Cu2ZnSnS4 Nanoparticle Sensitized Metal ... - ACS Publications

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Cu2ZnSnS4 Nanoparticle Sensitized Metal−Organic Framework Derived Mesoporous TiO2 as Photoanodes for High-Performance Dye-Sensitized Solar Cells Rui Tang, Zhirun Xie, Shujie Zhou, Yanan Zhang, Zhimin Yuan, Luyuan Zhang,* and Longwei Yin* Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan 250061, People’s Republic of China S Supporting Information *

ABSTRACT: We present a facile hot injection and hydrothermal method to synthesize Cu2ZnSnS4 (CZTS) nanoparticles sensitized metal−organic frameworks (MOFs)-derived mesoporous TiO2. The MOFs-derived TiO2 inherits the large specific surface area and abundantly porous structures of the MOFs structure, which is of great benefit to effectively enhance the dye loading capacity, prolong the incident light traveling length by enhancing the multiple interparticle light-scattering process, and therefore improve the light absorption capacity. The sensitization of CZTS nanoparticles effectively enlarges the photoresponse range of TiO2 to the visible light region and facilitates photoinduced carrier transport. The formed heterostructure between CZTS nanoparticles and MOFs-derived TiO2 with matched band gap structure effectively suppresses the recombination rates of photogenerated electron/hole pairs and prolongs the lifespan of the carriers. Photoanodes based upon CZTS/MOFs-derived TiO2 photoanodes can achieve the maximal photocurrent of 17.27 mA cm−2 and photoelectric conversion performance of 8.10%, nearly 1.93 and 2.21 times higher than those of TiO2-based photoanode. The related mechanism and model are investigated. The strikingly improved photoelectric properties are ascribed to a synergistic action between the MOFs-derived TiO2 and the sensitization of CZTS nanoparticles. KEYWORDS: metal−organic framework, TiO2, Cu2ZnSnS4, heterostructure, dye-sensitized solar cells

1. INTRODUCTION

limited by rapid recombination rate of photogenerated carriers.6,7 To meet the urgent demand for high energy conversion efficiency solar cells, a variety of strategies have been taken to tune the band gap energy structure and optical response behavior of TiO2, such as doping with ions, coupling with other semiconductors, and employing the surface plasmon resonance (SPR) effect. Especially, great effort has been paid to investigate superior sensitizers.8−12 The ideal photosensitizer for DSSCs should be sensitive to the light in the full optical range and can efficiently inject the photogenerated electrons to TiO2.9−11 From this view, coupling with narrow band structure materials, such as CdS,13,14 CdTe,15 CuInS2,16 and PbS,17 can effectively

Dye-sensitized solar cells (DSSCs) have received lots of concern as one of the most promising power sources, because they can be inexpensive and easier to fabricate than traditional thin-film-based solar cells. TiO2, one of the most promising photoanode materials for DSSCs1−3 and has been mostly investigated because of its superior physiochemical property, photostability, and appropriate energy states structure.4 In addition, anatase TiO2 is traditionally considered to be a favorable photoanode material since it displays an inherently proper Fermi level compared to the other photoanode materials. However, there exist two major drawbacks for the performance improvement of DSSCs based on anatase TiO2 photoanode. First, the band gap of TiO2 is too large; only ultraviolet (UV) light could response to it.5 Furthermore, TiO2, as an intrinsic semiconductor with low quantum efficiencies, is © 2016 American Chemical Society

Received: May 25, 2016 Accepted: August 5, 2016 Published: August 5, 2016 22201

DOI: 10.1021/acsami.6b06183 ACS Appl. Mater. Interfaces 2016, 8, 22201−22212

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ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns of synthesized and simulated MIL-125 (Ti) and MOFs-derived TiO2. (b) XRD patterns of (1) MOFs-derived TiO2 and (2) CZTS and CZTS/TiO2 prepared under different concentrations of CZTS precursors: (3) 0.5, (4) 1, and (5) 2 M.

related mechanism of DSSCs based on CZTS nanoparticles sensitized MOFs-derived TiO2 photoanode. Herein, we for the first time design a facile hot injection and hydrothermal process to prepare Cu2ZnSnS4 nanoparticles (CZTS nanoparticles) sensitized MOFs-derived hierarchically porous TiO2 as photoanode for high-performance DSSCs. CZTS nanoparticles are uniformly grown on the outer surface of MOFs-derived TiO2. MOFs-derived TiO2 inherits the characteristics of hierarchically porous structure with high surface area. The porous structure with large specific surface area effectively ensures the dye loading capacity and compact contact between electrolyte and photoanode, while the mesoporous structure significantly enhances the incident light-scattering and the light-harvesting ability. The formation of heterostructures between CZTS nanoparticles and MOFsderived TiO2 with matched energy state structures can obviously separate the photogenerated exciton. The DSSCs based on CZTS nanoparticles/MOFs-derived TiO2 hybrid photoanode exhibit highly improved light-harvesting properties and enhanced photocurrent conversion efficiency, demonstrating an improved photocurrent density to 17.27 mA cm−2 and improved photoelectric conversion performance of up to 8.10%.

