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Enhanced Photocurrent in BiFeO3 Materials by Coupling Temperature and Thermo-Phototronic Effects for Self-Powered UV Photodetector System Jia Qi, Nan Ma, Xiaochen Ma, Rainer Adelung, and Ya Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02543 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

Enhanced Photocurrent in BiFeO3 Materials by Coupling Temperature and ThermoPhototronic Effects for Self-Powered UV Photodetector System ⊥

⊥

‖

Jia Qi†,‡ , Nan Ma†,‡ , Xiaochen Ma†,‡, Rainer Adelung , Ya Yang†,‡* †

CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese

Academy of Sciences, Beijing, 100083, China. ‡

School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049,

China ‖

Functional Nanomaterials, Institute for Materials Science, Kiel University, Kaiserstr. 2, D-24143, Kiel,

Germany ⊥

Jia Qi and Nan Ma contributed equally to this work.

*To whom correspondence should be addressed: Email: [email protected]. ABSTRCT: Ferroelectric materials can be utilized for fabricating phtotodetectors because of the photovoltaic effect. Enhancing the photovoltaic performance of ferroelectric materials is still a challenge. Here, a self-powered UV photodetector is designed based on ferroelectric BiFeO3 (BFO) material, exhibiting high current/voltage response to 365 nm light in heating/cooling states. The photovoltaic performance of BFO-based device can be well modulated by applying different temperature variations, where the output current and voltage can be enhanced by 60% and 75% in heating and cooling states, respectively. The enhancement mechanism of the photocurrent is associated with both the temperature effect and the thermo-phototronic effect in the photovoltaic process. Moreover, a 4×4 matrix photodetector array has been designed for detecting the 365 nm light distribution in the cooling state by utilizing photovoltage signals. This study clarifies the role of the temperature effect and the thermo-phototronic effect in the photovoltaic process of BFO material and provides a feasible route for pushing forward practical applications of self-powered UV photodetectors.

KEYWORDS: BiFeO3, UV photodetector, self-powered, photovoltaic effect, thermo-phototronic effect.

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1. INTRODUCTION Driven by the ever increasing demand on clean and renewable energy, various solar cells have been developed to convert photo energy into electric energy through the photovoltaic effect. Among them, ferroelectric-photovoltaic devices have attracted enormous attention because of their superior potentials, such as the anomalous photovoltage or the polarization-dependent photocurrent.1-5 Over the past few years, a considerable number of studies on the ferroelectric photovoltaic effect have been conducted on BiFeO3 (BFO) due to the narrow bandgap (2.67 eV). Thus, many photovoltaic-dependent factors, such as light intensity, electrode materials, polarization, domain walls, electrode configurations, oxygen vacancies and so on, have been well investigated on BFO films/crystals.6-11 However, most of these studies have been done at room temperature, and the effect of temperature on the photovoltaic effect of BFO was neglected. In fact, the surface temperature of BFO will be increased when the device is exposed to light illumination, suggesting that the generated current/voltage signals by photovoltaic effect are always influenced by the light-induced temperature variations. More interestingly, by applying a temperature gradient across the InP/ZnO solar cell, an enhanced output performance was obtained under light illumination, which can be attributed to the thermo-phototronic effect.12 Therefore, to design BFO device with high photovoltaic performance, it is extremely essential to further clarify the role of temperature and the temperature variation-induced thermo-phototronic effect on the BFO device in the photovoltaic process. Besides as energy harvester, BFO photovoltaic devices can be well utilized as self-powered photodetector to detect ultraviolet (UV) light by relying on the photo generated electric signals under illumination. The traditional UV photodetectors are usually focused on semiconductor materials such as ZnO, SnO2, TiO2, ZnS.13-16 The external power supplies are always needed to achieve a higher photosensing performance, greatly limiting the miniaturization and integration in larger-scale arrays. Such issues can be solved by utilizing the photovoltaic effect of ferroelectric BFO materials. In this study, a temperature modulated ITO/BFO/Ag (ITO: Indium tin oxide) device has been designed as a


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self-powered photodetector to detect 365 nm light. The photocurrent of ferroelectric BFO film can be dramatically enhanced by 60% after applying a heating temperature at the bottom of BFO film, where the corresponding enhancement mechanism is associated with both temperature and thermo-phototronic effects. Moreover, we also demonstrate that a 4×4 matrix photodetector array can be utilized to detect the distribution of 365 nm light illumination by analyzing the obtained voltage signals.

