nanorod photoanodes

Accepted Manuscript Improved performance of quantum dot-sensitized solar cells based on TiO2 nanoparticle/nanorod photoanodes Qiqian Gao, Xueyu Zhang, Lianfeng Duan, Xiaoju Li, Xuesong Li, Yue Yang, Qiang Yu, Wei Lü PII:

S0925-8388(17)31554-2

DOI:

10.1016/j.jallcom.2017.05.006

Reference:

JALCOM 41736

To appear in:

Journal of Alloys and Compounds

Received Date: 3 March 2017 Revised Date:

24 April 2017

Accepted Date: 1 May 2017

Please cite this article as: Q. Gao, X. Zhang, L. Duan, X. Li, X. Li, Y. Yang, Q. Yu, W. Lü, Improved performance of quantum dot-sensitized solar cells based on TiO2 nanoparticle/nanorod photoanodes, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.05.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Graphical Abstract In this paper, a TiO2 nanoparticle/nanorod architecture electrode termed as P/R was fabricated and applied as photoanode of QDSSCs. The P/R composite architecture

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electrodes showed improved photovoltaic performance compared to single crystal

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TiO2 nanorod arrays. The champion efficiency of 4.42 % has been achieved.

ACCEPTED MANUSCRIPT Improved performance of quantum dot-sensitized solar cells based on TiO2

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nanoparticle/nanorod photoanodes

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Qiqian Gao, Xueyu Zhang*, Lianfeng Duan, Xiaoju Li, Xuesong Li, Yue Yang,

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Qiang Yu, Wei Lü

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Key Laboratory of Advanced Structural Materials, Ministry of Education & Advanced

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Institute of Materials Science, Changchun University of Technology, Changchun

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130012, P. R. China

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[email protected]

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Fax: +86-431-85716426; Tel: +86-431-85716421

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* To whom all correspondence should be addressed.

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Abstract Herein, a TiO2 nanoparticle/nanorod composite architecture was suggested for

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improving performance of quantum dot sensitized solar cells (QDSSCs), which was

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fabricated on transparent conductive glass substrates (FTO) via hydrothermal method

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followed by spin-coating, and further used as photoanode of QDSSCs. Compared

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with the TiO2 nanorod photoanode, the power conversion efficiency (PCE) of the

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QDSSCs with this architecture was obviously improved. The champion efficiency of

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4.42 % was achieved, showing a 33.5% enhancement in PCE. The improved PCE

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mainly resulted from the increase of Jsc, which increased from 9.80 (nanorod

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architecture) to 15.48 mA/cm2 (nanoparticle/nanorod composite architecture). The

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advantages of the composite structure photoanode consist in the superior light

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scattering capability, faster electron transport and increased photogenerated electron

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concentration.

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Keywords: TiO2 nanoparticle; quantum dot-sensitized solar cell; TiO2 nanorod array;

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photoanode.

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1. Introduction Nowadays, new materials and devices for energy conversion and storage have

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attracted great attentions due to the excessive exhaustion of fossil fuels. [1] In the past

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few decades, quantum dot sensitized solar cells (QDSSCs) have been widely studied

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as one of the most potential photovoltaic devices because of its unique properties of

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size-tunable band gap effect, high absorption coefficient, quantum confinement effect

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and the multiple exciton generation.[2] As a derivative of dye sensitized solar cells

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(DSSCs), QDSSCs inherit its typical structure containing: photoanode, electrolyte and

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counter electrode.[3] The photoanode as the important component of QDSSCs is

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generally prepared by TiO2. It had been demonstrated that the morphology and

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assembling structures of TiO2 affect the photoelectric property of QDSSCs deeply. [4]

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Since the ground-breaking work on colloidal TiO2 films based low-cost DSSCs

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by Brian O’Regan and Michael Grätzel in 1991, [5] various morphologies of TiO2

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nanostructure have been fabricated and applied in DSSCs and QDSSCs, such as

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nanoparticle, nanowire, nanosheet, nanorod and nanotube. [6-10] The frequently-used

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porous TiO2 nanoparticle thin films possess large specific surface area and more

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position for quantum dots attachment. However, the existence of too many grain

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boundaries in porous TiO2 nanoparticle thin films leads to serious charge

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recombination and low electron diffusion coefficient.

