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We demonstrated the improved conversion efficiency (η) of dye-sensitized solar cells (DSSCs) using the textile-patterned polydimethylsiloxane (PDMS) antireflection layers prepared by metal-coated textile master molds by a simple soft imprint lithography. When light propagates through the textile-patterned surface of PDMS (i.e., textile PDMS) laminated on the outer glass surface deposited with fluorine-doped tin oxide (i.e., FTO/glass), both the transmitted and diffused lights into the photo-anode of DSSCs were simultaneously enhanced. Compared to the bare FTO/glass, the textile PDMS increased the total transmittance from 82.3 to 85.1% and its diffuse transmittance was significantly increased from 5.9 to 78.1% at 550 nm of wavelength. The optical property of textile PDMS was also theoretically analyzed by the finite-difference time-domain simulation. By laminating the textile PDMS onto the outer glass surface of DSSCs, the η was enhanced from 6.04 to 6.51%. Additionally, the fabricated textile PDMS exhibited a hydrophobic surface with water contact angle of ~123.15°.
© 2015 Optical Society of America
1. Introduction
Over the past decade, dye-sensitized solar cells (DSSCs) have attracted increasing interest in various application fields from roof solar collectors to building integrated photovoltaics because of their cost-effective production and high energy-conversion capability [1–3]. Accordingly, many research efforts have focused on the development of high-efficiency DSSCs for alternative energy sources in industrial and commercial products including the hybrid and portable electronic devices [4–6]. In a typical structure of DSSCs, it is essentially composed of transparent conductive substrate, photo-anode, and electrolyte. When dye molecules accessible to the photo-anode absorb photons from sunlight, electrons are excited and collected to the external load. Herein, the number of absorbed photons plays a key role in generating photocurrent which is closely related to the conversion efficiency [7,8].
In recent years, light managements, such as antireflection, light scattering, and light trapping, have been intensively considered as a promising approach to develop various optoelectronic and photovoltaic devices [9–11]. Especially, to enhance the efficiency of DSSCs, ordered or disordered architectures with these effects have been developed [12–21]. For instance, subwavelength structures (SWSs) with topological ordered arrays have been fabricated to realize a complete antireflection coating for various solar cell applications [22–24]. Owing to their graded effective refractive index profile, Fresnel reflection losses could be effectively suppressed between the surface of device and air, and thus the light transmission could be improved in transparent substrates or cover layers (i.e., glasses, polymers, etc.) used for solar cells or modules [25–28]. Therefore, it could increase the power conversion efficiency of DSSCs by transferring more external light to the photo-anode. Additionally, the light trapping can be enhanced in the device by employing micro-grating or surface plasmon resonance structures due to the extension of light path lengths or strong electromagnetic fields, which increase the light absorption of active materials [28–32]. But, these techniques require precise and complicated fabrication processes. Typically, the nanolithography and dry etching processes should be employed to manufacture the SWSs [22–26,33].
To solve these problems, in this work, we reported the facile and cost-effective soft imprint lithography to fabricate the textile-patterned polydimethylsiloxane (i.e., textile PDMS) film which exhibits antireflective and light diffusive scattering properties due to its rough surface with sinusoidal gratings. Furthermore, this textile PDMS can be readily employed as a cover layer into the DSSCs to enhance the conversion efficiency by a simple lamination on the outer surface of glass substrate. Furthermore, in real outdoor environments, the textile PDMS cover layer with a self-cleaning function, which can wash the dust particles or any contaminants on the surface by rolling down rain droplets, can be very useful. Thus, its surface wettability was also characterized. For theoretical analyses of optical light propagation and transmittance properties in the fabricated textile PDMS, finite-difference time-domain (FDTD) and rigorous coupled-wave analysis (RCWA) simulations were performed, respectively.