2. EXPERIMENTAL SECTION Synthesis of BiFeO3 Powders. BFO powders were obtained by hydrothermal method. Firstly, a certain amount of Bi(NO3)3·5H2O (99.0%, Aladdin Industrial Corporation) and Fe(NO3)3·9H2O (98-101.0%, Alfa Aesar Corporation) in molar ratio of 1.05:1 were adding into KOH solution (4 mol/L). After mixing uniformly, the mixture was poured into the Teflon reactor and reacted at 180 ºC for 6 h. Subsequently, the obtained powders were washed by HNO3 solution (10 wt.%) and deionized water for several times via suction filtration, then they were sintered at 850 °C for 1 h in the muffle furnace. Device Fabrication and Characterization. BFO powders were ground uniformly with several drops of polyvinyl alcohol (PVA, 2 wt.%) in a agate mortar for about 10 min. A disk with diameter of 10 mm was prepared by using tablet machine under pressure of 4 MPa. Subsequently, the sample was pretreated at 650 °C for 1 h to remove the PVA binder, and then sintered at 850 °C for 2 h to obtain BFO film. ITO electrode was deposited on the top surface of BFO by radio-frequency (RF) magnetron sputter (150 W, 10 min), and Ag electrode was deposited on the bottom surface of BFO by directcurrent (DC) magnetron sputter (100 W, 30 min). The thickness of ITO and Ag electrodes was determined by SEM image that being deposited on the glass substrate, about 90 nm and 1.6 µm, respectively (Figure S1, Supporting Information).To design 4 × 4 array of photodetectors, 16 same ITO/BFO/Ag devices were fixed on a cooler (40 mm × 40 mm) and then connected with the multichannel testing system. The microstructure of BFO powders and films were characterized by fieldemission scanning electron microscopy (FESEM, Hitachi SU8020). The crystallographic structure of


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the BFO film was identified by X-ray diffraction (XRD) with Cu Kα radiation (XRD, PANalytical X’Pert3 Powder). Electrical Measurements. The output current and voltage signals of ITO/BFO/Ag device were tested by utilizing a low-noise current preamplifier (Stanford Research SR570) and low-noise voltage preamplifier (Stanford Research SR560), respectively. The light intensity was identified by a power meter equipped with LED power sensor (OPHIR, NOVA Ⅱ). The temperature variation was supplied by a commercial cooler mounted below the ITO/BFO/Ag device, and it was recorded by an IR thermographic camera (PI400, Optris). The output voltage signals of 4×4 matrix of ITO/BFO/Ag array were simultaneously measured by the multi-channel data acquisition system (NI PXIe-6358).

3. RESULTS AND DISCUSSION Figure 1a illustrates a schematic diagram of the fabricated ITO/BFO/Ag device, which consists of top ITO electrode, BFO film and bottom Ag electrode. The specific fabrication process is displayed in the Experimental Section. In the measurement, a 365 nm light illumination was applied on the top ITO electrode, and a heating/cooling temperature was supplied at the bottom Ag electrode. The effective radiation area is around 0.7 cm2 for the ITO/BFO/Ag device, as demonstrated in Figure 1b. The scanning electron microscopy (SEM) image in Figure 1c shows that the as-synthesized BFO powders contain large layered agglomerates and small round platelet. The fabricated BFO ceramic is around 1.15 mm in thickness, as presented in Figure 1d. The crystallographic structure of the BFO ceramic was examined by X-ray diffraction (XRD) in Figure 1e, indicating a rhombohedral phase (JCPDS NO.742016) with a bit impurity of Bi2Fe4O9 introduced during the sintering process. To get further insight on the structural of BFO sample, a Raman study was carried out, as depicted in Figure S2 (Supporting Information). The peaks at 136.7, 169.8 and 219.0 cm-1 can be assigned to A1 (1TO), A1 (2TO) and A1 (3TO) mode for the rhombohedral BFO, respectively, which is in good agreement with the previous report.17 The current-voltage (I-V) curves of ITO/BFO/Ag device were measured when it was kept in ACS Paragon Plus Environment

