Introduction

π-Conjugated polymers have attracted great interest as semiconductor materials in the field of organic electronics owing to their potential for the development of various flexible electronic devices. Organic field-effect transistors (OFETs)1, 2 and organic solar cells (OSCs)3, 4 based on these semiconducting polymers can be fabricated using printing technologies by virtue of their solution processability and superior mechanical properties. In the last decade, the performance of such organic electronic devices has rapidly improved, accompanied by the development of high-performance p-type semiconducting polymers,5, 6, 7, 8, 9, 10, 11 including alternating electron-donor–acceptor (D–A) copolymers and electron-donor homopolymers. Currently, π-extended thienoacene derivatives, particularly benzo[1,2-b:4,5-b']dithiophene (BDT)12, 13, 14, 15, 16 and naphtho[1,2-b:5,6-b']dithiophene (NDT),17, 18, 19, 20, 21, 22, 23, 24 are regarded as important building units for the design of high-performance semiconducting polymers because of their high degree of backbone coplanarity, deep-lying highest occupied molecular orbital (HOMO) energy levels, and strong intermolecular interactions. While D–A copolymers combining various electron-donor and acceptor subunits have been extensively studied, only a limited number of electron-donor homopolymers consisting of such thienoacene units have been reported thus far, in spite of their simple structures. Recently, it was reported that BDT-based homopolymers exhibited high carrier-transport properties in OFETs and high photovoltaic properties in bulk heterojunction (BHJ) OSCs.25, 26, 27, 28, 29, 30 However, there have been no precedents of NDT-based homopolymers, even though NDT derivatives have been widely used as promising electron-donor units in various D–A copolymers.17, 18, 19, 20

In this work, we designed and synthesized new π-conjugated NDT-based homopolymers bearing alkylthio-thienyl or alkyl-thienyl side chains; PNDTS and PNDT (Scheme 1). The alkylthio-thienyl functionalization can stabilize the HOMO energy level of the resulting homopolymers, compared with alkyl-thienyl functionalization, which is beneficial for better air stability and higher open-circuit voltage (Voc) of BHJ OSCs. Herein, we disclose the synthesis and optical, charge-transport, and photovoltaic properties of PNDTS and PNDT.

Experimental procedure

Synthesis of polymers

PNDTS

To a stirred solution of Br–NDTS–Br (0.27 g, 0.31 mmol) and hexamethyldithin (0.11 g, 0.33 mmol) in a mixture of dry toluene (2 ml) and dry DMF (0.5 ml) was added Pd(PPh3)4 (7.2 mg, 6.2 μmol) at 120 °C. After stirring for 24 h, 2-(tributylstannyl)thiophene (0.47 g, 1.24 mmol) was added to the mixture. The reaction mixture was further stirred for 3 h at 120 °C. After cooling to room temperature, the reaction mixture was poured into methanol. The precipitate was filtered and subjected to Soxhlet extraction with methanol, acetone, hexane, and finally chloroform. The chloroform fraction was concentrated and reprecipitated into methanol. The precipitate was collected by filtration and dried under vacuum to afford PNDTS as a dark red solid (yield=0.17 g, 78%). GPC (eluent: THF): Mn=3.9 kg mol−1, Mw=8.3 kg mol−1, PDI=2.1. Anal calcd (%) for C38H44S6: C 65.85, H 6.40, S 27.75; found: C 65.10, H 5.96, S 27.43, Br 0.13.

PNTD

This polymer was prepared by a method similar to that of PNDTS, using Br–NDT–Br (0.31 g, 0.39 mmol), hexamethyldithin (0.14 g, 0.41 mmol), and Pd(PPh3)4 (9.0 mg, 7.8 μmol). PNDT was obtained as a dark red solid (yield=0.21 g, 85%). GPC (eluent: THF): Mn=6.8 kg mol−1, Mw=11.7 kg/mol, PDI=1.7. Anal calcd (%) for C38H44S4: C 72.56, H 7.05, S 20.39; found: C 70.30, H 6.61, S 19.32, Br 0.15.

