphology. 28-30

Many studies on the binary system of nonfullerene solar cells have shown that the use of

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fluorine-containing donors is helpful for OPVs because fluorine-containing donors can stabilize the polymer main chain to reduce molecular vibrations and enhance cell efficiency.3132

Zhang et al. used the fluorine-containing planar polymer benzodithiophene-alt-

fluorobenzotriazole copolymer (J51, a band gap of 1.91 eV) as the active layer donor and two different nonfullerene N2200 and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) as acceptors. The highest efficiency of solar cells based on J51:N2200 and J51:ITIC was 8.27%33 and 9.26%,34 respectively. Hou and coworkers reported that the highly crystalline donor

materials

poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-

b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)

benzo

[1′,2′-c:4′,5′-

c′]dithiophene-4,8-dione))] (PBDB-T) and ITIC could achieve an efficiency of up to 11.21%.35 Their study highlighted that compared with an amorphous-like polymer (such as PTB7-Th), a high crystalline polymer might have a more satisfactory molecular alignment in the active layer. Furthermore, they studied the ternary OPV composition of PBDB-T, IT-M (an ITIC derivative), and Bis[70]PCBM. This system improved light harvesting and film formation (an increase in the degree of material mixing) and demonstrated a performance of 12.2%.36 Liang et al. incorporated PffBT4T-2OD as a morphology modifier in the PTB7-Th and ITIC binary system. Through the separation of crystalline and amorphous phases, the best blend morphology, which produced an efficiency of 8.22%, could be obtained.37 Yang et al. added a

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highly crystalline small molecule material, DR3TSBDT, to the binary system of PTB7Th:PC71BM. The compatibility of this composition in the solution was favorable and resulted in a device performance of 12.10% because DR3TSBDT possessed a BDT-based molecular structure which is similar to PTB7-Th.38 Careful selection of P and N moieties can lead to the efficient performance of ternary OPVs. In the present study, a wide-bandgap polymer (J51) and a mid-bandgap polymer (PBDB-T) were blended with a low-bandgap nonfullerene (ITIC, Fig. 1). We investigated the optoelectronic properties, blend film morphology, and device performance of the ternary blend and compared them with their binary system. This is the first study to use the J51:PBDB-T:ITIC ternary blend for increasing the light absorption range and providing the cascade energy-level alignment of the ternary blend. This ternary blend may also improve the blend film morphology and molecular packing behavior of the active layer by using amorphous-like J51 with highly crystalline PBDB-T and altering their optoelectronic properties and device performance.

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Fig. 1 (a) Chemical structures of J51, PBDB-T, and ITIC. (b) Inverted device structure with illustration of the active layer morphology in ternary organic solar cells. (c) Energy levels of the device used in this study. Experimental section J51 and PBDB-T were purchased from 1-Materials (Canada, Quebec), and ITIC was purchased from Solarmer Incorporation (USA, California). Solutions of J51, PBDB-T, and ITIC were prepared at different ratios (1:0:1, 0.8:0.2:1, 0.5:0.5:1, 0.2:0.8:1, and 0:1:1) and stirred overnight in chlorobenzene (CB). Subsequently, different concentrations of 1,8diiodooctane (DIO; 0.5, 0.75, and 1.0 vol%) were added to the solution. Blend solutions were filtered through a 0.2-μm polytetrafluoroethylene filter and then spin coated (2000 rpm, 30 s) on the ZnO layer. Each device was completed by depositing an 8-nm-thick layer of MoO3 and a 100-nm-thick layer of Ag at less than 10−6 Torr. The active area of the devices was 10 mm2.

