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Electrochemical properties of TiO2 encapsulated ZnO nanorod aggregates dye sensitized solar cells
Journal of Alloys and Compounds 537 (2012) 159–164
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrochemical properties of TiO2 encapsulated ZnO nanorod aggregates dye sensitized solar cells C. Justin Raj, S.N. Karthick, A. Dennyson Savariraj, K.V. Hemalatha, Park Song-Ki, Hee-Je Kim, K. Prabakar ⇑ Pusan National University, Department of Electrical Engineering, San 30, Jangjeong-Dong, Gumjeong-Ku, Busan 609 735, South Korea
a r t i c l e
i n f o
Article history: Received 22 March 2012 Accepted 2 May 2012 Available online 17 May 2012 Keywords: Semiconductors SEM Photoconductivity and photovoltaics X-ray diffraction Precipitation Thin films
a b s t r a c t Dye sensitized solar cells based on TiO2 encapsulated ZnO nanorod (NR) aggregates were fabricated and electrochemical performance was analyzed using impedance spectroscopy as a function of forward bias voltage. Charge transfer properties such as electron life time (sn), electron diffusion coefficient (Dn) and electron diffusion length (Ln) were calculated in order to ensure the influence of TiO2 layer over the ZnO NR aggregates. It is found that the short circuit current density (Jsc = 5.8 mA cm2), open circuit potential (Voc = 0.743 V), fill factor (FF = 0.57) and conversion efficiency are significantly improved by the introduction of TiO2 layer over ZnO photoanode. A power conversion efficiency of about 2.48% has been achieved for TiO2/ZnO cell, which is higher than that of bare ZnO NR aggregate based cells (1.73%). The formation of an inherent energy barrier between TiO2 and ZnO films and the passivation of surface traps on the ZnO film caused by the introduction of TiO2 layer increase the dye absorption and favor the electron transport which may be responsible for the enhanced performance of TiO2/ZnO cell. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSCs) have attracted considerable attention and emerged as a promising candidate for the development of next generation solar cells due to their power conversion efficiency and low production cost without the necessity of high temperature and vacuum during the fabrication [1,2]. Their energy conversion is based on the injection of electrons from a photoexcited state of the sensitizer (dye) into the conduction band of the semiconductor. A charge mediator, and a suitable redox couple, must be added to the electrolyte to reduce the oxidized dye. The mediator also needs to be renewed in the counter-electrode, which provides the DSSC to be regenerated [3–5]. Light harvesting and electron collection are the two critical factors determining the efficiency of dye sensitized solar cells; which are directly related to the photoanode [6]. Apart from this, dye loading, light absorption and the electron transport path determining the photo-carrier collection are the crucial parts of photoanode [7]. Mainly the nanopores TiO2 materials are used as photoanode in DSSCs. The wide-band-gap (3.37 eV) and a large exciton binding energy (60 meV) of ZnO nanostructure at room temperature [8,9] are also attractive and is a proficient substitute semiconductor to TiO2. There are few fascinating facts which draw the attention of researchers on ZnO than TiO2, like the high electron mobility and the close band gap energy position. However, ⇑ Corresponding author. Tel.: +82 51 510 7334; fax: +82 51 513 0212. E-mail address: [email protected] (K. Prabakar). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.012
the overall conversion efficiency of ZnO photoanode DSSC is lower when compared to TiO2 based photoanodes. The main reason for this is attributed to excessive dye aggregation on the ZnO surface [10], which in turn causes slower electron injection from the dye to ZnO [11]. Efforts were made to improve the photovoltaic parameters such as the short circuit photocurrent (Jsc), open circuit photovoltage (Voc), and the fill factor (FF) to achieve high performance in ZnO based DSSCs. As a critical parameter, Voc is determined by the difference between the quasi-Fermi level of electrons in the semiconductor film and the energy of the redox potential of the redox couple in the electrolyte. The semiconductor’s conduction band edge potential and the recombination of charge carriers at the photoelectrode/electrolyte interface will influence the Voc [12–14]. Both chemical and physical methods have been carried out to improve the Voc and the performance of the DSSCs, such as forming core-shell structure of the photoanode [15,16], doping appropriate materials [17,18] and fabricating photoanode from different morphological ZnO nanoparticles [19–21]. Electrochemical impedance spectroscopy (EIS) has been a widely employed tool to study the kinetics of photoanode, electrochemical and photo-electrochemical processes occurring in the DSSC [22,23]. Bisquert et al. [24,25], Kern et al. [26] and Adachi et al. [27] have developed two different general formulations of impedance models in theoretical and experimental aspects for electron diffusion and recombination in a thin layer of porous electrodes in the case of DSSCs. They afforded reliable values of the parameters relating to electron transport in DSSCs by conducting different experiments by varying different parameters of the cells
