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Metal organic frameworks derived high-performance photoanodes for DSSCs
Journal Pre-proof Metal organic frameworks derived high-performance photoanodes for DSSCs Yanfeng He, Zhenyu Zhang, Weiyang Wang, Lipei Fu PII:
S0925-8388(20)30452-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.154089
Reference:
JALCOM 154089
To appear in:
Journal of Alloys and Compounds
Received Date: 29 September 2019 Revised Date:
22 January 2020
Accepted Date: 27 January 2020
Please cite this article as: Y. He, Z. Zhang, W. Wang, L. Fu, Metal organic frameworks derived high-performance photoanodes for DSSCs, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.154089. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit author statement Yanfeng He: Conceptualization, Methodology, Funding acquisition, Writing-Original draft preparation. Zhenyu Zhang: Data curation, Investigation. Weiyang Wang: Resources, Funding acquisition, Writing-Reviewing and Editing. Lipei Fu: Visualization, Investigation.
Metal organic frameworks derived high-performance photoanodes for DSSCs Yanfeng He1,2*, Zhenyu Zhang2, Weiyang Wang1, Lipei Fu2 1. Key Laboratory of Unconventional Oil&Gas Development, China University of Petroleum (East China), Ministry of Education, Qingdao, 266580, People's Republic of China 2. School of Petroleum Engineering, Changzhou University, Changzhou 213016, People's Republic of China † [email protected]
Tel/Fax: +86051986330800
Abstract Metal organic frameworks (MOFs) and reduced graphene oxide (RGO) are used to modify photoanodes of dyes sensitized solar cells (DSSCs) to improve their photovoltaic performances. The MOFs are found that bring about a significant enhancement of the BET area of photoanodes, which endows a high dye adsorption capacity. Moreover, the added MOFs introduces an improved incident photon to electron efficiency because of the remarkably enhanced scattering power for the incident light. On the other hand, in the presence of RGO provides a fast transport channel for photo-induced electrons, which depresses the dark current of the resulting devices. After optimizing the mass fraction of MOFs and RGO, the synergy has been achieved. Moreover, by using a graphene-based counter electrode, the photovoltaic performances of the resulting full-carbon DSSCs are detected. The short-circuit current, open-circuit voltage, fill factor (FF) and energy conversion efficiency (η) reach 18.6 mAcm-2, 682 mV, 0.608 and 7.67%, indicating the potential application prospection of the as-prepared photoanodes. Keywords: MOFs, reduced graphene oxide, DSSCs, photoanode
1
1. Introduction Photoanode, as one of the most important parts of dyes sensitized solar cells (DSSCs), has gained great attention to obtain high energy conversion efficiency (η) from solar energy due to the increasingly serious global energy crisis [1, 2]. The major functions of photoanode in the photochemical device include loading dyes, transporting photo-induced electrons from dye to conductive substrate and scattering incident light to promote the absorption by dye molecules. Therefore, a large BET area, a low resistance and a porous structure are the advantages for the potential photoanode materials. Moreover, the conduction band (or Fermi level) of the photoanode should be more positive than that of the LUMO of dye molecules to accept the transferred electrons. The first breakthrough is achieved by Graezel’s group at 1991, and the nano porous TiO2 was used as a photoanode for the first time, which initiating the era of rapid development of the DSSCs [3]. The remarkably improved BET area and consequently enhanced dye adsorption capacity bring about the outstanding performances, which has been proven the following reported nano ZnO and SnO2 based photoanodes [4-6]. However, the high electric resistance of these semiconductors limits the average lifetime of photo-generated electrons as well as the core photovoltaic parameter (short-circuit current, JSC). To conquer this shortcoming, some attempts including doping impurity atoms and hybridizing with modifiers who possess high conductivity have been carried out. Khan et al. incorporated Ni2+ in the host ZnO matrix, which results in the dramatic shape evolution of the resulting films from simple bullet like structures to complex punch like microstructures with increased surface area and improved photovoltaic properties [7]. Lee further reported Co modified ZnO photoanode, and the photocurrent density and charge transfer of the electrode are 2.2 and 2.4 times higher than the pristine ZnO photoanode, respectively [8]. On the other hand, adding high conductivity materials is widely used to avoid introducing new photo-generated electrons annihilating center (impurity atoms). Specially, various carbon allotrope such as carbon nanotubes and graphene are deemed as the proper candidates because of their large BET area and excellent electric 2
property [9-12]. Peiris et al. adopted the reduced graphene oxide (RGO) to modify TiO2 photoanode, and the resulting η increases from 4.4% to 6.2% resulting from the remarkably improved dye loading amount [10]. Kumar et al. further achieved the co-sensitization in the RGO-TiO2 photoanode, and the η reaches 6.9% [11]. Recently, Tang et al. utilized the low defect density and high integrity of the three-dimensional graphene networks (3DGNs) to play as the fast electron transport channel for photoanode, which significantly enhances the JSC and η [12]. In fact, as a versatile material, graphene is widely adopted in other photoelectrical devices by using its outstanding properties. Zang et al. reported RGO modified various composites to enhance their photoelectric properties, which can be utilized in the X-ray photon detection, in-vivo bioimaging and light-controlled conductive switching areas [13-15]. Moreover, Zang’s group found that the presence of a 3D graphene nanowalls in the photoelectrochemical anode remarkably improves the resulting performances [16]. Although the reported BET area of these photoanodes is as large as ~200 m2g-1, the dye loading ability is far from expectation. Based on the corresponding reports, a close chemical contact is the pre-condition to achieve the injection of electron from dye to TiO2, and the lifetime of electronically excited state of dye is short [12]. Therefore, how to further adsorb dye molecules as much as possible on the surface of photoanode is one of effective method to improve the photovoltaic performances of the resulting devices. In the past two decades, metal organic frameworks (MOFs) become one of hot issues because of their numerous topologies, controllable pore size and superior surface chemistry with adjustable functionalization [17]. The potential application fields include but not limited to drug delivery, electrochemistry, environmental recover and catalysis. The adsorption edge of MOFs can be tunable by the selected organic ligands, which enhancing the light harvesting characteristic by the ligand antenna [18]. More important, the reported BET areas of MOFs always around 1500~2000 m2g-1, which is meaningful to enhance the dye adsorption ability of photoanode. Kaur et al. fabricated Eu-MOF based photoanode, and the JSC and η increase 43% and 80% because of the enhanced dye adsorption ability [19]. Tang et al. 3
found that the added Cu-BTC significantly improves the dye adsorption amount of RGO/TiO2 composite [20]. Most recent, Ismail’s group study the charge transfer dynamics of the interface area between MOFs and TiO2 in the photoanode, and the injection efficiency reaches 86% from the TiO2 to Ru3(BTC)2 [21]. Therefore, the large BET area and close contact between MOFs and TiO2 endow MOFs a bright prospect in the photoanode area. However, MOFs, in general, are insulators or semiconductors, which creates a barrier for the high electronic characteristics of MOFs based photoanode. Naturally, how to utilize the advantages of MOFs and avoid their shortcoming deserve further research. In this study, MOFs (ZIF-8 and Uio-66) and RGO are employed to co-modify photoanode, and the graphene counter electrode are adopted to prepare the full-carbon DSSCs, simultaneously. The dye loading amount of the resulting photoanode reaches 1.92×10-7 molcm-2, which is 120.1% higher than the original TiO2 photoanode. Based on the specific photovoltaic parameters such as JSC and fill factor (FF), the functions of MOFs and RGO are taken full play after optimizing the preparation process and mass fractions. The proper energy band structures and synergy between the MOFs, RGO and TiO2 bring about a η as high as 7.67%, which is 39.7% and 67.5% higher than the RGO modified and pure TiO2 photoanode based devices. The presence of MOFs provides a large BET area to adsorb more dye, while the RGO acts as the electron collector to accelerate the electron transport from dye to conductive substrate. The corresponding mechanism have been proven by the results of IPCE, dark current and photocurrent.
