Array of solid-state dye-sensitized solar cells with micropatterned TiO2 nanoparticles for a high-voltage power source
© Cho et al.; licensee Springer. 2013
Received: 10 July 2013
Accepted: 6 November 2013
Published: 20 November 2013
We demonstrate an array of solid-state dye-sensitized solar cells (SS-DSSCs) for a high-voltage power source based on micropatterned titanium dioxide nanoparticles (TNPs) as photoanodes connected in series. The underlying concept of patterning the TNP of a few micrometers thick lies on the combination of the lift-off process of transfer-printed patterns of a sacrificial layer and the soft-cure treatment of the TNP for fixation. This sacrificial layer approach allows for high pattern fidelity and stability, and it enables to construct stable, micrometer-thick, and contamination-free TNP patterns for developing the SS-DSSC array for miniature high-voltage applications. The array of 20 SS-DSSCs integrated in series is found to show a voltage output of around 7 V.
KeywordsTitanium dioxide nanoparticle Micropatterning Solid-state dye-sensitized solar cell High-voltage source 81.16.Rf 85.40.Hp 84.60.Jt
Dye-sensitized solar cells (DSSCs) with mesoporous titanium dioxide (TiO2) nanoparticles (TNPs) have been considered as a promising alternative to conventional inorganic solar cells due to their relatively high power conversion efficiencies and low production cost. So far, much effort has been made toward the enhancement of the power conversion efficiency of the DSSCs[2–4]. Together with the improvement of the power conversion efficiency, the generation of high output voltage is one of the critical issues for practical applications. The issue of the high voltage generation of the DSSCs has been addressed only in a unit cell producing limited output voltages of around 1 V[5–7], which is far below the voltages required for most practical devices, for example, around 4 V for mobile phones. Thus, the integration of DSSCs needs to be pursued for high-voltage sources. Owing to the excellent electron transport characteristics, stability, and appropriate conduction band position, a TNP layer is promising for use as a photoanode in the DSSC. Therefore, for the integration of a DSSC array, a reliable patterning technique of the TNP layer should be developed.
In patterning the TNP, several methods such as solvent-assisted soft lithography, micromolding technique in capillaries, and imprint lithography have been typically employed, but they involve the difficulty of patterning multiple stacks of the TNP and eliminating the residual layer. In other words, these patterning methods are not applicable for constructing relatively thick (a few micrometers) and stable TNP patterns demanded for sufficiently high absorption of light in the DSSCs. Moreover, the DSSCs with liquid electrolytes encounter confinement problem, leakage, and evaporation of the liquid in the integration into the array. Therefore, it is extremely important to develop a versatile method of patterning a few-micrometer-thick TNP layer for fabricating an array of solid-state dye-sensitized solar cells (SS-DSSCs).
In this work, we demonstrate an array of SS-DSSCs for a high-voltage power source using micropatterned TNP as photoanodes connected in series. The basic concept relies primarily on a chemically compatible lift-off process of a fluorous sacrificial layer which has the complementary patterns of the TNP of a few micrometers thick on a substrate. This sacrificial layer approach allows for high pattern fidelity and stability, and it leads directly to stable, micrometer-thick, and contamination-free TNP patterns for developing the SS-DSSC array for miniature high-voltage applications.
Fabrication of TNP patterns
In preparing photoanodes connected in series for a high-voltage DSSC array, micropatterns of the TNP were constructed on a pre-patterned fluorine-doped tin oxide (FTO) glass. An array of 20 FTO electrodes, where each electrode has a width of 500 μm and a gap of 500 μm between two adjacent electrodes, was prepared using photolithography and a dry etching process. A glass substrate with pre-patterned FTO was cleaned with acetone, deionized water, and ethanol in sequence and dried with nitrogen flow. The cleaned substrate was then dried at 90°C in a vacuum oven for 10 min to remove any residual water and subsequently treated with ultraviolet ozone for 5 min. In order to improve the adhesion and the mechanical strength of the TNP layer, the treated FTO glass was soaked in an aqueous solution of 40 mM TiCl4 at 70°C for 30 min. The FTO glass was then cleaned in the same way described above.