prolong the optical response range of TiO2 into the visible range and is considered to be promising substitution for organic dyes of DSSCs.18−21 Among various narrow band gap semiconductors, Cu2ZnSnS4 (CZTS), displays a direct band gap of Eg = 1.4−1.6 eV, an absorption coefficient of the order of 104 cm−1, and favorable charge transport property. Comparing with other narrow band structure materials, CZTS nanoparticles are free from highly toxic elements of cadmium and lead, and rare element of indium, and should be a particularly uniquely potential sensitizer for DSSCs.22−25 Thus, it is particularly interesting to couple TiO2 with CZTS nanoparticles to tune band gap and optical response property, and thus enhance the light harvesting.26,27 On the other hand, the photoelectrical conversion performance of DSSCs based on TiO2 photoanode could be greatly improved by enhancing the utilization of incident light.28,27 It is generally believed that prolonging the light traveling length is an effective approach to enhance the incident light-harvesting efficiency.29 Mesoporous structure has been considered to be effective for enhancing light-harvesting capacity due to the improved surface area and multiple interparticle scattering behavior, leading to an enhancement of the photoelectrical conversion efficiency.30,31 Promoted by the above benefits, extensive works have been carried out to introduce optical scatter points into the photoanode to increase the lightharvesting ability.32,33 It is reported that, by adapting hierarchically porous structures containing both mesopores and micorpores, the light-harvesting ability can be greatly enhanced.34,35 The hierarchically porous structures can effectively enhance both high sensitizer adsorption and good light-scattering ability. Metal−organic frameworks (MOFs)derived materials, with hierarchically porous structures and high surface area, are regarded as promising replacement materials for enhancing the optical response performance of DSSCs.36−39 For example, Wei’s group reported that the photoanode for DSSCs based on MOFs-derived ZnO parallelepipeds achieves an obviously enhanced power conversion efficiency.37−41 Up to now, the DSSCs based on narrow band gap semiconductor nanoparticles sensitized MOFs-derived porous photoanode are rarely reported. It is of great challenge and fundamental importance to systematically investigate the performance and

2. EXPERIMENTAL DETAILS 2.1. Preparation of Cu2ZnSnS4 (CZTS)/TiO2 Heterostructure. 2.1.1. Synthesis of MIL-125(Ti) and MOFs-Derived TiO2. According to the work Fu et al. reported before,38 anhydrous MeOH, terephthalic acid BDC, and titanium isopropoxide were dissolved in anhydrous N,N-dimethylmethanamide and transferred to an autoclave and heated at 150 °C for 1 day. After cooling to the room temperature, the products were rinsed with MeOH for several times. The obtained MOFs product was annealed to 470 °C for 3 h under air to remove the organic components. 2.1.2. Synthesis of CZTS Nanoparticles/MOFs-Derived TiO2 Heterostructure. CZTS nanoparticles (NPs) were obtained through a hot injection strategy.23 Typically, Cu(OAc)2, SnCl2·5H2O, and Zn(OAc)2, were dissolved in oleylamine (OLA). The mixture was then pumped under vacuum conditions for 30 min to remove moisture or O2. Subsequently, the mixture was purged with N2 gas and heated to 150 °C. The temperature was kept at 120 °C for half an hour to form stable Cu−, Zn−, and Sn−OLA coordinating complex or metal precursors. After that, the mixture was heated rapidly to 150 °C, where the injection of sulfur precursors took place. The sulfur precursor was 22202


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Figure 2. (a) Raman spectra of the small-wavenumber range and (b) the overall spectra of (1) pure CZTS, (2) 1CZTS/TiO2, and (3) MOFsderived TiO2.

