Novel Photoanode for Dye-Sensitized Solar Cells with Enhanced Light

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A novel photoanode for dye-sensitized solar cells with enhanced light harvesting and electron collection efficiency Weixing Song, Yudong Gong, Jianjun Tian, Guozhong Cao, Huabo Zhao, and Chunwen Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02887 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Applied Materials & Interfaces

A novel photoanode for dye-sensitized solar cells with enhanced light harvesting and electron collection efficiency Weixing Song,† Yudong Gong, † Jianjun Tian,‡ Guozhong Cao,§ Huabo Zhao,† Chunwen Sun*,† †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing 100083, China ‡

Advanced Materials and Technology Institute, University of Science and Technology Beijing, Beijing 100083, China §

Department of Materials and Engineering, University of Washington, Seattle, WA 98195-2120, USA

ABSTRACT A novel photoanode structure modified by porous flowerlike CeO2 microspheres as a scattering layer with a thin TiO2 film deposited by atomic layer deposition (ALD) is prepared to achieve a significantly enhanced performance of dye-sensitized solar cells (DSSCs). The light scattering capability of the photoanode with the porous CeO2 microsphere layer is considerably improved. The interconnection of particles and electrical contact between bilayer and conducting substrate is further enhanced by an ALD deposited TiO2 film, which effectively reduces the electron recombination and facilitates electron transport and thus enhances the charge collection efficiency of DSSCs. As a result, the overall efficiency of the obtained TiO2-CeO2-based cells reaches 9.86%, which is 31% higher than that of the DSSCs with a conventional TiO2 photoanode.

1

ACS Paragon Plus Environment


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Keywords: flowerlike CeO2 microspheres, light harvesting, electron collection efficiency, atomic layer deposition, dye-sensitized solar cells

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have drawn great attention with the increasing concern on sustainable and renewable energies.1-4 The highest power conversion efficiency (PCE) of DSSCs has been achieved with a TiO2 nanoparticles (NPs) layer.5 Further improvement of the photovoltaic performance has been widely studied, such as the development of dyes with broad absorption band,6-7 increasing the surface area of the porous layer,8-9 and increasing the light harvesting capability by introducing a scattering layer.10-12 Among them, a scattering layer with various larger semiconductor oxide structures has been widely investigated because NPs around 20 nm are too small to scatter visible light efficiently.13-14 In general, larger semiconductor oxide structures were applied to serve as a top layer of a bilayer structured photoanode. Many larger structures composed of TiO2, ZnO,

SnO2,

CeO2

like

hollow

spherical

microspheres,15

large

particles,16-17

nanocrystalline spherical aggregates,18-24 and tailored spherical aggregates built by nanosheets or nanotubes,25-26 spherical voids,27 and composite structures28-29 were used as scattering materials for high-efficiency DSSCs. There is still much room to find some alternative and stable photoanodes and thus improve the performance of DSSCs. Among them, CeO2 has attracted increasing attention because it has a high refractive index, good transmission for visible light, as well as strong adhesion and high stability.30-33 Thus 2


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cubic shaped CeO2 nanoparticles offered a strong light-scattering ability for DSSCs due to their high refractive index for visible light and exposed mirror-like facets.34-35 Power conversion efficiency of the DSSCs was improved by 17.8%, but there is a decrease in the dye loading amount when the cubic CeO2 particles are introduced as the top layer. Although CeO2 displays a bandgap of 3.2 eV and generally used as an ultraviolet blocking material, CeO2 is not generally considered a semiconductor and is not regarded as a photoactive material, considering that this bandgap is usually attributed to an O2p→ Ce4f transition.36 Herein, hierarchically porous flowerlike CeO2 microspheres are studied for the first time as components of a scattering layer, as shown schematically in Figure 1. The porous microspheres consisting of many nanosheets as petals provide not only high light scattering but also high surface area, required for a good scattering material for DSSCs. The hierarchical CeO2 microspheres extend the light travelling distance by confining the light propagation within the photoanode. As we have learned, it is first reported that CeO2 microspheres are used in DSSCs, resulting in enhanced light-harvesting and thus photovoltaic efficiency.

