From Isoindigo to Dibenzonaphthyridinedione: A Building Block for

Aug 30, 2016 - Most wide-bandgap (WBG) conjugated polymers with Eg > 2.2 eV exhibit low power conversion efficiency (PCE) due to their limited absorpt...
0 downloads 0 Views 6MB Size
Download PDF
Article pubs.acs.org/cm

From Isoindigo to Dibenzonaphthyridinedione: A Building Block for Wide-Bandgap Conjugated Polymers with High Power Conversion Efficiency Mian Cai,†,‡ Xichang Bao,† Xiao Wang,*,† Huanrui Zhang,† Meng Qiu,† Renqiang Yang,*,† Chunming Yang,§ and Xiaobo Wan*,†,‡ †

CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy & Bioprocess Technology, Chinese Academy of Sciences, 189 Songling Road, Qingdao 266101, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, People’s Republic of China S Supporting Information *

ABSTRACT: Most wide-bandgap (WBG) conjugated polymers with Eg > 2.2 eV exhibit low power conversion efficiency (PCE) due to their limited absorption window. Here we report the synthesis of a novel tetracyclic fused building block dibenzonaphthyridinedione (DBND) from isoindigo and its application as an acceptor building block for wide-bandgap copolymers with improved PCE. The Stille copolymerization of this building block with 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (2T) and (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethane (TVT) results in two WBG polymers PDBND-2T (Eg 2.32 eV) and PDBND-TVT (Eg 2.23 eV), respectively. Both polymers act as excellent donors in high-performance organic solar cells (OSCs). When blended with phenyl-C71-butyric acid methyl ester (PC71BM), PDBND-2T based OSCs exhibit a PCE of 5.75%, which makes it the broadest bandgap OSCs with PCE over 5%. PDBDN-TVT based OSCs featured a high PCE up to 6.32%. Such efficiency is the highest reported to date for a conjugated polymer at such a broad bandgap. Moreover, without additives or annealing process, PDBND-TVT based OSCs exhibit an efficiency around 6.0% with a thick active layer (240 nm) and the performance shows little sensitivity to polymer:PC71BM weight ratios (range from 1:1.5 to 1:3), which makes PDBND-TVT a potential material for processable large-area tandem or ternary OSCs.

■

INTRODUCTION Solution-processed bulk heterojunction organic solar cells (OSCs) continue attracting considerable attention in both academia and industry in the prospect of producing cheap, high-efficient, large-area, and flexible low weight photovoltaic modules through roll-to-roll processing, inkjet printing, or spray-coating technology. Extensive research efforts led to the discovery of novel materials1,2 and innovative new device structures for better performance.3 To achieve optimal sunlight absorption, especially in the near-infrared region, most efforts have been focused on the narrow-bandgap (NBG) donor (D)− acceptor (A) conjugated polymers. The use of state-of-the-art NBG materials in OSCs boosted the power conversion efficiencies (PCE) to 10% or more.4−11 However, it is difficult to improve further PCE just by the design of NBG polymers with narrower Eg values because the great loss of open-circuit voltage (VOC) could not be compensated by the gain of shortcircuit current (JSC) when the absorption edge extends from the 900 to 1000 nm region. To solve this problem, tandem cells or tertiary blend cells using wide-bandgap (WBG) polymers as an additional donor with complementary absorption spectra to © 2016 American Chemical Society

NBG polymers have become a useful strategy to improve further PCE in recent years. Notably, several records PCE have been reported for tandem (or multijunction) devices.12−14 However, WBG polymer is a double-blade sword: although it usually provides a high VOC, it also results in a relatively low JSC, which eventually leads to unsatisfactory PCE. Although many efforts have been focused on the design of novel acceptor building blocks for high-performance WBG polymers, the satisfied results are limited, especially for those with the absorption onset below 600 nm. The wider the bandgap, the less the photons in the solar spectrum captured and converted into photocurrent, which eventually lead to worse performance. Only a few acceptor building blocks, such as thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]-isoquinoline-5,11-(4H,10H)dione,15 dihydrodithieno[3,2-c:3,2-h]-[1,5]naphthyridine-5,10dione,16 pyridinonedithiophene,17 and more recently 3,4difluoro-thiophene,18 have been used for the synthesis of Received: June 6, 2016 Revised: August 17, 2016 Published: August 30, 2016 6196

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials Scheme 1. Synthesis of DBND Based Polymersa

a

Reagents and conditions: the synthesis of 2a and 3a were followed by the literature.28 (a) Zn, CF3COOH, THF, r.t. 1 h, quantitative; (b) HCl (conc.):THF (1:2), 100 °C, 8 h, 61%; (c) K2CO3, DMF, bubbling air, r.t., 48 h, quantitative for 4a and 4b (with tautomer); (d) K2CO3, 11(bromomethyl)tricosane, DMF, 100 °C, 24 h, 70%; (e) Pd2(dba)3, P(o-tol)3, (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethene or 5,5′bis(trimethylstannyl)-2,2′-bithiophene, toluene, 110 °C, 72 h, 98% for PDBND-TVT and 84% for PDBND-2T.

solid-state π−π stacking hence better charge carrier mobility which makes it an excellent candidate for high-performance WBG OSCs.15,16,23−25 DBND was then copolymerized with 2,2′-bithiophene (2T), and a polymer (PDBND-2T) with a wide optical bandgap (2.32 eV) was obtained. OSCs based on PDBND-2T exhibit a high PCE up to 5.75%, which makes it the broadest bandgap polymer with PCE over 5%. When copolymerized with (E)-1,2-di(thiophen-2-yl)ethane (TVT), the obtained polymer (PDBND-TVT) also exhibits a wide optical bandgap of 2.23 eV, and the related solar cell exhibits a record high PCE up to 6.32% among WBG materials with such a board bandgap. Notably, PDBND-TVT possesses several merits for the fabrication of large-area PSC devices including (1) its high PCE shows little dependence on the ratios of donor polymer:PC71BM (from 1:1.5 to 1:3) or on the thick active layer (a PCE of 6.0% is still maintained when the active layer thickness is increased to 240 nm); (2) no additives or annealing process are necessary to achieve such high PCE. Such advantages make this novel conjugated polymer suitable for fabricating large-area tandem OSCs.