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disadvantages, Liu Hong et al. fabricated single crystal rutile phase TiO2 nanorod

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arrays thin film on transparent conductive glass substrates (FTO) by hydrothermal

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synthesis method and obtained power conversion efficiency (PCE) of 2.54% with CdS

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[11]

In order to avoid these

ACCEPTED MANUSCRIPT sensitizer. [12] TiO2 nanorod arrays provided a direct pathway for electron transparent

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from the CdS quantum dots to FTO and avoided particle-to-particle jumping that

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occurs in nanoparticle thin films. In addition, Lin Changjian et al. boosted the

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efficiency of DSSCs from 6.59% to 7.91% by optimizing TiO2 nanorod arrays to

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porous rutile TiO2 nanorod. [13] They certified that etched TiO2 nanorod arrays possess

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more excellent light-scattering ability than porous TiO2 nanoparticle thin film by

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diffused reflectance spectra. As discussed above, the nanoparticle and nanorod TiO2

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photoanodes had their own advantages and disadvantages, respectively. The

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combination of TiO2 nanoparticle/nanorod composite photoanodes could improve the

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PCE of DSSCs. However, there are few reports about the composite structures of

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TiO2 nanoparticle and nanorod applied as photoanodes of QDSSCs.

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In this work, a TiO2 nanoparticle/nanorod architecture electrode termed as P/R

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was fabricated and applied as photoanode of QDSSCs. The effect of TiO2 nanoparticle

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film’s thickness on PCE has been investigated systematically. Based on our results,

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the P/R composite architecture electrodes showed improved photovoltaic performance

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compared to single crystal TiO2 nanorod arrays. The champion efficiency of 4.42 %

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has been achieved.

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2. Experimental

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2.1 Materials Conducting glass (FTO) were supplied by Zhuhai Kaivo Optoelectronic

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Technology Co., Ltd. Tetrabutyl titanate was purchased from Shanghai Chemicals Co.,

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Ltd. Commercial P25, selenium powder and acetic acid were bought from Aladdin.

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Cadmium acetate, methanol, 2-propanol and acetone were purchased from Beijing

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Chemical Works. Na2S·9H2O, CuSO4 was acquired from Tianjin Guangfu technology

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development Co., LTD. Anhydrous sodium sulfite was obtained from Damao

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Chemical Reagent, Tianjin.

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2.2 Preparation of TiO2 nanorod array

Vertical TiO2 nanorod array was fabricated on transparent conductive glass [14]

substrates (FTO) via hydrothermal synthesis method reported previously.

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Specifically, 15 mL of hydrochloric acid was mixed with 15 mL deionized water

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under the condition of stirring. The mixture was then stirred for 20 minutes followed

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by the addition of 0.5 mL of tetrabutyl titanate dropwise. The mixture was transferred

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into a 50 mL Teflon reaction kettle. FTO glasses were rinsed with ethyl alcohol,

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acetone and deionized water respectively, then put into the autoclave with conducting

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plane down. The hydrothermal process was conducted at 150

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2.3 Preparation of TiO2 nanoparticle/nanorod architecture

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for 12 h.

The P25 paste was prepared using modified method reported previously.

[15]

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Typically, P25 paste was prepared by mixing α-terpinol(1.6g), ethye cellulose (0.2g)

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and P25 (0.4g) in anhydrous ethanol (4.25ml) and stirred for 24 h. P/R architecture

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was obtained by spin-coating P25 paste on the as-prepared TiO2 nanorod arrays. The

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thickness of nanoparticle film was controlled by spin-coating times, and termed as

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P(1)/R, P(2)/R and P(3)/R respectively. The as-prepared TiO2 P/R architecture was

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subjected to a sintering process in air at 450

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fabrication process is shown in Scheme 1.