OFET device fabrication and measurements

PNDTS and PNDT were employed for OFETs with a bottom-gate/top-contact geometry. Heavily doped n-type Si wafers with a thermally grown 300-nm-thick SiO2 layer were used as the substrates. The SiO2/Si substrates were pretreated with a piranha solution at 100 °C for 1 h, and the copiously cleaned by sonication in deionized water, acetone, and isopropanol. A self-assembled monolayer of n-octyltrichlorosilane was then deposited onto the SiO2/Si substrates as an additional dielectric to reduce charge trapping by the silanol groups on SiO2 in the devices. The polymer layer was then prepared by spin-coating from a chlorobenzene solution, after passing through a 0.45 μm PTFE membrane filter. The devices were completed by evaporating gold (thickness=50 nm) through a shadow mask to define the source and drain electrodes with a channel length of 50–100 μm. The output and transfer characteristics of the OFETs were measured using an Agilent B1500A semiconductor parameter analyzer (Agilent Technologies, Santa Clara, CA, USA) under ambient conditions. Field-effect mobilities (μ) of OFETs were calculated in the saturation regime using the flowing equation: ID=(W/2L)μCi(VG–Vth)2, where ID is the source-drain current, W and L are the channel width and length, respectively, Ci is the capacitance per unit area of the gate dielectric (11.1 nF cm−2), VG is the gate voltage, and Vth is the threshold voltage.

OSC device fabrication and measurements

Pre-patterned ITO-coated glass substrates were cleansed sequentially by sonicating in detergent solution, deionized water, acetone, and isopropanol for 15 min each, and then subjected to UV/ozone treatment for 30 min. A thin layer (~30 nm) of ZnO was prepared by spin-coating (at 5000 r.p.m.) a precursor solution of zinc acetate dehydrate (0.50 g) and ethanolamine (0.14 g) in 2-methoxyethanol (5 ml), followed by baking at 200 °C for 10 min under air. The photoactive layer was then prepared by spin-coating from a chlorobenzene solution containing the polymer and [6,6]-phenyl-C71-butyric acid methylester (PC71BM), after passing through a 0.45 μm PTFE membrane filter. Finally, a 10-nm-thick MoO3 layer and a 100-nm-thick Ag layer were thermally evaporated on top of the active layer under high vacuum through a shadow mask, defining an active area of 0.04 cm2 for each device. Current density–voltage (J–V) and incident photon-to-current conversion efficiency (IPCE) measurements for the fabricated OSCs were conducted on a computer-controlled Keithley 2400 source measure unit in air, under simulated AM 1.5G solar illumination at 100 mW cm–2 (1 sun), using a Xe lamp-based Bunko-Keiki SRO-25 GD solar simulator and IPCE measurement system (Bunko-Keiki, Tokyo, Japan). The light intensity was calibrated using a standard silicon photovoltaic reference cell.

Results and Discussion

Synthesis and characterization

For the synthesis of NDT-based homopolymers, we performed the dehalogenative polycondensation of the corresponding dibromo monomers [Br-NDT(S)-Br] using hexamethylditin as the condensation reagent31 in the presence of a catalytic amount of Pd(PPh3)4 (Scheme 1). The detailed synthetic processes for the dibromo monomers are described in Supplementary Information. The resulting polymers were purified by reprecipitation, followed by Soxhlet extractions with methanol, acetone, and hexane. The weight-average molecular weights (Mw) and polydispersity indices (PDI) of the THF soluble parts were Mw=8.3 kg mol−1 and PDI=2.1 for PNDTS and Mw=11.7 kg mol−1 and PDI=1.7 for PNDT, as determined by gel permeation chromatography analysis using a polystyrene standard in a THF eluent. The Mw values might be underestimated due to lower solubility of these polymers in THF. The end-capping of the polymers with thiophene was confirmed by elemental analysis for Br contents. These polymers showed enough high solubility in chlorobenzene and o-dichlorobenzene at room temperature, which ensured the OFET and OSC device fabrications. Thermogravimetric analysis of PNDTS and PNDT indicated 5% weight-loss temperatures (Td) of 326 and 314 °C, respectively, under N2 (Supplementary Figure S1). The present polycondensation method utilizing hexamethylditin is useful for the synthesis of these thienoacene-based homopolymers because the corresponding dibromo monomers can be used directly for the polymerization reactions without preparing any unstable organometallic intermediates. The widely used straightforward method for producing these π-conjugated homopolymers is polycondensation via Stille cross-coupling reactions using both distannyl and dibromo monomers, which requires two different monomers to be prepared independently.27, 28, 29

