Solar power generation enhancement of dye-sensitized solar cells using hydrophobic and antireflective polymers with nanoholes

We improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs) using polydimethylsiloxane (PDMS) patterned with negatively tapered nanoholes (NHs) as a protective antireflection layer of the external glass surface. The NHs PDMS layers are prepared by a soft lithography via silicon molds with conical nanopillars. The NHs PDMS with a NH depth of 320 nm decreases the surface reflection of fluorine doped tin oxide (FTO)-coated glass over a wide wavelength range of 350–800 nm at incident angles (qin) of 0–70 , exhibiting a lower solar weighted reflectance (RSW) value of 7.1% at qin 1⁄4 0 and a lower average RSW value of 8.5% at qin 1⁄4 20–70 than those (i.e., RSW z 10.1% at qin 1⁄4 0 and average RSW z 15.6% at qin 1⁄4 20–70 ) of the FTO glass. In DSSC device applications, it increases the short-circuit current density (JSC) from 15.69 to 16.52 mA cm , thus resulting in an enhanced PCE value of 7.56% compared to the reference DSSC (i.e., PCE 1⁄4 7.15%). For different NH depths, the optical reflectance characteristics of the NHs PDMS/FTO glass are theoretically investigated using a rigorous coupled-wave analysis method, showing similar trends between the calculated and measured results. For solar spectrum angle-dependent photocurrents, it also shows a remarkable device performance at qin 1⁄4 20–70 . Besides, the NHs PDMS exhibits a hydrophobic surface with a water contact angle of 115 .


Introduction
Over the past decades, research into alternative energy sources has been a global issue as a result of enhanced environmental pollution and fossil-fuel consumption.Solar energy, among several other alternative energy sources, has been attracting interest in various application elds because of its abundance, cost-effective production, and environmental friendliness.2][3] However, the power conversion efficiencies (PCEs) of DSSCs fall below 13%, mainly due to limitations including the light absorption of the dyes and the interface charge separation between dyes and titanium dioxide (TiO 2 ) electrodes. 2,3Hence, possibilities exist for further enhancements in PCEs of DSSCs.In order to boost the light absorption in the dyed TiO 2 layer of the devices, among various approaches for improving the PCE, one simple method is the employment of efficient antireection coatings (ARCs) on the front surface of the substrates.5][6][7] However, to directly fabricate the nanostructures on substrates, nanopatterning (e.g., electron-beam, nanoimprint, or laser interference lithography) and etching processes including chemical and heat treatments are required, which increase the fabrication cost and complexity.Therefore, it is necessary to develop efficient ARCs of glasses in the effective ways including cost-effective, simple, and high-throughput processes.][10][11] The fabricated molds and stamps can be also repeatedly used for pattern transfer, and thus would facilitate the mass production (i.e., roll-to-roll or roll-to-plate) in industry applications.][12][13][14] Besides, it can increase the optical transmission of transparent substrates including glasses, polymers, etc. because of its lower refractive index of $1.4-1.43. 14,15Furthermore, the PDMS layer can be strongly attached on the surface of planar transparent substrates with detachability from it.On the other hand, the hydrophobicity is useful in outdoor applications because it can function the self-cleaning for the pollutants and dusts on the surface of devices. 16,17Thus, it is very meaningful to investigate the optical and wetting behaviors of nanostructured PDMS layers as well as their feasibility on the device performance of DSSCs.In this work, we demonstrated the PCE enhancement of DSSCs via protective AR PDMS layers with nanoholes (NHs) patterns.The NHs on the PDMS layers were negatively transferred from the silicon (Si) molds with tapered nanopillars by the so lithography.Their optical reectance characteristics, together with a theoretical prediction using a rigorous coupledwave analysis (RCWA) method, were also studied at various incident angles of light, including the surface wettability.