2. Experimental details
Prior to the fabrication of the textile patterns on the surface of PDMS films via a simple soft imprint lithography, a commercially produced textile substrate (TS) was chosen as a master mold which is composed of the woven nickel-plated polyethylene terephthalate fibers. The TSs were cut into a size of ~5 × 5 cm2 and cleaned by ultrasonication with ethanol and de-ionized water for 10 min, respectively. After that, the TS was attached on the glass plate with the use of double-side adhesive carbon tape. To prepare PDMS mixture, the base and curing agent of silicone elastomer (Sylgard 184) was mixed in 10:1 mass ratio, and then it was degassed in a desicator for 50 min. Then, the mixture was slowly poured on the master mold until the covered thickness reached approximately 1 mm. After aging the PDMS for 10 min, the mixture was cured at 75 °C in an oven for 90 min. On the other hand, DSSCs were prepared by conventional fabrication processes. First, TiO2 particles (CCIC, PST-18NR) were deposited on fluorine doped tin oxide (FTO) coated soda lime glass (i.e., FTO/glass) by using a doctor-blade technique using polyimide tapes as the spacer, and then it was annealed at 500 °C for 2 h in a furnace. Here, the cell size was about 0.5 × 0.5 cm2. This procedure was repeated two times to achieve such working electrode with a suitable thickness of ~15 μm. Next, the TiO2 deposited FTO/glass was immersed in a 3 × 10−4 M solution of ruthenium (II) dye (Solaronix, N719) in ethanol and kept at room temperature for 24 h in dark. To prepare a counter electrode, platinum paste (Dyesol, counter PT-1) was coated onto FTO/glass with the same doctor-blade method and it was annealed at 500 °C for 2 h. By an injection of electrolyte (Dyesol, electrolyte HPE) and sealing with the help of a hot press, finally, the DSSCs were fabricated. In order to examine the morphological property of textile PDMS films, a field-emission scanning electron microscope (FE-SEM, LEO SUPRA 55, Carl Zeiss) and an optical macroview system (MVX10, Olympus) were utilized. The surface wetting behaviors were characterized by using a contact angle measurement system (Phoenix-300, SEO). The current density-voltage (J-V) characteristics of the DSSCs were evaluated by using a photocurrent system consisting of a solar simulator (ABET, SUN 3000) with 1000 W Xe short arc lamp and a source meter (Keithley 2400). For the measurements of optical reflectance and transmittance spectra, a UV-vis-NIR spectrophotometer (Cary 5000, Varian) was used. Incident photon to current conversion efficiency (IPCE) spectra were measured by using a 300 W xenon arc lamp as the light source coupled to a monochromater (TLS-300x xenon light source, Newport) with optical power meter (2935-c, Newport). After calibration using a silicon photodiode (Newport 818-UV), the IPCE data were taken by illuminating monochromatic light on the DSSCs (with a wavelength sampling interval of 10 nm in the range of 300-800 nm). For FDTD and RCWA optical calculations, commercial softwares (FullWave and DiffractMOD 3.1, Rsoft Design) were utilized, respectively.
3. Results and discussion
Figure 1(a) shows the schematic diagram of the fabrication process for the textile PDMS including preparation of textile master molds, simple coating of the PDMS, and separation of the textile PDMS from the master mold. From these three steps based on the facile soft imprint lithography, the textile PDMS could be conveniently fabricated. As shown in Fig. 1(a), the master mold was formed by attaching the TS onto the flat glass plate using the adhesive layer which maintains a flatten textile template. Therefore, the PDMS mixture could be evenly covered on the master mold during the aging process for 10 min. After curing, by using a razor, the PDMS film with proper sizes was readily taken off from the master mold. Figure 1(b) shows the (i) optical microscope image and (ii) tilted-view and (iii) high-magnification FE-SEM images of the fabricated textile PDMS. As can be seen in the microscope image of Fig. 1(b)(i), it is clearly observed that the surface of PDMS was well patterned from the textile.
Fig. 1 (a) Schematic diagram of the fabrication process for the textile PDMS: Preparation of textile master molds, simple coating of the PDMS, and separation the textile PDMS from the master mold, and (b) (i) optical microscope image and (ii) tilted-view and (iii) high-magnification FE-SEM images of the fabricated textile PDMS.
According to the shape and surface morphology of the master mold, the PDMS looked like a woven fabric with checked patterns. In the tilted-view FE-SEM image of Fig. 1(b)(ii), the textile patterns were formed inversely on the PDMS surface. From the magnified FE-SEM image of Fig. 1(b)(iii), the textile PDMS showed that the one-dimensional line-like micro-grating structures were partially distributed in disorder with a period of ~15 μm. Because of this novel structure, the textile PDMS can be expected to offer beneficial surface optical properties for photovoltaic applications.