Figure 3. Illustration of CZTS/TiO2 synthesis process. prepared by dissolving/mixing 1 mmol N-dodecyl mercaptan (NDM) with 1 mL of OLA at room temperature. The reaction temperature was kept at 250 °C for 1 h before cooling in air. The as-prepared CZTS NPs were washed with toluene and ethanol and then redispersed with n-hexane. The obtained TiO2 cubes were pretreated under mercaptopropionic acid solution at 50 °C overnight. The CZTS n-hexane solution was then added to the pretreated TiO2 to deposit CZTS nanoparticles onto the outer surface of MOF-derived TiO2. Corresponding to adjustable CZTS n-hexane solution concentrations of 0, 0.5, 1, and 2 M, the CZTS nanoparticles sensitized MOFs-derived TiO2 composites are denoted as samples of TiO2, 0.5CZTS/TiO2, 1CZTS/TiO2, and 2CZTS/TiO2, respectively. 2.2. Solar Cell Fabrication. According to the work our group reported before, the electrode paste was compose by solution A and solution B. Solution A was prepared by dissolving ethyl cellulose into ethanol to form a homogeneous solution. Solution B was prepared by dissolving obtained samples and terpineol into ethanol. Then solution A and solution B were mixed together and stirred for several hours. Finally, the paste was spin-coated onto the substrate. In order to remove the organic linkers, the obtained photoanode was annealed according to the annealing processes as we reported before.39,40 The obtained photoanodes were scratched into a square with an active area of 0.09 cm2 and put into a dye N719 and acetonitrile solution for 12 h. Subsequently, the photoanode and Pt counter electrode were assembled together and sealed with a sealing film. The electrolyte was injected through the counter electrode. The ingredient of electrolyte was similar to the work we reported before.39,40 2.3. Materials Characterization. The X-ray diffraction(XRD) spectrum was taken at 40 kV in a 2θ range of 10−90°. Raman spectra were carried out with JY HR800 equipment. Scanning electron microscopy (SEM) was obtained by an SU-70 equipped with an

energy-dispersive system (EDS), and transmission electron microscopy (TEM) was taken with a Tecnai 20U-Twin equipment. The UV−vis spectra were recorded on a TU-1900. The photoluminescence (PL) spectra were measured with a detection system of Hitachi U4100, and obtained photoanodes were directly applied to testify the PL spectra. Photocurrent density−voltage plots and incident photon to charge carrier efficiencies (IPCE) were recorded on Newport serial equipment. The electrochemical impedance spectra (EIS) were obtained in a Princeton 2273A electrochemical workstation in a standard two-electrode cell.

3. RESULTS AND DISCUSSION The phase components of the products are determined by XRD. Figure 1a confirms as-synthesized MIL-125(Ti) and MOFs-derived TiO2, showing a good agreement with that of simulated and standard results. The XRD pattern of obtained MOFs-derived TiO2 shows a typical anatase structure, corresponding to standard pattern of anatase TiO2 (PDF no. 211272).36 Figure 1b shows XRD patterns of TiO2, CZTS, and CZTS/TiO2 heterostructures with different CZTS NPs loading contents: 0.5, 1, and 2 M, respectively. The diffraction peaks at 25.3°, 48°, and 55° match well with the (101), (200), and (211) planes of anatase TiO2 (PDF no. 211272), respectively. The peaks at 30.3°, 39.0°, and 51.4° can be indexed to (002), (102), and (111) planes of CZTS, and these results have a good match with the structure of wurtzite CZTS reported before.36 Furthermore, with the content of CZTS NPs increasing, the diffraction peaks’ intensity at 30.3°, 39.0°, and 51.4° increases gradually. 22203


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Figure 4. Typical SEM images of (a and b) MIL-125(Ti), (c and d) MOFs-derived TiO2, and (e and f) 1CZTS/TiO2.

Figure 5. (a and b) Low-magnification TEM images of MOFs-derived TiO2. (c) Low-magnification TEM image of 1CZTS/TiO2 cube. (d and e) HRTEM lattice images of the 1CZTS/TiO2 heterostructure; the marked d-spacing of 0.24 and 0.33 nm is in agreement with that of the (004) plane of anatase TiO2 and the (100) plane of CZTS. (f) Selected area electron diffraction pattern from the 1CZTS/TiO2 heterostructure; the diffraction rings correspond to the (101), (004), (200), (211), and (004) planes of anatase TiO2, (001), (002), (111), and (200) planes of CZTS.

CZTS and CZTS/TiO2 samples, confirming the presence of CZTS in the CZTS/TiO2 sample,49 while such a peak around 326 cm−1 is not shown in the spectrum of TiO2. The formation process of CZTS NPs/MOFs-derived TiO2 is proposed and schematically illustrated in Figure 3. To transform MIL-125(Ti) precursor into mesoporous TiO2 without forming dead pores,50 the thermogravimetric analysis (TGA) is conducted, confirming that 480 °C is the suitable calcination temperature (Figure S1). After annealing treatment in air, the MOFs precursors transform to mesoporous TiO2 structure. After being pretreated with0.1 M 3-mercaptopropionic acid (MPA), the MOFs-derived TiO2 is then dispersed in the CZTS n-hexane solution. After the chemical deposition, CZTS nanoparticles are homogeneously grown on the surface of MOFs-derived TiO2.