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and suggested the requisites for making efficient DSSCs. In this present manuscript we report the simple one step synthesis of ZnO NR aggregates and their performance in dye sensitized solar cells with TiO2 encapsulation over it. The detailed studies of recombination mechanism and transport properties offered by the encapsulation of TiO2 over the ZnO NR DSSCs were analyzed for the first time based on diffusion-recombination model using the impedance spectroscopy. 2. Experimental methods 2.1. Synthesis of ZnO nanorod aggregates A simple co-precipitation technique was involved in the synthesis of ZnO NR aggregates. Zinc chloride dihydrate (Sigma–Aldrich) was dissolved in deionized water and ammonium hydroxide solution was added drop wise to precipitate their hydroxides. The zinc hydroxide precipitated solution was maintained at pH = 8 and continuously stirred and refluxed at 90 °C for 2 h. The solid mass after refluxing was found to be crystalline and free flowing as well. Then, the crystalline nanopowder formed was filtered, thoroughly washed, oven dried and heat treated at 400 °C for 1 h. 2.2. Fabrications of ZnO and ZnO/TiO2 photoanodes The photoanode was deposited on FTO (fluor- doped SnO2 glass) glass substrate by adopting doctor blading technique. The paste was prepared by adding as synthesized 0.4 g ZnO nanopowder to 2 ml of an ethanol-water mixture (3:1) with 0.08 g of poly-(ethylene glycol) and 0.2 ml of Titan X100 solution. The paste was grounded thoroughly and ultrasonicated for 1 h and doctor bladed (area = 0.25 cm2) over ultrasonically cleaned substrate and was sintered at 400 °C for 1 h. The TiO2 sol– gel were prepared by slowly adding 3 ml of titanium isopropoxide (Aldrich) into 30 ml of anhydrous ethanol under stirring followed by addition of 1 ml of water which turned the solution into milky white colour. To accelerate the process of hydrolysis, 1 ml of HCl was used as a catalyst which turned the milky white colour back to transparent solution. At the end of 24 h of aging, the solution was a yellowish transparent fluid ready to be used in sol–gel deposition processes [16]. The ZnO nanorods aggregates deposited photoanode was dipped and withdrawn from these TiO2 sol–gel solutions at 3 cm per min pulling rate followed by annealing at 450 °C for 30 min. 2.3. Fabrications of DSSCs The photo electrodes were soaked in a 0.2 mM N719 dye (cis - bis (isothiocyanato) bis (2, 20 – bipyridyl – 4, 40 – discarboxylato) – ruthenium (II) – bis -tetrabutylammonium RuC58H86N8O8S2) solution for overnight at 35 °C and the excess dye was removed by soaking the photoanode in 99.9% ethanol. Platinum counter electrodes were deposited from a commercial platinum paste (Solaronix, platinum catalyst T/SP) on a FTO glass substrate, which was subsequently heat treated at 400 °C for 30 min in air. Finally, the photo anodes and the counter electrodes were sealed using a 25 lm hot-melt sealing sheet (SX 1170-25, Solaronix) and the internal space was filled with a redox liquid electrolyte of 0.5 M LiI, 0.05 M I2 and 0.5 M 4-tertbutylpyridine in acetonitrile. 2.4. Characterizations The crystalline nature of the ZnO NR aggregate was analyzed by X-ray diffraction (XRD, D/Max-2400, Rigaku) using a Cu Ka source operated at 40 kV and 30 mA in the 2h range of 10–80°. The surface morphology and thickness measurements of the ZnO and ZnO/TiO2 thin films and composition were analyzed using field emission scanning electron microscope and Energy dispersive X-ray analysis (FE-SEM, S-4200, Hitachi) operated at 15 kV. The amount of dye absorbed in the photoanode layer was observed through optical absorption spectrum (OPTIZEN 3220 UV spectrophotometer) of dye desorbed using 0.1 M NaOH solution. The current–voltage characteristics of the DSSCs were studied under 1 sun illumination (AM 1.5G, 100 mW cm2) with San Ei Electric (XES 301S, Japan) solar simulator having the irradiance uniformity of ±3%. Electrochemical impedance spectroscopy (EIS) was performed using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) for different biasing voltage (0.4 V – Voc) and the charge transport properties including electron lifetime, diffusion coefficient, and effective diffusion length were measured under a dark condition.
