Enhanced Photovoltaic Performance of Tetrazine-Based Small

Aug 30, 2017 - Two two-dimensional (2D) conjugated tetrazine-based small molecules (SMs), named TBDT(TTzT)2, and TBDT(TTz2T)2, were newly synthesized ...
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Research Article pubs.acs.org/journal/ascecg

Enhanced Photovoltaic Performance of Tetrazine-Based Small Molecules with Conjugated Side Chains Chen Wang, Chang Li, Shanpeng Wen,* Pengfei Ma, Ge Wang, Changhao Wang, Huayang Li, Liang Shen, Wenbin Guo, and Shengping Ruan State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: Two two-dimensional (2D) conjugated tetrazine-based small molecules (SMs), named TBDT(TTzT)2 and TBDT(TTz2T)2, were newly synthesized for photovoltaic application as donor materials. They employed a molecular backbone of D2-A-D1-A-D2 in which D1 represents an alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, A represents a tetrazine (Tz) unit, and D2 is a bithiophene or terthiophene ending donor unit. These synthesized molecules showed relatively broad light harvesting range and proper energy levels with a fullerene derivative acceptor. Meanwhile, we try to explore how the molecular conjugation influences the opto-electrical properties and photovoltaic performance of the tetrazine-based SM family by making comparison with their non-2D analogues. Experimental results showed that extending main chain conjugated length broadens absorption spectra, whereas side chain conjugation extension leads to larger absorption coefficients, lower highest occupied molecular orbital energy levels, and more favorable blend morphology. The optimized 2D conjugated molecules achieved better device performance with the highest Voc of 1.03 V and FF of 65.3% after using trace amounts of additive. These results suggested that extending molecular conjugation is a feasible strategy for photovoltaic material design. KEYWORDS: Tetrazine, 2D conjugation, Small molecules, Low HOMO level, Organic solar cells

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achieving higher PCEs.17−19 Up to now PCEs over 11% have also been achieved by small molecule solar cells (SMSCs),20 which shows their promising potential comparable to polymerbased ones. However, developing new and efficient SMs donor material is still one of the most key factors in advancing this field. For the design of SM donors, it generally highlights reduced bandgap for broad absorption range, low-lying highest occupied molecular orbital (HOMO) energy level for high open-circuit voltage (Voc) and well-ordered molecular packing for improved carrier mobility. There are two ways to realize these targets including main chain engineering and side chain engineering. The former mainly refers to combining electron-deficient moiety (acceptor) and electron-rich moiety (donor) to

INTRODUCTION Organic photovoltaic (OPV) solar cells have been considered as a promising candidate in future energy source application since they are cost-effective, lightweight, solution-processable, and easily integrated on flexible substrates.1−4 In the past decade, the performance of OPV solar cells has been rapidly improved along with the emerging highly efficient materials and optimizations in device processing.5−9 Recorded power conversion efficiency (PCE) as high as 11.7% has been reported in polymer/fullerene combination.7 However, polymer donors suffer batch to batch variation issues thus leading to undesirable cell reproducibility.10,11 In contrast, solution processed small molecules (SMs) have the merits of welldefined structures,12−14 high purity, no batch to batch variation, and easier band structure control compared to their polymeric counterparts.15,16 These advantages have cemented SMs as the donors of choice not only for understanding the structureproperties-device performance relationship but also for © 2017 American Chemical Society

Received: May 6, 2017 Revised: August 10, 2017 Published: August 30, 2017 8684

DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692

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ACS Sustainable Chemistry & Engineering

Figure 1. Chemical structures of the tetrazine-based small molecules.

