A New Ladder-Type Germanium-Bridged Dithienocarbazole Arene

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A New Ladder-Type Germanium-Bridged Dithienocarbazole Arene and Its Donor−Acceptor Conjugated Copolymers: Synthesis, Molecular Properties, and Photovoltaic Applications Pei-Chi Jwo, Yu-Ying Lai, Che-En Tsai, Yun-Yu Lai, Wei-Wei Liang, Chain-Shu Hsu, and Yen-Ju Cheng* Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsin-Chu 30010, Taiwan S Supporting Information *

ABSTRACT: We have developed a new germanium-bridged heptacyclic arene, dithienogermolocarbazole (DTGC), in which two outer thiophene subunits are covalently fastened to the central 2,7-carbazole core by two dibutylgermanium bridges. The germole moieties embedded in the DTGC structure were successfully constructed by one-pot nucleophilic cyclization in a high yield of 88%. Because of the relatively lower polarity of carbon−germanium bonds, the DTGC unit is chemically stable under basic conditions, rendering its more versatile functionalization. Comparison of germanium-bridged DTGC with the carbon-bridged DTCC (dithienocyclopentacarbazole) and silicon-bridged DTSC (dithienosilolocarbazole) analogues reveals that the HOMO energy level of DTGC lies between those of DTCC and DTSC and so does the LUMO energy level of DTGC. Density functional theory (DFT) calculations suggest that DTSC and DTGC have more bent structures than DTCC, which plays an important role in determining their frontier orbital energies. The structural disparity could be amplified in their corresponding polymers. The DTGC unit was copolymerized with four different comonomers, including benzothiadiazole (BT), dithienylbenzothiadiazole (DTBT), difluorobenzothiadiazole (FBT), and dithienyldifluorobenzothiadiazole (DTFBT) to yield a series of new alternating donor−acceptor copolymers, poly(dithienogermolo-carbazole-alt-benzothiadiazole) (PDTGCBT), poly(dithienogermolocarbazole-alt-dithienylbenzothiadiazole) (PDTGCDTBT), poly(dithienogermolocarbazole-alt-difluorobenzothiadiazole) (PDTGCFBT), and poly(dithienogermolocarbazole-alt-dithienyldifluorobenzothiadiazole) (PDTGCDTFBT). Because of the two additional thiophene rings in the repeating units on the backbone to facilitate π-electron delocalization, PDTGCFDTBT showed a lower optical band gap than PDTGCFBT. Furthermore, PDTGCDTFBT also showed the lowerlying LUMO and HOMO energy levels than PDTGCDTBT as a result of the electron-withdrawing fluorine atoms. Consequently, the bulk heterojunction solar cell incorporating PDTGCDTFBT delivered the highest performance with Voc of 0.84 V, Jsc of 9.87 mA/cm2, FF of 48.8%, and PCE of 4.05%. By adding 3 vol % 1-chloronaphthalene to tailor the morphology, the solar cell using PDTGCDTFBT with higher molecular weight exhibited the improved efficiency of 4.50% with a Voc of 0.84 V, a Jsc of 11.19 mA/cm2, and an FF of 47.7%.

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INTRODUCTION Research effort devoted to the area of bulk heterojunction solar cells has brought this technique closer toward the objective of commercial utilization.1 In addition to the manipulation of device structure and the engineering of active-layer morphology, development of new p-type materials is particularly crucial to achieve further advancement.2 It is therefore desirable to create superior building blocks that can be used to construct donor−acceptor p-type polymers.3 One useful molecular design is focused on constructing ladder-type polyaromatic molecules with high degree of planarity and rigidity4 that lead to the reduction of the structural reorganization energy and thus improve the hole mobility of the p-type materials for solar cell applications.5 Leclerc and co-workers reported the copolymerization of an N-alkylated carbazole with dithienylbenzothiadiazole (DTBT) to form a donor−acceptor copolymer, poly(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT), © 2014 American Chemical Society

which exhibited promising photovoltaic properties (Scheme 1).6 On the basis of the dithienocarbzole conjugated skeleton of PCDTBT, we first adopted the conception of forced molecular planarity to develop a ladder-type structure dithienocyclopentacarbazole (DTCC), where the two outer thiophenes are covalently fastened with the central carbazole unit by two bridging carbon atoms (Scheme 1).7 A copolymer prepared by the Stille coupling of distannylated DTCC and 4,7dibromobenzo-1,2,5-thiadiazole gave promising performance in the conventional solar devices. Thereafter, the bridging-atom effect on the dithienocarbazole skeleton was systematically investigated by changing the carbon bridge in DTCC unit to silicon and nitrogen bridges.8 Therefore, the new building Received: September 6, 2014 Revised: October 5, 2014 Published: October 22, 2014 7386

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Scheme 1. Structures of PCDTBT, DTCC, DTSC, DTPC, PDTCCBT, PDTSCBT, and PDTPCBT

Scheme 2. Structures of PDTGCBT, PDTGCDTBT, PDTGCFBT, and PDTGCDTFBT in This Research

blocks, dithienosilolocarbazole (DTSC) and dithienopyrrolocarbazole (DTPC), were designed and synthesized (Scheme 1).8 Compared to the carbon-bridged DTCC, the silicon or nitrogen bridging atoms effectively alter the orbital interactions between aromatic groups. The silole units embedded in DTSC exert certain electron-accepting ability to lower highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, while the pyrrole groups with strong electron-donating character give rise to higher-lying frontier orbital energies. The three heptacyclic units were coupled with the benzothiadiazole (BT) unit to form the corresponding donor−acceptor copolymers: poly(dithienocyclopentacarbazole-alt-benzothiadiazole) (PDTCCBT), poly(dithienosilolocarbazole-alt-benzothiadiazole) (PDTSCBT), and poly(dithienopyrrolocarbazole-alt-benzothiadiazole) (PDTPCBT).

