Simultaneously composition and interface control for ZnO-based dye-sensitized solar cells with highly enhanced efficiency

Nano-Structures & Nano-Objects 10 (2017) 1–8

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Simultaneously composition and interface control for ZnO-based dye-sensitized solar cells with highly enhanced efficiency Dongting Wang ∗ , Xuehong Zhu, Yuzhen Fang, Jianhong Sun, Cong Zhang, Xianxi Zhang Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, PR China

graphical

article

abstract

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Article history: Received 10 November 2016 Received in revised form 20 December 2016 Accepted 22 January 2017

Keywords: Interfacial engineering Composition control Dye-sensitized solar cells ZnO aggregates Electron recombination

abstract In this work, we concentrate on the investigation of the fundamental effects of annealing temperatures on the synthesized polydisperse of ZnO aggregates for the exploration of effective electrode material for dye-sensitized solar cells (DSCs). Systematic characterizations including TEM, SEM, BET analysis, XRD, and r-PL were performed for the ZnO aggregates annealed at various temperatures, and the results revealed that the morphology of aggregates, crystal size of primary particle, crystallinity, specific surface areas and the surface states largely depends on the annealing temperature. The photoelectrical efficiency tests demonstrated that the ZnO aggregates annealed at 450 °C exhibited the highest efficiency of 5.10%. The excellent performance could mainly originate from the more efficient light harvesting property of the resulted ZnO photoanode indicated by the incident photon-to-current efficiency (IPCE) and the accelerated electron transport within the ZnO film testified by electrochemical impedance spectroscopy (EIS). Moreover, the combination of the composition control with the introduction of both a ZnO compact layer (CL) and a TiO2 protecting layer (PL) was simultaneously integrated into one photoanode film for the first time. As a consequence, an impressive power conversion efficiency of 5.83% was achieved with the new photoanode architecture, an improvement of 39.5% over the unmodified DSC. The synergistically reduced charge-transport resistance and suppressed electron recombination are regarded to be the key element for outstanding performance. © 2017 Elsevier B.V. All rights reserved.

1. Introduction

∗

Corresponding author. Fax: +86 635 8239001. E-mail addresses: [email protected] (D. Wang), [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.nanoso.2017.01.001 2352-507X/© 2017 Elsevier B.V. All rights reserved.

Dye-sensitized solar cells (DSCs) show great promise to solve current environmental and energy problems owing to their low production cost and relatively high energy-conversion efficiencies [1–3]. DSCs predominately consist of three parts: the semicon-

