Advances in OptoElectronicsVolume 2011 (2011), Article ID 424071, 5 pagesdoi:10.1155/2011/424071

Research Article

Comparative Study between Dye-Sensitized and CdS Quantum-Dots-Sensitized TiO₂ Solar Cells Using Photoinduced Absorption Spectroscopy

Idriss Bedja¹ and Anders Hagfeldt²

¹CRC, Department of Optometry, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia²Department of Physical Chemistry, Uppsala University, P.O. Box 579, 75123 Uppsala, Sweden

Received 15 June 2011; Accepted 6 September 2011

Academic Editor: Surya Prakash Singh

Copyright © 2011 Idriss Bedja and Anders Hagfeldt. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Two 8 μm thick TiO₂ photoelectrodes have been sensitized separately by N719 dye molecules and CdS quantum dots for a comparison study. Photoinduced absorption (PIA) spectroscopy was employed to investigate the mechanistic properties of electrons under illumination conditions comparable to sunlight. The PIA spectrum of both electrodes (in the presence of electrolyte) is due to electrons in TiO₂ and iodine radicals in the electrolyte. In the absence of redox electrolyte, both electrodes show long-lived photoinduced charge-separation with lifetime in a millisecond range (8.5 ms for Q-dot-sensitized TiO₂ and 11.5 ms for dye-sensitized TiO₂).

1. Introduction

Nanostructured solar cells sensitized by organic dyes (DSSCs) [1–6] or by inorganic short bandgap semiconductors (also called quantum dots, QDs) [7–10] have attracted a great deal of interest. They are capable to obtain efficient conversion of solar energy to electricity at a low cost comparative to conventional semiconductor photovoltaic devices [11, 12]. The approach of using semiconductor colloids for the design of optically transparent thin semiconductor films is considered as a unique and an alternative for the amorphous silicon solar cells. Using this approach, dye-sensitized solar cells based on bi- and polypyridyl ruthenium complexes have achieved solar-to-electrical energy conversion efficiencies of 10-11% under AM 1.5 irradiation [1–3]. On the other hand, wide bandgap semiconductors have also been sensitized by short bandgap quantum dots (CdSe/TiO₂ [6], CdS/TiO₂-SnO₂ [10]) as alternative to dye sensitization. Vogel and coworkers [8] have investigated the sensitization of nanoporous TiO₂, ZnO, and so forth by Q-sized CdS. Photocurrent quantum yields up to 80% and open-circuit voltages up to 1 V range were obtained. In contrast with the dye-sensitized solar cells, fundamental understanding of factors controlling the interfacial electron transfer reactions in QD sensitized solar cells is limited.

In this paper, we report photoinduced absorption spectroscopy of an organic (N719 dye)-sensitized and inorganic (Q-dot CdS) semiconductor-sensitized TiO₂ (8 μm) photoelectrodes under illumination conditions comparable to sunlight in order to compare the mechanistic properties of electrons.

2. Experimental

2.1. Preparation of Nanostructured TiO₂ Films

We take care more about similarity in TiO₂ thicknesses in order to insure the same length of e-transfer from different sensitizer to external circuit. Before coating conducting glass ITO with TiO₂ nanoparticles, first we coated a blocking layer by immersing ITO plates into 0.02 M TiCl₄ solution at 70°C for 30 min. TiO₂ layer was made with EPFL paste by the Doctor Blade technique. Working electrodes were then sintered at 450°C for 30 min using heat gun and cooled down to room temperature.

2.2. Surface Modification of TiO₂  by Quantum Dots CdS

TiO₂ metal oxide nanostructured electrodes were successively dipped into an aqueous solution of saturated Cd(ClO₄)₂ and 0.1 M Na₂S for 1 and 2 min, respectively. After each CdS layer deposition, the electrodes were heated at 125°C for 5 min.

2.3. Surface Modification of TiO₂  by N719 Dye

The relatively hot (~80°C) TiO₂ nanostructured electrodes were immersed in a 0.5 mM ethanolic solution of N719 dye ((TBA)₂-cis-Ru(Hdcbpy)₂-(NCS)₂, Solaronix, Switzerland) and left for about 2 hours only. (There was no need for overnight adsorption, just a reasonable adsorption to be able to investigate a good mechanistic study.)

