Journal of Nanomaterials & Molecular NanotechnologyISSN: 2324-8777

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Research Article, J Nanomater Mol Nanotechnol S Vol: 0 Issue: 1

Enhancement of Photocurrent in Dye-Sensitized Solar Cells Using Bismuth Doped TiO2- Graphene as a Hot Carrier Transport

Golam Moula, Md Abdul Mumin and Paul A Charpentier*
Department of Chemical and Biochemical Engineering, Western University,London, Ontario, N6A 5B9, Canada
Corresponding author : Dr. Paul A Charpentier
Department of Chemical and Biochemical Engineering, Western University, London, Ontario, N6A 5B9, Canada,
Tel: (519) 661-3466; Fax: (519) 661-3498
Received: August 08, 2013 Accepted: November 07, 2013 Published: November 11, 2013
Citation: Moula G, Mumin MA, Charpentier PA (2013) Enhancement of Photocurrent in Dye-Sensitized Solar Cells Using Bismuth Doped TiO2-Graphene as a Hot Carrier Transport. J Nanomater Mol Nanotechnol S1:002. doi:10.4172/2324-8777.S1-002


Enhancement of Photocurrent in Dye-Sensitized Solar Cells Using Bismuth Doped TiO2- Graphene as a Hot Carrier Transport

Dye-sensitized solar cells (DSSCs) are of tremendous current interest, but they suffer from a lack of ability to harness the majority of visible light, or require heavy metal addition such as Pb or Cd which are not considered environmentally friendly. By decorating doped metal oxides such as titania with earth abundant and friendly metals on functionalized graphene sheets (FGSs), functional mats are enabled for enhanced light harvesting. In this work, novel highly crystalline bismuth doped TiO2 nanocrystals were prepared successfully by a facile sol-gel hydrothermal process and attached to FGS’s to make functional catalyst mats.


TiO2 Nanocrystal; Bi-Doped TiO2; Graphene; DSSC; Optoelectric properties; Electric energy; Conversion efficiency; Electron hot carrier transport


Dye-sensitized solar cells (DSSCs) have been investigated as low cost next-generation photovoltaic (PV) devices because of their simple architecture and low manufacturing costs compared to conventional inorganic devices [1-4]. Although photovoltaics is a promising technology for converting sunlight into electricity, tremendous challenges remain for both lowering the device cost and simultaneously reducing the environmental impact of manufacturing. In DSSCs, when a photon from sunlight strikes the dye molecule, electrons in the dye become excited into higher energy states. The resulting photoexcited electrons and holes transfer to the semiconductor (e.g. TiO2) manifold of unoccupied states and to an electrolyte or hole conductor, respectively. The collected electrons in the photoelectrode are transported through an external circuit to the counter electrode with the circuit completed through regenerating the electrolyte at the counter electrode. The main challenge to gain higher efficiencies in DSSC’s is to boost the electron excitation and rapid electron injection into the conduction band of the semiconductor, followed by electron transfer to the photoanode, where the original state of the dye is restored by electron donation from the electrolyte [5-7].
To achieve this goal, two approaches are examined in this work. First, graphene was examined as the hot carrier to transport the excited electrons for enhancing DSSC performance while providing better light absorption and enhanced charge transfer. Strong electron-electron interactions in graphene are expected to result in multiple-excitation generation by the absorption of a single photon; as it’s zero band gap [8] provides excellent electrical conduction in two dimensions [9-13]. Graphene has emerged as a conductive nanomaterial for promising high efficiency in DSSCs [6,14-16], which has been progressively developing in the application of photoanodes [6,15,16]. The application of graphene sheets in DSSCs is very promising due to electron transfer mobility as well as their electrochemical activities. Graphene sheets can mix with TiO2 semiconductor to form a composite film to enhance the electron transfer from TiO2 to a photoelectrode. Recently TiO2-graphene was found truly different and more advantageous as a photocatalyst than other TiO2-carbon composite materials [17-20]. The performance enhancement of DSSCs as a result of the graphene incorporation was demonstrated by several groups [15,21-23]. The major bottleneck towards increasing efficiencies is enhancing transport of photogenerated electrons across the TiO2 nanoparticle network, which competes with the charge (electrons and holes) recombination [22]. To suppress the recombination and improve the electron transport and electron lifetime, incorporation of TiO2 into highly porous and highly conductive graphene is an excellent choice. In this work, functionalized graphene was prepared by oxidation and thermal expansion of graphite [24].
Then, bismuth doped titania was examined as the nanocrystalline semiconductor used for the photoanode in the DSSC. TiO2 only absorbs light energy in the UV region, therefore having limited value as a PV cell. A significant research effort has been directed towards efficiency improvement of solar energy conversion by shifting the energy response to the visible region [25,26]. Doping is one of the more established techniques to shift the photosensitivity into the visible region. As a dopant, Bismuth is earth abundant and earth friendly while having a lower band gap energy (1.77 eV) and its electron donor doping into TiO2 can easily contribute to increasing the photoactivity in the visible region. Previously, the hydrothermal approach has been used extensively for synthesis of TiO2 and doped Bi-TiO2, which takes advantages of a direct preparation at low crystallization temperatures [27-29]. Recently, there has been significant interest of Bi-TiO2 as a photocatalyst using their UV to visible light manipulation capability [30-33] but no significant paper reported on using these nanomaterials in DSSC’s. Hence, this work examines the effect of Bi doping into TiO2 nanocrystals to form functional catalyst mats suitable for DSSC production.


