Efficient Flexible White-Light Photodetectors Based on BiFeO3

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Efficient Flexible White-Light Photodetectors Based On BiFeO3 Nanoparticles Suchanda Mondal, Kajari Dutta, shibsankar dutta, Debnarayan Jana, Adam Kelly, and Sukanta De ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00123 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Applied Nano Materials

Efficient Flexible White-Light Photodetectors Based On BiFeO3 Nanoparticles Suchanda Mondal1, Kajari Dutta*2, Shibsankar Dutta1, Debnarayan Jana3, Adam G. Kelly4, and Sukanta De*1 1

Department of Physics, Presidency University, 86/1 College Street, Kolkata 700073, India.

2

Department of Physics, Amity University, Major Arterial Road, Action Area II, Rajarhat, New Town, Kolkata 700135 3

Department of Physics, University of Calcutta, 92 A. P. C. Road, Kolkata 700009

4

School of Physics, Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and BioEngineering Research (AMBER), Trinity College Dublin, Dublin 2, Ireland. Abstract: A heterostructured white-light photodetector was fabricated by sequential deposition of zinc oxide (ZnO), bismuth ferrite (BFO) and PEDOT:PSS onto a flexible PET substrate. Central to this unique structure is an n+-n junction at the ZnO-BFO interface which allows the

device to drive unusually large currents. The intrinsic BFO electric field, arising from the depletion region within the ferroelectric material, is reduced through the combined effect of two barrier fields formed at the BFO-Au and ZnO-BFO interfaces. The combination of these three fields reduces parasitic recombination and causes photo-excited electron-hole pairs to drift to the electrodes. The photocurrent sensitivity and photocurrent gain were determined to be 0.04 A/W and 105 respectively. The decay time extracted from the photoresponse curve was 6 s, whereby the photocurrent drops by a factor of 30. In addition, the device exhibits good flexibility and retains almost constant performance after 100 bending cycles through a 90 degree angle. Keywords: BiFeO3 nanoparticles, photodetector, heterojunction, n+-n junction, MSFM device, photoresponse, flexibility. * Corresponding authors: E-mail: [email protected], [email protected] 1

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1. Introduction The detection of visible and infrared (IR) light with high sensitivity and fast response speed plays a significant role in the fields of optical fiber communication, remote sensing, digital imaging, and medical diagnostics.1-3 To meet the demands of the next generation of photodetectors, low-dimensional nanomaterials have shown excellent promise. Due to quantum confinement within low-dimensional structures, the spectral response of a photodetector can be tuned by simply altering the particle size. Moreover, nanomaterials can now be easily synthesized and fabricated into devices further easing their manufacture. The efficiency of an optoelectronic device is controlled by two basic principles; the photogeneration of electron-hole (e-h) pairs in the active region and the separation and drift of these charges to the external circuit. Overall device efficiency is thus limited by visible light absorption in the active layer and the width of the depletion region at the p-n junction. A recent study has shown that ferroelectric nanomaterials are an excellent candidate to overcome these limitations as they are not band gap-limited.4 In ferroelectric materials, charge separation occurs within an internal electric field generated by ferroelectric polarization meaning photo generated e-h pairs are separated inside the material instead of creating a p-n junction. This internal field arises due to non-centrosymmetry, or the breaking of strong inversion symmetry in ferroelectrics. This can generate huge barrier fields within the ferroelectric particles and facilitate the charge separation. In addition, most ferroelectric materials have a wide band gap which minimizes light absorbed from the visible spectrum. Over the past decade, the ferroelectric material bismuth ferrite, BiFeO3 (BFO), has been widely explored as a promising candidate for memory elements, solar cells, and photodetectors.52


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9

A mid-range band gap (2.6–2.9 eV) and conductive particle walls make BFO unique among

