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1,3-Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dioneBased Small-Molecule Donor for Efficient Solution-Processed Solar Cells Zhongbo Zhang, Zichun Zhou, Qin Hu, Feng Liu, Thomas P. Russell, and Xiaozhang Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14572 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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ACS Applied Materials & Interfaces

1,3-Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno [3,4-c]pyrrole-4,6(5H)-dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells Zhongbo Zhang,†,‡ Zichun Zhou, †,‡ Qin Hu,

⊥

⊥

Feng Liu,$ Thomas P. Russell, and

Xiaozhang Zhu†,‡,* †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of

Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China $

Department of Physics and Astronomy, and Collaborative Innovation Center of IFSA

(CICIFSA), Shanghai Jiaotong University, Shanghai 200240, P. R. China ‡

School of Chemistry and Chemical Engineering, University of Chinese Academy of

Sciences, Beijing 100049, China ⊥

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA

94720, United States

ABSTRACT:

A

small

molecule

TBTT-1

with

5-(2-ethylhexyl)-1,3-bis(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c ]pyrrole-4,6(5H)-dione (TBTT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. TBTT-1 exhibits a broad

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absorption with a low optical bandgap of approximately 1.53 eV in the thin film. An optimized power conversion efficiency (PCE) of 7.47% with a high short-circuit current of 14.95 mA cm−2 was achieved with diphenyl ether (DPE) as additive, which is the highest PCE for TPD-based small-molecule solar cells. According to the detailed morphology investigations, we found that DPE processing helped to enhance π−π stacking and reduce the scales of phase separation, which led to improve exciton splitting and charge transport in BHJ thin film, and thus enhanced device performance.

1. Introduction Organic photovoltaics (OPVs) with a bulk-heterojunction (BHJ) architecture have been considered as a promising renewable and low-cost energy conversion technology owing to the advantages of light weight, solution processability, and high flexibility.1–3 The power conversion efficiency (PCE) of OPVs based on polymer donor materials has been improved rapidly, achieving high PCEs over 10%.4–7 Meanwhile, small-molecule OPVs have also received special attentions because of well-defined molecular structures, low batch-to-batch variation, and feasible tuning of the electronic structure.8–10 Small molecules with typical optical bandgaps (Egopt) of approximately 1.7 eV have shown promising PCEs over 9% in a conventional device structure.11–16 Light harvesting of OPVs requires materials to have a high absorption coefficient with a low Egopt.17, 18 Thieno[3,4-c]pyrrole-4,6-dione (TPD)19 is regarded as one of the most valuable electron-deficient moieties available for the construction of donor (D)-acceptor (A) 2 ACS Paragon Plus Environment

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conjugated polymers and small molecules for OPV applications. TPD moiety was copolymerized with the octyldodecyloxy-substituted benzodithiophene (BDT) moiety by Leclerc and coworkers, affording mid-bandgap (~1.8 eV) PBDTTPD copolymer with a PCE of 5.5% with a short-circuit current (Jsc) of 9.81 mA cm−2 in a conventional single-junction device structure.20 Further modulations of side alkyl chains and π bridges on BDT-TPD-based copolymers delivered the highest PCE of 8.5% among TPD-based polymer donors.21–28 Kim and coworkers synthesized a series of mid-bandgap and wide bandgap small-molecule donors based on the BDT-TPD combination, and the modifications of the substituents on BDT moieties delivered a moderate PCE of 4.98% with a Jsc of 10.6 mA cm−2.29–31 The incorporation of highly electron-rich moieties, such as dithieno(3,2-b;2’,3’-d)silole (DTS), reduced Egopt to 1.55 eV, but led to an even lower PCE of 4.0% with a Jsc of 8.6 mA cm−2.32 We recently designed composite acceptor moieties, TBTTs, TPD flanked with two unsymmetrical thieno[3,4-b]thiophene moieties regioselectively, that have been successfully applied for the development of high-performance n-type semiconductors, 2DQTT-o,33,