Titanium (IV) isopropoxide (99.99% trace metals basis), triethanolamine (≥ 99%), ethylenediamine (≥ 99.5%), bismuth (ΙΙΙ) nitrate pentahydrate (99.99% trace metals basis), graphite flakes, nitric acid (≥ 90%), sulfuric acid (95-98%), potassium chlorate (≥ 99%), hydrochloric acid (37%), titanium (IV) oxide anatage (P25) nanopowder (99.7% trace metal basis), iodine (99.8%), methanol chromasolv (≥ 99.5%) were purchased from Sigma-Aldrich Canada. Anhydrous ethyl alcohol was purchased from Commercial Alcohols, Inc., Canada. All products were used as received. ITO coated glass slides with a resistivity of 8-12 Ω/sq were purchased from Sigma- Aldrich and thoroughly washed with a soap solution, then with acetone under sonication.
Synthesis of graphene
Graphene oxide (GO) was prepared by the reaction of graphite with strong oxidizers such as sulfuric acid, nitric acid and potassium chlorate following the Staudenmaier method [34,35]. To prepare GO, 5 g of graphite was added slowly into a mixed solution of 45 mL concentrated nitric acid and 87.5 mL concentrated sulfuric acid with continuous stirring in a 500 mL reaction flask placed in an ice bath. Then, the potassium chlorate was added very slowly over 45 min to avoid sudden increases in temperature [Caution! Addition of the potassium chlorate results in the formation of Cl2 gas, which is explosive at high concentrations] [36]. An inert gas, argon, was purged continuously at the head space of the reaction vessel while the potassium chlorate was added slowly helping to remove any formed chlorine gas and minimize the risk of explosion. After completion of the reaction, the mixture was cooled to room temperature, washed with excess water followed by washing with 5% solution of HCl and then washed several times with water until the pH reached neutral. The slurry was then centrifuged to collect the sample, which was dried overnight in a vacuum oven at 100°C. The dried GO sample was stored in a vacuum oven at 60°C until use. The thermal exfoliation of GO was accomplished by placing 200 mg of GO into a 25 mm diameter and 1.3 m long quartz tube that was sealed at one end and the other end closed using a rubber stopper which was used to insert the argon tube. The GO sample was flushed with argon for 10 min, and the quartz tube was quickly inserted into a Lindberg tube furnace preheated to 1050°C and held in the furnace for 30 s.
Synthesis of TiO2 nanocrystals
A typical sol-gel hydrothermal approach was used to synthesize the TiO2 nanocrystals. Under continuous magnetic stirring, 8 mL of titanium (IV) isopropoxide was added into 7 mL of triethanolamine. Then, deionized water of 18.2 MΩ cm resistivity was added into the mixed solution to make 50 mL of clear Ti4+ aqueous solution. A 50 mL of 0.1 M ethylenediamine aqueous solution was added into the Ti4+ aqueous solution. The final mixed solution was then poured into a 125 mL Teflon-lined autoclave placed inside a furnace for 24 hours at 100°C to accomplish the gelation process. The growth of TiO2 crystallization was then completed by heating the autoclave at 165°C for 72 hours. The bismuth doped Bi-TiO2 nanocrystals were synthesized followed the same synthesis protocol except for adding 2 or 5 mole % of bismuth (ΙΙΙ) nitrate pentahydrate into the initial mixing step of titanium (IV) isopropoxide with triethanolamine preparation before carrying out the synthesis process. The TiO2 and Bi-TiO2 samples were recovered by washing three times with deionized water and centrifuged after each washing. The product was dried in a vacuum oven at 60°C overnight. The final TiO2 and doped Bi-TiO2 nanocrystals were then calcined in air for 2 hours at 400°C with a heating/cooling rate of 10°C/min.
Sensitisation GO by doped Bi-TiO2 for photovoltaic analysis
0.5 g of the calcined doped Bi-TiO2 nanocrystals and about 0.05 g of GO (ratio 10:1) were added to a three-neck flask containing 40 mL of anhydrous ethanol and sonicated for 2 hours at 60°C. This was done for the purpose of sensitisation or decoration of the GO by anchoring the Bi-TiO2 nanocrystals to make graphene bridges for electron transport into the nanocrystals.
