Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule

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Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule Organic Solar Cells with 7.0% Efficiency: In-Depth Investigation on the Effects of Structure Modification and Solvent Vapor Annealing Jin-Liang Wang, Zhuo Wu, Jing-Sheng Miao, Kai-Kai Liu, ZhengFeng Chang, Ru-Bo Zhang, Hong-Bin Wu, and Yong Cao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00848 • Publication Date (Web): 29 May 2015 Downloaded from http://pubs.acs.org on June 4, 2015

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Chemistry of Materials

Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule Organic Solar Cells with 7.0% Efficiency: In-Depth Investigation on the Effects of Structure Modification and Solvent Vapor Annealing Jin-Liang Wang,*,†,§ Zhuo Wu,†,§ Jing-Sheng Miao,‡,§ Kai-Kai Liu,† Zheng-Feng Chang,† Ru-Bo Zhang, † Hong-Bin Wu,*, ‡ and Yong Cao‡ †

Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry, Beijing Institute of Technology, 5 South Zhongguancun Street, Beijing, 100081, China

‡

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou, 510640, China ABSTRACT: a family of narrow-band gap extended π-conjugated D2-A2-D1-A1-D1-A2-D2 type small molecules based on diketopyrrolopyrrole derivatives as the stronger acceptor core (A1) coupled with indacenodithiophene (IDT) (D1), difluorobenzothiadiazole(A2) are synthesized and their properties as donor materials in solution-processed small-molecule organic solar cells are investigated. The impacts of replacing the thiophene ring by more electron-deficient of thiazole ring and inserting thiophene spacer between electron donating (D1) and electron accepting (A1 and A2) aromatic moieties on bulk properties, such as the photophysical properties, the HOMO/LUMO energy level, charge carrier mobilities and the morphologies of blend films, as well as optimization on device performance via solvent vapor annealing are investigated. NDPPFBT shows outstanding efficiencies up to 7.00% after THF vapor annealing for 60 s because of very-high FF of 0.73 and high Voc of 0.89 V. The reported efficiency is among one of the highest values for small-molecules-based organic solar cells from electron accepting unit as core and appears as the first diketopyrrolopyrrole-based small-molecule BHJ organic solar cells with PCE over 7% with high FF and Voc.

Introduction Solution-processed organic solar cells (OSCs) have attracted considerable attention as an effective technology for using sunlight because of their advantages such as light weight, flexibility, and low cost in recent years.1-7 Typically, OSCs are based on bulk-heterojunction (BHJ) device structures, in which the photo active layer consists of narrow band gap π-conjugated molecules and fullerene derivatives. Among the various approaches for the synthesis of highly efficient donor molecules, the most successful one have been demonstrated to be the push-pull chromophore approach. As a result of intense charge transfer absorption and tunable band gap can be readily achieved.8-16 Over the past two decades, a great numbers of research effort have been focused in enhancing the power conversion efficiencies (PCEs) of polymer solar cells to the milestone efficiency of 10% in single-junction solar cells.17-21 Despite the rapid progress in the efficiency of polymer solar cells, some unsolved problems, such as the reproducibility of the synthesis, purification, and electronic properties of the final active

materials, still represent as big challenges. Over the past few years, solution-processed narrow-band gap small-molecules have gained increasing research interest because of their unique advantages, such as simple purification, monodisperse structures, no end-group contaminants, and reduced batch-to-batch variability compared to polymer materials.22-26Furthermore, the specific structure of small molecules can allow more reliable analyses on the relationships between structure-properties-device performance, thus can provide a much more clear understanding on how to design high performance photovoltaic materials. Therefore, the performance of solution-processed BHJ-OSCs based on small-molecules with electron donating (D) moieties as core (roughly defined as A-D-A structure) or electron accepting moieties (A) as core (roughly defined as D-A-D structure) has been dramatically increased over the past few years, approaching that of the best polymer solar cells reported to date.27-42 Among a large number of molecular chromophores reported in the literature, the typical building block for efficient small-molecule donor

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materials that can deliver high performance (PCEs >7%) are still rather limited, including dithienosilole (DTS) core unit by Bazan and coworkers,43-49 benzodithiophene (BDT) core units by Chen and Yang et. al.,50-55 IDT core units

with

difluorobenzothiadiazole

by

our

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porphyrin core units by Peng et.al..57 Moreover, the overall performance of D-A-D structure (for example, porphyrin as core) based devices still lags behind that of these

56

group,

Chart 1. Extended π-conjugated backbone structures of DPPFB, DPPFBT, and NDPPFBT.

devices from A-D-A structure (for example, DTS or BDT as core), although D-A-D structure usually has narrower energy band-gap. Therefore, due to the lack of in-depth study on relationship of structure-properties-performance based on such donor materials, it remains a great challenge to design small-molecule donor materials, particular for D-A-D structure. To address these challenges and combine the most desirable properties from both small-molecules and polymers, one strategy is to design and synthesis new extended π-conjugated backbone structures composed of multiple electron donating (D) moieties and electron accepting moieties (A) with efficient light-harvesting and high carrier mobility, which are essential toward high-performance of solution-processed small-molecule solar cells. Diketopyrrolopyrrole (DPP), with two fused electron-deficient lactams, is a typical strong acceptor chromophore with high molar extinction coefficient. Thus it has been widely investigated in small molecular organic solar cells with high short circuit current density (Jsc).58-66 Most of the DPP core was flanked with two electron-rich five-membered hetero-aromatic ring (thiophene, furan, selenophene or thieno[3,2-b]thiophene), which can effectively reduce electron-with drawing effect of the DPP core. On the other side, open circuit voltage (Voc) of devices from these compounds is usually lower due to elevation of the HOMO energy level of the donor with DPP units. In the past few years, by attaching of weaker aromatic donor units (phenyl, benzofuran, and pyrene) or ester group as terminated group on DPP-based oligomers,

