Monolithic Two-Dimensional Photonic Crystal Reflectors for the

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

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Monolithic Two-Dimensional Photonic Crystal Reflectors for the Fabrication of Highly Efficient and Highly Transparent DyeSensitized Solar Cells Sujin Baek,† Su-Jin Ha,† Heechul Lee,‡ Kiwon Kim,† Dongchoul Kim,*,‡ and Jun Hyuk Moon*,† †

Department of Chemical and Biomolecular Engineering and ‡Department of Mechanical Engineering, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 04107, Republic of Korea S Supporting Information *

ABSTRACT: The transparent characteristic of dye-sensitized solar cells (DSCs) makes them suitable for building integrated photovoltaic (BIPV) devices. However, the diffusive scattering layer, which is usually used to increase the efficiency of these devices, greatly lowers the transparency of the DSC. This paper described a two-dimensional (2D) photonic crystal (PC) reflector with a sub-micrometer characteristic length that can improve the efficiency of these devices while maintaining transparency. This 2D PCs were fabricated directly onto TiO2 photoelectrodes using colloidal lithography and have the structure of a nanopillar array. A nanopillar with a height of 430 nm was observed to selectively reflect up to 40% of the light of 400−500 nm wavelength. The perceived transparency of the 2D PC electrode was 52%, which is much higher than 0.3% of the conventional scattering layer. The DSC fabricated using the 2D PC electrode demonstrated a maximum photon-to-electric conversion efficiency of 8.23%, which is 18% higher than the pristine electrode. The 2D PC is a highly efficient and wavelengthselective reflector that can be applied to various photoelectric conversion devices. KEYWORDS: photonic crystals, nanopillar, colloidal lithography, dye-sensitized solar cells, selective reflector, photoelectrodes

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INTRODUCTION Dye-sensitized solar cells (DSCs) are a next-generation category of solar cells wherein light absorption is achieved by dye molecules that are bound to the surface of a wide band gap semiconductor oxide (e.g., TiO2, ZnO) porous electrode.1−4 The most distinct feature of DSCs in comparison with commercialized silicon or thin-film solar cells is their transparency. The transparent characteristics of DSCs make them suitable for outdoor applications, particularly for use as building integrated photovoltaic devices (BIPV), where powersupplying devices replace conventional building materials such as window glass. Most of the previous studies that exhibited high photon-toelectric conversion efficiencies over 11% for this kind of solar cell have employed the use of a relatively thick (typically 5−10 μm) scattering layer on top of the porous oxide film.5−8 Particles with the size of several hundred nanometers have typically been used.9−12 This scattering layer reflects the transmitted light back to the electrode that was not initially absorbed by the electrode, thereby improving the light© XXXX American Chemical Society

harvesting efficiency of the electrode. The use of more efficient and functional scattering layers such as yolk−shell microparticles, microspheres, and hollow microspheres have been studied.12−17 Note that these particulate scattering layers diffusively scatter a wide range of visible light, which significantly reduces the transparency of the DSCs. Therefore, they are limited to apply to transparent BIPV. Recently, the use of a photonic crystal (PC) layer has been investigated.18,19 PCs possess a macroscopic crystal structure with a one-, two-, or three-dimensional (1D, 2D, or 3D) periodic lattice, and the lattice distance is comparable to the wavelength of visible light.20 The PC can selectively reflect light of a specific wavelength by a photonic band gap (PBG) instead of diffusing all the visible light via particle scattering, which may allow for the transmission of some range of visible light, and therefore the transparency of DSCs may be maintained.6,21−27 Received: July 7, 2017 Accepted: October 4, 2017

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DOI: 10.1021/acsami.7b09885 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

lation of the distribution of the electromagnetic energy density of the 2D TiO2 nanopillar PC was conducted using COMSOL software. The simulation cells were composed using the information obtained from the SEM morphology of the nanopillar with varying etching times. The distribution of the energy density was confirmed by a pulse of 450 nm wavelength incident on the lower side of the cell. The J−V characteristics of the DSCs were measured using a Source Meter (Keithley Instruments) under simulated solar light, which was provided by a solar simulator (1000 W Xe lamp with an AM 1.5 G filter). The light intensity was adjusted to 100 mW cm−2 using a Si reference cell (BS-520, Bunko-Keiki). The incident photon to current conversion efficiency (IPCE) was estimated using a 300 W Xe light source (Oriel) with a monochromator operated in direct current (dc) mode. The incident light intensity was estimated using a photodiode detector (calibrated silicon detector, Newport). The electron transit time and recombination lifetime were measured by intensitymodulated photocurrent spectroscopy (IMPS) and intensity-modulated voltage spectroscopy (IMVS), respectively. The measurements were carried out using a frequency response analyzer (XPOT, Zahner), which was used to drive a green light-emitting diode (LED) with a wavelength of 520 nm. The LED provided both the dc and alternating current (ac) components of the illumination. The modulation amplitude of the ac component superimposed onto the dc component was 10%.

