Optimization of dye adsorption time and film thickness for efficient ZnO dye-sensitized solar cells with high at-rest stability
© Chang et al.; licensee Springer. 2012
Received: 8 October 2012
Accepted: 12 December 2012
Published: 28 December 2012
Photoelectrodes for dye-sensitized solar cells were fabricated using commercially available zinc oxide (ZnO) nanoparticles and sensitized with the dye N719. This study systematically investigates the effects of two fabrication factors: the ZnO film thickness and the dye adsorption time. Results show that these two fabrication factors must be optimized simultaneously to obtain efficient ZnO/N719-based cells. Different film thicknesses require different dye adsorption times for optimal cell performance. This is because a prolonged dye adsorption time leads to a significant deterioration in cell performance. This is contrary to what is normally observed for titanium dioxide-based cells. The highest overall power conversion efficiency obtained in this study was 5.61%, which was achieved by 26-μm-thick photoelectrodes sensitized in a dye solution for 2 h. In addition, the best-performing cell demonstrated remarkable at-rest stability despite the use of a liquid electrolyte. Approximately 70% of the initial efficiency remained after more than 1 year of room-temperature storage in the dark. To better understand how dye adsorption time affects electron transport properties, this study also investigated cells based on 26-μm-thick films using electrochemical impedance spectroscopy (EIS). The EIS results show good agreement with the measured device performance parameters.
KeywordsZinc oxide Dye-sensitized solar cells Dye adsorption time Film thickness Conversion efficiency At-rest stability Electrochemical impedance spectroscopy
Dye-sensitized solar cells (DSSCs) are regarded as promising low-cost solar cells with high light-to-energy conversion efficiency. Systems based on titanium dioxide (TiO2) nanoparticle films sensitized with ruthenium (Ru)-based dyes have achieved a light-to-energy conversion efficiency of more than 11% [1, 2]. Other metal oxides, including tin dioxide, indium (III) oxide, niobium pentoxide, and zinc oxide (ZnO), have also been used as photoelectrode materials [3–5]. Among these materials, ZnO has attracted considerable attention because it has an energy-band structure similar to that of TiO2 but possesses a higher electron mobility and allows more flexibility in synthesis and morphologies [6, 7].
The photovoltaic performance of a DSSC relies on the characteristics of its photoanode, which plays a central role in converting light into electrical energy. A DSSC photoanode typically consists of a mesoporous oxide film on a transparent conducting glass substrate. Dye molecules that capture photons from light during device operation are attached to the surface of oxide film. Photoexcitation of the dye molecules leads to the injection of electrons into the oxide film. Therefore, an oxide film with a large interfacial surface area and superior electron transport properties is vital for strong light harvesting and efficient device performance. Consequently, numerous researchers have attempted to develop novel nanostructures with these desirable properties [8–12]. Another important strategy that has been widely adopted in DSSCs to boost optical absorption is light scattering . The basic principle of the light scattering method is to confine light propagation and extend the traveling distance of light within the oxide film. In this way, the opportunity of photon absorption by the dye molecules is increased, so is the cell conversion efficiency. In traditional DSSCs, the porous photoelectrode typically consists of nanocrystallites of approximately 20 nm in diameter to ensure a large interfacial surface area; to generate light scattering, submicron-sized particles are incorporated into the nanocrystalline film. These submicron-sized light scatterers can either be mixed into the nanocrystalline film [14, 15] or form a scattering layer on the top of the nanocrystalline film [16–20]. In addition to submicron-sized particles, some other nanostructures, such as nanowires [21–23] and nanotubes [24, 25] have also been studied as light scatterers in DSCCs. Recently, a promising three-dimensional nanostructure that has been developed to fulfill multiple functions in DSSCs is nanocrystallite aggregates [26–29]. These aggregates not only provide a large interfacial surface area, but also generate light scattering because they are composed of nanoparticles that assemble into submicron aggregates. Employing nanocrystallite aggregates can avoid the drawbacks of using large particles as light scatterers in conventional DSSCs. Mixing the large particles into the nanocrystalline film unavoidably causes a decrease in the interfacial surface area of the film, whereas placing the large particles on top of the nanocrystalline film brings about a limited increase in the interfacial surface area of the film.
