Silver-Nanowire-Embedded Transparent Metal-Oxide Heterojunction

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 14292−14298

Silver-Nanowire-Embedded Transparent Metal-Oxide Heterojunction Schottky Photodetector Sohail Abbas,†,‡ Mohit Kumar,†,‡ Hong-Sik Kim,†,‡ Joondong Kim,*,†,‡ and Jung-Ho Lee*,§ †

Department of Electrical Engineering and ‡Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute for Future Energies (MCIFE), Incheon National University, 119 Academy Road, Yeonsu, Incheon 22012, Republic of Korea § Department of Materials and Chemical Engineering, Hanyang University, Ansan, Kyunggido 426-791, Korea S Supporting Information *

ABSTRACT: We report a self-biased and transparent Cu4O3/ TiO2 heterojunction for ultraviolet photodetection. The dynamic photoresponse improved 8.5 × 104% by adding silver nanowires (AgNWs) Schottky contact and maintaining 39% transparency. The current density−voltage characteristics revealed a strong interfacial electric field, responsible for zero-bias operation. In addition, the dynamic photoresponse measurement endorsed the effective holes collection by embedded-AgNWs network, leading to fast rise and fall time of 0.439 and 0.423 ms, respectively. Similarly, a drastic improvement in responsivity and detectivity of 187.5 mAW−1 and of 5.13 × 109 Jones, is observed, respectively. The AgNWs employed as contact electrode can ensure high-performance for transparent and flexible optoelectronic applications. KEYWORDS: transparent, heterojunction, silver nanowires, metal oxides, multijunction, high performing

T

chemical and physical fabrication techniques.1,18−20 However, ensuring the transparent, self-powered and high-speed features altogether in a PD, remain a challenge. The fabrication of nanowrie (NW)-embedded metal-oxide heterojunctions with novel architecture can ensure the required features, altogether. As selective wide bandgap metal-oxides heterojunctions possess built-in potential to separate the photogenerated charge carriers. The well-connected porous metal nanowires (i.e., silver or copper nanowires) preserve the optimized transparency and effectively collect these charge carriers, which results in a very high photoresponse.9,21−23 In this work, we report the growth of transparent Cu4O3/ TiO2 heterojunction with strong built-in interfacial potential. The growth of the device was observed using scanning electron microscopy and X-ray diffraction. Under UV illumination, the interfacial electric field separates the photogenerated charge carriers without any external voltage. Further, we engineered the AgNWs Schottky contact to collect holes, which drastically improved the rise and fall time of device. Similarly, the other figures of merits, i.e., responsivity, detectivity, normalized photocurrent to dark current ratio, noise equivalent power, and signal-to-noise ratio are improved. The FTO (fluorine-doped tin oxide) glass (area = 2.5 × 2.5 cm2) was initially cleaned with acetone, methanol and DI water

he wavelength of light less than 400 nm is categorized as ultraviolet (UV) radiation. The optoelectronics devices, which convert this region of radiation into electrical signal are known as UV photodetectors (PDs). They are frequently used in ozone sensing, air purification, medical imaging, flame detection, space exploration, and optical communication.1,2 Generally, PDs powered from high external source require complex electrical circuit, which increases their cost and weight; however, some recent advancement in single metal oxide, i.e., ZnO3 or nanoheterojunctions,4 requires relatively small power of mW with high performance, which have comparatively better advantages. Similarly, the opaque property limits their usage in see-through devices like windows and display screens. Therefore, designing a transparent, self-powered and high-speed UV PDs has attracted the scientific community.1,2 The transparency can be ensured by exploring and optimizing novel materials. The offset in their work function can create interfacial electric field, which transports the photoexcited charge carriers without any external voltage.5,6 The effective collection of these charge carriers at electrodes result in a high speed photoresponse. However, the state-ofthe-art PDs are nontransparent due to bulky active layer and opaque contact electrodes.2 The wide bandgap metal oxides are considered the potential candidates for designing PDs with tunable features. Various, metal oxides (i.e., CuO,7,8 TiO2,9 V2O5,10 SnO2,7 NiO,11,12 ZnO,13 Cu4O314,15) have been employed on different substrates with conventional (i.e., Si8) and emerging materials (i.e., perovskites16,17) through various © 2018 American Chemical Society

