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Enhanced photovoltaic properties in graphene/polycrystalline BiFeO3/Pt heterojunction structure Yongyuan Zang, Dan Xie, Xiao Wu, Yu Chen, Yuxuan Lin et al. Citation: Appl. Phys. Lett. 99, 132904 (2011); doi: 10.1063/1.3644134 View online: http://dx.doi.org/10.1063/1.3644134 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i13 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 99, 132904 (2011)

Enhanced photovoltaic properties in graphene/polycrystalline BiFeO3/Pt heterojunction structure Yongyuan Zang,1,a) Dan Xie,2,b) Xiao Wu,2 Yu Chen,2 Yuxuan Lin,2 Mohan Li,3 He Tian,2 Xiao Li,4 Zhen Li,4 Hongwei Zhu,4,5,c) Tianling Ren,2,d) and David Plant1 1

Electrical and Computer Engineering, McGill University, Montreal, Quebec H3A 2T8, Canada Tsinghua National Laboratory for Information Science and Technology (TNList), Institute of Microelectronics, Tsinghua University, Beijing 100084, People’s Republic of China 3 Chemical Engineering, McGill University, Montreal, Quebec H3A 2B2, Canada 4 Key Laboratory for Advanced Manufacturing by Material Processing Technology and Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China 5 Center for Nano & Micro Mechanics, Tsinghua University, Beijing 100084, People’s Republic of China 2

(Received 18 July 2011; accepted 6 September 2011; published online 28 September 2011) We report the enhanced photovoltaic properties in polycrystalline BiFeO3 (BFO) thin films with graphene as top electrodes. The short circuit current density (Jsc) and open circuit voltage of the heterojunction are measured to be 25 lA/cm2 and 0.44 V, respectively, much higher than the reported values for polycrystalline BFO with indium tin oxide (ITO) as top electrodes. Influence of HNO3 treatment on the photovoltaic properties is studied, and a significant photocurrent density improvement from 25 lA/cm2 to 2.8 mA/cm2 is observed. A metal-intrinsic semiconductor-metal model is proposed to explain the graphene induced enhancement comparing with traditional ITO. C 2011 American Institute of Physics. [doi:10.1063/1.3644134] V

Multiferroic material BiFeO3 (BFO) with a high remnant polarization, coexistence of ferroelectric and antiferromagnetic orders, is considered as a perspective candidate for technologically demanding applications.1–3 Recently, photovoltaic effect is reported in single crystalline BFO bulk with high external quantum efficiencies of up to 10% when illuminated with the appropriate wavelength.4,5 This remarkable photovoltaic effect can be attributed to the small optical band gap and narrow domain walls. However, all the present studies are based on the ITO/BFO/metal (Au or Pt) structure, using ITO as top electrodes.6,7 Little research has been reported for a better replacement of ITO material. Recently, graphene has attracted much attention due to ballistic electronic transport, ultra high carrier mobility, and good mechanical properties.8–11 A number of electrical applications based on graphene including filters and nanoelectronics have been reported.12,13 Much research interest has been devoted to graphene based transparent conductive electrodes in optical domain, owing to its high electrical conductivity and optical transparency.12 Inspired by the unique optical properties of graphene, in this letter, we report the photovoltaic effect in a graphene/BFO heterojunction and study the improvement of the photovoltaic properties with nitric acid (HNO3) treatment. A metal-intrinsic semiconductormetal (MIM) model is proposed to understand the origin of the photovoltaic effect in graphene/BFO/Pt heterojunction. BFO thin film of 300 nm thick was prepared by a solgel/spin coating method. Detailed preparation process was reported in our previous work.14 A schematic illustration of the graphene/BFO/Pt heterojunction solar cell is shown in

Fig. 1. As-prepared BFO thin film was deposited on Pt(150 nm)/Ti(30 nm)/SiO2/Si(100) substrate. Single layer polymethyl methacrylate (PMMA) thin film (200 nm) with a circular window (3.5 mm in diameter) was then coated on the top of the BFO thin film as an isolation layer, followed by the deposition of graphene film derived by CVD method. Finally, silver paste was applied on the graphene film to make a contact for the electrical measurement. The area of the graphene top electrode is 0.096 cm2. Raman spectra, as shown in Figs. 2(a)–2(d), reveal the characteristic feature for the monolayer graphene. It can be indexed from Fig. 2(a) that the position of expected G and 2D peaks are 1587 cm1 and 2690 cm1 with the FWHM of 18 cm1 and 32 cm1, respectively. The peak intensity ratio of 2.6 between 2D and G confirms the presence of monolayer graphene.15 Both peaks can be fitted well by a single Lorentzian function. Moreover, the absence of D band implies the high quality of the as-grown graphene with fewer defects. The uniformity of the monolayer manner can be further concluded from the 2D micro-Raman mapping image, as shown

a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. b) Electronic mail: [email protected]. c) Electronic mail: [email protected]. d) Electronic mail: [email protected] 0003-6951/2011/99(13)/132904/3/$30.00

