Tellurophene-Based Random Copolymers for High Responsivity and

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Tellurophene-Based Random Copolymers for High Responsivity and Detectivity Photodetectors Kai Zhang, Lei Lv, Xiaofen Wang, Yang Mi, Ruiqing Chai, Xinfeng Liu, Guozhen Shen, Aidong Peng, and Hui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15245 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Applied Materials & Interfaces

Tellurophene-Based Random Copolymers for High Responsivity and Detectivity Photodetectors ⊥

⊥

⊥

Kai Zhang,†,‡, Lei Lv,†, Xiaofen Wang,†, Yang Mi,♯ Ruiqing Chai†,‡ Xinfeng Liu,♯ Guozhen Shen,*, †,‡ Aidong Peng*,† and Hui Huang*,† †

College of Materials Science and Opto-Electronic Technology&Key Laboratory of Vacuum Physic, University of Chinese Academy of Sciences, Beijing 100049, P. R. China. ‡

State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China.

♯

Division of Nanophotonics, CAS Key Laboratory of Standardization and Measurement for

Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China KEYWORDS: organic photodetectors, tellurophene, random copolymers, high responsivity and detectivity

ABSTRACT: Organic photodetectors (OPDs) have attracted great attention due to their advantages including tunable response range, easy processability and flexibility. Various conjugated polymers have been developed for high performing OPDs. Herein, a series of tellurophene-based random copolymers containing two typical electron-withdrawing units

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naphthalene diimide (NDI) and perylene diimide (PDI) are designed and synthesized. Through varying the ratio of PDI/NDI moieties of the analogous polymers, the opto-physical properties and film morphology, together with photodetector performances are systematically tuned. It was demonstrated that the photodetectors based on the polymer with molar ratio of PDI/NDI units of 70/30 possessed strong photo-induced absorption and favorable morphology via transient absorption spectra and atomic force microscopy (AFM) studies. As a result, a high responsivity about 19.1A/W at 600 nm and an excellent detectivity more than 1012 Jones ranging from 350600 nm were successfully achieved, which are among the highest values for organic photodetectors and comparable to inorganic counterparts.

1. INTRODUCTION Organic photodetectors (OPDs) have emerged as an attractive technology due to the excellent properties including tunable response spectrum, solution processability, large area detection, light weight and mechanical flexibility.1-6 Recently, OPDs with high external quantum efficiency, low dark current density, fast response and selective or broad detection from ultraviolet-visible (UV-vis) to near infrared (NIR) range have been widely reported.4,

7-8

However, challenges still remain when compared with commercially available silicon photodetectors in terms of spectral response and detectivity, which are the key parameters for their performances.9-10 Various efforts have been made to improve the responsivity and detectivity of OPDs, such as molecular engineering, tuning interfacial layers, and novel device structures.1, 4, 6, 11-12 Among them, the donor-acceptor (D-A) alternating strategy has been employed to construct conjugated polymers possessing high photo-responsivity from UV-vis to NIR spectrum.13-15 Additionally,


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random copolymerization of one electron-donating unit and two different electron-withdrawing units or two different electron-donating units and one electron-withdrawing unit have been adopted to tune the optoelectronic properties as well as the film morphology of the conjugated polymers for low dark current and high performing OPDs.16-19 For example, Wang et al. successfully achieved high performing random copolymers based OPDs with a specific detectivity as high as 1012 Jones ranging of 330-950 nm through tuning the side-chain contents of the analogous polymers.19 Furthermore, they revealed the relationship between the compositions of the polymer acceptors and the morphology, which is critical for the dark current density and detectivity.17 In 2017, Li et al. reported a series of random copolymers based on two different electron deficient building blocks and one electron efficient building block for OPDs. Through tuning the contents of the electron deficient building blocks, the photo-physical properties and morphologies of the conjugated polymers were systematically tuned, resulting in low dark currents and high detectivities over 1012 Jones.20 Photocurrent is an important parameter to influence the responsivity and detectivity,6,

