UV-Cross-linkable DonorâAcceptor Polymers Bearing a Photostable

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UV-Crosslinkable Donor-Acceptor Polymers bearing Photostable Conjugated Backbone for Efficient and Stable Organic Photovoltaics Si-Cheng Wu, Lisa T. Strover, Xiang Yao, Xue-Qiang Chen, Wen-Jing Xiao, LiNa Liu, Jiandong Wang, Iris Visoly-Fisher, Eugene A. Katz, and Wei-Shi Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11506 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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ACS Applied Materials & Interfaces

UV-Crosslinkable

Donor-Acceptor

Polymers

bearing Photostable Conjugated Backbone for Efficient and Stable Organic Photovoltaics Si-Cheng Wu,a Lisa T. Strover,b Xiang Yao,a Xue-Qiang Chen,a Wen-Jing Xiao,a Li-Na Liu,a Jiandong Wang,a Iris Visoly-Fisher,*,b Eugene A. Katz*,b and Wei-Shi Li*,a,c a

Key Laboratory of Synthetic and Self-assembly Chemistry for Organic Functional Molecules,

Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling road, Shanghai 200032, China. b

Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland

Environmental and Energy Research, The Jacob Blaustein Institutes for Desert Research (BIDR), Ben-Gurion University of the Negev, Sede Boqer Campus, Midereshet Ben-Gurion 8499000, Israel. c

Engineering Research Center of Zhengzhou for High Performance Organic Functional

Materials, Zhengzhou Institute of Technology, 6 Yingcai Street, Huiji District, Zhengzhou 450044, China.

KEYWORDS: organic photovoltaics, crosslinkable D-A copolymers, thermal stability, photostability, concentrated sunlight.

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ABSTRACT: High performance photovoltaic polymers bearing crosslinkable function together with a photo-robust conjugated backbone are highly desirable for organic solar cells to achieve both high device efficiency and long-term stability. In this paper, a family of such polymers is reported based on poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difuoro-4,7-di(thiophen-2yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT), a high performance photovoltaic donor-acceptor polymer, with different contents of terminal vinyl-appended side chains for crosslinking. The polymers were named PPDT2FBT-Vx and prepared by varying the feeding ratio (x mol%, x = 0, 5, 10, and 15) of the vinyl-appended monomer in polymerization. It was found that the vinyl integration did not sacrifice the original high photovoltaic performance of the polymers, as evidenced by comparable average power conversion efficiencies (6.95%, 7.02% and 7.63%) observed for optimized devices based on PPDT2FBT-V0, PPDT2FBT-V5, and PPDT2FBT-V10, respectively, in blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Unlike thermal-crosslinking that greatly reduced device efficiency, UV crosslinking has proven to be an effective way to achieve both high device efficiency and thermostability for PPDT2FBT-V10 solar cells. UV-crosslinked PPDT2FBT-V10 solar cells displayed an initial average PCE of 5.28% and almost no decrease upon heat treatment at 120 C for 40 h. Morphology studies revealed that UV-crosslinking not only did not alter initial nanophase separation but also suppressed morphology evolution by aggregation in bulk heterojunction blend films. Photo-crosslinking requires material photo-stability. It is therefore worthwhile to note that these polymers and their blends with PC71BM were found to be extremely photostable, even upon continuing exposure to concentrated sunlight (up to 200 suns), and UV-crosslinking does not hamper this photostability, Further studies found that devices fabricated with UV-crosslinked PPDT2FBT-V10/PC71BM


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active layer can endure continuous light exposure to a solar simulator without deterioration in their performance.

1. Introduction

Organic solar cells (OSCs) present a third-generation solar-to-electric conversion technology with advantages including flexibility, solution-processable materials, and suitability for manufacture of light-weight and large-area devices.1-4

In the past decade, OSCs have

achieved rapid increase in device power conversion efficiency (PCE),5-8 exceeding 14% for various cells including single junction-,9 tandem-10 and three-component-structure11 devices. With such high efficiency, insufficient device stability now becomes one of the primary obstacles for OSC commercialization.12 Analysing the complex factors that cause device degradation, it is apparent that microstructure evolution of the bulk heterojunction (BHJ) active layer is one of the major reasons.13-15 In general, a BHJ active layer is prepared by blending a donor and an acceptor material, which form a donor/acceptor nanophase-separated bicontinuous network required for high performance.16-17 In most reported works, high device efficiencies benefited from favourable microstructures of specific donor and acceptor materials, with careful optimization of the process details including the donor/acceptor weight ratio; processing solvent; heat treatment; and additive engineering.18-23 However, BHJs generally lack factors that can fix its component molecules in place and prevent their further diffusion, leading to undesirable microstructure evolution with time. Such evolution may be accelerated under heat and light stress during daily device operation. Therefore, it is important to find a way to suppress such microstructure evolution and stabilize the device for long term use.