The morphology and size of the synthesized TiO2/Bi-TiO2 nanocrystals and GO sheets were examined by scanning electron microscopy (SEM) using a JOEL 2010F, while single sheet GO and Bi-TiO2 decorated GO sheet images were obtained by transmission electron microscopy (TEM) with a FEI Titan 80-300 Cubed TEM at the Canadian Centre for Electron Microscopy (McMaster University, Ontario). Samples for SEM imaging were prepared by applying the nanopowder directly to a silicon wafer on carbon adhesive tape. The TEM samples were prepared by drop casting ethanol dispersed GO and Bi-TiO2 decorated GO onto carbon coated copper grids, then drying overnight prior to analysis. Investigating the crystalline structure of TiO2 and Bi-TiO2 nanocrystals was also performed by X-ray diffraction (XRD) on a Rigaku-Miniflex bench-top X-ray power diffractometer (Carlsbad, CA) using Cu Kα (αλ for Kα=1.54059Å) radiation at 30 kV and 15 mA. The XRD pattern was collected in step scan mode with a small grazing angle of incident X-rays with a 2θ scan range of 20-80° and a step size of 0.1°. Dry powder samples were used for XRD results. The composition analysis of the TiO2 and Bi- TiO2 nanocrystals was obtained by X-ray energy dispersive analysis (EDAX) with a LEO (Zeiss) 1540XB FIB/SEM. The FIB is used for micromachining by sputter milling to monitor the real time at high resolution with the electron column and in-situ sectioning of samples which were then imaged by SEM. In addition, the system is fitted with an Oxford Instrument X-ray system allowing for elemental mapping and analysis of the milled sections. The BET (Brunauer-Emmett- Teller) surface area was determined from nitrogen adsorption and desorption isotherm data obtained at 77 K with a constant-volume adsorption apparatus (Micromeritic Tristar II) using N2 gas (99.995% pure; obtained from Praxair, Canada). The prepared samples were degassed at 150˚C overnight before measurements. A Nicolet 6700 FTIR spectrometer equipped with a smart iTR diamond horizontal attenuated total reflectance (ATR) accessory was used to characterize the nanocrystals and multifunctional mats. Spectra were measured in the range of 400-4000 cm-1 using 32 scans at 4 cm-1 resolution for each sample. A UV-Vis spectrometer (Varian Carry 3000) was used to acquire absorbance spectra of the TiO2 and Bi-TiO2 nanocrystal dispersed in ethanol. Photoluminescence (PL) studies were done using a Quantamaster 50 spectrofluorometer equipped with a Xenonshort arc lamp. The emission scan spectra were recorded by a proper selection of excitation light source wavelength determined from the absorbance of the specific TiO2 and Bi-TiO2 nanocrystals. For photocurrent efficiency of the P25, TiO2, Bi-TiO2 nanocrystal and GO-Bi-TiO2, the photovoltaic cell was scaled and tested by a Keithley 2420 programmable SourceMeter and an Oriel solar simulator (92250A, AM 1.5 G) equipped with a 150 W Xe lamp.
Fabrications of dye-sensitized solar cells (DSSCs)
The photovoltaic activities of the Bi-TiO2 nanocrystals were evaluated by comparing the photocurrent efficiency using Gratzel cells. The cells were prepared from two transparent glass plates (indium tin oxide-ITO) that were pre-treated on one side with a transparent thin layer of conducting material (15 Ω cm resistivity). In order to fabricate the DSSCs, the conducting side of one plate was coated with graphite and the other plate was coated with nanocrystal film by using the Bi-TiO2 paste to prepare the photoactive layer to effectuate photocurrent characterization of our materials. This layer was achieved by the doctor blade technique i.e. the paste was spread onto the conducting side of the ITO glass substrate (2.5 cm2) using a glass rod with an adhesive tape (Scotch, 40 μm thickness) as spacer. The samples were then dried at room temperature for one hour and treated at 400°C for 30 minutes to achieve an active porous layer and rough solid surface. The resulting TiO2 film was then immersed in an ethanol solution of 0.3 mM N3 dye- cis-Bis(isothiocyanato) bis(2,2/ -biphenyl-4, 4/-dicarboxylato) ruthenium(ΙΙ) for about 16 hours. The graphite coated plate was prepared by simply colouring the glass surface with a graphite pencil. The finally prepared TiO2 and graphite coated plates were then carefully sandwiched together and secured using a paperclip. To complete the cell, a drop of iodide electrolyte was added between the plates. The same protocol was followed to fabricate the DSSCs of P25 as a reference, doped Bi-TiO2 cells and GO- Bi-TiO2 cells.

