Open structure ZnO/CdSe core/shell nanoneedle arrays for solar cells
© Chen et al.; licensee Springer. 2012
Received: 17 August 2012
Accepted: 9 September 2012
Published: 20 September 2012
Open structure ZnO/CdSe core/shell nanoneedle arrays were prepared on a conducting glass (SnO2:F) substrate by solution deposition and electrochemical techniques. A uniform CdSe shell layer with a grain size of approximately several tens of nanometers was formed on the surface of ZnO nanoneedle cores after annealing at 400°C for 1.5 h. Fabricated solar cells based on these nanostructures exhibited a high short-circuit current density of about 10.5 mA/cm2 and an overall power conversion efficiency of 1.07% with solar illumination of 100 mW/cm2. Incident photo-to-current conversion efficiencies higher than 75% were also obtained.
Since the first report on the dye-sensitized solar cell by O'Regan and Grätzel in 1991 , a great number of photovoltaic devices based on nanostructures have been proposed or developed, such as nanostructured dye-sensitized cells [2, 3], extremely thin absorber (ETA) cells , quantum dot cells , nanowire array cells , organic/inorganic nanostructured cells , and III-VI quantum ring solar cells . Nanostructured solar cells have several advantages over conventional bulk and thin film solar cells: large surface area, high efficiency for light harvesting, less expensive materials, and low process cost.
The two most frequently used window materials in nanostructured solar cells are highly porous nanocrystalline TiO2 and highly textured ZnO nanorod arrays. Porous nanocrystalline TiO2 particles can provide a large surface area for the absorber material. However, their slow trap-limited diffusion process and short effective diffusion length of electrons are big obstacles in making more efficient cells. ZnO nanowires have higher carrier concentration and electron mobility which favor the electron transport to the collection electrode. As the nanowires are not in direct contact with each other, the electrons transport only along the nanowire axis without any lateral transport, which will reduce the non-radiative recombination and carrier scattering loss dramatically. Solar cells sensitized by organic dye absorbers have shown impressive results, although their long-term stability and bandgap controllability need to be improved further. On the other hand, inorganic narrow bandgap semiconductors, such as Ag2S , In2S3, CdS , CuInS2, and CdSe , are also promising candidates as sensitizers for nanostructured solar cells.
It has been postulated that ZnO/CdSe can form a type II heterojunction which will accelerate the separation of photoexcited electron–hole pairs and improve the efficiency of solar cells. In a previous study, Leschkies et al. fabricated CdSe quantum dot sensitized ZnO nanowire solar cells . They recorded a power conversion efficiency of 0.4% and a short-circuit current density of 2.1 mA/cm2, which are still low compared with those of dye-sensitized solar cells. Lévy-Clément et al. prepared a nanostructured ZnO/CdSe/CuSCN ETA solar cell [15, 16], and a high energy conversion efficiency greater than 2% was demonstrated under a 340-W/m2 illumination using a halogen lamp. However, they did not report the energy conversion efficiency under the air mass (AM)1.5 full sun intensity. Luan et al. reported a CdS/CdSe co-sensitized solar cell using a facile solution growth which resulted in a power conversion efficiency of approximately 1% with a fill factor of 0.55 . Until now, there have been only a few reports published concerning ZnO/CdSe nanostructure-based solar cells. The mechanisms of such structures have not been systemically studied, and more fundamental researches should be conducted to provide further understanding of the electronic transporting process in these nanostructures. Herein, we reported the fabrication and characterization of open structure ZnO/CdSe core/shell nanoneedle array-based solar cells. High short-circuit current densities and power conversion efficiencies were obtained, which provided significant insight as to how to improve the photovoltaic performance of this type of solar cell.
Growth of ZnO nanoneedle arrays by solution deposition
ZnO nanoneedle arrays were grown using solution deposition method  on fluorine-doped SnO2 (SnO2:F) substrate covered with a ZnO seed layer. The ZnO seed layer was formed by spin coating a solution of zinc acetate and ethanolamine in 2-methoxy-ethanol at 3,000 rpm, followed by annealing in a furnace at 400°C for 1 h. Seeded substrates were placed vertically in aqueous solutions containing 20 mM zinc nitrate, 20 mM hexamethylene-tetramine, and 125 mM 1,3-diaminopropane at 70°C for 12 h. The sample containing ZnO nanoneedle arrays was rinsed with deionized water thoroughly and annealed at 500°C for 1 h to remove any residual organics and to improve the crystalline structure.
Deposition of CdSe shell layer using electrochemical technique
A CdSe coating layer was electrochemically deposited at room temperature on the ZnO nanoneedle arrays from an aqueous selenosulfate solution . A two-electrode electrochemical cell was used with the ZnO nanoneedle arrays as the cathode and a Pt wire as the counter electrode. CdSe was deposited under galvanostatic conditions with a current density of 1 mA/cm2 and a charge density of 0.25 C/cm2. The samples were annealed at 400°C for 1.5 h to increase the mean grain size, which can help to reduce the negative effects of grain boundary trap states.
