Light-Induced All-Transparent Pyroelectric Photodetector - ACS

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Light-induced all transparent pyroelectric photodetector Mohit Kumar, Malkeshkumar Patel, Gyeong-Nam Lee, and Joondong Kim ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00172 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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ACS Applied Nano Materials

Light-induced all transparent pyroelectric photodetector

Mohit Kumar a,b, Malkeshkumar Patel a,b, Gyeong-Nam Lee a,b, and Joondong Kima,b,* 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

In this work, we demonstrate a new concept for all oxide-based transparent photodetector by employing photo-induced pyro-electric effect. Particularly, a combination of n-type ZnO and ptype NiO heterostructure is used to design a red light-driven transparent photodetector. The device shows a high transmittance (>75%) and very low absorbance in the visible region. An open-circuit voltage of 1.8 V was measured across the detector with the pulsed light illumination (λ=650 nm, 7 mW cm-2), which is attributed to the photo-induced pyro-electric effect. The thermometry images confirmed an increment in the surface temperature from 22.9 to 25º C due to the illumination of pulsed 650 nm. The peak duration corresponding to pyro-phototronic effect was 40 µs. This study will open a new avenue to design future advanced transparent optoelectronics devices, including solar cell, photodetectors and transparent windows.

Keywords: ZnO/NiO heterostructure; Photodetector; All oxide-based transparent; Pyro-electric potential.

* Author to whom correspondence should be addressed. Electronic mail: *Joondong Kim ([email protected]).

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1. INTRODUCTION The basic operation of a photodetection device relies on photon-absorption-induced electron hole pairs generation and their collection.1 To employ this effectively, a significantly absorbing semiconducting material along with a proper interfacial band-bending design is essential, which in turn generates a spontaneous photoelectric signal, even under unbiased condition.2 In fact, designing of a photodetector suffers from the large number of limitations such as proper interfacial band-bending, significant absorption, etc., which demands a quest of an alternative for photo-induced energy conversion. Recently, a new kind of nanogenerator was demonstrated that works on the photoinduced pyro-electric effect and opened a new possibility to use it as a photodetector.3 In fact, a temperature change is likely due to instant photon illumination across a non-centrosymmetric material, which in turn generates a potential difference between two ends of the material – known as photo-induced pyroelectric potential or pyro-phototronic effect, even without any direct absorption.3,4,5 The generated electric potential is not only free from the direct photogeneration but also could govern the charge separation phenomenon.3,6,7 This innovative perspective can be used for electric potential generation and charge transportation during optoelectronic processes by coupling with the built-in one. However, the experimental observation and utilization of the photo-induced pyroelectric effect to generator electric signal for all oxidebased visible light transparent device is yet to be explored. Generally, a combination of well-known n-ZnO and p-NiO offers all oxide-based visible light transparent optoelectronic devices that can operate under ultra-violet (UV) illumination.8,9 In fact, the band gaps of NiO (Eg=3.6 eV) and ZnO (Eg=3.2 eV) are in the UV absorption range, which in turn offers a high transmittance in the visible region.8,10,11 A high transparency within


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the visible region ensures the advantage of NiO/ZnO-based device to be applied for see-through optical devices, which is known to the economical beneficial. Thus, transparent device, that can generate the electric signal, becomes the predominant material throughout the world. In addition, it is worth to mention here that ZnO shows a pyroelectric potential, which in turn opens a new possibility to use this material to design a photo-induced pyro-electric photodetector by choosing appropriate interfacial matching.12 Though, to the best of our knowledge, the true implementation of all oxide-based transparent device as an efficient photodetector is not studied with its full potential. Here we demonstrate all oxide-based transparent light-driven photodetector by employing photo-induced pyro-electric potential. A proper carrier selective p-type NiO with ntype ZnO is used to design a visible light transparent device.9 The present device shows a high transmittance (>75%) and very low absorbance in the visible region. Further, the device exhibits strong pyro-phototronic effect for the light (λ=650 nm,) illumination and generates an opencircuit voltage of 1.8 V, corresponding to the intensity of 7 mW cm-2. The observed result is attributed to the photo-induced pyro-electric potential. The changed in the temperature, due to illumination, is also confirmed by thermometry images. This study will be useful to design future advanced transparent photodetector.

