Preparation and characterization of Eosin B- and Erythrosin J-sensitized nanostructured NiO thin film photocathodes

Thin Solid Films 490 (2005) 182 – 188 www.elsevier.com/locate/tsf

Preparation and characterization of Eosin B- and Erythrosin J-sensitized nanostructured NiO thin film photocathodes F. Veraa, R. Schreblera, E. Mun˜oza, C. Suareza, P. Curya, H. Go´meza, R. Co´rdovaa, R.E. Marottib, E.A. Dalchieleb,* b

a Instituto de Quı´mica, Universidad Cato´lica de Valparaı´so, Valparaı´so, Chile Instituto de Fı´sica, Facultad de Ingenierı´a, Universidad de la Repu´blica, Herrera y Reissig 565, C.C. 30, 11000 Montevideo, Uruguay

Available online 23 May 2005

Abstract Nickel oxide (NiO) thin films were prepared onto ITO/glass substrates by spin-coating, dipping and electrochemically. Studies of the morphological and structural properties of the films were done by atomic force microscopy (AFM). Photoelectrochemical and optical experiments were carried out in order to characterize the semiconductor properties of the nanostructured NiO thin films. The experiments were also done for Eosin B- and Erythrosin J-sensitized nanostructured NiO films, with the aim to visualize their potential application as photocatodes in tandem dye-sensitized solar cells (TDSSC). The NiO grown by dipping was the one presenting the best morphological properties. The photoelectrochemical results for all the bare NiO, NiO – Eosin B and NiO – Erythrosin J/electrolyte (I2/I ) systems showed a p-type behavior. An enhancement in the photocurrent has been observed for the systems sensitized with the dyes. For the NiO/Erythrosin J system the enhancement of the current under illumination in comparison to the dark current was about 200%. D 2005 Elsevier B.V. All rights reserved. Keywords: NiO; Dye sensitization; Photocurrent; Solar cell

1. Introduction About 13 years ago, an efficient dye-sensitized solar cell (DSSC) was first reported by O’Regan and Gra¨tzel [1]. This type of cell consists of one photoactive anode and one passive cathode in order to regenerate the redox species oxidized at the dye-sensitized nanostructured photoanode. With the aim to improve the efficiency and stability of the DSSC, replacement of this passive electrode with a dye-sensitized NiO photocathode (giving a photoelectrochemical dyesensitized tandem cell) was first presented by He et al. [2]. The theoretical upper limit of overall solar to electrical conversion efficiency for a cell with only one photocathode dye-sensitized electrode is around 30%, whereas the corresponding for a tandem device with two photoactive semiconductors is around 43% [3]. So, the substitution of

* Corresponding author. Fax: +598 2 711 1630. E-mail address: [email protected] (E.A. Dalchiele). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.04.052

the passive cathode with a photoactive one seems to be an evident and simple way to improve the efficiency of this type of solar cell. NiO was chosen as the material for the cathode, as it is one of the few simple metal oxides which are stable and ptype semiconducting. In fact, NiO is known as a large bandgap (3.6 –4.0 eV) semiconductor with a tendency to be of p-type as grown [2]. The dyes for the NiO sensitization that have been investigated by He et al. included tetrakis(4carboxyphenyl)porphyrin (TPPC) and Erythrosin B [2,3]. In order to continue the efforts to search for more and more efficient sensitizers for NiO photocathodes, two different dyes, Eosin B and Erythrosin J, have been studied in this work. To the best of our knowledge, there are no works in the literature regarding the use of these two dye sensitizers for NiO. Fig. 1 shows the molecular structures of the two dye molecules under study. The NiO nanostructured thin films were prepared by spin-coating, dipping and electrochemically. Study of the morphological and structural properties of the films was done by atomic force microscopy

F. Vera et al. / Thin Solid Films 490 (2005) 182 – 188

COOH O2N

COOH

NO2

HO

O Br

EOSIN B

O Br

183

I

I

OH

O I

O

I ERYTHROSIN J

Fig. 1. Molecular structures of Eosin B and Erythrosin J.

(AFM). Photoelectrochemical and optical experiments were carried out in order to characterize the semiconductor properties of the nanostructured NiO thin films. Moreover, these types of experiments were done also on Eosin B- and Erythrosin J-sensitized nanostructured NiO films, with the aim to visualize their potential application as photocatodes in tandem DSSC.

