Enhancement in photovoltaic performance of phthalocyanine-sensitized solar cells by attapulgite nanoparticles

Electrochimica Acta 72 (2012) 40–45

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Enhancement in photovoltaic performance of phthalocyanine-sensitized solar cells by attapulgite nanoparticles Ling Jin, Dajun Chen ∗ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, 2999 North Renmin Road, Songjiang District, Shanghai 201620, PR China

a r t i c l e

i n f o

Article history: Received 7 December 2011 Received in revised form 28 March 2012 Accepted 28 March 2012 Available online 15 April 2012 Keywords: Attapulgite nanoparticles Zinc octacarboxylic phthalocyanine Dye-sensitized solar cells Photovoltaic performance

a b s t r a c t Attapulgite nanoparticles were used to improve photovoltaic performance of phthalocyanine-sensitized solar cells. The effects of attapulgite on the devices were investigated in details. Adding of attapulgite into TiO2 electrodes not only reduced the adsorption of zinc octacarboxylic phthalocyanine but also prevented phthalocyanine aggregation effect, which greatly improved photovoltaic performance of the dye-sensitized solar cell. The solar cell with 10 mg attapulgite nanoparticles dispersed in the dye solution exhibited nearly three times larger photoelectric conversion efficiency under simulated AM 1.5 G irradiation (100 mW cm−2 ) when compared to the pure dye, which was further characterized by the electrochemical impedance spectroscopy (EIS). The EIS studies showed that attapulgite decreased the charge-transfer resistances at the TiO2 /dye/electrolyte interface, which can promote electron transport. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSSCs) have received a considerable amount of attention since Gratzel et al. reported their pioneering research in 1991 [1,2] for a simple, efficient and economical solar energy-to-electricity conversion devices. Due to the remarkable advantages of the DSSCs with low costs, these devices appear to be good alternatives to traditional silicon-based photovoltaic devices [3–5]. A typical DSSC consists of three main components including a photoactive working electrode, a counter electrode and a redox-active electrolyte [6]. The working electrode is made of a mesoporous TiO2 film, which is sensitized by a small molecule chromophore. In general, the light-absorbing sensitizers are based on transition metal complexes that feature metal-to-ligand charge transfer excited states [7]. So far, The most successful charge transfer sensitizers employed in DSSCs are polypyridyl-type complexes of ruthenium such as N3 and N719, which have remained as the best commercial dyes showing solar energy-to-electricity conversion efficiency up to 11% at simulated AM 1.5 G irradiation and stable operation for millions of turnovers [8,9]. However, one of the drawbacks of ruthenium complexes is the low optical absorbance in the red/near infrared regions (NIR) and the efficiency of the DSSCs is significantly limited [10]. More importantly, considering ruthenium is a rare metal, novel dyes without metal or using inexpensive metal are desirable for high efficient dye-sensitized solar cells [11].

∗ Corresponding author. Tel.: +86 21 67792891; fax: +86 21 67792855. E-mail address: [email protected] (D. Chen). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.03.167

Hence, extensive research efforts are targeting new sensitizer dyes with stronger absorbance over the red/NIR [12]. Phthalocyanines (Pcs) are well known chromophores used in the DSSCs and other photovoltaic devices, due to their promising electrochemical, photochemical, thermal stability and intense absorption in the red/NIR [6]. Today, a large number of phthalocyanine dyes have been synthesized for applications in DSSCs [13]. However, the poor solubility of Pc dyes in organic solvents is an obstacle to its application in DSSCs. Zinc (Zn(II)) phthalocyanines (ZnPcs) have been studied widely by many groups to further enhance the solubility of phthalocyanines in common organic solvents [9,14–19]. Another major problem with Pcs is their strong tendency to aggregate on the semiconductor surface resulting in rapid deactivation of the dyes excited states and low photovoltaic performance, which requires a coadsorber to minimize the dyes aggregation [9,16,20]. Recently, it has been reported [6,11,21] that the utility of tri-tert-butyl substituted ZnPc not only avoided the formation of molecular aggregates but also arranged the excited states to permit directionality on the charge transfer. Adding chenodeoxycholic acid into TiO2 film electrode not only reduces the adsorption of phthalocyanine sensitizers but also prevents sensitizer aggregation, leading to the better photovoltaic performance. Xiong and Xu [22] utilized the layered double hydroxide (LDH) as a support for immobilizing the photosensitizer palladium phthalocyaninesulfonate (PdPcS) to retard the dye aggregation. The results showed that the immobilized PdPcS was well dispersed onto the clay precursor, with an obviously increased portion of monomer. Consequently, incorporation of PdPcS into the LDH accelerated photosensitized reaction. However, the LDH have