Experimental and numerical modelling details
Fabrication of NHs PDMS and DSSCs Fig. 1 shows the schematic diagram for the fabrication procedure of the NHs-patterned PDMS (i.e., NHs PDMS) from the Si mold with tapered nanopillars via the so lithography.The conical nanopillars on Si molds were prepared by using the thermally dewetted platinum (Pt) nanoparticles as an etch mask pattern and subsequent dry etching process, as reported in our previous work. 18As shown in Fig. 1, to fabricate the NHs on the surface of PDMS layers without any distortion and deformation, we performed two-step coating processes.Firstly, a mixture solution consisting of the trimethylsiloxy terminated vinylmethylsiloxane-dimethylsiloxane (VDT-731; Gelest, Inc.) and methylhydrosiloxane-dimethylsiloxane (HMS-301; Gelest, Inc.) copolymers was spin-coated on the Si molds, and then they were cured at a temperature of 75 C for 30 min.Aerwards, the Sylgard 184 PDMS (base : agent ¼ 10 : 1) solution was poured on the samples with curing at 75 C for 2 h.Finally, the PDMS layers were carefully separated from the Si molds, creating the NHs PDMS.To study the effect of depth of NHs on the optical reectance property, the NHs PDMS samples with NH depths of 150 and 320 nm were prepared, named NHs-A at 150 nm and NHs-B at 320 nm, respectively.On the other hand, DSSCs were made by a conventional fabrication process.A TiO 2 paste was deposited on uorine doped tin oxide (FTO)-coated glasses using a doctor-blade method.Then, it was annealed at 500 C for 2 h in a furnace.The deposition and annealing processes were repeated to x the thickness of the deposited TiO 2 paste by $15 mm.For the ruthenium(II) dye (Solaronix, N719), the TiO 2 coated FTO glass substrates were immersed in a 3 Â 10 À4 M solution with mixing dye for 24 h in dark state.By using the same doctor-blade process, a Pt paste (Dyesol, counter PT-1) as a counter electrode was coated onto the FTO glass substrates.Lastly, an electrolyte (Dyesol, electrolyte HPD) was injected, and thus the DSSCs with an active area of 0.25 cm 2 were fabricated.

Characterization instruments
A scanning electron microscope (SEM; LEO SUPRA 55, Carl Zeiss) was utilized for observing the surface morphologies and pattern proles of the fabricated samples.The surface wetting behaviors were evaluated by using a contact angle measurement system (Phoenix-300, SEO).A UV-vis-NIR spectrophotometer (Cary 5000, Varian) was used to characterize optical reectance properties.The current density-voltage (J-V) characteristics of DSSCs were measured 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).Incident photon to current conversion efficiency (IPCE) data were obtained by illuminating monochromatic light on the DSSCs using a 300 W xenon arc lamp as the light source coupled to a monochromator (TLS-300Â xenon light source, Newport) with optical power meter (2935-c, Newport).

Numerical modelling and simulations
The theoretical analysis on the optical reectance and the electric eld intensity distribution of the NHs PDMS were also studied by the RCWA method using a commercial soware (DiffractMOD, Rso Design Group).To design the numerical model, the shape of the negatively tapered nanohole structure (i.e., empty space, air) on the surface of the PDMS was represented by a periodic geometry in the Cartesian coordinate system using a scalar-valued function of three variables, f(x, y, z), for simplicity.We assumed that the incident light enters from air to the PDMS layer.The E y , i.e., amplitude of y-polarized electric eld, was calculated for the incident plane wave with a Gaussian beam prole that is normalized at l ¼ 530 nm.The period of NHs was kept at 280 nm.The thicknesses of constituent layers were set to be 150 mm for the PDMS, 500 mm for the glass, and 650 nm for the FTO lm, respectively.The calculated values at each wavelength were averaged to remove rapid uctuations caused by the interference of light reected at the top and bottom surfaces of the PDMS/glass/FTO.The refractive index and extinction coefficient of glass and FTO were acquired from index Web site. 19For the PDMS, the refractive index was assumed to be 1.43.nanopillars-B) without any large deformation and distortion, as shown in the SEM images of Fig. 2(a).For the formed NHs on the PDMS lms, the average NH depths were estimated to be $150 nm (i.e., NHs-A PDMS) and $320 nm (i.e., NHs-B PDMS), respectively.The PDMS with a at surface showed a hydrophobic property with a water contact angle (q CA ) of $99 . 20The introduction of the roughness on the surface enhances the hydrophobicity, as can be explained by the Cassie-Baxter theory. 21Thus, the larger q CA values were obtained for the NHs PDMS, exhibiting the q CA z 106 for the NHs-A and q CA z 115 for the NHs-B, respectively.This hydrophobic surface might self-clean the dusts and contaminants on the devices in outdoor environments.

