Enhancing efficiency of CdS/TiO2 nanorod arrays solar cell through improving the hydrophilicity of TiO2 nanorod surface

doi:10.1016/j.solmat.2015.01.022

Solar Energy Materials and Solar Cells

Volume 136, May 2015, Pages 206-212

https://doi.org/10.1016/j.solmat.2015.01.022 Get rights and content

Highlights

•
The amount of CdS nuclei on TiO₂ surface was improved by increasing hydrophilicity of TiO₂.
•
The bond between CdS nuclei and TiO₂ nanorod could be enhanced by increasing hydrophilicity of TiO₂.
•
The J_sc, V_oc and τ_e of CdS/TiO₂ NTAs heterostructure increased by increasing hydrophilicity of TiO₂.
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Well crystalline CdS nanocrystals were only deposited on TiO₂ nanorod (001) surface.

Abstract

CdS nanoparticles with well-defined crystallinity were assembled on vertically aligned TiO₂ nanorod arrays (TiO₂ NRAs) to form CdS/TiO₂ NTAs heterostructures by cyclic voltammetry electrochemical deposition. The morphology and structure of CdS/TiO₂ NTAs heterostructure were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the amount of CdS nuclei on TiO₂ surface and the bond between CdS nuclei and TiO₂ nanorod were improved by increasing hydrophilicity of TiO₂ nanorod surface, and well crystalline CdS nanocrystals were deposited on TiO₂ nanorod (001) surface with good bonding between CdS nanoparticle and TiO₂ nanorod. With increased hydrophilicity of TiO₂ nanorod surface, the J_sc, V_oc and τ_e of CdS/TiO₂ NRAs heterostructure were increased. Especially, PEC (2.29%) was increased by near three times. This is because that CdS nanoparicles were uniformly dispersed on TiO₂ NRAs and had good bond with TiO₂ nanorods.

Introduction

Inspired by high-efficiency and low-cost dye-sensitized solar cells [1], [2], [3], researchers have paid their attention to quantum dot-sensitized solar cells (QDSCs) as an alternative to dyes owing to their high stability, spectral tunability by particle size and multiple exciton generation effect [4], [5], [6]. In order to improve the performance of solar cell devices, strategies that focus on using nanostructures to improve the electron transport rate [7] and light harvesting efficiency [8], and decrease the degree of charge recombination [9] have been investigated. Using one-dimensional (1D) single crystal TiO₂ nanorods arrays (TiO₂ NRAs) to take the place of spherical TiO₂ nanoparticles can improve the rate of electron transport between electrode and QDs, thereby increasing the incident photon-to-current conversion efficiency [10].

Up to now, several types of QDs, such as CdS [11], CdSe [12], In₂S₃ [13], Ag₂S [14] and CuInS₂ [15], have been assembled onto the surface of TiO₂ nanorods to achieve good photovoltaic performance. CdS has been extensively investigated for its potential applications in photoelectrochemical (PEC) solar cells. The methods so far adopted for the preparation of CdS include electrodeposition, chemical bath deposition (CBD) [16], successive ion layer absorption and reaction (SILAR) [17] and linker-assisted adsorption (LA) using bifunctional molecules [18]. CBD provides high surface coverage of CdS QDs and direct contact with TiO₂, but the performance is limited by the high density of surface defects and poor size distribution. SILAR is time-consuming, which is not suitable for practical applications. The existence of bifunctional molecules in LA blocks the electron and hole separation and transport. Electrochemical deposition is a more straightforward technique, which is conducted by employing in-situ deposition of QDs on the wide band-gap semiconductor with the assistance of an electric field [19], [20], [21]. This method offers easy control over all the reaction factors, as well as gives good bonding between the semiconductors [22], [23]. It provides high surface coverage of QDs with good anchoring to the TiO₂ NRAs, but the QD size distribution is broad as a result of easy aggregation [24], [25]. Further, the QDs/TiO₂ heterostructure still suffers from quick recombination of photogenerated electrons and holes under light irradiation, which results in low quantum efficiency of targeted photocatalytic reactions [26], [27].

In order to alleviate the aggregation of QDs and recombination of photogenerated electrons and holes, an intimate heterojunction nanostructure between QDs and TiO₂ substrate is necessary. Fortunately, the surface of TiO₂ can be modified to form an intimate heterostructure with QDs by pro-hydrophobic conversion under UV irradiation [28], [29].