Previously, 1D PCs were fabricated using alternating layers of porous TiO2 and SiO2, or a periodic modulation of pore fractions of TiO2 films.28−35 Most of these 1D PC structures were fabricated with a thickness of 1−2 μm, which enabled the improvement of the photocurrent density by 10−30% (see Table S1 of the Supporting Information for more details). 3D PCs were prepared using colloidal crystal templates.6,29−33 Compared to 1D PCs, 3D PCs could be made of a relatively thick film (2−14 μm) and thereby were reported to have a high photocurrent improvement of 10−200% (see Table S1 for more details). Nevertheless, the maximum efficiency of electrodes containing 1D and 3D PC reflector was less than 5% and 6.46%, respectively, and thus reported a very low absolute efficiency relative to commercial scattering layer electrodes. Moreover, fabricating a thick scattering layer is difficult using the 1D structure as it requires a multiple stacking process. In the case of the 3D structure, there is considerable diffusive scattering due to the defective structure generated by the colloid self-assembly, and thus a high haze layer is likely to be produced.36−38 Thus, an easily fabricated, highly efficient PC reflector remains in high demand. In this study, we introduce the monolithic surface relief 2D PC film as a reflector layer. The 2D PCs with sub-micrometer patterns were fabricated by use of the colloidal lithography technique without any expensive lithography process.34,39,40 The 2D PC layer consisted of a nanopillar array of a submicrometer lattice distance is only less than 1 μm thick and thus much thinner than previous 1D and 3D PC layers. Nevertheless, we achieve a photocurrent improvement of 18% and report an efficiency of 8.23%. This efficiency is higher than that of the electrodes of conventional 1D or 3D PCs. Moreover, the nanopillar 2D PC-integrated electrode DSCs exhibit a high perceived transparency of 50%.

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RESULTS AND DISCUSSION Monolithic surface relief 2D PC electrodes were fabricated using colloidal lithography as described in Figure 1a. Colloidal lithography achieves sub-micrometer resolution patterning using a colloidal particle mask, providing it with the advantages of having low cost, covering a large area, and having a highthroughput patterning when compared to conventional photolithography.39−41 First, the polystyrene (PS) colloidal particles are assembled onto the as-prepared TiO2 nanoparticle (NP)

EXPERIMENTAL PROCEDURES

Fabrication of 2D Nanopillar Array Photonic Crystals. A TiO2 nanoparticle dispersion was coated on a substrate using a doctor blade and sintered at 500 °C to form a nanoparticle TiO2 electrode. Monodispersed polystyrene (PS) particles were coated on this electrode. PS particles were synthesized by the dispersion polymerization of styrene monomers in an ethanol medium using poly(Nvinylpyrrolidone) (Junsei Chemicals Co.) as a stabilizer. A monolayer of PS particles was formed for use as a physical mask in the colloidal lithography. The NP TiO2 was etched by reactive-ion etching. The etching was carried out in a mixture of CF4 and Ar gases at a ratio of 2:1 with a 200 W radio frequency power (PlasmaPro System100 Cobra). After etching, the PS particle array mask was removed by calcination at 500 °C for 2 h. Assembly of Dye-Sensitized Solar Cells. The area of the TiO2 NP electrodes was controlled to be 8−10 mm2. The TiO2 electrode was sensitized using D205 dye (Mitsubishi Paper Mills) by soaking the electrode in 0.5 mM D205 acetonitrile solution. A Pt counter electrode was prepared by coating a 0.7 mM H2PtCl6 solution in anhydrous ethanol onto an FTO substrate. The TiO2 electrode and the counter electrode were assembled, and the gap between the two electrodes was fixed using a 60 μm thick polymeric film (Surlyn, DuPont). Finally, the electrolyte solution was injected into the gap; the electrolyte solution was prepared by mixing 0.05 M LiI (Sigma−Aldrich), 0.1 M guanidine thiocyanate (Wako), 0.03 M I2 (Yakuri), 0.5 M 4-tert-butylpyridine (Aldrich), and 0.7 M 1-butyl-3-methylimidazolium iodide (BMII) (Sigma−Aldrich) in a solution containing acetonitrile (Aldrich) and valeronitrile (85:15 v/v). Characterization. Scanning electron microscopy (SEM) images were obtained using a field-emission scanning electron microscope (SUPRA 55VP). Absorption spectra were recorded using a UV−vis spectrophotometer (UV-2550, Shimadzu). The computational simu-

