Highly Efficient Non-Fullerene Organic Photovoltaics Processed from

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Highly Efficient Non-Fullerene Organic Photovoltaics Processed from O-Xylene Without Using Additives Yang-Yen Yu, Tzung-Wei Tsai, Chun-Chen Yang, and ChihPing Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07867 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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The Journal of Physical Chemistry

Highly Efficient Non-Fullerene Organic Photovoltaics Processed from O-Xylene without Using Additives

Yang-Yen Yu1,2*, Tzung-Wei Tsai1, Chun-Chen Yang3 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 3

Department of Chemical Engineering, Ming Chi University of Technology, Taipei 243,

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

ACS Paragon Plus Environment


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Abstract Most efficient organic photovoltaic (OPV) devices are fabricated using halogenated solvents, which are hazardous and environmentally unfriendly. From an industrial perspective, green solvents are necessary for the roll-to-roll production of OPV modules. In this study, we fabricated non-fullerene (NF) OPV devices that are based on the blend films of PTB7-Th 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′]-di thiophene (ITIC) by using o-xylene and tetrahydrofuran (THF) as the processing solvents and chlorobenzene (CB) and o-dichlorobenzene as the control solvents. We compared the variations in the ultraviolet visible

absorption,

atomic

force

microscopy–derived

phase

morphologies,

space-charge-limited current carrier mobilities, and power conversion efficiency (PCE) of the related OPV devices. The high solubility of ITIC and PTB7-Th in the solvents yielded PCE of 8.11% and 6.79% for the o-xylene- and THF-derived devices, respectively. The PCE of 8.11% is among the highest performance reported to date for NF OPV devices fabricated using a green solvent (without additives or post-treatment). Furthermore, this PCE was suppressed in the CB-based device (PCE: 7.41%) because of the clearly defined morphology and higher and balanced carrier mobility.


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Introduction Organic photovoltaics (OPVs) composed of p-type conjugated polymers or small molecules and n-type fullerene or nonfullerene (NF) acceptors have been considerably successful and have gained considerable attention because of their power conversion efficiency (PCE; >12%).1-8 In high-performance OPV devices, most active layers are deposited from halogenated solvents such as chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (DCB). These solvents are environmentally unfriendly and cause health hazards; thus, they are unsuitable for commercial large-scale fabrication.9-10 Hence, determining a more favorable solvent for high-performance OPVs is imperative. The most used nonhalogenated solvents are methylbenzenes (MBs), such as toluene, xylene, and trimethylbenzene (TMB).11 Nevertheless, this system still typically requires using additives such as 1,8-diiodooctane (DIO)12 to yield high efficiency. DIO is a halogen-containing compound. Nonhalogenated solvent mixtures, including MB–cyclopentyl methyl ether,13 acetophenone–mesitylene,14 and carbon disulfide–acetone15 systems, have been used in OPV applications. For example, Chu and coworkers reported a PCE of 8.10% for a small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) OPV device by using MB–cyclopentyl methyl ether as the additive.13 Moreover, Dong fabricated an isoindigo-based conjugated polymer–[6, 6]-phenyl C61-butyric acid methyl ester (PCBM) device from o-xylene (with 2% of octane-1,8-dithiol) and demonstrated a PCE of 7.46%.16 Hou et al. reported PCE of 8.8% and 9.5% for PTB7-Th–PC71BM and PBDT-TS1–PC71BM, respectively, by using o-xylene–2% N-methylpyrrolidone.17 Yen et al. reported the most efficient fullerene-based OPV device fabricated from nonhalogenated solvents18 They demonstrated a PCE of 11.7% for a PffBT4T-C9C13–PC71BM device fabricated from a solution of 1,2,4-TMB, with 1-phenylnaphthalene as the additive. Because of tunable energy levels and wide light absorption, particularly in the longer



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wavelength near-infrared region, n-type small molecule-based NF materials have been widely studied in the past 2 years.19 Zhan and coworkers first reported an indacenodithieno [3,2-b] thiophene

(IT)-based

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:5,6-b′]-di

thiophene

(ITIC);

Fig.