significant progress has been made in decreasing the HOMO of these materials, resulting in for high photovoltaic performance (with a the best PCE of 5.9%).58-66 However, compared with that of those from DTS or BDT core-based small molecules, the devices from DPP-based small molecules showed less efficient PCE and only moderate fill factor (FF), mainly due to strong non-geminate recombination of DPP-based small molecules.64 Thus, further improvement of the PCE requires enhancement in both Voc and FF while deteriorating in Jsc can be effectively avoided. Meanwhile, incorporation of electron with drawing groups (fluorine atoms, nitrogen atoms of pyridine) to the electron-deficient subunits of conjugated polymer molecules has emerges as an popular strategy to improve the PCE, because of its capability in decreasing the HOMO energy level.67-74 Moreover, as compared with thiophene, thiazole is more electron deficient, thus thiazole functionalized DPP are expected to possess a more deeper HOMO and LUMO energy levels.75-77 These pioneering studies inspired us to synthesize new extended alternate D-A π-conjugated backbone structures with narrow band gap and low-lying HOMO energy levels, in which the existing merits of these two acceptor units (DPP being flanked with thiophene or thiazole as stronger acceptor unit and difluorobenzothiadiazole as weaker acceptor unit) can be combined in one single compound. However, systematic studies on how such specific chemical structures of multiple D-A pair and connected bridges of D-A pair can affect the properties of the


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materials and their photovoltaic performances is still very limited and immature, especially when more than four chromophores have been incorporated together in such extended materials.

as the central stronger acceptor unit (A1), two IDT-based π-conjugated bridges as the donor (D1), with two difluorobenzothiadiazole as the weaker acceptor (A2), and two n-hexyl-substituted bithiophene as the terminal groups (D2). We focus our studies on evaluating how structural variations affect bulk properties of desired in two aspects: (1) variation of electron affinities of the stronger acceptor unit (A1) through replacing the two thiophenes by

To illuminate the construction of the high performance extended alternate D-A backbone structures, we systematically describe the design, and synthesis a series of extended π-conjugated small-molecules based on D2-A2-D1-A1-D1-A2-D2 molecular skeletons, as shown in Chart 1. All of the target molecules have a DPP-based core R

F

a) n-BuLi, THF, -78 oC, 1 h. b) THF, (CH3)3SnCl, -78 oC to RT, 10 h, 95%

R S

R

R S

R

R

R

a) LDA, THF, -78 oC, 1 h. b) THF, (CH3)3SnCl, -78 oC to RT, 10 h, 91%

C6H13

R

F

F

N

S

S Br

Br

3

C6H13

Sn

Br

R

N

S

+

R

N R

R

a) n-BuLi, THF, -78 oC, 1 h. b) THF, (CH3)3SnCl, -78 oC to RT, 10 h, 94%

R S S

S S R

R

7 F

S

S

N

3 S Pd2(dba)3, P(o-tolyl) 3, toluene, reflux,12 h, 44% R N S O

R

4

R

DPPFB

R S

Sn

S

S S R

R

R

9

R

F

R = 2-ethylhexyl

X 11 X = SnMe3

S R

S N R N

S DPPFBT

O

O S

N R

a) LDA, THF, -78 oC, 1 h. b) THF, (CH3)3SnCl, -78 oC to RT, 10 h, 93%

10 X = H S

S

S N

Br

R S

N

S N R 6

C6H13

S

11, Pd2(dba)3, P(o-tolyl) 3, toluene, reflux, 12 h, 64%

O

R

N

C6H13 F

N

S

S

8

F

Br

S

S

6

S R

R

Pd2(dba)3, P(o-tolyl)3, toluene, reflux, 12 h, 81% Br

S

O

5

O

R

F

N S

R N

R

S

Pd(PPh3)4, Na2CO3, H2O, 2-thiopheneboronic acid, THF, reflux, 12 h, 91%

R

S

N

Pd2(dba)3, P(o-tolyl)3, toluene, reflux, 12 h, 67%; S

S N S

R

R

C6H13

S

F

Sn

2

1

Br

S

S

S

F

Br

R

NBS, CHCl3, DMF, 60 oC, Br 24 h, 85%

N

R N

S O

N

12

O

11, Pd2(dba)3, P(o-tolyl)3, Toluene, reflux,12 h, 55% S

N R 13

Br

NDPPFBT

N

Scheme 1. Synthesis of these π-extended small molecules DPPFB, DPPFBT, and NDPPFBT.

two thiazoles to control the molecular energy levels, optical properties, and open-circuit voltage (Voc); (2) variation of the length of π-bridge unit between two acceptor chromophores by an extra thiophene π bridge to change the light-harvesting ability, the HOMO/LUMO energy level, charge carrier mobility. These materials both exhibit good solubility in common organic solvents such as CHCl3, THF, and toluene owing to the ten 2-ethylhexyls and two hexyls, thus can be readily solution-processed and form excellent smooth films by spin-coating. In combination with using PC71BM as the acceptor, the BHJ-OSC devices based on NDPPFBT with an more extended conjugated length and stronger acceptor core, exhibited outstanding PCE of 7.00% with a high open circuit voltage (Voc) of 0.89 V, short circuit current density (Jsc) of 10.75 mAcm-2 and very-high FF of 0.73 for the device upon annealing by THF vapor for 60 s but without using any additives. To the best of our knowledge, this is

the highest PCE of solution-processed BHJ solar cells based on DPP-containing small-molecule donor materials, and also ranks to one of the best small-molecule donor materials based on electron acceptor unit as core (only porphyrin-based small molecule exhibited PCEs over 7%). Surprisingly, the device based on similar chemical structures DPPFB or DPPFBT and PC71BM give relative lower PCEs, respectively. The results herein provide a facile strategy to understand the impact of the intensity of acceptor units and conjugated spacer of two acceptor units when attached to an extended conjugated D2-A2-D1-A1-D1-A2-D2 framework. The reason that responsible to the impact is therefore of intense research interest and is investigated thoroughly in this paper. Results and Discussion The synthetic routes to these extended small molecules are shown in Scheme 1. Treatment of 178-83 with n-BuLi



