Electrolyte Interface Improving

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Fence constructed at semiconductor/electrolyte interface improving electron collection efficiency of photoelectrode for dye sensitized solar cell Hongzhen Liu, Yanyan Lou, Siriporn Jungsuttiwong, Shuai Yuan, Yin Zhao, Zhuyi Wang, Liyi Shi, and Hualan Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13069 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 8, 2017

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ACS Applied Materials & Interfaces

Fence constructed at semiconductor/electrolyte interface improving electron collection efficiency of photoelectrode for dye sensitized solar cell

Hongzhen Liua, Yanyan Loua, Siriporn Jungsuttiwongb, Shuai Yuan*c, Yin Zhaoc, Zhuyi Wangc, Liyi Shic, Hualan Zhou*d

a

Laboratory for Microstructures, Shanghai University, 99 Shangda Road, Shanghai

200444, China. b

Center for Organic Electronic and Alternative Energy, Department of Chemistry and

Center of Excellence for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani 34190, Thailand c

Research Center of Nanoscience and Nanotechnology, Shanghai University, 99

Shangda Road, Shanghai 200444, China. d

School of Medical Instrument and Food Engineering, University of Shanghai for

Science and Technology, Shanghai 200093, China *Corresponding

authors:

E-mail:

[email protected]

(S.

Yuan),

[email protected] (H. Zhou)

Abstract: The charge recombination and transfer at the TiO2/dye/electrolyte interface play a crucial role in DSSC. Here, a fine-controlled Au nanoparticles via electrodepositon incorporated into porous TiO2 photoanode and dodecanethiol molecules as assembled monolayer capping on Au nanoparticles was designed and prepared. The “fence-like” structure of Au-thiol molecules at the TiO2/dye/electrolyte interface can not only insulate electrolyte to suppress recombination but also make full use of the plasmon-enhanced light absorption of Au nanoparticles. The photoanodes were characterized by XPS, UV-Vis absorption and Mott–Schottky analyses. As compared to pure TiO2, the DSSC 1

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with interface “fence” structure achieved efficiency (η) of 8.17%, increasing by 10.4%. The enhancement results are essentially attributed to the increase of the light-harvesting and electron collection properties, accompanying a slightly promote in the Fermi level. Furthermore, after dodecanethiol molecules treatment, the Au nanoparticles with intensified near-field effect also acted as electron sinks to store more electrons and exhibited well electron transport performance from electrochemical impedance spectroscopy (EIS) analysis.

Keywords: Au nanoparticles, plasmon-enhanced solar cells, thiol molecules treatment, interface fence, EIS analysis.

1. Introduction Dye sensitized solar cells (DSSC) have attracted significant attention due to its easy fabrication and reasonably high photoelectric power conversion efficiency (PEC), or integration with batteries to form new devices.1-3 So far, the record highest efficiency reported for DSSC is over 14%. The light-harvesting efficiency (LHE) of the photoanode and charge recombination at the TiO2/dye/electrolyte interface are the two crucial factors hindering the development of high performance DSSC. In order to inhibit the charge recombination, the strategies of employing coadsorbent and coating materials have been used to modify TiO2.4-5 Recently, employing the localized surface plasmon resonances (LSPR) of metallic nanostructures into DSSC has been recognized as an available way to boost the LHE, especially Au,6-8 Ag.9-10 From previous studies, the improvement in photo-conversion efficiency can be attributed to a variety of explanations, including (i) increase the total light trapped from far-field scattering, (ii) enhance light absorption due to electromagnetic near-field, (iii) promote electron transfer through hot electron generated from metal energetic relaxation, (iv) develop coherent plasmon-to-semiconductor energy transfer via plasmon resonant energy transfer (PRET).11 2