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ductor photoanode, the redox couple, and the counter electrode i.e. platinized transparent conducting oxide (TCO) glass [4]. Apart from the sensitizer dyes and electrolytes optimization, considerable interest has been focused on the construction and/or modification of the photoanode architecture because of its vital role in both large quantities of sensitizers accommodation and photoexcited electrons transportation from the dye molecules to a TCO substrate [5,6]. Among various metal oxides, ZnO is regarded as a credible alternative semiconductor material owing to its distinct advantages of higher electron mobility than traditional TiO2 , facile fabrication routes of these nanostructures and easier modifications of the surface structure [7–10]. Compared to ordinary nanostructure frameworks, the exceptional 3-dimensional (3D) ZnO nanocrystalline aggregates have already been the preferred nanostructures to achieve high photon-to-current conversion efficiencies (PCEs) originating from their large internal surface area provided by the small primary nanocrystallites and their superior light-scattering ability stemmed from the sub-micrometer size [11–13]. So far, efficiencies as high as 7.5% have been achieved in scientific literature [14]. Nevertheless, the performance remains well below that of its counterpart TiO2 . This raises the question about the physical processes limiting the efficiency. Specifically, severe charge recombination within the electrode film because of the zigzag electron transport pathway and a large amount of grain boundaries is believed to be the crucial aspect for the inferior photovoltaic performance. In the past, several approaches have been put forward, mainly focusing on designing photoanodes based on multidimensional ZnO architectures consisting of distinct monodimensional nanostructures with specific features, in particular ZnO 1D nanorods/3D ZnO aggregates and 2D nanosheets/3D ZnO aggregates [15–17]. These modifications provided direct pathways for electron transport and thus led to substantial efficiency improvement. Nevertheless, the relatively complex production procedures and specific operation (i.e. film upside-down in a sealed bottle) make it a great challenge for mass production and practical applications. Alternatively, pre-treatment of the as-prepared ZnO seems provide another effective way to improve electron transport within the film via characteristic modulation and optimization. Recently, Xi et al. developed a two-step method to prepare submicrometer ZnO aggregates with different crystal sizes and found that optimal crystal size in the aggregates for photoanode led to higher PEC due to faster electron diffusion and less recombination [18]. Actually, except for crystal size, the concentration of surface states or defects, a common feature in those obtained by wet-chemistry synthetic methods, should also be considered. Particularly, the surface states could restrict charge transport in DSCs to some extent because trapping and detrapping of electrons are probably occur during the electron diffusion and transport processes [19]. Additionally, the recombination of plenty of electrons within the surfaces states with I− 3 ions in the electrolyte should also be considered [20,21]. Annealing treatment is expected to be an effective route to simultaneously control crystal size and surface defects, and thus cooperating all the factors for high efficiency. Indeed, earlier reports related to TiO2 have shown that the annealing treatment drastically affected the crystal size and grain boundaries, and thus exerted significantly influence on electron transport properties. For example, Peng et al. annealed the film electrodes at different temperatures and found that the surface states and crystal size of the m-TiO2 largely depended on the annealing temperature. To the end, great improvement of the photoelectrochemical properties of DSCs was obtained via balancing the crystal size, surface states and specific surface area [22]. In parallel, similar results were also reported by Snaith et al. [23]. However, it is great surprising that a thorough study of the impact of annealing temperatures on the promising ZnO

aggregates has not yet been reported. In such circumstances, the present work particularly highlights on the feasibility of achieving highly efficient ZnO aggregates based DSCs by specific physical parameters control through annealing conditions optimization. Significant interfacial charge recombination both occurred at the ZnO film/dye/electrolyte interface and in the electrolyte are regarded to be another factors leading to the poor photovoltaic performance. Mostly recently, it was reported by our group that a TiO2 layer obtained from TBOT yielded a great promotion on the overall characteristics of the cell due to the increased chargerecombination resistance [24]. Yu et al. reported the formation of a compact ZnO interlayer between the substrate and ZnO film and revealed its positive effect on suppressing the charge recombination [25]. Apparently, both the approaches are effective for interfacial modifications. However, separate application of these methods normally can only improve charge recombination to some extent. Considering the potentially positive effect of pre-thermal treatment and the acquired interfacial modification achievements, great improvement of electron transport property is expectable by simultaneously composition and interface control, which has yet to be reported. Herein we report the rational design of the ZnO photoanode film to achieve outstanding performance. In particular, we retard electron recombination both within the film and at the interfaces by regulating ZnO aggregates properties, depositing a ZnO compact layer, and constructing a TiO2 protective layer. As a result, an overall power conversion efficiency (PCE) of 5.83% was achieved for the triply-modified ZnO photoelectrode, with a significant efficiency enhancement of 39.5% in comparison with an unmodified ZnO-based DSSC. Various analyses showed that the PCE enhancement was due primarily to the improved quality of ZnO and the suppressed electron recombination at the interfaces. 2. Experimental 2.1. Materials Anhydrous lithium iodide (LiI), iodide (I2 ), tert-butylpyridine (t-BPy), 2,3-dimethyl-1-propyl imidazolium iodide (DMPII), 3-methoxypropionitrile, acetonitrile, and H2 PtCl6 were obtained from Sigma. Zinc acetate dehydrate (CH3 COO)2 Zn · 2H2 O, diethyleneglycol (HOCH2 CH2 )2 O, tetrabutyl titanate (TBOT), ethanol, and all other chemicals used were obtained from commercial sources, were of analytical grade, and were used as received. Ru-based N719 dye cis-bis(isothiocyanato)bis-(2,2-bipyridyl-4,4dicarboxylato) ruthenium(II) bis(tetrabutyl-ammonium) was obtained from Solaronix (Aubonne, Switzerland). 2.2. Preparation of ZnO aggregates ZnO aggregates were prepared via hydrolyzing 6 mmol