2. Experimental The porous nanocrystalline NiO thin film electrodes were prepared by the following three different thin film growth techniques, two chemical routes: dipping and spin-coating, and an electrochemical route: electrodeposition. The films were deposited onto Sn-doped In2O3 (ITO) coated glass substrates (DELTA-TECHNOLOGIES LIMITED, ¨ 10 V/g, and ca. 1.0 cm2 geometrical area ) that were first cleaned with soap, and successively washed in acetone, 2-propanol and deionized water in an ultrasonic bath. Afterwards, they were dried at room temperature. All solutions were prepared from deionized water (Millipore, 18.3 MV cm) and analytical reagents. 2.1. Dipping procedure A chemical route to prepare the NiO thin films was first explored: the dipping technique. Two aqueous precursor solutions were used: (A) 0.01 M Ni(NO3)2I6H2O and (B) 0.1 M NaOH, maintained at 25 -C. The dipping was conducted by alternatively and successively vertical immersion of the ITO substrates first in the precursor solution A and after that in the precursor solution B, for a total of 50 times. After deposition, substrates were washed with deionized water, dried and subsequently submitted to a heat treatment at 320 -C during 2 h in air atmosphere (Lindberg/BlueM, model tube furnace). 2.2. Spin-coating procedure For the development of this technique the preparation of a metal complex precursor of nickel is first necessary. Two

different solutions have been prepared: (A) 2.5 g of Ni(NO3)2I6H2O in 30 ml of deionized water, and a ligand solution (B): 2.78 g of benzoylacetone in 70 ml of methanol. Afterwards, under agitation, solution A was poured into solution B, and several drops of a 5 M ammoniacal solution was added in order to saturate the solution and then the Ni complex precipitated. Then, the precipitate was filtered and washed with deionized water in a Bu¨chner funnel and then dried at 50 -C during 24 h. Thereafter the complex was submitted to a purification process with dichloromethane, filtered and leaving the solvent to evaporate. In this way a Ni(II) phenyl methyl-h-diketonate (Ni[PhCOCHCOCH3]2) complex has been obtained [4], which has been proved to present a good solubility in methanol. Afterwards, a 0.75 mM solution of the nickel complex in methanol has been prepared (solution C). In order to prepare the NiO thin films by spin-coating, the cleaned ITO/glass substrate was placed in the center of the bench-top spinner. Then, one drop of the Ni complex solution C was dispersed onto the glass substrate and left to repose for 1 min, and then the spinner was rotated at a speed of 2400 rpm during 15 s, allowing the solution to spread. This procedure was applied two times. Later, the complex film system was irradiated with UV light (UV lamp, UVP, model UVS-28, 254 nm, 8 W) for 24 h. The film was then submitted to a heat treatment of 250 -C during 30 min in air. Finally, this whole procedure was applied 10 times for each sample. 2.3. Electrodeposition The electrodeposition was carried out in a three-electrode electrochemical cell, in which the ITO/glass substrate was the working electrode, and a Pt wire and a Hg/Hg2SO4 (Mercurous Sulfate Electrode, MSE, E MSE- = + 656 mV vs. NHE) as counter electrode and reference electrode, respectively, were used. The electrolyte was a 0.01 M Ni(NO3)2 solution, pH = 10, saturated with oxygen and thermostated at 25 -C. The ITO/glass substrate was held at a cathodic potential of 1.1 V during 3 h, under an oxygen atmosphere. In these conditions, the reduction of molecular O2 to OH takes place, leading to the formation of a Ni(OH)2 film onto the substrate. Afterwards, these films

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hν (eV)

were submitted to an annealing of 320 -C for 15 min in an air atmosphere.

2

50

3

4

2.4. Dye sensitization of nanostructured NiO thin films

2 25

2.5. Characterization setup Optical response of the resulting oxide films was studied by absorption spectroscopy in the 200 – 800 nm spectral regions. The spectra were obtained by means of an UV – Vis Perkin-Elmer spectrophotometer model Lambda EZ-201 using a bare ITO glass substrate as a reference. Morphology studies of nanostructured NiO thin films were performed ex situ with a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA, USA) in tapping mode.

0

Absorbance (Arb. Units)

Eosin B and Erythrosin J dyes were purchased from MERCK, and were used without purification. Sensitization by adsorption of dye onto the NiO nanostructured surfaces was carried out by soaking the films in a 10 mM eosin B or Erythrosin J ethanolic solutions for 1 to 4 days at room temperature. Excess dye was removed by subsequent rinsing with deionized water.