As in case the byproducts such as ZnS, Cu2S, and Cu2SnS3 with similar crystal structures to CZTS might occur, Raman spectra are used to further confirm the formation of CZTS.23,41,42 From the Raman spectrum of pure CZTS in Figure 2b, it is shown that the characteristic peak of CZTS at 326 cm−1 matches well with the that of wurtzite CZTS’s lattice vibration;43,44 no other obvious Raman peaks corresponding to ZnS (278 and 351 cm−1),45,46 Cu2S (475 cm−1), or Cu2SnS3 (298 and 356 cm−1) can be seen.23,47,48 Hence, the Raman spectra confirm the high crystallinity of as-synthesized CZTS, and no byproducts mentioned above are observed. Also, in order to better investigate whether the CZTS nanoparticles are successfully grown on TiO2, the wavelength region of 300−370 cm−1 is replotted in Figure 2a. It is obviously shown that the Raman peaks at 326 cm−1 can be found in the Raman spectra of 22204


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Figure 6. Nitrogen adsorption−desorption isotherm at 77 K and DFT desorption branch pore size distribution of (a and b) MIL-125(Ti) and (c and d) MOFs-derived TiO2 and CZTS/TiO2 samples.

tion TEM image of 1CZTS/TiO2 heterostructure hybrid in Figure 5c demonstrates that CZTS NPs are homogeneously spread on TiO2. The high-resolution TEM (HRTEM) lattice images (Figure 5, parts d and e) of 1CZTS/TiO2 confirm that the diameter of the CZTS NPs is about 8 nm. The lattice spacing of 0.33 and 0.24 nm can be indexed to the (100) and (004) planes of wurtzite CZTS and TiO2. The selected area electron diffraction (SAED) pattern further confirms the components of the 1CZTS/TiO2 sample (Figure 5f). The diffraction rings show good matches with the (001), (002), (111), and (200) planes of CZTS, and (101), (004), (200), (211), and (004) planes of TiO2, respectively. To confirm the textural properties of the samples, the nitrogen adsorption−desorption isotherms and pore size distribution curves are tested and depicted in Figure 6, with the detail information listed in Table 1. For the MIL-125(Ti) sample, the nearly vertically raised isotherm at the low-pressure range (P/P0 ∼ 0) indicates that there exist a lot of micropores, and the pore size distribution curve shows the pore size is around 1 nm. For the MOFs-derived TiO2, it displays a type IV isotherm with type H3 hysteresis, indicating a large specific surface area. There exist two types of pores in the obtained TiO2. One type is micropores with narrow pore size of about

Figure 4 displays the SEM pictures of the obtained MOFs, MOFs-derived TiO2, and 1CZTS/TiO2 hybrid heterostructure, respectively. The MOFs crystal reveals a well-defined cubic morphology with a side length of 400 nm (Figure 4, parts a and b). After the MOFs precursor is annealed for 3 h, cubelike hierarchically porous TiO2 can be obtained (Figure 4, parts c and d). The SEM images in Figure 4, parts c and d, clearly show the porous structure characteristic of the MOFs-derived TiO2, indicating that the MOFs-derived products are composed of interconnected nanoparticles. After the chemical deposition process, CZTS nanoparticles are heterogeneously grown on TiO2 (Figure 4, parts e and f). The EDS shows that elements of Ti, O, S, Zn, Sn, and Cu exist in the obtained samples (Figure S2). EDS element mapping images in Figure S3 clearly suggest all elements are uniformly spread among the obtained sample, indicating the CZTS NPs are uniformly grown on the MOFsderived porous TiO2; the corresponding EDS is shown in Figure S3h. The microstructures of the TiO 2 and CZTS/TiO 2 hierarchical structures are discussed by TEM. Parts a and b of Figure 5 show the TEM images of MOFs-derived TiO2, confirming MOFs-derived TiO2 is composed of nanoparticles, displaying typically mesoporous channels. The low-magnifica22205


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the optical response range of intrinsic TiO2 can obviously extend to the visible region. Previous reports show that the light-harvesting ability and light response can be greatly enhanced by adopting hierarchically porous architectures, in terms of porous multiple interparticle scattering behavior and effectively enhancing light-harvesting capacity of mesoporous structure, and thus lead to an improvement of the photoelectrical conversion efficiency.33,53 Herein, MOFs-derived TiO2 displays a hierarchically porous structure and a large surface area. Both the SEM and TEM confirm the MOFs-derived TiO2 is composed of interconnected nanoparticles (Figure S4). Abundant interconnected channels, scattering points, and cavities exist in the framework of MOFs-derived TiO2. Compared to P25 sample, MOFs-derived TiO2 shows greatly enhanced light-harvesting property (Figure 7a). As the light incident is injected to the photonaode, the interparticle multiple scattering processes takes place within the hierarchically porous framework of MOFs-derived TiO2; the mesoporous channels and the scattering cavities can significantly increase the incident light trapping and enhance the photoelectrical conversion by prolonging the light transporting length (as shown in Figure S5). From this point of view, the mesoporous structure of the MOFs-derived TiO2 is of great help in improving the incident light-harvesting capacity and could effectively enhance the light utilization. Therefore, synthesizing functional materials from MOFs precursors provides us an efficient way to construct porous-rich photoanode materials, which would efficiently enhance the light-harvesting performance of the materials. To further confirm the N719 dye is successfully loaded onto the obtained photoanode film, we test the UV−vis absorption spectrum of the photoanode within and without dye adsorption (Figure S6). The N719 solution exhibits three characteristic peaks in the UV−vis range at 534, 381, and 313 nm.54,55 For pure TiO2, it shows a narrow absorption range, which decays quickly around 390 nm. After being sensitized with N719, an obvious broadened light-harvesting range can be observed, retaining well all three characteristic peaks of N719, confirming N719 dye is successfully loaded on the TiO2-based photoanode film. In addition, as for the CZTS/TiO2 photoanode, comparing with pure TiO2, the light-harvesting range is markedly extended to the near-infrared (NIR) region,