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dark and under 365 nm light (271.4 mW/cm2), as demonstrated in Figure 1f. The Schottky contact between Ag electrode and BFO can be determined from the nonlinear I-V curve in dark. A straight line is observed under 365 nm light with short-circuit current (Isc) of 93.5 µA and open-circuit voltage (Voc) of 0.76 V, indicating a significant photoconductive response to 365 nm light. In the following study, the role of temperature in the photovoltaic process is explored on the ITO/BFO/Ag device by considering the high photoresponse to 365 nm light. At room temperature, the ITO/BFO/Ag device can deliver an output current of 66.5 µA and an output voltage of 0.47 V under illumination of 365 nm light (271.4 mW/cm2), as demonstrated in Figure 2a and Figure S3 (Supporting Information). Moreover, opposite current/voltage signals are obtained by reversely connecting the device to the measurement system, confirming that the measured current/voltage signals are generated by the photovoltaic effect. The maximum output power of ITO/BFO/Ag device under 365 nm light (271.4 mW/cm2) was determined by connecting a series of loading resistances, as presented in Figure 2b and Figure S4 (Supporting Information). The output current is decreased with increasing loading resistances, where the maximum output power is obtained at a loading resistance of about 5000 Ω with a corresponding power of about 6.48 µW. To characterize the photoresponse of ITO/BFO/Ag device, the light intensity-dependence of the output current/voltage was measured in the room temperature, as displayed in Figure 2c and Figure S5a (Supporting Information). Under illumination of 365 nm light, a temperature difference between the top ITO surface and the bottom Ag surface is generated, and it can be as high as 1.5 ºC at the intensity of 271.4 mW/cm2 (Figure S5b and 5c, Supporting Information). As the 365 nm light intensity increased from 3.0 mW/cm2 to 271.4 mW/cm2, more electron-hole pairs were separated by the Schottky barrier, leading to the larger output current/voltage signals at higher light intensity. Furthermore, the parameters of photoconductive gain (G), responsivity (R) and specific detectivity (D*) were calculated based on the output current data in Figure 2c to describe the photosensing performance of ITO/BFO/Ag device. The photoconductive gain is defined as G = (Iph/e)/(PS/hv), reflecting the ratio between the number of charges collected by


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the electrodes per unit time and the number of photons adsorbed by the BFO per unit time.18 Here, Iph is calculated as ǀIlightǀ-ǀIdarkǀ, P is the incident light intensity, hv is the energy of an incident photon, e is the electronic charge, and S is the effective irradiated area on the BFO. The responsivity R is utilized to express the sensitivity of the device to the light intensity, defining as R = Iph/(PS).19 To evaluate the ability of ITO/BFO/Ag photodetector to detect weak signal from noise environment, the specific detectivity D* is calculated according to the formula of D* = R/(2e·Idark/S)0.5.20 As demonstrated in Figure 2d and 2e, G, R and D* show a deceasing tendency with the increase of light intensity, maybe due to the hole-trapping saturation at high light intensity in the photovoltaic process.21-23 Owing to the high output current and suppressed dark current signals, the R and D* are as high as 6.0×10-4 A/W and 1×1012 Jones at the intensity of 3.0 mW/cm2, respectively. The responsivity of ITO/BFO/Ag is even larger than that of some traditionally semiconductor materials based UV photodetector.13,19 Moreover, due to the multiferroic nature of BFO, the response signals of ITO/BFO/Ag photodetector can be modulated by applying electric field or magnetic field. Therefore, the ITO/BFO/Ag photodetector possesses more opportunities for practical application. In addition, the response time and recovery time to 365 nm light were determined when the device was under the intensity of 271.4 mW/cm2, as demonstrated in Figure S6 (Supporting Information). The response time, defining as the time from 10% to 90% of the peak current value as light turn on, is about 10.8 s. The recovery time is about 0.6 s, which it is defined as the time from 90% to 10% of peak current value as light was turned off. To understand both temperature effect and thermo-phototronic effect on the photovoltaic performance of ITO/BFO/Ag device, different heating/cooling temperatures were applied on the bottom of the device, and the corresponding temperature profiles were recorded by an IR camera. Figure 3a-d display IR images and temperature profiles during the initial, lighting, heating and cooling states. The top surface temperature Ts and the bottom surface temperature Tb are same in the initial state, but a temperature difference ∆T is generated within ITO/BFO/Ag either by illuminating top ITO surface or by heating/cooling bottom Ag surface. When the 365 nm light intensity was increased from 3.0 mW/cm2 to


