Nitrogen doped TiO₂–Cu_xO core–shell mesoporous spherical hybrids for high-performance dye-sensitized solar cells

Abstract

We report on high-performance dye-sensitized solar cells (DSSCs) based on nitrogen doped anatase TiO₂–Cu_xO core–shell mesoporous hybrids synthesized through a facile and controlled combined sol–gel and hydrothermal process in the presence of hexadecylamine as the structure-directing agent. The matching of band edges between Cu_xO and TiO₂ to form a semiconductor heterojunction plays an important role in effective separation of light induced electrons and holes, providing a promising photoanode for DSSCs because of its wide absorption spectrum, high electron injection efficiency, and fast electron transference. DSSCs based on the mesoporous TiO₂–Cu_xO core–shell hybrids show a high short-circuit current density of 9.60 mA cm⁻² and a conversion efficiency of 3.86% under one sun illumination. While DSSCs based on the N-doped mesoporous TiO₂–Cu_xO hybrids exhibit the higher short-circuit current density of 13.24 mA cm⁻² and a conversion efficiency of 4.57% under one sun illumination. In comparison with un-doped TiO₂–Cu_xO hybrids, the doping of nitrogen into the lattice of TiO₂ can extend the light absorption in the ultraviolet range to the visible light region and effectively decrease the recombination rate of photo-generated electrons and holes. The presented N-doped mesoporous TiO₂–Cu_xO hybrids as photoanodes could find potential applications for high performance DSSCs.

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1. Introduction

Dye-sensitized solar cells (DSSCs) are attracting considerable attention because of their diverse advantages such as low cost, environmentally benign process and high efficiency.^1,2 Among the various types of oxide semiconductors, titanium oxide (TiO₂) is widely recognized as the most promising and versatile material for photocatalysis and solar cell applications due to its outstanding physical and chemical properties, including chemical stability, photostability, non-toxicity, inexpensiveness, and appropriate electronic band structure.^3,4 In the past decades, considerable efforts have been concentrated on practical applications for DSSCs based on varieties of nanostructured TiO₂, such as nanocrystals,^5,6 nanofibers,^7,8 nanotubes,⁹ inversed opals¹⁰ and mesoporous beads.^11,12 Especially, mesoporous TiO₂ spherical nanostructures have received significant research attention due to their abundant mesopores (2–50 nm), providing a high surface area to maximize the uptake of dye molecules, and enhancing the light-harvesting capability, thereby giving rise to a large current density and high photon-to-current conversion efficiency for the TiO₂ based DSSCs.^13,14

Unfortunately, one of the deficiencies of TiO₂ is its low efficiency of optical absorption in the visible light region due to its intrinsic wide band gap (E_g = 3.2 eV). Furthermore, the photo-generated electrons and holes can easily recombine, resulting in a large recombination rate for the photo-generated electron–hole pairs. To improve the photoelectron conversion energy efficiency, a variety of strategies have been developed to improve the optical response properties and photoelectron energy efficiency, such as coupling with low band gap semiconductors like PbS,¹⁵ CdTe,¹⁶ CdS,^17,18 CdSe¹⁹ and PbSe,²⁰ combining with noble metal nanoparticles, doping with non-metal^21,22 and metal ions,^4,23,24 and dye sensitization.²⁵ Especially, narrow band gap semiconductors acting as sensitizers can effectively facilitate the electron transfer to the conduction band of large band gap TiO₂ in the hybrids of semiconductor/TiO₂ heterojunction, thereby efficiently separating photogenerated charge carriers. As a result, visible light can be efficiently utilized and the separation rate of photo-generated electron–hole pairs can be substantially increased in the hybrids of narrow band gap semiconductor/TiO₂.

Monoclinic CuO and cubic Cu₂O, as the two main lattice structures of p-type copper oxide, display narrow band gap energy of 1.2–1.85 eV and 2.1 eV, respectively.^26,27 Because of various superiorities of copper oxides, such as low cost, low toxicity, abundance, and ability to be coupled with a wide band gap semiconductor, copper oxide compounds can be coupled with TiO₂ for application of photocatalysis,^28,29 hydrogen production,^30–32 sensors,^33,34 and solar cells.^35,36 It is of great importance to fabricate hybrids of copper oxide nanoparticles sensitized mesoporous anatase TiO₂ with a large surface area for their application in the field of solar cells.

In this work, we reported on nitrogen doped mesoporous TiO₂–Cu_xO core–shell hybrids as photoanodes of DSSCs. In comparison with pure TiO₂, the doping of nitrogen into the lattice of TiO₂ can extend the light absorption in the ultraviolet range to the visible light region and effectively decrease the recombination rate of photo-generated electrons and holes, and the formation of mesoporous TiO₂ and Cu_xO can provide a large surface area for dye loading, sufficient light harvesting and efficiently separated photogenerated charge carriers. DSSCs based on the as-synthesized core–shell hybrids of mesoporous TiO₂–Cu_xO core–shell hybrids show a high short-circuit current density of 9.60 mA cm⁻² and a conversion efficiency of 3.86% under one sun illumination. While DSSCs based on the mesoporous N-doped TiO₂–Cu_xO core–shell hybrids exhibit the higher short-circuit current density of 13.24 mA cm⁻² and a conversion efficiency of 4.57% under one sun illumination.

