Materials 2015, 8, 5715-5729; doi:10.3390/ma8095274

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materials ISSN 1996-1944 www.mdpi.com/journal/materials Article

Investigation of Coral-Like Cu2O Nano/Microstructures as Counter Electrodes for Dye-Sensitized Solar Cells Chih-Hung Tsai *, Po-Hsi Fei and Chih-Han Chen Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan; E-Mails: [email protected] (P.-H.F.); [email protected] (C.-H.C.) * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-3-863-4199; Fax: +886-3-863-4180. Academic Editors: Federico Bella and Claudio Gerbaldi Received: 31 July 2015 / Accepted: 26 August 2015 / Published: 31 August 2015

Abstract: In this study, a chemical oxidation method was employed to fabricate coral-like Cu2 O nano/microstructures on Cu foils as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The Cu2 O nano/microstructures were prepared at various sintering temperatures (400, 500, 600 and 700 ˝ C) to investigate the influences of the sintering temperature on the DSSC characteristics. First, the Cu foil substrates were immersed in an aqueous solution containing (NH4 )2 S2 O8 and NaOH. After reacting at 25 ˝ C for 30 min, the Cu substrates were converted to Cu(OH)2 nanostructures. Subsequently, the nanostructures were subjected to nitrogen sintering, leading to Cu(OH)2 being dehydrated into CuO, which was then deoxidized to form coral-like Cu2 O nano/microstructures. The material properties of the Cu2 O CEs were comprehensively determined using a scanning electron microscope, energy dispersive X-ray spectrometer, X-ray diffractometer, Raman spectrometer, X-ray photoelectron spectroscope, and cyclic voltameter. The Cu2 O CEs sintered at various temperatures were used in DSSC devices and analyzed according to the current density–voltage characteristics, incident photon-to-current conversion efficiency, and electrochemical impedance characteristics. The Cu2 O CEs sintered at 600 ˝ C exhibited the optimal electrode properties and DSSC performance, yielding a power conversion efficiency of 3.62%. The Cu2 O CEs fabricated on Cu foil were generally mechanically flexible and could therefore be applied to flexible DSSCs. Keywords: dye-sensitized solar cells; cuprous oxide; counter electrode; nanostructures

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1. Introduction Solar cell research is receiving an increasing amount of attention from academia and various governments in response to recent discussions on energy shortages and global environmental pollution. In 1991, Grätzel et al. [1] used high-surface-area TiO2 nanoparticles to develop a novel type of solar cell called the dye-sensitized solar cell (DSSC). Compared with conventional Si-based solar cells and thin-film solar cells, DSSCs are advantageous because of their simple structure, easy fabrication process, and low cost; hence, they promptly attracted the interest of the scientific community [2–10]. DSSC devices comprise a transparent conductive glass substrate, TiO2 nanoparticle thin-film electrode, dye, electrolyte, and Pt counter electrode (CE) [11–13]. Over the past 20 years, numerous scientists have endeavored to enhance the efficiency of DSSCs. Such efforts have included developing technologies for fabricating porous TiO2 nanoparticle electrodes, designing high-performance dyes, developing solid-state electrolytes, investigating flexible substrates, and improving device encapsulation technologies. Furthermore, several studies have contributed to major breakthroughs in the field. In a typical DSSC device, CEs are a crucial component facilitating redox reactions in the electrolyte, extracting electrons from the electrolyte, and reducing the dye from an excited state to a ground state, thereby completing the dye-regeneration process [14]. The CE is usually composed of a conductive catalytic layer. The function of the conductive catalytic layer is to catalyze the reduction of the I3 ´ ions in the electrolyte produced during the regeneration of the oxidized dyes (through the oxidation of iodide ions in the electrolyte). The requirements for the CE in a DSSC are thus low charge-transfer resistance and high exchange current densities for effective reduction of the oxidized species, and good chemical/electrochemical stability in the electrolyte systems used in DSSCs [15]. Although Pt has traditionally been used as a CE material [16–25], its high cost increases the DSSC fabrication cost considerably. Hence, identifying a highly efficient replacement for Pt in CEs is a crucial research topic. Several previous studies have endeavored to replace Pt CEs with other more cost-effective CE materials with favorable electrochemical properties such as conductive polymers [26], carbon black [27], graphene [28], and carbon nanotubes [29]. Cu2 O is a direct-energy gap p-type semiconductor with a band gap of approximately 2.0 eV. This compound has received considerable attention for application in photovoltaic devices because it offers several advantages such as its low cost, nontoxic nature, and abundant supply [30–32]. Moreover, it exhibits excellent optical absorption in the visible spectrum, can be deposited on thin films by using various coating methods, and can be fabricated using simple and economical processes. Therefore, the performance and application of solar cells can be improved by elucidating the properties of Cu2 O. Georgieva et al. [33] directly electrodeposited Cu2 O on transparent indium tin oxide (ITO) to obtain an ITO/Cu2 O/graphite structure with a thickness of 4–6 µm and optical band gap of 2.38 eV. Jeong et al. [34] electrodeposited ZnO and Cu2 O to fabricate heterojunction solar cells; however, the cells exhibited a maximal conversion efficiency of only 0.41%. Furthermore, numerous problems were unsolved, including how to achieve accurate control of the electrodeposition conditions for reducing Cu2 O defects. Mittiga et al. [35] injected nitrogen and oxygen into a tube furnace to oxidize Cu and fabricate high-quality Cu2 O; subsequently, an ion beam was employed to sputter transparent conductive oxide onto the Cu2 O and MgF2 was vapor-deposited onto antireflective films. The resulting Cu2 O/ZnO/ITO/MgF2 structure exhibited a conversion efficiency of 2.01%. Cu2 O was also sputtered

