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Side-chain engineering in naphthalenediimidebased n-type polymers for high-performance all-polymer photodetectors† Liuyong Hu,a,b Jinfeng Han,a,b Wenqiang Qiao,*a Xiaokang Zhou,a,b Canglong Wang,c Dongge Ma, a Yuning Li d and Zhi Yuan Wang

*a,e

A series of n-type semiconducting conjugated polymers based on naphthalene diimide (NDI) having three different side chains, 5-decylpentadecyl, 2-octyldodecyl and 4-(2-octyldodecyloxy)phenyl, were synthesized. Experimental results showed that the subtle structural changes at the side chains can influence the molecular packing, electron mobility, blend film morphology and thus performReceived 24th November 2017, Accepted 6th December 2017 DOI: 10.1039/c7py01980g rsc.li/polymers

ance of bulk-heterojunction polymer photodetectors using these NDI-based polymers as acceptors. The optimized all-polymer photodetector exhibited a specific detectivity (D*) of over 1013 Jones in the spectral region of 300–800 nm under −0.1 V bias, which is among the best D* values of the reported UV–vis–NIR all-polymer photodetectors and comparable to the best fullerene-based photodetectors.

Introduction Bulk-heterojunction all-polymer photodetectors (all-PPDs), containing a p-type semiconducting polymer as a donor and an n-type semiconducting polymer as an acceptor, have several attractive characteristics such as complementary absorptions of both donor and acceptor polymers, tunable energy levels and stable film morphology, in comparison with the fullerene based counterparts.1–4 On the basis of their spectra response, the PPDs could be divided into narrowband and broadband, which are used to detect a narrow range of light and a broad spectrum of light, respectively. 5–7 The development of all-PPDs is strongly dependent on the availability of donor and acceptor polymers. Thus, a tremendous effort has been devoted to designing and synthe-

a

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. E-mail: [email protected] b University of Chinese Academy of Sciences, Beijing 100049, P. R. China c Institute of Modern Physics, Chinese Academy of Science, Lanzhou 730000, P. R. China d Department of Chemical Engineering and Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 e Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6. E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional information about the CV data, absorption spectra and device characterization (dark J–V, R and D* data). See DOI: 10.1039/c7py01980g

This journal is © The Royal Society of Chemistry 2018

sizing novel electron-donating building blocks for donor polymers,8–16 and to a lesser extent electron-accepting building blocks for acceptor polymers.17–21 Therefore, it is necessary to further develop new n-type semiconducting polymers and investigate their optoelectronic properties in all-PPDs. Naphthalene diimide (NDI) is one of the most promising building blocks to construct n-type semiconducting polymers with high electron affinity and electron mobility.22–26 NDIbased n-type semiconducting polymers have been successfully used in photovoltaic, field effect transistor and photodetector areas.27–29 In some cases, the side chains of NDI-based polymers have been modified in order to alter the solubility, aggregation behavior and charge mobility and thus the device performance.30–32 For example, Cho et al. introduced the partially fluorinated alkyl side chains into NDI-based polymers and achieved remarkably high electron mobility values of up to 6.5 cm2 V−1 s−1, due to the high degree of order in the polymer backbone.33 Compared to the modification of backbones in NDI-based polymers with a random copolymerization strategy in our previous work,3 we introduced different side chains at the N-position of the NDI moiety to synthesize a series of n-type semiconducting polymers, namely PNDI-5DD, PDNI-2OD and PNDI-POD having the side chains of 5-decylpentadecyl, 2-octyldodecyl and 4-(2-octyldodecyloxy)phenyl, respectively (Fig. 1). The structural effect of these side chains on the molecular stacking, charge mobility, film morphology and photodetector performance was systematically studied.

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Fig. 1 Chemical structures of the acceptor polymers of PNDI-5DD, PNDI-2OD, and PNDI-POD and the donor polymer of PTB7-Th used in all-PPDs.

