Naphthodithiophenes: emerging building blocks for organic electronics.

Personal Account DOI: 10.1002/tcr.201402051

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Naphthodithiophenes: Emerging Building Blocks for Organic Electronics Kazuo Takimiya*[a,b] and Itaru Osaka[a] Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198 (Japan), E-mail: [email protected] [b] Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527 (Japan)

[a]

Received: May 31, 2014 Published online: ■■

ABSTRACT: Linear-fused naphthodithiophenes (NDTs) are emerging building blocks in the development of new semiconducting small molecules, oligomers, and polymers. The promising nature of NDT-based materials as organic semiconductors has been demonstrated by superior device characteristics in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs) in the last few years. In particular, it is quite impressive that a power conversion efficiency as high as 8.2% has been achieved for a single-junction OPV cell consisting of NDT-based semiconducting polymers and a fullerene derivative in such a short period of time. Here, we provide an overview of recent synthetic evolutions in NDT chemistry and progress in NDT-based materials, especially conjugated oligomers and polymers and their applications to OFETs and OPVs. Keywords: molecular electronics, naphthodithiophenes, organic field-effect transistors, organic photovoltaics, semiconductors

1. Introduction Acenes and oligo/polythiophenes are two major prototypical structures of p-type organic semiconductors, and they have been extensively studied since the dawn of organic electronics.[1] Thienoacenes, fused thiophene/benzene ring systems, on the other hand, can be regarded as “hybrids” of acenes and oligothiophenes, and have been focused on since the late 1990s with the expectation that they would afford high-performance p-channel organic field-effect transistors (OFETs).[2] In fact, superior p-channel OFETs have been achieved with thienoacene-based small-molecule organic semiconductors; recently, exceptionally high mobilities of up to 43 cm2 V−1 s−1 have been realized in solution-processed OFET devices.[3] These impressively high mobilities achieved with thienoacenes are rationalized by their rigid and planar molecular structures

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and the incorporated sulfur atoms with large atomic radii, both of which facilitate effective intermolecular orbital overlap in the thin-film or solid state.[2] For the development of materials for organic photovoltaics (OPVs), on the other hand, one of the most important material classes is semiconducting polymers and oligomers, which can act as p-type semiconductors when combined with fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), as the n-type semiconductor in the bulkheterojunction (BHJ) photoactive layer in OPVs.[4] Poly(3hexylthiophene) (P3HT) has been employed as a p-type semiconductor since the early stages of OPV research. However, its relatively large HOMO–LUMO gap (Eg), in other words, its narrow photoabsorption range up to ca.

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Fig. 1. Chemical structures of (a) benzo[1,2-b:4,5-b′]dithiophene (BDT), (b) PTB7, and (c) linear-fused naphthodithiophenes (NDTs) with the numbering schemes. Although different abbreviations for isomeric NDTs are used by several groups, the simple abbreviated expression NDTn (n = 1–4) for naphtho[2,3-b:6,7-b′]dithiophene (NDT1), naphtho[2,3-b:7,6-b′]dithiophene (NDT2), naphtho[1,2-b:5,6-b′]dithiophene (NDT3), and naphtho[2,1b:6,5-b′]dithiophene (NDT4), respectively, is used in this paper. Note that the number, n, has no scientific significance.

650 nm, and its high-lying HOMO energy level (EHOMO, ∼4.7 eV below the vacuum level), limit short-circuit current density (JSC) and open-circuit voltage (VOC), respectively, resulting in relatively low power conversion efficiency (PCE).[5] A current strategy for developing p-type materials is thus to tune the electronic structure so as to have a small Eg while keeping a low-lying EHOMO.[6] To realize such an electronic structure, the most successful molecular design is to construct a “donor–acceptor (D–A)” structure, where electron-donating (donor unit or D unit) and electron-deficient (acceptor unit or A unit) π units are alternately linked in the conjugated backbone.[7]

In the molecular design of semiconducting polymers and oligomers with the D–A architecture, the choice of each building unit is essentially important, because the electronic structure of each unit can determine the Eg and EHOMO of the resulting material.[8] Among a variety of D units so far examined in the development of p-type semiconducting polymers for OPV applications, one of the most familiar ones is tricyclic benzo[1,2-b:4,5-b′]dithiophene (BDT, Figure 1a),[9] one of the basic thienoacene structures.[2] The characteristic features of BDT are its rigid structure, relatively high-lying EHOMO, and ease of functionalization at the 4,8-positions allowing various molecular modifications. The most prominent example among

Kazuo Takimiya received his Ph.D. in 1994 from Hiroshima University under the supervision of Prof. Fumio Ogura. He then joined Prof. Tetsuo Otsubo’s research group at Hiroshima University, where he carried out research into organic conductors/superconductors. After returning from a stay with Prof. Jan Becher’s group at Odense University, Denmark (1997– 1998), he was promoted to associate professor in 2003. In 2007, he became full professor at Hiroshima University. In 2013, his group moved to the RIKEN Center for Emergent Matter Science (CEMS). His research interests include the synthesis and characterization of organic semiconductors and their application to organic electronics.

