200-nm long TiO2 nanorod arrays for efficient solid-state PbS quantum dot-sensitized solar cells

200-nm long TiO2 nanorod arrays for efficient solid-state PbS quantum dot-sensitized solar cells

Accepted Manuscript 200-nm long TiO2 nanorod arrays for efficient solid-state PbS quantum dot-sensitized solar cells http://www.journals.elsevier.co...

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Accepted Manuscript

200-nm long TiO2 nanorod arrays for efficient solid-state PbS quantum dot-sensitized solar cells

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

Zhengguo Zhang , Chengwu Shi , Kai Lv , Chengfeng Ma , Guannan Xiao , Lingling Ni PII: DOI: Reference:

S2095-4956(17)30472-2 10.1016/j.jechem.2017.08.017 JECHEM 391

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

30 May 2017 15 July 2017 21 August 2017

Please cite this article as: Zhengguo Zhang , Chengwu Shi , Kai Lv , Chengfeng Ma , Guannan Xiao , Lingling Ni , 200-nm long TiO2 nanorod arrays for efficient solid-state PbS quantum dot-sensitized solar cells, Journal of Energy Chemistry (2017), doi: 10.1016/j.jechem.2017.08.017

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Highlights



TiO2 array with length of 200 nm, diameter of 20 nm and areal density of 720 μm-2. The crystal size of PbS QDs remained unchanged with the increase of SILAR

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cycles.

Preparation of all solid-state PbS QD sensitized TiO2 nanorod array solar cells.



The best photoelectric conversion efficiency of all solid-state QDSSCs was

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4.12%.

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200-nm long TiO2 nanorod arrays for efficient solid-state PbS quantum dot-sensitized solar cells

Zhengguo Zhanga,b, Chengwu Shia,*, Kai Lva, Chengfeng Maa, Guannan Xiaoa, Lingling Nia a

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, Anhui, China

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School of Chemistry and Chemical Engineering, Beifang University of Nationalities, Yinchuan

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750021, Ningxia, China Abstract

To ensure the infiltration of spiro-OMeTAD into the quantum dot-sensitized photoanode and to consider the limit of the hole diffusion length in the spiro-OMeTAD layer, a rutile TiO2 nanorod

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array with a length of 200 nm, a diameter of 20 nm and an areal density of 720 μm−2 was successfully prepared using a hydrothermal method with an aqueous-grown solution of 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 °C for 75 min. PbS quantum dots were deposited by a spin coating-assisted successive ionic layer adsorption and reaction (spin-SILAR),

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and all solid-state PbS quantum dot-sensitized TiO2 nanorod array solar cells were fabricated using spiro-OMeTAD as electrolytes. The results revealed that the average crystal size of PbS

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quantum dots was ~7.8 nm using Pb(NO3)2 as the lead source and remain unchanged with the increase of the number of spin-SILAR cycles. The all solid-state PbS quantum dot-sensitized TiO2

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nanorod array solar cells with spin-SILAR cycle numbers of 20, 30 and 40 achieved the photoelectric conversion efficiencies of 3.74%, 4.12% and 3.11%, respectively, under AM 1.5G

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illumination (100 mW/cm2).

Keywords: TiO2 nanorod array; PbS quantum dot; Spiro-OMeTAD; All solid-state sensitized solar

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cell

* Corresponding author. Tel: +86 551 62901450; Fax: +86 551 62901450; E-mail: [email protected], [email protected] This work was supported by the National Natural Science Foundation of China (51272061, 51472071). 1. Introduction Quantum dot-sensitized solar cells (QDSSCs) are usually composed of quantum dot 2

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(QD)-sensitized metal oxide photoanodes, electrolytes and counter electrodes [1,2]. The microstructure of metal oxide photoanodes, the preparation methods of QDs and the chemical composition of electrolytes strongly affect the photovoltaic performance of QDSSCs. For PbS QDSSCs using liquid electrolytes, Tian et al. fabricated QDSSCs using mesoporous TiO2 thin films with a thickness of 10 μm and PbS QDs prepared by successive ion layer absorption and reaction (SILAR). The corresponding QDSSCs exhibited a photoelectric conversion efficiency