Experimental section General methods 1

H and 13C NMR spectra were recorded on a Bruker Avance 400 NMR spectrometer. UV–visible–near-infrared (UV–vis–NIR) absorption spectra were recorded on a Shimadzu UV-3700 spectrophotometer. The element analysis was conducted on a Vario EL Elementar Analysis Instrument. The molecular weight of the polymers was obtained by the gel permeation chromatography (GPC) method, using polystyrene as a standard and chloroform (CHCl3) as an eluent at room temperature. Differential scanning calorimetry (DSC) was performed on a TA-DSC Q100 from 20 °C to 400 °C under a nitrogen atmosphere with a heating/cooling rate of 10 °C min−1. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Pyris Diamond TG from 50 to 800 °C at a heating rate of 10 °C min−1 under a continuous nitrogen flow. Electrochemical properties of the polymers were characterized by cyclic voltammetry (CV). CV experiments were carried out on a CHI660b electrochemical workstation in an acetonitrile solution of n-Bu4NPF6 (0.1 M) at a scan speed of 50 mV s−1. Pt disk (2 mm diameter), Pt plate, and Ag/AgCl were used as a working electrode, a counter electrode, and a reference electrode, respectively. The polymer thin films for electrochemical measurements were coated from a chloroform solution, ca. 5 mg mL−1, onto a Pt disk electrode. Ferrocene was used as an internal standard of redox potentials for calibration. Atomic force microscopy (AFM) was performed on a SPA300HV instrument equipped with a SPI3800N controller (Seikoin Instruments, Japan) in tapping mode under ambient conditions using silicon cantilevers (Applied Nanostructures, nominal spring constant of 2.0 N m−1 and nominal resonance frequency of ∼75 kHz). The out-of-plane grazing incidence X-ray diffraction (GIXD) measurements were conducted on a Bruker D8 Discover reflector, with the scattering angle 2-theta ranging from 2° to 30° and a step-scan rate of 0.05° per 5 s. Synthesis of monomers and polymers Materials. 2,6-Dibromonaphthalene-1,4,5,8-tetracarboxydianhydride, 2-octyldodecyl-1-amine (1b), 5,5′-bis(trimethylstannyl)-2,2′-bithiophene and PTB7-Th were purchased from SunaTech Inc and used without further purification. 5-Decylpentadecyl-1-amine (1a) and 4-(2-octyldodecyloxy)