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Figure 1. Schematic of the solar cell structure based on the TiO2-CeO2 bilayer covered by an ALD-made film with scattered incident light. In order to fully utilize the dye absorbed on the CeO2, a conformal and compact TiO2 film deposited by atomic layer deposition (ALD) is used. ALD has emerged recently as a facile method to improve the photovoltaic performance of DSSCs.37-46 ALD is a self-limited growth technique capable of depositing a variety of thin film materials from the vapor phase.47-48 The ALD film may also be helpful to enhance the interconnection of nanostructures and the adhesion between TiO2 nanoparticles and a conducting substrate. Based on these considerations, photoanodes based on an ALD-modified TiO2-CeO2 bilayer structure with good scattering capability and facilitated electron transport are expected to improve the photovoltaic performance of DSSCs. 2. EXPERIMENTAL SECTION 2.1. Preparation of the photoanode. The flowerlike CeO2 microspheres were synthesized by a hydrothermal method according to a previous report.49 The CeO2 paste 4


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was prepared by mixing CeO2 microspheres powder, α-terpineol, ethyl-cellulose (30–50 mPa) and ethanol at a weight ratio of 2: 10: 1: 1. A commercial TiO2 paste (Yingkou Opvtech, China) covering on FTO glass plates by doctor-blade method and then was calcined at 450°C for 30 min, serving as a reference electrode (T-electrode). Another CeO2 layer was deposited on the TiO2 layer before calcination by coating the plates with CeO2 paste, and heated up to 450°C for another 30 min, marked as T-C-electrodes. The T-C-electrodes were subsequently covered by a conformal TiO2 film by ALD technique, and the obtained electrodes were marked as T-C-A electrodes. The deposition process was carried out inside a Picosun Sunale R-200 reactor using titanium tetrachloride (TDA, Aldrich, Germany, 25 °C) and deionized water (18.2 MΩ, 25 °C) as precursors. The reaction was performed at 300 °C with the parameter setting of 0.5 s pulse, 8 s duration, 6 s purge, respectively. The TDA or H2O was carried by nitrogen gas into the ALD system, and the nitrogen gas also served as a purge gas to remove all excess precursors and by-products of reaction. The precursor was confined to the reactor for 8 s in order to penetrate into complex 3D network and adsorb on all exposed surfaces of electrodes. The thickness of the film for each ALD cycle is ∼0.066 nm, measured by deposition of similar cycles on a silicon wafer via spectroscopic ellipsometry. The electrodes were marked as T-C-A-electrodes. These electrodes were heated up to 80 °C and subsequently immersed into 0.5 mM N719 dye (Yingkou Opvtech, China) in ethanol for 24 h at room temperature. 2.2. Fabrication of solar cells. The platinated FTO glass plates (Yingkou Opvtech, China)

were

used

as

counter

electrodes.

The

electrolyte

contains

0.5

M

1,3-dimethylimidazolium iodide, 0.03 M I2, 0.05 M LiClO4, 0.5 M 4-tert-butylpyridine, 5