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onto n-ZnO nanowire solar cells on ZnO–Ga/glass templates. These ZnO templates were n-type semiconductors with an energy gap of 3.37 eV. Cu2 O is a p-type semiconductor; therefore, depositing p-Cu2 O onto n-ZnO leads to the formation of a p–n junction [36,37]. However, previous studies have primarily adopted Cu2 O as the working electrode (WE) in solar cells, and reports on Cu2 O CEs in DSSCs are scarce. In the present study, we used a chemical oxidation method to fabricate coral-like Cu2 O nano/microstructures on Cu foils for use as CEs in DSSCs. The material properties of the Cu2 O CEs fabricated at various sintering temperatures were characterized, and subsequently, the influence of the sintering temperature on the DSSC efficiency was determined. 2. Experimental Section 2.1. Cu2 O Counter Electrode (CE) Fabrication Figure 1a depicts the process for fabricating the Cu2 O electrodes. The Cu foil substrates (0.5 mm thick) were chemically oxidized by being immersed in a solution containing 0.125 M (NH4 )2 S2 O8 (Sigma-Aldrich, St. Louis, MO, USA) and 2.5 M NaOH (Sigma-Aldrich, St. Louis, MO, USA). After reacting at 25 ˝ C for 30 min, the Cu substrates were converted to Cu(OH)2 nanowires. This intermediate product was sintered in a nitrogen atmosphere, leading to the dehydration of Cu(OH)2 and the formation of CuO nanosheets that were deoxidized into coral-like Cu2 O nano/microstructures. For fabricating the Cu2 O electrodes, Cu foil substrates were first ultrasonically washed in acetone, deionized water, 1 M HCl (Sigma-Aldrich, St. Louis, MO, USA), and then in deionized water again for 5 min each. Their surfaces were then blow-dried with nitrogen gas. Subsequently, 10 g of NaOH and 2.85 g of (NH4 )2 S2 O8 were added to 10 mL of deionized water, and the mixture was ultrasonically vibrated for 30 min to evenly dissolve the powders in the aqueous solution. The Cu substrates were placed in the solution for 30 min, extracted, dried at room temperature under ambient conditions, and then placed in a tube furnace. The tube furnace was evacuated to achieve an internal pressure of 7 mTorr and subsequently purged with nitrogen gas at a rate of 300 cm3 /min. The outlet of the nitrogen cylinder was maintained at a constant pressure, and the vacuum-purge process was performed for 3 min to ensure that the tube furnace was filled with nitrogen. Finally, the vacuum pump was shut down. When the internal nitrogen pressure reached 300 Torr, the nitrogen injection valve was closed and the Cu substrates were sintered in the nitrogen atmosphere to complete the fabrication of the Cu2 O. The Cu substrates were sintered for 4 h at different temperatures (400, 500, 600 and 700 ˝ C) to produce various Cu2 O CEs for DSSCs. 2.2. Cu2 O Counter Electrode (CE) Characterization The physical, chemical, and electrochemical properties of the Cu2 O CEs were comprehensively characterized using various analytical techniques. A scanning electron microscope (SEM) (JEOL JSM-7000F, JEOL Inc., Peabody, MA, USA) was used to observe the surface morphology of the Cu2 O CEs. A surface profiler was used to analyze the thickness of the Cu2 O CEs. An energy dispersive X-ray spectrometer (EDS) was employed to measure the O–Cu elemental ratios of the Cu2 O CEs; an X-ray diffractometer (XRD) (Rigaku D/Max-2500V, Rigaku Corp., Tokyo, Japan) was used to identify the crystal structures of the Cu2 O CEs; a Raman spectrometer was employed to verify the constituent

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materials of the Cu2 O CEs; and an X-ray photoelectron spectrometer (XPS) (Thermo K-Alpha, Thermo materials of the Cu2O CEs; and an X-ray photoelectron spectrometer (XPS) (Thermo K-Alpha, Fisher Scientific, Waltham, MA, USA) was used to analyze the elemental and chemical composition of Thermo Fisher Scientific, Waltham, MA, USA) was used to analyze the elemental and chemical the Cu2 O CE surfaces. The electrochemical activities of the Cu2 O CEs for I3 ´ reduction were examined composition of the Cu2O CE surfaces. The electrochemical activities of the Cu2O CEs for I3− reduction using a cyclic voltameter (CV) (CH Instruments 6116D, CH Instruments, Inc., Austin, TX, USA). The were examined using a cyclic voltameter (CV) (CH Instruments 6116D, CH Instruments, Inc., Austin, CV measurements were conducted using a three-electrode electrochemistry system. The Cu2 O CEs TX, USA). The CV measurements were conducted using a three-electrode electrochemistry system. under testing were used as the WE, the Pt foil was used as the CE, and Ag/Ag+ was used as a reference The Cu2O CEs under testing were used as the WE, the Pt foil was used as the CE, and Ag/Ag+ was electrode. The scan electrode. rate was set at scan 50 mV/s and set the at electrolyte an acetonitrile solution containing used as a reference The rate was 50 mV/s was and the electrolyte was an acetonitrile 10 mM LiI (Sigma-Aldrich, MO, USA),St.1 Louis, mM I2 MO, (Sigma-Aldrich, St. ILouis, MO, USA), solution containing 10 mM St. LiI Louis, (Sigma-Aldrich, USA), 1 mM 2 (Sigma-Aldrich, and 100 mM St. 4 Louis, MO, USA). the electrochemical 4 (Sigma-Aldrich, St. Louis, MO,LiClO USA), and 100 mM LiClO (Sigma-Aldrich, St. Furthermore, Louis, MO, USA). Furthermore, properties of the Cu O CEs were obtained from the electrochemical impedance spectroscopy (EIS) of 2 the electrochemical properties of the Cu2O CEs were obtained from the electrochemical impedance symmetrical structures.sandwich Two identical assembled sealant 2 O CEs were CEs spacer. were spectroscopysandwich (EIS) ofdevice symmetrical deviceCustructures. Two identicalwith Cua2O The electrolyte M 1-Butyl-3-methylimidazolium iodide (BMII) (Sigma-Aldrich, St. Louis, MO, assembled with(0.6 a sealant spacer. The electrolyte (0.6 M 1-Butyl-3-methylimidazolium iodide (BMII) USA), 0.05 M LiI,St.0.03 M IMO, M 4-tert-Butyloyridine St. Louis, MO, USA), 0.1 M 2 , 0.5USA), (Sigma-Aldrich, Louis, 0.05 M LiI, 0.03 M(Sigma-Aldrich, I2, 0.5 M 4-tert-Butyloyridine (Sigma-Aldrich, guanidine (Sigma-Aldrich, St. Louis, MO,(Sigma-Aldrich, USA), and a 5:1 St. mixture volume) St. Louis,thiocyanate MO, USA),(GuSCN) 0.1 M guanidine thiocyanate (GuSCN) Louis,(by MO, USA),of acetonitrile and valeronitrile (Sigma-Aldrich, Louis, MO, USA)) was injected St. between electrodes and a 5:1 mixture (by volume) of acetonitrileSt. and valeronitrile (Sigma-Aldrich, Louis,two MO, USA)) through a drilled hole. two Theelectrodes EIS was then conducted impedance (CH Instruments was injected between through a drilledusing hole.anThe EIS was analyzer then conducted using an 6116D, CH Instruments, Austin, TX, USA), a frequencyInc., range of 0.1TX, Hz to 1 MHz, a0V impedance analyzer (CHInc., Instruments 6116D, CHinInstruments, Austin, USA), in aunder frequency range ofan 0.1acHz to 1 MHz, a 0 V bias and an ac amplitude of 10 mV. bias and amplitude ofunder 10 mV.