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aniline (1c) were synthesized according to the reported procedures.34,35 N,N′-Bis(5-decylpentadecyl)-2,6-dibromonaphthalene-1,4,5,8-bis (dicarboximide) (2a). A mixture of 2,6-dibromonaphthalene1,4,5,8-tetracarboxydianhydride (4.0 g, 9.43 mmol), 1a (8.83 g, 24 mmol), o-xylene (36 mL), and propionic acid (12 mL) was stirred at 140 °C for 2 h. When the reaction was cooled to room temperature, the solvents were removed under reduced pressure, and the crude product was purified by column chromatography on silica gel with a mixture of dichloromethane and hexane (1 : 2, v/v) as the eluent. The resulting red solid was further purified by recrystallization from n-hexane to afford a slightly yellow solid as the product (3.7 g, 35% yield). 1 H NMR (400 MHz, CDCl3, 25 °C) δ ( ppm): 8.99 (s, 2H), 4.19 (t, 4H), 1.75–1.65 (m, 4H), 1.55 (m, 2H), 1.45–1.35 (m, 8H), 1.30–1.20 (m, 72H), 0.86–0.89 (m, 12H). 13C NMR (100 MHz, CDCl3, 25 °C) δ ( ppm): 160.66, 139.02, 128.30, 127.69, 125.35, 124.08, 41.63, 37.41, 33.63, 33.35, 31.92, 30.14, 29.72, 29.66, 29.36, 28.33, 26.71, 24.30, 22.68, 14.10 (note: some peaks in the 13C NMR spectrum overlap). MALDI-TOF MS: calculated for C64H104Br2N2O4 1125.33; found: 1123.6 (M+). N,N′-Bis(2-octyldodecyl)-2,6-dibromonaphthalene-1,4,5,8-bis (dicarboximide) (2b). Compound 2b (2.75 g, yield 30%) as a slightly yellow solid was obtained from 2,6-dibromonaphthalene-1,4,5,8-tetracarboxydianhydride (4.0 g, 9.43 mmol) and 1b (7.14 g, 24 mmol), using the same procedures as for 2a. 1 H NMR (400 MHz, CDCl3, 25 °C) δ ( ppm): 8.99 (s, 2H), 4.15 (d, 4H), 1.99 (m, 2H), 1.40–1.20 (m, 64H), 0.89–0.84 (m, 12H). 13 C NMR (100 MHz, CDCl3, 25 °C) δ ( ppm): 161.13, 160.98, 139.12, 128.34, 127.72, 125.28, 124.08, 45.44, 36.46, 31.90, 31.87, 31.58, 30.01, 29.61, 29.58, 29.53, 29.33, 29.28, 26.34, 22.67, 22.65, 14.09. MALDI-TOF MS: calculated for C54H84Br2N2O4 985.06; found: 983.5 (M+). N,N′-Bis(4-(2-octyldodecyloxy)phenyl)-2,6-dibromonaphthalene1,4,5,8-bis(dicarboximide) (2c). Compound 2c (3.3 g, 30%) as a yellow solid was obtained from 2,6-dibromonaphthalene1,4,5,8-tetracarboxydianhydride (4.0 g, 9.43 mmol) and 1c (9.35 g, 24 mmol), using the same procedures as for 2a. 1 H NMR (400 MHz, CDCl3, 25 °C) δ ( ppm): 9.01 (s, 2H), 7.20 (d, 4H), 7.06 (d, 4H), 3.90 (d, 4H), 1.85–1.75 (m, 2H), 1.35–1.25 (m, 64H), 0.86–0.89 (m, 12H). 13C NMR (100 MHz, CDCl3, 25 °C) δ ( ppm): 161.22, 161.13, 159.93, 139.40, 129.25, 128.90, 128.16, 126.30, 125.81, 124.56, 115.47, 71.12, 37.98, 31.94, 31.93, 31.41, 30.06, 29.71, 29.67, 29.63, 29.36, 26.89, 22.70, 14.13 (note: some peaks in the 13C NMR spectrum overlap). MALDI-TOF MS: calculated for C66H92Br2N2O6 1169.25; found: 1169.5 (M+). PNDI-2OD. Compound 2b (196.5 mg, 0.20 mmol) and 5,5′bis(trimethylstannyl)-2,2′-bithiophene (98.4 mg, 0.20 mmol) were dissolved in 4 mL of toluene in a flask protected by argon, and then 4 mg Pd2(dba)3 and 12 mg P(o-tol)3 were added into the flask. The reaction solution was heated to 110 °C gradually, and kept at 110 °C for 72 h under an argon atmosphere. After that, the reaction solution was cooled to room temperature, and precipitated into methanol. The precipitated solid was collected and purified by Soxhlet extraction


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sequentially with acetone (24 h), hexane (24 h), and chloroform (24 h). The chloroform solution was concentrated under reduced pressure and added dropwise into methanol. The precipitate was collected by filtration and dried under vacuum at room temperature for 8 h to afford PNDI-2OD as a dark solid (150 mg, yield: 76%). The polymer was thermally stable up to 450 °C (5% weight loss by TGA). 1H NMR (400 MHz, CDCl3, 25 °C) δ ( ppm): 8.86–8.80 (2H, br), 7.38–7.32 (4H, br), 4.24–4.03 (4H, br), 2.08–1.95 (2H, br), 1.50–1.16 (64H, br), 0.90–0.80 (12H, br). Anal. calcd: C, 75.26; H, 8.96; N, 2.83; S, 6.48%. Found: C, 74.97; H, 9.00; N, 2.75; S, 6.46%. Mw/Mn (GPC): 300.8k/203.9k. PNDI-5DD. Compound 2a (0.2 mmol, 225.1 mg) and 5,5′-bis (trimethylstannyl)-2,2′-bithiophene (0.2 mmol, 98.4 mg) were used. A dark solid was obtained (197 mg, 85% yield). 1H NMR (400 MHz, CDCl3, 25 °C) δ ( ppm): 8.87–8.79 (2H, br), 7.38–7.32 (4H, br), 4.25–4.08 (4H, br), 1.75–1.16 (86H, br), 0.90–0.80 (12H, br). Anal. calcd: C, 76.63; H, 9.91; N, 2.42; S, 5.53%. Found: C, 75.97; H, 10.08; N, 2.25; S, 5.46%. Mw/Mn (GPC): 123.3k/234.3k. PNDI-POD. Compound 2c (0.2 mmol, 233.8 mg) and 5,5′-bis (trimethylstannyl)-2,2′-bithiophene (0.2 mmol, 98.4 mg) were used. A dark solid was obtained (192 mg, 80% yield). 1H NMR (400 MHz, CDCl3, 25 °C) δ ( ppm): 8.88–8.84 (2H, br), 7.33–7.28 (4H, br), 7.25–7.15 (d, 4H), 7.08–6.98 (d, 4H), 3.91–3.81 (4H, br), 1.83–1.73 (2H, br), 1.50–1.16 (64H, br), 0.90–0.80 (12H, br). Anal. calcd: C, 75.83; H, 8.54; N, 2.33; S, 5.33%. Found: C, 75.63; H, 8.32; N, 2.43; S, 5.23%. Mw/Mn (GPC): 52.2k/ 140.9k.