So, we report an easy and quick electrochemical process to synthesize intimate CdS/TiO₂ NRAs heterostructures by pretreatment of TiO₂ NRAs under UV irradiation to improve its hydrophilicity. CdS nanocrystals are well distributed on the TiO₂ NRAs as the TiO₂ NRAs are irradiated by UV light. The CdS nanocrystals are well crystalline and have good bonding with TiO₂ nanorod. This structure may be able to enhance the photoresponse and photostability of CdS/TiO₂ NRAs heterostructures.

Section snippets

Preparation of TiO₂ NRAs on FTO

TiO₂ NRAs were prepared by a hydrothermal process, following the typical procedure [30]. In a typical synthesis, 40 mL of deionized water was mixed with 40 mL of concentrated hydrochloric acid (HCl, 37%, MERCK). The mixture was stirred at ambient conditions for 10 min before the addition of 1.3 mL of titanium butoxide (97% Aldrich), then 20 mL of above mixture was placed into a Teflon-lined stainless steel autoclave (30 mL volume, Parr Instrument Co.). After stirring for another 5 min, two pieces of

X-ray diffraction

The crystal structure of the TiO₂ NRAs before and after CdS deposition was confirmed by XRD analysis. The curve a in Fig. 2 shows the rutile structure of the TiO₂ nanorod arrays. All the diffraction peaks agree well with the tetragonal rutile phase (SG, P42/mnm; JCPDS no. 88-1175, a=b=0.4517 nm and c=0.2940 nm). From the powder diffraction pattern, the (002) diffraction peak is significantly enhanced, which indicates that the as-deposited film is highly oriented with respect to the substrate

Conclusion

CdS/TiO₂ NRAs heterostructures were prepared by the hydrothermal method combined with subsequent cyclic voltammetry electrochemical deposition. The hydrophilicity of TiO₂ nanorod surface played an important role in formation of the heterostructures. After UV irradiation, the amount of CdS nuclei on TiO₂ surface and the bond between CdS nuclei and TiO₂ nanorod were improved by increased hydrophilicity of TiO₂ nanorod surface. Well crystalline CdS nanocrystals were deposited on TiO₂ nanorod (001)

Acknowledgment

This work was supported by International Science &Technology Cooperation Program of China (2011DFA52290, 2012DFR50460), Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011), National Natural Science Foundation of China (51402209, 21176169), Shanxi Provincial Outstanding Innovation Project of Graduation (No. 2013303), and Youth Development Fund of Taiyuan University of Technology (2013Z033).

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Enhancing efficiency of CdS/TiO2 nanorod arrays solar cell through improving the hydrophilicity of TiO2 nanorod surface

Highlights

Abstract

Introduction

Section snippets

Preparation of TiO2 NRAs on FTO

X-ray diffraction

Conclusion

Acknowledgment

Electrochim. Acta

Chem. Phys. Lett.

Mater. Res. Bull.

J. Alloy. Compd.

Electrochim. Acta

J. Power Sources

J. Photochem. Photobiol. A

Photoelectrochemical cells

Nature

Solar energy conversion by dye-sensitized photovoltaic cells

Inorg. Chem.

Dye-sensitized solar cells

Chem. Rev.

Quantum-dot-sensitized solar cells

Chem. Phys. Chem.

Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals

Chem. Mater.

Exciton multiplication and relaxation dynamics in quantum dots: applications to ultrahigh-efficiency solar photon conversion

Inorg. Chem.

Direct synthesis of anatase TiO2 nanowires with enhanced photocatalytic activity

Adv. Mater.

A simple strategy for improving the energy conversion of multilayered CdTe quantum dot-sensitized solar cells

J. Mater. Chem.

Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells

J. Appl. Phys.

Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe–TiO2 architecture

J. Am. Chem. Soc.

TiO2 nanorod arrays functionalized with In2S3 shell layer by a low-cost route for solar energy conversion

Nanotechnology

Photoelectrical propertiesof Ag2S quantum dot-modified TiO2 nanorod arrays and their application for photovoltaic devices

Dalton Trans.

Solution fabrication and photoelectrical properties of CuInS2 nanocrystals on TiO2 nanorod array

ACS Appl. Mater. Interfaces

Enhancing efficiency of CdS/TiO₂ nanorod arrays solar cell through improving the hydrophilicity of TiO₂ nanorod surface

Preparation of TiO₂ NRAs on FTO

Direct synthesis of anatase TiO₂ nanowires with enhanced photocatalytic activity

Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe–TiO₂ architecture

TiO₂ nanorod arrays functionalized with In₂S₃ shell layer by a low-cost route for solar energy conversion

Photoelectrical propertiesof Ag₂S quantum dot-modified TiO₂ nanorod arrays and their application for photovoltaic devices

Solution fabrication and photoelectrical properties of CuInS₂ nanocrystals on TiO₂ nanorod array