(CH3 COO)2 Zn · 2H2 O in diethylene glycol (60 ml) sealed in a

Teflon-lined stainless steel autoclave (100 ml) at 160 °C for 8 h under vigorous stirring at a high heating rate of 10 °C min−1 [24]. After the mixture had been cooled to room temperature, the asobtained colloidal suspension was taken out of the autoclave, followed by repeating centrifugation. After drying at 80 °C in air for 4 h, the ZnO aggregates were then annealed at different temperatures, i.e. 350 °C, 450 °C, and 550 °C. For simplicity, these annealed samples are denoted as Z350, Z450, and Z550, respectively, according to the temperature used for synthesis.

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Fig. 1. HRTEM images of (a) ZnO; (b) Z350; (c) Z450, and (d) Z550.

2.3. Preparation of ZnO electrodes FTO conducting glass (NSG, 7 /sq, 84% transmittance) was used as the substrate for various ZnO photoelectrodes. Before electrode fabrication, the substrates were cleaned sequentially in HCl, acetone, ethanol, and water for 15 min each in an ultrasonic bath. To form the ZnO photoelectrode films based on Z450, 0.2 g Z450 was first mixed with ethyl cellulose (0.04 g) and terpineol (0.3 g), followed by doctor blading the as-prepared viscous Z450 paste on the FTO/glass substrate. Afterwards, the dried films were heated at 350 °C for 1 h to remove any residual organic matter from the ZnO surface. To fabricate the Z450 + CL + PL electrode, the preparation of ZnO CL on FTO substrate was first conducted as reported in the literatures [26]. Typically, 7.5 mmol Zn(Ac)2 · 2H2 O and 7.5 mmol monoethanolamine were first dissolved in 2-methoxyethanol (10 ml) via vigorous stirring (12 h) to obtain ZnO precursor solution. Then, the resulting solution was spin-coated onto clear FTO substrate, followed by calcination treatment at 300 °C for 20 min. After coating Z450 on the as-prepared FTO + CL electrode as described above, the obtained electrode was soaked in a 0.01 M ethanolic solution of TBOT at room temperature for 30 min and then washed with absolute ethanol. Subsequently, the obtained film was annealed at 500 °C for 1 h in the ambient atmosphere condition.

The presence of defect states in nanostructured ZnO aggregates were confirmed by photoluminescence (r-PL, RF-5301). The morphologies of the products were identified by high-resolution transmission electron microscopy (HR-TEM) images obtained on a JEOL-2010 microscope at an accelerating voltage of 200 kV. Energy-dispersive spectroscopy (EDS) was measured with an X-ray energy-dispersive spectrometer installed on the JEOL-2010 microscope. The structures of the ZnO films were identified by field emission scanning electron microscopy (FESEM, JEOL, JSM 6701-F). Dye loading amount of the ZnO films was quantitatively determined by UV–vis spectrophotometer (UV 2550, Shimadzu) after complete removing the adsorbed dye from the sensitized photoanode film (1 × 1 cm2 ) into a NaOH solution in water/ethanol (1.0 M, 50:50, v/v). The current–voltage (J–V) measurements were carried out using a Keithley 2400 source meter under simulated AM 1.5 G one sun (100 mW cm−2 ) illumination provided by a solar simulator (Newport Corporation). Before each device test, the power of the solar-simulated light was first calibrated with a NREL-calibrated Si solar cell. The electrochemical impedance spectroscopy (EIS) was obtained on an electrochemical workstation (CHI760, CH Instruments) in the frequency range from 10−1 Hz to 105 Hz at an ac amplitude of 10 mV. 3. Results and discussion

2.4. Solar cell fabrication

3.1. Morphological characterization

The resultant ZnO photoanode film with an active area of 0.25 cm2 (0.5 cm × 0.5 cm) was first put into anhydrous ethanol containing N719 dye for 1 h. Then, the dye-adsorbed ZnO electrode and the obtained Pt counter electrode were sealed with thermoplastic sealant to form a face-to-face cell. The voids in the assembled cells were filled with an electrolyte solution comprised of 1.0 M DMPII, 0.12 M I2 , 0.1 M LiI, and 0.5 M t-BPy in 3-methoxypropionitrile.