In order to investigate the surface wettability of the textile PDMS, its water contact angle was explored. Photographs of a water droplet on the surface of (i) the bare glass, (ii) the flat PDMS, and (iii) the textile PDMS, including measured contact angles (θc) are shown in Fig. 2(a). For the bare glass in Fig. 2(a)(i), the hydrophilic behavior was observed with a low contact angle (θc = 28.38°) because the glass generally possesses high critical surface tensions of > 45 dynes/cm [34]. Unfortunately, this hydrophilic window is not desirable for clean surface at outdoor environments because some microparticles of dust or salt are easily spread and attached by being contacted with rain or fog. On the contrary, the flat PDMS exhibited a higher contact angle (θc~98.71°) in Fig. 2(a)(ii), which indicates a hydrophobic surface. In fact, a PDMS material has an inherently hydrophobic property and its contact angle can be increased by creating a large surface roughness, which can be explained by the Cassie-Baxter theory [35,36]. In Fig. 2(a)(iii), the textile PDMS had a much larger θc value of ~123.15° than those of both the bare glass and flat PDMS. This can be mainly caused by a fact that the textile patterns effectively expand the interfacial area between the PDMS and water. When the textile PDMS film is used as a cover layer for photovoltaic devices, the self-cleaning effect can be expected [22]. Moreover, it enables to increase the conversion efficiency of solar cells due to the improved optical properties. Figure 2(b) shows the measured total transmittance and reflectance spectra of the FTO/glass, the flat PDMS on the FTO/glass, and the textile PDMS on the FTO/glass at normal incidence. Considering the incident direction of light for solar cells with transparent substrates, the PDMS cover layers were laminated on the glass surface of FTO/glass. As the light propagated from the glass to the FTO film, the FTO/glass exhibited a transmittance of 82.3% at 550 nm of wavelength, which is a general value of transparent conductive oxide coated soda lime glasses. For the flat PDMS on the FTO/glass, the transmittance was increased and the reflectance was reduced in the visible wavelength range, respectively. Since the refractive indices of the PDMS and glass are 1.4 and 1.52, respectively, the light experienced less change in refractive index between air and the glass [22]. On the other hand, the textile PDMS further increased the transmittance up to 85.1% at 550 nm of wavelength. This is mainly because the rough surface increases the possibility of reflected light to diffuse into the surface caused by the enhanced light path lengths, thus leading to an enhancement of light transmission [29].
Fig. 2 (a) Photographs of a water droplet on the surface of (i) the bare glass, (ii) the flat PDMS, and (iii) the textile PDMS, including measured contact angles (θc), and (b) measured total transmittance and reflectance spectra of the FTO/glass, the flat PDMS on FTO/glass, and the textile PDMS on FTO/glass at normal incidence.
In fact, it is hard to exactly calculate and optimize the light coupling and diffuse transmission of the textile PDMS because the corresponding geometric structure is very complicated. To roughly estimate the light propagation behavior in transmission for the textile PDMS, thus, we calculated the diffraction efficiency by performing a RCWA simulation using simple geometry approximation. Figure 3(a) shows the schematic diagram of a simple simulation model for textile PDMS. For approximation, it was assumed that the surface was formed by stamping with periodic arrays of cylinders. To investigate the size-dependent characteristics of textile PDMS on the diffraction efficiency, the period was defined as a diameter of cylinder as shown in the schematic diagram. Figure 3(b) shows the calculated total transmitted powers of flat PDMS and textile PDMS with 15 µm of period. Herein, the total transmitted power was obtained by the summation for all of the transmitted diffraction orders. Due to the rough surface of textile PDMS, the light coupling between air and the PDMS could be enhanced. Compared to the flat PDMS, it could be observed that the total transmitted power was increased over a whole range of wavelengths. Figures 3(c) and 3(d) show the contour plots for the calculated total and high-order transmitted power as a function of period of textile PDMS, respectively. The high-order transmitted power was extracted by subtracting the zeroth-order transmitted diffraction from the total transmitted power. For the textile PDMS with microscale periods of 5-25 µm, as shown in Fig. 3(c), the total transmitted power was not significantly changed in the visible wavelength region. As the period of textile PDMS was decreased, the total transmitted power was slightly increased. However, the high-order transmitted power was reduced with decreasing the period as shown in Fig. 3(d). This indicates that the diffusive transmitted light can be decreased because the light was less transmitted with large diffraction angle of high order. Therefore, for the textile PDMS, the period around 15 μm could be proper to enhance the light coupling and diffuse transmission between the PDMS and air.