of BDT core to realize the concept of 2D conjugated structure and obtained highly efficient polymer and SM donors.33,39 In addition, the positive effects of 2D conjugation toward blend morphology, that is reducing aggregation size and forming nanofibrils permeate the film, have also been discussed.41,42 These suggest us to extend the Tz-based SMs into 2D conjugated structure and further improve their photovoltaic performance. In this work, we optimized the previous molecular design by incorporating alkylthienyl side chain into the BDT central building block (TBDT) and synthesized two new 2D conjugated small molecules, TBDT(TTzT)2 and TBDT(TTz2T)2. The reference SMs with alkoxy side chains on BDT unit were also taken into consideration to help us correlate the optoelectronic performance of SMs with the molecular structure. Here we discussed the absorption properties, energy level structures, and photovoltaic performance of these four molecules (Figure 1) to investigate how the substitution of alkylthienyl side chain and the length of π-bridge and end groups affect these properties. The results showed that extending π-bridge and end groups length intensified and broaden absorption spectra regardless of the 2D conjugation, whereas the side chain alkylthienyl substitution leads to lower HOMO energy levels and more favorable blend morphology, both the VOC and fill factor (FF) of SMSCs were significantly improved. Consequently, the optimized conventional SMSCs employing TBDT(TTzT)2:PC71BM and TBDT(TTz2T)2:PC71BM blendings offered PCEs of 6.10% and 6.56% under 1 sun irradiation.

construct donor−acceptor (DA) architecture. This architecture provides flexible adjustability of bandgap, hole mobility and HOMO/lowest unoccupied molecular orbital (LUMO) energy levels by means of introducing various donor or acceptor moieties into the molecular backbone.21−23 Furthermore, selecting rigid and planar building blocks can also be used to enhance molecular planarity and enable facial π−π stacking for charge transport. Following this design guidelines, we previously synthesized and reported two SMs, BDT(TTzT)2 and BDT(TTz2T)2.24 These molecules adopted D2-A-D1-AD2 linear type main chains with benzo[1,2-b:4,5-b′]dithiophene (BDT, D1) and bithiophene or terthiophene (D2) as donor moiety and tetrazine (Tz) as acceptor moiety. The BDT units are among the most intensively studied building blocks because of their planar conjugated structure and outstanding charge transport properties, whereas the introduction of Tz into the main chains also makes these molecules exhibit deep HOMO energy levels originate from its strong electron withdrawing ability.25−29 As a result, the Tz-based SMSCs afford a large Voc of 0.98 V and also reasonable PCEs. On the other hand, it is also very important to manipulate the side chain engineering to optimize the molecular properties.30 We noted that both Tz-based SMs designed before adopted alkoxyl as side chains. The conjugated plane of these SMs was only along the main chain backbone, which limits the delocalization of π-electrons. It has been confirmed that attaching conjugated thiophene side chains instead of alkyl substituent to extend the lateral π-conjugation, thus build twodimensional (2D) conjugated structure is one successful side chain engineering in both PSCs and SMSCs.31−36 The 2D conjugation provides not only versatile bandgap and energy structure modulation but also enhanced intermolecular π−π interaction for realizing more efficient excitons diffusion and charge carrier transport.37−39 For instance, Chen et al. incorporated alkylthienyl-substituted BDT unit (BDTT) into the well-know PTB7 and developed the top-level photovoltaic material (names PTB7-Th).40 The 2D conjugated PTB7-Th polymer shows better device performance related to their stronger and red-shifted light absorption, deeper HOMO energy level, and substantially enhanced hole mobility. Recently, Cui and Li et al. used alkylthiothienyl as side chain

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EXPERIMENTAL SECTION

Measurements and Characterization. Nuclear magnetic resonance (NMR) spectra were conducted using Bruker AVANCE 500 MHz spectrometer or Varian Mercury 300 MHz spectrometer in CDCl3 using tetramethylsilane (TMS; δ 0 ppm) as an internal standard. We employed Shimadzu UV-3600 to measure the UV−vis absorption spectra. The thin film thicknesses can be determined by Veeco DEKTAK 150 surface profilometer. The electrochemical cyclic voltammogram were obtained on Bioanalytical Systems BAS 10 B/W electrochemical workstation, protected in N2 atmosphere and in supporting electrolyte of tetrabutylammoniumhexafluorophosphate (TBAPF6)/acetonitrile solution (0.1 M). A platinum wire, a platinum 8685

DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692

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ACS Sustainable Chemistry & Engineering Scheme 1. Synthetic Routes of TBDT(TTzT)2 and TBDT(TTz2T)2