Optical properties

UV–vis absorption spectra of PNDTS and PNDT in as-spun thin films are shown in Figure 1a. The relevant photophysical data are summarized in Table 1. These polymers exhibited three characteristic absorption bands. The absorption spectra in chloroform solutions are similar to those in the solid thin films (Supplementary Figure S2), implying the formation of aggregates even in the solution state. The absorption peaks of PNDTS in the thin film are slightly red-shifted compared to that of PNDT, suggesting the extension of the effective π-conjugation length and the increment of intermolecular interactions, which were presumably due to additional thioether groups in PNDTS. The optical bandgaps (Eg) of PNDTS and PNDT were estimated to be 2.29 and 2.35 eV, respectively, according to the onset positions of their lowest-energy absorption bands in the thin films. From the photoelectron yield spectroscopy (Figure 1b), the HOMO energy levels of PNDTS and PNDT were determined to be −5.25 and −4.97 eV, respectively, indicative of the deeper-lying HOMO energy level of PNDTS. The lowest unoccupied molecular orbital (LUMO) energy levels of PNDTS and PNDT (−2.96 and −2.62 eV, respectively) were sufficiently higher than that of [6,6]-phenyl-C71-butyric acid methylester (PC71BM; LUMO=−4.3 eV) to enable directed photoinduced charge transfer. Accordingly, both PNDTS and PNDT can be utilized as donor materials in OSC devices with PC71BM as an acceptor material, as discussed later.

Figure 1
figure 1

(a) UV–vis absorption spectra and (b) photoelectron yield spectra of PNDTS and PNDT in as-spun thin films.

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Table 1 Summary of photophysical properties of PNDTS and PNDT
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We also performed density functional theory calculations for the corresponding pentamers as models of PNDTS and PNDT at the B3LYP/6-31(d) level (Supplementary Figure S3). The distributions of the HOMO and LUMO wave functions in both molecules appeared to be very similar and were delocalized over their π-conjugated backbone. More importantly, replacing alkyl groups with thioalkyl groups indeed deepened the HOMO energy level because of the reduced electron-donating ability of the thioalkyl groups.32, 33, 34

Charge-transport properties

To evaluate the charge-transport properties, top-contact/bottom-gate OFETs based on PNDTS and PNDT were fabricated by spin-coating their chlorobenzene solutions onto n-octyltrichlorosilane-treated SiO2/Si substrates, followed by the vacuum-deposition of Au source and drain electrodes. The output and transfer characteristics of the PNDTS- and PNDT-based OFETs are shown in Figure 2, and the hole mobilities (μ) calculated according to the saturated regime, current on/off ratios (Ion/Ioff), and threshold voltages (Vth) are listed in Table 2. Both OFET devices exhibited typical p-type transistor behavior under ambient conditions. The hole mobility of PNDTS (μ=1.4 × 10−4 cm2 V−1 s−1) was substantially higher than that of PNDT (μ=3.8 × 10−5 cm2 V−1 s−1). Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements revealed that PNDTS had a stronger tendency to take an edge-on orientation on the substrate than PNDT (Supplementary Figure S4 and Supplementary Table S1). Thus, better charge-carrier transport along the parallel direction to the substrate was attained with PNDTS through π–π interactions in OFETs. The Vth value is generally proportional to the energy difference between the electrode work function and the HOMO level of the incorporated p-type semiconductor. PNDTS, with a deeper HOMO level of −5.25 eV, indeed required a negatively large gate bias (Vth=−24 V) to accumulate holes at the dielectric interface compared with PNDT (Vth=−10 V), which had a shallower HOMO level of −4.97 eV.

Figure 2
figure 2

Charge–carrier transport properties: (left) output characteristics at VD=−80 V and (right) transfer characteristics of OFETs based on (a, b) PNDTS and (c, d) PNDT with channel width/length of 1000/100 μm and 1000/60 μm, respectively.