34

and polymer donors (PBDT-TBTTs).35 Because of the enhanced

quinoidal resonance in D-A frameworks, PBDT-TBTTs showed significantly high Jsc of 18.15 mA cm−2 attributing to the fairly reduced Egopt of 1.56 eV. Herein we report the design and synthesis of a new TBTT-type small-molecule donor, TBTT-1 (Figure 1), consisting of a conjugated framework of TBTT with two bithiophenes terminated with 3-octyl-rhodanines. Because of its low Egopt of 1.53 eV with a high absorption coefficient of 1.09 × 105 M−1 cm−1. TBTT-1 exhibited a broad external quantum 3 ACS Paragon Plus Environment


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efficiency (EQE) spectral response of up to 800 nm. Moreover, TBTT-1-based OPVs showed a high PCE of 7.47% with a Jsc of 14.95 mA cm−2 after device optimizations, which is the highest PCE for TPD-based small-molecule solar cells.29–32, 36

Figure 1. Molecular structure of TBTT-1. 2. Experimental Section 2.1 Syntheses. All the starting materials were obtained from commercial sources and used as-received without further purification. PFN were bought from Luminescence Technology Corp.. Anhydrous THF and toluene were distilled over Na/benzophenone prior to use. All the reactions dealing with air- or moisture-sensitive compounds were carried out in a dry reaction vessel under a nitrogen atmosphere. 5-(2-Ethylhexyl)-1,3-bis(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3, 4-c]pyrrole-4,6(5H)-dione (TBTT). To an oven-dried round-bottomed flask loaded with tributyl(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)stannane 2 (1.10 g, 2.03 mmol) in anhydrous DMF (6 mL) and toluene (6 mL) under nitrogen atmosphere was added

1,3-dibromo-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione

(1)

(0.391 mg, 0.92 mmol) and Pd(PPh3)4 (53 mg, 0.046 mmol). The reaction was stirred and refluxed for 12 h under dark. The reaction mixture was then cooled to room 4 ACS Paragon Plus Environment

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temperature. Compound TBTT was collected by filtration and washed with MeOH as a brown solid (0.60 g, 85%). 1H NMR (400 MHz, CDCl3): δ 7.41 (s, 2H), 6.68 (s, 2H), 3.58 (d, 3J = 7.3 Hz, 2H), 2.79 (d, 3J = 7.0 Hz, 4H), 1.95−1.83 (m, 1H), 1.73 (t, 3J = 6.3 Hz, 2H), 1.46−1.30 (m, 24H), 0.92 (m, 18H);

13

C NMR (100 MHz, CDCl3): δ

163.1, 152.7, 147.6, 140.1, 135.5, 127.0, 118.9, 115.6, 115.0, 42.6, 40.5, 38.4, 36.1, 32.6, 30.8, 28.9, 28.7, 25.8, 24.1, 23.2, 23.1, 14.3, 14.2, 10.9, 10.6; MS (MALDI-TOF): 765.3 [M]+. 1,3-Bis(4-bromo-2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-5-(2-ethylhexyl)-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione (TBTT-Br). A solution of TBTT (0.40 g, 0.52 mmol) in CHCl3/DMF (3:1, 16 mL) was added NBS (0.204 g, 1.15 mmol) in one portion. The reaction mixture was stirred at room temperature in dark for 2 h. The reaction mixture was washed with saturated NaCl solution (20 mL), saturated NaHSO3 solution (20 mL), and saturated NaCO3 solution (20 mL) successively, and the organic layer was dried over MgSO4. After filtration, the solvent was removed under reduced pressure to afford crude product, which was further purified on a silica-gel column chromatography with CH2Cl2/petroleum ether (1:2) eluent. Compound TBTT-Br was obtained as a dark brown solid (0.42 g, 87%). 1H NMR (400 MHz, CDCl3): δ 6.59 (s, 2H), 3.55 (d, 3J = 7.3 Hz, 2H), 2.77 (d, 3J = 6.9 Hz, 4H), 1.91−1.81 (m, 1H), 1.72 (t, 3J = 6.2 Hz, 2H), 1.46−1.27 (m, 24H), 0.92 (m, 18H); 13C NMR (100 MHz, CDCl3): δ 162.9, 154.0, 147.4, 138.5, 134.2, 126.9, 120.1, 114.3, 103.6, 42.8, 40.4, 38.4, 36.2, 32.6, 30.8, 28.9, 28.8, 25.8, 24.2, 23.2, 23.1, 14.3, 14.2, 10.9, 10.7; MS (MALDI-TOF): 923.1 [M]+. 5 ACS Paragon Plus Environment