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41

Scheme 1. The preparation of TiO2 electrodes adsorbed ZnOCPc-AT nanoparticles.

higher charger density in the interlayer resulting in stronger interlayer electrostatic interactions, which makes it difficult to exfoliate the LDH. [23–25] Attapulgite (AT), generally denoted as (OH2 )4 (OH)2 Mg5 Si8 O20 ·4H2 O, a species of hydrated octahedral layered magnesium aluminum silicate [26,27]. AT owns dented surfaces and thus a relatively higher surface area, and moderate cation exchange capacity [28,29]. It provides many potential applications, such as environmental adsorbents [30], nanocomposites [31], and catalyst supports [32]. Compared with the LDH, AT with a rodlike morphology is readily controlled and well dispersed in solution without having to worry about the intercalation or exfoliation kinetics [33]. To the best of our knowledge, there is no report about its application in the DSSCs. In this work, AT nanoparticles were used as coadsorbent to reduce aggregation of zinc octacarboxylic phthalocyanine (ZnOCPc) and improve the photovoltaic performance of the DSSCs. The effects of ZnOCPc-AT loading onto TiO2 working electrode on the photocurrent density (Jsc ), the photovoltage (Voc ), the solar-to-electric power conversion efficiency () and electrochemical impedance of DSSCs were investigated.

immobilized onto AT was evidenced by a Hitachi H-800 transmission electron microscope (TEM). The morphology and distribution of attapulgite loading onto the TiO2 electrodes were examined with a Hitachis-4800 field-emission scanning electron microscope (FESEM) operating at 5 kV. UV absorption spectra of TiO2 electrodes sensitized by ZnOCPc-AT were recorded on a Lambda 950 UV–vis spectrophotometer. Photovoltaic properties of DSSCs were performed with a Keithley 2400 source meter under AM 1.5 G irradiation (100 mW cm−2 ) solar simulator. Electrochemical impendance spectra (EIS) were obtained from the potentiostat/galvanostat, equipped with a ZAHNER ZENNIUM module, under a constant light illumination (100 mW cm−2 ). The frequency range was from 200 mHz to 500 kHz. The applied bias voltage was set at the open-circuit voltage of the DSSCs between the ITO–Pt counter electrode and the FTO–TiO2 dye working electrode, starting from the short-circuit condition and by using a corresponding AC amplitude of 10 mV.

3. Results and discussion 3.1. Adsorption of ZnOCPc on AT nanoparticles

2. Experimental 2.1. Materials All reagents were used as received unless otherwise specified. ZnOCPc was available from our previous work [32]. The pristine AT was supplied by Jiangsu Junda Attapulgite Material Co., Ltd. with purity >90%, followed by treating with HCl and H2 O2 . After that, AT nanoparticles was obtained by filtering, rinsing, and drying. AT was dispersed by a SK 250 LH ultrasonic cleaning instrument at 59 kHz. 2.2. DSSCs fabrication The DSSC consists of a dye-adsorbed TiO2 electrode, a counter electrode, and an organic electrolyte. The electrolyte solution was a mixture of DMPII/LiI/I2 /TBP/GuSCN. The TiO2 electrodes with 0.25 cm2 working area were purchased from Dalian HeptaChroma SolarTech Co., Ltd., which were heated at 450 ◦ C for 30 min and then allowed to cool down to 80–90 ◦ C before immersing into the dye solutions. The resulting TiO2 electrodes were dipped into ZnOCPc solutions (32 ␮M in 50 ml DMSO with various contents of AT nanoparticles) and kept at room temperature for 8 h. Finally, the dye-adsorbed TiO2 electrodes were rinsed several times by DMSO and ethanol to remove unadsorbed dye and then dried quickly under N2 . The dye-adsorbed TiO2 electrode and counter electrode were assembled into a sandwich type cell with two clips. 2.3. Measurements The size distribution of AT was measured with a Zetasizer Nano 85 Particle Size and Zeta Potential Analyzer. ZnOCPc