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Figure 1. (a) The fabrication process of a coral-like Cu2 O counter electrode; and (b) the Figure 1. (a) The fabrication process of a coral-like Cu2O counter electrode, and (b) the schematic device structure of a dye-sensitized solar cells (DSSC) using the coral-like Cu2 O schematic device structure of a dye-sensitized solar cells (DSSC) using the coral-like Cu2O counter electrode. counter electrode.

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2.3. Dye-Sensitized Solar Cells (DSSC) Fabrication Figure 1b shows a structural diagram of a DSSC device fabricated using coral-like Cu2 O CEs (Section 2.1 describes the Cu2 O fabrication method). The DSSC WE was fabricated using the following method: First, a fluorine-doped tin oxide (FTO) substrate (2.2 mm thick) was cleaned and covered with 3M adhesives that were prepunched with 4-mm-diameter holes (area = approximately 0.126 cm2 ). The holes were prepunched to define the device coating area. The doctor blade method was applied to uniformly coat the area with 25-nm TiO2 nanoparticle paste. The coated samples were heated at 150 ˝ C for 10 min, and then a second coating was applied to obtain 12-µm-thick TiO2 WEs. Subsequently, the same method was employed to coat a 200-nm TiO2 nanoparticle layer for use as a scattering layer. The coated samples were then sintered at 500 ˝ C for 30 min. When cooled to 80 ˝ C, the TiO2 electrodes were immersed in a dye solution for 24 h. The dye solution was composed of 0.5 mM N719 dye (Sigma-Aldrich, St. Louis, MO, USA) and 0.5 mM chenodeoxycholic acid (Sigma-Aldrich, St. Louis, MO, USA) (used as a coadsorbant) in a 1:1 volumetric mixture of acetonitrile and tert-Butyl alcohol (Sigma-Aldrich, St. Louis, MO, USA). Subsequently, 60-µm encapsulation films were cut with outer and inner dimensions of 2.5 cm ˆ 2.5 cm and 0.8 cm ˆ 0.8 cm, respectively, for assembling the WEs and CEs. The upper and lower electrodes were sealed by applying a constant pressure of 3 kg/cm2 at 130 ˝ C for 3 min. After the electrodes had cooled, 5 µL of an electrolyte solution was injected into each device by using a micropipette. The electrolyte solution contained 0.6 M 1-Butyl-3-methylimidazolium iodide (BMII), 0.05 M LiI, 0.03 M I2 , 0.5 M 4-tert-Butyloyridine, 0.1 M guanidine thiocyanate (GuSCN), and a 5:1 mixture (by volume) of acetonitrile and valeronitrile. The remaining 0.8 cm ˆ 0.8 cm encapsulation films were used with glass plates to seal the CE holes by applying a pressure of 3 kg/cm2 , to prevent the leakage and evaporation of the electrolyte. Finally, alcohol was used to clean the surface of the solar cells in preparation for experimental measurements. 2.4. Dye-Sensitized Solar Cells (DSSC) Characterization The current density–voltage (J–V) characteristics of the DSSCs were measured under illumination of simulated AM 1.5G solar light from a 550-W xenon lamp solar simulator (ABET Technologies Sun 3000 Class AAA, Milford, CT, USA). To reduce the spectrum mismatch between the simulated light and AM 1.5G to below 2% in the region of 350–750 nm, the incident light intensity was calibrated to 100 mW/cm2 by using a reference Si photodiode equipped with an infrared-cutoff filter (KG-5, Schott, SCHOTT AG, Mainz, Germany) (the reference cell was certificated by Bunkoh-Keiki Co. Ltd., Tokyo, Japan). Photocurrent–voltage curves were obtained by applying an external bias voltage to the cell and measuring the generated photocurrent. The J–V characteristics of the DSSCs were used to determine the short-circuit current density (JSC ), open-circuit voltage (VOC ), fill factor (FF), and power conversion efficiency of the DSSCs. Incident monochromatic photon-to-current conversion efficiency (IPCE) spectra were measured using a 150-W xenon arc lamp (ABET Technologies, Milford, CT, USA) as the light source, which was coupled to a monochromator. The IPCE data were obtained by illuminating the solar cells with monochromatic light at a wavelength sampling interval of 5 nm between 300 and 750 nm and by measuring the short-circuit current of the solar cells. The IPCE measurements were performed under full computer control.

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Materials 2015, 8 5720 between 300 and 750 nm and by measuring the short-circuit current of the solar cells. The IPCE measurements were impedance performed under full computer Electrochemical spectroscopy (EIS) control. was employed to measure the cell impedance by Electrochemical impedance spectroscopy (EIS) was employed to measure the cell TX, impedance by using an impedance analyzer (CH Instruments 6116D, CH Instruments, Inc., Austin, USA) with using an impedance analyzer (CH Instruments 6116D, CH Instruments, Inc., Austin, TX, USA) with a a frequency range of 0.1 Hz to 1 MHz. During the impedance measurements, the cell was constantly frequency range of 0.1 Hz to 1 MHz. During the impedance measurements, the cell was constantly illuminated by simulated AM 1.5G solar light at an intensity of 100 mW/cm2 . The impedance of the cell illuminated by simulated AM 1.5G solar light at an intensity of 100 mW/cm2. The impedance of the (from 0.1 Hz to 1 MHz) was measured by applying a bias at the VOC of the cell (i.e., under no dc electric cell (from 0.1 Hz to 1 MHz) was measured by applying a bias at the VOC of the cell (i.e., under no dc current) and by using an ac amplitude of 10 mV. electric current) and by using an ac amplitude of 10 mV. 3. Results and Discussion 3. Results and Discussion 3.1. Cu2 O Counter Electrodes (CE) Characterization 3.1. Cu2O Counter Electrodes (CE) Characterization The SEM images of Cu2 O (Figure 2) revealed distinct morphologies among the samples fabricated The SEM images of Cu2O (Figure 2) revealed distinct morphologies among the samples fabricated at various sintering temperatures. Figure 2a shows that sintering the Cu2 O at 400 ˝ C caused nanowire at various sintering temperatures. Figure 2a shows that sintering the Cu2O at 400 °C caused nanowire structures of of 100–200 nmnm andand length of 1–2 µm.μm. Figure 2b shows that the 2O structurestotoform formwith witha adiameter diameter 100–200 length of 1–2 Figure 2b shows thatCu the nanowires agglomerated into 1–2into µm1–2 coral-like structures when sintered 500 ˝ C.atFigure 2c Figure shows that Cu2O nanowires agglomerated μm coral-like structures when at sintered 500 °C. 2c ˝ sintering the Cu O at 600 C produced 4–5-µm coral-like structures, and Figure 2d shows that when 2 shows that sintering the Cu2O at 600 °C produced 4–5-μm coral-like structures, and Figure 2d shows ˝ the reached 700 C, the becamebecame integrated and several cracks thatsintering when thetemperature sintering temperature reached 700Cu °C,2 Othestructures Cu2O structures integrated and several developed on the surface. thickness of the Cu2of O the CEsCu was measured using a surface profiler. The cracks developed on the The surface. The thickness 2O CEs was measured using a surface ˝ Cu 400,sintered 500, 600 and 700 of 5.54, 5.28, 5.13 2 O films, profiler. Thewhich Cu2Owere films,sintered which at were at 400, 500, C, 600have andthe 700thicknesses °C, have the thicknesses of and 5.01 µm, respectively. 5.54, 5.28, 5.13 and 5.01 μm, respectively.