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To address the above issue, a strategy utilizing crosslinkable materials as one of the active layer components has been proposed.14-15, 24-25 Crosslinking between the component molecules is activated after an appropriate microstructure is developed in the active layer, to “lock” the obtained microstructure in place and avoid its further evolution. A variety of photovoltaic polymers, including poly(3-hexylthiophene),25-28 poly(benzodithiophene-alt-thienothiophene),29 poly(benzodithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD),30 poly(thiophene-altquinoxaline)31 and poly(cyclopentadithiophene-alt-benzothiadiazole)32 have been modified with crosslinkable functionality by integrating vinyl,25, 30 bromo-,26, 33 oxetane,28 or azide,27 into the terminal units of their side chains. The so-afforded crosslinked devices displayed much better thermal stability than reference cells based on the same polymers without crosslinking, and/or those based on unmodified pristine polymers, unambiguously demonstrating the feasibility of this crosslinking strategy in stabilizing the active layer microstructure. However, the device efficiencies of cells incorporating these crosslinkable polymers were mostly inferior compared with state-of-the-art OSC efficiency levels over 10%. 33 Recently, we have endeavoured to design and develop crosslinkable photovoltaic materials that can afford solar cells combining long-term stability with high efficiency. Our approach is to combine the above crosslinking strategy with presently-known high performance materials. In a previous work, we integrated vinyl functionality into the side chain of PBDTTPD.30 The soafforded solar cells with crosslinked active layer displayed an initial efficiency of 6.06% and kept 91% of initial efficiency after heating at 150 C for 40 h, one of the highest PCEs of OSCs using crosslinked active layers.14 However, further studies found that these polymers are not photo-stable and undergo photo-bleaching upon exposure to natural or concentrated sunlight in Sede Boqer (in the Negev Desert in Israel, Lat. 30.8°N, Lon. 34.8°E, Alt. 475 m) (unpublished


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results). Obviously, the material's photo-stability is of utmost importance for efficient and stable OSCs. It is therefore surprising that the stability of cross-linked BHJ films was commonly tested only via heat- and solvent- stressing, while their photo-stability was only rarely characterized.28, 31

Herein we report a new family of donor-acceptor conjugated polymers, based on poly[phenylene-alt-(4,7-di(thiophen-2-yl)-5,6-difluorobenzo[c]-[1,2,5]thiadiazole)] with vinyl units on their side chains (PPDT2FBT-Vx, x%, x = 0, 5, 10, and 15, Scheme 1), which possess both crosslinkable function and photo-stable conjugated backbone. This polymer family, which was not cross-linked hitherto, has previously shown high photovoltaic performance with the best efficiency of 9.39%,34 To our delight, the integration of  10% content of vinyl units resulted in small influence on the original high photovoltaic performance. This is unlike most reported works,28 in which cross-linkable modification caused performance deterioration.

More

importantly, exposure to both natural and concentrated sunlight in the Negev Desert revealed that these polymers are photo-stable regardless of their vinyl-functionalization and crosslinking, either in its pristine form or blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in BHJ films. Excellent photo-stability encourages the applicability of photo-crosslinking of the polymers.

This is important since OSC device fabrication generally includes morphology

optimization by thermal annealing. In contrast with thermal crosslinking that might affect the thermal morphology optimization, photo-crosslinking allows independent control of both processes.14, 26, 27, 31 This is the case in this work, in which thermal crosslinking deteriorated PPDT2FBT-V10 solar cells at all, while UV crosslinking established efficient cells with initial



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average PCE of 5.28%. Moreover, such UV-crosslinked devices showed excellent stability toward to both heat and light exposure.

Scheme 1. Synthesis of PPDT2FBT-Vx (x = 0, 5, 10, and 15) polymers.

2. Results and discussion

2.1 Synthesis and characterization

As shown in Scheme 1, PPDT2FBT-Vx polymers were synthesized via Stille-coupling polymerization using the stannylated monomer M1, and two brominated monomers, M2 and M3. M1 and M2, the conventional monomers for PPDT2FBT polymer synthesis, were prepared according to literature-reported methods34-35 as described in details in Supporting Information. M3, a vinyl-appended monomer that endows the polymer with crosslinking functionality, was synthesized via a similar method to that of M2, and was structurally identified by 1H and

13

C

NMR (Figures S1 and S2). During the Stille coupling polymerization, the feeding ratio between M1, M2, and M3 was set to be 100 : 100-x : x, in which the value of x was 0, 5, 10, or 15. The afforded polymers, denoted as PPDT2FBT-Vx, were obtained in a yield of 58~76%. In order to determine the exact vinyl content incorporated in the polymers, 1H NMR characterization was


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carried out in 1,1,2,2-tetrachloroethane-d2 at 110 C. As shown in Figure 1, vinyl proton signals appear at 5.0 and 5.9 ppm for all these polymers except PPDT2FBT-V0, confirming the successful incorporation of vinyl-appended units into the polymer structure. From the integrals of these peaks, the vinyl content in the polymers was estimated to be 6% for PPDT2FBT-V5, 8% for PPDT2FBT-V10 and 11% for PPDT2FBT-V15 .The polymers were found to be highly soluble in chlorinated solvents like chloroform (CF), o-dichlorobenzene (OCB) and chlorobenzene (CB), which is consistent with the literature.34 By means of gel permeation chromatography (GPC), number-average molecular weights (Mn) were determined to be 9.3, 12.5, 19.3, and 12.2 kDa for PPDT2FBT-V0, PPDT2FBT-V5, PPDT2FBT-V10 and PPDT2FBT-V15, respectively, and their polydispersity indices (Ɖ) were found to be in the range of 2.07‒2.30, (Table 1). Moreover, the polymers possess good thermostability with 5%-weight-loss temperatures (Td), measured by thermogravimetric analysis (TGA, Figure. S3), around 400 C. In differential scanning calorimetry (DSC) within the temperature region of 0‒400 C (Figure S4), no thermal transition peaks were found for these polymers, implying an amorphous structure.

Table 1. Properties of the as-synthesized PPDT2FBT-Vx polymers. Polymer

M na (kDa)

Ɖa

Tdb (C)

λabs, onset (nm)

Eg, optc (eV)

PPDT2FBT-V0

9.3

2.09

398

704

1.76

0.60

-5.40

-3.64

PPDT2FBT-V5

12.5

2.07

399

709

1.75

0.58

-5.36

-3.61

PPDT2FBT-V10

19.3

2.57

399

701

1.77

0.60

-5.38

-3.61

PPDT2FBT-V15

12.2

2.30

410

695

1.78

0.49

-5.28

-3.50

Eox, onset HOMOd LUMOe (eV) (eV) (eV)

a

Measured by high temperature GPC using 1,2,4-trichlorobenzene and monodisperse polystyrene standards at 150 C. b The temperature of 5%-weight loss. c Calculated by 1240/λabs, + d e onset. HOMO = - e (Eox, onset - EFc/Fc + 4.8 V). LUMO = HOMO + Eg, opt.