Germanium is a main group IV element. In comparison with its neighbor carbon and silicon elements, the development of organogermanium molecules is synthetically much less explored for organic electronics. The electronic and steric effects of germanium moieties on the molecular properties of conjugated system are highly intriguing and worthy of indepth investigation. The germole-containing low-bandgap polymers have been recently demonstrated to show promising performance for polymer solar cells (PSCs) applications.9 With the intention of further investigating the bridging-atom effect in the dithienocarbazole-based system, we designed and synthesized a germanium-bridged heptacyclic dithienogermolocarbazole (DTGC) arene and compared its thermal, optical, electrochemical, and structural properties with DTCC and DTSC by thermogravimetry, cyclic voltammetry, absorption spectroscopy, and computation. Moreover, we have prepared four DTGC-based donor−acceptor copolymers poly(dithieno7387

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Scheme 3. Synthetic Route for Br-DTGC, Sn-DTGC, and DTGC

Scheme 4. Preparation of PDTGCBT, PDTGCDTBT, PDTGCFBT, and PDTGCDTFBT by Suzuki or Stille Coupling

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germolocarbazole-alt-benzothiadiazole) (PDTGCBT), poly(dithienogermolocarbazole-alt-dithienylbenzothiadiazole) (PDTGCDTBT), poly(dithienogermolocarbazole-alt-difluorobenzothiadiazole) (PDTGCFBT), and poly(dithienogermolocarbazole-alt-dithienyldifluorobenzothiadiazole) (PDTGCDTFBT), as shown in Scheme 2. The molecular and photovoltaic properties of these new germanium-bridged polymers will be discussed.

RESULTS AND DISCUSSION

Synthesis. Cross-coupling Suzuki and Stille reactions are conventional options for synthesizing p-type polymers used for PSCs. Brominated, stannylated, or boronated precursors are required for these reactions. In order to increase the synthetic practicability of DTGC, syntheses of Br-DTGC and Sn-DTGC are proposed and described in Scheme 3. To begin with, compound 1, prepared according to the procedure reported previously,8 underwent metal−halide exchange four times in 7388

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Table 1. Summary of the Molecular Properties for DTCC, DTSC, DTGC, and the Germanium-Bridged Polymers compound

Mn (kDa)

PDI

Td (°C)

EHOMO (eV)

ELUMO (eV)

Egele a (eV)

478 479 455 463

−5.32 −5.45 −5.36 −5.36 −5.28 −5.55 −5.40

−2.03 −2.19 −2.10 −3.63 −3.63 −3.66 −3.67

3.29 3.26 3.26 1.73 1.64 1.88 1.72

c

DTCC DTSCc DTGC PDTGCBT PDTGCDTBT PDTGCFBT PDTGCDTFBT

55.2 26.8 22.0 17.9

1.78 1.70 1.35 1.73

λmax(nm) toluene 394 395 396 416, 442, 408, 436,

457 576 592 565

λmax(nm) film

415, 444, 410, 438,

604 587 608 572

Egopt b (eV) 3.09 3.07 3.07 1.82 1.76 1.86 1.75

a ele Eg was determined from cyclic voltammetry by subtracting the HOMO energy value from the LUMO energy value. bEgopt was estimated from the onset of UV−vis absorption in the solid state. For DTCC, DTSC, and DTGC, the measurements were carried out in toluene. cThe listed values are from ref 10.

associated with the direct interactions induced by the fluorine atoms.12 Thermal Properties. Thermal properties of the polymers are summarized in Table 1. No glass transition or melting temperature was detected for these polymers by differential scanning calorimetry, indicating their amorphous nature. Evaluation of thermal stability of the polymers by thermogravimetry reveals that thermal decomposition temperatures (Td) of the polymers are all above 450 °C, confirming that they all possess sufficient thermal stability for PSC applications (Figure 1 and Table 1). It is worth noting that, in comparison

