Open Access

Dual effect of TiO2 and Co3O4 co-semiconductors and nanosensitizer on dye-sensitized solar cell performance

  • F. A. Taher1Email author,
  • Galila M. El-sayed1,
  • N. M. Khattab2 and
  • N. Almohamady3
Renewables: Wind, Water, and Solar20152:15

DOI: 10.1186/s40807-015-0015-z

Received: 30 May 2015

Accepted: 16 October 2015

Published: 4 November 2015

Abstract

Dye-sensitized solar cell (DSSC) was fabricated using nanosize of the dye sensitizer (Alizarin Yellow, AY) that was prepared by ball milling. The particle size and the composition of nano-Alizarin Yellow (nAY) was investigated using TEM and 1H- and 13C-NMR spectra, respectively. The effect of sensitizer size reduction on DSSC efficiency was studied. Co3O4 as a semiconductor in DSSC was prepared and confirmed by XRD. Also, composite of TiO2 and Co3O4 was used to improve the DSSC efficiency. In addition, the effect of terpineol as a solvent was tested. Photocurrent–photovoltage curves of all prepared DSSCs were investigated. Finally, to test the validity of the results, standard error was calculated.

Graphical Abstract

Keywords

DSSC Co3O4@TiO2 nanocomposites Nanosensitizer Alizarin Yellow

Background

DSSC is an alternative solution for the future energy crisis as a productive source for renewable energy (Kato et al. 2011; Zhuiykov 2014; Ludin et al. 2014). Excitation of dye sensitizer that was doped onto semiconductor or co-semiconductor by sun radiation to generate an electron and leave behind a hole is the initial photon-induced electron reaction in DSSC (Yum et al. 2014). After transition of the excited electron from semiconductor conduction band to a counter electrode through working electrode, the ground state of the dye is reached by electrolyte oxidation (Choi et al. 2013; Han and Ho 2014). The main issue is in returning some electrons back to the dye ground state or electrolyte causing an increase in the electron–hole recombination rate and then deficiency in DSSC efficiency (Lai et al. 2008; Akpan and Hameed 2009; Yamaguchi et al. 2010; Reda 2010; Kato et al. 2011; Tian et al. 2010; Kantonis et al. 2011; Sharma et al. 2010; Basheer et al. 2014a, b). Since, the efficiency of the DSSC relies on the sensitizer and semiconductor, the idea here is to increase the absorption band of the sensitizer by increasing its surface area or decrease the electron–hole recombination rate using darker co-semiconductor to achieve higher solar conversion efficiency.

Actually, Im and his co-worker have used the cocktail effect of TiO2 and Fe2O3 to increase the performance of DSSC. The efficiency of the DSSC has been developed by over 300 % (Im et al. 2011). Also, NiO/TiO2 nanocomposites were prepared and used as modified photoelectrodes in quasi-DSSC with 2.29 % conversion efficiency as by Mekprasart et al. (2011). To the best of our knowledge, so far, the effect of Co3O4 as a co-semiconductor was not previously reported therein. In this work, the dye sensitizer was converted to nanosize to investigate its size reduction on the DSSC efficiency. Also, a composite of TiO2 and Co3O4 was prepared to use as a semiconductor in DSSC. In addition, the effect of terpineol as a solvent was tested via I–V characteristic curves.

Methods

Preparation of nanodye

The chemical structure of Alizarin Yellow (AY, DyeStar) is shown in Fig. 1. Nano-Alizarin Yellow, nAY, was prepared by ball milling machine for 8 h (RETSCH PM 400, Germany), then heated at 40 °C for 24 h. The chemical composition of nAY was confirmed by 1H- and 13C-NMR spectroscopy (Bruker High-Performance Digital FT-NMR Spectrometer Avance III 400 MHz). TEM was measured to determine the particle size of nAY and its distribution (JEOL, TEM-1230).
Fig. 1

Chemical structure of Alizarin Yellow (AY)

