Chromatic Titanium Photoanode for Dye-Sensitized Solar Cells under

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Chromatic Titanium Photoanode for DyeSensitized Solar Cells under Rear Illumination Chih-Hsiang Huang, Yu-Wen Chen, and Chih-Ming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18351 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Chromatic Titanium Photoanode for Dye-Sensitized Solar Cells under Rear Illumination

Chih-Hsiang Huang, Yu-Wen Chen, and Chih-Ming Chen*

Department of Chemical Engineering, National Chung Hsing University, 145 Xingda Rd., South Dist., Taichung 402, Taiwan.

Corresponding Author: * Fax: +886-4-22854734; Tel: +886-4-22840510-511; E-mail: [email protected]

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ABSTRACT Titanium (Ti) has high potential in many practical applications such as biomedicine, architecture, aviation, and energy. In this study, we demonstrate an innovative application of dye-sensitized solar cell (DSSC) based on Ti photoanode that can be integrated into the roof engineering of large-scale architectures. A chromatic Ti foil produced by anodizing oxidation (coloring) technology (AOT) is an attractive roof material for large-scale architecture, showing a colorful appearance due to the formation of a reflective TiO2 thin layer on both surfaces of Ti. The DSSC is fabricated on the back side of the chromatic Ti foil by using the Ti foil as the working electrode, and this roof-DSSC hybrid configuration can be designed as an energy harvesting device for indoor artificial lighting. Our results show that the facet-textured TiO2 layer on the chromatic Ti foil not only improves the optical reflectance for better light utilization but also effectively suppresses the charge recombination for better electron collection. The power conversion efficiency of the roof-DSSC hybrid system is improved by 30-40 % with a main contribution from an improvement of short-circuit current density under standard one sun and dim-light (600-1000 lux) illumination. Keywords: electrodes; solar cells; microstructures; surface modification; titanium dioxide 2 ACS Paragon Plus Environment

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1. INTRODUCTION As a global issue, development of renewable energy has been attracting considerable attention in the past decades. Dye-sensitized solar cell (DSSC) is one of the most promising candidates as the source of renewable energy that can convert abundant solar energy to usable electricity.1-3 DSSC is assembled by sandwiching an electrolyte system in between a photoanode and a counter electrode.4-7 A conventional photoanode is composed of a TiO2 mesoporous film coated on a transparent conducting oxide (TCO) glass substrate, where the TiO2 mesoporous film adsorbs sensitizing dye that can generate electrons while exposing to light.8 The use of glass substrate ensures high light harvesting of dye molecules due to high optical transmittance of glass.9 In addition to glass, metal is also a promising candidate as the photoanode substrate due to several specific advantages.10-12 Firstly, metal has a high melting point as glass does, capable of sustaining the high-temperature sintering process for the construction of a TiO2 network film. Secondly, metal has good flexibility as its thickness is tens of micrometers, which is helpful in developing flexible DSSCs for broader applications. Efforts have been made on many types of metals including Ti,10,13-19 W,10,20 stainless steel,10,11,21 and Zn,10 where Ti is the most potential candidate due to its high 3 ACS Paragon Plus Environment


homogeneous compatibility with the TiO2 network film and superior resistance against corrosive electrolyte. Metal is opaque, so the resultant DSSC is illuminated from the transparent counter electrode side.10,14,22 However, rear illumination inevitably lowers light harvesting of sensitizing dye because the iodine-based electrolyte system intercepts part of the incident light. To compensate the energy loss due to rear illumination, efforts have been made in promoting the performance of the Ti substrate to make it not merely a supporting substrate. Surface modification of Ti is a useful approach which can change the microscopic texture of the Ti surface and improve the interfacial electron transport property.13,15-18,23-25 Tsai et al.13 used a pre-treatment of H2O2 to perform surface modification and obtained a porous TiO2 nanostructure composed of free-standing nanosheets on the Ti surface. The surface area of the Ti substrate was remarkably increased due to the formation of a porous TiO2 nanostructure, effectively facilitating the electron transfer and collection. Yun et al.15 reported that acid treatment of the Ti substrate with aqueous HNO3-HF removed the outermost fine-grained disordered oxidized layer, producing a high degree of crystallinity in the oxidized layer and therefore improving the optical reflectance, electron recombination, and electrochemical impedance. Rui et al.18 combined the above-mentioned two treatments (HNO3-HF and 4 ACS Paragon Plus Environment

