Carbon doped titanium dioxide (C-TiO2) is considered as a promising photocatalytic material due to its optical absorption extended in the visible region compared to pure TiO2. However, in the field of photovoltaic’s, use of C-doped nano-crystalline titanium dioxide (C-TiO2) electrodes for light absorption has been considered to be unnecessary so far. In this context, we report here on the use of C-TiO2 nano-crystalline electrodes in photochemical solar cells devices (PCSC). Carbon doping has reduced the band gap of TiO2 to 2.41 eV and 2.25 eV with increase in the doping extent for the 9 mM C-TiO2 and 45 mM C-TiO2 respectively. The C-TiO2 electrodes were first used as photo electrodes for solar cells, exhibiting JSC of 1.34651 mA/cm2, VOC 0.683 V, FF 50.23% and η 0.46%. for the 9 mM C-TiO2 and exhibiting JSC of 1.34651 mA/cm2, VOC 0.815 V, FF 54.3% and η 0.59% for the 45 mM C-TiO2. The fabricated solar cell devices have shown an increase in VOC of up to 0.815 V, which is higher than that of 0.7 V for dye sensitized solar cells. The doping of carbon in TiO2 lattice was closely studied by SEM, XRD, RS and UV-Vis spectroscopy.
Titanium dioxide (TiO2) is a typical n-type semiconductor that is extensively used for many applications, such as optical coating [
Use of these types of hybrid nano-crystalline titanium systems in DSSC to supplement light absorption has been considered to be unnecessary so far. Mainly because organic dyes used as solar sensitizers which are mainly Ru (II) complexes have been used to harvest solar radiation [
This section outlines the actual experimental procedures involved in the preparation of precursors solution and deposition of nano porous titanium dioxide thin films using ultrasonic spray pyrolysis.
In a synthesis procedure 781.24 μl of titanium tetraisopropoxide (TTIP) were carefully added to a 100 ml beaker containing 60 mls of pH adjusted double distilled water in an ice bath maintained at 0˚C. The precursor solution was added drop wise whilst simultaneously stirring to avoid the formation of white precipitates of titanium hy- droxides. The prepared solution was then carefully transferred into 250 ml volumetric flask containing 60 mls. of pH adjusted double distilled water in an ice bath. 0.8103 g of oxalic acid dihydrate (C2H2O∙2H2O Fluka) was then added to the volumetric flask and topped up to the mark whilst simultaneously stirring in an ice bath. The precursor solution was then sonicated for 360 minutes prior to ultrasonic spray. This resulted in preparation of a 9 mM Oxalic acid doped titanium dioxide precursor solution which was coded 9OA C-TiO2. Several other pre- cursor solutions of oxalic acid doped solutions were prepared in much the same way, the only difference was the level of dopant and type of dopant as illustrated here
In the prepared precursor solution was then decanted into a spray pyrolysis reaction vessels housing a 1.7 MHz ultrasonic nebulizer. Spray deposition begins when vapor of the precursor solution were carried by argon gas at 6 ml/minute into a split tube furnace. The pyrolysis if the precursor resulted in rapid growth of nano-particles which were deposited on fluorine doped tin oxide (FTO) glass substrate. Spray deposition was carried out for 30 minutes and at 450˚C.
