Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 92 (2008) 1358– 1365

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature Elias Stathatos a,, Yongjun Chen b, Dionysios D. Dionysiou b, a b

Department of Electrical Engineering, Technological-Educational Institute of Patras, M. Alexandrou 1, GR-26334 Patras, Greece Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221– 0071, USA

a r t i c l e in f o

a b s t r a c t

Article history: Received 7 January 2008 Received in revised form 20 May 2008 Accepted 20 May 2008 Available online 27 June 2008

Quasi-solid-state dye-sensitized solar cells with enhanced performance were made by using nanocrystalline TiO2 films without any template deposited on plastic or glass substrates at low temperature. A simple and benign procedure was developed to synthesize the low-temperature TiO2 nanostructured films. According to this method, a small quantity of titanium isopropoxide (TTIP) was added in an ethanolic dispersion of TiO2 powder consisting of nanoparticles at room temperature, which after alkoxide’s hydrolysis helps to the connection between TiO2 particles and to the formation of mechanically stable thick films on plastic or glass substrates. Pure TiO2 films without any organic residuals consisting of nanoparticles were formed with surface area of 56 m2/g and pore volume of 0.383 cm3/g similar to that obtained for Degussa-P25 powder. The structural properties of the films were characterized by microscopy techniques, X-ray diffractometry, and porosimetry. Overall solar to electric energy conversion efficiencies of 5.3% and 3.2% (under 1sun) were achieved for quasi-solid-state dye-sensitized solar cells employing such TiO2 films on F:SnO2 glass and ITO plastic substrates, respectively. Thus, the quasi-solid-state device based on low-temperature TiO2 attains a conversion efficiency which is very close to that obtained for cells consisting of TiO2 nanoparticles sintered at high temperature. & 2008 Elsevier B.V. All rights reserved.

Keywords: Nanostructured TiO2 Dye-sensitized solar cells Low temperature Quasi-solid state

1. Introduction Almost two decades ago, dye-sensitized solar cells (DSSCs) were proposed as low-cost alternatives to the conventional amorphous silicon solar cells [1], owing to the simplicity of their fabrication procedures, practically under ambient conditions with mild chemical processes. The overall efficiency of 10.4% placed DSSCs as potential inexpensive alternatives to solid-state devices [2]. For the successful commercialization of DSSCs two important factors have to be taken into account: (a) the low-cost fabrication process, which is approximately 10% (with current technology) of that needed for silicon solar cells [3], and (b) device stability, with lifetime expectancies of at least 10 years for outdoor use [3,4], which is also comparable with that of amorphous silicon. To ensure the durability of the cell, it was considered necessary to replace the liquid electrolyte with quasi-solid state [5], solid [6], polymer electrolytes [7] or p-type inorganic semiconductors [8]. Moreover, the low manufacturing cost by using roll-to-roll coating

 Corresponding author. Tel./fax.: +30 2610 369242.  Also to be corresponded to. Tel.: +1 513 556 0724; fax: +1 513 556 2599.

E-mail addresses: [email protected] (E. Stathatos), [email protected] (D.D. Dionysiou). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.05.009

process creates the need of replacing the glass substrate with light-weighted flexible plastic electrodes, expanding this way the area of DSSCs’ applications. Flexible plastic electrodes like polyethylene terephthalate sheet coated with tin-doped indium oxide (PET-ITO) appear to possess many technological advantages (no size/shape limitations, low weight, high transmittance) as they present very low production cost in relation to F:SnO2 (FTO) conductive glasses. The use of such plastic substrates requires that all processes needed for the fabrication of DSSC, including the formation of TiO2 nanocrystalline films, to be designed at temperatures lower than 150 1C. In the direction of replacing the glass substrates with flexible plastics, mesoporous TiO2 films have to be prepared at low temperature and also with nanocrystalline dimensions for better efficiency to energy conversion [9–12]. So far, the methods that obtain the most efficient TiO2 films for DSSCs have been based on high-temperature calcination. Hightemperature annealing, usually at 450–500 1C, is necessary to remove organic material needed to suppress agglomeration of TiO2 particles and reduce stress during calcination for making crack-free films with good adhesion on substrates [13–15]. Besides, high-temperature treatment of films promotes crystallinity of TiO2 particles and their chemical interconnection for better electrical connection. Low sintering temperature yields titania nanocrystalline films with high-active surface area but