Preparation of nanocobalt oxide

Cobalt oxide nanopowder was obtained by co-precipitation method. Drops of 0.3 M sodium hydroxide aqueous solution (98 %, Adwic) was stirred with aqueous solution of 0.01 M cobaltous chloride hexahydrate (98 %, Indiamart) for 2 h at room temperature. The obtained green precipitate, Co(OH)2, was washed several times with distilled water. Nanocobalt oxide (Co3O4) was obtained after drying at 80 °C and sintering at 900 °C. All theses steps can be represented by the following equations:
$$2{\text{NaOH}}_{{ ( {\text{aq)}}}} + \,{\text{CoCl}}_{ 2} \cdot 6{\text{H}}_{ 2} {\text{O}}_{{ ( {\text{aq)}}}} \to 2{\text{NaCl}}_{{ ( {\text{aq)}}}} + \,{\text{Co(OH)}}_{ 2} \cdot 6{\text{H}}_{ 2} {\text{O}}_{{ ( {\text{aq)}}}}$$
$${\text{Co(OH)}}_{2} \cdot6{\text{H}}_{2} {\text{O}}\xrightarrow{{ \triangleq 80^{\circ } {\text{C}}}}{\text{Co(OH)}}_{2} + 6 {\text{H}}_{2} {\text{O}}$$
$${\text{Co(OH)}}_{2} {\mkern 1mu} \xrightarrow{{ \triangleq 900^{\circ } {\text{C}}}}{\text{Co}}_{3} {\text{O}}_{4} + {\text{H}}_{2} {\text{O}}$$
Accordingly, the crystalline structure of Co3O4 was confirmed by powder X-ray diffraction (XRD: Empyrean, Holland). To obtain the particle size of Co3O4 and its distribution, TEM measurement was conducted (JEOL, TEM-1230).

Preparation of Co3O4@TiO2 composite

1.6 g Co3O4 and 5 g TiO2 (anatase 99.7 %, P25, Sigma-Aldrich) were mixed with 25 ml distilled water, and stirred for 48 h at room temperature. The resultant complex was sintered at 600 ℃ for 1 h.

Preparation of TiO2 and Co3O4@TiO2 pastes

To prepare the pastes, 2 g TiO2 and 2 g Co3O4@TiO2 composite were separately added into a solution of 0.5 g polyethylene glycol (20,000 g/mol, Sisco) dissolved in 7 ml of distilled water (as a binder to prevent the film from cracking during drying), 5 ml ethanol, and 15 ml terpineol (Sigma-Aldrich). The resultant two mixtures were thermally heated at 100 °C for 6 h.

Preparation of the working electrode

Fluorine-doped tin oxide glass (FTO, Pilkington Kappa Energy, 18 Ω/cm2) was cleaned with 95 % ethanol, 1-propanol and distilled water, then left to dry in open air. Before applying TiO2 and Co3O4@TiO2 pastes, FTO glass was heated in 0.2 M TiCl4 solution (99 %, Merck) at 70 °C for 30 min to make a nanocrystalline TiO2 film which prevents the electrolyte from approaching the conductive layer preventing the cell from the dark current. The previous pastes were coated onto FTO by the doctor blade technique using Scotch adhesive tape (thickness: 50 μm). The film was air dried for 10 min at room temperature and then annealed and sintered at 450 ℃ for 30 min. The loaded pastes on FTO were separately immersed in an aqueous solution of 1 × 10−4 M AY and 1 × 10−4 M nAY. The resultant working electrode was dried at room temperature overnight.

Preparation of the counter electrode

FTO glass was coated with Pt paste (Platisol, Solaronix) then dried at 70 °C for 3 h and sintered for 30 min at 450 °C under airflow of 30 ml/min. The counter electrode was then left to cool down to room temperature before usage.

Assembly of the DSSC

Between the counter and the working electrodes, the iodide/iodine electrolyte solution (0.5 M potassium iodide mixed with 0.05 M iodine in water-free ethylene glycol) was located and then binder clipped to immobilize each part. The area of the DSSC was fixed to be 2.25 cm2.