perovskite ferroelectrics10 but most notably, Clark and Robertson discovered using band structure modelling that the material contains both direct and indirect band gaps.11 Recently, it was found that BFO has a direct band gap of 2.55 eV at T = 295 K and an indirect band edge at 2.67 eV12 meaning BFO can be used as a visible-light photodetector. A typical metal-ferroelectric-metal (MFM) structure can produce a large depletion region due to nanoscale depletion regions across the inner insulating domains and conducting domain walls but these structures tend to produce low currents due to inefficient generation of e-h pairs.9 Similarly, a metal-semiconductor-metal (MSM) structure with ohmic contact develops a negligible depletion region but, as some semiconductors show high light absorptivity, they generate many e-h pairs and can create high current densities. Thus, a photodetector fabricated from a combined metal-semiconductor-ferroelectric-metal (MSFM) heterostructure could show superior performance through a large depletion region with high current densities. In such a structure, a large number of e-h pairs are generated under light illumination in the semiconductor and are separated in the high barrier field arising inside the ferroelectric material. The depletion region at the ferroelectric/semiconductor junction has a barrier field anti-parallel to the barrier field inside the ferroelectric meaning the effective barrier at the junction is reduced easing charge flow across the junction. Moreover, band bending at the junction further facilitates increased charge flow. To date, very little has been reported on BFO-based white-light photodetectors.13 A. Anshul et al. reported a visible-light photodetector based on in-plane BFO thin films grown on patterned interdigitated electrodes and found a photo-to dark-current ratio of ~ 40.14 In this work, 3



we present an MSFM heterostructure composed of solvothermally-synthesized BFO deposited on zinc oxide (ZnO) upon an ITO substrate. ZnO is an n-type semiconductor with strong UV light absorptivity, an efficient photoresponse, and reasonable electron mobility making it especially suitable as the semiconductor in the MSFM system. In our structure, the ZnO layer is thin compared to BFO layer meaning visible light is mostly absorbed via the BFO and the ZnO behaves as an electron transport layer due to its high electron mobility and transparency to visible light. In addition, the depletion region developed inside the BFO and at the BFO-ZnO junction plays a key role in separating the photo generated e-h pairs leading to a large photocurrent density. In this MSFM structure, ZnO and BFO show good energy band alignment which further assists the current flow. We also deposited a PEDOT:PSS layer on the BFO layer to facilitate hole transport to the metal electrode (Au) and electron drift to the ITO (to be discussed in detail later). Moreover, we present a device fabricated on a flexible PET substrate further adding to the versatility of the design.

2. Experimental Section Synthesis of BiFeO3: The BFO nanoparticles were synthesized via a solvothermal process. The precursor solution used in synthesizing pure BFO nanoparticles was prepared by mixing Bi(NO3).5H2O and Fe(NO3)3.9H2O with 1:0.8 molar ratio into DI water under constant magnetic stirring for 2 hrs. An aqueous solution of NaOH was prepared separately and Ethylenediamene was added to it and kept under magnetic stirring for 1 hr. The mixed solvent was then slowly added to the above precursor solution under continuous stirring. The resultant solution was further stirred for another 2 hr before being transferred to Teflon lined steel chamber filled to 4


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80% of its volume. The chamber was then closed and placed inside a preheated box furnace at 200˚C for 16 hrs. The crystalline brownish powder obtained after the reaction was collected by centrifugation and washed with DI water and ethanol and dried under vacuum. Device fabrication: The photodetector was fabricated on an ITO-coated PET substrate after cleaning by ultra-sonication in soap solution, water, and isopropanol and treatment in ultraviolet to remove carbon residues. A ZnO electron transport seed layer was grown by the sol-gel method. Briefly, zinc acetate dihydrate was dissolved in isopropyl alcohol to make 10 mM solution. The solution was then spin coated onto the ITO-coated PET substrate and subsequently annealed at 80˚C for 3 hrs to obtain the crystalline ZnO thin film. A uniform solution of 6.67 mg/ml of BiFeO3 nanoparticles was prepared by dispersing the nanoparticles ultrasonically in HPLC-graded methanol and drop cast on top of the ZnO layer. A PEDOT: PSS layer was then spin coated onto the top of the BFO film at 3000 rpm for 30 sec. The device was completed with the deposition of Au as a top electrode. Scheme 1 shows the schematic diagram of the crosssectional view of the device. The crystalline phases of the products were determined by X-ray powder diffraction by using a Bruker AXS D8SWAX diffractometer with Cu Kα radiation (λ = 1.54 Å). The crosssectional structures of the fabricated devices were investigated by a field emission scanning electron microscope (FESEM, JEOL, JSM- 6700F). Microstructural properties of the synthesized BFO were obtained using transmission electron microscope (TEM, JEOL 2010). For the TEM observations, the BFO powder was dispersed in 2-propanol and ultrasonicated for 15 min. A few drops of this ultrasonicated solution were taken on a carbon-coated copper grid. The optical property of the devices was studied by a UV–Vis–NIR spectrophotometer (U-4100 5



spectrophotometer, Hitachi). The devices were put into a chamber and illuminated under white light from the ITO side using a xenon lamp of 150W (Horiba Canada, S/N 2362-LPS220B, Solar Laser System makes Monochromator, model: M150). Current-voltage (I-V) data in dark and under illumination were recorded using Keithley 6487 Picoammeter. Capacitance-voltage (C-V) characteristics were measured using a HIOKI 3532-50 LCR HiTESTER.

