Concordantly fabricated heterojunction ZnO–TiO₂ nanocomposite electrodes via a co-precipitation method for efficient stable quasi-solid-state dye-sensitized solar cells

Abstract

This manuscript is concerned with the successful attempts we have made to fabricate nanostructured spheres composed of mixed metal oxides. ZnO/TiO₂ nanocomposites supported on an FTO substrate are used as the photoanode electrode for quasi-solid-state dye-sensitized solar cells (QS-DSSCs). The phase purity of the ZnO and TiO₂ phases of the composite shell has been studied by X-ray diffraction peak analysis. A novel gel polymer electrolyte based on a poly(acrylamide)–poly(ethylene glycol) composite and a binary organic solvent was prepared. The polymer gel electrolyte based on the composite of poly(acrylamide)–poly(ethylene glycol), the binary organic solvent of ethylene carbonate and propylene carbonate and the additive of 4-tert-butylpyridine has been employed to fabricate a quasi-solid-state dye-sensitized solar cell. The conversion efficiency of the dye-sensitized solar cells with nanocomposites is 6.5% which is more than double compared with that of bare ZnO nanoparticle photoanodes (3.8%). We believe that this improvement comes from the synergetic effect between ZnO and TiO₂, which increases dye absorption, electron transport and electron lifetime, as discussed with the EIS and loaded absorption results. From the current–voltage and incident photon-to-current conversion efficiency (IPCE) measurement, the conductivity of the nanocomposite ZnO–TiO₂ was shown to be higher compared to the nanostructured ZnO itself. This simple method can be universally adopted for all quasi-solid-state electrolyte-based DSSCs in order to improve their performance and durability.

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Introduction

After the discovery of the photoelectrochemical properties of nanostructured titanium oxide (TiO₂), it has been recognized as one of the most promising wide bandgap semiconducting materials for photocatalysis, dye-sensitized/quasi-solid-state dye-sensitized solar cells (DSSCs/QS-DSSCs) and lithium ion batteries.^1–6 The DSSC is a molecular approach to photovoltaic solar energy conversion technology. This is one of the emerging photovoltaic technologies that offers the potential to reduce the cost of photovoltaic electricity production. During the past two decades, nanoporous polycrystalline titania has been extensively used in DSSCs, which have demonstrated to be a promising alternative to silicon-based solar cells due to their relatively high solar-to-electric power conversion efficiency at a low cost.

ZnO has similar energy band levels, superior electron mobility,³ and a functioning high specific surface area, which results in it being an effective dye-adsorption and light-scattering layer. To achieve this we have developed a co-precipitation wet-chemical deposition synthesis route for ZnO–TiO₂ nanocomposite structures with a well-defined shape and size. Such novel nanocomposite powders provide not only an effective surface area but are also helpful for effective light harvesting in DSSCs.^27–29