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Figure 1. 1H-NMR spectra of PPDT2FBT-Vx in 1,1,2,2-tetrachloroethane-d2 at 110 C. The UV-vis absorption spectra in dilute CF solutions and as films are displayed in Figure 2a and Table 1 for detailed data. One can observe that these spectra are almost identical, with three absorption peaks in the region of 350-800 nm. The first peak appeared around 400 nm, which is attributable to the -* transition of the conjugated backbone. In solution, the second and third peaks were observed around 590 and 640 nm respectively. These peaks are assignable to 0-0 and 0-1 transitions of the intramolecular charge transfer (ICT) absorption band.30 In films, red shifts were clearly observed for these ICT peaks, suggesting the occurrence of - stacking


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among polymer chains in the solid state. The extent of these red shifts was found to depend on the polymer structure. For polymers PPDT2FBT-V0, PPDT2FBT-V5, and PPDT2FBT-V10, the red shifts for the 640 nm-peak were larger than 15 nm, while that of the corresponding PPDT2FBT-V15 peak was only 8 nm. This implies that the amount of vinyl content may have a subtle influence on the polymer chain packing in the solid state.

PPDT2FBT-V0 PPDT2FBT-V5

in solution

PPDT2FBT-V10

Fc

PPDT2FBT-V15

Normalized Absorbance

PPDT2FBT-V15

Current

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


in film

PPDT2FBT-V10 PPDT2FBT-V5 PPDT2FBT-V0

300

400

500

600

700

800

-1.5

-1.0

Wavelength (nm)

-0.5

0.0

0.5

Potential vs Ag/Ag+(v)

1.0

1.5

Figure 2. (a) UV-vis absorption spectra of PPDT2FBT-Vx polymers in dilute chlorobenzene solution and as films. (b) Cyclic voltammetry of PPDT2FBT-Vx films.

The bandgap and energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are highly relevant for photovoltaic materials and can be estimated from the onset redox potentials and light-absorption edge of film samples. As shown in Figure 2a and Table 1, these polymer films showed a light-absorption edge around 700 nm, i.e., bandgaps (Eg, opt) around 1.76 eV. In cyclic voltammetry (Figure 2b) PPDT2FBT-Vx films displayed decreasing onset oxidation potentials (Eox,onset) from 0.60 V for PPDT2FBT-V0 to



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0.49 V for PPDT2FBT-V15 vs. Ag/Ag+ reference electrode. Since ferrocene/ferrocenium (Fc/Fc+) redox couple having a standard energy level of -4.8 eV showed up at 0.02 V under the same conditions, the HOMO energy level was estimated to be -5.40 eV for PPDT2FBT-V0, -5.36 eV for PPDT2FBT-V5, -5.38 eV for PPDT2FBT-V10 and -5.28 eV for PPDT2FBT-V15.

This

indicates that the polymer modification with vinyl units tends to raise the HOMO energy level, which may be unfavourable to their photovoltaic performance. Using the data of optical bandgaps and HOMO energy levels, the LUMO energy levels of the polymers were calculated and found to be in the range of -3.64 to - 3.50 eV.

2.2 Solar cell performance before crosslinking

To evaluate the effect of vinyl modification on the photovoltaic performance, BHJ solar cells with a configuration of ITO/PEDOT:PSS/active layer/Ca/Al were fabricated and investigated. The active layer was prepared by spin-casting from a solution containing a mixture of the tested polymer and PC71BM, without crosslinking. All OSC devices were subjected to careful optimization (Table S1‒S4) and found similar optimized fabrication conditions: CB containing 2% (v/v) diphenyl ether (DPE) as processing solvent, polymer/PC71BM with a weight ratio of 1:1.5, thermal annealing at the optimized temperature for 10 min., and methanol treatment before deposition of the top electrode. Current density–voltage (J‒V) characteristics of the optimized cells are shown in Figure 3a, and their photovoltaic parameters (averaged for 5 cells) are listed in Table 2. It can be seen that devices based on PPDT2FBT-V0, i.e., without vinyl modification, displayed an average PCE of 6.95% with open-circuit voltage (VOC) of 0.815 V, short-circuit current density (JSC) of 12.2 mA cm-2, and fill factor (FF) of 69.7 %. Compared with PPDT2FBT-V0 device, the best PPDT2FBT-V5 cell displayed a slightly increased VOC and


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FF value. Together with an almost unchanged JSC, the PPDT2FBT-V5 cell gave an average PCE of 7.02%, slightly higher than that of the PPDT2FBT-V0 device. In the case of PPDT2FBT-V10, enhanced JSC and FF values resulted in the largest PCE with an average value of 7.63%, although the VOC showed a small decrease.

However, in the case of PPDT2FBT-V15, all device

parameters were inferior to those of PPDT2FBT-V0 devices, resulting in a drop in the averaged PCE (4.95%). It is clear from the external quantum efficiencies (EQE, Figure 3b) spectra that all these devices showed photo-response in the region from 300 to 730 nm, coinciding with their light absorption spectra.

Consistent with the highest JSC, the PPDT2FBT-V10-based cell

displayed the highest EQE values over the entire photo-response region. The photocurrents integrated from these EQE spectra are estimated to be 11.3, 11.5, 12.5 and 11.4 mA cm-2 for the cells based on PPDT2FBT-V0, PPDT2FBT-V5, PPDT2FBT-V10 and PPDT2FBT-V15, respectively, within 10% difference of those obtained from J-V curves.