Scanning electron microscopy morphology studies was done using a Jeol JSM-5600 scanning electron micro- scopy (SEM) microscope which was equipped with an EDX attachement for elemental analysis. X-ray diffrac- tion patterns were gathered using a Philips Xpert powder diffractometer equipped with a CuKα wavelength of 0.154184 nm and graphite crystal monochromator. Survey scans from 2θ = 20˚ to 80˚ were performed with a step size of 0.05˚. Regional high-resolution XRD scans were also conducted to investigate C-doped TiO2 lattice parameters via the most prominent reflections, using a 0.002˚ step size. Region 1, from 2θ = 24˚ - 30˚, aimed to capture the anatase (101) and rutile (110) reflections. Region 2, from 2θ = 35˚ - 43˚, examined the A(004), A(112), R(101) and R(111) peaks. Raman spectroscopy was carried out using a Jobin-Yvon T6400 Raman spectrograph operated in triple subtractive mode using, 514.5 nm line. Diffuse reflectance spectra of the result- ing nano-powders was obtained using a double beam Lamda 25 Perkin Elemer UV-Vis spectrometer, which was equipped with an intergrating sphere. A given amount of the nano-powder was uniformly pressed in a powder holder (jasco) and placed in the sample holder an intergrated sphere for reflectance measurements. Incident was swept from 100 - 1500 nm. I-V characteristics of the PEC devices were perfomed on a Keithely 2400 source meter under simulated sunlight. I-V and controlled by in house Labview applications. Lamp intestity data was
. Preperation of oxalic acid (OA) & tetra butyl amonium doped (TBA) TiO2 precursor solutions
Titanium (IV) isopropoxide (TIP) [1 × 10−2 M] | |||
---|---|---|---|
Sample Code | Mass of Oxalic Acid (OA) g | Sample Code | Tetra Butyl Ammonium (TBA) (ml) g |
9OA C-TiO2 | 0.8103 | 9TBA C-TiO2 | 5.58 |
18OA C-TiO2 | 1.6205 | 18TBA C-TiO2 | 11.17 |
27OA C-TiO2 | 2.4308 | 27TBA C-TiO2 | 22.07 |
36OA C-TiO2 | 3.2411 | 36TBA C-TiO2 | 22.34 |
45OA C-TiO2 | 4.05142 | 45TBA C-TiO2 | 27.92 |
54OA C-TiO2 | 5.66147 | 54TBA C-TiO2 | 56.60 |
It shows actual photo graph of the horizontal ultrasonic spray pyrolysis system utilized in the study
collected using a Molectron EPM 200 power meter with PM3 thermopile head. Scans were performed under constant power conditions with a 150 W Xe lamp (Oriel). I-V characteristics were at room temperature and were obtained under 1 sun intensity (100 mW/cm2). A glass filter cuvette filled with water was used as an IR Filter to minimize heating and a UV Filter was used to filter UV bandgap irradiation. The cell was placed in the path of the incident light and scanned from ISC (0 V) to VOC (720 mV) at 68 mV/s with 16 mV increments and 10 point averaging at each increment.
To fabricate the PCSCs, two holes were made within 1mm from the edges of FTO platinized electrodes prior to the doctor blading of the pt paste. USP developed working electrodes of COA-TiO2, CTBA-TiO2 and the platinized counter electrodes were put together in a sandwich configuration using meltonix gasket and hot presser to fabri- cate a PCSC device as shown here in
The surface morphology of the as synthesized thin films has been analyzed by SEM. Clearly
It illustrates the assembly of PCSC starting with (a) drilling of hole on bare photocathode (b) development of platinized photocathode (c) & (d) PCSC device in sealed configuration with redox electrolyte filled in between the inter electrode space
SEM image of surface morphology of nano-crystalline and EDS spectra for (a) 9TBAC-TiO2; (b) 18TBAC-TiO2 thin film
other elements (i.e.) gold (Au) from the gold coating in SEM sample preparation, aluminum (Al) from the alu- minum reaction vessel or substrate holders used in the study. Sodium (Na) and magnesium (Mg) originate from the FTO glass substrate. The EDS spectrum also reveals that the titanium atom has different oxidation states. Titanium atom appears at 0.52 KeV, 2.75 KeV, 4.3 KeV and 4.9 KeV. This is due to different binging energies of the core electrons. The first peak of titanium at 0.52 KeV overlapping with oxygen might be due to the high oxidation state of Ti(IV) and other peaks at 4.2, 4.3 & 4.9 KeV are due to titanium in oxidation state of plus three Ti(III). The decline in titanium oxidation state has been ascribed due to carbon doping.
XRD was used to investigate the crystalline structure of TiO2 samples as well as the structural effects of car- bon doping.
Raman spectroscopy was used to analyze the samples and complement XRD results for phase identification. The Rutile phase of TiO2 is tetragonal and exhibits symmetry characters of the space group
It is well known that the photo sensitivity of semiconductors is related to its band gap [
The 9 mM oxalic acid doped titanium dioxide (9(OA) C-TiO2) showed a 6 nm shift absorption in visible spec- trum with an absorption edge at 402 nm, whilst the 36(OA) C-TiO2, 45(OA) C-TiO2 showed absorption edges at 404 nm and 403 nm respectively. The USP synthesized samples for oxalic acid doped samples for 27(OA) C-TiO2 showed a band gap equal or lower than that of pure un-doped TiO2 (spectra not shown). The blue shift of the absorption edge observed with 27(OA) C-TiO2 can be attributed to charge transfer transition between the metal ion d electrons and the conduction or valence band of TiO2 [
XRD pattern for the as synthesized thin films (1) un doped TiO2; (2) 9 mM C-TiO2; (3) 36 mM C-TiO2, (4) 45 mM C-TiO2
Depicts the Raman spectra for the as synthe- sized thin films
It shows the DRS spectra of the OA doped thin films (a) Un doped TiO2; (b) 9OAC-TiO2; (c) 36OAC-TiO2; (d) 45OAC-TiO2. Also it shows the extended DRS spectra in the range 350 - 500 nm
It shows the DRS spectra of the TBA doped thin films Un doped TiO2, 9TBAC-TiO2 36TBAC-TiO2, and 45TBAC-TiO2. Also shows the extended DRS spectra in the range 350 - 500 nm
The samples doped with TBA doped TiO2 showed an enhanced shift than those of OA. This is mainly because TBA has a higher carbon composition than oxalic acid despite its lower diffusion mobility into the TiO2 and large molecular size as compared to OA. The enhanced absorption might also be due to the presence of ammonium ion which contains the nitrogen atom in TBA. It is well known that nitrogen doping as compared to carbon doping shows enhanced doping hence a massive shift in the absorption edge. Nitrogen doping has proven more efficient at improving visible light activity. It is theorized that the carbon states are too deep within the band gap to significantly increase visible light activity, unlike nitrogen doping 1 s state, which successfully overlap with the band O2p states [
where λedge is the wavelength of the absorption edge.