Synthesis. Synthesis of TBDT(TTzT) 2 . Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (0.0173 mmol) was added in to the blend of compounds 2 (165 mg, 0.25 mmol) and 3 (90 mg, 0.10 mmol). Then anhydrous DMF (2 mL) and anhydrous toluene (7 mL) were added by a syringe in N2 successively. Upon refluxing for 28 h, the mixture was cooled and poured into water. The organic phase was extracted with chloroform and then was dried by anhydrous magnesium sulfate (MgSO4). The solvent was removed using a rotary evaporator, and the crude product was finally purified by silica gel chromatography (petroleum ether/chloroform = 2:1 as eluant) to afford pure product as dark red solid (75 mg, 43%). 1 H NMR (300 MHz, CDCl3, TMS): δ (ppm) 8.03 (s, 4H), 7.78 (s, 2H), 7.40 (d, J = 3.3 Hz, 2H), 7.10 (d, J = 3.6 Hz, 2H) 6.96 (d, J = 3.6 Hz, 2H) 6.77 (d, J = 3.6 Hz, 2H), 2.92 (d, J = 6.6 Hz, 4H), 2.87−2.76 (m, 4H), 1.79−1.65 (m, 10H), 1.19−1.45 (m, 64H), 1.03−0.79 (m, 30H). 13C NMR (125 MHz, CDCl3) δ 160.76, 160.46, 147.76, 146.06, 142.25, 140.35, 139.23, 138.95, 138.21, 136.77, 136.55, 135.55, 133.89, 133.51, 133.40, 132.74, 132.11, 128.13, 126.63, 125.56, 124.67, 123.60, 122.46, 41.47, 36.15, 34.37, 32.62, 31.70, 31.67, 31.57, 31.48, 30.24, 30.16, 29.95, 29.71, 29.51, 29.37, 29.27, 28.98, 28.87, 25.71, 23.11, 22.69, 22.65, 22.60, 14.22, 14.16, 14.09, 10.91. Anal calcd for C98H126N8S10: C, 67.77; H, 7.31; N, 6.45; S, 18.46. Found: C, 67.87; H, 7.44; N, 6.54; S, 18.24. Synthesis of TBDT(TTz2T)2. Pd(PPh3)4 (0.0069 mmol) was added in to the mixture of compounds 6 (132 mg, 0.133 mmol) and 3 (52.3 mg, 0.058 mmol). Then, anhydrous DMF (1 mL) and anhydrous toluene (4 mL) was added by a syringe under nitrogen atmosphere successively. After refluxing for 28 h, the mixture was cooled and poured into water. The organic phase was extracted with chloroform and then was dried over anhydrous MgSO4. The solvent was removed using a rotary evaporator and the crude product was finally purified by silica gel chromatography (petroleum ether/chloroform = 2.5:1 as eluant) to afford pure product as dark red solid (57 mg, 41%). 1 H NMR (500 MHz, CDCl3, TMS): δ (ppm) 8.06 (s, 4H), 7.70 (s, 2H), 7.38 (d, J = 3.0 Hz, 2H), 7.15 (s, 2H), 7.12 (s, 2H), 6.98 (d, J = 3.5 Hz, 2H), 6.94 (d, J = 3.0 Hz, 2H), 6.75 (d, J = 3.5 Hz, 2H), 2.92− 2.81 (m, 16H), 2.76 (t, J = 7.5 Hz, 4H), 2.10−1.93 (m, 6H), 1.79− 1.65 (m, 24H), 1.48−1.21 (m, 72H), 0.96 (t, J = 7.5 Hz, 6H), 0.94− 0.79 (m, 36H). 13C NMR (125 MHz, CDCl3) δ 160.40, 146.43, 145.51, 140.70, 140.43, 139.07, 138.33, 138.17, 137.17, 136.60, 136.40, 133.98, 133.19, 132.91, 132.68, 132.52, 132.32, 129.69, 129.32, 127.98, 125.52, 125.29, 124.34, 122.81, 120.65, 41.46, 34.42, 32.70, 31.94, 31.78, 31.72, 31.60, 31.47, 30.46, 30.21, 30.13, 29.99, 29.87, 29.71,

