Enhanced Performance of Wideband Room Temperature

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Enhanced Performance of Wideband Room Temperature Photodetector Based on Cd3As2 Thin Film/Pentacene Heterojunction Ming Yang, Jun Wang, Jiayue Han, Jiwei Ling, Chunhui Ji, Kong Xiao, Xianchao Liu, Zehua Huang, Jun Gou, Zhijun Liu, Faxian Xiu, and Yadong Jiang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00727 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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Enhanced Performance of Wideband Room Temperature Photodetector Based on Cd3As2 Thin Film/Pentacene Heterojunction Ming Yang1, Jun Wang1,3,*, Jiayue Han1, Jiwei Ling2, Chunhui Ji1, Kong Xiao1, Xianchao Liu1, Zehua Huang1, Jun Gou1, Zhijun Liu1,3, Faxian Xiu2,4 and Yadong Jiang1,3 1

School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P.R. China

2 State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, P.R.China. 3 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, P.R. China 4

Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

Abstract 3D Dirac semimetal Cd3As2, as an ideal candidate photosensitive material, has attracted widespread attention due to its excellent characteristics of high carrier mobility and zero band gap. Although photodetector based on Cd3As2 crystal material has been made, there are still huge significances for the utilization of Cd3As2 thin film in commercial devices. In this paper, we demonstrated a wide-band photodetector based on heterojunction of Cd3As2 thin film and pentacene for the first time. This photodetector can detect the radiation wavelength from 450 nm (visible region) to 10600 nm (long wave infrared region) at room temperature, exhibiting high current responsivity (36.15 mA/W) and external quantum efficiency (7.29%) at 650 nm, of which Ri is more than six times as high as previously reported that of crystalline Cd3As2 devices. Most interestingly, the far infrared current responsivity of this detector at 10600 nm can reach 1.55 mA/W, which is extremely difficult for other 2D material detectors. Overall, the wide-wavelength photodetector based on the combined film using Cd3As2 and Pentacene is proved to possess superior performance for photodetector device application. Moreover, Cd3As2 thin film/pentacene heterojunction has an advantage in the manufacturing array devices, thus providing various possibilities for application of thin-film photodetector. The use of Cd3As2 thin film and organic molecules opens up a new path for the practical application of Cd3As2 materials. Keywords: 3D Dirac Semimetal, Wide-band, Photodetector, Heterojunction, Organic Molecules

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Introduction In the past decade, graphene has been widely studied because of its advantages such as large surface area, good heat transfer performance and strong conductivity.1-4 However, some inherent obstacles (easy aggregation, low absorption, as well as susceptible surface) limit its application in high performance photodetector.5-7 Therefore, advanced technologies and experimental conditions have created extremely favorable conditions to discover new classes of quantum materials, topological insulators, and Weyl and Dirac semimetals,8-11 which bring a promising prospect for electronic applications. In 3D Dirac semimetals fields, such as Cd3As2 and Na3Bi, electrons disperse linearly in momentum space, as they do in two dimensions of graphene. As 3D analogues of graphene, Cd3As2 shows ultrahigh mobility of 9×107 cm2/Vs at 5 K and up to 1.5×104 cm2/Vs at room temperature, where the node at zero energy is protected against gap formation by crystalline symmetry.12, 13 Even the mobility of suspended graphene cannot reach this order of magnitude.14

15, 16

More interestingly, Cd3As2 electronic properties are well-protected

against changes to the surface or environment compared with graphene. Owing to all these properties, the 3D Dirac semimetal Cd3As2 provides a great platform for manufacturing high-performance photodetectors.17-19 Previous studies mainly focus on the transport and the characteristics of 3D Dirac semimetals Cd3As2, such as giant magnetoresistance, nontrivial quantum oscillations, and Landau level splitting under a high magnetic field.20-23 Therefore, to explore the development of Cd3As2 in the photoelectric detection is completely necessary. In this new field, there only exist some studies on the preparation of nano-crystal Cd3As2 photodetectors. Wang et al. systematically studied the effects of temperature on the photoelectric current in crystalline Cd3As2 nanowire and nanoplate devices, and the response of Metal-Semiconductor-Metal (MSM) structure Cd3As2 detector to different wavelengths from visible to infrared region.24 The prototype metal-Cd3As2-metal photodetector exhibited a responsivity of 5.9 mA/W at low temperature at 650 nm. Yavarishad et al. used the Seeback theory to study crystal Cd3As2 platelet at room temperature and successfully tested its photoresponse at light modulations up to 6 kHz.25 Although the super-fast response of Cd3As2 was proved, the room-temperature responsivity of the sensor was very low as 0.27 mA/W. In comparison with nanoscale crystal Cd3As2, the Cd3As2 film possesses much better relaxation time, which can enable the feature of excellent response speed.26 Also, Cd3As2 thin film is suitable 2