100

PPDT2FBT-V0

0

PPDT2FBT-V0 PPDT2FBT-V10 PPDT2FBT-V15

-6

PPDT2FBT-V5

80

PPDT2FBT-V5

-3

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


-9


60

40

20

-12

-15 0.0

0.2

0.4

0.6

0.8

0 300

400

Voltage (V)

500

600

700

800

Wavelength (nm)

Figure 3. (a) Current density–voltage curves of the optimized PPDT2FBT-Vx/PC71BM based solar cells under simulated sunlight with an AM 1.5G spectrum and 100 mW cm-2 light intensity and (b) their EQE spectra.



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Table 2. Device parameters of PPDT2FBT-Vx/PC71BM-based solar cells a

a

Polymer

VOC (V)

JSC (mA cm-2)b

FF (%)

PCE (%)c

PPDT2FBT-V0

0.815  0.004

12.2  0.9 (11.3)

69.7  0.7

6.95  0.44 (7.49)

PPDT2FBT-V5

0.822  0.003

12.2  0.7 (11.5)

70.1  0.4

7.02  0.41 (7.70)

PPDT2FBT-V10

0.797  0.002

13.7  0.6 (12.5)

69.5  1.6

7.63  0.22 (7.87)

PPDT2FBT-V15

0.718  0.002

11.3  0.4 (11.4)

60.4  0.8

4.95  0.15 (5.20)

b

Data were averaged from 5 devices. Data in parenthesis are integrated JSC based on EQE spectra shown in Figure 3b. c Data in parenthesis are the highest efficiencies among the tested devices.

2.3 Cross-linking and photostability of PPDT2FBT-V10

On the basis of the above vinyl content-dependent photovoltaic performances prior to crosslinking, PPDT2FBT-V10 was chosen as the focus for further investigations of crosslinking and of photo-stability in this family of polymers. The crosslinking reaction of the vinyl functionality in the polymer side chains can be triggered by heating and UV irradiation.14 In this work, both methods have been studied. The as-prepared polymer films (by spin-casting from CB solutions onto glass substrates) were subjected either to heating at 160 C (on thermocouple contacted hot-stage), or exposed to 365 nm UV light in a glove box with N2 atmosphere for various times. The films were then dipped into CF to wash away the removable polymer fraction. The crosslinking effectiveness, expressed as the weight percentage of the polymer left on the glass substrate compared to the initial film volume, as determined with UV-vis absorption spectroscopy by calculating A/A0,


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where A and A0 are the absorbance of the film after CF washing and of the initial film, respectively.

0.6 0.5

100

PPDT2FBT-V10 UV 2h immersed into CF UV 3h immersed into CF UV 4h immersed into CF UV 5h immersed into CF

Remaining fraction (%)

0.7

Absorbance

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


0.4 0.3 0.2 0.1 0.0 300

80

60

40

20

0 400

500

600

700

800

2.0

2.5

Wavelength (nm)

3.0

3.5

4.0

4.5

5.0

Time (h)

Figure 4. UV-crosslinking effectiveness of PPDT2FBT-V10. (a) UV-Vis light absorption spectra of as-prepared PPDT2FBT-V10 film and of remaining films after UV exposure for various times and CF washing. (b) Plot of remaining polymer fraction on the glass substrates after CF washing against UV exposure time. It was found that heating PPDT2FBT-V10 films at 160 C for 8 min can result in 90% polymer fraction remaining on the glass substrate (Figure S5).

Moreover, the film

absorption spectrum remained unchanged before and after heat treatment and solvent washing. This indicates that the heating treatment is an effective and quick method to achieve crosslinking between PPDT2FBT-V10 polymer chains. When 365 nm UV light was used as the activation tool, crosslinking took place at a much slower rate. As shown in Figure 4, the remaining polymer fraction gradually increased with prolonging the exposure time. After 4 h, only 62% of the polymer became insoluble. In contrast, when PPDT2FBT-V0 films were treated by heating or UV exposure, no polymer was found on the glass substrate after CF washing, regardless of treatment time.



a

b

1.0

0.8

0.8

0.6

0.6

Abs

Abs

1.0

0.4

0.4

PPDT2FBT-V0

0.2

PPDT2FBT-V0

0.2

PPDT2FBT-V10

PPDT2FBT-V10 PPDT2FBT-V10(4hr UV crosslinked)

0.0 0

10

20

30

40

50

60

c

PPDT2FBT-V10(4hr UV crosslinked)

0.0

70

80

0

90

Sun hours

400

800

1200

1600

2000

Sun hours

d 1.0

1.0

0.8

0.8

0.6

Abs

0.6

Abs

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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PPDT2FBT-V0/PC71BM(680 nm) 0.4

PPDT2FBT-V10/PC71BM(680 nm)


0.4

PPDT2FBT-V10/PC71BM(680 nm) PPDT2FBT-V10(4hr UV crosslinked)/PC71BM(680 nm)

PPDT2FBT-V10(4hr UV crosslinked)/PC71BM(680 nm) PPDT2FBT-V0/PC71BM(385 nm)

0.2


0.2


PPDT2FBT-V10/PC71BM(380 nm) PPDT2FBT-V10(4hr UV crosslinked)/PC71BM(380 nm)

0.0 0

10

20

30

40

50

60

70

80

90

PPDT2FBT-V10(4hr UV crosslinked)/PC71BM(381 nm)