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followed by trimethyltin chloride facilely gave the monotin reagent 2 as light-yellow oil that was directly used in the next step without any further purification. 4 using was prepared through the Stille coupling reactions between 2 and monobromide intermediate 356 using a Pd2(dba)3/P(o-tolyl)3 catalytic system. Then 4 can be lithiated by LDA followed by quenching with trimethyltin chloride to afford monotin reagent 5. DPPFB were obtained through two-fold Stille coupling reaction between the monotin reagent 5 and dibromide 684 as dark solid in 81% isolated yield. 8 was prepared by a Suzuki coupling reaction between 7 and 2-thiopheneboronic acid in 91% yield. Treatment of 8 with n-BuLi followed by (a) 0.35

(b)

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trimethyltin chloride facilely gave the monotin reagent 9 as yellow oil that was directly used in the next step without any further purification. 10 was prepared through the Stille coupling reactions between 9 and monobromide intermediate 356. Then 10 can be lithiated by LDA followed by quenching with trimethyltin chloride to afford extend monotin reagent 11 that was directly used in the next step without any further purification. Treatment of 1275 with NBS in mixture solvents of chloroform and DMF to give dibromide 13 in high isolate yield. Finally, DPPFBT and NDPPFBT were obtained through two-fold Stille coupling reaction between monotin reagent 11 and dibromide 6 or 13 (c) 0.5

1

NDPPFBT:PC71BM=1:2-DCM-0s NDPPFBT:PC71BM=1:2-DCM-30s NDPPFBT:PC71BM=1:2-THF-60s

0.3

0.4

0.8 0.25 0.2 0.15

0.6

0.3

0.4

0.2

0.1

0 300

0.2

DPPFB DPPFBT NDPPFBT

0.05

400

500 600 700 Wavelength (nm)

800

0 300

0.1

DPPFB DPPFBT NDPPFBT 400

500 600 700 800 Wavelength (nm)

900 1000

0 300

400


800

900

-6

Figure 1. (a)The absorption spectra of these small molecules in chloroform solutions (2×10 M); (b) The absorption spectra of these small molecules in thin films; (c) Absorption spectra of NDPPFBT:PC71BM (1:2, w/w) blend films and the film after treatment of CH2Cl2 vapor for 30 s and THF vapor for 60 s. Table 1. Photophysical and electrochemical properties of these D2-A2-D1-A1-D1-A2-D2 small molecules in solutions and in thin films. a

Compd

a

Ered(onset) Eox(onset) λmaxabs. λmaxabs. (V) (V) (sol) (nm)(log ε) (film) (nm) 0.39 -1.55 DPPFB 390(4.84),551(sh), 398,660, 721 657(5.14), 696(5.15) -1.53 DPPFBT 433(4.97),554(sh), 443,600, 661,728 0.36 641(5.13),681(sh) -1.43 NDPPFBT 441(5.19), 550(5.20), 447,581, 653, 725 0.45 645(sh), 701(5.18) a + potentials are measured relative to a Fc/Fc redox couple as an external reference onset of thin-film absorption.

as dark solid in 64% and 55% isolated yield, respectively. All compounds were purified by silica gel column chromatography, and their structures and purity were verified by 1H and 13C NMR, elemental analysis, and ESI/MALDI-TOF MS. The thermal property of these extended small molecules was investigated by thermogravimetric analysis (TGA)(Figure S1). Under N2 atmosphere, the onset temperature with 5% weight-loss is about 410oC for DPPFB, 411oC for DPPFBT, 381oC for NDPPFBT, respectively, which indicated that the thermal stability of these molecules is adequate for application in organic solar cells.

b

EHOMO (eV) -5.19

ELUMO (eV) -3.25

Eg(cv) (eV) 1.94

Eg(opt) (eV) 1.59

-5.16

-3.27

1.89

1.57

-5.25

-3.37

1.88

1.56

b

(-4.8 eV in vacuum). estimated from the

Figure 1 shows the absorption spectra of the resulted extended small-molecules both in diluted chloroform solutions and in thin films, where all molecules showed two distinct absorption bands (Band I: 300-450 nm; Band II: 450-900 nm) in solution and solid state due to π-π* transition of conjugated backbone and the intra molecular charge transfer (ICT) between two molecular donor and two acceptor units. Such absorption profiles are typical of donor-acceptor type π-conjugated molecules. For absorption band I, the maximum absorption peak (λmax) of DPPFBT and NDPPFBT is located at 433 nm and 441 nm, respectively. These peaks display a red-shift by 43-51 nm and enhancement of intensity in comparison with that of


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DPPFB due to extended π-π* transition of conjugated backbone by thiophene spacers. For absorption band II, the absorption spectrum of DPPFB exhibited two peaks with almost equal intensity at about 657 nm and 696 nm as well as one obvious shoulder peak at about 551 nm. As compared with DPPFB, the maximum absorption peaks (λmax) of DPPFBT at about 641 nm and 681 nm caused dramatically blue shift by ca.15-16 nm. Such blue shift is probably due to reducing the interaction the DPP and IDT groups by longer thiophene spacer. In contrast, all of maximum absorption peak (λmax) of NDPPFBT at absorption band II exhibited obviously red-shift in comparison with DPPFBT due to stronger acceptor ability of thiazoles compared with thiophene groups. Meanwhile, the molar extinction coefficient of the absorption bands in NDPPFBT increased obviously relative to these of DPPFB and DPPFBT. Such broad absorption features is probably due to enhancing the interaction the donor and

acceptor groups by inserting thiophene spacer or thiazole groups instead of thiophene group as well as with their shaped-persistent conjugated backbones. In contrast, the absorption spectra of these extended small molecules in thin films are obvious red-shifted in comparison with those of in diluted solution. For example, compared with those absorption maximum peaks at about 700 nm of each materials in solutions, DPPFB and NDPPFBT showed red-shift of 25 nm, and 24 nm, respectively, whereas the thin films of DPPFBT displayed the largest red-shift of 47 nm on the maximum absorption peak (λmax). Moreover, compared to DPPFB that reserved shoulder features at about range of 500-600 nm, such region of DPPFBT and NDPPFBT exhibited obvious peaks with higher molar extinction coefficient, which might be attributed to the increase in molecular rigidity and

