Enhanced Organic Solar Cell Performance by Lateral Side Chain

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Enhanced Organic Solar Cell Performance by Lateral Side Chain Engineering on Benzodithiophene-Based Small Molecules Dhananjaya Patra, Widhya Budiawan, Tzu-Yen Huang, Kung-Hwa Wei, PenCheng Wang, Kuo-Chuan Ho, Mohammed Al-Hashimi, and Chih Wei Chu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00415 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Applied Energy Materials

Enhanced Organic Solar Cell Performance by Lateral Side Chain Engineering on Benzodithiophene-Based Small Molecules Dhananjaya Patraa,b‡, Widhya Budiawana, c, d‡, Tzu-Yen Huanga, e, Kung-Hwa Weif, Pen-Cheng Wangc, Kuo-Chuan Hoe, g, Mohammed Al-Hashimib and Chih-Wei Chua,h,* a

Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan (R.O.C.) 11529 b

c

Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar

Department of Engineering and System Science, National Tsing-Hua University, Hsinchu, Taiwan (R.O.C.)30013

d

Nanoscience and Technology Program, Taiwan International Graduate Program, Academia Sinica and National Tsing-Hua University, Taiwan (R.O.C.) 11529

e

Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan (R.O.C.) 10617

f

Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu, Taiwan (R.O.C.) 30013

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 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

g

Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan (R.O.C.) 10617 h

‡

College of Engineering, Chang Gung University, Taoyuan 33302, Taiwan

D.P. and W.B. contributed equally to this paper.

*email : [email protected] Keywords: solution-processed, small molecules organic solar cell, bulk heterojunction, side chain, benzodithiophene


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ABSTRACT:

The three novel acceptor–donor–acceptor (A–D–A) conjugated small molecules were synthesized, each featuring a benzodithiophene (BDT) core presenting lateral flexible side chains:TB-BDT6T substituted with 2-ethynyl-5-octylthiophene, TS-BDT6T substituted with 2(octylthio)thiophene, and TT-BDT6T substituted with 2-(2-ethylhexyl)thieno[3,2-b]thiophene groups. The lateral incorporation of functionalized π–conjugated flexible side chains, without altering the end-capped acceptor (cyanoacetate) moieties, amended the optoelectronic properties of these BDT-based small molecules. X-ray diffraction spectroscopy revealed that these small molecules possess high crystallinity; moreover, the optimized blend film morphologies, recorded using atomic force microscopy, revealed miscibility with PC61BM and turn out nano-scale phase separations. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of these small molecules were allowed, leading to high open-circuit voltages (VOC) for their solar cell devices. The bulk heterojunction small molecules solar cell based on TT-BDT6T:PC61BM blend presented the highest power conversion efficiency (5.80%) with a high value of Voc of 0.98 V, a short circuit density of 9.49 mA cm–2 and a fill factor of 62.44% under AM 1.5G irradiation (100 mW cm–2).



Introduction Global environmental concerns and potential scarcity of resources mean that future energy demands and security will require the development of accessible, sustainable, and scalable energy technologies.1–4 Intense research efforts from both academia and industry have led to state-of-the-art solution-processed small molecule organic solar cells (SMOSCs) becoming alternatives to polymer solar cells (PSCs), because they have distinct molecular structure, higher in purity, the absence of contaminations from end group, high crystallinity and charge carrier mobility, and easy to reproduce.4–7 Although SMOSCs have not been investigated for their power conversion efficiencies (PCEs) as deeply as PSCs, ground breaking results have been achieved recently with the PCEs of SMOSCs reaching approximately 10% and over, suggesting that small molecules might soon become versatile replacements for polymer materials.8–14 Several obstacles must be overcome—including improved light absorption, higher fill factors (FFs), improved film quality and surface morphology, also more balanced charge transport—if SMOSCs are to improve their PCEs relative to those of PSCs.5,6 Recent reports by Wei,6 Chen,8,9 and Peng11 have suggested a means of improving the FFs of SMOSCs through the use of small molecules possessing multiple alkylthienyl substituents flanking rigid conjugated cores and relatively highly conjugated systems. Among the three major factors necessary to boost the PCEs of PSCs and SMOSCs, namely the short-circuit current density (Jsc), the open-circuit voltage (Voc), and the FF, the most important is the value of Jsc. In general, the optimized values of Voc for SMOSCs are higher than those of PSCs, while their FFs are almost identical.15,16 The short-circuit current densities of SMOSCs still have room for improvement because, at present, their blend films display poor morphologies and narrow solar spectrum coverage. Consequently, improving the value Jsc while maintaining 4 ACS Paragon Plus Environment