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271.4 nW/cm2, the temperature variations caused by light illumination and by heating/cooling in the illuminating process are demonstrated in Figure 3e and Figures S7,8 (Supporting Information). The temperature of bottom surface is only affected by the applied heating/cooling temperature, regardless of the light illumination. However, the top surface temperature is determined by both the 365 nm light illumination and the applied heating/cooling temperature at the bottom of the device. Therefore, by applying a heating/cooling temperature on the 365 nm light-illuminated ITO/BFO/Ag device, not only a rapid temperature increase/decrease occurs on the top and bottom surface (Figure 3e and Figure S8a, Supporting Information), but also a temperature difference is observed between the two surfaces (Figure S7 and Figure S8b, Supporting Information). The influence of temperature effect and thermophototronic effect on the photovoltaic performance of ITO/BFO/Ag device is demonstrated in Figure 3f, g and Figure S8 (Supporting Information) by exposing it in different intensities of 365 nm light. A significant enhancement in photocurrent is achieved by heating the ITO/BFO/Ag device, but the voltage signal is reduced in heating state. Oppositely, the photocurrent can be decreased and the voltage can be increased in cooling state. The results indicate that the output current/voltage signals of ITO/BFO/Ag device can be well controlled by applying heating/cooling temperature. The light intensity-dependence of output current/voltage signals in heating, cooling and the room temperature are summarized in Figure 4a,c. It can be seen that the output current/voltage signals gradually increase as the increase of light intensity, regardless of the testing conditions. In a wide range of light intensity, the ITO/BFO/Ag photodetector has the largest current response in heating state but the largest voltage response in cooling state. As displayed in Figure S9 (Supporting Information), the output current/voltage in the room temperature is labelled as Iphoto/Vphoto, and in heating/cooling states as Ithermo+photo/Vthermo+photo, respectively. The corresponding enhanced ratios of output current/voltage by heating/cooling were calculated in Figure 4b,d. With increasing light intensity, the enhanced current ratio of (Ithermo+photo - Iphoto)/Iphoto displays an increasing/decreasing tendency in heating/cooling states, reaching to 60%/-37.7% at intensity of 271.4 mW/cm2. However, the enhanced voltage ratio of


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(Vthermo+photo - Vphoto)/Vphoto exhibits the same tendency but opposite sign as that of enhanced current ratio (Ithermo+photo - Iphoto)/Iphoto, having values of -64.3%/75% at intensity of 3.0 mW/cm2 under heating/cooling conditions. Therefore, based on the current response in the heating state, the photosensing properties of ITO/BFO/Ag should be greatly improved, as demonstrated in Figure S10 of Supporting Information. The G, R and D* is enhanced 11～67% in heating state as compared with that at room temperature under light intensity of 3.0～271.4 mW/cm2. To explore the origin of the enhanced photovoltaic currents at higher temperatures, we investigated the output current and voltage signals of ITO/BFO/Ag device at different heating/cooling temperatures in dark environment, as demonstrated in Figure S11 and S12 (Supporting Information). The temperature difference between the top ITO surface and the bottom Ag surface is increased as the increasing heating/cooling temperature, leading to larger output current and voltage signals at higher temperature difference. However, it should be noticed that the generated current/voltage signals by the temperature difference in dark are almost thousands of times lower than that in 365 nm illumination. Thus, we can conclude that the heat-induced photocurrent enhancement in 365 nm light-illuminated ITO/BFO/Ag device is associated with both the temperature effect and the temperature difference caused thermosphotoronic effect. The energy band diagrams are plotted in Figure 4e-h as the ITO/BFO/Ag photodetector in the room temperature, heating and cooling states. As is known, BFO film can be thought as an n-type semiconductor due to the naturally existed oxygen vacancies that act as donor impurities.24 The oxygen vacancies can introduce donor states, leading to significant photo absorption, even for sub-band incoming photons.25 The work function of Ag, ITO and BFO are taken as 4.26 eV, 4.5 eV and 4.7 eV, respectively.11,26 When the device is illuminated by 365 nm light, the photogenerated electron-hole pairs are separated by the built-in electric field due to the Schottky contact between BFO and Ag (Figure 1f), resulting in the flow of current from Ag to ITO electrode in the external circuit (Figure 4e, f). When a heating temperature was applied at the bottom of the BFO, the increase of temperature in the BFO can result in that more electrons are released from oxygen vacancies