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H2O2 in sequence) to prepare a three-dimensional TiO2 network on a Ti foil for all-flexible DSSC. They found that the surface treatment increased the surface roughness of Ti which enhanced interfacial electrical contact and mechanical stability. An et al.16 also found that the formation of a TiO2 nanoforest structure on the Ti surface, produced by a three-step chemical treatment, was beneficial for electron transport and collection. In this study, surface modification of Ti was performed using a unique anodizing oxidation (coloring) technology (AOT).26-30 The AOT treatment produced a uniform, dense Ti oxide layer with a faceted microscopic texture on the Ti surface, significantly improving the optical reflection and electron collection of the Ti-based photoanode. The power conversion efficiency (PCE) of DSSC was therefore improved with an improvement in the short-circuit current density (JSC) and fill factor (FF). More attractively, manipulating the AOT voltage changed the microscopic texture and thickness of the surface Ti oxide layer and affected its optical response to the incident light,26,29,31,32 resulting in an attractive appearance with a high color variety. The chromatic feature of AOT-treated Ti creates a potential application of Ti-based DSSC in the architectural roof engineering. Chromatic Ti foil has been applied in the roof engineering as an attractive roof material in large-scale architectures such as world-famous Marques de Riscal winery in 5 ACS Paragon Plus Environment


Spain33 to enhance its virtual effect. Inspired by this attractive application, we propose an innovative architectural/power integrated system by integrating chromatic Ti foil with a DSSC to form a roof-DSSC hybrid structure that can be applied in the roofing engineering of large-scale architectures such as gym (or museum, exhibition hall) as shown in Figure 1. The roof in Figure 1 is a Ti-based DSSC system which is made of a Ti foil on the topside, an electrolyte system in the middle, and a transparent counter electrode on the backside. In addition to serving as an architectural roofing material, the chromatic Ti foil simultaneously works as the working electrode of DSSC. The back side of the Ti foil, also with a chromatic Ti oxide layer on the surface, was assembled with a transparent electrode as the illumination side. It has demonstrated that DSSC performed better as compared with amorphous Si (a-Si) and organic photovoltaic (OPV) solar cells in a low intensity lighting environment due to its high harvesting capability to the diffuse light.34 Therefore, indoor lighting as a reusable power source is an emerging application for DSSCs35-38 and our innovative roof-DSSC hybrid system is a promising candidate. In addition to chromatic Ti foil, the use of Ti in the roofing application has another typical configuration, titanium composite material (TCM). The TCM panel is a tri-layer structure composed of a Ti sheet on the topside, a stainless steel sheet on the backside, and a non-combustible layer in between them. For the TCM application, the Ti sheet is 6 ACS Paragon Plus Environment

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free of the AOT treatment and shows its original metallic luster. Its function is only for mechanical and constructional purposes. In contrast, chromatic Ti foil (AOT-treated) used in the present study is used not only for constructional shielding but also for decorative purpose.

2. EXPERIMENTAL SECTION 2.1 Preparation of Chromatic Ti-based Photoanode The preparation of chromatic Ti-based photoanode includes the following steps. Firstly, a 50-µm-thick Ti foil (99.9 % purity, RBS-TS-L, TDP, Taiwan) was rinsed with 95 % ethanol (J.T. Baker) and then was cleaned in an ultrasonic bath containing deionized water for several times. After preliminary cleaning, the Ti foil was immersed in a 0.5 wt.% HF solution for 3 min to remove the native oxidized layer on the Ti surface. Secondly, the Ti foil was immersed in an anodizing bath containing a commercial electrolyte (Titan-color, Poligrat, Germany) and served as the anode. The anodizing oxidation process was performed at 30 οC with a constant rotating rate of 120 rpm. The AOT voltage was controlled at 5 to 20 V and the reaction duration was 5 min. After anodizing oxidation, the Ti foil was rinsed with deionized water and dried up using an air gun. Finally, a commercial TiO2 paste (Ti-Nanoxide T/SP, Solaronix SA, Switzerland) 7 ACS Paragon Plus Environment


was screen-printed39 on the AOT-treated Ti foil with a screen-printed area of 0.16 cm2. The TiO2-coated Ti foil was then put into an oven to perform the sintering process sequentially at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min. After sintering, the thickness of the TiO2 mesoporous film was measured to be 10±1 µm. 2.2 Preparation of Counter Electrode Indium tin oxide (ITO) glass (10 Ω/□, 1.1 mm thick, Gem Tech., Taiwan) was used as the substrate of counter electrode. The ITO glass was cleaned in an ultrasonic bath with 4 % glass cleaner (PK-LCG545, Parker, Japan) for 30 min and with deionized water for another 30 min. A polyvinylpyrrolidone (PVP)-capped Pt nanocluster layer was formed on the as-cleaned ITO glass substrate as the catalyst using a two-step dip-coating process mentioned below.40 The as-cleaned ITO glass substrate was firstly dipped in 2 % conditioner (ML371, OM Group, USA) at 70 °C for 5 min, and then was immersed in a PVP-capped Pt solution at 40 °C for 5 min. Finally, the ITO glass was rinsed with deionized water, dried in air, and then sintered in an oven at 325 °C for 30 min. 2.3 Assembly of DSSC The as-prepared TiO2/AOT-Ti photoanodes were dipping into a 0.42 mM N719 (D719, Everlight Chemical Industrial Co., Taiwan) dye solution (solvent: 99.9 % ethanol) 8 ACS Paragon Plus Environment

