Effect of Heterocyclic Anchoring Sequence on the ... - ACS Publications

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

The Effect of Heterocyclic Anchoring Sequence on the Properties of Dithienogermole- Based Solar Cells Bright Walker, Daehee Han, Mijin Moon, Song Yi Park, Ka-Hyun Kim, Jin Young Kim, and Changduk Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14804 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

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

ACS Applied Materials & Interfaces

The Effect of Heterocyclic Anchoring Sequence on the Properties of Dithienogermole- Based Solar Cells Bright Walker,†⊥Daehee Han,‡⊥ Mijin Moon, ‡⊥ Song Yi Park, † Ka-Hyun Kim, §* Jin Young Kim†* and Changduk Yang‡* †

Department of Energy Engineering, School of Energy and Chemical Engineering, Low

Dimensional Carbon Materials Center, Perovtronics Research Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, South Korea. ‡

Department of Energy Engineering, School of Energy and Chemical Engineering, Perovtronics

Research Center, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, South Korea. §

KIER-UNIST Advanced Center for Energy, Korea Institute of Energy Research, Ulsan 44919,

South Korea. KEYWORDS Dithienogermole, small molecule, bulk heterojunction, solar cell, photovoltaic, organic electronics.

ABSTRACT

ACS Paragon Plus Environment

1


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

Page 2 of 28

The synthesis and characterization of two new small molecular donor materials, DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 are presented for application in organic solar cells. These two materials represent structural evolutions of the high-efficiency, dithienogermole (DTGe)-cored small molecule DTGe(FBTTh2)2, in which the conjugation length in the backbone was extended by incorporating additional thiophene units. Using the same molecular framework, we have evaluated how the anchoring sequence of heterocyclic units influences material properties and function in solar cell devices. It was found that incorporating additional thiophene units into the backbone, regardless the position in the molecular platform, caused a small reduction in band gaps, however, both highest occupied molecular orbitals and lowest unoccupied molecular orbital energy bands were at lower energies when the thiophenes were incorporated near the terminus of the molecule. The film morphologies of both materials could be controlled by either thermal or solvent vapor annealing to yield phase separation on the order of tens of nanometers and improved crystallinity. Peak power-conversion efficiencies of 3.6% and 3.1% were obtained using DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2, after solvent vapor treatment and thermal annealing, respectively. Our study provides a detailed analysis of how the ordering sequence of heterocyclic building blocks influences the properties and function of organic solar cells.

Introduction Solution-processed conjugated small molecules have been intensely studied for a number of years and attracted considerable interest in the field of organic semiconductors.1-4 Compared to conjugated polymers, organic small molecule donor materials offer advantages in terms of welldefined material structures along with simplified synthesis and purification procedures; these desirable attributes have spurred intense research in the field of solution-processed, small


2

Page 3 of 28

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


molecule bulk heterojunction solar cells (SMBHJs). Such efforts have led to a successive increase in power conversion efficiency (PCE) with the highest reported PCE values now over 10%.5-7 Structural engineering of the donor skeleton in SMBHJs has proven to be an effective pathway to improve the performance of SMBHJs.4,

8-9

Among the most effective structural

design strategies includes incorporating alternating combinations of electron donating (D) and electron accepting (A) units into the conjugated backbones, which allows fine-tuned control of the energy band structure in this type of molecule and has led to a variety of successful solar cell materials.4, 10-14 In this perspective, we have developed a dithieno[3,2-b;2',3'-d]germole (DTGe)-based D-A compound, namely DTGe(FBTTh2)2, which exhibits outstanding material characteristics for SMBHJ devices and impressive PCEs of up to 7%.15 In this work, we seek to further advance and understand this system by investigating the effect of increased conjugation length, via the introduction of additional thiophene units into different points in the DTGe(FBTTh2)2 backbone. There are several compelling motivations to introduce additional thiophene units in this topology; (i) increasing the π-conjugation length is expected to have desirable impacts on the material properties of this molecule – notably, longer conjugation length typically correlates to an increased optical absorption range, offering the potential for increased photocurrents,14, 16 (ii) extending the planar π-conjugated structure by increasing the oligothiophene length may improve π-π interactions and packing in the material, offering the potential for increased charge carrier mobility in the material,17-18 (iii) the DTGe core imparts excellent solubility in organic solvents, allowing increased π-conjugation length without the need to add additional solubilizing alkyl chains,9 (iv) incorporating heterocylic thiophene units in different sequences in the


3


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

Page 4 of 28

conjugated backbone allows new structure-property relationships to be identified, allowing new materials to be designed more effectively.

a)

Donating bundle Accepting bundle

`

b)

Figure 1. Schematic diagrams. (a) Molecular design strategy highlighting changes to the electron donating structure. (b) Synthetic approach illustrating the synthesis of FBTTh3 and ThFBTTh2, showing the molecular structure of each precursor. The syntheses were carried out by microwave-assisted Stille coupling of the DTGe core with either FBTTh3 or ThFBTTh2 using Pd(PPh3)4 as a coupling catalyst in toluene.

These concepts, and the topological design strategy in this work are shown in Fig. 1a, illustrating how the compounds DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 were designed and synthesized based on the parent compound DTGe(FBTTh2)2. Herein, we focus our studies on evaluating how varying the position of the additional thiophene subunits within the molecular framework


4

Page 5 of 28

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


enables tuning of the molecular energy levels, optical and electrochemical properties, crystal structures and solar cell characteristics. Results and discussion Synthetic procedures. The design motif and synthesis of DTGe(FBTTh3)2 and DTGe(ThFBTTh2)2 are shown in Fig. 1, and the full synthetic and characterization details are provided in the Experimental Section. The synthesis involves a combination of the repetitive palladium-catalyzed Stille coupling and bromination with N-bromosuccinimide. A useful quality associated with 4,7-dibromo-5fluorobenzo[c][1,2,5]thiadiazole is that the fluorine atom imparts asymmetry such that near quantitative site-selective cross-coupling can be achieved. This allowed for facile synthetic access

to

the

key

intermediates

4-bromo-5-fluoro-7-(5''-hexyl-[2,2':5',2''-terthiophen]-5-

yl)benzo[c][1,2,5]thiadiazole (FBTTh3) and 4-(5-bromothiophen-2-yl)-5-fluoro-7-(5'-hexyl-[2,2'bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (ThFBTTh2), respectively by changing the order in the synthetic routes, as shown in Fig. 1b. The desired DTGe(FBTTh3)2 and DTGe(ThFBTTh2)2 were built up via a microwave-assisted Stille coupling between a bis-stannylated DTGe building block and the corresponding mono-brominated counterparts (FBTTh3 and ThFBTTh2) (Fig. 1a). Their structures and purity were determined by solution NMR spectroscopy, mass spectrometry, and elemental analysis. Both the molecules exhibited good solubility in organic solvents such as chloroform, chlorobenzene, and dichlorobenzene. Density Functional Theory (DFT).