The morphological changes of the as-prepared ZnO samples were first characterized by HRTEM and FE-SEM, respectively. As one can see in Fig. 1(a), the as-prepared ZnO composed of packed nanocrystallites are polydisperse aggregates with nearly spherical shape. Magnified TEM image of an individual ZnO aggregate (see Fig. S1, Supporting Information) revealed that the crystal size of the primary nanoparticles was less than 15 nm without annealing process. With additional 350 °C heat treatment performed on ZnO, the surface of the ZnO aggregates becomes smooth in comparison with pristine ZnO, which may imply the increased primary nanoparticles (Fig. 1(b)). When the temperature is further elevated to 450 °C, a continuous increase for the primary nanoparticles is revealed in Fig. 1(c) (also see Fig. S2b, Supporting Information). Moreover, improved connectivity between the

2.5. Material characterization and photoelectrochemical measurements The crystal structures of the ZnO powders were recorded on a D/MAX-rA diffractometer (Rigaku) using Cu Kα radiation.

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Fig. 2. Top-view SEM images of (a) ZnO compact layer and (b) Z450 + CL + PL electrode. (c) EDS spectrum obtained by scanning Z450 + CL + PL electrode film. (d) Cross section SEM image of Z450 + CL + PL electrode.

neighbor nanoparticles could be observed at the same time, which is probably induced by the coalescence processes of the ZnO nanocrystallites when subjected to high temperature treatment, as reported by Cao et al. [27]. Despite these substantial changes in primary nanocrystallites for the above tested conditions, it is noteworthy that the aggregates, themselves, have almost the same morphologies; sizes in the range of 50–500 nm and spherical shape regardless of annealing temperature. However, upon continuously increasing the annealing temperature to 550 °C, the morphology of the aggregates changes distinctly (Fig. 1(d), also see Fig. S2c, Supporting Information), resulting in the formation of disordered ZnO nanoparticles with a clearly discernible diameter of ∼50 nm. The change of primary nanoparticles size and microstructure (i.e. packing density and connectivity) would eventually give rise to the variation of dye adsorption capability and charge transfer property, which has been well accepted to be two main factors influencing the DSCs efficiency. As such, dye loading amounts and electron transport behaviors will be experimentally investigated below to systematically explore the origination of the enhanced performance. Fig. 2(a) shows the surface morphology of the ZnO CL. It can be seen that ZnO layer derived from Zn source sol is flat and compact, and no FTO trace is observed, demonstrating the successful deposition of a dense and homogeneous layer on FTO substrate. The SEM image in Fig. 2(b) reveals that the TiO2 -coated ZnO aggregate film is comprised of poly-disperse spherical aggregates with a size distribution of 50–500 nm, which is found to be almost the same with that of untreated one (see Fig. S3), indicating the well retention of aggregate structure after TiO2 PL post-treatment. Energydispersive spectroscopy (EDS) spectrum (Fig. 2(c)), consisting of the peaks of Ti, O, and Zn, confirms the presence of TiO2 nanocrystallites in the corresponding film. By scanning the film electrode in different regions, an average Ti concentration about 1.3 mol% was achieved. Moreover, the magnified TEM image of TiO2 -coated ZnO (Z450) was also characterized and presented after peeling off the ZnO sample from the aforementioned film (see Fig. S4). From Fig. 2(d) we could notably recognize the ZnO compact layer between the Z450 film and the FTO substrate, with a thickness of approximately 100 nm.