Itaru Osaka received his doctoral degree from the University of Tsukuba in 2002 under the supervision of Prof. Kazuo Akagi and Prof. Hideki Shirakawa. After four years of research at Fujifilm Co. Ltd., he joined the group of Prof. Richard D. McCullough at Carnegie Mellon University as a postdoctoral fellow. He started his professional career in Prof. Takimiya’s group at the Department of Applied Chemistry, Hiroshima University, as an assistant professor in 2009, and moved to RIKEN CEMS as a senior research scientist in 2013. His current research interests include the molecular design and synthesis of functional conjugated polymers, self-assembly, and printable organic thin-film devices, such as field-effect transistors and solar cells.

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the BDT-based p-type semiconductors for OPV applications would be PTB7 (Figure 1b) developed by Yu and co-workers, affording among the highest PCEs (>9%).[10] From the viewpoint of molecular structure, BDT is classified as an acenedithiophene (AcDT), where two thiophene rings sandwich one acene core,[2] and AcDTs including pentacyclic anthra[2,3-b:6,7(7,6)-b′]dithiophenes (ADT)[11] and additional larger analogues[12] have been investigated as an important platform for developing small-molecule organic semiconductors. To our surprise, however, naphthodithiophenes (NDTs, Figure 1c),[13] tetracyclic AcDTs, had been a less studied molecular system until 2010. This was because of their poor accessibility due to a lack of suitable and reliable syntheses. During the course of our synthetic studies on thienoacenes,[14] we were interested in the development of methods for the synthesis of NDTs, and established an effective synthesis of naphtho[1,2b:5,6-b′]dithiophene (NDT3) derivatives in 2010.[15] In addition, our initial trials of their application as organic semiconductors demonstrated that NDT3 is a promising building block for the development of p-type organic semiconductors.[15] Other isomeric NDTs, i.e., naphtho[2,3b:6,7-b′]dithiophene (NDT1) and naphtho[2,1-b:6,5b′]dithiophene (NDT4) were also examined and revealed to be promising building blocks in small-molecule p-channel organic semiconductors and semiconducting polymers for OFET applications.[16] Since then, not just OFETs, but high-performance OPVs derived from NDT-based oligomers and polymers have come to be developed by several groups. Thus, it is worthwhile to overview the recent rapid progress in NDT-based organic semiconductors and their devices. Since we have already discussed in depth the NDT-based small-molecule organic semiconductors and simple NDT–bithiophene copolymers for OFET applications,[17] this review mainly focuses on semiconducting oligomers and polymers with the D–A architecture, together with the synthesis of NDT-based building blocks.

2. Key Synthetic Achievements for the Development of NDT-Based Materials As mentioned above, only very limited methods for the synthesis of linear-fused NDTs had been reported before 2010.[15] Although Tilak described the synthesis of NDT4 via an acidinduced cyclodehydration using polyphosphoric acid (PPA) to form fused thiophene rings from 2,6-bis(2,2dimethoxyethylthio)naphthalene (Scheme 1a) in 1951,[18] the low isolated yield (16%) after tedious workup seems to prevent its use as a building block for organic electronic materials. Tobe and co-workers also reported the synthesis of various linearfused NDT derivatives via flash vacuum pyrolysis (FVP; Scheme 1b).[19] However, this method afforded a mixture of a range of NDT isomers, including NDT3 or NDT4 depending

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Scheme 1. Synthesis of linear-fused NDT derivatives reported before 2010.

on the starting material, and gave only moderate yields, and thus cannot deliver practical amounts of material suitable for further applications. Since 2010, different approaches to the synthesis of linearfused NDT derivatives have been independently developed by several groups, each of which includes a distinct key reaction such as: (1) sodium sulfide (Na2S)-promoted thienannulation reaction of an o-ethynyl-halobenzene substructure (Scheme 2),[15a,16a,20] (2) acid-induced cyclodehydration of alkyloxy-substituted naphthalenes to form fused thiophene rings (Scheme 3),[21] (3) intramolecular Knoevenagel condensation and dehydration to form fused thiophene rings on a naphthalene core followed by a decarboxylation reaction (Scheme 4),[22] and (4) base-induced tandem 6π cyclization to form the central naphthalene ring from enediyne moieties (Scheme 5).[23]



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Scheme 2. Na2S-promoted thienannulation reactions to form four isomeric NDTs with or without substituents at the thiophene α-positions (2,7-positions on the NDT cores).