electrode, and Ag/AgNO3 combination were used as the counter, working, and reference electrodes, respectively. Ferrocene/ferrocenium (Fc/Fc+) internal standard was used for calibration. The surface morphology images were obtained from atomic force microscopy (AFM, Veeco Dimension 3100) in tapping mode. Fabrication and Characterization of Organic Solar Cells. All the SMSCs in this work were fabricated using a conventional configuration. The film deposition started with the ITO substrates cleaning process. ITO (15 Ω/square) was ultrasonically washed using detergent, deionized water, acetone, and isopropyl alcohol. As the anode buffer layer, the PEDOT:PSS (Clevious P VP AI 4083) aqueous dispersion were dropped onto the ITO and spin-coated at 5000 rpm followed by annealing at 140 °C for 30 min in air to remove water. The PEDOT:PSS layer was at a thickness of ∼40 nm. Then the PEDOT:PSS layer were used for active layer preparation in glovebox. The solvent used for dissolving active materials were chlorobenzene and chlorobenzene with 0.5% v/v 1,8-diiodooctane. The SMs donor/ PC71BM acceptor weight ratio changed from 2:1 to 1.5:1 to 1:1 then 1:1.5. The donor concentration remained 8 mg/mL for both two SMs. The active layer solutions were placed in glovebox with stirring for no less than 24 h for completely dissolving. After spin-coating the TBDT(TTzT)2:PC71BM at the spin rate of 1000 rpm for 60 s (1200 rpm and 60 s for TBDT(TTz2T)2) in argon-filled glovebox, the devices were directly transferred to vacuum deposition system and were deposited 0.6 nm LiF and 100 nm aluminum cathode under vacuum degree ≈5 × 10−4 Pa. The effective area of device is around 0.064 cm2, which is defined by the shadow mask. We measured the current density−voltage (J−V) performance of devices by Keithley 2601 Source Meter. The measure condition contains dark and AM 1.5 G solar irradiation, which was checked with a calibrated Si. We employed Crowntech QTest Station 1000 AD to test the external quantum efficiency (EQE) of solar cells. It was in ambient atmosphere and room temperature to carry out all these measurements Materials. We used all the starting materials provided by Aldrich Chemical and Acros Co and used as received unless particular explanation. Toluene, THF and diethyl ether were dried by adding sodium/benzophenone and distillation in N2 atmosphere before used for synthetic preparations. Dichloromethane (DCM) and N,Ndimethylformamide (DMF) were dried by calcium hydride (CaH2) under nitrogen. Trimethyl(5-hexylthiophen-2-yl)stannane,43 tributyl(4-hexylthiophen-2-yl)stannane,44 and compounds 1−6 can be synthesized by referring to literature reports (Scheme 1).28,31,43,45 8686

DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692

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Figure 2. UV−vis absorption spectra of SMs in solution (left) and absorption coefficient spectra of SMs in solid film (right).

Table 1. Detailed Optical and Electrochemical Parameters of Four SMs BDT(TTzT)2 BDT(TTz2T)2 TBDT(TTzT)2 TBDT(TTz2T)2

λsol max (nm)

εsol (M−1 cm−1)

479 489 457 490

× × × ×

7.29 1.27 7.36 1.46

4

10 105 104 105

λfilm max (nm)

εfilm (cm−1)

Eopt g (eV)

Eonset (V) ox

HOMO (eV)

Eonset red (V)

LUMO (eV)

Eec g (eV)

452/528 508 471/516 550

× × × ×

1.91 1.77 1.99 1.82

0.97 0.76 1.05 0.83

−5.51 −5.30 −5.59 −5.37

−0.87 −0.90 −0.85 −0.89

−3.67 −3.64 −3.69 −3.65

1.84 1.66 1.90 1.72

3.49 4.09 3.98 5.84

4

10 104 104 104

29.54, 29.33, 29.01, 28.90, 25.71, 23.14, 22.78, 22.69, 22.61, 14.19, 14.11, 14.06, 10.89. Anal calcd for C138H182N8S14: C, 69.01; H, 7.64; N, 4.67; S, 18.69. Found: C, 69.18; H, 7.88; N, 4.66; S, 18.54.

