Polymer Schottky

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Graphene Quantum Dot-Sensitized ZnO Nanorod/Polymer Schottky Junction UV Detector with Superior External Quantum Efficiency, Detectivity, and Responsivity Saurab Dhar, Tanmoy Majumder, and Suvra Prakash Mondal* Department of Physics, National Institute of Technology, Agartala, India 799046 S Supporting Information *

ABSTRACT: Graphene quantum dot (GQD)-sensitized ZnO nanorods/poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) Schottky junction has been fabricated for visible-blind ultraviolet (UV) photodetector applications. Schottky diode parameters such as ideality factor, effective work function, and series resistance have been studied for GQD-modified and pristine ZnO nanorod-based devices. Under illumination of broadband light of intensity 80 mW/cm2, GQD-sensitized samples showed 11 times higher photocurrent value compared to pristine ZnO at −0.75 V external bias. GQD-modified detector demonstrated maximum photocurrent at UV region (wavelength ∼340 nm) for all reverse bias voltages. ZnO nanorods/polymer Schottky junction UV detectors revealed high external quantum efficiency (EQE) more than 100%. Interestingly, GQD sensitized nanorod-based device demonstrated high EQE value of 13,161% at −1 V bias (wavelength ∼340 nm), which is eight times higher than pristine ZnO NR-based detector. GQD-modified detectors also showed superior responsivity (36 A/W), detectivity (1.3 × 1012 Hz1/2/W) at −1 V bias under incident of light of wavelength 340 nm. Even at very low intensity of UV light (0.07 mW/cm2), GQD-modified UV detectors showed high photocurrent (∼7.0 mA/cm2). KEYWORDS: ZnO nanorods, graphene quantum dots, PEDOT:PSS, Schottky diode, UV detector, external quantum efficiency (EQE)

1. INTRODUCTION ZnO thin films and nanostructured materials have been studied extensively for ultraviolet (UV) detector because of its high optical band gap (3.2−3.4 eV), low cost, and easy synthesis process compared to conventional GaN- or SiC-based detectors.1−6 Particularly, ZnO nanowire based detectors are attractive and subject of extensive investigation for large surface to volume ratio and existence of deep level surface traps, which enhances the photocarrier lifetime and photoconductive gain.7−10 Soci et al.8 have studied ZnO nanowire-based visible-blind UV photodetectors with internal photoconductive gain as high as G ≈ 1 × 108. Single nanowire photodetector was fabricated on oxidized silicon substrate using optical lithography technique. Zhou et al.11 fabricated UV detector with high response and fast reset time using single ZnO nanowire/Pt Schottky barrier. The UV sensitivity of the sensor was improved by 4 orders of magnitude and the reset time has been drastically reduced from ∼417 to ∼0.8 s than a conventional ZnO/Au contact. Although, single ZnO nanowire based UV detectors are highly sensitive with fast response time, these sensors are limited to our daily application for high fabrication cost. On the other hand, UV detectors made of ZnO/conducting polymer hybrid structure are attractive and gained much attention due to high sensitivity, easy fabrication process and suitable for flexible electronics.12−18 Guo et al.12 fabricated solution-processed UV detector using ZnO nano© 2016 American Chemical Society

particles blended with semiconducting polymers. The detector demonstrated high responsivity of ∼721−1001 A W−1 and detectivity of 3.4 × 1015 Jones at 360 nm under 1), Sato and Yasumura had modified Norde’s approach to calculate the diode parameters and series resistance from forward I−V characteristics.38 However, both methods are tedious and require two experimental I−V measurements conducted at two different temperatures. We have calculated the series resistance as well as the diode parameters using Cheung and Cheung method,39 which is quite simple and the parameters (η, ϕB, Rs) can be estimated from one single forward I−V characteristics. The Cheung’s functions can be written as follows39