Scheme 3. Acid-induced cyclodehydration to synthesize 5,10-dialkyloxy NDT1 and 4,9-dialkyloxy NDT3.

The first approach can selectively afford four isomeric NDTs from dihydroxynaphthalene derivatives via a selective halogenation, triflation, Sonogashira cross-coupling reaction, and the final Na2S-promoted thienannulation reaction. The merits of this approach are: (1) easy availability of the starting materials, (2) high reproducibility of the thienannulation reaction, and (3) scalability of the synthesis. It is also important to mention that, of the four approaches, only this method allowed the synthesis of the parent system. On the other hand, the unsubstituted NDT units render the resulting oligomers and polymers less soluble. In this context, a

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method for selective functionalization at the 5,10-positions of NDT1, NDT3 and NDT4 was recently developed that makes the NDT units more valuable for material development (vide infra). The second approach (Scheme 3) reported by Loser and co-workers features acid-induced cyclodehydration, which is basically the same as Tilak’s early synthesis of parent NDT4 (Scheme 1),[18] and is well designed to take advantage of the presence of alkyloxy groups introduced on the starting naphthalene derivatives. In fact, the alkyloxy groups do not just act as solubilizing groups in the resulting NDT derivatives, but regulate the reaction sites in the cyclodehydration reaction, allowing selective formation of 5,10-alkyloxy-substituted NDT1 and 4,9-alkyloxysubstituted NDT3 structures. The key reaction in the third approach (Scheme 4), an intramolecular Knoevenagel condensation, has often been employed in the synthesis of fused thiophene structures on various aromatic rings, such as benzene and thiophene.[24] It is interesting to note that the 4,9-dialkyloxy NDT1 synthesized by this approach, which is an isomer of the 5,10dialkyloxy NDT1 synthesized by the second approach, is very difficult to synthesize by the second approach, and vice versa. Thus, it can be said that these two methods are complementary in the materials synthesis of isomeric dialkyloxy NDT1s.



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HS

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O OEt K2CO3

Br

OHC Br

CHO O

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O S CO2Et

EtO2C S

55%

67% O

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1) LiOH 2) Cu, Δ

O

9 S

S O

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4 R

Scheme 4. Intramolecular Knoevenagel condensation and dehydration to form 4,9-dialkyloxy NDT1.

C10H21

C10H21 C10H21 10

9 C10H21 S

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DBU

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DBU

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C10H21 9 C10H21 DBU

S

S S

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DBU

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73%

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4 C10H21

C10H21 10 S

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83%

5C H 10 21 C10H21

C10H21

Scheme 5. Base-induced tandem 6π cyclization to form 4,9- and 5,10-dialkyl-substituted NDT3 and NDT4.

As discussed above, the selective introduction of substituents on NDT cores is not an easy task. However, Cheng and co-workers have recently reported straightforward and selective syntheses of a series of 4,9- and 5,10-dialkylated NDT3 and NDT4 derivatives, where they employed a base-induced tandem 6π cyclization from enediyne derivatives with two thiophene rings at appropriate positions (Scheme 5). Regardless of the NDT core or substitution positions, the formation of the central naphthalene part proceeded smoothly, affording dialkylated NDT3 and NDT4 in excellent yields. Note that this approach is a unique one for the practical synthesis of NDTs from two outer thiophene rings, involving construction of the central naphthalene moiety at the final stage. In addition to these new synthetic approaches to the NDT cores, a method for selective functionalization of the parent NDT systems was also developed, the basic concept of which is demonstrated with the NDT3 core in Scheme 6.[25] After the protection of the thiophene α-positions by introducing triisopropylsilyl (TIPS) groups, the direct borylation reaction catalyzed by an Ir complex in the presence of bis(pinacolato)diboron afforded 5,10-bis(4,4,5,5-tetramethyl1,3,2-dioxaborolan-2-yl)-2,7-bis(triisopropylsilyl)-NDT3.[26] The boryl groups on the tetrafunctionalized NDT3 can act as the handle for functional group conversions to 5,10-