Paper

active area of the device. The photocurrent signal was amplified by using a low-noise current amplifier (DLPCA-200, Femto) and then detected using a lock-in amplifier (SR830, Stanford Research Systems). A Keithley 236 Source Measure unit was applied to measure the dark current density–voltage ( J–V curve) characteristics of the devices. Field-effect transistor fabrication and characterization The n-doped silicon substrate and a 300 nm silicon oxide layer were applied as the insulating layer and gate electrode, respectively. The substrates were cleaned sequentially in ultrasonic baths with deionized water, acetone and isopropyl alcohol for 30 min each, dried under a nitrogen gas flow and baked at 100 °C for 1 h. After that, the substrate was treated with UVozone for 10 min. The polymer PNDI-R in the chloroform solution (8 mg L−1) was directly spin-coated onto the substrate at 1500 rpm for 60 s under a nitrogen atmosphere. The film was annealed at 180 °C for 10 min, and then the source/drain electrodes were deposited by evaporation under high vacuum of Au with a channel width (W) of 3000 μm and length (L) of 100 μm. Current–voltage characteristics of the fabricated transistors were measured in a nitrogen-filled glovebox. The saturation region field-effect mobility (μ) and the threshold voltage 1=2 (Vth) were acquired from the plots of IDS vs. VGS in a forward scan with VDS under 80 V through the saturation-region transistor equation: IDS = (μWCi)(VGS − Vth)2/(2L).

Results and discussion

Device fabrication and characterization

Synthesis of monomers and polymers

The devices with the structure of ITO/PEDOT:PSS/acceptor polymer:PTB7-Th/ZnO/Al were fabricated with the following steps. ITO-coated glass substrates were washed sequentially in ultrasonic baths with acetone, deionized water, and isopropyl alcohol, and then baked at 120 °C for 1 h. A thin layer of PEDOT:PSS (Baytron P VP Al 4083) was spin-coated on the top of UV-ozone treated ITO at 3000 rpm for 60 s, and then baked for 30 min at 120 °C under ambient conditions. Each active blending solution (total of 16 mg mL−1, acceptor polymer: PTB7-Th, 1 : 1 w/w) for spin-coating contained 3% of diphenyl ether (DPE) by volume as the additive in chloroform. The polymer blending solution was spin-coated at 1500 rpm for 40 s under a nitrogen atmosphere and the active layer thicknesses were 101, 109 and 103 nm for PNDI-5DD:PTB7-Th, PNDI-2OD: PTB7-Th and PNDI-POD:PTB7-Th, respectively. Then the n-butanol solution of ZnO was spin-coated on the active layer at 1500 rpm for 40 s, affording the ZnO layer with the thickness of 20 nm. The Al (100 nm) layer was subsequently evaporated on the surface of ZnO under high vacuum (3 × 10−4 Pa) as the cathode electrode. Two pixels, each with the active area of 0.16 cm2, were fabricated per ITO. EQE measurements were performed under ambient conditions with the equipment from Beijing 7-Star Optical Instruments Co., Ltd. Incident light from a 250 W halogen lamp passing through two cascade monochromators was chopped at 25 Hz and focused on the