Table 1. Structural Parameters of the MIL-125(Ti), TiO2, and CZTS/TiO2 Samples samples

BET surface area (m2 g−1)

pore size (nm)

total pore volume (cm3 g−1)

MIL-125(Ti) TiO2 0.5CZTS/TiO2 1CZTS/TiO2 2CZTS/TiO2

1326.8 303.6 68.45 40.43 36.38

0.95 2.75 3.84 4.91 4.72

3.27 0.47 0.29 0.57 0.37

0.5 nm by the HK method; another type is mesopore with pore sizes of about 4 nm (Figure 6d) by the BJH method. The MOFs-derived TiO2 displays hierarchically porous structures and high surface area of 303.6 m2 g−1, which should be beneficial for the sensitizer adsorption and multiple incident light-scattering processes. Compared with pure TiO2, the specific surface area of CZTS/TiO2 undergoes a great decrease, and the micropores almost disappear after deposition of CZTS NPs. It is believed that porous structure with large surface area is helpful to the interfacial carrier transport and dye adsorption, and to light-harvesting ability.51 Figure 7a depicts the UV−vis diffuse reflectance spectroscopy (DRS) spectra of obtained MOFs-derived TiO2, CZTS/ TiO2, and CZTS, respectively. The absorption edge of pure MOFs-derived TiO2 less than 400 nm indicates a wide band gap semiconductor property and displays almost no optical response to the visible light. However, for the CZTS NPs/TiO2 samples, except the typical TiO2 absorption edge, another absorption edge occurs beyond 800 nm, which corresponds to the intrinsic absorption property of wurtzite CZTS, indicating that coupling of TiO2 with CZTS nanoparticles can effectively broaden the light-harvesting range. The band gap of the CZTS/ TiO2 and CZTS can be roughly estimated by the Kubelka− Munk (K−M) function: F(R ∞) = (1 − R )2 /(2R )

where R refers to the reflectance intensity (Figure 7b).52 The band gaps of the xCZTS/TiO2 samples (x = 0.5, 1, 2) are calculated to be 1.53, 1.51, and 1.49 eV, respectively. Hence, it can be concluded that, by coupling of TiO2 with CZTS NPs,

Figure 7. (a) UV−vis diffuse reflectance spectra of MOFs-derived TiO2, CZTS/TiO2, CZTS, and P25 samples. (b) The plots of the transformed Kubelka−Munk function vs the photon energy of CZTS/TiO2 and CZTS samples. 22206


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ACS Applied Materials & Interfaces demonstrating introduction of CZTS is of great help to the incident light absorption. After being loaded with N719 dye, the light harvesting in the visible light region is further enhanced. It is worth mentioning that all three N719 characteristic peaks remain, confirming the N719 dye can also be successfully loaded onto the CZTS/TiO2-based photoanode. Photoluminescence spectroscopy is applied to analyze the charge separation behavior of the samples, as shown in Figure 8. The relative quenching intensity of PL spectroscopy is an

Figure 9. Photocurrent density−voltage curves measured under AM 1.5G, at 1 sun light intensity with a shadow mask (MOFs-derived TiO2 and CZTS/TiO2 samples). Inset: the cross-sectional SEM image of 1CZTS/TiO2 heterostructures photoanode film on FTO substrate by spin-coating method.