forming a micron sized spherical ZnO aggregates. A thin layered structure was observed on the surface of TiO2/ZnO aggregates film (Fig. 1b) which shows that the TiO2 forms a thin layer over ZnO NR aggregates. Further it was confirmed through energy dispersion Xray analysis shown in Fig. 2a, about 5.82% of titanium was observed on the surface of ZnO NR aggregates. Fig. 2b shows the XRD pattern of the ZnO NR aggregates and identified with those of ZnO wurzite (hexagonal phase, space group P63mc) structure crystallization as reported in JCPDS card (No. 36-1451). The photocurrent density (Jsc) and open circuit voltage (Voc) characteristics of ZnO and TiO2/ZnO are shown in Fig. 3 with an anode thickness of 18 ± 0.25 lm (bare ZnO), 16 ± 0.5 lm and active area of 0.25 cm2 under 1 sun. The overall fill factor (FF) and conversion efficiency (g) can be calculated using the relations FF = Pmax/ (Voc Jsc) and g = [(FF Voc Jsc)/Pin] 100, respectively, where Pmax = (Vmax Imax) and Pin are the maximum power density and input power density, respectively. The solar cell parameters of these DSSCs are summarized and inserted in figure. The TiO2 encapsulated ZnO NR aggregate DSSC shows an enhanced performance than that of ZnO NR aggregate DSSCs. Fig. 4 shows the optical absorption spectrum of the desorbed dye from the photoanode. From this spectrum, it is confirmed that the dye absorption is maximum for the TiO2/ZnO photoanodes than that of bare ZnO NR [28]. This is supposed to be the formation of large surface area due to thin layer of TiO2 nanoparticles over the ZnO NR which improve the dye loading in the photoanode. The high improvement in the current density (Jsc), open-circuit voltage (Voc) and fill factor (FF) of the TiO2/ZnO DSSCs compared with ZnO aggregates based DSSC, shows a positive influence of TiO2 layer over ZnO photoanodes. This seems to make the TiO2 layer highly effective to increase the surface stability of ZnO photoanode against the acidic dye and improve performance of the DSSCs. Electrochemical impedance spectroscopy (EIS) was used to characterize the internal resistance, recombination properties and charge-transfer kinetics of ZnO and TiO2/ZnO photoanodes [23]. Under bias voltage condition, the electrons are injected into the photoanode (dye loaded semiconductor network) through the FTO substrate, and the photoanode gets charged by transport of the injected electrons. At the same time, some of the injected electrons in the conduction band of photoanode are recombined with the redox electrolyte. These processes are denoted as semicircles at different frequency ranges in a Nyquist plot: charge transfer at FTO/semiconductor interface and charge transfer at counter electrode/electrolyte interface are in the higher frequency and in the mid-frequency range, charge transfer at photoanode/electrolyte interface and ion diffusion in the electrolyte are in lower frequency range [25,29,30]. The Nyquist plots (Z⁄) for the TiO2/ZnO and ZnO DSSC analyzed in open circuit potential (Voc) under dark condition in the frequency range of 0.1 Hz to 1 MHz are shown in Fig. 5. The Z⁄ plot of ZnO and TiO2/ZnO photoanodes measured under open circuit potential consists of three semicircle with a nonzero intercept at high frequency end. Impedance analysis software Z VIEW was used to model the impedance spectra based on diffusion-recombination model, the equivalent circuit [25,31] consisting of a series of parallel RC circuit and a Warburg element as shown in the inset of Fig. 6. A series resistor (Rs) is added to the circuit to account for the nonzero intercept on the real axis of the impedance plot which represents FTO and the contact resistance of FTO/ZnO. The Warburg element can be used to monitor mass transport limitations in the DSSC through the observation of diffusion resistance Rd with the characteristic frequency xd as given in Eq. (1)
3. Results and discussion
Z d ¼ Rd tanh½ðix=xd Þ1=2 =ðix=xd Þ1=2 Fig. 1a and b show the SEM image of ZnO NR aggregates and TiO2 encapsulated ZnO nanorod aggregate films, respectively. Fig. 1a and the inset show a bunch of well defined hexagonal ZnO nanorods
ð1Þ
where i = (1)1/2 and x is the frequency of the potential perturbation. The electron transfers resistance (Rct) at counter electrode/
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Fig. 1. SEM images of (a) ZnO NR aggregate (b) TiO2/ZnO NR aggregate.
Fig. 2. XRD pattern of (a) ZnO NR aggregate (b) EDAX spectrum TiO2/ZnO NR aggregate film.
Fig. 3. I–V Behaviour of solar cells consisting of (a) ZnO NR aggregate photoanode (b) TiO2/ZnO NR aggregate photoanode.
electrolyte, charge transfer resistance (Rk) at photoelectrode/electrolyte interface, the diffusion resistance (Rd) of electrolyte and the chemical capacitance (Cl) were directly obtained from the fit. The Rs component, correlated to the FTO sheet resistance and ZnO/FTO contact resistance, was 18.32 X for TiO2/ZnO which is much less than that of bare ZnO contact resistance which is 36.83 X. This decrease in contact resistance for the TiO2 encapsulated ZnO aggregates electrode could be due to the better contact of the coatings to the substrate [32] through heat treatment with the fine nanoparticle layer of TiO2.
Fig. 4. Absorption spectrum of dye desorbed from photoanodes.
