Organic-inorganic broadband photodetector

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Organic-inorganic broadband photodetector

Xianguang Yang, Baojun Li

Xianguang Yang, Baojun Li, "Organic-inorganic broadband photodetector ," Proc. SPIE 10622, 2017 International Conference on Optical Instruments and Technology: Micro/Nano Photonics: Materials and Devices, 106220B (12 January 2018); doi: 10.1117/12.2288873 Event: International Conference on Optical Instruments and Technology 2017, 2017, Beijing, China Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 1/15/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Organic-inorganic broadband photodetector Xianguang Yang* and Baojun Li Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Nanophotonics, Jinan University, Guangzhou 511443, China *E-mail: [email protected] ABSTRACT The capability to detect optical signals over a broad wavelength band is highly important for practical device applications. However, high speed responsive across entire wavelength band within a single photodetector remains challenge. Here we demonstrated a broadband photodetector using a single quantum-dot-doped polyaniline nanowire with a broadband responsive at 350-700 nm (see schematic). The high responsivity is attributed to the high density of trapping states at the enormous interfaces formed in polyaniline and quantum dots. The interface trapping can effectively reduce the recombination rate, promote the separation of photogenerated carriers, and then enhance the efficiency for optical detection. AL

Schematic shows the operation principle of nanowire photodetector under broadband illumination. KEYWORDS: tunability, broadband, interface, photodetector, single nanowire, quantum dot 1. INTRODUCTION Recently, various types of photodetectors have been fabricated [1-14], among which organicinorganic hybrid photodetectors have attracted great attention in recent years [11-14]. In contrast to other types of photodetectors, organic-inorganic hybrid photodetectors have not only the merits of 2017 International Conference on Optical Instruments and Technology: Micro/Nano Photonics: Materials and Devices, edited by Baojun Li, Xingjun Wang, Ya Sha Yi, Proc. of SPIE Vol. 10622, 106220B · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2288873 Proc. of SPIE Vol. 10622 106220B-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 1/15/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

organic based devices such as function tunability and easy-formation characteristics [15-17], but also the features of inorganic based devices, containing the broadband absorption and the excellent intrinsic carrier mobilities [18-21]. Meanwhile, the organic-inorganic hybridization circumvents their own disadvantages [22]. Moreover, the unique reaction at the interface between the organic polymers and inorganic semiconductors is beneficial to the photoconductive applications [23-25]. Organic-inorganic hybrid nanowire (NW) is a rational choice for combing the unique advantages of one-dimensional nanostructures with the abundant trap states of organic-inorganic interfaces [26]. Compared with two- and three-dimensional nanostructures, one-dimensional organic-inorganic hybrid NW represents the optimal dimension to study the mechanism for effective transport of carriers. In this work, we demonstrated an organic-inorganic broadband photodetector based on a single polyaniline NW doped with quantum dots (QDs). The organic-inorganic hybrid polyaniline NW doped with QDs, was fabricated by physical drawing method at low-cost. Different from high-cost sandwich type of photodetector, our device geometry does not require the utilization of transparent electrodes and allow the incident light to efficiently reach the active region. The constructed photodetector exhibits large responsivity (105 A/W) and high external quantum efficiency (107%) across the entire spectral range of 350-700 nm. 2. METHODS CdSe-ZnS core-shell QDs and transparent conductive polyaniline were purchased from Zkwy Bio-Tech (Beijing, China) and Alfa Aesar (Tianjing, China) companies, respectively. Polyaniline NW doped with QDs was fabricated by a direct drawing method as follows. First, 900 mg of polyaniline was dissolved in 1 mL of dimethylbenzene to form a homogeneous polyaniline dimethylbenzene solution. Second, polyaniline-QD blend solution was obtained by adding QD dimethylbenzene solution (concentration 8 μM/L) into the polyaniline dimethylbenzene solution. The QD concentration of the polyaniline-QD blend solution was controlled from 0 to 60 wt % with respect to the polyaniline content by adjusting the volume of added QD solution. The blend solution was stirred at room temperature for 3 h, followed by 50 min of ultrasonication to form a homogeneous solution with an appropriate viscosity for drawing. Third, the tip of a silica fiber was immersed into the solution for 1−5 s and then removed at a speed of 0.1−5 m/s, leaving a polyaniline wire extending between the solution and the fiber tip upon rapid evaporation of the dimethylbenzene. The diameter of the polyaniline wire varied from 200 to 500 nm. The diameter and surface morphology of fabricated polyaniline NW doped with QDs can be roughly inspected and selected under an optical microscopy (HIROX, KH-7700). After inspection, the selected polyaniline NW doped with QDs was transferred to a pre-cleaned SiO2 substrate. Then, the substrate was spin-coated with methyl methacrylate and poly(methyl methacrylate), and electrode patterns were defined by electron beam lithography (JEOL 6510 with NPGS). Cr/Au (25/85 nm) electrodes were prepared by metal evaporation. Electrical characteristics of the single NW photodetector were measured by a semiconductor characterization system (Keithley 4200) and a probe station (Micromanipulator 6150) in a clean box at room temperature. For comparison, photodetector based on a single pristine polyaniline NW was also prepared and characterized via a similar process. Monochromatic illumination light was obtained from a 200 W xenon lamp by the use of a monochromator (WDG15-Z).