Preparation of a DSSC array
Each patterned TNP used as an individual photoanode for a unit cell was connected in series for a high-voltage DSSC array. The patterned TNP layer was immersed in a solution of 3 mM Z907 dye (Solaronix SA) dissolved in a 1:1 mixture of acetonitrile and tert-butyl alcohol for 24 h. The dye-coated TNP layer was simply washed with acetonitrile. For the solid-state hole transport material (HTM), spiro-OMeTAD (American Dye Source, Inc., Baie D'Urfé, Quebec, Canada) dissolved in chlorobenzene was mixed with a lithium bis(trifluoromethylsulfonyl)imide salt ionic dopant dissolved in acetonitrile. The solution was placed on the whole TNP-patterned FTO glass, and the pores in the TNP layer were filled with the solution by capillary action for 1 min. The TNP-patterned FTO glass was then spun at the rate of 2,000 rpm. For the preparation of a cathode, Au of 100 nm thick was thermally deposited at the rate of 1 Å/s through a shadow mask to connect 20 cells in series. The array of 20 DSSCs connected in series has a total active area of 1.4 cm2.
An optical microscope and a field emission scanning electron microscope (FE-SEM; SU-70, Hitachi, Ltd., Chiyoda, Tokyo, Japan) were used for taking the images of the patterned TNP layer. In order to examine the existence of any residual fluorous solvent in the patterned TNP layer which may deteriorate the photovoltaic performance of the SS-DSSCs, solid fluorine-nuclear magnetic resonance (19 F-NMR) spectra were measured with a Bruker AVANCE II (500 MHz) spectrometer (Bruker, Billerica, MA, USA) with a 2.5-mm probe at the spin rate of 20 kHz. A current–voltage curve was obtained using a source measure unit (model 2400, Keithley Instruments Inc., Cleveland, OH, USA) under the illumination of a solar simulator with air mass 1.5 global (AM 1.5 G) filters at 100 mW/cm2. The light intensity of the solar simulator was calibrated with a standard silicon diode.
Results and discussion
The optical microscopic image of the TNP patterns in the FTO regions on the substrate is shown in Figure 2b where TNP patterns isolated from the neighboring patterns were clearly seen. Each isolated TNP pattern, which is 500 μm wide and 14 mm long in the interval of 500 μm, represents an individual photoanode for a unit cell in the SS-DSSC array[14, 15]. Figure 2c shows the FE-SEM image of the cross-sectional TNP pattern. According to the FE-SEM image, each TNP pattern was about 2.5 μm thick. This is a typical thickness of the TNP photoanode for a whole SS-DSSC. Moreover, as shown in Figure 2d, the TNPs were highly packed in the multistacks of a few micrometers, and the surface roughness was about a few tens of nanometers. It should be noted that our micropatterning method based on the SL lift-off process is very simple and effective to produce a wide range of the TNP patterns by varying the thickness of the doctor-bladed TNP layer and the dimension of the SL patterns transfer-printed by the PDMS stamp.
For lifting-off the SL, the FTO substrate with the TNP patterns was exposed to a fluorous solvent. From the measurements of the 19 F-NMR spectrum of the TNP sample treated by a fluorous solvent, no extra peak was observed when compared to an empty rotor, as shown in Figure 2f. This tells us that no remnant solvent exists after annealing the TNP sample at 450°C, and thus, the SL lift-off process is contamination free for patterning the multistacks of TNPs in the fabrication of the array of the SS-DSSCs.
We presented how a functional layer of the nanoparticles can be patterned for use in hybrid electronic and optoelectronic devices in a simple, cost-effective, and contamination-free way. The underlying concept comes from the lift-off process of the transfer-printed patterns of a fluorous sacrificial layer and the soft-cure treatment of the nanoparticles for fixation. As an example, an array of the SS-DSSCs with a micropatterned TNP layer of several micrometers thick was demonstrated for high-voltage source applications. The array of 20 SS-DSSCs connected in series showed an open-circuit voltage exceeding 6 V. It is concluded that the micropatterning approach presented here will be applicable for a wide range of diverse nanoparticles to be employed in optical, electronic, and sensing devices.
- AM 1.5 G:
Air mass 1.5 global
Dye-sensitized solar cell
Field emission scanning electron microscope
Fluorine-nuclear magnetic resonance
Fluorine-doped tin oxide
Hole transport material
- J sc :
Short-circuit current density
Power conversion efficiency
- R s :
Solid-state dye-sensitized solar cell
Titanium dioxide nanoparticles
- V oc :
This work was supported by the National Research Foundation of Korea under the Ministry of Education, Science and Technology of Korea through the grant 2011–0028422.
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