The light-to-electric energy conversion efficiencies of DSSCs based on liquid electrolytes using organic compounds, such as acetonitrile, propylene carbonate and ethylene carbonate as solvent and the iodide/triiodide (I⁻/I³⁻) redox couple as electrolyte, have reached 10–11% under irradiation of AM 1.5.^2–7 However, this type of liquid-junction cell still has some problems including low long-term stability, which is caused by organic solvent evaporation and leakage of liquid electrolytes, high temperature instability, and difficulties in sealing the devices.⁸ To overcome these problems, much effort has been made to replace the liquid electrolytes with solid or quasi-solid type charge transport materials.^9–12 Compared with other kinds of charge transport materials, the gel polymer electrolytes have some advantages including high ionic conductivities, which are achieved by trapping the liquid electrolyte in polymer cages formed in a host matrix, and good contacting and filling properties with the nanostructured electrode and counter electrode. Up to now, several types of gel electrolytes based on different kinds of polymers have already been used in quasi-solid-state dye-sensitized solar cells.^12–25 Poly(acrylamide) possess a carbonyl group and an amine group on its molecular chain,²⁶ and also poly(ethylene glycol) has many hydroxyl groups on its molecular chain. It is expected that the interaction between the sensitized dye and the matrix of the gel polymer can be improved based on the hydrogen bond interaction between the carbonyl group on sensitized dye and the carbonyl group, amine group and hydroxyl group on the matrix of the gel polymer electrolyte. On the other hand, the ionic conductivity of the gel polymer electrolyte can be enhanced according to the complexation from the carbonyl group, amine group and hydroxyl group on poly(acrylamide) and poly(ethylene glycol) to K⁺ ions in the electrolyte. Consequently, the overall conversion efficiency of the DSSC can be enhanced. The present study is focused on using poly(ethylene glycol) as both a reactant and plasticizer, amd a novel homogeneous poly(acrylamide)–poly(ethylene glycol) composite without phase separation was synthesized. The key innovation in the present study is to use the composite as a matrix, with 4-tert-butylpyridine as an additive, and the binary organic compounds ethylene carbonate and propylene carbonate as solvent, and a gel polymer electrolyte with a quasi-solid-state was prepared. Furthermore, a dye-sensitized solar cell was fabricated by sandwiching the gel polymer electrolyte. Meanwhile, the structure and band analysis were examined by X-ray diffraction patterns (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. The structure and band analysis was examined using XRD and XPS. The morphological study of the obtained powders was carried out using FESEM and TEM. Optical characterization was performed using UV-vis-NIR spectroscopy. The electrochemical properties, such as the charge transport in the ZnO–TiO₂-based photoanodes, and the ZnO–TiO₂ /electrolyte interfacial properties, were investigated in detail using current–voltage characteristics and IPCE measurements.

Experimental section

Poly(ethylene glycol) with an average molecular weight of 400 (PEG-400), acrylamide monomer, ammonium persulfate, potassium iodide, iodine, ethylene carbonate (EC), propylene carbonate (PC) and γ-butyrolactone (γ-BL) were all of A.R. grade and all purchased from Sigma-Aldrich Chemicals. PEG-400 was dried at 120 °C for 12 h before use. Other reagents were used without further treatment before use.

Conducting glass plates (FTO glass, with a fluorine-doped tin oxide over-layer, and a sheet resistance of 15 Ω sq⁻¹, purchased from Hartford Glass Co., USA) were used as a substrate for precipitating the TiO₂ porous film on. The sensitizing dye cis-[(dcbH₂)₂Ru(SCN)₂] was purchased from Solaronix, SA.

Preparation of pure ZnO nanoparticles and ZnO–TiO₂ nanocomposites

In a typical procedure for co-precipitation,³⁰ zinc nitrate (ZnNO₃·5H₂O) (Sigma-Aldrich) as a source of Zn²⁺ ions in the case of pure ZnO nanoparticles, or zinc nitrate and titanium trichloride (TiCl₃) (Sigma Aldrich) as a source of Zn²⁺ and Ti²⁺ ions in the case of ZnO–TiO₂ nanocomposites, are dissolved with an aqueous sodium hydroxide solution (5 M) (Fluka) and the pH is adjusted to the desired value of 10. Furthermore, the preparation of the TiO₂ nanoparticles is mentioned in the ESI.† The resulting milky white precipitate in each case was collected, filtered, washed with distilled water several times to remove impurities, and then dried at 80 °C for 24 h. The as-prepared ZnO nanopowders and ZnO–TiO₂ nanocomposites were annealed at 600 °C for 2 h. After cooling down, the thermally treated powders were collected, packed and kept in a desiccator for further physicochemical investigations.

Synthesis of the poly(acrylamide)–poly(ethylene glycol) in situ composite

The poly(acrylamide)–poly(ethylene glycol) in situ composite was synthesized by adding 3 g PEG-400 into 6 g acrylamide monomer. The mixture was heated at 70–75 °C to melt the monomer and to mix the two components homogeneously. The mixture was marked as (A). A polymerization initiator (ammonium persulfate, 1 wt% of acrylamide monomer) and PEG-400 (1 g) were mixed and stirred until the mixture dissolved entirely at room temperature, and the mixture was marked as (B). Under vigorous stirring and being kept at a temperature of 70–75 °C in a water bath heater, mixture (B) was added into mixture (A) slowly, the polymerization reaction took place, and a homogeneous mixture formed; the mixture was marked as (C). It is noticeable that as soon as the polymerization reaction begins, mixture (C) should be taken out of the water bath heater and cooled down to room temperature to prevent implosion because the reaction is exothermic. When mixture (C) became a viscous gel in the ambient environment, it was heated again at 60–65 °C for 30 min to complete the polymerization reaction. After that, the poly(acrylamide)–poly(ethylene glycol) in situ composite was synthesized.