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However, metal nanoparticles directly incorporated into TiO2 nanocrystalline are exposed to iodide/triiodie electrolyte, which results in charge recombination and corrosion.12-14 For most of the previous efforts, core–shell structures, such as Au@SiO215-16, Ag@SiO2,17-18 Au@TiO219-20 and Ag@TiO2,21-23 have been designed to prevent metal nanoparticles from electrolyte. However, the shell of SiO2 would induce the loss of the photogenerated carriers owing to the insulating properties of SiO2.14, 24 The shell of TiO2 with electron transport properties as space isolation layer between the dyes and metal nanoparticles weaken LSPR enhancement.25 In order to control the thickness of TiO2 very precisely, the complex and expensive technique must be employed.26 Self-assembled monolayers (SAMs) of thiolates on metals as a form of nanotechnology has been studied from the 90s. Due to the high affinity of thiol, molecules with sulfur headgroup can form a closely arranged and firm self-assembled monolayer on the exposed surface of metals.27-28 It is attributed to the formation of strong covalent Au(I)-S ligation, which bound to the Au surface as thiolates with thermodynamic stability.29-30 Molecules with long alkyl ancillary group can exhibit favorable hydrophobicity and corrosion resistance properties.31-32 In the present study, Au NPs were incorporated into TiO2 nanocrystalline by electrodeposition. Then, dodecanethiol molecules were introduced to form a self-assembled organic shell on the bare Au NPs. (Scheme.1) Dodecanethiol molecules with long alkyl ancillary group will combine with Au NPs stably as an isolating layer to prevent the contact with electrolyte. Herein, Au NPs capped dodecanethiol molecules can achieve a win–win result of both taking full advantage of plasmon-enhanced light absorption of Au NPs and suppressing charge recombination with the triiodide ions in electrolyte. Meanwhile, Au NPs exhibit good electron-transport and electrons storage property, which promote the Fermi level affecting the open voltage (VOC). In addition, the variation of short current (JSC) essentially contacts with the monochromatic incident photon-to-electron conversion efficiency (IPCE) and the electron transfer mechanism at 3



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TiO2/dye/electrolyte interface have been investigated and discussed in detail.

Scheme. 1 The scheme of (a) TiO2-Dye, (b) Au-TiO2-Dye and (c) AuDodecanethiol-TiO2-Dye.

2. Experimental 2.1. Materials TiO2 paste (20 nm and 200 nm) and electrolyte (Heptachroma Co.); N719 (J&K Chemical, 99%); TiCl4 and Dodecanethiol (chemically pure, Sinopharm Group Co. Ltd); H2PtCl6·6H2O and HAuCl4·4H2O (Aladdin Co.); Surlyn film (60 µm) and FTO (8Ω/sq) (Wuhan Georg Science Instrument Co.). 2.2. Preparation of photoelectrodes The pure TiO2 photoanode was prepared as previous studies.33 Fluorine-doped tin oxide (FTO) glass substrates were cleaned by sonication in acetone, ethanol and deionized water, respectively. After drying treatment, the cleaned FTO glass substrates were treated for 15 min through ultraviolet and ozone pretreatment. TiO2 pastes of transport layer and scattering layer were coated onto the FTO substrates by the doctor-blade method. Then, FTO glass coated TiO2 were dried at 125oC for 6 min and sintered at 500oC for 30 min to remove organics. After sintering, these films were cooled and pretreated with aqueous solution (50 mM) of TiCl4 at 70oC for 30 min. Then, the treated films were rinsed with deionized water and sintered at 500oC for 30 min again to get the TiO2 photoelectrode. In order to prepare TiO2-Au photoelectrode, firstly, the TiO2 photoelectrode was wetted by chloroauric acid (HAuCl4) aqueous solution (0.3 mM) at 70oC and vacuumed to 4