2. Experiments 2.1 Materials Nano TiO2, Indoline, iodine and potassium iodide were purchased from Aladdin, and the TiO2 was sintered at 4000C for 2h to remove the organics. ZrCl4, Acetonitrile, terephthalic acid (H2BDC), polytetrafluoroethylene, HCl, dimethyl formamide (DMF), chloroplatinic acid, natural graphite and sodium dodecyl sulfate were purchased from the Shanghai chemical reagent plant (Shanghai, China). 4
Zn(NO3)2·6H2O
and
2-methylimidazole
are
obtained
commercially
from
Sigma-Aldrich. Deionized water was utilized to prepare all aqueous solutions. 2.2 Preparation The preparation of Uio-66 and ZIF-8 have been reported by Joly and Zhang [22, 23]. The graphene oxide was fabricated by modified Hummer’s method, and the RGO was obtained by a reduction progress with hydrazine at 98
for 4h [24]. Pure TiO2
photoanode, RGO-TiO2 photoanode and 3DGNs counter electrode have been reported by Tang’s reports [25-27]. The Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 were prepared by a physical mixture process, a certain amount of Uio-66 (ZIF-8) and 0.1g RGO/TiO2 were added into 20mL ethanol solution with 30min stirring. Then, the resulting paste was deposited on a conductive glass by the doctor-blade approach. The as-prepared photoanodes were immersed into 0.5 mmol indoline dye solution with a 1:1 (volume) mixture of acetonitrile and butanol and kept for 24 h. The 3DGNs counter electrode was prepared by Hu’s report [28]. These photoanodes and graphene electrodes were assembled into sandwich type cells by an alligator clip. The electrolyte including acetonitrile solution, 0.5 mol LiI and 0.05 mol I2 was introduced between the electrodes.
2.3 Characteristic The morphology images were obtained by the scanning electron microscope (SEM) (FEI Sirion 200 scanning electron microscope). BET surface areas were measured on a Nova 100 by using N2 as the adsorption gas. Raman spectra were recorded by a LabRam-1B Raman microspectrometer at 514.5 nm (Horiba Jobin Yvon, France). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance (Cu Kα radiation 0.154 nm). Photocurrent measurements were performed on a CHI 660D electrochemical analyser (Shanghai CH Instrument Company, China). The J-V curves were recorded by a PGSTAT 30 potentiostat (Netherlands). The incident photon to current conversion efficiency (IPCE) profiles were recorded on a Newport 1918-c 5
power meter. 3. Results and discussion Morphologies of Uio-66 and ZIF-8 are shown in Fig. 1a and 1b. SEM images of various photoanodes are displayed in Fig. 1c-1e, TiO2 nano-particles disperse uniformly on RGO surface, which is in line with previous reports [25-27, 29]. After hybridizing with MOFs, no obvious change can be seen from both the morphologies of Uio-66 (ZIF-8) and RGO/TiO2, suggesting that the physical mixture process does not exert a negative influence to the resulting composites. Moreover, the partial TiO2 particles are loading on the surface of MOFs, which provides a pre-condition to the transport of photo-induced electrons from the adsorbed dye with a high efficiency. As for the 3DGNs counter electrode, the stratified structure can be observed by the cross-section view (Fig. 1f). The continuous structure of graphene basal planes and low defect density endow it a high conductivity, and the good catalytic performance for I-/I3- redox reaction make 3DGNs a promising candidate to replace Pt electrode [28]. Raman curves of various samples are listed in the Fig. 2. There are three fingerprint peaks can be seen from the adopted RGO. G band, inducing by the breath vibration mode of honeycomb carbon ring, located at 1580 cm-2 is associated with the E2g phonon at Brillouin zone center [30]. Its position, intensity and full width at half maximum are closely related to the strain, thickness, doping level and fluctuation of temperature. The D band and 2D band are one- and two-order signals of disorder peak, which are located at 1350 cm-2 and 2700 cm-2 [31, 32]. Interesting, the D band is caused by defects, while the 2D band is free of defects but introduced by the double resonant Raman scattering with a two-phonon emission process. Therefore, the intensity of D band is in direct proportion to the defect density of graphene. Contrarily, the presence of 2D peak indicates a relatively high quality of the sample, and the specific shape is dependent on the number of graphene layers [32]. The characteristic peaks located at ~173 cm-1, 425 cm-1, 516 cm-1 and 670 cm-1 are ascribed as the Eg, B1g and A1g vibration modes of the TiO2 [33]. The major signals belonged to Uio-66 6
are located at 700 cm-1, 1440 cm-1 and 2930 cm-1, which is in agreement with previous reports [34]. As for the ZIF-8, the characteristic peaks include 860 cm-1, 1160 cm-1, 1430 cm-1 and 1620 cm-1 [35]. The Raman profiles demonstrate the successfully prepared MOFs. The corresponding signals of the MOFs, graphene and TiO2 are almost invariable from the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 samples, manifesting that no conspicuous change takes place during the combination process, which is in line with the morphology analysis. XRD curves of various samples are displayed in the Fig. 3. Both the RGO and 3DGNs show two fingerprint peaks at 26.50 and 43.10, which are induced by the (001) and (200) crystal planes of graphene (JCPDS card: 41-1487) [36]. The more sharp of the latter suggests the larger average size and lower defect density. The signals of (101), (004), (200), (105), (211) and (204) crystal planes of TiO2 are located at ~260, ~380, ~470, ~540, ~560 and ~620 (anatase phase, JCPDS card: 84-1286) [25]. The corresponding signals of graphene are buried by noise due to its discontinuous structure and low mass fraction, which has proven by the previous report [37]. The corresponding signals belonged to Uio-66 are located from 50~600, while the characteristic peaks belonged to ZIF-8 range from 50~400 [38, 39]. After hybridizing with RGO/TiO2, no distinct difference of these signals can be found from the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2. Based on the Scherrer equation, the average size of MOFs can be calculated as following [40]: (1) therein,
, ϴ and λ represent width of half maximum, diffraction angle of the
characteristic peak and wavelength of X-ray. The average sizes of pristine Uio-66 and ZIF-8 are 12.4 um and 8.9 um, and the values are consistent with the resulting photoanodes (12.2 um and 8.9 um from the Uio66-RGO/TiO2 and ZIF-8-RGO/TiO2), proving the morphology and structure of the MOFs is changeless, which is quite important to utilize the large BET area of the MOFs. IR patterns are recorded to further analyse the microstructure of these specimens (Fig. 4). The absorption peak at 1600 cm-1 is induced by the skeletal vibration of 7