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2.4 Preparation of ZnS/CdSe/CdS/TiO2 nanoparticle/nanorod photoanode

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for 30 min. The P/R architecture

The TiO2 P/R architecture was further decorated by CdSe/CdS QDs and ZnS

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passivation layer. The CdS QDs and ZnS passivation layers were prepared by

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successive ionic layer adsorption and reaction method (SILAR). [16] In detail, The

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TiO2 P/R architecture was immersed in a 0.05 M cadmium acetate methanol solution,

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methanol, 0.05 M Na2S methanol and deionized water (v/v, 1:1) solution and

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methanol for 30 seconds respectively. The four-step dipping procedure was termed as

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one cycle. According to our group’s previous study, 10 cycles of CdS showed the best

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efficiency.

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In detail, Na2SeSO3 solution was prepared by refluxing 80 mM Se power in an

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aqueous solution of 200 mM Na2SO3 at 120

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80 mM Cd(NO3)2·4H2O solution and 120 mM nitrilotriacetic acid solution were

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mixed together to prepare Cd source in a volume ratio of 1:1. The prepared Na2SeSO3

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solution and Cd source solution were mixed together (v/v, 1;1). Then the CdS/P/R

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architecture electrodes were dipped into the mixed solution in darkness at room

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temperature for about one and a half hours. The as-prepared photoanode structure

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named as CdSe/CdS/P/R. Finally, Two layers of ZnS were deposited on

The CdSe QDs were deposited by chemical bath deposition (CBD). [18]

for about 3 hours and used as Se source.

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ACCEPTED MANUSCRIPT CdSe/CdS/P/R by immersing photoanodes into a 0.1M Zn(CH3COO)2·2H2O

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methanol solution and a 0.1M Na2S methanol/deionized water (v/v, 1:1) solution

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sequentially.

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2.5 Preparation of CuS counter electrodes

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CuS counter electrodes were prepared by chemical bath deposition (CBD)

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method. [19] Na2S2O3 aqueous solution (1M) and CuSO4 aqueous solution (1M) were

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mixed together (v/v, 4:1). The PH of the mixed solution was adjusted to 2 with acetic

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acid. Then, the FTO glasses were immersed into 100 mL as-prepared mixed solution

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heated to 70

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and kept for 4 h. After cooling down to the room temperature in the air,

the substrates were washed and dried in air and then heated to 130

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min.

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2.6 Fabrication of QDSSCs

and kept for 30

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The as-prepared ZnS/CdSe/CdS/P/R photoanode, polysulde electrolyte that

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consisted of 1 M Na2S and 1M S in methanol and deionized water solution (v/v=7:3)

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and CuS counter electrode were assembled to a sandwich-type cell.

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2.7 Characterization

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The cross-sectional microstructures and thickness of P/R were observed by field

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emission scanning electron microscopy (FESEM, S4800, Hitachi). The phase

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structure was analyzed by X-ray diffraction (XRD) (D-MAX II A X-ray diffractmeter).

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Electro-chemical impedance spectroscopy (EIS) was acquired with an electrochemical

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workstation of Solartron Impedance Analyzer. The diffuse reflectance spectra and

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adsorption spectra of P/R and ZnS/CdSe/CdS/P/R were measured on a UV/Vis-NIR

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ACCEPTED MANUSCRIPT spectrophotometer (UV-3150). The X-ray photoelectron spectroscopy (XPS) was

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performed by a VG ESCALAB MKII spectrometer to confirm chemical composition

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features. IPCE was measured by Solar Cell Scan 100 (Solar Cell QE/IPCE

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Measurement System, Zolix Instruments Co.,Ltd.). The J-V performances were

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measured using a Keithley 2400 source meter under AM 1.5 illumination with Zolix

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ss150 solar simulator (Zolix Instruments Co.,Ltd., Beijing).

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3 Results and discussion

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Fig. 1 shows the microstructures and thickness images of R and P/R photoanode.

