Hydrothermal Growth and Application of ZnO Nanowire Films with ZnO and TiO2Buffer Layers in Dye-Sensitized Solar Cells
© to the authors 2009
Received: 26 April 2009
Accepted: 18 August 2009
Published: 16 September 2009
This paper reports the effects of the seed layers prepared by spin-coating and dip-coating methods on the morphology and density of ZnO nanowire arrays, thus on the performance of ZnO nanowire-based dye-sensitized solar cells (DSSCs). The nanowire films with the thick ZnO buffer layer (~0.8–1 μm thick) can improve the open circuit voltage of the DSSCs through suppressing carrier recombination, however, and cause the decrease of dye loading absorbed on ZnO nanowires. In order to further investigate the effect of TiO2buffer layer on the performance of ZnO nanowire-based DSSCs, compared with the ZnO nanowire-based DSSCs without a compact TiO2buffer layer, the photovoltaic conversion efficiency and open circuit voltage of the ZnO DSSCs with the compact TiO2layer (~50 nm thick) were improved by 3.9–12.5 and 2.4–41.7%, respectively. This can be attributed to the introduction of the compact TiO2layer prepared by sputtering method, which effectively suppressed carrier recombination occurring across both the film–electrolyte interface and the substrate–electrolyte interface.
Dye-sensitized solar cells (DSSCs) based on a dye-sensitized wide-band-gap nanocrystalline semiconductor (typically TiO2) film have attracted widespread attention as a potential, cost-effective alternative to silicon solar cells since they were first introduced by O’Regan and Grätzel in 1991 . As one of the key components of dye-sensitized solar cells, the photoelectrode, composed of nanocrystalline semiconductor materials accumulated on a transparent conducting glass, has a very important influence on the photovoltaic performance [2, 3]. It is well known that the energy conversion efficiency of DSSCs depends on the electron transport in the photoelectrode. Therefore, one-dimensional structure such as rods or wires of semiconductor materials can greatly improve DSSCs efficiency by offering direct electrical pathways for photogenerated electrons, thus enhancing the electron transport in the photoelectrode. Recently, considerable efforts have been devoted to the synthesis of such 1D materials used as the photoelectrodes of DSSCs [4–7].
Among various emerging 1D nanomaterials, ZnO, a wide-band-gap (3.37 eV) semiconductor with a large exciton binding energy of 60 meV at room temperature, is a promising alternative semiconductor to TiO2. This is because that the band gap and the energetic position of the valence band maximum and conduction band minimum of ZnO are very close to that of TiO2 and that the wurtzite structure of ZnO favors the formation of ordered 1D structures, moreover, presenting better electron transport compared with TiO2. Consequently, the solar cell using nanowire arrays as the photoelectrodes shows a higher conversion efficiency compared to those using the disorderedly structured ZnO films . In order to further improve the cell efficiency, the effective approaches currently applied are to control the morphology of ZnO nanostructure films, which can significantly increase dye loading and light harvesting [8, 9], and to modify the surface of ZnO nanostructure films that can suppress carrier recombination . However, by introducing a blocking layer at the base of the ZnO films, the influence of the blocking layer on the performance of ZnO DSSCs is an ongoing debate .
In this study, we report that the ZnO nanowire films with high aspect ratios and different thicknesses of ZnO buffer layers, which formed at the base of the nanowire films during growth, were prepared from different ZnO seed preparation methods. We also show that carrier recombination in ZnO nanowire-based dye-sensitized solar cells can be effectively suppressed and the photovoltaic conversion efficiency enhanced by introducing the TiO2buffer layer prepared by sputtering method.
Polyethyleneimine (PEI, M.W.: 600) was purchased from Aldrich and used as received.Cis-bis (isothiocyanato) bis (2,2′-bipyridy1-4,4′-dicarboxylate) ruthenium (II) bistetrabutylammonium (also called N719) was from Solaronix SA, Switzerland. Other chemicals (Beijing Chemical Co.) used in our experiments were of analytical reagent grade without further purification. Fluorine-doped tin oxide substrates (FTO TEC-8, LOF) were first cleaned through sonication in acetone/ethanol for 30 min and then hydrolyzed in boiling deionized water at 100 °C for 30 min followed by air-drying.
ZnO Nanowire Array Synthesis
ZnO nanowire arrays were made in aqueous solution, using a two-step process described elsewhere . To study the effect of a thin compact TiO2 film on FTO substrate on the solar cell performance of ZnO array film, it was prepared at room temperature by using reactive DC magnetron sputtering.