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exhaust air in the porous TiO2 film. Secondly, the electrodeposition method was adopted to synthesize Au NPs in an aqueous solution containing 0.3 mM HAuCl4. Electrodeposition experiments were completed in a three-electrode configuration (the pure TiO2 photoelectrode with 0.25cm2 active area as the working electrode, Ag/AgCl electrode as the reference electrode and Pt foil as the counter electrode). The charge density was 1000 mA/cm2 with constant current on-time 0.002s. After electrodeposition, TiO2-Au photoelectrodes were rinsed with ethanol and deionized water. In order to prepare TiO2-Au-SR photoelectrode, TiO2-Au photoelectrode were immersed in an alcoholic solution containing 1.0 mM dodecanethiol (C12H26S) for 5h at 70oC. Then the photoelectrodes were subsequently rinsed by ethanol and acetone. Above three kinds of photoelectrodes were dried at 80oC before immersed into 0.5 mM N719 dye solution (acetonitrile: TBA=1:1) for 24h at room temperature. The counter electrode was prepared through spin-coating isopropyl alcohol solution of chloroplatinic acid (20.0 mM) on FTO substrates and then annealed at 450oC for 30 min. 2.3. Fabrication of photovoltaic devices The dye-adsorbed photoelectrodes and Pt catalytic electrodes were assembled using Surlyn film by thermo-compressor. The liquid electrolyte was introduced through a pre-punctured hole on the counter electrode. A hole of Pt electrode were punched by drilling machine and the liquid electrolyte was injected into the interspace at vacuum state. At last, a piece of glass was used to seal the hole. The active area of the DSSC was 0.5cm×0.5cm. These cells were labeled as TiO2-cell, TiO2-Au-cell and TiO2-Au-SR-cell, respectively. 2.4. Characterizations The microstructure and morphology of the films were examined by field emission scanning electron microscope (FESEM; JEOL JSM-7500F) and high resolution transmission electron microscopy (HRTEM; JEOL JEM-2010F). The content of Au element were measured by Inductively Coupled Plasma technique (ICP, Optima 7300DV). 5



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X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Scientific ESCALAB 250Xi equipment with Al Kα radiation. Alkyl contaminants of C 1s was set as referenced binding energies at 284.6 eV. The absorption spectroscopy was determined by UV-vis spectrophotometer (Shimadzu UV-2600). Mott–Schottky measurement was completed in a three-electrode configuration (TiO2, TiO2-Au, TiO2-Au-SR photoelectrodes (ca. 3 µm) as the working electrode respectively, the saturated calomel electrode (SCE) as the reference electrode, and the Pt foil as the counter electrode). The active area was 0.5cm×0.5cm. The electrolyte was composed of 0.1M potassium iodide (KI) and 0.01M iodine (I2) dissolving in ethylene carbonate and propylene carbonate a solution (1:1 volume ratio). After soaked in 0.1M NaOH solutions for 8h, the desorption solution was analyzed by UV-vis spectrophotometer. Photocurrent density–voltage (J–V) measurements were carried out by a solar light simulator (Newport, 94063A), which was adjusted with a standard silicon reference cell. The monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra were obtained by IPCE system (QEX10, PV Measurements). Electrochemical impedance spectra (EIS) were performed by electrochemical workstation (Autolab 320, Metrohm), which worked from 0.1 Hz to 1×105 Hz at different bias in the dark. The impedance data were fitted using the build-in Nova software of Autolab. Because of the potential drop, the voltage need to be rectified through the equation: Vcor=Vapp－JARts (Vcor: the corrected voltage; Vapp: The applied voltage; J: the dc current density; A: the geometric area; Rts: the total series resistance). Meanwhile, Rts=Rs+Rpt+(1/3)Rt (Rs: the series resistance; Rpt: the electron transfer resistance of Pt electeode; Rt: the transport resistance of photoanode) 34 The open circuit voltage decay (OCVD) was carried out by above electrochemical workstation with a shutter equipment. Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) was carried out on 470 nm blue LED, frequency range from 3 KHz to 0.1 Hz and 5% modulated light intensity condition by Zahner electrochemical workstation. The effects of size and amount of Au NPs on DSSC 6


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have been discussed in Supporting Information.

3. Result and discussion 3.1 Microstructure and component analyses

Figure. 1 (a)(b) The FESEM-BSE image of pure TiO2 nanoparticles and Auelectrodeposited TiO2 nanoparticles films respectively; (c) The STEM image of Au electrodeposited TiO2 nanoparticles; (d) The lattice resolved HRTEM image of the Au NPs on TiO2.