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The nanorods are highly ordered and vertical on the substrate (Fig 1a, b). The inset in

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Fig. 1(a) is a typical SEM image of the TiO2 nanorod. It reveals that the nanorods are

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tetragonal in shape and the sides of the nanorods are smooth. There are many step

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edges on the top surface of nanorods, which could be the substrates for further growth

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of the TiO2. Fig. 1(b) is FESEM image of TiO2 nanorods in cross-sectional view,

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confirming that the nanorods are nearly vertical to FTO glass and about 4 µm in

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length. Fig. 1(c) shows the image of TiO2 P(2)/R in top view, as can be seen, the

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surface of spin-coated P25 thin film is rough and rugged, which is beneficial to

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adsorption of quantum dot sensitizers. Typical cross-sectional view of TiO2 P(1)/R,

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P(2)/R and P(3)R are shown in Fig. 1(d), (e) and (f). It shows that TiO2 nanoparticles

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were spin-coated on TiO2 nanorods tightly without cracks. The thickness of each

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spin-coated layer was about 5 µm.

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XRD was employed to investigate the crystalline features of the as-prepared

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samples as shown in Fig. 2. For FTO substrates, several characteristic peaks attributed

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ACCEPTED MANUSCRIPT to SnO2 could be observed. The TiO2 nanorod grown on FTO exists extra rutile phase

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crystal TiO2 diffraction peaks at 36.3° and 64.72° corresponding to (101) and (002)

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plane respectively. [20] The (002) peak located at 64.72° indicates that TiO2 nanorods

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were well crystallized and grew in [001] direction. Compared with TiO2 nanorod, the

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additional peaks appeared in TiO2 particle/nanorod/FTO can attributed to (101), (200),

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(105) and (211) planes of anatase phase TiO2. [21] This result is coincident with P25

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(99.8%, metals basis, 25nm, anatase phase) obtained from Aladdin.

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Fig. 3 shows cross-sectional EDS elemental mapping of CdSe/CdS/P(2)/R sample,

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where Cd, S and Se were homogeneous distribution in the P(2)/R film, indicating both

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TiO2 nanoparticles and nanorods had been deposited CdSe/CdS quantum dots

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successfully.

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XPS was carried out to further study the chemical environment and composition

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of the as-prepared P/R photoanode (Fig. 4). The XPS survey scan spectra in Fig. 4a

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affirms the formation of CdSe/CdS quantum dots on the TiO2 P(2)/R sample. The Ti

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2p1/2, 2p3/2 centered at 464.1 eV, 458.3 eV and O1s peak centered at 531.2eV (Fig. 4 b

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and c) are in agreement with pure TiO2. [22] The peaks located at 411.6 eV and 404.9

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eV correspond to Cd3d3/2 and Cd3d5/2 respectively (Fig. 4d). [23] The 6.7 eV difference

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between the binding energies of Cd3d5/2 and Cd3d3/2 peaks affirms the form of Cd2+.

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The spectrum of sulfur element in Fig. 4e shows one peak at 161.1 eV which is

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ascribed to S2− in CdS. Fig. 4f shows two peaks situated at 54.3 eV and 53.6 eV

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associated with the Se3d3/2 and Se3d5/2, which may be due to Cd-Se bonds.[24]

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Fig. 5 shows the J-V curves of QDSSCs with different photoanode structures.

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ACCEPTED MANUSCRIPT The detailed photovoltaic parameters are summarized in Table 1. Compared with

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CdSe/CdS/R structure, the PCE of QDSSCs based on CdSe/CdS/P(1)/R and

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CdSe/CdS/P(2)/R increased from 3.31% to 3.94% and 4.42% respectively. The

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enhancement was about 33.5%. The PCE improvement of the cells mainly comes

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from the increase of Jsc. The thickness of TiO2 nanoparticle thin films affects the

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photovoltaic performance of QDSSCs obviously. The max Jsc of 15.48 mA/cm2 and

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PCE of 4.42% have been achieved with the thickness ~9 µm, which could be

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attributed to more quantum dot sensitizers adsorbed on the photoelectrode, increasing

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concentration of photogenerated electrons which leads to higher photocurrent.