the presence of n-BuLi at −78 °C to give a lithiated intermediate, which was further reacted with dibutylgermanium dichloride through nucleophlic substitution to furnish TMSDTGC with the formation of two germole units in a satisfactory yield of 88%. It is noteworthy that the bromination of TMS-DTGC by N-bromosuccinimide (NBS) in CH2Cl2 afforded Br-DTGC in 75% yield, whereas the reaction yield decreased to 45% when THF was used as the solvent due to the inferior regioselectivity. Sn-DTGC was obtained from BrDTGC through lithiation by t-BuLi and quenching with trimethyltin chloride at −78 °C. Column chromatography of Sn-DTGC on neutral Al2O3 resulted in destannylated DTGC compound. DTGC was used to compare with the carbon (DTCC) and silicon (DTSC) analogues to understand the intrinsic difference stemmed from the bridging effect for the carbon IV group. Br-DTGC and Sn-DTGC were employed to copolymerize with benzothiadiazole (BT) (2), difluorobenzothiadiazole (FBT) (3), dithienylbenzothiadiazole (DTBT) (4), and dithienyldifluorobenzothiadiazole (DTFBT) (5) monomers by Suzuki or Stille coupling to form a series of D−A copolymers PDTGCBT, PDTGCFBT, PDTGCDTBT, and PDTGCDTFBT, respectively (see Scheme 4). It should be noted that the silicon-based DTSC monomer and its PDTSCBT copolymer decompose in basic conditions due to the more vulnerable C−Si bonds. On the contrary, the DTGC structure is very stable in the basic conditions used for the Suzuki cross-coupling polymerization due to the relatively lower polarity of carbon−germanium bonds. The relatively high stability of the C−Ge bond is one of the most notable advantages, allowing versatile functional group transformation of the organogermanium compound.10 It is thus envisaged that the resulting germanium-based copolymers would also have better stability than their silicon-based counterparts for the PSC applications. The molecular weights of the polymers are listed in Table 1. Their solubility is influenced significantly by the comonomers. For instance, although PDTGCBT has higher average molecular weight (Mn) than PDTGCDTBT, it still exhibits superior solubility than PDTGCDTBT in THF or toluene. A similar phenomenon was also observed for PDTGCFBT and PDTGCDTFBT, suggesting that with two more thiophenes on DTBT or DTFBT segments make the polymers less soluble. It may be rationalized by a general opinion that the thiophene unit is capable of increasing the quinoidal character of polymers, thus resulting in the enhancement of molecular planarity and intermolecular interactions, which would decrease the solubility of polymers.11 Furthermore, we found that the fluorinated polymers exhibited poorer solubility, which may be

Figure 1. Thermogravimetric analyses of PDTGCBT, PDTGCDTBT, PDTGCFBT, and PDTGCDTFBT.

to PDTCCBT (carbon analogue) and PDTSCBT (silicon analogue) (Scheme 1), PDTGCBT with germanium as the bridging atom exhibits the highest Td, which is in resonance with the superior stability of the C−Ge functionality.11c,12 Electrochemical Properties. Cyclic voltammetry (CV) measurements were carried out to estimate the electronic properties of DTGC and the corresponding polymers (Figure 2). The HOMO and LUMO energies for DTCC, DTSC, and DTGC are listed in Table 1 and Figure 3. DTCC has the highest HOMO energy level (−5.32 eV), followed by DTGC (−5.36 eV) and DTSC (−5.45 eV), implying that DTGC’s electron-donating ability lies between DTCC and DTSC. The LUMO energy level of DTGC is also situated between those of DTCC and DTSC. As a result, DTGC and DTSC have analogous electrochemical band gap (Egele) of 3.26 eV and DTCC has a slightly higher Egele value of 3.29 eV. As for the polymers, they all exhibited reversible redox abilities, being 7389

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Figure 4. Normalized absorption spectra of DTGC, DTSC, and DTCC in toluene solution. Figure 2. Cyclic voltammograms of PDTGCBT, PDTGCDTBT, PDTGCFBT, and PDTGCDTFBT in thin film at a scan rate of 50 mV s −1.

were evaluated and the corresponding absorption spectra are illustrated in Figure 5. PDTGCBT and PDTGCFBT have similar absorption profile and the absorption pattern of PDTGCDTBT is analogous to that of PDTGCDTFBT. As deduced from the onset of solid-state absorption, PDTGCDTBT and PDTGCDTFBT have smaller Egopt than the corresponding PDTGCBT and PDTGCFBT, confirming again that the presence of additional thiophenes connecting to BT or FBT units is capable of lowering the energy band gap of the corresponding polymers. However, the effect of fluorine atoms on the Egopt of the polymers in films is less pronounced than that to the Egele. As mentioned previously, the existence of the fluorine atom in the polymer chain may enhance intermolecular interactions between polymers, which would result in the morphology change. Therefore, the change of polymer solid-state morphology may cancel out the fluorine electron-withdrawing effect, thus resulting in insignificant alternation in the solid-state Egopt. Theoretical Calculations. DFT calculations at the B3LYP/ 6-311G(d,p)/PCM=Toluene level of theory were carried out to investigate the electronic and structural discrepancy between DTCC, DTSC, and DTGC. The frontier orbitals for the three molecules are illustrated in Figure 6, and the computational results are summarized in Table 2. The three compounds have similar frontier-orbital patterns. For all HOMOs, electron densities are distributed uniformly along the molecular backbone. As for LUMOs, electron densities are allocated in a more dispersed manner, resulting in more nodal planes between orbitals. Among all, DTSC has the lowest HOMO and LUMO energies, followed by DTGC and DTCC. The trend correlates well with the CV data. A closer look into the electron-density distribution of the frontier orbitals reveals that the bridging atoms do not significantly participate in these orbitals, whereas the corresponding orbital energies do vary as the change of bridging atom between the thienyl and carbazole units. We thus reason that the energy variation may come from the structural disparity brought by certain intrinsic properties, such as the atomic-size difference between C, Si, and Ge. Selected bond lengths and angles for DTCC, DTSC, and DTGC calculated at the B3LYP/6-311G(d,p)/PCM=Toluene level of theory are listed in Table 3. It is evident that when the bridging atom

Figure 3. Energy diagram (HOMO and LUMO energy levels) of DTCC, DTSC, DTGC, and the germanium-bridged polymers estimated by cyclic voltammograms.