Figure. 1(a) and (b) are the field-emission scanning microscope backscattered electron image (FESEM-BSE) of pure and Au- electrodeposited TiO2 films respectively. Au NPs deposited on TiO2 with the diameter about 10 nm was expressed as bright white spots. Figure. 1(c) shows the STEM image of Au NPs on TiO2 NPs clearly. The lattice resolved HRTEM image of the Au NPs on TiO2 reveals the (101) planes of TiO2 and (200) planes of Au distinctly.8 The content of Au incorporated TiO2 measured through ICP technique was about 0.23wt%. From the red marked square in Figure. 1(d) we got the Fast Fourier transform (FFT) pattern (Figure. S1(a)) with the electron diffraction spots of TiO2 and Au. Through the soft of Digital Micrograph, the observed d-value of 2.007Å of Au 7



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(200) planes could calculated from Figure. S1(b).

Figure. 2 (a) Au4f XPS spectra of the TiO2-Au and TiO2-Au-SR films; (b) S2p XPS spectra of the TiO2-Au and TiO2-Au-SR films. The green curve is attributed to thiolates (Au(I)-SR). The purple curve is attributed to disulfide dimers.

In order to determine the surface composition and chemical states, XPS analyses were used to detect the TiO2-Au and TiO2-Au-SR films. As shown in Figure. 2(a), the XPS peaks at 84.45 eV and 84.03 eV correspond to Au 4f7/2 with satellite peaks of Au 4f5/2(∆≈3.7eV) at 88.15eV and 87.70 eV, respectively. When Au capped by thiol molecules, Au(I)-S bonds are formed, and Au atoms lose 5d electrons.35 As a result, the Au 4f peaks of TiO2-Au-SR film shift to higher binding energies(BE).36 This shift hints the surface of Au NPs exists in an oxidized state Au(I). In Figure. 2(b), The S 2p spectrum was fitted with two doublets with S2p3/2 and S2p1/2 peaks. The two doublets show in green (S2p3/2:162.01 eV, S2p1/2:163.24 eV) and purple (S2p3/2:162.65 eV, S2p1/2:163.84 eV) respectively, which both with an area ration of S 2p3/2:S 2p1/2 = 2:1 and a split of 1.2 eV. 37 The green curve with BE 162.01 eV indicates the dodecanethiol was bound to the surface of Au NPs as thiolates (Au(I)-SR)， which provides evidence for the successful formation of self-assembled organic shell.

38-39

According to previous reports, the unbound or free

thiol molecules might appear around at BE 164 eV.30, 38-40 However, the doublet peaks in purple don’t comply with the above rules. Considering the films were rinsed with ethanol and acetone carefully, the appearance of purple curve should be attributed to the disulfide 8


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dimers generated by X-rays induced damage instead of unbound thiol molecules.41 In contrast, no characteristic peaks can be found in XPS spectra of TiO2-Au films. 3.2 Photovoltaic performances

Figure. 3 (a) J–V measurements and (b) IPCE characteristics for TiO2-cell, TiO2-Au-cell and TiO2-Au-SR-cell.

Table. 1 The photovoltaic parameters of DSSC Sample

JSC

VOC

FF

η

Dye loading

(mA/cm2)

(V)

(%)

(%)

(mol/cm2)

TiO2-cell

16.58

0.69

64.0

7.32

2.13×10-7

TiO2-Au-cell

17.06

0.69

64.6

7.60

2.02×10-7

TiO2-Au-SR-cell

18.16

0.70

64.3

8.17

1.91×10-7

In order to explore the influence of Au and Au-S structure on the photovoltaic performance of DSSC. The current density–voltage (J–V) characteristics of the cells measured with an active area of 0.5cm×0.5cm are shown in Figure. 3(a) and Table. 1. The cell based on pure TiO2 photoanode demonstrates a photoelectric conversion efficiency (η) of 7.32 % (JSC=16.58 mA/cm2, VOC=0.69V, FF=64.0%). With the deposited Au NPs, η increases slightly to 7.60% which mainly due to the increase of Jsc. Subsequently, with the Au NPs treated by thiol molecules in TiO2-Au-SR-cell, a remarkable efficiency of 8.17% ( JSC=18.16 mA/cm2, VOC=0.70V, FF=64.3%) attributed 9