1)}

and

demonstrated its application in NF OPVs, with a PCE of more than 8%.20-22 Recently, Hou et al. reported a PCE of more than 12% for ITIC derivative-based OPV devices.23 In addition to high performance, an NF-type OPV was reported to show excellent long-term thermal stability.24 These developments have increased the importance of NF-based OPVs in the past 2 years.25-28 Similar to the PCBM system, few studies have reported the use of nonhalogenated solvents in the fabrication of NF OPVs. According to our review of the relevant literature, few studies have developed efficient OPVs by using a single solvent system such as tetrahydrofuran (THF) and o-xylene. Recently, Welch et al. reported a PCE of 4.8% for NF OPVs based on blend films of PTB7-Th:PDI derivatives by using 2-methyl (2Me) THF.29 Sharma et al. developed a small molecule–PCBM-based OPV by using THF as the processing solvent and reported a PCE of 2.65%.30 The PCE of their device increased to 4.9% after adding chloronaphthalene, a toxic additive. Hou et al. fabricated a PBQ-4F–ITIC OPV device yielding a PCE of 5.1% by using THF as the processing solvent.31 After isopropanol (IPA; 95:5, v/v mixture of THF:IPA) was added, the PCE markedly increased to 11.3%. In the current study, we fabricated NF OPV devices with high PCE by using nonhalogenated solvents, namely THF and o-xylene, without using additives or post-treatment solvents or thermal annealing, which significantly reduced the fabrication complexity. We compared the optoelectronic properties, active layer morphology, and space-charge-limited current (SCLC) mobility of devices processed from o-xylene, CB, o-DCB, and THF. We determined PCEs of more than 8.11% and 6.79 % for the


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PTB7-Th:ITIC OPV devices processed from o-xylene and THF, respectively. This performance is among the highest PCE reported to date for NF OPV devices processed from nonhalogenated solvents without any post treatment or additives. The use of additives requires additional processing steps for their removal, thus resulting in low reproducibility. Hence, using a single green solvent without any post-treatment to achieve high performance is more attractive and economical.32 R2 PTB7-Th

S S

S

S S

S

F

O O R2

R2

R2

ITIC

O S

NC CN

R1 R1 S S R1 R1

NC

CN

S O

R1

Figure 1. Structure of PTB7-Th and ITIC and inverted device structure used in this study. Experiment PTB7-Th and ITIC were purchased from 1-Materials, Canada and Solarmer Incorporation, USA, respectively. Solutions of PTB7-Th and ITIC were prepared at defined ratios (1:1.3) and were stirred overnight in o-DCB, CB, xylene, and THF. The blend solutions were filtered through a 0.2-µm polytetrafluoroethylene filter and then spin-coated (1000–6000 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. Results and Discussion



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Figure 1 shows the device architecture and molecular structures of PTB7-Th and ITIC, which were used as the bulk heterojunction (BHJ) active layers. The OPV device composition of PTB7-Th:ITIC was first reported by Zhan et al., who obtained a PCE of 6.80%; they did not indicate the solvent used.22 Hou and coworkers revealed a PCE of 11.22% for the blend films of PBDB-T–ITIC by using CB and DIO as the solvent and additive, respectively.23 In this study, o-DCB and CB were used as the control solvents, whereas THF (nonaromatic) and o-xylene (nonhalogenated) were selected as the processing solvents. Typically, o-xylene (boiling point: 144°C), a less-toxic aromatic solvent, is favorable for roll-to-roll fabrication. Moreover, THF, a green solvent, can readily dissolve active layer materials. Because of the high solubility of PTB7-Th and ITIC, they can readily dissolve in the selected solvents. All the blend films were fabricated under the same conditions, with the donor:acceptor (D:A) ratio of PTB7-Th:ITIC being 1:1.3. The optimized thickness of the blend films was nearly 100 nm. Figure 2 illustrates the ultraviolet visible (UV-vis) absorption and photoluminescence (PL) spectra of the blend films deposited from different solvents. As shown in Figure 2a, the absorption characteristic in the UV region (300–400 nm) was attributed to the localized transitions of the benzodithiophene and IT units.22 The main peaks at 700 nm corresponded to the intramolecular charge transfer within ITIC and PTB7-Th. The UV-vis spectra for the blend films deposited from CB and o-xylene were nearly identical. The slight blue shift in the spectrum of the THF-derived film at the band edge was because of the low boiling point of THF (66°C). The film formation process was faster for the THF sample; therefore, the π–π stacking degree of this film was lower than those of the other blend films. We observed a slight red shift in the absorption characteristic of the o-DCB-derived blend film, indicating more favorable molecular packing and higher crystalline domains, which were because o-DCB had the lowest evaporation rate (boiling point: 180.5°C) compared with the other solvents. Figure 2b presents the PL spectra of the