Results and Discussion

Morphology of Bi-TiO2 nanocrystals and decorated Graphene
The X-ray diffraction (XRD) patterns were used to determine the crystalline phase identity of the TiO2 and Bi-TiO2 nanocrystals. Figure 1 shows the XRD patterns of the bismuth doped and undoped TiO2 powder samples. All the apparent diffraction peaks in both cases are assigned to the tetragonal structure as referenced in JPCDS 10- 0445, with the strong and sharp diffraction peaks revealing a high degree of crystallinity. When examining the diffraction peaks of the bismuth doped TiO2 nanoctystals, it is clearly shown that there are no other peaks besides the undoped TiO2 nanocrystals. This suggests that the doped crystals may be formed from bismuth substituting for Ti atoms in the lattice, with Ti-O-Bi being formed. In general, the intensity of the diffraction peaks decreases with an increase of doping concentration, indicating a loss of crystallinity due to lattice distortion. When Bi+ ions are incorporated into the periodic crystal lattice of TiO2, a strain is induced into the system, resulting in the alteration of the lattice periodicity, decrease in crystal symmetry with a significantly change in grain boundary properties.
Figure 1: XRD diffraction patter spectra from tetragonal phases acquired from powder a) TiO2, b) 2% doped Bi-TiO2 and c) 5% Bi-TiO2 nanocrystals.
Scanning electron microscopy (SEM) was used for investigating the morphology of the graphene mats before and after decorating with the Bi-TiO2 nanocrystals as shown in Figure 2. SEM images of the prepared Graphene sheets are shown in Figure 2a while a TEM image of typical single graphene sheet morphology is shown in Figure 2b. The morphology of the porous graphene sheets was studied by BET (nitrogen adsorption and desorption) surface analysis giving a surface area of 1189 m2/g, pore volume 3.39 cm3/g and average pore width 114 Å. This is an excellent morphology of graphene for decoration or sensitization by semiconductor nanocrystals to achieve hot electron transport in DSSCs. Figure 2c provides an SEM image of Bi-TiO2, which was essentially identical for the nanocrystal TiO2 formed (not shown). The particles were observed with an average 60-120 nm in size. Both nanocrystals have a prism-like shape, indicating uniform growth using the hydrothermal one pot sol-gel synthesis route. From the TEM results in Figure 2d, it can be clearly seen that the graphene sheet is well decorated by the Bi-TiO2 crystals. It is observed that the graphene forms a sac-like structure, in that Bi-TiO2 is embedded inside, with the resultant morphology signifying the distinct surface morphology and crystal growth. The experimental finding in the present context shows how small changes in the preparative strategy change the resultant product. In the formed metal oxide graphene mats, intimate interaction by electrostatic attraction offers increasing electron transfer. Previously it was shown by Medina-Gonzalez et al when examining CdS and CdTeS quantum dots decorated on TiO2 nanowires, that direct attachment without a linker molecule gave the best electron injection efficiency in Gratzel cells [37]. Short range forces associated to strong van der Waals forces are present at this nanoscale level between the nanocrystals and the graphene mats [38], accentuated by permanent dipole moments existing on the TiO2 surface originated by undercoordinated Ti atoms [39]. These dipoledipole interactions would be strong driving forces for direct assembly in our systems.
Figure 2: (a) SEM image of graphene sheets, (b) TEM micrograph of single graphene sheet, (c) SEM image of Bi-TiO2 nanoparticles, and (d) TEM image of Bi-TiO2 adsorbed on graphene sheet.
Chemical composition analysis of TiO2 and Bi-TiO2 nanocrystals
The chemical identity or elemental analysis of the samples was confirmed by energy-dispersive X-Ray spectroscopy (EDAX) and FTIR spectroscopy. Using EDAX to analyze the TiO2 and Bi-TiO2 nanocrystals, pronounced peaks were obtained with the presence of titanium and oxygen, while the Bi-TiO2 examined by EDAX confirmed the composition of titanium, oxygen and bismuth. The EDAX spectra are shown in Figure 3a for TiO2 and Figure 3b for Bi- TiO2 nanocrystals. In all spectra, the carbon signal originates from the sample holder, since the sample was mounted on conducting double sided carbon tape and osmium was used to cover the sample to make it conductive. No other impurities were detected in the EDAX analysis, indicating that both nanocrystals types are free from impurities or other trace elements.
Figure 3: SEM-EDAX analysis of (a) TiO2 and (b) Bi-TiO2.
FTIR spectroscopy is a nondestructive characterization method of choice for the vibrational identification of the chemical structure of a compound. The transmittance FTIR spectra of the TiO2 and Bi-TiO2 nanocrystals were measured and the spectra are shown in Figure 4. The structural analysis of the undoped and doped TiO2 revealed by the spectra indicates an extra peak at frequency 1000 cm-1 due to the inclusion of Bi ions in the nanocrystals as a dopant [40]. Hence, both the EDAX and FTIR results confirm the composition integrity TiO2 and Bi-TiO2 nanocrystals.