Figure 1. (a) Schematic diagram of colloidal lithography for fabricating 2D nanopillar array PCs. The surface SEM images of the (b) PS monolayer colloidal mask and (c) nanopillar 2D PC. (d) Magnified SEM image of nanopillars. (e) Large-area nanopillar 2D PCs: inset shows their digital camera image. B


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Figure 2. High-magnification cross-sectional SEM images of nanopillar 2D PCs with various heights of (a) 150 nm, (b) 430 nm, and (c) 750 nm.

film by using a floating transfer method.42,43 Second, RIE using CF4 gas is performed using the PS particle array as a mask. Etching occurs only on the TiO2 surface that is exposed through the cavities between particles. Removal of the particles by calcination leaves a surface relief of a TiO2 nanopillar array pattern. The nanopillar pattern has a triangular lattice structure similar to the arrangement of the particles and a diameter similar to that of the colloidal particles. The SEM image of a close-packed monolayer of approximately 550 nm diameter PS spheres that were coated onto a TiO2 NP film are shown in Figure 1b. A SEM image of the surface relief nanopillar array obtained by RIE and calcination is shown in Figure 1c. The lattice distance between the nanopillars is 550 nm and is equal to the lattice distance of the PS spheres. As can be seen in Figure 1d, the high-magnification SEM image shows that the nanopillars are very porous and possess morphology similar to that of the unetched TiO2 NP film. A low-magnification SEM photograph shows a large-area nanopillar array with a few defects, as shown in Figure 1e. The 2D nanopillar PCs with high refractive indices function as back reflectors. Previously, the surface relief PC reflector (e.g., grating) has been used for vertical-cavity surface-emitting lasers and silicon solar cells.44,45 The light of a wavelength similar to the period of the PCs is diffracted. Thus, a nanopillar array with a 550 nm period was fabricated, which is similar to the wavelength that is strongly absorbed by the D205 dye used in DSCs (see Figure S1). Meanwhile, the intensity of the reflection diffraction by the 2D nanopillar PC is determined by the phase difference of the light passing through the top and bottom of the nanopillar. For example, when the phase difference is nπ radians (where n is an integer), the transmitted light is suppressed and all the energy is reflected.45,46 In the experiment, the reflective diffraction was optimized by controlling the height of the nanopillar. The height was controlled by controlling the RIE time. The cross-section and surface SEM images of the TiO2 nanopillar monolithic electrodes with various etching times are shown in Figure 2; the height of the nanopillars are shown to be linearly increasing with increasing etching time. More specifically, since the PS spheres are also etched during the etching of TiO2 film, the diameter of the nanopillar cylinders are also decreased during the etching. The detailed diameter, height, and filling fraction of the 2D nanopillars obtained by using various etching times are provided in Table S2. We confirm that the RIE and additional

calcination does not affect the crystallinity of the nanocrystalline TiO2 (see Figure S2). The transmittance values of the monolithic 2D PC TiO2 electrode films prepared using various etching times were compared in Figure 3a. The transmittance of the pristine TiO2

Figure 3. (a) Measured transmittance spectra and (b) a relative transmittance spectrum of 2D PCs with various nanopillar heights. (c) Electromagnetic energy density distribution when the electromagnetic wave (EM) passes through the nanopillar PCs with various heights.

NP electrode film was also displayed for comparison. To confirm the reflection occurs via the 2D PC layer only, the relative reflection spectrum excluding the reflectance of the pristine sample is displayed as shown in Figure 3b. As the etching time increases, that is, as the height of the nanopillar increases, the transmittance is decreased, and the relative reflectance is increased accordingly in the 400−500 nm wavelength range. However, when the height of the nanopillar is too large, the relative reflectance is decreased. When the height of the nanopillar is 430 nm, the reflectance at the wavelength of 465 nm is about 35% higher than that of the pristine sample. As the etching time increases, the reflected wavelength shifts slightly, which can be accounted for by the decreasing nanopillar diameter and the changing average refractive index of the PC. To analyze the reflectivity of the nanopillar, the wave propagation was simulated through use of C