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for large area and uniform preparation and meets the requirements of array devices. Additionally, Cd3As2 thin film shows a good saturation absorption property for light and this amazing feature has recently been used for preparing the laser. Meng et al. revealed remarkable saturable absorption effects of Cd3As2 thin film at 1-2 µm and took use of Cd3As2 thin film to engage a Tm fiber laser experiment.27 However, the wide-waveband photodetector fabricated by Cd3As2 thin film and the combination of Cd3As2 thin film heterojunction have not been studied yet. There have also been some studies using graphene and other two-dimensional materials to form heterogeneous junction, with good broadband detection effect.28,29 Recently, some organics and inorganic materials have been used as photodetectors, such as CdS/P3HT, CdS/rGO, Cd3P2/PCBM , Zn3As2/P3HT, ZnO/P3HT and graphene/quantum dot materials.30-34 The success of above studies has provided an inspiration for the preparation of a new Cd3As2 thin film and pentacene heterojunction photodetector. Furthermore, the Cd3As2 thin film used in this experiment is proved to be n-type semiconductor material by Hall effect measurement. Considering that its mobility is high, Cd3As2 thin film causes very small resistance, limiting the detector for the application of the small current response. So we propose to apply p-n junction effect to prepare a new type of photodetector. For this reason, a suitable p type semiconductor material should be chosen. Compared with the inorganic, organic materials is more promising for the combination with Cd3As2 thin film because of cheap, easy to make membrane. Studying organic polymer materials and organic small molecule materials carefully, we find that organic polymer materials as the solution properties is generally prepared by spin coating method while small molecule organic material is adopted by the thermal evaporation method. According to the metal properties of Cd3As2 thin film, it is very difficult for polymers to form film uniformly on its surface. Among small molecule organic material, pentacene is wellknown due to high conductivity and wide response band. Therefore, a high performance photodetector using pentacene and Cd3As2 thin film should be very constructive and meaningful. In this paper, we firstly prepared a wide-band photodetector with a heterogeneous junction structure, which was fabricated by Cd3As2 thin film with the pentacene. Thus, we reported the photoelectric detection features in details. The device consisting of metal-Cd3As2/pentacene–metal

has

a

vertical

heterojunction

structure.

This

photodetector has wide-waveband response (from the visible to long wave infrared 3


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region) and far better performance than photodetector fabricated with Cd3As2 crystal nanostructure at room temperature. In addition, it not only eliminates adverse factor of low current response of pure Cd3As2 detector, but also increases the current response by an order of magnitude (up to 36.15 mA/W). With comprehensive consideration of the above factors, Cd3As2 thin film is undoubtedly a promising choice to make optoelectronic devices in the future. The use of Cd3As2 thin film and organic molecules also opens up a new path for the practical application of Cd3As2 materials. Experimental detail Production process of Cd3As2 thin film In this study, molecular-beam epitaxy (MBE) technique is employed to produce high-quality Cd3As2 thin film. The film was grown in mica substrates under the ultrahigh vacuum in a MBE system, as described in previous reports. 23,35,36 Preparation of composite film devices fabricating Cd3As and pentacene The schematic diagram of the fabrication process of Cd3As2/pentacene composite thin film and photodetector device is shown in Figure S1. The schematic diagram of fabrications of Cd3As2/pentacene heterojunction photodetector is illustrated as follows. In addition, the Cd3As2 thin film photodetector and the pentacene photodetector were also prepared on mica in the same way as above. The mica substrate with 100 nm Cd3As2 thin film was placed into the evaporation equipment with metal mask to evaporate a 100 nm thick layer of pentacene thin film. Then, electron beam evaporation was used to evaporate Au electrodes plated on the upper sides of the respective materials. Finally, the photodetectors of Cd3As2 thin film as well as Cd3As2 thin film/pentacene were fabricated with a channel of 3 mm long and 400 µm wide. Characterization The morphology of the Cd3As2 thin film was observed by field emission scanning electron microscope (SEM) (Manufacturer Specifications - Inspect F, FEI). To test the stoichiometry of the thin film, energy-dispersive X-ray spectroscopy (EDS) (INCA PentaFETx3) was used to collect chemical compositions. The surface morphology of pentacene thin film was studied by atomic force microscopy (AFM) (Bruker Dimension Edge).