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dihalogen, nitrile, hydroxyl, alkyl, ester, and alkyloxy derivatives (Scheme 6a). NDT1 and NDT4 could also be functionalized in a similar manner, though the NDT2 core did not allow the introduction of two bulky boryl groups on the naphthalene peri positions owing to the steric hindrance caused by the peri hydrogens (Scheme 6b). These newly established syntheses allow us practical access to various NDT building blocks and indeed have paved the way to the development of new NDT-based materials, in particular conjugated oligomers and polymers that are applicable to OFET and OPV devices. Incorporation of the NDT cores having a rigid, extended π system with four fused aromatic rings, however, generally reduces the solubility of the resulting material, and thus the introduction of solubilizing groups is very important both for the synthesis/ purification and application of the resulting oligomers and polymers to the devices. From this viewpoint, the NDT units with solubilizing alkyl or alkyloxy substituents are particularly useful. On the other hand, electronic effects from these substituents can have an impact on the energy levels of the HOMO (EHOMO) and LUMO (ELUMO) of the resulting materials. In the case of NDT3, the EHOMO of 5,10-dialkyloxy derivatives shifts upward by as much as 0.4 eV, whereas the 5,10-dialkyl



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(a) [Ir(OMe)(COD)]2 (5 mol %), dtbpy

1) BuLi 2) TIPS-Cl TIPS

NDT3

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CuBr2 CuCl2 Zn(CN)2, CsF, Cu(NO2)3 Oxone

OH

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yield / % 96 94 22 91

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C16H33 (from X = Br) CO2CH3 (from X = Br) OC12H25 (from X = OH)

9-BBN, 1-hexadecene 90 BuLi, ClCO2CH3 41 K2CO3, C12H25Br quant

(b) 10

BPin 10

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5

TIPS

TIPS

NDT1

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S

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BPin

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BPin

TIPS 5 BPin

NDT2

NDT4

Scheme 6. (a) Synthesis of orthogonally functionalized NDT3 and selective functional group conversion, and (b) molecular structures of functionalized NDT1, NDT2, and NDT4.

derivatives show a smaller shift (∼0.1 eV), reflecting the stronger electron-donating character of the alkyloxy group than that of the alkyl group.[25] It should also be noted that the position of the substituents also affects the extent of the EHOMO shift. When comparing the 4,9- and 5,10-dialkyl derivatives, a smaller shift of EHOMO was observed for the 5,10-derivative.[23] This can be qualitatively explained by the fact that the electron density of the HOMO of NDT3 is smaller at the 5- and 10-positions than at the 4- and 9-positions. Although these electronic effects from substituents at the stage of NDT units can be diluted by incorporation into large conjugated systems such as oligomers and polymers (vide infra), the EHOMO of electronic materials often plays a vital role in determining open-circuit voltage (VOC) in OPVs and air stability and threshold voltage (Vth) in OFETs, and thus one should pay attention to the choice of the position and type of substituents.

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3. Application of NDT Units to Organic Semiconductors: From Small Molecules to Polymers Small Molecules As the parent NDT building blocks have EHOMO values of 5.3–5.8 eV below the vacuum level, NDT-based materials can act as p-type organic semiconductors.[8] In fact, 2,7-diphenyl NDT derivatives and NDT-based copolymers with bithiophene co-monomer units exhibited field-effect hole mobilities as high as 1.5 cm2 V−1 s−1 and 0.77 cm2 V−1 s−1, respectively, in thin-film transistor settings, indicating experimentally that the NDT cores are useful building units for p-type organic semiconductors.[16] Furthermore, we investigated in depth the isomeric effects of the NDT cores in developing small molecular or polymeric semiconductors; the



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Fig. 2. (a) Two dicyanomethylene-terminated quinoidal NDT3 derivatives with low-lying LUMO energy levels and (b) typical n-channel OFET characteristics obtained from quinoidal NDT3 derivatives (left, output, and right, transfer curves). Adapted from reference [27b] with permission (© 2014 American Chemical Society).

NDT1 core is suitable for the small molecular organic semiconductors, whereas the NDT3 core is appropriate for polymeric semiconductors.[17] As these results have already been reviewed thoroughly,[17] here we show n-type semiconductors that are derived from the NDT3 and NDT4 cores by incorporating quinoidal strands terminated with dicyanomethylene moieties (Figure 2a).[27] Although the colors of the quinoidal NDT3 derivatives are greatly different depending on the manner of introduction of the quinoidal strand, their ELUMO values are ca. 4.6 eV below the vacuum level, which is suitable for stable n-channel FET operation under ambient conditions. In fact, relatively high electron mobilities (μe) of up to 0.1 cm2 V−1 s−1 were reported for the top-gate, bottom-contact OFETs using one of the NDT-based quinoidal compounds as the active layer (Figure 2b).