Measurement of the photophysical and electrochemical properties

UV–Vis spectrophotometer was used to record the absorption spectra of AY, nAY, TiO2 and Co3O4@TiO2 solutions; emission spectra of AY and nAY solutions; and photoluminescence spectra of AY, nAY, AY–TiO2 and nAY–TiO2 solutions (Perkin Elmer, lambada 35, USA). I–V characteristics were measured using a photocurrent–voltage (I–V) curve analyzer (Peccell Technologies, Inc., PECK2400-N, version 2.1) under AM 1.5 (950 mW/cm2) irradiation with a solar simulator (Peccell Technologies, PEC-L11).

Results and discussion

Effect of the size reduction on the characteristics of nAY

The effect of size reduction on the particle size, chemical composition and spectral analyses was investigated by TEM image, NMR and UV–Vis spectra, respectively. Figure 2 shows TEM photograph of the as prepared nAY. A homogenous rod-like structure was observed with diameters and lengths less than 20 and 100 nm, respectively.
Fig. 2

TEM image of a AY and b nAY

To confirm whether the ball milling process results in the partial decomposition of some AY molecules or not, the 1H- and 13C-NMR spectra of nAY were measured (Fig. 3). As can be seen in Fig. 3a, the aromatic ring protons multiplet of nAY molecule were observed from 7.11 to 8.70 ppm. The resonance signal due to OH proton singlet was observed at 9.80 ppm. While for the corresponding carbon resonance, Fig. 3b, the carbonyl carbon signal appears in a characteristic field of 146.02 ppm which confirms that there is no partial chemical decomposition of nAY molecule and the ball milling process does not affect its composition.
Fig. 3

Experimental a 1H and b 13C NMR spectra of nAY in CDCl3

UV–Vis absorption spectra were obtained from nAY and AY solution (Fig. 4). When comparing the maximum absorption wavelength, λ max, a bit of bathochromic shift of AY λ max from 400 to 445 nm for nAY was observed which corresponds to the transition from HOMO to LUMO. This red shift of λ max of nAY can be attributed to its smaller particle size (17–35 nm) that reflects a strong electron donation ability of nAY (increasing the delocalization of the π* orbital of AY), i.e., the absorption energy was shifted to lower frequency with decrease of the particles’ diameter. This was readily observed from the reflected color change of AY from brilliant yellow to mustard yellow of nAY passing through canary yellow, where each color corresponds to the different particle size of nAY. After sensitization of AY and nAY on TiO2, λ max was shifted to red region by 100 and 55 nm, respectively, due to J-aggregation on TiO2 surface (curves not inserted). In addition, the emission spectra of AY and nAY at 300 nm showed a broad spectral peak at the same position corresponding to relaxation to lower energy level (Fig. 5). Also, the lower emission intensity of nAY indicates the delay in recombination rate of e− and h+ that emphasizes the advantage of using nAY in DSSC fabrication. This can also be noticed in the hypochromic shift of the photoluminescence spectra of AY to lower intensity for nAY (Fig. 6). This hypochromic effect indicates the decrease of photons number coming from electrons and holes recombination (Balraju et al. 2010). In addition, the adsorption of AY and nAY on TiO2 showed the decrease in these photons numbers.
Fig. 4