TBDT(TTzT)2 and TBDT(TTz2T)2 were estimated to be 1.99 and 1.82 eV, respectively. The alkylthienyl substitution also significantly enhanced the absorption coefficients of molecules in film, guaranteeing more complete light absorption. Electrochemical Properties. To evaluate the energy structures of these molecules, we carried out the redox curves of them in solid state using cyclic voltammetry (CV) measurement, as shown in Figure 3. The redox curve of Fc/

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RESULTS AND DISCUSSION Synthesis and Characterization. The Tz based small molecules’ synthesis routes are presented in Scheme 1. Componds 1 and 3 were synthesized by referring reported literatures.28,31 Compound 5 were synthesized via a two-step reaction: stille coupling reaction between tributyl(4-hexylthiophen-2-yl)stannane and compound 1 and then bromination with N-bromosuccinimide (NBS). Compounds 1 and 4 reacted with trimethyl(5-hexylthiophen-2-yl)stannane to afford compounds 2 and 6,43,45 respectively. At last, compound 2 or 6 was reacted with compound 3 to give TBDT(TTzT)2 and TBDT(TTz2T)2, respectively. These TBDT-based SMs can be well processed due to their good solubility in various solvents like chlorobenzene, chloroform, and DCM, guaranteeing their solution processability in OPVs applications. Figure S1 shows the thermal stability of two synthesized SMs, studied by using thermogravimetric analysis (TGA). The two SMs show good thermal stability for the photovoltaic application. Optical Properties. The UV−vis absorption spectra of four SMs in dilute chloroform solutions and solid state are presented in Figure 2. Detailed parameters of optical properties are listed in Table 1. In solution, the absorption spectra of molecules with longer π-bridge as well as end group (BDT(TTz2T)2 and TBDT(TTz2T)2) showed obvious red-shift by ca. 10 nm, which was caused by the extended conjugated length. Relatively, the 2D SMs displayed similar absorption with non-2D ones. Compared with solution, the thin films showed a lot broadened and red-shifted absorption spectra than that in solution.46 Shoulder peaks at long wavelength region also indicated more aggregated state in film. Moreover, the effect of conjugated side chain on spectra was clearly observed. The absorption edge of TBDT(TTzT)2 and TBDT(TTz2T)2 were blue-shifted 25 and 20 nm than their BDT-based analogues. Calculated from the absorption edges, the optical bandgap of

Figure 3. CV curves of four SMs film on platinum electrode in 0.1 mol/L TBAPF6 in CH3CN solution.

Fc+ was also measured under the same condition to calibrate the potential of Ag/Ag+ reference electrode. On the premise that the energy level of Fc/Fc+ is 4.8 eV below the vacuum level, the equations of refs 47 and 48 could be used to determine the HOMO/LUMO energy levels and the bandgap of these SMs onset HOMO(eV) = −e(Eox − E Fc + 4.8)

onset LUMO(eV) = −e(Ered − E Fc + 4.8) onset onset Egec(eV) = e(Eox − Ered )

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DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692

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Figure 4. DFT-calculated LUMO and HOMO orbitals of SMs in the optimized geometry structures.

Table 2. Photovoltaic Parameters Based on Different Photoactive Layer and D/A Weight Ratio photoactive layer

weight ratio

film thickness (nm)