2. Experimental

2.1 Preparation of N-doped TiO₂–Cu_xO hybrids

Amorphous precursor TiO₂ beads were synthesized according to literature procedures.^11,37 At room temperature, 2.69 g of hexadecylamine was dissolved in 400 mL of ethanol under vigorous stirring, followed by the addition of 1.6 mL of 0.1 M KCl solution in the reaction solution. Then, 8.8 mL of tetraisopropyl titanate was dropwise added into the solution keeping constant the stirring speed at ambient temperature. The white TiO₂ suspension was kept still at the same temperature for 18 h. Finally, the TiO₂ beads were collected on a filter, washed with ethanol three times and dried at room temperature.

TiO₂–Cu_xO hybrids were synthesized through a solvothermal process in the presence of copper acetate monohydrate. 0.4 g of precursor TiO₂ was dispersed in the solution containing 10 mL of ultrapure water and 20 mL of 0.025, 0.05 and 0.1 M concentrations of copper acetate monohydrate in ethanol solution (the corresponding TiO₂–Cu_xO hybrids are denoted S2, S3 and S4 samples, respectively). Afterwards, 1.2 mL of 25 wt% ammonia solution was added to each one. For comparison, the pure mesoporous anatase TiO₂ beads were also obtained. 1.6 g of precursor TiO₂ beads was added into a 30 mL ethanol–water mixture (2 : 1 by volume) containing 0.5 mL of 25 wt% ammonia solutions (denoted sample S1). This mixed solution was stirred for 10 min. After that, the mixture was transferred into a Teflon-lined autoclave and heated to 160 °C for 16 h. The resulting precipitates were collected by centrifugation, washed with ethanol and dried in air at room temperature. Finally, the products were calcined at 550 °C for 2 h in air to remove the organic components and produce the good crystallinity for characterization.

Nitrogen doped mesoporous TiO₂–Cu_xO core–shell nanostructures were synthesized according to the above-mentioned experimental procedure. The only change is adding 1.8 mL of hydrazine hydrate solution after 8.8 mL of tetraisopropyl titanate was dropwise added into the mixed solution, the others are unchanged. The N-doped TiO₂–Cu_xO hybrids prepared in 0.025, 0.05 and 0.1 M concentrations of copper acetate monohydrate in ethanol solution are denoted NS2, NS3 and NS4 samples, respectively.

2.2 Characterization

X-ray diffraction (XRD) patterns were obtained using a Philips Rigaku D/Max-kA X-ray diffractometer equipped with a Cu Kα source at 40 kV and 30 mA. The surface microstructure and chemical components of the products were analyzed using a SU-70 field-emission scanning electron microscope (FESEM) and an attached X-ray energy dispersive spectrometer (EDS). The absorption spectrum and the UV/visible diffuse-reflectance spectra (DRS) were recorded using a UV-vis spectrophotometer (TU-1900). Nitrogen adsorption–desorption isotherms were determined at 77 K using a Gold APP V-Sorb 2800P surface area and porosity 60 analyzer. The surface area measurements were performed according to the Brunauer–Emmett–Teller (BET) method, while the pore size distribution was obtained from the adsorption branch of isotherm using the corrected form of the Kelvin equation by means of the Barrett–Joyner–Halenda (BJH) method. Dye uptake per unit area (1 cm²) was investigated using a UV-Vis spectroscope (TU-1900) by the dissolution of dye adsorbed onto the sample film membrane in 0.2 M NaOH in water and ethanol (50 : 50, v/v) solution. The selected-area electron diffraction (SAED) characterization and microstructural analyses were carried out on a Philips Tecnai 20U-Twin high-resolution transmission electron microscope at an acceleration voltage of 200 kV. The cells were tested using a solar simulator (Newport, Class 3A, 94023A) at one sun (AM1.5G, 100 mW cm⁻²) using a Keithley 2420 sourcemeter equipped with a calibrated Si-reference cell (certified by NREL). The incident photon to current efficiency (IPCE) measurement was carried out using a QEX 10 system (PV measurement). A reference Si photodiode calibrated for spectral response was used for the monochromatic power-density calibration. The electrochemical impedance spectra (EIS) were recorded using a Princeton Parstate 2273A in a two-electrode design; the sample films served as a working electrode and the Pt-coated ITO or FTO glass as a counter electrode at an applied bias of the open circuit voltage under one-sun irradiation. The frequency range was 10 mHz to 100 kHz; the magnitude of the alternating potential was 20 mV. The EIS data were analyzed using an appropriate equivalent circuit using simulation software.

2.3 Fabrication of solar cells

For the working electrode, the fabrication process of the sample paste was described in detail as follows.³⁸ 0.12 g of ethyl cellulose powder (Aladdin-reagent, China) was dissolved in ethanol to yield a 10 wt% solution. The obtained mixture was added into 0.2 g of the calcined sample and 0.8 g of terpineol (Aladdin-reagent, China) which was diluted with 1.0 mL of ethanol. The mixture was then stirred in a magnetic field and sonicated by an ultrasonic horn three consecutive times. At last, the obtained paste was spin-coated onto a FTO glass substrate using spin coating apparatus at 2000 rpm for several seconds to obtain a film of required thickness repetitively several times. In order to remove the polymer template and organic compounds, the TiO₂ photoelectrode was dried at 125 °C, and gradually heated under flowing air at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min and at 500 °C for 15 min.

