Isomeric Fluorene-based Heteroundecenes with Different Side Chains Anchoring Positions for Small Molecule Acceptors

WANG Wei, ZHAN Chun, XIAO Shengqiang

(State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070,China)

Abstract: Two isomeric fluorene-based heteroundecenes of bis(thienocyclopenthieno[3,2-b]thieno)fluorene (BT2T-F) and bis(dithieno[3,2-b:2',3'-d]thiophene)cyclopentafluorene (B3T-F) have been designed and synthesized. The side chains of 4-hexylphenyl anchor on the 5th and 8th positions in B3T-F while on the 4th and 9th positions in BT2T-F, in which the former is closer to the center of the fused ring. The corresponding acceptor-donor-acceptor (A-D-A) type small molecule acceptors (SMAs) of BT2T-FIC and B3T-FIC were prepared by linking BT2T-F and B3T-F as fused ring donor units with the acceptor unit of 2-(5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC-2F), respectively. B3T-FIC presents a superior crystallinity with intense face-on π-π stacking in its neat film while BT2T-FIC is more disordered. When blended with PBDB-T-2Cl as a polymer donor, the optimized PBDB-T-2Cl:BT2T-FIC device exhibits an averaged power conversion efficiency (PCE) of 10.56% while only 7.53% in the PBDB-T-2Cl:B3T-FIC device. The improved short-circuit current (Jsc) and fill factor (FF) of the PBDB-T-2Cl:BT2T-FIC device are the main contribution of its higher performance, which is attributed to its more efficient and balanced charge transport and better carrier recombination suppression. Given that BT2T-FIC blend and B3T-FIC blend films both take a preferential faceon orientated π-π stacking with comparable distances, the suitable SMA domain size obtained in the BT2T-FIC blend could account for its more efficient photovoltaic performance. These results highlight the importance of side-chain strategy in developing efficient SMAs with huge fused ring cores.

Key words: polymer solar cells; small molecule acceptor; side chains; morphology

1 Introduction

Solution-processing bulk-heterojunction (BHJ)polymer solar cells (PSCs) have drawn tremendous attention in the past few decades due to their huge potential to perform mass production with low processing cost[1-3]. Small molecule acceptors (SMAs),generally featuring as the acceptor-donor-acceptor(A-D-A) structure, have rapidly emerged from various acceptor materials with their overwhelming photovoltaic performance in recent years. The fusedring core and side chains in their donor (D) units as well as ending acceptor (A) groups all can be flexibly modified by molecular engineering. Such highly tunable molecular structures of SMAs realize their absorption complementary and energy level alignment with abundant donor polymers, beneficial for achieving favorable short-circuit current (Jsc) and open-circuit voltage (Voc)[4-8]. With the synchronized development of donor polymers and processing technologies, the record power conversion efficiency (PCE) of singlejunction PSCs has been up to 18% based on Y6-type acceptors[9-11]. For a D unit, its fused-ring core with side chains has been frequently witnessed to play a critical role in determining the optoelectronic and crystal characteristics of SMAs[12-14]. In a fused-ring core,the fusion of arena building blocks constructs largersize conjugation systems for facilitating π-electron delocalization. When bonding with ending acceptor groups such as the popular 1,1-dicyanomethylene-3-indanone (IC) and its derivatives, the push-pull electron system in SMAs causes strong intramolecular charge transfer (ICT) for enhancing light harvesting capability.Besides, SMAs with larger size of rigid fused-ring D unit could exhibit smaller reorganization energies for better charge recombination suppression and more efficient charge transport. It could improve fill factor(FF) and lower energy loss in PSC devices, which are the key photovoltaic photoelectric parameters for better PCE[15,16]. Therefore, extending the fused-ring cores has been one of the most important strategies to improve the photovoltaic performance of SMAs.

With the expansion of conjugated fused rings,the challenge on side chain engineering has arisen in the design and synthesis of SMAs. The huge fused aromatic rings usually cause excessive self-aggregation by their strong intermolecular interaction not only in target SMAs but also their intermediate compounds in synthesis. Given the convenience of synthesis and the processability of SMAs, the out-of-plane side chains anchoring on thesp3-hybridized bridging atoms have to be adopted in gigantic fused-ring cores[17,18].Although the optional anchoring positions for suchsp3 side chains would increase as the fused ring expands,the number ofsp3 side chains has been limited due to the negative effect of too manysp3 side chains in SMAs on the crystallinity and the phase separation in BHJ films[19,20]. Thus, there are multitudinous possible structure designs for a large-sized fused-ring core,in which the number and anchoring positions of side chains are various. It brings difficulties to the structure design and screening of SMAs because such subtle differences in side chains have often led to various photovoltaic characteristics of SMAs. For example, the SMA named IUIC has a heteroundecene-based core with four pairs of 4-hexylphenyl staggering onsp3-hybridized carbon atoms of both sides. When blended with donor polymer PTB7-Th, IUIC offered a PCE of 10.2%[21]. In its isomeric SMA named IUIC2, both sides of the heteroundecene-based core introduced two pairs of 4-hexylphenyl on the adjacentsp3-hybridized carbon atoms respectively. The optimized PTB7-Th:IUIC2 devices exhibited an inferior PCE of only 4.06% with a decreasedJSCof 10.77 mA cm-2from 21.51 mA·cm-2for that of PTB7-Th:IUIC devices. It resulted from the low efficiency of exciton dissociation in the PTB7-Th:IUIC2 blend, minorly contributing to photocurrent generation in its BHJ active film[16]. The SMAs with over 10-membered fused-ring cores are seldom reported. Moreover, the anchoring positions of outermostsp3 side chains have been particularly noteworthy in side chains engineering. The out-of-planesp3 side chains lead that the close intermolecular packing is mainly carried out between the molecular terminal regions, consisting of the ending groups and part of conjugated cores outwards the outermostsp3 side chains planes[22,23].Within these close intermolecular packing caused by intermolecular interactions (e g, π···π, O···S and N···S interaction), the resulted orbital overlap of π-electrons is the important channel for intermolecular charge transfer in a BHJ active film[24-26]. The modulation on outermostsp3 side chains has thus been suggested to effectively tune the intermolecular interactions between SMAs for optimizing the aggregation and phase separation behavior in BHJ films, which could boost charge transport and improve photoelectric conversion for SMAs[27,28]. Briefly, how to reasonably design the side chain has become the key to fully enjoying the advantages of large-sized fused rings for developing efficient corresponding SMAs.