Preparation of the gel polymer electrolyte

A mixture of organic solvent was made by mixing ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolactone (γ-BL). Potassium iodide (KI = 10 wt% of the total weight of the composite and the mixture solvent) and iodine (I₂ = 10 wt% of KI) were dissolved in the mixture of organic solvent to form a liquid electrolyte. A suitable amount of the poly(acrylamide)–poly(ethylene glycol) composite was added into the liquid electrolyte under continuous stirring at room temperature to form a gel polymer electrolyte with a quasi-solid-state.

Assembly of the quasi-solid-state dye-sensitized solar cell

Photoanodes and solar cells were prepared as reported elsewhere.^31–35 Pastes of ZnO, TiO₂ or ZnO–TiO₂ were coated onto the FTO substrate (15 Ω sq⁻¹) through mixing with distilled water and absolute ethanol. The coated substrates were heated at 450 °C for 30 min. After cooling, the coated substrates (electrodes) were immersed in a dye bath containing 0.3 mM N719 dye (Solaronix) in ethanol solution for 24 h to absorb the dye adequately, the other impurities were washed up with anhydrous ethanol, and they were dried in moisture-free air. After that, a porous film electrode of ZnO, TiO₂ or ZnO–TiO₂ with absorbed dye was prepared. A quasi-solid-state dye-sensitized solar cell was assembled by dropping a drop of the gel polymer electrolyte into the aperture between the ZnO, TiO₂ or ZnO–TiO₂ porous film electrode (anode electrode) and a Pt plated conducting glass sheet (cathode electrode, prepared by adding a 50 nm layer of platinum (Pt) on the ITO surface using electrodeposition). The two electrodes were clipped together and a cyanoacrylate adhesive was used as a sealant to prevent the electrolyte solution from leaking.

Surface/structure characterization

The crystallite phases were identified by X-ray diffraction (XRD) on a Bruker axis D8 diffractometer. The particle morphologies were examined using field emission scanning electron microscopy (FESEM, JSM-6700FT, JEOL). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed with a JEOL-JEM-1230 microscope. The UV-vis transmission spectrum was measured by a UV-vis-NIR scanning spectrophotometer (Jasco-V-570 Spectrophotometer, Japan). The Fourier transform infrared (FTIR) absorption spectrum was obtained using a JASCO 3600 spectrophotometer in the wave number range 200–4000 cm⁻¹. XPS (ESCA-3400, Shimadzu) was used to analyze the chemical composition of the surfaces. Photocurrent–voltage J–V characteristic curve measurements were performed using solar simulation and a Keithley 2601 multimeter. IPCE analyses were carried out using a QE/IPCE measurement system from Oriel at 10 nm intervals between 300 and 700 nm, where a monochromator was used to obtain the monochromatic light from a 300 W Xe lamp. A calibrated photodiode (S1227-1010BQ from Hamamatsu) was used before each IPCE analysis.

Results and discussion

Growth and characterization of the pure ZnO and TiO₂ nanoparticles and the ZnO–TiO₂ nanocomposites

ZnO nanoparticles were fabricated on FTO. The XRD data (Fig. 1a) show that the nanoparticles grew as crystalline ZnO with the hexagonal wurtzite structure (space group P63mc (186); a = 0.3249 nm, c = 0.5206 nm). The data are in agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) card for ZnO (JCPDS 070-8070). The prominence of the peak assigned to the (002) plane of ZnO is consistent with predominant ZnO growth along the c-axis, perpendicular to the substrate, giving the nanorod morphology. Furthermore, TiO₂ nanoparticles were synthesized by an autoclave hydrolysis method and ultimately dispersed in water. On the basis of a Scherrer analysis of the (010) peak at 26° of the X-ray powder diffraction pattern (as shown in Fig. 1b), the crystallite size of the TiO₂ nanoparticles was found to be 16 nm, and only the anatase phase was observed. The particle size is generally similar to the crystallite size for this synthesis method.³⁶ Full profile pattern fittings were applied to confirm the crystal phases of the samples as well as to determine the crystallite sizes. No impurity peaks are detected, indicating that the grown nanostructures were of high purity and good crystallinity.