Regardless of the film nanoarchitecture employed, film thickness and dye adsorption time are two important factors that must be considered during photoanode fabrication. Increasing the total interfacial surface area of the porous film by raising the film thickness is simple, which boosts the amount of dye adsorbed and, thus, light absorption. Thus, raising the film thickness can increase the short-current density (JSC) [21, 30]. However, a thick film also aggravates unwanted charge recombination and poses more restrictions on mass transfer. Consequently, both the open-current voltage (VOC) and overall conversion efficiency decline [14, 21, 30, 31]. Therefore, film thickness must be optimized to obtain efficient cells.
Another key fabrication factor is the dye adsorption time, which determines the quantity and the nature of the adsorbed dye molecules. The dye adsorption time should be sufficiently long so that the interfacial surface of the oxide film is completely covered with a monolayer of dye molecules. In fabricating TiO2-based photoanodes, the length of the dye adsorption time is first determined and then applied to all film thicknesses during the subsequent thickness optimization process [32–34]. This is because TiO2 is insensitive to prolonged sensitization times because of its higher chemical stability. Conversely, a prolonged dye adsorption time in ZnO-based photoanodes often significantly deteriorates cell performance. Thus, varying film thicknesses may require different dye adsorption times for optimal cell performance. Compared to TiO2, ZnO is less stable with acidic dyes, such as Ru-based N3 and N719 dyes. The formation of Zn2+/dye aggregates is a result of ZnO dissolution in these acidic dye solutions [32, 35–37]. The formation of dye aggregates has also been reported for indoline dyes . Ideally, the oxide surface should be covered with a monolayer of dye molecules to achieve efficient electron injection. When dye molecules undergo aggregation, electron injection becomes less efficient, and overall conversion efficiency declines. However, Yan et al. , on the other hand, observe the surface etching of ZnO nanoflowers after a long sensitization time. Surface etching also leads to a significant loss in overall conversion efficiency. For ZnO-based cells, it is essential to optimize the dye adsorption time to minimize the formation of dye aggregates and the damage to ZnO surfaces. Because the dye molecules must penetrate the mesoporous oxide film before they attach to the interfacial surface, the optimal dye adsorption time likely depends on the thickness of the ZnO film. Thus, this study investigates both the film thickness and the dye adsorption time. Although these two factors have been individually investigated before and certain studies have reported the influences of dye concentration and adsorption time on DSSC performance [32, 36], a detailed and systemic study of the effects of film thickness and dye adsorption time for ZnO-based DSSCs is lacking.
This study reports the preparation of DSSC photoelectrodes using commercially available ZnO nanoparticles sensitized with the acidic N719 dye. This study also systematically investigates the influences of ZnO film thickness and dye adsorption time on the performance of the resulting DSSCs. To further understand the effect of dye adsorption time, electrochemical impedance spectroscopy (EIS) was used to investigate the electron transport characteristics of the fabricated cells. This study shows the correlation between JSC and dye loading as a function of the dye adsorption time and reports the at-rest stability of the best-performing cell.