0.0 0

400

800

1200

1600

2000

Sun hours

Sun hours

Figure 5. Absorbance changes of (a, b) PPDT2FBT-Vx polymer films monitored at 650 nm and (c, d) PPDT2FBT-Vx/PC71BM blend films monitored at 680 and ~380 nm as a function of exposure dose (expressed by the intensity/ number of suns multiplied by the exposure time) to (a, c) natural sunlight and (b, d) concentrated sunlight with a 200-sun intensity. Photostability is one of the basic requirements for photovoltaic materials. In order to check the photostability of this family of polymers and to investigate the influence of crosslinking on photostability, encapsulated films of PPDT2FBT-V0 and PPDT2FBT-V10 without crosslinking treatment and with 4 h UV-crosslinking treatment, were exposed to either natural (1 sun intensity) or concentrated sunlight and followed the previously suggested experimental methodology.36-39 Natural sunlight exposure was performed over


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multiple days between ~10:00-16:00 in late June and July, 2017, in the Negev Desert, while concentrated sunlight exposure tests were conducted at ~200 suns intensity (1 sun = 100 mW/cm2). As shown in Figure 5a, 5b, and S6‒S11, no obvious photobleaching was observed for these samples, regardless of vinyl-functionalization and crosslinking, or lack thereof. It was further found that even when blended with PC71BM to form BHJ films, both PPDT2FBT-V10 and PPDT2FBT-V0 do not change their photo-stability, as evidenced by absorbance change monitored at 680 nm (Figure 5c and 5d) and the whole spectral change (Figure S12‒S17). All these experiments clearly demonstrated that the PPDT2FBT polymers have excellent absorbance photostability, and that crosslinking and blending with acceptor materials does not hamper this stability.

This polymer

characteristic is closely related with its backbone structure, which does not have any photolabile units, such as benzodithiophene in PBDTTPD polymers, and possesses strong intra- and intermolecular dipole-dipole and hydrogen-bonding interactions among C-F, CH, and C-O moieties.34 It is worthy to point out that the absorbance of PPDT2FBT-Vx/PC71BM monitored around 380 nm also kept stable during the exposure to natural and concentrated solar lights (Figure 5c and 5d).

Since it is mainly contributed by PC71BM, the observation implies the acceptor

component is also stable in these blend films.



Page 16 of 34

2.4 Solar cell performance after crosslinking and device stability

Since both heating and UV exposure can trigger the crosslinking of PPDT2FBT-V10, solar cells with crosslinked active layers were prepared either using heat treatment or UV exposure for active layer cross-linking before top electrode deposition. However, the devices with heattreated (160 C for 8 min.) crosslinked active layers displayed extremely poor performance with PCE drop to 0.31% (Figure S18). Optical microscopy found the appearance of large aggregates following the heat treatment, suggesting that significant phase separation occurred in the blend film, degrading the cell performance (Figure S19). Fortunately, solar cells with UV-crosslinked (subjected to UV light for 4 h) active layers showed an average PCE of 5.28% with the highest value of 5.41% (VOC = 0.74 V, JSC = 11.0 mA cm-2, FF = 65.9%). Compared with devices without exposure to UV light, the crosslinked ones showed decreases in all three parameters, from 0.81 to 0.74 V for VOC, from 13.7 to 11.0 mA cm-2 for Jsc, and from 69.5% to 65.9% for FF. JSC suffered the largest deterioration, probably due to a decrease in - interactions between polymer chains, as crosslinking generally requires polymer backbones to move around and find a more suitable configuration.

In order to check the thermostability of the UV-crosslinked active layer, a number of halfcells just after UV-crosslinking performed on their active layers were heated at 120 C for different times, followed by top electrode deposition for completing the cell structure. As shown in Figure 6, no significant changes in average VOC, JSC, and FF parameters were found for the UV-crosslinked PPDT2FBT-V10-based cell, affording a very stable PCE during heat treatment up to 40 h. In comparison, a PPDT2FBT-V0-based solar cell having a non-crosslinked active layer showed a sharp decrease in JSC and FF parameters, leading to a gradual drop in PCE. After


Page 17 of 34

40 h heating, only 60% of the initial efficiency remained. This contrasting result indicates that active layer crosslinking is key to achieving a thermostable device.

b

1.2

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0

8

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Time (h)

Time (h)

Figure 6. Variations of the normalized (a) PCE, (b) VOC, (c) JSC and (d) FF value for PPDT2FBT-V10 solar cells with UV-crosslinked active layers upon heating at 120 C for various times. Normalized PV parameters of PPDT2FBT-V0-based solar cells with noncrosslinked active layers are also shown for comparison.



Page 18 of 34

a

b

c

d

e

f

g

h

i

j

k

l

Figure 7. (a–d) AFM topography and (e–h) phase images and (i–l) TEM of (a, b, e, f, i, j) PPDT2FBT-V0/PC71BM (1/1.5 wt/wt) and (c, d, g, h, k, l) UV-crosslinked PPDT2FBTV10/PC71BM (1/1.5 wt/wt) blend films (a, e, c, g, i, k) before and (b, f, d, h, j, l) after 120 C heating for 40 h.

Optical microscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were performed to understand changes in the active layers during heat treatment. large aggregates were observed in the blend film of PPDT2FBT-V0/PC71BM (Figure S20), while almost no aggregation appeared in the UV-crosslinked PPDT2FBT-V10/PC71BM blend film during the heat treatment. AFM images (Figure 7a‒7h) show that both PPDT2FBTV0/PC71BM and UV-crosslinked PPDT2FBT-V10/PC71BM films presented a smooth morphology


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with surface root-mean-square (RMS) roughness of 6.26 and 2.36 nm, respectively. After heating at 120 C for 40 h, the PPDT2FBT-V0/PC71BM blend film became rougher with an increased RMS of 14.04 nm. In comparison, the increase in RMS roughness after heat treatment of the UV-crosslinked PPDT2FBT-V10/PC71BM film (to 3.45 nm) was significantly smaller. Such different changes were also observable in TEM characterization on the active layers of the cells. As shown in Figure 7i‒7l, both PPDT2FBT-V0/PC71BM and UV-crosslinked PPDT2FBTV10/PC71BM BHJ films exhibited fibrous morphology before thermal treatment. However, after thermal treatment, fibrous structure evolved into large aggregates in PPDT2FBT-V0/PC71BM film, but basically maintained in the UV-crosslinked PPDT2FBT-V10/PC71BM film. These results unambiguously demonstrate that crosslinking can effectively suppress the evolution of large aggregates in BHJ blend films, thus rendering a device with much better thermostability.

