CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402143

Metal–Organic Frameworks at Interfaces in Dye-Sensitized Solar Cells Yafeng Li,[a, b, c] Caiyun Chen,[a, b] Xun Sun,[a, b] Jie Dou,[a, b] and Mingdeng Wei*[a, b] ZIF-8, a kind of widely studied metal-organic frameworks, was used for the interfacial modification of dye-sensitized solar cells by a facile post-treatment strategy for the first time, which solved the problem of severely decreased short-circuit photocurrent in previous report. After the surface treatment, the performance of cells was obviously improved. The conditions for the deposition of ZIF-8 were optimized. The best photovoltaic property was obtained when the growth time of ZIF8 was 7 min and the TiO2 photoanode was post-treated for 2 times. Besides the energy barrier effect of ZIF-8 that improved the open-circuit photovoltage and electron lifetime, the dyes adsorbed tightly on TiO2 surface was found to be a key point for the efficient electron injection and improved performance.

Dye-sensitized solar cells (DSSCs) are a potential “green” alternative to traditional silicon solar cells because they offer several advantages, such as low manufacturing costs, being more environmentally friendly, and high power conversion efficiencies.[1, 2] The photoanodes employed in the best-performing solar cells comprise TiO2 nanoparticles, which serve the dual purpose of facilitating dye uptake and charge transport. However, the use of TiO2 nanoparticles increases the density of surface defects in the photoanode and leads to increased charge recombination, resulting in device performances that deviate strongly from predicted values.[3] Therefore, interfacial charge recombination processes in DSSCs should be effectively inhibited in order to realize efficiency enhancements.[4, 5] Insulators or wide-band-gap semiconductors, such as Al2O3, ZrO2, Zn2SnO4 and carbonates, are widely used to coat the TiO2 photoanode and retard charge recombination through an energy barrier effect of energy barrier.[6–9] A sub-nanometer Ga2O3 overlayer even led to an ultrahigh open-circuit photovoltage (Voc) of 1.1 V.[10] In our previous work we reported the first use of ZIF-8, [a] Dr. Y. Li, C. Chen, X. Sun, J. Dou, Prof. M. Wei State Key Laboratory of Photocatalysis on Energy and Environment Fuzhou University Fuzhou, Fujian 350002 (PR China) E-mail: [email protected] [b] Dr. Y. Li, C. Chen, X. Sun, J. Dou, Prof. M. Wei Institute of Advanced Energy Materials Fuzhou University Fuzhou, Fujian 350002 (PR China) [c] Dr. Y. Li State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences Fuzhou, Fujian 350002 (PR China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402143.

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a type of metal–organic frameworks (MOF), to improve photovoltaic performance. This remains the only example of using MOFs for interfacial modification of DSSCs so far.[11] MOFs, which are porous crystalline materials made from inorganic nodes and organic linkers, have attracted extensive attention during recent years. The features of MOFs enable investigations in the fields of gas storage, chemical separation, catalysis, sensing, drug delivery, and others.[12–16] Recently, studies on MOFs have been extended towards applications related to clean and renewable energy owing to their interesting electrochemical properties; well-documented in recently published Review articles.[17–19] Unfortunately, although energy migration in networks of MOFs has been investigated,[20] studies on DSSCs that employ MOFs are still rare. To date, only a handful of studies have explored MOFs in photovoltaic devices: as electrolyte additive,[21] dye sensitizer,[22] or photoanode material,[23, 24] but only rather low efficiencies were achieved. As mentioned above, we have used ZIF-8 to coat a TiO2 photoanode for enhanced Voc values. However, we found that the short-circuit photocurrent (Jsc) and power conversion efficiency (h) values decreased severely with increasing ZIF-8 growth time.[11] Therefore, we should adapt our strategy of employing ZIF-8 to enhance the functioning of the DSSCs and increase their overall performance. Post-modification with Al2O3 after dye-sensitization of photoanodes is an interesting way of achieving performance enhancements.[25, 26] Herein, we present the first report of a posttreatment approach to decorate a TiO2 photoanode interface. In this strategy, the TiO2 photoanode is first sensitized by dyes and then further treated by ZIF-8 overlayer growth and dyesensitization. The TiO2/dye/ZIF-8/dye electrode obtained in this manner can be considered to have undergone one post-treatment, and hence is designated as TiO2-P1. The post-treatment procedures were also repeated more than once. In order to passivate surface states and block back-reactions, coating layers should typically be very thin.[10] The thickness of the coating layer can be easily controlled by adjusting the ZIF8 growth time.[27] That ZIF-8 can be successfully coated onto a TiO2 photoanode is described in our previous report.[11] Herein, we analyzed TiO2 photoanodes with different posttreatment times by X-ray photoelectron spectroscopy (XPS). As shown in Figure S1 (Supporting Information), two characteristic peaks occur at around 1044.7 and 1021.7 eV, which can be ascribed to Zn 2p1/2 and Zn 2p3/2, respectively. With increasing post-treatment time the intensities of these peaks increase gradually, suggesting growth of the ZIF-8 layer with increasing post-treatment numbers. The growth time of ZIF-8 in each cycle was optimized, and a growth time of 7 min according was found to be optimal, acChemSusChem 0000, 00, 1 – 4

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Figure 1. I–V curves of the cells with different post-treatment times. &: TiO2 (reference), *: 1 post-treatment, ~: 2 post-treatments, !: 3 post-treatments.