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Moreover, nanoparticle films with stronger light-scattering effects would also improve

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the light harvesting efficiency resulting in a higher Jsc, which will be confirmed by

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UV-vis diffuse reflectance spectra later. Further increase of thickness to 15µm induces

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the decay of cell performance. The Jsc of CdSe/CdS/P(3)/R cell decreased from 15.48

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mA/cm2 to 9.19 mA/cm2, which can be attributed to more recombination centers and

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defects in the thicker nanoparticle film. The photogenerated electrons would loss

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during long distance transporting. The open-circuit voltage decreases slightly with the

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thickness increasing. It can be ascribed to the enlargement of the surface area, which

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provides additional charge-recombination centers for higher recombination rates

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within the upper TiO2 nanoparticle thin films.

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Incident photon-to-current conversion efficiency (IPCE) had been measured in

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order to evaluate electron generation characteristics of QDSSCs. Fig. 6 shows the

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IPCE spectra of QDSSCs based on CdSe/CdS/R and CdSe/CdS//P(2)/R as a function

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ACCEPTED MANUSCRIPT of the illumination wavelength. The IPCE of CdSe/CdS//P(2)/R was higher than that

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of CdSe/CdS/R in the whole wavelength region especially in 350-600 nm. The IPCE

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was improved from visible to near infrared radiation region apparently, which was

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consistent with the Jsc parameters as shown in Table 1. The improvement in IPCE

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results came from sufficient quantum dot sensitizers adsorption and light

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harvesting.[25]

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Moreover, in order to explore the reasons of higher Jsc, UV-vis diffuse

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reflectance spectra had been tested. As shown in Fig. 7, the reflectance of the TiO2

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P(2)/R is higher than those of the TiO2 R thin film. This result indicates that TiO2

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P(2)/R possesses stronger light-scattering capabilities which can directly lead to

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higher light-harvesting efficiency. [26]

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Fig 8 shows the absorption spectra of CdSe/CdS/R and CdSe/CdS//P(2)/R

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electrodes. The CdSe/CdS//P(2)/R electrodes show increased absorption capacity

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(350-650nm) and extended absorption region (650-800nm) compared to the

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CdSe/CdS/R electrodes. This absorption spectra further certifies the enhanced

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light-harvesting properties of the CdSe/CdS//P(2)/R electrodes.

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Based on the results shown above, the QDSSCs prepared with TiO2 P/R

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composite structure photoanodes show a remarkable enhancement of the photovoltaic

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performance compared to the single TiO2 nanorods photoanode. The highly improved

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PCE can be mainly attributed to the increase of Jsc. The advantages of the TiO2 P/R

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composite structure photoanode consist in the superior light scattering capability,

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faster electron transport and increased photogenerated electron concentration. The

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ACCEPTED MANUSCRIPT TiO2 nanorods serve as direct transport route for photogenerated to FTO substrate,

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avoiding the scattering among TiO2 nanoparticles. The additional coated TiO2

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nanoparticles provided more effective space for QD deposition, which would increase

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coverage of QDs on anodes, thus increasing the light harvesting. In addition, as shown

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in Fig.1, the TiO2 nanoparticle layer could serve as an effective light scattering layer

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which decrease the light reflection as shown in Fig.7. Therefore, the further coating of

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TiO2 nanoparticles on TiO2 nanorods induces the increase of incident light utilization

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and further adsorbed QDs would enhance the photogenerated electron concentration.

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However, as indicated above, only suitable thickness of TiO2 nanoparticle layer can

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enhance PCE effectively. The thinner layer is not enough for effective light scattering.

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The thicker layer would induce decrement in cell performance because

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photogenerated electrons would be lost during transport between boundary of adjacent

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TiO2 nanoparticles.

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4. Conclusions

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In summary, a combined photoanode consisting of TiO2 nanorod and

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nanoparticle had been fabricated on FTO substrate via hydrothermal synthesis and

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spin-coating method, which shows higher photoelectric properties especially in Jsc

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compared to pure TiO2 nanorod array structure. In addition, the effect of different

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thickness of nanoparticle layer on cell performance had been investigated through J-V

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curve. The QDSSCs prepared with the CdSe/CdS//P(2)/R photoelectrode achieved the

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best PCE of 4.42%. The enhanced performance is mainly ascribed to enhanced light

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scattering effect based on the investigation of the IPCE, diffuse reflectance spectra

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and absorption spectra. Present work provides a new direction for improving

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performance of QDSSCs.

