Full paperIn situ assembly of well-defined Au nanoparticles in TiO2 films for plasmon-enhanced quantum dot sensitized solar cells
Introduction
Quantum dot-sensitized solar cell (QDSC), as a promising solar cell, has attracted much attention because of their low cost, easy fabrication and high theoretical power conversion efficiency (can reach up to 44%) [1], [2], [3]. QDSCs use semiconductor quantum dots (QDs) (such as CdS, CdSe, CdTe and PbS) as the photosensitizer instead of organic dyes used in traditional dye-sensitized solar cells (DSCs) because of their versatile optical and electrical merits, such as (1) size-dependence of the optical properties with strong light response, (2) tunable band gap, (3) larger extinction coefficient, and (4) generation of multiple excitons with single-photon absorption [4], [5], [6], [7], [8], [9]. Unfortunately, the power conversion efficiency (PCE) of QDSCs remains lower than that of DSCs (13%) and hybrid perovskite solar cells (22.1%) [10], [11], [12].
The cell structure of a QDSCs consists of a wide-band-gap mesoporous oxide film (a photoelectrode, such as the commonly used TiO2), QDs (the sensitizer), an electrolyte, and a counter electrode [13], [14], [15], [16], [17], [18]. The mesoporous photoanode plays a predominant role in determining the PCE, as a support matrix, which determines how much the QDs can be loaded and thus affects the short-circuit current density [19], [20], [21]. Meanwhile, it also contributes to the charge injection from an excited QD and the transport of electrons to the collecting electrode surface [22], [23]. The porous nature of nanocrystalline TiO2 films in combination with its electronic configuration drives their wide use in QDSCs due to the large surface area available for QDs adsorption [24], [25]. However, many surface defects from the oxygen vacancies in TiO2 nanocrystals promote the surface charge recombination, resulting in the decrease of the photo-current density [26]. In addition, the weak light harvesting capability and the severe carrier recombination in photovoltaic devices have opposite relations with the photosensitive layer thickness, which collaboratively influence the PCE [27], [28], [29], [30]. Therefore, to improve the light trapping or harvesting and simultaneously facilitate the electron transport in the electrode are considered as a beneficial approach for the improvement of the device performance.
The combination of TiO2 nanomaterials with noble metals to form metal/semiconductor hybrid nanostructures has been investigated to increase the photovoltaic efficiency [31], [32], [33], [34]. This is because surface plasmon resonance from noble metal nanostructure can give rise to unique properties, such as an intense absorption feature and enhanced local electromagnetic field [35], [36]. Noble metal nanoparticles (e.g. Au, Ag and Pt NPs) are usually chosen to form such hybrid structures to obtain enhanced photovoltaic activity, with Au/TiO2 having been particularly well-studied and employed into various optoelectronic devices owing to its high stability and tunable absorption over a wide solar spectrum range [37], [38], [39]. Derkacs and co-workers reported a remarkable enhancement in short-circuit current density and energy conversion efficiency in amorphous silicon p-i-n solar cells with the inclusion of Au NPs above the amorphous silicon film [40]. Chen et al. fabricated Au NPs inlaid mesoporous titania nanoparticles (Au@MTNs) thin films as photoanodes in all-plastic-based DSCs and found that the flexible DSCs fabricated with Au@MTNs photoanode have better performance (5.62%) compared to that fabricated with only TiO2 MTNs photoanode (4.93%) [41]. Yang et al. blended Au NPs into the interconnecting layer in an inverted tandem polymer solar cell configuration that connects two subcells and demonstrated both the top and bottom subcells' efficiency simultaneously improved by the plasmonic effects of Au NPs [42]. Besides, Wang and co-workers applied the Au/TiO2 thin film as working electrode in DSCs and obtained an enhancement of 63% in photocurrent and 84% in efficiency compared to the cell with the pure TiO2 photoanode [43]. Nevertheless, there have been a few reports where Au NPs have been incorporated into QDSCs and the preparation methods mainly focus on the aqueous chemical growth and subsequent in situ formation of Au anchored to TiO2 matrix, which will inevitably cause Au NPs aggregation and thus lead to the reduction of solar cell performance. Additionally, decreased open-circuit photovoltage was also observed upon the incorporation of “bare” Au NPs into the TiO2, and attributed to the metal both being corroded in electrolyte and acting as a charge recombination center [39], [43], [44]. Therefore, a study on in situ assembly of well-defined monodisperse Au NPs into TiO2 mesoporous films as well as subsequent optical absorption and transport efficiency simultaneously improved in QDSCs is of particular importance for further improvement of the Au/TiO2 plasmonic photovoltaic performance.
In this study, a photoanode film composed of homogeneously distributed Au NPs within TiO2 mesoporous film via an in situ assembly process for plasmon-enhanced CdS/CdSe QDSCs is developed. The TiO2/Au film exhibits broad light absorption and effective charge injection, as characterized by UV–Vis absorption spectra and electrochemical impedance spectroscopy (EIS), the absorbed photon-to-electron conversion efficiency (APCE) as well as photoluminescence (PL) decay curves, which is originating from the incorporation of Au NPs. Notably, the core-shell structure of CdS and CdSe QDs with sizes from 5 nm to 10 nm assembled on the surface of Au NPs was also achieved by the successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) process. The Au NPs in photoelectrode work as electron relay, facilitating the photogenerated electron injected to the conduction band (CB) of TiO2 and inhibiting the recombination of electrons in the CB of TiO2 photoanode transfer back to react with Sn2− in electrolyte. As a result, an overall efficiency of 6.0% under simulated AM 1.5, 100 mW/cm2 was obtained for the TiO2/Au QDSC, which is 50% higher than that of the pure TiO2 cell. The physicochemical properties of TiO2/Au nanocomposites and their photovoltaic performances as photoanodes in QDSCs have been systematically studied, which would enable us to identify some key aspects of the enhancement mechanism with which the performance of QDSCs is influenced by the Au NPs.