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(PCE) of 4.01% [3]. Liu et al. assembled QDSSCs using TiO2 nanotube arrays with lengths of 5.4 μm and deposited PbS QDs using the electric field assisted chemical bath method, obtaining a PCE of 3.41% [4]. Jia and Yi prepared TiO2 nanorod arrays with lengths of 2 μm and diameters of 150 nm for QDSSCs, resulting in a PCE of 0.77% [5]. For PbS QDSSCs using solid-state

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electrolytes, Seok et al. applied P3HT and PEDOT:PSS as the electrolytes, and the corresponding solid-state QDSSCs with a 1-μm thick mesoporous TiO2 thin film and oleic acid-capped PbS QDs achieved a PCE of 2.9% [6]. When the 1-μm thick mesoporous TiO2 thin film and oleic acid-capped PbS QDs were replaced with a 2-μm thick mesoporous TiO2 thin film and PbS QDs

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via SILAR, the corresponding solid-state QDSSCs PCE decreased to 2.0% [7]. Kim et al. applied an aqueous solution of an ammonium hexafluorotitanate/boric acid mixture and TiCl4 aqueous

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solution to sequentially grow 2-μm long ZnO nanorod arrays and obtained a TiO2-coated ZnO nanorod array with a length of 1 μm and a diameter of 150 nm. The corresponding QDSSCs with

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P3HT and PEDOT:PSS reached a PCE of 3.9% [8]. To the best of our knowledge, the rutile TiO2 nanorod array with a length of 200 nm, a diameter of 20 nm and an areal density of 720 μm−2 has

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been rarely reported to fabricate all solid-state QDSSCs by considering the infiltration of spiro-OMeTAD into the QD-sensitized photoanode and the limit of the hole diffusion length in

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the spiro-OMeTAD layer. In this work, the 200-nm long rutile TiO2 nanorod arrays were prepared using hydrothermal

methods with an aqueous-grown solution containing 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 °C for 75 min. The PbS QDs were deposited on the TiO2 nanorod array using the step-by-step repeated spin-coating of Pb(NO3)2, Na2S and 1,2-ethanedithiol (EDT) solution via spin-SILAR. The influence of the number of spin-SILAR cycles on the average crystal sizes of PbS QDs was investigated, and the photovoltaic performance of the corresponding 3

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solid-state PbS QD-sensitized TiO2 nanorod array solar cells was evaluated. 2. Experimental 2.1. Preparation of 200-nm long rutile TiO2 nanorod arrays The 200-nm long rutile TiO2 nanorod arrays were prepared on a 60-nm TiO2 compact layer coated FTO glass using an aqueous-grown solution with 38 mM titanium isopropoxide and 6 M

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hydrochloric acid in accordance with our previous reports [9–11]. The arrays were grown at 170 °C for 75 min. The as-prepared TiO2 nanorod arrays were then annealed at 450 °C for 30 min in air prior to use.

2.2. Deposition of PbS QDs on 200-nm long rutile TiO2 nanorod arrays by spin-SILAR

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PbS QDs were deposited on the 200-nm long rutile TiO2 nanorod arrays using the step-by-step repeated spin-coating of Pb(NO3)2, Na2S9H2O and EDT solution via spin-SILAR methods. The detailed deposition procedures were the same as our previous reports [10,11] and were carried out in ambient conditions. The spin-SILAR cycle numbers were 20, 30 and 40.

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2.3. Fabrication and characterization of all solid-state PbS QDSSCs

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The incident photon-to-current conversion efficiency (IPCE) was measured as a function of wavelength from 300 to 1100 nm under the DC mode without bias light using a specially designed

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IPCE system (PV measurement Inc.) equipped with a dual Xenon/quartz halogen light source. The system was calibrated with a certified silicon solar cell (Fraunhofer ISE) prior to measurements.

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The preparation of the 60-nm thick TiO2 compact layer, the spiro-OMeTAD layer, the Au electrode, and the characterization of the UV-Vis, UV-Vis-NIR, XRD, FESEM and the

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photovoltaic performance measurements were the same as our previous reports [10,11]. 3. Results and discussion 3.1. Morphology, crystal phase and UV-Vis absorption of 200-nm long rutile TiO2 nanorod arrays

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spectrum (d) of the TiO2 nanorod array.