Fluorene exhibits unique advantages in developing large-sized fused-ring cores. On one hand, side chains anchoring on thesp3-hybridized carbon atom of fluorene could guarantee sufficient solubility for the preparation of the intermediates as well as the processability of the target SMAs. Thesesp3 side chains are usually at the center of fluorenebased SMAs, which avoids hindering the effective intermolecular packing of molecular terminal regions. On the other hand, some fluorene-based intermediates have been reported in recent years and they provide convenience to extend fused rings with multifarious aromatics[29,30]. In this work, fluorene was extended for heteroundecenes and two pairs of bulky 4-hexylphenyl side chains were introduced to ensure their solubility. Two isomeric ladder-type fused rings of bis(thienocyclopenthieno[3,2-b]thieno) fluorene(BT2T-F) and bis(dithieno[3,2-b:2’,3’-d]thiophene)cyclopentafluorene (B3T-F) has been designed and synthesized. As shown in Scheme 1, a pair of linear octyl was anchored on the central 15th positions of BT2T-F and B3T-F respectively while two pairs of 4-hexylphenyl groups have symmetrically introduced on the other side of fused-ring cores with octyl side chains. The 4-hexylphenyl groups were anchored on the 5th and 8th positions in B3T-FIC, which are closer to the center of heteroundecenes than those of BT2T-FIC (on the 4th and 9th positions). Combining with 2-(5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC-2F) as the ending groups,the corresponding isomeric SMAs (BT2T-FIC and B3T-FIC) were obtained and they both have good solubility in common producing solutions. B3TFIC shows a much better crystallinity and molecular packing ordering than BT2T-FIC. When blended with the representative donor polymer PBDB-T-2Cl, B3TFIC exhibited an optimized averaged PCE of 7.53%whereas a superior PCE of 10.56% for BT2T-FIC.

Scheme 1 Synthetic routes of BT2T-FIC and B3T-FIC

2 Experimental

The key intermediates of 2,9-bis(trimethyltin)-12,12-bis(2-octyl)-12H-dithieno-[2′,3′:4,5]thieno[3,2-b:2′,3′-h]fluorene (compound 1) and dimethyl 2,7-diiodo-9,9-dioctyl-9H-fluorene-3,6-dicarboxylate(compound 6) were prepared according to the literatures respectively[29,30]. The other chemicals in the synthesis were purchased from Sinopharm, Derthon,and Energy. Tetrahydrofuran (THF) was distilled before being used. General experimental information on material characterizations, device fabrication, and performance measurement, BHJ blend characterizations can be found in our previous work[29].

2.1 Dimethyl-2,2’-(12,12-bis(2-octyl)-12Hdithieno-[2′,3′:4,5]thieno[3,2-b:2′,3′-h]fluorene-2,7-diyl)bis(thiophene-3-carboxylate) (compound 2)

Compound 1 (1.41 g, 1.5 mmol), methyl 2-bromothiophene-3-carboxylate (0.83 g, 3.8 mmol),Pd(PPh3)4(0.06 g, 0.15 mmol, 10% equiv.) were added to a 100 mL flask under nitrogen. The mixture was dissolved with anhydrous toluene (20 mL) and DMF(3 mL). The solution was heated to reflux and stirred for 12 hours. After cooled down to room temperature,the reaction solution was poured into water. The solution was then extracted with dichloromethane and the combined organic layer was dried over MgSO4.The solvent was removed under reduced pressure and the obtained crude product was purified via silica gel column chromatography with petroleum ether/ethyl acetate (10:1,v/v) as the eluent to obtain 1.68 g of the target compound (yield, 75%).1H NMR (500 MHz,CDCl3),δ(ppm): 8.19 (s, 2H), 7.79 (s, 2H), 7.77 (s,2H), 7.55(d,J= 5.45 Hz, 2H), 7.27 (d,J= 0.70 Hz,2H), 3.91 (s, 6H), 2.14-2.10 (m, 4H), 1.19-1.02 (m,20H), 0.78 (t,J= 7.13 Hz, 6H), 0.69-0.63 (m, 4H).13C NMR (125 MHz, CDCl3),δ(ppm): 163.49, 148.91,143.54, 142.05, 138.60, 137.93, 136.73, 135.84,132.04, 130.63, 127.60, 127.59, 124.26, 122.34,115.37, 114.69, 54.57, 51.87, 41.65, 31.73, 30.04,29.23, 29.20, 23.88, 22.52, 13.96. MALDI-TOF MS for C49H50O4S6: calcd. 894.20, found: 894.1 (M+).