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relatively small nanocrystals with many defects and poor interconnection, thus lower conductivity [16,17]. High sintering temperature for TiO2 films is then the most efficient method for the preparation of high-performance DSSCs, but it is also a cost-intensive process. In addition, high-temperature treatment of TiO2 films cannot be applied to flexible plastic electrodes which in recent years emerge as an important technological quest. Different approaches appear in the literature to avoid hightemperature annealing of thick and porous TiO2 films. Among a variety of methods used for the low-temperature treatment of TiO2 films like hydrothermal crystallization [18], chemical vapor deposition of titanium alkoxides [19], microwave irradiation [20], ultraviolet light irradiation treatment [21] and sol–gel method [22], the efficiency of DSSCs employing ITO-PET substrates was in the range of 2–3% at standard conditions of 100 mW/cm2 light intensities at AM 1.5 [23]. Herein, we present a very simple and benign method for the formation of pure TiO2 nanoparticles in films without any further treatment (e.g., hydrothermal). This is an easy method for producing surfactant-free films of nanocrystalline TiO2 at room temperature with excellent mechanical stability when deposited on glass or plastic substrates for DSSCs. According to this procedure, a small amount of titanium isopropoxide (TTIP) is added to an alcoholic dispersion of commercially available P25-TiO2 (surface area of 55 m2/g, mean average particle size of 25 nm and 30/70% rutile/anatase crystallinity) powder. The hydrolysis of the alkoxide after its addition helps to the chemical connection between titania particles and their stable adhesion on plastic or glass substrate without sacrificing the desired electrical and mechanical properties of the film. In this study, we demonstrate, for the first time, the development of a completely quasi-solid-state DSSC prepared under ambient conditions. Although many reports have focused on one aspect of the cell, such as room temperature preparation of TiO2 films on flexible substrates [24] or solid electrolytes [25], there are no studies on the combination of DSSCs with flexible anodes employing quasi-solid-state electrolytes. We believe that the idea of a quasi-solid-state DSSC prepared at low temperature is quite attractive for industrial applications as it could bring merit to the cost reduction by manufacturing the entire cell through a roll-to-roll process. 2. Experimental 2.1. Materials Titanium (IV) isopropoxide (TTIP), was purchased from Fluka. Poly(propylene glycol)bis(2-aminopropyl) ether MW 230, 3-isocyanatopropyltriethoxysilane, 1-methyl-3-propylimidazolium iodide, potassium iodide, iodine, hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6) and all solvents were purchased from Aldrich and used as received. Titania powder P25 was provided by Degussa, (Germany, 30% Rutile and 70% Anatase) and Cisbis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato)-ruthenium (II) (RuL2(NCS)2–N3) was provided by Solaronix SA (rue de l’Ouriette 129, 1170 Aubonne VD, Switzerland). SnO2:F transparent conductive electrodes (FTO, TEC15) 15 O/sq were purchased from Hartford Glass Co., USA and indium-tin Oxide/poly(ethylene terephthalate) (ITO-PET) 60 O/sq from IST. Double distilled water (Mega Pure System, Corning, conductivity: 18.2 mS cm1) was used in all experiments. 2.2. Preparation of the TiO2 solution One gram of P25-TiO2 powder was added in 10 ml of ethanol followed by magnetic stirring to obtain a homogeneous disper-