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2-(5-Bromo-4-octylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3-dioxolane

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(4).

A

mixture of 5-bromo-4-octylthiophene-2-carbaldehyde 3 (2.0 g, 6.6 mmol), pinacol (2.51 g, 13.19 mmol), and 4-methylbenzenesulfonic acid monohydrate (0.039 g, 0.33 mmol) was refluxed in toluene (65 mL) for 4 h using a Dean-Stark apparatus. After cooling to room temperature, sodium hydroxide (0.1 g, 2.5 mmol) in ethanol (2 mL) was added to the mixture and stirred for 30 min. The suspension was filtered and the residue was washed with toluene (125 mL). The filtrate was washed with brine (3 × 100 mL), dried over sodium sulfate. After filtration, the solvent was removed under reduced pressure to afford crude product, which was further purified on a silica-gel column chromatography with CH2Cl2/petroleum ether (1:1) eluent. Compound 4 was obtained as pale yellow oil (2.5 g, 94%). 1H NMR (400 MHz, CDCl3): δ 6.83 (s, 1H), 6.06 (s, 1H), 2.52−2.45 (t, 2H), 1.56−1.51 (m, 2H), 1.28 (m, 22H), 0.90−0.86 (m, 3H); 13

C NMR (100 MHz, CDCl3): δ 143.1, 141.6, 127.2, 110.2, 96.8, 83.2, 77.5, 77.2,

76.8, 32.0, 29.7, 29.7, 29.5, 29.4, 29.3, 24.3, 22.8, 22.2, 14.2; EI-MS: 402 [M]+. 4,4,5,5-Tetramethyl-2-(3-octyl-[2,2’-bithiophen]-5-yl)-1,3-dioxolane

(5).

A

solution of compound 4 (1.0 g, 2.48 mmol) and 2-(tributylstannyl) thiophene (1.02 g, 2.73 mmol) in anhydrous DMF (15mL) and toluene (15 mL) was deaerated twice with argon followed by the addition of Pd(PPh3)4 (0.143 g, 0.12 mmol). After being stirred at 100 °C for 24 h under argon, the reaction mixture was poured into water (100 mL) and extracted with CH2Cl2. The organic layer was washed with water and then dried over Na2SO4. After removal of solvent, the crude product was purified on a silica-gel column chromatography using CH2Cl2/petroleum ether (1:1) eluent. Compound 5 was 6 ACS Paragon Plus Environment

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obtained as pale yellow oil (0.9 g, 89%). 1H NMR (400 MHz, CDCl3): δ 9.83 (s, 1H), 7.58 (s, 1H), 7.07 (d, 3J = 3.9 Hz, 1H), 7.02 (d, 3J = 3.9 Hz, 1H), 2.74 (t, 3J = 7.8 Hz, 2H), 1.66 (q, 3J = 7.5 Hz, 2H), 1.39-1.25 (m, 11H), 0.88 (t, 3J = 6.6 Hz, 3H);