After calcination, the TiO₂ electrodes with an active working area of 0.4 × 0.4 = 0.16 cm² were immersed into a 0.5 mM dye N-719 ethanol solution in a 1 : 1 (v/v) mixture of acetonitrile (Aladdin-reagent, China) and tert-butanol (Aladdin-reagent, China) at room temperature for 24 h. Subsequently, the dye loaded photoanode and the Pt-counter electrode (Dyesol) were assembled into a sandwich type cell and sealed with a spacer of 25 μm thickness (Surlyn, DuPont). The internal space of the cell was filled with a liquid electrolyte, which consisted of 0.6 M 1-methy-3-propylimidazolium iodide (PMII), 0.05 M LiI, 0.05 M I₂, and 0.5 M 4-tertbutyl pyridine (TBP) in a (85 : 15 vol%) mixture of acetonitrile (Aladdin-reagent, China) and valeronitrile (Aladdin-reagent, China).

3. Results and discussion

3.1 Structural characterization

3.1.1 XRD pattern analysis. The crystalline structure and phase component of the synthesized products were examined by X-ray powder diffraction (XRD). Fig. 1 shows the XRD patterns of TiO₂ and TiO₂–Cu_xO hybrids prepared at different concentrations of copper acetate monohydrate: (a) 0 M, (b) 0.025 M, (c) 0.05 M and (d) 0.1 M. Fig. 1a depicts typical wide-angle XRD patterns of the synthesized materials. It is indicated that the synthesized materials after annealing at 550 °C in air are composed of highly crystalline TiO₂ with an anatase structure (PDF: 21-1272) (space group: I41/amd) with lattice constants of a = 3.785 Å and c = 9.514 Å. The diffraction peaks at 25.3°, 37.8°, 48°, 53.9° and 55° can be well assigned to (101), (004), (200), (105) and (211) planes of crystalline TiO₂, respectively. The XRD patterns in Fig. 1b–d demonstrate the crystalline structure and phase components of the products obtained after treatment in copper acetate monohydrate ethanol solution. For the XRD patterns of the products, besides anatase TiO₂, the presence of the CuO phase with a monoclinic lattice structure and a cubic Cu₂O structure can also be determined in the synthesized products. The peaks at 35.5°, 38.7°and 48.7°, correspond well with (002), (111) and (−202) planes of CuO (PDF: 45-0937) (space group: C2/c). The peaks located at 37° and 62.4° can be indexed as (111) and (220) planes of cubic Cu₂O (PDF: 34-1354) (space group: Pn3̄m), respectively. The average size of CuO and Cu₂O nanoparticles in the as-prepared nanomaterials is estimated from the Debye–Scherrer formula³⁹ to be about 23.50 ± 1.0 nm and 19.6 ± 1.0 nm, respectively. For the complex of copper oxides composed of Cu₂O and CuO phases, it can be denoted Cu_xO in the TiO₂–Cu_xO hybrids.

Fig. 1 XRD patterns of TiO₂ and TiO₂–Cu_xO hybrids prepared at different concentrations of copper acetate monohydrate: (a) S1, (b) S2, (c) S3 and (d) S4.

3.1.2 XPS characterization. As is known, pure TiO₂ can absorb only the UV light, which is only a small part of the overall solar energy. Furthermore, in order to extend the light response from the ultraviolet range to the visible light region to further improve the power conversion efficiency of the DSSCs,^40,41 anion doping (C, N, F, and S) is proved to be effective to efficiently shift the optical absorption to the visible range of TiO₂.^42–44

For this reason, we choose hydrazine as the nitrogen source to dope mesoporous TiO₂ into the TiO₂–Cu_xO core–shell hybrid photoanodes for DSSCs. XPS characterization is conducted to clarify the chemical composition component and the chemical bonding state of the N-doped TiO₂–Cu_xO core–shell hybrids (NS3). As shown in Fig. 2a, the general survey spectrum of the N-doped TiO₂–Cu_xO nanoparticles contains C, N, Cu, Ti, and O elements. The carbon could have resulted from adventitious hydrocarbons from the XPS instrument itself and can be taken as the standard signal for the correction of other peaks. For the Ti 2p spectrum (Fig. 2b), two main peaks of Ti 2p_3/2 and 2p_1/2 at bonding energies of 458.5 and 464.3 eV, respectively, reveal that Ti ions exist in the form of Ti⁴⁺ in the lattice of TiO₂.^41,45Fig. 2c depicts a significant N 1s peak at 400.9 eV, which can be attributed to the formation of Ti–O–N or Ti–N–O bonding.⁴⁶ In the present work, hydrazine was used as the nitrogen source.⁴⁷Fig. 2d shows the representative XPS spectra of Cu 2p3 and Cu 2p1. Two fitting peaks for Cu 2p3 at around 933.2 and 932 eV can be assigned to the Cu(II)⁴⁸ state and the Cu(I)⁴⁹ state, respectively. In addition, the shakeup satellite peaks around 942.7 and 940.5 eV suggest the existence of fully oxidized CuO and incompletely oxidized Cu₂O.⁵⁰ The XPS results above obtained for Cu reveal that two types of phases of CuO or Cu₂O coexist for the copper oxides, which can be denoted Cu_xO, in good agreement with the XRD results. Fig. 2e displays core-level high-resolution XPS spectra of O 1s for the representative nitrogen doped TiO₂–Cu_xO hybrids. The XPS spectral peak deconvolution of O 1s shows a large peak at 529.3 eV, indicating O=O bonding linked to the Ti–O or Cu–O structure, and two smaller shoulders located at around 530.6 eV (Ti–O) and 229.2 eV (Cu–O).