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sion. No surfactants were used as templates to the solution. TTIP was mixed in the previous dispersion in various concentrations (varied from 0.017 to 0.18 M). After several minutes, the dispersion was ready to be used either on glass or plastic substrates. 2.3. Film preparation and materials characterization Films with effective surface area of around 1 cm2 were formed on glass substrate by dip coating (12.370.5 cm min1 withdrawal velocity) within 1 h after solution preparation by using an adhesive tape as a mask cut in the appropriate shape. After the adhesive tape was removed and the round area of the films was cleaned from any spoiled material, the remaining active area of our films was 0.8 cm2. Films could also be formed by casting but we chose the dip-coating method for better reproducibility of film properties. All the films were left to dry in air for 5 min and then were thoroughly rinsed with distilled water several times to wash out any loose material that could be detached from the rest of the film (i.e., as-prepared films). If necessary, to remove any humidity, the TiO2 films were left to dry in a programmable furnace (Paragon HT-22-D, Thermcraft) at 100 1C for 30 min and cooled down naturally. A Kristalloflex D500 diffractometer (Siemens) with CuKa (l ¼ 1.54 A˚) radiation was employed for X-ray diffraction (XRD) study of TiO2 crystallinity. A porosimetry apparatus (TriStar 3000, Micromeritics) was used to examine the structural characteristics of TiO2 material. The proper quantity of TiO2 particle samples (i.e., 0.1 g) was scraped from the TiO2 as-prepared film and was characterized. The film morphology was examined with scanning electron microscope (SEM, Hitachi S-4000) which was also used to measure film thickness. High-resolution transmission electron microscope (HR-TEM) with field emission gun at 200 kV was used to examine the film nanostructure. Samples were dispersed in methanol (HPLC grade, Pharmco) using an ultrasonic cleaner (2510R-DH, Bransonic) for 5 min and fixed on a carbon-coated copper grid (LC200-Cu, EMS). In addition, film roughness and nanoparticle morphology were further examined with atomic force microscopy (AFM). The images were obtained with a Digital Instruments Microscope in the tapping mode. 2.4. Fabrication characterization of dye-sensitized photoelectrochemical cell 2.4.1. Dye sensitization of TiO2 films The TiO2 films prepared by the above procedure on FTO glass or ITO-PET plastic substrates were immersed into an 1 mM ethanolic solution of RuL2(NCS)2 and were left there overnight. The dye-coated electrodes were copiously washed with ethanol and dried in a stream of nitrogen. Before their use in DSSC, the electrodes were left in the oven (100 1C) for 15 min to remove any ethanol or humidity that could be present in the pores of the films. 2.4.2. Preparation of gel electrolyte For the gel electrolyte applied to the DSSCs, we used a hybrid organic–inorganic material which was prepared according to the following procedure: poly(propylene glycol)bis(2-aminopropyl ether) of molecular weight 230 and 3-isocyanatopropyltriethoxysilane (ICS; molar ratio ICS/diamine ¼ 2) were mixed in tetrahydrofuran (THF) under reflux conditions (64 1C) for 6 h. Under such conditions, the isocyanate group of ICS reacts with the amino groups of poly(propylene glycol)bis(2-aminopropyl ether) (acylation reaction), producing urea connecting groups between the polymer units and the inorganic part. After evaporation of THF under vacuum, a viscous precursor was obtained, which was

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Mass (AM 0 and AM 1.5 direct) filters were used in combination to simulate AM 1.5 direct solar irradiance. The spectral output of Xe lamp was matched in the region of 350–800 nm with the presence of Prinz IR3 heat-reflecting sunlight filter (Prinz Optics, GmbH) so as to reduce the mismatch between the simulated and the true solar spectrum to o3%. The number of incident photons was calculated by employing a radiant power/energy meter (Oriel-70260). Photodiode detector was individually calibrated against NIST traceable standards. In addition, the calibration was corrected for different wavelengths. The accuracy in measurement was generally 73%. J– V curves were recorded by connecting the cells to a Keithley Source Meter (model 2601) which was controlled by Keithley computer software (LabTracer). Cell active area for these measurements was finally 0.8 cm2. Illumination intensity was varied between 12.5 and 100 mW/cm2 at 1.5 AM conditions by using wire grids in front of the lamp source. Cell performance parameters including short-circuit current density (JSC), opencircuit voltage (VOC), maximum power (Pmax), fill factor (ff) and overall cell conversion efficiency, were measured and calculated from the J– V characteristic curve.

stable at room temperature for several months [26,27]. The abbreviated name of the precursor used in the present work is PPG230, referring to molecular weight 230. The gel electrolyte was synthesized by the following procedure. The functionalized alkoxide precursor PPG230 of 0.7 g, shown in Scheme 1, were dissolved in 2.4 g of sulfolane (also in Scheme 1) under vigorous stirring. Then we added 0.6 ml glacial AcOH followed by 0.3 M 1-methyl-3-propylimidazolium iodide, 0.1 M KI and 0.05 M I2. 1-Methyl-3-propylimidazolium iodide was used in order to avoid crystallization of KI, while the presence of KI was necessary since these small mobile ions allow increase in ionic conductivity. After 6 h of stirring, one drop of the obtained sol was placed on the top of the titania electrode covered with the dye and a slightly platinized F:SnO2 or ITO-PET counterelectrode was pushed by hand on the top. The platinized FTO glass was made by casting few drops of H2PtCl6 solution (5 mg/1 ml of EtOH) followed by heating at 450 1C for 10 min. In the case of plastic substrates, the Pt thin layer was formed by thermal evaporation. The two electrodes tightly stuck together by–Si–O–Si–bonds developed by the presence of PPG230. 2.4.3. Photophysical and electrical characterization of the DSSCs The absorption spectrum of the dye was measured with a Cary 1E spectrophotometer. Incident photon-to-current efficiency (IPCE%) values were measured by illumination of the samples with a Newport 300 W Xenon lamp through a filter monochromator (Oriel-7155). The Xe lamp spectrum satisfactorily simulates solar radiation at the surface of the earth. A water filter with fused silica windows was used to cut infrared irradiation. Newport Air EtO EtO Si(CH2)3 EtO