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for 4 h. After dye adsorption, the TiO2/AOT-Ti photoanode and the PVP-capped Pt counter electrode were assembled face-to-face and sealed with a 25-µm-thick Surlyn film using hot pressing at 115 °C for 10 s. A proper amount of liquid electrolyte (0.1 M LiI, 0.05 M I2, 0.2 M PMII, 0.5 M TBP and 0.2 M TBAI in acetonitrile/valeronitrile, volume ratio = 85:15) was injected into the sealed DSSC to fill the gap between two electrodes. A control DSSC was also fabricated using the above-mentioned process except the AOT treatment. To study the feasibility of chromatic Ti in the module applications, two DSSC modules were fabricated using FTO glass and ITO-coated polyethylene naphthalate (PEN) sheet, respectively, as the counter electrodes. 2.4 Characterization and Photovoltaic Performance Measurements The microscopic texture of AOT-treated Ti surface was characterized using a field-emission scanning electron microscopy (FE-SEM, JSM-7401F/6700F, JEOL, Japan). To analyze the depth of anodizing oxidation, the AOT-treated Ti substrate was characterized

using

a

x-ray

photoelectron

spectroscope

(XPS,

PHI

5000

VersaProbe/scanning ESCA microprobe, ULVAC-PHI). Atomic force microscopy (AFM, SPA400, Seiko, Japan) was used to image the surface morphology and measure the mean surface roughness (Ra) of AOT-treated Ti substrates. Phase identification of the AOT-treated Ti substrates was carried out using an in-plane grazing incidence x-ray 9 ACS Paragon Plus Environment


diffraction (GID-XRD, D8 DISCOVER, BRUKER). The optical reflection spectra of AOT-treated Ti substrates were measured using an integrating sphere equipped to a QE (quantum efficiency) measurement system (QE-3000, Titan Electro-Optics Co., Ltd., Taiwan). The AOT-treated Ti substrate was further cross-sectioned using a focused ion beam (FIB, JIB-4601F, JOEL, Japan) to prepare an ultrathin sample for observation using a field-emission transmission electron microscope (FE-TEM, JEM-2100F, JEOL, Japan). The as-assembled DSSCs were evaluated under AM1.5G (one sun, 100 mW/cm2) illumination with a solar simulator (YSS-E40, Yamashita Denso Corp, Japan). The photovoltaic performance evaluation of DSSCs under indoor illumination was also performed using a dim light simulator (YUYI-LLS01, Yu-Yi Enterprise, Taiwan) including a miniature spectrometer pre-configured for general UV-VIS measurements (USB2000+UV-VIS, Ocean Optics, Inc., USA). A mask with an aperture area of 0.35 cm × 0.35 cm (for cell) or 1 cm × 5 cm (for module) was covered on the illuminated side of DSSC to define the illumination area. The spectra of incident photo to electron conversion efficiency (IPCE) were measured using a solar cell QE/IPCE measurement system (QE-3000, Titan Electro-Optics Co., Ltd., Taiwan). The electron transfer in the DSSC was characterized based on the analysis of electrochemical impedance spectroscopy (EIS) performed using 10 ACS Paragon Plus Environment

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a potentiostat instrument (Autolab PGSTAT302N, Metrohm Autolab B.V., Netherlands) with a bias potential range from -0.45 V to -0.75 V in dark condition. The EIS setting contained AC amplitude of 10 mV with a frequency range of 100 kHz to 0.1 Hz, a two-electrode configuration for the DSSC, and the photoanode as a working electrode. Intensity-modulated

photocurrent

spectroscopy

(IMPS)

and

intensity-modulated

photovoltage spectroscopy (IMVS) measurements were carried out under a modulated green light-emitting diode (λ = 530 nm) driven by a frequency response analyzer (Autolab PGSTAT302N, Metrohm Autolab B.V., Netherlands). The light intensity (5×10-3 W cm−2) was modulated by varying the LED voltage sinusoidally in a frequency range of 1-105 Hz.