5


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

Page 6 of 28

In order to understand the electronic structure of the molecules, DFT calculations were performed using the B3LYP/6-31G (d p) basis set for isolated, ground-state molecules in the gas phase. It should be noted that the fluoro-benzothiadiazole group (FBT) may be oriented with either syn or anti conformations (with the sulfur atom pointing either upwards or downwards) with respect to neighboring thienyl moieties. It is known that the difference in potential energy between the two conformations is similar to thermal energy at room temperature and there is little conformational preference in materials which contain these moieties, however, x-ray scattering data of solid-state films of FBT-Th containing polymers are more consistent with the anti configuration.19 Therefore, we are reporting DFT results based on the anti conformation. Frontier molecular orbital geometries of DTGe(FBTTh3)2 and DTGe(ThFBTTh2)2 are shown in Fig.

2.

The

highest

occupied

molecular

orbital

geometries

(HOMOs)

of

both

DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 bear close resemblance to the parent molecule DTGe(FBTTh2)2,15 being localized on the π-bonds of the thiophene moieties and π-bonds of the benzenoid rings. The additional thiophene units increase the size of the conjugated backbone and are expected to increase the size of HOMO orbital, however, the HOMO orbital lobes on the terminal thiophene units in both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 are relatively small compared to the lobes on central aromatic moieties. Thus, the HOMO orbitals are primarily localized near the center of the molecule, whether the additional thiophene units are inserted inside or outside of the FBT units, and the additional thiophene units slightly increase the conjugation length of the HOMO orbitals of both materials.


6

Page 7 of 28

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


Figure 2. Frontier orbital geometries of (a) DTGe(FBTTh3)2 and (b) DTGe(ThFBTTh2)2 calculated by density functional theory.

The lowest unoccupied molecular orbitals (LUMOs) of the new molecules also resemble the parent molecule DTGe(FBTTh2)2, being localized on the 3,1,2-thiadiazole moieties and double bonds arranged as a quinoid tautomer. In the case of the LUMO orbital, the additional thiophene ring in DTGe(ThFBTTh2)2 appears to have a larger effect on the orbital geometry than is the case in DTGe(FBTTh3)2. In both molecules, the LUMO is localized on the quinoid bonds in between the two FBT units, where the additional thiophene units in DTGe(ThFBTTh2)2 contribute two additional quinoid nodes to the LUMO and increase the size and conjugation length of the orbital. The

gas-phase

HOMO

energies

were

calculated

for

DTGe(ThFBTTh2)2

and

DTGe(FBTTh3)2 and found to be -4.60 and -4.67 eV, respectively while the LUMO energies


7


were found to be -2.79 and -2.84 eV, respectively. The HOMO and LUMO energies were calculated to be -4.69 and -2.80 eV, respectively for the parent compound DTGe(FBTTh2)2. These calculations suggest that the band gaps of both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 should be similar (1.81 and 1.82 eV, respectively) and be slightly narrower than DTGe(FBTTh2)2, (1.89 eV)15 while the deeper HOMO orbital of DTGe(FBTTh3)2, is likely to lead to a higher open circuit voltage (VOC) than DTGe(ThFBTTh2)2.15 Indeed, average VOCs were found to be about 70 mV higher for devices based on DTGe(FBTTh3)2, than devices based on DTGe(ThFBTTh2)2 (discussed in greater detail later), confirming these predictions.

Cyclic Voltammetry. To investigate frontier orbital energy levels of DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2, we performed cyclic voltammetry (CV) measurements. These plots are shown in Fig. 3. The CV and optical band gaps (Egopt) for DTGe(FBTTh2)2 are included for reference. The frontier molecular orbital energy levels corresponding to these results are listed in Table 1. The HOMO energy levels were measured

to be -5.11, -4.81 and -4.88 eV for DTGe(FBTTh2)2,15

DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 respectively. Both compounds which incorporate the extra thiophene units in the electron-donating bundle exhibited higher-lying energy levels, due to the well-known electron donating properties of thiophene. Fc/Fc+ DTGe(ThFBTTh2)2 DTGe(FBTTh2)2

Current (A)

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

Page 8 of 28

-2

-1

0

1

Potential(V)


8

Page 9 of 28

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


Figure 3. Cyclic voltammograms of DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 dissolved in chloroform with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte and ferrocene as a reference using a glassy carbon electrode and Pt electrode. Table 1. Frontier orbital energies measured by CV.

HOMO

LUMO

Egopt

[eV]

[eV]

[eV]

Material

Absorption Onset [nm]

DTGe(ThFBTTh2)2 -4.81

-3.22

1.59

778

DTGe(FBTTh3)2

-4.88

-3.29

1.59

781

DTGe(FBTTh2)2

-5.11

-3.46

1.65

775

Optical Properties. UV-vis spectra were collected for both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 as solutions in chlorobenzene as well as thin films. These plots are shown in Fig. 4. The absorption spectra in solution show that the onset of DTGe(FBTTh3)2 is slightly red-shifted relative to the DTGe(FBTTh2)2 control, while the absorption coefficient of DTGe(ThFBTTh2)2 is slightly increased. As thin films, the absorption onsets for all materials are red-shifted compared to solution, due to close packing and increased π-orbital overlap between molecules in the solid state. Similar to the solution absorption spectra, the film absorption onsets are slightly red-shifted for films of DTGe(FBTTh3)2 relative to the DTGe(FBTTh2)2 control, due to the incorporation of extra thiophene units. The absorption coefficient (α) also increased noticeably for both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 in the solid state, reaching a maximum of 63,000 cm1

at 616 nm and 54,600 cm-1 at 686 nm for DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2,

respectively.