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the high values of VOC and FF is the major challenges of SMOSCs. For high performance, SMOSCs require good molecular design from efficient building blocks to ensure improved light absorption, suitable energy levels between the donors and acceptors, and higher charge mobilities. It has been widely realized that manipulating side groups in polymers as well as SMOSCs are equally important to tuning the PCE, however the mechanisms are not well demonstrated in the literatures.3,4,17 Engineering of central electron donating core by π–conjugated flexible side chains are influenced the aggregation behavior which includes intermolecular interactions and molecular packing in the solid state. The 2- and 5-positions substituted Benzo[1,2-b:4,5b´]dithiophene (BDT) is the successful two-dimensional (2D) electron donor unit building block for both PSCs and SMOSCs because of its high coplanarity, strong π–π stacking, high charge mobility, and enhanced solubility.8–11,15–25 Furthermore, BDT has an off-axis dipole moment boosted through intermolecular interactions and ambient stability resulting from its low-lying highest occupied molecular orbital (HOMO).1,20 Substituting electron-donating flexible side chains laterally around the central core of BDT is a potentially useful approach for manipulating the solubility, absorption properties, and molecular energy levels of BDT-based conjugated polymers and small molecules.10,22,23,26–31 There were some homopolymer and copolymers polymers reported by substituting phenylethynyl groups on BDT in the literature due to greater electron delocalization and thus a lowering the band gap.28–31 The HOMO energy levels of conjugated BDT derivatives are influenced by the electron-donating and –accepting abilities of their flexible side chains connected to the central electron-donating core. In general, weakly electron-donating sulfur atoms, commonly thioalkyl-substituted side chains, appended to the conjugated backbone of BDT will result in low HOMO energy levels. Moreover, sulfur atoms 5 ACS Paragon Plus Environment


also exhibit π-accepting behavior through overlapping the pπ(C)–dπ(S) orbital, where the empty d-orbital of S atom takes π-electrons from the p-orbital of C=C bond.32–34 Thienothiophene substituted BDT also improved the ordered molecular packing, and hole mobility, which are helpful for maximizing the FF.35,36 Thus, lateral incorporation of functionalized π–conjugated flexible groups (2-ethynyl-5-octylthiophene (TB), 2-(octylthio)thiophene (TS) and 2-(2ethylhexyl)thieno[3,2-b]thiophene (TT)) in polymers as well as small molecules might display tunable optoelectronic behavior. Earlier, our group reported the synthesis of alkylated thiophene-flanked BDT and benzotrithiophene-based small molecules and their impressive photovoltaic performance.20,37 In order to have a comparative study on on the structure–property relationships and lateral manipulation of the electron-donating groups of BDT derivatives, herein we report the synthesis of three analogous small molecules—TB-BDT6T, TS-BDT6T, and TT-BDT6T (Figure 1). The resulted small molecules containing BDT as the central electron donor unit symmetrically connected by three octylthiophene units each side and end-capped with cyanoacetate acceptors. We have also investigated the physical, electrochemical, crystallinity, surface morphology, and photovoltaic properties of these lateral substituted BDT-based electron-donor small molecules.

Results and Discussions Synthesis The synthetic routes for TB-BDT6T, TS-BDT6T, and TT-BDT6T are presented in Scheme 1; the detailed procedures are described in the Supporting Information (SI). The syntheses of the intermediates 1 and 4 were performed using slightly modified versions of procedures reported in 6 ACS Paragon Plus Environment

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the SI.20,37,33,38,35 The introduction of side chains onto BDT, through the reactions of compound 6 with compounds 3–5, generated the compounds 7–9, respectively. Subsequent deprotonation and treatment

with

trimethyltin

chloride

yielded

the

distannyl

intermediates

10–12,

respectively.20,33,35 Symmetrical Stille coupling of compounds 10–12 with compound 1320 yielded the small molecules TB-BDT6T, TS-BDT6T, and TT-BDT6T. All these thermally stable small molecules shown the decomposition temperatures (Td; 5% weight loss) above 300°C under a N2 atmosphere (Figure S1). TB-BDT6T, TS-BDT6T, and TT-BDT6T have good solubility in organic solvents such as dichloromethane, chloroform, and THF; also easy to process into organic solar cell devices.