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

TBDT(TTzT)2:PC71BM

2:1 1.5:1 1:1 1:1.5 2:1 1.5:1 1:1 1:1.5

68 75 85 97 62 70 78 90

1.00 1.04 1.01 1.01 0.89 0.93 0.96 0.95

4.91 5.01 4.96 4.54 4.87 5.24 6.09 5.92

45.7 51.0 48.1 41.3 46.5 52.4 52.2 50.4

2.24 2.66 2.41 1.89 2.02 2.55 3.05 2.83

TBDT(TTz2T)2:PC71BM

conjugated side chain efficiently contributes to the π-electron delocalization. Besides, the HOMO orbitals of the four molecules are delocalized over the entire molecule skeleton, whereas their LUMO orbitals are localized on the Tz units and the adjacent thiophenes mainly. These are consistent with the observed electrochemical experiment results that four SMs demonstrated similar LUMO but very different HOMO energy levels. Photovoltaic Device and Characterizations. We fabricated SMSCs with conventional device configuration (ITO/ PEDOT:PSS/TTz-based SMs:PC71BM/LiF/Al) to optimize the photovoltaic properties of SMs. The active layer were bulkheterojunction fabricated by the blend of TBDT(TTzT)2 or TBDT(TTz2T)2 as donor and PC71BM as acceptor. Two synthesized SMs can be well dissolved in typical organic solvents of chlorobenzene (CB), toluene, and chloroform, and CB works relatively well for processing the cells. Therefore, different weight ratios of SMs:PC71BM ranging from 2:1 to 1:1.5 in CB solution were first scanned. From Table 2 we can see that the TBDT(TTzT)2/PC71BM and TBDT(TTz2T)2/ PC71BM blend systems provided the best performance at D/A weight ratio of 1.5:1 and 1:1, respectively, which is consistent with the reported BDT-based SMs. Due to the deep-lying HOMO energy level, SMSCs based on TBDT(TTzT)2 donor afforded very high Voc of over 1 V (1.04 V). If the low FF and short-circuit current density (Jsc) can be well optimized, this will be very promising to be applied as the wide-bandgap subcell for tandem cells design.50 Suggested by our previous work on BDT(TTzT)2 and BDT(TTz2T)2 based devices, the use of trace amount of 1,8-diiodooctane (DIO) in the blend system is conducive to improved component miscibility and favorable phase separation. So we processed the SMs:PC71BM solution with 0.5% v/v DIO as additive and try to realize better device performance.51,52 We found that the Jscs and FFs of SMSCs were remarkably improved after incorporating additive without much influence on Vocs. Typical current density−

Where Eonset and Eonset were the recorded onset potentials ox red relative to Ag/Ag+, and EFc was the formal potential of Fc/Fc+ relative to Ag/Ag+ that determined as 0.26 V. The estimated HOMO and LUMO values of these tetrazine-based molecules are also listed in Table 1. CV results showed that when using alkylthiophene to replace the alkoxy side chain, the LUMO energy levels were only slightly changed while the HOMO energy levels were down-shifted obviously. The LUMO and HOMO energy levels of TBDT(TTz2T)2 were 0.01 and 0.07 eV lower than those of BDT(TTz2T)2, respectively, leading to the bandgap increased from 1.66 to 1.72 eV. According to the reported studies, the lower HOMO energy levels are attributed to the weaker electron-donating ability of alkylthienyl group than that of alkoxyl group.38 On the other hand, comparison between TBDT(TTzT)2 and TBDT(TTz2T)2 consistently implied that extending the length of π-bridge and end groups in main chain results in narrow bandgap of molecules by significantly up-shifting the HOMO energy level (0.22 eV). The influence of altering side chain and extending conjugated main chain on HOMO levels will directly affect their final photovoltaic performance. The TBDT(TTzT)2 molecule exhibited the lowest HOMO energy level in this SMs family of −5.59 eV; therefore, high Voc could be expected since Voc increases in proportion to the energy offset between donor’s HOMO and acceptor’s LUMO.49 To further study the chemical structure’s effect on energy levels of these molecules, we further performed density functional theory (DFT) calculations at the DFT B3LYP/631G* level to investigate the electronic properties. The calculations were simplified by replacing all the alkyl side chains with methyl groups. The simulated results are shown in Figure 4, and the dihedral angles of four SMs are summarized in Table S2. From the HOMO wave functions of TBDT(TTzT)2 and TBDT(TTz2T)2, we found that the HOMO level’s electron density can be delocalized onto the side alkylthienyl groups, suggesting that the introduction of a 2D 8688

DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692

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ACS Sustainable Chemistry & Engineering voltage (J−V) curves of devices and the photovoltaic parameters were shown in Figure 5 and Table 3. For

Figure 6. XRD patterns of four SM blend films before and after using DIO additive. Figure 5. J−V curves of optimized SMSCs based on different active donor.