Preparation of ZnO Seeds on FTO Substrates
Spin-coating method. Zinc acetate dehydrate [Zn(CH3COO)2·2H2O] was dissolved in the mixed solution of ethanolamine and 2-methoxyethanol. The concentrations of both Zn(CH3COO)2·2H2O and ethanolamine in the resulting solution are 0.75 M. The coating solution was spin-coated onto FTO substrates at 3,000 rpm for several times. The FTO substrates were subsequently annealed at 300 °C in air for 15 min in order to convert Zinc acetate to ZnO.
Dip-coating method. The FTO substrates were dip-coated in a 2.5 mM ethanolic solution of zinc acetate dehydrate. Following dip-coating, the zinc acetate films on the FTO substrates were annealed at 300 °C in air for 15 min.
ZnO nanowire arrays were grown by placing vertically the ZnO-seeded FTO substrates in solutions with 25 mM Zn(NO3)2, 25 mM hexamethylenetetramine (HMT) and 7.3 mM polyethyleneimine at 92.5 °C. In order to obtain a constant nanowire array growth rate, the solutions were refreshed during the reaction period (solution turnover time 2.5 h). Subsequently, the substrates were washed with water/ethanol and annealed at 400 °C for 30 min to remove any residual organics.
The resulting substrates were immersed in dry ethanol containing 0.3 mM of N719 for 40 min. To assemble the solar cells, a Pt-coated conducting glass was placed on the ZnO nanowire array films separated by a 50-μm thin membrane spacer. The assembled cell was then clipped together as an open cell. An electrolyte, which was made with 0.1 M LiI (Aldrich), 0.1 M I2(Aldrich), 0.6 M dimethylpropylimidazolium iodide (DMPImI, Aldrich) and 0.5 Mtert-butylpyridine (Aldrich) in dry acetonitrile (Aldrich), was injected into the open cell from the edges by capillarity.
The morphology of the products was characterized with use of field-emission scanning electron microscopy (FESEM, Hitachi S-4800). XRD analysis was performed on a powder X-ray diffractometer (Rigaku D/max-2500 diffractometer using CuKα radiation,λ = 0.1542, 40 kV, 100 mA). Photocurrent–voltage measurements were performed using simulated AM 1.5 sunlight with an output power of 100 mW cm−2.
Results and Discussion
Effect of ZnO Crystal Seed Particles Prepared by Different Methods
Mean values of the nanowire dimensions, nanowire aspect ratio and array density for different ZnO seed preparation methods
ZnO seed preparation methods
Nanowire aspect ratio
Density (×109 wires cm−2)
Parameters of dye-sensitized solar cell based on ZnO nanowire array films with different ZnO seed preparation methods
Effect of TiO2Blocking Layer
Mean values of the nanowire dimensions, nanowire aspect ratio and array density for nanowire arrays on the bare and TiO2-coated FTO substrates
ZnO seed preparation methods
Nanowire aspect ratio
Density (×109 wires cm−2)
Bare FTO substrates
Parameters of dye-sensitized solar cell based on ZnO nanowire array films on the bare and TiO2-coated FTO substrates
In summary, the work presented here shows that the different ZnO seed preparation methods strongly influenced the morphology and density of ZnO nanowire arrays. The nanowire film growing from the ZnO seeds prepared by dip-coating had a thin ZnO buffer layer (~300–500 nm thick), which can less effectively suppress carrier recombination than the thick ZnO buffer layer (~0.8–1 μm thick) for spin-coating, resulting in the maximum loss ofV oc(about 40 mV). In order to further investigate the effect of a TiO2buffer layer on the performance of ZnO nanowire-based DSSCs, a TiO2blocking layer (about 50 nm thick) underneath the ZnO nanowire array film was prepared onto the FTO substrate by sputtering method. The two different ZnO nanowire films with and without the compact TiO2buffer layer (~50 nm) had the similar thickness of ZnO buffer layer (~300–500 nm) and were used to assemble the DSSCs. By introducing the compact TiO2layer (~50 nm thick), the photovoltaic conversion efficiency and open circuit voltage of the ZnO DSSCs were improved by 3.9–12.5 and 2.4–41.7%, respectively. This can be because that the compact TiO2layer effectively suppressed carrier recombination occurring across both the film–electrolyte interface and the substrate–electrolyte interface.
The authors would like to acknowledge financial support for this work from the Beijing Municipal Education Commission.
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