Fig. 2 XPS spectrum of the N-doped TiO₂–Cu_xO sample: (a) survey spectrum, (b) Ti 2p spectrum, (c) N 1s spectrum, (d) Cu 2p spectrum and (e) O 1s spectrum.

3.2 Microstructure characterization

3.2.1 SEM characterization. The microstructure and morphology of the synthesized mesoporous TiO₂/Cu_xO hybrids were characterized using a SU-70 FESEM. The low- and high-magnification SEM images of S2, S3, and S4 samples are depicted in Fig. 3a–f, respectively. The sample S2 after calcination at 550 °C shows a spherical shape with a mesoporous structure with an average diameter of 1.5 μm. According to XRD results, the S2, S3, and S4 samples are composed of anatase TiO₂, Cu₂O and CuO phases. Compared with S4 sample, relatively low amount of Cu₂O and CuO phases is deposited on the surface of TiO₂ in the S2 and S3 samples. With the concentration of copper acetate monohydrate increasing to 0.1 M, more and more Cu₂O and CuO phases are deposited on the surface of TiO₂, forming a typical core–shell hybrid structure of mesoporous TiO₂–Cu_xO hybrids (Fig. 3e and f). In comparison with the S2 and S3 samples, the thickness of the Cu_xO shell increases to about 50 nm. The high magnification SEM images in Fig. 3b, d and f show that the mesoporous TiO₂–Cu_xO hybrids are composed of the mesoporous anatase TiO₂ core and Cu_xO shell. The Cu_xO shell is composed of Cu₂O and CuO nanoparticles. The mesoporous core–shell structured hybrids could provide a large surface area and space to enhance the adsorption of dye and electrolyte, and be helpful to improve the photoelectric conversion efficiency of solar cells.

Fig. 3 FE-SEM images of TiO₂–Cu_xO mesoporous beads prepared using different concentrations of copper acetate monohydrate: (a and b) S2, (c and d) S3 and (e and f) S4.

The elemental energy-dispersive spectroscopy (EDS) mapping characterization was used to investigate the chemical composition component and elemental distribution of the TiO₂–Cu_xO core–shell nanostructures (S4). Fig. 4a shows a low FESEM image of the TiO₂–Cu_xO core–shell nanostructures. The EDS elemental mapping of Ti, O and Cu elements in Fig. 4b–d shows that Ti, O and Cu elements are homogenously distributed among the whole TiO₂–Cu_xO core–shell nanostructures, suggesting that Cu_xO nanoparticles are grown homogeneously on the surfaces of mesoporous anatase TiO₂ matrices. A typical EDS spectrum (Fig. 4e) shows that the products are composed of O, Ti and Cu elements.

Fig. 4 (a)–(d) SEM image and Ti, O, and Cu EDS mapping of TiO₂–Cu_xO core–shell hybrids (S4). (e) A typical EDS spectrum.

3.2.2 TEM characterization. Transmission electron microscopy (TEM) was used to further reveal the microstructures of mesoporous TiO₂–Cu_xO hybrids (S3). The morphology and size distribution of TiO₂–Cu_xO hybrids are depicted in the low magnification TEM image in Fig. 5a. A magnified TEM image taken from the edge of TiO₂–Cu_xO hybrids in Fig. 5a is illustrated in Fig. 5b. It is clearly shown that the mesoporous hybrid is composed of TiO₂ and Cu_xO nanocrystals. A HRTEM lattice image of the single TiO₂ nanoparticle is shown in Fig. 5c. The marked d-spacing of 0.13 nm and 0.10 nm corresponds well to that of (22−2) and (−231) planes of anatase TiO₂. Fig. 5d shows a monoclinic CuO nanocrystal with clear crystalline lattice fringes. The fringe spacing of 0.17 nm corresponds to the (311) planes, while the fringe spacing of 0.13 nm corresponds to the (020) planes. While in a HRTEM lattice image of the TiO₂–Cu_xO hybrid of Fig. 5e, the marked d-spacing of 0.35 nm and 0.23 nm is in agreement with that of (101) plane of TiO₂ and (111) plane of CuO. The diffraction rings in Fig. 5f correspond well with (101), (200) and (211) planes of anatase TiO₂ and (111) plane of monoclinic CuO, respectively.