3.2. XRD patterns and r-PL analysis XRD measurement was used to further characterize the crystalline of the ZnO samples (Fig. 3(a)). Notably, all samples annealed at different temperatures exhibits similar crystal compositions, primarily consisting of typical wurtzite hexagonal ZnO (JCPDS no. 36-1451). However, the intensities of the diffraction peaks increase gradually and the FWHM of the diffraction peaks becomes narrower with promoting the annealed temperature from 350 to 550 °C. Therefore, it is reasonable to speculate the increase of crystal size and crystallinity during thermal treatment process. The average sizes of the nanoparticles were calculated from the fullwidth at half-maximum (FWHM) of characteristic XRD peaks on the basis of the Scherer’s equation. As listed in Table 1, the crystal size is successively increased from 14.3 nm for the ZnO to 42.1 nm for Z550, which matches the particle sizes change tendency reflected by the TEM images in Fig. 1. Larger crystal size would lead to higher electron diffusion rate, hence reducing the chance of recombination of electron and redox species in electrolyte. Apart from crystal size, intrinsic defects difference between the as-prepared ZnO and typical high crystallinity sample (i.e. Z450) is also examined by room temperature photoluminescence. It can be seen from Fig. 3(b) that the peak intensity of the visible emission of Z450 that underwent annealing is much less than that of as-synthesized aggregates. As previously reported, the visible emission is commonly regarded to arise from the defects in the ZnO crystals [18,28]. As a result, it can be concluded that the ZnO aggregates annealed in high temperature have a much lower defect concentration, which is also another result of the improved crystallinity of the ZnO aggregates. 3.3. Cell performance and photoelectrochemical behaviors Solar cells based on ZnO electrode films fabricated with the four samples (ZnO, Z350, Z450, and Z550) were assembled and tested under the AM 1.5 simulated sunlight with a power density of 100 mW cm−2 . Fig. 4 shows the J–V curves of the DSCs based on the above ZnO products and the performance characteristics such as open-circuit voltage (Voc), short-circuit current density

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Table 1 Characteristics of the various ZnO samples together with the photovoltaic parameters of the corresponding DSCs. Samples

JSC (mA m−2 )

VOC (V)

FF

PCE%

Dye uptake (×10−7 mol/cm2 )

τe (ms)

Crystal size (nm)

ZnO Z350 Z450 Z550

12.6 13.0 13.9 8.86

0.545 0.558 0.578 0.599

0.608 0.623 0.636 0.642

4.18 4.51 5.10 3.41

1.15 1.03 0.96 0.78

4.9 9.0 9.6 12.3

14.3 20.2 27.5 42.1

Fig. 4. J–V curves of DSCs fabricated with ZnO, Z350, Z450, and Z550, respectively.

Fig. 3. (a) XRD patterns of ZnO, Z350, Z450, and Z550. (b) PL spectra of ZnO and Z450.

(Jsc), and efficiency (PCE) are given in Table 1. Clearly, the pristine ZnO aggregate cell achieves only a low efficiency of 4.18% with JSC of 12.6 mA cm−2 and a VOC of 0.545 V. In contrast to untreated ZnO, the JSC and VOC of the Z350 based cells both increase slightly, leading to a higher efficiency. High JSC of 13.9 mA cm−2 and VOC of 0.578 V are observed with the annealing temperature further elevating to 450, resulting in an impressive PCE of 5.10%, a 22% increment from that of the pristine ZnO aggregates cell. However, as the annealing temperature further increases to 550 °C, the JSC decreases to 8.86 mA cm−2 . As discussed above, the decreased photocurrent density is due to much low dye-loading ability in the Z550 as the primary nanoparticles are the largest among all the samples, which could cause a decrease in the specific surface area, thus resulting in a decreased JSC . In contrast to the drastically reduced JSC , however, the Voc increases to 0.599 V. As a result, only a low PCE of 3.41% was obtained for Z550. 3.4. Dye loading amount measurements To obtain an insight into the origin of the PCE enhancement, the dye loading amounts into the four different photoanodes were first characterized. As can be seen in Table 1, the asprepared ZnO exhibited the best capabilities in dye adsorption with the dye loading amount reaching 1.15 × 10−7 mol cm−2 . In contrast, the annealed ZnO displayed much dye adsorption lower capabilities with the dye loading amount reduced from 1.03 × 10−7 mol cm−2 to 0.78 × 10−7 mol cm−2 as increasing temperature from 350 to 550 °C. This significant variation can be explained as follows: On one hand, the crystal size independently governed