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Figure 2. SEM images of the Cu2 O counter electrodes sintered at (a) 400 ˝ C; (b) 500 ˝ C; Figure 2. SEM images of the Cu2O counter electrodes sintered at (a) 400 °C, (b) 500 °C, (c) 600 ˝ C; and (d) 700 ˝ C. (c) 600 °C, and (d) 700 °C.

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Figure Figure3a–d 3a–d Figure 33 shows shows the the EDS EDS results results for for the the Cu Cu22O O CEs CEs sintered sintered at at various various temperatures. temperatures. Figure ˝ show and 700 700 °C C yielded yielded atomic atomicweight weight(relative (relativeatomic atomicmass) mass) showthat thatsintering sinteringthe the Cu Cu22O O at at 400, 400, 500, 500, 600 600 and O–Cu (36.94:63.06), 12.48:87.52 12.48:87.52 (36.15:63.85) (36.15:63.85)and and O–Cu ratios ratios of of 11.45:88.35 11.45:88.35 (33.93:66.07), (33.93:66.07), 12.85:87.15 12.85:87.15 (36.94:63.06), 10.40:89.60 The EDS EDS results results show show that that the the oxygen oxygen content content of of the theCu Cu2O 2O 10.40:89.60 (31.55:68.45), (31.55:68.45), respectively. respectively. The ˝ ˝ samples C was higher than that that of of the the samples samples sintered sinteredatat400 400°C.C.However, However, samplessintered sintered at at 500 500 and and 600 600 °C ˝ when C, the oxygen oxygen content content dropped droppedsubstantially, substantially,whereas whereasthe the whenthe thesintering sintering temperature temperature reached reached 700 °C, Cu CuCu electrode increased. This observation waswas attributed to the oxygen atoms Cucontent contentofofthethe electrode increased. This observation attributed to loss the of loss of oxygen 2 O2O inatoms Cu2 OinatCu high sintering temperatures, leadingleading to a decrease in the in oxygen ratio and increase in the high sintering temperatures, to a decrease the oxygen ratioanand an increase 2O at Cu ratioCuinratio the Cu O electrodes. in the in 2the Cu2O electrodes.

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Figure 3. The energy-dispersive X-ray spectroscopy (EDS) analytic results of the Cu2 O Figure 3. The energy-dispersive X-ray spectroscopy (EDS) analytic results of the Cu2O counter electrodes sintered at (a) 400 ˝ C; (b) 500 ˝ C; (c) 600 ˝ C and (d) 700 ˝ C. counter electrodes sintered at (a) 400 °C, (b) 500 °C, (c) 600 °C and (d) 700 °C. Raman samples, enabling enabling the the interaction interactionbetween between Raman spectroscopy spectroscopy involves involves using using lasers lasers to excite test samples, photonsand andphonons phonons in in aa sample sample to to be be characterized characterized to photons to determine determine the the modes modes of ofvibration vibrationand androtation rotation latticesand and molecules. molecules. Figure Figure 44 shows ofoflattices shows the the Raman Raman spectra spectra of of the the Cu Cu22OO electrodes electrodessintered sinteredatat400, 400, ˝°C in a nitrogen atmosphere. The Raman signals of Cu2O were observed at 151 and 500, 600 and 700 500, 600 and 700 C in a nitrogen atmosphere. The Raman signals of Cu2 O were observed at 151 and −1 −1 220cm cm´1 for CuO, CuO, the the signals signals were were observed observed at at 284 284 and and 345 345 cm cm´1 .. The 220 ;; for TheRaman Ramanspectra spectraofofthe theCu Cu2O 2O electrodesrevealed revealedCuCu signals in the samples butCuO no CuO indicating the electrodes 2O electrodes signals in the samples but no signal,signal, indicating that thethat electrodes sintered 2O ˝ and 700 °C were converted to Cu2O. Additionally, the Raman spectra at 600 400,and 500, atsintered 400, 500, 700600 C were converted to Cu2 O. Additionally, the Raman spectra indicated that the indicated that the Cu O peak signal was most apparent in theatsample 2 Cu2 O peak signal was most apparent in the sample sintered 600 ˝ C.sintered at 600 °C. AnXRD XRDwas wasemployed employed to to analyze analyze the the crystal crystal structure structure of of the the Cu An Cu22O O CEs. CEs. Figure Figure55shows showsthe theXRD XRD measurement results, in which the Cu O signals correspond to the 2θ angles of 29°, 36°, 42°, 61° measurement results, in which the Cu22O signals correspond to the 2θ angles of 29˝ , 36˝ , 42˝ , 61˝and and 74°, ˝ which in turn correspond to the lattice planes of (110), (111), (200), (220) and (311), respectively 74 , which in turn correspond to the lattice planes of (110), (111), (200), (220) and (311), respectively (JCPDS card card No. No. 78-2076). diffraction peaks of the Cu Cu substrate correspond to 2θto (JCPDS 78-2076).Additionally, Additionally,thethe diffraction peaks of the substrate correspond angles of 44° and 51°. These results showed that the Cu O signals were apparent in the samples after 2θ angles of 44˝ and 51˝ . These results showed that the2 Cu2 O signals were apparent in the samples the chemical oxidation and sintering processes, thus confirming the conversion of the electrodes to after the chemical oxidation and sintering processes, thus confirming the conversion of the electrodes