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ACS Applied Materials & Interfaces the finite-difference time-domain method (FDTD) and can be observed in Figure 3c. When the electromagnetic wave (EM) passes through the nanopillar PC, the energy density distribution showed the strongest zero-order diffraction for the nanopillar with a 430 nm height. It is interpreted that the reflected light from the top and bottom of the nanopillar is in phase with the constructive interference, in comparison to samples of the other heights. This result is consistent with the results of the relative reflectance with changing nanopillar heights. The performance of a DSC with an installed monolithic 2D PC electrode film was also evaluated. Figure 4a shows the

photocurrent density versus voltage (J−V) curve of a 2D PC electrode DSC with nanopillars of various heights under AM 1.5 G illumination. The J−V curve of the pristine TiO2 electrode film was measured for comparison. Table 1 lists the short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF) from the J−V curve, and the calculated photon-to-electric conversion efficiency. Among the nanopillars of various heights, the highest current values and efficiencies were obtained for 430 nm height 2D PC electrodes. This result corresponds well with the previously described reflectance results. That is, the high light reflectance of a 430 nm nanopillar 2D PC increases the light absorption by the electrode, resulting in a higher current value and hence a higher efficiency. The efficiency of the 2D PC (430 nm height) electrode used in a DSC is 8.23%, which is 18% higher than that of the pristine electrode DSC. Most previous studies using 3D PCs have applied 3D inverse opal films of over 5 μm thickness on top of the electrode and reported DSC efficiencies of up to 6.5%.6,23,25 Note that since a 3D PC layer with a thickness similar to that of the pristine electrode was introduced, the contribution of light absorption by the dye adsorbed onto the PC was significant over the reflection by the PC film. The 1D PC were fabricated by alternately stacking porous silica and TiO2 layers or by stacking TiO2 layers with controlled pore fractions.28,30−33 The thickness of these 1D PC layers was about 1 μm. The efficiency of the fabricated DSCs was as low as 5%.28,31−33 The 2D PC DSC fabricated in this study is a thin film of less than 500 nm; it is much thinner than conventional 1D and 3D PC DSCs. Nonetheless, the 2D PC resulted in an efficiency improvement of 18%, and the maximum photoelectric conversion efficiency of the DSC was found to be 8.23%. This efficiency is higher than the efficiency of any previous electrode made using a 1D or 3D PC reflection layer. In other words, this nanopillar array PC is a more effective reflector than the existing 1D and 3D PC electrodes. Since the increase in efficiency achieved using a 2D PC is due to the increase of Jsc, the various efficiencies that determine Jsc were analyzed. The Jsc can be correlated to the efficiencies of the light harvested by the electrode, the electron injection from the dye into the electrode, and the electron collection by the conducting substrate:47,48 Jsc =

∫ qI0(λ)ηlh(λ)ηcolηinj dλ

(1)

First, ηcol is determined by the ratio of charge transport time (τt) and recombination lifetime (τr), namely, ηcol = 1 − (τt/ τr).49 The τt and τr values were obtained from the angularfrequency minima in the IMPS Nyquist plot (τt = 1/2πf min) and the IMVS Nyquist plot (τr = 1/2πf min), respectively, where f min is the frequency that corresponds to the minimum imaginary component.49 When comparing the 2D PC electrode to the pristine electrode, the τc and τr of both electrodes are observed to be almost the same, as shown in Figure 4b. Accordingly, the average ηcol for the 2D PC electrodes and the

Figure 4. (a) J−V curves of DSCs with various monolithic 2D PCs on the electrode surface. (b) Electron lifetime and transit time of 2D PC and bare electrode DSCs. (c) IPCE spectrum of 2D PC and bare electrode DSCs.

Table 1. Photovoltaic Parameters of the Pristine and 2D PC Electrode DSCs bare Jsc (mA/cm2) Voc (V) FF efficiency (%)

13.46 0.74 0.70 6.96

(±0.2) (±0.03) (±0.04) (±0.6)

2D PC (h = 150 nm) 14.47 0.73 0.72 7.55

(±0.2) (±0.01) (±0.00) (±0.02) D

2D PC (h = 430 nm) 15.55 0.75 0.71 8.23

(±0.7) (±0.02) (±0.02) (±0.6)

2D PC (h = 750 nm) 14.64 0.74 0.70 7.55

(±0.2) (±0.00) (±0.00) (±0.1)