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Results and discussion

Fig. 1 shows the chemical structures of J51, PBDB-T, and ITIC as well as the energylevel diagrams of materials and devices used in this study. These three materials showed different absorption ranges in visible light: 500–600 nm for J51, 550–650 nm for PBDB-T, and 650–750 nm for ITIC (Fig. 2a). To optimize the ternary blends, we altered their blend ratios, applied the additive DIO, and then examined their optoelectronic properties through UV–visible and photoluminescence (PL) spectroscopy. Fig. 2b shows the UV–visible spectra of the ternary blends, in which J51:PBDB-T:ITIC was mixed in five different weight ratios of 1:0:1, 0.8:0.2:1, 0.5:0.5:1, 0.2:0.8:1, and 0:1:1. The additive amount of DIO was fixed at 0.5 wt%. At a wavelength of 500 nm, when the amount of PBDB-T increased and that of J51 decreased, the spectrum was effectively red shifted. By varying the weight ratios of the ternary blends, we observed a controllable absorption of blends. For the blend film with a ratio of 0.8:0.2:1, we obtained an overall strong and balanced absorption in the range of 450‒800 nm. Furthermore, we investigated the effect of DIO (0.5, 0.75, and 1.0 vol%) on the light absorption of the blend film with a ratio of 0.8:0.2:1. As shown in Fig. 2c, we observed a change in the UV–visible spectra of this blend film. After an increase in the concentration of DIO, we observed a slight red shift of the spectra in the range of 700‒800 nm. This behavior might be attributed to the efficient molecular packing of ITIC and is preferred for OPV application.39-40

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a)

J51 PBDB-T ITIC

Absorbance (a.u.)

1.0

0.8

0.6

0.4

0.2

0.0 400

500

600

700

800

Wavelength (nm)

b)

1.0

Absorbance (a.u.)

0.8

1/0/1 0.8/0.2/1 0.5/0.5/1 0.2/0.8/1 0/1/1

0.6

0.4

0.2

0.0 400

c)

1.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

500

600

700

800

700

800

Wavelength (nm)

J51/PBDB-T/ITIC=0.2/0.8/1 0.5vol% DIO 0.75vol% DIO 1.0vol% DIO

0.6

0.4

0.2

0.0 400

500

600

Wavelength (nm)

Fig. 2 UV–visible spectra of (a) J51, PBDB-T, and ITIC; (b) blend films deposited from different ratios of J51:PBDB-T:ITIC; and (c) J51:PBDB-T:ITIC films with different concentrations of DIO. It is noticed that the UV-vis spectra of J51:PBDB-T:ITIC films as shown in Fig. 2c shows

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a red-shift adsorption at ~700 nm as the DIO content increases. The red-shift at ~700 nm in UV-vis spectra indicates that the DIO forms the agglomerate structure with ITIC. This agglomeration is caused by the higher boiling point of DIO that results in a slower rate of layer formation. The slower rate of layer formation facilitates the stacking of ITIC molecules and the production of larger sized agglomerates.

PL refers to the excitation of a photon received by a material so that electrons originally in the valence band transition to a higher energy level. At this time, a hole is left in the original valence band, leading to the formation of electron-hole pairs. These electron-hole pairs that are combined through radiation coupling again release a photon. For a solar cell device, the photoexcited electron-hole pair should be separated into free carriers for current generation. Superior PL quenching indicates efficient charge separation.41-42 Fig. 3 shows the PL spectra of J51, PBDB-T, and ITIC as well as the ternary blend films prepared at different ratios of J51:PBDB-T:ITIC and at various concentrations of DIO, respectively. At the excitation wavelength of 600 nm, the ternary blend film with a ratio of 0.2:0.8:1 showed the optimal PL quenching, indicating efficient charge transfer. The PL intensity decreased with the addition of DIO to up to 0.75 vol%, implying that the blend film with a ratio of 0.2:0.8:1 has the optimal morphology among all blend compositions This behavior is correlated with the UV–visible spectra, which showed that superior ITIC packing led to the red shift of absorbance and morphology.35 We hypothesized that a blend ratio of 0.2:0.8:1 in the presence of 0.75 vol% of

9



DIO might have the most favorable optoelectronic properties.

8

1.0x10

7

8.0x10

PL Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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7

6.0x10

pure J51 pure PBDB-T pure ITIC a 1/0/1 a 0.8/0.2/1 a 0.5/0.5/1 a 0.2/0.8/1 a 0/1/1 b 0.2/0.8/1 c 0.2/0.8/1

7

4.0x10

7

2.0x10

0.0 650

700

750

800

Wavelength (nm)