(1)

where V, η, q, IRs, K, Is, and T are the applied bias voltage, the ideality factor, the electronic charge, the voltage drop across series resistance Rs, Boltzmann’s constant, reverse saturation current and the absolute temperature in Kelvin, respectively. The reverse saturation current of Schottky diode can be written as ⎡ qϕ ⎤ Is = AA*T 2exp⎢ − B ⎥ ⎣ KT ⎦

q dV KT d(ln I )

dV KT = IR s + η d(ln I ) q

(5)

⎛ KT ⎞ ⎛ I ⎞ ⎟ H (I ) = V − η⎜ ⎟ln⎜ ⎝ q ⎠ ⎝ AA*T 2 ⎠

(6)

H(I ) = IR s + ηϕB

(7)

The plot of dV/d(ln I) vs I in eq 5 is a straight line. The series resistance (Rs) and the ideality factor (η) of the diode can be obtained from the slope and the y-axis intercept. Using the η value extracted from eq 5, a plot of H(I) vs I will also give a straight line with y-axis intercept equal to ηϕB. The slope of this plot also provides a second determination of RS, which can be used to check the consistency of this approach.

(2)

Where A and A* are device area and Richardson’s constant and ϕB is the effective barrier height. The value of A* was consider to be 36 Acm−2K−2 for ZnO.20,36 The ideality factor (η) of the Schottky contact was estimated from the linear region of I−V characteristics using the relation 31825

DOI: 10.1021/acsami.6b09766 ACS Appl. Mater. Interfaces 2016, 8, 31822−31831

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ACS Applied Materials & Interfaces Figure 2c shows the plot of dV/d(ln I) vs I for S1 device. The values of η and Rs were extracted using eq 5 and obtained as 2.8 and 15.3 Ω. The plot of H(I) vs I is shown in Figure 2d. The extracted values of ϕB and Rs for ZnO NRs/PEDOT:PSS junction was obtained from eq 7 and found to be 0.65 eV and 15.2 Ω. The series resistance calculated from eqs 5 and 7 are almost identical, which verify the consistency of Cheung’s method. We have also estimated the diode parameters and series resistance of GQD-sensitized ZnO NRs/PEDOT:PSS junction. It is worth mentioning that, GQD attachment time can influence the photosensing properties of S2 sample. In our study, GQDs have been deposited on ZnO NRs at time intervals of 30 min and 1, 2, and 3 h. To study the role of GQD attachment time, we have carried out J−V measurements and transient photocurrent measurements under dark and illumination of white light of intensity 80 mW/cm2 (Figures S5 and S6). Interestingly, the Jlight/Jdark ratio at reverse bias increases up to 1 h attachment time and decreases at 2 h, 3hours. As for example, the ratio Jlight/Jdark (−1 V bias) for 30 min and 1, 2, and 3 h GQD attachment times is 4, 15, 2.3, and 1.5, respectively. The decrease in photoresponse properties above 1 h GQD deposition time is attributed to the formation of thick GQD layers on the ZnO NR surface. Such kind of photosensing behaviors of GQD-sensitized semiconductor nanostructures have been reported by several researchers.29,30,40 The J−V measurements clearly demonstrated that, the optimum device performance can be obtained at 1 h GQD attachment time. In the later discussions, all photodetector measurements have been carried out with S2 samples of GQD attachment time for 1 h. We have also calculated the Schottky diode parameters η, ϕB and Rs of the same device. Figure 3a shows the dark J−V characteristics of device S2 in linear and semilogarithmic plot (inset). Interestingly, GQD-sensitized nanorods also demonstrated good Schottoky behavior with rectification ratio 56 at ±1 V bias. More importantly, the rectification ratio enhances 1.3 times compared to unmodified nanorods. The Schottky diode parameters η, ϕB, and Rs have been extracted from the Cheung and Cheung method and found to be 2.20, 0.69, and 163.5Ω, respectively (Figure S7). For both S1 and S2 samples, the diode ideality factors deviates from the ideal Schottky barrier value (η = 1). This deviation is due to the presence of surface traps at ZnO nanorod surfaces.41 The estimated barrier heights (ϕB) of S1 and S2 samples are close to the reported ZnO/PEDOT:PSS junctions.20 However, a little enhancement in ϕB for S2 sample is attributed to the surface modification of ZnO NRs after GQD attachment. At dark condition, the series resistance of S2 is also found to be higher than S1 device, because of the presence of GQD layer. The half-wave rectifier performance of GQD sensitized nanorod/PEDOT:PSS Schottky junction has been demonstrated using an input square wave signal as shown in Figure 3b. The GQD-modified device demonstrated excellent half-wave rectification behavior with minimal distortion of the output signal. From the above discussion, we have observed that, not only did ZnO NRs/PEDOT:PSS show Schottky junction behavior, the GQD-modified ZnO NRs/PEDOT:PSS sample also demonstrated good Schottky nature and could be attractive for UV detector applications. Figure 4 represents the J−V characteristics of S2 sample under the dark and illumination of broad band light of intensity 80 mW/cm2. The J−V plot of S1 sample is presented at the inset. Interestingly, under light the