3. Results and discussion To investigate the phase purity of the solvothermally-synthesized BFO, XRD measurements were carried out as shown in Figure 1a. As indicated, the spectrum reveals peaks that correspond to reflections from the (012), (104), (110), (006), (202), (024), (116), (018), (214)/(300) planes of rhombohedral BiFeO3, which are consistent with the standard reported values (JCPDS File No. 71-2494). Very weak signatures of two different phases of BFO (Bi36Fe2O57 and Bi2Fe4O9) were also detected.15 The inset of Figure 1a shows a histogram of the particle size distribution and a Gaussian fitting gives the average diameter of the nanoparticles to be ~ 9 nm. Figure 1b shows a representative TEM image from which the histogram statistics were extracted and the lower inset shows a HRTEM image of a BFO nanoparticle. The 0.29 nm spacing between two adjacent lattice planes corresponds to the (104)/(110) lattice planes of BFO. Figure 1c shows an FESEM image of a device cross-section wherein the discrete layers are clearly visible. The ZnO layer is quite thin (~ 70 nm) and acts as a charge transport layer whereas the BFO (~ 150 nm) serves as an active material layer. Approximately 70 nm of PEDOT:PSS was deposited on the BFO and acts as an electron blocking layer. The photograph of the device shown in Figure 1d indicates that the device is flexible and will be discussed later in more detail. 6


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Figure 2 shows the transmittance spectra of the ITO-coated PET with the ZnO and BFO layers. A decrease in transmittance is seen at ~ 330 nm corresponding to the ITO and ZnO band gaps of ~ 4 eV and 3.4 eV (at 300K). The ITO-coated PET transmits ~ 85% in the spectral range of 400-850 nm. A slight decrease in transmittance is observed in the range of 400-475 nm due to the ZnO coating. Hence, 85% of incoming light is transmitted to the BFO where 85% of the light is absorbed in the region 400 to 550 nm and nearly 40% of light is absorbed in the region 550 to 850 nm making this heterostructure suitable as a white-light photodetector. The I-V characteristics of the various heterostructures are shown in Figure 3a. For the ITO-ZnO-Au stack, Idark was 2.87 µA cm-2 at +2V and near-linear. This indicates Ohmic contact to the ZnO layer from the ITO and Au electrodes. The I-V curve of ITO-BFO-Au exhibits a Schottky junction at the metal-semiconductor interface (an enlarged view is shown inset in Figure 3a). The ITO-BFO-Au stack consists of two back-to-back Schottky diodes (Figure 3b) with a lower barrier at the ITO-BFO interface than at the BFO-Au interface. This offset arises as the work function (φ) of Au (φAu = 5.1 eV) is larger than that of ITO (φITO = 4.5eV). Notably, the barrier heights of both Au and ITO to ZnO are low compared to the BFO barriers, shown in Figure 3b. In addition, the band alignment also indicates that the Fermi level is closer to conduction band for ZnO compared to that of BFO. As a result, the ZnO behaves like an n+-type semiconductor and BFO acts as an n-type semiconductor meaning the ZnO layer is well suited as an electron transport material. In the combined ITO-ZnO-BFO-Au heterostructure, the dark current again increases as an n+-n junction is formed at the ZnO-BFO interface shown by the schematic diagram in Figure 3c.16, 17 The values of EF-ECB for ZnO and BFO were determined to be 0.9 and 1.4eV, respectively. Therefore it is again concluded that the ZnO in our device acts as a n+-type semiconductor whereas BFO behaves like a n-type semiconductor, considering that the 7