Fig. 3 PL spectra of J51, PBDB-T, ITIC, and J51:PBDB-T:ITIC films at different concentrations of DIO. a 0.5 vol% of DIO, b 0.75 vol% of DIO and c 1.0 vol% of DIO. To further confirm this hypothesis, we obtained AFM images of these blends. Fig. 4 shows the tapping-mode AFM images of blend films deposited with different ratios of J51:PBDBT:ITIC. The influence of different contents of J51 and PBDB-T on the surface morphology and phase separation degree of the polymer and ITIC phase can be studied. As shown in Fig. 4a, we observed an island-like aggregative for the J51:ITIC blend. The island-like structure became clearer as the amount of PBDB-T increased (Fig. 4b and 4c). As the PBDB-T ratio increased to 0.2:0.8:1 (J51:PBDB-T:ITIC), the proportion of the crystalline phase began to increase (Fig. 4d). Very large crystalline particles were formed on the blend film prepared using PBDB-T and ITIC (Fig. 4e) because PBDB-T is a highly crystalline polymer material.35, 43 The results of the AFM analysis also indicated that these active layers exhibited relatively low

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surface roughness. This advantage helps to reduce joint defects. Therefore, the blend ratio of PBDB-T exhibited a significant effect on film surface roughness. The film roughness increased from 3.35 to 5.23 nm as the ratio of J51:PBDB-T:ITIC increased from 1:0:1 to 0.5:0.5:1. The smallest value of surface roughness was 3.09 nm for the ternary mixed film obtained at a ratio of 0.2:0.8:1 (J51:PBDB-T:ITIC), indicating that the polymer and solvent were effectively mixed at such a ratio. It is known that an appropriate amount of crystal phase added to the planar binary system can effectively improve the film surface property. In this study, PBDB-T was used as crystal phase that can prevent excessive crystallization and partial uneven crystallization in the active layers. Therefore, a lowest roughness can be obtained under the optimal ratio of J51/PBDB-T/ITIC. In this study, we found that the optimal ratio of J51/PBDBT/ITIC is 0.2/0.8/1. The description has been added in the revised paper .44As shown in Fig. 4(f) and 4(g), AFM images obtained at 0.75 and 1 vol% of DIO at a J51:PBDB-T:ITIC ratio of 0.2:0.8:1 indicated that the roughness values were 5.03 and 6.15 nm, respectively, and the blend film with 0.75 vol% of DIO had a dense and relatively uniform grain size and grain arrangement compared with the blend film prepared using 0.5 vol% of DIO. Moreover, the grain size became less uniform when the DIO concentration was increased to 1.0 vol%. This result corresponds to those obtained from the UV–visible analysis that showed that unsatisfactory optical properties may be caused by aggregation resulting from the addition of ITIC and excessive DIO.

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Fig. 4 Tapping-mode AFM (1 × 1 μm) images (left: topographic images and right: phase images) of blend films deposited with different ratios of J51:PBDB-T:ITIC: (a) 1:0:1, (b) 0.8:0.2:1, (c) 0.5:0.5:1, (d) 0.2:0.8:1, (e) 0:1:1 with 0.5 vol% of DIO and the J51:PBDB-T:ITIC ratio of 0.2:0.8:1 with (f) 0.75 vol% of DIO and (g) 1.0 vol% of DIO. Fig. 5 and Table 1 summarize the J-V characteristics and external quantum efficiency (EQE) data of the binary and ternary blend films prepared at different ratios of J51:PBDBT:ITIC and at different concentrations of DIO. For J51:ITIC binary devices, the short-circuit current density (JSC) gradually increased when the PBDB-T content was increased from 11.13 mA cm−2 for the original J51:ITIC device to 15.47 mAcm−2 for the ternary blend prepared at

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a ratio of 0.2:0.8:1, indicating uniform mixing among J51, PBDB-T, and ITIC and succeeded in broadening the spectrum to increase light harvesting and enhance the JSC value. The PBDBT:ITIC binary device showed the lowest fill factor (FF) value after PBDB-T addition. This result indicates that adding a highly crystalline PBDB-T to the original smooth surface can slightly increase the surface roughness, resulting in a decrease in the FF value. In terms of PCE, the ternary systems of J51:PBDB-T:ITIC performed more effectively than did the binary systems of J51 and PBDB-T. The better efficiency of 7.82% was obtained for the device derived at a ratio of 0.2:0.8:1. Furthermore, we found that adding 0.75 vol% of DIO under this optimal condition could slightly increase the JSC value and significantly increase the FF value. A considerable improvement was observed in the FF because a moderate amount of DIO effectively reunited with ITIC, making the overall surface smoother. This enabled the FF to increase, resulting in a significant increase in device performance to up to 9.06%. However, when 1.0 vol% of DIO was added, all the JSC, VOC, FF, and PCE values dropped. The obvious difference in the FF is due to a rougher surface caused by the excessive clustering of ITIC by DIO. The optimal devices showed a PCEavg. of 8.75 ± 0.11%, with a JSC of 15.47 ± 0.07 mA cm−2, a VOC of 0.89 ± 0.01 V, and an FF of 0.64 ± 0.01%. Fig. 5b shows the EQE curves of binary and ternary blend films prepared using different concentrations of DIO. For the J51:PBDB-T:ITIC material, the EQE spectrum ranged from approximately 350 to 800 nm. This shows that the device prepared from the J51:PBDB-T:ITIC ratio of 0.2:0.8:1 (0.75 vol%