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Optical characterization of single polyaniline NW with and without QDs doping was performed under a Micro spectrophotometer (CRAIC 20/20 PV). The absorption and transmission spectra of single NWs were measured by the use of corresponding modules equipped in Micro spectrophotometer. A 473-nm continuous work laser was used to excite the single polyaniline NW doped with QDs. The excitation laser was focused to a spot with a size of 50 μm through an 80× objective (numerical aperture, 0.6). The photoluminescence (PL) signals were collected by the same objective and directed through a dichroic filter. The filtered light was split by a beam splitter and directed to a charge coupled device (CCD) camera and a spectrometer for image and spectrum measurement, respectively. 3. RESULTS AND DISCUSSION Figure 1a shows a representative scanning electron microscope (SEM, SH-5000M, HIROX) image of a 300-nm-diameter polyaniline NW doped with QDs. To clearly inspect the QD distribution in the polyaniline NW, energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM, Tecnai G2 F30), operating at an accelerating voltage of 300 kV, were performed, the obtained results are shown in Figure 1b. The fabricated NWs were placed onto a 200-mesh copper grid coated with carbon supported membrane. For comparison, TEM image of a 300-nm-diameter pristine polyaniline NW is shown at the bottom inset of Figure 1b. It can be seen from the two TEM images that the CdSe-ZnS core-shell QDs were successfully doped into the 300nm-diamter polyaniline NW. The measured maximum diameter variation of 15 nm is over a length of 1.7 μm. The EDS analysis confirm the presence of S (4.43 wt %), Zn (12.16 wt %), Se (1.39 wt %), and Cd (1.88 wt %) elements. Herein, CdSe-ZnS core-shell QD act as photoactive component, while QD size and optical characterization are shown in Figure 2. Figure 2a shows the TEM image of experimentally used QDs. The corresponding diameter distribution histogram is shown in the inset. It shows that the frequency of QDs in 6-nm-diameter is of more than 80%. Semiconductor QD material exhibits interesting optical properties, which can absorb and emit photons at a nanometer scale (1−10 nm). Figure 2b shows the absorption (blue line) and PL (red line) spectra of QDs. The absorption is monotonically decreasing in the wavelength region of 350−600 nm, whereas the 650 nm absorption peak is attributed to the self-absorption effect of QDs. This self-absorption peak is corresponding to the center wavelength of PL emission band. The full-width at half-maximum (FWHM) of the PL emission is approximately 36 nm.


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Figure 1. SEM, TEM, and EDS characterization of a single polyaniline NW doped with QDs. (a) SEM image of a straight polyaniline NW doped with QDs. (b) EDS spectrum and TEM image of a 300-nm-diameter polyaniline NW doped with QDs. Bottom inset shows the TEM image of a 300nm-diameter pristine polyaniline NW. Reproduced from permission of Reference [27].