DSSCs sensitized by ZnOCPc are obtained unimpressive photoelectric conversion efficiency because of aggregation and a lack of directionality in the excited state [6]. In these DSSCs, the Voc is determined by the difference between the quasi Fermi level of TiO2 and the redox potential of electrolyte, and on the other hand, the Jsc is mainly depended on light-harvesting capability and charge injection efficiency [6]. To improve performances of DSSCs, both for Voc and Jsc , AT nanoparticles was adopted as coadsorbent to minimize the aggregation of ZnOCPc. Scheme 1 shows the process of AT dispersed in ZnOCPc solution under ultrasonic treatment and the TiO2 electrodes immersed into ZnOCPc-AT solution for appropriate time. In this procedure, ZnOCPc molecules were adsorbed on the external surface of AT nanoparticles. It is clear that the original AT has smooth surface (Fig. 1a). After ultrasonic treatment, the external surface of AT is rough due to the adsorption of ZnOCPc molecules (Fig. 1b). The untreated AT nanoparticles has a relatively large agglomeration and a wide range of particle size distribution in the solvents. To obtain AT with a homogeneous size distribution, the effect of different ultrasonic time on size distribution was studied. It was found that ultrasonic treatment time had an obvious effect on size distribution of AT nanoparticles. As the ultrasonic time increased, the size distribution tends to narrow. Fig. 2 shows the size distribution of AT nanoparticles with diverse ultrasonic time. It is clear that the size distribution of AT nanoparticles with 60 min treatment time is the narrowest and smallest in all the ultrasonic time. Therefore, 60 min is a sufficient period for ultrasonic treatment on the AT nanoparticles. The DSSCs were finally assembled by the dye adsorbed TiO2 electrodes, and the current–voltage (J–V) characteristics of the devices

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L. Jin, D. Chen / Electrochimica Acta 72 (2012) 40–45

Fig. 1. TEM images of (a) original AT and (b) ZnOCPc-AT.

Table 1 Photovoltaic parameters for the DSSCs incorporating AT nanoparticles with different ultrasonic time under AM 1.5 G irradiation. Ultrasonic time (min)

Jsc (mA cm−2 )

Voc (V)

ff

Pmax (␮W)

 (%)

15 30 60

0.43 0.49 0.65

0.49 0.51 0.52

0.57 0.55 0.67

121.42 136.2 225.8

0.12 0.14 0.23

the homogeneous size distribution. This leads to dissociating the dye aggregation and improving the electron injection yield, thus Jsc increased. Therefore, in our subsequent work, the ultrasonic treatment time has been fixed at 60 min. 3.2. Effect of AT content on the photovoltaic performances of DSSCs

were measured. The resulting J–V curves of the DSSCs based on AT nanoparticles dispersed for different ultrasonic time are presented in Fig. 3, and the corresponding photovoltaic characteristics are summarized in Table 1. The DSSC incorporating AT nanoparticles dispersed for 60 min shows better photovoltaic properties with an open circuit voltage of 0.52 V, a short circuit photocurrent density of 0.65 mA cm−2 , a fill factor of 0.67, and a photoelectric conversion efficiency of 0.23% under AM 1.5 G irradiation (100 mW cm−2 ). The highest efficiency benefits from the highest short circuit photocurrent density, which can be explained by the adding AT with

3.2.1. Photophysical property ZnOCPc sensitizer loading onto the TiO2 surface was strongly dependent on AT content, and the absorption spectra of TiO2 electrodes with different contents of AT were investigated. As shown in Fig. 4, the decrease in absorbance with increasing contents of AT is due to the less adsorbed dye molecules. The inset of Fig. 4 shows the absorption peak around 550 nm can be assigned to H-aggregates and weakened with the increased AT contents. That means the extent of H-aggregation of ZnOCPc is diminished with the addition of AT into ZnOCPc solutions and adsorption onto the TiO2 electrode surface. When compared to the absorption spectra, the absorption peak of H-aggregation is the weakest with the addition of 10 mg AT. The low H-aggregation degree could result from high surface areas and moderate absorptive capacity of AT [34]. Moreover, the

Fig. 3. Current–voltage characteristics for the DSSCs incorporating AT nanoparticles with different ultrasonic treatment time under AM 1.5 G irradiation (100 mW cm−2 ).