Table 2. DSSCs Parameters of Photoanode Based on MOFsDerived TiO2 and CZTS/TiO2 Heterostructures

Figure 8. PL spectrum of MOFs-derived TiO2 and CZTS/TiO2 samples.

effective technique to analyze the electron−hole separation behavior. Better photoinduced carrier transporting performance indicates lower PL intensity. All the TiO2 and CZTS/TiO2 samples show a similar broad emission region from 350 to 500 nm. After being sensitized with CZTS nanoparticles, the emission peak intensity decreases first, and the 1CZTS/TiO2 heterostructure shows the most quenched PL intensity. With the CZTS concentration up to 2 M, the PL intensity increases drastically. It is believed the excessive CZTS loading will generate new carrier recombination sites, leading to negative effects in the photoinduced carriers transport processes.56 The photocurrent density (J)−voltage (V) characteristic plots of DSSCs based upon pure MOFs-derived TiO2 and CZTS NPs sensitized MOFs-derived TiO2 are measured and shown in Figure 9, and relative detail data are listed in Table 2. In the inset of Figure 9, the cross-sectional SEM image of assynthesized CZTS/MOFs−TiO2 film is given. It suggests that the thickness of the photoanode thin film is around 10 μm. For DSSCs based on pure TiO2 photoanode, the JSC and energy conversion efficiency (η) are 8.95 mA cm−2 and 3.67%, respectively. Coming to the 0.5CZTS NPs/TiO2, the η and JSC significantly increase to 5.65% and 12.57 mA cm−2. As for the 1CZTS/TiO2 heterostructure, the η and JSC further increase to 8.10% and 17.27 mA cm−2, respectively. However, for the 2CZTS/TiO2-based photoanode, the η and JSC decreases to 7.25% and 15.24 mA cm−2, still higher than those of pure TiO2. Considering the results of DRS and PL spectra, the greatly improved photoelectric conversion efficiency of intrinsic TiO2 should be assigned to the sensitizing of CZTS NPs. On the one hand, sensitizing TiO2 with CZTS NPs can effectively broaden the light response range of TiO2; on the other hand it can

samples

.JSC (mA cm−2)

VOC (V)

FF (%)

η (%)

TiO2 0.5CZTS/TiO2 1CZTS/TiO2 2CZTS/TiO2

8.95 12.57 17.27 15.24

0.76 0.79 0.81 0.83

54.21 56.76 58.18 57.24

3.67 5.65 8.10 7.25

markedly lead to improved photoinduced carrier separation by forming a uniquely matched heterojunction between TiO2 and CZTS nanoparticles. However, redundant CZTS nanoparticles will create new electron−hole recombination centers, which may reduce the power conversion efficiency. Figure 10 shows the IPCE spectroscopy based on pure MOFs-derived TiO2 and CZTS NPs sensitized MOFs-derived TiO2. From IPCE plots, the relationship between the photocurrent and incident light can be further revealed. The

Figure 10. IPCE spectra of DSSCs based on samples of MOFs-derived TiO2 and CZTS/TiO2 with different CZTS contents. 22207


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Figure 11. Impedance spectra of DSSCs based on MOFs-derived TiO2 and CZTS/TiO2 samples measured under illumination of 100 mW cm−2: (a) Nyquist plot and (b) Bode phase plots.

the Nyquist curve refers to the carrier transport resistance at the photoanode/dye/electrolyte interface. The 1CZTS/TiO2 sample displays the smallest recombination resistance (Rct2) of (156.4 Ω) among the samples (291.8, 190.2, and 217.3 Ω for TiO2, 0.5CZTS/TiO2, and 2CZTS/TiO2, respectively) and demonstrates the most enhanced photoinduced carrier transport performance, which is in accordance with the PL spectroscopy (Table 3).

IPCE is more direct than UV−vis spectra to investigate the nature between incident light and photogenerated carriers in DSSCs. Compared with pure TiO2, CZTS-sensitized TiO2 exhibits a broadened optical response range between 400 and 700 nm. Meanwhile, with the CZTS concentration ranging from 0.5 to 2 M, the IPCE intensity displays a rising tendency. When the CZTS concentration is 1 M, the IPCE achieves the maximum value of 60%, which is almost 4 and 4.5 times compared with MOFs-derived TiO2 in the UV and visible range, respectively. It is reported by Wu et al. that IPCE is a comprehensive result of electron injection efficiency and lightharvesting efficiency.57 Therefore, this IPCE enhancement should be connected to the sensitizing of CZTS nanoparticles, which leads to an improved optical response property and facilitated interfacial carrier transporting kinetics. Without the sensitizing of CZTS, though sensitized with organic dye, the IPCE of TiO2 in the visible light region is still limited. However, excessive CZTS nanoparticles will lead to a decreased exciton collection efficiency, which is in accord with the PL spectra, and consequently brings about the IPCE decreasing. To confirm both the N719 and CZTS can act as the photon acceptors, the IPCE in the wavelength range of 650−900 is further tested (shown in Figure S7). These peaks appearing beyond 800 nm should be attributed to the CZTS-induced optical response, considering the band gap of CZTS is 1.5 eV. It proves that within the as-prepared DSSCs, not only the N719 dye molecule but also the CZTS can act as the photon acceptors. EIS measurement is carried out to discuss the interfacial electron-transfer kinetics of photoexcited charges of DSSCs based on pure TiO2 and CZTS/TiO2 samples. Figure 11a depicts the Nyquist curves of photoanodes based on TiO2 and CZTS/TiO2 samples. The sheet resistance (RS) of the substrate, the carrier transport resistance at the counter electrode (Rct1), and the interfacial carrier transport resistance (Rct2) are fitted with an equivalent circuit using the ZSimpWin software (Figure 11a, inset). As the applied electrolyte and counter electrode are the same, DSSCs based on TiO2 and CZTS/TiO2 samples exhibit similar RS around 12.0 Ω, and Rct1 of 161.6, 127.9, 89.2, and 165.8 Ω for pure TiO2, 0.5, 1, 2 M CZTS/TiO2 hybrid samples, respectively. The second arc in