Received: March 29, 2018 Accepted: April 18, 2018 Published: April 18, 2018 14292

DOI: 10.1021/acsami.8b05141 ACS Appl. Mater. Interfaces 2018, 10, 14292−14298

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the device without and with AgNWs, (b) device original photograph and dotted blue square depicts spin-coated AgNWs network, (c) XRD of Cu4O3/glass, and (d) energy band diagram.

sources. It could illuminate an area of 1 cm2 by placing 0.5 cm away from window layer. The light is energized using a function generator (MFG-3013A), and a photometer (TES-1333 solar power meter) was used to calibrate the varying light intensity. The Cu4O3 and TiO2 sequentially deposited on the FTOcoated glass substrate to design the platform for transparent Schottky devices. These bottom-up approaches allow us to employ the AgNWs Schottky contact for the metal-oxide heterojunction. Figure 1a shows the schematics of as prepared [named, @No-AgNWs] and AgNW-embedded [named, @ AgNWs] Cu4O3/TiO2 devices. Figure 1b shows the device original photograph, in which the dotted blue square depicts the spin-coated AgNW network. Prior to the device performance, growth of Cu4O3 was confirmed by performing XRD measurements and observed results are presented in Figure 1c. It is one of the crystal phase of copper oxide formed under careful oxygen condition.14 The observed peaks at 30.5 and 36° can be attributed to (200) and (400) planes of a tetragonal Cu4O3 structure, having lattice parameters of a = 5.837 Å, c = 9.932 Å, and c/a = 1.7016, (Crystallography open database: cod:9000603).15 In addition, presence of (026) peak at 64.35° indicates the polycrystalline nature of copper oxide. The crystallite size of 14 nm corresponding to the (200) peak was calculated using the Scherrer formula: as the size of crystallite, D = 0.94λ/(Bcos θ); where B is the fwhm (full width at half-maximum) in radians; λ is wavelength in nm; and θ is the Braggs’ angle in degree. Similarly, the XRD analysis of TiO2 can be found in our previous work.24 The energy band diagram of AgNWs-embedded device is depicted in Figure 1d, proposing the charge transport

in an ultrasonic machine (Power sonic 505) and dried with nitrogen gas, subsequently. The sample was placed in the sputtering machine (SNTEK, Korea) to grow Titanium (Ti) with a 3 mm width kepton tape mask. In the presence of a 99.99% Ti target, the sample was sputtered for 5 min under condition of 300-W DC, 5 rpm rotation, 50 sccm Ar gas flow and 5 m-Torr of working pressure. Further, the sample was oxidized via RTP (rapid thermal processing system, sntek s/ n;125N50) at a temperature of 700 °C for 5 min with constant oxygen flow, forming a transparent TiO2 layer on the FTO glass.24 Moreover, 140 nm Cu4O3 was deposited by placing the prepared TiO2 sample in sputtering machine with a 99.99% copper target, at room temperature, with 100 W DC under 15 and 5 sccm mixture of argon and oxygen gas, with a constant 5 rpm rotation.14,15 Finally, a well-connected AgNWs network was formed over Cu4O3 by dropping AgNWs solution (S32IKNS6A7) via micropipette (Witeg DEM 15-Germany) and subsequently spinning with a rotation of 3000 rpm for 30 s on a spin coater (spin coater EF-4op).25 The crystallography of the Cu4O3 structure was analyzed using the X-ray diffraction (XRD, Rigaku, D/Max 2500) with Cu Kα radiation in the θ-2θ scan mode. The surface behavior of the device was examined using scanning electron microscopy (FE-SEM/EDS-7800F Jeol). The optical properties, namely, transmittance, reflectance, and absorbance, were characterized using a UV−visible−NIR spectrometer (Shimadzu, UV-2600). The J−V characteristics and time-based photoresponse were measured by using linear voltammetry and dynamic chronoamperometry function of the potentiostat/galvanostat (Zive SP1, ZIVELAB). The LEDs (365, 460, 520, 620, and 740 ± 5 nm, LEDENGIN) were used as varying wavelength light 14293


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Figure 2. SEM images of the surface morphology at low- and high-magnification scales: (a, b) without AgNWs; (c, d) with AgNWs. Optical analysis: (e) transmittance and reflectance spectra of the full device with and without AgNWs, (f) absorbance with and without AgNWs.