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˝ the (111) lattice plane exhibited the highest peak CuCu For the Cu samples sintered atat600 600 °C, 2O. 2O toCu Forthe theCu Cu samplessintered sinteredat 600°C, C,the the(111) (111)lattice latticeplane planeexhibited exhibitedthe thehighest highestpeak peak 2 O. 2 Osamples For 2O. 2O intensity,indicating indicatingthe theoptimal optimalcrystal crystal structure structure of of the the Cu Cu22O. O. The The results results also showed that the Cu peak intensity, results also also showed showed that thatthe theCu Cupeak peak intensity, indicating the optimal crystal structure ˝ increased with the sintering temperature from 400 to 700 °C, ˝ intensity at 44° which was attributed toto intensity 400 to to 700 700 °C, C, which which was wasattributed attributedto intensityatat44 44°increased increased with with the the sintering sintering temperature from 400 thegradual gradualloss lossof ofoxygen oxygenatoms atoms in in the the Cu Cu222O O at at high high sintering sintering temperatures. temperatures. The loss of oxygen atoms the temperatures. The Theloss lossof ofoxygen oxygenatoms atoms the gradual loss of oxygen atoms in the Cu reducedthe theoxygen oxygenratios ratiosand andincreased increased the the Cu Cu ratios ratios of of the the Cu Cu222O O electrodes. electrodes. reduced electrodes. reduced the oxygen ratios and increased the Cu ratios

o

Intensity Intensity(a.u.) (a.u.)

400oC 400oC 500oC 500oC 600 C 600ooC 700oC 700 C

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300 300

400 400

500 500

600 600 −1 -1

Raman Shift Shift (cm (cm−1-1)) Raman

700 700

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Figure electrodes sintered sintered at at various various temperatures. temperatures. Figure4.4. 4.Raman Raman spectra spectra of of the the Cu Cu222O O counter counter electrodes electrodes Figure Raman spectra of the Cu O counter sintered at various temperatures.

Figure 5. The X-ray diffraction (XRD) analytic results of the Cu2 O counter electrodes Figure 5. 5. The The X-ray X-ray diffraction diffraction (XRD) analytic results of the Cu2O counter electrodes Figure sintered at various temperatures. (XRD) analytic results of the Cu2O counter electrodes sintered at at various various temperatures. temperatures. sintered ˝ Figure 600 and and 700 700°C. C.Because Because O samples at 400, 400, 500, 500, 600 600 and 700 °C. Because Figure666shows showsthe theXPS XPSresults results for for the the Cu Cu222O O samples sintered sintered at Figure shows the XPS results for the Cu the electrode substrates werewere Cu foils, the aqueous chemical oxidation process initially theCuCu Cu electrode substrates Cu foils, the aqueous chemical oxidation process converted initially 2O 2O the 2O electrode substrates were Cu foils, the aqueous chemical oxidation process initially converted the Cu Cu to to Cu(OH) When the samples samples were sintered sintered in aa nitrogen nitrogen atmosphere, the Cu(OH) Cu(OH) the Cu to Cu(OH) When 22the samples were sintered in a nitrogen atmosphere, the Cu(OH) 2 2 . Cu(OH) 2 was converted the .. When the were in atmosphere, the 2 was dehydrated dehydrated to produce produce CuO, which which was subsequently subsequently deoxidized to The Cu22O. O. The XPS revealed results dehydrated to produce CuO, which was subsequently deoxidized to Cu2 O. XPS results was to CuO, was deoxidized to Cu The XPS results revealed Cu 2p 2pat signal eV at 932.2 932.2 eV and Cu 2p1/2 signal ateV, 952.1 eV, corresponding corresponding to an an oxidation oxidation 3/2 signal 1/2 arevealed Cu 2p3/2aasignal and aeV Cuand 2p1/2 signal atsignal 952.1at corresponding to an oxidation state of Cu at aa Cu 2p 952.1 eV, to 3/2 932.2 stateThe of XPS +1. The The XPS show spectra show that the intensities intensities of 2p the3/2Cu Cu 2pCu Cusignals 2p1/2 signals were similar similar 3/2 and 1/2 signals +1. spectra that the that intensities of the Cu and 2p1/2Cu were similar for the state of +1. XPS spectra show the of the 2p 2p were 3/2 and for the the sintered samples atsintered sintered at 400, 400, 500˝ C, andindicating 600 °C, °C, that indicating that these these samples were dehydrated, dehydrated, samples 400, 500 and 600 these samples were dehydrated, deoxidized, for samples at 500 and 600 indicating that samples were deoxidized, and converted from Cu(OH) to Cu O through forming the intermediate product CuO. 2 2 and convertedand from Cu(OH)2from to CuCu(OH) the intermediate product CuO. However, the deoxidized, converted Cu2O through forming the intermediate product for CuO. 2 to forming 2 O through ˝ However, for the samples sintered at 700 °C, the Cu 2p and Cu 2p signal intensities decreased, 3/2 1/2 samples sintered 700 C, sintered the Cu 2pat3/2700 and°C, Cu the 2p1/2Cusignal which wasdecreased, due to the However, for theat samples 2p3/2 intensities and Cu 2pdecreased, 1/2 signal intensities which was was due to the the gradual gradual loss ofCu oxygen atoms in the the Cu Cutemperatures. 2O at high sintering temperatures. gradual lossdue of oxygen atoms loss in the high sintering which to of oxygen in 2O at high sintering temperatures. 2 O at atoms Figure777shows showsthe thecyclic cyclicvoltammograms voltammograms (CV) (CV) of of the the III33´−−/I /I−−´ redox couple for various Cu OOCEs. CEs. Figure shows the cyclic voltammograms (CV) of the Figure redox couple couple for for various various Cu Cu222O CEs. 3 /I redox The results obtained with the CV showed that redox reactions occurred in the voltage ranges of −0.05 Theresults resultsobtained obtainedwith withthe theCV CVshowed showedthat thatredox redoxreactions reactionsoccurred occurredininthethevoltage voltage ranges −0.05to The ranges ofof ´0.05 +0.05 V (oxidation) and ´0.15 to ´0.25 V (reduction). In the cyclic voltammograms, the cathodic