O N

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R = 2-ethylhexyl R O

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Several NDT-based oligomers with acceptor–donor–acceptor (A–D–A) or donor–acceptor–donor–acceptor–donor (D–A– D–A–D) motifs, in which NDT was used as the donor unit, were synthesized and evaluated as p-type organic semiconductors for OFET and OPV applications (Figures 3 and 4, Table 1). Loser and co-workers developed the A–D–A-type

N

S

Oligomers

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O

S

C10H21 O3

Fig. 3. NDT-based oligomers with the A–D–A motif.



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C10H21 S

N S N

N

S

S

S N

C10H21 C10H21

C10H21

S

S S

N

N

O4

S S

S

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C10H21

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C10H21

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C10H21 O5

C10H21

C10H21 C10H21 N

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C10H21 O6

C10H21 S

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C10H21

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S

S

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S

S S

S C10H21

C10H21

C10H21

C10H21 N S

S N

S N

C10H21 O7

Fig. 4. NDT-based oligomers with the D–A–D–A–D motif.

Table 1. Electronic structure, power conversion efficiency of OPVs, and mobility in OFETs of O1–O7. Compound

EHOMO / eV[a]

ELUMO / eV[a]

Egopt / eV[c]

PCE / %[d]

μhFET / cm2 V−1 s−1

Ref.

−5.21 −5.11 −5.16 −5.34 −5.2 −5.22 −5.23

−3.60 −3.39 −3.18 −3.34 (−3.21)[b] (−3.2)[b] (−3.23)[b]

1.70 1.70 2.16 1.96 1.99 2.02 2.00

4.0[e] 4.7[e] 0.98 2.20 1.09 1.62 1.44,0.87[e]

0.046 0.057 – – 1.5×10−5 2×10−6 2×10−6

[21a,b] [21b] [28a] [28a] [28b] [28b] [28c]

O1 O2 O3 O4 O5 O6 O7

Electrochemically estimated EHOMO and ELUMO unless otherwise stated. [b]Values in parentheses were determined by the following equation: ELUMO = EHOMO + Egopt. Estimated from the absorption onset. [d]PC71BM was used as the n-type semiconductor unless otherwise stated. [e]PC61BM was used as the n-type semiconductor.

[a] [c]

oligomers O1 and O2 using 4,9-dialkyloxy NDT1 and NDT3, respectively, in combination with strongly electronaccepting thiophene-capped diketopyrrolopyrrole (TDPP) moieties (Figure 3).[21] It is interesting to point out that the EHOMO of O2 with the NDT3 core (−5.11 eV, Table 1) is higher than that of O1 with the NDT1 core, even though the EHOMO of the parent NDT3 (−5.8 eV) is much lower than that of NDT1 (−5.3 eV). As a similar trend was already observed for the NDT-based copolymers,[16,17] the extent of conjugation, i.e., how effectively the NDT unit can conjugate with the neighboring π system, plays a vital role for determining EHOMO

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values. The NDT3 unit tends to delocalize its frontier orbital into neighboring units, resulting in relatively high-lying EHOMO values compared with the NDT1 unit having less effective delocalization. Although the VOC of the O2-based BHJ solar cell (0.755 V) with PC61BM is lower than that of the O1-based one (0.844 V), reflecting their EHOMO values, the overall solar cell performance of the former is higher with a PCE of 4.7%, which is among the highest for solar cells using NDT-based oligomers as the p-type semiconductor material. The high PCEs of the O2-based devices are understood partially as a consequence of their higher fill factor (FF, 50.1%) than the