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The absorptivity in visible to near infrared region was obtained by integrating sphere instruments (Eflection type integral ball IS-20-5-R, Transmission type integrating ball IS-20-5-T and Fiber Spectrometer NIR2500). The middle infrared absorptivity was recorded using Fourier Transform Infrared Spectroscopy (FTIR Spotlight400). The photocurrent was measured with Source meter (KETHELEY 2636B System Source meter). A test platform, which consists of oscilloscope (InfiniiVision DXO-X 2024A), low noise current amplifier (MODEL SR570) and a chopper controller (MODEL SR540 OPTICAL CHOPPER - Stanford Research Systs), detected the response time of Cd3As2/pentacene film. In this experiment, different types of lasers were used, particularly including a wavelength of 2800 nm as QCL laser and a wavelength of 10600 nm as carbon dioxide laser, while the remaining wavelengths were used in semiconductor lasers. All the photoelectric performance measurements were carried out at room temperature. Results and discussions As the fundament of detector, the detailed characterization of Cd3As2/pentacene thin film is displayed in Figure 1. Figure 1a shows the energy-dispersive X-ray spectroscopy (EDS) and its surface morphology with SEM image of the Cd3As2 thin film. The EDS curve proves that the chemical atomic ratio (close to 59.3:40.7) of Cd: As. The Cd3As2 thin film is slightly As-enriched, deviated from the ideal 60:40 ratio of the polycrystalline Cd3As2 precursor. It is possible that the growth rate of the two elements is different. The SEM image of the Cd3As2 thin film suggests that the Cd3As2 thin film has a good denseness and crystallinity. Figure 1b shows 2x2 μm AFM image of pentacene film with an average thickness of 100 nm deposited onto Cd3As2 thin film at 120 ºC. Figure S2 is flexible array Cd3As2 thin film/pentacene heterojunction photodetector display diagram. Due to the use of a certain flexible mica substrate, the prepared photodetector has flexibility. Meanwhile, it is possible to prepare the array photodetector because of the convenient preparation of Cd3As2 thin film. Large-scale array photodetectors are also the focus of our next study.

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Figure 1. The characterization of the Cd3As2 and pentacene thin film (a) the EDS and SEM of its surface morphology the inset characterizations of the Cd3As2 thin film. (b) 2x2 µm AFM image of a pentacene film.

After showing inherent properties of the thin film, the structure of the detector needs to be explained to better understand how the detector works. Figure 2a shows the schematic of Cd3As2/pentacene heterojunction photodetector. From the crosssection view, Cd3As2/pentacene heterojunction has a thickness of nearly 200nm and the thickness of the two kinds of film is basically equivalent. It is also illustrated that the as-prepared Cd3As2/pentacene heterojunction has excellent dense structure and uniform thickness. On the other hand, the energy band diagram of Cd3As2/pentacene heterojunction under bias voltage and illumination is briefly shown in Figure 2b. In our view, electrons flow from the n-type Cd3As2 into the lower energy states in p-type pentacene, leaving the positively charged empty state within a depletion region in the n-type Cd3As2. Then the energy band bends upward in the vicinity of the Cd3As2/pentacene heterojunction to form a built-in field.

Figure 2. (a) the schematic of photocurrent measurement of Cd3As2 /pentacene heterojunction and cross-section view of Cd3As2 /pentacene heterojunction. (b) the energy-band diagrams of the ideal Cd3As2/pentacene heterojunction under bias voltage and illumination.