3. RESULTS AND DISCUSSION Figure 2 shows the SEM micrographs of the surface morphologies of the Ti foils before and after the AOT treatment. To take into consideration the sintering process used to form the mesoporous TiO2 film, the AOT-treated Ti foils were also placed in an oven to conduct the same sintering treatment. The surface of non-treated Ti foil (0 V) was roughly flat with some shallow ditches (Figure 2a). After surface treatment (AOT and sintering), the Ti foils showed very different surface morphologies as seen in Figure 11 ACS Paragon Plus Environment


2(b)-2(e). Faceted, sharp-edged grains were developed after the AOT treatment as a result of different etching rates of dissimilar crystal structures between neighboring Ti grains. A magnification in Figure 2(c-1) showed that these faceted grains contained many ledges as marked by the arrows. The faceted structure with many ledges increased the surface area of the Ti substrate which was beneficial for the electrical contact between the screen-printed TiO2 mesoporous film and the AOT-treated Ti substrate. The AFM measurement results (Figure S1 in the ESI) also showed that the mean surface roughness (Ra) increased significantly for the AOT-treated samples (5 V - 20 V: 115.8 nm - 151 nm) in comparison with that without the AOT treatment (0 V: 11.9 nm), showing that the AOT treatment increased the surface roughness of the Ti substrate by creating a faceted microstructure on the Ti substrate. The highest surface roughness (Ra = 151 nm) was obtained as the AOT voltage was 5 V but decreased with increasing the AOT voltage from 10 to 20 V. Additional SEM observation at higher magnifications was performed (Figure S2). It was found that the AOT treatment produced many voids on the Ti surface but their quantity and size depended upon the AOT voltage. When the voltage was lower than 20 V, the void formation was not obvious as seen in Figure S2a-S2d. With a much higher magnification, the voids can be roughly observed as marked by the arrows in Figure S2f-S2i. These voids are tens of nanometers. When the AOT voltage was as high 12 ACS Paragon Plus Environment

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as 80 V (Figure S2e-S2j), the void formation was clearly seen and the void size was much larger (hundreds of nanometers). The Ti surface became porous and was unfavorable for electron transport and collection, which led to a worse DSSC performance. Therefore, the focus was mainly placed on the lower AOT voltages from 5 V to 20 V. The insets in Figure 2 are the digital photos of the Ti foils, showing a high color variety in their appearances. For the non-treated Ti foil (0 V), a metallic luster was observed. However, the AOT-treated Ti foils showed orange-yellow (5 V), purple (10 V), blue (15 V), and light-blue (20 V) with the change of the AOT voltage. The high color variety was attributed to the formation of a faceted morphology (Figure 2) and nano-sized voids (Figure S2) on the Ti surface which affected the light reflection. The optical reflection of the Ti foils in the wavelength range of 300-800 nm was examined (Figure S3 in the ESI). The non-treated Ti foil showed a small reflection peak around 400 nm. After the AOT treatment, the optical reflection behavior of the Ti foils changed. For the AOT-treated Ti foils with a voltage of 5 V and 10 V, the optical reflection separated to the two ends of the wavelength range. However, the optical reflection still concentrated around the 400-450 nm wavelength for the AOT-treated Ti foils with a voltage of 15 V and 20 V. Overall, the AOT treatment enhanced the optical reflection of the Ti foils. 13 ACS Paragon Plus Environment


The Ti foils were also examined using XPS. As shown in Figure 3, only the Ti peaks were observed on the Ti surface without any surface treatment (0 V). The oxygen signal was detected at the binding energy of around 530.9 eV for the Ti-O bond (as pointed by the arrow) when the Ti foils underwent the surface treatment (5, 10, 15, and 20 V), revealing that some Ti oxides might form on the Ti surface after the AOT and sintering treatment. The XPS depth-profile analysis of the AOT-treated Ti foil (10 V) was performed and the results were shown in Figure 4. In Figure 4, the XPS measurement was confined within the binding energy range of 450-470 eV and 525-540 eV for the 2p orbit of Ti and 1s orbit of O, respectively. Two characteristic peaks of Ti were observed on the AOT-treated Ti foils (Figure 4a), while their binding energies shifted to lower values with increasing the etching depth. On the upmost surface, the two characteristic peaks located at 465 eV and 459 eV which corresponded to the 2p1/2 and 2p3/2 orbit of Ti4+, respectively. In Figure 4(b), one characteristic peak at the binding energy of 530.6 eV was observed and it corresponded to the 1s orbit of O. Based on the XPS examination, the oxide layer formed on the AOT-treated Ti foils was identified as the TiO2 phase. The phase identification of TiO2 on the other AOT-treated Ti foils with different voltages was also confirmed based on the XPS measurement (Figure S4 in the ESI).