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and 0.05 M guanidine thiocyanate in acetonitrile. The electrolyte was sandwiched by an as-prepared photoanode and a counter electrode with two strips of 3M tape. The active areas of the solar cells were limited by a circular mask in a diameter of 5 mm. For each type of device, six samples were assembled and measured and a typical device of each type was chosen to analysis. 2.3. Characterization. The morphology of the as-synthesized porous flowerlike CeO2 microspheres was characterized by SEM (Hitachi, SU-8020) and TEM (FEI, F20). The structure of CeO2 microspheres was characterized by X-ray diffraction (XRD, Panalytical, X’pert3 powder). UV-Vis spectroscopy (Shimadzu, UV-3600) was used to measure the dye loading amount and diffuse reflectance of the photoanodes. Dye was desorbed by immersing the photoanodes in 0.01 M KOH solution of 1:1 water/ethanol for 12 h and then measured. The diffuse reflectance spectra were obtained using an integrating sphere with BaSO4 as a reference. The specific surface area was measured with a Quantachrome Instruments NOVA4000 by the BET method and the pore size was calculated by the BJH method using the desorption branch of the nitrogen isotherm. Both CeO2 spheres and TiO2 nanoparticles (Yingkou Opvtech, China) were measured. The photovoltaic characteristics of the solar cells were measured using an electrochemical workstation (Zahner, Zennium) under simulated AM 1.5 solar illumination (Crowntech, SOL02 series). EIS was performed with an impedance analyzer (Zahner CIMPS) under dark conditions at a forward bias of 0.75V. Intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) were carried out under a modulated green light emitting diode driven by a Zahner (PP211) source supply. The light intensity was modulated by varying the LED voltage sinusoidally. 6


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3. RESULTS AND DISCUSSION 3.1. Synthesis and Materials Characterization. The surface and structures of the photoanode containing TiO2 layer and CeO2 layer were observed using scanning electron microscopy (SEM). As shown in the cross sectional image of the photoanode (Figure 2a), the bilayer structure indicates that a flowerlike CeO2 microsphere layer covers the surface of the TiO2 nanoparticles (NPs) layer with a thickness of about 10 µm. For the reference device, the thickness of the TiO2 NPs layers was about 12～13 µm. The diameters of CeO2 microspheres are in the range from 2 µm to 3 µm, as shown in Figure 2b. It can be clearly observed in Figure 2c that each flowerlike CeO2 microsphere consists of many petal-like nanosheets. The Transmission electron microscopy (TEM) image (Figure 2d) indicates that the CeO2 microspheres possess hollow structures in favor of light transmission the light transmission and reflectance and thus the light scattering as well as dye absorption. As seen in Figure 2e, the interplanar spacings of 0.31 nm and 0.27 nm are consistent with the (111) and (200) lattice planes of face-centered cubic CeO2.20, 49-50 The XRD pattern also reveals that the CeO2 microspheres consist of face-centered cubic CeO2 (Figure 3). The mean size of the CeO2 nanocrystallites determined from the (111) plane by Scherrer equation is 9.4 nm, consistent with the observation from TEM (Figure 2e).

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Figure 2. a) Cross-sectional and b) surface SEM images of the TiO2-CeO2 bilayer with a CeO2 scattering layer on the top of TiO2 nanoparticles, c) larger magnification of a CeO2 sphere, d) TEM image of a typical porous CeO2 microsphere and e) HRTEM image of a nanosheet.

220

400 200

200 0 20

311

600

111

800

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30

40

50

2 Theta (degree)

60

Figure 3. XRD pattern of the porous CeO2 microsphere powder.

8


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The thickness of the ALD-deposited TiO2 film is measured by simultaneously depositing a conformal TiO2 film on a Si wafer, since it is difficult to measure the thickness of the film on the surface of microspheres. The surface morphology of CeO2 microspheres before and after being covered by an ALD film was compared in Figure S1a, b. The film on the Si wafer was observed under AFM, and the image (Figure S1c) indicates that the film was conformal and free of pinholes. The 10 nm ALD TiO2 film deposited at 300°C is composed of polycrystalline anatase, since the TiCl4-H2O process to deposit TiO2 nearly resulted at 250–350℃ in randomly oriented polycrystalline anatase according to the previous report.47 but The 10 nm ALD TiO2 film is quite thin compared to the CeO2 or TiO2 layer with several micrometers in thickness, so it is difficult to detect the polycrystalline anatase in the XRD pattern of the TiO2 and CeO2 bilayer with ALD film on glass under strong signal of CeO2 and TiO2 layer. (Figure S2). The Nitrogen adsorption–desorption isotherm of the flowerlike CeO2 microspheres exhibits a type IV isotherm curve, as shown in Figure 4a. The Brunauer-Emmett-Teller (BET) surface areas of CeO2 microspheres and commercial TiO2 particles were measured to be 86.3 m2 g-1 and 64.6 m2 g-1, respectively. The Barret–Joyner–Halenda (BJH) pore size of the as-synthesized microspheres determined from the desorption isotherm shows a distribution from 4.0 nm to 9.2 nm in Figure 4b. Furthermore, compared to the TiO2 nanoparticles, the porous features of the CeO2 microspheres with a larger specific surface area indicates that it is beneficial for increasing dye loading, and thus offers an enhanced light harvesting ability of DSSCs.