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For the heterojunction based photodetector, the I-V curves with and without illumination can demonstrate the the rectification effect of the photodetector and select appropriate bias voltage. Figure 3a shows the I-V curves of the Cd3As2 thin film/pentacene heterojunction photodetector with and without illumination at the wavelength of 650nm. In order to find out the best Vbias, Iill/Idark is used to calculate and combined with the actual test results. According to the above principles, the photodetector uses 0.5mV as the main test Vbias. Photocurrent curves of the Cd3As2/pentacene heterojunction, as the most important parameters, are shown in Figure 3b and 3c, which was measured under different wavelengths (450 nm, 520 nm, 650 nm, 780 nm, 808 nm, 980 nm, 1310 nm, 1550 nm, with the laser power of 10 mW while 2800 nm and 10600 nm whose laser powers are nearly 5.8 mW and 50 mW with the bias voltage at 0V and 0.5 mV). It can be seen from Figure 3b, 3c that the current at Vbias= 0 V is lower than that at Vbias= 0.5 mV. Apparently the photodetector has obvious photoelectric current at all these wavelengths. The wide-waveband photoelectric response should be correlated with the Cd3As2/pentacene heterojunction absorption. The detailed mechanism is explained below. As shown in Figure 3d, the photocurrent increases approximately linearly with increasing Vbias, proving the stable and consistent features of this device.

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Figure 3. (a) the I-V curves of the Cd3As2 thin film/pentacene heterojunction photodetector with and without illumination at the wavelength of 650nm. (b),(c) Photocurrent curves of the Cd3As2 thin film/pentacene heterojunction photodetector are measured under different wavelengths without and with bias voltage. (d) the curve of variation of photocurrent with bias voltage at the wavelength of 450 nm, 520 nm,650 nm,780 nm,980 nm, 1310 nm and 1550 nm. (e) the Ri curve of Cd3As2 thin film/pentacene heterojunction photodetector and Cd3As2 thin film photodetector. (f) the EQE curve of Cd3As2/pentacene heterojunction photodetector under different wavelength.

The responsivity (Ri) is calculated to evaluate the photodetector properties of Cd3As2/pentacene heterojunction photodetector. In general, Ri is defined as the photocurrent generated per unit power of the incident light on the effective area of a photodetector. The formula is as follows:

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I (1) P Iph = Ilight -Idark (2)

R =

where Iph is the photocurrent per unit device area, denotes the current variation when the laser switches on and off, Ilight is the device current under the illumination, Idark is the dark current of device, and p is the power of illumination light on the detector channel.37 The efficiency of light energy utilization can be described by external quantum efficiency (EQE), which can be calculated by following formula:

I hv I hv = = R (3) q q p q∅ Above-mentioned Iph is the photocurrent; ∅ was the photon flux, which is defined as EQE =

the number of incident photons; q is the unit electron charge; hν is the energy of one single photon; p is the power of incident light, and Ri is the responsivity.37-39 Based on the above formula, the Ri of Cd3As2 thin film/pentacene heterojunction photodetector and Cd3As2 thin film photodetector with a bias voltage of 500 mV is calculated and then shown in Figure 3e. The largest Ri of Cd3As2 thin film photodetector is 12.5 mA/W obtained at 520 nm, indicating that the film itself has a good current response. Moreover, the current response of Cd3As2 thin film/pentacene heterojunction photodetector has been greatly improved at every frequency after combining such kind of thin film with the organic pentacene. The largest Ri appears at 650 nm, i.e. 36.15 mA/W. Additionally, the Ri of pentacene has a maximum value of Ri 8.6 mA/W at 650 nm. The inset figure is the Cd3As2 thin film/pentacene heterojunction photodetector and Cd3As2 thin film photodetector Ri at the wavelength of 2800 nm and 10600 nm. The EQE curve of Cd3As2/pentacene composite film photodetector is shown in Figure 3f. Accordingly, the largest value is 7.29% at 650 nm. The most valuable point of Cd3As2 thin film/pentacene heterojunction photodetector has good infrared response, for instance, the photodetector has the outstanding Ri of 10.1 mA/W, 17.03 mA/W, 13.2 mA/W, 3.17 mA/W and 1.55 mA/W at the wavelength of 980 nm, 1310 nm, 1550 nm, 2800 nm and 10600 nm, respectively, which is usually lacked in graphene-based photodetector. The communication band is located at 980 nm and 1550 nm,17, 24 hence this photodetector can be very useful in information communications. These features are good enough to make Cd3As2 thin film/pentacene heterojunction photodetector applicable to various applications. The Ri at the wavelength 10600 nm, suggest that the photodetector will 9