Fig. 1 Left: XRD patterns. Right: 3D simulation of the pure ZnO, the pure TiO₂ and the ZnO/TiO₂ nanocomposite.

More complicated features are observed in the spectrum of the mixed oxide system (Fig. 1). The reflections, due to the hexagonal ZnO crystal phase, vanished. The spectrum form suggests a high degree of amorphous fraction. The broad band at 29.6° and the clearly seen maxima at 30.9°, 44°, 53.2°, 69.5°, 73° and 77° are assigned to ZnTiO₃ (cubic symmetry; JCPDS 00 025 1164). The other two peaks at 34.3° and 36.1° can be referred to as anatase ZnO. The peak located at 64.2°, which is most probably related to cubic TiO₂ for the XRD line at the same position, appeared in the spectrum of the ZnO film (Fig. 1).

Morphological properties

FTIR studies of the ZnO/TiO₂ film show the characteristics of the formation of a high purity material. The FTIR spectrum in Fig. S2 (see ESI†) clearly shows the peaks corresponding to ZnO and TiO₂. Peaks located in the area of 1000–1250 cm⁻¹ and 1750 cm⁻¹ correspond to the vibration of Ti–O and the Ti–O–O bridging stretching mode.³⁷ A sharp strong band around 669 cm⁻¹ was observed corresponding to the stretching mode of the Zn–O nanoparticles. On the other hand, the absence of peaks at 1400 cm⁻¹ proves the absence of peroxo groups. It is noted that the examination of the film in IR was made on silicon wafer. It is also clear from the data in Fig. S3 that there are no IR absorption peaks corresponding to impurities like organic residues, –CH and –CH₂ at 1400–2900 cm⁻¹ and C–O–C at 1000–1500 cm⁻¹. Furthermore, the FTIR spectrum firmly suggests the presence of –OH groups, which is absolutely necessary for dye adsorption in DSSC fabrication. The samples show peaks corresponding to the stretching vibration of O–H and bending vibrations of adsorbed water molecules at around 3300–3450 cm⁻¹ and at 1640 cm⁻¹ respectively.³⁸ Thus, the as-prepared ZnO/TiO₂ films after washing with pure water consisted of pure ZnO and TiO₂ particles without any significant organic contaminants, which is an important advantage in the evaluation of the DSSC efficiency made with these ZnO/TiO₂ electrodes.

The FESEM images in Fig. 2a and b show that the ZnO and ZnO/TiO₂ films exhibit a uniform surface morphology without any cracks. The figures show an approximately spherical morphology with the agglomeration of particles, which is the core material. In the case of adding ZnO to the TiO₂ core material, the size of the particles is slightly increased which is related to the shell thickness as presented in Fig. 2b. The chemical compositional analysis of the samples is essential for knowing the exact concentration of elements, added dopants, and defects, if any, present in the samples. Fig. 2c shows a cross sectional view of a QS-DSSC consisting of an FTO layer covered with a porous, 1.2 μm thick 20 nm crystallite TiO₂ film. N719 Ru dye and polymer gel electrolyte fill the pores of the film and form a 110 nm thick over-layer. Moreover, Fig. 2d shows a schematic diagram of the QS-DSSC shown in Fig. 2c. Furthermore, it is interesting to note (according to Fig. S1, left, ESI†) that the TiO₂ material is highly crystalline, as expected, and the particles are connected between them. There is no indication of isolated particles, thus we believe that the formation of the TiO₂ film is accompanied by good mechanical stability. Apparently, there is no measurable fraction of the amorphous TiO₂ phase because the quantity of titanium(IV)-iso-propoxide (TTIP) used was relatively low. The nanoparticles had an average size of 25–30 nm, confirming the X-ray pattern behavior.