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Fig. 1. Surface (a) and cross-sectional (b) SEM images, XRD pattern (c) and UV-Vis absorption

Fig. 1 shows the SEM images, XRD pattern and the UV-Vis absorption spectrum of the TiO2 nanorod array. As shown in Fig. 1(a, b), the TiO2 nanorod array had a length of 200 nm, a diameter

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of 20 nm, and an areal density of 720 μm−2. The length of 200 nm was markedly shorter than that of 1~3 μm generally used in QDSSCs [5,8,12], which should be beneficial to the infiltration of

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spiro-OMeTAD into the PbS QD-sensitized TiO2 nanorod array and would ensure hole transport in the spiro-OMeTAD layer, since the thickness of the spiro-OMeTAD layer is typically 200~300

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nm for the high-efficiency perovskite solar cells [13–15]. The diameter of 20 nm is also smaller than that (100~300 nm) usually used in QDSSCs [5,8,12], which implied that the TiO2 nanorod

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arrays with a high areal density can be obtained. Then, the areal density of 720 μm−2 can provide

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sufficient loading of PbS QDs although the length of the TiO2 nanorod array was only 200 nm. From Fig. 1(c), the weak diffraction peaks at 2θ = 36.1° and 62.8° were observed, corresponding to the spacing of the (101), (002) planes of the tetragonal rutile phase (JCPDS: 71-0650), and a preferred orientation along the (101) plane was observed [9]. From Fig. 1(d), the absorption onset of the TiO2 nanorod array was 390 nm, and the optical band gap was estimated to be 3.2 eV. 3.2. Deposition of PbS QDs on 200-nm long rutile TiO2 nanorod arrays

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Fig. 2. Low (a) and high (b) resolution XRD patterns of the PbS QDs on the 200-nm long rutile

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TiO2 nanorod array with spin-SILAR cycle numbers of 20, 30, and 40.

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Fig. 2 displays the XRD patterns of PbS QDs using Pb(NO3)2 as a lead source on 200-nm long rutile TiO2 nanorod arrays. For the spin-SILAR cycle numbers of 20, 30 and 40, the three samples in Fig. 2(a) all exhibit the diffraction peaks of 2θ = 30.0°, corresponding to the spacing of the

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(200) plane of the cubic PbS (JCPDS: 78-1057). This result is also in accordance with the

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literature [5,16]. Interestingly, there was no obvious difference of the full width at half maximum (FWHM) at 2θ = 30.0° in the spin-SILAR cycle numbers of 20, 30 and 40, as seen in Fig. 2(b),

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although the intensity was enhanced with the increase of spin-SILAR cycle number. The average crystal size of PbS QDs was all 7.8 nm when estimated by Scherrer's equation and remained unchanged with the increase of the spin-SILAR cycle number. The average crystal size of 7.8 nm using Pb(NO3)2 as the lead source is larger than that of 6.5 nm using Pb(Ac)2 as the lead source [11]. This is because Pb(NO3)2 can ionize dissociative Pb2+, and Pb(Ac)2 is a covalent compound.

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Fig. 3. Surface and cross-sectional SEM images of the PbS QDs on the 200-nm long rutile TiO2

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nanorod array with spin-SILAR cycle numbers of 20 (a, d), 30 (b, e), 40 (c, f).

Fig. 3 presents the surface and cross-sectional SEM images of PbS QDs on the 200-nm long rutile TiO2 nanorod arrays with spin-SILAR cycle numbers of 20, 30 and 40. Compared with Fig. 3(a, d) and Fig. 1(a, b), it can be found that a compact PbS QD thin film was deposited on the TiO2 nanorod arrays with a spin-SILAR cycle number of 20, which agrees with our previous

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report [10]. When the spin-SILAR cycle number increased from 20 to 30 and 40, the diameters of

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the PbS QD-sensitized TiO2 nanorods slightly increased from 36 nm to 39 nm and 43 nm, respectively, and the loose and porous PbS QD covering layer on the top of the TiO2 nanorod

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arrays appeared. Their thicknesses were 100 and 150 nm for the spin-SILAR cycle number of 30 and 40, respectively. The increased diameter of the PbS QD-sensitized TiO2 nanorods should

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enhance the photocurrent of the QDSSCs due to increasing their light harvesting. However, the appearance of the loose and porous PbS QD covering layer was not benefit to the infiltration of