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to increase both in JSC and VOC was achieved, in spite of the decrease on the dye loading. The

corresponding

incident

monochromatic

photon-to-electron

conversion

efficiency (IPCE) spectra are shown in Figure. 3(b). Compare to pure TiO2 photoanode, the peak value of IPCE increased about 4% after deposited Au NPs. Furthermore, after the TiO2-Au photoanode was deal with thiol molecules, the peak maximum increased about 11.5%, which confirms the significant photoelectric benefits of the fence-like interface structure in the cell. Meanwhile, the trend of IPCE is consistently correlated with the trend of JSC and the enhancement of IPCE mainly concentrates upon 400-700 nm which is same as other reports.42 The JSC variation fundamentally result from the IPCE, which can be calculated according to following equations:43 λmax

J sc = qe

∫

I photo ( λ )IPCE ( λ ) d ( λ )

(1)

λmin

IPCE = LHE ×ηinj ×ηcc

(2)

where q is the elementary charge, LHE is the light-harvesting efficiency, ƞinj is the injection efficiency of electrons from the dye lowest unoccupied molecular orbital (LUMO) level to the TiO2 conduction band (CB), and ƞcc is the collection efficiency of the injected electrons at the conductive substrate (FTO glass). Firstly, LHE is directly related to the kinds and amounts of dye. Since Au NPs are directly deposited on the surface of TiO2, it is inevitable that the reduction of TiO2 active area will result in the decrease of dye loading (Table 1) to some degree. However, due to the localized surface plasmon resonances of Au NPs, the Au NPs serves as a role of something like an “antenna" to enhance the extinction coefficient (ε) of dye and help dye absorbing the visible spectrum more efficiently.6, 11 The variation of light absorption will be analyzed in UV-Vis absorption later. Secondly, since the over-potential (-∆G) between the CB of TiO2 and LUMO level of N719 is about 0.35 eV which is much larger than 0.2 eV that sufficient driving forces of electron injection from the dye to the CB of TiO2,43 10


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the ƞinj can be approximated as 100%.44 Finally, ƞcc relevant with the charge recombination at TiO2/dye/electrolyte interfaces will be discussed from EIS analysis.

3.2.1 UV-Vis absorption and Mott–Schottky analyses

Figure. 4 (a) The optical absorption spectra of the TiO2, TiO2-Au, TiO2-Au-SR films; (b) The net change of absorption of the TiO2-Dye, TiO2-Au-Dye, TiO2-Au-SR-Dye films and the

calculation

is:

Abs∆(TiO2-Dye)=Abs(TiO2-Dye)-Abs(TiO2);

Abs∆(TiO2-Au-Dye)=Abs(TiO2-Au-Dye)-Abs(TiO2); Abs∆(TiO2-Au-SR-Dye)=Abs(TiO2-Au-SR-Dye)-Abs(TiO2).

In Figure. 4(a), there are broad LSPR peaks centered at 538 nm resulting from the Au NPs. In addition, the LSPR of Au NPs was not influenced by thiol molecules absorbed on the surface of gold. Resonant wavelength of Au NPs (10 nm) approximately is 490 nm in air and it will increases with the increasing Au NPs size.45 It should be noted that the LSPR peaks of Au NPs deposited in porous TiO2 films are rather broad and red-shifted (538 nm) due to the high refractive index value (n=2.49) of the TiO2 substrate.46-48 Figure. 4(b) shows the relative changes of the dye N719 absorption with Au NPs and Au NPs after the dodecanethiol treatment. The relative increase of the absorption results from the LSPR of Au NPs, which can augment the dye excitation to enhance the light harvesting.28, 46, 49.

11



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Figure. 5 Mott-Schottky plots for TiO2, TiO2-Au and TiO2-Au-SR photoelectrodes.