2.4. Characterization Methods

UV-Vis spectra were recorded using a Hewlett-Packard 8453 diode array spectrometer. For PIA spectroscopy (Figure 1), excitation of the sample was provided by light from a blue LED (Luxeon Star 1 W, Royal Blue, 470 nm), which was square-wave modulated (on/off) by electronical means using an HP 33120 A waveform generator and a home-built LED driver system. The beam, with an intensity in the range of 0.5–30 mW/cm², excited a sample area of about 1 cm². White probe light was provided by a 20 W tungsten-halogen lamp. A cutoff filter (Schott RG715) was used to minimize excitation of the sample by the probe light where indicated. The transmitted probe light was focused onto a monochromator (Acton Research Corporation SP-150) and detected using a UV-enhanced Si photodiode, connected to a lock-in amplifier via a current amplifier (Stanford Research Systems models 830 and 570, resp.). For the time-resolved studies the output of the current amplifier was connected to a data acquisition board (National Instruments PCI-6052E). All PIA measurements were done at room temperature.

Figure 1: Schematic drawing of the photoinduced absorption spectroscopy setup.

3. Results and Discussion

3.1. UV-Vis Absorption Spectra

Figure 2 shows UV-Vis absorption spectra of bare TiO₂ film, CdS-sensitized, and N719-sensitized TiO₂ films. It is clear that CdS and N719 dye both have extended further the absorption into the visible up to 600 and 750 nm, respectively.

Figure 2: Absorption spectra of (a) TiO₂ particulate film, (b) CdS-adsorbed TiO₂ film, and (c) N719 dye-adsorbed TiO₂ film.

3.2. PIA Spectroscopy of CdS/TiO₂ System

Figure 3 shows typical PIA spectra of the CdS-nanostructured TiO₂ system. In the absence of the redox electrolyte, the PIA spectrum clearly reflects the differential spectrum of CdS upon formation of the oxidized CdS (or hole formation), with a bleach of the main absorption band at 470 nm. The PIA spectrum after the onset of 550 nm should reveal the spectrum of injected electrons into TiO₂ conduction band with a peak around 650 nm compared to CdS alone on ITO. In the later, large recombination occurs when it failed to be transported far away form the existing created holes. When the redox electrolyte is added, the PIA spectrum changes significantly. The oxidized CdS is rapidly reduced by the electrolyte, and thus little bleaching is observed under 530 nm. The brad absorption at wavelengths larger than 600 nm can be attributed to injected electrons in TiO₂.

Figure 3: Photoinduced absorption (PIA) spectra of quantum dots CdS-modified TiO₂-NP electrode (circle) in air and CdS on conducting glass ITO (square). The spectra were recorded using blue light (460 nm) excitation (42 mW·cm⁻²) with a modulation frequency of 9 Hz.

In Figure 4, PIA spectra of CdS-adsorbed TiO₂ electrodes in the presence of electrolyte and under open circuit are shown. When the redox electrolyte, composed of 0.7 M LiI and 0.05 M I2 in 3MPN, is added, the PIA spectrum changes significantly. The absorption increases up to 600 nm and remains almost flat up to 800 nm. We attribute the PIA signals to electrons in nanostructured TiO₂ and iodine radicals () [13]. Triiodide does not absorb at wavelengths larger than 600 nm, while exhibits a broad absorption peak at 750 nm with an extinction coefficient of 2200 M⁻¹ [14].

Figure 4: PIA spectra of CdS adsorbed at a nanostructured TiO₂ film. Filled squares: in air; filled circles: in the presence of electrolyte. The spectra were recorded using blue light (460 nm) excitation (42 mW cm⁻²) with a modulation frequency of 9 Hz.

Study of the kinetics in semiconductor sensitizing solar cells is not only feasible by laser flash photolysis but also possible using time-resolved PIA measurements. Figure 5 shows such a typical PIA transient decay recorded at 700 nm. Pseudo-first-order rate constant for e-injection from CdS conduction band to TiO₂ conduction band was about 8.5 ms approximately which does not follow simple first-order kinetic but is characterized by a range of injection times. This relatively fast decay proves at least a well pore filling of TiO₂ film by ultrafine CdS particles.

Figure 5: PIA decay transient absorption of Q-dots CdS-modified TiO₂ electrode after excitation with blue light (11 mW/cm²) recorded at 600 nm using a sampling rate of 10³ s⁻¹ and averaged 100 times.