9


0.04

a

dV/dD (cm3.g-1.nm-1)

200

Adsorption Desorption

160 120 80 40 0

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

b

0.03 0.02 0.01 0.00

1.0

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0

10

20

30

Pore Diameter (nm)

40

Figure 4. a) N2 adsorption–desorption isotherms and b) the pore size distribution of flowerlike CeO2 microspheres. 3.2. Light Scattering Analysis. The flowerlike CeO2 could absorb more dye molecules than the NP TiO2 in the same layer thickness through comparing dye loading amounts. (Figure S3a). As shown in Figure 5a, both the T-C-electrodes and T-C-A-electrodes thus possess better dye loading capability whereas the T-C-electrodes have the highest dye loading amount rather than the T-C-A-electrodes (Table 1). The decreasing dye loading caused by the ALD TiO2 film also indicates that the compact film may narrow the pores or even fill the small pores, and thus reduce the surface area of the TiO2-CeO2 layer.

Diffuse Reflectance (%)

TiO2

a

TiO2-CeO2

Adsorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume adsorbed (cm3/g,STP)


TiO2-CeO2-TiO2/ALD

400

500

600

Wavelength (nm)

700

80

b

TiO2 TiO2-CeO2

60

TiO2-CeO2-TiO2/ALD

40 20 0 300

400

500

600

Wavelength (nm)

10


700

800

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Figure 5. a) Normalized absorption spectra of dye loading on the different electrodes. b) Diffuse reflectance of the photoanodes based on TiO2 NPs, with CeO2 layer and TiO2 film deposited by ALD. Table 1. Photovoltaic properties of the DSSCs with different electrodesa Jsc Electrode

[mA cm-2]

Voc

PCE Dye-loading FF

[V]

[%]

[nmol cm-2]

T

15.728 0.742 0.644 7.52

101

T-C

17.904 0.744 0.678 9.03

123

T-C-A

18.654 0.764 0.692 9.86

111

a

T, T-C, and T-C-A note the typical DSSCs fabricated with a T-electrode, a T-C-electrode and a T-C-A electrode, respectively.

Considering the hierarchical structure of the flowerlike CeO2 microspheres with the high surface area, large porosity, excellent thermal stability31 and especially the cubic nanostructure with exposed mirror-like facets and high refractive index30, 51, the CeO2 microspheres would be expected to be a good scattering material and can serve as a scattering layer in order to enhance light harvesting in DSSCs34-35. The diffuse reflectance spectra of these different photoanode layers were measured to study the light scattering effect. As shown in Figure 5b, the light reflection of the photoanode layer including CeO2 microspheres at the region 300-800 nm remarkably increases compared with the photoanode layer made of only TiO2 nanoparticles using the same BaSO4 as the reference. The increase indicates that the porous CeO2 microspheres improve the light scattering effect due to its exposed petal structure and porous hierarchical morphology. The 11



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sensitized TiO2-CeO2 photoanode presents reduced diffuse reflectance after introducing a modified TiO2 thin film by ALD, as the mirror-like facets of the face-centered cubic CeO2 have better light scattering property than TiO2.35,