Figure 4. J−V plot of S2 sample and S1 sample (inset) in semilogarithmic scale. J−V characteristics were measured under dark and illumination of broadband light of intensity ∼80 mW/cm2.

change in reverse saturation current is much higher in S2 compared to S1 sample. As for example, the photocurrent change at −1 V bias is 1.08 mA/cm2 and 13.42 mA/cm2 for sample S1 and S2, respectively. Figure 5a shows the transient photocurrent of sample S1, S2 and S3 under broadband light source of illumination intensity 80 mW/cm2 at −0.75 V bias. As pristine ZnO NRs are also good UV detector, the device FTO/ ZnO NRs/Ag (sample S3) has been tested for comparison. Figure 5b shows the transient photocurrent of sample S1 and S3 for better visibility of the results. At identical condition, the values of (Jlight − Jdark) for S1, S2, and S3 are 1.8, 19.1, and 0.1, respectively. It is evident that GQD plays an important role for superior photoconductive behaviors of ZnO/PEDOT:PSS Schottky junction. Figure 6 shows the transient photocurrent of sample S2 at −0.5, −0.75, and −1 V bias and sample S1 at −1 V bias under illumination of monochromatic light of wavelength 300, 320, 340, 360, 380, and 400 nm. At a particular wavelength, the transient photocurrent increases with increasing reverse bias. It can be mentioned that, we did not observe any significant change in photocurrent at forward bias under identical condition (Figure S8). Interestingly, the photocurrent attains maximum value at 340 nm at all reverse bias condition. More importantly, the photocurrent value (Jlight − Jdark) at 340 nm (−1 V bias), for S2 sample enhances 8.4 times compared to sample S1, which is attributed to the strong UV absorption of GQDs. To characterize the wavelength dependent gain of the photodetectors, we carried out external quantum efficiency (EQE) measurements at bias voltages −0.5, −0.75, and 1 V. The EQE has been calculated using the following equation.41,42 1240Jλ EQE(%) = 100 λPλ (8) Where, Jλ is photocurrent density (mA/cm2), λ is monochromatic wavelength (nm), and Pλ is the monochromatic light intensity (mW/cm2). Figure 7 shows the EQE (%) vs wavelength plot of sample S2 at −0.5, −0.75, and −1.0 V bias. The same plots for S1 and S2 samples at −0.5 V bias is shown at the inset for comparison. Interestingly, EQE value for all samples exceeds 100% at all bias voltages and the highest EQE value is obtained at 340 nm wavelength. As for example, the maximum values of EQE for device S1 at bias voltage −0.5 31826


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Figure 5. (a) Transient photocurrent (J−t plot) of S1, S2, S3 samples. (b) Transient photocurrent (J−t plot) of S1, S3 samples. All photocurrent response were measured under broadband light source of intensity 80 mW/cm2 and at −0.75 V bias.