essential for applications in PSCs. Comparison of PDTGCBT and PDTGCDTBT or PDTGCFBT and PDTGCDTFBT reveals that the HOMO energy level rises with the presence of two additional thiophenes connecting to BT or FBT units, whereas the LUMO energy level is less affected, thus resulting in the lower Egele values of PDTGCDTBT and PDTGCDTFBT (Table 1). This result is likely related to the increased quinoidal character of the polymers induced by the existence of additional thiophene units.11 The effect of introducing fluorine atom in the polymer chain is also significant. It can be clearly seen from Figure 3, with the extra fluorine atoms on the BT units, the HOMO energy levels of the polymers are downwardly shifted (−5.36 eV vs −5.55 eV for PDTGCBT and PDTGCFBT; −5 . 2 8 e V v s −5 . 4 0 e V f o r PD T G C D T B T a n d PDTGCDTFBT) while the LUMO energy level remains less affected. Optical Properties. The absorption spectra of DTCC, DTSC, and DTGC are depicted in Figure 4. Analogous to the electrochemical band gaps, DTCC (3.09 eV), DTGC (3.07 eV), and DTSC (3.07 eV) also show very similar optical band gaps. Afterward, the optical properties of the four polymers 7390

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Figure 5. Normalized absorption spectra of PDTGCBT, PDTGCFBT, PDTGCDTBT, and PDTGCDTFBT in (a) toluene solution and (b) in thin film.

Figure 6. Illustrations of frontier orbitals for DTCC, DTSC, and DTGC.

Table 2. Summary of Details for Frontier Orbitals for DTCC, DTSC, and DTGC excitation energy compound

HOMO (eV)

LUMO (eV)

λmax,exp (nm)

λcalc (nm)

oscillator strength

configurationb

DTCC DTSC DTGC

−1.58 −1.70 −1.67

−5.13 −5.24 −5.22

394 395 396

392 396 395

1.46 1.32 1.36

H→L H→L H→L

a

a Experimental values were measured for nonsimplified DTCC, DTSC, and DTGC in toluene solution. bOnly configurations with largest coefficients in the CI expansion of each state are listed.

changes from C to Si, the bond lengths of C3−X (L1) and C6− X (L2) increase significantly from 1.52 and 1.54 Å to 1.88 and 1.89 Å, although the increase of bond lengths is less explicit as the atom varies from Si to Ge (1.96 and 1.97 Å) due to the small difference in the atomic size between Si and Ge. The elongation of the C3−X (L1) and C6−X (L2) bonds may reduce the electronic repulsion within the frontier orbitals, thus resulting in the lower-lying HOMO and LUMO energies of DTSC and DTGC. Moreover, the bond elongation can further influence other structural parameters, such as the bond angle of C2−C3−X (θ1), the bond lengths of C2−C3 (L3) and C4−C5 (L4), and the curvature of the conjugated backbones (Table 3). The slightly higher HOMO and LUMO values for DTGC may result from the minor difference of these structural parameters between DTGC and DTSC. It should be emphasized that the curvature of molecules may not merely affect the orbital energy but also the polymer conformation when these heptacyclic monomers are polymerized with an electron acceptor. As listed in Table 3, DTSC

Table 3. Selected Calculated Bond Lengths and Angles for DTCC, DTSC, and DTGCa DTCC DTSC DTGC

X

L1 (Å)

L2 (Å)

L3 (Å)

L4 (Å)

θ1 (deg)

θ2 (deg)

C Si Ge

1.52 1.88 1.96

1.54 1.89 1.97

1.42 1.43 1.42

1.45 1.47 1.47

136 141 140

149 120 116

Definition of L1, L2, L3, L4, θ1, and θ2; X = O, Si, or Ge; L1 = bond length of C3−X; L2 = bond length of C6−X; L3 = bond length of C2− C3; L4 = bond length of C4−C5; θ1 = bond angle of C2−C3−X; θ2 = included angle of H1−C1 and H8−C7 bonds. a

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Table 4. Summary of the Device Characteristics polymer

blend ratio (wt %)

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

PDTGCBT PDTGCDTBT PDTGCFBT PDTGCDTFBT PDTGCDTFBT with 3 vol % CNa H-PDTGCDTFBTb

1:3 1:2 1:4 1:4 1:4 1:4

0.80 0.76 0.90 0.84 0.82 0.84

8.44 5.78 4.27 9.87 10.16 11.19

44.9 37.3 51.1 48.8 49.3 47.7

3.03 1.63 1.96 4.05 4.11 4.50

mobilityc (cm2 V−1 s−1) 3.20 8.63 1.20 3.97

× × × ×

10−4 10−4 10−3 10−3

a 1-Chloronaphthalene. bHigh molecular-weight PDTGCDTFBT. cThe charge mobility of polymers was determined by organic field-effect transistors.

