Enhanced Photovoltaic Performance of Tetrazine-Based Small

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Enhanced Photovoltaic Performance of TetrazineBased Small Molecules with Conjugated Side Chain Chen Wang, Chang Li, Shanpeng Wen, Pengfei Ma, Ge Wang, Changhao Wang, Huayang Li, Liang Shen, Wenbin Guo, and Shengping Ruan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01420 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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ACS Sustainable Chemistry & Engineering

Enhanced Photovoltaic Performance of TetrazineBased Small Molecules with Conjugated Side Chain 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 E-mail: [email protected].

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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 molecular backbone of D2-A-D1-A-D2 in which D1 represents alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, A represents tetrazine (Tz) unit, and D2 is bithiophene or terthiophene ending donor unit. These synthesized molecules showed relatively broad light harvesting range and proper energy levels with fullerene derivative acceptor. Meanwhile, we try to explore how the molecular conjugation influences the optoelectrical properties and photovoltaic performance of the tetrazine-based SMs 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 coefficient, lower HOMO 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 amount 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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INTRODUCTION Organic photovoltaic (OPV) solar cells have been considered as a promising candidate in the future energy source application since they are cost-effective, lightweight, solutionprocessable and easy 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 issue thus leading to undesirable cells reproducibility.10,11 In contrast, solution processed small molecules (SMs) have the merits of well defined structures,12-14 high purity, no batch to batch variation and easier band structure control compared to their polymeric counterparts15,16. These advantages have cemented SMs as the donors of choice not only for understanding the structure-properties-device performance relationship but also for 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 polymer-based 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 construct donor-acceptor (D-A) architecture. This architecture provides flexible adjustability of bandgap, hole mobility and HOMO/lowest unoccupied molecular orbital


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(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-A-D2 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 two-dimensional (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


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energy level and substantially enhanced hole mobility. Recently, Cui and Li et al. used alkylthiothienyl as side chain 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 towards 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 one sun irradiation.


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

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 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 (0.1M). A platinum wire, a platinum 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 morphologies images were obtained from an atomic force microscopy (AFM, Veeco Dimension 3100) in tapping mode.


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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 (15Ω/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 glove box. 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 glove box 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 glove box, 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


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Materials We used all the starting materials provided by Aldrich Chemical and Co 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,N-Dimethylformamide (DMF) were dried by Calcium hydride (CaH2) under nitrogen. trimethyl(5-hexylthiophen-2-yl)stannane,43 tributyl(4-hexylthiophen-2yl)stannane,44 and compound 1 ~ 6 can be synthesized by referring reported literatures (Scheme 1).28,31,43,45 Synthesis Synthesis of TBDT(TTzT)2 tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (0.0173 mmol) was added in to the blend of compound 2 (165 mg, 0.25 mmol) and compound 3 (90 mg, 0.10 mmol). Then anhydrous DMF (2mL) and anhydrous toluene (7mL) was added by a syringe in N2 successively. Upon refluxed 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 sulphate (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,


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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 compound 6 (132 mg, 0.133 mmol) and compound 3 (52.3 mg, 0.058 mmol). Then anhydrous DMF (1mL) and anhydrous toluene (4mL) was added by a syringe under nitrogen atmosphere successively. After refluxed 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), 1.481.21 (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, 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.


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RESULTS AND DISCUSSION Synthesis and characterization

Scheme 1. Synthetic routes of TBDT(TTzT)2 and TBDT(TTz2T)2. The Tz based small molecules’ synthesis routes are presented in Scheme 1. Compond 1and compound 3 were synthesized by referring reported literatures.28,31 Compound 5 were synthesized via two-step reaction: stille coupling reaction between Tributyl(4-hexylthiophen-2yl)stannane and compound 1 and then bromination with N-bromosuccinimide (NBS). Compound 1 and compound 4 reacted with trimethyl(5-hexylthiophen-2-yl)stannane to afford compound 2 and compound 6,43,45 respectively. At last, compound 2 or compound 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.


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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 nm and 20 nm than their BDT-based analogues. Calculated from the absorption edges, the optical bandgap of 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.

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


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Table 1. Detailed optical and electrochemical parameters of four SMs.