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to conduction band under the UV light illumination, leading to the increase of the carrier density27, as displayed in Figure 4g. Meanwhile, the temperature difference (∆T) between top and bottom surface of BFO can result in the electron transfer from high temperature Ag side to the low temperature ITO side due to an inner thermo-potential (VTE). The produced thermo-potential enhance the transfer of lightinduced electron-hole pairs, which is the thermo-phototronic effect.12 In fact, the temperature difference can be decreased with increasing the light intensity (Figure 3e), indicating that the temperature effect not thermo-phototronic effect is dominated in enhancing the photocurrent of the BFO film. Conversely, when a cooling temperature was applied at the bottom of the BFO film, the temperature effect can decrease carrier density in the BFO and the temperature difference across the BFO film can also decrease the current due to the reversed direction of the photocurrent, as illustrated in Figure 4h. Thus, the observed photocurrent of the device can be decreased with decreasing the applied temperatures. To demonstrate the potential applications of BFO devices as a photodetector system, a matrix composed of 16 ITO/BFO/Ag units was designed to detect the light intensity distribution from the measured output voltage signals. A multichannel data acquisition system was utilized to process the obtained data and record the real-time mapping figure. The photodetector array has high uniformity under 365 nm illumination (19.7 mW/cm2), manifesting similar voltage signals for 16 units as the light turn on and turn off (Figure S13, Supporting Information). Figures 5a and 5b clearly display the photographs of the fabricated 4×4 photodetector array under 365 nm light as well as it was covered by a L-shaped mask, respectively. When the photodetector array is illuminated by 365 nm light through square annular-shaped mask, N-shaped mask and diagonal line-shaped mask, the corresponding mapping images and output voltage signals are displayed in Figure 5c and Figure S14a-d (Supporting Information). The illuminated pixels demonstrate similar color distribution in the mapping image, indicating similar output voltage signals. Therefore, the light intensity distribution can be determined by comparing the voltage data of pixels in the mapping image. As decreasing the applied light intensity on the system, the output voltage signals of illuminated pixels are reduced (Figure 5d and Figure S15e,


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Supporting Information). By cooling/heating the photodetector array under illumination condition, the corresponding increased/decreased output voltage signals can be obtained in Figure S15 (Supporting Information), showing the darker/lighter color of pixels in the mapping image (Figure 5e and 5d).

4. CONCLUSION In summary, we have demonstrated how to enhance the photocurrent in ferroelectric BFO film by applying different external temperature variations at the bottom of the device. The corresponding photocurrent of the device can be enhanced by 60% at a 365 nm light intensity of 271.4 mW/cm2 after applying a heating temperature of about 42.5 ºC at bottom of the device. The enhancement mechanism of the photocurrent is associated with both the temperature effect-induced change in carrier concentration and the thermo-phototronic effect induced electron transfer in the BFO film, where the temperature effect is dominated. Moreover, a 4×4 matrix photodetector array has been designed to detect the distribution of the 365 nm light illumination by simultaneously recording the corresponding voltage data in 16 channels. These investigations are important for understanding the coupled temperature and thermo-phototronic effects on enhancing photocurrent in ferroelectric materials and have potential applications in self-powered photodetector systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Raman spectrum, output voltage, current change with external loading resistance, output voltage and temperature fluctuation under different light intensities, response time and recovery time, temperature difference in heating state, temperature difference and current/voltage signals in cooling state, current/voltage signals under intensity of 271.4 mW/cm2, photosensing performance, current/voltage signals in heating/cooling states, performance of photodetector array, voltage signals of photodetector ACS Paragon Plus Environment

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array in cooling/heating states.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key R&D Project from Minister of Science and Technology in China (Grant No. 2016YFA0202701), the National Natural Science Foundation of China (Grant No. 51472055), External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. 121411KYS820150028), Qingdao National Laboratory for Marine Science and Technology (Grant No. 2015ASKJ01), the 2015 Annual Beijing Talents Fund (Grant No. 2015000021223ZK32), and the "thousands talents" program for the pioneer researcher and his innovation team, China.