Table 2. A summary of the device performances and the charge carrier mobilities from blend films of these extended small-molecules and PC71BM before/after CH2Cl2 or THF vapor annealing. Compd

Jsc 2 (mA/cm )

Voc (V)

FF (%)

PCEavg (PCEmax) (%)

μe 2 -1 -1 (cm V s )

DPPFB 7.14±0.12 0.84±0.01 38.77±0.39 2.31±0.04(2.32) (4.06±1.28)×10-4 a -4 DPPFB 9.88±0.23 0.76±0.01 69.00±0.80 5.21±0.09(5.32) (1.6±0.4)×10 b DPPFB 5.80±0.17 0.79±0.01 65.89±1.99 3.03±0.13(3.22) -4 DPPFBT 8.71±0.39 0.81±0.01 49.58±1.52 3.51±0.21(3.83) (2.27±0.21)×10 a -4 DPPFBT 10.51±0.99 0.76±0.01 60.85±4.12 4.83±0.26(5.24) (3.8±0.4)×10 NDPPFBT 9.38±0.14 0.92±0.01 54.04±0.59 4.67±0.08(4.63) (1.26±0.21)×10-4 a NDPPFBT 11.00±0.28 0.88±0.01 63.03±3.51 6.05±0.35(6.55) (1.26±0.06)×10-4 b NDPPFBT 9.52±0.26 0.89±0.01 68.97±0.55 5.84±0.18(6.11) c NDPPFBT 10.71±0.16 0.88±0.01 71.60±0.41 6.86±0.14(7.00) (1.67±0.21)×10-4 a b c treatment with CH2Cl2 vapor for 30 s; treatment with THF vapor for 50 s; treatment with THF values calculated from 20 devices with standard deviation for the measurements.

planarity through interactions between benzothiadiazole and IDT. Such features are attributed to a more planar conjugated backbone and more ordered structure in the solid state, therefore a higher π-electron delocalization through the whole molecular backbone and enhanced interchromophore interactions is expected, which could be beneficial to a higher hole mobility. From the onset of absorption, the optical band gap of thin films were estimated to be 1.59 eV for DPPFB, 1.57 eV for DPPFBT, and 1.56 eV for NDPPFBT, respectively. It is clear that NDPPFBT has the relative broad Absorption among these three materials in thin films and that is preferable to high photovoltaic performance as discussed below. In order to get insight into the relationship between the chemical structures and the electrochemical properties of the desired materials, the cyclic voltammetry (CV) experiments of these three materials in thin films were conducted. The CV curves of these compounds showed one quasi-reversible p-doping process and n-doping process (Figure S2). The HOMO (often addressed by the ionization potential values) and LUMO (often addressed

μh 2 -1 -1 (cm V s ) -6

(3.43±0.52)×10 -4 (1.37±0.22)×10

-6

(5.83±1.10)×10 -4 (1.42±0.31)×10 -6 (5.66±0.50)×10 -5 (3.94±0.30)×10 -4

(2.40±0.23) ×10 vapor for 60 s; The average

by the electron affinity values) levels are -5.19 eV/-3.25 eV for DPPFB, -5.16 eV/-3.27 eV for DPPFBT, and-5.25 eV/-3.37 eV for NDPPFBT, respectively, according to the following equation of EHOMO = -e(Eox+4.80) (eV) and ELUMO = -e(Ered+4.80) (eV). Introduction of the thiophene units on IDT moiety caused slightly increase in the HOMO energy levels of DPPFBT with respect to the DPPFB due to enhancing the donor ability by thiophene units. Moreover, NDPPFBT showed markedly deeper HOMO energy levels (ΔE = 0.09 eV) relative to that of DPPFBT as a consequence of replacing the two thiophenes by two thiazoles on the stronger acceptor units (diketopyrrolopyrrole) to enhance the electron-deficient property.85 Moreover, NDPPFBT also showed slight deeper LUMO energy levels compared with that of DPPFBT due to the similar reason as that for the HOMO energy levels. The electrochemical band gaps (Eg(cv)) albeit slightly larger than the corresponding optical band gaps (Eg(opt)) because states measured in the electrochemical experiments and the optical results (neutral states) are somewhat different. Such deep-lying HOMO energy level



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for NDPPFBT is expected to afford High Voc of the resultant organic solar cells. To demonstrate the potential of these extend small-molecules as promising donor materials for organic solar cells, we fabricated BHJ-OSCs with a device structure ITO/PEDOT:PSS/small molecule donor:PC71BM/PFN/Al. Blend ratio for all three donor materials was 1:2 (donor materials: PC71BM) by weight. In addition, solvent vapor annealing from with two different solvents (CH2Cl2 and THF) was applied in the fabrication of the BHJ-OSC sin order to optimize the morphology of the active layer.86-89 It is worthy to note that Bäuerle and Chen independently reported that chloroform vapor annealing gave positive results for their small molecule based devices during the preparation of this manuscript.41,54 As expected, compared with the devices from the other two materials, all of the devices based on NDPPFBT exhibited higher Voc (≥0.88 V), which is consistent with much lower-lying HOMO energy levels and is very impressive to such high Voc for DPP-based small-molecule organic solar cells.

