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Silver nanowires-embedded Transparent MetalOxide Heterojunction Schottky Photodetector Sohail Abbas, Mohit Kumar, Hong-Sik Kim, Joondong Kim, and Jung-Ho Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05141 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018
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ACS Applied Materials & Interfaces
Silver nanowires-embedded Transparent Metal-Oxide Heterojunction Schottky Photodetector
Sohail Abbasa,b, Mohit Kumara,b, Hong-Sik Kima,b, Joondong Kima,b,* and Jung‐Ho Lee c,*
a
Department of Electrical Engineering, Incheon National University, 119 Academy Rd. Yeonsu,
Incheon, 22012, Republic of Korea b
Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute
for Future Energies (MCIFE), Incheon National University, 119 Academy Rd. Yeonsu, Incheon, 22012, Republic of Korea c
Department of Materials and Chemical Engineering, Hanyang University, Ansan, Kyunggido,
426‐791, Korea.
ABSTRACT We report a self-biased and transparent Cu4O3/TiO2 heterojunction for ultraviolet photodetection. The dynamic photoresponse improved 8.5×104% by adding 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; Multi-junction; High performing.
*Author to whom correspondence should be addressed. Electronic mails: J. Kim (
[email protected]), J.-H. Lee (
[email protected])
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The 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 of recent advancement in single metal oxide i.e. ZnO3 or nanoheterojunctions4 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-of-art PDs are nontransparent due to bulky active layer and opaque contact electrodes.2 The wide bandgap metaloxides 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 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 NWs-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 2 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
the photo-generated 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 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 minutes under condition of 300-watt 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 minutes 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 watts 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 (S32I-KNS6A7) via micropipette (Witeg DEM 15-Germany) and subsequently spinning with a rotation of 3000 rpm for 30 seconds 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 timebased
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 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.
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The Cu4O3 and TiO2 sequentially deposited on the FTO-coated 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 AgNWs-embedded [named, @AgNWs] Cu4O3/TiO2 devices. While, the Figure 1b shows the device original photograph, in which dotted blue square depicts spin coated AgNWs 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/(Bcosθ); 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 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 eV 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 multi-junction 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 4 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
band-bending by the diffusion 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 (Supporting information). 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 in high photo-generation of charge carriers. Further, the strong interfacial electrical field at heterojunction transports the photo-generated electrons and holes to external circuit through FTO and AgNWs, respectively. The TiO2 grown on FTO at high temperature has granular structure.24 Similarly, Figures 2a and 2b 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 boundaries can trap the photo-generated 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 280-1400 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