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As noted in Figure 4(a), the binding energy shift of two Ti-based peaks implied a transition of electron configuration from Ti4+ to Ti with increasing the etching depth, which was used to determine the thickness of the TiO2 phase. According to the etching rate used in the XPS measurement, the thickness of the TiO2 phase layer formed on the AOT-treated Ti foil was estimated and the results were shown in Figure 5. As expected, the TiO2 thickness increased with increasing the operation voltage of AOT. The inset at the upper-left corner shows the cross-sectional TEM image of a Ti foil after the AOT treatment with a voltage of 10 V. A compact and dense layer was formed on the Ti surface and its thickness was about 40 nm which was consistent with the XPS measurement result. This surface layer exhibited an amorphous structure without any noticeable crystalline (lattice fringe) characteristics as seen in the inset at the lower-right corner. The EDX analysis indicated that the surface layer contained Ti and O with an atomic ratio close to 1 : 2 (Ti : O = 32 : 68 in at.%), confirming again the surface amorphous layer was the TiO2 phase. GID-XRD analysis was used to perform the phase identification of the TiO2 surface layer (Figure S5). It was found that only the Ti peaks were observed but no any crystalline phase (anatase and rutile) was formed on the AOT-treated Ti surface, which was consistent with the microstructural examination of TEM. 15 ACS Paragon Plus Environment


Figure 6(a) shows the J-V curves of the DSSCs based on different AOT-treated Ti photoanodes under standard one sun illumination. The corresponding photovoltaic parameters were deduced from the J-V curves and were tabulated in Table 1. The control DSSC based on non-treated Ti photoanode (0 V) exhibited a power conversion efficiency of 3.6 % with a short-circuit current density (JSC) of 6.163 mA/cm2, an open-circuit voltage (VOC) of 0.77 V, and a fill factor (FF) of 0.756, respectively. When the AOT-treated Ti photoanodes were used for DSSC, the efficiency was remarkably improved. The optimal operation voltage was 10 V and the resulting DSSC reached the highest efficiency of 4.93 % which was improved by 38 % in comparison with the control DSSC. Figure 6(b)-(d) show the variation of three important photovoltaic parameters, JSC, VOC, and FF, respectively, with the change of the AOT voltage. Compared with the superimposed efficiency data, the efficiency improvement was mainly attributed to the improved JSC because the variation of efficiency was in good agreement with that of JSC (Figure 6b). Similar results were obtained for the DSSCs under dim light (1000 and 600 lux) illumination (Figure S6, Table S1, and Table S2 in the ESI). The optimal AOT voltage was also 10 V and the resulting DSSCs exhibited the highest efficiency of 8.22 % and 5.3 % under 1000 and 600 lux illumination, respectively, and the main contribution also came from the improvement of JSC (Figure S7 in the ESI). Compared with the 16 ACS Paragon Plus Environment

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control DSSC, the efficiency of the DSSC employing the AOT-treated Ti foil (10 V) was improved by 42 % and 31 % under the illumination of 1000 and 600 lux, respectively. In general, JSC can be expressed as an integral function of IPCE:41,42

JSC = ‫ ׬‬q×Fሺλሻ×[1-rሺλሻ]×IPCEሺλሻdλ

(1)

where q is the electron charge, F(λ) is the incident photon flux density at wavelength λ, and r(λ) is the incident light loss due to absorption and reflection by TCO glass. Because q, F(λ), and r(λ) are constant for all DSSCs due to identical illumination condition and TCO glass, JSC is affected by IPCE alone. Figure 7 plots the IPCE spectra showing that the AOT treatment can improve the IPCE of DSSCs by around 10 % in the visible spectrum range. The best IPCE response was obtained at the AOT voltage of 10 V which was consistent with the efficiency and JSC performance shown in Figure 6. IPCE(λ) can be further expressed by the following equation:43,44

IPCEሺλሻ=LHEሺλሻ∙ϕinj∙ηcoll

(2)

where LHE(λ) is the light harvesting efficiency of dye molecules, ϕinj is the electron injection efficiency from the excited dye molecules to TiO2, and ηcoll is the charge collection efficiency of the injected electrons by the Ti substrate. The influence of ϕinj was negligible because all DSSCs used the same dye (N719). So, the improvement of IPCE for the DSSCs with the AOT-treated Ti photoanodes should come from LHE(λ) 17 ACS Paragon Plus Environment