52

The diffuse reflectance of the

electrodes made of only CeO2 layer and TiO2 layer on FTO film are measured separately, and as shown in Figure S3b, the CeO2 layer shows higher diffuse reflectance from the onset of 430 nm due to lower bandgap of the CeO2 than that of the TiO2.36 3.3. Photovoltaic Characterization and electron collection analysis of Complete Cells. Typical current-voltage (I-V) curves of DSSCs with and without a scattering layer and ALD TiO2 layer were shown in Figure 6. As expected, the bilayer of TiO2 NPs and CeO2 microspheres shows better functional performances than the pure TiO2 NPs. A typical TiO2 based device without the scattering layer shows a lower short circuit current density (Jsc) and power conversion efficiency (PCE) (Table 1). A typical device with the flowerlike CeO2 microspheres as a scattering layer yields obviously increased Jsc and PCE. The overall efficiency was remarkably improved. With an additional thin and conformal 10 nm TiO2 film deposited by ALD, the photovoltaic performance of the fabricated solar cells increases further in terms of photocurrent density, fill factor, and resulting PCE. Different films deposited by ALD with thicknesses of 5 nm, 10 nm, and 15 nm for DSSCs were measured and the I-V curves are presented in Figure S4. The short-circuit current (Jsc) increases as the film thickness increases until the film reaches 10 nm thickness, and thicker films may reduce the pore size and thus the dye loading amount, leading to a decreased performance. A typical ALD-modified TiO2-CeO2 based device has the best performance of 9.86% efficiency. The efficiency of the T-C-A-electrode based device increases by 31.1%, compared with 7.52% PCE of the 12


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device composed of an electrode fabricated by only TiO2 NPs layer (T-electrode). The increase ratio of 31% is better than the result with other materials or structures as scattering layers reported in the literatures.28-29, 34 The DSSCs based on T-C-electrodes also show higher PCE than that with T-electrode, and increases by 20.1%. Moreover, the increase ratio of PCE of CeO2 microspheres based devices is greater than that of cubic CeO2 nanoparticles-based cells (17.8% improvement) reported previously34. The result may be caused by the fact that the petal-like nanosheets offer higher surface area than the cubic-shaped nanoparticles, which is favorable to absorbing more dye molecules, and more possibilities of light reflection between nanosheets, causing more light propagation and harvest. The larger fill factor shows that the ALD modification on the surface reduces the loss of photoexcited carriers being extracted from the device due to the suppression of surface electron recombination. The increase in short-circuit current density is mainly attributed to the highly compact and interconnected ALD-made thin TiO2 film and the

16 12 8 4 0

0.2

0.4

Voltage (V)

0.6

0.8

-0.5 -1.0 -1.5

100

b

0.0

0.2

0.4

Voltage (V)

c

TiO2

80

IPCE (%)

a

Dark Current Density (mA/cm2)

20

D L A / 2 O i T 2 2 O O e e C C 2 2 2 O i O O i i T T T

Current Density (mA/cm2)

light scattering caused by flowerlike structures in spite of ALD treatment.

D L A / 2 O i T 2 2 O O e e C C 2 2 2 O i O O i i T T T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2-CeO2 TiO2-CeO2-TiO2/ALD

60 40 20

0.6

0.8

0

400

500

600

700

800

Wavelength (nm)

Figure 6. The current-voltage curves measured under a) AM 1.5 G solar irradiance (100 mW/cm2) and b) dark, for DSSCs based on TiO2 NPs, with CeO2 layer and with ALD TiO2 film, respectively. c) Incident-photon-to-current efficiency (IPCE) spectra of the DSSCs with different photoanodes. 13



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In order to quantify the influence of the porous flowerlike CeO2 microspheres and the ALD thin film on the photovoltaic performance, the incident photon-to-current conversion efficiency (IPCE) was measured for solar cells with different photoanodes, as seen

in

Figure

6c.