9


120000

70000

100000 80000

(b) 60000

DTGe(TFBTTh3)2 DTGe(ThFBTTh2)2

50000

DTGe(FBTTh2)2

α (cm-1)

(a) 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

Page 10 of 28

60000 40000

DTGe(ThFBTTh2)2 DTGe(FBTTh2)2

40000 30000 20000

20000

10000

0 300

DTGe(FBTTh3)2

400

500

600

700

800

0 300

Wavelength (nm)

400

500

600

700

800

900

Wavelength (nm)

Figure 4. Absorbance of DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 as (a) solutions in chlorobenzene and (b) thin films. The spectra of the reference compound DTGe(FBTTh2)2 are included for comparison.15

Solar Cell Characteristics. Solar cells were fabricated from mixtures of DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 with PC71BM as an acceptor. Blend ratios of 3:7, 4:6, 5:5, 6:4 and 7:3 were explored for mixtures of both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 with PC71BM, while annealing temperatures in the range of 80 to 120 oC were investigated. J-V curves and external quantum efficiency (EQE) spectra corresponding to different blend ratios of each material with PC71BM are reported in Fig. 5, while detailed device characteristics for DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 are reported in Tables S1 and S2, respectively. A brief summary of optimized device parameters is included in Table 2.


10

Page 11 of 28

6

(a)

6

3:7 4:6 5:5 6:4 7:3

4 2

(b)

J (mAcm )

-6

EQE (%)

-4

-2 -4 -6

-8

40 30 20

-8

-10

DTGe(ThFBTTh2)2

-12 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.4

-0.2

Potential (V) 6

2

0.0

0.2

0.4

0.6

0.8

0 300

1.0

400

Potential (V)

(e)

(f)

4

700

800

70

DTGe(FBTTh3)2 60 50

-2 -4 -6

-2 -4 -6

EQE (%)

As-cast o 80 C o 100 C o 120 C o 130 C

-2

-2

J (mAcm )

0

40 30

5:5 As-cast 5:5 Annealed 5:5 SVA

20

-8

-8

-10

-10

DTGe(FBTTh3)2

-12

10

DTGe(FBTTh3)2

-12 -0.2

600

2

0

-0.4

500

Wavelength (nm)

6

3:7 4:6 5:5 6:4 7:3

4

7:3 As-cast 7:3 Annealed 7:3 SVA

10

-10

DTGe(ThFBTTh2)2

-12

(d)

DTGe(ThFBTTh2)2

50

-2

-2

70 60

0

-2

J (mAcm )

(c)

As-cast o 80 C o 100 C o 120 C

4 2

0

J (mAcm )

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


0.0

0.2

0.4

0.6

0.8

1.0

-0.4

-0.2

Potential (V)

0.0

0.2

0.4

0.6

0.8

1.0

0 300

400

Potential (V)

500

600

700

800

Wavelength (nm)

Figure 5. Solar cell characteristics. J-V characteristics of DTGe(ThFBTTh2)2:PC71BM bulk heterojunctions using (a) different blend ratios (b) the 7:3 blend ratio at different annealing temperatures and (c) EQE spectra of 7:3 DTGe(ThFBTTh2)2:PC71BM devices after thermal and solvent vapor annealing. J-V characteristics of DTGe(FBTTh3)2:PC71BM bulk heterojunctions using (d) different blend ratios (e) the 5:5 blend ratio annealed at different temperatures and (f) EQE spectra of 7:3 DTGe(ThFBTTh2)2:PC71BM devices after thermal and solvent vapor annealing.

Table 2. Summary of DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 solar cell characteristics.*

D:A Material

DTGe(ThFBTTh2)2

DTGe(ThFBTTh2)2

VOC

FF

PCE

(mAcm-2)

(V)

4.40

0.585

0.284

0.73

(4.29)

(0.575)

(0.286)

(0.71)

8.33

0.670

0.522

2.92

(8.21)

(0.66)

(0.520)

(2.84)

11.84

0.540

0.562

3.60

(11.87)

(0.563)

(0.491)

(3.28)

Treatment Ratio

DTGe(ThFBTTh2)2

JSC

7:3

7:3

7:3

As-cast TAa (100 oC) SVA (CH2Cl2, 80s)

(%)


11


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

DTGe(FBTTh3)2

DTGe(FBTTh3)2

5:5

5:5

As-cast TAa (120 oC)

Page 12 of 28

6.33

0.675

0.279

1.19

(5.22)

(0.640)

(0.317)

(1.05)

10.64

0.753

0.404

3.24

(10.51)

(0.743)

(0.386)

(3.02)

9.55 0.613 0.518 3.03 SVA (CH2Cl2, (9.30) (0.600) (0.473) (2.65) 80s) * Average values are reported in parentheses underneath champion device characteristics. DTGe(FBTTh3)2

a

5:5

TA = Thermal annealing.