2.2 Compound 3

The Grignard reagent of 4-(octyloxy)benzene-1-magnesium bromide was prepared by the following procedures: magnesium turnings (0.54 g, 22.5 mmol)was added in dry THF (15 mL) under nitrogen atmosphere and 4-5 drops of 1,2-dibromoethane were then slowly added into this suspension with stirring.After adding 1-bromo-4-octylbenzene (5.25 g, 19.5 mmol) slowly in the solution, the mixture was heated to reflux for 3 hours. Under nitrogen atmosphere,compound 2 (1.25 g, 1.5 mmol) was dissolved in dry THF (60 mL) and the as-prepared 4-(octyloxy)benzene-1-magnesium bromide (15 mL, 15.6 mmol) was added dropwise in this solution at room temperature. The mixture was heated to reflux for 16 hours with stirring.The reaction solution was extracted with diethyl ether and the combined organic layer was dried over MgSO4.After concentrating the organic layer, the crude product was purified by column chromatography on silica gel(hexane/ethyl acetate,v/v, 15/1) to give a yellow solid of 1.77 g (yield, 80%).1H NMR (500 MHz, CDCl3),δ(ppm): 8.10 (s, 2H), 7.62 (s, 2H), 7.22-7.18 (m, 10H),7.13 (d,J= 8.10 Hz, 8H), 6.77(d,J= 0.55 Hz, 2H), 6.53(d,J= 5.40 Hz, 2H), 3.28 (d,J= 0.55 Hz, 2H), 2.61(t,J= 7.70 Hz, 8H), 2.08-2.05 (m, 4H), 1.64-1.58 (m,8H), 1.35-1.29 (m, 24H), 1.18-1.01 (m, 20H), 0.89 (t,J= 6.48 Hz, 12H), 0.78 (t,J= 7.08 Hz, 6H), 0.61 (s, 4H).13C NMR (125 MHz, CDCl3),δ(ppm): 148.76, 146.40,144.61, 142.17, 141.52, 138.35, 137.78, 137.35, 135.76,131.94, 131.92, 131.61, 128.01, 127.51, 124.10, 122.04,115.10, 114.57, 80.56, 54.53, 41.62, 35.55, 31.75,31.71, 31.34, 30.05, 29.24, 29.22, 28.98, 23.84, 22.61,22.54, 14.09, 14.00. MALDI-TOF MS for C95H114O2S6:calcd. 1 478.71, found: 1 478.6 (M+).

2.3 BT2T-F (Compound 4)

Compound 3 (0.60 g, 0.40 mmol) was added to a 500 mL flask and dissolved by 130 mL of acetic acid with stirring. Sulfuric acid (0.50 mL, 10 mmol) was added slowly to this solution and the mixture solution was then heated to 75 ℃ for 2 hours. The reaction solution was then poured into ice water slowly. The solution was neutralized with NaOH solution and extracted with dichloromethane. The collected organic layer was dried and then concentrated by rotary evaporation. The crude product was quickly purified by silica gel chromatography with hexane/ethyl acetate (v/v, 15/1) as the eluent to produce compound 4 of 0.44 g(yield, 75%). Due to the instability of this compound,it was quickly put into the next synthesis step.1H NMR(500 MHz, CDCl3),δ(ppm): 8.03 (s, 2H), 7.69 (s, 2H),7.26 (d,J= 3.20 Hz, 2H), 7.20 (d,J= 8.20 Hz, 8H), 7.13(d,J= 4.90 Hz, 2H), 7.07 (d,J= 8.25 Hz, 8H), 2.55(t,J= 7.80 Hz, 8H), 2.12-2.09 (m, 4H), 1.59-1.56 (m, 8H),1.34-1.27 (m, 24H), 1.17-0.99 (m, 20H), 0.88-0.86 (m,12H), 0.76 (t,J= 7.10 Hz, 6H), 0.61-0.58 (m, 4H).

2.4 Compound 5

DMF (4.0 mL, 51.6 mmol) was added to a dry 25 mL flask under nitrogen and then cooled to 0℃. Phosphorus oxychloride (POCl3) (3.0 mL, 31.7 mmol) was added to the flask and the solution was stirred for 1 hour to prepare the Vilsmeier reagent.Compound 4 (0.87 g, 0.60 mmol) was dissolved in 1,2-dichloroethane (20 mL) in 100 mL flask under nitrogen and the solution of Compound 4 was then added to the Vilsmeier reagent slowly at 0 ℃. The mixture solution was then heated to 80 ℃ with stirring for 12 hours. The reaction solution was poured into ice water and neutralized with dilute NaOH solution slowly.The solution was then extracted with dichloromethane.The collected organic layer was washed with water and then dried over anhydrous MgSO4. After removing the solution by rotary evaporation, the crude product was chromatographed on silica gel with petroleum ether/ethyl acetate (v/v, 8/1) as eluent, obtaining 0.585 g of yellow solid (yield, 65%).1H NMR (500 MHz, CDCl3),δ(ppm): 9.84 (s, 2H), 8.07 (s, 2H), 7.74 (d,J= 9.15 Hz, 4H), 7.19 (d,J= 8.25 Hz, 8H), 7.10 (d,J= 8.30 Hz, 8H), 2.55 (t,J= 7.80 Hz, 8H), 2.14-2.11 (m, 4H),1.59-1.54 (m, 8H), 1.34-1.27 (m, 24H), 1.13-1.05 (m,20H), 0.86 (t,J= 6.78 Hz, 12H), 0.75 (t,J= 6.58 Hz,6H), 0.61-0.60 (m, 4H).13C NMR (125 MHz, CDCl3),δ(ppm): 182.34, 158.08, 152.98, 149.23, 147.19, 144.19,142.43, 142.12, 139.00, 138.59, 138.13, 137.00,133.38, 132.46, 131.61, 128.75, 127.60, 114.66, 62.37,54.70, 41.55, 35.56, 31.72, 31.67, 31.23, 29.95, 29.68,29.18, 29.15, 29.06, 23.81, 22.55, 22.50, 14.03, 13.96.MALDI-TOF MS for C97H110O2S6: calcd. 1 498.68,found: 1 499.5 (M+H+).