13

C

NMR (100 MHz, CDCl3): δ 182.8, 141.4, 140.6, 140.5, 139.0, 135.0, 127.9, 127.7, 127.5, 32.0, 30.5, 29.6, 29.5, 29.4, 29.3, 22.8, 14.24; EI-MS: 406 [M]+. Tributyl(3’-octyl-5’-(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)-[2,2’-bithiophen]-5-yl )stannane (6). Compound 5 (0.50 g, 1.23 mmol) was dissolved in anhydrous THF (10 mL) under nitrogen atmosphere and cooled to −78 °C and then n-butyllithium (0.85 mL, 1.35 mmol, 1.60 M in hexane) was added via syringe under stirring. After stirring at −78 °C for 30 min, tributyltin chloride (0.37 mL, 1.35 mmol) was added in one portion and stirred at −78 °C for another 30 min. The clear reaction solution was warmed to room temperature for 30 min. A few drops of saturated NH4Cl solution was added to the reaction mixture and extracted three times with CH2Cl2. The organic layer was separated, dried over MgSO4, and concentrated under reduced pressure. 0.85 g of compound 6 was obtained as pale yellow oil in 99% yield and was directly used for the next step without further purification. 1H NMR (300 MHz, CDCl3): δ 7.21 (d, J = 3.3 Hz, 1H), 7.08 (d, 3J = 3.3 Hz, 1H), 6.96 (s, 1H), 6.12 (s, 1H), 2.73−2.65 (m, 2H), 1.64−1.57 (m, 6H), 1.38−1.25 (m, 30H), 1.15−1.01 (m, 6H), 0.90 (t, 3J = 7.2 Hz, 12H); MS (MALDI-TOF): 696.3 [M]+. 5-(2-Ethylhexyl)-1,3-bis(2-(2-ethylhexyl)-4-(3’-octyl-5’-(4,4,5,5-tetramethyl-1,3-di oxolan-2-yl)-[2,2’-bithiophen]-5-yl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]p yrrole-4,6(5H)-dione (7). A solution of TBTT-Br (300 mg, 0.32 mmol) and 7 ACS Paragon Plus Environment


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compound 6 (497 mg, 0.71 mmol) in anhydrous DMF (8mL) and toluene (8 mL) was deaerated twice with argon followed by the addition of Pd(PPh3)4 (19 mg, 0.016 mmol). After being stirred at 100 °C for 24 h under argon, the reaction mixture was poured into water (100 mL) and extracted with chloroform. The organic layer was washed with water and then dried over magnesium sulfate. After removal of solvent, the crude product was purified on a silica-gel column chromatography using chloroform/petroleum ether (1:1) eluent. Compound 7 was obtained as a red solid (400 mg, 78%). 1H NMR (400 MHz, CDCl3): δ 7.32 (d, 3J = 3.8 Hz, 2H), 7.07 (d, 3J = 3.8 Hz, 2H), 7.00 (d, 3J = 1.8 Hz, 4H), 6.14 (s, 2H), 3.56 (d, 3J = 7.4 Hz, 2H), 2.82 (d, 3

J = 7.0 Hz, 4H), 2.74 (t, 3J = 7.8 Hz, 4H), 1.90 (d, 3J = 6.4 Hz, 1H), 1.75 (q, 3J = 6.3

Hz, 2H), 1.69−1.61 (m, 4H), 1.41−1.26 (m, 68H), 0.97−0.85 (m, 24H); 13C NMR (100 MHz, CDCl3): δ 163.1, 153.6, 142.8, 142.0, 141.1, 139.7, 137.1, 135.7, 134.6, 131.1, 129.3, 127.5, 126.7, 125.8, 117.3, 115.6, 96.8, 83.3, 42.8, 40.7, 38.3, 36.4, 32.6, 32.0, 30.8, 30.6, 29.7, 29.7, 29.6, 29.4, 29.0, 28.7, 25.8, 24.4, 24.1, 23.3, 23.1, 22.8, 22.2, 14.34, 14.26, 11.0, 10.6; HRMS (MALDI-TOF): m/z calcd for C88H119NO6S9 [M]+: 1573.6524, found 1573.6518. 5-(2-Ethylhexyl)-1,3-bis(2-(2-ethylhexyl)-4-(3’-octyl-5’-((Z)-(3-octyl-4-oxo-2-thiox othiazolidin-5-ylidene)methyl)-[2,2’-bithiophen]-5-yl)thieno[3,4-b]thiophen-6-yl)4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TBTT-1). A solution of compound 7 (400 mg, 0.25 mmol) in HCl (10%):THF (1:2) was heated to 60 °C under an inert atmosphere for 12 h. After cooling to room temperature, the suspension was filtered and washed with methanol. The red solid obtained was dissolved in chloroform. Then, 8 ACS Paragon Plus Environment