2. RESULTS AND DISCUSSION A schematic of the NiO/ZnO/FTO device is illustrated in Figure 1a, with the growth detailed in the experimental section. The device architecture comprises columnar grown ZnO thin film sandwiched between a FTO and NiO films. The presence of NiO will not only work as a carrier selective contact but also used for top electrode.9,13 The band diagram at the NiO/ZnO



interface is depicted in Figure 1b, revealing that this junction not only transparent to visible light but also could be a proof-of-concept UV-operated photodetector. In addition, one can note that due to interfacial band-bending this junction is highly carrier selective and can effectively block the flow of electrons and holes towards the NiO and ZnO, respectively.14 A cross-sectional FESEM view of the device is depicted in Figure 1c, showing the material distribution across the device and closely packed vertically grown columnar nature of ZnO. On the other hand, planar-view FESEM is used to measure the average lateral size of ZnO columnar and same is found to be ~125 nm [Figure 1d]. The presence of different layers is also confirmed by performing the cross-sectional elemental line profile of NiO and ZnO thin films, deposited on ITO substrate under similar conditions, and presented in Figure 1e.13 The optical transmittance spectra of NiO/glass, ZnO/glass and full device are depicted in Figure 2a. From this Figure, one can note that the average transmittance of individual layers as well as full device is greater than 75% in the visible range. In fact, the device shows an excellent transmittance ~83.5% at a wavelength of 550 nm, at which the human eyes have the maximum luminosity curve.9 In addition, the observed sharp decrease in the transmittance around 360 nm can be attributed to the fundamental absorption edges of all oxides.10 It is interesting to note that the average transmittance of full device decreases at higher wavelengths, which can be attributed to the free-electron absorbance in FTO.15 On the other hand, the optical absorption of all films and full device are measured, as depicted in Figure 2b and its inset. Note that a slight absorbance is taking place in the visible region, indicating that the present device is highly optically transparent. For clarity, an absorption spectrum of NiO/glass is presented in the inset of Figure 2b. The figure 2c shows a photograph of a full device that is indeed largely transparent for


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visible light and preserving the outside color, which are two major criteria to be applied it for see-through optical device.16 The device shows nonlinear current-voltage (I–V) characteristics with varying bias voltage under dark as well as with 650 nm illumination, indicating the rectifying behavior of the same [Figure 3a].17 The ideality factor, n, was estimated using the relation =

, where q,

K, and T are the elementary charge, Boltzmann’s constant, and absolute temperature, respectively.8 The ideality factor of the device was found 1.12. An ideality factor close to one indicates that transport across the device is dominated by diffusion rather than a recombination process likely because of the low density of trap states. In fact, the trap density (Ntrap) of the device was calculated using trap-limited conduction and used formula: VTFL=8eNtrapL2/(9ɛɛ0),18 where VTFL is trap-filled limit voltage, L is the thickness of ZnO film, ɛ is the relative dielectric constant of ZnO (8.5),19 and ɛ0 is the vacuum permittivity.18 VTFL is found to be 1.02 V. After substituting all required parameters, the trap density of the device is found to be 3.37×1015 cm-3. This low trap density confirms the formation of high quality junction. As expected from the band diagram, the device does not show any changes in the I-V characteristics under 650 nm illumination, confirming the absence of direct photogeneration. The device shows high optical transmittance in the visible region and thus, it is expected that this architecture does not respond for the same. On the other hand, the device shows considerable response for UV light. The UV response of the device is well studied and reported somewhere else.10,9,20 Since ZnO is a pyro-electric material and thus, it is likely that this device can interact to the pulsed incidence photon and in turn can generates potential by the pyro-electric effect.21 As a trial, the response of the device was measured at 650 nm with an intensity of 7 mW cm-2, as presented in Figure 3b. It is interesting to note that this device shows peaks in the open-circuit