(a) 12

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For the untreated devices, the best PCE is 2.32% for DPPFB, 3.83% for DPPFBT, and 4.63% for NDPPFBT with PC71BM as acceptor, respectively. The significantly improved PCE in NDPPFBT devices are mainly due to their higher FF of 0.54 and higher Voc of 0.93 V when compared with the other two donor materials. Indeed, the strategy of replacing the two thiophenes by two thiazoles on the stronger acceptor units (diketopyrrolopyrrole) in NDPPFBT resulted in an increase of 0.12 V in Voc despite its lowest band gap, while reduction in Jsc was effectively avoided. When a CH2Cl2 vapor annealing treatment (30 s) was applied to the active layer, the overall performance of the devices from all of the donor materials were dramatically enhanced as a result of significant enhancement of Jsc and FF (Table 2), although Voc slightly decreased after solvent vapor annealing. For example, the best performance device based on NDPPFBT and PC71BM results in a Jsc = 11.07 mAcm-2, Voc = 0.88 V, FF = 0.67, and PCE = 6.55%, while the DPPFB devices doubled their efficiency upon such solvent vapor annealing treatment (5.32% vs. 2.32%).

(c) 50

(b) 50

40

40

8

30

30

4

0

-4 -0.2

DPPFB-DCM-0s DPPFB-DCM-30s DPPFBT-DCM-0s DPPFBT-DCM-30s NDPPFBT-DCM-0s NDPPFBT-DCM-30s NDPPFBT-THF-60s 0

0.2 0.4 0.6 Voltage (V)

20

20

0.8

1

0 300

400


DPPFB-DCM-0s DPPFB-DCM-30s DPPFB-THF-50s

10

NDPPFBT-DCM-0s NDPPFBT-DCM-30s NDPPFBT-THF-60s

10

800

0 300

400

500 600 Wavelength (nm)

700

800

Figure 2. Current density-voltage (J-V) characteristics of the best OSC devices based on these D2-A2-D1-A1-D1-A2-D2 small-molecules and PC71BM (before/after solvent vapor treatment); (b) EQE plots of the best OSC devices based on NDPPFBT blend with PC71BM before/after CH2Cl2 vapor annealing for 30 s and THF vapor annealing for 60 s; (c) EQE plots of the best OSC devices based on DPPFB blend with PC71BM before/after CH2Cl2 vapor annealing for 30 s and THF vapor annealing for 50 s.

To further optimize the device performance of the devices from NDPPFBT by using more environmentfriendly solvent for the treatment, we investigated the influence of THF vapor annealing on the devices performance, which can provide much better solubility for the donor materials in THF, thus can be capable to tune the film morphology. We hypothesized that the better solubility of the donor material will led to the fast vapor penetration into the active layer. Indeed, we found the PCEs of the device of NDPPFBT:PC71BM were also enhanced in some extent upon THF solvent vapor annealing. As a result, a best PCE of 7.00% (Jsc = 10.75

mAcm-2, Voc = 0.89 V, FF =0.73) and an average PCE of 6.86% was reached by accounting over twenty individual devices, in which THF vapor annealing for 60 s but with the absence of any additive. It is important to note that the reported ca. 7% efficiency, as well as the very-high FF is one of among the best for OSCs from organic small-molecules with electron acceptor unit as core to date, and appeared to be more efficient than that of the best DPP-based small-molecules in literatures.63-64 Our novel and systematic work makes DPP-based small molecules become another high-efficiency (ca. 7%) and solution-processed small-molecule donor materials. In


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addition, shorter THF vapor annealing time (50 s) lead to a slightly reduced PCE of 6.11%. While the longer annealing time (80 s) can basically remain the optimal device performance, with PCE of 6.92% (See Table 2). The current density versus voltage (J–V) characteristics of the best device from these three materials before and after solvent vapor annealing was shown in Figure 3a for comparison. To shed light on the origin of the observed photovoltaic performance differences between DPPFB, DPPFBT, and NDPPFBT as well as the effect of solvent vapor annealing on the active layer, we carried out more detailed investigations on absorption properties, external quantum efficiency (EQE), their charge mobilities, and morphology of blend films by atomic force microscopy (AFM) to draw structure-property-device performances relationship, as described below. As shown in Figure 1, compared to the absorption spectra of as-cast the blend film of NDPPFBT with PC71BM, the spectra of the film with solvent vapor annealing showed more well-defined features across the entire visible spectrum. Especially, the absorption intensity in 350-800 nm region was found to increase obviously and the peak at range of 600-800 nm be slightly red-shifted. This is even more notable after THF vapor annealing for 60 s, which should be related to the enhanced π-π stacking and interaction of inter molecules by the increase in molecular planarity between difluorinated benzothiadiazole and IDT units. Such similar enhanced absorption features also take place in the blend film of DPPFB:PC71BM or DPPFBT:PC71BM after treatment of CH2Cl2 vapor for 30 s (Figure S3 and S4). Therefore, the observed improvement in Jsc (9.23 mAcm-2 vs. 10.07 mAcm-2 for NDPPFBT) upon solvent vapor annealing can be attributed to enhanced absorption at this stage, and should be clearly indicated in the EQE value of devices. To further understand the device performance and verify the positive effect of the enhanced absorption after solvent vapor annealing on the EQE of devices, the EQE spectra of these devices were measured and shown in Figure 2. All the EQE spectra covered a broad wavelength range from 300-800 nm for all the devices based on these materials. It is apparent that exposure to CH2Cl2 and THF vapor leads to increased EQE in the entire photoactive region, particular for longer wavelength region of 500-800 nm for NDPPFBT-based devices, which is consistent with the increase in Jsc. For example, the EQE value of original NDPPFBT-based devices is about 48% at 560 nm and 22% at 730 nm, which is increased to about 51% at 560 nm and 32% at 730 nm after THF vapor annealing for 60 s, respectively. Such results indicate that the photo-to-electron conversion process is more efficient after solvent vapor annealing for NDPPFBT-based devices. However, although the EQE curve of DPPFB-based