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O1-based one (42.7%), which is related to the higher hole mobility of the O2 thin films than the O1 thin films as evaluated by OFETs and the space-charge limited current (SCLC) plots using the hole-only diodes.[21b] In contrast to the relatively high solar cell performances of the O1- and O2-based devices, the O3-based solar cells were reported to afford poor performances with PCEs of less than 1.0%. This could be partially attributed to the Eg of greater than 2.0 eV, indicating that the choice of acceptor unit in the oligomer with the A–D–A-type structure is crucial. On the other hand, Dutta and co-workers have also developed a series of NDT3-based oligomers (O4–O7) with the D–A–D–A–D architecture, where benzo[c][1,2,5]thiadiazole (BTz), 2,2′-bisthiadiazole, or thiazolo[5,4-d]thiazole (TzTz) were employed as the A unit and triphenylamine or thiophene moieties as the outer D unit (Figure 4).[28] The oligomers were examined as the active semiconducting material in p-channel OFETs and as the p-type semiconductor in the BHJ active layer combined with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) or PC61BM in OPVs. Although the structural motif and electronic structures (EHOMO, ELUMO, and Eg, see Table 1) of O1/O2 and O3–O7 are similar, the reported mobilities in OFET devices (∼10−5 cm2 V−1 s−1) and PCEs of OPVs (up to 2.2% for solar cells of O4 combined with PC71BM) of the latter are not very outstanding compared to the devices with O1 and O2. The implication from these results is that the selection and combination of co-monomer units, which determine the electronic structure of the resulting material, are essentially important in designing NDT-based electronic materials. In particular, the introduction of strong electron-accepting units, such as TDPP, effectively reduces the Eg values, even in the A–D–A oligomers. Thus, further molecular developments with appropriate electron-accepting units, both for the A–D–A and D–A–D–A–D oligomers, are anticipated. Polymers The first NDT-based semiconducting polymers with the D–A architecture, P1, consisting of 5,10-dialkyloxy NDT1 and thieno[3,4-c]pyrrole-1,4-dione (TPD) units,[22] and P4 and P5, consisting of the NDT3 unit and BTz and naphtho[1,2b:5,6-c′]bis[1,2,5]thiadiazole (NTz) units,[29] respectively, were reported in 2012 (Figure 5). These three polymers showed promising performance as p-type semiconductors both in OPV and OFET devices. In particular, P5 presents its versatility as a high-performance p-type semiconducting material; P5-based OFETs showed hole mobility as high as 0.54 cm2 V−1 s−1, and its BHJ solar cells with PC61BM afforded a PCE close to 5% (Table 2). These superior characteristics demonstrated by the early NDT-based D–A polymers prompted researchers to develop new analogues with different A units, such as

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thieno[3,4-b]thiophene (TT), 2,5-dihydropyrrolo[3,4c]pyrrole-1,4-dione (diketopyrrolopyrrole, DPP), TzTz, quinoxaline (QX), and benzo[c][1,2,5]oxadiazole (BOz) (Figure 5).[30–37] Compared with the NDT3-based polymers, NDT1-based polymers have been less studied so far. However, Lee and co-workers developed polymers with the 5,10-dialkyloxy NDT1 unit combined with the TT units (P2 and P3) following their initial work on P1.[30] Introduction of TT units into the polymer backbone effectively lowered EHOMO, especially for fluorine-substituted TT (P3) compared to P1 possessing the TPD unit (Table 2). In fact, the low-lying EHOMO levels were beneficial to achieving higher VOC values in OPVs; the P2- and P3-based OPVs showed VOC values of ∼0.75 and ∼0.79 V, respectively, which are higher than that for P1-based OPVs (∼0.69 V). Thanks to the high VOC values, better solar cell performances with PCEs of up to 5.16% were reported for the P2- and P3-based OPVs. NDT3-based polymers with various A units were synthesized and examined as p-type semiconducting polymers (Figure 5). Among the polymers consisting of the unsubstituted NDT3 unit, e.g., P4–P12,[31,32] except for P4 and P5, the performances of the OPVs were not very high (PCE < 2.0%, Table 2). On the other hand, very high mobility (1.32 cm2 V−1 s−1) in OFETs was achieved for P6 with the DPP unit.[31] Since the DPP unit generally affords strong intermolecular interaction in the solid state[33] and consequently low solubility, particularly when combined with the largely π-extended NDT3 building block, the very large branched 2-dodecylhexadecyl (DH) group was introduced onto the nitrogen atoms in the DPP unit to enhance the solubility of the resulting polymer. In fact, this modification turned out to be very fruitful; compared to its 2-octyldodecyl (OD)-substituted counterpart, P6 with DH groups came to have a high molecular weight (Mn = 30.8 kDa), fourfold higher than that of P6 with OD. As observed for other polymers, higher molecular weight is important to achieve high mobility in OFETs,[34] and in fact, OFETs based on P6 (DH) afforded higher mobility than that of P6 (OD) by almost one order of magnitude. From these results, we can conclude that the design of polymers using the unsubstituted NDT3 unit requires not only appropriate choice of the A unit, but also careful design of the alkyl side chains in terms of their length, shape (i.e., linear or branched), number and placement for enabling solubility and good device performances. In contrast, several soluble D–A polymers containing alkyloxy- or alkyl-substituted NDT3 with DPP (P13, P14), TPD (P15), TT (P16, P17), BTz (P18), BOz (P19), and NTz (P20) as the A unit have also been reported.[35–39] Of these polymers, a comparison of P13 and P6, both of which have the same backbone structure consisting of NDT3 and DPP as the D and A units, respectively, would afford an interesting insight