band gaps of BFO nanoparticles and polycrystalline ZnO thin film are of ~ 2.5618 and 3.3eV, respectively. A Schottky contact and an Ohmic contact are formed at the BFO-Au and ITO-ZnO interfaces, respectively, establishing the barrier fields Ebs, at the BFO-Au junction, and Ebj, at the ZnO-BFO junction - shown in the shaded region of Figure 3c. The effective barrier field, Eeff, is a combination of three barrier fields and expressed as Eeff = Ebs + Ebj – EBFO, where EBFO is the barrier field inside BFO arising due to the intrinsic depletion region within the ferroelectric material. This EBFO is antiparallel to the combined barrier field (Ebs + Ebj) at the depletion regions at the n+-n interface and the Schottky junction. This effective barrier field plays a key role in controlling parasitic recombination and moving photogenerated e-h pairs to the electrodes when the device is under illumination. In addition, the deposition of a PEDOT:PSS layer hinders electron transport and as a consequence the holes move towards Au electrode and electrons move in the direction of ITO. In the final ITO-ZnO-BFO-PEDOT:PSS-Au structure, the current increases to 38µA.cm-2 at +2V, nearly 3 times larger than that of the BFO device. Under white-light illumination, the current increases as shown in Figure 3d with a significant Ilight/Idark of 105 at 2V. The photocurrent sensitivity, S, is the photocurrent (Iph) per unit optical power and is determined from the I-V plots in Figure 3d where Iph = Ilight - Idark and Popt is the optical power at the device (note: Popt= light intensity on device times area of the device). The photocurrent gain, G, is the ratio of the number of electrons collected per unit time to the number of absorbed photons per unit time, can be expressed as19

8


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‫=ܩ‬

಺೛೓ ೜ ು೚೛೟ ೓ഔ

=

ௌ

೜ ೓ഔ

(1)

where q is elementary charge and υ is the frequency of the photon. Considering that all of the incident photons have been absorbed by the device, S and G are calculated as 0.04 A/W and 0.1, respectively for the current at +2 V. This initial I-V characterization suggests the heterostructure acts as a photodetector with good sensitivity. The responsivity curve of the final device shown in Figure 4a indicates the device shows a stable photosensitivity of ~ 0.037 A/W over the whole visible range (450 – 650 nm), in good agreement with the theoretically-derived sensitivity of S = 0.04 A/W. To verify the wavelength independence of the device, the I-V measurements (Figure 4b) were performed under illumination at various wavelengths in the region from 350 to 800 nm, as white light is the combination of all wavelengths in region 430 – 770 nm. The photocurrents for these different wavelengths in the visible region are observed to have almost same value (~1.2 mA.cm-2). Further, a four-fold increase in current is seen for composite white light compared to its discrete components demonstrating our heterostructure is especially well suited for use as a white-light photodetector Flexibility is fast becoming a sought-after feature in modern electronic hardware. To this end, we investigated the behaviour of our device under a variety of bending conditions (Figure 4c). A complete device was bent to 90o, then relaxed, and this process was repeated up to 100 cycles to verify the reliability of the device. Figure 4c shows that the photocurrents remain almost unchanged before and after flexing 100 cycles, demonstrating excellent flexibility and a robust bending durability. This indicates that the device could be used on both planar and curved surfaces. 9



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The transport phenomena under white-light illumination can be clarified through capacitance-voltage (C–V) measurements from which we can determine the thickness of the depletion layer. In Figure 5, the C–V characteristic of the device (ITO-ZnO-BFO-PEDOT:PSSAu) is plotted as 1/ C2 vs. V. The C-V curve can be analysed by using the following expression20 ଵ

஼మ

=

ଶ(∅್೔ ି௏)

(2)

௤ఌೞ ேವ

where ϕ is the total built-in potential at n+-n and Schottky junction, ND and εs are the donor concentration and the effective dielectric constant of the n-type BFO layer, respectively. The projected intersection of the 1/C2 with the horizontal axis curve gives a value of ϕ = 0.47 V. Putting the value for ϕ into Equation (2), we further obtain ND ~ 4.9 × 1016 cm−3, using εs=325 at 100 kHz.18 Using these data in the following equation, we can now calculate the depletion layer width (W):20 ܹ=ට

ଶఌೞ (∅್೔ ି௏)

(3)

௤ேವ

From this expression, the depletion width, W, of the final device is calculated as ~197 nm (at 0 V). This result indicates that the depletion regions at n+-n junction of ZnO-BFO interface and Schottky junction at BFO-Au interface extend across the entire thickness of the active layer BFO (~150 nm from FESEM image) at zero bias. The inset of Figure 6a shows the time-resolved photoresponse at 2 V bias with 55 on/off cycles, in which the white-light source turns “on” and “off” for 1 min each. In addition, the on/off cycles (~ 11) shown in Figure 6a provide clear information about the photoresponse. Under illumination, the photocurrent reaches a maximum value of 4 mA cm-2, with a photo- to dark-current ratio, ILight/IDark, of ~ 107, in agreement with the ratio extracted from the I-V 10