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play a role in the middle infrared and long wave infrared detection area. From the obvious Ri improvement of Cd3As2 thin film/pentacene heterojunction photodetector compared with pure pentacene photodetector, we can conclude that pentacene should play a key role. Figure 3e can be used to analyze the effect of pentacene on the Cd3As2 thin film/pentacene heterojunction photodetector. It can be found from the figure that the action of pentacene can be divided into two kinds: the first is in the visible band and the second is in the infrared band. In the visible spectrum, pentacene has good absorption. Hence, when the light is incident in the junction area of pentacene and Cd3As2, both absorbs the light and then produces photoproduction carriers. Therefore, in the visible band, the Ri of the Cd3As2 thin film/pentacene heterojunction photodetector is higher than that of the pure Cd3As2 thin film photodetector. Because pentacene has a single peak exciton fission effect at about 660 nm,20 more conductive carriers are provided when the light is irradiated in this band, which is the reason of Ri maximum at 650nm. As for infrared wavelengths, although the pentacene do not absorb the wavelengths of light (the absorption spectra of pentacene are shown in the figure S3, over 800 nm absorption is basically zero), the existence of pentacene for Cd3As2 film, rather than a layer of great resistance medium, can have the effect of limiting dark current. Consequently, it can also be seen that in the infrared band, the Ri of the Cd3As2 thin film/pentacene heterojunction photodetector is higher than that of the pure Cd3As2 thin film photodetector. In general, the photoelectric response of device is closely related to the absorption rate of material. Figure 4 shows the absorption rate curves of Cd3As2 thin film/pentacene heterojunction which are obtained in the regions of visible region, near-infrared region and mid-far infrared region. The dashed line is the actual measured data, and the solid line is a smoothed curve. Due to the influence of substrate mica in the region of infrared light, the measured curve will oscillate, presented by the dashed line. These curves indicate that the mixed thin film possess an obvious absorption rate at wide wavelengths of 400-9000 nm. More concretely, the absorptivity starts to increase first, then reaches the maximum value of 80% at 670 nm, finally decreases rapidly in visible region. The absorption peak at 650 nm originates from singlet exciton fission of pentacene.40 Singlet exciton fission is particularly applied in solar cells and photodetectors because it can double their quantum efficiency. Indeed, singlet exciton fission has recently been exploited in a photodetector based on pentacene to yield an external quantum efficiency (EQE) over 10


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100%.41,42 This broad band absorption brings about the wide spectrum’s current response of Cd3As2 thin film/pentacene heterojunction photodetector. The absorption of the wide spectrum of Cd3As2 /pentacene composite thin film is mainly due to the zero band gap of Cd3As2 materials.

Figure 4. The absorption spectrum of Cd3As2 /pentacene composite thin film from visible to long wave infrared. (a) the absorption spectrum of Cd3As2 /pentacene composite thin film at visible region. (b) the absorption spectrum at near infrared region. (c) the absorption spectrum at long wave infrared region.

As expected, p-n junction can be formed by n-type Cd3As2 thin film with p-type pentacene. When the beam illuminates the junction, a large number of bare hole carriers and electron carriers are produced. Subsequently, some of the electrons will move to the conduction band of Cd3As2 whereas some of the holes will enter the valence band of pentacene, just as the previous band analysis. It results in the increase of both the negative carriers in the Cd3As2 and the positive carriers in the pentacene, which can improve the photoelectric current. Therefore, the device photocurrent generation mechanism is based on photovoltaic effect. Photocurrent generation always occurs in the p-n junction area locating between Cd3As2 and pentacene interface where the built-in electric field exists. However, when the waveband changes to infrared band range, the rise time of the photocurrent is obviously slower than that of the visible band. Since photovoltaic effect cannot interpret this phenomenon, the photothermoelectric (PTE) effect comes into play.24 In general, VPTE can be calculated by integrating the optically induced electron temperature gradient together with a spatially varying Seebeck coefficient: VPTE =∫ S∇Te dx, where S depends on carrier’s temperature and density.24,43 When the wavelength is over 700 nm, the mica is less capable of transmitting heat than gold electrode and Cd3As2 thin film. The gold electrode is the major heat propagation area 11