Fabrication of solar cells
ZnO films (active area 0.28 cm2) of various thicknesses (14 to 35 μm) were deposited on fluorine-doped tin oxide (FTO) substrates (8 to 10 Ω/□, 3 mm in thickness, Nippon Sheet Glass Co. Ltd, Tokyo, Japan) by screen printing. Screen-printable ZnO paste was prepared by dispersing commercially available ZnO nanoparticles (UniRegion Bio-Tech, Taiwan) in an equal proportion of α-terpineol (Fluka, Sigma-Aldrich, St. Louis, MO, USA) and ethyl cellulose. Before dye adsorption, the ZnO films were sintered at 400°C for 1 h to remove any organic material in the paste. This thermal treatment sintered the nanoparticles together to form an interconnecting network. Dye sensitization was achieved by immersing the sintered ZnO films in a 0.5 mM solution of cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis(tetrabutylammonium) (N719, Solaronix; Solaronix SA, Aubonne, Switzerland). The solvent used to prepare the dye solution consisted of equal parts of acetonitrile and tert-butanol. Dye sensitization was performed at room temperature, and the adsorption time varied from 0.5 to 4.5 h. The electrodes loaded with the N719 dye were then washed with acetonitrile and dried in air. Platinum (Pt)-coated FTO glass (Nippon Sheet Glass, 8–10 Ω/□, 3 mm in thickness) served as the counter electrode, which was prepared by placing a drop of H2PtCl6 solution on an FTO glass and subsequently sintering the glass at 400°C for 20 min. The ZnO photoanode and the counter electrode were sealed together with a 60-μm-thick hot-melting spacer (Surlyn, DuPont, Wilmington, DE, USA), and the inner space was filled with a volatile electrolyte. The electrolyte was composed of 0.1 M lithium iodide, 0.6 M 1,2-dimethyl-3-propylimid-azolium iodide (PMII, Merk Ltd., Taipei, Taiwan), 0.05 M I2 (Sigma-Aldrich), and 0.5 M tert-butylpyridine (Sigma-Aldrich) in acetonitrile.
The morphologies of the ZnO nanoparticle films were examined by field-emission scanning electron microscopy (FE-SEM; Nova230, FEI Co., Hillsboro, OR, USA). The crystalline phases of the ZnO films were determined by X-ray diffraction (XRD) using a diffractometer (X'Pert PRO, PANalytical B.V., Almelo, The Netherlands) with Cu Kα radiation. The thickness of the ZnO nanoparticle film was measured using a microfigure-measuring instrument (Surfcorder ET3000, Kosaka Laboratory Ltd., Tokyo, Japan). Dye loading of the photoelectrode was estimated by desorbing the dye in a 10 mM NaOH aqueous solution and then measuring the absorbance of the solution using UV–vis spectroscopy (V-570, Jasco Inc., Easton, MD, USA). Photovoltaic characterization was performed under a white light source (YSS-100A, Yamashita Denso Company, Tokyo, Japan) with an irradiance of 100 mW cm−2 at an equivalent air mass (AM) of 1.5 on the surface of the solar cell. The irradiance of the simulated light was calibrated using a silicon photodiode (BS-520, Bunko Keiki Co., Ltd, Tokyo, Japan). Current–voltage (J-V) curves were recorded with a PGSTAT 30 potentiostat/galvanostat (Autolab, Eco-Chemie, Utrecht, The Netherlands). The evolution of the electron transport process in the cell was investigated using EIS, and the impedance measurements were preformed under AM 1.5 G illumination. The applied DC bias voltage and AC amplitude were set at open circuit voltage (VOC) of the cell and 10 mV between the working and the counter electrodes, respectively. The frequency range extended from 10−2 to 105 Hz. The electrochemical impedance spectra were recorded using an electrochemical analyzer (Autolab PGSTAT30, Eco-Chemie) and analyzed using Z-view software with the aid of an equivalent circuit.
Results and discussion
Characteristics of ZnO films
Photovoltaic characteristics of fabricated DSSCs
Figure 3b presents a comparison of VOC values of the fabricated devices. This figure shows that the VOC values first increase with the dye adsorption time. After reaching a maximum VOC value, a further increase in the adsorption time leads to a decline in the VOC value. Similar to the JSC plot, the adsorption time required to achieve the respective maximum VOC increases as the film thickness increases. Figure 3b also shows that the maximum VOC values decrease slightly as the film thickness increases. This is likely the result of increased charge recombination and more restricted mass transfer with thick films. As the film thickness increases, electrons encounter a longer transport distance and recombine more easily with I3−. This results in a stronger electron transfer resistance and a shorter electron lifetime in the ZnO film . The FF values shown in Figure 3c exhibit no clear trends. The FF values vary between 0.67 and 0.72, which are relatively high compared to those reported for ZnO-based DSSCs [37, 41].