devices was obviously enhanced by the CH2Cl2 vapor annealing for 30 s, the intensity of EQE spectra was obviously decreased in the entire photoactive region by the THF vapor annealing for 50 s, which is consistent with the change of Jsc. The calculated Jsc values obtained by integration of the EQE data for these devices only showed minor mismatch (2-5%) with the experimental Jsc. To verify the positive effect of solvent vapor annealing on charge transport and reveal their dependence on the structure of these extended small molecules, we compared the hole and electron mobility in the actual device before and after solvent vapor annealing procedure using a space charge limited current (SCLC) method.90-93 The structures of hole-only and electron-only device are ITO/PEDOT/donor materials:PC71BM/MoO3/Al and ITO/ZnO/PFN/donor materials: PC71BM/Ca/Al, respectively. Table 2 displays the hole and electron mobilities deduced from the SCLC measurements and Figure S5-7 displays J-V characteristics of the hole-only and electron-only devices as obtained in dark. The hole mobility for device fabricated from these three materials was enhanced more than one order of magnitude after solvent vapor annealing. For example, the hole mobility for original device fabricated from NDPPFBT is (5.66±0.50)×10-6cm2V-1s-1. While it was enhanced to -5 2 -1 -1 (3.94±0.30)×10 cm V s after treatment with CH2Cl2 vapor for 30 s and further enhanced to (2.40±0.23) ×10-4 cm2V-1s-1 after treatment with THF vapor for 60 s, which is about correspond to a 40-fold enhancement. In addition, the electron mobility for original device fabricated from NDPPFBT/PC71BM was slightly increased from -4 2 -1 -1 -4 2 -1 -1 (1.26±0.21)×10 cm V s to (1.67±0.21)×10 cm V s after THF vapor annealing for 60 s. As a result, the imbalance between the charge carrier mobilities (the ratio between electron and hole, μe/μh) was dramatically improved after solvent vapor annealing. For example, μe/μh was about 22 for original device fabricated from NDPPFBT/PC71BM, which dramatically improved to ~3.2 after THF vapor annealing for 30 s and further improved to ~0.79 after THF vapor annealing for 60 s, respectively. These results clearly indicate that THF vapor annealing is beneficial for improvement of FF. As a result of high and balanced charge transporting in NDPPFBT/PC71BM blend, the best device show a high FF up to 0.72 after THF vapor annealing for 60 s, which should be associated with the best balanced charge carrier mobility therein. As processes of the accumulation of space charges and recombination of charge carriers are that closely related with the carrier mobilities of electron and hole, and the ratio between them, 94-95 we attributed that THF vapor annealing is mainly responsible for the improvement in Jsc and overall performance as a resulted of much better charge transport and less charge recombination loss. Given the important role of film morphology of the active in determining the device performance of OSCs, we

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carried out tapping mode AFM study to reveal the morphology change upon the solvent annealing. The samples were prepared by spin-coating NDPPFBT:PC71BM blend under the same condition for device fabrication. As can be seen in Figure 3, for the film without solvent annealing, large parts of the film surface were smooth and featureless with a root-mean-square roughness (Rrms) of about 0.7 nm and smaller phase domains were observed, indicating all of the donor materials have a good miscibility with PC71BM. After THF vapor annealing for 60 s or 80 s, the blend films were more phase-separated, featured with larger surface aggregates and more notable phase domain. As a result of forming a well interpenetrating network with clear donor-acceptor phase separation, the roughness of the film surface increased to 1.2 nm and 1.1 nm, respectively, when after a 60 s’ annealing and a 80 s’ annealing. Given its good solubility, we anticipated that THF vapor annealing provided a driving force to facilitate domain grow, resulting in more obvious phase segregation between donor/acceptor, and the more ordered film morphology of the active layer in nanoscale.96 (a)

(b)

RMS = 0.7 nm

(c) RMS = 1.2 nm

RMS = 1.1 nm

10.0 nm 5.0 nm (d)

(e)

(f) 0.0 nm

Figure 3. Tapping mode AFM height (a, b, c) and phase (d, e, 2 f) images 5×5 μm of NDPPFBT/PC71BM (1:2, w/w) without (a, d) and with THF vapor annealing for 60 s (b, e) and 80 s (c, f).

Conclusions In summary, a family of DPP core-based, extended conjugated D2-A2-D1-A1-D1-A2-D2 type small molecules, DPPFB, DPPFBT and NDPPFBT have been designed, synthesized, characterized photophysical and electrochemical properties, and utilized to prepare highly efficiency BHJ-OSCs as small-molecule donor materials. All of three materials exhibited strong absorption range from 300-900 nm and relative low-lying HOMO energy level, in particular for NDPPFBT, due to the extended conjugated structure composed of multiple electron donating moieties and electron accepting moieties. Therefore, the BHJ-OSC devices based on these three materials and PC71BM exhibited dramatically increase of the performance and all of the best PCE are over 5% upon annealing by CH2Cl2for 30 s but without using any