3.3. PIA Spectroscopy of N719/TiO₂ System

Figure 6 shows typical PIA spectra of the dye-sensitized TiO₂ system. In the absence of the redox electrolyte (Figure 6(a)), the PIA spectrum clearly reflects the differential spectrum of N719 upon formation of the oxidized dye, with a bleach of the ground state MLCT (metal-to-ligand charge transfer) main absorption band at 540 nm and an absorption peak at 780 nm (Figure 6(a)) which corresponds to MLCT transitions. Electrons in the TiO₂ will also be present in the PIA. When the redox electrolyte composed of 0.7 M LiI and 0.05 M I₂ in 3MPN is added (Figure 6(b)), the PIA spectrum changes significantly. The oxidized dye is rapidly reduced by the iodide in the electrolyte, presumably according to the following reactions [15–17].

Figure 6: PIA spectra of N719 adsorbed at a nanostructured TiO₂ film. (a) In air; (b) in the presence of electrolyte, 0.7 M LiI and 0.05 M I₂ in 3MPN. The spectra were recorded using blue light (460 nm) excitation (42 mW cm⁻²) with a modulation frequency of 9 Hz. With redox electrolyte (0.7 M LiI and 0.05 M I₂ in 3MPN); Hz.

The resulting PIA spectrum should reveal the spectrum of electrons in TiO₂ electrons in TiO₂ as well as triiodide in the electrolyte. The gradually increasing absorption with wavelengths larger than 600 nm can be attributed to electrons in TiO₂ and iodine radicals [13]. Electrons in nanostructured TiO₂ exhibit a very broad absorption and have an extinction coefficient of about 1000 M⁻¹ cm⁻¹ at 800 nm [18]. Triiodide does not absorb at wavelengths larger than 600 nm, while exhibits a broad absorption peak around 750 nm with an extinction coefficient of 2200 M⁻¹ cm⁻¹. Recently, it was shown in pulsed nanosecond laser studies that is an important long-lived intermediate in the dye-sensitized solar cell [17, 19]. The PIA spectrum of Figure 6(b) has contributions of both electrons and radicals.

Figure 7 shows typical PIA transient decay recorded at 750 nm. Pseudo-first-order rate constant for e-injection from dye excited state to TiO₂ conduction band was about 12 ms. This does not follow simple first-order kinetic, but is characterized by a range of injection times. This has been explained by trapping of electrons within the TiO₂ nanocrystals [16]. Similar pseudo-first-order rate constant of 11 ms has been measured using PIA transient decay for N719/TiO₂ for electron recombination [20]. This relatively fast decay proves at least a well pore filling of TiO₂ film by N719 dye molecules. The CdS/TiO₂ system shows its first-order rate constant for e-injection of 8.5 ms which is relatively faster than that of the N719/TiO₂ system (12 ms). This relatively faster kinetic process seen in CdS/TiO₂ system may reflect its faster process of high kinetic processes in the range of picoseconds already observed with this system [21]. For solar cell performance the pseudo-first-order rate constant under steady-state conditions is a relevant parameter, as it can give direct information on possible recombination losses due to the reaction of electrons with oxidized dye molecules.

Figure 7: PIA decay transient absorption of N719 dye-modified TiO₂  electrode in the absence of electrolyte after excitation with blue light (11 mW/cm²) recorded at 750 nm, using a sampling rate of 10³ s⁻¹ and averaged 100 times.

4. Conclusion

Photoinduced absorption spectroscopy where the excitation is provided by an on/off monochromatic light source can give direct information on electron-injection and hole-electron recombination rates using spectra of transient species, and their kinetics can be explored using time-resolved techniques. PIA can monitor slow processes and is cheaper compared to laser flash photolysis. The relatively faster kinetic process observed in CdS/TiO₂ system compared to N719/TiO₂ system could reflect its faster process of high kinetic processes in the range of picoseconds already observed.

Acknowledgments

The research project is funded by the National Plan for Science and Technology Program, Grant no. 09-NAN859-02, King Saud University, Riyadh, Saudi Arabia. Id. Bedja would like to thank King Abdullah Institute for Nanotechnology at King Saud University, Riyadh, Saudi Arabia, for a financial support. Thanks also go to Dr. Gerrit Boschloo for his helpful discussions.

References

B. O'Regan and M. Grätzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO₂ films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991.

A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, “Dye-sensitized solar cells,” Chemical Reviews, vol. 110, pp. 6595–6663, 2010.

M. Grätzel, “Recent advances in sensitized mesoscopic solar cells,” Accounts of Chemical Research, vol. 42, no. 11, pp. 1788–1798, 2009.