21-22

which is dependent on exciton generation, diffusion, and dissociation in optoelectronics.23-25 Thus photocurrent could be improved through tuning the morphology of the interfacial and active layers to extend singlet exciton diffusion length26-27 or introducing heavy atoms to facilitate the singlet to triplet intersystem crossing for achieving long exciton diffusion length.2829

Tellurophene is attracting much attention to construct organic semiconductors for

optoelectronic devices due to its metalloid characteristics of tellurium and its large spin-orbit coupling, which may lead to long-lived triplet excited states through intersystem crossing.30-33 Thus, tellurophene based conjugated systems may be excellent materials to harness high photocurrent.34-35 For example, Grubbs reported organic photovoltaic devices based on



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tellurophene-containing polymers and achieved a comparable PCE of 4.4% with its thiophene analogues but a much higher photocurrent.30 McCulloch et al. constructed a series of DPPTTbased copolymers with chalcogenphene comonomers units (thiophene, selenophene, and tellurophene). Upon tuning the chalcogenphene moieties, the photovoltaic performances were systematically tuned. As a result, the PCE of 7.1% was achieved for tellurophene based copolymers.36 In 2016, our group reported three novel n-type tellurophene-based copolymers, which were applied for all polymer solar cells for the first time. Upon molecular engineering, a PCE of 4.3% was achieved.37 However, tellurophene based conjugated polymers have never been employed for OPDs. Furthermore, naphthalene diimide (NDI) and perylene diimide (PDI) are widely used electron-withdrawing building blocks to construct D-A conjugated copolymers for optoelectronic devices such as organic field effect transistors,38 photovoltaics37,

39

and

photodetectors.6 Random copolymerization of these two building blocks (PDI and NDI) with other electron donating building blocks has already been employed to finely tune the optophysical properties and the morphology of high performing conjugated polymers.40-41 In this contribution, we designed and synthesized a series of conjugated polymers through random copolymerization of tellurophene and PDI/NDI building blocks. With varying the ratios of the two electron-deficient building blocks, the electronic energy levels, light absorption, and especially the film morphology of the polymers were systematically tuned. As a result, a high responsivity of 19.1 A/W at 600 nm and an excellent detectivity more than 1012 Jones ranging from 350-600 nm were achieved for the copolymer with the molar ratio of PDI/NDI units of 70/30 based OPDs, which was attributed to the optimal morphology and high photocurrent, supported by the transient absorption spectra and atomic force microscopy (AFM).

2. RESULTS AND DISSCUSSION


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2.1 Synthesis and Characterization

Figure 1. Synthesis of the copolymers P0, P30, P50, P70, and P100 The synthesis of the tellurophene-based random copolymers P0, P30, P50, P70, and P100 was performed via Still coupling of 2,5-bis(trimethylstannyl)tellurophene42 with 2Br-PDI(OD) 43 and 2Br-NDI(HD)

55

under different molar ratios (Figure 1). The polymers possess excellent

solubility in organic solvents including chloroform, chlorobenzene, and dichlorobenzene. The structures of the copolymers were confirmed by 1H NMR (Figure S1-S5) and elementary analysis. The number-average molecular weights (Mn) and polydispersity index (Mw/Mn) of the polymers P0, P30, P50, P70, and P100 were measured by high temperature gel permeation chromatography (GPC), which are 67.2 kDa (2.48), 51.2 kDa (2.42), 39.6 kDa (2.20), 21.7 kDa (1.79) and 13.8 kDa (1.85), respectively. The thermal properties were investigated by thermal gravimetrical analysis (TGA) (Figure S6) and different scanning calorimetry (DSC) (Figure S7). The thermolysis onset temperatures for all the polymers are above 400℃ (with a 5% mass loss defined as the decomposition temperature), indicating excellent thermal stability. Furthermore, there are no apparent thermal transitions in the heating and cooling DSC scans for all the polymers, suggesting amorphous characteristics. 2.2 Optical and Electrochemical Properties