and ηcoll. As shown in Figure 2, the AOT treatment increased the surface roughness of Ti by forming a faceted microscopic texture. The faceted microscopic texture increased the optical reflection of incident light (Figure S3 in the ESI) which might improve the light harvesting efficiency of dye molecules and therefore improved LHE(λ). In addition to LHE(λ), we still need to discuss the contribution of ηcoll. Before discussing ηcoll, impedance analysis of DSSCs based on different Ti photoanodes was performed in dark condition to gain an insight of the electron transport property inside the DSSC. The charge transfer resistance (Rct) on the photoanode can be determined by fitting the impedance spectra based on an equivalent circuit.45,46 The fitted Rct values of all DSSCs were plotted as a function of the applied bias in Figure 8. In dark condition, the applied bias drives electrons to move from the Ti substrate to the electrolyte which is in the direction opposite to the electron flow under light illumination. Basically, there are two routes for the movement of electrons from the Ti substrate to the electrolyte. For the first route, electrons pass through the TiO2 mesoporous film coated on the Ti substrate and go into the electrolyte by way of the TiO2/electrolyte interface. On this route, charge recombination occurs mainly at the TiO2/electrolyte interface. The second route is directly from uncovered Ti substrate to the electrolyte without passing through the TiO2 mesoporous film and, therefore, charge recombination occurs at the Ti/electrolyte 18 ACS Paragon Plus Environment

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interface. Herein, the discussion of electron transport should be focused on the second route because the AOT treatment alters the surface condition of the Ti substrate which may affect the electron transfer behavior at the Ti/electrolyte interface, or more precisely, the Ti/AOT-TiO2/electrolyte interface. To understand the electron transfer behavior at the Ti/electrolyte interface, the bias potential was set at a low and intermediate range (−0.45 V to −0.6 V) within which the TiO2 mesoporous film behaves as an electrical insulator and the second route becomes the primary one for the movement of electrons. In other words, the Rct values shown in Figure 8 correspond to the charge transfer resistances at the Ti/AOT-TiO2/electrolyte interface. It was found that the AOT treatment with a voltage of 10 V yielded the highest Rct value, meaning that the movement of electrons through the AOT-TiO2 layer produced by 10 V was the most difficult. In other words, charge recombination due to back flow of electrons from the Ti substrate to the electrolyte could be significantly suppressed on the AOT-treated Ti photoanode. The AOT-TiO2 layer acted as a blocking layer and effectively suppressed the back flow of electrons. In addition, the AOT-TiO2 layer offered a larger surface area due to its faceted microscopic texture (Figure 2) and high surface roughness (Figure S1 in the ESI) which could increase the electrical contact area between the TiO2 mesoporous film and the Ti substrate. The larger contact area was advantageous for the electron transfer from the 19 ACS Paragon Plus Environment


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TiO2 mesoporous film to the Ti substrate. The larger contact area and better blocking performance of the AOT-TiO2 layer yielded a synergistic effect and effectively improved the electron transfer on the photoanode, and therefore the electron collection efficiency (ηcoll) was improved as well as IPCE. IMPS/IMVS measurements were also performed to explore the ηcoll in the DSSCs. Based on the spectral response to the green light irradiation (Figure S8 in the ESI), the electron lifetime (τn) and transit time (τd) can be calculated by using the following equations.6,47,48 ߬௡ = ଶగ௙ ߬ௗ = ଶగ௙

ଵ

(3)

಺ಾೇೄ,೘೔೙

ଵ

(4)

಺ಾುೄ,೘೔೙

where fIMVS,min and fIMPS,min are the characteristic frequency minimum of the imaginary components of the semicircles in the spectra (Figure S8 in the ESI). The electron collection efficiency (ηcoll) can be further calculated as follows. ߟ௖௢௟௟ = 1 −

ఛ೏ ఛ೙

(5) Figure 9 summarizes the above three parameters relating to the electron transport property in the DSSCs, showing that the DSSC based on the AOT-treated Ti foil (10 V) had the highest ηcoll due to its highest τn and lowest τd (The calculated values of τn and τd 20 ACS Paragon Plus Environment

