Photoanodes Consisting of Mesoporous Anatase TiO2 Beads with

Article pubs.acs.org/JPCC

Photoanodes Consisting of Mesoporous Anatase TiO2 Beads with Various Sizes for High-Efficiency Flexible Dye-Sensitized Solar Cells Chun-Ren Ke, Li-Chieh Chen, and Jyh-Ming Ting* Department of Materials Science and Engineering, National Cheng Kung University, Tainan, 70101, Taiwan ABSTRACT: The synthesis and characterization of TiO2 beads and the use of them in the photoanodes of all-plastic flexible dye-sensitized solar cells (FDSCs) are reported. Pure anatase TiO2 beads having different sizes and characteristics were first made using a novel two-step chemical method under different conditions. Photoanodes consisting of the beads as scattering layers were then fabricated. The use of beads largely enhances the dye loading and gives highly effective light scattering, leading to improved light absorbance. The resulting cells were evaluated for the electron diffusion time, electron lifetime, charge collection efficiency, incident photon-to-electron conversion efficiency, electron injection efficiency, and IV characteristics. The pure anatase TiO2 beads, having low oxygen vacancy concentrations and directional attached grains, lead to more photoelectrons and enhance the electron diffusion, giving very short diffusion times. We have demonstrated for the first time that the use of beads, having diameters ranging from 250 to 750 nm, enhances the light-to-electricity conversion efficiency of FDSCs having plastic substrates by as much as 28%. The cell conversion efficiency is also enhanced from 4.3 to 5.5%.

1. INTRODUCTION Dye-sensitized solar cell (DSC) is a promising power source on account of its low cost, facile fabrication, and relatively high efficiency (∼11%).1−3 Flexible DSC (FDSC), having plastic substrates, is currently attracting much attention due to not only the increasing need for flexible electronics but also the fact that the large-scale roll-to-roll process can be applied for cell fabrication.4−6 However, the fabrication temperature of TiO2 photoanodes on plastic substrates is limited to 150 °C, giving poor particle connections. Since one of the deciding factors affecting the cell performance is the photoanodes,7−9 various approaches have been investigated in an attempt to resolve this issue.10−14 For example, an approach to improve the connections among TiO2 nanoparticles and to improve the adhesion between TiO2 photoanode and substrate is the mechanical compression technique.14 In the meantime, nanoparticles of TiO2 are used for the fabrication of photoanodes to obtain nonporous structures having high surface areas. Therefore, the photoelectrons need to pass thought numerous grain boundaries to reach the transparent conductive-oxide (TCO) substrate; also the transport of electrolyte is limited owing to the irregular pores in the nonporous photoanode.15 As a result, the so-called TiO2 beads were developed and demonstrated for use in rigid DSCs (RDSCs).16 The beads sizes are large enough to produce Miescattering. The beads are composed of nanoparticles that are ideal for dye-loading. Also, the specific bead structure results in the formation of regular mesopores. Ordered and large pores are desired for the mass transport of redox species.15 There have been several studies on using such TiO2 beads in, exclusively, RDSCs. TiO2 beads were synthesized using a onepot solution method followed by high temperature calcination.17 The obtained beads were used as scattering layers for in RDSCs. The resulting cell efficiency was enhanced by about © 2012 American Chemical Society

30% as compared to cells using P25 photoanodes. TiO2 beads were used to replace 400 nm TiO2 powders as scattering layers, leading to a 12% increase in the cell efficiency.18 The beads were synthesized using a sol−gel process followed by solvethermal process and an additional calcination at the end.16 A 40% efficiency increase was obtained by replacing P25 with TiO2 beads synthesized using a diglycol-mediated process followed by a hydrothermal process.19 TiO2 beads were fabricated into photoanodes for use in RDSCs by a soft template based approach.20 Common in these studies is that they all focused on the synthesis and characterizations of TiO2 beads, followed by investigating the resulting cell performance. However, from bead synthesis to cell assembly, there is fabrication of photoanode whose characteristics are decisive. These include, for example, optical properties, electron transport, and the effect of bead size on the characteristics. Unfortunately, little or no attention has been paid to this important respect. Also, as mentioned above, TiO2 beads have only been used in RDSCs so far and have not yet been demonstrated in FDCSs. As a result, we have fabricated TiO2 beads with different sizes for the fabrication of photoanode on plastic substrates. The optical, electrical, and electrochemical properties of the resulting photoanodes are first examined. The effect of bead characteristics on the photoanode performance is presented and discussed. This is important since the understanding of such effect would lead to the fabrication of desired beads, especially for use in FDSC. It is also noted that the beads are synthesized using a novel two-step chemical method. The obtained beads do not have to undergo multifarious postReceived: August 23, 2011 Revised: January 6, 2012 Published: January 9, 2012 2600