WBG polymers with Eg values around 2.0 eV; to the best of our knowledge, for those WBG polymers with Eg values greater than 2.2 eV, the best PCE reported to date are only about 5%.16 Could we further improve the PCE of WBG polymers by the design of novel acceptor building blocks? Besides the demands for novel WBG polymers with higher PCE, to fabricate large-area OSCs (whether it is single-junction or tandem cells) with high reproducibility, a robust conjugated polymer that is not sensitive to processing parameters is required. Normally, to achieve high PCE, strict processing conditions have to be well-controlled: (1) the addition of additives or annealing process is needed to achieve nanoscale phase separation; (2) the control of the thickness of the active layer is also important, and in many cases, high PCE were obtained at a thickness around 100 nm; and (3) the dedicate balance of the donor/acceptor ratio in the active layer is also necessary. However, the addition of additives and the annealing process for better phase separation imply that the long-term maintenance of such nanostructures might be questionable, the thin active layer might lead to many structural defects when applied on large surface areas, and the fluctuation of donor/ acceptor ratio during large film formation might lead to the illreproducible PCE. All these factors impede the large-area OSCs fabrication. Although many efforts have been put into this research area,16,19−21 the high-performance conjugated polymers that are not sensitive to all these processing conditions were rarely reported, especially for WBG polymers. Here, we report the isomerization of isoindigo, which leads to a novel acceptor building block, namely dibenzo[c,h][2,6]naphthyridine-5,11(6H,12H)-dione (DBND). Isoindigo has been known to be an acceptor building block for NBG materials for OSCs, with the related copolymer absorption onset about 800 nm.22 Different from isoindigo, DBND is a fused aromatic system with moderate electron-withdrawing ability. Furthermore, its fully planar structure favors better

■

RESULT AND DISCUSSION Monomer Synthesis and Polymerization. In our continuous efforts in manipulating the isoindigo core,26,27 we noticed that the double bond of isoindigo 1a could be reduced under Zn/trifluoroacetic acid conditions in quantitative yield and the product 2a could be easily isomerized in HCl/H2O/ THF solution to give a saturated fused bis-lactam 3a (Scheme 1, steps 1−2).28 We envisioned that the double bond could be reinstalled by oxidation if a proper base and oxidant are chosen,29 which would lead to a fully fused conjugated molecule. Fortunately, compound 3a could be easily oxidized in air in the presence of K2CO3, and compound 4a (DBND) was obtained as a yellow solid in almost quantitative yield. It is 6197

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials Table 1. Molecular Weights, Optical Properties, and Electrical Properties of PDBND-TVT and PDBND-2T filmb

solution polymer PDBND-2T PDBND-TVT

Mna

(kDa)

72.9 29.7

PDI

a

2.5 2.1

λmax (nm)

λedge (nm)

λmax (nm)

λedge (nm)

Egopt (eV)

IPc (eV)

474,505 485,523

528 546

467,501 485,523

535 555

2.32 2.23

5.51 5.32

Determined by GPC at 150 °C using TCB as the eluent. bFilms are prepared by dropping casted the polymer solution on the piezoid. cFilms are prepared by dropping casted the polymer solution on the working electrode.

a

2T exhibited a higher Mn (72.9 kDa) than PDBND-TVT (29.7 kDa), which is consistent with the observation of its gelation ability at lower temperature. The much larger Mn of PDBND2T implies that in the reaction media, PDBND-2T shows much better solubility than PDBND-TVT with the similar molecular weight, which allows further chain growth. The thermal properties of both polymers were measured with TGA under nitrogen atmosphere. PDBND-2T and PDBND-TVT showed high thermal stability with the decomposition temperatures up to 300 and 325 °C, respectively (Figure S2). Both polymers meet the requirements of thermal stability for photovoltaic application. Optical and Electrochemical Properties. The normalized UV−vis absorption spectra of DBND monomer, PDBND2T and PDBND-TVT in diluted o-DCB and as a thin film are shown in Figure 1, and the related data are listed in Table 1.

worthwhile to mention that DBND is a known compound, and a different approach toward it was reported by Gopinath et al.30 However, no studies on its application in conjugated polymers were reported thereafter. Compared to Gopinath’s method, which suffers from the low yield in the bis-lactam ring formation step, our methodology provides a better way toward this fused conjugated molecule. This methodology was then applied to the synthesis of dibromo-DBND 4b, which was obtained with no difficulties. The solubility of 4a and 4b in most of organic solvents is rather low, which makes the fully characterization difficult. Only welldefined 1H NMR spectra were obtained. Compound 4a exists in bis-lactam form; interestingly, tautomerization of compound 4b was observed: it exists as a mixture of bis-lactam form and lactam/quinolinol form in a 48:52 ratio (for details, see the Supporting Information). Alkylation using 2-decyl-tetradecyl bromide on the core structure in K2CO3/DMF increased their solubility greatly and made the full characterization possible. Detailed analysis of 13C-NMR spectra reveals that O-alkylated product was obtained as the major product (the carbon signal with chemical shift at 70 ppm validated that the alkyl chain is grafted onto the oxygen atom). The yield of the O-alkylated product is around 70%, and detailed analysis of the side products revealed that there is around 10% of half-O-alkylated/ half-N-alkylated product, and also a trace amount of all Nalkylated product (see the Supporting Information for details). The preference of O-alkylation may be driven by aromatization, similar to the cases reported by Andersson31 and Liu.32 With the monomer in hand, Stille-coupling polymerizations between DBND and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (2T) and (E)-1,2-bis(5-(trimethylstannyl)thiophen-2yl)-ethane (TVT) were carried out in toluene, using Pd2(dba)3 as the catalyst and P(o-tol)3 as the ligand. The resultant polymer was precipitated in methanol, and then collected and purified via Soxhlet extraction using methanol and hexane to remove impurities and oligomers. The purified polymer was redissolved in hot chloroform, concentrated, and precipitated in methanol to give the final polymer suitable for device fabrication. Both PDBND-2T and PDBND-TVT were obtained in high yield (84% for PDBND-2T and 98% for PDBNDTVT). PDBND-2T was a yellowish solid and PDBND-TVT an orange solid, both showing good solubility in organic solvents such as chlorobenzene or o-dichlorobenzene (o-DCB). The solubility of PDBND-2T in o-DCB is around10 mg mL−1 at 100 °C, but the solution would gelate when cooled down gradually to room temperature. PDBND-TVT shows excellent solubility in o-DCB with a concentration of 17 mg mL−1 at 100 °C, and the solution would not gelate at room temperature, which permits the fabrication of thick active layers. The molecular weight of both polymer were determined via gel permeation chromatography (GPC) using 1,2,4-trichlorobenzene as the eluent at 150 °C after calibration against polystyrene standards. The average molecular weights (Mn) and polydispersity index (PDI) are shown in Table 1. PDBND-

Figure 1. UV−vis absorption spectra of DBND monomer, PDBNDTVT and PDBND-2T in diluted o-DCB and as the thin film.

