出版時のタイトル：Fabrication of Cu2O/γ-FeOOH heterojunction solar cells using electrodepositionapplication/pdfjournal articl

1000030203110

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Ichimura, Masaya

Vequizo, Junie Jhon Magdadaro

ビッキーゾ, ジュニ　ジョン　マグダダロ

English

出版時のタイトル：Fabrication of Cu2O/γ-FeOOH heterojunction solar cells using electrodepositionjournal articl

Nagoya Institute of Technology Repository System

1Fabrication of Cu2O/γ-FeOOH Heterojunction Solar Cells by ElectrodepositionJunie Jhon M. Vequizo1, 2 and Masaya Ichimura1, *1Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology,Gokiso, Showa, Nagoya 466-8555, Japan2Department of Physics, College of Science and Mathematics, Western Mindanao State University,Zamboanga City 7000, PhilippinesCu2O/γ-FeOOH heterojunction solar cells were fabricated by galvanostatic-potentiostaticelectrodeposition methods. The γ-FeOOH films showed n-type conductivity with band gap of 2.2eV. The electrodeposited Cu2O/γ-FeOOH heterojunction exhibited photovoltaic characteristics withshort circuit current density of 0.95 mA/cm2 and  open  circuit  voltage  of  0.11  V.  From core  levelspectroscopy, the Cu2O/FeOOH heterostructure displayed a type II junction with valence bandoffset of 0.8 eV. The conduction band minimum of Cu2O was predicted to be higher than that ofγ-FeOOH by 0.7 eV. With this finding, the γ-FeOOH material may be regarded as a suitablecandidate for hetero-partner of other p-type absorbers.* Corresponding author: ichimura.masaya@nitech.ac.jp2The interesting characteristics of oxide semiconductors such as copper oxide (Cu2O) andiron oxide (Fe2O3) have stimulated extensive research on the potential materials for wide range ofdevice applications (e.g. photocatalyst, solar cells, etc.) due to the abundance of their components,environmentally-friendly character, and suitable electronic and optical properties. In addition, manykinds of oxide thin films can be prepared with ease by employing simple electrodepositiontechnique. Cu2O (with direct band gap, Eg = 2.0 - 2.1 eV) has been regarded as an attractive p-typeabsorber material in heterojunction solar cells.1-5) Numerous efforts have been carried out tofabricate heterostructure solar cells based on Cu2O  with  several  window  layers  such  as  ZnO6-14),Ga2O315) and SnO216). Meanwhile, Fe2O3 is an n-type semiconductor and has been popular as aphotocatalytic material, but has never been applied for heterostructure solar cells. Iron oxidehydroxide (FeOOH) has been utilized as a starting precursor to produce γ-Fe2O3 and α-Fe2O3 viathermal annealing at different temperatures.17) Additionally, γ-FeOOH material has also shownpromising contributions as photocatalyst combined with TiO218) and adsorbent materials for heavyions removals19). Recently, we reported the n-type semiconductor properties of γ-FeOOH thin filmselectrodeposited from oxygen-bubbled aqueous solutions at room temperature.20) It was found thatthe electrodeposited γ-FeOOH exhibits n-type conductivity with a band gap > 2 eV.In  this  work,  the  Cu2O/γ-FeOOH heterostructure is fabricated bygalvanostatic-potentiostatic electrodeposition methods, and the photovoltaic performance as well asthe band alignment at the interface are evaluated and investigated for the first time. To ourknowledge, no studies have been performed to utilize Fe-based compounds as the n-type layer in ap-n heterojunction. Thus, this paper is the first report on the fabrication of p-n junction using ironoxide/oxyhydroxide compound as one of the components. Our results demonstrate that the usage ofγ-FeOOH as a buffer layer in heterojunction solar cells is possible. Thus, since iron is non-toxic andone of the most abundant elements on earth, γ-FeOOH can also be a good hetero-partner for otherp-type absorbers as an alternative replacement to the popular but highly toxic CdS.All the electrodeposition experiments were carried out in a