coefficient and more favorable blend morphology that will be discussed later. On the other hand, the Voc of TBDT(TTz2T)2 based devices were ca. 90 mV lower than that of TBDT(TTzT)2, but PCE turn out to be better mainly compensated by the improved Jsc and FF. The improvement of Jsc benefited from the broader light absorption as well as the mobility/ morphology evolution induced by extended molecular conjugation. The above photovoltaic results are first supported by the AFM topographic test for different active layers. Figure 7 shows corresponding height images before (a, b, c, d) and after (a1, b1, c1, d1) incorporating DIO. At first glance, samples without DIO exhibited very rough surface, large size phase separation with aggregations on 100−300 nm length scales. So that the exciton migration toward the D/A interface for charge separation will be severely limited. This morphology feature should be also responsible for the poor Jsc and FF of solar cells fabricated by CB only. Upon DIO incorporation, the surface morphology was improved significantly with reduced aggregation sizes and root-mean-square roughness (Rq) values. In this work, it can be found that when the alkoxyl on BDT unit was replaced by alkylthienyl, the SMs TBDT(TTzT)2 and TBDT(TTz2T)2 showed better miscibility with PC71BM and phase separation size further reduced. Especially for the TBDT(TTz2T)2/PC71BM blend, the formation of more obvious and continuous interpenetrating networks afforded charge transport pathway toward electrodes. This coincides with the highest FF and efficiency values obtained experimentally in this SMSCs family. In addition, the smoother surface of TBDT-based blend films was favorable for the interfacial contact at the cathode side, which also helps to reduce the Voc loss at the interface.55 Transmission electron microscope (TEM) tests of SMs:PC 71 BM films are also carried out for a more comprehensive performance study, as shown in the Figure S2. From this we observed homogeneous nanoscale phase separation. BDT(TTzT) 2 , BDT(TTz2T) 2 , and TBDT-

TBDT(TTz2T)2 based cells, a PCE of 6.56% was recorded with Voc of 0.94 V, Jsc of 10.69 mA/cm2, and FF of 65.3%. TBDT(TTzT)2 based devices also demonstrated an reasonable efficiency of over 6%. X-ray diffraction (XRD) study is carried out for confirming the effects of DIO additive, as shown in Figure 6. It is clear that after adding trace amount of DIO, there are obviously enhanced diffraction peaks located at 2θ equals 4.3°, 5.2°, 5.2°, and 5.1° for BDT(TTzT)2, BDT(TTz2T)2, TBDT(TTzT)2, and TBDT(TTz2T)2, respectively. Furthermore, for the alkylthienyl substituted 2D SMs, except for the first-order XRD peaks at 2θ around 5°, very clear high-order diffraction peaks from lamella layer stacking are also emerged at 2θ equals 13.8° for TBDT(TTzT)2:PC71BM and 10.1°, 15.1° for TBDT(TTz2T)2:PC71BM film, indicating that the films processed with DIO have higher crystallinity.42,53,54 In addition, the photovoltaic data of BDT(TTzT)2 and BDT(TTz2T)2 are also added to help us understand the role of alkylthienyl side chain and extended conjugated main chain in photovoltaic properties. It is quite clear that TBDT(TTzT)2 and TBDT(TTz2T)2 based devices showed much similar Jscs as their non2D conjugated ones, whereas the Vocs and FFs were obviously influenced. There is about 50−60 mV increase in Voc after TBDT incorporation, which coincides with the obtained change in HOMO energy levels. High Voc of 1.03 V was realized by the TBDT(TTzT)2/PC71BM based device, also suggesting the energy losses at both sides of the electrodes is relatively low. As is reported, the extension of 2D conjugated structure often showed higher FF value in device.30,37 We also observed similar trend in TBDT-based SMs with significantly increased FFs from 53.5% and 57.2% to 61.9% and 65.3%. Resultantly, PCE increment of over 20% can be obtained. It should be noted that although the light absorption range has narrowed upon alkylthienyl substitution, the potential Jsc losses were probably offset by the relatively higher absorption

Table 3. Photovoltaic Parameters of Devices Based on Four SM Donors and Fabricated under Optimal Conditions PCE (%) photoactive layer BDT(TTzT)2:PC71BM BDT(TTz2T)2:PC71BM TBDT(TTzT)2:PC71BM TBDT(TTz2T)2:PC71BM

Voc (V) 0.98 0.87 1.03 0.94

± ± ± ±

0.01 0.01 0.01 0.01

Jsc (mA cm−2) 9.48 10.54 9.50 10.65

± ± ± ±

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0.11 0.13 0.10 0.05

FF (%)

max

aver

± ± ± ±

5.01 5.29 6.10 6.56

4.91 5.22 6.04 6.49

53.0 56.8 61.6 65.1

0.7 0.8 0.7 0.7

DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692

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ACS Sustainable Chemistry & Engineering

Figure 7. AFM height images of different active layers without (a, b, c, d) and with (a1, b1, c1, d1) DIO, in which the a and a1, b and b1, c and c1, d and d1 correspond to BDT(TTzT)2/PC71BM, BDT(TTz2T)2/PC71BM, TBDT(TTzT)2/PC71BM, and TBDT(TTz2T)2/PC71BM, respectively.