Fig. 6a and b shows the impedance spectra for the ZnO and TiO2/ ZnO DSSCs under different applied bias in dark condition. From the plot it is observed that the diameter of the lower frequency semicircle decreases with increase in bias voltage. This can be explained as; at bias condition, the electrons are injected into conduction band of the ZnO photoanode and recombine with the I3 of the redox electrolyte. The rate of this back reaction is faster as the bias voltage tends towards the Voc. Therefore, the decrease of lower frequency semicircle under different applied bias can be explained as the change of the number of electrons recombining with the electrolyte
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high chemical capacitance (Cl) indicating faster charge transport at the interfaces of TiO2/ZnO/electrolyte and the dye. The charge transfer resistance slightly decreases for the TiO2 encapsulated photoanode which may be due to the variation of surface roughness over the ZnO NR aggregates that tends to large recombination of the electron with the electrolyte. This tends to increase the chemical capacitance of the photoanode due to large injection and accumulation of photo exited electron in the conduction band of the photoanode. The high value of charge transfer resistance and diffusion resistance (increase in internal resistance) and lower value of chemical capacitance of ZnO photoanode implies a reduction of electrons transfer with poor efficiency [23,34]. Fig. 9a–c shows the variation of charge transfer properties such as electron life time (sn), electron diffusion coefficient (Dn) and diffusion length (Ln) with respect to the applied bias voltage. These parameters are calculated from the charge transfer resistance (Rk), diffusion resistance (Rd) and chemical capacitance (Cl) as follows [30] Fig. 5. Nyquist plots of the ZnO and TiO2/ZnO NR aggregate photoanode based DSSCs under open circuit voltage condition.
sn ¼ Rk C l 2
at the ZnO/electrolyte interface. Thus the semicircular arc related closely to the short circuit current due to the back reaction; expansion of semicircle corresponds to a decrease in the short circuit current. TiO2/ZnO photoanode DSSC shows that the lower frequency arc is less than that of ZnO photoanode due to high short circuit current than ZnO DSSC [30,33]. This can be also explained using Bode plot as shown in Fig. 7a and b, the mid frequency peak of the Bode plot was attributed to the electron transfer process at the ZnO/ dye/electrolyte interface. The minimum angular frequency (xp) associated with the charge transfer resistance and chemical capacitance are defined as xp = (Rk Cl)1, i.e. xp = (sn)1. As the bias voltage increases the amplitude of the mid frequency peak decreases and the xp shifted towards the higher frequency due to large back reaction of electron with the electrolyte at high bias potential and which tends to reduce the life time of electron (fast back reaction with electrolyte) [26]. Fig. 8a–c shows the variation of charge transfer resistance (Rk), chemical capacitance (Cl) and diffusion resistance (Rd), respectively with respect to the applied bias voltage calculated from the fitted results. The value Rk, Cl, and Rd shows an exponential dependent on the applied bias voltage. The results showed that the TiO2/ZnO DSSC photoanode exhibited the smallest charge transfer resistance Rk and
Dn ¼ L =Rd C l pffiffiffi Ln ¼ sn Dn
ð2Þ ð3Þ ð4Þ
where L is the thickness of the film. From Fig. 9a and b the electron life time shows an exponential variation with respect to the bias voltage. The longer electron life time and slower electron diffusion coefficient are associated with the trapping-detrapping of the electron in the conduction band of ZnO [30,35,36]. The electron life time was higher for the bare ZnO DSSC up to a particular bias voltage (0.55 V) due to the large recombination resistance (Rk) and the sn increases within higher bias voltage in the case of TiO2/ZnO DSSC. This is attributed to the variation in the chemical capacitance owing to large accumulation of photo-injected electrons in the conduction band of TiO2/ZnO at higher applied potential. The improvement in the (Dn) of TiO2/ZnO DSSCs is associated with the declination of diffusion resistance and variation in chemical capacitance as per Eq. (3). From Fig. 9c it is observed that the diffusion length (Ln) depends on the bias voltage. The Ln of both the cells show nearly same in lower bias voltage and it tends to improve for TiO2/ZnO cell at higher bias potential. Since Ln is directly related with the charge collection efficiency of the DSSC [30]. Thus large current density of TiO2/ZnO DSSC tends to enhance the diffusion length of the electron than that of bare ZnO DSSC.
Fig. 6. Nyquist plots of the (a) ZnO and (b) TiO2/ZnO NR aggregate DSSCs under dark for various bias voltages.
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Fig. 7. Bode plot of (a) ZnO and (b) TiO2/ZnO NR aggregate DSSCs under dark for various bias voltages.
Fig. 8. Variation of (a) charge transfer resistance, (b) chemical capacitance and (c) diffusion resistance with respect to bias voltages.
Fig. 9. Variation of (a) effective electron life time, (b) electron diffusion coefficient and (c) electron diffusion length with respect to bias voltages.