The solutions of PDBND-2T and PDBND-TVT both exhibit strong absorption in the 400−550 nm region with a high maximum extinction coefficient of 1.1 × 105 M−1 cm−1 at 505 nm and 0.96 × 105 M−1 cm−1 at 523 nm, respectively (Figure S3). Compared to DBND monomer, both polymers exhibit a large red-shift of about 120−140 nm, which reflects the internal charge transfer effect due to the formation of the D−A structure. In the thin film state, PDBND-TVT shows a similar optical property to that in solution, whereas PDBND-2T exhibits a 4 nm hypsochromic shift compared to its solution, indicating that H-aggregate is formed in the solid state. In both cases, no obvious red shift of the absorption onset was observed, unlike many other reported D−A type conjugated polymers. The results indicate that both polymers might already have adopted a tightly packed aggregation form in solution that will not alter in solid state, presumably due to the highly planar DBND structure that favors better intermolecular interaction even in solution state. Similar results were reported for other conjugated polymers with large fused-ring systems.6,23 Nevertheless, both polymers in film state show an obvious increase of the relative intensity of the 0-0 vibrational peak 6198

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials

Figure 2. Theoretical calculation of molecular conformation and orbitals of PDBND-2T (a, c, e) and PDBND-TVT (b, d, f) dimers at ωB97XD functional. (a, b) Energy-minimized structure; (c, d) HOMO orbitals; (e, f) LUMO orbitals.

long alkyl chains were replaced by methyl groups. Computationally predicted energy-minimized polymer backbone structures are shown in Figure 2. For both polymers, the dihedral angle between DBND and the thiophene ring is around 30° (28.42° to 31.04° for PDBND-TVT and 26.23° to 30.55° for PDBND-2T), indicating the existence of steric hindrance. However, TVT units in PDBND-TVT adopt a much more planar conformation with the dihedral angle between the thiophene rings and the vinylene group less than 5° (0.30° to 4.55°), whereas 2T units in PDBND-2T exhibit a much larger dihedral angle (about 30°) between the two thiophene rings. Overall, PDBND-TVT shows a more planar conformation than PDBND-2T, as is shown from the top sight of structures in Figure S5. The more planar structure would promote a higher crystallinity, which would result in higher charge transfer mobility. Our calculation also indicates that the electron density in the HOMOs of both dimers is delocalized over almost the entire backbones. Surprisingly, unlike isoindigo based copolymers,33 the LUMOs of which are usually located on the electrondeficient cores, one can see very clearly from Figure 2 that the 2T or TVT units between two DBND units contribute greatly to the electron density of the LUMOs of the dimers, implying that the electron density associated with LUMOs of both PDBND-2T and PDBND-TVT is delocalized along the polymer. This result indicates that the LUMOs of both polymers are influenced by the donor building blocks and exhibit a strong π* character.39 This also indicates that DBND is not a strong acceptor unit in the D−A type polymer, so the difference between energy levels of LUMO of both acceptor and donor building blocks is not large, which allows the reorganization of the LUMO of DBND units with that of 2T or TVT units. Photovoltaic Properties. Solution-processed bulk heterojunction devices based on PDBND-2T:PC71BM were first fabricated with a conventional device structure of ITO/ PEDOT:PSS/polymer:PC71BM/Ca/Al. Very encouragingly, a PCE as high as 4.66% (average 4.57%) was obtained on the ascast film, with VOC of 0.89 V, JSC of 8.01 mA cm−2 and FF of 0.66 (Table 2, entry 1), which is among the best results in the

(compared to 0-1 peak) than that of in solution state, which indicates that the polymer backbone becomes more planar in the film state.33,34 The absorption edge of two polymers is 535 nm for PDBND-2T and 555 nm for PDBND-TVT, respectively. The optical bandgap was calculated from the edge absorption of the film using the formula Egopt(eV) = 1240/λedge(nm). The calculated Egopt of PDBND-2T and PDBND-TVT are 2.32 and 2.23 eV, respectively. The optical bandgap is smaller than the fundamental gap, since the energy needed for electron−hole pair separation is not included in the consideration.35 The actual HOMO/LUMO energy level is hard to be accurately determined, and electrochemical cyclic voltammetry (CV) was used to measure the ionization potential (IP) as well as the electron affinity (EA) of the polymers (Figure S4), which partially reflect the HOMO/ LUMO energy level. The IP values of the polymers are determined from the onset of the oxidation peak using equation IP = (Eonsetox − ferrocene) + 4.8 eV. The results are summarized in Table 1. The onset of the oxidation peak of PDBND-2T and PDBND-TVT are at 1.05 and 0.90 V (vs Ag/ AgCl), and the relative IP values are 5.51 and 5.32 eV, respectively, suggesting that TVT is a stronger electron donating building block compared to 2T. Unfortunately, no well-defined reduction peak was observed when the potential was scanned to −2.0 V vs Ag/AgCl, so we could not determine the EA of the polymers at this time. The higher IP of PDBND2T indicates that solar cells based on it could exhibit a higher VOC than that based on PDBND-TVT. Theoretical Calculation. Density functional theory (DFT) calculation with B3LYP functional is commonly used to discuss the energy-minimized structures as well as the HOMO/LUMO energy levels of conjugate molecules. However, B3LYP overestimates the delocalization the wave functions and favors more planar structures.36 Thus, ωB97XD method developed by the Head-Gordon group37 and later applied in the calculation of conjugated polymers 38 was used to minimize the delocalization error in our case to compare the planarity and the HOMO/LUMO distribution along PDBND-2T and PDBND-TVT. For the sake of computational simplicity, only the conformation and orbitals of dimers were calculated and the 6199

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials Table 2. Performance Parameters of OSCs Based on PDBND-2T with Different Polymer:PC71BM Ratiosa

a b

buffer layer

ratio

PEDOT:PSS PEDOT:PSSb V2O5 V2O5 V2O5 V2O5

1:2 1:2 1:1 1:1.5 1:2 1:2.5

VOC (V) 0.89 0.89 0.93 0.91 0.93 0.93

± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01

JSC (mA cm−2) 8.19 8.08 7.43 8.67 8.39 6.43

± ± ± ± ± ±

0.18 0.08 0.27 0.11 0.19 0.22

FF 0.63 0.52 0.65 0.68 0.72 0.74

± ± ± ± ± ±

PCE (%) 0.02 0.02 0.01 0.02 0.00 0.00

4.66(4.57 3.82(3.72 4.60(4.48 5.36(5.28 5.75(5.61 4.58(4.46

± ± ± ± ± ±

0.09) 0.10) 0.12) 0.08) 0.14) 0.12)

The averages and standard derivations were calculated from at least five devices. Best PCEs are obtained with the film thickness around 80 nm. OSCs prepared with 2% DIO as the additive.

Figure 3. Photovoltaic characteristics: J−V curves and EQE plots of PDBND-2T:PC71BM (a, c) and PDBND-TVT:PC71BM (b, d) optimal solar cells with different weight ratios under the illumination of AM 1.5G 100 mW cm−2.

led to a slight increase of VOC from 0.89 V to above 0.90 V (0.91−0.93 V) at all PDBND-2T:PC71BM blending ratios (from 1:1 to 1:2.5), and a large improvement of FF from 0.63 to about 0.7. The best performance was achieved when the blending ratio was 1:2, with the average PCE of 5.61%, and the peak efficiency (5.75%) was obtained with VOC of 0.94 V, JSC of 8.52 mA cm−2 and FF of 0.72. Such performance makes PDBND-2T the best WBG polymer with Eg exceeding 2.30 eV but holding a PCE close to 6.00%. Further increase of the content of PC71BM (to 1:2.5 ratio) led to a further improvement of FF value (to 0.74); however, the JSC value decreased, leading to an average efficiency of 4.46%. The current density−voltage (J−V) curves and the external quantum efficiency (EQE) of the optical devices are shown in Figure 3a,b. As can be seen from the EQE curves, PDBND2T blend film shows a response above 50% from 390 to 540 nm, and a strong EQE peak (60%−64%) in the 480−520 nm regions, which is in consistent of the UV−vis absorption. The calculated EQE matches well with the JSC with 2−4% mismatch with different polymer:fullerene ratios. Several factors might contribute to the outstanding performance of PDBND-2T based OSCs: (1) a high Mn (73 kDa) of PDBND-2T, which promotes a high carrier motilities46−48 and a proper morphology with decreased domain size and a welldeveloped fiber network which increase the number of excitons