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additives in comparison with the original devices. Notably, the best PCE of 7.00% for NDPPFBT with very-high FF of 0.73 and high Voc of 0.89 V was achieved in simple device configuration after THF vapor annealing for 60 s, which is among one of the highest reported for small-molecules-based solar cells with electron acceptor unit as core. These amazing results have revealed how large impacts on the photovoltaic performances can be achieved by replacing the thiophene ring by more electron-deficient of thiazole ring and IDT flanked with two thiophene moieties or not as well as solvent vapor annealing. Exposure to CH2Cl2 or THF vapor allows for a re-organization of the blend, which increased the intensity and vibrational feature of absorption, improved the balance of charge carrier mobility and PCE. The results marks the first time in which DPP core unit has been used in small-molecule structure to prepare BHJ solar cells with an efficiency over 7% with the high Voc(up to 0.93) and very-high FF (up to 0.74). Experimental Section Materials and Characterization: All air and water sensitive reactions were performed under nitrogen atmosphere. Tetrahydrofuran (THF) was dried over Na/benzophenoneketyl and freshly distilled prior to use. The other materials were of the common commercial level and used as received. 1H and 13C NMR spectra were recorded on a Bruker ARX-400 (400 MHz) or ARX-500 (500 MHz) spectrometer, using CDCl3, except where noted. All chemical shifts were reported in parts per million (ppm). 1H NMR chemical shifts were referenced to CDCl3 (7.26 ppm), and 13C NMR chemical shifts were referenced to CDCl3 (77.23 ppm). MALDI-TOF-MS was recorded on a Bruker BIFLEX III mass spectrometer. Thermal gravity analyses (TGA) were carried out on a TA Instrument Q600 analyzer. Elemental analyses were performed using a German Vario EL III elemental analyzer. Absorption spectra were recorded on PerkinElmer Lambda 750 UV-vis spectrometer. Cyclic voltammetry (CV) was performed on BASI Epsilon workstation. Glassy carbon electrode was used as a working electrode and a platinum wire as a counter electrode, These films were drop-cast on a glass carbon working electrode from THF at a concentration of 5 mg/mL. Measurements were carried out at a scan rate of 50 mV/s in CH3CN containing 0.1 M n-Bu4NPF6 as the supporting electrolyte. All potentials were recorded versus Ag/AgCl reference electrode and calibrated with the redox couple of Fc/Fc+ under the same experimental conditions. BHJ-OSC fabrication. Device preparation and characterization were carried out in clean room conditions with protection against dust and moisture. The active area of OPV devices is 0.16 cm2 (~2×8 mm2, as defined by a shadow mask). The fabrication of OSCs followed the procedures described in our previous paper.97

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Characterization and measurement. The values of power conversion efficiency were determined from J-V characteristics measured by a Keithley 2400 source-measurement unit under AM 1.5G spectrum from a solar simulator (Oriel model 91192). Masks made from laser beam cutting technology with well-defined area of 16.0 mm2 were attached to define the effective area for accurate measurement. Solar simulator illumination intensity was determined using a monocrystal silicon reference cell (Hamamatsu S1133, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). The active layer was spin-coated from blend chloroform solutions with weight ratio of three materials and PC71BM at 1:2 and then were placed in a glass petri dish containing 1 mL CH2Cl2 for 30 s or 1 mL THF for 50 s, 60 s, and 80 s for solvent vapor annealing. The film morphology was studied by atomic force microscopy (AFM, VeecoMultiMode V) operating in tapping mode. EQE values of the encapsulated devices were measured by using an integrated system (Enlitech, Taiwan, China) and a lock-in amplifier with a current preamplifier under short-circuit conditions. The devices were illuminated by monochromatic light from a 75 W xenon lamp. The light intensity was determined by using a calibrated silicon photodiode. DPPFB: In a 100 mL two-neck round-bottom flask, 5 (156 mg, 0.12 mmol), 6 (32.0 mg, 0.050 mmol), and Pd2(dba)3 (2.3 mg, 0.0025 mmol), tri(o-tolyl)phosphine (3.1 mg, 0.010 mmol) was added. The flask was evacuated and back-filled with N2 three times, and then degassed toluene was injected into the mixture. The resulting solution was stirred at refluxing temperature for 12 h under the N2 atmosphere. After being cooled to room temperature, the solvents were then removed under reduced pressure. The dark residue was purified by silica gel chromatography, eluting with petroleum ether (PE)-CH2Cl2 (3:1) to give dark solid (115 mg, 81%).1H NMR (CDCl3, 400 MHz, ppm): δ 8.96-9.01 (m, 2H, Th-H), 8.21-8.27 (m, 4H, Th-H), 7.46 (s, 2H, Ph-H), 7.35-7.37 (s, d, 4H, J = 3.6 Hz, Ph-H, Th-H), 7.25-7.26 (m, 4H, Th-H), 7.16-7.17 (d, J = 3.6 Hz, 2H, Th-H), 6.74-6.75 (d, J = 3.6 Hz, 2H, Th-H), 4.10 (m, 4H, N-CH2), 2.82-2.85 (t, J = 7.6 Hz, 4H, CH2), 2.01-2.07 (m, 18H, CH2,CH), 1.70-1.74 (m, 4H, CH2), 1.33-1.43 (m, 28H, CH2),0.91-0.97 (m, 84H, CH2), 0.57-0.73 (m, 54H, CH2, CH3).13C NMR (CDCl3, 100 MHz, ppm): δ 162.0, 156.6, 155.7, 154.1, 153.5, 149.2, 146.7, 146.6, 144.3, 143.5, 141.5, 141.4, 139.4, 137.5, 137.1, 136.4, 136.2, 134.5, 133.3, 131.9, 131.8, 130.3, 127.7, 126.5, 125.3, 124.3, 124.2, 123.5, 120.7, 114.9, 114.6, 112.7, 111.0, 110.9, 108.6, 54.3, 46.2, 44.3, 43.8, 39.6, 35.3, 34.5, 34.3, 34.1, 31.8, 30.5, 29.0, 28.9, 28.4, 27.5, 24.0, 23.4, 23.1, 22.8, 14.4, 14.3, 14.2, 10.94, 10.89, 10.8, 10.6. MALDI-TOF MS (m/z):calcd for C166H216F4N6O2S12:2787. Found: 2787 (M+). Elemental Analysis: calcd for C166H216F4N6O2S12: C, 71.50; H, 7.81; N, 3.01. Found:C, 71.10; H, 7.65; N, 2.92.

