Two dimensional graphitic-phase C3N4 as multifunctional protecting layer for enhanced short-circuit photocurrent in ZnO based dye-sensitized solar cells
Graphical abstract
Two dimensional g-C3N4 was, for the first time, adopted in a solar energy conversion device and the DSSC with ZnO–C3N4 composite demonstrates a ∼57% Jsc enhancement compared with that derives from pure ZnO.
Introduction
Dye-sensitized solar cells (DSSCs) have been intensively studied for decades because of their low cost, high efficiency and facile preparation process [1], [2], [3]. Photoanode is one of the key components in the configuration and nano-sized TiO2 is the most commonly used photoanode materials. Due to the similar electron band structure, excellent bulk electron mobility and the richest family of nanostructures, ZnO is also regarded as promising candidate for the photoanode in DSSC [4], [5], [6]. Although many researchers have been devoted to prepare various ZnO structures for DSSC application, the highest power conversion efficiency (PCE) of ZnO-based devices merely reaches to ∼7.5%, which is far less than the TiO2 based DSSCs with PCE of ∼13% [7], [8], [9], [10]. In addition, the ZnO nanoparticles are vulnerable in the ruthenium based dyes, because the acid function groups in the dye molecules can detach Zn2+ from the host lattice to form Zn2+-dye agglomerations. The agglomerations on the ZnO surface not only result in low electron injection efficiency but also trigger massive charge recombination opportunities [11], [12], [13]. Therefore, surface modification with protecting layers such as SiO2 Al2O3 or TiO2 were widely adopted to enhance the stability of ZnO and suppress the charge recombination [14], [15], [16], [17]. In these researches, the additional coating layers function as physical passivating layer to minimize the direct contact between ZnO and the dye molecules, which could significantly improve the fill factor (FF) and open circuit voltage (Voc) of the devices. However, the introduced semiconducting or insulating layers could not directly improve the harvesting efficiency of the incident light and usually bring forth injection problem of the photo-generated electrons. Therefore, these strategies seldom increases the short circuit current (Jsc) of the devices.
Recently, two-dimensional crystals with thicknesses in atom scale attracted extensive attentions because of their excellent electronic, optical, and biocompatible properties [18], [19], [20]. Metal-free graphitic-phase C3N4 (g-C3N4) possesses graphite-like structure with layer distance of ∼0.33 nm and band gap of ∼2.69 eV [21]. Heretofore, g-C3N4 with different topological structures and g-C3N4 based heterogeneous materials have been applied in various fields [22]. The foremost research interests are focused on the photo-degradation for environmental remediation [23], [24], [25], [26], [27], [28], [29], [30], [31] and water splitting for H2 fuel [32], [33], [34]. In addition, the potential application of g-C3N4 were also expanded to many territories such as adsorption [35], fuel cell [36], sensors [37], bio-imaging [38], and Li ion batteries [39]. However, so far as we know, g-C3N4 has been seldom studied to explore its potential application in solar energy conversion devices [40].
In this work, ZnO–C3N4 composites were prepared by an ultrasonic irradiation assisted monolayer dispersion and the effects of g-C3N4 coating layer on the performances of ZnO-based DSSCs were interpreted (Scheme 1). It was found that the g-C3N4 could expand the absorption spectra of ZnO based photoanodes to visible region and enhance the harvest of low energy photons. Moreover, the compatible band structure of g-C3N4 builds a stepwise energy gradient at the ZnO/C3N4/dye interface, which improves the injection efficiency of photo-generated electrons and finally leads to ∼57% Jsc enhancement. Therefore, a higher PCE of 4.5% was demonstrated, indicating ∼20% improvement compared with the pure ZnO based device.
Section snippets
Graphitic-phase C3N4
The g-C3N4 nano-sheet is prepared through an ultrasonic assisted liquid phase exfoliation. The bulk C3N4 was prepared by heating melamine at 550 °C for 2 h in N2 atmosphere. 0.1 g of the yellowish product are grounded and ultrasonicated in 100 mL distilled water for 1 h and centrifuged for 15 min at 4000 rpm to remove the unexfoliated g-C3N4 and obtain the light yellow suspension for later use.
ZnO nanoparticles
The ZnO nanoparticles were prepared by decomposing Zn(Ac)2·2H2O in a solvothermal method. Briefly, 0.3 g Zn(Ac)
Results and discussion
The SEM image displayed in Fig. 1a shows the as-prepared ZnO nanoparticles have prism-like shapes and the particle size is about 20–50 nm. The TEM image in Fig. 1b shows well-defined lattice fringes and the interplanar spacing could be measured as ∼0.26 nm, which corresponds to the (0 0 0 1) plane. After coated with ∼3% (wt) g-C3N4, many wrinkle-like patterns appear on each ZnO nanoparticle, which is obviously different from the clear surface condition of pure ZnO, indicating the homogenous coverage
Conclusions
Two dimensional g-C3N4 was incorporated as multifunctional protecting layer on ZnO nanoparticles via a monolayer dispersion method. The as-prepared ZnO–C3N4 (3%) composite shows enhanced photovoltaic performances than pure ZnO when used in DSSCs. Based on the optical and electrochemical investigations, the enhancement could be ascribed to the following reasons: (1) the g-C3N4 reduces the band gap of the composite, which expands the absorption spectrum and enhances the light harvesting of the
Acknowledgments
This work is supported by National Natural Science Foundation of China (61204078, 61176004 and U1304505), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) (Department of education, P.R. China), Program for Innovative Research Team (in Science and Technology) in University of Henan Province (No. 13IRTSTHN026), Basic and Frontier Research Programs of Henan Province (No. 142300410420) and Key Project of Science and Technology of Henan Province (Nos. 13A150517 and
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