graphene basal plane, and the wide peak ranges from 3000 cm-1 to 3700 cm-1 is the stretching vibration of O-H from surface adsorbed water [41]. The 780 cm-1 signal is ascribed to the Ti-O-Ti vibration of TiO2, and this peak becomes wider of the RGO/TiO2 sample because of the form of Ti-O-C bond, which can be used to confirm the chemical contact between RGO and TiO2 [42]. As for MOFs, all the major absorption peaks are marked in the Fig. 4. According to Shao’s group report, the peaks located at ~400 cm-1, ~660 cm-1 and ~740 cm-1 are induced by the mixture of O-C, C-H and Zr-O of Uio-66 [43]. The signal located at 1100 cm-1 is ascribed to the stretching vibration of Zr-O band, while the 1420 cm-1 and 1590 cm-1 peaks are attributed to the asymmetric and symmetric stretching of O-C=O [44]. As for the ZIF-8 sample, the peak at 670 cm-1 is assigned to Zn-O band, while the peak at 420 cm-1 is induced by the Zn-N band [45]. After combing with RGO/TiO2, no change can be seen from both the position and relative intensity of absorption signals because of the physical mixture process, which is in agreement with the SEM, Raman and XRD results. J-V curves of the resulting devices by using these as-prepared photoanodes are shown in the Fig. 5. The JSC and VOC of the pure TiO2 photoanode based DSSCs are 11.6 mAcm-2 and 680 mV, and the JSC increases to 13.1 mAcm-2 when the RGO/TiO2 is adopted because of the decreased resistance and enhanced electron transport ability of the photoanode. The JSC further increases to 17.8 mAcm-2 and 17.2 mAcm-2 by using the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 photoanodes, and the VOC almost keeps at a constant (the specific parameters are listed in the Table 1). Furthermore, the FF of pure TiO2, RGO/TiO2, Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 are 0.581, 0.619, 0.606 and 0.611, demonstrating that the added MOFs do not significantly degrade the conductivity. The resulting η of the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 based devices are 7.33% and 7.11%, which are 34.0% and 29.9% higher than that of the RGO/TiO2 based sample, proving the remarkably positive effect of the added MOFs. Moreover, the corresponding comparison of the obtained photovoltaic performances from the related reports and this work has been 8
summarized in the Table 2 [46-62]. Considering the specific changes of varying photovoltaic parameters, some possible reasons can be proposed. Firstly, the significantly improved JSC can be achieved by the increased photo-induced electrons and decreased dark current. Therein, a higher incident photon to current conversion efficiency (IPCE) and an increased adsorption amount of dyes will introduce the more photoelectrons. According to the change of concentration of dye before and after adsorbing by various photoanodes, the loading amount of dye molecules can be abstracted. The specific adsorption amount by using the Uio-66-RGO/TiO2 photoanode reaches 1.92×10-7 molcm-2, which is 29.7% and 120.1% higher than that of the RGO/TiO2 and pure TiO2 photoanodes. As for the ZIF-8-RGO/TiO2 photoanode, the loading capacity further increases to 2.08×10-7 molcm-2. The remarkably improved dye adsorption ability is caused by the large BET areas of MOFs. Based on N2 adsorption isotherm, the BET areas of pristine Uio-66 and ZIF-8 reach 1876.6 and 2005.4 m2g-1, and the values of the resulting Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 still maintain a high level (605.8 m2g-1 and 659.4 m2g-1), which are 3.06 times and 3.37 times higher than that of RGO/TiO2 photoanode. Adsorbing more dye molecules means more photo-generated electrons can be produced, leading to the significantly enhanced JSC. Considering Uio-66 (ZIF-8) cannot provide the electrons fast transport function that RGO can do (the conductivities of MOFs are too low to play as the electron transport channel), the large BET areas of them should be the major reason for the enhanced performances. Although the ZIF-8 possesses a more larger BET area, the resulting photovoltaic performance of the Uio-66-RGO/TiO2 is better. The core reason is that the pore diameter of the ZIF-8 is mesoporous rather than microporous, leading that the adsorption capacity for micro-molecule is not as good as that of Uio-66. Moreover, the IPCE profiles of various samples are displayed in the Fig. 6. Compared with that of pure TiO2 photoanode based device, the RGO/TiO2 assisted sample shows the higher conversion efficiency at the long wavelength range because of the visible-light activity of RGO and the comparable size of the RGO/TiO2 composite to the 9
visible-light. As for the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2, the efficiency increases over the entire wavelength range, demonstrating the scattering ability of the resulting photoanodes increases remarkably during the entire incident light spectrum, resulting to the multiple absorption effect of dye molecule and the consequently enhanced JSC. A possible reason is that the presence of RGO, Uio-66 and ZIF-8 depresses the agglomeration of TiO2 nanoparticles, endowing a higher effective BET area to TiO2. Therefore, more photons in the UV-light range can be absorbed by these composites, which leads the corresponding enhancement in the short wavelength area. To further confirm this point, photocurrents of various photoanodes are detected. According to the recorded curves, the specific values of the Uio-66 and ZIF-8 modified photoanodes are 6.23 times and 5.74 times higher than those of pure TiO2 photoanode, and their fast and uniform photocurrent response manifests the great electrical and optical performances (Fig. 7). The above results prove the higher yield of photo-induced electrons by using the MOFs assisted photoanodes. Beside yield, the obtained JSC is also closely related to the annihilation of electrons. Dark current is the major consummation of photoinduced electrons, which is induced by the annihilation of electrons and I3- ions on the ITO substrate. As shown in the Fig. 8, the dark current of the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 assisted samples are smaller than that case of the pure TiO2 when the potential is higher than 0.6 V, and the onset of the dark current is 12 mV and 11 mV higher than the pure TiO2, respectively. The RGO/TiO2 based device displays the lowest dark current when the potential higher than 0.58 V, and the onset of dark current shifts to the highest potential. The phenomenon indicating that the dark current is depressed by the presence of RGO because of the fast transport of photoinduced electrons. Although the added MOFs increase the dark current slightly, the wastage is limited in a low level. The degraded performance is due to the poor conductivities of these MOFs, and the negative effect of them on the dark current is similar with that of pure TiO2. Therefore, the high yield and low loss of photo-generated electrons bring about the significantly increased JSC of the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 based devices. 10