Characterization of ZnO nanoneedle arrays and ZnO/CdSe core/shell nanostructures
The crystal structure of the samples was examined by X-ray diffraction (XD-3, PG Instruments Ltd., Beijing, China) with Cu-Kα radiation (λ = 0.154 nm) at a scan rate of 2° per min. X-ray tube voltage and current were set at 40 kV and 35 mA, respectively. The morphologies of the different nanostructures were investigated by scanning electron microscopy (SEM) (FEI Sirion, FEI Company, Hillsboro, OR, USA). The high-resolution transmission electron microscopy (HRTEM) images were taken with a Technai F-20 microscope (FEI Company, Hillsboro, OR, USA) at an acceleration voltage of 200 kV. The HRTEM specimens were prepared by drop casting the sample dispersion onto copper grid with holey carbon film and were dried under room temperature. The room temperature photoluminescence (PL) spectra of the ZnO/CdSe core/shell nanostructures were measured by exciting the samples with a YAG solid state laser at a wavelength of 532 nm. The UV-visible absorption spectra were obtained using a UV-visible spectrometer (TU-1900, PG Instruments, Ltd., Beijing, China).
ZnO/CdSe core/shell solar cell assemble and performance measurement
The solar cells were assembled using the ZnO/CdSe core/shell nanoneedle array-covered SnO2:F glass as the photoanode and a SnO2:F glass coated with a thin platinum layer (approximately 10 nm) as the counter electrode. A 100-μm-thick spacer was sandwiched between these two electrodes to prevent electrical shorts. A polysulfide electrolyte containing 1 M Na2S and 1 M S was injected into the space between the nanoneedle arrays and the platinized SnO2:F cathode to complete the cell assembly. The solar cell current–voltage characteristics were measured using a Keithley 2400 sourcemeter (Keithley Instruments Inc., Cleveland, OH, USA) while illuminating the solar cells with a solar simulator (model 94022A, Newport, OH, USA) at one sun (AM1.5, 100 mW/cm2). The measurements were carried out with respect to a calibrated OSI standard silicon solar photodiode. The incident photon-to-current conversion efficiency (IPCE) measurements were carried out with a custom measurement system consisting of a 150-W Xe lamp (LSH-X150, Zolix, Beijing, China), a monochromator (7ISW30, 7 Star Optical Instruments Co., Beijing, China) and a sourcemeter (2400, Keithley Instruments Inc.).
Results and discussion
Morphology and crystal structure of ZnO nanoneedle arrays and ZnO/CdSe core/shell nanostructures
Optical properties of the ZnO/CdSe core/shell nanostructures
Photovoltaic performance of ZnO/CdSe core/shell solar cells
From the high short-circuit current density and the IPCE values, we can conclude that the ZnO/CdSe interface forms an ideal type II heterojunction with suitable band alignment, which is essential to efficient charge transfer. ZnO nanoneedles have good electron conductivity and form very open structures, which is advantageous over the short effective diffusion length of electrons and the diffusion problems associated with the redox couples in the porous TiO2 network. The short-circuit current density can be further improved by increasing the length of the ZnO/CdSe core/shell nanoneedles. The drawback limiting the energy conversion efficiency of this type of solar cells is a rather poor fill factor of 0.22, which limits the energy conversion efficiency. This low fill factor may be ascribed to the lower hole recovery rate of the polysulfide electrolyte, which leads to a higher probability for charge recombination . Although the I−/I3− redox couple has ideal kinetic properties in regeneration of the oxidized dye and in inhibition of the recombination of an excited electron with the electrolyte, it is corrosive to the CdSe semiconductor, which will cause a rapid degradation of the solar cell performance. To further improve the efficiency of these nanoneedle array solar cells, a new hole transport medium with suitable redox potential and low electron recombination at the semiconductor and electrolyte interface should be developed. Recently Li et al. reported a very high fill factor of 0.89 in CdS quantum dot sensitized solar cells based on a modified polysulfide electrolyte . If this electrolyte is suitable for our ZnO/CdSe core/shell solar cells, a much better photovoltaic performance can be expected. Moreover, as reported by Soel et al., other contributions such as the counter electrode material may also have an influence in the fill factor .
In summary, we have prepared open structure ZnO/CdSe core/shell nanoneedle arrays on SnO2:F glass by solution deposition and electrochemical techniques. Optical measurements indicate that these nanostructures are very favorable for the use in photovoltaic devices. Nanoneedle array-based solar cells were assembled using a polysulfide electrolyte. A much higher short circuit current and IPCE (76%) are obtained in these solar cells, showing a promising alternative to existing dye-sensitized solar cells.
This work is supported by the State Key Research Development Program of China (2010CB833103), the National Natural Science Foundation of China (60976073), the National Found for Fostering Talents of Basic Science (J1103212), and the Foundation for Outstanding Young Scientist in Shandong Province (BS2010CL036). Jun Jiao thanks the financial support from the Oregon Nanoscience Microtechnologies Institute (ONAMI) and the National Science Foundation.
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