4. Conclusion Electron transfer properties of ZnO nanorod aggregates and TiO2 encapsulated ZnO NR aggregates based DSSCs were analyzed using electrochemical impedance spectroscopy under various forward
bias voltage conditions. The high chemical capacitance and electron diffusion length are attributed to large electron collection and transfer in the TiO2/ZnO DSSCs than that of bare ZnO DSSC. The TiO2/ZnO DSSCs show high efficiency (2.48%) and enhanced electrochemical properties compared to that of bare ZnO NR aggregate
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DSSCs. The presence of TiO2 nanoparticles over the ZnO NR improve the surface contact of the ZnO with the FTO substrate, enhances the dye adsorption which proved to be an excellent photovoltaic cell performance. Acknowledgement The author C.J.R. would like to thank the BRAIN KOREA21 (BK21) for its financial support. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0009749). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrochemical properties of TiO2 encapsulated ZnO nanorod aggregates dye sensitized solar cells C. Justin Raj, S.N. Karthick, A. Dennyson Savariraj, K.V. Hemalatha, Park Song-Ki, Hee-Je Kim, K. Prabakar ⇑ Pusan National University, Department of Electrical Engineering, San 30, Jangjeong-Dong, Gumjeong-Ku, Busan 609 735, South Korea
a r t i c l e
i n f o
Article history: Received 22 March 2012 Accepted 2 May 2012 Available online 17 May 2012 Keywords: Semiconductors SEM Photoconductivity and photovoltaics X-ray diffraction Precipitation Thin films
a b s t r a c t Dye sensitized solar cells based on TiO2 encapsulated ZnO nanorod (NR) aggregates were fabricated and electrochemical performance was analyzed using impedance spectroscopy as a function of forward bias voltage. Charge transfer properties such as electron life time (sn), electron diffusion coefficient (Dn) and electron diffusion length (Ln) were calculated in order to ensure the influence of TiO2 layer over the ZnO NR aggregates. It is found that the short circuit current density (Jsc = 5.8 mA cm2), open circuit potential (Voc = 0.743 V), fill factor (FF = 0.57) and conversion efficiency are significantly improved by the introduction of TiO2 layer over ZnO photoanode. A power conversion efficiency of about 2.48% has been achieved for TiO2/ZnO cell, which is higher than that of bare ZnO NR aggregate based cells (1.73%). The formation of an inherent energy barrier between TiO2 and ZnO films and the passivation of surface traps on the ZnO film caused by the introduction of TiO2 layer increase the dye absorption and favor the electron transport which may be responsible for the enhanced performance of TiO2/ZnO cell. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSCs) have attracted considerable attention and emerged as a promising candidate for the development of next generation solar cells due to their power conversion efficiency and low production cost without the necessity of high temperature and vacuum during the fabrication [1,2]. Their energy conversion is based on the injection of electrons from a photoexcited state of the sensitizer (dye) into the conduction band of the semiconductor. A charge mediator, and a suitable redox couple, must be added to the electrolyte to reduce the oxidized dye. The mediator also needs to be renewed in the counter-electrode, which provides the DSSC to be regenerated [3–5]. Light harvesting and electron collection are the two critical factors determining the efficiency of dye sensitized solar cells; which are directly related to the photoanode [6]. Apart from this, dye loading, light absorption and the electron transport path determining the photo-carrier collection are the crucial parts of photoanode [7]. Mainly the nanopores TiO2 materials are used as photoanode in DSSCs. The wide-band-gap (3.37 eV) and a large exciton binding energy (60 meV) of ZnO nanostructure at room temperature [8,9] are also attractive and is a proficient substitute semiconductor to TiO2. There are few fascinating facts which draw the attention of researchers on ZnO than TiO2, like the high electron mobility and the close band gap energy position. However, ⇑ Corresponding author. Tel.: +82 51 510 7334; fax: +82 51 513 0212. E-mail address: [email protected] (K. Prabakar). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.012
the overall conversion efficiency of ZnO photoanode DSSC is lower when compared to TiO2 based photoanodes. The main reason for this is attributed to excessive dye aggregation on the ZnO surface [10], which in turn causes slower electron injection from the dye to ZnO [11]. Efforts were made to improve the photovoltaic parameters such as the short circuit photocurrent (Jsc), open circuit photovoltage (Voc), and the fill factor (FF) to achieve high performance in ZnO based DSSCs. As a critical parameter, Voc is determined by the difference between the quasi-Fermi level of electrons in the semiconductor film and the energy of the redox potential of the redox couple in the electrolyte. The semiconductor’s conduction band edge potential and the recombination of charge carriers at the photoelectrode/electrolyte interface will influence the Voc [12–14]. Both chemical