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blend films and PTB7-Th and ITIC thin films; the excitation wavelength was set at 600 nm. As shown in Figure 2b, the PL intensity of the PTB7-Th and ITIC thin films were high in the 720–820 region. The enlarged graph in Figure 2b shows the different PL quenching behaviors of these blend films. An effective PL quenching behavior was observed in the active layer, implying that the nanoscale phase separation of blend film morphology is more favorable for efficient exciton dissociation.33 The films can be sorted in descending order in terms of their PL intensity levels as follows: o-DCB, THF, CB, and o-xylene. The o-DCB sample showed more favorable intermolecular (π–π) stacking in the active layer, as revealed by the UV-vis spectra, indicating the higher crystallinity of p- and n-type materials in the active layer. Nevertheless, the PL intensity of this blend film was the highest, signifying that the phase separation degree or domain size was not ideal. PTB7-Th and ITIC thin films may self-aggregate into a larger domain, and the generated excitons cannot diffuse efficiently to the D:A interfaces, thus yielding a higher PL intensity. By contrast, we observed that the o-xylene-derived film yielded the lowest PL intensity, indicating efficient exciton dissociation within the blend film. This result suggests that o-xylene-derived blend films may have a more adequately defined BHJ morphology and can yield a higher OPV performance level. a) 1.2 1.0 0.8

ABS (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


ITIC PTB7-Th PTB7-Th:ITIC in CB PTB7-Th:ITIC in DCB PTB7-Th:ITIC in THF PTB7-Th:ITIC in o-xylene


0.6 0.4

600

650

700

Wavelength (nm)

0.2 0.0 400

500

600

700

800

Wavelength (nm) ACS Paragon Plus Environment

750


b)

6


6

7x10

6

6x10

6

5x10

6

4x10

PL Intensity

8x10

PL Intessity

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6

3x10

5x10

5

4x10

5

3x10

5

2x10

5

1x10

5

CB DCB THF o-xylene

0

6

2x10

720

760

Wavelength (nm)

6

1x10

0 720

740

760

780

800

820

840

Wavelength (nm) Figure 2. a) UV-vis and b) PL spectra of blend films deposited from various solvents. To understand our hypothesis of blend film morphology from the PL data, we subjected the blend films to tapping-mode atomic force microscopy (AFM). For direct comparison, the BHJ films used for morphological analysis were prepared in an identical manner to the OPV devices. As shown in Figure 3 (left, topographical images; right, phase images), a significant difference in morphology was observed for the BHJ of the films deposited from different solvents. The root mean square surface roughness values were 1.8, 5.1, 2.2, and 1.5 nm for the films prepared from CB, o-DCB, THF, and o-xylene, respectively. The rougher surface morphology observed for the o-DCB-derived film indicates a higher possibility of larger phase segregation for this film. The phase morphology of the active layers can provide information on the D:A distribution.34 The phase image of the o-DCB-derived film (Figure 3b, right) shows two regions: bright regions, attributable to PTB7-Th-rich domains, and dark aggregations, attributable to ITIC domains.35 Compared with the other blend films, the o-DCB-derived blend film displayed a higher phase separation level, as evidenced by the segregation of irregular PTB7-Th-rich domains (10–200 nm) surrounded by an ITIC-rich domain. This phenomenon can be tracked to its inadequate PL quenching behavior (Figure


800

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2b), which is attributed to larger D:A phase separation domains. Figure 3a presents the interpenetrating network morphology observed for the CB-derived blend film, revealing that the sample comprised PTB7-Th-rich domains (grain size: approximately 50–100 nm) surrounded by ITIC domains. We thoroughly examined the bright PTB7-Th-rich domains and observed that smaller ITIC domains (10 nm) were effectively distributed within these PTB7-Th-rich domains; a high degree of phase segregation between PTB7-Th and ITIC might provide more favorable D:A interfaces for efficient charge segregation.36-37 Figure 3c displays the morphology of the THF-based blend film, indicating ITIC-rich domains (dimensions: approximately 20–40 nm) surrounded by continuous PTB7-Th rich domains. The o-xylene-derived blend film showed different morphologies, with the D:A ratio resulting in favorable phase segregations with an island-like morphology (Figure 3d). The observed nanoscale phase separation revealed spherical ITIC-rich domains (dimensions: approximately 8%

Non-fullerene OPV


Highly Efficient Non-Fullerene Organic Photovoltaics Processed from

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