DPPFBT: In a 100 mL two-neck round-bottom flask, 11(0.13 g, 0.09 mmol), 6 (22.8 mg, 0.035 mmol), and Pd2(dba)3 (1.9 mg, 0.0018 mmol), tri(o-tolyl)phosphine (2.2 mg, 0.0072 mmol) was added. The flask was evacuated and back-filled with N2 three times, and then degassed toluene was injected into the mixture. The resulting solution was stirred at refluxing temperature for 12 h under the N2 atmosphere. After being cooled to room temperature, the solvents were then removed under reduced pressure. The dark residue was purified by silica gel chromatography, eluting with PE-CH2Cl2 (2:1) to give dark solid (0.070 g, 64%).1H NMR (CDCl3, 400 MHz, ppm): δ 8.97-8.98 (d, J = 3.6 Hz, 2H, Th-H), 8.21-8.25 (m, 4H, Th-H), 7.32 (m, 8H, Ph-H, Th-H), 7.32 (m, 4H, Th-H), 7.23-7.24 (d, 2H, J = 3.6 Hz, Th-H), 7.15-7.16 (m, 6H, Th-H), 6.74-6.75 (d, J = 3.6 Hz, 2H, Th-H), 4.08 (m, 4H, N-CH2), 2.81-2.85 (t, J = 7.6 Hz, 4H, CH2), 2.01 (m, 18H, CH2,CH), 1.70-1.73 (m, 4H, CH2), 1.33-1.41 (m, 28H, CH2), 0.90-0.96 (m, 84H, CH2), 0.54-0.76 (m, 54H, CH2, CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ 161.9, 156.1, 156.0, 153.2, 151.3, 151.1, 149.0, 148.9, 148.7, 148.0, 146.7, 142.7, 142.3, 141.7, 139.9, 139.4, 138.1, 137.7, 137.1, 136.1, 135.9, 134.5, 134.3, 134.2, 132.1, 130.1, 128.4, 126.2, 125.3, 124.7, 124.4, 123.9, 123.4, 120.0, 114.3, 111.5, 108.7, 54.3, 46.3, 44.3, 43.9, 39.6, 35.3, 34.6, 34.5, 34.2, 34.0, 31.8, 30.5, 29.0, 28.9, 28.4, 27.7, 27.52, 27.45, 24.0, 23.4, 23.2, 23.1, 22.8, 22.3, 14.4, 14.3, 11.0, 10.9, 10.6. MALDI-TOF MS (m/z):calcd for C182H224F4N6O2S16:3113. Found: 3114 ([M+H+]+). Elemental Analysis: calcd for C182H224F4N6O2S16: C, 70.13; H, 7.24; N, 2.70. Found:C, 69.79;H, 7.19; N, 2.61. NDPPFBT: In a 100 mL two-neck round-bottom flask, 11(0.13 g, 0.09 mmol), 13 (23 mg, 0.0035 mmol), and Pd2(dba)3 (1.9 mg, 0.0018 mmol), tri(o-tolyl)phosphine (2.2 mg, 0.0072 mmol) was added. The flask was evacuated and back-filled with N2 three times, and then degassed toluene was injected into the mixture. The resulting solution was stirred at refluxing temperature for 12 h under the N2 atmosphere. After being cooled to room temperature, the solvents were then removed under reduced pressure. The dark residue was purified by silica gel chromatography, eluting with PE-CH2Cl2 (1:1) to give dark solid (0.60 g, 55%).1H NMR (CDCl3, 400 MHz, ppm): δ 8.20-8.24 (m, 4H, Th-H), 8.09 (s, 2H, Tz-H), 7.33-7.37 (d, m, J = 3.6 Hz, 8H, Th-H), 7.28 (s, 2H, Th-H), 7.22-7.23 (d, 2H, J = 3.6 Hz, Th-H), 7.15-7.19 (m, 6H, Th-H), 6.73-6.74 (d, J = 3.6 Hz, 2H, Th-H), 4.37 (m, 4H, N-CH2), 2.81-2.85 (t, J = 7.6 Hz, 4H, CH2), 2.01 (m, 18H, CH2,CH), 1.70-1.73 (m, 4H, CH2), 1.33-1.39 (m, 28H, CH2), 0.86-0.98 (m, 84H, CH2), 0.64-0.77 (m, 54H, CH2, CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ161.5, 156.2, 153.3, 152.6, 151.3, 151.1, 149.0, 148.7, 148.5, 146.8, 142.6, 142.3, 141.7, 140.9, 140.2, 138.1, 137.9, 137.4, 137.1, 136.2, 135.8, 134.5, 132.1, 130.6, 130.1, 128.5, 125.3, 124.4, 124.0, 123.5, 120.2, 114.3, 111.5, 110.9, 54.3, 47.0, 44.3, 43.9, 39.6, 35.3, 34.5, 34.1, 31.8, 30.8, 30.5, 29.9, 29.0, 28.9, 28.4, 27.7, 27.5, 27.3, 24.2, 23.4, 23.1, 22.8, 14.3, 11.0,

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10.9, 10.6. MALDI-TOF MS (m/z):calcd for C180H222F4N8O2S16:3118. Found: 3118 (M+). Elemental Analysis: calcd for C180H222F4N8O2S16: C, 69.32; H, 7.17; N, 3.59. Found:C, 69.10; H, 7.23; N, 3.49.

ASSOCIATED CONTENT Supporting Information. The synthesis and characterization of the precursors and desired materials, the 1 13 copies of H NMR and C NMR are described in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected] Author Contributions §J.-L. Wang, Z. Wu, and J.-S. Miao contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the grants from the National Natural Science Foundation of China (21472012, 21202007, 51225301, 91333206); the Thousand Youth Talents Plan of China; Beijing Natural Science Foundation (2152027). The Development Program for Distinguished Young and Middle-aged Teachers and Special programs to cultivate major projects of Beijing Institute of Technology.

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TOC: 0.5

12 10

0.4

NDPPFBT NDPPFBT-DCM-30s NDPPFBT-THF-60s

8 6

0.3

NDPPFBT NDPPFBT-DCM-30s NDPPFBT-THF-60s

4

0.2

Jsc = 10.75 mA cm-2 2

Voc = 0.89 V

0

FF = 73.1%

0.1

PCE = 7.00% -2 -0.2

0

0.2 0.4 Voltage (V)

0.6

0.8

1

0 300

400


800

900

13


Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule

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