and physical methods have been carried out to improve the Voc and the performance of the DSSCs, such as forming core-shell structure of the photoanode [15,16], doping appropriate materials [17,18] and fabricating photoanode from different morphological ZnO nanoparticles [19–21]. Electrochemical impedance spectroscopy (EIS) has been a widely employed tool to study the kinetics of photoanode, electrochemical and photo-electrochemical processes occurring in the DSSC [22,23]. Bisquert et al. [24,25], Kern et al. [26] and Adachi et al. [27] have developed two different general formulations of impedance models in theoretical and experimental aspects for electron diffusion and recombination in a thin layer of porous electrodes in the case of DSSCs. They afforded reliable values of the parameters relating to electron transport in DSSCs by conducting different experiments by varying different parameters of the cells
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and suggested the requisites for making efficient DSSCs. In this present manuscript we report the simple one step synthesis of ZnO NR aggregates and their performance in dye sensitized solar cells with TiO2 encapsulation over it. The detailed studies of recombination mechanism and transport properties offered by the encapsulation of TiO2 over the ZnO NR DSSCs were analyzed for the first time based on diffusion-recombination model using the impedance spectroscopy. 2. Experimental methods 2.1. Synthesis of ZnO nanorod aggregates A simple co-precipitation technique was involved in the synthesis of ZnO NR aggregates. Zinc chloride dihydrate (Sigma–Aldrich) was dissolved in deionized water and ammonium hydroxide solution was added drop wise to precipitate their hydroxides. The zinc hydroxide precipitated solution was maintained at pH = 8 and continuously stirred and refluxed at 90 °C for 2 h. The solid mass after refluxing was found to be crystalline and free flowing as well. Then, the crystalline nanopowder formed was filtered, thoroughly washed, oven dried and heat treated at 400 °C for 1 h. 2.2. Fabrications of ZnO and ZnO/TiO2 photoanodes The photoanode was deposited on FTO (fluor- doped SnO2 glass) glass substrate by adopting doctor blading technique. The paste was prepared by adding as synthesized 0.4 g ZnO nanopowder to 2 ml of an ethanol-water mixture (3:1) with 0.08 g of poly-(ethylene glycol) and 0.2 ml of Titan X100 solution. The paste was grounded thoroughly and ultrasonicated for 1 h and doctor bladed (area = 0.25 cm2) over ultrasonically cleaned substrate and was sintered at 400 °C for 1 h. The TiO2 sol– gel were prepared by slowly adding 3 ml of titanium isopropoxide (Aldrich) into 30 ml of anhydrous ethanol under stirring followed by addition of 1 ml of water which turned the solution into milky white colour. To accelerate the process of hydrolysis, 1 ml of HCl was used as a catalyst which turned the milky white colour back to transparent solution. At the end of 24 h of aging, the solution was a yellowish transparent fluid ready to be used in sol–gel deposition processes [16]. The ZnO nanorods aggregates deposited photoanode was dipped and withdrawn from these TiO2 sol–gel solutions at 3 cm per min pulling rate followed by annealing at 450 °C for 30 min. 2.3. Fabrications of DSSCs The photo electrodes were soaked in a 0.2 mM N719 dye (cis - bis (isothiocyanato) bis (2, 20 – bipyridyl – 4, 40 – discarboxylato) – ruthenium (II) – bis -tetrabutylammonium RuC58H86N8O8S2) solution for overnight at 35 °C and the excess dye was removed by soaking the photoanode in 99.9% ethanol. Platinum counter electrodes were deposited from a commercial platinum paste (Solaronix, platinum catalyst T/SP) on a FTO glass substrate, which was subsequently heat treated at 400 °C for 30 min in air. Finally, the photo anodes and the counter electrodes were sealed using a 25 lm hot-melt sealing sheet (SX 1170-25, Solaronix) and the internal space was filled with a redox liquid electrolyte of 0.5 M LiI, 0.05 M I2 and 0.5 M 4-tertbutylpyridine in acetonitrile. 2.4. Characterizations The crystalline nature of the ZnO NR aggregate was analyzed by X-ray diffraction (XRD, D/Max-2400, Rigaku) using a Cu Ka source operated at 40 kV and 30 mA in the 2h range of 10–80°. The surface morphology and thickness measurements of the ZnO and ZnO/TiO2 thin films and composition were analyzed using field emission scanning electron microscope and Energy dispersive X-ray analysis (FE-SEM, S-4200, Hitachi) operated at 15 kV. The amount of dye absorbed in the photoanode layer was observed through optical absorption spectrum (OPTIZEN 3220 UV spectrophotometer) of dye desorbed using 0.1 M NaOH solution. The current–voltage characteristics of the DSSCs were studied under 1 sun illumination (AM 1.5G, 100 mW cm2) with San Ei Electric (XES 301S, Japan) solar simulator having the irradiance uniformity of ±3%. Electrochemical impedance spectroscopy (EIS) was performed using a BioLogic potentiostat/galvanostat/EIS analyzer (SP-150, France) for different biasing voltage (0.4 V – Voc) and the charge transport properties including electron lifetime, diffusion coefficient, and effective diffusion length were measured under a dark condition.