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of DIO) exhibited the highest EQE values with the maximum of 78% at 700 nm. Furthermore, we calculated the current density of the devices by integrating the EQE spectra as a reference to the current density of AM 1.5 G. The calculated JSC values were 11.05 mAcm−2 for 1:0:1 (0.5 vol% of DIO), 12.12 mAcm−2 for 0.8:0.2:1 (0.5 vol% of DIO), 13.40 mA cm−2 for 0.5:0.5:1 (0.5 vol% of DIO), 15.06 mAcm−2 for 0.2:0.8:1 (0.5 vol% of DIO), 14.42 mAcm−2 for 0:1:1 (0.5 vol% of DIO), 15.23 mAcm−2 for 0.2:0.8:1 (0.75 vol% of DIO), and 15.11 mAcm−2 for 0.2:0.8:1 (1.0 vol% of DIO). The calculated values are close to the JSC values determined using the solar simulator. Table 1 Summary of the photovoltaic parameters and EQE data of J51:PBDB-T:ITIC binary and ternary blend organic solar cells with different J51:PBDB-T:ITIC ratios and DIO concentrations

J51/PBDB-

Jsc(Jcalc.)a)

Jsc -2

[mA cm-

Voc

FF

PCEbest

[%]

[%]

T/ITIC

[mA cm ]

1/0/1 b)

11.13 ± 0.20

11.05

0.83 ± 0.00

0.65 ± 0.01

6.00 ± 0.09

6.10

0.8/0.2/1 b)

12.42 ± 0.32

12.12

0.85 ± 0.00

0.61 ± 0.00

6.46 ± 0.20

6.67

0.5/0.5/1 b)

13.93 ± 0.18

13.40

0.82 ± 0.04

0.56 ± 0.01

6.39 ± 0.25

6.71

0.2/0.8/1 b)

15.27 ± 0.27

15.06

0.87 ± 0.00

0.58 ± 0.02

7.67 ± 0.16

7.82

0/1/1 b)

14.59 ± 0.26

14.42

0.89 ± 0.00

0.58 ± 0.02

7.46 ± 0.16

7.67

0.2/0.8/1 c)

15.47 ± 0.07

15.23

0.89 ± 0.01

0.64 ± 0.01

8.75 ± 0.11

9.06

0.2/0.8/1 d)

15.19 ± 0.30

15.11

0.89 ± 0.01

0.55 ± 0.01

7.43 ± 0.16

7.65

a)

2

]

[V]

The Jsc calculated from EQE spectrum; b) with 0.5vol% DIO; c) with 0.75vol% DIO; d)

a)

with 1.0vol% DIO

PCEavg.

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4

b) 80 70

0

a

1/0/1 a 0.8/0.2/1 a 0.5/0.5/1 a 0.2/0.8/1 a 0/1/1 b 0.2/0.8/1 c 0.2/0.8/1

-4

-8

60

EQE (%)

Current Density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 a

1/0/1 a 0.8/0.2/1 a 0.5/0.5/1 a 0.2/0.8/1 a 0/1/1 b 0.2/0.8/1 c 0.2/0.8/1

40 30 20

-12

10 -16

0

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

400

Voltage (V)

500

600

700

800

Wavelength (nm)