and DTGC have comparable curvature (θ2 = 120° and 116°) and are both more bent than DTCC (θ2 = 149°). It can be envisaged that a polymer containing a bent fragment may have lower degree of main-chain linearity. Consequently, the disparity in curvature between DTCC, DTSC, and DTGC is expected to induce diverse intramolecular conformations and intermolecular packing for the resultant polymers. Photovoltaic Properties. PDTGCBT, PDTGCFBT, PDTGCDTBT, and PDTGCDTFBT were evaluated in inverted solar devices based on the ITO/ZnO/polymer:PC71BM/MoO3/Ag configuration. The blending ratios of polymer:PC71BM were optimized by using 1,2-dichlorobenzene as the processing solvent. The measurements were carried out under a simulated AM 1.5 G illumination of 100 mW/cm2. The obtained device parameters are listed in Table 4. First of all, it was found that the HOMO energy value correlates qualitatively well with the Voc value. This correlation is consistent with the empirical observation that lowering the HOMO energy level of donors often results in the enhancement of Voc values.13 However, no apparent correlation was found between Jsc or FF with other polymer properties, such as UV-absorption ability (Table 1) or charge mobility (Table 4), suggesting that the active-layer morphology influenced by the electronic and steric properties of polymers may play an important role in determining the device efficiency. PDTGCBT/PC17BM (1:3 in wt %)-based device delivered a moderate efficiency of 3.03%. In comparison to PDTGCBT, introduction of either fluorine atom (PDTGCFBT) or additional thiophene units (PDTGCDTBT) in the polymer chain does not result in better PSC efficiency. Nevertheless, when both variables are combined together, i.e., use of DTFBT as the acceptor unit, the corresponding polymer PDTGCDTFBT exhibited a high PCE of 4.05%. Further device optimization by using 1-chloronaphthalene (CN)14 as the active-layer additive gave a PCE of 4.11%. Subsequently, high-molecular-weight PDTGCDTFBT (H-PDTGCDTFBT) was employed as well. The solubility of H-PDTGCDTFBT in THF is so poor that the attempt to determine its molecular weight by gel permeation chromatography was not successful. The device using H-PDTGCDTFBT as the donor material still produced the highest PCE of 4.50% among all the tested devices, with a Voc of 0.84 V, a Jsc of 11.19 mA cm−1, and an FF of 47.7%. J−V curves of all the studied devices are depicted in Figure 7. Organic Field Effect Transistors. To investigate the mobilities of the polymers, organic field-effect transistors (OFETs) were fabricated in the bottom-gate/top-contact geometry as described in the Experimental Section (Table 4 and Figure 8). The hole mobilities were deduced from the transfer characteristics of the devices in the saturation regime. The polymer-based OFETs using SiO2 as the gate dielectric were thermally annealed at 120 °C. We found that the

Figure 7. J−V characteristics of the devices based on the ITO/ZnO/ DTGC-based polymer:PC71BM/MoO3/Ag configuration under a simulated AM 1.5 G illumination of 100 mW cm−2.

fluorinated copolymers exhibited higher mobilities than the corresponding non-fluorinated counterparts (i.e., 1.20 × 10−3 vs 3.20 × 10−4 cm2 V−1 s−1 for PDTGCFBT and PDTGCBT; 3.97 × 10−3 vs 8.63 × 10−4 cm2 V−1 s−1 for PDTGCDTFBT and PDTGCDTBT). PDTGCDTFBT exhibited the highest mobility of 3.97 × 10−3 cm2 V−1 s−1, which is in good agreement of its best solar cell performance.

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CONCLUSIONS A new heptacylic ladder-type dithienogermolocarbazole (DTGC) structure has been developed. Comparison of DTGC with its carbon (DTCC) and silicon (DTSC) analogues reveals that DTCC has the highest-lying HOMO energy level, followed by DTGC and DTSC, indicating that DTGC’s electron-donating ability lies between DTCC and DTSC. The LUMO energy level of DTGC is also situated between those of DTCC and DTSC. DFT calculations suggest that the atomicsize difference between C, Si, and Ge could be an important factor in causing the difference in the frontier-orbital energy between DTCC, DTSC, and DTGC. Moreover, the bridging atom would also influence the molecular curvature, and the disparity in curvature between DTCC, DTSC, and DTGC is expected to induce diverse conformations for the resultant polymers. Br-DTGC and Sn-DTGC were further employed to copolymerize with BT, FBT, DTBT, and DTFBT monomers by Suzuki or Stille coupling to form PDTGCBT, PDTGCFBT, PDTGCDTBT, and PDTGCDTFBT. DTGC and its copolymers show better chemical and thermal stability, indicating the advantage of employing the germole functionality into organic materials. Electronic and optical properties of the polymers 7392

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Figure 8. Typical output curves (a, c, e, g) and transfer plots (b, d, f, h) of the OFET devices based on PDTGCBT, PDTGCDTBT, PDTGCFBT, and PDTGCDTFBT, respectively. 7393