2.5 BT2T-FIC

Compound 5 (300 mg, 0.20 mmol) and IC-2F(138 mg, 0.60 mmol) were added to a 100 mL flask under nitrogen. Chloroform (30 mL) and pyridine (2.0 mL) was added in the flask and dissolved the chemicals with stirring. The reactants were heated to reflux for 12 hours. The reaction mixture was concentrated under reduced pressure and then purified by silica gel chromatography with chloroform as the eluent. 0.21 g of pure product was afforded as a dark blue solid(yield, 78%).1H NMR (500 MHz, CDCl3),δ(ppm):8.89 (s, 2H), 8.55-8.53 (m, 2H), 8.09 (s, 2H), 7.81 (s,2H), 7.75 (s, 2H), 7.67 (t,J= 7.48 Hz, 2H), 7.18 (d,J= 8.30 Hz, 8H), 7.11 (d,J= 8.30 Hz, 8H), 2.57 (t,J=7.75 Hz, 8H), 2.15 (t,J= 7.85 Hz, 4H), 1.59-1.54 (m,8H), 1.34-1.28 (m, 24H), 1.15-0.99 (m, 20H), 0.86 (t,J= 6.73 Hz, 12H), 0.76 (t,J= 7.08 Hz, 6H), 0.63-0.55(m, 4H).13C NMR (125 MHz, CDCl3),δ(ppm): 186.19,159.57, 158.29, 157.82, 156.01, 155.42, 155.31,153.35, 153.24, 149.62, 143.18, 142.75, 142.43,139.47, 139.37, 139.30, 138.23, 137.64, 137.54,136.60, 134.46, 133.73, 132.45, 128.90, 127.62,119.80, 115.23, 114.98, 114.85, 114.52, 112.51, 112.35,68.44, 62.29, 54.83, 41.55, 35.56, 31.72, 31.66, 31.21,29.93, 29.20, 29.14, 29.04, 23.85, 22.54, 22.50, 14.02,13.96. MALDI-TOF MS for C121H114F4N4O2S6: calcd.1 923.62, found: 1 923.5 (M+).

2.6 Dimethyl-2,7-bis(dithieno[3,2-b:2’,3’-d]thiophen-2-yl)-9,9-dioctyl-9H-fluorene-3,6-dicarboxylate (Compound 7)

Compound 6 (2.27 g, 3.0 mmol), Pd(PPh3)4(0.14 g, 0.15 mmol) and tributyl(dithieno[3,2-b:2’,3’-d]thiophen-2-yl)stannane (2.50 g, 7.0 mmol) were added to a 100 mL two-necked flask. Under a nitrogen atmosphere, 50 mL of anhydrous toluene was poured in the flask. The mixture was heated to reflux and stirred overnight. The reaction mixture was concentrated under reduced pressure. The concentrate was purified by silica gel chromatography with petroleum ether/ethyl acetate (v/v, 6/1) as the eluent. The pure product (1.61 g) was obtained with a yield of 65%.1H NMR (500 MHz, CDCl3),δ(ppm): 8.15 (s, 2H), 7.52 (s, 2H), 7.40(d,J= 5.20 Hz, 2H), 7.32 (d,J= 6.85 Hz, 2H), 5.30(s, 2H), 3.84 (s, 6H), 2.03-2.00 (m, 4H), 1.22-1.07 (m,20H), 0.81 (t,J= 7.15 Hz, 6H), 0.69 (s, 4H).13C NMR(125 MHz, CDCl3),δ(ppm): 169.01, 154.16, 142.86,141.47, 141.13, 139.74, 133.73, 131.15, 130.96,130.95, 126.12, 125.80, 121.61, 120.73, 120.02, 56.18,52.56, 52.53, 39.99, 31.73, 29.81, 29.14, 29.07, 23.81,22.58, 14.05. MALDI-TOF MS for C49H50O4S6: calcd.894.20, found: 894.3 (M+).

2.7 B3T-F (Compound 8)

Compound 7 (0.90 mmol, 0.81 g) was dissolved in anhydrous THF (10 mL) and the as-prepared 4-(octyloxy)benzene-1-magnesium bromide (12 mL, 12 mmol) was slowly added to the solution under stirring.The reaction solution was heated to reflux overnight.The reaction solution was then poured into water and extracted with ethyl acetate. The organic layer was dried with anhydrous MgSO4and concentrated. The crude product was purified by column chromatography eluting with petroleum ether/ethyl acetate (v/v, 40/1)quickly. Since the pure product is unstable, it was put into the next reaction directly. The intermediate obtained in the previous step was mixed with octane in a 250 mL two-necked flask and dissolved at 60 °C with stirring. Then, 15 drops of concentrated sulfuric acid were added dropwise. The reaction was for around 3 hours and it was monitored by TLC. The reaction solution was slowly poured into water and extracted with dichloromethane. The organic layer was washed several times to remove excess acid with water. The remaining acid was then neutralized with an appropriate amount of NaOH solution and the collected organic layer was dried over anhydrous MgSO4. After removal of solvent, the concentrate was subjected to column chromatography with petroleum ether/dichloromethane(v/v, 50/1). 0.34 g of a yellow solid was obtained with a total yield of about 50% in two steps.1H NMR (500 MHz, CDCl3),δ(ppm): 7.60 (s, 2H), 7.39 (s, 2H), 7.30(d,J= 5.15 Hz, 2H), 7.21-7.18 (m,10H), 7.07 (d,J= 8.15 Hz, 8H), 2.54 (t,J= 7.85 Hz, 8H), 2.07-2.04(m, 4H), 1.61-1.56 (m, 8H), 1.34-1.27 (m, 24H),1.21-1.13 (m, 20H), 0.87-0.79 (m, 22H).13C NMR (125 MHz, CDCl3),δ(ppm):152.57, 151.40, 147.48, 142.08,141.62, 141.22, 140.32, 139.50, 136.71, 136.15,132.37, 131.70, 128.39, 128.12, 125.31, 120.66,117.10, 113.42, 63.04, 54.58, 40.54, 35.57, 31.80,31.67, 31.18, 30.10, 29.23, 29.18, 23.98, 22.63, 22.59,22.55, 14.07, 14.05. MALDI-TOF MS for C95H110S6:calcd. 1 442.69, found: 1 442.9 (M+).