Optical properties The normalized UV–Vis absorption spectra of TB-BDT6T, TS-BDT6T, and TT-BDT6T in dilute CHCl3 and in the film spin-coated onto glass substrate are presented in Figure 2, and the correlating absorption properties are listed in Table 1. The absorption spectra of the thin film of donor material also were provided in SI (Figure S2). The absorption maxima of dilute solutions of TB-BDT6T, TS-BDT6T, and TT-BDT6T in CHCl3 appeared at 451, 486, and 462 nm, respectively; for their solid films, they appeared at 579, 573, and 565 nm, respectively. The signals for the solid films of TB-BDT6T, TS-BDT6T, and TT-BDT6T were all significantly red-shifted (by 128, 87, and 103 nm, respectively) compared with the absorption profile in solution, presumably because planar structure and strong intramolecular charge transfer (ICT) between the electron donating BDT and electron-accepting cyanoacetate (CN) units.2,6,20 A shoulder peak appeared near 616 nm for TS-BDT6T, suggesting stronger π-stacking of its molecular backbone in the solid state, compared with that of TB-BDT6T and TT-BDT6T. The 7 ACS Paragon Plus Environment


optical band gaps (Egopt) of TB-BDT6T, TS-BDT6T, and TT-BDT6T, calculated from the absorption edges of their thin films, were 1.76, 1.75, and 1.78 eV, respectively. Thethioalkylsubstituted thiophene unit in TS-BDT6T resulted in the lowest value of Egopt owing to the electron deficient characteristic of the sulfur atom.33,34 We infer that the presence of electronwithdrawing CN end groups (acceptors) in all of these small molecules facilitated charge separation, leading to the highly red-shifted ICT bands and lower values of Egopt. The broad absorption profiles of TB-BDT6T, TS-BDT6T, and TT-BDT6T from 400 to 700 nm suggested that each of these compounds would be a suitable candidate for use in bulk heterojunction (BHJ) solar cells.

Electrochemical properties Cyclic voltammetry (CV) was used to investigate the electrochemical properties of TBBDT6T, TS-BDT6T, and TT-BDT6T and the oxidation and reduction cyclic voltammograms are depicted in Figure 3 and the data is summarized in Table 1.; TB-BDT6T, TS-BDT6T, and TT-BDT6T each underwent one quasi-reversible oxidation, in positive potential and one quasireversible or reversible reduction in negative potential ranges. This can be attributed to the BDT and CN units. The onset oxidation potentials (Eoxonset) and reduction potentials (Eredonset) for TBBDT6T, TS-BDT6T, and TT-BDT6T were measured to be Eoxonset 0.96, 1.04, and 1.0 V, respectively, and the Eredonset were –0.72, –0.68, and –0.73 V, respectively. The HOMO and LUMO energy levels were calculated using the equation20,37,38

EHOMO/ELUMO = [–(Eonset – Eonset (FC/FC+ vs. Ag/Ag+)) – 4.8] eV


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whereby the energy level of ferrocene below the vacuum level is 4.8 eV and the formal potential Eonset(FC/FC+vs. Ag/Ag+) is considered to be 0.45 V. The HOMO for TB-BDT6T, TS-BDT6T, and TT-BDT6T were found to be 5.31, 5.40 and 5.35 eV, and the LUMO energy levels –3.63;–3.67 –and –3.64 eV, respectively. The electrochemical band gaps (Egec) were found to be in the range of 1.68-1.73 eV. TS-BDT6T possessed lower energy levels relative to those of TB-BDT6T and TT-BDT6T; this can be attributed to the onset oxidation potential which is relatively high. The electron accepting behavior of the thioalkyl-substituted thiophene units in TS-BDT6T was another reason for its low-lying HOMO and LUMO energy levels.7,33,39 From the CV studies, there was a minor change in the HOMO and LUMO energy levels of the small molecules. We projected the pronounced decrease in the HOMO energy levels of these three BDT-based small molecules would result in higher Voc values owing to their incorporation in BHJ solar cells.