of carriers, resulting in an electric field oriented toward p-side. The built-in potential at Schottky contact is calculated by using φbi=φp-φm, which is equal to 0.13 V in this case. The current− voltage (I−V) analysis of mere AgNWs/Cu4O3 Schottky device confirms a weak electric field oriented in opposite direction to Cu4O3/TiO2 heterojunction. In fact, the I−V characteristics of mere AgNWs/Cu4O3 Schottky device are shown in (Figure S1). Under illumination, the minor photoexcitation of carriers is possible at the AgNWs/Cu4O3 Schottky junction, but at the heterojunction the photons (hv), having energy higher than the energy bandgap of Cu4O3 (2.34 eV), contribute to the high photogeneration of charge carriers. Further, the strong interfacial electrical field at heterojunction transports the photogenerated electrons and holes to external circuit through FTO and AgNWs, respectively. The TiO2 grown on FTO at high temperature has granular structure. 28 Similarly, Figures 2a, b show the surface morphology of bare Cu4O3/TiO2 device at low and high magnification, respectively. One can note that the growth of well-grown granular structures, where the presence of grain

mechanism. It was drawn by keeping in view, the built-in potentials, work functions (energy required to remove an electron from Fermi level to vacuum level), energy band gaps (the forbidden gap between conduction and valence energy band edges) and electron affinities (energy required to remove electron from conduction band to vacuum level) of TiO2, Cu4O3 and Ag.14 The TiO2 energy bandgap (Eg) and work function (qφn) are reported to 3.2 and 4.5 eV, respectively.26 The FTO has a work function (qφ) of 4.7 eV. Similarly, work function (qφm) of AgNWs, employed as electrode, was reported to be ∼4.69 eV27 but in the present case the work function was measured via Kelvin probe force microscope (KPFM) and it is 4.57 eV (data not shown). The bandgap for the Cu4O3 is 2.3 eV28 with a work function of ∼4.7 eV(qφp), contrary to 5.3 eV.28 This variation may be due to multijunction interaction. At a p−n junction, the offset of work functions creates a space charge region through the diffusion of carriers with a built-in potential (Vbi) of 0.2 V, which induces an electric field oriented toward p-side. Similarly, the offset of AgNWs and p-Cu4O3 work functions causes a band-bending by the diffusion 14294


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Figure 3. Current density−voltage (J−V) characteristics under dark and varying light intensity of the device: (a) without AgNWs and (b) its semilog representation; (c) with AgNWs and (d) its semilog representation. Transient photoresponse: (e) rise and fall time without AgNWs and (f) at varying light intensities; (g) rise and fall time with AgNWs rise and (h) at varying light intensities.

boundaries can trap the photogenerated change carrier, resulting in low response. On the other hand, the Figures 2c and d show the device morphology after AgNWs decoration where well-connected AgNWs network will provide the smooth charge collection, which in turn could improve the device performance.29 Moreover, the porous AgNWs networks provides a high transmittance, compared to opaque-metal layer and also ensure the inlet of the incident light on the metal oxide Schottky device.30

The optical profiles for Cu4O3/TiO2 and AgNWs-embedded Cu4O3/TiO2 devices were obtained from 280 to 1400 nm. Figure 2e illustrates the transmittance and reflectance of both devices. In the 280−500 nm range, low average transmittance (5% for AgNWs-embedded and 8% without AgNWs), confirms the active photon absorption for ultraviolet photodetection. Meanwhile, for the middle wavelengths (500 < λ < 850 nm), transmittance was 39% (AgNW-embedded Cu4 O 3 /TiO 2 device) and 52% (Cu4O3/TiO2 device), which verify the transparent Schottky devices for optoelectronic applications.30 14295


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Figure 4. With the varying light intensities: responsivity, R (mA W−1), and detectivity, D (Jones), (a) without AgNWs and (b) with AgNWs; normalized photocurrent to dark current ratio NPDR (W−1) and noise equivalent power NEP (W Hz−1/2) (c) without AgNWs and (d) with AgNWs; signal-to-noise ratio (e) without AgNWs and (f) with AgNWs.