The UV-vis absorption spectra of the polymers in dilute 1,2-dichlorobenzene and as thin films spin-coated on glass are demonstrated in Figure 2 with the data summarized in Table 1. In solution, all the polymers showed two distinct absorption bands at 300-400 nm and 450-700 nm, which are attributed to the π-π* transition and intramolecular charge transfer (ICT) from tellurophene unit to PDI/NDI, respectively.37, 40 Also, both two distinct bands especially the low energy band significantly red-shifted along with the increase of the composition of NDI units, which are usually observed for NDI-based polymers.44 Moreover, the thin films of the polymers showed large red-shifting of the ICT band in comparison to that of the solution due to the interchain aggregation and π-π stacking.45-46 The optical bandgaps calculated from the low-edge onset absorption are 1.51, 1.59, 1.63, 1.67, and 1.72 eV for P0, P30, P50, P70, and P100, respectively. Obviously, the bandgap increased along with the increase of the composition of PDI unit in the copolymers, which may be ascribed to the increasing steric hindrance from large PDI unit and the reduced planarity of the polymer backbone.

Figure 2. Optical absorption spectra of the polymers (a) in DCB solution; (b) as thin-films casted on glass.


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Electrochemical redox behaviors of these n-type copolymers were investigated by cyclic voltammetry (CV) employing ferrocene as the internal standard as shown in Figure S8. The corresponding data are summarized in Table 1. Based on the onset reduction potentials of the polymers, the lowest unoccupied molecular orbital (LUMO) energy levels were determined to be -3.94, -3.97, -3.99, -4.06, and -4.09 eV for P0, P30, P50, P70, and P100, respectively according to ELUMO=-4.8-eEred-1/2 (eV) below the vacuum. It was obvious that the LUMO energy levels decreased gradually with increasing the amount of PDI moiety in the polymer backbone. By subtracting the optical bandgap from the LUMO energy levels, the HOMO energy levels of the polymers P0, P30, P50, P70, and P100 were estimated to be -5.45, -5.56, -5.62, -5.73, and -5.81 eV, respectively. Table 1. Thermal, optical absorption, and electrochemical properties of the polymers λmaxsolution

λmaxthin-film Egopt(a)

Ered-1/2

LUMO(b)

HOMO(c)

Td

(nm)

(nm)

(eV)

(V)

(eV)

(eV)

(℃)

P0

704

706

1.51

-0.50

-3.94

-5.45

415

P30

626

670

1.59

-0.47

-3.97

-5.56

436

P50

539

598

1.63

-0.45

-3.99

-5.62

441

P70

525

576

1.67

-0.38

-4.06

-5.73

443

P100

524

541

1.72

-0.35

-4.09

-5.81

438

Polymer

(a) Determined from equation: Eg = 1240/ λ , λ is on-set wavelength of optical spectra; (b) Estimated from equation: ELUMO = -4.44 eV-eEred-1/2; (c) Calculated from: EHOMO = ELUMO-Eg. Furthermore, photoluminescence measurements of the copolymers were also performed to exploit the generation of long-lived triplet excited states. However, both the solutions and the powders of the polymers were almost non-emissive, which may be reasonably ascribed to the



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“heavy-metal effect”47 that the heavy atom promotes inter system crossing, and thus generates non-emissive triplets.31, 48-49 2.3 Fabrication and Characterization of Photodetectors

Figure 3. (a) Device architecture for the photodetectors. (b) Current density-voltage curves under illumination. Table 2. Characteristics of the photodetectors at 600 nm under -5V Polymer

Jph(A/cm2)

Jd(A/cm2)

Rλ(A/W)

D*(Jones)

P0

4.54×10-4

7.42×10-6

1.34

7.81×1010

P30

1.40×10-4

4.93×10-6

0.51

3.25×1010

P50

3.24×10-4

3.74×10-6

1.23

8.93×1010

P70

8.80×10-3

4.17×10-6

19.11

1.75×1012

P100

2.37×10-4

1.28×10-5

0.46

2.47×1010

In order to investigate the optoelectronic performance, photodetectors with a device configuration of SiO2/ZnO/Polymer/Au were fabricated (Figure 3a). The current density-voltage (J-V) characteristics of the photodetectors under illumination and in the dark are demonstrated in