(TTz2T)2 based films show a ∼10 nm domain size whereas the TBDT(TTzT)2 clearly shows a smaller domain of ∼7 nm, which can provide a larger interface area for efficient excitons dissociation. In addition, the TBDT(TTz2T)2 based film shows the most obvious and continuous nanoscale interpenetrating networks, providing pathways for the charge transport. This morphology evolution coincides with the superior FFs and efficiencies of 2D SM based devices. To get further insight into the device performance of four tetrazine-based SMs, we used the space-charge-limited-current (SCLC) method to determine the hole mobilities (μh) of SMs. Dark J−V curves and derived μh recorded from SM pure films are shown in the Supporting Information (Figure S3 and Table S1). And, Figure 8 shows the μh determination of SM blend

bridge and end group lengths show higher mobilities than those of (TTzT)2 based molecules, which also explains their better photovoltaic performance. The EQE were carried out to further evaluate the device reliability of the fabricated SMSCs. Resulting EQE curves of devices based on four SMs were shown in Figure 9. From the

Figure 9. EQE curves of SMSCs based on four SMs donors.

EQE curves, it can be seen that all of the devices showed photoresponse in the 300−700 nm region with slightly different cutoff edges, which were consistent with the absorptions spectra of these molecules. The peak value of TBDT(TTzT)2 and TBDT(TTz2T)2 based devices reached 64% at 500 nm and 66% at 550 nm, respectively. The integrated Jscs calculated by EQE spectra with a standard solar spectrum were in good agreement with the results obtained from J−V measurement.

Figure 8. Dark J−V curves of the hole-only device based on four SM based active layers.

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CONCLUSIONS In conclusion, here we introduced alkylthienyl side chain into BDT core building block, and synthesized two new SMs, TBDT(TTzT)2 and TBDT(TTz2T)2. Compared with their alkoxyl substituted BDT core based analogues, these SMs possessed main chain and side chain π-conjugation thus formed 2D conjugated backbones. Meanwhile, they also have different numbers of thiophenes on π-bridge and end groups thus varied π-conjugation length. Our experimental results showed clear relationship between chemical structure and optoelectronic

films. Calculated μh values of BDT(TTzT)2, BDT(TTz2T)2, TBDT(TTzT)2, and TBDT(TTz2T)2 are 8.76 × 10−5, 2.14 × 10−4, 4.63 × 10−4, and 7.25 × 10−4 cm2/(V s), respectively. Compared to BDT-based films, TBDT-based films show significantly improved charge carrier migration. The hole mobility in the TBDT(TTz2T)2 mixture is almost 1 order of magnitude larger than in BDT(TTzT)2. Enhanced hole mobilities are consistent with the TEM morphology results and beneficial to higher FFs and efficiencies of devices. Additionally, (TTz2T)2 based molecules with extended π8690

DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692

Research Article

ACS Sustainable Chemistry & Engineering properties, and finally affect the device performance. On one hand, the alkylthienyl side chain substitution leads to lower HOMO energy levels and optimized blend nanomorphology with fullerene. Both the Vocs and FFs were improved. On the other hand, the extending π-conjugation length results in upshifted HOMO levels but narrower bandgap, and better device performance was achieved due to the larger Jscs. Taken together, extending conjugation either in main or side chains was proven to be one feasible strategy for optimizing SM donor design.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01420. TGA plots, TEM images of blend films, hole mobilities of pure SM films, DFT calculations information, and supplementary photovoltaic parameters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shanpeng Wen: 0000-0001-5114-6307 Wenbin Guo: 0000-0002-7105-0908 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51303061, 11574110), the Project of Science and Technology Development Plan of Jilin Province (Grant Nos. 20140204056GX, 20160204013GX), the Project of Science and Technology Plan of Changchun City (Grant No. 13KG49), and the China Postdoctoral Science Foundation (Grant No. 2014T70288, 2013M541299, 2016M600231).

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DOI: 10.1021/acssuschemeng.7b01420 ACS Sustainable Chem. Eng. 2017, 5, 8684−8692