Fig. 3 (a) Schematic diagram of a simple model for textile PDMS, (b) calculated total transmitted powers of flat PDMS and textile PDMS with 15 µm of period, and contour plots for the calculated (c) total and (d) high-order transmitted power as a function of period of textile PDMS.
Figure 4(a) shows the measured diffuse transmittance spectra of the FTO/glass, the flat PDMS on FTO/glass, and the textile PDMS on FTO/glass. Here, the diffuse transmittance was characterized by subtracting the specular transmittance from the total transmittance. The inset of Fig. 4(a) also shows the photographic images of the flat and textile PDMS films on FTO/glass. For the bare FTO/glass, the diffuse transmittance was observed as 5.9% at 550 nm of wavelength. Due to the rough surface of the sputtered FTO film on glasses, incident light was somewhat scattered and diffused during the passing through the FTO/glass, especially, at short wavelengths of < 500 nm. When the incident light passed the flat PDMS on FTO/glass, there was no distinguishable change of diffuse transmittance spectra. For the textile PDMS on FTO/glass, in contrast, the diffuse transmittance was significantly increased in the entire range of wavelengths because of their microscale-roughened surface. In the wavelength range from 460 to 680 nm, it exhibited high diffuse transmittance values of > 76.7%. As shown in the photographic images of the inset of Fig. 4(a), it can be clearly seen that the textile PDMS on FTO/glass exhibited a high haze optical property with screening the Kyung Hee University seal. Indeed, this transmittance haze factor is important for enhancing the efficiency of photovoltaic devices with improved light absorption property [37,38]. For theoretical analysis of light propagation properties, the FDTD numerical calculation was performed for (i) the flat PDMS and (ii) the textile PDMS in Fig. 4(b). In FDTD calculations, for the incident light, it was assumed that the linear polarized plane wave at 550 nm is propagated along the normal direction with normalized Gaussian beam profile. For the textile PDMS, the electric field distribution was estimated for the two dimensional sinusoidal grating with considering the FE-SEM image of Fig. 1. When the light passed through flat PDMS, as shown in Fig. 4(b)(i), the propagation direction did not change. On the contrary, in the textile PDMS of Fig. 4(b)(ii), the calculated electric fields showed the large diffraction of light, which mainly caused high diffuse transmittance. These light diffuse and antireflection properties enable it to be used as a cover layer on the window of photovoltaic devices including the DSSC to improve the efficiency.
Fig. 4 (a) Measured diffuse transmittance (T) spectra of the FTO/glass, the flat PDMS on FTO/glass, and the textile PDMS on FTO/glass (i.e., diffuse T = total T - specular T) and (b) theoretical analysis of light propagation properties using the FDTD numerical calculation for (i) the flat PDMS and (ii) the textile PDMS. The inset of (a) also shows the photographic images of the flat and textile PDMS films on FTO glass.