Fig. 5 (a) TEM image of a TiO₂–Cu_xO hybrid (S3) prepared with 0.05 M copper acetate monohydrate. (b) A high-magnification TEM image taken from the edge of TiO₂–Cu_xO nanostructures. (c) A HRTEM lattice image of anatase TiO₂. The marked d-spacing of 0.13 nm and 0.10 nm corresponds well to that of (22−2) and (−231) planes. (d) A HRTEM lattice image of Cu₂O nanoparticles. The marked d-spacing of 0.17 nm and 0.13 nm corresponds well to that of (311) and (0−20) planes. (e) A HRTEM lattice image shows the TiO₂–Cu_xO nanostructures, the marked d-spacing of 0.35 nm and 0.23 nm is in agreement with that of (101) plane of TiO₂ and (111) plane of CuO. (f) Electron diffraction pattern of the TiO₂–Cu_xO nanostructures, the diffraction rings correspond to the (101), (200) and (211) planes of anatase TiO₂, and (111) plane of CuO.

TEM examination was carried out on the sample S4 to investigate the effects of concentration of copper acetate monohydrate on the microstructures of the TiO₂–Cu_xO hybrid samples (Fig. 6). It can be seen that the sample S4 displays a typical core–shell structure with mesoporous anatase TiO₂ acting as the core section and the Cu_xO as the shell section (Fig. 6a), and this is consistent with the FESEM images in Fig. 3f. Fig. 6b depicts a high magnification TEM image taken from the shell's edge of TiO₂–Cu_xO core–shell nanostructures, showing Cu_xO nanoparticles with the size ranging from 20 to 30 nm to form a Cu_xO shell with a thickness of 150 nm. Fig. 6c demonstrates a HRTEM lattice image of an anatase TiO₂ nanocrystal and a monoclinic CuO nanoparticle. The marked d-spacing of 0.35 nm and 0.35 nm correspond well to (−101) and (01−1) planes of anatase TiO₂. The marked d-spacing of 0.20 nm and 0.16 nm correspond well to (1−1−2) and (02−1) planes of monoclinic CuO. Fig. 6d displays a typical SAED pattern of the samples. The diffraction rings are in agreement with (101), (200) and (211) planes of anatase TiO₂ and (111) planes of monoclinic CuO, respectively.

Fig. 6 (a) TEM image of TiO₂–Cu_xO core–shell hybrids prepared with 0.1 M copper acetate monohydrate. (S4) (b) A magnified TEM image taken from the shell's edge of TiO₂–Cu_xO core–shell nanostructures. (c) A HRTEM lattice image of anatase TiO₂ nanocrystals and Cu_xO nanoparticles. The marked d-spacing of 0.35 nm and 0.35 nm corresponds well to (−101) and (01−1) planes of anatase TiO₂. The marked d-spacing of 0.20 nm and 0.16 nm corresponds well to (1−1−2) and (02−1) planes of monoclinic CuO. (d) Electron diffraction pattern of TiO₂–Cu_xO core–shell hybrids, the diffraction rings correspond to the (101), (200) and (211) planes of anatase TiO₂, and the (111) plane of CuO.

3.2.3 Optical properties. The optical properties of mesoporous TiO₂ and TiO₂–Cu_xO core–shell hybrids were investigated by UV-visible diffuse reflectance spectroscopy. Fig. 7a shows the diffuse reflectance spectra (DRS) of (a) pure TiO₂ (S1) and TiO₂–Cu_xO samples (S2–S4), respectively. The pure TiO₂ sample shows a sharp absorption edge at around 380 nm, which is typical for anatase TiO₂. It is interesting to observe that the core–shell TiO₂–Cu_xO hybrids show continuous absorption in the visible range, which can be primarily ascribed to the coupling of the narrow band gap Cu_xO nanoparticles with TiO₂, effectively extending the optical response to the visible region from the ultraviolet region. Furthermore, the intensity of UV-visible diffuse reflectance spectra becomes stronger after the Cu_xO shell is introduced to form the core–shell TiO₂–Cu_xO hybrids. The band gap energy of the mesoporous TiO₂ and TiO₂–Cu_xO hybrids can be roughly determined according to the plots in Fig. 7b, which are obtained via the transformation based on the Kubelka–Munk function (F(R∞) = (1 − R)²/(2R), where R is the reflection coefficient).⁵³ The estimated band-gap energy of mesoporous TiO₂–Cu_xO hybrids with different Cu_xO loading contents (S1, S2, S3, and S4 samples) corresponds approximately to the light response with an energy band gap of 3.20, 2.98, 2.73 and 2.94 eV, respectively. Therefore, it can be concluded that the sample S3 has the best light absorbing capacity and the transition of light response from UV to visible light of the TiO₂–Cu_xO samples can be realized by controlling the contents of Cu_xO nanoparticles. It is obvious that the optical properties of the TiO₂–Cu_xO samples can be tuned by adjusting the concentration of copper acetate monohydrate.

Fig. 7 (a) UV-vis diffuse reflectance spectra (DRS) of the TiO₂ and TiO₂–Cu_xO hybrid samples. (b) The plots of the transformed Kubelka–Munk function versus the energy of light. (c) UV-vis diffuse reflectance spectra (DRS) of the N-doped TiO₂ and N-doped TiO₂/Cu_xO samples. (d) The plots of the transformed Kubelka–Munk function versus the energy of light.