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Figure 3b and Figure S9, with the corresponding data summarized in Table 2. As shown in Figure 3b, the photocurrent density (Jph) decreased gradually from P0 to P50-based photodetectors within the same order of magnitude. Whereas P70-based OPDs could reach as high as 8.80×10-3 A/cm2 under -5V, which is about one order of magnitude higher than those of the other OPDs. The photocurrent decreased to the same order of magnitude with P0, P30, P50based photodetectors when the ratio of PDI moiety was increased to 100%. However, the dark current density (Jd) of the random copolymers-based photodetectors were obviously smaller than those of the reference polymers (P0, P100). The enhanced Jph and reduced Jd of the random copolymers especially for P70 may be attributed to the morphology of the polymer and interfaces, which will be discussed later. The responsivity Rλ, which represents the ratio between the photocurrent and imping light power, was calculated according to the equation50 below:

Rλ =

(I

ph

− Id )

(1)

p∗S

where Iph and Id represent the photocurrent and dark current respectively, p is the average light power intensity, and S is the effective illuminated area. As shown in Figure 4a, all the devices exhibited photovoltaic response from 350 to 800 nm with high responsivities ranging from 350600 nm. Significantly, the responsivity of P70-based OPDs reached as high as 19.11 A/W at 600 nm under -5V, which is among the highest value for the organic photodetectors,51-52 comparable to its inorganic counterpart.53 As demonstrated in Figure 4b, the spectra responsivity based on P70 is dependent on the applied reverse bias, which is enhanced when the bias improved from 2V to -5V. Similarly, all the other devices exhibited the same tendency (Figure S10). It was believed that variation in the responsivity is mainly governed by dark current, which is related to the film morphology and device configuration (Figure 4d).6, 19



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The specific detectivity (D*), a figure of merit for photodetectors, was also investigated to evaluate the device sensitivity to incident light. The corresponding data in Table 2 were obtained from the equation22 as follows: 1/ 2

 S∆f   D = Rλ   2eI dark  ∗

(2)

where e is the elemental electronic charge, ∆f represents the electrical bandwidth in Hz.

Figure 4. (a) Calculated responsivity of the basic device based on P0, P30, P50, P70, and P100 under a voltage of –5 V. (b) Calculated responsivity of the device based on P70 at different applied voltages. (c) Calculated specific detectivity spectra for the devices based on different


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polymers under a voltage of –5 V. (d) The dark current density (red strips) and calculated responsivity (blue squares) of the devices for each polymer at 600 nm under –5 V. Accordingly, the D* values of all the OPDs at -5V are shown in Figure 4d. It is found that the D* plots of the OPDs almost had the same outlines as their Rλ, with the calculated detectivities decreasing from short wavelength to long wavelength but exceeding 1010 Jones from 350 to 600 nm. In addition, P70 based OPDs achieved the highest specific detectivity over 1012 jones, which was 1-2 orders of magnitude higher than those of other copolymers. Overall, the performance of P70-based devices is among the best reported OPDs and comparable to the silicon photodetectors.54 It was found that the specific detectivity also depended on the electricity field (Figure S11). According to the equation above, both the responsivity and dark current contribute to the value of D*, so the specific detectivities demonstrated diverse with different biases. Moreover, the OPDs with and without interfacial layer were fabricated to understand the role of ZnO. As shown in Figure S12, the OPDs with ZnO as the interfacial layer demonstrated a high photocurrent, which is 2-3 orders of magnitude higher than that of the OPDs without ZnO. This suggests that the ZnO layer plays an important role to enhance the photocurrent. 2.4 Transient absorption analysis



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Figure 5. Transient absorption spectra of P70 (a) and P100 (b); Dynamics (c) at probe wavelength of 690 nm of P70 and P100 as thin films on quartz glasses excited at 400 nm. The photodetectors based on P70 and P100 are selected as examples for further investigation since they exhibited the best and worst performance, respectively, in terms of responsivity and detectivity. It is believed that efficient charge generation from photo-excitation in the active layer accounted for the high performance, especially for responsivity.6 Therefore ultrafast transient absorption (TA) measurements of P70 and P100 over timescales of 1000 ps were conducted to probe the generation of charge-separated states and estimate their lifetimes under the excitation of 400 nm laser with a pump fluence of ～10µJ/cm2 (Figure 5). Table 3. Decay parameters for the organic photodetectors excited at 400 nm τ1 (ps)