UV–Vis absorption spectra of AY and nAY

Fig. 5

Emission spectra of AY and nAY

Fig. 6

Photoluminescence spectra for AY, nAY, AY–TiO2 and nAY–TiO2

Characteristics of Co3O4

The X-ray diffraction characteristic peaks in Fig. 7a were analyzed to determine the structure and crystallite size of the as-prepared Co3O4. Nanopowder XRD peaks of Co3O4 are well consistent with the data of the JCPDS file (card no. 78-1970) of phase-pure Co3O4 with cubic spinel structure (Co2+ ions occupy the tetrahedral sites and Co3+ ions the octahedral sites) showing the main Bragg’s reflection peak in the (311) plane. The peaks at 2θ value of 18.9°, 31.2°, 36.9°, 38.6°, 44.7°, 55.6°, 59.4° and 65.1° correspond to the crystal planes of (111), (220), (311), (222), (400), (422), (511) and (440) of well-crystallized Co3O4, respectively (Xiao et al. 2014; Kong et al. 2014). It is clear that Co3O4 is the only phase after the decomposition of the green precipitate, Co(OH)2, at 900 °C without any other diffraction peak. The crystal size of Co3O4 is deduced through (311) plane using Scherrer equation \(D\, = \,{{ 0. 94\,\lambda } \mathord{\left/ {\vphantom {{ 0. 9 4\lambda } {\beta \;{ \cos }\theta }}} \right. \kern-0pt} {\beta \;{ \cos }\theta }}\) where D is the crystal size of Co3O4, λ is the wavelength of incident X-ray (0.154 nm), θ is the half diffraction angle of peak in degree, and β is the full width half maximum of a reflection located at 2θ. The average crystal size of Co3O4 is 154 nm which in a good agreement with TEM result of the cubic Co3O4 nanoparticles (Fig. 7b). Furthermore, the UV–Vis absorption spectra of TiO2 and Co3O4@TiO2 composite, Fig. 8, showed a band gap of 3.2 eV of the absorption band of TiO2. Co3O4@TiO2 composite exhibits a continuous absorption band of the dark composite toward higher wavelength in the range 300–750 nm (Kim et al. 2014). This red shift of the edge of the absorption peak implies band gap narrowing. The smaller band gap reflects the advantage of using Co3O4 in decreasing the recombination rate of e and h+. The electrons that would transfer to the electrolyte or dye (AY or nAY) can be confined in the conduction band of Co3O4@TiO2 composite (i.e., Co3O4@TiO2 composite captures electrons from TiO2 conduction band).
Fig. 7

a X-ray diffraction pattern of nano-Co3O4 b TEM of cubic Co3O4

Fig. 8

UV–Vis absorption spectra of TiO2 and Co3O4@TiO2 composite

Photocurrent–voltage behavior of the DSSCs

Three effects of photosensitizer size reduction, nAY, oxide co-semiconductor, Co3O4, and presence of terpineol as a solvent on the efficiency of the prepared DSSCs were investigated under 950 mW/cm2 by analyzing their photocurrent density–voltage curves in Fig. 9 with error bars of photocurrent density. As for a comparison, the photovoltaic properties of the solar cells fabricated with commercial dye, AY, and its nanosize, nAY, in absence and presence of Co3O4 on the working electrode were measured. The values of open-circuit photovoltage, V oc, short-circuit photocurrent, I sc, fill factor FF and overall energy conversion efficiency are presented in Table 1.
Fig. 9

I–V curves of DSSCs (A–E)

Table 1

The cell performance parameters of the prepared DSSCs

DSSCs

Dye

Semiconductor

Solvent

Voc (V)

Isc (mA)

SE (±)

FF

η (%)