Figure 2. TEM and optical characterization of CdSe-ZnS QDs. (a) TEM image of QDs with average diameter of 6 nm. The inset shows the diameter distribution histogram of QDs. (b) Absorption and PL spectra of QDs. Reproduced from permission of Reference [27]. Transparent conductive polyaniline was chosen as polymer matrix to fabricate organicinorganic hybrid NW (i.e., polyaniline NW doped with QDs). Figure 3a shows the absorption spectrum of polyaniline NW doped with QDs (blue line) and the transmission spectrum of pristine polyaniline NW (red line). It shows that the pristine polyaniline NW is optical transparent in the wavelength region of 350−700 nm (transmission > 90%). The absorption of polyaniline NW doped with QDs is in agreement with the absorption of QDs, as shown in Figure 2b. These results demonstrate that the CdSe-ZnS QDs has been successfully doped into the polyaniline NW. Figure 3b shows the PL spectrum of a 500-nm-diameter polyaniline NW doped with QDs, excited by 473nm blue light illumination. The corresponding dark-field optical microscope image is shown in the inset. Both the red PL emission from QDs doped in polyaniline NW and the blue scattering spots


from NW and substrate can be imaged by charge coupled device (CCD) camera. The FWHM of the red PL emission is approximately 35 nm, which is in agreement with the FWHM of QD emission (Figure 2b). The result indicates that the red PL emission of organic-inorganic hybrid NW (i.e., polyaniline NW doped with QDs) is indeed from the doped QDs in polyaniline NW. 1.0

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Figure 3. Optical characterization of a single polyaniline NW doped with QDs. (a) Absorption spectrum of polyaniline NW doped with QDs and transmission spectrum of pristine polyaniline NW. (b) PL spectrum of polyaniline NW doped with QDs. The inset shows the dark field optical image taken under 473-nm illumination excitation. Reproduced from permission of Reference [27]. To construct a photodetection device, the fabricated single polyaniline NW doped with 40 wt % QDs is placed on a pre-cleaned substrate. Then, two Cr/Au electrodes were thermally deposited on each end of the NW. Figure 4a shows typical I−V characteristics of 300-nm-diameter polyaniline NW doped with QDs (blue line) and pristine polyaniline NW (red line). The measurement configurations are schematically illustrated in the insets. These results were measured under 350 nm light illumination at intensity of 20 mW/cm2. The measurement was conducted in air and at room temperature. It can be seen from Figure 4a that the photocurrent is largely enhanced with the doping of QDs into polyaniline NW. The I−V characteristics of the polyaniline NW doped with QDs under seven given wavelengths of 350, 395, 473, 532, 600, 650, and 700 nm light illumination are shown in Figure 4b. The inset shows the SEM image of the single NW photodetector. It is worth noting that the red curve of Figure 4b was measured without light illumination, corresponding to the red curve of Figure 4a. The photocurrent under dark condition is related to the intrinsic conductivity of polyaniline because the doped QDs cannot be excited. The red curve of Figure 4a was measured under 350 nm light illumination, but no QDs doped into the pristine polyaniline NW. The photocurrent is also attributed to intrinsic conductivity of pristine polyaniline. The nonlinear I−V curves indicate the Schottky contact between NW and electrodes. The photogenerated currents are much higher than dark current in the dark condition, indicating that the photodetector responds to the incident light with a relatively high sensitivity. For the photodetection mechanism, the organic-inorganic interface of polyaniline and QD plays a key role in charge dissociation and transportation. When the hybrid NW is under light illumination, the excitons (hole-electron pairs) are generated from QDs and trapped by the organicinorganic interface. The dissociation of trapped excitons occur efficiently at the interface between polyaniline and QD, as reported in the interface between an organic polymer of poly(3hexylthiophene) and an inorganic semiconductor of CdSe [10,22]. Specifically, the photogenerated


holes can be captured by the polyaniline because of the lower ionization potential and the photogenerated electrons can be accepted by the CdSe-ZnS QDs because of the higher electron affinity. Thus, the polyaniline is an effective channel for hole transportation, while the CdSe-ZnS QDs are as the channel for electron transportation. As a result, the recombination of photogenerated carriers is largely reduced. In addition, in the hybrid NW, the CdSe-ZnS QDs are highly dispersed in the polyaniline matrix, resulting in the formation of a one-dimensional interconnected network. Such a network structure leads to enormous organic-inorganic interfaces for exciton trapping and charge separation. Therefore, the efficient charge separation and transportation are achieved in the hybrid NW photodetector. 350 nm light with intensity of 20 mW /cmz