2.8 Compound 9

DMF (3.0 mL, 38.7 mmol) was added to a dry 25 mL flask under nitrogen and then cooled to 0 ℃. Then,POCl3(2.7 mL, 28.5 mmol) was also added to the flask and stirred for 1 hour. The solution of compound 8 (0.72 g, 0.50 mmol) in 1,2-dichloroethane (30 mL) was then added dropwise in the flask. The reaction solution was heated to 80 ℃ and stirred for 12 hours. The reaction solution was poured into water and extracted with dichloromethane. The organic layer was dried with MgSO4and concentrated under rotary evaporation. The concentrate was subjected to column chromatography with petroleum ether/ethyl acetate (v/v, 10/1) as eluent and 0.17 g of orange solid were obtained in a yield of 60%.1H NMR (500 MHz, CDCl3),δ(ppm): 9.91 (s,2H), 7.84 (s, 2H), 7.63 (s, 2H), 7.46 (s, 2H), 7.16 (d,J= 8.20 Hz, 8H), 7.09 (d,J= 8.20 Hz, 8H), 2.55 (t,J=7.85 Hz, 8H), 2.07 (m, 4H), 1.61-1.55 (m, 8H), 1.34-1.26 (m, 24H), 1.20-1.12 (m, 20H), 0.87-0.78 (m,22H).13C NMR (125 MHz, CDCl3),δ(ppm): 182.57,153.17, 151.84, 147.45, 146.22, 142.93, 141.97,140.86, 140.70, 140.26, 139.77, 138.81, 136.29,132.23, 130.07, 128.54, 127.99, 117.37, 114.20, 63.11,54.73, 40.44, 35.55, 31.78, 31.65, 31.17, 30.02, 29.21,29.20, 29.14, 24.00, 22.58, 22.54, 14.05, 14.03. MALDI-TOF MS for C97H110O2S6: calcd. 1 498.68, found:1 498.88 (M+).

2.9 B3T-FIC

Compound 9 (0.30 g, 0.20 mmol) and IC-2F (0.13 g, 0.60 mmol) were added to a 100 mL two-necked flask under nitrogen. The mixture was dissolved by chloroform (50 mL) and added pyridine (1.6 mL).The reaction solution was heated to reflux and stirred at for 12 hours. The solvent was evaporated under reduced pressure to obtain a concentrated solution of the reactant. The concentrated solution was separated by column chromatography with chloroform. A dark blue solid pure product (0.17 g) were obtained in a yield of 75%.1H NMR (500 MHz, CDCl3),δ(ppm):8.91 (s, 2H), 8.55-8.51 (m, 2H), 7.98 (s, 2H), 7.69-7.65(m, 4H), 7.53 (s, 2H), 7.16 (d,J= 8.25 Hz, 8H), 7.10(d,J= 8.25 Hz, 8H), 2.56 (t,J= 7.83 Hz, 8H), 2.12-2.09 (m, 4H), 1.61-1.55 (m, 8H), 1.34-1.27 (m, 24H),1.21-1.13 (m, 20H), 0.87-0.79 (m, 22H).13C NMR(125 MHz, CDCl3),δ(ppm): 186.08, 158.09, 153.87,152.24, 149.83, 147.91, 147.00, 143.82, 142.77,142.21, 140.99, 139.37, 138.25, 137.87, 137.62,136.26, 134.42, 133.99, 132.63, 128.66, 127.92,120.93, 117.63, 114.74, 114.25, 69.46, 63.13, 54.85,40.47, 35.55, 31.77, 31.64, 31.16, 30.02, 29.20, 29.14,24.03, 22.59, 22.54, 14.06, 14.03. MALDI-TOF MS for C121H114F4N4O2S6: calcd. 1 922.72, found: 1 923.0 (M+).

3 Results and discussion

3.1 Material synthetic and optoelectronic properties

The synthetic routes of BT2T-FIC and B3TFIC are presented in Scheme 1. BT2T-FIC and B3TFIC both show good solubility in common processing solvents, such as chlorobenzene (46 mg mL-1and 35 mg mL-1respectively) and chloroform (37 mg mL-1and 30 mg mL-1respectively). In thermogravimetric analysis, 5% weight loss temperatures (Td) of BT2TFIC and B3T-FIC are 331.9 and 342.0 ℃ respectively under nitrogen atmosphere, suggesting their good thermal stability. The ultraviolet-visible (UV-vis)absorption spectra of BT2T-FIC and B3T-FIC are presented in Fig.1(a). In dilute chlorobenzene (CB)solution (with 10-5mol·L-1), BT2T-FIC and B3TFIC both exhibit absorption mainly ranging from 650 to 750 nm. The maximum molar extinction coefficient (εmax) of BT2T-FIC solution is 2.53 × 105M-1cm-1at 701 nm, which is stronger than that of B3T-FIC solution (2.01×105M-1cm-1at 697 nm).The absorption of BT2T-FIC and B3T-FIC films are both slightly broadened to the near-infrared region than their corresponding solution absorption. The film absorption peaks are mainly assigned to the 0-0 and 0-1 vibrionic transitions of π-π* transition. BT2T-FIC film offers an εmaxof 1.10 × 105cm-1at its 0-0 transition peak (at 720 nm) with a redshift of 19 nm. The εmaxof B3T-FIC film is as high as that of BT2T-FIC at its 0-0 transition peak (at 728 nm) while B3T-FIC film shows an enhanced redshift of 31 nm, which hints the stronger aggregation and ordering of BT2T-FIC. The optical gaps (Egopt) of BT2T-FIC and B3T-FIC are 1.56 and 1.55 eV respectively, estimated from the absorption edge of their thin film (Table S1). In addition, cyclic voltammetry (CV) was performed for the energy levels of the lowest unoccupied molecular orbital (LUMO)and the highest occupied molecular orbital (HOMO).The estimated HOMO and LUMO levels of BT2TFIC are -3.98 and -5.56 eV respectively while B3TFIC shows an uplifted HOMO of -5.54 eV and a downshifted LUMO of -4.00 eV (Fig.1(b)). The slight shifts between their HOMO and LUMO (about 0.02 eV) suggest that switching the anchoring position of side chains has a weak influence on their energy levels.