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mixture was quenched with water and extracted with diethyl ether (50 mL × 3) and water (50 mL). After removal of the solvent and residual chlorotrimethylstannane under reduced pressure distillation (0.1 Torr, 60 °C) for 2 h, Sn-DTGC was obtained as yellow product (0.13 g, 69%). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.18 (s, 1 H), 8.15 (s, 1 H), 7.68 (s, 1 H), 7.52 (s, 1 H), 7.23 (s, 2 H), 4.60 (m, 1 H), 2.36− 2.34 (m, 2 H), 1.99−1.96 (m, 2 H), 1.59−1.51 (m, 8 H), 1.46−1.34 (m, 8 H), 1.28−1.17 (m, 32 H), 0.91 (t, J = 7.2 Hz, 12 H), 0.81 (t, J = 6.4 Hz, 6 H), 0.44 (S, 18 H). MS (FAB, C59H95Ge2NS2Sn2): calcd, 1265.22; found, 1264.0. Synthesis of DTGC. Sn-DTGC (0.13 g, 0.10 mmol) was passed through a column chromatography on neutral aluminum oxide (hexane) to give a yellow sticky product DTSC (78 g, 81%). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.16 (s, 1 H), 8.13 (s, 1 H), 7.63 (s, 1 H), 7.47 (s, 1 H), 7.32 (d, J = 4.4 Hz, 2 H), 7.14 (d, J = 4.4 Hz, 2 H), 4.60−4.56 (m, 1 H), 2.39−2.23 (m, 2 H), 2.02−1.87 (m, 2 H), 1.55− 1.43 (m, 8 H), 1.40−1.33 (m, 8 H), 1.31−1.03 (m, 32 H), 0.87 (t, J = 7.2 Hz, 12 H), 0.78 (t, J = 6.4 Hz, 6 H). 13C NMR (CDCl3, 100 MHz, ppm): δ 155.07, 143.39, 141.22, 140.68, 140.19, 140.02, 131.44, 130.01, 125.28, 124.68, 124.36, 122.79, 121.41, 104.87, 102.21, 56.46, 33.84, 31.93, 29.71, 29.44, 29.18, 27.71, 26.82, 26.12, 22.68, 14.47, 14.02, 13.67 (multiple carbon peaks result from phenomenon of atropisomerizm). MS (FAB, C53H79Ge2NS2): calcd, 939.60; found, 939.7. Synthesis of PDTGCBT. Br-DTGC (0.21 g, 0.189 mmol), 2 (0.0733 g, 0.189 mmol), tris(dibenzylideneacetone)dipalladium (6.9 mg, 0.008 mmol), tris(o-tolyl)phosphine (18.4 mg, 0.060 mmol), and Aliquat 336 (two drops) were dissolved in deoxygenated toluene/1.0 M Na2CO3 (10 mL, 5:1, v/v). The mixture was degassed by bubbling argon for 10 min at room temperature. The reaction mixture was refluxed at 90 °C for 72 h. The solution was dropwise added into methanol (200 mL). The precipitate was collected by filtration and washed by Soxhlet extraction with acetone (24 h) and hexane (48 h) sequentially. The product was redissolved in THF (100 mL). The Pdthiol gel (Silicycle Inc.) was added to the above THF solution to remove the residual Pd catalyst at room temperature for 24 h. After filtration of the solution and removal of THF, the polymer was added into methanol to reprecipitate. The purified polymer was collected by filtration and dried under vacuum for 1 day to give a purplish-black solid (130 mg, yield 71%, Mn = 55.2 kDa, PDI = 1.78). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.21 (br, 4 H), 8.00 (br, 2 H), 7.78 (br, 1 H), 7.64 (br, 1 H), 4.67 (br, 1 H), 2.40 (br, 2 H), 2.06 (br, 2 H), 1.59−1.04 (m, 48 H), 0.91 (br, 12 H), 0.79 (br, 6 H). Synthesis of PDTGCDTBT. Br-DTGC (0.21 g, 0.191 mmol), 3 (0.12 g, 0.191 mmol), tris(dibenzylideneacetone)dipalladium (8.8 mg, 0.010 mmol), tris(o-tolyl)phosphine (23.3 mg, 0.077 mmol), and deoxygenated chlorobenzene (7 mL) were introduced to a 25 mL round-bottom flask. The mixture was then degassed by bubbling nitrogen for 10 min at room temperature. The round-bottom flask was placed into the microwave reactor and reacted at 180 °C for 50 min under 270 W. The solution was added into methanol dropwise. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone (24 h) and hexane (48 h) sequentially. The product was redissolved in hot THF. The Pd-thiol gel (Silicycle Inc.) was added to above THF solution to remove the residual Pd catalyst at 50 °C for 24 h. After filtration and removal of the solvent, the polymer was redissolved in hot THF again and added into methanol to reprecipitate. The purified polymer was collected by filtration and dried under vacuum for 1 day to give a dark green solid (215 mg, yield 91%, Mn = 26.8 kDa, PDI = 1.70). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.20 (br, 1 H), 8.17 (br, 1 H), 8.09 (br, 2 H), 7.88 (br, 2 H), 7.64 (br, 1 H), 7.48 (br, 1 H), 7.38 (br, 2 H), 7.34 (br, 2 H), 4.61 (br, 1 H), 2.35 (br, 2 H), 2.03 (br, 2 H), 1.56−1.07 (m, 48 H), 0.92 (br, 12 H), 0.81 (br, 6 H). Synthesis of PDTGCFBT. Sn-DTGC (0.26 g, 0.242 mmol), 4 (0.10 g, 0.242 mmol), tris(dibenzylideneacetone)dipalladium (11.1 mg, 0.012 mmol), tris(o-tolyl)phosphine (29.5 mg, 0.097 mmol), and deoxygenated chlorobenzene (8 mL) were introduced to a 25 mL round-bottom flask. The mixture was then degassed by bubbling nitrogen for 10 min at room temperature. The round-bottom flask was

have been investigated. Comparison of PDTGCBT and PDTGCDTBT or PDTGCFBT and PDTGCDTFBT reveals that the HOMO energy level rises with the presence of additional thiophenes, whereas the LUMO energy level is less affected, thus resulting in the lower Eg of PDTGCDTBT and PDTGCDTFBT. With the extra fluorine atoms on BT unit, the HOMO energy level is shifted downwardly while the LUMO energy level remains less affected. All polymers have been tested in the inverted devices based on the ITO/ZnO/ polymer:PC71BM/MoO3/Ag configuration. The PDTGCDTFBT-based device exhibited a higher PCE of 4.05%. Furthermore, the device by using high-molecular-weight PDTGCDTFBT as the donor material gave the highest PCE of 4.50%, with a Voc of 0.84 V, a Jsc of 11.19 mA cm−1, and an FF of 47.7%. We expect that DTGC-based molecules will demonstrate even more applications in the field of organic photovoltaics.