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Figure 4. Electrical characterization of single NW photodetectors. (a) I-V curves of photodetectors based on a pristine polyaniline NW (red line) and a polyaniline NW doped with QDs (blue line). The schematic insets present the TEM images of corresponding NWs. (b) I-V curves of a photodetector based on a polyaniline NW doped with QDs under seven given wavelengths illumination. The inset shows the SEM image of the NW photodetector. (c) EQE versus wavelength under bias voltages of 2.4, 1.8, and 1.2 V. (d) Responsivity versus wavelength under bias voltages of 2.4, 1.8, and 1.2 V. Reproduced from permission of Reference [27]. To evaluate the performance of a photodetector, the spectral responsivity (R) and external quantum efficiency (EQE) are usually calculated. Responsiviy (R) can indicate the photocurrent generated by per unit power of light illuminated on the effective area of the NW photodetector. EQE is defined as the ratio of the number of photogenerated carriers inducing the photocurrents to


the number of incident photons effectively absorbed by the photodetector. These two significant parameters of R and EQE can be calculated according to the following equations[28]: R= EQE = R ×

ΔI PS

(1)

hc hc ΔI = × eλ PS eλ

(2)

where ΔI is the difference between the photogenerated current and dark current, P is the light power density illuminated on a single NW, and S is the effective illumination area of a single NW. The constants: h is Planck’s constant (6.6 × 10−34 J·s), e is the electronic charge (1.6 × 10−19 Coulombs), and c is the velocity of light in vacuum (3.0 × 108 m/s). The physical variable λ is the wavelength of illumination light. Figure 4c shows the EQE as a function of wavelength under bias voltages of 2.4, 1.8, and 1.2 V. The maximum EQE of 350 nm wavelength measured under 2.4 V bias voltage reaches up to 3.25 × 106 (3.25 × 108%). Even the minimum EQE of 700 nm wavelength measured under 1.2 V bias voltage still reaches up to 1.33 × 105. The high EQE is attributed to the enhanced light-NW interaction, the high density of trap states, and the efficient dissociation of trapped excitons at the enormous organic-inorganic interfaces formed between polyaniline and QDs. The three reasons are equally important. In detail, the enormous organic-inorganic interfaces give rise to the enhanced light-NW interaction and the high density of trap states, which can facilitate exciton generation and trapping. Then, the efficient dissociation of trapped excitons is occurred, which reduce the recombination of photogenerated carriers. Figure 4d shows the responsivity as a function of wavelength under bias voltages of 2.4, 1.8, and 1.2 V. The large responsivity (105 A/W) across entire waveband indicates the effective interaction of studied light with QDs. The broad spectral response range of 350-700 is due to the broadband absorption of QDs doped into polyaniline NW. The curve profiles of Figures 4c and 4d are similar to the blue curve of Figure 3a, which is the absorption spectrum of polyaniline NW doped with QDs. The above results demonstrate that the spectral response range of single NW photodetector is determined by the absorption properties of QDs doped into the polyaniline NW. Thus, the spectral response range can be changed to a desired wavelength band by carefully choose the experimentally doped QDs.

lox

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Figure 5. Heterojunction based diode. Reproduced from permission of Reference [29].


4. CONCLUSIONS High-quality polyaniline NW doped with QDs were successfully fabricated via a low-cost physical drawing method. Broadband photodetector using a single organic-inorganic hybrid NW on a rigid SiO2 substrate was prepared and characterized. The photodetector showed excellent light response in the spectral range of 350 nm to 700 nm. Moreover, the spectral range could be tuned via the size change of QDs. The external quantum efficiency value reaches up to 106, the responsivity value reaches up to 105 A/W. These performance indexes are in the high level across entire spectral range, which is attributed to enormous interface traps. Significantly, single crossed heterojunction can be assembled with the organic-inorganic hybrid NW to achieve diode type photodetector (see Figure 5) [29]. These results indicate that organic-inorganic hybrid polyaniline NW has significant potentials for nanoscale photodetection and might be effectively integrated into optoelectronic circuits. ACKNOWLEDGMENT The authors thank Dr. Yuchao Li and Dr. Zaizhu Lou from Institute of Nanophotonics at Jinan University for fruitful discussions. This work was supported by the National Natural Science Foundation of China (Nos. 11274395 and 21703083), the Natural Science Foundation of Guangdong Province (No. 2017A030313026), and the Fundamental Research Funds for the Central Universities (No. 21617334). REFERENCES (1)

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