High-Performance All-Polymer Photodetectors via a Thick Photoactive

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Organic Electronic Devices

High Performance All-Polymer Photodetectors via Thick Photoactive Layer Strategy Zhiming Zhong, Kang Li, Jiaxin Zhang, Lei Ying, Ruihao Xie, Gang Yu, Fei Huang, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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ACS Applied Materials & Interfaces

High Performance All-Polymer Photodetectors via Thick Photoactive Layer Strategy

Zhiming Zhong,†,‡ Kang Li,† Jiaxin Zhang,† Lei Ying,*,†,‡ Ruihao Xie,† ,‡ Gang Yu,† Fei Huang*,†,‡ and Yong Cao†

†

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ South

China Institute of Collaborative Innovation, Dongguan 523808, China

KEYWORDS: all-polymer photodetectors, thick photoactive layer, naphtho[1,2-c:5,6c’]bis([1,2,5]thiadiazole), high detectivity, noise power density

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ABSTRACT: To achieve high detectivity in all-polymer photodetectors (all-PPDs), a thick-film photoactive layer is favored because it can effectively suppress the dark current density. However, if the photoactive layer of the film is too thick, it leads to reduced responsivity owing to increased recombination loss. We developed high performance all-PPDs by using a narrow bandgap p-type polymer NT40 and an n-type polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5′-(2,2′bithiophene)} as the photoactive layer. The high charge carrier mobility of both copolymers enabled a photoactive layer thickness of 300 nm, leading to an ultra-low dark current density of 4.85×10−10 A cm−2, a detectivity of 2.61×1013 Jones, a high responsivity of 0.33 A W−1 at 720 nm and a bias of −0.1 V. The detectivity achieved >1013 Jones in a wide range from 360 to 850 nm, which is among the highest values so far reported for all-PPDs without extra gains. More importantly, the resultant all-PPDs exhibited a high working frequency over 10 kHz associated with a large linear dynamic range. These findings demonstrate the great potential for practical applications of the all-PPDs developed in this work.


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INTRODUCTION Photodetectors that can convert light signals into electric signals are crucial elements in future wearable and flexible electronics, such as imaging,1,2 artificial retinal implants,3 monitoring and sensing devices.4-12 In recent years, bulk-heterojunction (BHJ) organic photodiodes have attracted wide attention due to their great advantages of wide spectrum response, highly tunable energy level, low-temperature and low-cost fabrication compared to their inorganic counterparts (e.g., Si and GaN).13-21 Among the reported BHJ organic photodetectors, particular interest has been devoted to developing all-polymer photodetectors (all-PPDs) that contain a p-type semiconducting polymer and an n-type semiconducting polymer in the photoactive layer.22-24 The specific merits of the all-PPDs include the morphological and mechanical stability related to their counterparts, consisting of fullerene derivatives or small-molecule non-fullerene acceptors.25-31 However, the overall performances of all-PPDs are generally less impressive than those of based on inorganic semiconductors or organic photodetectors using fullerene derivatives as the electron-acceptors. In principal, the key figure-of-merit parameters that define the performance of photodetectors are responsivity (R) and specific detectivity (D*), where R is the electrical output per optical input, related to external quantum efficiency (EQE); and D* is defined as follows, 𝐷 ∗ = 𝑅 × 𝐴 𝑆𝑛

(1)

where A is the device area, Sn is the noise spectral density. Although the intrinsic reasons that organic photodetectors generate Sn have not been fully understood, it is 3 ACS Paragon Plus Environment


generally believed that the noise current that determines Sn is closely related to the leaking current.32 From the abovementioned equation, one learns that the enhancement of D* could be realized by increasing R and/or reducing the leaking current of the photodetectors. Even though increasing the film thickness of the photoactive layer can effectively suppress the leaking current, which in turn leads to an obviously enhanced D*,33,34 the R is typically limited by film thickness. A photoactive layer that is too thick will sacrifice external quantum efficiency (EQE) and R as a result of the increased recombination loss.35,36 To address such a trade-off between R and D*, the optimal thickness of the BHJ photoactive layer is typically limited to 200 nm, owing to the relatively low charge carrier mobility of the organic semiconducting polymers. An effective strategy to address this issue is to use photoactive polymers with high charge carrier mobility, which can facilitate the transportation of the photo-generated charge carriers to the respective electrodes.37 Recently, a range of new high mobility donor-acceptor type π-conjugated polymers have been developed based on an electron-deficient naphtho[1,2-c:5,6c’]bis([1,2,5]thiadiazole) (NT) moiety.38-40 These polymers have been used to fabricate organic photovoltaic devices with a high tolerance of photoactive layer thickness up to 1 μm. Although these NT-copolymers have presented high charge carrier mobility, they have exhibited poor solubility due to the enlarged planarity and rigidity of the NT unit. This disadvantage can be overcome by incorporating a certain amount of benzodithiophene derivatives into the polymer backbone, leading to a set of random terpolymers that are very compatible with the n-type copolymer N2200 for organic 4 ACS Paragon Plus Environment