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3-octyl-rhodanine (187 mg, 0.76 mmol) and 3 drops pyridine were added. The mixture was stirred at 70 °C for 12 h. After cooling to room temperature, the reaction mixture was poured into water (50 mL) and extracted with chloroform. The organic layer was washed with water and then dried over magnesium sulfate. After removal of solvent, the crude product was purified on a silica-gel column chromatography using chloroform/petroleum ether (1:1) eluent. TBTT-1 was obtained as a black solid (380 mg, 82%). 1H NMR (400 MHz, CDCl3): δ 7.46 (s, 2H), 7.01 (s, 2H), 6.95 (d, 3J = 8.7 Hz, 4H), 6.82 (s, 2H), 3.98 (t, 3J = 7.7 Hz, 4H), 3.63 (d, 3J = 7.2 Hz, 2H), 2.90 (d, 3J = 7.1 Hz, 4H), 2.67 (s, 4H), 1.98 (d, 3J = 6.9 Hz, 1H), 1.88−1.80 (m, 2H);

13

C NMR

(100 MHz, CDCl3): δ 192.0, 167.2, 162.6, 154.2, 142.7, 140.8, 139.5, 137.3, 136.9, 135.4, 135.0, 134.2, 126.8, 125.1, 124.1, 120.6, 118.4, 115.5, 45.0, 43.0, 40.8, 38.6, 36.8, 33.0, 32.1, 32.0, 31.1, 30.2, 30.1, 30.10, 30.07, 30.03, 29.8, 29.6, 29.34, 29.28, 29.15, 28.9, 27.1, 27.0, 26.0, 24.3, 23.43, 23.33, 23.31, 22.9, 22.8, 14.4, 14.27, 14.25, 14.18, 11.10, 10.9; HRMS (MALDI-TOF): m/z calcd for C98H129N3O4S13 [M]+: 1828.6386, found 1828.6381. 2.2

Device

Fabrication

and

Characterization.

The

solution-processed

small-molecule BHJ solar cells were fabricated and characterized according to the previously reported methods.18 2.3 Microstructure Investigation. Grazing incidence X-ray scattering (GIXD) and resonant soft x-ray scattering (RSoXS) characterization of the thin films were performed as previously reported.18,37



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3. RESULTS AND DISCUSSION 3.1 Syntheses. The synthetic route of target molecule TBTT-1 is shown in Scheme 1. The intermediates 2 and TBTT-Br were synthesized according to our previously reported procedures.33, 37 The double Stille coupling reaction of compounds TBTT-Br and 6 afforded compound 7 in 78% yield, which was subjected to pinacol deprotection and Knoevenagel condensation to generate TBTT-1 in 84% yield. TBTT-1 is a black solid and exhibits good solubility in common organic solvents such as chloroform, toluene, chlorobenzene, etc. Thermogravimetric analysis (TGA) indicates the good thermal stability of TBTT-1 with a 5 % weight loss at 372 °C under a N2 atmosphere (see Figure S1 in Supporting Information).

a

Reagents and conditions: (a) Pd(PPh3)4 (5 mol%), toluene/DMF, 100 °C, 12 h; (b)

NBS (2.1 equiv.), chloroform/DMF, r.t., 2 h, 87%; (c) pinacol (2.0 equiv.), TsOH·H2O (5 mol%), toluene, reflux, 2 h; (d) (i) n-BuLi (1.1 equiv.), THF, −78 °C, 30 min; (ii) n-Bu3SnCl (1.1 equiv.), −78 °C to r.t., 2 h, 95%; (e) (i) THF/10% 10 ACS Paragon Plus Environment