To investigate the steric hindrance effect of the side chains of NDI-based polymers on polymer properties and device performance, three polymers were designed and synthesized as shown in Scheme 1. The side chains of 5-decylpentadecyl, 2-octyldodecyl, 4-(2-octyldodecyloxy)phenyl are grafted on the NDI moiety. Together with the bithiophene building block, three conjugated polymers, PNDI-5DD, PNDI-2OD and PNDI-POD, with different side chains were synthesized. The number-average molecular weight (Mn) and the polydispersity index (PDI) were found to be 123.3 kDa (PDI = 1.9) for PNDI-5DD, 203.9 kDa (PDI = 1.5) for PNDI-2OD and 52.2 kDa (PDI = 2.7) for PNDI-POD. The structures of three polymers were also characterized by 1H NMR spectroscopy and element analysis. PNDI-5DD and PNDI-2OD exhibit good solubility in common solvents, such as chloroform, chlorobenzene and tetrahydrofuran (THF), while PNDI-POD is only soluble in chloroform.


Thermal properties Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to study the thermal properties of the polymers. The three polymers show good thermal stability with decomposition temperatures (Td) for 5% weight loss over 440 °C under a nitrogen flow (Fig. S1a†). The second heating and cooling DSC traces of these polymers are

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Scheme 1 A synthetic route to PNDI-5DD, PNDI-2OD and PNDI-POD. Reagents and conditions: (i) o-Xylene, propionic acid, 140 °C, 2 h. (ii) 5,5’-Bis (trimethylstannyl)-2,2’-bithiophene, Pd2(dba)3, P(o-tol)3, toluene, 100 °C.

shown in Fig. S1b.† The DSC traces of PNDI-2OD and PNDI-5DD exhibited an endothermic peak at 270 °C and 310 °C upon heating, and an exothermic peak at 200 °C and 220 °C upon cooling, corresponding to the melting and crystallization transitions, respectively.36 However, no clear thermal transition was observed for PNDI-POD, due to its rigid backbone and large steric hindrance in the side chain. Optical and electrochemical properties The absorption spectra of the three NDI-based polymers in dilute chloroform and as thin films are shown in Fig. S2† and Fig. 2a, respectively. These polymers exhibit similar spectral features as other known NDI-based conjugated polymers with broad absorptions at 300–500 nm and 500–900 nm, corresponding to π–π* transitions and intramolecular charge transfer opt (ICT), respectively.37 The optical band gaps (Eg ) of the poly-

Fig. 2 (a) The optical absorption of the polymer films spin-coated on a quartz substrate; (b) the HOMO and LUMO energy levels of the acceptor polymers and PTB7-Th.

Table 1

mers were estimated from the absorption edges of the polymer thin films to be 1.37, 1.38 and 1.43 eV for PNDI-2OD, PNDI-5DD and PNDI-POD, respectively. The slight large optical bandgap of PNDI-POD suggests that the phenyl group in the side chains does not effectively extend the overall π-conjugation length. To better gain insight into the bandgap and molecular structures of PNDI-R, density functional theory (DFT) calculations were performed with the DMol3 code,38,39 and the Generalized Gradient Approximation (GGA) Perdew–Burke– Ernzerhof exchange–correlation functional (PBE) was used.40 The long alkyl side chains of the polymers were replaced with methyl groups and the polymers were simplified to three repeating units. Compared with the alkyl side chains, the phenyl group exhibited a large torsional angle with the backbones of the polymers (Fig. S3†). This result indicates that the introduction of alkoxyphenyl side chains would slightly affect the bandgap of the polymers. Furthermore, the electrochemical properties of the polymers were investigated by cyclic voltammetry (CV) (Fig. S4†). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the onset of the oxidation and reduction waves, respectively (Table 1). The LUMO energy levels of the polymers were determined to be −3.97, −3.97, and −3.95 eV and the corresponding HOMO energy levels were −5.93, −5.96, and −5.92 eV. Accordingly, the side chains have no influence on the energy levels of the polymers. Furthermore, the HOMO and LUMO levels of the acceptor polymer matched well with those of the donor polymer (PTB7Th), which are desirable for the efficient separation and transfer of the excitons in the BHJ system (Fig. 1b).41,42

Characterization of the polymers

PNDI-5DD PNDI-2OD PNDI-POD

opt: d

Mn a (kDa)

PDIa

Td b (°C)

c λfilm max (nm)

Eg

123.3 203.9 52.2

1.9 1.5 2.7

445 450 440

706 701 657

1.38 1.37 1.47

(eV)

HOMOe (eV)