forming a micron sized spherical ZnO aggregates. A thin layered structure was observed on the surface of TiO2/ZnO aggregates film (Fig. 1b) which shows that the TiO2 forms a thin layer over ZnO NR aggregates. Further it was confirmed through energy dispersion Xray analysis shown in Fig. 2a, about 5.82% of titanium was observed on the surface of ZnO NR aggregates. Fig. 2b shows the XRD pattern of the ZnO NR aggregates and identified with those of ZnO wurzite (hexagonal phase, space group P63mc) structure crystallization as reported in JCPDS card (No. 36-1451). The photocurrent density (Jsc) and open circuit voltage (Voc) characteristics of ZnO and TiO2/ZnO are shown in Fig. 3 with an anode thickness of 18 ± 0.25 lm (bare ZnO), 16 ± 0.5 lm and active area of 0.25 cm2 under 1 sun. The overall fill factor (FF) and conversion efficiency (g) can be calculated using the relations FF = Pmax/ (Voc Jsc) and g = [(FF Voc Jsc)/Pin] 100, respectively, where Pmax = (Vmax Imax) and Pin are the maximum power density and input power density, respectively. The solar cell parameters of these DSSCs are summarized and inserted in figure. The TiO2 encapsulated ZnO NR aggregate DSSC shows an enhanced performance than that of ZnO NR aggregate DSSCs. Fig. 4 shows the optical absorption spectrum of the desorbed dye from the photoanode. From this spectrum, it is confirmed that the dye absorption is maximum for the TiO2/ZnO photoanodes than that of bare ZnO NR [28]. This is supposed to be the formation of large surface area due to thin layer of TiO2 nanoparticles over the ZnO NR which improve the dye loading in the photoanode. The high improvement in the current density (Jsc), open-circuit voltage (Voc) and fill factor (FF) of the TiO2/ZnO DSSCs compared with ZnO aggregates based DSSC, shows a positive influence of TiO2 layer over ZnO photoanodes. This seems to make the TiO2 layer highly effective to increase the surface stability of ZnO photoanode against the acidic dye and improve performance of the DSSCs. Electrochemical impedance spectroscopy (EIS) was used to characterize the internal resistance, recombination properties and charge-transfer kinetics of ZnO and TiO2/ZnO photoanodes [23]. Under bias voltage condition, the electrons are injected into the photoanode (dye loaded semiconductor network) through the FTO substrate, and the photoanode gets charged by transport of the injected electrons. At the same time, some of the injected electrons in the conduction band of photoanode are recombined with the redox electrolyte. These processes are denoted as semicircles at different frequency ranges in a Nyquist plot: charge transfer at FTO/semiconductor interface and charge transfer at counter electrode/electrolyte interface are in the higher frequency and in the mid-frequency range, charge transfer at photoanode/electrolyte interface and ion diffusion in the electrolyte are in lower frequency range [25,29,30]. The Nyquist plots (Z⁄) for the TiO2/ZnO and ZnO DSSC analyzed in open circuit potential (Voc) under dark condition in the frequency range of 0.1 Hz to 1 MHz are shown in Fig. 5. The Z⁄ plot of ZnO and TiO2/ZnO photoanodes measured under open circuit potential consists of three semicircle with a nonzero intercept at high frequency end. Impedance analysis software Z VIEW was used to model the impedance spectra based on diffusion-recombination model, the equivalent circuit [25,31] consisting of a series of parallel RC circuit and a Warburg element as shown in the inset of Fig. 6. A series resistor (Rs) is added to the circuit to account for the nonzero intercept on the real axis of the impedance plot which represents FTO and the contact resistance of FTO/ZnO. The Warburg element can be used to monitor mass transport limitations in the DSSC through the observation of diffusion resistance Rd with the characteristic frequency xd as given in Eq. (1)
3. Results and discussion
Z d ¼ Rd tanh½ðix=xd Þ1=2 =ðix=xd Þ1=2 Fig. 1a and b show the SEM image of ZnO NR aggregates and TiO2 encapsulated ZnO nanorod aggregate films, respectively. Fig. 1a and the inset show a bunch of well defined hexagonal ZnO nanorods
ð1Þ
where i = (1)1/2 and x is the frequency of the potential perturbation. The electron transfers resistance (Rct) at counter electrode/
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Fig. 1. SEM images of (a) ZnO NR aggregate (b) TiO2/ZnO NR aggregate.
Fig. 2. XRD pattern of (a) ZnO NR aggregate (b) EDAX spectrum TiO2/ZnO NR aggregate film.
Fig. 3. I–V Behaviour of solar cells consisting of (a) ZnO NR aggregate photoanode (b) TiO2/ZnO NR aggregate photoanode.
electrolyte, charge transfer resistance (Rk) at photoelectrode/electrolyte interface, the diffusion resistance (Rd) of electrolyte and the chemical capacitance (Cl) were directly obtained from the fit. The Rs component, correlated to the FTO sheet resistance and ZnO/FTO contact resistance, was 18.32 X for TiO2/ZnO which is much less than that of bare ZnO contact resistance which is 36.83 X. This decrease in contact resistance for the TiO2 encapsulated ZnO aggregates electrode could be due to the better contact of the coatings to the substrate [32] through heat treatment with the fine nanoparticle layer of TiO2.
Fig. 4. Absorption spectrum of dye desorbed from photoanodes.