Table 3. Series Resistances and Electron Lifetime of Photoanode Based on MOFs-Derived TiO2 and CZTS/TiO2 samples

RS

Rct1

Rct2

frequency (Hz)

τ (ms)

TiO2 0.5CZTS/TiO2 1CZTS/TiO2 2CZTS/TiO2

10.7 12.4 11.8 13.0

161.6 127.9 89.2 165.8

291.8 190.2 156.4 217.3

67.02 35.83 9.57 19.87

2.3 4.3 16.4 7.9

Through the Bode phase plots, the lifetime information on the interfacial electrons can be discussed (Figure 11b). Kern et al. reported that the carrier lifetime (τ) can be determined by the function τ = 1/2πf max, and f max is the highest peak in the medium-frequency range.58 The τ value of photoanodes based on TiO2, 0.5CZTS/TiO2, 1CZTS/TiO2, 2CZTS/TiO2 electrodes are calculated to be 2.3, 4.3, 16.4, and 7.9 ms (Table 3). Larger τ means lower recombination rate of electron−hole pairs during carrier transport across the interfaces, which matches well with the result of Rct2. In accordance with Ding et al.’s earlier work, proper pore structure and high specific surface area are of great importance to prolong the electron lifetime via increasing the contact sites between the electrolyte/electrode interface.59 Herein, longer τ for photoanodes based on CZTS/ TiO2 is observed, indicating CZTS/TiO2 more effectively suppresses the interfacial electron back-reaction with electrolyte, and leads to better carrier injection efficiency, in good agreement with the IPCE result. The proposed charge-transfer process for the DSSCs based on CZTS/TiO2 heterostructures is illustrated in Figure 12, where all energy state position values are comparing with the normal hydrogen electrode (vs NHE). As the conduction band (CB) of N719 (−1.1 V) position is higher than that of CZTS 22208


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Additionally, it should be pointed that the as-synthesized MOFs-derived TiO2 with mesoporous structure comprehensively enhances the photoelectrical conversion performance of the photoanode. The pore-rich structured TiO2 on the one hand can effectively extend the incident light traveling length via enhancing the multiple light-scattering processes. On the other hand, the porous structure provides enough electrolyte infiltration channels, which guarantees the full contact of the electrode/electrolyte interfaces and provides sufficient reaction sites. Hence, the photoelectrical performance of TiO2-based photoanode can be thoroughly enhanced by applying the MOFs-derived TiO2 as the photoanode material. In a word, MOFs-derived mesoporous materials provide us a new method to synthesize high-performance photoanode materials. The photoelectric performance of the CZTS NPs sensitized MOFs-derived TiO2 developed here can be comparable with some latest works. For instance, as TiO2 thin film and CdSsensitized TiO2 are applied as photoanode, power conversion efficiencies of 0.82% and 2.15% can be achieved, respectively.61 For the CdS−PbS cosensitized TiO2 nanorod array, an improved power conversion performance of 1.30% is achieved, higher than that of 0.44% for the CdS-sensitized TiO2 nanorod array.62 The study by Santra et al. indicates that the power conversion efficiency of CuInS2−CdS cosensitized TiO2 photoanode can be enhanced to 3.91% from 1.14% for the CuInS2-sensitized TiO2.63 According to Luo et al.’s work, the power conversion efficiency of CuInS2−CdS cosensitized TiO2 can be further improved to 5.38% after doping CdS with manganese.64 As for the dye/nanoparticles cosensitized solar cells, Guo et al. reported N719/CdS cosensitized TiO2 achieves an improved photoelectric conversion performance of 3.06% comparing with 0.66% of CdS/TiO2.40 For the N719/PbS cosensitized TiO2 photoanode, an improved energy conversion performance of 6.35% is achieved, which is better than that of 5.95% for the N719-sensitized TiO2.65 Therefore, we have achieved an outstanding photoelectric performance through constructing CZTS nanoparticles sensitized MOFs-derived TiO2 heterostructured photoanode. Adjustable sensitizing MOFs-derived TiO2 with CZTS nanoparticles offers us an efficient method to study the influence of CZTS in enhancing the photoelectric performance of the CZTS/TiO2 heterostructure. Sensitizing TiO2 with CZTS nanoparticles pronouncedly enhances the optical response property. Obtained CZTS/ TiO2 heterostructure offers us a novel structure with uniquely matched energy state structure that can effectively suppress the photoinduced exciton recombination and displays a porous structure with high surface area for improving the interfacial carriers’ transporting ability.