LUMOe (eV)

f EEC (eV) g

−5.96 −5.93 −5.92

−3.97 −3.97 −3.95

1.99 1.96 1.97

a

Measured by GPC with polystyrene standards in chloroform at room temperature. b Onset temperature for 5% weight loss in nitrogen by TGA. Film spin-cast from 10 mg mL−1 chloroform solution on a quartz substrate. d Optical band gap. e Calculated from ELUMO = −e(Ered on + 4.43) and f EHOMO = −e(Eox on + 4.43). Electrochemical band gap is derived from EHOMO subtracting ELUMO.

c

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Photodetector characteristics

Fig. 3 The out-of-plane GIXRD diagrams of the thin films of acceptor polymers.

Molecular stacking To study the influence of the side chains on the local orientation and molecular packing of the polymer thin films, the out-of-plane grazing incidence X-ray diffraction (GIXRD) was measured. As shown in Fig. 3, PNDI-2OD only displayed a strong (010) diffraction at 22.75° along the out-of-plane direction, indicating the preferential face-on orientation in thin films. However, the thin films of PNDI-5DD displayed the prominent higher-order scattering peaks up to the second order (200), implying the dominating edge-on orientation. In addition, PNDI-POD exhibited a disordered structure in thin films with no obvious scattering peaks, presumably due to the large steric hindrance in side chains. Thus, the side chains on the NDI moieties can significantly affect the molecular packing and orientation of the polymer chains, which are critical for charge transfer and separation in the BHJ film. Field-effect electron mobility In order to study the influence of side chains on electron mobility, the field-effect transistors (FET) with a bottom-gate (BG) and top-contact (TC) geometry were fabricated and measured. The transfer characteristics are displayed in Fig. S5† and the data are summarized in Table S1.† PNDI-5DD, PNDI-2OD and PNDI-POD exhibited the electron mobility (μe) values of 9.6 × 10−4, 1.9 × 10−4 and 4.2 × 10−5 cm2 V−1 s−1, respectively. The highest electron mobility was achieved with the PNDI-5DD fabricated FET. In the thin film of PNDI-5DD, polymer chains take the preferential edge-on orientation, which is proved to be in favor of charge hopping between adjacent polymer chains along the lateral direction in the polymer films.43


To evaluate and compare these polymers as electron acceptors in all-PPDs, the devices with the structure of ITO/PEDOT:PSS/ polymer blend/ZnO/Al were fabricated (Fig. 4a). The active layer consists of the blends of PTB7-Th:PNDI-2OD, PTB7-Th: PNDI-5DD and PTB7-Th:PNDI-POD. A well-known donor polymer of PTB7-Th has relatively strong absorption in the UV–vis–NIR region and preferential face-on molecular orientation and has led to the development of efficient photodetectors and photovoltaic devices with other acceptor polymers.44,45 To promote the desired film morphology of the active layers, a small amount (3 vol%) of diphenyl ether was utilized as a solvent additive. A thin layer of ZnO was used to modify the cathode to improve the electron extraction and block the hole injection.46 The dark current density–voltage characteristics of the PPDs are shown in Fig. S6–8,† and the relevant parameters are given in Table 2 and Fig. 4b. All the photodetectors exhibited relatively low dark current density and varied slightly with the types of side chains on the NDI moieties. The lowest dark current density ( Jd) of 1.2 × 10−10 A cm−2 was offered by PNDI-5DD based devices. On increasing the sizes of side chains, Jd increased for PNDI-2OD and PNDI-POD based PPDs, being 8.6 × 10−9 and 7.1 × 10−9 A cm−2, respectively. Therefore, by finely tuning the side chains of PNDI, the current in the dark could be reduced. The external quantum efficiency (EQE) of the all-PPDs measured under −0.1 V is shown in Fig. 4c and the EQE spectra measured under other biases are given in Fig. S6–8.† The PPDs based on PNDI-2OD displayed the highest efficiency in converting a photon to an electron with an EQE of ∼40% at the wavelengths of 300–800 nm, which can be attributed to the face-on orientation in the film for promoting charge carrier transfer in the vertical direction. The device based on PNDI-5DD displayed an EQE of ∼35% at 300–800 nm, while the PNDI-POD device exhibited a low EQE of 25%. The figure-of-merit of PPDs is mainly determined by specific detectivity (D*), which is related to Jd and EQE. Assuming Jd as the major contributor to the shot noise, D* can be calculated with the equation of D* = EQE × (λ/1240)/ (2qJd)1/2, where λ is the wavelength in nm, q is the absolute charge of 1.6 × 10−19 C and Jd is the dark current density in A cm−2. The D* values of the all-PPDs at −0.1 V are shown in Fig. 4d and Table 2. Owing to the lowest dark current (1.2 × 10−10 A cm−2) and relatively high EQE (30.6% at 400 nm and 30.6% at 700 nm), the photodetector based on PNDI-5DD displayed the highest D* of 1.7 × 1013 Jones at 400 nm and 3.0 × 1013 Jones at 700 nm. The device based on PNDI-2OD and PNDI-POD exhibited lower D* values due to their relatively high dark current and low EQE values, respectively. Finally, the all-PPD based on PNDI-5DD achieved the D* value over 1013 Jones in the spectral region of 300–800 nm under −0.1 V bias, which is among the best D* values of all the known UV– vis–NIR all-PPDs and comparable to the best fullerene-based PPDs.47,48