Fig. 4. UV/vis spectra of ZnOCPc adsorbed on TiO2 electrodes obtained with different contents of AT. And the inset is magnified figure around 550 nm.

Fig. 2. Effects of ultrasonic treatment time on the size distribution of AT nanoparticles. The samples are with 10 mg AT nanoparticles dispersed in 50 ml ZnOCPc solutions.

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43

Fig. 5. FESEM images of the TiO2 electrode surface used (a) 0 mg AT, (b) 1 mg AT, (c) 10 mg AT, and (d) 50 mg AT in ZnOCPc solutions.

adsorption amount of AT on the TiO2 electrodes surface can be easily observed from the FESEM images shown in Fig. 5. Obviously, the adsorption amount of AT on the TiO2 electrodes surface is maximum in Fig. 5c. However, the adsorption amount of AT on the TiO2 electrode surface does not increase with the increasing contents of AT in ZnOCPc solutions. On the contrary, the absorption amount of AT reduced when the contents of AT is 50 mg in comparison with 10 mg AT, which can be attributed to the reunion of AT in ZnOCPc solution. 3.2.2. Photovoltaic property The J–V characteristics of DSSCs based on different contents of AT in ZnOCPc solutions were evaluated under AM 1.5 G irradiation and shown in Fig. 6. The photovoltaic performance parameters of DSSCs in terms of Jsc , Voc , ff, Pmax , and  were summarized in Table 2. The DSSC sensitized by ZnOCPc without AT showed Jsc of 0.44 mA cm−2 , Voc of 0.48 V, ff of 0.44 and Pmax of 93.01 ␮W leading to  of 0.09%. From Fig. 6 and Table 2, it is obvious that the

DSSCs based on different contents of AT have significantly enhanced photovoltaic performance. Both Jsc and Voc increase considerably, suggesting that there are advantages of using AT in ZnOCPc solutions. These advantages include reduced aggregation degree of ZnOCPc and enhanced charge injection efficiency. The highest photovoltaic performance was achieved by the DSSC based on 10 mg AT in ZnOCPc dye solutions yielding a Jsc of 0.60 mA cm−2 , Voc of 0.56 V and  of 0.22%. This result confirms the trends noted before and clearly demonstrates that the photoelectric conversion efficiency of DSSCs is improved due to the homogeneous size distribution of AT. For a regenerative photoelectrochemical system of the DSSCs, the value of Voc is given by the difference of the quasi-Fermi level of the electrons in the metal oxide (TiO2 ) and the potential of the counter electrode which is equal to the redox potential of electrolyte [35]. The quasi-Fermi level depends on the accumulated charge in the semiconductor and approaches the conduction band edge when the concentration of conduction band electrons is high. The Voc could be described according to the following equation [5]: Voc =

ECB + q

k T  B q

ln

N  C

n

−

Ered q

(1)

where ECB is the potential of the conduction band edge,  is a characteristic constant of TiO2 tailing states, kB is the Boltzmann constant, T is the temperature, q is the elementary charge, NC is the effective density of states at the TiO2 conduction band edge, n is the number of electrons in TiO2 , and Ered is the chemical potential of redox species in the electrolyte. In this work, the same electrolyte has Table 2 Summary of photovoltaic performance of the DSSCs based on different contents of AT in ZnOCPc solutions under AM 1.5 G irradiation.

Fig. 6. J–V characteristics of DSSCs based on different contents of AT in ZnOCPc solutions under AM 1.5 G irradiation (100 mW cm−2 ).

