WS2 Photodetectors with High

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Bilayer Heterostructured PThTPTI/WS2 Photodetectors with High Thermal Stability in Ambient Environment Zhiwen Jin,† Dan He,‡ Qing Zhou,† Peng Mao,† Liming Ding,*,‡,§ and Jizheng Wang*,†,§ †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ National Center for Nanoscience and Technology, Beijing 100190, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Organic-based photodetectors (PDs) have great potential applications in next-generation portable, low-cost, large-area displays and optical communications. However, for practical applications, they are facing big challenges due to their instabilities in ambient environments, especially under high temperatures. Robust materials and device architectures are highly demanded to overcome the problem. In this report, we employed a donor conjugated polymer PThTPTI and realized thinfilm PDs which can stably operate in ambient air under temperatures as high as 300 °C. By adding a discontinuous thin layer of WS2 beneath the PThTPTI film, the device photosensitivity is significantly enhanced without loss of the high thermal stability. This work provides new insights in designing novel and stable organic-based devices for future optoelectronic applications. KEYWORDS: heterojunction, thermal stability, photodetector, WS2, organic

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INTRODUCTION Photodetectors (PDs) have great applications in large-area displays and optical communications.1,2 Next-generation PDs are required to be portable and cheap; meanwhile, they should be able to operate in a harsh ambient air environment.3−5 Organic semiconductor materials are selected as potential candidates to meet these emerging demands,6,7 owning to their high light absorption coefficients and solution processabilities.8,9 However, organic PDs normally show poor stabilities in ambient air, especially under high temperatures.10 Organicbased PDs capable of working at high temperature (higher than 200 °C) are rare.3 The instability is originated from two aspects: (1) organic materials: under ambient condition, O2 and H2O may easily intrude into the materials and gradually degrade them,10,11 especially at high temperatures. (2) device structure: hierarchical or multilayer structures (with each layer having a specific purpose for efficient exciton dissociation and carrier transport) are usually employed to achieve high sensitivity;12,13 high temperature can induce severe molecule diffusion and hence mixing between the layers, which will detrimentally damage the device.14,15 Thereby, stable organic materials and suitable device architectures must both be designed in order to realize robust organic-based PDs. Recently, we designed and synthesized a novel p-type copolymer PThTPTI for solar cell applications,16 and thermogravimetric analysis (TGA) indicates its decomposition temperature is above 400 °C. This inspired us to design and fabricate PThTPTI-based PDs for stable and high-temperature © XXXX American Chemical Society

operations in an ambient environment. Meanwhile, we need to find a suitable acceptor material to form a high-quality donor/ acceptor interface to efficiently dissociate photoexcitons generated in the PThTPTI. Transition-metal dichalcogenides (TMDs) possess tunable band gaps, high electron mobilities, and high thermal stabilities, and their monolayered structures have been intensively exploited in the past decade for a wide range of potential optoelectronic applications.17−21 Among the well-investigated TMDs, WS2 (conductive band edge at −3.8 eV and valence band edge at −5.8 eV) is a very good match with the PThTPTI for exciton dissociation (conductive band edge at −2.8 eV and valence band edge at −5.4 eV),22,23 which motivated us to design a PThTPTI/WS2 bilayer structure. Impressively, the fabricated PThTPTI/WS2 bilayer films and devices both show high thermal stability up to 300 °C in ambient air.

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RESULTS AND DISCUSSION The surface roughness for fabricated PThTPTI film is about 2 nm. We first fabricated field-effect transistors (FETs) based on PThTPTI (shown in Figure 1) and tested them in ambient air (Figure 1a,b); the extracted hole mobility is 0.18 cm2 V−1 s−1. We then annealed the device in ambient air for 60 min at different temperatures on a hot-plate heater in the range of 25− Received: September 22, 2016 Accepted: November 14, 2016 Published: November 14, 2016 A

DOI: 10.1021/acsami.6b12090 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Properties of the PThTPTI FET in ambient air. (a) Transfer curve. (b) Output curves. (c) Transfer curves measured after annealing the device at different temperatures. (d) Hole mobility versus annealing temperature.