Figure 4: FTIR spectra of TiO2 and doped Bi-TiO2.
Optoelectronic analysis of TiO2 and Bi-TiO2 nanocrystals
The TiO2 and Bi-TiO2 nanocrystals optical properties were studied by UV/Vis absorption spectroscopy (UV/Vis) and photoluminescence (PL) spectroscopy. The UV–Vis absorption spectrum of both TiO2 and Bi-TiO2 nanocrystals are shown in Figure 5 in the wavelength range of 300-1200 nm. TiO2 absorbs UV A light with an absorption maxima 380 nm, while 2% Bismuth doped Bi-TiO2 nanocrystals absorb at maxima 460 nm while 5% Bismuth doped Bi-TiO2 nanocrystals absorbs at maxima 530 nm. This indicates that Bismuth doping increases the absorption maxima wavelength by red shifting due to lower band gap energy of Bismath compared to virgin TiO2. The absorption peaks become wider with increased Bi doping was increased. Wider absorption of the nanocrystals facilities capturing of solar energy with a broad range of UV A to Vis light which is promising for increasing the photocurrent efficiency of DSSCs. Also the obtained absorption spectra indicate good crystallinity and particle shape. To support the absorption and emission optoelectronic properties, photoluminescence of both TiO2 and doped Bi-TiO2 nanocrystals was examined with the resulting spectrums shown in Figure 6. The excitation wavelengths for photoluminescence analysis were 380 nm for TiO2, 460 nm for 2% Bi-TiO2 and 530 nm for 5% Bi- TiO2 nanocrystals respectively that obtained from absorption study. The luminescence observed in the Vis-NIR region complements the absorption results. One of the possible reasons for variations in the band-edge emission is for the native defects, which depend on the dimension of the doping atom and its orientation in the crystal phase modulating new quantum effects [41]. In consequence, the defects could affect the position of the band-edge emission as well as the shape of the emission spectrum. In general, doping with different donors-acceptors produces broadening of the emission peak, with the peak shift dependent on the individual dopant [42].
Figure 5: Electronic Absorption spectra of TiO2 and doped Bi-TiO2.
Figure 6: Photoluminescence spectra of TiO2 and doped Bi-TiO2.
Photovoltaic efficiency characterization
The photovoltaic cells were prepared using bare TiO2 nanocrystals, Bi-TiO2 nanocrystals and GO sensitized by Bi-TiO2 to test in DSSCs. To compare the cell efficiencies, commercial TiO2 (P25) was used as a reference for photovoltaic characterization. To characterize the efficiency of these PV devices, they were illuminated using ~1 sun condition under simulated 140 W/m2, where the effects of slow diffusion were less severe. The obtained J-V characteristic curves are shown in Figure 7 and the obtained characteristic parameters are summarized in Table 1. The main test parameters such as the short circuit photocurrent density (Jsc), an open circuit photovoltage (Voc), the field factor (FF), the conversion photocurrent efficiency (η) of the photovoltaic cells and comparing the cell efficiencies with the commercial TiO2 are presented in this table. When comparing the efficiencies, it is vividly demonstrated that the efficiency was increased 9 fold compared to the bare TiO2 nanocrystals, 21 fold in 2% doped Bi-TiO2, 26 fold in 5% doped Bi-TiO2, 34 fold in GO sensitized by 2% doped Bi-TiO2 and 46 fold in GO sensitized by 5% doped Bi-TiO2. A remarkable boost in efficiency was observed when TiO2 nanocrystals were used in the DSSC instead of using commercial TiO2. A further increase in efficiency was achieved in the case of the doped Bi- TiO2 nanocrystals due to their improved optoelectronic properties as shown in the Figures 5 and 6. A significant enhanced efficiency was attained when GO was used as a hot electron transporter in the cells to minimize the recombination of electron and hole after photo excitation. The resultant electrons were conducted to the ITO through conducting GO to the photoanode and the holes were recovered by the electrolyte to the photocathode graphite layer.
Figure 7: Photocurrent density-voltage (J-V) curves for dye-sensitized solar cells using: (a) Commercial TiO2 –P25 (b) bare TiO2 nanocrystals, (c) doped 2% Bi-TiO2 nanocrystals, (d) doped 5% Bi-TiO2 nanocrystals, (e) doped 2% Bi-TiO2 nanocrystals in GO and (f) doped 5% Bi-TiO2 nanocrystals in GO.
Table 1: Photovoltaic efficiency in the DSSCs examined by J-V measurement in solar simulator for photovoltaic cells of: (a) Commercial TiO2–P25 (b) bare TiO2 nanocrystals, (c) doped 2% Bi-TiO2 nanocrystals, (d) doped 5% Bi-TiO2 nanocrystals, (e) doped 2% Bi-TiO2 nanocrystals in GO and (f) doped 5% Bi-TiO2 nanocrystals in GO.