3. Results and discussion 3.1. Titania film deposition on FTO glass and plastic electrodes In this work, we examined a novel procedure for synthesizing mesoporous TiO2 thick films at low temperature. Furthermore, this straightforward method gives the additional advantage of

O

O

NH C NH CH

CH2

[OCH2CH]n

CH3

NH C NH

(CH2)3Si

CH3

OEt OEt OEt

S O

PPG 230

Sulfolane

O Scheme 1

Table 1 Cell performances measured for both glass and plastic substrates for different TTIP concentrations measured under 100 mW/cm2 and AM 1.5 simulated solar light Substrate

TTIP (M)

TTIP:P25-TiO2 (Molar ratio)

Q (%)a

IPCE (%)b

Jsc (mA/cm2)

Voc (mV)

ffc

n (%)d

FTO glass

0.180 0.110 0.070 0.053 0.035 0.017

0.14 0.084 0.056 0.042 0.028 0.014

0 0 0 0 2.5 4.0

31 46 58 62 59 48

6.9 11.2 11.7 12.5 9.7 8.4

673 711 720 720 700 688

0.58 0.59 0.59 0.59 0.60 0.59

2.7 4.7 5.0 5.3 4.1 3.4

ITO-PET plastic

0.180 0.110 0.070 0.053 0.035 0.017

0.14 0.084 0.056 0.042 0.028 0.014

0 0 0 0 2.4 5.3

20 27 34 41 37 32

4.6 5.9 6.7 6.9 6.8 5.5

697 736 741 750 722 694

0.59 0.59 0.60 0.60 0.61 0.60

1.9 2.6 3.0 3.2 3.0 2.3

a

% mass loss after water rinsing of the film. Incident photon to current efficiency according to the equation: IPCE ¼ (1241(eV  nm)  J(mA/cm2))/(wavelength (nm)  P(W/m2)). c Fill factor: ff ¼ ((V– J)maxp)/(VOC–JSC). d Overall efficiency calculated from the equation: n ¼ (VOC  JSC  ff)/Plight. b

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fabricating nanocrystalline TiO2 films without surfactant templating, thus avoiding any organic content that limits electron diffusion and DSSC overall performance. We varied the concentration of TTIP in P25-TiO2 solution in the direction of less quantity needed for the application of stable films on glass or plastic substrates. We also examined the mass loss of each film after rinsing with pure water as a way of controlling the adherence of each film on any of the substrates. The TTIP/P25TiO2 molar ratio was varied from 0.05 to 1 (always the P25-TiO2 quantity was kept constant) and the results are summarized in Table 1. When the TTIP/P25-TiO2 molar ratio was less than 0.02, the adherence of the film on substrate was poor. The best adherence of the films was achieved for molar ratios in the range from 0.02 to 0.1. This result is expected as a certain amount of TTIP is needed for the interconnection of P25-TiO2 particles due to–O–Ti–O–network formation after alkoxide’s hydrolysis/ condensation in ambient humidity according to the reaction TiO2 þ nTiðORÞ4 þ 2nH2 O ! ðn þ 1ÞTiO2 þ 4nROH;