300 °C. After each annealing, the heater is turned off and the device was naturally cooled down to room temperature and was then retested (Figure 1c). The extracted room-temperature carrier mobility versus the annealing temperature is given in Figure 1d. It is seen that, in the range of 25−150 °C, the polymer shows almost constant mobility (about 0.18 cm2 V−1 s−1), whereas, in the range of 150−300 °C, the mobility decreases slightly with the annealing temperature (from 0.18 to 0.05 cm2 V−1 s−1), which should be induced by the increased level of carrier trapping and scattering caused by hightemperature produced unintentional doping and/or charge accumulation in the film.9,24 The very little variation of hole mobility (0.18−0.05 cm2 V−1 s−1) in such a wide annealing temperature range (25−300 °C) hints that PThTPTI could be a very stable organic semiconductor for high-temperature operations.25 The WS2 films were grown on quartz substrates using the chemical vapor deposition (CVD) method, and the coverage ratio of WS2 film on the quartz substrate was varied by controlling the growth time (Figure S1). Figure 2a,b presents the optical image and the AFM image of the WS2 film with a coverage ratio of 90%, from which discontinuous atomically thin WS2 flakes can be clearly seen and the surface roughness is about 5 nm. Figure 2c shows the TEM image of one WS2 flake, which is actually composed of countless thin WS2 single crystal nanosheets (∼5 nm). Figure 2d gives the HRTEM image of one WS2 nanosheet, which shows a 0.27 nm interplane spacing, matching well with that of the WS2 (110) plane.26 The inset in Figure 2d is the selected area electron diffraction (SAED), which exhibits the crystalline nature of the WS2 film, and the diffraction rings from inside to outside can be ascribed to the (100) and (110) planes, respectively.26

Figure 2e presents the optical absorption spectrum of the WS2 film. The two characteristic absorption peaks at 636 nm (1.95 eV) and 530 nm (2.34 eV) are clearly observed, which are arising from the direct transitions from valence band to conduction band at the K-point of the Brillouin zone (due to the spin−orbital splitting of the valence band, two valence− conduction transitions exist, corresponding to the two observed peaks.27). The room-temperature Raman spectrum for the WS2 film is given in Figure 2f, which shows two typical prominent phonon vibration modes of WS2: E12g (at 355.8 cm−1) and A1g (at 422.7 cm−1).28 Figure 3a presents the schematic structure of the bilayer PThTPTI/WS2 PD (single PThTPTI layer PD and single WS2 layer PD were also fabricated for comparisons), the molecular structure of PThTPTI, the atomic structure diagram of WS2, and the energy band diagram of PThTPTI and WS2. Owing to the relative positions of their energy levels, photoelectrons in PThTPTI would flow into WS2. The WS2 film is consisted of discontinuous atomically thin WS2 flakes (Figure 2); thereby, photoelectrons that flowed into the flakes could not be smoothly transported to and collected by the electrode. Hence, they would be severely trapped by the WS2 flakes, which could significantly increase the lifetime of the photoelectrons and as a result would lead to greatly enhanced photogain. The absorption spectra of the three films used for PD fabrications (the WS2 film (90% coverage ratio), the PThTPTI film, and the PThTPTI/WS2 film) are given in Figure 3b. The PThTPTI film shows broad absorption mainly in a visible range from 400 to 700 nm, whereas the WS2 film exhibits very poor absorption, and no obvious difference between the PThTPTI film and the PThTPTI/WS2 film can be observed. This indicates that the WS2 film is ultrathin and the absorption of the PThTPTI/WS2 film is dominantly from the PThTPTI film. B


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Figure 2. WS2 film. (a) Optical image. (b) AFM image. (c) TEM image. (d) High-resolution TEM image and typical SAED pattern. (e) Absorption spectrum. (f) Raman spectrum.

To test the thermal stabilities of the three films, we annealed the films at 300 °C in ambient air for 60 min. Raman spectra (Figure 3c) and optical images (Figure S2) of the three films were taken before the annealing and after the 300 °C annealing, from which we can reliably judge that the annealing does not change microcosmic morphologies (seen in Figure S2) and chemical bonding structures (Figure 3c) of all the three films.29 Combining the TGA test of the PThTPTI/WS2 PD (Figure 3d), we conclude that the films (and the devices) possess excellent thermal stabilities in ambient air under temperatures as high as 300 °C. For a PD, the photocurrent is closely related to responsivity (R) and photogain (G), which are defined by the following equations30 R=