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of the photodetector. The PTV effect of infrared band is higher than visible light band, leading to the sluggish rise of photocurrent.44 For the photodetector device, the rise and decay time of the photocurrent are of the same importance. Generally speaking, the rise time was defined as the time for the current increased to 63% of the peak value after turning on the light, and the decay time was defined as the time for the current to decrease to the value of 37% of peak value after turning off the light.24,38 The detector response time tested by self-built platform that the rise and decay times were about 44 ms and 124 ms, 60 ms and 147 ms at the wavelength of 450 nm and 1550 nm, respectively. With the wavelength over 700 nm, the mica substrate does not conduct heat as efficiently as metal and semimetallic Cd3As2, the gold contact is the major heat dissipation channel of the device. The PTV effect of infrared band is higher than visible light band, leading to the rise of photocurrent in the infrared band slowly. Figure S4 illustrates this very well. The capacitance of the device was nearly 300 pF, and the resistance was 1851 Ω, according to the response time calculation formula τ=2πRC.45,46 The theoretical response time of the device was nearly 3.49 µs, while the actually response time of the device was quite slow compared with the theoretical one. On the macro level, device size and channel width have great influence on response time. In other words, the larger the device size is, the longer the response time is. The lattice mismatch between Cd3As2 and pentacene could lead to dislocation or defect to bind the carrier and delay the response time.47-49 The comparison of Cd3As2 phototransistors’s performance in previous report among two-dimensional photoelectric materials and other topological insulator is shown in Table 1. The NO.1 photodetector in the table is the nanoplate and nanowire photodetector of Cd3As2 crystal. It has a wide spectrum response, and its maximum Ri is 5.9 mA/W, under 150 K conditions.24 NO.2 photodetector is based on the Cd3As2 crystal, its maximum Ri is 0.27 mA/W, and its maximum working wavelength is 1064 nm

25

. NO.6 and 7 in the table are the Cd3As2 thin film detector prepared by this

experiment and the Cd3As2/pentacene thin film detector prepared by this experiment. The Ri of the photodetector in NO.6 and NO.7 is 2.1 and 6.1 times, 46 and 134 times of NO.1 and No.2 respectively. By comparing NO.3-5 photodetectors, there is still plenty of room for improvement in Ri.50-52 But the photodetector of this experiment has a wide-wavelength response capability, which can reach far-infrared band. Besides, one advantage is that such detector is no need for refrigeration, which can 12


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bring a promising future for it. The maximum Ri of this work is achieved when the laser power is 2.64 mW. Furthermore, the higher value of Ri may achieve in lower laser power because of low concentration of photogenerated carriers. The enhancement of photogenerated carrier concentration along with the increasing incident laser power will reinforce the scattering of photogenerated carriers, in turn leading to the generation of local heat. This can evidently reduce the Ri of the system. The value of Ri obtained at 2.64 mW is higher than the previous reports of Cd3As2 photodetector and other topological insulator-based photodetectors. In a word, the first attempt to combine Cd3As2 /pentacene photodetector with organic molecules reveals that it can detect ultra-wide bandwidth. Table 1. The comparison of Cd3As2 phototransistors’ performance in previous report among the organic molecules as well as two-dimensional photoelectric materials. NO.

Active materials

Ri

Wavelength range (nm)

(mA/W) 1 2 3

Cd3As2 crystal nanowire/plate Cd3As2 crystal platelets Gr-WS2-Gr

Test temperature (K)

Ref

5.9

532~10600

150

[24]

0.27

1064

300

[25]

488~633

300

[50]

473~1064

300

[51]

100 -5

4

Ag-SnSe-Ag

8.8x10

5

ITO-SnSe-ITO

0.006

White light

300

[52]

6

Pure Cd3As2 thin film

12.48

450~10600

300

This

Cd3As2/ Pentacene thin film

36.15

7

work 450~10600

300

This work

Figure 5. A set of related data of Cd3As2/pentacene heterojunction device under the condition of current on/off at the wavelength of 1550 nm (a) with the different laser power intensity (1.59 mW/cm2, 3.18 mW/cm2, 6.37 mW/cm2, 8.92 mW/cm2), where the Vbias is 0.5 mV. (b)

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with the different Vbias (0.5 mV, 1 mV,2 mV, 5 mV), where the laser power intensity is 3.18 mW/cm2.