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hybrochloric acid, 60 °C, 12 h; (ii) 3-octyl-rhodanine (3.0 equiv.), piperidine, chloroform, reflux, 12 h, 84 %. Scheme 1. Synthesis of TBTT-1.a 3.2 Photophysical and Electrochemical Properities. Figure 2a shows normalized UV-vis absorption spectra of TBTT-1 in dilute chloroform solution (1.0 × 10–5 M−1 L–1) and in thin film. TBTT-1 displayed a maximum absorption at 609 nm with a high absorption coefficient (ε) of 1.09 × 105 M−1 cm−1 in chloroform. The maximum absorption of TBTT-1 in thin film was bathochromically shifted to 645 nm by 36 nm compared with that in solution, which could be attributed to the planarized molecular structure or effective π-π interaction in the condensed thin-film state. In thin film absorption, an absorption hump at 750 nm was seen, which may be contributed from intermolecular aggregation. The planarity of conjugated molecule can enhance π-delocalization and intermolecular π−π interaction, which is beneficial for charge carrier transport in the optoelectronic devices. The absorption onset of TBTT-1 film is 808 nm corresponding to an optical bandgap of 1.53 eV. We applied cyclic voltammetry measurements to evaluate the frontier orbital energy levels of TBTT-1 (Figure 2b). The potentials were internally calibrated using the ferrocene/ferrocenium (Fc/Fc+) redox couple (4.8 eV below the vacuum level) and the energy levels were calculated from the onset oxidation potential (0.22 eV) and reduction potential (–1.46 eV). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were −5.02 and −3.34 eV, respectively, which well matched the LUMO energy level of PC71BM for efficient charge separation. 11 ACS Paragon Plus Environment


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Figure 2. (a) UV−vis absorption spectra of TBTT-1 in chloroform solution and in thin film. (b) Cyclic voltammogram of TBTT-1 in dichloromethane solution (0.1 M Bu4NPF6). 3.3 Photovoltaic Performance. To explore the potential of TBTT-1 in photovoltaic applications, BHJ solar cells were fabricated from a chloroform solution using TBTT-1 as the donor and PC71BM as the acceptor with the conventional device structure of ITO/PEDOT:PSS/TBTT-1:PC71BM/PFN/Al, which were tested under AM 1.5G solar illumination (100 mW/cm2). Figure 3b shows the current density versus voltage (J−V) curves of solar cells with and without 3% v/v DPE additive. Table 1 summarized the photovoltaic parameters. The photovoltaic performance is highly related to the additive [diphenyl ether (DPE)] used in the film processing. The devices based on TBTT-1:PC71BM without any additive showed a low PCE of 3.22% with an open-circuit voltage (Voc) of 0.74 V, a short-circuit current (Jsc) of 7.38 mA cm−2, and a fill factor (FF) of 59.1%. By contrast, the active layer processed with 3% v/v DPE additive delivered a significantly improved PCE of 7.47%, with a Voc of 0.75 V, Jsc of 14.95 mA cm−2, and a FF of 66.6%. External quantum efficiency (EQE) curves of the optimal TBTT-1-based devices covered a wide wavelength range from 12 ACS Paragon Plus Environment

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300 to 800 nm (Figure 3a), which agrees well with its absorption spectrum. In TBTT-1-based devices processed with 3% v/v DPE, high EQE values over 50% were observed in a wide range of 405 to715 nm, with a peak value of 63% at 580 nm. The Jsc values calculated from the EQE curves under the standard solar spectrum (AM 1.5 G) were consistent with the current densities obtained from the J-V measurement with a small deviation of less than 5%.

Figure 3. (a) External quantum efficiency spectra of TBTT-1:PC71BM (1:0.8) with and without 3% v/v DPE. (b) The corresponding J−V characteristics. Table 1. Photovoltaic Performance of TBTT-1-Based Solar Cells under the Illumination of AM 1.5G, 100 mW/cm2. Voc

Jsat

FF

PCE

(V)

(mA cm−2)

(%)

(%)a

0%

0.74(0.738)

7.38(7.22)

59.1(57.2)

3.22(2.98)b

3%

0.75(0.756)

14.95(14.83)

66.6(65.2)

7.47(7.35)b

DPE Content

a

The values in parentheses indicate the average values of PCEs obtained from more than ten

devices. bDevice structure: ITO/PEDOT:PSS/TBTT-1:PC71BM (1:0.8 weight ratio)/PFN/Al.