dx.doi.org/10.1021/jp208116s | J. Phys. Chem. C 2012, 116, 2600−2607

The Journal of Physical Chemistry C

Article

Figure 1. SEM images of the products obtained from (A) the sol−gel and (B) 120 °C and 6 h hydrothermal processes. The amount of hexamine was 0.75 g. (The scale bars are 100 nm.).

photoanodes were also fabricated. The latter consisted of 3 and 5 μm thick layers. The photoanodes were immersed in an ethanol solution of cis-bis(isothio-cyanato)bis(2,20-bipyridyl4,40-dicarboxylato)-ruthenium(II)bis- tetrabutylammonium (N719) (5 × 10−4 M) overnight for dye loading. Cells were assembled using Pt-coated PEN as the counter electrode and an electrolyte consisting of 0.1 M LiI, 0.05 M I2, 0.6 M 1,2dimethyl-3-propylimidazolium iodide (DMPII), and 0.5 M 4tert-butylpyridine (TBP) in 3-methoxypropionitrile (MPN) system. 2.3. Characterization. The crystalline phase of TiO2 beads were identified by X-ray diffraction (XRD) (Rigaku D-max, Cu Kα 0.154 nm), and high-resolution transmission electron microscopy (HRTEM) (JEOL JEM 2100F). The particle size and morphology were examined using both scanning electron microscope (SEM) (JEOL 6701F) and HRTEM. A α-step was used to determine the thickness. Specific surface area, pore diameter, and pore volume were determined by the Brunauer− Emmett−Teller (BET) (Micromeritics ASAP2010, 77 K) method. The surface chemistry of TiO2 photoanodes was examined using X-ray photoelectron spectrometer (XPS) (PHI 5000 VersaProbe X, referenced to the C1s signal with binding energy 284.6 eV). The amount of dye loading and the light absorbance of the photoanodes were examined using UV− visible spectroscopy (UV−vis) (PerkinElmer LAMBDATM 950 UV/vis/NIR). Cell performance was evaluated using a solar simulator (100 mW/cm2, the equivalent of one sun at AM1.5G) and incident photon-to-electron conversion efficiency (IPCE) was conducted under 10 mW/cm2 light in the range of 400 to 800 nm. The electron transports were investigated using electrochemical impedance spectroscopy (EIS) (AutoLab, 100 mW/cm2 AM1.5G and bias frequency range ranges from 0.05 to 105 Hz) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) (XPOT Zahner, 15 mW/cm2 and 455 nm light and bias light frequency ranges from 0.1 to 105 Hz).

treatment processes due to the use of a steric agent that can be easily removed by water. Finally, FDSCs are made and evaluated. The effect of photoanode characteristics on the cell performance is presented and discussed. Furthermore, we demonstrate for the first time that the use of beads, having diameters ranging from 250 to 750 nm, enhance the light-toelectricity conversion efficiency of FDSCs having plastic substrates.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Mesoporous Anatase TiO2 Beads. Mesoporous TiO2 beads were made using a two-step method, involving a sol−gel and a hydrothermal process. The sol−gel solution was first prepared by adding x grams (x = 0.25−0.75) of hexamine in 200 mL of anhydrous alcohol to serve as the steric and surfactant agent. The solution was then mixed with 1 mL of KCl (0.1 M). Subsequently, 4.4 mL of titanium(IV) isopropoxide (TTIP) was poured into the mixture. Powders precipitated from the mixture were therefore obtained at the completion of the sol−gel process. These particles were mixed with deionized (DI) water and subjected to a hydrothermal process at various temperatures including 120, 160, and 200 °C for 6 h. The resulting powders were then characterized and used for the fabrication of photoanodes without any postprocess calcination. 2.2. Preparation of FDSCs. TiO2 beads and commercial P25 TiO2 powders were both used for the fabrication of FDSCs. Paste consisting of TiO2 powders (0.6 g), acetic acid (100 μL), DI water (0.5 mL), ethanol (2.5 mL), and t-butanol (4 mL) was prepared by mixing the constituents. The paste of P25 was first coated on indium tin oxide (ITO)-coated polyethylene naphthalate (PEN) by spin coating followed by the paste of beads. The dimensions of the ITO-coated substrate are 2 cm × 2 cm × 200 μm and have a sheet resistance less than 15 ohm/m2. The two-layer coatings thus formed were and then subjected to mechanical compression without any heat treatment to obtain photoanodes. As a result, the bead layer served as a scattering layer. The thicknesses of the P25 and bead layers were 3 ± 0.15 and 5 ± 0.25 μm, respectively. The thickness was controlled by the spinning speed, the spinning time, and the known linear shrinkage after compression. To ensure the desired thicknesses were obtained, all the photoanodes thus made were screened to reject the ones having undesired thicknesses. The selection of the thickness was predetermined. For comparison, 3 and 8 μm thick pure P25