Secondly, the difference in resistance of various photoanodes can lead to the obtained varying of JSC and FF. Electrochemical impedance spectroscopy (EIS) is a useful tool to abstract resistance of photoanode and counter electrode. According to the EIS patterns shown in the Fig. 9, two semicircles can be seen from all the samples. The middle-frequency semicircle reveals the impedance of the electron transport at the interface between the electrolyte and counter electrode, and the semicircle in the high-frequency reflects the impedance of charge transfer at the electrolyte-photoanode interface [21]. The similar semicircles at high-frequency field of all the specimens is because of the identical 3DGNs counter electrode. Based on previous report, the 3DGNs counter electrode displays good catalytic ability for I-/I3- redox, which is comparable with that of the Pt electrode [24]. On the contrary, the scale of semicircle in the middle-frequency area is closely related to the adopted photoanodes. The pure TiO2 photoanode based device possesses the largest semicircle, indicating the high resistance of electron transport on the interface area between the electrolyte and photoanode. In the presence of RGO significantly decreases the semicircle resulting from the high conductivity of graphene, which is in line with the previous reports [12, 21, 22]. After adding MOFs, the corresponding semicircle enlarges slightly, implying that the added Uio-66 and ZIF-8 do not increase the impedance significantly. Therefore, the enlarged BET area of the resulting photoanode is the vital reason for the obtained high photovoltaic performances, and the high yield and low loss of photo-induced electrons resulting from the high dye loading amount and small dark current lead to the outstanding JSC and η. The mass fraction of MOFs in the photoanode exerts a remarkable influence on the photovoltaic performances of the as-prepared DSSCs. The positive effect (providing a large BET area to adsorb more dye molecules on the surface of photoanode, endowing the better scattering and absorption abilities to incident photon and producing more photo-induced electrons) cannot be given full play when the ratio of MOFs is too low, which will lead to unsatisfied photovoltaic performances. On the other hand, excess proportion of MOFs means the lower mass fractions of RGO and TiO2. The decreased 11
TiO2 is negative to obtain a high JSC (reducing the adsorption activity sites), while the decreased amount of RGO will bring about a severe agglomeration phenomenon of TiO2 nanoparticles. After corresponding optimizing (the ratio of Uio-66, RGO and TiO2 is 40:8:52), the JSC, VOC, FF and η reach 18.6 mAcm-2, 678 mV, 0.608 and 7.67% for the Uio-66-RGO/TiO2 photoanode based device, achieving the synergy between the MOF, RGO and TiO2. Similarly, the η as high as 7.22% is obtained after optimizing the mass fraction of ZIF-8 in the photoanode.
4. Conclusion MOFs includes Uio-66 and ZIF-8 are adopted to modify photoanode, and RGO is also employed to provide a better electron transport network and ameliorate the conductivity of the photoanode. The large BET area of MOFs enhances the dye loading amount significantly, which brings about a high yield of photo-induced electrons. Although a high loading amount of dye molecules introduces more photogenerated electrons, the resulting JSC is also determined by the wastage. The high IPCE of MOFs assisted device is resulting from the boosted scattering capacity of incident light, which further improves the resulting photovoltaic performances. Moreover, the relatively small dark current indicates the low loss ratio of photo-generated electrons, which is also beneficial to the resulting performances. The EIS profile manifests the impedance of the MOFs added photoanode is comparable with that of RGO/TiO2 photoanode, implying the negative influence of the MOFs on the impedance can be ignored. Therefore, the large BET area of MOFs endows a high loading amount of dye molecules and a high yield of photoinduced electrons, while the negative influence of MOFs on the dark current, impedance as well as the loss of photogenerated current can be ignored. On the other hand, the 3DGNs is used as the counter electrode to fabricate the full-carbon DSSCs, and the resulting η reaches 7.67% for the Uio-66-RGO/TiO2 photoanode based sample, which is 39.7% and 67.5% higher than the cases of using RGO/TiO2 and pure TiO2 photoanodes.
12
Acknowledges This work is supported by the Open Fund of Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum, East China, 19CX05005A-2) and the Fundamental Research Funds for the Central Universities.
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19
Figures
Fig. 1 SEM images of (a) Uio-66 (b) ZIF-8 (c) RGO/TiO2 (d) Uio-66- RGO/TiO2 (e) ZIF-8RGO/TiO2 and (f) 3DGNs.
20
Fig. 2 Raman curves of various samples.
Fig. 3 XRD patterns of various samples.
21
Fig. 4 IR profiles of various samples.
Fig. 5 J-V curves of the resulting devices by using various photoanodes.
22
Fig. 6 IPCE profiles of the resulting devices by using various photoanodes.
Fig. 7 Photocurrent signals of various photoanodes. 23
Fig. 8 Dark current profiles of the resulting devices by using various photoanodes.
Fig. 9 EIS profiles of the resulting devices by using various photoanodes, and the equivalent circuit diagram are shown as the inset.
24
Tables
Table 1 Photovoltaic properties of the resulting devices with various photoanodes. BET areas and dye loading amounts of various photoanodes are also listed. All measurements were recorded under AM-1.5G one sun and the active areas were ca. 0.15 cm2 for all cells. Photoanodes
JSC
VOC
FF
η
Dye
BET area
(mAcm-2)
(mV)
(%)
(%)
(×10-7 molcm-2)
(m2g-1)
TiO2
11.6
675
58.1
4.55
0.87
49.5
RGO/TiO2
13.1
674
61.9
5.47
1.48
197.9
ZIF-8-RGO/TiO2
17.8
679
60.6
7.33
1.92
605.8
Uio-66-RGO/TiO2
17.2
677
61.1
7.11
2.08
659.4
Optimized
18.6
678
60.8
7.67
1.98
640.5
Uio-66-RGO/TiO2
25
Table 2 The performances of reported DSSCs with various photoanodes and counter electrodes. Photoanode
Counter electrode
η (%)
Reference
TiO2
Graphite and carbon
6.67
46
TiO2
Pt/PProDOT-Et2
6.69
47
TiO2
graphene/PEDOT/PET
6.26
48
TiO2
Graphene-PPy
5.27
49
TiO2
CNTs-WS2
6.41
50
TiO2
CNTs-TiN
5.41
51
TiO2
Graphene
2.8
52
TiO2
Porous graphene
5.2
53
TiO2
GO
2.1
54
Carbon black-TiO2
Pt
5.6
55
Nanoporous
Pt
3.4
56
Carbon fibers-TiO2
Pt
1.28
57
MWCNTs-TiO2
Pt
6.21
58
Acid treated
Pt
2.16
59
Graphene-TiO2
Pt
4.28
60
Graphene oxide
Pt
4.65
61
Pt
2.7
62
ZIF-8-RGO/TiO2
3DGNs
7.33
This work
Uio-66-RGO/TiO2
3DGNs
7.67
This work
carbon-TiO2
SWCNT-TiO2
nanosheets-TiO2 Nafion-coated graphene--TiO2
26
1. MOFs are adopted to modify photoanode to enhance dye loading capacity. 2. MOFs and RGO play as different roles in the photoanode, and the synergy is achieved. 3. The 3DGNs is employed as counter electrode to prepare full-carbon DSSCs. 4. The JSC, VOC, FF and η reach 18.6 mAcm-2, 678 mV, 0.608 and 7.67%.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0925-8388(20)30452-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.154089
Reference:
JALCOM 154089
To appear in:
Journal of Alloys and Compounds
Received Date: 29 September 2019 Revised Date:
22 January 2020
Accepted Date: 27 January 2020
Please cite this article as: Y. He, Z. Zhang, W. Wang, L. Fu, Metal organic frameworks derived high-performance photoanodes for DSSCs, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.154089. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit author statement Yanfeng He: Conceptualization, Methodology, Funding acquisition, Writing-Original draft preparation. Zhenyu Zhang: Data curation, Investigation. Weiyang Wang: Resources, Funding acquisition, Writing-Reviewing and Editing. Lipei Fu: Visualization, Investigation.