Without thermal annealing or other processing treatments, both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 yielded modest solar cell performance. The best performance of as-cast DTGe(ThFBTTh2)2:PC71BM devices was achieved with a 7:3 blend ratio and included a PCE of 0.73%, while the best performance with a 5:5 blend ratio for as-cast DTGe(FBTTh3)2:PC71BM devices was 1.19%. The formation of desirable film morphologies is critical to the performance of bulk heterojunction solar cells and is not often achieved in as-cast films. Thus, many processing techniques and device architectures20-22 have been developed to control film morphologies and improve semiconducting properties; three highly effective methods include thermal annealing,23-24 solvent additives25-28 and solvent vapor annealing (SVA).29-31 CH2Cl2 was used for solvent vapor annealing as it has a high vapor pressure, allowing the liquid to quickly equilibrate with the vapor phase, and has been used successfully to improve the performance of small molecule solar cells in the past.32-33 All three of these strategies were investigated as routes to improve device performance. Thermal annealing was found to dramatically improve the device characteristics of both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 for all blend ratios. Optimal performance with


12

Page 13 of 28

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


DTGe(ThFBTTh2)2 was achieved using the 7:3 ratio annealed at 100 oC for 10 min. These conditions led to devices with a short-circuit current (JSC), VOC and fill factor (FF) of 8.3 mAcm2

, 0.670 V and 52.2%, respectively, corresponding to a significantly improved PCE of 2.9%

relative to the as-cast device. Optimal performance with DTGe(FBTTh3)2 after thermal annealing was slightly better than in DTGe(ThFBTTh2)2 and was achieved upon thermal annealing of the 5:5 ratio at 120 oC for 10 min. These conditions led to devices with a JSC, VOC and FF of 10.1 mAcm-2, 0.752 V and 41.6%, respectively, corresponding to a dramatically improved PCE of 3.16%. 1-Chloronaphthalene, 1,8-diiodooctane and diphenyl ether were investigated as solvent additives for both DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 however, no significant improvement in photovoltaic parameters was observed. SVA with CH2Cl2, however, was found to significantly improve the performance of both materials relative to pristine films, and in the case of DTGe(ThFBTTh2)2 yielded better performance than thermal annealing. Device characteristics for both materials processed by SVA with CH2Cl2 vapor for various amounts of time are reported in Table S3. Optimal SVA processing time for DTGe(ThFBTTh2)2 devices was 80 seconds, yielding devices with a JSC, VOC and FF of 11.8 mAcm-2, 0.540 V and 56.2%, respectively, corresponding to a dramatically improved PCE of 3.60%. Optimal SVA processing time for DTGe(FBTTh3)2 devices was also 80 seconds, however the performance of these devices was somewhat less than the devices processed by thermal annealing, including a JSC, VOC and FF of 9.5 mAcm-2, 0.613 V and 51.8%, respectively, corresponding to a PCE of 3.03%. It is noteworthy that the photocurrent onsets for optimized SVA devices (Fig.s 5c, f) showed significant red-shifts relative to the parent compound DTGe(FBTTh2)2. Whereas absorption onsets were made less distinct by light scattering, photocurrent onsets were much sharper and


13


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

Page 14 of 28

showed a distinct red-shift. Optimized devices of the parent material DTGe(FBTTh2)2 exhibited a photocurrent onset at 742 nm, whereas optimized DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 devices exhibited onsets at 768 and 761 nm, confirming the decrease in band gap predicted by DFT. Microstructure Analysis. In order to clearly understand the origins of improved device performance upon thermal annealing and SVA, we investigated how these processing treatments affected the film microstructure by transmission electron microscopy (TEM). Samples were prepared using processing conditions optimized for solar cell devices (either annealed at 120 oC for 10 min, or SVA with CH2Cl2 vapor for 80 s) and compared to untreated samples. The float-off technique was used to remove the films from ITO/PEDOT:PSS substrates and mount them on TEM grids. Bright field TEM images for these films are shown in Fig. 6.


14

Page 15 of 28

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


Figure 6. Transmission electron microscope images of 7:3 DTGe(ThFBTTh2)2:PC71BM (a-c) and 5:5 DTGe(FBTTh3)2:PC71BM (d-f) bulk heterojunctions. Images (a, d) correspond to ascast films; images (b, e) correspond to films thermally annealed at 120 oC while images (c, f) correspond to SVA films.

For as-cast DTGe(ThFBTTh2)2:PC71BM films, large scale phase separation can be seen with light-colored globular features having horizontal dimensions in the range of 100 to 300 nm. This kind of large-scale phase separation is known to be detrimental to SMBHJ device performance.23 In contrast, films annealed at 120 oC show regular diamond / almond shaped crystal-like features with domain sizes in the range of 50 to 100 nm. This change in morphology corresponds well with the dramatic improvement in solar cell performance, including a notable increase in JSC from 4.4 to 8.3 mAcm-2 and increase in FF from 0.28 to 0.52. These observations are consistent with increased interfacial area between donor and acceptor phases, which facilitates photoinduced electron transfer, as well as improved material crystallinity, which facilitates efficient charge carrier extraction. SVA of DTGe(ThFBTTh2)2 also had a notable effect on the morphology of DTGe(ThFBTTh2)2; notably, the SVA films exhibited bright rod-like domains with widths on the order of 10 to 50 nm. These reduced feature sizes correspond well with a significantly increased JSC of 11.8 mAcm-2 and PCE of 3.6%, which can again be attributed to optimal donor-acceptor interfacial area. In order to develop a deeper understanding of the film microstructure and confirm the qualitative changes in crystallinity observed in TEM images, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed using the same processing conditions used in the TEM samples. These data are shown in Fig. 7. For comparison, GIWAXS data were collected for pristine materials as well and are reported in the Supplementary Information (Fig. S1).


15


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

Page 16 of 28

Figure 7. GIWAXS patterns of 7:3 DTGe(ThFBTTh2)2:PC71BM (a-c) and 5:5 DTGe(FBTTh3)2:PC71BM (d-f) bulk heterojunctions. Images (a, d) correspond to as-cast films; images (b, e) correspond to films thermally annealed at 120 oC while images (c, f) correspond to SVA films.