ILight − IDark Pill

= EQE

λq G hc

G=

(μn + μp )τE L

(2)

where ILight is the current under the illumination, IDark is the dark current, Pill is the incident illumination power on the effective area (channel area), EQE is the external quantum efficiency, λ is the wavelength of interest, q is the electron charge, h is the Planck constant, and c is the speed of light. μn is the electron mobility, μp is the hole mobility, τ is the photocarrier lifetime, E is the electrical field, and L is the device channel length. It is clear that carrier mobility and carrier lifetime are very important to the photocurrent Iph (Iph = Ilight − Idark). The dark current and photocurrent (under 650 nm light illumination with an intensity of 0.1 mW cm−2) as a function of applied voltage for the three devices are given in Figure 4a. It is seen that the WS2 PD has negligible dark current and photocurrent (Figures S3−S6): the WS2 film is consisted of

(1) C


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Figure 3. (a) Schematic illustration of the PThTPTI/WS2 PD, the molecular structure of PThTPTI, the atomic structure diagram of WS2, and the energy band diagram of PThTPTI and WS2. (b) The absorption spectra. (c) Raman spectra. (d) TGA curve of the PThTPTI/WS2 PD.

Figure 4. Properties of the three PDs. (a) I−V curves in dark and under 650 nm light illumination with an intensity of 0.1 mW cm−2. (b) Photocurrent as a function of the WS2 coverage ratio. (c) Photocurrent as a function of the PThTPTI hole mobility. (d) Spectral responsivity.

because the PThTPTI film is continuous and has strong light absorption ability. However, as we know, excitons in organic materials have large binding energies and cannot be thermally dissociated into free charge carriers at room temperature.31 The

discontinuous atomically thin WS2 flakes, which cannot form smooth carrier transport paths across the channel, thereby seriously limiting the currents. The PThTPTI PD has higher dark current and photocurrent than that of the WS2 PD: this is D


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Figure 5. On/Off switching properties, On−off cycling tests and time-resolved photocurrents of the PDs under an incident light density of 0.1 mW cm−2 at 650 nm (at a bias of 10 V): (a−c) for the PThTPTI PD and (d−f) for the PThTPTI/WS2 PD.

Figure 6. Properties of the PDs measured at different temperatures: (a) Photocurrent as a function of the test temperature, (b) time-resolved photocurrent of the PThTPTI PD, and (c) time-resolved photocurrent of the PThTPTI/WS2 PD. Properties of the PDs measured after being annealed at different temperatures. (d) Photocurrent as a function of the annealing temperature, (e) time-resolved photocurrent of the PThTPTI PD, and (f) time-resolved photocurrent of the PThTPTI/WS2 PD.

carriers freed by the external electric field and impurity inside the film are very limited;32 hence, high EQE and photocurrent cannot be achieved in a single material film. The WS2/ PThTPTI bilayer architecture provides a rich donor−acceptor interface (PThTPTI/WS2 interface), which helps free the tightly bound electron−hole pairs. The photoelectrons drop into the WS2 flakes and are then severely trapped there. This spatially separates photoelectrons and holes and, hence,

significantly reduces carrier recombination and prolongs carrier lifetime. As a result, the photocurrent is largely enhanced. We also investigated the effect of WS2 coverage ratio on the device performance, which is shown in Figure 4b. It is seen that the 90% coverage ratio offers the best device performance: the larger the WS2 coverage ratio, the more the donor/acceptor interface, and hence the higher the photocurrent. Figure 4c gives the photocurrent as a function of hole mobility in the PThTPTI film (annealed at different temperatures). It is E


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upon light on/off (trap/release mechanism39). For the PThTPTI/WS2 PD, the rise/decay time increases with the temperature (Figure 6c, (rise time: 17 ms to 6.89 s and decay time: 38 ms to 8.43 s from 25 to 300 °C)), which is opposite to the behavior of the PThTPTI PD. The abnormal trend can be explained by the electron diffusion process in the WS2 flakes (which do not exist in the PThTPTI PD): electrons dropped into the WS2 flakes will diffuse away from the PThTPTI/WS2 interface into other areas of the WS2 flakes, gradually reaching equilibrium by forming a stable concentration distribution from the interface to inside the WS2 flakes. The higher the temperature, the more electrons generated at the PThTPTI/ WS2 interface and distributed in the WS2 flakes. Hence, longer time is needed for the carrier distribution to reach equilibrium, leading to larger rise time. Upon light turning off, the trapped electrons gradually return back to the PThTPTI/WS2 interface to recombine with holes in the PThTPTI film. The higher the temperature, the more electrons trapped in the WS2 flakes. Thereby, longer time is needed for them to diffuse back to the interface, resulting in larger decay time. After the devices were tested under each temperature (25− 300 °C), they were naturally cooled down to room temperature and were tested again. The retested room-temperature performance was compared with that originally tested before any high-temperature treatment. We found that, when the temperature is lower than 150 °C, the devices performed almost the same as they initially did. When the devices were tested at temperatures higher than 150 °C, the roomtemperature photocurrents of both the PThTPTI PD and the PThTPTI/WS2 PD slightly decrease, which should be caused by the high-temperature induced hole mobility reduction (from 0.18 to 0.05 cm2 V−1 s−1). Remarkably, the time-resolved photocurrents of both the PThTPTI PD and the PThTPTI/ WS2 PD do not exhibit any obvious changes. The On/Off switching properties of the PThTPTI PD and the PThTPTI/ WS2 PD are given in Figures S7−S10; it is seen that both the PThTPTI PD and the PThTPTI/WS2 PD present reproducible photoresponses. This further demonstrates that PThTPTI is a very stable organic semiconductor, and the PThTPTI/WS2 interface does not degrade upon the high-temperature treatments. The successful combination of PThTPTI and the two-dimensional WS2 offers new insights and opens a path for designing novel organic-based devices with high stability in ambient environment.