To further investigate the real contribution for the novel structured textile PDMS in the dye-sensitized photo-anode layer of DSSCs, their absorption (i.e., 1-R-T) was extracted by the measured total reflectance (R) and transmittance (T). Figure 5 shows the (a) measured total reflectance and transmittance spectra and (b) estimated absorption (1-R-T) spectra of the dye-sensitized photo-anode/FTO/glass with the bare surface, the flat PDMS, and the textile PDMS. As shown in Fig. 5(a), for all the samples, there is no significant variation in the transmittance spectra. On the other hand, the introduction of the flat PDMS into the dye-sensitized photo-anode/FTO/glass led to the reduction in the reflectance. Moreover, the textile PDMS further reduced the reflectance. From these results, as shown in the absorption spectra of Fig. 5(b), it can be observed that the dye-sensitized photo-anode/FTO/glass with the textile PDMS had the higher absorption spectrum compared to ones with the bare surface and the flat PDMS over a wide wavelength region of 350-800 nm. To clarify the increased absorption of dye-sensitized photo-anode/FTO/glass, the absorption enhancement percentage for the dye-sensitized photo-anode/FTO/glass with the flat PDMS and the textile PDMS relative to one with the bare surface as a function of wavelength is also shown in Fig. 5(b), respectively. Compared with the dye-the sensitized photo-anode with the bare surface, the textile PDMS further improved the light absorption (i.e., textile PDMS/bare, blue-dotted line), exhibiting the larger average enhancement percentage of ~4.3% (i.e., ~1.6% for the flat PDMS/bare, red-dotted line) in the wavelength range of 350-800 nm. This is attributed to the enhanced light trapping caused by the efficient antireflection (or increased total transmittance) and the strong diffuse light scattering effects, as verified in Figs. 2(b) and 4(a). Thus, by employing the textile PDMS onto the DSSCs, an enhanced efficiency could be obtained.
Fig. 5 (a) Measured total reflectance and transmittance spectra and (b) estimated absorption (1-R-T) spectra of the dye-sensitized photo-anode/FTO/glass with the bare surface, the flat PDMS, and the textile PDMS. Absorption enhancement percentage for the dye-sensitized photo-anode/FTO/glass with the flat PDMS and the textile PDMS relative to the one with the bare surface as a function of wavelength is also shown in (b), respectively.
Figure 6(a) shows measured J-V curves of the bare DSSC, the flat PDMS laminated on DSSC (i.e., flat PDMS on DSSC), and the textile PDMS on DSSC. The insets show the (i) schematic diagram and (ii) photographic image of the textile PDMS on DSSC, respectively. As shown in Fig. 6(a)(i), the 0.5 × 0.5 cm2 of textile PMDS was placed on the window of DSSC and positioned at the cell area. Due to Vande Walle's force between the PDMS and glass, the textile PDMS was laminated spontaneously as can be seen in the photographic image of Fig. 6(a)(ii). The fabricated DSSC exhibited a general device performance, indicating the conversion efficiency (η) of 6.04% and the fill factor (FF) of 65.32%, respectively. The short-circuit current density (Jsc) and open-circuit voltage (Voc) were 12.13 mA/cm2 and 0.763 V, respectively. For the flat PDMS on DSSC, the η slightly increased up to 6.15%. This means that the flat PDMS is not effective to improve the light absorption of the dye-sensitized photo-anode. In contrast, for the textile PDMS, larger enhanced η of 6.36% was obtained by the mainly increased Jsc value of 12.74 mA/cm2 due to the improved absorption in the dye-sensitized photo-anode layer of DSSCs in Fig. 5(b), which is caused by its efficient antireflection and light diffuse properties, exhibiting the Jsc and η enhancement percentages of ~5 and 5.3%, respectively, compared to those of the bare DSSC. Additionally, there are no significant changes in both the Voc and FF because the textile PDMS did not interfere between the photo-anode and electrolyte. Device characteristics of the corresponding DSSCs are summarized in Table 1.
Fig. 6 (a) Measured J-V curves and (b) IPCE spectra of the bare DSSC, the flat PDMS on DSSC, and the textile PDMS on DSSC. The insets of (a) show the (i) schematic diagram and (ii) photographic image of the textile PDMS on DSSC. IPCE enhancement percentages for the flat PDMS on DSSC and the textile PDMS on DSSC relative to the bare DSSC as a function of wavelength are also shown in (b), respectively.
Table 1. Device characteristics of the bare DSSC, the flat PDMS on DSSC, and the textile PDMS on DSSC at 0.5 × 0.5 cm2 of PDMS area.