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photovoltaic applications. Here, we used the NT-based random copolymer NT40 containing the electron-rich thiophene and benzothiophene derivative in the backbone as the electron-donor, and we used N2200 as the electron-acceptor (Figure 1a) to construct the all-PPDs.41 The high hole mobility of NT40 enabled us to achieve a thick photoactive layer up to 300 nm, which could effectively suppress the dark current while simultaneously maintaining high responsivity. The resulting visible-near infrared allPPDs with optimized photoactive layer thickness presented impressively high detectivity (D*) and responsivity in a broad spectral region of 350-800 nm. In particular, the champion D* reached 2.61×1013 Jones (1 Jones = 1 cm Hz1/2 W−1) at 720 nm and −0.1 V bias, which was among the highest values so far reported for allPPDs.13,22-31 Owing to the ultra-low dark current density, a linear dynamic range (LDR) of 96.6 dB could be achieved. This device also enabled high operating frequency and exhibited fast response speed, with a cut-off frequency of > 10 kHz and rise and fall times of 11 and 12 μs, respectively.

These findings clearly highlight the prominent

role of using thick-film photoactive layer for suppressing dark-current, and may open a new approach for construction high performance all-PPD toward practical applications based on high mobility conjugated polymers.



Figure 1. (a) Chemical structures of polymers NT40 and N2200. (b) Normalized UVvis absorption spectra of the pristine films of NT40 and N2200, and the blended films of NT40:N2200 (2:1, wt:wt); (c) the energy levels alignment of the related polymers.

RESULTS AND DISCUSSION Optical and Electrochemical Properties As shown in Figure 1b, the p-type copolymer NT40 showed a broad absorption in the range of 300-750 nm. This could compensate for the absorption valley of N2200 that has dual absorption characteristics at wavelengths of 300-450 nm and 600-860 nm. The photoactive layer consisting of NT40:N2200 with a weight ratio of 2:1 presents a panchromic absorption profile from 300-850 nm. This is favorable for response of the resulting photodetectors. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of NT40 and N2200 are –5.34/– 3.22 eV and –5.81/–3.84 eV, respectively (Figure 1c), providing sufficient offsets for 6 ACS Paragon Plus Environment

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exciton separation.

All-Polymer Photodetectors Characteristics The all-PPDs were fabricated with a conventional structure of indium tin oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/active layer/poly[(9,9-bis(3’-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt2,7-(9,9-dioctylfluorene)] dibromide (PFN-Br)/Al. The PEDOT:PSS AI 4083 layer was used to modify the surface of ITO for a better work function, wettability and roughness. The active layers with an optimized NT40:N2200 ratio (2:1, wt:wt) were spin-casted from the environmentally-friendly solvent 2-methyltetrahydrofuran (2-MeTHF) solutions and then thermally annealed at 120 oC for 10 min. A thin layer of PFN-Br was used as the cathode interlayer layer to facilitate charge collection and prevent the potential diffusion of aluminum during evaporation. We initially investigated the effects of film thickness on the performance of the resulting devices. Figure 2a shows the current density – voltage (J-V) characteristics measured in the dark and at illumination with a light intensity of 100 mW cm−2. Note that the dark current density (Jd) gradually decreased from 5.38×10−8 A cm−2 for the device with the photoactive layer thickness of 110 nm to 4.85×10−10 A cm−2 for the photoactive layer thickness of 300 nm, whereas the photocurrent densities of the devices at various film thickness were nearly identical. Notably, the resulting all-PPDs based on NT40:N2200 exhibited excellent photodiode characteristics with rectification (± 2 V) over 105, which is the highest value so far reported for all-PPDs.13,22-31 7 ACS Paragon Plus Environment


Moreover, it is worthy of noting that the NT40:N2200 blends with different thickness exhibited similar surface morphology, as indicated by atomic force microscopy images (Figure S3, see the SI).