In order to further improve the photoelectron conversion efficiency (η) of solar cells, we synthesized mesoporous N-doped TiO₂–Cu_xO core–shell hybrids. Fig. 7c shows the UV-visible diffuse reflectance spectra (DRS) demonstrating the optical properties of the N-doped TiO₂–Cu_xO hybrids of (a) NS1, NS2, NS3, and NS4 samples. It is observed that the N-doped TiO₂–Cu_xO hybrids show continuous absorption in the visible range. The absorption edge for pure TiO₂ is observed at 385 nm (∼3.2 eV), while the absorption edge of the N-doped TiO₂ sample (NS1) is at around 415 nm (∼3.0 eV). Compared to un-doped TiO₂–Cu_xO core–shell hybrids (S2, S3, and S4 samples), a drastic red-shift takes place towards the visible spectral range in the N-doped TiO₂–Cu_xO core–shell hybrids (NS2, NS3 and NS4 samples). It could be ascribed to the combined effect of the doping-induced mid-gap electronic states and the lattice disorder effects due to the nitrogen doping. In the N–TiO₂ system, the visible-light response arises due to the occupied localized N 2p states above the valence band. The doping also creates localized states below the conduction band edge.^51,52

The band gap energy of the samples can be roughly confirmed according to the plots in Fig. 7d, which is obtained via the transformation based on the Kubelka–Munk function (F(R∞) = (1 − R)²/(2R), where R is the reflection coefficient).⁵³ The N-doped TiO₂ sample exhibits a smooth absorption edge at around 415 nm, corresponding to the band gap energy of 3.0 eV. The estimated band gap value of the N-doped TiO₂–Cu_xO hybrids corresponds approximately to 3.00, 2.92, 2.81 and 2.70 eV for NS1, NS2, NS3, and NS4 samples, respectively.

3.2.4 Nitrogen adsorption–desorption curves. The nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves determined at 77 K by the BJH method of the TiO₂ (S1), TiO₂–Cu_xO of S2, S3, and S4 materials prepared at different concentrations of copper acetate monohydrate: (a) 0 M, (b) 0.025 M, (c) 0.05 M and (d) 0.1 M, and the N-doped TiO₂–Cu_xO of NS1 and NS3 are all shown in Fig. 8. As shown in the isotherms, all the materials exhibit a type IV characteristic (based on the IUPAC classification) and the typical H1 hysteresis loop. The capillary condensation occurs at a relative pressure (P/P₀) of ≈ 0.60–0.95, indicating a uniform mesoporous diameter distribution. The textural parameters such as specific surface area, pore diameter and pore volume of the TiO₂ and TiO₂–Cu_xO samples are summarized in Table 1. The TiO₂ sample displays a specific surface area of about 73.08 m² g⁻¹ and a pore volume of 17.80 cm³ g⁻¹, respectively. It is shown that the coupling of mesoporous TiO₂ and Cu_xO nanoparticles results in an increase of the surface area and a decrease of the pore volume of the core–shell hybrids. This can be confirmed by the increased condensation step of the TiO₂–Cu_xO hybrid samples in Fig. 8a. With the concentration of copper acetate monohydrate, the specific surface area of the TiO₂–Cu_xO hybrids increases from 51.92 for the S2 sample, 62.64 for the S3 sample, and finally to 75.28 m² g⁻¹ for the S4 sample. While the pore size of the TiO₂–Cu_xO hybrids deceases from 25.12 nm for the S2 sample to 16.33 nm for the S4 sample. For the N-doped TiO₂–Cu_xO of NS1 and NS3, as shown in Table 1, the specific surface area is larger than that of S1, S3, and S4 samples, while the pore diameter and pore volume of NS1 and NS3 samples are larger than those of S1–S4 samples.

Fig. 8 (a) N₂ adsorption–desorption curves and (b) pore size distribution plots of TiO₂, TiO₂–Cu_xO and N-doped TiO₂–Cu_xO samples (open symbols: adsorption; closed symbols: desorption).

Table 1 Structural properties of TiO₂, TiO₂–Cu_xO and N-doped TiO₂–Cu_xO samples

Samples	BET surface area/m^2 g^−1	Pore diameter/nm	Total pore volume/cm^3 g^−1
S1	73.08	17.80	0.395
S2	51.92	25.12	0.587
S3	62.64	21.99	0.478
S4	75.28	16.13	0.469
NS1	68.56	27.29	0.612
NS3	54.67	39.13	0.591

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3.3 Performance of solar cells

3.3.1 Electrochemical impedance spectroscopy (EIS) analysis. In order to better understand the kinetics of electrochemical and photoelectrochemical processes occurring in DSSCs, the analysis of electrochemical impedance spectroscopy (EIS) of DSSCs was performed under illumination and at an open-circuit voltage. Fig. 9 demonstrates the Nyquist plots displaying two semicircles with a contact series resistance (R_s) on the FTO substrate. The smaller and larger semicircles in the Nyquist plots are attributed to the charge transfer at the electrode/electrolyte interface and the working electrode/dye/electrolyte interface, respectively. The sheet resistance (R_s) of the substrate, charge transfer resistance of the counter electrode (R₁) and recombination resistance (R₂) were analyzed using ZSimpWin software using an equivalent circuit containing a constant phase element (CPE) and resistances (R) (Fig. 9, inset). The cells based on the P25, S1, S3, NS1 (N denotes nitrogen-doped) and NS3 samples possess almost the same value of 4.0 Ω for R_s, and R₁ of 28.7, 30.1, 29.9, 24.2 and 29.1 Ω, respectively, due to the use of the same counter electrode (Pt/FTO glass) and electrolyte. The recombination resistance (R₂) of the samples is 67.7, 67.9, 56.8, 56.7 and 36.9 Ω, respectively.

