Facile synthesis of composition-tuned ZnO/Zn x Cd1-x Se nanowires for photovoltaic applications
© Luo et al.; licensee Springer. 2015
Received: 2 February 2015
Accepted: 28 March 2015
Published: 15 April 2015
ZnO/Zn x Cd1-x Se coaxial nanowires (NWs) have been successfully synthesized by combining chemical vapor deposition with a facile alternant physical deposition method. The shell composition x can be precisely tuned in the whole region (0 ≤ x ≤ 1) by adjusting growth time ratio of ZnSe to CdSe. As a result, the effective bandgaps of coaxial nanowires were conveniently modified from 1.85 eV to 2.58 eV, almost covering the entire visible spectrum. It was also found that annealing treatment was in favor of forming the mixed crystal and improving crystal quality. An optimal temperature of 350°C was obtained according to our experimental results. Additionally, time resolved photo-luminescence spectra revealed the longest carrier lifetime in ZnO/CdSe coaxial nanowires. As a result, the ZnO/CdSe nanowire cell acquired the maximal conversion efficiency of 2.01%. This work shall pave a way towards facile synthesis of ternary alloys for photovoltaic applications.
One-dimensional nanostructures have attracted considerable attention due to their unique advantages and potential applications in photovoltaic devices [1-3]. In particular, nanostructured oxide semiconductors, such as ZnO nanowires (NWs) and TiO2 nanocrystals, have been widely applied to photo-electrochemical (PEC) cells or solar cells owing to the low cost and high stability against photocorrosion, and mature fabrication techniques [4-13]. However, these oxide semiconductors have a relatively wide bandgap and can not efficiently absorb sunlight in visible region, yielding a low efficiency. A series of semiconductor nanocrystals, such as ZnSe , CdSe , CdS , CdSeTe , ZnCdSe , and ZnCdTe , have been coated onto the surface of ZnO or TiO2 to expand photoresponse. As compared with binary alloys, ternary materials are the more efficient sensitizers due to their tunable bandgaps and band structures [17-26]. Many efforts have been devoted to tune their compositions by different fabrication methods. For instance, Xu et al. fabricated type II ZnO/Zn x Cd1-x Se nanocables via an ion-exchange approach ; Ruchi et al. prepared TiO2/Zn x Cd1-x Se nanotubes through a successive ionic layer adsorption and reaction technique ; Li et al. synthesized Zn x Cd1-x Se shell layer on ZnO NWs by chemical vapor deposition (CVD) method . In spite of these efforts, the composition of ternary alloys can not be conveniently controlled because of the different ion concentrations in solution method or the different saturated vapor pressures of elements in vapor method. Hence, up to date, it is still a challenge to develop a simple and facile route to fabricate composition-tuned ternary alloys.
In this work, we successfully synthesized Zn x Cd1-x Se shell layers on ZnO NWs with tunable compositions (0 ≤ x ≤ 1) by an alternant physical deposition method. The scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), transmission analysis and time resolved photo-luminescence (TRPL) were performed to investigate their morphologies, crystal structures, compositions, and optical properties, respectively. It has been found that the composition of ZnCdSe shell could be conveniently and precisely controlled by adjusting growth time ratio of ZnSe to CdSe. Meanwhile, solar cells based on different ZnO/ZnCdSe coaxial NWs were assembled and their performances were evaluated as well. This work opens a novel avenue for facile synthesis of sophisticated ternary alloys.
Synthesis of ZnO NWs and Zn x Cd1-x Se shells
For photovoltaic applications, the as-prepared ZnO/Zn x Cd1-x Se coaxial NWs were used as the working electrodes. Nanostructured counter electrode was prepared by sputtering a thin layer of Cu2S on aluminum zinc oxide (AZO) glass. The two electrodes were sealed together with a 60-μm-thick polypropylene spacer (Surlyn, DuPont, Wilmington, USA), and the internal space of the cell was filled with a polysulfide electrolyte (1.0 M S, 1.0 M Na2S, and 0.1 M NaOH in deionized water). The active area of the solar cell was about 0.5 cm2.
The morphologies of the as-prepared ZnO and ZnO/Zn x Cd1-x Se NWs were measured with a field emission SEM (LEO 1530, Zeiss, Thornwood, USA). The structures and compositions were characterized by XRD (X’Pert PRO, PANalytical, Chapel Hill, USA) and TEM (Tecnai F30, Fei, Hillsboro, USA). The transmission spectra were measured using a Varian Cary 5000 UV-vis NIR spectrophotometer (Agilent, Santa Clara, USA). TRPL measurements were carried out in an Edinburgh FLS920 spectrofluorometer (Edinburgh Instruments Ltd, Livingston, UK) at room temperature. The detected energies for different samples were in accordance with their estimated bandgaps (1.85, 1.98, 2.05, 2.18, 2.34, and 2.58 eV). Current density-voltage (J-V) characteristics of solar cells were recorded under AM1.5 solar illumination (100 mW · cm−2). The incident photon-to-current conversion efficiency (IPCE) was measured on a broadband spectroscopy system consisting of a grating monochromator (Spectra Pro-750i, Acton Research Corporation, Trenton, USA), a 100 W bromine-tungsten lamp, and a lock-in amplifier (SR830 DSP, Stanford Research Systems, Sunnyvale, USA), by comparing with a reference Si and Ge cells.
Results and discussion
Performances of solar cells measured under AM1.5 (100 mW · cm −2 )
J sc (mA · cm −2 )
V oc (V)
In summary, we demonstrated that the ZnO/Zn x Cd1-x Se coaxial NWs with tunable shell compositions could be facilely synthesized by combining CVD with an alternant physical deposition method. Morphological studies by SEM show that the entire ZnO nanowire can be coated with a relatively uniform shell. XRD and TEM results disclosed that the composition x can be precisely controlled by adjusting growth time ratio of ZnSe to CdSe; meanwhile, annealing treatment under a suitable temperature is beneficial for forming ternary alloys and improving crystal quality. Transmission analysis indicated that the effective bandgap of ternary alloys could be modified in a wide range from 1.85 eV to 2.58 eV by composition tuning. Time resolved photo-luminescence spectra revealed that carrier lifetime of ZnO/Zn x Cd1-x Se coaxial NWs exponentially decreases with the increase of Zn content. A maximal conversion efficiency of 2.01% was achieved in ZnO/CdSe nanowire cell. This work provides a facile method to synthesis ternary alloys with tunable composition.
The work was supported by ‘973’ Program (No. 2012CB619301 and 2011CB925600), the National Natural Science Foundations of China (No. 61106008 and 61227009), the Natural Science Foundations of Fujian Province, and the fundamental research funds for the central universities.
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