Figure 2. UV/Vis absorption spectra of dyes desorbed from TiO2 films. c: TiO2 (reference), c: 1 post-treatment, c: 2 post-treatments, c: 3 post-treatments.

cording to Figure S2 and Table S1 (Supporting Information), and the discussion presented there. The procedure was repeated to establish the influence of post-treatment time on photovoltaic performance. The I–V characteristics of the cells prepared with different post-treatment times are illustrated in Figure 1, and the parameters obtained from I–V curves are listed in Table S2 (Supporting Information). The results demonstrate that the Voc shows a steady improvement with increasing post-treatment time. For the TiO2-P3 photoanode, the Voc increased by 67 mV from 722 to 789 mV. At the same time, the Jsc slightly decreased, by 0.33 mA cm2. The obvious increase in Voc surpasses the shortage caused by the decreased Jsc and dominates the cell performance, resulting in an enhancement of its efficiency h, from 5.84 % to 6.35 %. The post-treatment approach shows its unique advantage as compared to the pretreatment procedure used in our previous study, which greatly reduced h (by 15.7 %) owing to a considerable decrease in Jsc.[11] Based on the photovoltaic properties listed in Table S2, the TiO2 photoanode treated twice showed the best overall performance: its efficiency is 14.4 % higher than the untreated one. The incident-photon-to-converted-electron (IPCE) spectra of the cells are shown in Figure S3 (Supporting Information), and show the same tendency as the I–V curves. To investigate possible reasons for the improved cell performance, the amounts of adsorbed dye were determined by UV/Vis spectroscopy. For DSSCs, a larger amount of adsorbed dye is beneficial for light-harvesting. Therefore, much attention has been paid to the fabrication of TiO2 photoanode with high specific surface areas, to enable as much dye adsorption as possible.[28, 29] However, it is very difficult to further increase the surface area of TiO2 nanoparticles. On the contrary, MOFs provide a great opportunity to acquire porous inorganic–organic hybrid materials with impressively high surface areas; one to two orders of magnitude higher than state-of-the-art TiO2.[30–32] These important, distinguishing features of MOFs (porosity and surface area) inspired us to use them in our studies on DSSCs. According to the UV/Vis spectra in Figure 2, the amount of dye

adsorbed onto TiO2-P1 (i.e., the response to the peak absorption at a wavelength of 431 nm) increases by 30 %, and if more ZIF-8 is employed, more dye is adsorbed. This illustrates that the ZIF-8 overlayer indeed enhances the amount of dye anchored, owing to its large surface area. The resulting enhanced light-harvesting ability of TiO2 photoanodes could be considered as one significant factor in improving the Jsc. As a matter of fact, the Jsc in a DSSC device can be calculated as follows:

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Jsc ¼ q hlh hinj hcc I0

ð1Þ

where I0 is the incident photon flux, q is the elementary charge, hlh is the light harvesting efficiency, hinj is the electron injection efficiency, and hcc is the charge collection efficiency.[33] After post-treatment, hlh and hcc are increased because of the larger amount of dye adsorbed and the prolonged electron lifetime, as discussed below, while hinj is decreased due to the presence of a ZIF-8 overlayer.[10] The Jsc decreased for the TiO2P3 photoanode, which means that a too-thick ZIF-8 layer reduces hinj sharply. This is precisely why the thickness of the overlayer should be restricted. Moreover, the decreased Jsc may also be due to by retarded dye regeneration, in turn due to obstructed electrolyte penetration. Interfacial charge recombination is a well-known limiting factor in the performance of DSSCs. To investigate the interfacial electron transport properties of the cells, we performed electrochemical impedance spectroscopy (EIS) measurements of the cells before and after surface treatment. Nyquist plots of the impedance data, from which the degree of interfacial charge recombination can be evaluated,[34] are shown in Figure 3. According to the equivalent circuit shown in the inset of Figure 3, the impedances calculated for the charge recombination (Rrec) of TiO2, TiO2-P1, TiO2-P2, and TiO2-P3 photoanodes are to be 14.3, 15.5, 23.9 and 48.2 W, respectively. The successive increase in Rrec implies that interfacial charge recombination is effectively inhibited by deposition of ZIF-8,[11] and the effect of the energy barrier is more prominent when the ZIF-8 ChemSusChem 0000, 00, 1 – 4