3. RESULTS AND DISCUSSION The particles obtained from the sol−gel process are basically spherical, as shown in Figure 1A for that obtained at a hexamine amount of 0.75 g. The average diameter of these particles is 375 nm. After the hydrothermal process, beads were obtained as shown in Figure 1B for that obtained at a hydrothermal temperature of 120 °C. The average diameters are 375, 750, and 500 nm for the beads obtained at 120, 160, 2601



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Table I. Result of XPS and BET Analysis for Various TiO2 Powdersa bead

hydrothermal temp (°C)

grain size (nm)

bead size (nm)

Ti4+ (%)

Ti3+ (%)

Ti4+/ Ti3+

BET surface area (m2/g)

average pore diameter (nm)

total pore volume (cm3/g)

c d e f

(P25) 120 160 200 200

14 18 20 20

375 750 500 250

54.9 65.1 90.6 93.3 93.4

45.1 34.9 9.4 6.7 6.6

1.22 1.87 9.64 13.93 14.15

54 95 69 62 63

9.70 11.6 16.0 16.1 16.5

0.13 0.27 0.27 0.25 0.26

a

The powder and grain sizes determined on the SEM image and the Scherrer equation, respectively, are also given. The hexamine amount used in the sol−gel process is 0.75 g for beads c−e and 0.50 g for bead f.

and 200 °C, respectively. The bead size depends not only on the temperature but also is influenced by the ζ potential such that the correlation between the bead size and temperature is weak.21−23 On the other hand, the average diameter of beads increases with the amount of hexamine used in the sol−gel process. For hexamine amounts of 0.25, 0.50, and 0.75 g, the average diameters of the powders obtained at 200 °C are 20, 250, and 500 nm, respectively. It is noted that the powders obtained using 0.25 g of hexamine are actually nanoparticles. Beads were only obtained when the hexamine amounts were 0.50 and 0.75 g. Nevertheless, we have fabricated beads with various sizes, as shown in Table I, in the range of several hundreds of nanometers, which are appropriate for the occurrence of Mie scattering in the range of N719 absorption spectrum, as to be discussed further later. XRD patterns indicate that the powders obtained from sol− gel process are amorphous as shown in Figure 2A. After the

observed, as shown in Figure 3. The sizes of the nanoparticles shown in parts A−C of Figure 3 are 13, 18, and 21 nm.

Figure 2. XRD patterns of the products obtained from (A) the sol− gel, (B) 120 °C, (C) 160 °C, and (D) 200 °C hydrothermal processes. The amount of hexamine was 0.75 g, and the hydrothermal time was 6 h.

hydrothermal process, the amorphous powders become essentially anatase TiO2 beads, as shown in parts B−D of Figure 2 for samples obtained at 120, 160, and 200 °C. The anatase structure was also confirmed by Raman analysis. It was also found that the full width at half-maximum (fwhm) of the (101) peak decreases with the temperature, indicating that the TiO2 grain size increases with the temperature. Furthermore, the diffraction peak intensity increases with the temperature, suggesting better crystallinity at a higher temperature. The average grain sizes estimated based on the Scherrer equation are 14, 18, and 20 nm for 120, 160, and 200 °C hydrothermally synthesized TiO2, respectively. The grain sizes are summarized in Table I. The calculated grain sizes are quite similar to that

Figure 3. The HRTEM images of beads obtained at (A) 120, (B) 160, and (C) 200 °C. The amount of hexamine was 0.75 g and the hydrothermal time was 6 h. (The lattice defects are circled and the oriented attachments are squared. The scale bars are 10 nm.).