To further investigate the isomeric effect of side chains on molecular geometries and electronic properties, the density functional theory (DFT)calculations were conducted on BT2T-FIC and B3TFIC with Gaussian 09 suite15 at B3LYP/6-31G(d,p)level. The optimal geometries of BT2T-FIC and B3TFIC are presented in Fig.1(c). The conjugate plane of BT2T-FIC twists with 1.57odihedral angles at thesp3-hybrid carbons anchoring bulky phenylhexyl chains respectively while that of B3T-FIC only twisted with a 2.01odihedral angle at the centralsp3-hybrid carbon anchoring alkyl side chains. It results in a higher dipole moment of 1.96D in BT2T-FIC than B3T-FIC(0.746D), which could strengthen the intramolecular charge transfer and light capture ability of BT2T-FIC.As shown in Fig.1(c), the distances from the edge of the IC-2F to the firstsp3 hybrid carbon anchoring side-chains were calculated to be 11.52 and 15.41 Å in BT2T-FIC and B3T-FIC respectively. Such longer molecular terminal regions in B3T-FIC offer much more space for close intermolecular packing without the steric repulsion from side chains. For BT2TFIC, its shorter molecular terminal regions and more intramolecular twisting are both unfavorable for intermolecular packing.

3.2 Photovoltaic device characterization

Fig.1 (a) Absorption spectra of the dilute solutions of BT2T-FIC and B3T-FIC in CB with 1 × 10-5 M and the absorption of BT2T-FIC and B3T-FIC film at room temperature; (b) Energy levels of donor and acceptor materials; (c) DFT optimized conformation and the dipole moment of BT2T-FIC and B3T-FIC

Table 1 Optimized average and maximum photovoltaic performance of BHJ PSCs with PBDB-T-2Cl as the donor material a

As presented in Fig.1(a), the representative donor polymer PBDB-T-2Cl exhibits a complementary absorption to BT2T-FIC and B3T-FIC. Besides, the suitable HOMO/LUMO offsets between PBDB-T-2Cl and the two SMAs (about 0.03/0.42 eV respectively)suggest the potential for good carrier transfer efficiency(Fig.1(b)). PBDB-T-2Cl was thus chosen as a donor to manufacture the PSC devices with an inverted structure of glass/ZnO/PBDB-T-2Cl:SMA/MoO3/Ag to investigate the photovoltaic properties of BT2T-FIC and B3T-FIC. With the optimization of film thickness,the optimal weight ratios for PBDB-T-2Cl:BT2T-FIC and PBDB-T-2Cl:B3T-FIC devices were confirmed to be 1:0.8 and 1:0.6 respectively according to a fixed PBDB-T-2Cl concentration of 10 mg mL-1in CB.Thermal annealing (TA) has also been carried out to further improve their photovoltaic performance. The optimal photovoltaic parameters are summarized in Table 1 and the correspondingJ-Vcurves are presented in Fig.2(a). The optimized PBDB-T-2Cl:BT2T-FIC device offers an averaged PCE of 10.56% with aVocof 0.983 V, aJscof 17.37 mA·cm-2and a FF of 0.618.Hindered by the inferiorJsc(14.62 mA·cm-2) and FF(0.516), the optimized PBDB-T-2Cl:B3T-FIC device only provides an averaged PCE of 7.53% with aVocof 0.998 V. The photon energy loss (Eloss) in PBDB-T-2Cl:BT2T-FIC and PBDB-T-2Cl:B3T-FIC devices can be simply estimated to be 0.577 and 0.552 eV byEloss=-eVoc(e is the elementary charge), respectively.Their lowElossvalues suggest the weak radiation and non-radiation recombination in these devices, leading to their goodVocs (about 1 V). It may benefit from the small reorganization energies in such large-size rigid fused ring cores of the heteroundecenes. Furthermore,the external quantum efficiency (EQE) spectra of the optimized devices are shown in Fig.2(b). PBDBT-2Cl:BT2T-FIC and PBDB-T-2Cl:B3T-FIC blend films both mainly exhibit a photoresponse from 400 to 800 nm. The EQE values of the PBDB-T-2Cl:BT2TFIC device overwhelm that of the PBDB-T-2Cl:B3TFIC device in the almost whole range. The maximum EQE value in the BT2T-FIC device is 78.63% at 563 nm while that of the B3T- FIC device is only 64.12%at 555 nm. The calculated EQE-integratedJscvalues are 17.18 and 14.61 mA cm-2for BT2T-FIC and B3TFIC devices respectively, which both matched well with their measuredJscvalues (under the irradiation of simulated AM 1.5 G sunlight) within a 5% mismatch.