Since

the

IPCE

peak

of

(Bu4N)2-[Ru(4,4’-(COOH)-2,2’-bipyridine)2(NCS)2] (N719) dye-sensitized devices is obtained at around 530 nm, the maximum value of IPCE of the solar cells was observed and compared. The introduction of porous CeO2 microsphere layer and ALD thin TiO2 film leads to the enhancement of the IPCE values, respectively. The IPCE value can be deconvoluted into the light harvesting efficiency (LHE), charge injection efficiency (CIE), and charge collection efficiency (CCE) according to Equation (1).42 IPCE=LHE×CIE×CCE

(1)

LHE is mainly determined by the optical and electronic nature of the dye molecules and dye loading amount on the photoanode layer. Therefore the dye loading amount directly affects the LHE of the samples. As shown in the UV–vis absorption spectra (Figure 5a), the amount of dye loading rises with a CeO2 layer introduced into the photoanode due to the high surface area of CeO2 microspheres. However, in case of the cell with ALD-modified thin TiO2 film, the dye-loading capacity of the porous electrode decreases to some degree, as the ALD film completely covering the CeO2 microspheres unavoidably reduces the pore volume. To understand the kinetic processes of electron transport and recombination and thus compare the charge collection efficiency of the DSSCs, IMPS/IMVS was further characterized.53 The time constant of electron transport (τt) and electron recombination time (τr) are calculated from the IMPS and IMVS frequency data according to the 14


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following Equations (2) and (3).43 Charge collection efficiency (CCE) was derived from τt and τr according to Equation 4. τt =1/(2πfmin, IMVS )

(2)

τr =1/(2πfmin, IMPS )

(3)

CCE =1-τt /τr

(4)

It can be obviously observed in Figure 7a that τt for the TiO2-CeO2-ALD-based DSSC is markedly shorter than the corresponding value for TiO2-CeO2-based and TiO2-based DSSC. Thus, the result indicates, that the electron transport rate increases in the conformal ALD TiO2 film modified TiO2-CeO2 layer. The conformal ALD layer has a smaller number of defects and less grain boundary compared to the NPs,47,

54

and

therefore the percolation and transport of photoelectrons are particularly fast. Furthermore, ALD treatment improves interconnection between TiO2 nanoparticles, thus leading to the enhanced photocurrent.

15



102

a

103

TiO2

TiO2

Lifetime (ms)

Transit time (ms)

b

TiO2-CeO2

TiO2-CeO2 TiO2-CeO2-TiO2/ALD

TiO2-CeO2-TiO2/ALD

102

101

Charge Collection Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

0.2

0.4

Voltage (V)

0.6

0.8

-80

c

80

0.4

Voltage (V)

0.6

0.8

1

d

-60

60

-40

TiO2 TiO2-CeO2

40 20

0.2

1

-20

TiO2-CeO2-TiO2/ALD

0.2

0.4

0.6

0.8

1

0

20

40

60

80

100

120

140

Voltage (V)

Figure 7. a) Electron transit time and b) electron lifetime for the DSSCs based on different photoanodes, and c) charge collection efficiency calculated by a) and b), d) typical Nyquist plots of the DSSCs with different photoanodes under dark condition at a bias voltage of −800 mV. The values of τr were compared in Figure 7b. The typical ALD-modified solar cell has a larger value of τr than the solar cells without ALD film, implying longer electron life time in the TiO2-CeO2-ALD layer. The higher τr for ALD modified solar cells could also account for higher Voc than the solar cells without ALD film. As shown in Figure 6b and Table 1, the Voc of the TiO2-CeO2-ALD solar cells is obviously increased. The variation of Voc can be clarified by observing the dark current-voltage characteristics of these devices (Figure 6b). The dark current onset starts between 0.20 V and 0.22 V for the solar 16