FIG. 1. (Color online) (a) Schematic illustration of graphene/BFO/Pt solar cell configuration. (b) Cross section view of the graphene/BFO/Pt solar cell. (c) Photographic image of the graphene/BFO/Pt solar cell.

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FIG. 2. (Color online) (a) Raman spectrum of as-deposited graphene showing G and 2D peaks. 2D micro-Raman intensity mapping of (b) G band (1587 cm1) and (c) 2D band (2690 cm1). (d) 2D micro-Raman intensity ratio (I2D/IG) mapping from the same region as (c) and (d). (e) UV-Vis spectrum of transferred graphene film on quartz substrate.

in Figs. 2(b)–2(d). The peak intensity and ratio distribution between 2D and G band further confirms the high percentage of monolayer with fewer multilayer islands in the film achieved. The optical UV-vis transmission spectrum for the transferred graphene film on a quartz substrate is shown in Fig. 2(e), exhibiting a high transparency of 97%. Since the optical transmission is usually reduced by 2.2%–2.3% for an additional graphene layer,16 this high transparency also confirms the monolayer pattern of our graphene film. Figure 3(a) shows the XRD pattern of as-deposited BFO thin film. Diffraction peaks such as (110), (012), and (024) can be indexed with some secondary phases. The dark and light illumination I-V curves are shown in Fig. 3(b). The power of illumination source is 100 mW/cm2 (AM 1.5). A significant increase in photocurrent can be observed by applying illumination to the cell. The dark current density at zero bias is 1.8 lA/cm2, indicating a substantial current. The short circuit photocurrent density (JSC) is measured to be 25 lA/cm2, and the open circuit voltage (Voc) is about 0.44 V, much higher comparing with that reported in pulsed

FIG. 3. (Color online) (a) XRD pattern of as-grown BFO thin film. (b) J-V curves of the graphene/poly-BFO/Pt heterojunction in the dark and with white light illumination. (c) Time dependence of zero bias photocurrent density with light ON and OFF.

Appl. Phys. Lett. 99, 132904 (2011)

laser deposition (PLD) derived BFO (40 mV)7 and sol-gel achieved BFO with ITO electrode (0.08 V).6 Time dependence of the photocurrent at zero bias voltage is shown in Fig. 3(c), indicating a repeatable and stable instantaneous response to the light illumination. The obvious ON/OFF photocurrent density ratio makes BFO a possible candidate in the photosensitive resistor application. The power efficiency of our graphene/polycrystalline BFO/Pt heterojunction can be calculated by the ratio of output electrical power Pe to the incident optical power Po, namely, Pe/Po, where Pe ¼ JV (mW/cm2). The maximum value is calculated to be 2.5  103%,13 indicating a weak conversion efficiency in BFO material. The significant photovoltaic properties improvement in graphene/BFO/Pt heterojunction comparing with the traditional ITO electrode can be explained by three factors. First, monolayer graphene processes less absorption comparing with ITO.13 Since the band gap (Eg) of BFO is about 2.2 eV,5 the photon which can stimulate the electrode-hole pairs must process a wavelength less than 560 nm (k ¼ hc/ Eg). It can be observed that graphene exhibits less than 7% absorption, comparing with 25% in ITO around the desired wavelength region.17 Second, as a flexible material, graphene can provide a conformal coating on BFO thus a better electrical contact, comparing with ITO electrode, where electrical contact is degraded by the internal stress. Third, the theoretical intrinsic mobility of graphene can reach 105 cm2/Vs at room temperature, much higher than the value of ITO.18 Recently, graphene deposited on the ferroelectric thin film layer has been reported with a mobility of 7  104 cm2/ Vs, indicating better electrical properties. Photovoltaic properties of graphene/BFO/Pt structure can be improved by HNO3 treatment. The graphene/BFO unit is immersed into 65% HNO3 for varied time (1–9 min). The Jsc and Voc variation before and after HNO3 treatment are shown in Fig. 4(a). It can be concluded that Jsc increases remarkably (more than two magnitudes of orders) from 25 lA/cm2 to 2.8 mA/cm2 after HNO3 treatment for 7 min and keeps constant afterward. Voc drops slowly from 0.44 to 0.20 V for 0–3 min HNO3 treatment and remains steady after then. A comparison between J-V curves before and after HNO3 treatment is also presented, as shown in the inset of Fig. 4(b). The HNO3 induced resistance variation of graphene can be attributed as an important factor to the significant photocurrent increase. During the HNO3 treatment, the strong oxidizing property of HNO3 withdraws the electrodes in the graphene and thus p-dopes the graphene sheet.16 Since the sheet resistance of graphene is calculated by q ¼ 1/r ¼ 1/ q(nln þ plp), where l is the carrier mobility and n (or p) is the carrier concentration, HNO3 induced p-doping can help to increase the carrier concentration and thus reduce the sheet resistance of graphene. Fig. 4(b) shows the graphene sheet resistance variation with different HNO3 treating time. Meanwhile, HNO3 treatment may also help to remove the interface state and impurity across the graphene/BFO interface and reduce the contact resistance correspondingly. The Voc variation can be understood based on the graphene/BFO/Pt energy band diagrams as shown in Fig. 4(c). To construct the band diagram, the band gap (Eg) and electron affinities (v) of BFO is estimated to be 2.2 eV and