Results and discussion
Fig. 3 shows (a) the measured reectance spectra of the FTO glass, the at PDMS/FTO glass, the NHs-A PDMS/FTO glass, and the NHs-B PDMS/FTO glass, (b) the contour plot of the variation of the calculated reectance spectra of NHs PDMS/FTO glass as functions of wavelength and depth of NHs, and (c) the electric eld (E y ) intensity distributions for the corresponding structures at l ¼ 530 nm.As shown in Fig. 3(a), by laminating the at PDMS on the glass surface of the FTO glass, the surface reectance of the FTO glass was reduced because of the step gradient refractive index (GRIN) prole in constituent materials of air (n ¼ 1)/PDMS (1.43)/glass (1.52)/FTO (1.98).On the other hand, the NHs PDMS further decreased the reectance over a wide wavelength range of 350-800 nm.This is attributed to the linear and continuous GRIN prole via negatively tapered NHs (i.e., moth-eye effect). 22For the NHs-B PDMS with a larger NH depth of $320 nm, its solar weighted reectance (R SW ) value of $7.1%, which is the ratio of the usable photons reected to the total usable photons, 23 at wavelengths of 350-800 nm is lower than those of the other samples (i.e., R SW z 10.1%, 9%, and 7.9% for  the FTO glass, the at PDMS/FTO glass, and the NHs-A PDMS/FTO glass, respectively).In RCWA simulations of Fig. 3(b), the calculated reectance results of NHs PDMS at different depths of NHs were roughly similar to the experimentally measured spectra in Fig. 3(a).The R SW values of NHs PDMS/FTO glass as a function of depth of NHs are also shown in the inset of Fig. 3(b).The R SW values are abruptly decreased from 9.6% at 0 nm (i.e., at PDMS/FTO glass) to 7.6% at 200 nm and they are minimally saturated at NH depths of >300 nm, exhibiting R SW values of $7.4-7.5%.5][26] Therefore, it can be considered that the NHs PDMS with the NH depth of $320 nm is an optimized sample that has the lowest R SW value of $7.1% in the NHs PDMS/FTO glass structure.Additionally, the antireection characteristics of the NHs PDMS can be observed in the calculated E y intensity distributions.In Fig. 3(c), for both the glass and at PDMS, the E y intensities at l ¼ 530 nm are high in the air region due to their strong surface reection.For the NHs PDMS, on the other hand, there exist relatively weaker E y intensities in the air region due to the suppressed reectivity caused by the GRIN prole of the NHs on the surface of PDMS.
It is also important to study incident light angle-dependent reectance characteristics due to the change of the sun position in a day and seasons.Fig. 4 shows (a) the R SW values for the measured reectance spectra of the FTO glass, the at PDMS/FTO glass, and the NHs-B PDMS/FTO glass as a function of incident angle (q in ) in the wavelength range of 350-800 nm and (b) the contour plots of variations of the calculated angledependent reectance spectra for the corresponding structures.As shown in Fig. 4(a), for all the samples, the R SW values gradually increased with increasing the q in value from 20 to 70 .However, as expected, the NHs-B PDMS with a superior AR ability further decreased the R SW values of FTO glass compared to the at PDMS at each q in value, exhibiting a lower average R SW value of $8.5% at q in ¼ 20-70 (i.e., average R SW values of $15.6% and $11.9% for the FTO glass and the at PDMS/FTO  In order to demonstrate the AR effect of the NHs PDMS in DSSCs, the device characteristics of DSSCs employed with NHs PDMS as a protective AR layer were investigated.Fig. 5 shows the (a) J-V curves and (b) IPCE spectra of the reference DSSC without PDMS and the DSSCs with the at PDMS, the NHs-A PDMS, and the NHs-B PDMS.Photograph of the DSSC incorporated with NHs-B PDMS protective AR layer is also shown in the inset of Fig. 5(b).It can be observed that the PDMS is well laminated on the external glass surface of the DSSC due to van der Waals force between the PDMS and glass.The device characteristics of the corresponding DSSCs are summarized in Table 1.As shown in Fig. 5(a), by laminating the PDMS on the glass surface of DSSCs, the increased short-circuit current density (J SC ) values were observed while there was no large variation in both the open-circuit voltage (V OC ) and ll factor (FF).The DSSC with the NHs-B PDMS especially exhibited a higher J SC value of 16.52 mA cm À2 than those of the other DSSCs (i.e., J SC ¼ 15.69, 15.92, and 16.22 mA cm À2 for the reference DSSC and the DSSCs with the at PDMS and the NHs-A PDMS, respectively).This is ascribed to the reduced surface reection by the linear GRIN prole via NHs on the PDMS in Fig. 3(a), as mentioned above.The increased photocurrent can be also conrmed in the IPCE data of Fig. 5(b).The DSSC with the NHs-B PDMS showed a higher IPCE spectrum compared to the other DSSCs in the wavelength range of 400-800 nm.As a result, the PCE value of the DSSC with the NHs-B PDMS was improved by 7.56% (i.e., PCE ¼ 7.15% for the reference DSSC), indicating the PCE enhancement percentage of $5.8%.
To study the angle-dependence of solar power generation in DSSCs with the AR NHs PDMS, their J SC values were measured and the enhancement percentage in J SC was estimated for the DSSC with the at PDMS and the NHs-B PDMS relative to the reference DSSC at q in ¼ 20-70 , as shown in Fig. 6.For all the DSSCs, the J SC values were generally reduced with increasing the q in value.This is attributed to the reduction of the projection area where the incident light enters into the DSSC due to the tilted DSSC from the normally incident light source of the solar simulator as well as the increased surface reection losses.However, the DSSC with the NHs-B PDMS showed the higher J SC value at each q in value, maintaining a larger average enhancement percentage value of $8.8% compared to the reference DSSC at q in ¼ 20-70 (i.e., $3.9% for the at PDMS DSSC/ reference DSSC).Thus, the use of the NHs PDMS AR layer with NH depths of >300 nm can lead to the enhancement of the solar energy harvesting in DSSCs for an entire day and the seasons.