Fig. 9 Nyquist impedance plots of (a) P25, (b) S1, (c) S3, (d) NS1 and (e) NS3 under one-sun irradiation. The frequency range was 10 mHz to 100 kHz; the magnitude of the alternating potential was 20 mV. The EIS spectra were fitted using ZSimpWin software using an equivalent circuit.

3.3.2 Performance of solar cells. The dye adsorbing capacity and the photovoltaic performance of DSSCs based on (b) S1, (c) S2 and (d) S3 film electrodes were comparatively examined with that of the (a) P25 photoelectrode, as listed in Table 2. The photocurrent density–voltage (J–V) characteristics of these DSSCs are shown in Fig. 10. In comparison with the P25 cell, the S1 cell exhibits a similar open-circuit voltage (V_oc) and FF, yet a larger short-circuit current density (J_sc) and higher conversion efficiency (η). This is mainly attributed to the larger surface area of S1, helpful to adsorb more dye molecules onto the surface.

Table 2 Performances of solar cells based on the P25 and synthesised samples under simulated AM 1.5 illumination

Samples	V oc (V)	J sc (mA cm^−2)	Fill factor (%)	η (%)	Dye adsorption (nmol cm^−2)
a η (%) = JscVocFF/Pin, where Pin = 100 mW cm^−2 (AM 1.5). Each η is an average value obtained from 5 samples.
P25	0.69	6.94	55.82	2.67	65.51
S1	0.67	8.01	61.49	3.30	78.65
S2	0.63	8.62	64.52	3.54	60.92
S3	0.62	9.60	64.85	3.86	68.50
S4	0.68	9.43	59.55	3.82	86.27
NS1	0.64	11.70	56.55	4.23	70.45
NS2	0.65	12.62	51.71	4.24	58.67
NS3	0.66	13.24	52.26	4.57	59.40
NS4	0.66	13.02	52.38	4.50	73.74

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Fig. 10 J–V curves of solar cells based on (a) P25, (b) S1, (c) S2, (d) S3, (e) S4, (f) NS1, (g) NS2, (h) NS3, and (i) NS4.

For the S2, S3 and S4 cells, the photoelectron conversion efficiency (η) can be maintained as 3.54%, 3.86%, 3.82%, respectively. It is suggested that for the Cu_xO sensitized TiO₂ samples, the short-circuit current density (J_sc) and conversion efficiency (η) of the S2–S4 cells are higher than the S1 cell, which is attributed to the large amount of dye adsorption, sufficient light harvesting in the visible region, and fast charge transport. Firstly, the capacity of the adsorbed dye exerts a profound influence on the photocurrent density. In this regard, the amount of adsorbed N719 dyes can be estimated by measuring the eluted dye molecules using UV-vis absorption spectroscopy.⁴⁰ The TiO₂–Cu_xO core–shell hybrids display typical mesoporous characteristics with a large specific surface area and a narrow pore diameter (Table 1), which can adsorb a larger amount of dye molecules.

Secondly, after coupling Cu_xO nanoparticles with the mesoporous anatase TiO₂, the TiO₂–Cu_xO core–shell nanohybrids display a distinct red shift to the visible light region with a longer wavelength for the absorption edge (Fig. 7a). The S3 sample shows the mostly enhanced ability to absorb visible-light, and a stronger scattering is revealed from the diffuse reflectance measurement for the S3 sample in comparison with other samples (Fig. 7a), suggesting an improved light harvesting efficiency, and higher short circuit current J_sc.^54–57

Finally, the electrochemical impedance spectroscopy (EIS) analysis of DSSCs fabricated based on P25 and TiO₂–Cu_xO nanohybrids of S1, S2, S3, and S4 samples was performed to elucidate the characteristics of the charge transfer ability. As shown in Fig. 9, it is clearly shown that the sheet resistance (R_s) of the substrate for the three samples is almost the same (4 Ω). The R₁ values of the samples are 28.7, 30.1, 29.9 Ω, however, the recombination resistance (R₂) of the samples are 67.7, 67.9, 56.8 Ω, respectively. The EIS analysis suggested that as compared to pure TiO₂ (P25), mesoporous TiO₂ (S1 sample), the formation of mesoporous TiO₂–Cu_xO core–shell hybrids facilitates the charge and electron transfer,⁵⁸ which indicates that electrons are easier to move at the surface and contribute to the charge transport at the photoanode.