evident that the photocurrent is proportional to hole mobility in the PThTPTI film, which is consistent with eqs 1 and 2. Spectral R of the two devices is given in Figure 4d (under a bias of 10 V and an illumination intensity of 10.6 μW cm−2); it is seen that R follows quite well with the absorption spectrum of the PThTPTI film: the stronger the absorption, the more photogenerated carriers, and hence the larger the R. Figure 5 gives the On/Off switching properties, On−off cycling tests, and time-resolved photocurrents of the PThTPTI PD and the PThTPTI/WS2 PD. The On/Off time duration is 10/10 s, and each photoresponse was repeated more than 50 times. For the PThTPTI PD, a small portion of electrons in the photoexcitons will be caught by traps in the PThTPTI film, leaving holes free. To keep the neutrality of the device, each hole reaching the electrode is replenished by another one entering from the other electrode. The more the trapped electrons, the larger the gain and hence the photocurrent.33 Both the process of electrons filling in the traps and the process of electrons being released by the traps take long times;34 thereby, the rise speed and decay speed of the photocurrent are both slow (Figure 5a,b). Shown in Figure 5c, the rise time and decay time are 6.64 s and >10 s, respectively. (The rise time is defined as the time for the current to rise to 90% of the peak value, and the decay time is defined as the time for the current to decay to 10% of the peak value.35,36) For the PThTPTI/WS2 PD, photoexcitons (generated in the PThTPTI film) are quickly separated at the PThTPTI/WS2 interface, and electrons drop into the WS2 flakes and are then trapped there. This significantly reduces carrier recombination and hence largely prolongs carrier lifetime. The freed holes are then transported in the PThTPTI film under the external voltage bias. After switching off the light, holes in the PThTPTI film quickly recombine with electrons trapped in the WS2 flakes via the PThTPTI/WS2 interface (carrier separation and recombination via a donor/acceptor interface is much faster than that via a trap state32,37). Hence, the photocarrier generation and carrier recombination processes via the PThTPTI/WS2 interface are fast upon light on/off, leading to short photocurrent rise time and decay time, which are 17 and 38 ms, respectively. The PThTPTI/WS2 PD can also be switched on/off repeatedly (Figure 5d−f). In order to test their abilities of operating at high temperatures, the two devices were directly measured between 25 and 300 °C in ambient air. Figure 6a shows the photocurrent as a function of the test temperature for the PThTPTI PD and the PThTPTI/WS2 PD. In the range of 25− 150 °C, for both devices, the photocurrent is nearly linearly dependent on the temperature. As we know, the photocurrent is proportional to carrier mobility and EQE. In the temperature range of 25−150 °C, the polymer shows almost constant mobility (about 0.18 cm2 V−1 s−1), while the photoexciton dissociation probability is linearly related to the temperature: the higher the temperature, the higher the exciton dissociation probability.38 In the high-temperature range of 150−300 °C, the photocurrent exhibits a slight decrease, which should be a collective result of the reduced carrier mobility and the enhanced exciton dissociation probability. Figure 6b shows the time-resolved photocurrent of the PThTPTI PD; it is seen that the rise/decay time decreases with increasing the test temperature (rise time: 6.64 s to 1.28 s and decay time: >10 s to ∼10 s from 25 to 300 °C). This should be induced by the temperature-enhanced trap-assisted photoexciton dissociation process and carrier recombination process