2

3

4

30

(αl . hν)2 [eV2]

4

(a)

1.0

(b)

0.5

15 0.0 2

3

4

15 10

0.30

(c)

0.15 0.00

5 0 300

2 400

500

3 600

4 700

Wavelength (nm) Fig. 2. Absorption spectra and bandgap energy determination (insets) for: (a) Ni(OH)2 (thin line) and NiO prepared by dipping (thick line), (b) NiO prepared by spin-coating, and (c) electrodeposited NiO.

2.6. Photoelectrochemical measurements The photoelectrochemical measurements were carried out in a conventional three-electrode photoelectrochemical cell, with a Pt counter electrode, a MSE reference electrode and the nanostructured NiO film as the working electrode. The redox electrolyte was composed of 0.5 M KI and 0.003 M I2 in a methanol/propylene carbonate mixture (1:3 by volume), pH = 7.8. Prior to each experiment, the cell solution was purged for 30 min by bubbling purified argon through it; argon was continuously passed over the surface solution during experiments. The cell was connected to a BAS-ZANNER IM6 model potentiostat. Optical excitation of the electrodes was obtained using a 75 W halogen lamp, in combination with a mechanical chopper. The Mott – Schottky plots were obtained from electrochemical impedance spectroscopy (EIS) measurements, using the IM6 model BAS-Zhaner equipment interfaced with a PC. A typical three-electrode electrochemical cell, similar to that described above, was used to do the EIS measurements.

3. Results and discussion 3.1. Optical characterization of the NiO thin films For the oxides optical characterization and bandgap energy determination, the absorption spectra in the visible and UV region were measured and are shown in Fig. 2. It is seen that the NiO has absorption maxima close to k = 300

nm [5] for the three different techniques used for its preparation. This UV absorption is assigned to electronic excitation from the valence states to the conduction band (i.e. above bandgap); meanwhile at the visible spectrum region the semiconductor is almost transparent [2,5,6]. Fig. 2a shows absorption spectra for Ni(OH)2 (thin line) and NiO (thick line) prepared by the dipping technique. They are almost similar in the visible region, but the Ni(OH)2 has not such a strong absorption at the UV region as the NiO has. Such differences among spectra imply that the thermal treatments were appropriately done. Actually, this difference confirms that the NiO is obtained by the dehydration of the nickel hydroxide. In transition metal oxides, as NiO, the electronic structure is highly influenced by strong correlation effects [5,7 – 10] and for that reason is classified as a chargetransfer insulator [11]. In spite of that, for all spectra shown in Fig. 2, the absorption onset can be assimilated as a transition from a direct band edge [12]; then the bandgap energy can be estimated as it was done at the insets of Fig. 2. Thus the bandgap energy estimated for the films prepared by the chemical methods were 3.69 eV for the dipping technique and 3.66 eV for the spincoating technique. Meanwhile the electrochemical route resulted in a bandgap energy of 3.60 eV. All these values are in the range of 3.6 – 4.0 eV which are usually reported in the literature [2,3,5,6,13]. Moreover, the discrepancies in the values for the bandgap energies deduced previously for the different deposition techniques follow the absorption peak position closely. This fact confirms the bandgap energy determination method.

F. Vera et al. / Thin Solid Films 490 (2005) 182 – 188

3.2. Morphological study of the NiO thin films Surface topography of the NiO thin films were analyzed by AFM. Fig. 3 shows typical AFM images taken of NiO samples obtained by the three different growth techniques studied in this work. For the sake of comparison the AFM of an ITO glass substrate is also shown. In general, the films present a very high covering degree, homogeneity and a very good adherence. Moreover, they also present nanostructured morphology. Typical particle size and root mean square (RMS) roughness values obtained from these AFM images, for the three NiO electrodes under study, are presented in Table 1. The NiO films prepared by dipping and spin-coating are those presenting lower particle sizes, ‘‘Q particles’’ (particles with sizes between 5 and 25 nm [14]). Therefore, constituting these nickel oxide films a mesoporous structure, being the main factor in order to obtain high efficient dye-sensitized solar cells. However, the nanostructured NiO films obtained by dipping were those presenting the highest rugosity (as demonstrated by the highest RMS roughness value), so we have chosen the

5.00

185

Table 1 Particle size and root mean square (RMS) roughness values obtained by analysis of AFM images of Fig. 3 for ITO substrate and NiO thin films grown with different techniques Growth technique

Particle size (nm)

RMS roughness (nm)

Bare ITO Dipping Spin-coating Electrodeposition

100 14 7 43

4 17 9 7

dipping coating method as the main preparation technique for the remaining part of the present work. 3.3. Optical properties of the dye-sensitized NiO electrodes For the research of a dye-sensitized cathode the chemical adsorption of a dye into the semiconductor is required, to improve the optical absorption at the visible region. Two different dyes were studied: Eosin B and Erythrosin J, whose optical absorption maxima lie at k = 526 nm and 532 nm, respectively.