voltage measurement during the light ON and OFF states. The peak duration for increasing and decreasing was ~40 µs for and it can be considered as a response speed of the device. The possible reason behind these peaks can be the pyro-phototronic effect, without any photovoltaic one.12,22 One can note that this device does not show any potential during under a continuous illumination condition. Since, ZnO is known for photo-induced pyro-electric potential,5 however, the observed high voltage of ~1.8 V can be most likely due to presence of NiO and/or high resistive ZnO (~MΩ). Therefore, to understand the role of NiO, the device of ITO/ZnO/FTO was prepared and analyzed, Figure S1 in the supporting information. Interestingly, the device shows the generation of open-circuit voltage and the magnitude of the same is in the range of mV, indicating that the high voltage (1.8V) generation in the NiO/ZnO/FTO is due to the p-n junction. Yet, the further experimentation is needed to confirm the crucial role of NiO thickness in the high voltage generation. In addition, it is worth to mention here that recent reports depict that a high voltage can be generated by utilizing pyro-electric potential with a proper contact.22,23 The main feature of pyro-electric potential is photo-induced temperature changes and thus, the thermal images of the device in the dark and with pulsed (10 kHz) 650 illuminations are capture and depicted in Figure 3c. Under dark, the surface temperature of the device was 22.9º C, like the room temperature, while it subsequently increased to 25.0º C, once the device is illuminated with pulsed 650 nm, 7 mW cm-2. This change in the temperature is can be attributed to the photoinduced heating effect.12 In fact, these images are clearly confirming that a pulsed red light can generate a heating within the device and in turn electric potential across the device. The key figure of merits for a pyro-electric photodetector is to have significant response to the incidence illumination with a relevant signal generation.9 Thus, the transient responses at various intensities, increasing from 1 to 7 mW cm-2, were measured under zero bias conditions.


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Out of hundreds of cycles, four cycles are systematically depicted in Figure 3d. Clearly, for all the intensities, pyro-potential peaks are observed and the magnitude of the same varies with the intensities. This increase in the pyro-potential with light intensity can be attributed to the enhanced temperature-generation under higher light intensities. The stability and durability of our pyroelectric photodetector were also analyzed by measuring hundreds of cycles of current−time curves under light wavelength of 650 nm, as shown in Figure 3d and S2 in the supporting information. For clarity, a magnified view is depicted in Figure 3e. The intensity−dependent voltage generation is presented in Figure 3f. The light pulse having higher light intensity can generate relatively more temperature and in turn improved pyro-voltage.12 The generated voltage is increased monotonically while it gets saturated for the higher light intensity, more likely due to inherent nature of the carrier selective NiO. In addition, the detection band is one of the most important factor to use the present device for practical application and therefore, the response of the device for different wavelengths were measured and presented in Figure S3 (supporting information). It is worth to mention here that the device shows pyro-peaks for broad range.24 To depict the underlying governing dynamics, schematics are presented figures 4a-c, showing the behavior of voltage-generation. Generally, the photo-induced response of a device can be understood by a combination of two mechanisms: (1) direct electron-hole pair generation and (2) photon-induced pyroelectric potential. These two processes are fundamentally different; the first is directly photogenerated, while the second process is due to the photo-induced heat generation across ZnO.5 Since no direct photogeneration is taking place within the device and thus, the first effect can be ruled out. On the other hand, the observed peaks during the ON and OFF states confirming the presence of pyro-electric effect. In fact, once the light is ON, the



pyroelectric potential (Epy) is generated in the direction of the built-in potential (Eb), as shown in Figure 4b, which in turn effectively generates the voltage, resulting in a sharp peak in the output voltage, as shown in Figure 3b. A continuous illuminance of the used low intensity cannot generate a sufficient temperature difference between the two ends. Thus, the pyroelectric potential is gradually reduced until it disappears, and consequently, the output reaches zero voltage, which is mainly because of the steady temperature generation. On the other hand, a reverse sharp response peak is observed once the light is turned OFF, originating from the reverse pyro-potential induced by the prompt temperature decrease in ZnO, as shown in Figure 4c. An effective opposite electric field is generated once the light is switched OFF, which in turn shows an opposite peak in the voltage-time characteristics, which also decays until reaching a zero level. These observations indicate that the magnitude and direction of the instant photo-induced voltage depend absolutely on the pyroelectric potential generation. It is also interesting to note that the magnitude of the pyro-peaks during light ON and OFF states is similar. Most probably due to equivalent blocking efficiencies of NiO (∆Ec=3.2 eV for electrons) and ZnO (∆Ev=2.9 eV for holes). This new concept-based all transparent metal oxide device will open new avenue to use it for future see-through optoelectronic devices.