Therefore, individual nanoparticles inside a bead are single crystals, and a bead is a polycrystal composed of a large number of closed packed single crystals. Also shown in Figure 3 is the interplanar spacing (0.35 nm) between the {101} planes. 2602



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obtained at 200 °C. After deconvolution, four peaks were identified. These four peaks are assigned to Ti4+2p3/2, Ti3+2p3/2, Ti4+2p1/2, and Ti3+2p1/2, located at 458.6, 457.9, 464.3, and 463.6 eV, respectively.26 From the Ti2p core level spectra, the concentrations of Ti4+2p3/2 and Ti3+2p3/2 and their ratio can be obtained. XPS analysis was also performed on P25 powders and other TiO2 beads obtained at 120 and 160 °C. The results are given in Table I. Table I shows that the higher hydrothermal temperature the higher the Ti4+ concentration or the larger the Ti4+/Ti3+ ratio is, matching the trend of the crystallinity found by XRD analysis shown in Figure 2. As a result, a higher hydrothermal temperature leads to a lower oxygen vacancy concentration. Figure 4B shows the high-resolution core level spectra of O1s for the TiO2 obtained at 200 °C. It is seen again that Ti2O3 is nearly absent. The specific surface area, as shown in Table I, reduces with the hydrothermal temperature as a result of increasing grain size. However, the surface areas of all the beads are higher than that of commercial P25 powders. It was also found that the variations in the BET surface area, pore diameter, and grain size are more significant between 120 and 160 °C than between 160 and 200 °C. The same trend happens in the crystallinity observed in TEM images (Figure 3) and the oxidation state determined from XPS analysis (Table I) where the concentration of Ti4+ changes only by 2.7% as the temperature increases from 160 to 200 °C. We elucidate that the formation of low-defect TiO2 beads is nearly completed at a hydrothermal temperature as low as 160 °C. The pore diameters and volumes of beads are higher than those of the commercial P25 powders. In particular, the pore volumes are nearly more than twice those of the commercial P25. It is also noted that the TiO2 beads obtained at different hexamine amounts exhibit very similar BET data. At the present time, it is neither our intention nor purpose to discuss the formation mechanism of the beads. By use of the obtained beads and commercial P25 powders, FDSCs having a total of 6 different photoanodes were fabricated and tested. It is apparent that the cells (C to F) having the obtained beads as scattering layers exhibit better efficiencies (Figure 5and Table II). In general the Jsc, Voc, and FF all increase due to the use of beads. Among them, the

Defects were found in the TiO2 obtained at a hydrothermal temperature of 120 °C, as circled in Figure 3A. Such defects were not observed in both the 160 (Figure 3B) and 200 °C (Figure 3C) hydrothermally synthesized TiO2. Both the XRD (Figure 2) and HRTEM (Figure 3) analyses therefore indicate improved crystallinity of hydrothermally synthesized powders with the temperature; while HRTEM analysis also shows more defects in the sample obtained at the lowest hydrothermal temperature of 120 °C. Furthermore, an interesting finding is that two nanoparticles are tightly bonded together through the {101} planes, as squared in parts B and C of Figure 3. Generally, such oriented attachment rarely occurs on the {101} planes due to their low surface energy.24 Oriented attachment serves to reduce the total surface energy by eliminating surfaces. However, the attachment of {101} planes can take place in certain conditions, such as in hydrothermal process.25 {101} planes are expected to adsorb less functional groups like hydroxyl groups during the synthesis than {001} planes and therefore can be more tightly bonded than {001} planes. Nevertheless, the strong attachment means a decreased number of barriers or boundaries for electrons transport, giving faster electron transport rates. XPS analysis was further performed to examine the existence of the defects on particle surfaces, such as oxygen vacancies usually found in oxides. Figure 4A shows the high-resolution core level spectra of Ti2p for the TiO2 beads

Figure 5. J−V curves of the FDSCs obtained. The Jsc, Voc, and FF are summarized in Table II.