Besides thermostability, light stabilities toward daily exposure to were investigated by two methods. One method is similar to the above thermostability experiments, i.e., a number of halfcells with similarly UV-crosslinked active layers were firstly exposed to simulated sunlight at 100 mW cm-2 intensity for various times, and then followed by top electrode deposition for completing the cell structure. The other method is that several finished solar cells bearing a structure of ITO/PEDOT:PSS/active layer/Al (Here, Ca layer was removed because it is instable) based on UV crosslinked PPDT2FBT-V10/PC71BM active layer were continuously exposed to simulated solar light and measured their J-V curves time-by-time. As shown in Figure 8, all device parameters, including VOC, JSC, FF and PCE, almost kept stable for all the solar cells in both cases. These results indicate that solar cells fabricated with crosslinked PPDT2FBTV10/PC71BM active layer have excellent photostability and can endure daily light exposure. The



observation is consistent with the above-mentioned photostability of neat polymer and BHJ

1.2

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blend films.

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Page 20 of 34

12

14

16

0

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8

10

Time (h)

Figure 8. Variations of the normalized (a) PCE, (b) VOC, (c) JSC and (d) FF value for half and whole cells with UV-crosslinked PPDT2FBT-V10/PC71BM active layers upon exposure to simulated solar light with 1.5 G spectrum and 100 mW cm-2 intensity for various times.

3. Conclusions

In this work, we demonstrate a family of crosslinkable photovoltaic polymers, PPDT2FBTVX (x = 0, 5, 10, 15%) bearing various amounts of vinyl units, which are not only photo-stable but also afford organic solar cells with good thermo- and light-stability as well as high efficiency.


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It was found that vinyl content of  10 % has little influence on polymer photovoltaic performance.

Bulk heterojunction solar cells based on these crosslinkable polymers and

PC71BM, but without crosslinking, displayed record PCE of 7.87 %, which was achieved by an optimized PPDT2FBT-V10-based device. Although both heat and UV exposure proved to be effective for triggering the crosslinking reaction between PPDT2FBT-V10 polymer chains, the latter is the preferable option as thermal crosslinking might interfere with thermal morphology optimization. Indeed, thermally crosslinked cells displayed poor performance, while cells with a UV-crosslinked active layer displayed an initial efficiency of 5.41 % and extraordinarily excellent thermostability.

Its device parameters (VOC, JSC, FF and PCE) remained almost

unchanged during heating at 120 C for 40 h, while a reference PPDT2FBT-V0 device with a non-crosslinked active layer quickly degraded. Photo-crosslinking requires material photo-stability. It is therefore worthwhile to note that this family of polymers can endure sunlight irradiation even under a dose of 2000 sun hours. Moreover, solar cells with UV-crosslinked active layer showed no decrease in PCE after light irradiation for 16 h. Thus, this extremely good absorbance photostability together with the above observed photovoltaic behaviors in terms of both efficiency and device stability make this crosslinkable polymer family very attractive for photovoltaic applications. In association with the recent rapid progress in high-performance non-fullerene acceptors,40-42 more efficient and stable organic solar cells can be expected by combinations of this family of crosslinkable polymers with suitable high performance non-fullerene acceptors.



Page 22 of 34

4. Experimental Section

4.1 Materials and methods

Commercial reagents except solvents, which were dehydrated with standard methods prior to use, were used directly without further purification. Synthesized compounds and polymers were structurally characterized by 1H and

13

C NMR were recorded on a Varian Mercury 400

MHz spectrometer using tetramethylsilane as an internal reference and CDCl3 as solvent for small molecular compounds at room temperature, while 1,1,2,2-tetrachloroethane-d2 was used as solvent for polymers at 110 °C. To estimate polymer weight and its polydispersity, hightemperature gel permeation chromatography (GPC) was carried out at 150 °C with 1,2,4trichlorobenzene eluent and monodisperse polystyrene standards. Polymer thermal properties were evaluated by means of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), performed under nitrogen atmosphere with a TGA Q500 and a a Q2000 modulated DSC instrument, respectively. The heating rate was 10 °C min-1, while the cooling rate was 15 °C min-1. Polymer optical properties were investigated by UV-vis absorption spectroscopy carried on a Hitachi U-3310 or Cary 5000 UV-Vis-NIR (Agilent Technologies) spectrophotometers. Polymer redox potentials were measured by cyclic voltammetry (CV) on a CHI 660C instrument. A glassy carbon (28.26 mm2) was used as working electrode, while a platinum wire and an Ag/AgNO3 non-aqueous electrode were used as counter and reference electrodes, respectively. The measurements were performed on film samples prepared by casting their CB solutions onto glassy carbon electrodes. Dehydrated acetonitrile was used as solvent and 0.1 M Bu4NPF6 was used as electrolyte. The scan rate was 50 mV s-1. Film morphology was observed by atomic force microscopy (AFM) on a JPK NanoWizard AFM instrument in