family of WBG organic solar cells with bandgap about 2.2 eV. For example, a terpolymer based on benzobisoxazole core exhibits a bandgap around 2.1−2.2 eV, and the best PCE that could be obtained based on this polymer is 2.8%.40 An anthracene based polymer with a bandgap of 2.10 eV exhibits an average PCE of 4.51%.41 A tetracyclic lactam based polymer reported by Andersson et al. exhibits a bandgap of 2.2 eV, with the average PCE of 5% (best PCE 5.2%).16 To improve further the performance, 2% of diiodooctance (DIO), the commonly used additive was added. However, no improvement was observed. On the contrary, a drastic decrease in FF to 0.52 was observed and the PCE dropped to 3.72% (Table 2, entry 2). It seems that the PDBND-2T:PC71BM blended film already adopted the optimized nanoscale structure once casted into films (see TEM and AFM discussion section for details). We then focused our attention on the optimization of the holetransport layer for further improvement.42−44 O2 plasma treated V2O5 layer was chosen as the anode buffer layer to replace the PEDOT:PSS layer not only due to its suitable optical and electrical properties, high work function and ambient stability but also due to that O2 plasma treatment provides better interfacial contact between V2O5 and the active layer that hence minimizes the contact resistance, which might lead to better performance.45 To our delight, an obvious improvement was observed (Table 2, entries 3−6). Using O2 plasma treated V2O5 6200

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials Table 3. Performance Parameters of OSCs Based on PDBND-TVT with Different Polymer:PC71BM Ratiosa

a b

buffer layer

ratio

PEDOT:PSS PEDOT:PSSb V2O5 V2O5 V2O5 V2O5

1:1.5 1:1.5 1:1.5 1:2 1:2.5 1:3

VOC (V) 0.81 0.84 0.81 0.80 0.81 0.82

± ± ± ± ± ±

0.04 0.02 0.00 0.01 0.01 0.00

JSC (mA cm−2) 8.34 8.92 11.11 11.30 11.17 10.3

± ± ± ± ± ±

0.61 0.36 0.14 0.26 0.10 0.21

FF 0.57 0.56 0.64 0.67 0.62 0.67

± ± ± ± ± ±

PCE (%) 0.04 0.03 0.01 0.02 0.03 0.01

4.43 4.43 5.79 6.32 5.78 5.84

(3.98 (4.16 (5.72 (6.15 (5.58 (5.62

± ± ± ± ± ±

0.45) 0.27) 0.07) 0.17) 0.20) 0.22)

The averages and standard derivations were calculated from at least five devices. Best PCEs are obtained with the film thickness around 180 nm. OSCs prepared with 2% DIO as the additive.

Figure 4. (a) Dependence of average PCE, VOC, Jsc, and FF on weight ratios of PDBND-TVT:PC71BM solar cells with an active layer thickness about 180 nm. (b) Influence of the active layer thickness on the average PCE of PDBND-TVT:PC71BM solar cells.

reach to the polymer/fullerene interface (Figure 5b);8,46,49−52 (2) the fully planar structure of DBND building block might lead to improved crystallinity and hence a better π−π stacking of the polymer, which therefore increases the carrier motilities and promote a better nanophase separation thus increase chances for the dissociation and diffusion of excitons.16 PDBND-TVT:PC71BM based solar cells were also fabricated and optimized and the results are shown in Table 3. Similar to PDBND-2T, when PEDOT:PSS was used as the hole transport layer, an average PCE of 3.98% (highest PCE of 4.43% with VOC of 0.85 V, JSC of 8.89 mA cm−2, and FF of 0.59) was observed. After the addition of 2% DIO, the results only showed a marginal increase on the average PCE to 4.16% (but with the same highest PCE compared to the one without DIO), showing that adding additives only has neglectable effects on the PDBND-TVT:PC71BM system. Replacing PEDOT:PSS by O2 plasma treated V2O5 as the hole transport layer again led to a great improvement of the device performance. All devices with the blending ratio ranging from 1:1.5 to 1:3 exhibited an average PCE over 5%. Especially for the device with 1:2 blending ratio, an average PCE of 6.13% was achieved, with the highest PCE of 6.32%. The PCE dropped back to 5.62% when the blending ratio decreased to 1:3. The J−V curves and the EQE of the devices with different blending ratios (from 1:1.5 to 1:3) are shown in Figure 3b,d. As shown in the EQE curves, except for the device with 1:3 blending ratio, which has the least amount of PDBND-TVT in the active layer, other devices exhibit an EQE around 60−65% in the 400−450 nm region, and the EQE even reaches 70% in the 500−560 nm region, which matches well with the UV−vis absorption spectra of PDBND-TVT films, indicating that the photon harvesting and charge collection ability of PDBND-TVT is very high. Similar to PDBND-2T, the EQE curves of PDBND-TVT exhibit an obvious response between 560 and 700 nm due to the absorption of PC71BM in this region. The calculated JSC values

integrated from the EQE curves also matches well with the JSC values determined by J−V curves (only a 3−6% mismatch). More interestingly, performance of the devices based on PDBND-TVT:PC71BM with an optimized active layer thickness (around 170 to 180 nm) show less dependence on the blend ratio (ranging from 1:1:5 to 1:3), as shown in Figure 4a and Table 2. The average PCE is 5.72%, 6.15%, 5.58%, and 5.62% for 1:1.5, 1:2, 1:2.5, and 1:3 blend ratios, respectively. Such results mean that an average PCE of 5.77% with a mean square error (MSE) of 0.23% could be obtained even when the PDBND-TVT:PC71BM blend ratio was doubled. The JSC initially increased with the increase of the content of PC71BM in the active layer and reached the largest value at the blending ratio of 1:2 (11.3 mA cm−2), and then the JSC slightly decreased to 10.3 mA cm−2 at ratio of 1:3. The FF varies little from 0.62 to 0.67 with different ratios which is also in accordance with their stable performance. The reason for such insensitivity will be discussed later. This insensitivity toward blending ratio is beneficial toward large area fabrication of such BHJ devices because the performance fluctuations caused by the concentration variation of donors and/or acceptors would be minimized. Another interesting feature is the influence of the active layer thickness on the performance of the devices based on PDBNDTVT:PC71BM. A thicker active layer is preferred in the fabrication of large area OSCs because more uniformed film structure with less defects could be obtained, which results in a more reproducible performance. However, to obtain a thick active layer with high performance is difficult. Although the increase of the thickness of active layers increases, the number of absorbed photons that could be converted to light current, however, also greatly increases the chances for charge recombination. Thus, most of the high-performance OSCs show the best efficiency with film thickness of around 100 nm.53 The relationship between the film thickness of the 6201

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials

Figure 5. AFM phase images and TEM images of the active layers containing PDBND-2T:PC71BM (a, c) and PDBND-TVT:PC71BM (b, d) in weight ratio of 1:2.