The as-cast, SMBHJ films of both materials show symmetric, diffuse rings at q ≈ 1.3 Å-1 corresponding to diffraction from the fullerene cages in the PC71BM. Both materials also show sharper rings at q ≈ 1.7 Å-1 which correspond to diffraction from ordered π-π stacking in the materials. DTGe(ThFBTTh2)2 exhibits a fairly uniform ring, indicating that the π-π stacking in the material is fairly anisotropic, whereas DTGe(FBTTh3)2 shows the most intense π-π diffraction in the qxy plane, indicating stronger isotropic π-π stacking in the horizontal direction and a preference for edge-on packing relative to the substrate. DTGe(ThFBTTh2)2 shows a series of diffraction rings at q = 0.3 to 0.7 Å-1 which correspond to diffraction due to lamellar packing along the long axes of the molecule. The rings are more intense along the vertical axis, which indicates that lamellar packing preferentially occurs in planes oriented parallel to the


16

Page 17 of 28

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


surface with the molecules oriented edge-on. DTGe(FBTTh3)2 shows an even stronger preference to undergo edge-on packing. The GIWAXS patterns of both materials became dramatically sharper upon thermal annealing, indicating an increase in crystallinity, concurrent with the appearance of well-defined structural features in TEM images. DTGe(ThFBTTh2)2 shows two discreet rings at q = 1.67 and 1.74 Å-1 indicating two different π-π stacking modes with inter-planar distances of 3.77 and 3.61 Å. These two rings could be explained by the presence of two separate crystal phases with different π-π stacking distances, or a unit cell in which two parts of the molecule overlap with different interplanar distances. We have not been able to obtain single crystals suitable for X-ray crystallography, and therefore the unit cell(s) and specific diffraction plane assignments remain unknown at this time. However, the intensity of the π-π diffraction peaks is highest at an angle of ≈ 45o relative to the origin, suggesting that π-π stacking in DTGe(ThFBTTh2)2 preferentially occurs at an oblique angle relative to the substrate, as opposed to edge-on or face-on orientations which are common in organic semiconductors. Annealed DTGe(ThFBTTh2)2 exhibits a number of diffraction peaks in the range of q = 0 to 1.74 Å-1 which correspond to d-spacing values in the range of 1 − 5 nm, consistent with the length or width of the molecules. The

annealed

DTGe(FBTTh3)2

exhibits

fewer

peaks

than

the

corresponding

DTGe(ThFBTTh2)2, suggesting a more symmetric crystal structure. A single ring at q = 1.67 Å1

indicates a π – π stacking distance of 4.07 Å, which is significantly longer than the as-cast film

or DTGe(ThFBTTh2)2 either before or after annealing. The π-π diffraction peak in this material is more intense near the x-axis, suggesting that the material preferentially stacks in the horizontal direction and preferentially orients edge-on relative to the substrate. Several diffraction peaks


17


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

Page 18 of 28

occur at q = 0.2 to 1.0 Å-1 corresponding to lamellar diffraction. A peak at q = 0.300 Å-1 shows much stronger intensity than all of the other peaks and corresponds to a d-spacing of 2.09 nm. SVA films of both DTGe(FBTTh3)2 and DTGe(ThFBTTh2)2, bulk heterojunctions exhibit peak intensities which are intermediate between the as-cast and annealed films. These observations are consistent with the intermediate-sized structural features apparent in TEM images of SMBHJ films of both materials. The diffraction pattern of SVA DTGe(ThFBTTh2)2 exhibits peaks at similar locations as the annealed film, indicating that the same crystal structure is obtained whether DTGe(ThFBTTh2)2 bulk heterojunctions are processed by either thermal or SVA. In contrast, DTGe(FBTTh3)2 exhibits peaks with different q-values than observed in the thermally annealed films. Notably, SVA DTGe(FBTTh3)2 SMBHJs show π-π stacking and intense lamellar peaks corresponding to π-π stacking and lamellar d-spacing values of 3.542 Å and 1.43 nm, respectively. These observations lead to the interesting conclusion that different crystal structures are obtained with DTGe(FBTTh3)2 SMBHJs if the film is processed by thermal or SVA. Conclusions A series of DTGe core-based D-A small molecules was designed and synthesized with specific focus on the influence of the heterocyclic unit ordering on material and device properties. By incorporation of additional thiophene rings in different sequence of the DTGe platform we were able to extend the conjugation length of the molecules and tune the energy band structure. The optical band gaps decreased slightly upon incorporation the thiophene rings, leading to extended photocurrent onsets, while the HOMO levels increased in energy. A greater increase in HOMO level was observed when the thiophene ring was inserted between the DTGe and FBT groups than when the thiophene was added to the terminus of the molecule. This increase in HOMO


18

Page 19 of 28

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


energy had very important implications in device performance, as it resulted in a decrease in VOC of the new materials, DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2 compared to the parent material, DTGe(FBTTh3)2 and lower overall device performance. The increase in HOMO energy was greater for DTGe(ThFBTTh2)2 than for DTGe(FBTTh3)2, indicating that incorporating the additional, electron-rich thiophene moieties next to the dithienogermole core of the molecule had a more profound effect on the energy band structure than when they were incorporated at the outer periphery of the molecule. Film processing by thermal and SVA could be used to control the phase separation and crystallinity of the films. Both materials exhibited improved crystallinity with SVA and thermal annealing. Interestingly, DTGe(FBTTh3)2 was found to adopt two different crystal structures depending on whether the films were SVA or thermally annealed. Both materials were effective donors in SMBHJ type solar cell devices: Optimal processing conditions included a 7:3 donor:acceptor ratio and CH2Cl2 SVA, in the case of DTGe(ThFBTTh2)2 and a 5:5 donor:acceptor ratio and thermal annealing at 120 oC in the case of DTGe(FBTTh3)2. Both materials exhibited nano-crystalline features and phase separation on the order of tens of nanometers in optimized films, with peak efficiencies of 3.60 and 3.24 % for DTGe(ThFBTTh2)2

and

DTGe(FBTTh3)2,

respectively.