During the J-V measurement, four parameters such as JSC, VOC, FF and η were obtained of the fabricated PCSCs.
. It shows the determined absorption edge λ (nm) and the estimated energy band gap for the OA doped thin films
Sample Code | Absorption Edge (λ) nm | Energy Band Gap (eV) |
---|---|---|
9OA C-TiO2 | 346 | 3.12 |
18OA C-TiO2 | 404 | 3.06 |
27OA C-TiO2 | 396 | 3.12 |
36OA C-TiO2 | 403 | 3.06 |
45OA C-TiO2 | 405 | 3.05 |
. It shows the determined absorption edge λ (nm) and the estimated energy band gap for the TBA doped thin films
Sample Code | Absorption Edge (λ) nm | Energy Band Gap (eV) |
---|---|---|
Un doped TiO2 | 346 | 3.12 |
9TBA C-TiO2 | 496 | 2.49 |
36TBA C-TiO2 | 512 | 2.41 |
45TBA C-TiO2 | 550 | 2.25 |
J-V characteristics of the fabricated PCSCs
of 0.462%. 45TBA C-TiO2 9 TBA doped photo-electrode showed photo electrode showed a maximum conversion with an open circuit voltage of (VOC) of 0.817 V, photocurrent JSC of 1.37639 mA/cm2, a fill factor of 54.5% and an efficiency of 0.613%. The abrupt increase in JSC by 49% was particularly notable. Light absorption as was noted from the DRS spectra of 9OA C-TiO2 showed an absorption edges around 404 as compared to the TBA doped photo-electrodes which showed an absorption edges at around 551 nm. This increase in photo current generation as the dopant type change might due to (1) increase in (2). The chemical structure of oxalic acid (
In addition pure un-doped titanium dioxide contains oxygen defects which cause carrier trapping at the sur- face of pure un-doped TiO2 nano-particles leads to a low electron diffusion coefficient (7 × 10−6 cm−2∙V−1∙s−1) [
We have also demonstrated that C-TiO2 nano-structured photo electrodes with optical band gaps in the visible section of the solar spectrum can be obtained with oxalic acid and tetrabutyl ammonium bromide as the carbon sources using spray pyrolysis technique. Our analysis has also shown that the TBA doped samples exhibit a greater shift in absorbance as compared to OA doped sample. The results are evidenced by extended absorption edges of 45TBA C-TiO2 photo electrode which were well above the 500 nm mark. Furthermore the respective so- lar cells have shown higher conversion efficiencies as compared to the OA doped photo electrodes. The PCSC device with a 45TBA C-TiO2 photo electrode showed the highest solar conversion with an open circuit voltage of (VOC) of 0.817 V, photocurrent JSC of 1.37639 mA/cm2, a fill factor of 54.5% and an efficiency of 0.613%. Ad- mittedly the overall solar cell efficiency for the PCSC devices employing TBA doped titanium dioxide is still very low ~0.6% as compared to the dye sensitized counterpart fabricated by Grätzel in 1991 with an efficiency of ~11%. Therefore, C-doped TiO2 nano-crystals prepared by our approach are ideal semiconductor materials for DSSC and PCSC devices. This study has revealed that our method of fabricating PCSC devices using ultra- sonic spray pyrolysis has low cost, simple processing, excellent reproducibility, easy to extend on a large scale production and is applicable in the field of solar energy conversion.
We are grateful for financial support from our sponsor ESKOM and Govani Beki Research and Development centre (GMRDC) of the university of Fort Hare.