Figure 7. External quantum efficiency (EQE) % vs wavelength plot for sample S2 at bias −0.5, −0.75, and −1.0 V. EQE (%) vs wavelength plot for sample S1 and S2 at bias −0.5 V (inset).

Figure 6. Transient photocurrent (J−t) plots of S2 sample under incident of monochromatic light of wavelengths 300, 320, 340, 360, 380, and 400 nm at bias −0.5, −0.75, and −1.0 V. J−t plots of S1 sample under incident of same monochromatic light at −1.0 V bias.

(Figure 8b). The unpaired electrons are collected at the electrodes and increase the photocurrent of the sample. This hole-trapping through oxygen adsorption and desorption at ZnO NR surface creates high density of trap states which enhances the photoresponse. In a Schottky junction photodetector, hole-trapping usually occurs at the metal−semiconductor interface under reversed-bias condition which results the shrinking of depletion region. Electron tunneling can occurs after certain reverse bias because of depletion layer thinning. If electrons pass multiple times, this mechanism yields EQE greater than 100%. The presence of GQD layer at ZnO NRs surface enhances the UV absorption, which results the efficient hole trapping and reduce the electron−hole recombination compared to pristine ZnO nanorods. For this reason, EQE for GQD-sensitized nanorod is much higher than ZnO nanorodbased schottky junction. The operating bias voltage is an important perquisite for the fabrication of energy efficient photodetector. Higher operating bias voltage requires more

V, −0.75 V and −1 V are 481, 1042, and 1617%, respectively (Figure S9a). The EQE value for device S2 at bias voltage −0.5, −0.75, and −1 V are 3800, 8244, and 13167%, respectively (Figure 7). The large EQE value above 100% is attributed to the surface trap induced carrier injection on nanorods.8 Figure 8 shows the schematic representation of photoconduction mechanism due to the hole trapping at the nanorod surface. It has been observed that, in ZnO semiconductor, the ambient oxygen molecules are adsorbed at the surface due to capture of free electrons [O2(g) + e− → O−2 (ad)]. As a result, a depletion layer is formed at the surface (Figure 8a). Under illumination of UV light, electrons-holes pairs are generated and holes migrate to the nanorod surface due to the band bending and release the negatively charged oxygen ions [h+ + O−2 (ad) → O2(g)] 31827


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Figure 8. Schematic representation of carrier transport mechanisms in ZnO nanorod based device. (a) Formation of depletion region due to adsorbed oxygen molecules at the surface of nanorods. (b) Generation electrons-holes pairs under illumination of light. Migration of holes to the nanorod surface and releasing of negatively charged oxygen ions. (c) Carrier transport mechanism in GQD sensitized ZnO nanorod/PEDOT:PSS junction under reverse bias.

power for the operation of a photodetectors. In our study, GQD-sensitized ZnO NRs/PEDOT:PSS Schottky junction detectors demonstrated superior UV-sensing properties and EQE value at −1 V, which is much less operating bias compared to the reported ZnO-based UV detectors.8,12 The responsivity (Rλ) of the samples has been determined using the following relation.41 Rλ =

Jlight − Jdark Popt

(9)

Where Jlight and Jdark denote the current density at light and dark. Popt is the intensity of incident light at a particular wavelength λ. Figure 9 shows the responsivity of the sample S2 at −0.5, −0.75, and −1.0 V. The responsivity plots for S1 and S2 samples at −0.5 V bias is shown at the inset. For both the devices, the responsivity has been found to be maximum at wavelength 340 nm. The maximum responsitivity enhances with applied bias and the maximum value for S2 device always higher than S1 sample (Figure S9b). As for example the Rλ value of S2 sample at −1 V bias is 36A/W, which is 8 times greater than S1 sample. The photodetector figure of merit is the noise equivalent power (NEP) which is actually the incident light power that a detector can distinguish from the noise. The detectivity of a photodetector can be obtained by the following relation.41−43 Dλ =