Fig. 5 (a) J-V characteristics and (b) EQE curves of binary and ternary blend films of the device used in this study. awith 0.5 vol% of DIO, bwith 0.75 vol% of DIO, cwith 1.0 vol% of DIO. We prepared SCLC devices for measuring electron and hole mobilities to understand the effect of different ratios of the ternary system on the performance of active layers. We prepared the device with a structure of ITO/ZnO/blends/Al for measuring electron mobility and ITO/PEDOT:PSS/blends/MoO3/Ag for measuring hole mobility.45-46 These SCLC devices were prepared under optimized conditions of OPV devices, and the resulting hole and electron mobilities are shown in Fig. S1 and summarized in Table S1. As shown in Fig. S1, the SCLC electron mobility (μe) and hole mobility (μh) for devices prepared using different blend ratios and DIO concentrations were 7.18 × 10−5 and 1.18 × 10−4 for 1:0:1 (0.5 vol% of DIO), 4.68 × 10−5 and 1.17 × 10−4 for 0.8:0.2:1 (0.5 vol% of DIO), 2.95 × 10−5 and 9.87 × 10−5 for 0.5:0.5:1 (0.5 vol% of DIO), 5.16 × 10−5 and 1.20 × 10−4 for 0.2:0.8:1 (0.5 vol% of DIO), 9.06 × 10−5 and 1.13 × 10−4 for 0:1:1 (0.5 vol% of DIO), 4.59 × 10−5 and 1.34 × 10−4 for 0.2:0.8:1 (0.75 vol% of DIO), and 3.73 × 10−5 and 1.26 × 10−4 cm2 V−1 s−1 for 0.2:0.8:1 (1.0 vol% of DIO), respectively. The values of electron and hole mobilities did not differ considerably for these

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devices with different J51:PBDB-T:ITIC ratios and DIO concentrations. Moreover, the series resistance (Rs) and shunt resistance (Rsh) of each component were calculated and are listed in Table S1. We observed that the larger the FF value was, the smaller was the Rs value and the larger was the Rsh value. The device blend film prepared using a J51:PBDB-T:ITIC ratio of 0.2:0.8:1 (0.75% of DIO) had a relatively higher μh than did other ternary devices and a superior optoelectronic performance, which can be attributed to its higher FF and JSC values.47

Fig. S2 illustrates the 2D GIWAXS patterns of blend films that shows the molecular stacking behavior of binary and ternary systems. No obvious out-of-plane and in-of-plane molecular packing for the J51:ITIC binary blend corresponded to the amorphous feature of J51. Intermolecular interactions between J51 and ITIC moieties further disrupted the efficient packing behavior of ITIC crystals. The ternary film had a more obvious molecular arrangement and a lamellar diffraction peak at 0.6 Å−1 for the out-of-plane morphology; the position of this diffraction peak is the same as that of pure PBDB-T. This diffraction disappeared for the PBDB-T:ITIC binary film, and ITIC suppressed the formation of the out-of-plane arrangement of PBDB-T.16 The addition of J51 may improve the crystallization of PBDB-T; thus, the intermolecular alignment tended to the preferred direction, thereby ensuring efficient hole transport. This is also the reason for the higher hole mobility of the ternary system than of the binary system.30 These results highlight the feature of ternary blends: judicious selection of conjugated polymers promotes efficient light harvesting and energy alignment in the blend film.

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Moreover, the morphologies of the active layer could be altered by varying the third component of the ternary blend film.

3.Conclusion

We used small (ITIC), middle (PBDB-T), and wide (J51) bandgaps as conjugated materials for building ternary OPV devices. The blend film morphology of the ternary device was optimized by incorporating 0.75 vol% of DIO, which delivered a PCE of 8.75 ± 0.11%, a JSC of 15.47 ± 0.07 mA cm−2, a VOC of 0.89 ± 0.01 V, and an FF of 0.64 ± 0.01; this PCE was higher than the corresponding values of 6.00 ± 0.09 and 7.46 ± 0.16% for the J51- and PBDBT-based binary devices, respectively. From the results of the morphology (from AFM and GIWAXS) and UV–visible, PL, and EQE spectra, we conclude that the improved performance of PBDB-T:J51:ITIC ternary devices was due to complementary solar light harvesting, cascade energy alignment, and optimized blend film morphology. Appropriate selection of Ptype and N-type conjugated materials can result in ternary blends having efficient performance compared with their binary counterparts.

Acknowledgements

We thank the Ministry of Science and Technology of Taiwan (MOST 104-2221-E-131-025MY3 and and MOST 107-2221-E-131-010) for the financial support. References

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TOC Graphic

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Current Density (mAcm )

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Ternary OPV

PCE>9 %

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Voltage (V)

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Efficient Ternary Organic Photovoltaics Using Two-Conjugated

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