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tapping mode. Transmission electron microscopy (TEM) was recorded on a JEM-2100F instrument. For photostability studies, encapsulated samples were exposed to natural sunlight concentrated outdoors and transmitted indoors using an optical fiber. Transmitted sunlight was focused onto the sample through a hexagonal kaleidoscope (area = 10.4 mm2). Samples were placed on a metallic thermal bath set to 25 °C. Alternatively, samples were exposed to natural sunlight outdoors mounted on a fixed angle (30 ) stand facing south. The sunlight spectrum measured at noontime ± (2–3) hours at Sede Boqer is very close to the AM 1.5G spectrum.43

4.2 Material synthesis

4,7-Bis(5-trimethylstannylthiophen-2-yl)-5,6-difluoro-benzo[c][1,2,5]thiadiazole (M1). A solution of 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (600 mg, 1.791 mmol) in anhydrous THF (50 mL) was cooled down to -78 °C under argon and added to 2 M lithium diisopropylamide (LDA, 2.33 mL, 4.66 mmol). The resulting mixture was stirred at -78 °C for 1 h, and then naturally warmed to room temperature and stirred for another 15 min. Afterwards, the mixture was cooled down to -78 °C again and 1 M trimethylstannyl chloride (4.66 mL, 4.66 mmol) was added dropwise. After stirring at room temperature for 24 h, the reaction was quenched by the addition of water (20 mL). After extraction with ethyl acetate 3 times, the collected organic layers were combined, washed with brine, dried over anhydrous NaSO4, and concentrated under reduced pressure. The obtained residue was subjected to size-exclusion chromatography to separate M1. Yield: 600 mg (50.5%). 1H NMR (400 MHz, CDCl3) δ: 8.33 (d, J = 7.2 Hz, 2H), 7.33 (d, J = 6.0 Hz, 2H), 0.43 (s, 18H). 1,4-Dibromo-2,5-bis(2-hexyldecyloxy)benzene

(M2).

A

mixture

of

2,5-

dibromohydroquinone (268 mg, 1 mmol), 1-bromo-2-hexyldecane (760 mg, 2.5 mmol), and



Page 24 of 34

K2CO3 (276 g, 2 mmol) was added to a 50 mL flask under argon. After addition of 25 mL DMF, the reaction mixture was stirred at room temperature for 30 min, and then at 100 °C for 24 h. Afterwards, the reaction mixture was concentrated under reduced pressure and the residue was subjected to silica column chromatography using hexane as the eluent, affording 512 mg of compound M2 as a colorless oil. Yield: 71.5%. 1H NMR (400 MHz, CDCl3) δ: 7.01 (s, 2H), 3.44 (d, J = 3.0 Hz, 4H), 1.84-1.72 (m, 2H), 1.27 (s, 48H), 0.88 (t, J = 3.0 Hz, 12H). 1,4-Dibromo-2,5-bis(undec-10-en-1-yloxy)benzene (M3). Compound M3 was obtained as white powder (550 mg, yield 48.1%) following the same method as for compound M2 but using 10-bromodec-1-ene in place of 1-bromo-2-hexyldecane. 1H NMR (400 MHz, CDCl3) δ: 7.09 (s, 2H), 5.86-5.78 (m, 2H), 5.02-4.92 (m, 4H), 3.95 (t, J = 5.0 Hz, 4H), 2.07-2.02 (m, 4H), 1.83-1.78 (m, 4H), 1.49-1.27 (m, 24H). 13C-NMR (100 MHz, CDCl3) δ: 150.13, 139.24, 118.54, 114.13, 111.18, 70.35, 33.81, 29.48, 29.40, 29.27, 29.12, 29.12, 28.94, 25.93. HR MS (ESI, m/z): [M+H]+ calcd. for C28H45O2Br2, 571.1779; found, 571.1781. PPDT2FBT-V0.

M1 (244.1 mg, 0.369 mmol), M2 (263.9 g, 0.369 mmol),

tris(dibenzylideneaceton)dipalladium(0)

(Pd2(dba)3,

13.5

mg,

0.0147

mmol),

tri(o-

tolyl)phosphine (P(o-tol)3, 8.97 mg, 0.0295 mmol) and chlorobenzene (2.5 mL) were added into a microwave vial (10 mL) in a glove box with N2 atmosphere. After the vial was put into a microwave reactor, the polymerization was carried out at 80 °C (65 W) for 10 min, then at 100 °C (70 W) for 10 min, and finally at 140 °C (200 W) for 40 min.

Afterwards, 2-

tributylstannylthiophene (50 μL) and then 2-bromothiophene (64 μL) were added sequentially and individually allowed to react for an additional 20 min for the purpose of polymer endcapping.

The crude PPDT2FBT-V0 polymer was obtained by pouring the polymerization

mixture into a mixture of 350 mL methanol and 10 mL aq. HCl (36%) solution, followed by


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filtration. After Soxhlet extraction with acetone, hexane, and chloroform, the pure PPDT2FBTV0 was precipitated from the chloroform fraction with MeOH and dried under vacuum at 50 °C overnight. Yield 210 mg (64%). Mn = 9.3 kDa, Ɖ = 2.09. PPDT2FBT-V5. PPDT2FBT-V5 was obtained following the same method for PPDT2FBTV0 using M1 (157.4 mg, 0.238mol), M2 (161.7 mg, 0.226 mmol) and M3 (6.8 mg, 0.012 mmol) in the presence of Pd2(dba)3 (8.7mg, 0.0095mmol) and P(o-tol)3 (5.78 mg, 0.019 mmol) in a yield of 76% (160 mg). Mn = 12.5 kDa, Ɖ = 2.07. PPDT2FBT-V10.