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Figure 3. Nyquist plots showing impedance data of the cells. Inset: the equivalent circuit of the cells. &: TiO2 (reference), *: 1 post-treatment, ~: 2 post-treatments, !: 3 post-treatments.

coating layer grows thicker. The trend agrees well with the results from the pretreatment procedure, suggesting an identical function of ZIF-8 in both approaches. The effect of ZIF-8 in prohibiting charge recombination was further proved by darkcurrent measurements, shown in Figure S4 (Supporting Information).[35] Therefore, coating with ZIF-8 as a post-treatment results in an interface structure that is superior in terms of suppressing charge recombination, and is favorable for the improvement of Voc, as observed above. Bode plots of the cells are shown in Figure S5 (Supporting Information). The ZIF-8 coating layer introduced by post-treatment reduces the peak frequency in the mid-frequency region, and longer post-treatment times lead to lower frequencies. The electron lifetime tn reflects the interfacial charge recombination rate and can be determined by tn = (2p fmin)1, where fmin is the peak frequency in Bode plots.[34] Thus, the electron lifetime increases with increasing post-treatment time. This implies reduced charge recombination, and matches well with the arguments discussed above. Apparently, the strategy of employing MOFs to decorate the interface in a TiO2 photoanode, developed herein, is an efficient approach towards improving the photovoltaic properties of DSSCs. We asked ourselves why the post-treatment strategy gives a much better improvement of cell performance than the pretreatment reported earlier. In both cases, the ZIF-8 overlayer functions as energy barrier and improves both the Voc and electron lifetime of the cells. Therefore, a change in Jsc is the most likely reason for the difference in performance. A comparison between photoanodes from the two procedures is shown in Figure 4. When using the pretreatment step, a ZIF-8 overlayer exists between TiO2 and dye, and hampers the injection of photo-excited electrons into the conduction band of TiO2 and thereby reduces Jsc.[11] Although electron injection is also hampered for dyes adsorbed on the ZIF-8 overlayer created by post-treatment, the dyes tightly anchored onto the TiO2 surface ensure the efficiency of photocurrent output. The additional dyes adsorbed onto ZIF-8 can contribute to the en 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. The possible photoanode statuses after sensitization in pretreatment and post-treatment approaches.

hancement of Jsc, thereby resulting in a markedly enhanced performance. In conclusion, we propose an effective strategy for efficient modification of interfaces in dye-sensitized solar cells (DSSCs). The strategy is based around the use of metal–organic frameworks (MOFs). By using a post-treatment approach, a severe decrease in Jsc is avoided due to the tightly adsorbed dyes on TiO2 photoanode in comparison to the pretreatment method. After surface treatment, the short-circuit photocurrent (Jsc) increases owing to the increased amount of dye adsorbed. Moreover, the energy barrier effect of ZIF-8 enhances the open-circuit voltage (Voc) and electron lifetime, resulting in an improved overall performance of the DSSCs. The results suggest that the use of MOFs should follow a careful design, as a subtle change in the treatment process can give rise to vastly different cell performances. This work provides useful information for the effective application of MOFs in the area of DSSCs.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21073039, 21303020), the Research Fund for the Doctoral Program of Higher Education of China (20123514120004, 20133514110002), the State Key Laboratory of Structural Chemistry (20120001), the Fuzhou University Fund (2012-XQ-13), and the Key Laboratory of Novel Thin Film Solar Cells, CAS. Keywords: electrochemistry · metal-organic frameworks · photovoltaics · solar cells · surface treatment [1] A. Yella, H. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. Nazeeruddin, E. W. Diau, D. C.-Y. Yeh, S. M. Zakeeruddin, M. Grtzel, Science 2011, 334, 629. [2] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grtzel, Nature 2013, 499, 316. [3] N.-G. Park, J. Phys. Chem. Lett. 2013, 4, 2423. [4] J. Zhao, B. Sun, L. Qiu, H. Caocen, Q. Li, X. Chen, F. Yan, J. Mater. Chem. 2012, 22, 18380.

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Received: March 4, 2014 Revised: May 5, 2014 Published online on && &&, 0000

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COMMUNICATIONS Timing is everything: ZIF-8, a widely studied metal–organic framework material, is used to modify titania/dye/electrolyte interfaces in dye-sensitized solar cells. The use of a facile post-treatment strategy, instead of a pretreatment one, solves the problem of severely decreased short-circuit photocurrents. The tight adsorption of dyes onto the TiO2 surface is a key point for efficient photocurrent output, and leads to an improved performance.

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Y. Li, C. Chen, X. Sun, J. Dou, M. Wei* && – && Metal–Organic Frameworks at Interfaces in Dye-Sensitized Solar Cells

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Metal-organic frameworks at interfaces in dye-sensitized solar cells.

ZIF-8, a kind of widely studied metal-organic frameworks, was used for the interfacial modification of dye-sensitized solar cells by a facile post-tre...
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