by temperature-induced crystallization increases gradually in the full temperature range, and hence an accessible lower surface area with a higher annealing temperature [29,30]. On the other hand, apart from crystal size variation, improved connectivity between the primary nanoparticles are also observed (see Fig. 1), which consequently limited the amount of dye absorbed to a low level. It is now well-accepted that both a high internal surface area for light harvesting and a specific microstructure for fast electron transport are required for a high-efficiency DSCs. In this regard, the significant increase in the PCE of Z450 cell can be dominantly attributed to the enhanced electron transport, which will be experimentally confirmed by electrochemical impedance spectroscopy measurements. 3.5. EIS analysis To understand the interfacial charge transfer properties of all fabricated photoelectrodes, we performed electrochemical impedance spectroscopy (EIS) on all these different samples in the dark at an applied bias of Voc and 10 mV AC with frequency range from 0.1 Hz to 100 kHz. As shown in Fig. 5(a), in addition to the small semicircle associated with the charge transfer process occurring at the Pt electrode/electrolyte interface in the frequency range of 105 Hz–103 Hz, a larger semicircle related to the charge recombination process at the ZnO/dye/electrolyte interface can be clearly distinguished in the middle frequency range of 103 Hz–100 Hz in all the EIS spectra [31]. Based on the reported analysis on EIS spectra of the DSCs, the extent of charge recombination resistance in the photoanode can be judged by the impedance value ranging in the middle frequency, which is defined by the diameter of the semicircle [16,32,33]. Apparently, an increasing trend of the central arc diameter in the sequence Z550 > Z450 > Z350 > ZnO is observed from Fig. 5(a), implying the gradually increased interfacial charge recombination resistances of all annealed samples in comparison with that of ZnO. The weakened charge recombination can be ascribed to the following two reasons. Firstly, it is reported that the surface states can trap injected electrons, hence making electrons difficult to transfer to the oxidized species [22,34,35]. The gradual improvement of crystallinity for the ZnO samples accompanying with enhanced the annealing temperature leads to the decrease of the surface states density (see Fig. 3(b)). As a result, the charge

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Fig. 6. IPCE spectra as a function of wavelength for the fabricated DSCs based on ZnO, Z350, Z450, and Z550.

Fig. 5. Nyquist plots (a) and Bode phase plots (b) of DSCs based on ZnO, Z350, Z450, and Z550 in the dark. Fig. 7. J–V curve of the Z450 + CL + PL cell.

recombination is restrained to some extent. Secondly, improved connectivity of primary nanoparticles along with increased crystal size could be observed during the annealing treatment (see Fig. 1 and Table 1), which gives rise to the reduced the grain boundary and increased electrons diffusion coefficients [36–38]. Consequently, charge transport is accelerated within the ZnO film, decreasing the possibility of charge recombination. Since the injected electrons from the sensitizer into the oxide semiconductor should be recombined by I− 3 ions at the oxide/dye/electrolyte interface in the open circuit condition, the gradually increased interfacial charge recombination resistances from pristine ZnO to Z550 will eventually result in a longer electron lifetime, which is supported by the corresponding Bode plots (see Fig. 5(b)) drawn from the Nyquist plots. According to the reported EIS model, the electron lifetimes (τe ) can be calculated on the basis of the following expression: τe = 1/ωmax = 1/(2π fmax ), where fmax is the maximum angular frequency of the middle frequency region [39,40]. It can be seen from Table 1 that the τe for the cells changed remarkably after annealing, with values increasing from 4.9 ms for the as-prepared ZnO aggregates to 9.0 ms, 9.6 ms, and 12.3 ms for Z350, Z450, and Z550, respectively. This result suggests longer lifetime for electrons in the annealed ZnO based cells than that in pristine ZnO based cell. In other words, higher annealing temperature employed for the ZnO post treatment generates faster electrons diffusion in the tested cells. For the ZnO-based cells, the longer electron lifetime is regarded to be favorable for electron passing through a longer distance with less diffusion resistance and, thus, probably leads to for more effective electron capture and collection.

absorption wavelength of the sensitizer dye (i.e., N719). Besides, the IPCE is almost proportional to the annealing temperature in the full range of 350–550 nm except for the cell based on Z550. It has been revealed that the IPCE is predominantly determined by the light-harvesting efficiency (LHE), injection efficiency (Φinj ), and electron-collection efficiency (ηc ) and can be expressed as the following equation: IPCE = LHE (λ) ∗ φinj ∗ ηc .