PPDT2FBT-V10 was obtained following the same method for

PPDT2FBT-V0 using M1 (155.1 mg, 0.234mol), M2 (151mg, 0.211 mmol) and M3 (13.4 mg, 0.023 mmol) in the presence of Pd2(dba)3 (8.58mg, 0.00937mmol) and P(o-tol)3 (5.7 mg, 0.0187 mmol) in a yield of 76% (120 mg). Mn = 19.3 kDa, Ɖ = 2.57. PPDT2FBT-V15. A mixture of M1 (117.5 mg, 0.177 mmol), M2 (108.0 mg, 0.151 mmol) and M3 (15.2 mg, 0.027mmol) were dissolved in dry chlorobenzene (3 mL) under an Ar atmosphere. After the solution was frozen by liquid nitrogen, Pd2(dba)3 (6.5 mg, 0.0071 mmol) and P(o-tol)3 (4.3 mg, 0.0143 mmol) were quickly added. The resulting mixture was thoroughly degassed again by three freeze–pump–thaw cycles and backfilled with Ar. After reacting at 130 °C for 36 h, the polymer was end-capped by subsequently adding 2-tributylstannylthiophene (25 μL) and 2-bromothiophene (64 μL) into the vial and reacting for 1 h after each addition. The crude PPDT2FBT-V15 polymer was obtained by pouring the polymerization mixture into 350 mL methanol and 10 mL aq. HCl (36%), and subsequently Soxhlet-extracted with acetone, hexane, and chloroform. Finally, the pure PPDT2FBT-V15 polymer was obtained by precipitation from



Page 26 of 34

the chloroform fraction with MeOH and dried under vacuum at 50 °C overnight. Yield: 110 mg (71.3%). Mn = 12.5 kDa, Ɖ = 2.07.

4.3 Fabrication and characterization of photovoltaic devices

The studied solar cells' structure was ITO/PEDOT:PSS/active layer/Ca/Al. For their fabrication, ITO glasses were firstly cleaned by ultra-sonication in deionized water, acetone, and isopropanol, and then subjected to UV−ozone treatment for 15 min. After that, a filtered PEDOT:PSS solution (Heraeus Clevios PVP. Al 4083) was spin-casted at 4000 rpm and baked at 140 °C for 15 min, affording a thin PEDOT:PSS layer (25 nm) on top of cleaned ITO glasses. After the substrates were transferred into a dry nitrogen glovebox (O2 < 1 ppm, H2O < 1 ppm), the active layers were deposited by spin-casting a chlorobenzene solution containing both tested polymer and PC71BM at a weight ratio of 1/1.5 and a total concentration of 27.8 mg mL-1. In the solution, a 2% volume fraction of diphenyl ether (DPE) was added as an additive to tune the active layer morphology. After stirring at 120 °C for 8 h, the active layers were spin-casted using heated solution at optimized temperature (ITO glasses and pipette tips were heated too), and then the ITO glasses were subjected to thermal annealing at optimal temperature for 10 min. After cooling to room temperature, 200 μl methanol (filling the surface of the ITO glass) were spin-deposited on the top of the active layer (1000 rpm, 40 s). If desired, the active layer was crosslinked by a thermal or UV exposure method as described in the main manuscript. Finally, 10 nm-thick Ca and 100 nm-thick Al cathode were deposited by thermal evaporation under vacuum of 10-5 mbar to complete the device. The effective working area was 0.07069 cm2. A Veeco Dektak 150 profilometer was used to estimate the thickness of each layer deposited. Device performance was evaluated by measuring current density–voltage (J–V) curves with a


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Keithley 2420 source meter. An Oriel AAA solar simulator (Oriel 94043A, 450 W) was used to provide simulated solar irradiation with a AM 1.5G spectrum and 100 mW cm-2 light intensity. A standard NREL-certified silicon cell was used to for light intensity adjustment. External quantum efficiency (EQE) spectroscopy was performed using a self-assembled system composed of a 75 W Xe lamp, an Oriel monochromator (74125), an optical chopper, a lock-in amplifier, and a NREL-calibrated crystalline silicon cell. For thermostability checking, the half cells after UV-crosslinking of their active layers were heated at 120 °C in dark in a N2-glovebox having O2 < 1 ppm and H2O < 1 ppm for different times (from 0 to 40 h), followed by top electrode deposition and device performance characterization.

ASSOCIATED CONTENT Supporting Information. 1H and

13

C NMR spectra of monomer M3, TGA, DSC, UV-Vis

light absorption spectra of as-prepared PPDT2FBT-V10 film and of remaining films after heating at 160 C for 8 min and CF washing，J-V curve of the PPDT2FBT-V10 device after thermal crosslinking and optical microscopy images, UV-vis absorption spectra of PPDT2FBT-Vx polymer films and PPDT2FBT-Vx/PC71BM blends during sunlight exposure, tables of PPDT2FBT-Vx/PC71BM based solar cell devices optimization process. Funding Sources Financially support by the National Natural Science Foundation of China (Grant Nos. 21474129, 21674125, and 51761145043), International Science and Technology Cooperation Program of China (Grant No. 2015DFG62680), Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB20020000), Science and



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Technology Open Cooperation Projects of Henan Province (Grant Nos. 162106000018 and 172106000067) and Zhengzhou Institute of Technology, is gratefully acknowledged. LTS, IVF and EAK acknowledge support by the Blaustein Center for Scientific Cooperation's postdoctoral fellowship, Israel’s Ministry of Science and Technology China-Israel Cooperative Scientific Research fund (Grant number 3–12387), and the Adelis Foundation. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interests. REFERENCES (1) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-based Organic Solar Cells. Chem. Rev. 2007, 107, 1324-1338. (2) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868-5923. (3) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736-6767. (4) Yang, J.; Yan, D.; Jones, T. S. Molecular Template Growth and Its Applications in Organic Electronics and Optoelectronics. Chem. Rev. 2015, 115, 5570-5603.


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