Photovoltaic performance of dye-sensitized solar cell (DSSC) devices based on a titania nanocrystals, doped titania nanocrystals and doped titania–graphene paste were investigated using a photo sun solar simulator technique. The results provide key information about device performance, clarifying that photocurrent increment is not related to lower charge transfer resistance, neither with a downshift of the Fermi level of the graphene titania semiconductor. The goals of extending both the nanocrystalline TiO2 and doped Bi-TiO2 spectral response to the visible region and improving the photocurrent efficiency were realized with higher photoreactivity when compared with the commercial TiO2–P25 DSSC. Doping of TiO2 with bismuth resulted in a broad electronic absorption and consequently increased efficiency in DSSCs. The DSSC devices constructed using TiO2– graphene paste reach even higher enhancement photocurrent and therefore higher efficiency than devices made with a nanocrystalline TiO2 paste due to extra photocurrent generation, because the TiO2– graphene paste presents higher light harvesting in the visible region of the solar spectra combined with a large scattering effect. In summary, graphene is a material of general interest for optoelectronic devices, and it was introduced it into the working electrode of DSSC successfully. The short-circuit current density and the conversion efficiency of the DSSC was significantly increased. The enhancement of the light harvesting graphene titania paste is what generate extra photocurrent without any energetic and structural change in the semiconductor.


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