Metal organic frameworks derived high-performance photoanodes for DSSCs Yanfeng He1,2*, Zhenyu Zhang2, Weiyang Wang1, Lipei Fu2 1. Key Laboratory of Unconventional Oil&Gas Development, China University of Petroleum (East China), Ministry of Education, Qingdao, 266580, People's Republic of China 2. School of Petroleum Engineering, Changzhou University, Changzhou 213016, People's Republic of China † [email protected]
Tel/Fax: +86051986330800
Abstract Metal organic frameworks (MOFs) and reduced graphene oxide (RGO) are used to modify photoanodes of dyes sensitized solar cells (DSSCs) to improve their photovoltaic performances. The MOFs are found that bring about a significant enhancement of the BET area of photoanodes, which endows a high dye adsorption capacity. Moreover, the added MOFs introduces an improved incident photon to electron efficiency because of the remarkably enhanced scattering power for the incident light. On the other hand, in the presence of RGO provides a fast transport channel for photo-induced electrons, which depresses the dark current of the resulting devices. After optimizing the mass fraction of MOFs and RGO, the synergy has been achieved. Moreover, by using a graphene-based counter electrode, the photovoltaic performances of the resulting full-carbon DSSCs are detected. The short-circuit current, open-circuit voltage, fill factor (FF) and energy conversion efficiency (η) reach 18.6 mAcm-2, 682 mV, 0.608 and 7.67%, indicating the potential application prospection of the as-prepared photoanodes. Keywords: MOFs, reduced graphene oxide, DSSCs, photoanode
1
1. Introduction Photoanode, as one of the most important parts of dyes sensitized solar cells (DSSCs), has gained great attention to obtain high energy conversion efficiency (η) from solar energy due to the increasingly serious global energy crisis [1, 2]. The major functions of photoanode in the photochemical device include loading dyes, transporting photo-induced electrons from dye to conductive substrate and scattering incident light to promote the absorption by dye molecules. Therefore, a large BET area, a low resistance and a porous structure are the advantages for the potential photoanode materials. Moreover, the conduction band (or Fermi level) of the photoanode should be more positive than that of the LUMO of dye molecules to accept the transferred electrons. The first breakthrough is achieved by Graezel’s group at 1991, and the nano porous TiO2 was used as a photoanode for the first time, which initiating the era of rapid development of the DSSCs [3]. The remarkably improved BET area and consequently enhanced dye adsorption capacity bring about the outstanding performances, which has been proven the following reported nano ZnO and SnO2 based photoanodes [4-6]. However, the high electric resistance of these semiconductors limits the average lifetime of photo-generated electrons as well as the core photovoltaic parameter (short-circuit current, JSC). To conquer this shortcoming, some attempts including doping impurity atoms and hybridizing with modifiers who possess high conductivity have been carried out. Khan et al. incorporated Ni2+ in the host ZnO matrix, which results in the dramatic shape evolution of the resulting films from simple bullet like structures to complex punch like microstructures with increased surface area and improved photovoltaic properties [7]. Lee further reported Co modified ZnO photoanode, and the photocurrent density and charge transfer of the electrode are 2.2 and 2.4 times higher than the pristine ZnO photoanode, respectively [8]. On the other hand, adding high conductivity materials is widely used to avoid introducing new photo-generated electrons annihilating center (impurity atoms). Specially, various carbon allotrope such as carbon nanotubes and graphene are deemed as the proper candidates because of their large BET area and excellent electric 2
property [9-12]. Peiris et al. adopted the reduced graphene oxide (RGO) to modify TiO2 photoanode, and the resulting η increases from 4.4% to 6.2% resulting from the remarkably improved dye loading amount [10]. Kumar et al. further achieved the co-sensitization in the RGO-TiO2 photoanode, and the η reaches 6.9% [11]. Recently, Tang et al. utilized the low defect density and high integrity of the three-dimensional graphene networks (3DGNs) to play as the fast electron transport channel for photoanode, which significantly enhances the JSC and η [12]. In fact, as a versatile material, graphene is widely adopted in other photoelectrical devices by using its outstanding properties. Zang et al. reported RGO modified various composites to enhance their photoelectric properties, which can be utilized in the X-ray photon detection, in-vivo bioimaging and light-controlled conductive switching areas [13-15]. Moreover, Zang’s group found that the presence of a 3D graphene nanowalls in the photoelectrochemical anode remarkably improves the resulting performances [16]. Although the reported BET area of these photoanodes is as large as ~200 m2g-1, the dye loading ability is far from expectation. Based on the corresponding reports, a close chemical contact is the pre-condition to achieve the injection of electron from dye to TiO2, and the lifetime of electronically excited state of dye is short [12]. Therefore, how to further adsorb dye molecules as much as possible on the surface of photoanode is one of effective method to improve the photovoltaic performances of the resulting devices. In the past two decades, metal organic frameworks (MOFs) become one of hot issues because of their numerous topologies, controllable pore size and superior surface chemistry with adjustable functionalization [17]. The potential application fields include but not limited to drug delivery, electrochemistry, environmental recover and catalysis. The adsorption edge of MOFs can be tunable by the selected organic ligands, which enhancing the light harvesting characteristic by the ligand antenna [18]. More important, the reported BET areas of MOFs always around 1500~2000 m2g-1, which is meaningful to enhance the dye adsorption ability of photoanode. Kaur et al. fabricated Eu-MOF based photoanode, and the JSC and η increase 43% and 80% because of the enhanced dye adsorption ability [19]. Tang et al. 3
found that the added Cu-BTC significantly improves the dye adsorption amount of RGO/TiO2 composite [20]. Most recent, Ismail’s group study the charge transfer dynamics of the interface area between MOFs and TiO2 in the photoanode, and the injection efficiency reaches 86% from the TiO2 to Ru3(BTC)2 [21]. Therefore, the large BET area and close contact between MOFs and TiO2 endow MOFs a bright prospect in the photoanode area. However, MOFs, in general, are insulators or semiconductors, which creates a barrier for the high electronic characteristics of MOFs based photoanode. Naturally, how to utilize the advantages of MOFs and avoid their shortcoming deserve further research. In this study, MOFs (ZIF-8 and Uio-66) and RGO are employed to co-modify photoanode, and the graphene counter electrode are adopted to prepare the full-carbon DSSCs, simultaneously. The dye loading amount of the resulting photoanode reaches 1.92×10-7 molcm-2, which is 120.1% higher than the original TiO2 photoanode. Based on the specific photovoltaic parameters such as JSC and fill factor (FF), the functions of MOFs and RGO are taken full play after optimizing the preparation process and mass fractions. The proper energy band structures and synergy between the MOFs, RGO and TiO2 bring about a η as high as 7.67%, which is 39.7% and 67.5% higher than the RGO modified and pure TiO2 photoanode based devices. The presence of MOFs provides a large BET area to adsorb more dye, while the RGO acts as the electron collector to accelerate the electron transport from dye to conductive substrate. The corresponding mechanism have been proven by the results of IPCE, dark current and photocurrent.