5.00

100.0 nm

250.0 nm

Height

(b)

(a)

125.0 nm

50.0 nm

0.0 nm

2.50

0

2.50

0 5.00

2.50

2.50

250.0 nm

Height

0.0 nm

0 5.00

5.00

250.0 nm

(d)

(c) 4.00 125.0 nm

125.0 nm

3.00 0.0 nm 2.50

0.0 nm

2.00

1.00

0 0

1.00

2.00

3.00

4.00

2.50

0 5.00

Fig. 3. AFM images of: (a) ITO substrate; and NiO nanostructured thin films obtained by: (b) dipping; (c) spin-coating; and (d) electrodeposition.

F. Vera et al. / Thin Solid Films 490 (2005) 182 – 188

(a)

(b)

12

6

0 400

500

600

500

600

Wavelength (nm) Fig. 4. Absorption spectra comparison for different ethanol solution dyes (dotted line) and dye-sensitized NiO (thick line): (a) Eosin B and (b) Erythrosin J.

Fig. 4 shows the optical absorption spectra for each dye in an ethanol solution and the corresponding dye-sensitized NiO. The optical absorption increase (of the dye-doped NiO with respect to the undoped semiconductor, i.e. pure NiO with no dye in its pores) is very low in both cases. However, comparing the dye in ethanol solution against the dyesensitized NiO, there is a clear increase of the optical absorption width (broader spectra) in both cases. Moreover, the optical absorption of the NiO/Erythrosin J system is the highest one (see Fig. 4b), together with a slight red shift of the optical absorption maximum with respect to the one corresponding only to the Erythrosin J in ethanol solution. The broadening of the optical absorption spectra and its eventual red-shifting are indications of adsorption interaction between the dye and the semiconductor surface [2]. Then the chemical adsorption is more important in the case of the Erythrosin J. The low dye to oxide adsorption may be due to the low porosity in the morphology of the NiO films. Moreover, taking in consideration that the adsorption of the dye to the oxide surface occurs through the carboxyl groups, it also may be due to the fact that the sensitizing dye molecules under study have only one carboxyl group within its structure. 3.4. Photoelectrochemical properties of the dye-sensitized NiO electrodes The current– voltage characteristics of a bare nanoporous NiO electrode recorded voltammetrically in a threeelectrode photoelectrochemical cell under chopped visible light are shown in Fig. 5a. The illumination was periodically interrupted to show both the light current and dark current densities (the difference j light j dark is just the photocurrent density j ph). The cathodic sense of the current under illumination is a very clear indication of ptype behavior for this nanoporous NiO electrode with an onset of photocurrent at ca. 0.2 V vs. MSE. However, only an almost negligible photocurrent density ( j ph å 0.06

AA/cm2) was observed, since visible light (majority composed of sub bandgap energy, hm < Eg) cannot excite the semiconductor. The NiO prepared by dipping also shows the largest bandgap of 3.69 eV. In spite of this, it presents a small visible light absorption in the 400– 550 nm range as can be seen in Fig. 2a. This small absorption may be responsible (together with some component of light with hm > Eg) of the photocurrent observed under visible light illumination. The cathodic dark current can be related to reactions with the solvent as well with the redox couple in solution. Under similar illumination condition but with the dye-sensitized NiO electrodes, significant photocurrents can be observed, see Figs. 5b and c. The p-type behavior is again unambiguously demonstrated. The cathodic photocurrents are now much higher, approximately 0.4 AA/cm2 and 0.8 AA/cm2 for the Eosin B- and Erythrosin J-sensitized NiO electrodes, respectively. Therefore, this enhancement in photocurrent must be attributed exclusively to charge carrier injection by photoexcited dye molecules. For instance, the increase in percentage of the current under illumination with respect to the dark current, at a potential of 0.350 V vs. MSE, is about 20% for the bare NiO, and 60% and 200% for 0.0 Light off

(a) -0.4

Light on

-0.8 0.0

(b) j (µA.cm-2)

Absorbance (Arb. Units)

186

Light off

-1.5 Light on

-3.0 -4.5 0.0

(c)

Light off

-1.5 Light on -3.0 -4.5 -0.5

-0.4

-0.3

-0.2

E ( V vs. MSE) Fig. 5. Current density vs. potential curves for dipping-obtained nanostructured NiO films in the redox electrolyte comprised of 0.5 M KI and 0.003 M I2 in a methanol/propylene carbonate mixture (1:3 by volume) using intermittent light. (a) bare NiO; (b) Eosin B-sensitized NiO and (c) Erythrosin J-sensitized NiO thin films. Scan rate was 5 mV/s. The illumination was a visible broad band 75 W halogen lamp.

