Flexible Dye-Sensitized Solar Cell Based on Vertical ZnO Nanowire Arrays
© Chu et al. 2010
Received: 1 July 2010
Accepted: 13 September 2010
Published: 26 September 2010
Flexible dye-sensitized solar cells are fabricated using vertically aligned ZnO nanowire arrays that are transferred onto ITO-coated poly(ethylene terephthalate) substrates using a simple peel-off process. The solar cells demonstrate an energy conversion efficiency of 0.44% with good bending tolerance. This technique paves a new route for building large-scale cost-effective flexible photovoltaic and optoelectronic devices.
Flexible solar cells have attracted tremendous interest because of its potential for wearable electronics and other versatile applications . Dye-sensitized solar cell (DSSC) is well known as one competitive alternative to conventional inorganic photovoltaic cells due to its simplicity, low cost, and good power conversion efficiency . Typically, the photoanodes of flexible DSSCs consist of titanium oxide nanocrystals on a plastic substrate that are produced by utilizing low-temperature processes, such as sintering , mechanical pressing , hydrothermal crystallization , electrophoretic deposition , microwave irradiation , or film transfer . However, low-temperature treatments usually result in poor material crystallinity and fragile characteristic, thus limit the potential of DSSCs as flexible electronics. Recently, significant progress in photovoltaics has been achieved based on one-dimensional materials owing to the improved crystalline quality, efficient charge separation/transport process, and mechanical flexibility [9–15].
ZnO, an n-type semiconductor, is known for its high electric conduction and carrier mobility . ZnO has demonstrated as a good photoanode in DSSCs since its energy band structure is similar to that of TiO2 [17, 18]. Additionally, the syntheses of one-dimensional ZnO materials with controllable structures have been well developed by using either vapor or solution phase growth methods [11, 19]. In this work, vertically aligned ZnO nanowire arrays with high aspect ratio are synthesized by vapor phase growth. A robust peel-off technique is employed to transfer vertically aligned nanowire arrays onto indium tin oxide (ITO)-coated poly(ethylene terephthalate) (PET) flexible substrate. Bendable ZnO nanowire-based DSSCs are then assembled following polymer packaging with nanowires as electrodes. The fabricated device shows good tolerance under strong mechanical bending.
Aligned ZnO nanowire arrays are synthesized via seed growth scheme . A ~500 nm ZnO seed layer is first deposited onto an n-type Si (100) substrate (resistivity in the range of 1–10 Ω cm) in a molecular beam epitaxy system. Pure zinc powder is then placed at the center of a horizontal tube furnace for the ZnO nanowire array growth. Oxygen carried by Ar (1:1000) is kept flowing during the synthesis. The typical growth and characterization of ZnO nanowires can be found in our previous reports [19, 21, 22].
Polydimethylsiloxane (PDMS) solution, made by mixing the base and curing agent (10:1 w/w) (Sylgard 184), is diluted (5:1 w/w) by a hexamethylcyclotrisiloxane solution in methylene chloride (Alfa Aesar). The solution is subsequently dropped onto the nanowire film and spun at a rate of 2,000 rpm for 1 min to allow uniform distribution of a thin PDMS layer at the bottom of ZnO nanowire arrays. After a heat treatment at 150°C for 1 h, the PDMS layer is cured at the bottom of the ZnO nanowire array forming a matrix. This PDMS matrix together with the embedded ZnO wires can be then removed from the wafer mechanically. The free-standing nanowire/PDMS film is then mounted onto an ITO/PET substrate using silver paste for subsequent device fabrication process.
Before the DSSC device packaging, the prepared ZnO/PDMS/Ag/ITO/PET film is dipped into a 0.3 mM N-3 dye (cis-Bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II)) in t-butanol/acetonitrile solution (1:1 volume ratio) for 24 h of dye loading. A parafilm is used as a spacer layer between the nanowire array and a Pt (2 nm)-coated ITO/PET counter electrode. The electrolyte containing 0.5 M LiI, 0.05 M I2, 0.5 M tert-butylpyridine in acetonitrile and valeronitrile (1:1 volume ratio) is filled in the space between ZnO nanowire and the Pt/ITO counter electrode to construct the solar cell.
Results and Discussion
Figure 2b is the SEM image of ZnO nanowire/PDMS film while the nanowire remains good directional alignment. The bent film indicates the excellent mechanical flexibility through utilizing PDMS polymer matrix. The bottom view of ZnO/PDMS film (Figure 2c) shows that the ZnO nanowires are interconnected by PDMS whereas the exposed wire tails will be implemented as the contact electrodes. Figure 2d shows the top view of ZnO/PDMS composite in which the nanowires maintain good vertical alignment after transferring onto ITO substrate.
The incident photon-to-current conversion efficiency (IPCE) value, referred as the quantum efficiency (QE), is the ratio of the observed photogenerated charge carriers to the incident photon flux, uncorrected with reflective loss for optical excitation through the conducting transparent electrode. Figure 3c plots the quantum efficiency of the flexible ZnO nanowire DSCC with and without bending. The maximum efficiency of 38% is achieved at 515 nm, which is comparable to the competitive solar cells built from ZnO nanoporous film on rigid substrates . A tail in the UV region close to ZnO band gap is observed owing to the light harvesting directly from ZnO. The QE of the device under bending shows a reduced value (~23%) with the same peak position at 515 nm. The degraded performance is due to the loss of light absorption and the bending induced mechanical defects.
Large scale single crystalline ZnO nanowire arrays have been successfully transferred onto flexible substrates maintaining vertical orientation by employing PDMS matrix. This technique offers a simple and robust way to realize the application of vertically aligned nanowire array on arbitrary substrates via low-temperature process. The ZnO nanowire array-based DSSCs constructed on flexible ITO/PET substrate demonstrate reasonable energy conversion efficiency and good mechanical bending tolerance. However, the performance of the presented DSSC device has deficiencies. The current light illumination direction from the Pt-coated ITO/PET counter electrode limits the cell performance since photons will be partially reflected by Pt coating. Second, a shortcoming of the DSSC is the usage of silver paste that does not provide good electrical contact between the nanowire array and ITO electrode substrate. More technical enhancement is required to improve the device functionality.
We thank Dr. Dongshe Zhang for helpful discussions. The authors acknowledge support from DOE EFRC program.
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