The incident photon to current efficiency (IPCE) measurement was carried out using a QEX 10 system (PV measurement). The corresponding incident monochromatic photon-to-electron conversion efficiency (IPCE) spectra of DSSCs based on P25, S1, S3, NS1, and NS3 samples are displayed in Fig. 11. The DSSC samples show a similar feature along the entire wavelength in the range of 400–750 nm. The IPCE results were consistent with the photovoltaic performance of the DSSC samples. Especially, the DSSC samples based on NS1 and NS3 exhibit a higher IPCE along the scanned wavelength in comparison with DSSCs based on S1, S2, and P25. The variation trend of IPCE spectra of DSSCs based on an N-doped photoanode clearly reveals the effect of the N doping on the photovoltaic performance of DSSCs based on the N-doped sample. It is shown that the photoelectric response of the N-doped photoanodes is enhanced and the electron density increases. Also, it is demonstrated that the IPCE of DSSCs based on the NS3 sample displays the highest IPCE over the whole wavelength among all the samples. The improvement of IPCE can be attributed to the decreased recombination rate of photogenerated electrons and holes, due to N-doping and heterostructure formation between TiO₂ and copper oxide.

Fig. 11 IPCE spectra of solar cells based on samples of (a) P25, (b) S1, (c) S3, (d) NS1 and (e) NS3.

A comparison of the J–V characteristics of DSSCs based on (e) NS1, (f) NS2, (g) NS3 and (i) NS4 film electrodes is shown in Fig. 10. The open-circuit photovoltage (V_oc), corresponding short-circuit photocurrent density (J_sc), fill factor of the cell (FF), power conversion efficiency (η), and dye adsorption are listed in Table 2. Compared to the conversion efficiency of undoped TiO₂–Cu_xO core–shell hybrids, the N-doped TiO₂–Cu_xO hybrids display greatly improved solar cell performance, with a conversion efficiency of 4.23, 4.24, 4.57, and 4.5 for NS1, NS2, NS3, and NS4 samples, respectively. In comparison to the mesoporous TiO₂–Cu_xO hybrids, the improved performance of the solar cells based on the N-doped TiO₂–Cu_xO core–shell hybrids can be attributed to the higher light scattering ability, which enhances the utilization of solar light;^59,60 the band gap of the nitrogen doped TiO₂–Cu_xO which is more narrow than the pure TiO₂–Cu_xO; the faster electron transport of the interfaces, which is confirmed by the electrochemical impedance spectroscopy (EIS) (Fig. 9); and the recombination resistance (R₂) of S1, NS1, S3 and NS3 which is 67.9, 56.7, 56.8 and 36.9 Ω, respectively. It is well shown that NS3 has the smallest recombination resistance (R₂); namely, NS3 has the highest open-circuit. The performance of the solar cell based on the N-doped TiO₂–Cu_xO hybrid materials can be comparable to the related materials prepared via the other methods. For example, Sun³⁸ reported that the optimal short-circuit photocurrent and EQE values of the Cu_xO modified TiO₂ nanorod arrays can increase by more than five and nine times compared to the pristine TiO₂, respectively. The performance of DSSCs based on the 0.3 wt% Cu₂O–N-doped TiO₂ hybrid is better than the undoped TiO₂-based DSSCs by Koo.⁶¹

The highest power conversion efficiency of DSSCs based on the N-doped TiO₂–Cu_xO material (NS3) photoanode is 4.57%, which is higher than 3.86% for S3, 3.30% for S1, and 2.67% for P25, respectively. A great enhancement in power conversion efficiency of the NS3 photoanode can be obtained compared to that of S3. Hence, the higher short-circuit current density and conversion efficiency of the solar cells based on N-doped TiO₂–Cu_xO core–shell hybrids could be attributed to the larger surface area for adsorbing more dyes, and higher light scattering ability for enhancing the utilization of solar light, which were confirmed by the aforementioned dye adsorption, IPCE and UV-vis diffuse reflectance measurements. As excited by the incident photons, the photoelectrons in Cu_xO migrate to the conduction band of TiO₂, and the holes gather in the valence band of Cu_xO. During this process, the lifetime of the charge carriers can increase, thus, the recombination of electron–hole pairs is further inhibited, finally resulting in an improved photoelectrical performance.³⁸

4. Conclusions

We for the first time fabricated N-doped mesoporous TiO₂–Cu_xO core–shell hybrids. The matching of band edges between Cu_xO and TiO₂ to form a semiconductor heterojunction plays an important role in the effective separation of light induced electrons and holes, providing a promising photoanode for its wide absorption spectrum, high electron injection efficiency, and fast electron transference. DSSCs based on the mesoporous TiO₂–Cu_xO core–shell hybrids show a high short-circuit current density of 9.60 mA cm⁻² and a conversion efficiency of 3.86% under one sun illumination. Furthermore, DSSCs based on the N-doped mesoporous TiO₂–Cu_xO hybrids exhibit the higher short-circuit current density of 13.24 mA cm⁻² and a conversion efficiency of 4.57% under one sun illumination. The performance improvement of the solar cell based Cu_xO nanoparticles/mesoporous anatase TiO₂ beads nanostructures can be attributed to the larger surface area adsorbing a large amount of dye molecules; the improved light harvesting efficiency by a distinct red shift moving to the visible light region and the electron transport facilitated by the heterojunction of TiO₂–Cu_xO; and the decrease of the recombination rate of photogenerated electron–holes due to the N-doping into the lattice of TiO₂ and the semiconductor heterostructure between TiO₂ and Cu_xO.

Acknowledgements

We acknowledge support from the National Natural Science Funds for Distinguished Young Scholars (No. 51025211), National Natural Science Foundation of China (No. 51272137), and the Tai Shan Scholar Foundation of Shandong Province.

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