The present work aims at optimizing titanium dioxide morphology for dye-sensitized solar cells applications. Five different anatase phase mesoporous titanias were prepared and tested as photoanodes in dye-sensitized solar cells. The materials were prepared by using a template approach. Two materials were synthesized by using monodisperse silica nanospheres and the other three using two different organic templating agents (Pluronic P123 and Brij 58). A complete characterization of the obtained materials was performed by powder XRD, FEG-SEM, UV-Vis reflectance spectroscopy, BET surface area measurements, and TG-DTA. Several cells were assembled using N719 as dye and a nonvolatile electrolyte based on benzonitrile. The cells were tested by means of
Since they appeared in 1991 [ simple design; ease of fabrication; low cost; no need of the extremely expensive equipment such as that necessary for conventional photovoltaics.
In spite of these advantages, 23 years passed without a commercial large scale implementation of DSSCs and this was mostly due to two most important drawbacks, that is, relatively low efficiency (about 12% for the highest performance, lab scale devices [
Notwithstanding their weak points, DSSCs remain attractive because of their design, open to several improvements in each one of the main parts, that is, wide band gap semiconductor, sensitizer, electrolyte, and counter electrode. In this light, our recent work was addressed to modify the photoanode to improve electron diffusion through it by inserting multiwalled carbon nanotube in the anatase nanoparticles building in this way conductor-semiconductor Schottky junctions [ to improve the electronic properties of anatase by controlled doping with Sc3+ ions [ to develop new and high performance electrolytic compositions based on benzonitrile as solvent, being characterized by low volatility and high stability [
If we now focus our attention on the semiconductor improvement, it is evident that its properties can be tailored not only by modifying it chemically or physically but also by optimizing its morphology.
In a DSSC, the morphology of the semiconductor is of key importance for determining its efficiency. In effect, dye-sensitized semiconductors shifted from a mere laboratory curiosity to a promising level when dye-sensitized titanium dioxide was used as a nanocrystalline powder instead of a bulk form [
Titanium (IV) isopropoxide (TIP) (Vertec, 97 + %), titanium (IV) fluoride (98%), lithium iodide (ultradry, 99.999%), and tetrafluoroboric acid (48% in water) were purchased from Alfa Aesar. Titanium (IV) chloride (99.9%), titanium (IV) propoxide (98%), iodine (99.999%), 4-
3 mm thick FTO glass slides with sheet resistances of 10 Ω/□ and 15 Ω/□ for the photoanode and cathode, respectively, were purchased from XOP Fisica (Spain).
Aeroxide VP P90 fumed titanium dioxide was kindly given as a free sample by Evonik.
Surlyn (Dupont) hot melt thermoplastic was used to seal the cells (25
Cerasolzer CS246-150 soldering alloy was purchased from MBR Electronics (Switzerland).
Kynar PVDF 502-CUH-HC film was used as antireflection and UV blocking layer (<400 nm) and was kindly given as free sample by Arkema Inc.
Three different synthetic approaches were adopted [
Powder X-ray diffraction analysis of the produced solids was performed by using a Panalytical X’Pert PRO MPD diffractometer employing a Cu K
By using the MAUD software package [
Low angle scans (0.5–10°) were performed on P123-A, P123-B, and TiO2-Brij samples by using a beam knife, 20 mm mask, a 1/8° divergent slit, and a 5 mm antiscatter slit. The soller slit and the collimator were the same as above.
Scanning electron microscope images of the samples were obtained by using a Zeiss Auriga FESEM microscope.
The surface area of powder samples was determined by using a Quantachrome Monosorb single point BET system.
The band gap of the materials was calculated using the UV-Vis reflectance spectra of the samples. The spectra were collected in the range 220–800 nm (5.64–1.55 eV), using a double-beam spectrophotometer (Shimadzu, Japan, model UV2600) equipped with a diffuse reflectance integrating sphere accessory. Baseline spectra were collected using pressed BaSO4 powder compacts that were placed in the sample and reference beams. Data were collected at a scan rate set in slow mode and a slit width of 0.5 nm. Band gap values were extrapolated from the Tauc plots of the Kubelka-Munk function calculated for the indirect interband transition [
Simultaneous TG-DTA analyses were performed on the samples prepared by making use of organic templating agents (P123-A, P123-B, and TiO2-Brij) before calcination. The measurements were performed in an Ar/O2 mixture (80/20 vol/vol) in the range RT-800°C with a scan rate of 5°C/min.
Dye-sensitized solar cells were prepared according to the procedure reported in [
The electrolytic composition used was 0.6 M EMII, 0.5 M TBP, 0.1 M GuSCN, 0.1 M LiI, and 0.03 M I2 in benzonitrile [
Using a MBR Electronics USS-9210 Ultrasonic Soldering System with the Cerasolzer CS246-150 as soldering alloy, the contacts to the cells were soldered.
The 1286 Electrochemical Interface from Solartron Analytical, UK, using the Full Combo ZPLOT/CorrWare software by Scribner Associates Inc., USA, was used to collect the
Dark current curves were collected with the same settings of those collected under illumination.
The incident photon to current conversion efficiency (IPCE) was measured in DC mode with a white light bias by a custom-made apparatus [
The diffraction patterns of all the samples after calcination are shown in Figure
Rietveld refinement results of the X-ray diffraction patterns of the samples under study. Unit cell axes and estimated crystallite size values are given.