where R ¼ C3 H7 : Less alkoxide content resulted in poor film adhesion as a large fraction of the TiO2 film was peeled off the substrate. On the other hand, higher alkoxide content in the sol creates more amorphous TiO2 due to the presence of TTIP, and consequently limits the performance of the DSSC. In addition, the presence of alkoxide in high wt/wt percentage could cover the surface P25-TiO2 particles with amorphous phase and make it inactive. Therefore, it seems reasonable that the amount of TiO2 to be formed from TTIP has to be significantly much less than the amount of P25-TiO2 added as nanocrystalline powder dispersion in EtOH. The film on plastic or glass substrate was made by dip-coating. The adhesion of TiO2 film on both substrates was also examined by subjecting the films to ultrasonication. The behavior of films prepared with variable quantities of TTIP to ultrasonication was poor. Most of the part of the film was detached from any substrate tested. On the other hand, the durability of the same films in water rinsing (sometimes violently) was remarkable. The effect of water rinsing to the amount of TiO2 on the film for all TTIP molar ratios used in the sol is presented in Table 1. The results are reported as percentage of mass loss (Q%) detached from the as-prepared film after water rinsing and film drying. From the third column of Table 1, it is obvious that the mass loss, even at low concentrations of TTIP, was at very low values.

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3.2. Structural properties of the nanocrystalline TiO2 films FT-IR studies of the TiO2 film show the characteristics of the formation of high-purity material. The FT-IR spectrum of Fig. 1 clearly shows the peaks corresponding to TiO2. Peaks located in the area of 400–650 cm1 correspond to the vibration of Ti–O and Ti–O–O bonds. On the other hand, the absence of peaks at 1000 cm1 proves the absence of peroxo groups. It is noted that the examination of the film in IR was made on silicon wafer. It is also clear from data of Fig. 1 that there are no IR absorption peaks corresponding to impurities like organic residues,–CH and–CH2 in 1400–2900 cm1 and C–O–C in 1000–1500 cm1. Furthermore, FT-IR spectrum firmly suggests the presence of–OH groups, which is absolutely necessary for dye adsorption in DSSCs’ fabrication. Thus, the as-prepared TiO2 films after washing with pure water consisted of pure TiO2 particles without any significant organic contaminants, which is an important advantage in the evaluation of DSSCs’ efficiency made with these TiO2 electrodes. The film with the optimum TTIP content was examined with microscopy techniques to determine the homogeneity of the film (SEM), the surface roughness and the particle size (AFM). The shape, crystallinity and size of the particles were examined with HR-TEM. In Fig. 2a, SEM analysis at low magnification shows that the TiO2 film exhibits a uniform surface morphology without any

100

Transmittance %

80

-OH Adsorbed CO2

60 40 -OH 20 0 4500

Ti-O 4000

3500

3000

2500

2000

1500

1000

500

wavenumber (cm-1) Fig. 1. FT-IR spectrum of the as-prepared film on silicon wafer rinsed multiple times with water and dried at 100 1C. There are no organics (area of 1500–1000 cm1) and the Ti–O bond is clearly presented at short wavenumbers.

Fig. 2. (a) SEM image of the TiO2 film on FTO glass prepared at low temperature and (b) cross-section SEM image of the same film.

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cracks. The thickness of the film, which is an important parameter for the evaluation of DSSC efficiency constructed with this film, was measured to be 2 mm (Fig. 2b). Film thickness was not affected by TTIP concentration as this quantity even at its higher value, was very low compared to P25-TiO2 content. It is obvious that if we want to make thicker films we have to make multiple dip-coatings since each additional dip-coating layer adds 2 mm TiO2 film. Fig. 3a is a HR-TEM micrograph of TiO2 sample scraped from the support. It is interesting to note (according to Fig. 3b; higher magnification of the selected area of Fig. 3a) that the material is highly crystalline, as it is expected, and particles are connected between them. There is no indication for isolated particles, thus we believe that the formation of TiO2 film is accompanied by good