Section snippets
Preparation of TiO2/Au hybrid films
All chemicals used were analytical grade reagents without further purification. The TiO2 mesoporous film electrodes were firstly prepared by the coating of TiO2 paste (Degussa P25) on cleaned fluorine-doped tin oxide (FTO) glass substrates via a typical doctor blading method, followed by sintering at 500 ◦C for 30 min to improve the crystallinity and remove impurities. The thickness of the TiO2 film, measured from the cross-sectional image from a scanning electron microscope (SEM), was about 14
Characterization of the photoanodes
The SEM images representing the top view of the photoanode films are shown in Fig. 2. Figs. 2a and 2b show the surface morphology of TiO2 and TiO2/Au with Au NPs fabricated via the in situ self-assembly method. After the alternative self-assembling of PEI and HAuCl4 and thermal treatment, the structural features including surface morphology of film as well as particle sizes of the TiO2/Au NPs system remain constant. A little increase in the particle size with the CdS and CdSe QDs deposition is
Conclusions
In summary, TiO2/Au photoanodes with 10–25 nm Au NPs homogeneously assembled in TiO2 mesoporous films are designed and fabricated as the working electrode to construct QDSCs. The core-shell structure of CdS and CdSe QDs conformally covering on the surface of Au NPs was also achieved by the SILAR and CBD process. The incorporation of Au NPs and construction of the core-shell structure show improvements mainly in the photocurrent density which is mainly attributed to the typical plasmon-induced
Acknowledgments
This work was financially supported in part by National Science Foundation (NSF, DMR-1505902), the Fundamental Research Funds for the Central Universities (No. JBX171413) and the Starting Research Funds of Xidian University (No. 20103176410), and Yuan Wang would like to acknowledge the scholarship from China Scholarship Council.
Yuan Wang received her Ph.D. from State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology. She is currently working at School of Advanced Materials and Nanotechnology at Xidian University. Her current research is focused on development of visible-light-responsive photocatalysts and quantum dot sensitized solar cells.
References (64)
- et al.
Nano Energy
(2016) - et al.
Nano Energy
(2017) - et al.
Nano Energy
(2017) - et al.
Nano Energy
(2016) - et al.
Nano Energy
(2017) - et al.
Nano Energy
(2016) - et al.
Nano Energy
(2015) - et al.
Nano Energy
(2015) - et al.
Sci. Bull.
(2015) - et al.
J. Power Sources
(2015)
Sci. Bull.
Appl. Catal. B
J. Power Sources
Nano Energy
Nano Energy
Appl. Phys. Lett.
ACS Nano
Nano Rev.
Sci. China Mater.
Sci. China Mater.
J. Phys. Chem. Lett.
Nat. Chem.
Science
J. Am. Chem. Soc.
J. Am. Chem. Soc.
Sci. China Mater.
Sci. China Mater.
Phys. Chem. Chem. Phys.
Nanoscale
Acc. Chem. Res.
Sci. China Mater.
J. Phys. Chem. Lett.
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Yuan Wang received her Ph.D. from State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology. She is currently working at School of Advanced Materials and Nanotechnology at Xidian University. Her current research is focused on development of visible-light-responsive photocatalysts and quantum dot sensitized solar cells.
Qifeng Zhang is currently working at University of Washington as a Research Assistant Professor. His research interests involve engineering applications of nano-structured materials on electrical devices including solar cells, UV light-emitting diodes (LEDs), Field-effect transistors (FETs), and gas sensors. His current research focuses on the synthesis of nanomaterials and the application of nanomaterials in electronic and optoelectronic devices, such as dye-sensitized solar cells (DSCs) and organic/inorganic hybrid solar cells.
Fei Huang received her Ph.D. from Donghua University, China. She is currently a post-doctoral fellow under the supervision of Prof. Jianjun Tian at University of Science and Technology Beijing. Her recent research mainly focuses on interface modification, photoanode material synthesis of quantum dot sensitized solar cells.
Zhimin Li received his Ph.D. in Materials Science from Northwestern Polytechnical University in 2008. He worked at National Institute for Materials Science (NIMS) in Japan from 2010.07 to 2011.07 as a postdoctoral research fellow. He is now a professor at School of Advanced Materials and Nanotechnology at Xidian University. His current research interest is dielectric materials, and energy materials and devices.
Yan-Zhen Zheng is an associate professor of Beijing University of Chemical Technology. Her current research is focused mainly on new energy materials and devices including organic-inorganic hybrid solar cells, lithium batteries, supercapacitors and photo-electrocatalytic hydrogen evolution.
Xia Tao is deputy director of State Key Laboratory of Organic-Inorganic Composites, and professor of chemical engineering at Beijing University of Chemical Technology. She has published more than 80 peer-reviewed articles. Her current research mainly focuses on optoelectric functional materials and nanostructured photocatalytic materials for energy related applications in photocatalysis and solar cells.
Guozhong Cao is Boeing-Steiner Professor of materials science and engineering, professor of chemical engineering, and adjunct professor of mechanical engineering at the University of Washington, and also a professor at Dalian University of Technology. His current research is focused on chemical processing of nanomaterials for energy related applications including solar cells, rechargeable batteries, supercapacitors, and hydrogen storage.