Optimal dye adsorption times and photovoltaic characteristics of best-performing cell at each film thickness
Film thickness (μm)
Optimal dye adsorption time (h)
Conversion efficiency (%)
Short-circuit photocurrent density (mA/cm2)
Open circuit voltage (V)
The Nyquist plots in Figure 6b show the experimental impedance data obtained at various dye adsorption times. The impedance spectra of DSSCs generally exhibit three semicircles. The semicircle in the high-frequency range corresponds to charge transfer behavior at the Pt/electrolyte (RPt and CPt), the FTO/electrolyte (RFTO and CFTO), and the FTO/ZnO (RFZ and CFZ) interfaces. The semicircle in the mid-frequency range (the central arc) is assigned to the electron transfer at the ZnO/dye/electrolyte interfaces, which is related to Rw, Rk, and Cμ. The semicircle in the low-frequency range represents the Warburg diffusion process of I−/I3− in the electrolyte (ZN) [42–45].
Effects of dye adsorption time on electron transport properties of fabricated cells
Dye adsorption time (h)
Mean electron lifetime (ms)
Effective electron diffusion time (ms)
Charge collection efficiency (%)
Effective electron diffusion coefficient (×10−3 cm2 s−1)
Effective electron diffusion length (μm)
The effective electron diffusion time (τd) in the photoanodes is given by τd = τeff/(Rk/Rw). The lowest τd also occurs at the optimal dye adsorption time of 2 h, indicating that the optimal dye adsorption time enhanced electron transport in the ZnO photoanode. Charge collection efficiencies (ηCC) were estimated using the relation ηCC = 1 − τd/τeff. Again, ηCC reaches its maximum value at the optimal dye adsorption time of 2 h, suggesting that using an appropriate dye adsorption time minimizes charge recombination.
The parameter Deff was then calculated using the relation Deff = (Rk/Rw)(L2/τeff), where L is the thickness of the ZnO film (26 μm). The highest Deff value (8.05 × 10−3 cm2 s−1) was also obtained at the optimal dye adsorption time of 2 h. This high Deff value can be explained by more injected electrons and induced faster transport of electrons. The parameter Leff, calculated by the relation Leff = (Deff × τeff)1/2, reflects the competition between the collection and recombination of electrons. A cell fabricated using the optimal dye adsorption time of 2 h achieved the highest Leff value of 111.6 μm, which exceeds the thickness of the photoelectrode (26 μm). This indicates that most of the injected electrons reached the FTO substrate before recombination occurred. This Leff trend shows good agreement with that of JSC. Increased recombination can explain the significant drop in JSC values at other dye adsorption times. Overall, the EIS analysis results are in good agreement with the measured device performance parameters.
In summary, this study reports the successful fabrication of DSSC photoelectrodes using commercially available ZnO particles sensitized with acidic N719 dye. The effects of two fabrication factors, the film thickness and the dye adsorption time, were systematically investigated. The results show that to obtain efficient ZnO/N719-based DSSCs, the dye adsorption time must be varied with the photoanode thickness. This is because the dye adsorption time suited for a particular film thickness does not apply to other film thicknesses. This is primarily because prolonged dye sensitization times lead to significant deterioration in the performance of ZnO-based cells. This is in contrast to the typical behavior for TiO2-based cells, which usually adopt a single sufficiently long dye adsorption time for all film thicknesses. This is because TiO2-based cells are generally insensitive to prolonged sensitization times because of the higher chemical stability of TiO2. Through systematic optimization of the film thickness and the dye adsorption time, the highest overall conversion efficiency achieved in this study was 5.61%, obtained from a 26-μm photoelectrode sensitized for 2 h. The best-performing cell also showed remarkable at-rest stability, retaining approximately 70% of its initial efficiency after more than 1 year of room-temperature storage in the dark.
Dye-sensitized solar cells
Electrochemical impedance spectroscopy
Field-emission scanning electron microscopy
Fluorine-doped tin oxide
- J SC :
Short-circuit photocurrent density
- V OC :
Open circuit voltage
The authors acknowledge the financial support from the Bureau of Energy, Ministry of Economic Affairs, Taiwan (project no. B455DR2110) and National Science Council, Taiwan (project no. NSC 101-2221-E-027-120). The authors also thank Professor Chung-Wen Lan at the Department of Chemical Engineering, National Taiwan University for instrument support.
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