Fig. 2 FESEM patterns of the pure ZnO (a), the ZnO/TiO₂ nanocomposite (b), and the cross section of a full cell (c), and a schematic diagram of a dye-sensitized solar cell (d).

The shape, crystallinity and size of the particles were examined with HRTEM. Fig. 3a–d show the TEM images of the ZnO and ZnO/TiO₂ films. Both of them show a spherical morphology with good monodispersity of the nanoparticles. Fig. 3b of ZnO/TiO₂ infers that ZnO was effectively reacted as a composite with the core material TiO₂. The high resolution image of each sample is shown in Fig. 3c and d. The lattice-resolved image in Fig. 3c exhibits a typical spacing of about 2.40 Å, corresponding to the (101) plane of the wurtzite ZnO structure. For ZnO–TiO₂ composite systems, along with TiO₂, a lattice spacing of about 2.35 Å corresponding to the (101) plane of wurtzite ZnO and a lattice spacing of 3.45 Å corresponding to the (101) plane of TiO₂ anatase was observed, as shown in Fig. 3d. Here, it is possible to observe the interference between the lattice corresponding to ZnO and TiO₂, which may be due to the good formation of the nanocomposites between both of them.

Fig. 3 TEM patterns of the pure ZnO (a) and the ZnO/TiO₂ nanocomposite (b), and HRTEM patterns of the pure ZnO (c) and the ZnO/TiO₂ nanocomposite (d).

UV-vis spectroscopy, bandgap energy and chemical components

Fig. 4 (left) shows transmittance UV-vis spectroscopy of the ZnO nanoparticles and the ZnO/TiO₂ nanocomposites in the wavelength range of 400–800 nm. There is an obvious red shift compared with the ZnO nanoparticles, possibly due to differences in the surface state of the ZnO/TiO₂ composites. In addition, the absorbance of the spectrum increases in the case of ZnO compared with that of TiO₂ (as shown in Fig. S1, right, in the ESI†), due to the surface states of ZnO. To obtain the bandgap, the spectra shown in Fig. 4 (left) are replotted in Fig. 4 (right). The theory of optical absorption defines the relationship between the absorption coefficient and the photo-energy. The relevant equation for a direct transition semiconductor is shown as follows:³⁹ α = (hν − E_g)^1/2/hν, where α = A/t, and α is a coefficient, A is absorbance, t is the film thickness and hν is the photo-energy. Using this equation, curves of hν versus (αhν)² were plotted and the extrapolated liner portion corresponded to the E_g value. The estimated bandgap energy was 2.58 eV for the ZnO nanoparticles and 3.00 eV for ZnO/TiO₂. To the authors’ best knowledge, the band energy of ZnO is 3.37 eV and that for anatase TiO₂ is 3.20 eV. The band energy of the ZnO/TiO₂ composites is lower than that for the ZnO and TiO₂ nanoparticles. That is because the presence of ZnO can modify the optical properties, to extend the range of the excited spectrum and favor the absorption of solar energy in the visible region. It also has been found that a red shift occurs as the ZnO deposition time increased, causing an increase in the amount of Zn. The chemical components and chemical changes due to the modification of the amount of ZnO on the surface of the TiO₂ nanoparticles were examined by XPS, as shown in Fig. 5. The ZnO/TiO₂ nanocomposites were selected to analyze changes of the surface state. Fig. 5(a) shows the spectra of the TiO₂ nanoparticles and the ZnO/TiO₂ composites over a wide scan range. Zn was observed for the composites, because of the ZnO coating on the TiO₂ nanoparticles. A high resolution XPS spectrum of Zn on the surface of the ZnO/TiO₂ composites is shown in Fig. 5(c). The Zn 2p^3/2 peak at approximately 1021.6 eV is assigned to the Zn–O bonds,⁴⁰ which indicates the formation of a ZnO composed with TiO₂ nanoparticles by a hydrothermal method. The narrow scan of the Ti 2p peaks for the TiO₂ nanoparticles is shown in Fig. 5(b). A slight shoulder peak is observed at approximately 459.8 eV. This is due to the formation of Ti³⁺. The strong chemical activity of Ti³⁺ can transfer excess electrons to the carboxylate group of the dye molecules, which form coordination bonds and may increase the amount of dye adsorption.⁴¹ Moreover, the formation of Ti³⁺ can increase oxygen vacancies, which support the transport of electrons and holes between the dye molecules and the photoanode. The Ti(2p) peak of the ZnO/TiO₂ composites shifts to a lower binding energy. It also shows that the composed ZnO can change the energy band of TiO₂. In Fig. 5(d), it can be seen that a good symmetric O 1s peak was obtained for the pure TiO₂ nanoparticles, while the O 1s peak for the ZnO/TiO₂ composites is asymmetric.