The flat band potential Efb and the charge carrier concentration can be determined by the following equation:

( Csc )

−2

K T  2  E − E fb − B e  =  N Dεε 0 eA2

(3)

Where CSC is the capacitance of the space charge region, e is the elementary charge, ε is the relative dielectric constant of the TiO2 layer (dielectric constant for anatase TiO2=55) 50

, ε0 is the vacuum permittivity, E is the applied potential, KB is Boltzmann's constant, and

T is the absolute temperature. So, we can get the value of Efb through the fitted linear part of curve intersect X axis. The Efb values of the TiO2, TiO2-Au and TiO2-Au-SR extracted from Figure. 5 are -0.530V, -0.532V and -0.540V respectively. The flat-band potential shift to more negative values after deposited Au NPs and reach the minimum value with thiol treatment. This result should be attributed to metal-semiconductor junctions.51 The Fermi level under the conduction band of TiO2 has the same trend with the variation of Efb. Since the Voc of DSSC is dependent on the difference between the Efb of TiO2 and the redox potential of the electrolyte,33 the negative shift of Efb is favorable for the increase of Voc.

12


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3.2.2

Electron transport and recombination process

Electrochemical impedance spectra (EIS) is a powerful technique that has been employed to obtain the information regarding the electron transport and recombination kinetics of many functional systems, especially in DSSC.52-55 EIS was performed in dark condition at different applied voltages. The typical Nyquist plots are presented in Figure. S2(a) in the Supporting Information. Two semicircles obviously appear in the Nyquist plots. The first one in high frequency region corresponding to the electron transfer process at the I-/I3- electrolyte /Pt electrode interface is represented as the resistance (RPt) and capacitance (CPt). The second one in low-middle frequency is due to the charge recombination resistance (Rct) and chemical capacitance of the nanoparticles (Cµ) across the TiO2/dye/electrolyte interface. A straight-line segment about 45° can be find in the intermediate frequency range, which represent transport resistance (Rt) of electron diffusion process in porous TiO2 nanoparticles.55 The transmission line model (shown in Figure. S2(b)) with an extended distributed element (DX) was used to fit EIS data.53

Figure. 6 (a) electron transport resistance Rt and recombination resistance Rct, (b) the chemical capacitance Cµ, (c) transport time τtr and electron lifetime τn and (d) the electron 13



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collection efficiency ηcc as a function of the corrected voltage

By

compare

Rt

with

Rct,(Figure.

6(a))

due

to

the

excellent

metal

electro-conductivity,9 the cell with Au NPs exhibited smaller Rt, which was conducive to electron transport in porous TiO2 structure. And after the treatment of thiol molecules with long-chain-alkyl ligands, thiol molecules anchoring Au NPs firmly through Au-S bonds protected Au NPs from direct contacting with I-/I3- electrolyte (Scheme. 2(a)). Rct plays a significant role in suppressing the charge recombination at TiO2/dye/electrolyte interface. As shown in Figure. 6(b), it can be expressly observed that in the presence of the Au NPs, Cµ was enhanced to different degrees. It is commonly believed that Cµ represent the trap states localized below the conduction band minimum in DSSC. Cµ values can be described by the following equation.56

 qVcor  C µ = C 0, µ exp α  kBT 

(4)

Where kB is the Boltzmann constant, T is the absolute temperature, q is the elementary charge and α is a parameter that represents the depth of the trap energy distribution. In our three kind of cells, α values are similar and equal to 0.315 between the most literature reported range 0.2～0.4.55

Scheme. 2 (a) Dodecanethiol- Au NPs incorporated TiO2; (b) energy level scheme with electron transport and electron storage mechanism of Au NPs. 14