three-electrode electrochemicalcell with indium tin oxide (ITO)-coated glass substrate, platinum (Pt) sheet, and saturated calomelelectrode (SCE) as working, counter and reference electrodes, respectively. Initially, γ-FeOOH thinfilms were potentiostatically electrodeposited on the pre-cleaned ITO substrate from acidic aqueoussolutions containing 0.05 M FeSO4 and 0.1 M Na2SO4. The applied potential was -0.9 (V vs SCE).3Modifications in the deposition condition as reported in our previous work on electrodeposition ofγ-FeOOH20) were made in the current experimental conditions with the following considerations toimprove the properties of the film: (a) Na2SO4 was used instead of KCl to ensure homogeneity inthe  anions  and  avoid  the  possible  inclusion  of  Cl- during  the  formation  of  γ-FeOOH film and  (b)solution was stirred during the entire deposition process. As a result, an enhanced current densityand thicker films were easily obtainable. A 50-nm thick γ-FeOOH film was used in the fabricationof the p-n junction. Cu2O layer was galvanostatically deposited onto the as-prepared γ-FeOOH filmfrom the solution containing 0.2 M CuSO4 - 1.6 M C3H6O3 (pH was adjusted to 12.5 using KOH)with constant cathodic current density (-1.0 mA/cm2) at solution temperature of 40oC. Thedeposition duration for the Cu2O layer was maintained at 10 min. This Cu2O deposition condition issimilar to our previous report on Cu2O/ZnO heterojunction.14) Since there are numerous studies thatperformed and reported extensive characterizations on Cu2O thin films, only the γ-FeOOH materialundergone additional characterizations such as compositional, optical transmission, andphotoelectrochemical (PEC) analyses as part of our continuous investigation to improve itsproperties. Compositional analysis was performed using a JEOL JAMP-9500F field-emissionAuger electron spectroscopy (AES). Argon ion etching was done using an acceleration voltage of 2kV with ion current of 2.6 μA to sputter the film’s surface. The chemical state of the elements wasdetermined by performing the X-ray photoelectron spectroscopic (XPS) analysis using the XPSPHI-5000 (ULVAC-PHI) with Al Kα as the X-ray source. The optical characterization was carriedout using a JASCO U-570 ultraviolet/visible/near infrared (UV/vis/NIR) spectrometer from300-1000 nm in referenced to the substrate. To determine the conductivity type and photoresponse,photoelectrochemical (PEC) measurement was performed in a three-electrode electrochemical cellusing 0.1 M Na2SO4 as the electrolyte and the voltage was scanned linearly towards the forwardbias region. The sample was illuminated from the substrate side to evaluate the photoresponse byusing a Xe lamp as the light source with radiation power of 100 mW/cm2.After the fabrication of Cu2O/γ-FeOOH heterojunction, indium (In) electrodes with 1 mm2area each were evaporated on top of the heterostructure for electrical contacts in currentdensity-voltage (J-V) characterization. Thus, the structure of the solar cell isIn/Cu2O/γ-FeOOH/ITO. The photovoltaic properties of the Cu2O/γ-FeOOH solar cell wereevaluated using a solar simulator under AM1.5 (100 mW/cm2) illumination condition. The4heterostructure were exposed to the irradiation from the ITO substrate side. Additionally, theinterface of the heterojunction was studied by performing the X-ray photoelectron spectroscopicanalysis using the XPS PHI-5000 (ULVAC-PHI) with Al Kα as the X-ray source.The galvanostatically electrodeposited Cu2O thin film displays polycrystalline cubicstructure with preferred orientation along (111) similar to the reported XRD data for Cu2O in Ref.14, while the as-prepared γ-FeOOH film shows nanocrystalline structure as reported in our previouswork on this material (Ref. 20). The composition ratio (Fe/O) calculated from the AES data usingthe standard Fe2O3 compound as a reference is about 0.51. Figure 1 shows the Fe 2p and O 1s XPSspectra