AT contents (mg)

Jsc (mA cm−2 )

Voc (V)

ff

Pmax (␮W)

 (%)

0 1 10 50

0.44 0.52 0.60 0.51

0.48 0.51 0.56 0.54

0.44 0.54 0.64 0.59

93.01 144 217.26 162.62

0.09 0.14 0.22 0.16

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L. Jin, D. Chen / Electrochimica Acta 72 (2012) 40–45

second semicircles correspond to the charge-transfer resistances at the electrolyte/Pt interface (Rct1 ) and the TiO2 /dye/electrolyte interface (Rct2 ), respectively; the third semicircle corresponds to the Warburg diffusion process of I− /I3 − in the electrolyte (Zw ) [1]. However, in this work, Rct1 is negligible owing to Rct1 is not obvious and it is apparently overlapped by Rct2 . Rct2 representing the interfacial charge-transfer resistance between TiO2 and electrolyte redox species follows an exponential behavior expressed as follows [37,38].



Rct = R0 exp −ˇ

Fig. 7. (a) Equivalent circuit of the DSSCs, (b) EIS Nyquist polts of the DSSCs fabricated using variant TiO2 electrodes with the different contents of AT.

been used in all the DSSCs, and therefore Ered can remain the same for the DSSCs. The difference of the Voc can be determined based on the following expression. Voc =

ECB + q

k T  B q

ln

N  C

n

(2)

From Eq. (2), it is clearly that the Voc is affected by ECB and n. In addition, n is determined by the balance between electron injection and charge recombination. An improvement in the electron injection efficiency will increase the number of injected electrons in TiO2 and then will increase the Voc , while the charge recombination process will reduce the electron population in TiO2 and thus will decrease the Voc [5]. Note that ECB and n are closely related to the surface charge and charge recombination, respectively. Voc increased from 476 mV to 563 mV for DSSC with 10 mg AT in ZnOCPc solution as compared to ZnOCPc only. The reason must be a higher n due to either a better injection of electrons into TiO2 from the excited dye or suppression of loss reactions. Apparently, AT applied in the DSSCs has a dramatic effect on the rate of interfacial electron transfer from the conduction band of TiO2 to electrolyte. 3.2.3. EIS analysis The effect of the TiO2 electrodes with different contents of AT on the electron transport at the interfaces in the DSSCs can be investigated with the aid of electrochemical impedance spectroscopy (EIS) measurements. The EIS is a useful tool for characterizing significant interfacial charge-transfer processes in the DSSCs, such as the charge recombination at the TiO2 /dye/electrolyte interface, electron transport in the TiO2 electrode, electron transfer at the counter electrode, and diffusion process of I− /I3 − in the electrolyte [36]. The electrochemical impedance analysis of the DSSCs were performed over the frequency range of 200 mHz to 500 kHz with an amplitude of 10 mV under illumination of simulated solar light (AM 1.5 G, 100 mW cm−2 ) in this study. In addition, the applied bias voltage was set at the open-circuit voltage of the DSSCs. In general, the EIS spectrum of a DSSC having a configuration FTO/TiO2 /dye/electrolyte/Pt/ITO displays three semicircles in the measured frequency range. The equivalent circuit used to model the impedance of the DSSCs is shown in Fig. 7a. The ohmic serial resistance (Rs ) corresponds to the overall series resistance. The first and

qV kB T



 = R0 exp −



ˇ (EFn − Ered ) kB T

(3)