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C10H21

C10H21

C8H17 O

R O

N S

X X

S O R

C8H17

O C10H21

S

C10H21

N

S O C12H25

R

O

X=H X=F

C6H13 N

S N

S

S

S n

C12H25

n

R = 2-ethylhexyl P16 P17

P15

C12H25

S

S


S O R

X

X

S

S

n

R = 2-ethylhexyl

N S

O

O

O R

X=S X=O

R O

S

N

S

S


O

R O

S

n

N

R

O C8H17 R O

X=S X=O

n N

C12H25

S

N C6H13

C10H21

P20

Fig. 5. NDT-based D–A semiconducting polymers.

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Table 2. Electronic structure, power conversion efficiency of OPVs, and mobility in OFETs of P1–P20. Compound

EHOMO / eV[a]

ELUMO / eV[a]

Egopt / eV[d]

PCE / %[e]

μhFET / cm2 V−1 s−1

Ref.

−5.30 −5.44 −5.51 −5.15[b] −5.25[b] −5.34 −5.26 −5.14 −5.22 −5.24 −5.14 −5.29 −5.19 −5.20 −5.38 −4.86 −5.03 −5.15 −5.30 −5.22

(−3.50)[c] (−3.79)[c] (−3.87)[c] – – (−3.96)[c] (−3.26)[c] (−3.13)[c] (−3.50)[c] (−3.47)[c] (−3.14)[c] (−3.55)[c] −3.90 −3.75 −3.43 (−3.26)[c] (−3.34)[c] −3.43 −3.38 –

1.80 1.65 1.64 1.7 1.6 1.38 1.78 1.73 1.51 1.74 1.70 1.73 1.29 1.45 2.04 1.60 1.69 1.76 1.76 1.69

4.0 4.49 5.16 3.8[f ] 4.9[f ] – 0.76 1.62 1.42 1.5 1.13 1.23 – – 5.26 2.51 4.88 3.22 5.07 8.2,7.5[f ]

1.4×10−5 3.5×10−4 5.7×10−4 0.08 0.54 1.3 7.9×10−5 2.7×10−3 2.8×10−4 5.0×10−5 7.6×10−4 7.6×10−5 0.019 0.11 0.067 5.8×10−4 1.2×10−3 0.43 0.34 0.1

[22] [30] [30] [29] [29] [31] [32a] [32a] [32a] [32b] [32b] [32b] [35] [35] [36] [37] [37] [38] [38] [39]

P1 P2 P3 P4 P5 P6 (DH) P7 P8 P9 P10 P11 P12 P13 P14 P15 P16 P17 P18 P19 P20

[a] Electrochemically estimated EHOMO and ELUMO unless otherwise stated. [b]PESA was used. [c]Values in parentheses were determined by the following equation: ELUMO = EHOMO + Egopt. [d]Estimated from the absorption onset. [e]PC71BM was used as the n-type semiconductor unless otherwise stated. [f ]PC61BM was used as the n-type semiconductor.

into the effect of substitution on the electronic structure of the polymer and its packing structure in the solid state. The electrochemically estimated EHOMO of P13 (−5.19 eV) is 0.15 eV higher than that of P6, which is consistent with the presence of the alkyloxy groups with their strong electron-donating nature.[35] On the other hand, XRD patterns for the thin films of P6 with the parent NDT3 core clearly showed a series of peaks up to the fourth order assignable to a well-defined edge-on lamellar structure. In addition, the π–π stacking distance was determined to be 3.65 Å, which is quite short for these kinds of conjugated polymers.[16b] Such structural features are consistent with the high mobility (>1.0 cm2 V−1 s−1) of the P6-based OFETs. On the contrary, only the first lamellar peak was observed for the thin film of P13, indicating that the crystallinity of P13 thin films was greatly reduced. Although no peak assignable to π–π stacking was observed for the P13 thin film, a large π–π stacking distance (4.14 Å) was observed for the closely related polymer P14 thin film. These structural data obviously indicate that introduction of the branched alkyloxy group on the NDT3 core in the P6 backbone reduces the crystallinity and intermolecular interaction, which indeed explains the reduction in the mobility of the P13-based OFETs (∼0.019 cm2 V−1 s−1) by more than one order of magnitude compared to P6. It is also interesting to note that P15, consisting of the alkyloxy NDT3 and TPD units, has an isomeric backbone