I. Bedja, S. Hotchandani, and P. V. Kamat, “Preparation and photoelectrochemical characterization of thin SnO2 nanocrystalline semiconductor films and their sensitization with bis(2,-bipyridine)(2,-bipyridine-4,-dicarboxylic acid)ruthenium(II) complex,” Journal of Physical Chemistry, vol. 98, no. 15, pp. 4133–4140, 1994.

I. Bedja, S. Hotchandani, and P. V. Kamat, “Electrochemical induced Fluorescence quenching and photocelectrochemical behavior of chlorophyll a-modified SnO2 films,” Journal of Applied Physics, vol. 80, no. 8, pp. 4637–4643, 1996.

T. A. Heimer, E. J. Heilweil, C. A. Bignozzi, and G. J. Meyer, “Electron injection, recombination, and halide oxidation dynamics at dye-sensitized metal oxide interfaces,” Journal of Physical Chemistry A, vol. 104, no. 18, pp. 4256–4262, 2000.

D. Liu and P. V. Kamat, “Electrochemically active nanocrystalline SnO2 films: surface modification with thiazine and oxazine dye aggregates,” Journal of the Electrochemical Society, vol. 142, no. 3, pp. 835–839, 1995.

R. Vogel, P. Hoyer, and H. Weller, “Quantum-sized PbS, CdS, Ag2S, Sb2S3, and Bi2S3 particles as sensitizers for various nanoporous wide-bandgap semiconductors,” Journal of Physical Chemistry, vol. 98, no. 12, pp. 3183–3188, 1994.

J. Rabani, “Sandwich colloids of ZnO and ZnS in aqueous solutions,” Journal of Physical Chemistry, vol. 93, no. 22, pp. 7707–7713, 1989.

I. Bedja, S. Holchandani, and P. V. Kamat, “Photosensitization of composite metal oxide semiconductor films,” Physical Chemistry Chemical Physics, vol. 101, no. 11, pp. 1651–1653, 1997.

A. Hagfeld and M. Grätzel, “Light-induced redox reactions in nanocrystalline systems,” Chemical Reviews, vol. 95, no. 1, pp. 49–68, 1995.

A. Hagfeldt and M. Grätzel, “Molecular photovoltaics,” Accounts of Chemical Research, vol. 33, no. 5, pp. 269–277, 2000.

G. Boschloo and A. Hagfeldt, “Photoinduced absorption spectroscopy of dye-sensitized nanostructured TiO2,” Chemical Physics Letters, vol. 370, no. 3-4, pp. 381–386, 2003.

G. L. Hug, “National Bureau of Standards, National standard reference data system,” NSRDS-NBS, vol. 69, p. 541, 1981.

S. Pelet, J. E. Moser, and M. Grätzel, “Cooperative Effect of Adsorbed Cations and Iodide on the Interception of Back Electron Transfer in the Dye Sensitization of Nanocrystalline TiO₂,” Journal of Physical Chemistry B, vol. 104, no. 8, pp. 1791–1795, 2000.

C. Nasr, S. Hotchandani, and P. V. Kamat, “Role of iodide in photoelectrochemical solar cells. Electron transfer between iodide ions and ruthenium polypyridyl complex anchored on nanocrystalline SiO2 and SnO2 films,” Journal of Physical Chemistry B, vol. 102, no. 25, pp. 4944–4951, 1998.

C. Bauer, G. Boschloo, E. Mukhtar, and A. Hagfeldt, “Interfacial electron-transfer dynamics in Ru(tcterpy)(NCS)3-sensitized TiO2 nanocrystalline solar cells,” Journal of Physical Chemistry B, vol. 106, no. 49, pp. 12693–12704, 2002.

G. Boschloo and D. Fitzmaurice, “Electron accumulation in nanostructured TiO2 (anatase) electrodes,” Journal of Physical Chemistry B, vol. 103, no. 37, pp. 7860–7868, 1999.

I. Montanari, J. Nelson, and J. R. Durrant, “Iodide electron transfer kinetics in dye-sensitized nanocrystalline TiO2 films,” Journal of Physical Chemistry B, vol. 106, no. 47, pp. 12203–12210, 2002.

G. Boschloo and A. Hagfeldt, “Photoinduced absorption spectroscopy as a tool in the study of dye-sensitized solar cells,” Inorganica Chimica Acta, vol. 361, no. 3, pp. 729–734, 2008.

P. V. Kamat, “Picosecond charge-transfer events in the photosensitization of colloidal TiO2,” Langmuir, vol. 6, no. 2, pp. 512–513, 1990.