mechanical stability. Apparently, there is no measurable fraction of amorphous TiO2 phase because of the quantity of TTIP used was relatively low. The film also consists of nanoparticles with an average size of 25–30 nm in agreement with results obtained from AFM microscopy. The size of the particles according to AFM image presented in Fig. 4, varied from 25 to 30 nm as in the case of HRTEM but the roughness of the surface was relatively high. The XRD spectrum seen in Fig. 5 clearly shows that the crystal phase of the TiO2 in the as-prepared films was very similar to that obtained for pure P25-TiO2 powder. This is a result which was expected since most of the material in the film consisted of P25TiO2 powder. This result is also in agreement with HR-TEM image which shows the presence of crystallites with mean diameter of 25–30 nm. The as-prepared TiO2 films were also examined with N2 adsorption–desorption analysis in order to compare the textural properties of these films with those obtained for the commercial P25 powder. The measurements were carried out on powder which was carefully scratched from the thick films. The N2 adsorption–desorption isotherms of as-prepared TiO2 film with the optimum TTIP/P25-TiO2 molar ratio are presented in Fig. 6. The shape of the isotherms is a typical for mesoporous materials. For comparison, we present the same isotherms for the commercially available P25 powder. All samples were degassed for 2 h (at 105 1C) before N2 adsorption–adsorption analysis. The Brunauer, Emmett and Teller (BET) specific surface area and Barrett, Joyner and Halenda (BJH) pore volume of as-prepared TiO2 films were relatively high at 56 m2/g and 0.383 cm3/g, respectively, and almost the same for all the TTIP/P25-TiO2 molar ratios referred in Table 1. The pore size distribution plot (as an insert in Fig. 6) for room temperature prepared TiO2 film showed an average pore size of 27.3 nm. The pore size distribution is wider in the case of the as-prepared TiO2 which is due to the presence of TTIP. Basically, any TTIP/P25-TiO2 ratio (within the range studied) did not have any significant effect on the surface area and pore volume of the films.

800

600

400

200

0

Fig. 3. HR-TEM image of TiO2 film prepared at low temperature: (a) at low magnification and (b) at high magnification of the selected area.

200

400

600

0 800 nm

Fig. 4. AFM image of the as-prepared TiO2 film at low temperature. The surface of the film is relatively rough and the particle size varies between 25 and 30 nm.

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Quasi-solid-state DSSCs employing 2 mm thick TiO2 electrodes prepared at low temperature with optimum TTIP quantity were compared with those that were made with TiO2 films fabricated at high temperature [28]. The efficiency of the cells with the as-prepared films in room temperature was 23% lower than that of the cells with TiO2 sintered at high temperature. This result was expected as TiO2 particles treated at high temperature give improved charge collection efficiencies because of firm necking between them. However, the disadvantage of somewhat lower

3.3. Characterization of the dye-photoelectrochemical solar cell performance The as-prepared TiO2 films on FTO glass and plastic ITO-PET substrates were examined as negative electrodes in quasi-solidstate DSSCs. The corresponding characteristic J– V parameters are presented in Table 1. The maximum overall efficiency was obtained in the case of 0.045 M TTIP in the solution which maintains crystallinity of the film and also keeps amorphous TiO2 phase at very low values. The overall light to electric energy conversion efficiency of 5.3% at 1sun that was obtained in the present study is among the highest ever measured for room temperature prepared TiO2 films. The current–voltage (J– V) characteristic curves of quasi-solid-state DSSCs for the room temperature TiO2 films with optimum TTIP content prepared on glass and plastic conductive substrates are presented in Fig. 7.

13 12 (1)

11 Current Density (mA/cm)2

10

1000

800

Intensity

1363

600

9 8 7

(2)

6 5 4

Active area 0.8 cm2 1) ff=0.59 2) ff=0.6 Voc=720mV Voc=750mV Jsc=12.5mA/cm2 Jcs=6.95mA/cm2 n%=5.3 n%=3.2

3

400

2 200

1 0

0

0.0 20

25

30

35

40

45

50

55

0.1

0.2

60

0.3

0.4

0.5

0.6

0.7

Voltage (volt)

2 Theta angle (degree) Fig. 7. J– V curves at 1sun for the DSSCs containing the as-prepared lowtemperature nanocrystalline TiO2 film with quasi-solid-state electrolyte employing (1) FTO glass and (2) ITO-PET plastic substrate.

Fig. 5. XRD spectrum of TiO2 film prepared at room temperature (powder sample obtained from the coating and dried for 1 h).

300 0.010

250

Pore volume (cm3/g.nm-1)

Adsorbed volume (cm3/g,STP)

0.008

200

150

100

0.006

0.004

0.002

0.000 0

10

20

30

50

40

50

60

70

80

90

100

Pore diameter (nm)

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative pressure (P/Po) Fig. 6. Nitrogen absorption–desorption isotherms and pore-size distribution plots (insert) for low temperature prepared TiO2 film (-K-) and commercially available P25TiO2 powder (-’-).