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EXPERIMENTAL SECTION

Synthesis of TMS-DTGC. Compound 1 was synthesized by following a previously reported procedure.8 A 2.5 M solution of n-BuLi in hexane (2.7 mL, 6.75 mmol) was added dropwise to a solution of 1 (1.28 g, 1.24 mmol) in dry THF (50 mL) at −78 °C. After stirring at −78 °C for 1 h, the cooling bath was removed and the mixture was stirred at room temperature for 1 h. Subsequently, dibutylgermanium dichloride (0.99 g, 3.84 mmol) was added at −78 °C. After stirring at −78 °C for 0.5 h, the mixture was stirred at room temperature for 4 h. The mixture was quenched with water and extracted with diethyl ether (60 mL × 3) and water (60 mL). The combined organic layer was dried over MgSO4. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on neutral aluminum oxide (hexane) to give a yellow sticky product TMSDTGC (1.18 g, 88%). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.16 (s, 1 H), 8.13 (s, 1 H), 7.66 (s, 1 H), 7.50 (s, 1 H), 7.24 (s, 2 H), 4.59−4.57 (m, 1 H), 2.34−2.31 (m, 2 H), 1.99−1.93 (m, 2 H), 1.56−1.49 (m, 8 H), 1.41−1.32 (m, 8 H), 1.26−1.14 (m, 32 H), 0.88 (t, J = 7.2 Hz, 12 H), 0.79 (t, J = 6.4 Hz, 6 H), 0.38 (s, 18 H). 13C NMR (CDCl3, 100 MHz, ppm): δ 160.80, 143.57, 142.86, 141.00, 140.69, 140.16, 137.04, 132.05, 124.84, 124.52, 122.94, 121.55, 105.29, 102.63, 56.51, 33.96, 31.88, 29.54, 29.42, 29.31, 27.83, 26.91, 26.28, 22.71, 14.58, 14.14, 13.77, 0.27 (multiple carbon peaks result from phenomenon of atropisomerism7b). MS (FAB, C59H95Ge2NS2Si2): calcd, 1083.97; found, 1084. Synthesis of Br-DTGC. N-Bromosuccimide (0.29 g, 1.69 mmol) was added in one portion to a solution of TMS-DTGC (0.82 g, 0.76 mmol) in CH2Cl2 (82 mL). The reaction was stirred under dark for 6 h at room temperature. The mixture was extracted with diethyl ether (60 mL × 3) and water (60 mL). The combined organic layer was dried over MgSO4. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on neutral aluminum oxide (hexane) to give a yellow sticky product BrDTSC (0.63 g, 75%). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.14 (s, 1 H), 8.11 (s, 1 H), 7.50 (s, 1 H), 7.33 (s, 1 H), 7.08 (s, 2 H), 4.55−4.51 (m, 1 H), 2.25−2.23 (m, 2 H), 1.99−1.93 (m, 2 H), 1.53−1.41 (m, 8 H), 1.39−1.31 (m, 8 H), 1.29−1.03 (m, 32 H), 0.87 (t, J = 7.2 Hz, 12 H), 0.79 (t, J = 6.4 Hz, 6 H). 13C NMR (CDCl3, 100 MHz, ppm): δ 155.40, 143.39, 141.39, 141.77, 140.30, 140.03, 139.82, 132.64, 130.15, 124.77, 124.46, 122.93, 121.55, 111.33, 104.78, 102.15, 56.53, 33.80, 31.77, 29.39, 29.29, 29.19, 27.62, 26.78, 26.10, 22.61, 14.53, 14.04, 13.66 (multiple carbon peaks result from phenomenon of atropisomerism). MS (FAB, C53H77Br2Ge2NS2): calcd, 1097.39; found, 1097. Synthesis of Sn-DTGC. A 1.6 M solution of t-BuLi in hexane (0.45 mL, 0.72 mmol) was added dropwise to a solution of Br-DTGC (0.157 g, 0.14 mmol) in dry THF (12 mL) at −78 °C. After stirring at −78 °C for 1 h, 1.0 M solution of chlorotrimethylstannane in THF (0.79 mL, 0.79 mmol) was introduced by syringe to the solution. The mixture was warmed up to room temperature and stirred for 4 h. The 7394