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Table 2 Summary of device performances of PNDTS and PNDT
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Photovoltaic properties

BHJ OSCs were fabricated by spin-coating polymer/PC71BM blend solutions in chlorobenzene onto a ZnO-deposited ITO glass substrate, followed by the vacuum-deposition of MoO3 as a hole-extraction layer and Ag as an anode.35 The OSCs were optimized by changing the weight ratio of the polymer/PC71BM BHJ blends to 1:2, 1:1, and 2:1 (Supplementary Figure S5 and Supplementary Table S2), and both the PNDTS- and PNDT-based devices exhibited the best performance with the weight ratio of 1:1. Figure 3a shows the current density–voltage (J–V) characteristics of PNDTS- and PNDT-based BHJ OSCs with the optimal weight ratio, measured under simulated AM 1.5G illumination (100 mW cm−2). The key photovoltaic parameters are listed in Table 2. The PNDTS-based OSC showed a higher power-conversion efficiency (PCE) of 3.0% with an open-circuit voltage (Voc) of 0.84 V, a short-circuit current density (Jsc) of 7.83 mA cm−2, and a fill factor (FF) of 45%, while the PNDT-based OSC had a PCE of 2.5% with a Voc of 0.78 V, a Jsc of 7.22 mA cm−2, and a FF of 44%.

Figure 3
figure 3

Photovoltaic properties of the polymers: (a) J–V curves (inset: dark J–V curves) and (b) IPCE spectra for BHJ OSCs based on PNDTS:PC71BM (1:1, w/w) and PNDT:PC71BM (1:1, w/w), measured under AM 1.5G (100 mW cm−2).

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The incident photon-to-current conversion efficiency (IPCE) spectra of the optimal BHJ OSCs indicate intense photocurrent responses in the range of 350–550 nm (Figure 3b), which agrees well with the optical absorption of these polymers (Figure 1a). It should be noted that PNDTS with a deeper HOMO energy level exhibited a lager Voc of up to 0.90 V than that of PNDT. In general, the Voc of BHJ OSCs is closely correlated with the energy difference between the HOMO of the donor material and the LUMO of the acceptor material (PC71BM).36 Therefore, the introduction of alkylthio-thienyl substituents in PNDTS increased the Voc of the photovoltaic device.

To characterize the microstructures and molecular orientations in the solid states, GIWAXS measurements were performed for the polymer/PC71BM (1:1, w/w) BHJ blend films. The two-dimensional (2D) GIWAXS images of the blend films are shown in Figure 4a, and their line cut profiles for the out-of-plane (qz) and in-plane (qxy) directions are shown in Figure 4b, c respectively (see Supplementary Table S3 for detailed packing parameters). Both the PNDTS- and PNDT-based blend films showed a semi-crystalline nature and a preferred edge-on orientation on substrates with the lamellar spacings (d100) of 25.0 and 22.8 Å, respectively, suggesting that the incorporation of alkylthio-thienyl substituents in PNDTS slightly increased the lamellar spacing. The π–π stacking distance (d010) for PNDTS and PNDT was observed at 3.5 Å, which is comparable to that of BDT-based homopolymer28 but smaller than those of conventional thiophene-based polymers such as P3HT.37 Thus, PNDTS and PNDT appear to form relatively tight interchain packing in the BHJ blend films, owing to the high coplanarity of NDT units.

Figure 4
figure 4

(a) 2D GIWAXS patterns of blend films of PNDTS and PNDT with PC71BM (1:1, w/w). (b) Out-of-plane (qz-scan) and (c) in-plane (qxy-scan) profiles of the corresponding blend films.

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Conclusions

In conclusion, two novel π-conjugated NDT-based homopolymers, PNDTS and PNDT, were designed and synthesized. To simplify the synthesis route, we employed palladium-catalyzed dehalogenative polycondensation with hexamethylditin as the condensation reagent. Replacing alkyl-thienyl groups with alkylthio-thienyl groups resulted in the deepening of the HOMO energy level of the resulting polymers, which was beneficial for photovoltaic applications. OFET and BHJ OSC devices based on PNDTS exhibited a higher hole mobility and PCE than PNDT-based devices. Moreover, PNDTS-based BHJ OSCs showed a large Voc up to 0.90 V because the alkylthio-thienyl groups lowered the HOMO energy level of the polymer, which makes PNDTS a promising donor material for BHJ OSCs.

scheme 1

Synthesis of NDT-based homopolymers.