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3.4 Charge-Transport Properties. Hole and electron mobilities of solar cells were measured by using the space charge limited current (SCLC) method (Supplementary Figure S2). By fitting the J-V curves from both hole- and electron-only devices, the carrier mobilities of TBTT-1:PC71BM blends were determined; the results are summarized in Table 2. For the blends without DPE, the hole and electron mobilities were determined to be 1.48 × 10−4 and 2.33 × 10−5 cm2 V−1 s−1, respectively, with an imbalanced µe/µh ratio of 0.16. For the active layer processed with 3% v/v DPE, the hole and the electron mobilities increased to 2.24 × 10−4 and 8.30 × 10−4 cm2 V−1 s−1 respectively, with a more balanced µe/µh of 3.70, which is beneficial to enhance charge transport and the efficiency of charge carrier collection. 3.5 Charge separation and collection. To understand the enhanced performance of devices using 3% v/v DPE additive, the relationship between the net photocurrent (Jph = Jlight − Jdark) and effective voltage (Veff = V0 − V) has been carried out (Figure 4a), where Jlight and Jdark are the current density under illumination and in the dark, respectively, V is the applied voltage, and V0 is the compensation voltage when Jph = 0. The Jph in device without DPE showed strong electric-field dependence, Veff of 2.0 V is insufficient to saturate Jph. However, The Jph in device with 3% v/v DPE elevated sharply with effective voltage increase, and reached a saturated value (Jsat) at high effective voltages, indicating that all electron-hole pairs could be effectively dissociated. In addition, the ratio of Jph/Jsat can be used to judge the overall exciton dissociation efficiency and charge collection efficiency. At the maximal power output condition, the Jph/Jsat of the device without DPE was 74.6%, while the device with 3% 14 ACS Paragon Plus Environment

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v/v DPE gave a higher Jph/Jsat of 94.6%, confirming dramatically improved charge collection efficiency. We examined the photocurrent (Jsc) under different light intensity to study the recombination property under short current condition (Figure 4b). Both curves showed almost identical slope values in 0% and 3% DPE processed devices (0.94 vs 0.95), indicating a similar recombination dominated by geminate recombination process.38

Figure 4. (a) Photocurrent density versus effective applied voltage (Jph−Veff) plot of TBTT-1 with and without 3% v/v DPE under AM 1.5G, 100 mW cm−2 simulated solar illumination. (b) Jsc of TBTT-1 with and without 3% v/v DPE devices against the incident light power. Table 1. Photocurrent Data and the Charge Mobilities of TBTT-1:P71BM without and with DPE.

DPE

Jsat (mA cm−2)

Jph/Jsat

µh

µe

(%)

(cm2 V−1 s−1)

(cm2 V−1 s−1)

0%

9.65

74.6

1.48 × 10−4

2.33 × 10−5

3%

15.8

94.6

2.24 × 10−4

8.30 × 10−4



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3.6 Thin-film Morphology. The OPV performance is closely related to the morphology

of

BHJ

thin

films.