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measurement. Upon extinguishing the light source, the photocurrent drops rapidly. The photoresponse curve clearly shows that the decay of the photocurrent to its original dark value is a fast followed by a slow process. Figure 6b shows the magnification of the fast decay part in a single on/off cycle at +2 V. The fast decay time, in which the photocurrent decreases by a factor of 30, can be determined by fitting the curve using the expression ‫ܫ = ܫ‬௟௜௚௛௧ exp(− ‫ݐ‬ൗ߬௜ ) and was found to be 6 sec. Similarly, the rise time of the photocurrent is clearly defined by two regions. The fast response region is fitted using ‫ܫ = ܫ‬௟௜௚௛௧ (1 − exp൫− ‫ݐ‬ൗ߬௜ ൯) and Figure 6c shows both the experimental and fitted curves of the rise time, which was found to be 9 sec. The slow regions for both rise and decay may be due to trapping and defect-assisted recombination/generation of the charge carriers. The effective barrier field plays major role in controlling parasitic recombination and provides the exponential-type dependence of photocurrent and fast response. In particular, the photo- to dark-current ratio of ~ 105 is quite high compared to the literature.14 As a further note, J. Xing et al. fabricated a switchable photodetector based on a BFO thin film in a coplanar electrode configuration and reported peak sensitivity of 1.5 X 10-4 A/W at 365 nm using polarization flipping.21 Our devices show a sensitivity of ~ 0.04 A/W without polarization flipping so further optimization and a higher performance may be possible using this technique. In summary, a heterostructured white-light photodetector based on ZnO-BFOPEDOT:PSS thin films was developed using a simple fabrication method. The ZnO performs a dual role as an electron transport layer and as an n+-type material whereas the n-type BFO behaves like an active layer. The barrier field due to the nanoscale depletion regions intrinsic to the BFO reduces the fields formed at the n+-n junction of the ZnO-BFO interface and the Schottky junction at the BFO-Au interface. This reduction allows the device to generate much 11



larger currents than would be possible with either material alone and has the added effect of reducing parasitic recombination. The thickness of the depletion layer was found to be ~ 197 nm, calculated using C-V measurements, indicating that the depletion region extends across the entire thickness of the BFO layer (~150 nm) at zero bias. An additional deposition of a PEDOT:PSS layer on the top of the device blocks the electron flow and as a consequence the electrons can only move to the ITO electrode and holes can drift in the direction of Au. The device shows a photosensitivity of ~ 0.04 A/W, a photocurrent gain of ~ 0.1, and good stability under flexing making this heterostructure extremely promising as a flexible white-light photodetector.

Acknowledgements This work was financially supported by department of science and technology (DST), government of India through scheme for young scientists (DST fast track: SB/FTP/PS-190/2013) and DST-FIST (SR/FST/PSI-188/2013). Dr. De acknowledges Presidency University for funding through FRPDF scheme. Dr. Kajari Dutta (Das) is thankful to University Grants Commission (UGC), Govt. of India for providing the financial support through the D. S. Kothari Post Doctoral Fellowship.

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Photodetector

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Scheme 1: Schematic diagram of the hetero-structured (MSFM) device.

16


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70

Particle size 9 nm

* Bi36Fe2O57 (110)

(104)

No. of Particles

# Bi2Fe4O9

50 40 30 20 10

30

8

9

10

11

Particle diameter (nm)

BFO

12

(214)/(300)

7

(018)

(006)

(202)

6

(116)

(012)

0

*# 20

(b)

60

(024)

(a) Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

50

60

2θ (in degree)

Figure 1: (a) XRD pattern (inset shows the histogram plot for the particle size distribution corresponding to TEM image). (b) TEM image of BFO nanoparticles (inset shows the high resolution TEM image of a BFO nanoparticle). (c) FESEM image of the cross sectional view and (d) Real photograph of the device ITO-ZnO-BFO-PEDOT:PSS-Au showing good flexibility.