In order to further demonstrate the stability of the detector, the variable laser power and Vbias are used to test the Cd3As2 thin film/pentacene heterojunction photodetector shown in Figure 5. The photocurrent curve of the Cd3As2/pentacene heterojunction photodetector is measured under different laser power intensity (1.59 mW/cm2, 3.18 mW/cm2, 6.37 mW/cm2, 8.92 mW/cm2) at the wavelength of 1550 nm along with Vbias is 0.5 mV (Figure S5 shows 450nm measured under different laser power intensity under the same conditions.). In order to observe the effects of various laser power on the Cd3As2/pentacene thin film detector, the different curves are arranged and pulled down near 0. It is worth noting that the photocurrent value of Cd3As2/pentacene heterojunction photodetector is µA order of magnitude higher than dozens of nA order of magnitude of previous report of pure Cd3As2 crystal photodetectors.24,25 Along with the increase of laser power, the amplitude of light current is gradually increasing, showing the device’s good switching characteristics. Moreover, Figure 5b reveals that photocurrent curves of the Cd3As2/pentacene thin film photodetector under different Vbias (0.5, 1, 2, 5 mV) at the wavelength of 1550 nm with laser power intensity is 3.18 mW/cm2. With the increase of bias voltage, the amplitude of light current is increased. This result shows that the device have good periodicity and switching characteristics. In order to better display the stable performance of the detector under the laser irradiation, Figure S6 shows the detector with and without irradiation under the light at the wavelength of 450 nm and 1550 nm. The detector's current is basically unchanged without laser irradiation. For laser irradiation, as the laser power increases, the response current also increases together.

Conclusions In summary, for the first time, 3D Dirac semimetal Cd3As2 thin film and pentacene heterojunction photodetector has been successfully achieved. This device has been demonstrated a broadband photoresponse from visible light (450 nm) to far-IR (10600 nm), the highest Ri of the prototype device reaches 36.15 mA/W, with the corresponding EQE of 7.29% at room temperature. The superior equipment performance is in correlation with the high absorption of Cd3As2/pentacene film in the visible to far infrared. Additionally, the Cd3As2 thin film/pentacene heterojunction

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photodetector was further constructed onto ﬂexible mica substrates, which demonstrated great potential for smart wearable device. It is obvious that the photoelectric detector fabricated by combination of Cd3As2 thin film and organic molecules possesses much better bandwidth responsiveness in all aspects comparing with traditional and similar systems, which becomes one of the most efficient multipurpose hybrid systems for preparing optoelectronic devices in the future. This research opens up a valuable course for the application of 3D Dirac Semimetal and organic molecules in photodetectors and a feasible way for preparing the large-scale array photodetector.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected]

Notes The authors declare no competing financial interest.

Author Contributions M.Y. wrote this manuscript. J.W., M.Y. conceived and supervised the project. M.Y., J.H., J.L., X.K. performed the photocurrent measurements and data analysis. C.J., X.L., F.X. provided help and support for the revision of the manuscript. J.W., F.X., Z.L. provided theoretical support for the manuscript. All authors discussed the results and commented on the manuscript.

ASSOCIATED CONTENT Supporting Information Additional schematic diagram of the fabrication process of Cd3As2/ pentacene composite thin film and photodetector device, flexible array Cd3As2 thin film/pentacene heterojunction photodetector display diagram, absorption spectrum of pentacene thin film and so on.

Acknowledgements

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This work is partially supported by National Science Funds for Creative Research Groups of China (No. 61421002), the National Natural Science Foundation of China (NSFC) (No.61501092 and 61674040)

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For Table of Contents Only

Enhanced Performance of Wideband Room Temperature Photodetector Based on Cd3As2 Thin Film/Pentacene Heterojunction Ming Yang, Jun Wang, Jiayue Han, Jiwei Ling, Chunhui Ji, Kong Xiao, Xianchao Liu, Zehua Huang, Jun Gou, Zhijun Liu, Faxian Xiu and Yadong Jiang

The Cd3As2 thin film/pentacene heterojunction photodetector can detect the radiation wavelength from 450 nm (visible region) to 10600 nm (long wave infrared region) at room temperature. The wide band detection performance is in correlation with the high absorption of Cd3As2 thin film in the visible to far infrared, and Cd3As2/pentacene heterojunction promotes the photo-excited carrier separating process. The Cd3As2 thin film/pentacene heterojunction photodetector will play a key role in the middle infrared and long wave infrared detection area.

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Enhanced Performance of Wideband Room Temperature

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