2. Experiments 2.1 Materials Nano TiO2, Indoline, iodine and potassium iodide were purchased from Aladdin, and the TiO2 was sintered at 4000C for 2h to remove the organics. ZrCl4, Acetonitrile, terephthalic acid (H2BDC), polytetrafluoroethylene, HCl, dimethyl formamide (DMF), chloroplatinic acid, natural graphite and sodium dodecyl sulfate were purchased from the Shanghai chemical reagent plant (Shanghai, China). 4
Zn(NO3)2·6H2O
and
2-methylimidazole
are
obtained
commercially
from
Sigma-Aldrich. Deionized water was utilized to prepare all aqueous solutions. 2.2 Preparation The preparation of Uio-66 and ZIF-8 have been reported by Joly and Zhang [22, 23]. The graphene oxide was fabricated by modified Hummer’s method, and the RGO was obtained by a reduction progress with hydrazine at 98
for 4h [24]. Pure TiO2
photoanode, RGO-TiO2 photoanode and 3DGNs counter electrode have been reported by Tang’s reports [25-27]. The Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 were prepared by a physical mixture process, a certain amount of Uio-66 (ZIF-8) and 0.1g RGO/TiO2 were added into 20mL ethanol solution with 30min stirring. Then, the resulting paste was deposited on a conductive glass by the doctor-blade approach. The as-prepared photoanodes were immersed into 0.5 mmol indoline dye solution with a 1:1 (volume) mixture of acetonitrile and butanol and kept for 24 h. The 3DGNs counter electrode was prepared by Hu’s report [28]. These photoanodes and graphene electrodes were assembled into sandwich type cells by an alligator clip. The electrolyte including acetonitrile solution, 0.5 mol LiI and 0.05 mol I2 was introduced between the electrodes.
2.3 Characteristic The morphology images were obtained by the scanning electron microscope (SEM) (FEI Sirion 200 scanning electron microscope). BET surface areas were measured on a Nova 100 by using N2 as the adsorption gas. Raman spectra were recorded by a LabRam-1B Raman microspectrometer at 514.5 nm (Horiba Jobin Yvon, France). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance (Cu Kα radiation 0.154 nm). Photocurrent measurements were performed on a CHI 660D electrochemical analyser (Shanghai CH Instrument Company, China). The J-V curves were recorded by a PGSTAT 30 potentiostat (Netherlands). The incident photon to current conversion efficiency (IPCE) profiles were recorded on a Newport 1918-c 5
power meter. 3. Results and discussion Morphologies of Uio-66 and ZIF-8 are shown in Fig. 1a and 1b. SEM images of various photoanodes are displayed in Fig. 1c-1e, TiO2 nano-particles disperse uniformly on RGO surface, which is in line with previous reports [25-27, 29]. After hybridizing with MOFs, no obvious change can be seen from both the morphologies of Uio-66 (ZIF-8) and RGO/TiO2, suggesting that the physical mixture process does not exert a negative influence to the resulting composites. Moreover, the partial TiO2 particles are loading on the surface of MOFs, which provides a pre-condition to the transport of photo-induced electrons from the adsorbed dye with a high efficiency. As for the 3DGNs counter electrode, the stratified structure can be observed by the cross-section view (Fig. 1f). The continuous structure of graphene basal planes and low defect density endow it a high conductivity, and the good catalytic performance for I-/I3- redox reaction make 3DGNs a promising candidate to replace Pt electrode [28]. Raman curves of various samples are listed in the Fig. 2. There are three fingerprint peaks can be seen from the adopted RGO. G band, inducing by the breath vibration mode of honeycomb carbon ring, located at 1580 cm-2 is associated with the E2g phonon at Brillouin zone center [30]. Its position, intensity and full width at half maximum are closely related to the strain, thickness, doping level and fluctuation of temperature. The D band and 2D band are one- and two-order signals of disorder peak, which are located at 1350 cm-2 and 2700 cm-2 [31, 32]. Interesting, the D band is caused by defects, while the 2D band is free of defects but introduced by the double resonant Raman scattering with a two-phonon emission process. Therefore, the intensity of D band is in direct proportion to the defect density of graphene. Contrarily, the presence of 2D peak indicates a relatively high quality of the sample, and the specific shape is dependent on the number of graphene layers [32]. The characteristic peaks located at ~173 cm-1, 425 cm-1, 516 cm-1 and 670 cm-1 are ascribed as the Eg, B1g and A1g vibration modes of the TiO2 [33]. The major signals belonged to Uio-66 6
are located at 700 cm-1, 1440 cm-1 and 2930 cm-1, which is in agreement with previous reports [34]. As for the ZIF-8, the characteristic peaks include 860 cm-1, 1160 cm-1, 1430 cm-1 and 1620 cm-1 [35]. The Raman profiles demonstrate the successfully prepared MOFs. The corresponding signals of the MOFs, graphene and TiO2 are almost invariable from the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 samples, manifesting that no conspicuous change takes place during the combination process, which is in line with the morphology analysis. XRD curves of various samples are displayed in the Fig. 3. Both the RGO and 3DGNs show two fingerprint peaks at 26.50 and 43.10, which are induced by the (001) and (200) crystal planes of graphene (JCPDS card: 41-1487) [36]. The more sharp of the latter suggests the larger average size and lower defect density. The signals of (101), (004), (200), (105), (211) and (204) crystal planes of TiO2 are located at ~260, ~380, ~470, ~540, ~560 and ~620 (anatase phase, JCPDS card: 84-1286) [25]. The corresponding signals of graphene are buried by noise due to its discontinuous structure and low mass fraction, which has proven by the previous report [37]. The corresponding signals belonged to Uio-66 are located from 50~600, while the characteristic peaks belonged to ZIF-8 range from 50~400 [38, 39]. After hybridizing with RGO/TiO2, no distinct difference of these signals can be found from the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2. Based on the Scherrer equation, the average size of MOFs can be calculated as following [40]: (1) therein,
, ϴ and λ represent width of half maximum, diffraction angle of the
characteristic peak and wavelength of X-ray. The average sizes of pristine Uio-66 and ZIF-8 are 12.4 um and 8.9 um, and the values are consistent with the resulting photoanodes (12.2 um and 8.9 um from the Uio66-RGO/TiO2 and ZIF-8-RGO/TiO2), proving the morphology and structure of the MOFs is changeless, which is quite important to utilize the large BET area of the MOFs. IR patterns are recorded to further analyse the microstructure of these specimens (Fig. 4). The absorption peak at 1600 cm-1 is induced by the skeletal vibration of 7