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were listed in Table S3 and S4, respectively, in the ESI). Therefore, it was confirmed that the optimal AOT voltage was 10 V and the resulting TiO2 surface layer was a superior blocking layer for effective suppression of charge recombination and efficient electron collection. The use of a thin TiO2 blocking layer on the photoanode has been confirmed to effectively improve the photovoltaic performance of DSSCs.49,50 Cameron et al.49 prepared a TiO2 blocking layer on fluorine-doped tin oxide (FTO) coated glass by spray pyrolysis. Alberti and co-workers50 fabricated a thin TiO2 blocking layer on a highly transparent Al-doped Zn oxide (AZO) substrate using reactive sputtering at temperature below 200 οC. Compared with the above published works, the TiO2 blocking layer fabricated in the present study was grown by direct surface oxidation of the base Ti foil using a solution-based method. This intrinsic growth behavior assures “seamless” connection between the TiO2 blocking layer and the base Ti foil, which can significantly improve the electron transport and collection on the photoanode. Two DSSC modules based on different counter electrodes, rigid FTO glass and flexible ITO-coated PEN sheet, were fabricated to study the feasibility of the chromatic Ti in the module applications. As shown in Table 2, the DSSC modules based on FTO glass and ITO/PEN sheet exhibited PCE of 4.37 % and 3.1 %, respectively, under one sun illumination. A higher PCE of 5 % to 8 % was even obtained under dim light 21 ACS Paragon Plus Environment


illumination (Table S5 in the ESI), showing that the chromatic Ti has high potential in practical module applications.

4. CONCLUSION A chromatic Ti foil produced by the AOT treatment was successfully used as an efficient photoanode substrate for DSSC. The AOT treatment produced a reflective and facet-textured TiO2 layer on both surfaces of the Ti foil. The faceted microscopic texture improved the light harvesting of dye molecules by improving the optical reflection of the Ti photoanode. Besides, its higher surface area also increased the electrical contact area between the TiO2 mesoporous film and the Ti substrate, improving the electron transfer and collection. The impedance analysis in dark condition showed that charge transfer resistance was increased at the Ti/electrolyte interface, implying that the AOT-TiO2 layer formed on the Ti surface effectively suppressed the back flow of electrons from the Ti substrate to the electrolyte and thereby improved the electron collection on the Ti photoanode. Benefitting from improved optical reflection and electron collection, the IPCE was improved for the DSSCs based on the AOT-treated Ti photoanodes in comparison with that based on non-treated Ti photoanode. The improvement of IPCE leaded to an improvement of short-circuit current density, and therefore the power 22 ACS Paragon Plus Environment

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conversion efficiency of DSSC was improved significantly. Considering that the chromatic Ti is a promising architectural (roof) material, we proposed an innovative application for DSSC by integrating the DSSC in the roof engineering. The DSSC-roof hybrid system is believed to be an efficient energy harvesting device for indoor dim-light applications.

ACKNOWLEDGMENTS

The authors thank the financial support of the Ministry of Science and Technology (MOST), Taiwan, R.O.C. through Grants 105-2119-M-005-001. The authors also thank the Green Energy and Environment Research Laboratories of Industrial Technology Research Institute (ITRI), Taiwan, R,O.C. for their help in module assembly.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxxxxxxxx. Photovoltaic parameters of DSSCs based on different AOT voltage under illuminance of 1000 and 600 lux; IMVS and IMPS results of DSSCs based on different AOT voltage; 23 ACS Paragon Plus Environment


AFM images, SEM images, and optical reflection spectra to characterize the surface morphology and optical performance of the AOT-treated Ti foils; XPS and XRD analysis to characterize the phase formation of the AOT-treated Ti foils; Variation of photovoltaic parameters of DSSCs as a function of the AOT voltage; Bode plots of DSSCs assembled with Ti photoanodes with and without the AOT treatment.


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TABLE CAPTIONS

Table 1 Photovoltaic parameters deduced from the J-V curves in Figure 6(a). The DSSCs based on different AOT-treated Ti photoanodes were measured under one sun illumination.

Table 2

Photovoltaic parameters of two DSSC modules under one sun illumination (deduced from Figure S9), where the aperture area of the mask used for the efficiency measurement is 1 cm × 5 cm. Both DSSC modules use chromatic Ti as the photoanode, but their counter electrodes are different, one is rigid FTO glass and the other is flexible PEN/ITO sheet.



FIGURE CAPTIONS

Figure 1

Schematic of an innovative application of DSSC in the roof engineering of large-scale architectures such as gym (or museum, exhibition hall) by using a chromatic Ti foil, where the chromatic Ti foil simultaneously works as the architectural roof material and the working electrode of DSSC. This innovative DSSC-roof hybrid configuration can be a promising energy harvesting device for indoor artificial lighting.

Figure 2

SEM micrographs of the surface morphologies of the Ti foils (a) before and (b-e) after the AOT and sintering treatment. The inset pictures show the colors of the Ti foils.

Figure 3

XPS full spectra of the Ti foils before and after the AOT and sintering treatment.

Figure 4

XPS depth-profile analysis of the AOT-treated Ti foil. The AOT voltage is 10 V. The XPS spectra in (a) and (b) are Ti and O, respectively.