Fig. 4 UV-vis T% (left) and bandgap (right) of the pure ZnO (a) and the ZnO/TiO₂ nanocomposite (b).

Fig. 5 XPS of (a) a wide scan range, (b) and (c) a narrow scan range of the TiO₂ and ZnO nanoparticles, respectively, and (d) the binding energy of oxygen particles.

Photoelectrochemical properties: J–V characteristics

For the quasi-solid-state DSSCs with the low fluidity electrolyte, the electron collection yield is low because of the short electron diffusion length in the ZnO/TiO₂ film.^42,43 Reducing the thickness of the ZnO/TiO₂ film permits more electrons to be collected by the FTO before recombination by triiodide ions in the electrolyte.⁴⁴ However, a thinner ZnO/TiO₂ film would incur a lower amount of adsorbed dye and limit the absorption of incoming light. In this study, we employ Ru N719 organic dyes with a (PA/PEG) hybrid polymer gel electrolyte to construct quasi-solid-state DSSCs.

The photocurrent density–voltage (I–V) curves of the PA/PEG-based quasi-solid-state solar cells sensitized by Ru N719 using ZnO and/or the ZnO–TiO₂ nanocomposites, illuminated at a light intensity of 100 mW cm⁻², are displayed in Fig. 6 (left).

Fig. 6 Comparison of the I–V characteristics (left) and IPCE (right) of the DSSCs made from pure ZnO (a), and the ZnO/TiO₂ nanocomposites (b).

For comparison, I–V curves of the solar cells sensitized by Ru N719 using TiO₂ nanoparticles measured under the same conditions are also presented in Fig S3(a) (discussed in the ESI†). The detailed photovoltaic performance parameters of V_oc, J_sc, FF, and η under a certain light intensity are summarized in Table 1. For the ZnO–TiO₂ nanoparticle-based cells, the presence of PA/PEG can obviously improve the photovoltaic performance in terms of an increased V_oc, J_sc, and η, as compared to that of the pure ZnO or pure TiO₂-based cells. As mentioned above, PA/PEG can be adsorbed onto the ZnO, TiO₂ and ZnO–TiO₂ surface through Ti–O and Zn–O bonds and can suppress back electron transfer from the nanoparticles to I₃⁻,⁴⁵ consequently raising the quasi-Fermi level of the conduction band electrons in the nanoparticle film and increasing the V_oc. As a result, the η value is improved from 3.8% (cell with ZnO) to 6.5% (cell with ZnO–TiO₂) and from 5.6% (cell with TiO₂ in the ESI†) to 6.5% (cell with ZnO–TiO₂). For the cell with ZnO–TiO₂ nanoparticles, a more negative lowest unoccupied molecular orbital level of the dye and a lower back reaction rate at the ZnO–TiO₂/electrolyte interface between the injected electrons and I₃⁻ together result in a higher J_sc as well as V_oc (discussed in the ESI†). The considerable efficiency and excellent long-term stability of the cell with the ZnO–TiO₂ nanoparticles reveal that the development of this type of ruthenium dye-sensitized quasi-solid-state solar cell is promising for commercial applications.