A

AY

TiO2

0.0041

2.0464

0.1482

0.3767

0.0033

B

AY

Co3O4@TiO2

0.1489

9.9752

0.6135

0.3546

0.5548

C

nAY

TiO2

0.2120

3.4553

0.1986

0.3040

0.2344

D

nAY

Co3O4@TiO2

0.1496

28.2245

2.1594

0.4670

2.0765

E

nAY

Co3O4@TiO2

Terpineol

0.5036

12.8855

0.4437

0.3458

2.3625

The reduction of the original macro-size of AY to less than 100 nm of nAY, Fig. 2, has a great effect on the DSSC efficiency that increased by 70 % (C). This could be related to the nanosize of AY. As the size of a dye crystal decreases to nanometer regime, the size of AY particles begin to modify the properties of the crystal, so the electronic structure is altered from the continuous electronic bands to discrete or quantized electronic levels. Therefore, the nanomaterial becomes size dependent, and the electronic excitations shift to higher energy, and the oscillator strength is concentrated into just a few transitions. Actually, the presence of Co3O4 as a co-semiconductor in DSSCs electrode (B and D) increased their efficiency by 165 and 620 times in comparison with DSSC (A), respectively. This can be explained due to (1) TiO2–Co3O4 composite that is darker in color and has a high absorption in the visible region of the solar spectrum. (2) The hexagonal crystal structure of Co3O4 which is a p-type semiconductor due to O2-deficiency in its lattice and consequently the electron charge is fast injected into the conduction band (CB) of Co3O4. (3) Co3O4 shows different types of band gaps: direct allowed 2.06 and 1.44 eV, direct forbidden 1.38 and 1.26 eV, indirect allowed 1.10 eV (energy of phonon assisting indirect transition = 0.02) and indirect forbidden 0.75 eV (energy of phonon assisting indirect transition = 0.27) (Kabre 2011). The efficiency of DSSC (D) increased by 13-fold in the presence of terpineol as a solvent (E). It can be seen that the film prepared using terpineol as the solvent has exhibited the highest energy conversion efficiency. When water was used as the solvent, only polyethylene glycol exhibited a stable film formation on FTO conductive glass. This exhibited a cracked surface that was recognized by naked eye observation. On the other hand, when terpineol—an organic solvent with the hydroxyl functionalities that can accept or donate hydrogen bonds—was used as solvent, most of the semiconductor dispersions enabled us to form uniform thin films. Accordingly, the increased gap between ground state of dye and redox potential electrolyte would lead to higher performances of DSSCs in the following direction: (A) < (C) < (B) < (D) < (E). Figure 10 shows the standard error (SE) of the photocurrent density mean of DSSCs (A–E) to test the validity. A valid mean is reliable mean if it is at least 2.5 times standard deviation (Bevington and Robinson 2002). However, by calculating the DSSCs efficiency and considering these standard errors values, it was found the efficiency values did not change for four digits.
Fig. 10

Standard error analysis of photocurrent density of DSSCs (A–E)

The predicted mechanism for the conversion of photons to current for the DSSCs could be interpreted to pass through the following stages, Fig. 11. The electrons are excited by solar energy from HOMO to LUMO level of dye (AY/AY+) that is adsorbed on TiO2–Co3O4 composite surface, owing to the intermolecular ππ* transition (Stage 1). These excited electrons diffuse immediately into the CB of TiO2 (Stage 2); then move to the CB of Co3O4 (Stage 3) which decrease the electrons flow back to the HOMO (Stage 4) or \(I_{3}^{ - }\) electrolyte in recombination (Stage 5) (i.e., reduce the electron trapping effect—by increasing the contact surface area of the TiO2–Co3O4 composite with AY or electrolyte) (Anta 2012). These electrons go forward to the FTO of the working electrode (Stage 6). Consequently, these electrons reach the counter electrode through the external wiring (Stage 7). The oxidized dye (AY+) accepts electron from I redox mediator, regenerating the HOMO of the dye (AY) and I is oxidized to \(I_{3}^{ - }\). The oxidized redox mediator, \(I_{3}^{ - }\), is reproduced to I at the counter electrode.
Fig. 11

DSSCs’ predicted conversion mechanism

Conclusion

Five DSSCs were prepared to investigate the effects of their construction on their solar conversion efficiency. The nanosize of AY (less than 100 nm) has a great effect on the DSSC efficiency that increased by 70 %. Actually, the presence of Co3O4 as a co-semiconductor in DSSCs electrode increased their efficiency by 165 and 620 times for the cells modified by TiO2 + Co3O4 only and TiO2 + Co3O4 with nAY, respectively. The presence of solvent (terpineol) increased the efficiency of DSSC by 13-fold. Finally, the predicted mechanism for the conversion of photons to current for the DSSCs was discussed.

Declarations

Authors’ contributions

FAT carried out the electrochemical studies of the DSSCs, participated in the sequence alignment, and drafted the manuscript and also the revision process. GME conceived the study, and participated in its design and helped to draft the manuscript. NK measured all the photophysical properties of DSSC, also participated in the study design and coordination. NA prepared all the as-obtained compounds and assembled the DSSCs. All authors read and approved the final manuscript.

Competing interests

The authors declare that they have no competing interests.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

(1)
Department of Physical Chemistry, Faculty of Science (Girls), Al-Azhar University
(2)
Solar Energy Department, National Research Center
(3)
Faculty of Science (Girls)

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Copyright

© Taher et al. 2015