The

higher

efficiency

of

DTGe(ThFBTTh2)2 stemmed from a larger JSC and FF, than DTGe(FBTTh3)2, which were more significant than the negative effect that the high-lying HOMO band had on the VOC. The relative JSC and FF can be rationalized by the optimized morphologies of each material; the relatively large and discontinuous almond-shaped donor-rich domains apparent in TEM images of DTGe(FBTTh3)2 films apparently led to inefficient hole extraction and increased charge carrier recombination compared to the cross-hatched network of material that is observed in


19


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

Page 20 of 28

DTGe(ThFBTTh2)2. This work offers a clear and detailed picture of how the placement of thiophene moieties in the conjugated backbone affects the structure, optoelectronic properties and solar cell performance of conjugated small molecules and may aid in the future design of molecular SMBHJ materials. Experimental All starting materials were purchased from Aldrich and Acros and used without further purification and FBTTh3 and ThFBTTh2 were synthesized by a slightly modified method previously reported by our group.15 Anhydrous THF, Toluene, and DMF were obtained by distillation from sodium/benzophenone prior to use. 1H NMR and

13

C NMR spectra were

recorded on a VNMRS 400 (Varian, USA) spectrophotometer using CDCl3 as solvent and tetramethylsilane (TMS) as the internal standard and MALDI MS spectra were obtained from Ultraflex III (Bruker, Germany). UV-VIS-NIR spectra were taken on Cary 5000 (Varian USA) spectrometer at room temperature. Absorption (and photocurrent) onsets were determined by fitting the steepest part of the onset with a line and finding the intersection of this line with the background (defined as line tangent to where the data crossed the x-axis). Element analysis was carried out with a Flash 2000 (Thermo Scientific, Netherlands). DFT calculations were employed using Gaussian software with a B3LYP/6-31G* basis set and solubilizing alkyl chains were truncated to methyl groups to allow convergence on a minimum energy conformation. Cyclic voltammetry (CV) measurements were performed on Solartron electrochemical station (METEK, Versa STAT3) with three-electrodes: a silver wire pseudo-reference electrode, a platinum wire counter electrode and a glassy carbon working electrode. The glassy carbon electrode was polished with alumina (1, 0.3 µm) before use. Chloroform was used as a solvent with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. The


20

Page 21 of 28

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


measurements were done at an analyte concentration of 1~2 mg/ml with a scan rate of 0.1 V/s under a flow of nitrogen bubbles. The HOMO energy levels were obtained from the equation HOMO= –(Eoxonset – E(ferrocene)onset + 4.8) eV. The LUMO levels of small molecule were obtained from optical bandgap and HOMO energy level. Atomic force microscopy was carried out using a Veeco Multimode microscope in tapping mode with Si tips having a resonant frequency of 300 kHz. TEM images were collected using a JEM-2100F (Cs corrector) HR-TEM using lacey carbon grids (purchased from Ted Pella) using defocusing values of ~20 − 30 µm. Synthesis of 7,7'-(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6fluoro-4-(5''-hexyl-[2,2':5',2''-terthiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTGe(FBTTh3)2): In a N2 filled glove box, a 30mL microwave tube was charged with the compound 5,5 ′ bis(trimethylstannyl)-3,3′-di-2-ethylhexylgermolo-2,2′-bithiophene (Me3Sn-DTGe-SnMe3, 300 mg, 0.38), 4-bromo-5-fluoro-7-(5''-hexyl-[2,2':5',2''-terthiophen]-5-yl)benzo[c][1,2,5]thiadiazole (FBTTh3), Pd(PPh3)4, and sealed with a Teflon cap. The reaction mixture was gradually heated to 100 oC, (1 min), 125 oC (1 min), 140 oC (10 min), 150 oC (10 min), and 160 oC (60 min) using a Biotage microwave reactor. Upon cooling, the crude product was washed with methanol and purified by flash chromatography (hexane/chloroform 3:2). As for further purification, the solid was sonicated for 1 hour and stirred overnight with mixture of solvents (methanol/hexane 3:1). The products were filtrated with acetone and recrystallized with hexane and methylene chloride. The metallic purple solid product was obtained (150 mg, isolated yield = 27%). 1H NMR (400MHz, CDCl3, δ): 8.35 (s, 2H), 8.03 (d, 2H), 7.74 (d, 2H), 7.23 (d, 2H), 7.18 (d, 2H), 7.04 – 6.98 (m, 4H), 6.70 (d, 2H), 2.80 (t, 4H), 1.76 – 1.57 (m, 8H), 1.47 – 1.17 (m, 30H), 0.87 (dd, 18H).

13

C NMR (150MHz, CDCl3, δ): 153.98, 152.56, 152.24, 152.12, 148.44, 147.88, 147.67,

141.98, 140.04, 139.13, 138.50, 137.58, 137.07, 136.89, 136.74, 132.79, 131.43, 127.41, 127.23,


21


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

Page 22 of 28

126.69, 126.08, 126.05, 39.77, 38.37, 34.22, 34.12, 32.83, 32.34, 31.79, 31.46, 25.78, 25.24, 23.58, 16.88, 16.73, 13.59. Elemental Analysis Calc. for C72H76F2GeN4S10: C 60.53, H 5.36, N 3.92, S 22.44. Found: C 60.47, H 5.21, N 3.79, S 22.75. MALDI-TOF MS: Calc. for 1428.25; Found: 1428.28. Synthesis of 7,7'-(5,5'-(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(thiophene-5,2-diyl))bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTGe(ThFBTTh2)2): In a N2 filled glove box, a 30mL microwave tube was charged with the compound 5,5 ′ -bis(trimethylstannyl)-3,3 ′ -di-2-ethylhexylgermolo-2,2 ′ -bithiophene (Me3Sn-DTGe-SnMe3, 300 mg, 0.38), 4-(5-bromothiophen-2-yl)-5-fluoro-7-(5'-hexyl-[2,2'bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (ThFBTTh2), Pd(PPh3)4, and sealed with a Teflon cap. The reaction procedure and purification were performed with the same method as the DTGe(FBTTh3)2. DTGe(ThFBTTh2)2 was afforded as purple solid (163mg, isolated yield = 30%). 1H NMR (400MHz, CDCl3, δ): 8.17 (d, 2H), 8.00 (dd, 2H), 7.70 (d, 2H), 7.29 (s, 2H), 7.22 (d, 2H), 7.16 (dd, 2H), 7.11 (d, 2H), 6.72 (dd, 2H), 2.82 (t, 4H), 1.78 – 1.64 (m, 10H), 1.46 – 1.13 (m, 28H), 0.99 – 0.77 (m, 18H). 13C NMR (100MHz, CDCl3, δ): 159.69, 151.68, 151.39, 149.33, 146.03, 145.69, 144.94, 144.66, 139.84, 138.20, 135.68, 134.35, 130.68, 130.64, 128.52, 126.41, 126.35, 124.80, 123.68, 123.29, 122.76, 111.00, 37.05, 35.66, 31.60, 31.44, 30.20, 29.10, 28.87, 23.62, 23.22, 22.62, 20.82, 14.35, 14.10, 11.00. Elemental Analysis Calc. for C72H76F2GeN4S10: C 60.53, H 5.36, N 3.92, S 22.44. Found: C 60.59, H 5.36, N 5.40, S 22.74. MALDI-TOF MS: Calc. for 1428.25; Found: 1428.17. Solar Cell Device Fabrication and Testing: Solar cell devices were fabricated on glass/ITO substrates which were cleaned with detergent, then ultra-sonicated in acetone and isopropyl alcohol and subsequently dried in an oven overnight at 100 oC. PEDOT:PSS layers (Baytron