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Macromolecules

Article

placed into the microwave reactor and reacted at 180 °C for 50 min under 270 W. The solution was added into methanol dropwise. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone (24 h) and hexane (48 h) sequentially. The product was redissolved in THF. The Pd-thiol gel (Silicycle Inc.) was added to above THF solution to remove the residual Pd catalyst at room temperature for 24 h. After filtration and removal of the solvent, the polymer was redissolved in THF again and added into methanol to reprecipitate. The purified polymer was collected by filtration and dried under vacuum for 1 day to give a dark red solid (54 mg, yield 20%, Mn = 22.0 kDa, PDI = 1.35). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.40 (br, 2 H), 8.26 (br, 1 H), 8.23 (br, 1 H), 7.83 (br, 1 H), 7.69 (br, 1 H), 4.69 (br, 1 H), 2.42 (br, 2 H), 2.08 (br, 2 H), 1.59−1.18 (m, 48 H), 0.92 (br, 12 H), 0.78 (br, 6 H). Synthesis of PDTGCDTFBT. Br-DTGC (0.13 g, 0.122 mmol), 5 (0.08 g, 0.122 mmol), tris(dibenzylideneacetone)dipalladium (5.6 mg, 0.006 mmol), tris(o-tolyl)phosphine (14.8 mg, 0.049 mmol), and deoxygenated chlorobenzene (4 mL) were introduced to a 25 mL round-bottom flask. The mixture was then degassed by bubbling nitrogen for 10 min at room temperature. The round-bottom flask was placed into the microwave reactor and reacted at 180 °C for 50 min under 270 W. The solution was added into methanol dropwise. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone (24 h) and hexane (48 h) sequentially. The product was redissolved in hot toluene. The Pd-thiol gel (Silicycle Inc.) was added to above toluene solution to remove the residual Pd catalyst at 50 °C for 24 h. After filtration and removal of the solvent, the polymer was redissolved in hot toluene again and added into methanol to reprecipitate. The purified polymer was collected by filtration and dried under vacuum for 1 day to give a dark green solid (33 mg, yield 21%, Mn = 17.9 kDa, PDI = 1.73). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.23 (br, 2 H), 8.20 (br, 1 H), 8.17 (br, 1 H), 7.66 (br, 1 H), 7.50 (br, 1 H), 7.42 (br, 2 H), 7.39 (br, 2 H), 4.63 (br, 1 H), 2.36 (br, 2 H), 2.04 (br, 2 H), 1.56−1.18 (m, 48 H), 0.90 (br, 12 H), 0.80 (br, 6 H). Fabrication of Inverted PSCs. Preparation of the ZnO precursor: The sol−gel ZnO precursor was prepared before spin-coating.15 Zinc acetate dehydrate (Zn(CH3COO)2·H2O, Aldrich, 99.9%, 1 g) and ethanolamine (NH2CH2CH2OH, Aldrich, 99.5%, 0.28 g) were dissolved in 2-methoxyethanol (CH3OCH2CH2OH, Aldrich, 99.5%, 0.28 g) and stirred for 8 h for the hydrolysis reaction. Inverted solar cells were fabricated on ITO-coated glass substrates. The ITO-coated glass substrates were first cleaned with detergent and ultrasonicated in water, acetone, and isopropyl alcohol. The ZnO precursor solution was spin-cast on top of the ITO-glass substrate. The film was annealed at 180 °C for 15 min under ambient atmosphere. The ZnO film thickness was approximately 30 nm. The ZnO-coated substrates were transferred into a glovebox. A solution containing a mixture of polymer/PC71BM (1:2, 1:3, or 1:4 wt %) in dichlorobenzene with a concentration of 6 mg/mL was spin-cast on top of ZnO film (700, 1000, and 1400 rpm). The H-PDTGCDTFBT/PC71BM system was dissolved in hot dichlorobenzene. Then a thin layer of MoO3 film (ca. 7 nm) was evaporated on top of the active layer. Finally, the anode of Ag (ca. 150 nm) was deposited through masks by thermal evaporation. The pressure is lower than 10−6 Torr. The active area of device was 4 mm2. J−V characteristics were measured using a Keithley 1400 source measure unit. Solar cell performance was measured using an Air Mass 1.5 Global (AM 1.5 G) solar simulator with an irradiation intensity of 100 mW cm−2. OFET Fabrication. An n-type heavily doped Si wafer with a SiO2 layer of 325 nm and a capacitance of 10.6 nF/cm2 was used as the gate electrode and dielectric layer. Thin films (40−60 nm in thickness) of polymers were deposited on SiO2/Si substrates by spin-coating of their toluene solutions (10 mg/mL). Then, the thin films were annealed at temperature of 120 °C for 10 min. Gold source and drain contacts (40 nm in thickness) were deposited by vacuum evaporation on the organic layer through a shadow mask, affording a bottom-gate, topcontact device configuration. Electrical measurements of OTFT devices were carried out at room temperature in air using a 4156C, Agilent Technologies. The field-effect mobility was calculated in the saturation regime by using the equation Ids = (μWCi/2L)(Vg − Vt)2,

where Ids is the drain-source current, μ is the field-effect mobility, W is the channel width (1 mm), L is the channel length (100 μm), Ci is the capacitance per unit area of the gate dielectric layer, Vg is the gate voltage, and Vt is threshold voltage.

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ASSOCIATED CONTENT

S Supporting Information *

Computational details and 1H and 13C NMR spectra of the new compounds and copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (Y.-J.C.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science & Technology and the Ministry of Education, and the Center for Interdisciplinary Science (CIS) of the National Chiao Tung University, Taiwan, for financial support. We thank the National Center of Highperformance computing (NCHC) in Taiwan for computer time and facilities. Y.J.C. thanks the support from the Golden-Jade fellowship of the Kenda Foundation and the Foundation of the Advancement of Outstanding Scholarship (FAOS) in Taiwan.

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REFERENCES

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dx.doi.org/10.1021/ma5018499 | Macromolecules 2014, 47, 7386−7396