in the respective semilog plot. In addition, with the illumination of different intensities, ranging from 0.7 to 6 mW cm−2, the device shows a systematic shift away from the dark line, with an increased current, confirming its effectiveness for the application in UV-photodetection. The Voc of 0.3 V and Jsc of 0.25 μA cm−2 is observed for the 6 mW cm−2 light intensity. On the other hand, there is a remarkable enhancement of the photocurrent (1.01 mA cm−2) after applying the AgNWs networks over the Cu4O3 layer, shown in Figure 3c and Figure 3d (semilog plot). The Voc values were decreased slightly by 0.1 V, most likely due to the presence of the opposite electric field created at AgNWs/Cu4O3 Schottky junction. The other reason for Voc reduction beside multijunction could be the AgNWs electrode penetration into Cu4O3 layer, which is evident at the bias photoresponse, and there is low on/off ratio compare to the self-biased operation. However, the maximum output power enhanced from 0.075 × 10−6 W cm−2 to 2.02 × 10−3 W cm−2 at an intensity of 6 mW cm−2, which further demonstrates the

On the other hand, reflectance of the devices is smooth over the whole wavelengths except for a hump in a UV region, which may be caused by the metal properties below the plasma frequency. Figure 2f shows the increased absorbance below 600 nm because of the effective absorption of shorter wavelength photons into the wide bandgap metal-oxides for the generation of charge carriers. It is worth to mentioned here that for the longer wavelength range (1000 < λ < 1400 nm), a significant decrease in transmittance and obvious increase of absorbance indicate the free electron absorption. Figures 3a−d illustrates the electrical properties of the devices in voltage range of −1.5 V to +1.5 V under varying UV intensities, showing the open circuit voltage (Voc) and a shortcircuit current density (Jsc). Figure 3a shows the J−V characteristics of bare Cu4O3/TiO2 device, while Figure 3b is its semilog representation. The presence of photocurrent at zero-bias indicates that the device can be a self-biased photodetector under the action of Voc, which is clearly visible 14296


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ACS Applied Materials & Interfaces Table 1. Comparison of Transparent UV Photodetectors S. N. 1 2 4 5 6 7 8 9 10 11 a

materials a

ZnO UNN 3DNHa NiO/ZnO NiO/ZnO/ITO NiO/ZnO/ITO Cu4O3/ZnO/ITO Ni/NiO/ZnO/FTO Cu/TiO2/FTO Ni/ZnO/AgNWs Cu4O3/TiO2/FTO AgNWs/Cu4O3/TiO2/FTO

power density (W/cm2)

applied bias (V)

τr (ms)

τf (ms)

Ra (A/W)

wavelength (nm)

ref

86 μ 80 μ 12.1 μ 3m

0.2 m 1 0 0 0 −5 −1 −3 0 0

∼25 0000 5000 ∼323 0.041 33 24.2 0.99 0.987 148 0.439

∼15 0000 9000 ∼12 0.071 89 212 1.49 2.59 150 0.423

14 m 13 0.19 20 μ

340 and 370 370 370 365 365 400 365 365 365

3 4 11 12 15 19 24 25 This work

80 μ 100 μ 1.06 μ 6m 6m

3.85 0.897 14600 0.37 n 187 m

UNN = ultraporous nanoparticle network and 3DNH = three-dimensional nanohetrojunction.

Further, to affirm AgNWs effectiveness the signal-to-noise ratios (SNR = Ip/Id) are calculated for both devices. A higher value of the SNR is desired for the better performance of the PD. For all input intensities the AgNW-embedded Cu4O3/ TiO2 device ensured improved values, which are displayed in Figures 4e, f, respectively. Moreover, the device sensitivity to the only UV light in whole spectral region is depicted in the spectral response at Figure S3. Finally, to authenticate enhanced performance, the recently proposed transparent UV photodetectors are compared (Table 1). In conclusion, we demonstrated a high-performing transparent Schottky photodetector on the base of metal oxide heterojunction. The performance drastically enhanced by using porous AgNWs networks onto the metal-oxide heterojunction. The J−V characteristics revealed the presence of strong interfacial electric field at heterojunction, which was transporting the photogenerated charge carriers without any external voltage. The bare Cu4O3/TiO2 device demonstrates rise and fall times of 148 and 150 ms, respectively. However, the AgNWs-embedded device respond to UV light in a very short time of 0.439 and 0.423 ms. Similarly, the other figures of merit also improved due to AgNWs decoration on Cu4O3/TiO2 device: increase in the responsivity from 0.3 to 187.5 mA W−1; and enhanced detectivity from 6.48 × 104 to 5.13 × 109 Jones. Thus, the active fabrication of a metal oxide heterojunction with highly conductive metal nanowires can efficiently harvest the photogenerated carriers in transparent PDs.