For the textile PDMS on DSSC, the increased photocurrents can be also verified in IPCE data. Figure 6(b) shows the IPCE spectra of the bare DSSC, the flat PDMS on DSSC, and the textile PDMS on DSSC and the IPCE enhancement percentages for the flat PDMS on DSSC and the textile PDMS on DSSC relative to the bare DSSC as a function of wavelength, respectively. The textile PDMS on DSSC showed a higher IPCE spectrum compared to the other DSSCs over an entire spectra range of 300-800 nm, indicating the IPCE peak value of ~67.6% at 530 nm of wavelength (i.e., ~65.5% for the bare DSSC and ~66.6% for the flat PDMS on DSSC, respectively). Furthermore, compared with the bare DSSC, its IPCE was averagely enhanced to be ~4.7% at wavelengths of 300-800 nm while the flat PDMS on DSSC had an IPCE enhancement percentage of ~1.9%. For the textile PDMS/bare DSSCs, this IPCE average enhancement percentage of ~4.7% is similar to the absorption average enhancement percentage (i.e., ~4.3%) for the textile PDMS/bare dye-sensitized photo-anode/FTO/glass in Fig. 5(b).
To investigate the size effect of the textile PDMS on the device performance of DSSCs, the J-V curves were characterized for DSSCs covered by PDMS layers with different sizes. Figure 7 shows (a) the measured J-V curves of the textile PDMS on DSSCs for different textile PDMS areas from 0.3 × 0.3 to 1.3 × 1.3 cm2 and (b) the calculated electric field distribution of the textile PDMS on glass. The inset of Fig. 7(a) shows the η of the textile PDMS on DSSCs as a function of area of textile PDMS. As the area was increased from 0.3 × 0.3 to 1.3 × 1.3 cm2, the η was gradually increased from 6.19 to 6.51%. This enhancement is probably caused by two reasons. The first one is the light diffraction at the edge of PDMS as represented in Fig. 7(b). When the incident light meets the edges of textile PDMS or between the textile PDMS and glass (i.e., marked by blue arrows), the light was curved inside around the edge parts due to the diffraction. Hence, the photo-anode enables to absorb photons from direct incidence of light as well as indirect incidence of light at outer side by the textile PDMS, leading to the larger photocurrents. The second reason is the increased area of diffuse light. Since the light was diffracted with several directions after passing through the textile PDMS, the possibility to absorb the photons could be increased. However, as can be seen in the inset of Fig. 7(a), the η was almost saturated at areas of above 1 × 1 cm2 (i.e., η = 6.51%) because the diffracted and diffused light at the far area from the cell did not reach to the photo-anode. From these results, the textile PDMS with the optimized area of 1 × 1 cm2 can effectively enhance the conversion efficiency of DSSCs, including the protection of DSSCs by the hydrophobic cover layer. Device characteristics of DSSCs with different textile PDMS areas are summarized in Table 2.
Fig. 7 (a) Measured J-V curves of the textile PDMS on DSSCs for different textile PDMS areas from 0.3 × 0.3 to 1.3 × 1.3 cm2 and (b) calculated electric field distribution of the textile PDMS on glass. The inset of (a) also shows the η of the textile PDMS on DSSC as a function of area of textile PDMS.
Table 2. Device characteristics of DSSCs with different textile PDMS areas.
4. Conclusion
We fabricated the textile PDMS via a facile soft imprint lithography using the textile master mold. Due to the woven fiber surface of master mold, the PDMS exhibited inverse textile patterns with sinusoidal micro-grating structures. This textile PDMS provided an improved hydrophobic property as well as advanced optical characteristics. For a wetting behavior, the textile PDMS showed a high θc of ~123.15° due to the expanded interfacial area between the PDMS and water, and thus it can be expected to use as a self-cleaning functioned protection cover layer. In addition, it also increased total and diffuse transmittance spectra, simultaneously, when the light passed through the textile PDMS on FTO/glass. In particular, compared with the bare glass and flat PDMS, the textile PDMS considerably enhanced the diffuse transmittance over a wide wavelength range of 350-1100 nm. Using the FDTD simulation, it could be explained theoretically that high diffuse transmittance was mainly caused by the large diffraction of light due to the sinusoidal grating structure. To demonstrate the feasibility of device applications, the textile PDMS cover layers with various sizes were incorporated into DSSCs. As a result, for the area of 1 × 1 cm2, the improved η of 6.51% was obtained compared to the bare DSSC with the η of 6.04%. This facile and cost-effective fabrication process as well as the resulted highly-transparent textile PDMS cover layers with high haze and hydrophobicity can be useful in various photovoltaic applications.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013- 068407).
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