1,3

The small

molecules exhibited a LUMO energy level of approximately 0.3 eV above to the PC61BM, which can be attributed to the photoinduced electron transfer from the BDT to the CN units

X-ray diffraction (XRD) We were used X-ray diffraction (XRD) to study the molecular order and packing of these small molecules. The XRD patterns of pristine films of TB-BDT6T, TS-BDT6T, and TTBDT6T and its blends film with PCBM processed from CHCl3 in the range 2θ from 2° to 14°are displayed in Figure 4. We also scanned 2θ up to 30° as shown in SI (Figure S3) to ensure any peaks in higher 2θ, but no peaks were observed above 14°.The XRD patterns of TB-BDT6T, TS-BDT6T, and TT-BDT6T exhibited strong first-order (100) diffraction peaks at values of 2θ of 3.5, 3.6, and 3.6°, respectively, corresponding to d100-spacings of 24.5, 25.2, and 25.2 Å, 9 ACS Paragon Plus Environment


respectively; these values are similar to those found recently for some other BDT-based SMOSCs.8,20 Only for TS-BDT6T and TT-BDT6T we observed (200) diffraction peaks at a value of 2θ of 7.4°, corresponding to a d200-spacing of 11.8 Å. The high crystallinity of TSBDT6T and TT-BDT6T film probable correlated to the strong self-assembly into an ordered lamellar structures.8 We also observed similar diffraction (100) peak in the blend film of TBBDT6T:PC61BM, TS-BDT6T: PC61BM, and TT-BDT6T : PC61BM at 2θ of 3.4, 3.6, and 3.7°, corresponding to d100 25.9, 24.5, 23.8 Å. The (200) diffraction also can be observed in the blend film of TB-BDT6T:PC61BM and TS-BDT6T:PC61BM at 2θ of 6.9 and 7.4° corresponding to d200 12.8 and 11.8 Å, respectively, indicating that the lamellar structure of conjugated molecule still maintained even when blended with PC61BM. This ordered structure in the active layer should benefit the charge transportation. TS-BDT6T, TT-BDT6T, and its blend film exhibited relatively well-organized lamellar structures, as evidenced from their XRD data, that would probably remain in active layers of devices; thus, we suspected that they would be good candidate materials for use in solar cells.

Photovoltaic properties BHJ solar cells were fabricated by incorporating the three BDT-based small molecules and PC61BM was selected as electron acceptor. The photovoltaic properties were investigated using conventional structure indium tin oxide (ITO)/ PEDOT:PSS/small molecule:PC61BM/Ca/Al. Table 2 presents the optimized device performance of the BHJ solar cells incorporating TBBDT6T, TS-BDT6T, and TT-BDT6T under simulated AM 1.5 G illumination (100 mW cm–1). Chloroform was selected as the solvent for preparing the BDT-based organics small-molecule BHJ solar cells. 10 ACS Paragon Plus Environment

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Figure 5 displays the J-V characteristics of the BDT-based small molecules and PC61BM in optimized donor/acceptor blend weight ratio. The detailed device performances are also shown in Table 2. The devices incorporating TS-BDT6T and TT-BDT6T with PC61BM displayed relatively high values of Voc, due to their low-lying HOMO energy levels. The best photovoltaic device containing TB-BDT6T was prepared at a donor/acceptor weight ratio of 1:0.75; a value of Voc of 0.84 V, a value of Jsc of 7.37 mAcm-2, an FF of 58.0%, and a PCE of 3.59 were obtained. The performance of photovoltaic devices fabricated using TB-BDT6T:PC61BM with different donor-acceptor weight ratio are presented in the Supporting Information (Figure S4, Figure S5, and Table S1). The device performances of TS-BDT6T: PC61BM are presented in Figure S6 and Table S2. A PCE of 3.67% with a Voc of 0.96 V, a Jsc of 6.92 mA cm–2, and an FF of 55.28% were observed at weight ratio of 1:0.25. The highest PCE of TS-BDT6T: PC61BM was 4.98% at a weight ratio of 1:0.5, with a Voc of 0.96 V, a Jsc of 8.45 mA cm–2, and an FF of 61.35%. Notably, the value of Voc of the device containing TS-BDT6T was relatively high due to its low-lying HOMO energy level,33,40 presumably because the electron-deficient thiothienyl-substituted flexible side chains flanking the central BDT unit in the conjugated backbone increased the ionization potential.37,38 As a result, the value of Voc of the device containing TS-BDT6T is one of the best obtained among recently reported small-molecule and polymer solar cells incorporating thiothienylsubstituted BDT as a building block.33,39,40,36,41–46 The device performance of TT-BDT6T:PC61BM at a weight ratio 1:0.25 exhibit PCE of 4.87% (Figure S8 and Table S3) and increased significantly to 5.05% after the donor-acceptor ratio increased to 1:0.40, with a value of Voc of 0.97 V, Jsc of 9.09 mA cm-2, and an FF of 57.3%.