Supporting Information Supporting Information is available online from the Elsevier InterScience or from the author.

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Crystalline CdTe Nanowire Field Effect Transistors as Nanowire-Based Photodetector. Phys. Chem. Chem. Phys., 2014, 16, 22687. [24] Wang C.; Jin K. J.; Xu Z. T.; Wang L.; Ge C.; Lu H. B.; Guo H.; He M.; Yang, G. Z. Switchable Diode Effect and Ferroelectric Resistive Switching in Epitaxial BiFeO3 Thin Films. Appl. Phys. Lett. 2011, 98, 192901. [25] Liu F.; Fina I.; Gutiérrez D.; Radaelli G.; Bertacco R.; Fontcuberta J. Selecting Steady and Transient Photocurrent Response in BaTiO3 Films. Adv. Electron. Mater., 2015, 1, 1500171. [26] Zhang J.; Su X.; Shen M.; Dai Z.; Zhang L.; He X.; Cheng W.; Cao M.; Zou, G. Enlarging Photovoltaic Effect: Combination of Classic Photoelectric and Ferroelectric Photovoltaic Effects. Sci. Rep. 2013, 3, 2109. [27] Yang Y.; Xu W.; Xu X.; Wang Y.; Yuan G.; Wang Y.; Liu Z. The Enhanced Photocurrent of Epitaxial BiFeO3 Film at 130 °C. J. Appl. Phys. 2016, 119, 044102.


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FIGURE CAPTIONS

Figure 1. Fabrication and characterization of ITO/BFO/Ag photodetector. (a) Schematic diagram of ITO/BFO/Ag photodetector. (b) Photograph of BFO film. (c) SEM image of as-synthesized BFO powders. (d) Thickness of BFO film. (e) XRD patterns of BFO film. (f) Current-voltage (I-V) curves of ITO/BFO/Ag under dark and 365 nm illumination.


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Figure 2. Performance of ITO/BFO/Ag photodetector. (a) Measured output current signals of the ITO/BFO/Ag device at forward connection and reversed connection to the measurement system under 365 nm light (271.4 mW/cm2). (b) Dependence of output current and output power on the external loading resistance for ITO/BFO/Ag device under illumination (271.4 mW/cm2). (c) Time-dependent output current for the device under different light intensities. (d,e) Dependence of output current and photoconductive Gain G (d), responsivity R and detectivity D* (e) on the intensity of 365 nm light.


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Figure 3. Output current and voltage signals of ITO/BFO/Ag photodetector in heating state. (a-d) IR image taken during the measurements and corresponding temperature change upon (a) initial, (b) light, (c) heating and (d) cooling conditions. (e-g) In heating state the temperature variation on the top and bottom surfaces of ITO/BFO/Ag and the 365 nm light-induced temperature variation as the light intensities increased from 3.0 to 271.4 mW/cm2 (e), and the corresponding output current (f) and voltage (g) signals of the device.


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Figure 4. Temperature and thermo-phototronic effects on the photo-sensing performance of ITO/BFO/Ag photodetector. (a-d) Dependence of output currents (a) and voltages (c) on the light intensity under different conditions, and the corresponding enhancement of output current (b) and voltage (d) signals due to the applied temperatures. (e-h) Illustrations of energy band diagram for ITO/BFO/Ag in different conditions: (e) initial state, (f) under 365 nm illumination, (g) under 365 nm illumination and heating, (h) under 365 nm illumination and cooling.


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Figure 5. Performance of ITO/BFO/Ag photodetector array. (a) Photograph of the fabricated 4×4 ITO/BFO/Ag photodetector array under 365 nm light. (b) Photograph of the device illuminated by 365 nm light through L-shaped mask. (c) Mapping images of output voltage signals when the array is illuminated by 365 nm light through square annular-shaped mask, N-shaped mask and diagonal lineshaped mask. (d-f) Mapping images of output voltage signals as the photodetector array illuminated by different intensities of 365 nm light through L-shaped mask in the room temperature (d), cooling (e) and heating (f) states.


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Enhanced Photocurrent in BiFeO3 Materials by ... - ACS Publications

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