(1)

Generally, the LHE (λ) is largely dependent on the dye adsorption amount of the photoanodes, while Φinj and ηc are believed to be related to crystallinity of nanomaterials. In addition, ηc is more directly decided by electron lifetime and the film resistance [12,22]. In our system, since the amount of dye absorption on the annealed ZnO samples is lower than that of pristine ZnO, as given in Table 1, the higher IPCE for Z350 and Z450 is most likely given rise by the latter two aspects. On one hand, the improvement of crystallinity may result in effective excited-state electrons injection from dye into the conduction band of ZnO, which means high Φinj . On the other hand, the growth and coalescence of the primary nanocrystallites could enhance electron collection efficiency (ηc ) due to increased electron lifetime (see Table 1) and decreased grain boundary. Compared to Z350, the increase in IPCE for Z450 could result predominantly from enhanced electron transfer and collection efficiency. As for Z550, though higher crystallinity and larger crystal size endow them faster electron diffusion and less recombination characteristics, the lower capability of dye adsorption probably made the major contribution to IPCE.

3.6. IPCE analysis 3.7. Combination with dual interfacial modifications The incident photon-to-current conversion efficiency (IPCE) spectra provide direct evidence for the cooperative effects of all the factors on the cells. Fig. 6 shows IPCE spectra for the annealed ZnO and pristine ZnO cells as a function of the illumination wavelength. As can be seen, the maxima in photoactivity for all photoanodes are achieved at approximately 530 nm, coinciding with the typical

To simultaneously minimize photoanode charge recombination processes at both the FTO/ZnO interface and the ZnO/electrolyte interface, we further integrated interfacial modifications i.e. a ZnO CL and a TiO2 PL into the Z450 electrode film, and the photovoltaic performance of Z450 + CL + PL is given in Fig. 7. Noticeable, in

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Fig. 8. Schematic diagrams of electron transport and recombination of photoelectrodes based on ZnO and Z450 + CL + PL.

comparison with that of the Z450 cell, a higher JSC of 14.9 mA cm−2 and VOC of 0.626 V were achieved for the Z450 + CL + PL cell, resulting in an outstanding PCE up to 5.83%, a 14.3% increment from that of the Z450 cell. Subsequently, J–V measurement and EIS analysis were performed for Z450 + CL and Z450 + PL cells in dark condition, respectively (Fig. S5, Supporting Information). As shown in Fig. S5a, one can clearly see that after adding the ZnO CL layer, the increase of dark current is restrained and it indicates the reduction of electron movement from FTO to oxidized dye species [25,41]. Moreover, an increased interfacial charge-transfer resistance of Z450 + PL cell is observed in comparison with that of Z450 (see Fig. S5b). This is understandable because the TiO2 protective layer on top of the ZnO photoanode can effectively reduce unwanted charge recombination occurring at the ZnO/dye/electrolyte interface [24,42]. Moreover, the dye loading amounts for Z450 + CL, Z450 + PL and Z450 + CL + PL were measured and the values were 1.03 × 10−7 , 1.05 × 10−7 , and 1.10 × 10−7 mol/cm2 , respectively. Since the discrepancy of dye absorption on the three samples is also negligible, the enhanced JSC as well as VOC and thus PCE for Z450 + CL + PL can be mainly ascribed to the suppressed interface charge recombination. 3.8. Schematic views of electron transfer and recombination The schematic of the influence of composition and interface controls on electron transfer and recombination is shown in Fig. 8. Firstly, the nature of the as-prepared ZnO aggregates decides the existence of extensive grain boundary and defects, and hence lowering the transportation of electron across the ZnO photoanode film. Moreover, in the absence of neither of the interfacial engineering of the photoanode, the ZnO aggregates film would not only inevitably expose a portion of FTO surface to electrolyte but also unavoidable lead to the transfer of injected electron from ZnO to the electrolyte. Consequently, undesired charge transfer (i.e., the back-electron transfer) and unwanted electron recombination from ZnO to the dye and/or electrolyte could probably also occur. After rationally regulating the annealing temperature, electron transfer across ZnO film is greatly improved due to the provided favorable high pathways arising from the increased primary particle size and decreased defects. Meanwhile, back electron transfer from FTO to electrolytes and electron recombination can be effectively suppressed by introducing the CL and PL. Obviously, integrating all aforementioned favorable designs, i.e. the composition control and interface modifications into one photoanode film could simultaneously achieve the purpose of accelerating electron transport within the film and suppressing charge recombination at the interfaces, and hence giving rise to the markedly enhanced efficiency.