graphene basal plane, and the wide peak ranges from 3000 cm-1 to 3700 cm-1 is the stretching vibration of O-H from surface adsorbed water [41]. The 780 cm-1 signal is ascribed to the Ti-O-Ti vibration of TiO2, and this peak becomes wider of the RGO/TiO2 sample because of the form of Ti-O-C bond, which can be used to confirm the chemical contact between RGO and TiO2 [42]. As for MOFs, all the major absorption peaks are marked in the Fig. 4. According to Shao’s group report, the peaks located at ~400 cm-1, ~660 cm-1 and ~740 cm-1 are induced by the mixture of O-C, C-H and Zr-O of Uio-66 [43]. The signal located at 1100 cm-1 is ascribed to the stretching vibration of Zr-O band, while the 1420 cm-1 and 1590 cm-1 peaks are attributed to the asymmetric and symmetric stretching of O-C=O [44]. As for the ZIF-8 sample, the peak at 670 cm-1 is assigned to Zn-O band, while the peak at 420 cm-1 is induced by the Zn-N band [45]. After combing with RGO/TiO2, no change can be seen from both the position and relative intensity of absorption signals because of the physical mixture process, which is in agreement with the SEM, Raman and XRD results. J-V curves of the resulting devices by using these as-prepared photoanodes are shown in the Fig. 5. The JSC and VOC of the pure TiO2 photoanode based DSSCs are 11.6 mAcm-2 and 680 mV, and the JSC increases to 13.1 mAcm-2 when the RGO/TiO2 is adopted because of the decreased resistance and enhanced electron transport ability of the photoanode. The JSC further increases to 17.8 mAcm-2 and 17.2 mAcm-2 by using the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 photoanodes, and the VOC almost keeps at a constant (the specific parameters are listed in the Table 1). Furthermore, the FF of pure TiO2, RGO/TiO2, Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 are 0.581, 0.619, 0.606 and 0.611, demonstrating that the added MOFs do not significantly degrade the conductivity. The resulting η of the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 based devices are 7.33% and 7.11%, which are 34.0% and 29.9% higher than that of the RGO/TiO2 based sample, proving the remarkably positive effect of the added MOFs. Moreover, the corresponding comparison of the obtained photovoltaic performances from the related reports and this work has been 8
summarized in the Table 2 [46-62]. Considering the specific changes of varying photovoltaic parameters, some possible reasons can be proposed. Firstly, the significantly improved JSC can be achieved by the increased photo-induced electrons and decreased dark current. Therein, a higher incident photon to current conversion efficiency (IPCE) and an increased adsorption amount of dyes will introduce the more photoelectrons. According to the change of concentration of dye before and after adsorbing by various photoanodes, the loading amount of dye molecules can be abstracted. The specific adsorption amount by using the Uio-66-RGO/TiO2 photoanode reaches 1.92×10-7 molcm-2, which is 29.7% and 120.1% higher than that of the RGO/TiO2 and pure TiO2 photoanodes. As for the ZIF-8-RGO/TiO2 photoanode, the loading capacity further increases to 2.08×10-7 molcm-2. The remarkably improved dye adsorption ability is caused by the large BET areas of MOFs. Based on N2 adsorption isotherm, the BET areas of pristine Uio-66 and ZIF-8 reach 1876.6 and 2005.4 m2g-1, and the values of the resulting Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 still maintain a high level (605.8 m2g-1 and 659.4 m2g-1), which are 3.06 times and 3.37 times higher than that of RGO/TiO2 photoanode. Adsorbing more dye molecules means more photo-generated electrons can be produced, leading to the significantly enhanced JSC. Considering Uio-66 (ZIF-8) cannot provide the electrons fast transport function that RGO can do (the conductivities of MOFs are too low to play as the electron transport channel), the large BET areas of them should be the major reason for the enhanced performances. Although the ZIF-8 possesses a more larger BET area, the resulting photovoltaic performance of the Uio-66-RGO/TiO2 is better. The core reason is that the pore diameter of the ZIF-8 is mesoporous rather than microporous, leading that the adsorption capacity for micro-molecule is not as good as that of Uio-66. Moreover, the IPCE profiles of various samples are displayed in the Fig. 6. Compared with that of pure TiO2 photoanode based device, the RGO/TiO2 assisted sample shows the higher conversion efficiency at the long wavelength range because of the visible-light activity of RGO and the comparable size of the RGO/TiO2 composite to the 9
visible-light. As for the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2, the efficiency increases over the entire wavelength range, demonstrating the scattering ability of the resulting photoanodes increases remarkably during the entire incident light spectrum, resulting to the multiple absorption effect of dye molecule and the consequently enhanced JSC. A possible reason is that the presence of RGO, Uio-66 and ZIF-8 depresses the agglomeration of TiO2 nanoparticles, endowing a higher effective BET area to TiO2. Therefore, more photons in the UV-light range can be absorbed by these composites, which leads the corresponding enhancement in the short wavelength area. To further confirm this point, photocurrents of various photoanodes are detected. According to the recorded curves, the specific values of the Uio-66 and ZIF-8 modified photoanodes are 6.23 times and 5.74 times higher than those of pure TiO2 photoanode, and their fast and uniform photocurrent response manifests the great electrical and optical performances (Fig. 7). The above results prove the higher yield of photo-induced electrons by using the MOFs assisted photoanodes. Beside yield, the obtained JSC is also closely related to the annihilation of electrons. Dark current is the major consummation of photoinduced electrons, which is induced by the annihilation of electrons and I3- ions on the ITO substrate. As shown in the Fig. 8, the dark current of the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 assisted samples are smaller than that case of the pure TiO2 when the potential is higher than 0.6 V, and the onset of the dark current is 12 mV and 11 mV higher than the pure TiO2, respectively. The RGO/TiO2 based device displays the lowest dark current when the potential higher than 0.58 V, and the onset of dark current shifts to the highest potential. The phenomenon indicating that the dark current is depressed by the presence of RGO because of the fast transport of photoinduced electrons. Although the added MOFs increase the dark current slightly, the wastage is limited in a low level. The degraded performance is due to the poor conductivities of these MOFs, and the negative effect of them on the dark current is similar with that of pure TiO2. Therefore, the high yield and low loss of photo-generated electrons bring about the significantly increased JSC of the Uio-66-RGO/TiO2 and ZIF-8-RGO/TiO2 based devices. 10