Conclusions
The inversely tapered NHs were fabricated on the surface of the PDMS by the so lithography via the Si mold with conical nanopillars and their optical reectance properties were experimentally and theoretically investigated.For the 320 nm-depth NHs-B PDMS/FTO glass with the hydrophobic surface (i.e., q CA z 115 ), it showed the superior AR characteristics at wavelengths of 350-800 nm and q in ¼ 0-70 , exhibiting the lower R SW z 7.1% at q in ¼ 0 and average R SW z 8.5% at q in ¼ 20-70 .By incorporating the NHs-B PDMS into the outer glass of DSSCs as a protective AR layer, the J SC was not only enhanced to 16.52 mA cm À2 , but also the IPCE spectrum was increased, thus leading to the improved PCE value of 7.56% (i.e., PCE ¼ 7.15% for the reference DSSC).For incident light angle-dependent photocurrents, it also showed a superior solar power generation ability at q in ¼ 20-70 .These results can provide a deep insight into the nanostructured PDMS for omnidirectional broadband ARCs in various photovoltaic systems that use transparent substrates and covers including glasses and plastics.

Fig. 2
Fig. 2 shows (a) the 40 -tilted oblique-view SEM images of the Si mold with nanopillars and the fabricated PDMS with nanoholes (NHs) and (b) the photographs of a water droplet on the surface of corresponding PDMS samples.Using the so lithography technique, the negatively tapered nanohole arrays were relatively well transferred onto the surface of the PDMS lms from conical nanopillar-structured Si molds (i.e., average height z170 nm for the nanopillars-A and z380 nm for the

Fig. 1
Fig. 1 Schematic diagram for the fabrication procedure of the nanoholes-patterned PDMS (i.e., NHs PDMS) from the Si mold with tapered nanopillars via the soft lithography.

Fig. 2
Fig. 2 (a) 40 -tilted oblique-view SEM images of the Si mold with nanopillars and the fabricated PDMS with NHs and (b) photographs of a water droplet on the surface of corresponding PDMS samples.

Fig. 3
Fig. 3 (a) Measured reflectance spectra of the FTO glass, flat PDMS/FTO glass, NHs-A PDMS/FTO glass, and NHs-B PDMS/FTO glass, (b) contour plot of the variation of the calculated reflectance spectra of NHs PDMS/FTO glass as functions of wavelength and depth of NHs, and (c) E y intensity distributions for the corresponding structures at l ¼ 530 nm.The R SW values of NHs PDMS/FTO glass as a function of depth of NHs are also shown in the inset of (b).

Fig. 5
Fig. 5 (a) J-V curves and (b) IPCE spectra of the reference DSSC without PDMS and the DSSCs with the flat PDMS, the NHs-A PDMS, and the NHs-B PDMS.Photograph of the DSSC incorporated with NHs-B PDMS protective AR layer is also shown in the inset of (b).

Fig. 4
Fig.4(a) R SW values for the measured reflectance spectra of the FTO glass, the flat PDMS/FTO glass, and the NHs-B PDMS/FTO glass as a function of incident angle (q in ) in the wavelength range of 350-800 nm and (b) contour plots of variations of the calculated angledependent reflectance spectra for the corresponding structures.

Fig. 6 J
Fig. 6 J SC of values the reference DSSC and the DSSCs with the flat PDMS, and the NHs-B PDMS and estimated enhancement percentage in J SC for the DSSCs with the flat PDMS DSSC and the NHs PDMS relative to the reference DSSC at q in ¼ 20-70 .

Table 1
Device characteristics of the corresponding DSSCs