22

Page 23 of 28

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


Clevios AI4083) were next deposited by filtering through a 0.45 µm cellulose acetate syringe filter and spin-coating at 4000 rpm for 40 s, followed by baking at 140 oC for 15 min in air and transferring into a nitrogen filled glove box. Unless otherwise noted, active layers were deposited by spin coating mixed solutions of each donor material with phenyl-C71-butyric acid methyl ester (PC71BM) at overall solids concentrations of 33 mg / mL. Active layers were allowed to dry for 5 min at room temperature followed by 5 min at 70 oC on a hotplate. Aluminum electrodes (100 nm thick) were deposited by thermal evaporation. The deposited electrodes defined the active area of the devices as 13 mm2. Current density–voltage (J-V) measurements were collected with a Keithley 2635A source measurement unit, carried out under a nitrogen atmosphere using simulated (AM 1.5G) solar radiation from a Xenon arc lamp equipped with a high quality optical fiber to guide the light. J-V curves were measured under 100 mWcm-2 simulated solar illumination, calibrated with a standard Si photodiode. Eight solar cell devices were fabricated for each condition yielding at least five functioning devices in each case; average values of these devices are reported in parentheses underneath the best observed values. EQE measurements were conducted in ambient air using an EQE system (Model QEX7) by PV measurements Inc. (Boulder, Colorado). Spectral mismatch factors between currents produced under the Xenon arc lamp and standard AM1.5 G spectrum were found to be 10% or less.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Extended tables of device data, GIWAXS data of neat films (PDF) AUTHOR INFORMATION


23


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

Page 24 of 28

Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ⊥These authors contributed equally.

Funding Sources Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2015R1A2A1A10053397, 2015H1D3A1062473, 2014K1A3A1A19066591) and the Ulsan National Institute of Science and Technology (Research Funds 1.160092.01). Research and Development program of the Korea Institute of Energy Research (B6-2432). GIWAXD measurements at PLS-II 6D UNIST-PAL beamline and 9A beamline were supported in part by MEST, POSTECH, and UNIST UCRF. REFERENCES 1. Lin, Y.; Li, Y.; Zhan, X., Small Molecule Semiconductors for High-Efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41 (11), 4245-4272. 2. Roncali, J.; Leriche, P.; Blanchard, P., Molecular Materials for Organic Photovoltaics: Small Is Beautiful. Adv. Mater. 2014, 26 (23), 3821-3838.


24

Page 25 of 28

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


3. Walker, B.; Kim, C.; Nguyen, T.-Q., Small Molecule Solution-Processed Bulk Heterojunction Solar Cells. Chem. Mater. 2011, 23 (3), 470-482. 4. Ni, W.; Wan, X.; Li, M.; Wang, Y.; Chen, Y., A-D-A Small Molecules for SolutionProcessed Organic Photovoltaic Cells. Chem. Commun. 2015, 51 (24), 4936-4950. 5. Liu, Y.; Chen, C.-C.; Hong, Z.; Gao, J.; Yang, Y.; Zhou, H.; Dou, L.; Li, G.; Yang, Y., Solution-Processed Small-Molecule Solar Cells: Breaking the 10% Power Conversion Efficiency. Scientific Reports 2013, 3, 3356. 6. Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y., Solution-Processed Organic Solar Cells Based on Dialkylthiol-Substituted Benzodithiophene Unit with Efficiency near 10%. J. Am. Chem. Soc. 2014, 136 (44), 1552915532. 7. Kan, B.; Li, M.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Long, G.; Yang, X.; Feng, H.; Zuo, Y.; Zhang, M.; Huang, F.; Cao, Y.; Russell, T. P.; Chen, Y., A Series of Simple Oligomer-Like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency. J. Am. Chem. Soc. 2015, 137 (11), 3886-3893. 8. Welch, G. C.; Perez, L. A.; Hoven, C. V.; Zhang, Y.; Dang, X.-D.; Sharenko, A.; Toney, M. F.; Kramer, E. J.; Nguyen, T.-Q.; Bazan, G. C., A Modular Molecular Framework for Utility in Small-Molecule Solution-Processed Organic Photovoltaic Devices. J. Mater. Chem. 2011, 21 (34), 12700-12709. 9. Coughlin, J. E.; Henson, Z. B.; Welch, G. C.; Bazan, G. C., Design and Synthesis of Molecular Donors for Solution-Processed High-Efficiency Organic Solar Cells. Acc. Chem. Res. 2013, 47 (1), 257-270. 10. Ko, H. M.; Choi, H.; Paek, S.; Kim, K.; Song, K.; Lee, J. K.; Ko, J., Molecular Engineering of Push-Pull Chromophore for Efficient Bulk-Heterojunction Morphology in Solution Processed Small Molecule Organic Photovoltaics. J. Mater. Chem. 2011, 21 (20), 72487253. 11. Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.; Brabec, C., Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications. Macromolecules 2007, 40 (6), 1981-1986. 12. Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L., Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115 (23), 12666-12731. 13. Walker, B.; Liu, J.; Kim, C.; Welch, G. C.; Park, J. K.; Lin, J.; Zalar, P.; Proctor, C. M.; Seo, J. H.; Bazan, G. C.; Nguyen, T.-Q., Optimization of Energy Levels by Molecular Design: Evaluation of Bis-Diketopyrrolopyrrole Molecular Donor Materials for Bulk Heterojunction Solar Cells. Energy Environ. Sci. 2013, 6 (3), 952-962. 14. Ajayaghosh, A., Donor–Acceptor Type Low Band Gap Polymers: Polysquaraines and Related Systems. Chem. Soc. Rev. 2003, 32 (4), 181-191. 15. Moon, M.; Walker, B.; Lee, J.; Park, S. Y.; Ahn, H.; Kim, T.; Lee, T. H.; Heo, J.; Seo, J. H.; Shin, T. J.; Kim, J. Y.; Yang, C., Dithienogermole-Containing Small-Molecule Solar Cells with 7.3% Efficiency: In-Depth Study on the Effects of Heteroatom Substitution of Si with Ge. Adv. Energy Mater. 2015, 5 (9), 1402044. 16. Bundgaard, E.; Krebs, F. C., Low Band Gap Polymers for Organic Photovoltaics. Sol. Energy Mater. Sol. Cells 2007, 91 (11), 954-985. 17. Fuchigami, H.; Tsumura, A.; Koezuka, H., Polythienylenevinylene Thin‐Film Transistor with High Carrier Mobility. Appl. Phys. Lett. 1993, 63 (10), 1372-1374.