Materials 2015, 2015, 88 Materials 99 Materials 2015, 8 5723 to +0.05 +0.05 V V (oxidation) (oxidation) and and −0.15 −0.15 to to −0.25 −0.25 V V (reduction). (reduction). In In the the cyclic cyclic voltammograms, voltammograms, the the cathodic cathodic to − current peaks peaks correspond correspond to to the the reduction reduction of of III333´− ions ions through through interaction interaction with with the the CE. CE. In In general, general, current to the reduction of the magnitude magnitude of of the cathodic current peak represents the catalytic capability and/or thethe surface area of the of the the cathodic cathodiccurrent currentpeak peakrepresents representsthe thecatalytic catalyticcapability capabilityand/or and/or surface area the surface area of − CE toward reduction ofofII33I−3 ´in ininDSSCs. DSSCs. The cathodic current peaks ofofthe the Cu OO CEs CEs were in the of a CE towardreduction reductionof DSSCs.The Thecathodic cathodiccurrent currentpeaks peaksof the Cu Cu222O CEs were were in in the aa CE toward the ˝ descending order orderofof of600, 600,500, 500, 400 and 700 °C, indicating indicating thatcatalytic the catalytic catalytic capability and/or the descending order 600, 500, °C, that the capability and/or the descending 400400 andand 700700 C, indicating that the capability and/or the surface surface area of the Cu Cu O CEs CEs were were altered bytemperatures. the sintering sinteringAmong temperatures. Among the four CEs, surface area the by the temperatures. four CEs, area of the Cu2of O CEs were by thealtered sintering the four Among CEs, the the Cu2 O sintered 22Oaltered the600 Cu22˝O O sintered atthe 600largest °C exhibited exhibited the largest cathodic current the peak. By contrast, the400 Cuand O sintered the Cu 600 °C largest cathodic current peak. the Cu at C sintered exhibitedat cathodicthe current peak. By contrast, CuBy sintered at 700 ˝ C 22O sintered 2 O contrast, at 400 400 and and 700 °C °Cpeak revealed reduced peakIncurrent current densities. In this this current study, the the cathodic current peaks of at 700 revealed reduced peak densities. In study, cathodic peaks of revealed reduced current densities. this study, the cathodic peaks of the current Cu2 O CEs were the Cu Cu22to O both CEs were were related related to both both catalytic property and surface area ofthe theCu CuO CEs since since the the Cu Cu22O O 2O CEs the O CEs to and the Cu related catalytic property andcatalytic surface property area of the Cusurface sinceof 2 O CEsarea 2 2Onano/microstructures nano/microstructures exhibited different different morphology as sintering at at various various temperatures. temperatures. nano/microstructures exhibited morphology sintering exhibited different morphology as sintering at various as temperatures. o

Intensity Intensity(a.u.) (a.u.)

400oC C 400 o oC 500 500oC 600oC C 600 o oC 700 700 C

930 930

940 940

950 950

Binding Energy Energy (eV) (eV) Binding

960 960

22

Current CurrentDensity Density(mA/cm (mA/cm))

Figure 6. X-ray X-rayphotoelectron photoelectronspectroscopy spectroscopy(XPS) (XPS) analytic results of the 2 O counter Figure 6. 6. analytic results of the the Cu22Cu O counter counter Figure X-ray photoelectron spectroscopy (XPS) analytic results of Cu O electrodes electrodes sintered sintered at at various various temperatures. temperatures. electrodes sintered at various temperatures. 0.8 0.8

o

400oC C 400 o 500oC C 500 o oC 600 600oC 700oC C 700

0.4 0.4 0.0 0.0

-0.4 -0.4 -0.8 -0.8

-0.4 -0.4

-0.3 -0.3

-0.2 -0.2

-0.1 -0.1

0.0 0.0

+ +

Voltage vs. vs. Ag/Ag Ag/Ag Voltage

0.1 0.1

Figure 7. The cyclic voltammograms (CV) of the Cu2 O counter electrodes sintered at Figure 7. 7. The The cyclic cyclic voltammograms voltammograms (CV) (CV) of of the the Cu Cu22O O counter counter electrodes electrodes sintered sintered at at Figure various temperatures. various temperatures. temperatures. various Furthermore, properties of the of Cu O fromobtained the electrochemical O obtained CEs were from the Furthermore, the theelectrochemical electrochemical properties theCEs Cuwere Furthermore, the electrochemical properties of 2 the Cu 22O CEs were obtained from the impedance spectroscopy (EIS) of symmetrical structures. identical Cuidentical were 2 O CEs Cu electrochemical impedance spectroscopy (EIS) of ofdevice symmetrical deviceTwo structures. Two O electrochemical impedance spectroscopy (EIS) symmetrical device structures. Two identical Cu22O assembled with a sealant spacer, and the electrolyte was the same as used for a DSSC in this research. CEs were were assembled assembled with with aa sealant sealant spacer, spacer, and and the the electrolyte electrolyte was was the the same same as as used used for for aa DSSC DSSC in in this this CEs Figure 8 shows the impedance spectra of symmetrical cells fabricated using the Cu O CEs sintered 2 the Cu2O CEs research. Figure Figure 88 shows shows the the impedance impedance spectra spectra of of symmetrical symmetrical cells cells fabricated fabricated using using research. the Cu2O CEs at various temperatures. The larger semicircle in theinhigher frequency rangerange corresponded to the sintered at various temperatures. The larger semicircle the higher frequency corresponded to sintered at various temperatures. The larger semicircle in the higher frequency range corresponded to charge-transfer resistance at the at interface; the smaller in the lower the charge-transfer charge-transfer resistance atCE/electrolyte the CE/electrolyte CE/electrolyte interface; the semicircle smaller semicircle semicircle in frequency the lower lower the resistance the interface; the smaller in the range was associated with ion diffusion resistance in the electrolyte. Among the four Cu2 O CEs, the

Materials 2015, 8 Materials 2015, 8

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frequency range was associated with ion diffusion resistance in the electrolyte. Among the four Cu2O ˝ Cu sintered at 600 C exhibited theexhibited smallest the charge-transfer resistance at the CE/electrolyte 2 O CEs CEs, the Cu sintered at 600 °C smallest charge-transfer resistance at the 2O CEs ˝ interface. By contrast, at2O 700 C showed resistance at 2 O sintered CE/electrolyte interface.the ByCu contrast, the Cu sintered at 700the °C largest showedcharge-transfer the largest charge-transfer the CE/electrolyte interface. resistance at the CE/electrolyte interface. 160

o

400 C o 500 C o 600 C o 700 C

- Z" (ohm)

120

80

40

0

0

50

100

150

200

250

Z' (ohm)

Figure 8. Electrochemical impedance spectra of symmetrical device structures fabricated Figure 8. Electrochemical impedance spectra of symmetrical device structures fabricated using usingthe theCu Cu2 O O CEs CEs sintered sintered at at various various temperatures. temperatures. 2