where R0 is a constant, ˇ is the transfer coefficient, EFn is the position of Fermi level of electrons and V is the potential at the electrode. As shown in Fig. 7b, the EIS Nyquist plots (i.e. minus imaginary part of the impedance −Z vs the real part of the impedance Z when sweeping the frequency) for DSSCs sensitized by ZnOCPc solutions with the different contents of AT. It is clear that the EIS Nyquist plots all show a bigger semicircle, which corresponds to the chargetransfer resistances at the TiO2 /dye/electrolyte interface and show a difference in the semicircle at intermediate-frequency region. It is generally assumed that this frequency region represents the recombination process between electrons in TiO2 and electrolyte. It is found that the radius of the semicircle for the DSSCs with AT is much smaller than that without AT, which indicates a smaller Rct2 exists in the TiO2 /dye/electrolyte interface for the DSSC with AT, possibly due to the wider energy gap between the dye and TiO2 providing a powerful drive for electron injection from the dye to the conduction band of TiO2 . This result demonstrates that electron transport and injection into the conduction band of TiO2 in the DSSC based on ZnOCPc-AT is faster than that for ZnOCPc only. At the same time, the result suggests AT can reduce the tendency to aggregate on the TiO2 electrodes surface and improve the photovoltaic performance of the DSSCs. The charge-transfer resistance assumes a clear order 10 mg AT < 50 mg AT < 1 mg AT < 0 mg AT, indicating that the charge transport resistance have significantly difference at the interface of the TiO2 /dye/electrolyte due to the introduction of different contents of AT. It is worth noting that, the charge-transfer resistance of DSSC with 10 mg AT is smaller than that of the other DSSCs, which can be explained the homogeneous size distribution of AT onto TiO2 electrode may facilitate the electron transfer. This is in agreement with the photovoltaic performance as presented in the photocurrent–voltage characteristic measurement. The EIS Bode phase plots of the DSSCs sensitized by ZnOCPc solutions with the different contents of AT are shown in Fig. 8. As for the frequency range between 1 and 1000 Hz investigated, all the EIS Bode plots exhibit a prominent peak feature corresponding to the charge-transfer at the TiO2 /dye/electrolyte interface [36]. Moreover, the difference in the EIS bode plots of all cells mainly occurs in the mid-frequency range and slight changes are observed at the low-frequency and high-frequency range. The electron lifetime ( e ) can be extracted from the peak frequency (fp ) at the mid-frequency peak in the bode phase plot using  e = 1/2fp [5]. As it can be seen in Fig. 8, the mid-frequency peaks of the DSSCs of variant TiO2 electrodes with different contents of AT are shifted to higher frequency when compared to those of the DSSC without AT. This corresponds to a decrease in the electron lifetime for the DSSCs with AT at the TiO2 /dye/electrolyte interface. Despite the enhancement in the electron lifetime, the lower device efficiency is observed for the DSSCs based on ZnOCPc only, which is attributed to the lower number of photoinduced electron generation and lesser electron injection efficiency of ZnOCPc.

L. Jin, D. Chen / Electrochimica Acta 72 (2012) 40–45

Fig. 8. EIS Bode-phase plots of the DSSCs fabricated using variant TiO2 electrodes with the different contents of AT.

4. Conclusions In this work, the nanocrystalline TiO2 electrodes sensitized by ZnOCPc and the adding AT nanoparticles were prepared and the effects of AT nanoparticles on the photovoltaic properties of the dye-sensitized solar cells were investigated and evaluated by UV–vis, current–voltage, EIS measurements. The results showed that incorporation of AT nanoparticles into the TiO2 electrodes can effectively suppress the aggregation of ZnOCPc molecules and also facilitate the electron transfer, which improve the photovoltaic performance of the DSSCs. AT contents and treatment time are also important influencing factors. The values of Jsc and Voc increased as AT contents added up to 10 mg and then became decreased. In particular, AT nanoparticles were dispersed in ZnOCPc solutions under ultrasonic treatment for 60 min, which is the optimal ultrasonic condition. Acknowledgment This work was supported by grants from the Program of Introducing Talents of Discipline to Universities (No. 111-2-04). References [1] A. Baheti, P. Singh, C.-P. Lee, K.R.J. Thomas, K.-C. Ho, The Journal of Organic Chemistry 76 (2011) 4910. [2] B. O’Regan, M. Gratzel, Nature 353 (1991) 6346. [3] C.O. Avellaneda, A.D. Gonc¸alves, J.E. Benedetti, A.F. Nogueira, Electrochimica Acta 55 (2010) 1468. [4] T. Le Bahers, F.d.r. Labat, T. Pauporté, P.P. Lainé, I. Ciofini, Journal of the American Chemical Society 133 (2011) 8005. [5] J.A. Mikroyannidis, P. Suresh, M.S. Roy, G.D. Sharma, Electrochimica Acta 56 (2011) 5616.

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