increase in the cell efficiency is primarily controlled by the Jsc. The improvement is as high as 28% (cells E vs B). The contributions from Voc and FF are less. As mentioned above, the beads are pure anatase TiO2 and have lower amounts of

Figure 4. HRXPS spectra of the beads obtained at a hydrothermal temperature of 200 °C and a time of 6 h. (A) Ti2p and (B) O1s spectra. 2603



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Table II. Short Circuit Current (Jsc), Open Circuit Voltage (Voc), Fill Factor (FF), and Efficiency (η) of Various FDSCs cell

Jsc (mA/cm2)

Voc (V)

FF (%)

η (%)

A: P25 (3 μm) B: P25 (8 μm) C: P25 (3 μm) + Bead c (5 μm) D: P25 (3 μm) + Bead d (5 μm) E: P25 (3 μm) + Bead e (5 μm) F: P25 (3 μm) + Bead f (5 μm)

6.58 9.69 10.10 10.30 11.20 10.03

0.75 0.69 0.69 0.74 0.74 0.73

64.6 64.1 65.0 65.7 66.2 65.7

3.21 4.29 4.57 5.01 5.48 4.84

oxygen vacancy concentrations. Although there are variations in the degree of crystallinity among the beads, the phase of the beads is pure anatase, which exhibits higher electron transport rates than rutile-containing P25 powders. The bead layers have lower electrical resistance than the P25 layers. As a result, the FF of bead-containing cells is in general better than that of P25only cells. The cell performance was then further evaluated using UV− vis spectroscopy, IMPS/IMVS, EIS, and IPCE, as summarized in Table III. The use of beads largely enhances the dye loading due to the much larger specific surface areas and pore volumes of the beads as compared to the P25. Figure 6 shows the light absorbance of TiO2 photoanodes used in cells A−F. The total absorbance and the light-harvesting efficiency (LHE) at 455 nm obtained from this figure are shown in Table III. Between cells A and B where only P25 powders were used, the former is thinner and therefore has less LHE. For cells B−F in which the TiO2 photoanode thicknesses are the same, the cells having beads, i.e., C−F, show apparently better LHE. Among them, cells D and E exhibit the best and the second-best LHE, respectively, followed by cells C and then F. This is attributed to the difference in the bead size, i.e., a larger bead size gives more effective light scattering due to size-dependent Mie scattering.27 As a result, it was found that the total absorbance or the percentage absorbance increase over cell B is proportional to the bead size linearly, as shown in Figure 7. The result indicates that the bead size has a significant effect on scattering behavior. Here, the bead size for the most effective light scattering is 750 nm, which occurs without sacrificing the dye loading. IMPS measurements, as shown in Figure 8A for cell E, were performed to determine the electron diffusion time τd and the resulting data are shown in Table III. The τd of cell B is obviously higher than that of cell A due to the thicker TiO2 photoanode. However, we also found that the τd per micrometer of cell B (1.55 ms/μm) is shorter than that of cell A (2.59 ms/μm). Cell B has better light absorbance, a

Figure 6. UV−visible spectra of N719-sensitized TiO2 photoanodes.

Figure 7. Correlation between light-harvesting ability and the bead size.

much higher dye loading, and slightly better ηinj than cell A; more photoelectrons are therefore generated in cell B than cell A. As the surface states in both P25 photoanodes are the same, more traps can be occupied in the photoanode of cell B, leading to the trap-free mode electron diffusion.28 This particular mode of diffusion is often found in thick photoanodes.29 The use of beads further reduces the τd. This is attributed to a complex interplay among the light absorbance, dye loading, and ηinj. In comparison to cell B, cells C−F have higher values of these three factors and therefore more photoelectrons. This coupled with the fact that the bead-containing photoanodes have less traps or oxygen vacancies than the P25-only photoanode; the values of τd of these cells are therefore reduced, as discussed above. Furthermore, the beads exhibit pure anatase phase,

Table III. Information Obtained from UV−Vis, IMPS/VS, EIS, and IPCE Analysesa cell

dye loading (×10−7 mol/cm2)

total absorbance

LHE at λ = 455 nm

τd,IMPS (ms)

Dn,IMPS (μm2/ms)

τn,IMVS (ms)

τEIS (ms)

ηcc,IMPS/VS (%)

IPCE at λ = 455 nm (%)

ηinj, calculate at λ = 455 nm (%)