τ2 (ps)

P70

1.27±0.08 (65.4%)

34.6±1.3 (34.6%)

P100

1.17±0.06 (68.3%)

36.6±1.2 (31.7%)

As shown in Figure 5a and 5b, broad positive ∆A bands were observed spanning over 550-750 nm, which were assigned to photo-induced absorption (PIA) due to transition from newly occupied states to higher levels.55 Moreover, the maximum PIA intensity decreased from P70 to P100, suggesting fewer charges (excitons) were created and thus the photocurrent decreased. Figure 5c shows that the kinetics of the PIA signals at 690 nm were fitted with a biexponential decay function. The parameters are listed in Table 3. The PIA signals at 690 nm for P70 and P100 afforded nearly identical time constants, suggesting the similar lifetime.56 Thus, we assigned the different photocurrent to the intensity of PIA signals. The higher PIA intensity of P70-based OPDs may lead to the stronger photocurrent (Table 2). Based on the TA study, it can


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be conclude that (1) photo-generated carriers in P70 and P100 have similar lifetimes and the same recombination pathway, (2) and the P70 based OPDs afford a higher Jph(Table 2). Obviously, these observations are consistent with the photocurrent data. 2.5 Film Morphology The morphology of the film was critical for the performance of optoelectronic devices.57-58 Tapping-mode atomic force microscope (AFM) was employed to study the morphology of the corresponding films (Figure 6). The root-mean-square (RMS) roughness was found to be 0.78 nm, 0.76 nm, 0.65 nm, 0.61 nm, and 2.24 nm for P0, P30, P50, P70, and P100, respectively. Therefore the different compositions affect mainly the morphology of the thin film in OPDs, which determined the dark current and photocurrent.17 It is obvious P70-based OPDs exhibited the smoothest surface, which could prohibit the penetration of the top electrode into the active layer and ensure the best contact, leading to the lowest Jd.59

Figure 6. AFM topography images of the polymer films spin-coated on SiO2/ZnO (a) P0; (b) P30; (c) P50; (d) P70; (e) P100



To probe the influence of the ZnO interfacial layer on the photodetector performances, the substrate with and without ZnO was investigated with AFM. As shown in Figure S13, the substrate modified by ZnO exhibited smoother surface with a RMS value of 0.99 nm, smaller than that of the substrate without ZnO (2.43 nm). Therefore, an optimal interaction between substrate/ZnO and active layer can ensure a higher photocurrent.

3. CONCLUSIONS In summary, a series of random copolymers based on one electron-donating moiety tellurophene and two different electron-deficient moieties PDI/NDI were designed and synthesized for high performance organic photodetectors. Through changing the composition of the two electrondeficient moieties, the opto-physical properties of the materials and the morphology of the polymer film were systematically tuned. As a result, photodetector based on P70 afforded high responsivity about 19.1 A/W at 600 nm and excellent detectivity more than 1012 Jones ranging from 350-600 nm, which are ideal for UV-vis light detection. This contribution presents tellurophene-based materials for OPDs application for the first time and shed lights on understanding the relationship between the materials properties and device performances.

Supporting Information The Supporting Information is available free of charge on the ACS Publication website Experimental section including synthesis details and characterizations: 1HNMR, TGA, DSC, CV curves and devices measurements.

Author Information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]


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*E-mail: [email protected] Author Contributions ⊥

Kai Zhang, Lei Lv, and Xiaofen Wang contributed equally to this work

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the NSFC (51303180 and 21574135), Beijing Natural Science Foundation (2162043), One Hundred Talents Program of Chinese Academy of Sciences, and University of Chinese Academy of Sciences for financial support

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Tellurophene-Based Random Copolymers for High Responsivity and

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