overwhelmed the adverse effect of the increase of the chances for charge recombination in thick layers. It might be concluded that the high efficiency for PDBND-TVT to capture photons (as reflected by its high EQE) is vital for its excellent performance in thick layers. With optimized film thickness at 180 nm, PDBND-TVT based OSCs exhibits peak PCE of 6.32% with VOC of 0.80 V, JSC of 11.49 mA cm−2, and FF of 0.69. The excellent characters of PDBND-TVT make it a promising candidate for processable large-area tandem or ternary cells. Morphology Characterization. To understand better the excellent photovoltaic performance of PDBND-2T and PDBND-TVT, the morphologies and phase separation characteristics of the optimal active layers were investigated using atomic force microscopy (AFM) and bright field transmission electron microscopy (TEM) techniques, and the results are shown in Figure 5. The AFM phase images shown in Figure 5 reflect that both PDBDN-2T and PDBND-TVT blended films showed well-developed nanowire-like fibrils networks, indicating an enhanced phase separation that is an advantage for OSCs.49,56 The AFM height images (Figure S6) shows that the film of PDBND-2T and PDBND-TVT blended with PC71BM are smooth and uniform with a root-mean-square (RMS) surface roughness of 6.63 and 3.55 nm, respectively.

PDBND-TVT:PC71BM active layer and the corresponding PCE was studied. Devices at three film thicknesses (∼140, ∼180, and ∼240 nm) with different PDBND-TVT:PC71BM blending ratios (from 1:1.5 to 1:3) were compared, and the PCE-film thickness relationship is shown in Figure 4b. In all cases, the thinnest active layer gave the worst performance, and the active layer around 180 nm gave the best results. These phenomena were not observed for the PDBND-2T:PC71BM active layer, the best PCE of which was obtained with a film thickness of around 80 nm. When the film thickness was further increased to around 240 nm, the PCE dropped back a little, but was still higher than that at around 140 nm thickness. Especially for an active layer with 1:2 blending ratio, the average PCE at about 240 nm is 6.0%, very close to that at 180 nm (6.15%) thickness. It is worthwhile to point out that such a thickness is beyond the requirement (200 nm) to fabricate large-area OSCs.16,54,55 When the film thickness increased from ∼140 to ∼240 nm, little FF variation but a large JSC increase was observed. For example, for devices with 1:2 blending ratio, the JSC value was increased from 9.11 to 11.30 mA cm−2 (24% increase). Even when the thickness increased to 240 nm, a large JSC up to 11.17 mA cm−2 was still maintained. The large increase of JSC value is attributed to the increase of photons that could be harvested in thicker layers, the effect of which 6202

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials

Figure 6. 2D GIWAXS patterns of (a) PDBND-2T:PC71BM and (b) PDBND-TVT:PC71BM blended film (1:2 w/w). (c) Out-of-plane cutlines of PDBND-2T and PDBND-TVT neat and blended films.

was used as the donor materials due to its better solubility compared with PDBND-2T, which also enable it to harvest a larger amount of photons. GIWAXS Analysis. The crystallinity, local molecular ordering/orientation, and aggregation state of both polymers in the optimized neat and blended films were further studied using two-dimensional grazing incidence wide-angle X-ray scattering (2D) GIWAXS technique to cast a light on their outstanding performance. In the neat and blended PDBND-2T film (Figure S9a and Figure 6a), the polymer exhibits relatively low crystallinity with only (100) lamellar stacking peaks visible. On the contrary, a higher degree of molecular order was observed of PDBND-TVT in both neat and blended film, as evidenced by lamellar (100)-(300) reflection peaks shown in Figure S9b and Figure 6b. Two of the neat polymer films evidence an obvious face-on backbone orientation relative to the substrate interface as indicated by the obvious (010) π−π stacking peak shown on the out-of-plane linecut. The out-ofplane (010) peaks of the PDBND-2T and PDBND-TVT neat film are shown in q = 1.68 Å−1 and q = 1.86 Å−1, respectively, and the corresponding π−π distances are calculated to be 3.74 and 3.38 Å. Owning to the planar structure of DBND and TVT units as well as their better π−π overlap, the π−π distance of PDBND-TVT backbones is among the smallest values for conjugated polymers despite the long alkyl chains used.7,9 When blended with PC71BM (polymer:fullerene weight ratio is 1:2), the PDBND-2T crystallinity was disrupted, as evidenced by the lose of (010) Bragg reflection. On the contrary, the faceon character of PDBND-TVT was retained. The π−π stacking of PDBND-TVT was relatively less influenced by the addition of PC71BM and may be due to its smaller π−π distance and stronger π−π interaction. The higher crystallinity and smaller π−π distance of PDBND-TVT than that of PDBND-2T is consistent with its higher hole mobility and better OSCs performance.

The morphology observed in TEM images are consistent with that in AFM pattern. Bicontinuous interpenetrating networks with nanofibrils are observed for both polymer blended films, which reflect favorable nanoscale phase separation. These polymeric fibrils are only several nanometers in width, which is less than the typical exciton diffusion lengths for conjugated polymers (∼20 nm).25,57 Hence, the percentage of excitons that reach the polymer/fullerene interface where charge generation occurs would be largely increased, and the favorable exciton diffusion/dissociation would lead to a high JSC and FF.58 Moreover, TEM images of PDBND-TVT:PC71BM active layers at different blending ratios (from 1:1.5 to 1:3) were also compared, and the TEM patterns are listed in Figure S7. In all cases, bicontinuous interpenetrating networks with fine nanofibrils are observed with similar nanoscale polymeric fibrils, indicating that the insensitivity of FF toward blending ratio of PDBND-TVT:PC71BM based OSCs might be derived from the stable and favorable nanoscale phase separation. Janssen and co-workers studied the influence of blending ratio on the formation of nanofibril networks, and came to the conclusion that for those fibers with a width more than 30 nm, the amount of fullerenes does not significantly influence fiber width.59 Our results further confirmed that this phenomenon might still be true when the fiber width decreases to a few nanometer scales. This might be attributed to the high crystallinity of PDBNDTVT. The hole mobility of PDBND-2T and PDBND-TVT was measured in the architecture ITO/PEDOT:PSS/polymer:PC71BM (1:2)/MoO3/Ag, and the corresponding results are 4.48 × 10−5 and 8.19 × 10−5 cm2 V−1 s−1, respectively (Figure S8). The PDBND-TVT blended film shows mobility twice that of PDBND-2T, which is consistent with its higher JSC. Nevertheless, both polymers exhibit a hole carrier mobility well-matched to the electron mobility in fullerenes (also in 10−5 to 10−4 cm2 V−1 s−1 range). The balanced hole/electron mobility might account for the excellent PCE for both polymers. It was worthwhile to mention that even with a much smaller Mn, PDBND-TVT gains a great improvement of EQE as well as JSC in comparison with PDBND-2T. The reason for the higher light current of PDBND-TVT than PDBND-2T base OSCs could be summarized as follows: (1) as predicted by DFT calculation, PDBND-TVT has a more planar structure than PDBND-2T, which favors better solid stacking and smaller π−π distance and facilitates better crystallinity (which will be discussed in the GIWAXS result) and charge mobility; (2) PDBND-TVT has a slightly narrower bandgap and exhibits a slightly bathochromic shifted absorption in sunlight region, which enable it to harvest more photons from solar spectrum; and (3) a thicker layer could be fabricated when PDBND-TVT