Fig.2 (a) J-V curves of optimized PBDB-T-2Cl:SMA devices; (b) EQE curves of the PBDB-T-2Cl:SMA blends and the corresponding integrated Jscs

Fig.3 (a) Photoluminescence (PL) of the PBDB-T-2Cl neat film and the PBDB-T-2Cl:SMA blend films excited at 610 nm; (b) the PL of BT2T-FIC; (c) B3T-FIC and their blend films excited at 720 and 730 nm respectively as well as the corresponding PL quenching efficiency

3.3 Charge generation, transport and recombination

Further measurements on carrier generation,transport and recombination behaviors have been carried out to investigate the superior PCE of the PBDB-T-2Cl:BT2T-FIC device with both about 20%increase ofJscand FF from the PBDB-T-2Cl:B3TFIC device. As presented in Fig.3, when PBDB-T-2Cl was excited at 620 nm, the photoluminescence (PL) of PBDB-T-2Cl in the BT2T-FIC blend is quenched by BT2T-FIC for around 90.5%. In the B3T-FIC blend, the quenching yield for PBDB-T-2Cl decreases to 85.1%.The stronger quenching efficiency demonstrates a more efficient electron transfer from BT2T-FIC to PBDB-T-2Cl. When BT2T-FIC and B3T-FIC are excited at 720 and 730 nm respectively, the BT2T-FIC blend yields a slightly improved quenching (90.5%) than that of the B3T-FIC blend (85.1%). It implies that the hole transfer from SMA to PBDB-T-2Cl is ameliorated in the BT2T-FIC blend. In addition, the exciton dissociation efficiency in the devices was investigated by the measurement of the photocurrent density (Jph)versusthe effective voltage (Veff) (Fig.4(a)). The higher exciton dissociation probabilities (P(E,T), defined asP(E,T)=Jph/Jph,sat) for the BT2T-FIC device (92.8%) is obtained than that for the B3T-FIC device (88.6%),suggesting the more efficient exciton dissociation in the BT2T-FIC blend. It could partly account for the better performance of BT2T-FIC in PL quenching and EQE measurements. In addition, a lower leakage current is also observed in the BT2T-FIC blend (Fig.4(b)).Besides, the higher ratification factor in the BT2T-FIC blend at reverse bias indicates its superior film quality with improved efficiency of charge transport.

Fig.4 (a) Photogenerated current density versus effective voltage curves under AM 1.5G illumination; (b) current J-V curves from -3 V to+3 V

Fig.5 Dependence of Jsc (lg Jsc ∝ αlgPlight) (a) and Voc (Voc ∝ (nKBT/q)lnPlight) (b) on Plight within the PBDB-T-2Cl:SMA PSCs

The electron and hole mobilities of these two blends were measured with the space-charge-limited current (SCLC) model through the corresponding electron-only device and hole-only device, which were constructed by the device structures of ITO/ZnO/BHJ film/Ca/Al and ITO/PEDOT:PSS/BHJ film/Ca/MnO3/Ag respectively. The PBDB-T-2Cl:B3T-FIC blend exhibits the electron mobility (μe) and hole mobility(μh) for 1.90 × 10-4cm2V-1s-1and 0.78 × 10-4cm2V-1s-1respectively and the value ofμe/μhis 2.44. Theμeandμhin the PBDB-T-2Cl:B3T-FIC blend are improved to 3.21 × 10-4and 1.47 × 10-4cm2V-1s-1respectively with a smallerμe/μhof 2.18. These more efficient and balance carrier mobilities in the BT2T-FIC blend are consistent with the results in PL measurement. To reveal the recombination mechanism in the BT2T-FIC and B3T-FIC blend films, theJscversuslight density(Plight) curves were obtained in line with the relationship ofJsc∝(Plight)α(Fig.5(a)). The fitted slope value for the BT2T-FIC blend (0.944) is higher than the B3TFIC blend (0.912). It suggests that the bimolecular recombination was better suppressed in the BT2TFIC blend with less carrier loss during transportation.As presented in Fig.5(b), the measurement ofVocas a function ofPlightwas also performed according to the lawVoc∝(nkBT/q)ln(Plight), wherekBis the Boltzmann constant,Tis the temperature in Kelvin, andqis the elementary charge. The fittednvalue for the B3T-FIC blend is 1.50, further deviating from the 1 than that of the BT2T-FIC blend (1.17). It reveals more singlemolecule recombination occurs in the B3T-FIC blend film with higher trap density, which may result from its inferior morphology.

3.4 Structure characterization of neat polymer and BHJ blend films

Fig.6 GIWAXS scattering patterns (a) and IP and OOP profiles (b) of PBDB-T-2Cl, BT2T-FIC and B3T-FIC neat films as well as their PBDB-T-2Cl:SMA blend films