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CONCLUSIONS In conclusion, a highly stable organic donor material PThTPTI was used to fabricate PDs. By setting a discontinuous WS2 flake film beneath the PThTPTI layer, photoexciton dissociation and carrier recombination are both greatly enhanced via the PThTPTI/WS2 interface, leading to significantly improved photocurrent and light response speed in comparison to the single PThTPTI layer device. Importantly, both the PThTPTI PD and the PThTPTI/WS2 PD show high thermal stability up to 300 °C in ambient air. Other two-dimensional materials such as MoS2 could also work well with PThTPTI. Our results here should be of great value in exploring thermally stable organicbased optoelectronic devices for applications in harsh ambient environments.

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EXPERIMENTAL SECTION

Material Preparation. All of the materials were purchased from Sigma-Aldrich and were used as received without further purification. F


ACS Applied Materials & Interfaces Quartz and N-doped silicon with 300 nm silicon dioxide were used as the substrates. The WS2 were grown on clean quartz substrates using a typical CVD process.40,41 In such a method, WO3 powders in a quartz boat were put in the center of a tube furnace. Then, the S powers were put in a separated quartz boat at the upper stream and the quartz substrates were placed at the downstream next to WO3 powders. To start the growth, Ar/H2 gas flow was used and the heating zone for WO3 was raised to 900 °C in 30 min, and the temperature for S was raised to 240 °C. After the furnace tube reached the growth temperature, the growth lasted for different times to control the coverage ratio of WS2 flakes on the quartz substrate, which was followed by a natural cooling down to room temperature. The PThTPTI was synthesized using previously reported methods.23 The PThTPTI was dissolved in mixed solvent (chlorobenzene:chloroform = 1:4 by volume) with a concentration of 5 mg/ mL and was vigorously stirred for 12 h. Device Fabrication. PDs were fabricated on the quartz substrates, while FETs were fabricated on the SiO2 (300 nm)/Si+ substrates. The films were prepared by spin-casting the PThTPTI solution on the relevant substrates, followed by a soft-baking process (125 °C for 10 min in ambient air). The PThTPTI film thickness is about 40 nm. The devices (WS2 PDs, PThTPTI PDs, PThTPTI/WS2 PDs) were completed by the thermal evaporation of 60 nm thick gold electrodes through a shadow mask, which results in a channel width/length of 2000/10 μm. Characterization. All electrical characterizations were recorded with a Keithley 4200 in ambient air, and the temperature was controlled by a hot plate. The monochromatic light was from a Newport Oriel 200. Prior to the utilization of the light, the spectral and the light intensity were calibrated using a monosilicon detector. The optical images were obtained on an Olympus BX51 optical microscope. The TEM images were examined by a JEOL JEM-2011. The AFM image was acquired using a Veeco NanoScope IV with a silicon cantilever in tapping mode. UV−vis spectra were recorded using a JASCO V-570 spectrophotometer. Raman spectroscopy measurement was taken using a LabRAM HR800 (Horiba Jobin Yvon) with the excitation energy of 2.62 eV (473 nm). The device annealing was done for 60 min at each temperature. To test the device at high temperatures, the devices were put on a hot plate at each temperature for 60 min before the characterization.

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ACKNOWLEDGMENTS

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REFERENCES

Z.J. and D.H. contributed equally to this work. The authors acknowledge the financial support by the 973 Program (Grant Nos. 2014CB643600 and 2014CB643503), the National Natural Science Foundation of China (61405208), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12030200). L.D. appreciates the National Natural Science Foundation of China (U1401244, 21374025, 21372053, 21572041, and 51503050), the Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), the State Key Laboratory of Luminescent Materials and Devices (2016-skllmd-05), the Youth Association for Promoting Innovation (CAS), and the Center for Excellence in Nanoscience (CAS) for financial support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12090.

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Additional data, including optical images of used films and the properties of the fabricated PDs (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.D.). *E-mail: [email protected] (J.W.). ORCID

Zhiwen Jin: 0000-0002-5256-9106 Author Contributions

Z.J., D.H., Q.Z., and P.M. performed the experiments. Z.J. performed the data analysis and experimental planning. The project was conceived, planned, and supervised by Z.J., L.D., and J.W. The manuscript was written by Z.J., L.D., and J.W. All authors reviewed the manuscript. Notes

The authors declare no competing financial interest. G


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WS2 Photodetectors with High

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