A B C D E F

0.84 2.37 4.85 4.64 4.52 4.48

77 108 118 147 127 116

0.48 0.58 0.60 0.72 0.67 0.60

8.1 12.4 9.3 5.6 8.1 8.1

1.1 5.2 6.9 11.4 7.9 7.9

38.0 65.1 23.0 23.0 27.1 28.3

8.8 16.6 5.8 7.2 8.8 8.8

78.7 81.0 59.6 75.7 70.1 71.4

16.3 24.5 30.0 31.2 34.0 28.4

43.6 51.9 83.6 57.4 72.5 66.1

a τd, electron diffusion time; Dn, electron diffusion coefficient; τn and τEIS, electron lifetime; ηcc, charge-collection efficiency; ηinj, electron injection efficiency. The values of total absorbance were obtained by integrating UV−vis spectra (Figure 6), and the values of LHE were obtained from the following equation, LHE = 1−10−A, where A represents absorbance.

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although has similar absorbance to that of cell F, exhibit the longest τd. While the photoanodes D, E, and F have similar oxygen vacancies, photoanode C has the largest amount of oxygen vacancies, which contributes to its longer τd. Moreover, the beads used in cell C do not have the oriented attachments among the small nanoparticles (Figure 3) and therefore give a higher transport resistance, thus increasing the τd. Another factor represents the electron transport is the electron diffusion coefficient Dn = d2/τd, where d is film thickness.31 The values of Dn are also given in Table III. Since cells B−F have the same thickness, Dn is inversely proportional to τd. In general, the value of Dn of RDSC ranges from 10−4 to 10−8 cm2 s−1.32 The flexible version (plastic substrate), which must be fabricated at low temperatures, often gets lower Dn than RDSC. In our case, an excellent Dn of 1.14 × 10−4 cm2 s−1 has been obtained for FDSCs. This value is as good as that of RDSCs. IMVS measurements, as shown in Figure 8B for cell E, were performed to determine the electron diffusion time τn, and the resulting data are shown in Table III. The τn of cell B is nearly twice that of cell A. Along the light path, the photon flux reduces with the path length, i.e., the photoanode thickness. Near the current collector where the photon flux and therefore the electron density n are high,33 less recombination occurs for a thicker photoanode due to the difficulty in diffusion for the redox couple.34 Reduced recombination means an enhanced lifetime. On the other hand, at the region far from the current collector, the photon flux is low, giving a low electron density.33 This is particularly true for a thicker photoanode. The recombination rate has been found to vary with n2.35−37 From this viewpoint, a reduced recombination and thus a longer lifetime are expected for cell B. Furthermore, as shown in Table II, cell B has a smaller Voc than cell A. Therefore, it is believed that the overall electron density in cell B is lower than that in cell A,38 making cell B have a longer lifetime. Comparing the cells with and without beads, the electron lifetimes of the cells having beads are all lower than that of cells having only P25 powders. As mentioned above, there are more photoelectrons in the bead-containing cells. As a result, these cells have higher recombination rates and thus shorter electron lifetimes. However, the greater numbers of photoelectrons lead to enhanced Voc for the bead-containing cells, as shown in Table II, except cell C. Cell C has the smallest Voc as it has the largest amount of oxygen vacancies and low photoelectrons, thus giving a lower quasi Fermi level. EIS measurements, as shown in Figure 8C for cell E, were performed to determine the electron diffusion time τEIS under simulated sun illumination and the resulting data are shown in Table III. The electron lifetime τEIS obtained from EIS is shorter than the electron lifetime τn obtained from IMVS due to the different light intensity and wavelength. The light intensity used in EIS measurement is 10 times that used in IMVS measurement, leading to a higher electron density and thereby a shorter electron lifetime as mentioned above. Nevertheless, the electron lifetimes obtained using both techniques show the same trend against the cells. From the obtained τd and τn, the charge collection efficiency ηcc was determined by: ηcc = 1 − τd/τn.39 Cell B has a larger ηcc than cell A since cell B has a τn that is nearly twice that of cell A, as mentioned above. Since more photoelectrons are generated in cell B than cell A and cell B has a larger ηcc than cell A, a higher Jsc in cell B than in cell A was obtained. The cells containing bexads all have lower ηcc than the cells having only P25 powders. Although the electron diffusion time or diffusion