Figure 2. (a) J-V characteristics of the photodetectors measured (scanning from –2 to +2 V) in the dark condition and illumination of AM 1.5G (100 mW cm–2). (b) The EQE curves at –0.1 V with various active layers thicknesses. (c) EQE curves and (d) the corresponding responsivities spectra of the devices based on 300 nm thick NT40:N2200 layer measured at various bias voltage.

The external quantum efficiency (EQE) was also recorded to evaluate the photoresponse of the all-PPDs. As shown in Figure 2b, these devices with various 8 ACS Paragon Plus Environment

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thicknesses of active layers exhibited a high response, with an EQE over 50% from 360 to 750 nm at −0.1 V. A set of biases were applied to boost the photoconductive gain. However, the EQE only increased slightly under an additional reversed electric field (Figure 2c and Table S1, SI), especially when the thickness was < 200 nm (see Figure S2, SI). This indicates that the built-in electric field was strong enough to extract most of the charge, as evidenced from the fill factors. As the intensity of the built-in electric field gradually faded away with the increased thickness of the photoactive layer, it became favorable for charge carrier extraction under reversed bias. The R of the photodetector can be derived from the measured EQE according to the following equation, 𝑅=

𝐸𝑄𝐸 × 𝜆 1240

(2)

(𝐴/𝑊)

where λ is the wavelength (nm). The relevant responsivity spectra are plotted in Figure 2d, with the corresponding data summarized in Table S1 (SI). The optimized devices exhibited a maximal responsivity of 0.33 A W−1 at 720 nm under –0.1 V. This is comparable to those obtained from organic photodetectors based on small molecule acceptors such as phenyl C61 butyric acid methyl ester, rhodanine-benzothiadiazolecoupled indacenodithiophene, etc., which operate at high voltage.2,42 The R slightly increased and reached 0.33 A W−1 upon the increased applied bias at a film thickness of 300 nm (Table 1).



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Table 1. Detailed parameters of all-PPDs measured at different active layers under – 0.1 V bias. Thicknessa

Jdb

Rectification EQE720 nmb

R720 nmb

D*720 nmb

(nm)

(A cm−2)

(±2V)

(%)

(A W−1)

(Jones)

110

5.38×10−8

1.3×105

51.2

0.29

2.26×1012

160

4.89×10−8

1.2×105

45.7

0.27

2.12×1012

200

2.40×10−8

2.2×105

44.1

0.26

2.92×1012

240

6.05×10−9

5.3×105

50.4

0.29

6.65×1012

270

1.54×10−9

6.8×105

55.8

0.32

1.46×1013

300

4.85×10−10 9.4×105

56.0

0.33

2.61×1013

350

1.24×10−9

53.1

0.31

1.55×1013

a

3.8×105

Thickness of the photoactive layer; b obtained under −0.1 V bias.

Detectivity Analysis It is widely accepted that equation (1) can be simplified as follows, 𝐷 ∗ = 𝑅 2𝑞𝐽𝑑

(3)

where q is the value of the absolute charge (1.6×10−19 C), and Jd is the dark current density.4-9 The D* curves with different active layer thicknesses at −0.1 V are shown in Figure 3a, with relevant D* values at a wavelength of 720 nm summarized in Table S1 (SI). The D* values measured under −0.1 V increased gradually when the thickness increased from 110 to 300 nm. The devices with a photoactive layer thickness of 300 nm exhibited the highest D* value of 2.61×1013 Jones at a wavelength of 720 nm, 10 ACS Paragon Plus Environment

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originating from the high responsivity and lower Jd value. To our knowledge, this is the highest D* value so far reported for all-PPDs (using the same calculation method), except for the photomultiplication type of photodiodes with additional gain working under high voltage. We also calculated the D* values at various reverse biases from 0 to −5 V (Figure 3b, and Figure S4 in the SI) and realized that the D* values gradually decreased with increased reverse bias. This observation is understandable because the dark current dramatically increased with increased bias, whereas the increase in responsivity was less pronounced. (a)

10

10

(b) 13

13

10

12

@ -0.1 V 110 nm 160 nm 200 nm 240 nm 270 nm 300 nm 350 nm

11

300

D* (Jones)

10 D* (Jones)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

500

600

300 nm

12

10

0V -0.1 V -0.5 V -1.0 V -2.0 V -5.0 V

11

10

700

800

300

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Figure 3. (a) The specific detectivity (D*) curves under –0.1 V with different active layer thicknesses and (b) measured under various biases.