Table 1 I–V characteristics results of the DSSCs manufactured using the pure ZnO, the pure TiO₂ and the ZnO/TiO₂ nanocomposite, respectively. Furthermore, a detailed simulated value of recombination resistance (R2) and electron lifetime value (τ_r) from the EIS spectra, calculated by an equivalent circuit as shown in Fig. 7

Sample	Voc	Jsc (mA cm^−2)	FF	η (%)	Active area (cm^2)	R2 (Ω)	τr (s)
Pure ZnO	0.812	7.2419	65.86	3.80	0.25	256.5	0.170
Pure TiO2	0.711	9.267	67.07	5.60	0.25	158.2	0.138
ZnO/TiO2 nanocomposite	0.740	13.017	67.48	6.50	0.25	96.04	0.094

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The incident photoelectric conversion efficiency and electrochemical impedance spectroscopy (EIS)

The I–V characteristic results are further confirmed by the IPCE values obtained for the DSSCs. The corresponding IPCE values for the DSSCs employing different nanomaterials are mentioned below. The IPCE values obtained for the cells with ZnO–TiO₂ and with ZnO are 80% and 54% respectively at 532 nm, as shown in Fig. 6 (right), which are compared to cells with TiO₂ that were of 64%, as shown in Fig S3(b).† The highest photocurrent response is attained for the ZnO–TiO₂-based DSSCs. The corresponding IPCE values are in good agreement thus confirming the improved pore filling obtained as the liquid electrolyte is utilized together with the gel polymer electrolyte.

Charge transfer dynamics. To obtain better insight into the dynamics of the interfacial charge transfer process within the DSSCs, electrochemical impedance spectroscopy (EIS) was performed. Fig. 7 shows the Nyquist plots of the DSSCs based on the above three photoelectrodes. Generally, two semicircles in the Nyquist plots could be observed. The small semicircles in the high frequency region and large semicircles in the low frequency region⁴⁶ correspond to the electron recombination at the photoelectrode/dye/electrolyte interface, and this shows that the recombination resistance decreases in the order of pure ZnO > pure TiO₂ > ZnO/TiO₂ photoelectrode prepared by the co-precipitation route implying a fast recombination reaction (or shorter electron lifetime) for the above photoelectrode order.^47–52 The high frequency semicircle (1k–100k Hz) is related to the capacitance (CPE1) and charge transfer resistance (R1) between the counter electrode and electrolyte, while the impedance in the middle frequency region is associated with the capacitance (CPE2) and interfacial charge transfer resistance (R2) between ZnO/TiO₂ (or ZnO or TiO₂)/dye and the electrolyte. Fig. 7 provides the impedance spectra of the DSSCs with ZnO/TiO₂ (or ZnO or TiO₂) photoelectrodes. The corresponding R2 values, by fitting the impedance spectra based on the equivalent circuit, are shown in the inset of Fig. 7 and are listed in Table 1. The interfacial charge transfer (recombination) resistance (R2) of the DSSCs decreased significantly from 256.5 Ω to 96.04 Ω in the order of pure ZnO > pure TiO₂ > ZnO/TiO₂ photoelectrode-based DSSCs, indicating that a faster recombination rate occurred within the DSSCs based on the TiO₂ NPs. This may result in a smaller photovoltage which is in agreement with the above photovoltaic data. Specifically, there was a huge difference shown in the large semicircles. This result indicates that the ZnO/TiO₂ nanocomposite improves the efficient charge transport in the mesoporous layer.

Fig. 7 Nyquist diagrams of the impedance spectra of (a) the pure ZnO, (b) the pure TiO₂, and (c) the ZnO/TiO₂ nanocomposite. Furthermore, the inset refers to the equivalent circuits of the DSSCs. R1: the serial resistance, R2: the charge transfer resistance of FTO ZnO/TiO₂ (or ZnO or TiO₂) and the counter electrode/electrolyte interfaces, CPE1: the constant phase element of FTO ZnO/TiO₂ (or ZnO or TiO₂) and the counter electrode/electrolyte interfaces, R3: the electron transfer in the ZnO/TiO₂ (or ZnO or TiO₂)/dye/electrolyte interfaces, and CPE2: the constant phase element of the ZnO/TiO₂ (or ZnO or TiO₂)/dye/electrolyte interfaces.