HOMO

LUMO

(nm)

(M-1 cm-1)

(nm)

(cm-1)

(eV)

(V)

(eV)

(V)

(eV)

(eV)

BDT(TTzT)2

479

7.29 × 104

452/528

3.49 × 104

1.91

0.97

-5.51

-0.87

-3.67

1.84

BDT(TTz2T)2

489

1.27 × 105

508

4.09 × 104

1.77

0.76

-5.30

-0.90

-3.64

1.66

TBDT(TTzT)2

457

7.36 × 104

471/516

3.98 × 104

1.99

1.05

-5.59

-0.85

-3.69

1.90

TBDT(TTz2T)2

490

1.46 × 105

550

5.84 × 104

1.82

0.83

-5.37

-0.89

-3.65

1.72

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/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:47,48 could be used to determine the HOMO/LUMO energy levels and the bandgap of these SMs HOMOeV = −eE!"#$ − % + 4.8 LUMOeV = −e − % + 4.8

eV = e −

Where and were the recorded onset potentials relative to Ag/Ag+, and % 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


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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. 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

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


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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/6-31G* 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 levels’ electron density can be delocalized onto the side alkylthienyl groups, suggesting that the introducing of 2D conjugated side chain efficiently contributes to the π-electrons 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.

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

Photovoltaic Device and Characterizations


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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 bulk-heterojunction 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 firstly 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 sub-cell 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-voltage (J-V) curves of devices and the photovoltaic parameters were shown in Figure 5 and Table 3. For 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


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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 non-2D 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 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.


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Figure 5.J-V curves of optimized SMSCs based on different active donor.

Figure 6.The XRD patterns of four SM blend films before and after using DIO additive.

Table 2.Photovoltaic parameters based on different photoactive layer and D/A weight ratio.

Photoactive layer

Weight ratio

Film thickness (nm)

Voc(V)

Jsc (mAcm-2)

FF(%)

PCE (%)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TBDT(TTzT)2: PC71BM

TBDT(TTz2T)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

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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

Table 3. Photovoltaic parameters of devices based on four SM donors and fabricated under optimal condition.

Photoactive layer

Voc (V)

PCE(%)

Jsc (mA cm-2)

FF (%) Max.

Aver.

BDT(TTzT)2:PC71BM

0.98±0.01

9.48±0.11

53.0±0.7

5.01

4.91

BDT(TTz2T)2:PC71BM

0.87±0.01

10.54±0.13

56.8±0.8

5.29

5.22

TBDT(TTzT)2:PC71BM

1.03±0.01

9.50±0.10

61.6±0.7

6.10

6.04

TBDT(TTz2T)2:PC71BM

0.94±0.01

10.65±0.05

65.1±0.7

6.56

6.49

The above photovoltaic results are firstly 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 excitons migration towards 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


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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 towards 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 favourable for the interfacial contact at the cathode side, which also helps to reduce the Voc loss at the interface.55

Figure 7.AFM height images of different active layer without (a, b, c, d) and with (a1, b1, c1, d1) DIO, in which the a&a1, b&b1, c&c1, d&d1 corresponds to BDT(TTzT)2/PC71BM, BDT(TTz2T)2/PC71BM, TBDT(TTzT)2/PC71BM, and TBDT(TTz2T)2/PC71BM, respectively. Transmission Electron Microscope (TEM) tests of SMs:PC71BM 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(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, TBDT(TTz2T)2 based film shows most obvious and continuous nanoscale


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interpenetrating networks, providing pathways for the charge transport. This morphology evolution coincides with the superior FFs and efficiencies of 2D SMs 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 pure SM pure films are shown in the Supporting Information (Figure S3 & Table S1). And Figure 8 shows the µh determination of SM blend films. Calculated µh of BDT(TTzT)2, BDT(TTz2T)2, TBDT(TTzT)2, and TBDT(TTz2T)2 are 8.76×10-5 cm2/Vs, 2.14×10-4 cm2/Vs, 4.63×10-4 cm2/Vs and 7.25×10-4 cm2/Vs, 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 π-bridge and end groups length show higher mobilities than those of (TTzT)2 based molecules, also explains their better photovoltaic performance.

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


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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 EQE curves, it can be seen that all of the devices showed photo-response in the 300-700 nm region with slightly different cut-off edge, 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 9.The EQE curves of SMSCs based on four SMs donors.

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 πconjugation 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 properties, and


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finally affect the device performance. On one hand, the alkylthienyl side chain substitution leads to lower HOMO energy levels and optimized blend nano-morphology with fullerene. Both the Vocs and FFs were improved. On the other hand, the extending π-conjugation length results in up-shifted HOMO levels but narrower bandgap, better device performance was achieved due to the larger Jscs. Taken together, extending conjugation either in main chain or side chain was proven to be one feasible strategy for optimizing SM donor design. ASSOCIATED CONTENT Supporting Information The Supporting Information: TGA plots, TEM images of blend films, Hole mobilities of pure SM films, DFT calculations information and Supplementary photovoltaic parameters AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 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


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For TOC only

Increase molecular conjugation along the main chain and side chain in tetrazine-based small molecules family towards improved photovoltaic performance.


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Enhanced Photovoltaic Performance of Tetrazine-Based Small

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