25


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

Page 26 of 28

18. Garnier, F.; Horowitz, G.; Peng, X. Z.; Fichou, D., Structural Basis for High Carrier Mobility in Conjugated Oligomers. Synth. Met. 1991, 45 (2), 163-171. 19. Huang, C.-F.; Chang, J.-Y.; Huang, S.-H.; Wu, K.-Y.; Jheng, J.-F.; Chuang, W.-T.; Hsu, C.-S.; Wang, C.-L., Polymorphisms and Morphological Studies of a Difluorobenzothiadiazole Conjugated Copolymer with 7.8% Polymer Solar Cell Efficiency. J. Mater. Chem. A. 2015, 3 (7), 3968-3974. 20. Kim, H.; Byun, J.; Bae, S.-H.; Ahmed, T.; Zhu, J.-X.; Kwon, S.-J.; Lee, Y.; Min, S.-Y.; Wolf, C.; Seo, H.-K.; Ahn, J.-H.; Lee, T.-W., On-Fabrication Solid-State N-Doping of Graphene by an Electron-Transporting Metal Oxide Layer for Efficient Inverted Organic Solar Cells. Adv. Energy Mater. 2016, 6 (12), 1600172. 21. Lim, K.-G.; Ahn, S.; Kim, Y.-H.; Qi, Y.; Lee, T.-W., Universal Energy Level Tailoring of Self-Organized Hole Extraction Layers in Organic Solar Cells and Organic-Inorganic Hybrid Perovskite Solar Cells. Energy Environ. Sci. 2016, 9 (3), 932-939. 22. Lim, K.-G.; Park, S. M.; Woo, H. Y.; Lee, T.-W., Elucidating the Role of Conjugated Polyelectrolyte Interlayers for High-Efficiency Organic Photovoltaics. ChemSusChem 2015, 8 (18), 3062-3068. 23. Walker, B.; Tamayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q., Nanoscale Phase Separation and High Photovoltaic Efficiency in Solution‐ Processed, Small‐Molecule Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2009, 19 (19), 3063-3069. 24. Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J., Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15 (10), 1617-1622. 25. Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J., Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2008, 130 (11), 3619-3623. 26. Su, M. S.; Kuo, C. Y.; Yuan, M. C.; Jeng, U.; Su, C. J.; Wei, K. H., Improving Device Efficiency of Polymer/Fullerene Bulk Heterojunction Solar Cells through Enhanced Crystallinity and Reduced Grain Boundaries Induced by Solvent Additives. Adv. Mater. 2011, 23 (29), 33153319. 27. Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C., Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6 (7), 497-500. 28. Park, J. K.; Kim, C.; Walker, B.; Nguyen, T.-Q.; Seo, J. H., Morphology Control of Solution Processable Small Molecule Bulk Heterojunction Solar Cellsviasolvent Additives. RSC Advances 2012, 2 (6), 2232-2234. 29. Wessendorf, C. D.; Schulz, G. L.; Mishra, A.; Kar, P.; Ata, I.; Weidelener, M.; Urdanpilleta, M.; Hanisch, J.; Mena‐Osteritz, E.; Lindén, M., Efficiency Improvement of Solution‐Processed Dithienopyrrole‐Based a‐D‐a Oligothiophene Bulk‐Heterojunction Solar Cells by Solvent Vapor Annealing. Adv. Energy Mater. 2014, 4 (14), 1400266. 30. Verploegen, E.; Miller, C. E.; Schmidt, K.; Bao, Z.; Toney, M. F., Manipulating the Morphology of P3HT–PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing. Chem. Mater. 2012, 24 (20), 3923-3931. 31. Miller, S.; Fanchini, G.; Lin, Y.-Y.; Li, C.; Chen, C.-W.; Su, W.-F.; Chhowalla, M., Investigation of Nanoscale Morphological Changes in Organic Photovoltaics During Solvent Vapor Annealing. J. Mater. Chem. 2008, 18 (3), 306-312.


26

Page 27 of 28

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


32. Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R., Solvent-Annealed Crystalline Squaraine: PC70BM (1:6) Solar Cells. Adv. Energy Mater. 2011, 1 (2), 184-187. 33. Viterisi, A.; Gispert-Guirado, F.; Ryan, J. W.; Palomares, E., Formation of Highly Crystalline and Texturized Donor Domains in DPP(TBFu)2:PC71BM SM-BHJ Devices Via Solvent Vapour Annealing: Implications for Device Function. J. Mater. Chem. 2012, 22 (30), 15175-15182.


27


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

Page 28 of 28

TOC Graphic:


28

Effect of Heterocyclic Anchoring Sequence on the ... - ACS Publications

Recommend Documents