Moreover, this result is in good agreement with the changing trends of the PCE and the amount of dye loading as shown in Fig. 8, which is demonstrated by the optical absorbance with the wavelength by dissolving out the adsorbed dye molecules from the photoanode in 10 mL NaOH. The probable reason is that a low dye loading on the ZnO anode may generate a low electron density and interfacial charge transfer rate, causing a high R2 value. With an appropriate coating of TiO₂ on ZnO–TiO₂, the amount of adsorbed dye increases and significantly reduces the R2 value. This fact may be partly attributed to the penetration limitation of dye molecules due to the blocking effect induced by the coated TiO₂ and/or ZnO–TiO₂.

Fig. 8 Optical absorbance of the dissolved-out dyes of the pure TiO₂, the pure ZnO and the ZnO/TiO₂ nanocomposite, respectively loaded on Ru N719 dye.

Since the V_oc value of the DSSCs was correlated with the charge recombination in the conduction band of TiO₂ (ref. 53) the electron lifetime (τ_r) values calculated by fitting the equation τ_r = R2 × CPE2 (CPE2, chemical capacitance) was used to differentiate the difference of V_oc in the DSSCs based on various photoanodes. Obviously, τ_r of the DSSCs decreased with the order of pure ZnO > pure TiO₂ > ZnO/TiO₂ photoelectrodes, which was in agreement with the significant reduction of V_oc displayed in the J–V results. Accordingly, one can conclude that the alternate co-precipitated nanocomposites introduced in the present work markedly augmented the specific surface area of ZnO–TiO₂, while on the other hand simultaneously provided additional recombination sites at the surface of the TiO₂ NPs for recombination of the photo-generated electrons with I₃⁻ in the redox polymer electrolyte. It is worth noting that the electron lifetime for the ZnO–TiO₂ cell was longer than that of the TiO₂ and ZnO-based DSSCs since the former (nanocomposite) effectively suppresses the charge recombination. ZnO-based DSSCs exhibited the shortest electron lifetime, suggesting that photo-generated electrons within such cells may suffer a series of trapping events or serious recombination with oxidizing species in the electrolyte as they undertook a random network pathway through the mesoporous nanoparticle film.

Conclusions

Quasi-solid-state DSSCs were successfully fabricated using the composite poly(acrylamide)–poly(ethylene glycol)-based polymer gel electrolyte and Ru N719-based organic dyes. The hybrid polymer gel is beneficial to the entrapment of a large volume of liquid electrolyte. The electrolyte gelatinized by the hybrid PA/PEG exhibits considerable ionic conductivity and a triiodide ionic diffusion constant. By optimization of the electrolyte composition and the ZnO, TiO₂ and ZnO/TiO₂ film thickness, the fabricated quasi-solid-state solar cells sensitized by N719 dye achieved an overall energy conversion efficiency of 3.8, 5.6 and 6.5%, respectively at a light intensity of 100 mW cm⁻². Moreover, we have successfully synthesized a composite nanostructure consisting of ZnO nanoparticles composed with TiO₂ nanoparticles, using facile wet-chemical methods. The SEM images indicated that the composites reveal a well-ordered and good size distribution of the particles and they were used for photocatalytic dye decolorization. In addition, the nanocomposites showed no aggregation in comparison with other samples. It is important for solar cell activity that the particle size of the photocatalyst should be homogeneous. The XRD study exhibited that there were no impurities in the samples. Further addition of ZnO resulted in an increase in the particle size. This nanocomposite photoanode points to a class of structures that can provide both fast transport and a high surface area, enabling DSSCs that are tolerant of electrolytes with lower recombination rates than those found with the conventional iodide/triiodide couple. The high efficiency and excellent long-term stability of the quasi-solid-state DSSCs assembled with organic dye and the PA/PEG hybrid gel electrolyte indicate that they are promising for industrialization.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors would like to extend their sincere appreciation to Central Metallurgical Research and Development Institute, Egypt for its financial support to pursue the work.

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Footnote

† Electronic supplementary information (ESI) available: Additional SEM images, XRD patterns, optical characterization, a I–V curve and IPCE measurements of various products. See DOI: 10.1039/c5ra21822e

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