As the foothold to perform each photoelectric conversion step, multiscale microstructures (e g, molecular packing and phase separation) in BHJ blend films are highly coupled with the final photovoltaic performance of PSCs[31-37]. Thus,structure characterization has also been carried out to qualitatively investigate the superior performance of BT2T-FIC. Grazing incidence wide-angle X-ray scattering (GIWAXS) measurement was performed on PBDB-T-2Cl，BT2T-FIC and B3T-FIC neat films as well as their polymer:SMA BHJ blend films. The GIWAXS diffraction patterns of these neat and blend films are presented in Fig.6 with the corresponding profiles along both the out-of-plane (OOP) and inplane (IP) directions. In the PBDB-T-2Cl neat film，the lamellar peak atqr= 0.297 Å-1in IP direction and the π-π stacking peak atqz= 1.62 Å-1in OOP direction suggest the face-on orientation of PBDB-T-2Cl crystallites. Meanwhile, the edge-on orientation also existed in the PBDB-T-2Cl neat film according to the OOP lamellar peak atqr= 0.320 Å-1and IP π-π stacking peak atqz= 1.63 Å-1. The B3T-FIC neat film does not exhibit the coexistence of edgeon and face-on orientation like PBDB-T-2Cl. Its unobservable IP π-π stacking peak and corresponding OOP lamellar peak imply that there is almost no edgeon orientation of B3T-FIC crystallites. The sharp and strong OOP π-π stacking peak atqz= 1.73 Å-1(d= 3.63 Å) with the IP lamellar peak atqr= 0.233 Å-1(d= 27.0 Å) suggest that B3T-FIC crystallites nearly all take a face-on orientation in its neat film with high crystallinity. Comparing with the B3T-FIC neat film, the characteristic lamellar and π-π stacking peaks in the BT2T-FIC neat film are so weak that it is difficult to identify them from other diffraction signals accurately. It indicates the much lower crystallinity and worse ordering of BT2T-FIC, which may result from its shorter molecular terminal regions and more intramolecular twisting. When blending with PBDBT-2Cl, the diffraction characteristics in the B3T-FIC blend presented the fusion of the stacking signals of both PBDB-T-2Cl and B3T-FIC neat films. According to the strong OOP π-π stacking peak atqz= 1.71 Å-1(d= 3.67 Å) with the corresponding IP lamellar peak atqr= 0.264 Å-1(d= 23.8 Å), the face-on orientation is predominantly taken in the B3T-FIC blend. Besides,this IP lamellar diffraction intensity is observed to be evidently decreased in the B3T-FIC blend than that in the B3T-FIC neat film, implying the weakened crystallinity of face-on oriented B3T-FIC crystals in the B3T-FIC blend. In addition, the low ordering of BT2T-FIC did not destroy the effective intermolecular stacking in its blend film. As exhibited by the dashed lines in Fig.6, the diffraction characteristics of the PBDB-T-2Cl:BT2T-FIC blend film are inherited from those in PBDB-T-2Cl neat film. The strengthened OOP π-π stacking peak atqz= 1.68 Å-1(d= 3.74 Å)suggests that the face-on orientation is preferable in the PBDB-T-2Cl:BT2T-FIC blend film instead of the balanced edge-on and face-on orientation in the PBDBT-2Cl neat film, which is beneficial for the promotion of vertical charge transport. Briefly, the face-on orientation is both preferably taken by crystallites in the BT2T-FIC blend and B3T-FIC blend films with comparable π-π stacking distances. Given the great difference in photovoltaic performance between BT2TFIC and B3T-FIC devices, these slight deviations in molecular packing and orientation may not the main reason for their various performance.

Fig.7 GISAXS in-plane profiles of the PBDB-T-2Cl:SMA blend films and their model fittings with the Debye-Anderson-Brumberger (DAB) model

The phase separation behaviors between donor polymer and SMA in BHJ blend films were also estimated by grazing incidence small-angle X-ray scattering (GISAXS). Fig.7 shows the GIWSAXS inplane intensity profiles of the two PBDB-T-2Cl:SMA blends by fitting their 2D patterns with the Debye-Anderson-Brumberger (DAB) model respectively[38,39].The averaged sizes of the amor-phous polymer: SMA mixture domain and pure SMA domain in these BHJ blend films were roughly estimated. In the PBDB-T-2Cl:B3T-FIC blend, the size of the amorphous mixture domain is 31.0 nm and the BT2T-FIC domain is 21.1 nm. The PBDB-T-2Cl:BT2T-FIC blend exhibits an amorphous mixture domain size of 32.8 nm, close to that of the PBDB-T-2Cl:B3T-FIC blend. Nevertheless,the BT3T-FIC domain size decreases to 13.3 nm in the PBDB-T-2Cl:BT2T-FIC blend, which could attribute to the inferior crystallinity of BT2T-FIC. The smaller SMA domain size in the BT2T-FIC blend could not only decrease the exciton transport distance to the interface with the polymer domain but also increase such interfacial area. It is beneficial for more efficient exciton transport with less recombination probability as well as promoting exciton dissociation efficiency at the phase interface in the BT2T-FIC blend, which could improve itsJsc. It may be one of the main reasons for the superior performance of the BT2T-FIC blend than the B3T-FIC blend.

4 Conclusions

Two isomeric SMAs, BT2T-FIC and B3T-FIC,have been designed and synthesized in this work.Comparing with B3T-FIC, the side chains further away from the center of the heteroundecene core in BT2TFIC has a minor effect on the energy levels but leads to more intramolecular twisting and shorter molecular terminal regions. BT2T-FIC thus exhibits weakened aggregation with more disordering whereas B3T-FIC has high crystallinity and ordering with intensive faceon orientation. When paired with the polymer donor PBDB-T-2Cl to fabricate PSC devices, theJscand FF values of the PBDB-T-2Cl:BT2T-FIC device were both significantly improved about 20% than those of the PBDB-T-2Cl:B3T-FIC device while the highVoc(about 1 V) were both obtained in the two devices. The PBDBT-2Cl:BT2T-FIC device thus shows a surperior PCE of 10.56% than that of the PBDB-T-2Cl:B3T-FIC device(7.53%). The improved photovoltaic performance of the PBDB-T-2Cl:BT2T-FIC device is mainly ascribed to its more suitable phase separation to promote exciton separation, enhance and balance charge transfer as well as suppress charge recombination in the BHJ blend.This study demonstrates the importance of side chains design in constructing SMAs with large-size fused ring cores, which provides a meaningful experience for further developing such SMAs with high performance.