3.2.Dye-Sensitized Dye-Sensitized Solar Solar Cells Cells (DSSC) (DSSC) Characterization Characterization 3.2. Inthis thisstudy, study,Cu Cu22O O was was sintered sintered at at various various temperatures temperatures to In to fabricate fabricate coral-like coral-like Cu Cu22OOCEs CEsfor forDSSC DSSC devices. Figure Figure 9a 9a shows shows the the J–V J–V curves curves of devices. of the the DSSCs DSSCs fabricated fabricated using using various various Cu Cu22OO CEs. CEs. Table Table11 showsthe the measured measured J–V J–V characteristics characteristics of of the the DSSCs DSSCs with shows with Cu Cu22O O CEs CEs sintered sintered atat 400, 400, 500, 500,600 600and and ˝ 700 °C. SixDSSC DSSC devices devices were were prepared prepared and and tested tested for 700 C. Six for each each sintering sintering condition. condition. The Thedeviations deviationsofofthe the photovoltaic characteristics are also listed in Table 1. The corresponding Jsc values were 6.61, 8.64, photovoltaic characteristics are also listed in Table 1. The corresponding Jsc values were 6.61, 8.64, 2 2; the VOC values were 0.19, 0.68, 0.68 and 0.63 V; the FF values were 0.34, 11.35and and3.94 3.94mA/cm mA/cm 11.35 ; the VOC values were 0.19, 0.68, 0.68 and 0.63 V; the FF values were 0.34, 0.53, 0.53, 0.47 and 0.35; and theefficiencies device efficiencies were3.11%, 0.43%, 3.11%, results 0.47 and 0.35; and the device were 0.43%, 3.62% and3.62% 0.88%.and The0.88%. resultsThe showed that 2 showed thatwith the the DSSCs with thesintered Cu2O CEs sintered at 600 °Cthe exhibited (11.35 ˝ 2 mA/cm ) the DSSCs Cu2 O CEs at 600 C exhibited optimalthe Jscoptimal (11.35 Jsc mA/cm ) and power and power efficiency conversion(3.62%). efficiencyThe (3.62%). DSSCs with the Cu2O CEs sintered at 400 ˝and 700 °C conversion DSSCsThe with the Cu 2 O CEs sintered at 400 and 700 C exhibited exhibited comparatively lowerand Jsc power valuesconversion and powerefficiencies. conversion The efficiencies. Thewere J–Vconsistent results were comparatively lower Jsc values J–V results with consistent with the electrode analysis and suggested that the Cu2O CEs fabricated at different sintering the electrode analysis and suggested that the Cu2 O CEs fabricated at different sintering temperatures may temperatures may affect the electrode properties and DSSC characteristics. Figure 9b shows the IPCE affect the electrode properties and DSSC characteristics. Figure 9b shows the IPCE results for the DSSCs results for the DSSCs with various Cu2O CEs. In the figure, the IPCE wavelength peak values of the with various Cu2 O CEs. In the figure, the IPCE wavelength peak values of the DSSCs are approximately DSSCs are approximately 530 nm, corresponding to the N719 dye. The results also showed that the 530 nm, corresponding to the N719 dye. The results also showed that the IPCE peak values of the IPCE peak values of the DSSCs with CEs sintered at 400, 500, 600 and 700 °C are 40.5%, 55.6%, DSSCs with CEs sintered at 400, 500, 600 and 700 ˝ C are 40.5%, 55.6%, 65.6% and 22.1%, respectively. 65.6% and 22.1%, respectively. The sintering temperature for fabricating these devices can be The sintering temperature for fabricating these devices can be arranged in the following descending order arranged in the following descending order according to the IPCE peak values: 600, 500, 400 and according to the IPCE peak values: 600, 500, 400 and 700˝ C. The IPCE results were consistent with the 700°C. The IPCE results were consistent with the Jsc values derived from the J–V measurements. Jsc values derived from the J–V measurements. Figure 10 shows the EIS Nyquist plots for the various DSSC devices. EIS is a useful tool for Figure 10 shows the EIS Nyquist plots for the various DSSC devices. EIS is a useful tool for characterizing critical interfacial charge-transfer processes in DSSCs such as the electron characterizing interfacialatcharge-transfer processes in interface, DSSCs such as the electron transfer/chargecritical recombination the TiO2/dye/electrolyte electron transporttransfer/charge in the TiO2 recombination at the TiO /dye/electrolyte interface, electron transport in the TiO electrode, electron − 2 2 electrode, electron transfer at the CE, and I3 transport in the electrolyte. For the investigated frequency ´ transfer at the CE, and I3 transport in the electrolyte. For the investigated frequency range (0.1 Hz to 1 MHz), three regions were generally distinguished: a small semicircle in the lowest frequency

Materials 2015, 8

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Materials 2015, 8 5725 range (0.1 Hz to 1 MHz), three regions were generally distinguished: a small semicircle in the lowest frequency range (approximately 0.1 Hz to 1 Hz), a larger semicircle in the middle frequency range range (approximately 0.1 Hz to 1 Hz), a larger semicircle in the middle frequency range (approximately (approximately 1 Hz to 1 kHz), and a smaller semicircle in the highest frequency range (>1 kHz) [38]. 1 Hz to 1 kHz), and a smaller semicircle in the highest frequency range (>1 kHz) [38]. With the bias With the bias illumination and voltage applied, the small semicircle at the lowest frequencies was illumination and voltage applied, the small semicircle at the lowest frequencies was associated with associated with ion diffusion in the electrolyte; the larger semicircle at the middle frequencies ion diffusion in the electrolyte; the larger semicircle at the middle frequencies corresponded to the corresponded to the charge-transfer processes at the TiO2/dye/electrolyte interface; and the smaller charge-transfer processes at the TiO2 /dye/electrolyte interface; and the smaller semicircle at the highest semicircle at the highest frequencies corresponded to the charge-transfer processes at the frequencies corresponded to the charge-transfer processes at the CE/electrolyte interface [39]. In this CE/electrolyte interface [39]. In this study, the TiO2/dye/electrolyte interface impedance was study, the TiO2high; /dye/electrolyte impedance high; therefore, impedances of considerably therefore, interface the impedances of was the considerably CE/electrolyte interface, TiOthe 2/dye/electrolyte the CE/electrolyte interface,were TiO2overlapped. /dye/electrolyte and electrolyte were overlapped. The results interface, and electrolyte The interface, results showed that the DSSCs with the Cu 2O CEs ˝ showed the °C DSSCs with the the lowest Cu2 O CEs sintered at 600 C exhibited850 theΩ), lowest impedance value sinteredthat at 600 exhibited impedance value (approximately indicating that these (approximately 850optimal Ω), indicating that these CEs possessed the characteristic optimal number of CEs photogenerated CEs possessed the number of photogenerated carriers; this of the shows that ˝ carriers; this characteristic of the CEs shows that the Cu O CEs sintered at 600 C exhibited the optimal the Cu2O CEs sintered at 600 °C exhibited the optimal2 catalytic activity and material properties. The ˝ catalytic activity and2Omaterial The DSSCs with impedance the Cu2 O sintered at 700 C showed the sinteredproperties. at 700 °C showed the highest value, which resulted from the DSSCs with the Cu highest value, which resulted from the oxygen atoms, inferior loss of impedance oxygen atoms, inferior crystal structure, andloss lowof electrocatalytic activity of crystal the Cu2structure, O sinteredand at low electrocatalytic activity of the Cu O sintered at high temperatures. The EIS results were in agreement 2 high temperatures. The EIS results were in agreement with the short-circuit currents and overall power with the short-circuit currents overall power conversion efficiencies of the DSSCs. conversion efficiencies of the and DSSCs.