for as-deposited γ-FeOOH film after 1.6 min of Ar ion bombardment. The sputtering iscarried out to remove unwanted carbon impurities, which are detected at the surface located atbinding energy (BE) of 285 eV. As displayed in Fig. 1, Fe 2p peaks are located at BE near 711.5and 724 eV with satellite peaks near 715 and 730 eV, which signify the characteristics of Fecompounds. Furthermore, the O 1s spectrum seems to have two obvious peaks at BE closer to 530.2and 531.9 eV. These peaks could be associated to oxide (O2-) and hydroxyl (OH) components ofoxygen. Therefore, we could most likely consider that the as-deposited film is iron oxide hydroxideconsistent with the composition obtained by AES and also with the Raman shifts reported in ourprevious studies on γ-FeOOH.20Figure 2 shows the optical transmission and the band gap estimation for the as-preparedγ-FeOOH. The transmission is close to 65 % in the wavelength region from 800 – 1000 nm and theabsorption edge is very apparent near 600 nm. By extrapolation of the linear region in the plot of(αhv)2 versus hv, as depicted in the Tauc’s plot in Fig. 2(b), the band gap is approximately 2.2 eV,which is similar to the reported band gap of the most popular iron compound, α-Fe2O3.Fig 3 depicts the PEC response of the γ-FeOOH film deposited at -0.9 (V vs SCE) underlinearly increasing forward bias. As displayed in the figure, photocurrent density increases whenillumination is turned on and decreases when in dark condition, i.e. generation of carriers by photoirradiation occurs. Since the photocurrent is positive, the photogenerated minority carriers are holes,i.e., the conductivity type is n-type.Figure 4(a) shows the current-voltage characteristics for Cu2O/γ-FeOOH heterojunctionmeasured under dark and light conditions. As shown in this figure, the rectification property and theincrease in the current density under illumination are observed. In addition, the Cu2O/γ-FeOOH5heterostructure displays some photovoltaic characteristics as depicted in Figure 4(b). The solar cellparameters: open circuit voltage, VOC and short  circuit  current density,  JSC are approximated to be0.11 V and 0.95 mA/cm2, respectively. This result signifies that the γ-FeOOH could be applicableas n-type material in heterojunction solar cells.Core-level spectroscopy is commonly used as a quantitative tool to study the interface between theindividual components of the heterostructures.14, 21-23 In  view  of  the  XPS  signal  intensity,  the  Fe2p3/2 and Cu 2p3/2 levels are the better choice as core levels (CL). However, in the Cu2O/γ-FeOOHconfiguration, the overlapping of Fe 2p3/2 peak with CuLM2,3M2,3 Auger signals can be expectedand this makes the use of (Fe and Cu) 2p levels difficult.24  Hence, the (Fe and Cu) 3p levels arechosen as another option for CL. The valence and conduction band offsets of the Cu2O/γ-FeOOHheterostructure can be determined from the following relations:( ) ( ) )( 3333 22 heteropFeheteropCuFeOOHVFeOOHpFeOCuVOCu pCuV EEEEEEE -----=D -- gg , (1)VFeOOHgOCugC EEEE D+-=D- )( 2 g . (2)The expressions inside the parentheses in relation (1) are the energy difference betweenthe core level and the valence band maximum (VBM) of individual materials and CL energydifference at the interface of the heterojunction.Figures 5 and 6 depict the XPS spectra of the individual materials. The VBM spectrum foreach layer with linear extrapolations of the leading edge for VBM determination (1.0 and 0.7 eV forγ-FeOOH and Cu2O, respectively) is  also shown. The obtained VBM for Cu2O agrees well  in theprevious studies on Cu2O-based heterostructures (0.34 – 0.70 eV)14, 25). The energy differencebetween CL and VBM are estimated to be 55.2 eV for γ-FeOOH and 75.4 eV for Cu2O. Figure 7shows  the  CL  energy  difference  (equal  to  19.4  eV)  at  the  Cu2O/γ-FeOOH hetero-interface.According to relation (1), ΔEV is approximately 0.8 eV and considering the