For each of the three systems, the values of Voc were high compared with those of TT-BDT– containing polymers and other small molecules. Devices incorporating TS-BDT6T, and TTBDT6T all exhibited high values of Voc, due to the fine degree of engineering of their HOMO energy levels; they provided results among the best reported recently for other small-molecule and polymer solar cells.8,35,47 However, TB-BDT6T has lower Voc than others small molecules even though the HOMO energy level almost similar. We proposed that the lower Voc in TBBDT6T due to lower miscibility with PC60BM which supported by AFM image. Due to the promising photovoltaic performance of TT-BDT6T devices, we further investigated the device of the system based on the best TT-BDT6T:PC61BM weight ratio (1:0.40) using polydimethylsiloxane (PDMS) as additive in blend solution. A macromolecular additive (PDMS) has been shown to be good additive in SMOSCs via morphological optimization.10,48–50 We obtained the highest PCE in this study (5.8%) by using additive 0.05 mg/ml of PDMS in active layer with a value of VOC of 0.98 V, a value of JSC of 9.49 mAcm-2, and an FF of 62.44%. The better device performance after using PDMS is mainly owing to the enhanced JSC and FF which might be associated to its higher and more balanced charge mobilities and better morphological surface. We used atomic force microscopy (AFM) to investigate the active layer for the morphologies, miscibility, and distribution formed from the small molecules.. Figure 6 and Figure S10 present the AFM morphologies of the optimized blend and the pristine films. The pristine films of TBBDT6T, TS-BDT6T, and TT-BDT6T had root mean square roughnesses (Rrms) of 2.46, 6.36, and 2.1 nm, respectively; for the blend films of the small molecules and PC61BM at their optimized weight ratios [TB-BDT6T:PC61BM (1:0.75), TS-BDT6T:PC61BM (1:0.5), and TTBDT6T:PC61BM (1:0.4)], the Rrms values were 1.54, 0.85, and 0.71 nm, respectively. Thus, the 12 ACS Paragon Plus Environment

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blend films of the small molecules TB-BDT6T, TS-BDT6T, and TT-BDT6T were smooth and uniform, with surface roughness lower than their pristine films. The roughness of TT-BDT6T:PC61BM blend film decreases from 0.71 nm to 0.52 nm using PDMS additive as shown in Fig. 6d, which has more uniformly distributed and smoother surface than film without PDMS. The nano-fibrillar features were observed in the image of the film with PDMS forming interpenetrating network. On other hand, the large domain size in organics solar cell is a challenging issues because the length of exciton diffusion is small and the exciton in active layer without PDMS may not dissociate into free carrier because exciton may not reach the donor/acceptor interface.10 Therefore, the addition of 0.05 mg/ml PDMS to the optimized TT-BDT6T:PC61BM active layer (wt. ratio 1:0.4) that exhibited the highest PCE (5.80%) in this study. The less coarse surfaces of these blended film of small molecules and PC61BM suggested less phase separation, which would presumably reduce the probability of diffusional escape for mobile charge carrier and lower the degree of charge recombination.38 We conclude that the small molecules were highly miscible with PC61BM in the blend films and may form a finer interpeneterating network that the optimized blends featured nano-scale phase separation and, therefore, the best photovoltaic device performance. To further confirmed the high performance of these small molecules, the external quantum efficiencies (EQEs) curves of the optimized BHJ devices based on TB-BDT6T/PC61BM (1:0.75), TS-BDT6T/PC61BM (1:0.5), and TT-BDT6T/PC61BM (1:0.4) under monochromatic light are shown in Figure 7. The EQE spectra of these small molecules were similar to their absorption spectra, with the devices exhibiting photo responses from 380 to 700 nm. The EQEs were observed near 556 nm around 50, 54, and 52%, for TB-BDT6T/PC61BM(1:0.75), TS13 ACS Paragon Plus Environment