Although the calculated D* based on equation (3) was reasonable when the photodetector was performed at a high reverse bias,43 it is worth noting that equation (3) assumes that the total noise of the photodetector is dominated by the shot noise in the Jd, and other noises (such as the Johnson noise and the flicker noise) are not taken into account.13,32-44 To address this issue, we carefully measured the noise current of 11 ACS Paragon Plus Environment


the optimized devices according to equation (1), with the relevant noise power density spectra shown in Figure 4a. The noise power density curves of the devices fit a ~1/f law at low frequency and gradually became independent of the frequency at a higher frequency. This observation was different from the assumption for equation (3), which assumed that the noise power density should be an independent constant. By plotting the D* obtained from equation (1) and (3) in Figure 4b, we noticed that the D* values obtained from equation (3) were more than one order of magnitude higher than those obtained from equation (1). For instance, from equation (3) the maximal D* was calculated to be (2.98 ± 0.88)×1012 Jones at a bias of 0 V and a frequency of 100 kHz, which is much lower than the 2.61×1013 Jones calculated from equation (1). A similar trend has also been realized in previous report.45,46 Moreover, we also noted that the variation in the calculated D* values from the noise power density at the high frequency was less pronounced than it was at the low frequency. The detailed D* values at a wavelength of 720 nm for the devices derived from noise spectral density are summarized in Table S2 (SI).


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Figure 4. (a) Noise power density spectrum and (b) comparison of D* derived from equation (1) and (3) at 720 nm for devices with a photoactive layer thickness of 300 nm.

Linear Dynamic Range and Cut-Off Frequency The operating condition of photodetectors is typically below 100 μW cm−2, which is several orders of magnitude lower than the AM 1.5G (100 mW cm−2) for organic photovoltaic devices. We studied the light intensity dependent J-V characteristics of the resulting devices, with the corresponding curves plotted in Figure 5a and 5b. The LDR, which is a significant figure of merit to quantify the detectable linear range of photodetectors, could be calculated from the following equation (4).15 LDR = 20 × log (𝐼𝑚𝑎𝑥 𝐼𝑚𝑖𝑛)

(4)

where Imax is the upper limit and Imin is the lower limit of the incident light intensity. Due to the weak intensity of the light source used, the measure-limited LDR of the optimized device was 96.6 dB, corresponding to the incident 720 nm light intensity range from 41.9 μW cm−2 to 0.619 nW cm−2. The large LDR values should benefit from the high EQE and D*. From the perspective of practical applications, such as imagers, the response speed and frequency are important parameters for photodetectors. For instance, the standard for IMAX® high frame rate movies require 48 frames per second (corresponding to the working frequency of 48 Hz). In the current case, as shown in Figure 5c, the resultant device exhibited a very high cut-off frequency of > 10 kHz, which is far beyond the 13 ACS Paragon Plus Environment


requirements of standard applications. More to this point, we also evaluated the rise (trise) and fall time (tfall) of the resulting device. As illustrated in Figure 5d, the resulting devices exhibited fast responses, with trise and tfall of 11 μs and 12 μs at –0.1 V, defined as the time space when the photocurrent signal went from 10% to 90% and 90% to 10%, respectively. These findings clearly indicate the great potential of these NT40:N2200 devices for practical applications.

Figure 5. (a) The 720 nm light intensity dependent J-V characteristics of NT40:N2200 device. (b) Linear dynamic range for the NT40:N2200 device at –0.1 V under 720 nm illumination. (c) The NT40:N2200 device responses to a light pulse with a frequency of 10 kHz. (d) The cut-off frequency at –3 dB of the NT40:N2200 device. 14 ACS Paragon Plus Environment

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CONCLUSION In summary, we developed high performance all-polymer photodetectors based on a narrow bandgap p-type polymer NT40 and an n-type copolymer N2200 as the photo active layer. This combination of photoactive layers presented a broad spectral response from 300 to 900 nm. The high charge carrier mobility of both NT40 and N2200 allowed us to achieve a thick photoactive layer of over 300 nm. This led to the obviously suppressed dark current density of 4.85×10−10 A cm−2, associated with a remarkable detectivity of 2.61×1013 Jones, and responsivity of 0.33 A W−1 at a bias of –0.1 V with a wavelength of 720 nm. To our knowledge, these observed performances are among the highest value so far reported for all-PPDs without extra gains. Further investigation produced a linear dynamic range of the optimized device of 96.6 dB associated with a fast response time and a cut-off frequency of >10 kHz. These findings demonstrated that the all-polymer photodetectors developed have great potential for practical applications.