AΔf NEP

=

Figure 9. Responsivity vs wavelength plot for sample S2 at bias −0.5, −0.75, and −1.0 V. Responsivity vs wavelength plot for sample S1 and S2 at bias −0.5 V (inset).

contribution in the total noise current, the detectivity can be written as Rλ Dλ = 2qJdark (11)

Rλ AΔf In

Where, q is the charge of electron and Jdark is the dark current density. Figure 10 shows the detectivity plot of sample S2 at different bias voltage. The maximum detectivity at bias −0.5, −0.75, and −1.0 V are found to be 6.2 × 1011, 1.01 × 1012, and 1.29 × 1012 Hz1/2/W. It can be mentioned that, at all bias voltages, sample

(10)

where D is measured in cm Hz1/2/W (or Jones), A is the effective detector area in cm2, Δf is the electrical bandwidth in Hz, In is the noise current in A, and Rλ is the responsivity in A/ W. Considering the shot noise due to dark current is the major 31828


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saturation nature for the S2 sample at the same intensities is attributed to the presence of GQDs.

4. CONCLUSIONS In summary, we have fabricated an inorganic/organic Schottky junction using ZnO NRs/PEDOT:PSS polymer and GQDsensitized ZnO NRs/PEDOT:PSS polymer. The Schottky diode parameters such as ideality factor, effective work function, and the series resistance have been calculated from the I−V characteristics using Cheung and Cheung method. The photosensing behaviors have been studied using a broadband light of intensity 80 mW/cm2. GQD-sensitized nanorod showed higher ON/OFF ratio compared to pristine ZnO nanorods. The transient photocurrent measurements at 340 nm showed a strong photoconductivity under −0.5, −0.75, and −1.0 V external bias. Both samples (S1 and S2) showed EQE value more than 100%, which has been explained as being due to the surface trap induced carrier injection in nanorods. More importantly, GQD-sensitized device demonstrated superior EQE value at all bias voltage (−0.5 to −1.0 V) compared to control samples. GQD-modified nanorods also showed improved responsivity and detectivity compared to unmodified samples. Our GQD modified photodetector showed high photocurrent even at very low intensity of UV light (0.07 mW/ cm2). The present study demonstrates the potential use of GQD-modified ZnO nanorod/PEDOT:PSS Schottky junction for visible blind UV detector applications.

Figure 10. Detectivity vs wavelength plot for sample S2 at bias −0.5, −0.75, and −1.0 V.

S2 shows higher directivity near 340 nm compared to the S1 sample (Figure S10). Figure 11 shows the variation of photocurrent density (calculated as Jlight − Jdark) with incident light intensity (λ = 340

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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.6b09766. Figures S1−S10 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This present research work was partially funded by CSIR Extramural Research Scheme, Sanction 03(1316)/14/EMR-II dated 16/04/2014, Government of India. We acknowledge the central research facility (CRF) of NIT Agartala for SEM, UV− vis−NIR spectroscopy and XRD characterizations.

Figure 11. Photocurrent density (Jlight − Jdark) vs incident light intensity plot for sample S1 and S2, at applied bias −0.5 V. The wavelength of incident light was 340 nm.

nm) studied under an applied bias −0.5 V. The photcurrent values for S2 sample are always larger than S1 sample in all measured intensity range (0.07−0.31 mW/cm2). This is due to the dominant role of GQDs for UV absorption, which has been observed from the previous characterizations. At low intensities (0.07 to 0.15 mW/cm2), the photocurrent increases almost linearly for both samples, which is due to the proportional variation of charge carrier photogeneration efficiency with the absorbed photon flux.8 However, at higher light intensities, it deviates from linear nature. More importantly, the photocurrent for S1 sample has been found to be invariant at the intensity range 0.15 to 0.31 mW/cm2. But sample S2 showed the gradually increasing nature of photocurrent with light intensities. The saturation of photocurrent of S1 sample beyond 0.15 mW/cm2 illumination intensity is due to the reducing hole-trap states at the NR surface. Interestingly, the non-

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