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Fig. 4 (a) The structure of an all-PPD device; (b) dark current density ( Jd) and specific detectivity (D*) at 400 nm and 700 nm of the polymers. (c, d) The EQE spectra and detectivity of the all-PPDs under −0.1 V bias.

Characteristics of all-PPDs under −0.1 V bias

Table 2

Polymer

Jd a (A cm−2)

EQE400 nm (%)

EQE700 nm (%)

D*400 nm (Jones)

D*700 nm (Jones)

PNDI-5DD PNDI-2OD PNDI-POD

1.2 × 10−10 (1.5 × 10−10) 8.6 × 10−9 (9.8 × 10−9) 7.1 × 10−9 (8.3 × 10−9)

30.6 (29.8) 34.8 (34.1) 22.2 (20.9)

30.6 (30.3) 37.7 (37.4) 22.5 (21.6)

1.7 × 1013 (1.4 × 1013) 2.1 × 1012 (1.9 × 1012) 1.5 × 1012 (1.3 × 1012)

3.0 × 1013 (2.5 × 1013) 4.0 × 1012 (3.7 × 1012) 2.7 × 1012 (2.3 × 1012)

a

The values in brackets are the average values of 5 nominally identical devices.

Film morphology To better understand the influence of polymer structures on the electrical and photovoltaic properties in all-PPDs, the film morphology of the active layers was studied by GIXRD and

Fig. 5

atomic force microscopy (AFM). Fig. S9† shows the out-ofplane GIXRD of the films of PNDI-2OD:PTB7-Th, PNDI-5DD: PTB7-Th and PNDI-POD:PTB7-Th blends. There are similar π–π stacking (010) peaks at 22.5° and the peak intensity gradually decreases from PNDI-2OD to PNDI-5DD and PNDI-POD. The

The AFM height images (2.0 μm × 2.0 μm) of the films of polymer blends.

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stacking distance is known to be able to influence the charge transport in the blend films and thus the EQE.49,50 Furthermore, these blend films exhibited quite different surface roughness as revealed by the AFM measurements (Fig. 5). The root-mean-square (RMS) roughness was 3.7, 9.8 and 6.8 nm for PNDI-5DD, PNDI-2OD and PNDI-POD, respectively. The smoother surface of the PNDI-5DD:PTB7-Th blend film could prevent the top electrode penetrating into the active layer and is ideal for achieving a low dark current.51,52 Therefore, the device performance is affected by the film morphology, which is in turn controlled by the polymer side chains.

Conclusions The side chains of NDI-based acceptor polymers can significantly affect the electron mobility, blend film morphology and then device performance. An all-PPD based on PNDI-5DD with a less steric hindrance group in the side chain showed the relatively high EQE and lowest dark current density due to its smoother surface and preferred molecular stacking. Consequently, this all-polymer photodetector exhibited the specific detectivity of over 1013 Jones in the spectral region of 300–800 nm under −0.1 V bias.

Conflicts of interest The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21134005, 21474102 and 21474105), the International Cooperation Foundation of China (2015DFR10700), and the Natural Science and Engineering Research Council of Canada.

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