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Acknowledgements

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The authors are grateful for funding by the National Natural Science Foundation of

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China (grant nos. 61376020, 61574021 and 61604017). Science and Technology

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Department of Jilin Province (20140414024GH).

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[21] H. J. Koo, Y. J. Kim, Y. H. Lee, et al., Nano-embossed Hollow Spherical TiO2 as

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Bifunctional Material for High-Efficiency Dye-Sensitized Solar Cells, Adv.

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Mater. 20 (2008) 195-199.

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[22] F. Chang, J. Zhang, Y. C. Xie, et al., Fabrication characterization and

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photocatalytic performance of exfoliated g-C3N4-TiO2 hybrids, Appl. Surf. Sci.

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311 (2014) 574-581.

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[23] L. J. Liu, J. N. Hui, L. L. Su, et al., Uniformly dispersed CdS/CdSe quantum dots

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co-sensitized TiO2 nanotube arrays with high photocatalytic property under

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visible light, Mater. Lett. 132 (2014) 231-235.

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[24] C. Li, H. F. Zhang, C. W. Cheng, CdS/CdSe co-sensitized 3D SnO2/TiO2 sea

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urchin-like nanotube arrays as an efficient photoanode for photoelectrochemical

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hydrogen generation, RSC Adv. 6 (2016) 37407-37411. [25] M. L. Wang, C. G. Huang, Y. G. Cao, et al., Dye-sensitized solar cells based on

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nanoparticle-decorated ZnO/TiO2 core/shell nanorod arrays, J Phys. D Appl.

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Phys. 42 (2009) 155104.

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[26] J. Y. Wang, S. H. Qu, Z. C. Zhong, et al., Fabrication of TiO2

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nanoparticles/nanorod composite arrays via a two-step method for efficient

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dye-sensitized solar cells, Prog. Nat. Sci. 24 (2014) 588-592.

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ACCEPTED MANUSCRIPT Figure captions

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Scheme 1. The fabrication processes of P/R architecture.

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Fig. 1: FESEM images of TiO2 nanorods in top view (a) and cross-sectional view (b)

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and the inset in panel (a) is a typical HR-SEM image of the TiO2 nanorod. FESEM

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image of TiO2 P(2)/R in top view(c). Typical cross-sectional view of TiO2 P(1)/R (d),

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P(2)/R (e) and P(3)/R (f).

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Fig. 2: The XRD patterns of FTO substrate, TiO2 nanorod/FTO and TiO2

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particle/nanorod/FTO.

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Fig. 3: FESEM images of CdSe/CdS/P(2)/R in cross section (a) and EDS elemental

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mapping of Ti(b), O(c), Cd(d), S(e), Se(f).

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Fig. 4 XPS spectra of CdSe/CdS/P(2)/R photoanode: survey (a), Ti2p (b), O1s (c),

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Cd3d (d), S2p (e), Se3d (f).

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Fig. 5: J-V curves of QDSSCs based on different layers of nanoparticle thin films (2

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cycles of ZnS).

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Fig. 6: IPCE curves of QDSSCs(passivated with 2 cycles of ZnS) based on

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CdSe/CdS/R and CdSe/CdS//P(2)/R.

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Fig. 7: Normalized diffuse reflectance spectra of TiO2 P(2)/R and R thin films

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deposited on FTO

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Fig. 8: Absorption spectra of CdSe/CdS/R and CdSe/CdS//P(2)/R.

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Voc (mV)

Jsc (mA/cm2)

FF (%)

PCE(%)

CdSe/CdS/R

633

9.80

53.4

3.31

CdSe/CdS/P(1)/R

629

14.05

44.6

3.94

CdSe/CdS/P(2)/R

623

15.48

45.9

4.42

CdSe/CdS/P(3)/R

578

9.19

50.0

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2.66

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ACCEPTED MANUSCRIPT HIGHLIGHT A TiO2 nanoparticle/nanorod composite architecture was prepared for QDSSCs.




A suitable thickness of TiO2 nanoparticle layer can enhance PCE effectively.




The highly improved PCE can be mainly attributed to the increase of Jsc.

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