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efficiency is overcome by the lower anticipated cost of preparation for the TiO2 electrode. Manufacturing processes which truly represent significant cost savings will only be realized when high speed, roll-to-roll coating operations are devised. This requires low cost, flexible polymer film, and therefore rapid, low-temperature deposition techniques for TiO2 films. Furthermore, gel electrolytes are a promising approach to combine the high-ionic conductivity of liquids while reducing the risk of leaks and minimizing sealing problems in DSSCs. The performance of the cell with optimum TTIP content was examined at different light intensities and the results for both glass and plastic substrates are presented in Table 2. When the light intensity increased, Jsc and Voc were also increased; in contrast ff decreased due to Ohmic losses in the conducting substrate [29]. The increase in the open-circuit voltage was expected and it is due to an increased charge generation rate resulting in an increased chemical potential within the cell [30]. The lower efficiency of plastic solar cells than that of cells made with FTO glasses was expected as the conducting material on the plastic is indium-tin-oxide instead of fluorine-doped tin oxide in the case of glass. F:SnO2 always gives better efficiencies to the dye-solar cell employing such kind of conducting material on top of the glass. Besides, the resistance of plastic conducting material is 60 O/sq instead of 15 O/sq in the case of F:SnO2. Consequently, the overall conversion efficiencies were remained close to 5.6 for glass and 3.4 for plastic substrates for a wide range of light intensities. Internationally accepted ageing tests have not yet been established for organic-based solar cells considering the stage of development of these cells [3]. As presented in Fig. 8, we performed stability tests for both DSS cells (on FTO glass and ITO-PET) prepared with room temperature titania films. The cells were all unsealed and were used without any further treatment after electrolyte was gelled. According to the data obtained from Fig. 8, both devices showed good stability when subjected to 1sun light irradiance from a Xe lamp as solar simulated light source. After 1000 h of light soaking (6 weeks) under open-circuit conditions there was a 10% and 12.5% drop of overall efficiency for cells employing FTO and ITO-PET, respectively. These results are in agreement to data obtained from literature [25,31,32]. Furthermore, three multiple samples for each device (on FTO glass or ITO-PET substrate) were tested resulting in reproducible results with slight deviations in short-circuit current density (70.06 mA/cm2), open-circuit voltage (710 mV) and overall efficiency (70.04%).

5.1 4.8 4.5 FTO substrate ITO-PET substrate

4.2 3.9 3.6 3.3 3.0 2.7 0

100 200 300 400 500 600 700 800 900 1000 Time (hours)

Fig. 8. Overall conversion efficiency evolution with time under successive 1sun illumination soaking for DSS cells prepared with FTO glass substrate (-’-) and ITO-PET plastic substrate (-K-).

electrodes in DSSCs. The TiO2 films exhibited good mechanical stability and high photoelectrochemical performances when they were applied to quasi-solid-state DSSCs. The preparation method for TiO2 electrodes was accomplished in a relatively short time comparing to that made at high temperature with comparable overall efficiencies. The ITO-PET and FTO DSSC yielded an energy conversion efficiency of 3.2% and 5.3% (1sun), respectively. As the 5.3% overall efficiency is absolutely comparable with that obtained for conventional quasi-solid-state devices with TiO2 electrodes prepared at high temperature, this fabrication method is advantageous to the reduction of process costs.

Acknowledgments The authors acknowledge Dr. V. Karoutsos for AFM and Dr. V. Drakopoulos for FE-SEM images. Prof. D.D. Dionysiou acknowledges financial support from the Center of Sustainable Urban Engineering (SUE) at the University of Cincinnati and also partial funding from the National Science Foundation through a CAREER award (BES, # 0448117). References

4. Conclusions This study presents a new method for the preparation of lowtemperature TiO2 nanocrystalline films for the fabrication of TiO2 Table 2 Cell performances of DSSCs on FTO glass and ITO-PET plastic substrates for TTIP:P25-TiO2 ¼ 0.042 molar ratio illuminated at various light intensities Substrate

Light Intensity (mW/cm2) Jsc (mA/cm2) Voc (mV) ff

n (%)

FTO glass

12.6 (1/8 sun) 25.1 (1/4 sun) 49.7 (1/2 sun) 99.8 (1 sun)

1.68 3.22 6.43 12.5

657 689 702 720

0.63 0.62 0.60 0.59

5.6 5.5 5.4 5.3

ITO-PET plastic 12.6 (1/8 sun) 25,1 (1/4 sun) 49.7 (1/2 sun) 99.8 (1 sun)

0.98 1.87 3.75 6.95

676 693 725 750

0.65 0.64 0.61 0.60

3.4 3.3 3.3 3.2

The film thickness was 2 mm.

5.4

n%

1364

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