3. CONCLUSIONS In summary, it is demonstrated that light can be used to trigger the voltage across all oxide-based transparent device by utilizing the photo-induced pyro-electric effect. The combination of n-type ZnO and p-type NiO was found to very suitable for the transparent photodetector. The transmittance of the device was found more than 75% for the entire visible range. Appearance of the potential peaks during the ON and OFF states, confirming pyro-electric


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potential and generate an open-circuit voltage of 1.8 V at 650nm, 7 mW cm-2. The thermometry images confirm the charge in surface temperature with pulsed 650 nm illumination. The observed results are well attributed to the photo-induced pyro-electric potential. This study will open a new avenue to design future advanced building materials and transparent optoelectronics devices.

4. EXPERIMENTAL SECTION A commercial available fluorine-doped tin oxide (FTO)-coated glass was used as a substrate. Prior to the device fabrication the substrate was ultrasonically cleaned sequentially in acetone, methanol and deionized water. Zinc oxide (ZnO) thin films having a thickness of ~400 nm was grown using a commercially available 99.99% pure ZnO target. Ultra-pure (99.999%) argon gas was injected into the chamber with a flow rate of 50 sccm to maintain a working pressure of 5 mTorr during sputtering. An RF power of 300 W was applied to the target and the substrates were rotated with a speed of 5 rpm to achieve uniform film thickness. To form p-type NiO layer, a pure Ni target (purity, 99.999%) was reactive sputtered with simultaneous flowing of Ar (30 sccm) and O2 (4 sccm) gases. A DC power of 50 W was applied to the Ni target for 15 min at a working pressure of 3 mTorr. All metal oxides were deposited at room temperature. The cross section and morphology of the device were studied using a field emission scanning electron microscope (FESEM, JEOL, JSM_7800F). Optical characterization was carried out using an ultraviolet-visible-near-IR spectrophotometer (Shimadzu, UV-2600). Dark I–V measurements were performed using a source meter unit (SMU, Keithley 2440). The transient behavior was studied by recording the open circuit voltage under pulsed light conditions. Different LED sources (λ= 365, 520, 650, and 740 nm) were used to generate the light pulse using a function



generator (MFG-3013A, MCH Instruments). The intensity of the source was calibrating with a power meter (KUSAM-MECO, KM-SPM-11). The transient open circuit voltage of the device was obtained using a digital oscilloscope (TBS 1102B-EDU, Tektronix). IR camera (PerfectPrime IR 0002) was used to monitor the temperature features of the transparent device.

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 (NRF2015R1D1A1A01059165), Korea Research Fellowship Program through the NRF by the Ministry of Science and ICT and Future Planning (NRF-2015H1D3A1066311). Mohit Kumar and Malkeshkumar Patel are equally contributed to this work.


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Figure 1. Device architecture and morphologies: (a) cross-sectional schematic of the NiO/ZnO/FTO device. (b) Band-bending at the NiO/ZnO interface. (c) Cross-sectional SEM image of the device, exhibiting materials distribution. (d) Planar-view SEM image, depicting vertically aligned columnar ZnO nanostructures. (e) Cross-sectional elemental line profile of NiO and ZnO thin films, deposited on ITO substrate under similar condition.


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Figure 2. Optical properties: (a) Transmittance and (b) absorbance spectra of the ZnO/glass, NiO/glass and full device, confirming a very low absorption across the device. Inset shows the magnified view of NiO/glass absorbance. (c) Original photograph of the device.



Fig. 3 (color online). Charge transport properties: (a) I-V characteristic of the NiO/ZnO heterojunction under dark and illuminations (λ=650 nm, 7 mW cm-2) at room-temperature. (b) Transient responses of the device of the illumination at zero bias, presenting two peaks during the ON and OFF states. Input signal is corresponding to the voltage applied to the LED. (c) Thermometric images of the sample under dark and with of the illumination. (d) Transient voltage-time characteristics for four cycles with different intensities from 1 to 7 mW cm-2, showing the dynamic behavior of Vpy with increasing the incidence light intensity. (e) magnified view of the (d). (f) The change in the peak voltage as a function of intensity.


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Fig. 4 (color online). Schematic illustrations of the working mechanism of the pyro-phototronic effect under ON and OFF light illuminations: (a) Built-in potential of the junction and its direction in the device in the absence of light. (b) Evolution of the junction width and generation of the pyroelectric potential during illumination condition. (c) Evolution of the reverse pyroelectric potential with an opposite direction to the built-in potential. Various charge symbols indicate that pyro-electric charges are different from the electric one.



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Light-Induced All-Transparent Pyroelectric Photodetector - ACS

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