We

characterized

the

nanostructure

of

TBTT-1:PC71BM thin films by grazing incidence X-ray diffraction (GIXD), transmission electron microscopy (TEM), and resonant soft X-ray scattering (RSoXS) methods. Shown in Figure 5 were the GIXD patterns and line-cut profiles of BHJ thin films processed with and without additive. As seen from Figure 5a and 5b, BHJ blends processed in both conditions showed an edge-on orientation. 0% DPE processed film showed a (100) diffraction in 0.31 Å−1 (distance of 2.02 nm) with a crystal coherence length of 15.5 nm in out-of-plane direction. A π−π stacking peak was seen in in-plane direction at 1.74 Å−1 (0.361 nm) with a coherence length of 3.59 nm. 3% DPE processed film showed similar crystalline features. The (100) peak coherence length was estimated to be 15.7 nm; and π−π stacking peak coherence length was estimated to be 4.40 nm. Thus, π−π stacking was improved upon adding small amount of DPE additives, which helped charge transport in devices. Figure 6a showed the TEM images of 0% DPE processed BHJ thin films. Large domains were seen (hundreds of nanometers), which were PC71BM aggregations.39 The bright regions correspond to TBTT-1-rich domains, in which we could see some weak features still. When using DPE, BHJ thin film showed dramatic changes in phase image. Although there were still dark regions constituted of PC71BM rich region, the morphology of BHJ thin film became smoother and texturized. The smoothed morphology and smaller length scale of phase separation led to enhanced Jsc and FF in solar cell devices. RSoXS was further used to estimate the size statistics of 16 ACS Paragon Plus Environment

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phase-separated morphology. 0% DPE processed film showed a strong scattering peak at 0.0028 Å−1, corresponding to a distance of 224 nm, which is correlated to the center-to-center distances of PCBM aggregations. A shoulder peak was seen at 0.0062 Å−1 (distance of 100 nm), coming from the refined morphology in TBTT-1 rich region. Such a combination gave rise to a 3.22% PCE in device. 3% DPE processed thin film showed a reduced PCBM aggregation peak at 0.0048 Å−1, and thus the PCBM-rich region distance is 131 nm. The much-reduced intensity of scattering signal indicate the extent of phase separation is much weaker comparing to 0% DPE processed film. A shoulder peak was seen at 0.0105 Å−1, corresponding to a distance of 60 nm, coming from refined texture of phase-separated morphology and helping with charge separation and transport,40, 41 all of which help to boost the performance of solar cell devices.

Figure 5. (a, b) GIXD diffraction patterns of TBTT-1:PC71BM blend films without and with 3% v/v DPE, respectively; (c) GIXD line cut profiles (solid line: out-of-plane; dotted line: in-plane). 17 ACS Paragon Plus Environment


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Figure 6. (a, b) TEM images of TBTT-1:PC71BM blend films without and with 3% v/v DPE, respectively; (c) RSoXS of BHJ thin films. The scale bar is 200 nm in TEM. 4. CONCLUSION In summary, a small-molecule donor TBTT-1 with TBTT as the central unit, bithiophene as the π bridge, and 3-octyl-rhodanine as the terminal acceptor units, was designed and synthesized. The obtained material shows good absorption features (intensive 300−800 nm absorption) and the proper frontier energy levels. TBTT-1 cannot mix well with PC71BM, leading to large size PC71BM aggregations. However, when the DPE additive was introduced, a much smoother thin film was obtained, showing a multi-length scaled morphology with a PC71BM-rich domain spacing of 131 nm and a refined texture spacing of 60 nm. The improved morphology helped to increase device performances largely, and we obtained a champion efficiency of 7.47 % with a Jsc of 14.95 mA cm−2 and a FF of 66.6%, which is so far the highest for TPD-based small-molecule OPVs. DPE processing helped to enhance π−π stacking as 18 ACS Paragon Plus Environment

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well as reduce the length scales of phase separation, which improved exciton splitting and charge transport in BHJ thin film, corresponding well with performance improvement. Our work indicates that TBTT moiety should be very promising for the development of high-performance small-molecule donor materials for OPV applications.

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. TGA plot of TBTT-1 with a heating rate of 10 °C/min under nitrogen atmosphere, details of device optimization, semiconducting properties, and NMR charts.

AUTHOR INFORMATION

Corresponding Author *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

We thank the National Basic Research Program of China (973 Program) (No. 2014CB643502), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010200), and the National Natural Science Foundation of China 19 ACS Paragon Plus Environment


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(91333113, 21572234) for financial support. F FL and TPR were supported by the U.S. Office of Naval Research under contract N00014-15-1-2244. Portions of this research were carried out at beamline 7.3.3 and 11.0.1.2 at the Advanced Light Source, Molecular Foundry, and National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

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