Sample |
|
|
|
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MSC-1 | 3.794 ± 0.002 | 9.502 ± 0.008 | ~66 |
MSC-2 | 3.790 ± 0.001 | 9.505 ± 0.002 | ~90 |
P123-A | 3.788 ± 0.002 | 9.506 ± 0.008 | ~13 |
P123-B | 3.791 ± 0.001 | 9.513 ± 0.006 | ~13 |
TiO2-Brij | 3.788 ± 0.002 | 9.51 ± 0.01 | ~10 |
XRD patterns of the samples after calcination (a) and reference pattern (b).
While the crystallite size of the MSC-1 and MSC-2 samples is consistent with their relatively low specific surface areas, P123-A, P123-B, and TiO2-Brij crystallite sizes seem to contradict the values of the first two which are practically 1.5 times higher than that of the latter ones (see below). This apparent discrepancy is easily solved by the SEM images, in which, as shown later, the morphology of TiO2-Brij appears clearly much more compact than in the case of P123-A and P123-B samples.
Low angle scans (0.5-10° in 2
Low angle diffraction pattern of P123-A sample before calcination (green curve) and after calcination (red curve). In the inset, a magnification of the green curve in the lowest angle region (up to 1.6°) is given to better appreciate the peak centered at 0.711°.
SEM micrographs for samples MSC-1, MSC-2, P123-A, P123-B, and TiO2-Brij are shown in Figures
SEM micrograph of sample MSC-1. The well-defined crystal structure possesses a mesoporous pattern which is a replica of the silica template (see Figure
SEM micrograph of sample MSC-2.
SEM micrograph of the silica template used for the synthesis of the sample MSC-1.
SEM micrograph of sample P123-A.
SEM micrograph of sample P123-B.
SEM micrograph of sample TiO2-Brij. The morphology appears clearly more compact, with less interparticle voids than for P123-A and P123-B samples.
The specific surface area values are summarized in Table
Specific surface area values of the different mesoporous titania samples.
Sample | Specific surface area/m2 g−1 |
---|---|
MSC-1 | 13 ± 2 |
MSC-2 | 16 ± 2 |
P123-A | 120 ± 5 |
P123-B | 120 ± 4 |
TiO2-Brij | 75 ± 3 |
The band gap values are reported in Table
Band gap values of the samples under study extrapolated from the Tauc plots of the Kubelka-Munk function calculated for the indirect interband transition.
Sample | Band gap/eV |
---|---|
MSC-1 | 3.154 ± 0.003 |
MSC-2 | 3.190 ± 0.002 |
P123-A | 3.123 ± 0.003 |
P123-B | 2.997 ± 0.003 |
TiO2-Brij | 3.065 ± 0.002 |
TG and DTA curves for noncalcined P123-A, P123-B, and TiO2-Brij samples are shown in Figures
TGA profiles for P123-A, P123-B, and TiO2-Brij samples in an Ar/O2 atmosphere 80/20 vol/vol analyzed before calcination.
DTA curves for P123-A, P123-B, and TiO2-Brij samples analyzed before calcination.
In Figure The cells using MSC-1 and MSC-2 titanias show the lowest performances of the group, and this can be justified considering that they have (a) the highest series resistance values; (b) the abnormally high The cells using P123-A, P123-B, and TiO2-Brij titanias all behave in a very similar way, possessing similar efficiencies, short circuit current densities, open circuit voltages, fill factors, series resistances, saturation current densities, and ideality factors. The slight performance superiority of P123-
DSSC parameters extrapolated from
Sample |
|
|
|
|
|
|
|
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MSC-1 | 12.5 ± 0.1 | 0.7311 ± 0.0001 | 6.0 ± 0.1 | 0.65 ± 0.01 | 104 ± 3 | 108 ± 3 | 2.41 ± 0.01 |
MSC-2 | 10.9 ± 0.1 | 0.7422 ± 0.0001 | 5.7 ± 0.1 | 0.70 ± 0.01 | 94 ± 3 | 90 ± 4 | 2.78 ± 0.01 |
P123-A | 13.4 ± 0.1 | 0.7122 ± 0.0001 | 6.7 ± 0.1 | 0.70 ± 0.01 | 84 ± 2 | 1.7 ± 0.1 | 1.76 ± 0.01 |
P123-B | 14.5 ± 0.1 | 0.7088 ± 0.0001 | 6.8 ± 0.1 | 0.66 ± 0.01 | 86 ± 1 | 0.61 ± 0.04 | 1.59 ± 0.01 |
TiO2-Brij | 13.5 ± 0.1 | 0.7122 ± 0.0001 | 6.4 ± 0.1 | 0.66 ± 0.01 | 76 ± 2 | 1.3 ± 0.1 | 1.66 ± 0.01 |
IPCE curves of the DSSC devices.
Mesoporous titania samples, prepared from both hard and soft template approaches, were used as dye-sensitized solar cells’ photoanodes. While being structurally more perfect and so apparently more suitable for an easy electron transport through the photoanode, mesoporous titanias prepared from hard template are the least suitable for application in DSSCs because of their low specific surface area. A substantial performance improvement is expected by optimizing the dimensions of the particles that can be achieved by tailoring the synthesis conditions. On the other hand, titanias prepared from soft template approaches give a better operation and the similarity of their performances allows one to choose the synthesis on the basis of lower costs and shorter preparation times. Further improvements can be envisaged also for this class of materials, for example, by finding a way to retain the order of mesopores through thermal treatments and improving interparticle connections, which are expected to improve the dye loading and electron transport, respectively.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to acknowledge Dr. Francesco Mura (Laboratorio di Nanotecnologie e Nanoscienze della Sapienza Università di Roma) for FEG-SEM analyses and Dr. Sergio Brutti (Dipartimento di Scienze, Università degli Studi della Basilicata) for BET specific surface area measurements.