merit of AgNWs-embedded Cu4O3/TiO2 device for sensitive photodetection under tiny light intensity condition. Moreover, the power law fitting was performed and presented in Figure S2. Figure 3e−h compares the transient photoresponse of bare Cu4O3/TiO2 and AgNW-embedded Cu4O3/TiO2 devices. The rise time (τr, time taken by photocurrent to reach 90% from 10%) and fall time (τf, time taken by photocurrent to decay from 90 to 10%) are the important features of a photodetector, which defines how fast the detector will sense and display the response to the input signals.2 The bare Cu4O3/TiO2 device shows a rise time of 148 ms and fall time of 150 ms, as shown in Figure 3e. Likewise, as the intensity increased from 1.8 mW cm−2 to 6 mW cm−2, it exhibited an increase in photocurrent, as shown in Figure 3f. However, the AgNW-embedded device provided the significantly enhanced performances of rise time (0.439 ms) and fall time (0.423 ms), as shown in Figure 3g and h, respectively. Which, clearly endorses the effectiveness of AgNWs in the collection of holes. The other important figures of merit, e.g. responsivity, detectivity, normalize photocurrent to dark current ratio, noise equivalent power and signal-to-noise ratio, required for efficient photodetection are plotted with changing intensities in Figure 4.25 The responsivity (R = Jp/Pin, where Jp is photocurrent density and Pin is input power per area) and detectivity (D = R/ 2qJd , where q = electronic charge, and Jd is the dark current density) are the prime features of a photodetector.2,5 Figures 4a and b show the R and D for a bare Cu4O3/TiO2 and AgNWembedded Cu4O3/TiO2 devices, respectively. Both the devices show a decreasing behavior with increasing intensity from 1 to 6 mW cm−2. The as prepared device shows 0.37 nA W−1 responsivity, which enhanced to 187.5 mA W−1 after decorating it with AgNWs, for the intensity of 6 mW cm−2. This affirms that the drastic increase in photocurrent is due to excellent conductivity of AgNWs. Similarly, the detectivity increased from 6.48 × 104 to 5.13 × 109 Jones due to embedded AgNWs. The normalized photocurrent to dark current ratio (NPDR = R/Id, where Id is dark current) and noise equivalent power (NEP = 1/NPDR 2q/Id ), of bare and AgNWs-embedded device, with varying intensity are shown in Figure 4c, d, respectively. Even though, taking both desired and undesired current into consideration, the NPDR of AgNWsembedded device drastically increased from 36 W−1 to 1.88 × 105 W−1, for a 6 mW cm−2 intensity.2 The NEP also verified AgNWsembedded device being capable of detecting power as low as picowatt, contrary to the microwatt level detection by bare Cu4O3/TiO2 device.

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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.8b05141. Current and voltage characteristics of AgNWs/Cu4O3− Schottky device and it is semilog representation (Figure S1); photocurrent density with light intensities (a) without AgNWs and (b) with AgNWs embedded devices (Figure S2; spectral response (a) without AgNWs and (b) with AgNWs embedded device (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.K.). *E-mail: [email protected] (J.-H.L.). ORCID

Joondong Kim: 0000-0002-9159-0733 Jung-Ho Lee: 0000-0002-6731-3111 14297


Letter

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support the Basic Science Research Program through the National Research Foundation (NRF) of Korea by the Ministry of Education (NRF- 2015R1D1A1A01059165) and Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (KETEP-20168520011370). Prof. J.-H. Lee has special appreciation of the Human Rescources Development program (KETEP-20154030200680). Authors acknowledge the helpful discussion and useful informaiton provided by Prof. Ralf B. WEHRSPOHN, Fraunhofer IMW Halle, Germany. Sohail Abbas and Mohit Kumar are equally contributed to this work.

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