Figure 12. Energy band structure and photogenerated charge-transfer mechanism in CZTS nanoparticles/MOFs-derived TiO2.

(−0.7 V), the electron excited from the N719 molecule will be immediately transferred to that of CZTS,60 which make the CZTS acts as electron acceptors. Meanwhile, anatase TiO2 as a wide band gap semiconductor (3.2 eV), has a valence band (VB) of 2.91 V and CB of −0.3 V. Either the CB or the VB of TiO2 is more positive than the VB (0.8 V) and CB (−0.7 V) of CZTS.44 This matched energy structure between anatase TiO2 and CZTS forms a typical type II heterojunction. Under incident light, the excited electrons on the CB of CZTS will easily transfer to the CB of TiO2 and the photoinduced holes in the VB of TiO2 will transfer to the VB of CZTS. The as-formed multijunction between the CZTS/TiO2/N719 interfaces will obviously enhance the transport driving force of photogenerated carriers. As the CB position of CZTS is at the middle of that of TiO2 and N719, CZTS will act as the electron acceptor and effectively receive the photoinduced electrons from N719 and then pass the electrons to TiO2, which eliminates carrier recombination caused by the surface trap of intrinsic TiO2. However, if the content of CZTS nanoparticles is too high, the electrons accepted by CZTS may be stuck, and thus increase the carrier recombination rate, which has been proved by the optical performance described above. In addition, CZTS, as a narrow band gap semiconductor (1.5 eV), not only acts as the photoinduced electron-transfer medium, but also absorbs the incident photon as the photon acceptors. Compared with simply sensitizing TiO2 with N719, modifying TiO2 with CZTS can further broaden the light absorption range near to the NIR region. Therefore, the decoration of CZTS is very important in the light-harvesting processes. On the basis of the experiment results, the best photoelectric conversion performance of the DSSCs achieved by 1CZTS/ TiO2 photoanodes should be assigned to three issues. First, CZTS NPs sensitizing TiO2 greatly enhanced the optical response in the visible region, which obviously increases the photogenerated current. Second, the matched band edge between anatase TiO2 and Cu2ZnSnS4 forming a typical type II heterojunction significantly suppresses the carriers’ recombination and can be proved by the PL spectra. Third, the DRS measurements confirm the porous structure with large specific surface area of the MOFs-derived TiO2 can effectively enhance the light-harvesting capacity through the multiple interparticle scattering processes.

4. CONCLUSIONS In summary, Cu 2 ZnSnS 4 -sensitized MOFs-derived TiO 2 mesoporous structure is successfully applied as photoanode for high-performance DSSCs for the first time. The relationships between the morphology, photoresponse, carrier transport, and photoelectric conversion performance are systematically investigated. When serving as the photoanode of DSSC, 1CZTS/TiO2 heterostructure exhibits the highest photocurrent density of 17.27 mA cm−2 and photoelectric conversion performance of 8.10%. The enhanced energy conversion performance of CZTS/TiO2 heterostructure-based DSSCs should be assigned to the following reasons: wide optical response range of CZTS/TiO2 heterostructure owing to the sensitizing of CZTS nanoparticles, the facilitated photoinduced 22209


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carriers’ transport due to the formation of heterojunctions, and the improved multiple interparticle scattering processes attributed to the porous structure with high surface area characteristic of MOFs-derived TiO2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06183. TGA of MIL-125(Ti), EDS spectra of as-synthesized MIL-125(Ti), MOFs-derived TiO2, and 1CZTS/TiO2, EDS elemental mapping images of 1CZTS/TiO2, surface TEM image of the MOFs-derived TiO2, illustration of CZTS/TiO2 multiple interparticle light-scattering processes, absorption spectra of N719, MOFs-derived TiO2, TiO2/N719, 1CZTS/TiO2, and 1CZTS/TiO2/N719 samples, IPCE spectra of DSSCs based on samples of MOFs-derived TiO2 and CZTS/TiO2 with different CZTS contents tested under the incident light region of 650−900 nm (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: + 86 531 88396970. Fax: + 86 531 88396970. E-mail: [email protected]. *Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the project supported by the State Key Program of National Natural Science of China (no. 51532005), the National Nature Science Foundation of China (nos. 51472148, 51272137), and the Tai Shan Scholar Foundation of Shandong Province.

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REFERENCES

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