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in the BFO body and thus increases the photocurrent in the application. In summary, photovoltaic effect is reported in graphene/ polycrystalline BFO/Pt heterojunction structure. The short circuit current and open circuit voltage are measured to be 25 lA/cm2 and 0.44 V, respectively, higher than reported values in polycrystalline BFO with ITO as top electrodes. Photocurrent increases from 25 lA/cm2 to 2.8 mA/cm2 after HNO3 treatment, indicating a significant enhancement effect. To understand the origin of HNO3 induced enhancement and the advantage of graphene comparing with ITO electrode, a MIM model is proposed. Our result suggests that graphene/ BFO/Pt structure is a promising candidate in BFO based photo-sensitive and energy related application. FIG. 4. (Color online) (a) Photocurrent density (Jsc) and open circuit voltage (Voc) variation with different HNO3 treatment time. Inset: graphene sheet resistance variation with different HNO3 treatment time. (c) Energy band diagram of the graphene/BFO/Pt heterojunction structure upon illumination.

3.3 eV.5,20 The work function (U) of graphene and Pt are assumed to be 4.8 and 5.6 eV.21 The width of the two depletion regions in BFO thin film can be estimated as follows: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2eo eUGFBFO 2eo eUPtBFO þ ; W¼ W GFBFO þW PtBFO ¼ eN eN (1) where eo is the vacuum permittivity, e is the relative dielectric constant of BFO material, U is the contact potential barrier height, and N is the carrier density in BFO. As reported in the literatures,22 N and e can be estimated to be  1017 cm3 and 100, and thus W can be determined to 500 nm, which is larger than the BFO thickness in the experiment. So, the entire BFO thin film is depleted without any neutral region, forming a metal-intrinsic semiconductormetal (MIM) structure. The external potential barrier is determined by the work function difference of graphene and Pt electrode (Uexternal ¼ UPTUGS ¼ 5.64.8 eV ¼ 0.8 eV) rather than the graphene/BFO interface. In principle, the maximum Voc equals to the external potential barrier voltage. However, due to the contact resistance and parasitic effects, the measured value is 0.44 V before HNO3 treatment. By applying HNO3, the work function of the graphene is reduced by 0.13 eV due to the p-doping effect,16 and thus the external potential barrier and measured Voc should be also reduced by 0.13 V, which is in good agreement with our experimental result (0.2 V). Moreover, in a traditional Schottky contact solar cell, the built-in electric field is localized near the interface. Photo carriers generated around the interface can be accelerated by the built-in electric field, but those produced inside the body will be recombined during the diffusion process before reaching the depletion region. However, in a MIM based solar cell, entire BFO thin film is depleted in the longitudinal direction, and the built-in voltage drops proportionally across the thin film. Such “global” electric field can separate all the photo carriers produced in the graphene/BFO interface and

The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC), le Fonds que´be´cois de la recherche sur la nature et les technologies (FQRNT), Natl. NSFC (61025021, 60936002, 60729308, 50972067, and 51072089), 2009ZX02023-001-3, Tsinghua Natl. Lab. Info. Sci. & Tech. (TNList) Cross-discip. Fndn. & int’l. Coop. Prj. Ministry Sci. & Tech. Chin. (2008DFA12000) for financial support. 1

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