F. Vera et al. / Thin Solid Films 490 (2005) 182 – 188

(A/C)2(cm4µF-2)

0.020

0.015

0.010

Pt

Dye

p-NiO -4 ECB -3

*

D /D

-2

+

e

-1

-

hν

0 h

+

1 2

EVB

-

I 3 /I D/D

-

e

-

+

Electrolyte LOAD e-

Fig. 7. Schematic band energy representation illustrating the dye-sensitized p-NiO solar cell. The Erythrosin J dye (D) has been taken as an example.

decrease of ca. 10 –15% in the photocurrent has been observed.

4. Conclusions Nanocrystalline nanoporous NiO thin films have been obtained by dipping and spin-coating after a post-growth heat treatment. Bandgap energies of 3.65 T 0.05 eV were obtained for all samples prepared by the different techniques. The adsorption of the Erythrosin J to the NiO prepared by dipping was better than the one presented by the Eosin B. Photoelectrochemical measurements showed a p-type behavior for the bare NiO films and for the Eosin B- and Erythrosin J-sensitized NiO electrodes. A very clear enhancement of the photocurrent for the NiO sensitized electrodes with respect to the bare NiO ones has been observed. A 60% and a 200% increase of the current under illumination with respect to the dark current have been observed for the Eosin B- and Erythrosin J-sensitized NiO electrodes, respectively. It is expected that the combination of these types of photocathodes with a wellmatched photoanode couple in a tandem solar cell will lead to a significant increase of the overall conversion efficiency.

Acknowledgments

0.005

0.000

-5

E (V vs. MSE)

the Eosin B- and Erythrosin J-sensitized electrodes, respectively. This larger percentage value of Erythrosin J than Eosin B may be due to its better adsorption to the nanostructured NiO, as deduced from the optical properties of the dye-sensitized NiO system. Fig. 6 shows the Mott –Schottky plot of the bare NiO/ electrolyte junction. This plot justifies the photocathodic performance of the nanostructured NiO electrodes (p-type NiO behavior), in the photoelectrochemical measurements previously discussed. Moreover, an extrapolated flatband potential of å 130 mV vs. MSE has been obtained, which is in agreement with the previously reported results [2,15]. Furthermore, in order to explain and visualize the processes involved in the photocurrent generation, the energy level diagram of a dye-sensitized nanostructured pNiO solar cell, in which we take the energetic of Erythrosin J as an example, is shown in Fig. 7. This diagram has been constructed from the NiO flatband potential value obtained from Fig. 6, and the redox potential of the dye (determined from a cyclic voltammetry experiment of an Erythrosin J solution, results not shown). It can be seen that for the Erythrosin J, the HOMO level is located below the energy level of the top of the valence band, while the LUMO level is above the energy level of the redox system I3 /I , and well below the bottom energy level of the NiO conduction band. Then the energy level diagram clearly demonstrates that both processes, (i) electron transfer from the excited dye to the oxidized species (I3 ) in the electrolyte and (ii) regeneration of dye by hole injection from HOMO level into the valence band, are thermodynamically feasible, further explaining the origin of the observed cathodic photocurrent. Another very important point to be taken into account in the development of DSSC is the cell stability. The adsorbed dye molecule should be stable enough in the working environment (at the semiconductor – electrolyte interface), to sustain several years of operation maintaining non-degrading performance. In our case, for both studied systems and after a series of measurements carried out during 45 days, a

187

EFB=+0.130 V -0.6

-0.4

-0.2

0.0

0.2

0.4

E (V vs. MSE) Fig. 6. Mott – Schottky plot of a bare NiO electrode in the redox electrolyte comprised of 0.5 M KI and 0.003 M I2 in a methanol/propylene carbonate mixture (1:3 by volume), pH = 7.8. Measurement frequency: 2 kHz.

The authors are thankful to FONDECYT-Chile for financial support of this work (proyect No: 1040658) and are also grateful to DGI-Universidad Cato´lica de Valparaı´so, Chile. E.A.D. and R.E.M. would like to thank CSIC— Universidad de la Repu´blica and PEDECIBA-Fı´sica (Uruguay) for financial support. C.S. and E.M. specially thank CONICYT for their Doctoral Scholarships. Special thanks to Sara Green for the revision of the manuscript.

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