BDT6T/PC61BM (1:0.5), and TT-BDT6T/PC61BM (1:0.4), respectively. The best device containing TT-BDT6T/PC61BM (1:0.40) with 0.05% of PDMS shown EQE 59% at 558 nm, which exhibited in a highest value of Jsc (9.50 mAcm–2); this behavior suggests that the maximum number of exciton were efficiently dissociated to extract the charge carriers in this device. This indicate that PDMS additive enhanced free carrier generation effectively because the domain size decreased in the morphology resulting more effective donor/acceptor interface10. We measured the hole (µh) and electron (µe) mobilities using the space charge limited current (SCLC) of the optimized blends (Supporting Information: Figure S11, Table S3). The device of TT-BDT6T/PC61BM (1:0.4) showed a hole and electron mobility of 1.38 × 10–4 and 1.12 × 10–4, cm2V–1s–1 respectively; these values provided the most well balanced µe/µh ratio among this series of BDT-based small molecules. Subsequent addition of 0.05 mg/ml of PDMS to the TTBDT6T/PC61BM (1:0.4) blend led to enhancement in the values of both µe of 1.94 cm2V–1s–1 and µh of 2.22 cm2V–1s–1 which showed more balanced hole and electron mobility. Therefore, the improvement of the hole and electron mobilities by adding PDMS indicates that a finer morphological structure in the active layer were formed, which is helpful to the charge separation and transport 49,51 and led to highest value of JSC (9.50 mAcm-2). We conclude that the optimized blends of TT-BDT6T displayed high mobility, but both TS-BDT6T and TB-BDT6T low mobility, and that the TT-BDT6T/PC61BM (1:0.4) blend provided the highest PCE (5.80%) because of its better-defined morphology, higher EQE response, and superior charge transport properties among this series of examined devices.

Conclusions


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The three novel BDT-based π-conjugated small molecules of A–D–A architecture—TBBDT6T, TS-BDT6T, and TT-BDT6T— have been synthesized through side chain engineering of the BDT core using different electron-donating groups. Therefore, the Voc of their devices were observed high value (up to 0.98 V) because of deep HOMO and LUMO energy level of small molecules. TS-BDT6T and TT-BDT6T underwent good packing, in contrast to TBBDT6T, in the solid state, as confirmed from XRD measurements; TS-BDT6T and TT-BDT6T also exhibited superior charge transport properties and well-ordered separation networks and distributions and, thus, provided relatively high PCEs. The best PCE 5.80% was reached in this study for TT-BDT6T:PC61BM using 0.05 mg/ml PDMS as additive, with high Voc (0.98 V) and Jsc(9.49 mAcm–2) and a remarkable FF (62.44%). Thus, high values of Voc can be obtained after lateral introduction of electron-donating moieties for further structural engineering of BDT-based small molecules. Further modification of the end groups structure, chemical structure and device optimizations are currently underway in the search for high-performance BDT-based SMOCS; we believe there remain great opportunities for further improvements in device performance.

Author Information Corresponding Author *E-mail: [email protected]. Phone: 886-2-2787-3183 ‡

D.P. and W.B. contributed equally to this paper

ORCID Dhananjaya Patra : 0000-0002-2471-5057 Widhya Budiawan : 0000-0001-7016-7154 Tzu-Yen Huang : 0000-0001-8473-9459 15 ACS Paragon Plus Environment


Kung-Hwa Wei: 0000-0002-0248-4091 Kuo-Chuan Ho: 0000-0001-7501-1271 Mohammed Al-Hashimi : 0000-0001-6015-2178 Chih-Wei Chu : 0000-0003-0979-1729

Acknowledgment We would like to thank for the financial support from the National Science Council of Taiwan (NSC 100-2120-M-009-004, 101-2120-M-009-001, 101-2221-E-001-010) and the Thematic Project of Academia Sinica, Taiwan (AS-100-TP-A05)

Supplementary Information The Supporting Information is available on the ACS Publications website at DOI: ……. Materials, experiments detail, measurements and characterization, J-V curves, photovoltaic parameters and its distribution, AFM images of pristine film of small molecules, and NMR spectra. References (1)

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Ni, W.; Wan, X. J.; Li, M. M.; Wang, Y. C.; Chen, Y. S. A-D-A Small Molecules for

Solution-Processed Organic Photovoltaic Cells. Chem. Commun 2015, 51, 4936−4950.


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Figure1. Chemical structures of the BDT-based small molecules TB-BDT6T, TS-BDT6T, and TT-BDT6T.



Scheme 1. Synthesis of the BDT-based small molecules TB-BDT6T, TS-BDT6T, and TTBDT6T.