4. Conclusions In summary, the influence of the annealing temperature on the properties of ZnO aggregates was firstly studied. Various materials and device characterization demonstrated that the crystal size and the crystallinity of the ZnO aggregates have significant influence on electron transport within the film, which correspondingly influences the Jsc and Voc of DSCs. As a consequence, a distinguished PCE of 5.10% is obtained at optimal annealing temperature of 450 °C due to the cooperative effect of several characteristics of ZnO samples. Moreover, a further enhanced PCE up to 5.83% was achieved by integrating a ZnO compact layer and TiO2 protective layer into the Z450 based cell to simultaneously suppress possible interfacial charge recombination in the photoanode. This work provides an effective method for outstanding cell efficiency by accelerating electron transport within the film and suppressing charge recombination at the interfaces at the same time. Acknowledgments This work was supported by Shandong Province Natural Science Foundation of China (ZR2015PB015) and Liaocheng University Funds for Young Scientists (31805). Appendix A. Supplementary data Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoso.2017.01.001. References [1] K. Meng, P.K. Surolia, K.R. Thampi, BaTiO3 photoelectrodes for CdS quantum dot sensitized solar cells, J. Mater. Chem. A 2 (2014) 10231–10238. [2] J. Choi, G. Kang, T. Park, A competitive electron transport mechanism in hierarchical homogeneous hybrid structures composed of TiO2 nanoparticles and nanotubes, Chem. Mater. 27 (2015) 1359–1366. [3] T.P. Chou, Q.F. Zhang, G.E. Fryxell, G.Z. Cao, Hierarchically structured ZnO film for dye-sensitized solar cells with enhanced energy conversion efficiency, Adv. Mater. 19 (2007) 2588–2592. [4] O’R. Brian, G. Michael, A low-cost, high-efficiency solar cell based on dyesensitized, Nature 353 (1991) 737–740. [5] Q.F. Zhang, K. Park, J.T. Xi, D. Myers, G.Z. Cao, Recent progress in dyesensitized solar cells using nanocrystallite aggregates, Adv. Energy Mater. 1 (2011) 988–1001. [6] Q.F. Zhang, C.S. Dandeneau, X.Y. Zhou, G.Z. Cao, ZnO nanostructures for dyesensitized solar cells, Adv. Mater. 21 (2009) 4087–4108. [7] A. Oudhia, N. Shukla, P. Bose, R. Lalwani, A. Choudhary, Effect of various synthesis protocols on doping profile of ZnO:Eu nanowires, Nano-Struct. Nano-Objects 7 (2016) 69–74. [8] V. Manthina, A.G. Agrios, Single-pot ZnO nanostructure synthesis by chemical bath deposition and their applications, Nano-Struct. Nano-Objects 7 (2016) 1–11. [9] S.P. Mucur, T.A. Tumay, S. Birdoğan, S.E. San, E. Tekin, Triangular-shaped zinc oxide nanoparticles enhance the device performances of inverted OLEDs, Nano-Struct. Nano-Objects 1 (2015) 7–14.

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