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spiro-OMeTAD into the compact PbS QD thin film-sensitized TiO2 nanorod array or the electron transportation from the PbS QDs to the TiO2. Fig. 4 shows the UV-Vis-NIR absorption spectra of the PbS QDs on the 200-nm long rutile TiO2

nanorod array. With the increase of the spin-SILAR cycle number, the absorbance of the PbS QDs on the TiO2 nanorod arrays increased. This result is ascribed to the increase of the PbS QD loading quantities, which agrees with the SEM results. The redshift of the absorption onsets is likely due to the simultaneous increase of the optical absorption and the decrease of optical reflection and transmission [17]. 7

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Fig. 4. UV-Vis-NIR absorption spectra of the PbS QDs on the 200-nm long rutile TiO2 nanorod array with spin-SILAR cycle numbers of 20, 30 and 40.

3.3. Photovoltaic performance of all solid-state PbS QD-sensitized TiO2 nanorod array solar cells

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Table 1 lists the photovoltaic performance parameters for the all solid-state PbS QD-sensitized TiO2 nanorod array solar cells and the corresponding photocurrent-photovoltage characteristics, as well as the IPCE spectra shown in Fig. 5. With the increase of the spin-SILAR cycle number from 20 to 30 and 40, the open-circuit voltage (Voc) and fill factor (FF) gradually decreased from 0.51 V

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and 0.65 to 0.50 V and 0.62, 0.43 V and 0.56, respectively. This result may be because the loose and porous PbS QD covering layer appeared to hinder the infiltration of spiro-OMeTAD into the

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PbS QD-sensitized TiO2 nanorod array and suppressed the charge separation in the interface of the spiro-OMeTAD/PbS QD-sensitized TiO2 nanorod array. With the increase of the spin-SILAR

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cycle number from 20 to 30, the short-circuit photocurrent density (Jsc) increased from 11.29 mA/cm2 to 13.30 mA/cm2. This result is likely because the diameter of the PbS QD-sensitized

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TiO2 nanorods increased and the conduction band electrons of the PbS QDs can easily inject into the conduction band of the TiO2 nanorods. Meanwhile, the partial conduction band electrons in the

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loose and porous PbS QD covering layers may inject into the conduction band of the TiO2 nanorods. With the increase of the spin-SILAR cycle number from 30 to 40, the Jsc decreased from 13.30 mA/cm2 to 12.94 mA/cm2. This result is likely related to the increased thickness of the loose and porous PbS QD covering layer, which hinders the partial conduction band electrons from transporting into the conduction band of the TiO2 nanorods. Therefore, the all solid-state PbS QD-sensitized TiO2 nanorod array solar cells with a spin-SILAR cycle number of 30 achieved the highest PCE of 4.12%, along with a Jsc of 13.30 mA/cm2, a Voc of 0.50 V and an FF of 0.62. The IPCE spectra of the corresponding QDSSCs were measured and the calculated photocurrent was 8

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13.21 mA/cm2, which agrees well with the measured photovoltaic performance results. Table 1. Photovoltaic performance parameters of all solid-state PbS QD-sensitized TiO2 nanorod

Spin-SILAR cycle number

Voc (V)

Jsc (mAcm-2)

FF

PCE (%)

20

0.51

11.29

0.65

3.74

30

0.50

13.30

0.62

4.12

40

0.43

12.94

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array solar cells.

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0.56

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Fig. 5. Photocurrent-photovoltage characteristics (a) and the IPCE spectra (b) of the all solid-state PbS QD-sensitized TiO2 nanorod array solar cells. 4. Conclusions The rutile TiO2 nanorod array with a length of 200 nm, a diameter of 20 nm and an areal density of 720 μm−2 was successfully prepared using the hydrothermal method with an aqueous-grown solution of 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 °C for 75 min. The influence of the spin-SILAR cycle number on the deposition process of the PbS QDs was investigated, and the results revealed that the average crystal sizes of PbS QDs were all ~7.8 nm 9

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using Pb(NO3)2 as the lead source and remained unchanged with the increase of the spin-SILAR cycle number from 20 to 30 and 40. The all solid-state PbS QD-sensitized TiO2 nanorod array solar cells with the spin-SILAR cycle number of 30 achieved a PCE of 4.12%. References [1] S.A. Mcdonald, G. Konstantatos, S. Zhang, P.W. Cyr, E.J.D. Klem, L. Levina, E.H. Sargent,

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