(a)

(b)

Figure 9. The (a) current density–voltage (J-V) and (b) incident monochromatic Figure 9. The (a) density–voltage (J-V) and (b) incident monochromatic photon-to-current photon-to-current conversion efficiency (IPCE) curves of DSSCs using various Cu2 O conversion efficiency (IPCE) curves of DSSCs using various Cu2O counter electrodes. counter electrodes. Table 1. The characteristics of device short-circuit current density (Jsc), open-circuit voltage Table The characteristics of device short-circuit current density open-circuit (Voc),1.fill factor (FF), and device efficiency based on various Cu2O(Jsc), counter electrodes.voltage (Voc), fill factor (FF), and device efficiency based on various Cu2 O counter electrodes. Cu2O Counter (°C) CuElectrode 2 O Counter 400(˝ C) Electrode 500 400 600 500 700

Jsc (mA/cm2) 2) Jsc (mA/cm 6.61 ± 0.24 8.64 ± 0.22 6.6111.35 ˘ 0.24 ± 0.25 8.643.94 ˘ 0.22 ± 0.28

Voc (V) Voc (V) 0.19 ± 0.02 0.68 ± 0.01 0.190.68 ˘ 0.02 ± 0.01 0.680.63 ˘ 0.01 ± 0.02

600

11.35 ˘ 0.25

0.68 ˘ 0.01

Fill 0.34Factor ± 0.03 0.53 ± 0.02 0.34 0.47˘±0.03 0.02 0.53 0.35˘±0.02 0.03 0.47 ˘ 0.02

700

3.94 ˘ 0.28

0.63 ˘ 0.02

0.35 ˘ 0.03

Fill Factor

Efficiency (%) Efficiency 0.43 ± 0.10(%) 3.11 ± 0.25 0.43±˘0.31 0.10 3.62

3.11±˘0.16 0.25 0.88 3.62 ˘ 0.31 0.88 ˘ 0.16

Materials Materials 2015, 2015, 88

5726 12 700 400oC 500oC 600oC 700oC

- Z" (ohm)

600 500 400 300 200 100 0

0

200

400

600

800

1000

Z' (ohm)

Figure 10. The EIS Nyquist plots of DSSCs using various Cu2 O counter electrodes. Figure 10. The EIS Nyquist plots of DSSCs using various Cu2O counter electrodes. 4. Conclusions 4. Conclusions In study, we we used used aa chemical chemicaloxidation oxidationmethod methodtotofabricate fabricatecoral-like coral-like nano/microstructures In this this study, CuCu nano/microstructures on 2O2 O on for as useCEs as CEs in DSSCs. foil substrates were first immersed in an aqueous Cu Cu foilsfoils for use in DSSCs. The CuThe foilCu substrates were first immersed in an aqueous mixture ˝ mixture solution containing (NH ) S O and NaOH and allowed to react at 25 C for 30 min, which 4 2 NaOH 2 8 solution containing (NH4)2S2O8 and and allowed to react at 25 °C for 30 min, which resulted in resulted in their conversion to Cu(OH) nanostructures. Through nitrogen-sintering, the Cu(OH) was 2 2 their conversion to Cu(OH)2 nanostructures. Through nitrogen-sintering, the Cu(OH)2 was dehydrated dehydrated produce CuO,was which wasdeoxidized then deoxidized to form coral-like structures.The The sintering to produce toCuO, which then to form coral-like CuCu structures. sintering 2O2 O ˝ processes were conducted at 400, 500, 600 and 700 °C, C, and the Cu2 O CEs produced were individually characterized using SEM, EDS, XRD, Raman spectrometer, XPS, and CV to determine the influence of the sintering temperature temperature on on the the material material properties properties of of the the CEs. CEs. The Cu2 O CEs were subsequently employed totofabricate andand the the properties of theofDSSCs with the various Cu2 O CEs analyzed were fabricateDSSCs, DSSCs, properties the DSSCs with the various Cuwere 2O CEs to determine the J–V, IPCE, andIPCE, EIS characteristics. The resultsThe showed the DSSCs with the Cu analyzed to determine the J–V, and EIS characteristics. resultsthat showed that the DSSCs with 2O ˝ CEs sintered at 600 C exhibited electrode properties and device efficiency Because the Cu sintered at 600 the °C optimal exhibited the optimal electrode properties and (3.62%). device efficiency 2O CEs Cu foil substrates Cu2 O CEs from substrates canCu be employed to fabricate CEsfoil produced from foil substrates can be (3.62%). Becauseare Cuflexible, foil substrates are produced flexible, Cu 2O Cu flexible DSSCs. employed to fabricate flexible DSSCs. Acknowledgments The authors gratefully acknowledge financial financial support from Ministry of Science and Technology of Taiwan Taiwan (MOST (MOST 103-2221-E-259-007 103-2221-E-259-007 and and MOST MOST 104-2221-E-259-020). 104-2221-E-259-020). Author Contributions Contributions Chih-Hung Tsai generated ideas, ideas, designed experiments, experiments, analyzed results, supervised the entire research Po-Hsi Fei Fei and and Chih-Han Chih-Han Chen Chen performed research work, work, and and wrote wrote the the manuscript. manuscript. Po-Hsi performed the the experimental experimental work and and characterized characterized the the materials materials and and devices. devices. work

Conflicts of of Interest Interest Conflicts The authors authors declare declare no no conflict conflict of of interest. interest. The

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