Figure 5

Thickness of the TiO2 layer produced by the AOT treatment as a function of the AOT voltage. Insets are TEM (upper left) and high resolution TEM


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images (lower right) of the cross section of the AOT (10 V)-treated Ti substrate. Figure 6

(a) J-V curves of the DSSCs based on different AOT-treated Ti photoanodes under one sun illumination. (b)-(d) Variation of three main photovoltaic parameters, JSC, VOC, and FF, respectively, with change of the AOT voltage.

Figure 7

IPCE spectra of the DSSCs employing the Ti-based photoanodes: 0 V refers to a Ti foil without the AOT treatment, 5-20 V refer to the Ti foils with the AOT treatment of different operation voltages.

Figure 8

Fitted Rct values for the DSSCs employing the Ti-based photoanodes with and without the AOT treatment as a function of the applied bias in dark condition. The inset schematic shows an equivalent circuit of the DSSCs for the EIS fitting.

Figure 9

Collection of three characteristic parameters, τn, τd, and ηcoll, relating to the electron transport property in the DSSCs. The red circle symbol refers to τn, the blue triangle symbol refers to τd, and the green column refers to ηcoll.



Table 1

AOT

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Photovoltaic parameters deduced from the J-V curves in Figure 6(a). The DSSCs based on different AOT-treated Ti photoanodes were measured under one sun illumination.

JSC 2

VOC

Fill

Efficiency

Voltage

(mA/cm )

(V)

Factor

(%)

0V

6.163 ± 0.02

0.77 ± 0.01

0.756 ± 0.01

3.60 ± 0.03

5V

7.283 ± 0.29

0.76 ± 0.02

0.759 ± 0.01

4.20 ± 0.10

10 V

8.834 ± 0.30

0.76 ± 0.01

0.737 ± 0.02

4.97 ± 0.18

15 V

7.525 ± 0.06

0.76 ± 0.01

0.757 ± 0.01

4.32 ± 0.09

20 V

7.330 ± 0.34

0.75 ± 0.01

0.767 ± 0.01

4.21 ± 0.15

Table 2

Photovoltaic parameters of two DSSC modules under one sun illumination (deduced from Figure S9), where the aperture area of the mask used for the efficiency measurement is 1 cm × 5 cm. Both DSSC modules use chromatic Ti as the photoanode, but their counter electrodes are different, one is rigid FTO glass and the other is flexible PEN/ITO sheet. Counter

JSC 2

VOC

Fill

Efficiency

electrode

(mA/cm )

(V)

Factor

(%)

FTO glass

8.374

0.699

0.747

4.37

PEN/ITO

6.668

0.636

0.731

3.10


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Figure 1 Schematic of an innovative application of DSSC in the roof engineering of large-scale architectures such as gym (or museum, exhibition hall) by using a chromatic Ti foil, where the chromatic Ti foil simultaneously works as the architectural roof material and the working electrode of DSSC. This innovative DSSC-roof hybrid configuration can be a promising energy harvesting device for indoor artificial lighting.



Figure 2 SEM micrographs of the surface morphologies of the Ti foils (a) before and (b-e) after the AOT and sintering treatment. The inset pictures show the colors of the Ti foils.


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Figure 3 XPS full spectra of the Ti foils before and after the AOT and sintering treatment.



Figure 4 XPS depth-profile analysis of the AOT-treated Ti foil. The AOT voltage is 10 V. The XPS spectra in (a) and (b) are Ti and O, respectively.


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Figure 5 Thickness of the TiO2 layer produced by the AOT treatment as a function of the AOT voltage. Insets are TEM (upper left) and high resolution TEM images (lower right) of the cross section of the AOT (10 V)-treated Ti substrate.



Figure 6 (a) J-V curves of the DSSCs based on different AOT-treated Ti photoanodes under one sun illumination. (b)-(d) Variation of three main photovoltaic parameters, JSC, VOC, and FF, respectively, with change of the AOT voltage.


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Figure 7 IPCE spectra of the DSSCs employing the Ti-based photoanodes: 0 V refers to a Ti foil without the AOT treatment, 5-20 V refer to the Ti foils with the AOT treatment of different operation voltages.



Figure 8

Fitted Rct values for the DSSCs employing the Ti-based photoanodes with and without the AOT treatment as a function of the applied bias in dark condition. The inset schematic shows an equivalent circuit of the DSSCs for the EIS fitting.


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Figure 9 Collection of three characteristic parameters, τn, τd, and ηcoll, relating to the electron transport property in the DSSCs. The red circle symbol refers to τn, the blue triangle symbol refers to τd, and the green column refers to ηcoll.



For Table of Contents Only


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Chromatic Titanium Photoanode for Dye-Sensitized Solar Cells under

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