Figure 8. (A) IMPS, (B) IMVS, and (C) EIS plots for cell E.

which is also known to give faster electron transport than P25 powders that has both anatase and rutile phases.30 On the other hand, the values of τd among the cells with beads are also different. As mentioned above, a complex interplay among several factors determines the value of τd. Among the beadcontaining cells, the dye loadings are quite similar. It appears that the light absorbance plays the major role in enhancing the generation of photoelectrons. The trend is that the diffusion time τd becomes faster with the absorbance, indicating again enhanced light harvesting due to the scattering effect of the beads. Higher light absorbance enables the photoanode to generate more photoelectrons to occupy surface traps, giving a larger photoelectron/trap ratio or even a trap-free condition. This therefore results in a shorter τd. Among them, cell D exhibits the best light scattering effect (Table II or Figures 6 and 7) and therefore has the fastest electron transport. Cell C, 2605



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Taking also the IPCE at 455 nm as an example, the Jsc was found to be proportional to the IPCE. Although the light intensities used for the IV and the IPCE measurements are different, it is known that the IPCE value does not fluctuate with light intensity in reasonable light-intensity range.35 From the IPCE measurements, the light-harvesting efficiency, and the charge collection efficiency, the values of injection efficiency ηinj were obtained35 and are shown in Table III. Cells A and B, having P25-only photoanodes with different thicknesses, exhibit similar ηinj due to the same material and the similar dye loading per volume. It is obvious that the bead-containing cells have better ηinj than P25-only cells except cell D. During the firststep of the bead synthesis process, hydroxylic groups form on the particle surfaces from sol−gel derivatives owing to the associated incomplete condensation process.40 The existence of hydroxylic groups on the surfaces favors the adsorption of carboxyl groups on the N719 dye, which also allows the absorbed dye molecules to be oriented in a proper way for effective electron injection upon excitement of the dye.41,42 As a result, the bead-containing cells, except cell D, exhibit higher ηinj.

coefficient of the bead-containing cell has been enhanced, the much reduced electron lifetime overshadows this positive effect. We believe that a large amount of generated photoelectrons from beads might have been blocked in P25 layer before reaching the charge collector due to limited electron transport ability of the rutile-containing P25 powders. Among cells C−F, cell C has the lowest ηcc owing to its worst crystallinty (Figure 2) and highest amount of oxygen vacancies in the beads used (Table I). For cells D−F, the photoanodes not only exhibit better crystllinity and lower oxygen vacancies but also contain orient attachments as shown in Figure 3. Therefore cells D−F have higher ηcc than cell C. Nevertheless, as mentioned above, all the bead-containing cells have lower ηcc than the cells having only P25 powders. However, because of their excellent dye loadings, better absorbance, and IPCE performance as discussed below, the bead-containing cells still exhibit higher Jsc than the P25-only cells. The IPCE curves are shown in Figure 9A; while the IPCE values at 455 nm are shown in Table III. It is obvious that beadcontaining cells have better IPCE performance. It is seen that

4. CONCLUSIONS We have obtained high-efficiency FDSCs through the use of mesoporous anatase TiO2 beads as the scattering layer in photoanodes. The pure anatase TiO2 beads, having low oxygen vacancy concentrations and directional attached grains, lead to more photoelectrons and enhance the electron diffusion in the photoanodes. The electron diffusion is also enhanced as beads exhibit short diffusion times. The large amounts of photoelectrons are attributed to the high dye loading and sizedependent Mie scattering of bead layers. We have demonstrated for the first time that the use of beads, having diameters ranging from 250 to 750 nm, enhances the solar conversion efficiency of all-plastic FDSCs by as much as 28% (5.5 vs 4.3% of cell efficiency).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS The work was supported by the National Science Council in Taiwan under Grant Nos. NSC 98-2622-E-006-012-CC2 and NSC 99-2622-E-006-010-CC2.

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REFERENCES

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Figure 9. (A) IPCE spectra of FDSCs and (B) normalized spectra of cells B and E.

the enhancement primarily occurs near 455 nm as shown in Figure 9B. This figure shows the normalized IPCE curves of cells E, exhibiting the best IPCE, and B. The IPCE of cell E is about 38% higher than that of cell B. In fact, it was further observed that the performance trend of the IPCE is proportional to the increasing trend of Jsc values (Table II). 2606



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