EXPERIMENTAL SECTION Materials: All reagents and chemicals were purchased from commercial sources (Aldrich, Alfa Aesar and Stream) and used without further purification unless otherwise specified. The copolymer NT40 and N2200 were prepared according to the procedure reported in the references.41 Fabrication of All-PPDs: The all-PPDs were fabricated with a conventional 15 ACS Paragon Plus Environment


configuration: ITO/PEDOT:PSS/active layer/PFN-Br/Al. The indium tin oxide (ITO) coated glass substrates were cleaned by sonication in detergent, deionized water, acetone, and isopropyl alcohol and dried in an oven at 75 °C for 10 h. After the substrates were treated by oxygen plasma for 2 min, PEDOT:PSS (CLEVIOSTM P VP AI 4083) layer as a anode buffer layer were spin-coated on ITO-coated glass substrates at 3000 rpm for 30 s, then thermally annealed at 150 °C on a hotplate for 15 min to obtain about 30 nm thin film. The blend polymers (NT40:N2200, 2:1 wt:wt) were dissolved in 2-methyl-tetrahydrofuran (2-MeTHF) with the total concentration of 6 mg mL−1 for NT40. By varying the rotate speed from 500 to 4000 rpm, the film thickness was kept in the range of 110~350 nm, and then treated by thermal annealing at 120°C for 10 min. An atomic force microscopy (Bruker Multimode 8) was used to scan the height images in scanasys mode. The PFN-Br interlayer material was dissolved in methanol with the concentration of 0.5 mg mL−1, which were spin-coated on the top of the active layer at 2000r for 30s to produce 5 nm thin film. Finally, the devices were finished after an aluminum cathode (about 90 nm) were deposited by thermal evaporation under vacuum (about 1.0×10−6 mbar). The device area was defined to be 0.0516 cm2 through a shadow mask. Characterization of All-PPDs: All the measurements were carried out under ambient conditions. The thickness of the organic films was determined by a Dektak 150 surface profiler. The absorption spectra of films were measured by Shimadzu UV3600. The photocurrent density-voltage (Jlight-V) characteristics of the devices under AM 1.5G (100 mW cm–2, generated from a solar simulator, Enlitech SS-F5-3A) were measured 16 ACS Paragon Plus Environment

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on a Keithley 2400 source-measurement unit. The dark current density-voltage (JdarkV) characteristics of the devices were measured on a Keithley 2400 source-meter in an electrically and optically shielded box. The J-V characteristics under illumination of 720 nm beam (generated from a Zolix Omni-λ monochromators and tungsten-halogen lamp) were measured by a Keithley 236 source-meter in the electrically and optically shielded box. The intensities of the 720 nm beams could be controlled through neutral density filters and calibrated by a low-power calibrated photodetector (Newport 818UV/DB). The external quantum efficiency (EQE) spectrum was carried out on a commercial EQE measurement system QE-R3011 (Enlitech Co., Ltd.) equipped with DC module. The light intensity at each wavelength was calibrated using a standard single crystal Si photovoltaic cell before the testing. The noise spectral density characteristics of the devices were recorded by a semiconductor parameter analyzer (Platform Design Automation, Inc. FS380 ProTM). The pulsed red light (620-630 nm) for cut-off frequency measurement was generated by a red LED chip and a function generator (square wave, Aim-TTi TG120), and we used a 4.7 kΩ resistor to convert photocurrent signal of our sample to voltage signal and for the oscilloscope (Tektronix TDS3052B) to record. For the response time measurement, the frequency of pulsed red light (620-630 nm) was set to 1 kHz, and we also used the same 4.7 kΩ resistor and oscilloscope (Tektronix TDS3052B) to record data.

ASSOCIATED CONTENT Supporting information 17 ACS Paragon Plus Environment


The supporting information is available free of charge in the ACS Publication website. The responsivity curves under –0.1V with various active layers thicknesses. EQE curves under various bias voltages of devices with different thickness of active layers. The detailed parameters of optimized NT40:N2200 based photodetectors under different applied bias voltage. The detectivity data derived from noise spectral density.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (L. Ying) *E-mail: [email protected] (F. Huang)

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Z. Zhong and K. Li contributed equally to this work. This work was financially supported by the National Natural Science Foundation of China (No. 21822505, 21634004, 51521002 and 21574103), and Program for Science and Technology Development of Dongguan (2019622163009). Z. Zhong thanks the support from China Postdoctoral Science Foundation (No. 2018M643079).

REFERENCES 18 ACS Paragon Plus Environment

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High-Performance All-Polymer Photodetectors via a Thick Photoactive

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