Reagents and conditions: (i) n-BuLi, THF, Compound 6, Compound 3 or 4 or 5, SnCl2·2H2O and HCl (10%); (ii) n-BuLi, THF, SnMe3Cl, -78 °C; (iii) Pd(PPh3)4, toluene, reflux.


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(a)

(b)

Figure 2. Normalized UV–Vis absorption spectra of TB-BDT6T, TS-BDT6T, and TT-BDT6T as (a) dilute solutions in CHCl3 and as (b) thin films on glass surfaces at room temperature.



Figure 3. Cyclic voltammograms of thin films ofTB-BDT6T, TS-BDT6T, and TT-BDT6T (scan rate: 100 mV s–1).


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(a)

(b)

Figure 4. XRD patterns of (a) TB-BDT6T, TS-BDT6T, and TT-BDT6T and (b) TBBDT6T:PC61BM, TS-BDT6T:PC61BM, and TT-BDT6T:PC61BM blend films spin coated from CHCl3 on glass surfaces.



Figure 5. Current density-voltage (J-V) characteristics of small molecules TB-BDT6T, TSBDT6T, and TT-BDT6T solar cell with optimized donor/acceptor ratio.


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(b)

(a)

(c)

(d)

Figure 6. Tapping mode AFM height images of a blend film of (a) TBBDT6T:PC61BM (1:0.75), (b) TS-BDT6T:PC61BM (1:0.50), (c) TT-BDT6T:PC61BM (1:0.40), and (d) TT-BDT6T:PC61BM (1:0.40) prepared with added PDMS 0.05 mg/ml; all cast from CHCl3.



Figure 7. EQE curves of optimized BHJ solar cell devices of TB-BDT6T:PC61BM (1:0.75); TSBDT6T:PC61BM (1:0.50); TT-BDT6T:PC61BM (1:0.40), and TT-BDT6T:PC61BM (1:0.40) prepared with added PDMS 0.05 mg/ml.


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Table 1. Optical and Electrochemical Properties of TB-BDT6T, TS-BDT6T, and TT-BDT6T.

Solution Small molecule

λmax

a

Energy Levels

Solid filmb λmax

Band Gapsd Eonsetox (V)/

Eonsetred (V)/ Egec (eV)

Egopt (eV)

(nm)

(nm)

HOMOc (eV)

LUMOc (eV)

TB-BDT6T

451

579

0.96/–5.31

–0.72/–3.63

1.68

1.76

TS-BDT6T

486

573

1.04/–5.40

–0.68/–3.67

1.72

1.75

TT-BDT6T

462

565

1.0/–5.35

–0.73/–3.64

1.73

1.78

a

In CHCl3 (dilute solution).b Spin-coated from CHCl3 solution onto a glass surface. cEHOMO/ELUMO = [–(Eonset – Eonset (FC/FC+ vs. Ag/Ag+)) – 4.8] eV,where 4.8 eV is the energy level of ferrocene below the vacuum level and the formal potential Eonset(FC/FC+vs. Ag/Ag+) is equal to 0.45 V. d Electrochemical band gap: Egec= Eox/Onset – Ered/Onset; optical band gap: Egopt = 1240/λedge.



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Table 2. The photovoltaic properties of small molecules with optimized donor-acceptor ratioa. Molecules VOC (V)

JSC (mAcm-2)

FF (%)

PCE (%)

TB-BDT6T

0.85 + 0.01

6.87 + 0.30

58.72 + 0.60

3.41 + 0.12

(1:0.75)

(0.84)

(7.37)

(58.0)

(3.59)

TS-BDT6T (1:0.50) TT-BDT6T

0.96 ± 0.01 (0.96) 0.97 ± 0.01

8.47 ± 0.23 (8.45) 8.97 ± 0.17

60.50 ± 1.19 (61.35) 57.66 ± 0.98

4.89 ± 0.09 (4.98) 4.99 ± 0.05

(1:0.40)

(0.97)

(9.09)

(57.26)

(5.05)

TT-BDT6Tb

0.97 ± 0.01

9.40 ± 0.22

61.79 ± 1.25

5.64 ± 0.09

(1:0.40)

(0.98)

(9.46)

(62.44)

(5.79)

(D/A ratio)

a

The best devices are provided in parentheses. The values are the average of 10 devices in different batches; b0.05% of PMDS were added


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