Secondly, the difference in resistance of various photoanodes can lead to the obtained varying of JSC and FF. Electrochemical impedance spectroscopy (EIS) is a useful tool to abstract resistance of photoanode and counter electrode. According to the EIS patterns shown in the Fig. 9, two semicircles can be seen from all the samples. The middle-frequency semicircle reveals the impedance of the electron transport at the interface between the electrolyte and counter electrode, and the semicircle in the high-frequency reflects the impedance of charge transfer at the electrolyte-photoanode interface [21]. The similar semicircles at high-frequency field of all the specimens is because of the identical 3DGNs counter electrode. Based on previous report, the 3DGNs counter electrode displays good catalytic ability for I-/I3- redox, which is comparable with that of the Pt electrode [24]. On the contrary, the scale of semicircle in the middle-frequency area is closely related to the adopted photoanodes. The pure TiO2 photoanode based device possesses the largest semicircle, indicating the high resistance of electron transport on the interface area between the electrolyte and photoanode. In the presence of RGO significantly decreases the semicircle resulting from the high conductivity of graphene, which is in line with the previous reports [12, 21, 22]. After adding MOFs, the corresponding semicircle enlarges slightly, implying that the added Uio-66 and ZIF-8 do not increase the impedance significantly. Therefore, the enlarged BET area of the resulting photoanode is the vital reason for the obtained high photovoltaic performances, and the high yield and low loss of photo-induced electrons resulting from the high dye loading amount and small dark current lead to the outstanding JSC and η. The mass fraction of MOFs in the photoanode exerts a remarkable influence on the photovoltaic performances of the as-prepared DSSCs. The positive effect (providing a large BET area to adsorb more dye molecules on the surface of photoanode, endowing the better scattering and absorption abilities to incident photon and producing more photo-induced electrons) cannot be given full play when the ratio of MOFs is too low, which will lead to unsatisfied photovoltaic performances. On the other hand, excess proportion of MOFs means the lower mass fractions of RGO and TiO2. The decreased 11
TiO2 is negative to obtain a high JSC (reducing the adsorption activity sites), while the decreased amount of RGO will bring about a severe agglomeration phenomenon of TiO2 nanoparticles. After corresponding optimizing (the ratio of Uio-66, RGO and TiO2 is 40:8:52), the JSC, VOC, FF and η reach 18.6 mAcm-2, 678 mV, 0.608 and 7.67% for the Uio-66-RGO/TiO2 photoanode based device, achieving the synergy between the MOF, RGO and TiO2. Similarly, the η as high as 7.22% is obtained after optimizing the mass fraction of ZIF-8 in the photoanode.
4. Conclusion MOFs includes Uio-66 and ZIF-8 are adopted to modify photoanode, and RGO is also employed to provide a better electron transport network and ameliorate the conductivity of the photoanode. The large BET area of MOFs enhances the dye loading amount significantly, which brings about a high yield of photo-induced electrons. Although a high loading amount of dye molecules introduces more photogenerated electrons, the resulting JSC is also determined by the wastage. The high IPCE of MOFs assisted device is resulting from the boosted scattering capacity of incident light, which further improves the resulting photovoltaic performances. Moreover, the relatively small dark current indicates the low loss ratio of photo-generated electrons, which is also beneficial to the resulting performances. The EIS profile manifests the impedance of the MOFs added photoanode is comparable with that of RGO/TiO2 photoanode, implying the negative influence of the MOFs on the impedance can be ignored. Therefore, the large BET area of MOFs endows a high loading amount of dye molecules and a high yield of photoinduced electrons, while the negative influence of MOFs on the dark current, impedance as well as the loss of photogenerated current can be ignored. On the other hand, the 3DGNs is used as the counter electrode to fabricate the full-carbon DSSCs, and the resulting η reaches 7.67% for the Uio-66-RGO/TiO2 photoanode based sample, which is 39.7% and 67.5% higher than the cases of using RGO/TiO2 and pure TiO2 photoanodes.
12
Acknowledges This work is supported by the Open Fund of Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum, East China, 19CX05005A-2) and the Fundamental Research Funds for the Central Universities.
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19
Figures
Fig. 1 SEM images of (a) Uio-66 (b) ZIF-8 (c) RGO/TiO2 (d) Uio-66- RGO/TiO2 (e) ZIF-8RGO/TiO2 and (f) 3DGNs.
20
Fig. 2 Raman curves of various samples.
Fig. 3 XRD patterns of various samples.
21
Fig. 4 IR profiles of various samples.
Fig. 5 J-V curves of the resulting devices by using various photoanodes.
22
Fig. 6 IPCE profiles of the resulting devices by using various photoanodes.
Fig. 7 Photocurrent signals of various photoanodes. 23
Fig. 8 Dark current profiles of the resulting devices by using various photoanodes.
Fig. 9 EIS profiles of the resulting devices by using various photoanodes, and the equivalent circuit diagram are shown as the inset.
24
Tables
Table 1 Photovoltaic properties of the resulting devices with various photoanodes. BET areas and dye loading amounts of various photoanodes are also listed. All measurements were recorded under AM-1.5G one sun and the active areas were ca. 0.15 cm2 for all cells. Photoanodes
JSC
VOC
FF
η
Dye
BET area
(mAcm-2)
(mV)
(%)
(%)
(×10-7 molcm-2)
(m2g-1)
TiO2
11.6
675
58.1
4.55
0.87
49.5
RGO/TiO2
13.1
674
61.9
5.47
1.48
197.9
ZIF-8-RGO/TiO2
17.8
679
60.6
7.33
1.92
605.8
Uio-66-RGO/TiO2
17.2
677
61.1
7.11
2.08
659.4
Optimized
18.6
678
60.8
7.67
1.98
640.5
Uio-66-RGO/TiO2
25
Table 2 The performances of reported DSSCs with various photoanodes and counter electrodes. Photoanode
Counter electrode
η (%)
Reference
TiO2
Graphite and carbon
6.67
46
TiO2
Pt/PProDOT-Et2
6.69
47
TiO2
graphene/PEDOT/PET
6.26
48
TiO2
Graphene-PPy
5.27
49
TiO2
CNTs-WS2
6.41
50
TiO2
CNTs-TiN
5.41
51
TiO2
Graphene
2.8
52
TiO2
Porous graphene
5.2
53
TiO2
GO
2.1
54
Carbon black-TiO2
Pt
5.6
55
Nanoporous
Pt
3.4
56
Carbon fibers-TiO2
Pt
1.28
57
MWCNTs-TiO2
Pt
6.21
58
Acid treated
Pt
2.16
59
Graphene-TiO2
Pt
4.28
60
Graphene oxide
Pt
4.65
61
Pt
2.7
62
ZIF-8-RGO/TiO2
3DGNs
7.33
This work
Uio-66-RGO/TiO2
3DGNs
7.67
This work
carbon-TiO2
SWCNT-TiO2
nanosheets-TiO2 Nafion-coated graphene--TiO2
26
1. MOFs are adopted to modify photoanode to enhance dye loading capacity. 2. MOFs and RGO play as different roles in the photoanode, and the synergy is achieved. 3. The 3DGNs is employed as counter electrode to prepare full-carbon DSSCs. 4. The JSC, VOC, FF and η reach 18.6 mAcm-2, 678 mV, 0.608 and 7.67%.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.