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cells without ALD film. With the ALD-modified film, the onset is found to significantly shift to around 0.61 V. The dark current results from the recombination of photoexcited carriers at the electrolyte/FTO and electrolyte/photoanode interface.55 The recombination reaction is notably reduced as seen from the dark current onset for the ALD modified DSSCs. ALD was carried out using duration mode and the precursors were confined in the reactor for several seconds to allow sufficient surface adsorption inside the deep groove of pores. So it should be noted that the deposition of a uniform TiO2 film on the TiO2-CeO2 layer also covers the exposed surface of the conducting substrate. The devices made of TiO2 layer and ALD TiO2 film present better performance than these made of pure TiO2 layer (Figure S5). Shorter electron transit time and longer lifetime indicate that the ALD made TiO2 film would enhance not only the interconnection of TiO2 particles but also the adhesion between TiO2 layer and conducting substrate, leading to enhanced photovoltaic performance of the device with ALD TiO2 film (Figure S5a). Therefore, the ALD performance is a highly important factor for efficient DSSCs. The enhancement will increase the number of electron pathways and suppress the electron recombination at the electrolyte/FTO interface, which would significantly reduce the dark current. Increased electron transfer routes caused by the enhanced adherence further lead to smaller resistance for electrons to transfer to the FTO surface. The compact ALD TiO2 film also favors the electron accumulation at the FTO interface and therefore leads to a more negative Fermi level and an increased Voc. Both lower τt and higher τr would consequently increase the electron charge collection and CCE are plotted in Figure 7c. The result is consistent with the observed photo current density. Given the higher IPCE and lower LHE of the ALD-modified devices compared 17



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to the other TiO2-CeO2 devices, the contribution of the improved CCE to the IPCE is much greater than that of reduced LHE. Hence, increased photovoltaic performance in respect of photocurrent mostly originates from the enhanced interconnectivity by ALD deposition. Electrochemical impedance spectroscopy (EIS) analysis was carried out under dark with a bias voltage of −800 mV. The resistance of the back reaction from TiO2 or TiO2-CeO2 to the I-/I3- electrolyte is measured and analyzed in the Nyquist plot of Figure 7d. Compared to the reference device, ALD-modified devices display larger value of the resistance for TiO2-CeO2-ALD photoanodes derived from the larger radius of lower-frequency semicircle. The information refers to the bigger charge transfer resistance at the photoanode/electrolyte interface, indicating less electron recombination. The less defects and tight connectivity of nanoparticles induced by ALD coated film reduce the electron recombination site and probability.41-42, 47 The result is in accord with the IMVS analysis. A decrease in series resistance for the typical ALD-modified device was also recorded as seen from the intercepts on the real axis in the plot.43 The variation can be attributed to the reduced electron contact resistance at the TiO2/FTO interface. The TiO2 layer has loose connectivity of nanoparticles and adhesion with conducting substrate,42 The more electron contact sites and less grain boundary generated from the introduction of an ALD TiO2 film effectively reduce interfacial contact resistance. 4. CONCLUSION A new scattering layer composed of hierarchical flowerlike CeO2 microspheres and ALD deposition were successfully applied to notably improve the performance of 18


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TiO2-photoanode DSSCs. The introduction of porous flowerlike CeO2 microspheres leads to the increase in diffuse reflectance and dye loading capability. A faster electron transport rate and longer electron lifetime were achieved by covering the TiO2-CeO2 bilayer with a conformal and compact TiO2 film deposited by ALD. Therefore, both the light harvesting efficiency and charge collection efficiency of the DSSCs are significantly improved by the new photoanode structure. The DSSCs with ALD-modified TiO2-CeO2 photoanode show a 31% improvement of overall conversion efficiency compared with the reference TiO2-photoanode solar cell. Accordingly, various efficient porous structures or materials may have potential application in DSSCs via surface modification of TiO2 or ZnO. This novel structure will thus provide a promising approach to designing high-performance photovoltaic devices. ASSOCIATED CONTENT Supporting Information SEM images of a CeO2 microsphere and an AFM image of an ALD TiO2 film; the current-voltage curves for DSSCs with ALD TiO2 films. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected]. Tel: +86 10 8285 4649. Fax: +86 10 8285 4649. ACKNOWLEDGEMENTS This work was supported by the Thousands Talents Program for the pioneer researcher and his innovation team in China. The authors also acknowledge the financial support of the National Natural Science Foundation of China (No. 61404035). We thank Prof. 19



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Novel Photoanode for Dye-Sensitized Solar Cells with Enhanced Light

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