■

CONCLUSIONS A novel tetracyclic fused building block DBND was synthesized via the isomerization of isoindigo. On the basis of this unit, two WBG polymers PDBND-2T and PDBND-TVT were designed, synthesized, and characterized in detail. Bulk heterojunction solar cells fabricated with both materials exhibit excellent performances, showing the possibility of DBND to act as a more universal acceptor building block for WBG polymers. PDBND-2T (Eg 2.32 eV) based OSCs achieve a high PCE of 5.75%, which make it the broadest bandgap OSCs with PCE over 5%. With a slight narrower bandgap (2.23 eV), PDBNDTVT based OSCs exhibited a record efficiency of 6.32%. More encouragingly, no additive or annealing process was necessary 6203

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials

Electron Acceptor for All-Polymer Solar Cells. Angew. Chem., Int. Ed. 2016, 55, 1436−1440. (3) Gu, C.; Huang, N.; Chen, Y.; Qin, L.; Xu, H.; Zhang, S.; Li, F.; Ma, Y.; Jiang, D. π-Conjugated Microporous Polymer Films: Designed Synthesis, Conducting Properties, and Photoenergy Conversions. Angew. Chem., Int. Ed. 2015, 54, 13594−13598. (4) Bin, H.; Zhang, Z. G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138, 4657−4664. (5) Lin, Y.; Zhao, F.; He, Q.; Huo, L.; Wu, Y.; Parker, T. C.; Ma, W.; Sun, Y.; Wang, C.; Zhu, D.; Heeger, A. J.; Marder, S. R.; Zhan, X. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc. 2016, 138, 4955−4961. (6) Huo, L.; Liu, T.; Sun, X.; Cai, Y.; Heeger, A. J.; Sun, Y. Singlejunction Organic Solar Cells Based on a Novel Wide-bandgap Polymer with Efficiency of 9.7%. Adv. Mater. 2015, 27, 2938−2944. (7) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-efficiency Polymer solar cells. Nat. Commun. 2014, 5, 5293−5230. (8) Ma, W.; Yang, G. F.; Jiang, K.; Carpenter, J. H.; Wu, Y.; Meng, X. Y.; McAfee, T.; Zhao, J. B.; Zhu, C. H.; Wang, C.; Ade, H.; Yan, H. Influence of Processing Parameters and Molecular Weight on the Morphology and Properties of High-Performance PffBT4T2OD:PC71BM Organic Solar Cells. Adv. Energy Mater. 2015, 5, 1501400. (9) Zhao, J.; Li, Y.; Hunt, A.; Zhang, J.; Yao, H.; Li, Z.; Zhang, J.; Huang, F.; Ade, H.; Yan, H. A Difluorobenzoxadiazole Building Block for Efficient Polymer Solar Cells. Adv. Mater. 2016, 28, 1868−1873. (10) Kan, B.; Li, M.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Long, G.; Yang, X.; Feng, H.; Zuo, Y.; Zhang, M.; Huang, F.; Cao, Y.; Russell, T. P.; Chen, Y. A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency. J. Am. Chem. Soc. 2015, 137, 3886−3893. (11) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang, F.; Qu, Y.; Ma, W.; Yan, H. Terthiophene-based D-A Polymer with an Asymmetric Arrangement of Alkyl Chains that Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 14149−14157. (12) Chen, C. C.; Chang, W. H.; Yoshimura, K.; Ohya, K.; You, J.; Gao, J.; Hong, Z.; Yang, Y. An Efficient Triple-junction Polymer Solar Cell Having a Power Conversion Efficiency Exceeding 11%. Adv. Mater. 2014, 26, 5670−5677. (13) Yusoff, A. R. b. M.; Kim, D.; Kim, H. P.; Shneider, F. K.; da Silva, W. J.; Jang, J. A High Efficiency Solution Processed Polymer Inverted Triple-junction Solar Cell Exhibiting a Power Conversion Efficiency of 11.83%. Energy Environ. Sci. 2015, 8, 303−316. (14) Zhang, K.; Gao, K.; Xia, R.; Wu, Z.; Sun, C.; Cao, J.; Qian, L.; Li, W.; Liu, S.; Huang, F.; Peng, X.; Ding, L.; Yip, H. L.; Cao, Y. HighPerformance Polymer Tandem Solar Cells Employing a New n-Type Conjugated Polymer as an Interconnecting Layer. Adv. Mater. 2016, 28, 4817−4823. (15) Liao, Q.; Cao, J.; Xiao, Z.; Liao, J.; Ding, L. Donor-acceptor Conjugated Polymers Based on a Pentacyclic Aromatic Lactam Acceptor Unit for Polymer Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 19990−19993. (16) Kroon, R.; Diaz de Zerio Mendaza, A.; Himmelberger, S.; Bergqvist, J.; Backe, O.; Faria, G. C.; Gao, F.; Obaid, A.; Zhuang, W.; Gedefaw, D.; Olsson, E.; Inganas, O.; Salleo, A.; Muller, C.; Andersson, M. R. A New Tetracyclic Lactam Building Block for Thick, Broad-bandgap Photovoltaics. J. Am. Chem. Soc. 2014, 136, 11578−11581. (17) Schneider, A. M.; Lu, L.; Manley, E. F.; Zheng, T.; Sharapov, V.; Xu, T.; Marks, T. J.; Chen, L. X.; Yu, L. Wide Bandgap OPV Polymers Based on Pyridinonedithiophene Unit with Efficiency > 5%. Chem. Sci. 2015, 6, 4860−4866. (18) Wolf, J.; Cruciani, F.; El Labban, A.; Beaujuge, P. M. Wide Band-Gap 3,4-Difluorothiophene-Based Polymer with 7% Solar Cell

to achieve such high efficiency for both polymers. Furthermore, DBND-TVT based OSCs maintain a PCE around 6.0% even when active layer thickness reaches about 240 nm, and show little sensibility toward the weight ratios of polymer/fullerene (ranges from 1:1.5 to 1:3), which makes it an excellent candidate for the fabrication of large-area OSCs. A morphology study and (2D) GIWAXS analysis revealed that the fully planar DBND structure might account for the high crystallinity of the polymer, especially for PDBND-TVT, which favors the close packing of the polymer backbones and the formation of interpenetrating nanofibril networks, which leads to the high PCE. Despite their large Eg values and quite limited light absorption window, the realization of such high PCEs for both PDBND-2T and PDBND-TVT based OSCs is very encouraging, which implies there is still great potential to improve further the efficiency of WBG polymers by the careful design of the acceptor and donor building blocks and the device structures. The further manipulation on the DBND core structure, the copolymerization with other electron donating building blocks, and the fabrication of tandem and tertiary OSCs based on such materials are ongoing in our laboratory, which will be reported in due course.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02225. General procedures and experimental details; monomer and polymer synthesis; GPC, TGA traces; UV−vis spectra and cyclic voltammetry measurement; AFM, TEM, and GIWAXS patterns; SCLC results and 1H and 13 C NMR spectra (PDF).