Fig. 6a and b shows the impedance spectra for the ZnO and TiO2/ ZnO DSSCs under different applied bias in dark condition. From the plot it is observed that the diameter of the lower frequency semicircle decreases with increase in bias voltage. This can be explained as; at bias condition, the electrons are injected into conduction band of the ZnO photoanode and recombine with the I3 of the redox electrolyte. The rate of this back reaction is faster as the bias voltage tends towards the Voc. Therefore, the decrease of lower frequency semicircle under different applied bias can be explained as the change of the number of electrons recombining with the electrolyte
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high chemical capacitance (Cl) indicating faster charge transport at the interfaces of TiO2/ZnO/electrolyte and the dye. The charge transfer resistance slightly decreases for the TiO2 encapsulated photoanode which may be due to the variation of surface roughness over the ZnO NR aggregates that tends to large recombination of the electron with the electrolyte. This tends to increase the chemical capacitance of the photoanode due to large injection and accumulation of photo exited electron in the conduction band of the photoanode. The high value of charge transfer resistance and diffusion resistance (increase in internal resistance) and lower value of chemical capacitance of ZnO photoanode implies a reduction of electrons transfer with poor efficiency [23,34]. Fig. 9a–c shows the variation of charge transfer properties such as electron life time (sn), electron diffusion coefficient (Dn) and diffusion length (Ln) with respect to the applied bias voltage. These parameters are calculated from the charge transfer resistance (Rk), diffusion resistance (Rd) and chemical capacitance (Cl) as follows [30] Fig. 5. Nyquist plots of the ZnO and TiO2/ZnO NR aggregate photoanode based DSSCs under open circuit voltage condition.
sn ¼ Rk C l 2
at the ZnO/electrolyte interface. Thus the semicircular arc related closely to the short circuit current due to the back reaction; expansion of semicircle corresponds to a decrease in the short circuit current. TiO2/ZnO photoanode DSSC shows that the lower frequency arc is less than that of ZnO photoanode due to high short circuit current than ZnO DSSC [30,33]. This can be also explained using Bode plot as shown in Fig. 7a and b, the mid frequency peak of the Bode plot was attributed to the electron transfer process at the ZnO/ dye/electrolyte interface. The minimum angular frequency (xp) associated with the charge transfer resistance and chemical capacitance are defined as xp = (Rk Cl)1, i.e. xp = (sn)1. As the bias voltage increases the amplitude of the mid frequency peak decreases and the xp shifted towards the higher frequency due to large back reaction of electron with the electrolyte at high bias potential and which tends to reduce the life time of electron (fast back reaction with electrolyte) [26]. Fig. 8a–c shows the variation of charge transfer resistance (Rk), chemical capacitance (Cl) and diffusion resistance (Rd), respectively with respect to the applied bias voltage calculated from the fitted results. The value Rk, Cl, and Rd shows an exponential dependent on the applied bias voltage. The results showed that the TiO2/ZnO DSSC photoanode exhibited the smallest charge transfer resistance Rk and
Dn ¼ L =Rd C l pffiffiffi Ln ¼ sn Dn
ð2Þ ð3Þ ð4Þ
where L is the thickness of the film. From Fig. 9a and b the electron life time shows an exponential variation with respect to the bias voltage. The longer electron life time and slower electron diffusion coefficient are associated with the trapping-detrapping of the electron in the conduction band of ZnO [30,35,36]. The electron life time was higher for the bare ZnO DSSC up to a particular bias voltage (0.55 V) due to the large recombination resistance (Rk) and the sn increases within higher bias voltage in the case of TiO2/ZnO DSSC. This is attributed to the variation in the chemical capacitance owing to large accumulation of photo-injected electrons in the conduction band of TiO2/ZnO at higher applied potential. The improvement in the (Dn) of TiO2/ZnO DSSCs is associated with the declination of diffusion resistance and variation in chemical capacitance as per Eq. (3). From Fig. 9c it is observed that the diffusion length (Ln) depends on the bias voltage. The Ln of both the cells show nearly same in lower bias voltage and it tends to improve for TiO2/ZnO cell at higher bias potential. Since Ln is directly related with the charge collection efficiency of the DSSC [30]. Thus large current density of TiO2/ZnO DSSC tends to enhance the diffusion length of the electron than that of bare ZnO DSSC.
Fig. 6. Nyquist plots of the (a) ZnO and (b) TiO2/ZnO NR aggregate DSSCs under dark for various bias voltages.
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Fig. 7. Bode plot of (a) ZnO and (b) TiO2/ZnO NR aggregate DSSCs under dark for various bias voltages.
Fig. 8. Variation of (a) charge transfer resistance, (b) chemical capacitance and (c) diffusion resistance with respect to bias voltages.
Fig. 9. Variation of (a) effective electron life time, (b) electron diffusion coefficient and (c) electron diffusion length with respect to bias voltages.
4. Conclusion Electron transfer properties of ZnO nanorod aggregates and TiO2 encapsulated ZnO NR aggregates based DSSCs were analyzed using electrochemical impedance spectroscopy under various forward
bias voltage conditions. The high chemical capacitance and electron diffusion length are attributed to large electron collection and transfer in the TiO2/ZnO DSSCs than that of bare ZnO DSSC. The TiO2/ZnO DSSCs show high efficiency (2.48%) and enhanced electrochemical properties compared to that of bare ZnO NR aggregate
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DSSCs. The presence of TiO2 nanoparticles over the ZnO NR improve the surface contact of the ZnO with the FTO substrate, enhances the dye adsorption which proved to be an excellent photovoltaic cell performance. Acknowledgement The author C.J.R. would like to thank the BRAIN KOREA21 (BK21) for its financial support. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0009749). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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