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In order to further illustrate the variation of Cµ and the transport and recombination of electron, it is necessary to list an energy level scheme (Scheme. 2(b)) to explain the mechanism. Under illumination, photoelectrons is produced from excited dyes and it obey a rule from high energy level to low energy level generally. The Fermi level of Au NPs (Efg), though affected by the particle size, is expected to be close to the value of bulk Au (Ef =+0.45 V vs NHE), which is more positive than the conduction band of TiO2 (ECB =-0.5 V vs NHE).47 Since photoelectrons inject from LUMO level of dye to CB of TiO2, and Au NPs can act as electron sinks to store more electrons until the Fermi level of TiO2 and Au being equilibration.28, 42, 46 Meanwhile, the electron in Au NPs can also transfer to CB of TiO2, which is described as “hot electron transfer” process.11 Nevertheless, Au NPs exposed to I-/I3- redox couples can act as recombination sites of excited electrons14, 46. Here, we adopted thiol molecules capping Au NPs insulating electrolyte to retard charge recombination. The increase of Cµ of TiO2-Au-SR-cell indicates that Au NPs exhibited more capable electron storage ability after thiol molecules treatment. Compare with TiO2-Au-cell, the more stored electrons in TiO2-Au-SR-cell result in a more negative uniform Fermi level (Ef’) of TiO2-Au NPs, which is consisted with the results of Mott– Schottky analysis. It is the origin of the slightly improved VOC of device with Au NPs and thiol molecules. The accumulated electrons and recombination can also be interpreted by the open circuit voltage decay (OCVD) analysis, (Figure. S3) which also proved the electron storage ability of Au NPs, in accord with the EIS analyses. The kinetics of electron transport and recombination is usually discussed in terms of the electron transport time (τtr=RtCµ) and electron lifetime (τn=RctCµ) which can be extracted from EIS data.57-58 IMPS/IMVS also been discussed as shown in Figure. S4 and Table. S1. From Figure. 6(c), both TiO2-Au-cell and TiO2-Au-SR-cell exhibit shorter electron transport time than pure TiO2-cell, which should be attributed to better electron transport performance under depositing Au NPs. The TiO2-Au-SR-cell with maximum 15



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electron lifetime is due to the presence of thiol molecules retarding charge recombination and larger Cµ value. Ideally, electrons at CB of TiO2 should transport through the mesoporous TiO2 film to arrive at FTO as fast as possible and avoid reacting with acceptors in the electrolyte. Hence, electron transport and recombination is one pair of competitive processes which can be reflected on the electron collection efficiency (ηcc) calculated by equation (5).55

η cc =

1

τ τ

(1+

As

shown

in

tr

(5)

)

n

Figure.

6(d),

the

ηcc

values

are

sequenced

as

TiO2-Au-SR-cell>TiO2-Au>TiO2-cell. As a result, the enhancement of JSC not only own to the increase of LHE by LSPR effect of Au NPs, but also attribute to the increase of ηcc.

4. Conclusions In summary, the Au NPs was successfully incorporated into porous TiO2 nanocrystalline films by electrodeposition method. The structure of Au NPs capped dodecanethiol molecules was confirmed by SEM, TEM and XPS. In this work, the LSPR of Au NPs enhanced the light absorption obviously. The Au-SR bonds formed at the surface didn't decay the light absorption. The photoelectric conversion efficiency of TiO2-cell TiO2-Au-cell and TiO2-Au-SR-cell, is 7.32%, 7.60% and 8.17%, respectively. Dodecanethiol molecules can be anchored on Au NPs surface by Au-S bond, insulating the Au surface from electrolyte and retarding the charge recombination at

the

TiO2/dye/electrolyte interface. Compared to pure TiO2-cell, the photoelectric conversion efficiency of TiO2-Au-SR-cell increased by 10.4%, which is mainly ascribed to the increase of short current integrated by IPCE ( IPCE = LHE ×ηinj ×ηcc ). The Au NPs capped thiol not only enhanced the LHE but also favored electron transport and suppress recombination to increase ηcc. On the other hand, Au-SR with electrons storage property elevated the Fermi level and Voc. The research results confirmed an effective concept to 16


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construct a fence-like interface structure to improve the competition of electron transport than recombination and improve the photoelectric conversion efficiency of DSSC.

Acknowledgement The authors acknowledge the support of the National Natural Science Foundation of China (51472154, 51302164).

Supporting Information The fast Fourier transform pattern of TEM; typical Nyquist plots and the transmission line model with an extended distributed element; the open circuit voltage decay analysis; the IMPS/IMVS plots; the analyses of the size and amount of Au NPs on DSSC.

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TOC 32x13mm (300 x 300 DPI)


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