■

AUTHOR INFORMATION

Corresponding Authors

*X. Wang. E-mail: [email protected]. *R. Yang. E-mail: [email protected]. *X. Wan. E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the paper. M. Cai and X. Bao contributed equally to this paper. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the research financial support from “100 talents” program from Chinese Academy of Sciences, and also the supports from National Science Foundation of China (NSFC 51573204, 21402220, and 51573205). X. Bao thanks the Youth Innovation Promotion Association CAS for financial support (2016194).

■

REFERENCES

(1) Davy, N. C.; Man, G.; Kerner, R. A.; Fusella, M. A.; Purdum, G. E.; Sezen, M.; Rand, B. P.; Kahn, A.; Loo, Y. L. Contorted Hexabenzocoronenes with Extended Heterocyclic Moieties Improve Visible-Light Absorption and Performance in Organic Solar Cells. Chem. Mater. 2016, 28, 673−681. (2) Dou, C.; Long, X.; Ding, Z.; Xie, Z.; Liu, J.; Wang, L. An Electron-Deficient Building Block Based on the B←N Unit: An 6204

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials Efficiency: An Alternative to P3HT. Chem. Mater. 2015, 27, 4184− 4187. (19) Lin, Y.; He, Q.; Zhao, F.; Huo, L.; Mai, J.; Lu, X.; Su, C. J.; Li, T.; Wang, J.; Zhu, J.; Sun, Y.; Wang, C.; Zhan, X. A Facile Planar Fused-Ring Electron Acceptor for As-Cast Polymer Solar Cells with 8.71% Efficiency. J. Am. Chem. Soc. 2016, 138, 2973−2976. (20) Zhu, X.; Fang, J.; Lu, K.; Zhang, J.; Zhu, L.; Zhao, Y.; Shuai, Z.; Wei, Z. Naphtho[1,2-b:5,6-b′]dithiophene Based Two-Dimensional Conjugated Polymers for Highly Efficient Thick-Film Inverted Polymer Solar Cells. Chem. Mater. 2014, 26, 6947−6954. (21) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. Fluorine Substituted Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in Polymer-fullerene Solar Cells. J. Am. Chem. Soc. 2011, 133, 4625−4631. (22) Lei, T.; Cao, Y.; Zhou, X.; Peng, Y.; Bian, J.; Pei, J. Systematic Investigation of Isoindigo-Based Polymeric Field-Effect Transistors: Design Strategy and Impact of Polymer Symmetry and Backbone Curvature. Chem. Mater. 2012, 24, 1762−1770. (23) Deng, Y.; Liu, J.; Wang, J.; Liu, L.; Li, W.; Tian, H.; Zhang, X.; Xie, Z.; Geng, Y.; Wang, F. Dithienocarbazole and Isoindigo Based Amorphous Low Bandgap Conjugated Polymers for Efficient Polymer Solar Cells. Adv. Mater. 2014, 26, 471−476. (24) Cao, J.; Liao, Q.; Du, X.; Chen, J.; Xiao, Z.; Zuo, Q.; Ding, L. A Pentacyclic Aromatic Lactam Building Block for Efficient Polymer Solar Cells. Energy Environ. Sci. 2013, 6, 3224. (25) Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10−27. (26) Xu, S.; Ai, N.; Zheng, J.; Zhao, N.; Lan, Z.; Wen, L.; Wang, X.; Pei, J.; Wan, X. Extended Isoindigo core: Synthesis and Applications as Solution-processable n-OFET Materials in Ambient Conditions. RSC Adv. 2015, 5, 8340−8344. (27) Zhao, N.; Ai, N.; Cai, M.; Wang, X.; Pei, J.; Wan, X. Thiophenefused Isoindigo Based Conjugated Polymers for Ambipolar Organic Field-Effect Transistors. Polym. Chem. 2016, 7, 235−243. (28) Papageorgiou, C.; Borer, X. Acid-Catalyzed Rearrangements for a Diastereoselective Entry into a New Fused Hexacyclic Heterocycle: (5RS,7aRS,12RS,14aRS,)-4,5,7a,11,12,14,14a-octahydro- 5,12-dimethyl-diindolo [1,7-bc:1′,7′-gh][2,6] naphthyridine. Helv. Chim. Acta 1988, 71, 1079−1083. (29) Dubovyk, I.; Pichugin, D.; Yudin, A. K. Palladium-catalyzed Ring-contraction and Ring-expansion Reactions of Cyclic Allyl Amines. Angew. Chem., Int. Ed. 2011, 50, 5924−5926. (30) Gopinath, K. W.; Govindachari, T. R.; Rajappa, S. Synthesis of Calycanine. Tetrahedron 1960, 8, 291−292. (31) Kroon, R.; Diaz de Zerio Mendaza, A.; Himmelberger, S.; Bergqvist, J.; Backe, O.; Faria, G. C.; Gao, F.; Obaid, A.; Zhuang, W.; Gedefaw, D.; Olsson, E.; Inganas, O.; Salleo, A.; Muller, C.; Andersson, M. R. Correction to “A New Tetracyclic Lactam Building Block for Thick, Broad-bandgap Photovoltaics”. J. Am. Chem. Soc. 2015, 137, 550−550. (32) He, B.; Pun, A. B.; Klivansky, L. M.; McGough, A. M.; Ye, Y.; Zhu, J.; Guo, J.; Teat, S. J.; Liu, Y. Thiophene Fused Azacoronenes: Regioselective Synthesis, Self-Organization, Charge Transport and Its Incorporation in Conjugated Polymers. Chem. Mater. 2014, 26, 3920− 3927. (33) Lei, T.; Cao, Y.; Fan, Y.; Liu, C. J.; Yuan, S. C.; Pei, J. Highperformance Air-stable Organic Field-effect Transistors: Isoindigobased Conjugated Polymers. J. Am. Chem. Soc. 2011, 133, 6099−6101. (34) Lei, T.; Xia, X.; Wang, J. Y.; Liu, C. J.; Pei, J. ″Conformation Locked″ Strong Electron-Deficient Poly(p-phenylene vinylene) Derivatives for Ambient-Stable n-Type Field-effect Transistors: Synthesis, Properties, and Effects of Fluorine Substitution Position. J. Am. Chem. Soc. 2014, 136, 2135−2141. (35) Brédas, J. Mind the Gap! Mater. Horiz. 2014, 1, 17−19. (36) Korzdorfer, T.; Brédas, J. L. Organic Electronic Materials: Recent Advances in the DFT Description of the Ground and Excited States Using Tuned Range-separated Hybrid Functionals. Acc. Chem. Res. 2014, 47, 3284−3291.