Chem. Rec. 2014, ••, ••–••

with P1, consisting of the alkyloxy NDT1 and TPD units. Although the EHOMO levels of P15 and P1 are comparable, the VOC values in OPV devices (0.69 for the P1- and 0.93 V for the P15-based device) are very different. Another comparison between the isomeric NDT1/NDT3-based polymers also gives puzzling results: introduction of TT as the A unit in the NDT1 polymer, i.e., P2 and P3, shifted EHOMO downwards, whereas the same modification in the NDT3 polymer, i.e., P16 and P17, shifted EHOMO upwards (Table 2). The reason for these shifts caused by the TT units can be explained by the different tendencies for conjugation of the NDT1 and NDT3 units as discussed above; the NDT1 core can be stabilized by the peripheral 18π electron system, which tends to conjugate less with the neighboring units, whereas the NDT3 core can easily be involved in an effective conjugation path with the neighboring groups and efficiently extend the effective conjugation lengths over the polymer backbone.[8] Application of the alkyloxy-substituted NDT3-based polymers (P15–P19) to thin-film devices, in particular to OPVs, has generally afforded better performances than the unsubstituted NDT3-based polymers (P7–P12). For example, PCEs higher than 5% were reported for the P15[36] and P19[38] PC71BM-based OPVs by Li and co-workers. Although the alkyl-substituted NDT3 unit has not yet been fully exploited as the D unit in semiconducting polymers,[40] P20 consisting of the 5,10-dialkylated NDT3 and NTz units can afford OPVs



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Fig. 6. (a) Favorable face-on orientation of P20 in the thin-film state (left, 2D-GIWAX data of the spin-coated thin film of P20 on a Si/SiO2 substrate; and right, schematic representation of the face-on orientation of the polymer chains). (b) J–V (left) and EQE curves (right) of P20–PC71BM- and P20–PC61BM-based solar cells.

with impressively high PCEs of up to 8.2% with PC71BM and 7.5% with PC61BM, which are among the highest for OPVs based on recently developed new semiconducting polymers.[39] The characteristic feature of these OPVs is that the active BHJ layer has a relatively high crystallinity, which, in combination with the favorable face-on polymer orientation, facilitates transport of generated charges, resulting in higher PCEs (Figure 6). These results represent the great potential of the alkylated NDTs in developing new semiconducting polymers for high-performance OPVs.

4. Conclusion and Outlook Although the research on NDT-based π-conjugated materials was only initiated in 2010, several promising materials based on the D–A architecture and their OFET and OPV devices with impressive performances have already been reported in the last few years. We can thus expect further development of new NDT-based materials as well as superior devices, similar to their lower homologue, BDT, which is a prevailing D unit in the development of semiconducting polymers for OPV applications. The distinct merits of NDT units rely on their π-extended

Chem. Rec. 2014, ••, ••–••

structures, potentially affording strong intermolecular interactions and good crystallinity in the thin-film state, which is beneficial for efficient carrier transport. Another interesting point regarding the NDT units is that several structural isomers are available, affording a chance to control the electronic structure of the resulting semiconducting oligomers and polymers. Furthermore, new NDT3- and NDT4-based polymers linked at the 5,10-positions with co-monomer units have also been developed very recently, affording another means for controlling the electronic as well as the packing structures.[41] Although the low solubility of the materials based on unsubstituted NDT units could be an issue, as shown in the present review, NDT derivatives with solubilizing alkyl or alkyloxy groups as well as efficient methods for their synthesis have already been developed, paving the way to superior materials for optoelectronic applications. In particular, semiconducting polymers for OPV applications is one of the most promising fields, since the related heterocycles including BDT have afforded fruitful results. In fact, even after such a short period of time since the first synthesis of NDT-based electronic materials, impressively high OPV performance with PCE as high as 8.2% has already been reported.[42] Therefore, further progress in OPV research with NDT-based materials is expected in the near future.



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Acknowledgements

NDTs, where each thiophene ring shares two bonds with the naphthalene, affording an isoelectronic structure with pyrene.[44] Although these NDTs have been previously examined as small molecular organic semiconductors, we will also not deal with them in this review.

This work was financially supported by Grants-in-Aid for Scientific Research (Nos. 23245041, 24685030) from MEXT, Japan, and the Strategic Promotion of Innovative Research and Development from the Japan Science and Technology Agency.

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