17



90 80

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

70 60 50 40 30 ITO ITO-ZnO ITO-ZnO-BFO

20 10 0

350 400 450 500 550 600 650 700 750 800 850

λ (nm) Figure 3: (a) I-V characterization curves for ITO-

ZnO-Au, ITO-BFO-Au, ITO-ZnO-BFO-Au and ITO-ZnO-BFO-PEDOT:PSS-Au in dark; (Inset shows the enlarged view). (b) and (c) Energy band

18


Page 18 of 23

Page 19 of 23

40 -2

30 25

-2

20

0

(b)

)

(a)

14 12 10 8 6 4 2 0 -2 -4 -6

I (µA.cm

35

I (µ A.cm )

-2 -2

-1

0

1

ITO

ZnO

-4

ITO-ZnO-Au ITO-BFO-Au ITO-ZnO-BFO-Au ITO-ZnO-BFO-PP-Au

10

ITO Au

2

V (in volt)

15

BFO

Au

EF

EF

-6

5 0

-8

-5 -10 -1.0

-0.5

0.5

1.0

V (volt)

0

-2 -3

ZnO

--

BFO

ee

2.0

(c)

-

EF

(d)

1

Au

e

0.1 0.01 1E-3 1E-4 1E-5

EBFO

-6

+

h

-7

1E-6 ITO-ZnO-BFO-PEDPT:PSS-Au@dark ITO-ZnO-BFO-PEDPT:PSS-Au@light

1E-7 +

+

-8 -9

100 10

-4 -5

-10

-2

ITO

1.5

I (mA.cm )

-1

0.0

PEDOT:PSS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


h

h

E bs

1E-8 -2

-1

0

V (volt)

E bj Figure 5: C-V characterization curve of ITO-ZnO-BFO-PEDOT:PSS-Au.

19


1

2


0.05

350 nm 400 nm 450 nm 500 nm 550 nm 600 nm 700 nm 800 nm White light

ITO-ZnO-BFO-PEDOT:PSS-Au

(a)

4

0.04

3 -2

I (mA.cm )

0.03 0.02 0.01

2 1

(b)

0 0.00 400

500

600

700

800

-1 -1.0

-0.5

0.0

λ (nm)

4

(c)

White light (no bending) 0 White light (90 bending) 0 White light (90 bending, 50 cycles) 0 White light (90 bending, 100 cycles)

3 2 1 0

-1 -1.0

0.5

V (volt)

5

I (mA.cm-2)

Responsivity (A/W)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

-0.5

0.0

0.5

1.0

V (volt)

20


1.5

2.0

1.0

1.5

2.0

Page 21 of 23

1.2 1.0 0.8

2

15

-2

1/C (10 F )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4 0.2 0.0 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

V (volt)

21


2.5


10

0.1

(a)

55 cycles

0.1

1E-3

-2

-2

White light off

1E-5

0.01

Experimental Fitted

0.003

1E-4

I(A.cm )

White light on

I (A.cm

-2

)

1

(b)

0.004

0.01

I (A.cm )

0

2000

4000

6000

V (Volt)

1E-3

6 Sec

0.002

0.001 1E-4 1E-5 0

200

400

600

800

1000 1200 1400

0.000 170 175 180 185 190 195 200 205 210 215 220

Time (sec) 0.004

Time (sec) Experimental Fitted

(c)

0.003 -2

I (A.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

9 Sec

0.002

0.001

0.000 120

130

140

150

160

170

Time (sec) Figure 6: (a) Time resolved photoresponse curve with multiple on/off cycles for ITO-ZnO-BFOPEDOT:PSS-Au at +2V; (Inset shows the 55 cycles). The magnified portion of a single on/off cycle (b) decay part and (c) rising part.

22


Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC 0 -1 --

-2

ee

-3

e

-

-4 -5

EF

-6 +

h

-7 +

-8

+

h

h

-9

A heterostructured white-light photodetector was fabricated by sequential deposition of zinc oxide (ZnO), bismuth ferrite (BFO), and PEDOT:PSS onto a flexible PET substrate. Central to this unique structure is an n+-n junction at the ZnO-BFO interface which allows the device to drive unusually large currents. The intrinsic BFO electric field, arising from the depletion region within the ferroelectric material, is reduced through the combined effect of two barrier fields formed at the BFO-Au and ZnO-BFO interfaces. The device shows a fast response under white light and good stability under flexing making this device a promising flexible white-light photodetector.

23


Efficient Flexible White-Light Photodetectors Based on BiFeO3

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