difference in the bandgap between Cu2O (2.1 eV) and γ-FeOOH (2.2 eV as discussed above), ΔEC is approximately equalto 0.7 eV. Thus, a type II structure shown in Figure 8 is developed. As reported in Ref. 14, ΔEC is6about  0.5  eV  for  the  Cu2O/ZnO  hetero-interface,  with  the  conduction  band  minimum  (CBM)  ofCu2O higher than that of ZnO. Thus CBM position of γ-FeOOH is rather close to that of ZnO.In summary, γ-FeOOH thin films displayed suitable characteristics for solar cellapplications. As-deposited γ-FeOOH exhibited n-type conductivity and the optical band gap wasaround 2.2 eV. Successful fabrication of Cu2O/γ-FeOOH heterojunction was demonstrated byemploying facile galvanostatic-potentiostatic electrodeposition techniques. The formed p-n junctionbetween Cu2O and γ-FeOOH is considered to be the first p-n junction based on any Fe compounds.Photovoltaic characteristics were confirmed for Cu2O/γ-FeOOH heterojunction with VOC = 0.11 Vand JSC = 0.95 mA/cm2. Photoelectron spectroscopy predicted that the CBM of Cu2O is higher thanthat of γ-FeOOH by 0.7 eV. Hence, a type II heterostructure was developed. The above findingsopen alternative options in choosing oxide semiconductor as n-type or as buffer layer in thefabrication of heterojunction solar cells. The authors would like to thank Mr. Y. Moriguchi for his technical assistance in XPSmeasurement and Dr. M. Kato for his useful suggestions.71) V. Georgieva and M. Ristov: Sol. Energy Mater. Sol. Cells 73 (2002) 67.2) P. E. de Jongh, D. Vanmaekelbergh, and J. J. Kelly: Chem. Mater. 11 (1999) 3512.3) T.  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Miyata: Appl. Phys. Express 6 (2013) 044101.16) L. Panadimitrious, N. A. Economou, and D. Trivich: Solar Cells 3, (1981) 73.17) H. Naono and K. Nakai: J. Colloid Interface Sci. 128 (1989) 146.18) S. B. Rawal, A. K. Chakraborty, and W. I. Lee: Bull. Korean Chem. Soc. 30 (11) (2009) 2613.19) Y.  Jia,  T.  Luo,  X.-Y.  Yu,  Z.  Jin,  B.  Sun,  J.-H.  Liu,  and  X.-J.  Huang:  New J.  Chem. 37 (2013)2551.20) J. J. M. Vequizo and M. Ichimura: Appl. Phys. Express 6 (2013) 125501.21) E. A. Kraut, R. W. Grant, J. R. Waldrop, and S. P. Kowalczyk: Phys. Rev. B 28 (1983) 1965.22) B. Kramm, A. Laufer, D. Reppin, A. Kronenberger, P. Hering, A. Polity, and B. K. Meyer: Appl.Phys. Lett. 100 (2012) 094102.23) A. J. Nelson : J. Appl. Phys. 78 (1995) 5701.24) G. Polzonetti, V. Di Castro, and C. Furlani: Surf. Inter. Anal. 22 (1994) 211.25) M. Yang, L. Zhu, Y. Li, L. Cao, and Y. Guo: J. Alloys and Compounds 578 (2013) 143.8Fig. 1 (a) Fe 2p and (b) O 1s spectra for the as-deposited γ-FeOOH thin film after sputtering.Fig. 2 (a) Optical transmission and the (b) corresponding Tauc’s plot for the as-deposited γ-FeOOHthin film.Fig. 3 PEC response for the as-deposited γ-FeOOH thin film under linearly increasing bias.Fig. 4 (a) J-V characteristics for Cu2O/γ-FeOOH heterojunction with 50 nm-thick γ-FeOOH underdark and light conditions and (b) the region near 0 V of the J-V curve under illumination.Fig. 5 (a) Fe 3p and (b) VBM spectra for the as-deposited for the γ-FeOOH thin film deposited at-0.9 (V vs SCE).Fig. 6 (a) Cu 3p and (b) VBM spectra for the as-prepared Cu2O thin film deposited on ITO substrateat -1.0 mA/cm2.Fig. 7 (Cu-Fe) 3p spectrum at the interface of the electrodeposited Cu2O/γ-FeOOH heterojunction.Fig. 8 Band alignment diagram for the electrodeposited Cu2O/γ-FeOOH heterojunction. EC and EVare conduction band minimum and valence band maximum, respectively.9Fig. 110Fig. 211Fig. 312Fig. 413Fig. 514Fig. 615Fig. 716Fig. 8ΔECL = 19.4 eVECΔEC = 0.7 eVΔEV = 0.8 eVγ-FeOOHCu2OEgn = 2.2 eVEgp = 2.1 eVECEVEV

Applied Physics Express

Fabrication of Cu2O/γ-FeOOH Heterojunction Solar Cells by Electrodeposition

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Fabrication of Cu2O/γ-FeOOH Heterojunction Solar Cells by Electrodeposition

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