(37) Chai, J. D.; Head-Gordon, M. Long-range Corrected Hybrid Density Functionals with Damped Atom-atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (38) Do, K.; Saleem, Q.; Ravva, M. K.; Cruciani, F.; Kan, Z.; Wolf, J.; Hansen, M. R.; Beaujuge, P. M.; Bredas, J. L. Impact of Fluorine Substituents on π-Conjugated Polymer Main-Chain Conformations, Packing, and Electronic Couplings. Adv. Mater. 2016, DOI: 10.1002/ adma.201601282. (39) Ashraf, R. S.; Meager, I.; Nikolka, M.; Kirkus, M.; Planells, M.; Schroeder, B. C.; Holliday, S.; Hurhangee, M.; Nielsen, C. B.; Sirringhaus, H.; McCulloch, I. Chalcogenophene Comonomer Comparison in Small Band Gap Diketopyrrolopyrrole-based Conjugated Polymers for High-performing Field-effect Transistors and Organic Solar Cells. J. Am. Chem. Soc. 2015, 137, 1314−1321. (40) Bhuwalka, A.; Ewan, M. D.; Elshobaki, M.; Mike, J. F.; Tlach, B.; Chaudhary, S.; Jeffries-El, M. Synthesis and Photovoltaic Properties of 2,6-Bis(2-thienyl)benzobisazole and 4,8-Bis(thienyl)- benzo[1,2-b:4,5b′]dithiophene Copolymers. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 316−324. (41) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Anthracene-Based Medium Bandgap Conjugated Polymers for High Performance Polymer Solar Cells Exceeding 8% PCE Without Additive and Annealing Process. Adv. Energy Mater. 2015, 5, 1500065. (42) Kim, J.; Kim, H.; Kim, G.; Back, H.; Lee, K. Soluble Transition Metal Oxide/polymeric Acid Composites for Efficient Hole-transport Layers in Polymer Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 951−957. (43) Liu, J.; Kim, G. H.; Xue, Y.; Kim, J. Y.; Baek, J. B.; Durstock, M.; Dai, L. Graphene Oxide Nanoribbon as Hole Extraction Layer to Enhance Efficiency and Stability of Polymer Solar Cells. Adv. Mater. 2014, 26, 786−790. (44) Zhou, H.; Zhang, Y.; Mai, C. K.; Collins, S. D.; Nguyen, T. Q.; Bazan, G. C.; Heeger, A. J. Conductive Conjugated Polyelectrolyte as Hole-transporting Layer for Organic Bulk Heterojunction Solar Cells. Adv. Mater. 2014, 26, 780−785. (45) Bao, X.; Zhu, Q.; Wang, T.; Guo, J.; Yang, C.; Yu, D.; Wang, N.; Chen, W.; Yang, R. Simple O2 Plasma-processed V2O5 as an Anode Buffer Layer for High-performance Polymer Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 7613−7618. (46) Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Labban, A. E.; Tassone, C. J.; Toney, M. F.; Fréchet, J. M. J.; Beaujuge, P. M.; McGehee, M. D. Controlling Solution-Phase Polymer Aggregation with Molecular Weight and Solvent Additives to Optimize PolymerFullerene Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2014, 4, 1301733. (47) Intemann, J. J.; Yao, K.; Yip, H.; Xu, Y.; Li, Y.; Liang, P.; Ding, F.; Li, X.; Jen, A. K. Y. Molecular Weight Effect on the Absorption, Charge Carrier Mobility, and Photovoltaic Performance of an Indacenodiselenophene-Based Ladder-Type Polymer. Chem. Mater. 2013, 25, 3188−3195. (48) Lee, H. K. H.; Li, Z.; Constantinou, I.; So, F.; Tsang, S. W.; So, S. K. Batch-to-Batch Variation of Polymeric Photovoltaic Materials: its Origin and Impacts on Charge Carrier Transport and Device Performances. Adv. Energy Mater. 2014, 4, 1400768. (49) Hendriks, K. H.; Heintges, G. H.; Gevaerts, V. S.; Wienk, M. M.; Janssen, R. A. High-molecular-weight Regular Alternating Diketopyrrolopyrrole-based Terpolymers for Efficient Organic Solar Cells. Angew. Chem., Int. Ed. 2013, 52, 8341−8344. (50) Liu, C.; Wang, K.; Hu, X.; Yang, Y.; Hsu, C. H.; Zhang, W.; Xiao, S.; Gong, X.; Cao, Y. Molecular Weight Effect on the Efficiency of Polymer Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 12163− 12167. (51) Subbiah, J.; Purushothaman, B.; Chen, M.; Qin, T.; Gao, M.; Vak, D.; Scholes, F. H.; Chen, X.; Watkins, S. E.; Wilson, G. J.; Holmes, A. B.; Wong, W. W.; Jones, D. J. Organic Solar Cells using a High-molecular-weight Benzodithiophene-benzothiadiazole Copolymer with an Efficiency of 9.4%. Adv. Mater. 2015, 27, 702−705. (52) Kang, H.; Uddin, M. A.; Lee, C.; Kim, K. H.; Nguyen, T. L.; Lee, W.; Li, Y.; Wang, C.; Woo, H. Y.; Kim, B. J. Determining the Role of 6205

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206

Article

Chemistry of Materials Polymer Molecular Weight for High-performance All-polymer Solar Cells: its Effect on Polymer Aggregation and Phase Separation. J. Am. Chem. Soc. 2015, 137, 2359−2365. (53) Jao, M.; Liao, H.; Su, W. Achieving a High Fill Factor for Organic Solar Cells. J. Mater. Chem. A 2016, 4, 5784−5801. (54) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-efficiency Solution Processable Polymer Photovoltaic Cells by Self-organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. (55) Lee, J.; Sagawa, T.; Yoshikawa, S. Thickness Dependence of Photovoltaic Performance of Additional Spray Coated Solar Cells. Thin Solid Films 2013, 529, 464−469. (56) Li, W.; Hendriks, K. H.; Furlan, A.; Roelofs, W. S.; Wienk, M. M.; Janssen, R. A. Universal Correlation Between Fibril Width and Quantum Efficiency in Diketopyrrolopyrrole-based Polymer Solar Cells. J. Am. Chem. Soc. 2013, 135, 18942−18948. (57) Drees, M.; Hoppe, H.; Winder, C.; Neugebauer, H.; Sariciftci, N. S.; Schwinger, W.; Schäffler, F.; Topf, C.; Scharber, M. C.; Zhu, Z.; Gaudiana, R. Stabilization of the Nanomorphology of Polymer− fullerene “Bulk Heterojunction” Blends Using a Novel Polymerizable Fullerene Derivative. J. Mater. Chem. 2005, 15, 5158−5163. (58) Shaw, P. E.; Ruseckas, A.; Samuel, I. D. W. Exciton Diffusion Measurements in Poly(3-hexylthiophene). Adv. Mater. 2008, 20, 3516−3520. (59) van Franeker, J. J.; Heintges, G. H.; Schaefer, C.; Portale, G.; Li, W.; Wienk, M. M.; van der Schoot, P.; Janssen, R. A. Polymer Solar Cells: Solubility Controls Fiber Network Formation. J. Am. Chem. Soc. 2015, 137, 11783−11794.

6206

DOI: 10.1021/acs.chemmater.6b02225 Chem. Mater. 2016, 28, 6196−6206