Dual effect of TiO2 and Co3O4 co-semiconductors and nanosensitizer on dye-sensitized solar cell performance
© Taher et al. 2015
Received: 30 May 2015
Accepted: 16 October 2015
Published: 4 November 2015
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.
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.
Preparation of nanodye
Preparation of nanocobalt oxide
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
Characteristics of Co3O4
Photocurrent–voltage behavior of the DSSCs
The cell performance parameters of the prepared DSSCs
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.
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.
The authors declare that they have no competing interests.
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