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of each wavelength in the incident light spectrum and the eye sensitivity factor for each wavelength. Tv is calculated by the methodology prescribed in ISO 9050:2330:50

pristine electrodes were similar (60% and 56%, respectively). The value of ηinj is determined by the chemical adsorption between the dye and the electrode or the energy level difference.47 Therefore, the same ηinj can be expected in both electrodes. ηlh is influenced by the dye loading and optical path length of the electrode.48 In this experiment, a similar dye adsorption density was observed between the 2D PC and pristine electrodes (see Table S3). Although the nanopillar structure has a large specific surface area, they are formed only on the electrode surface, so the contribution of surface nanopillar to the overall surface area of the electrode may be negligible. On the other hand, the optical path length was expected to improve using 2D PCs. This is because the visible light reflected by the 2D PC can be absorbed again inside the electrode layer. From the analysis of the above efficiencies, the high Jsc value of the 2D PC electrode used in a DSC is due to the improvement of the ηlh due to reflection of the 2D PC. Meanwhile, the incident photon to current efficiency (IPCE) curves of the 2D PC electrode and the pristine electrode were compared, as shown in Figure 4c. The presence of the 2D PC structures significantly enhances the IPCE over the incident light range, specifically light with wavelengths in the range of 500−600 nm. This confirms that the enhancement of Jsc is attributed to the enhanced light harvesting originating from the reflection provided by the 2D PC structures. Finally, the transparency and photocurrent gain of a 2D PC and a commercially available particle scattering layer were compared. As shown in Figure S3, the morphology of the scattering layer includes agglomerated particles of 400 nm in size and has a thickness of 3 μm. A digital camera image of a 2D PC electrode clearly demonstrates these characteristics present in the background, whereas films coated with commercial scattering layers are opaque, as shown in Figure 5a. That is, the

780

Tv =

∑380 T (λ)DλV (λ)Δλ 780

∑380 DλV (λ)Δλ

(2)

where V(λ) is the photonic spectral luminous efficiency function that represents the wavelength-dependent sensitivity for the observer in photometry (ISO/CIE 10527), Dλ is the AM 1.5 G solar spectral irradiance, and T(λ) is the spectral transmittance of the sample. The Tv of the 2D PC electrode is 52% while that of the scattering layer-coated electrode is only 0.3%. Here, we evaluate the increase in the photocurrent while considering the Tv value. The commercial scattering layer improves Jsc by 12% while reducing Tv from 80% to almost 0%, as shown in Figure 5b. On the other hand, a 2D PC electrode achieves a Jsc of 15.55 mA/cm2, similar to a commercial scattering layer electrode, while the Tv value is decreased to 50% (Figure 5b). This result suggests that the 2D PC reflector overcome the correlation coefficient of inverse relationship between transmittance and photocurrent of conventional scattering layer. Therefore, it can be regarded as a new reflector which enables high light harvesting efficiency increase while maintaining transparency.

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CONCLUSIONS The use of 2D PC-based monolithic reflectors in DSCs to improve light absorption efficiency while maintaining transparency was demonstrated. 2D PCs in the form of a nanopillar array on a TiO2 photoelectrode surface were fabricated using colloidal lithography. The thin film nanofiller 2D PC layer of 430 nm thickness selectively and strongly reflects light of a 500 nm wavelength. As a result, an improvement of up to 18% in the photocurrent was achieved in comparison to a bare DSC. Interestingly, compared with the commercial scattering layer, the nanopillar 2D PC was able to demonstrate similar photocurrents while maintaining a much higher Tv. We believe 2D PCs can be applied to various photoelectric conversion devices as easily prepared and highly effective reflectors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09885. Tables showing a comparative list of change of transmittance and photovoltaic parameters of previous 1D and 3D PC electrodes, size parameters of 2D PCs with various nanopillar heights, and dye adsorption density of 2D PC with various nanopillar heights; figures showing absorbance of D205 dye, XRD results of nanocrystalline TiO2 without and with RIE or calcination treatment, and cross-sectional SEM image of conventional particulate scattering layer (PDF)

Figure 5. (a) Digital images of the bare, 2D PC layer, and conventional particulate scattering layer. (b) Photocurrent density vs perceived transparency of bare, conventional scattering layer, and 2D PC layer.

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digital camera image of the 2D PC electrode exhibits low diffusive transmittance of visible light while the commercial scattering layer-coated electrode is highly diffusive and also has low light transmittance. To quantify the transparency, the perceived transparency, Tv, was calculated. Tv is a standard measure of transparency, taking into account the transmittance

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.H.M.). *E-mail: [email protected] (D.C.K.). ORCID

Jun Hyuk Moon: 0000-0002-4776-3115 E


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ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Research Foundation of Korea (grant no. 20110030253 and 2016M3D3A1A01913254). J.H.M. acknowledge the support from Sogang University (201610050). The Korea Basic Science Institute is also acknowledged for the SEM measurement.

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