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Enhancement of Light Absorption in Photovoltaic Devices using Textured PDMS Stickers Inchan Hwang, Deokjae Choi, Sojeong Lee, Ji Hoon Seo, Ka-Hyun Kim, Ilsun Yoon, and Kwanyong Seo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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ACS Applied Materials & Interfaces

Enhancement of Light Absorption in Photovoltaic Devices using Textured PDMS Stickers Inchan Hwang, a,d,† Deokjae Choi,a,† Sojeong Lee,a Ji hoon Seo,a Ka-Hyun Kim,b,* Ilsun Yoon,c,* and Kwanyong Seo a,* a

Department of Energy Engineering, Ulsan National Institute of Science and Technology

(UNIST), Ulsan, 44919, Republic of Korea b

KIER-UNIST, Advanced Center for Energy, Korea Institute for Energy Research, Ulsan

44919, Republic of Korea c

Department of Chemistry, Chungnam National University, Daejeon 34134, Republic of

Korea d

Max Planck Center for Attosecond Science, Max Planck POSTECH/KOREA Res.

Initiative, Pohang, Gyeongbuk, 37673, Republic of Korea

KEYWORDS: Anti-reflection, Surface structuring, PDMS, Sticker, Light absorption, Solar cells

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ABSTRACT

We

designed

and

fabricated

a

random-size

inverted-pyramid-structured

polydimethylsiloxane (RSIPS-PDMS) sticker to enhance the light absorption of solar cells and thus increase their efficiency. The fabricated sticker was laminated onto bare-glass and crystalline-silicon (c-Si) surfaces; consequently, low solar-weighted reflectance values were obtained for these surfaces (6.88% and 17.2%, respectively). In addition, we found that incident light was refracted at the PDMS–air interface of each RSIPS, which redirected the incident power and significantly increased the optical path length in the RSIPS-PDMS sticker which was 14.7% greater than that in a flat-PDMS sticker. Moreover, we investigated power reflection and propagation through the RSIPS-PDMS sticker using a finite-difference timedomain method. By attaching an RSIPS-PDMS sticker onto both an organic solar cell (OSC) based on a glass substrate and a c-Si solar cell, the power conversion efficiency of the OSC and the c-Si solar cell were increased from 8.57% to 8.94% and from 16.2% to 17.9%, respectively. Thus, the RSIPS-PDMS sticker is expected to be universally applicable to the surfaces of solar cells to enhance their light absorption.

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1. INTRODUCTION

Light-absorption enhancement is one of the key research areas related to the development of high-efficiency solar cells. A number of studies have investigated how the light-absorption of solar cells can be maximized by forming an antireflection (AR) surface using, e.g., quarter-wavelength AR coating or surface structuring techniques.1–5 Although quarter-wavelength AR coating utilizing a dielectric thin-film layer can effectively reduce surface reflections by decreasing the difference between the refractive indices of air and an active media, reflectance is lowered only in a specific wavelength region; either the thickness or the refractive index of the dielectric thin film determine which region is affected.6,7 Surface structuring is an effective way to increase light absorption in a broadband wavelength range; this is done by reducing surface reflection and increasing the optical path length. Unfortunately, fabricating a desired surface structure on glass substrates used in dyesensitized solar cells (DSCs), organic solar cells (OSCs), and perovskite solar cells is far from straightforward. Furthermore, the surface structuring of crystalline silicon (c-Si) solar cells not only requires complex processes but also increases the surface area of the cell, which leads to a decrease in efficiency of the device owing to surface recombination.8–12 Another approach for enhancing the light-absorption of solar cells is to use surfacestructured transparent polymer films.13–19 These films can be formed repeatedly using a master mold and can be attached onto the surface of solar cells without damaging the original devices. Polydimethylsiloxane (PDMS) is widely used in solar cell applications because it has high transmittance, a low refractive index (1.43) when used as an AR film, and good adhesive properties; these characteristics allow it to be reversibly attached and detached from surfaces. 20,21 Therefore, if PDMS AR films could be fabricated as stickers, they could be

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used to enhance the light-absorption of all types of solar cells regardless of the type of substrate used. In this study, we fabricated random-size inverted-pyramid-structured (RSIPS) PDMS stickers to enhance the light-trapping efficiency of solar cells. We found that the fabricated RSIPS-PDMS stickers exhibited a reversible attachment/detachment characteristic during testing irrespective of the type of solar-cell substrate that it was applied to. We confirmed that a glass and a c-Si substrate showed significantly lower solar-weighted reflectance values (Rsw) of 6.88% and 17.2%, respectively, when RSIPS-PDMS stickers were applied to them (the bare glass and c-Si substrates showed values of 8.66% and 39.2%, respectively). By applying RSIPS-PDMS stickers to glass-based OSCs and c-Si solar cells, we were able to enhance the photovoltaic efficiency of the OSCs from 8.57% to 8.94% and that of the c-Si solar cells from 16.2% to 17.9%. To investigate the optical properties of the RSIPS-PDMS stickers, we used a finite-difference time-domain (FDTD) simulation.

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2. EXPERIMENTAL SECTION 2.1 Fabrication of random-size inverted-pyramid-structured polydimethylsiloxane (RSIPS-PDMS) sticker: To fabricate a micro-pyramid-structured silicon mold, we cleaned a 2x2 cm2 planar silicon substrate with acetone, methanol and de-ionized (DI) water. After removing the silicon native oxide by immersing buffered oxide etchant (BOE, HF: NH4F = 7: 1), the planar silicon substrate was dried by nitrogen (N2) gas flow. The cleaned silicon substrate was immersed into wet-etching solution of potassium hydroxide (KOH, 40 wt% solution): isopropyl alcohol (IPA, 99.9%): DI water = 1: 0.5: 8.5 vol% at 75 ℃ for 40 min to fabricate random-size pyramid-structured silicon mold. To remove the remaining potassium ions on the silicon substrate, a mixed acid solution of hydrochloric acid (HCl, 36%): hydrogen peroxide (H2O2, 30%): DI water = 1: 1: 5 vol% was used at 80 ℃ for 10 min. Next, we deposited trichloro-(1H,1H,2H,2H-perfluorooctyl) silane (FOTS) monolayer to detach the textured PDMS layer easily. A mixed solution of PDMS base resin and curing agent (Sylgard 184, Dow Corning Co.) was poured on the silicon master mold and annealed at 100 ℃ for 10 min. The cured RSIPS-PDMS sticker was detached from the silicon master mold. 2.2 Fabrication of inverted organic solar cells: Indium tin oxide (ITO) coated glass substrates were cleaned by ultrasonication in deionized (DI) water, acetone, and IPA, successively. And then the cleaned substrates were treated in a UV-ozone chamber for 20 min. For a formation of electron transport layer, zinc oxide (ZnO) sol-gel solution was coated on the cleaned substrate at a spin speed of 3000 rpm for 30 s. And then the substrate was heated at 200 ℃ for 10 min to form an amorphous ZnO layer. A mixed photoactive solution of

poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-

ethylhexyl) carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) of 8 mg and [6,6]-phenyl- C71butyric-acid-methyl-ester (PC71BM) of 12 mg dissolved in chlorobenzene of 970 µl, was

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coated on the ZnO layer by the spin-coating process for 40 s at a speed of 2500 rpm. To form a hole transport layer and a metal anode, molybdenum oxide (MoO3) of 5 nm and silver (Ag) of 100 nm were deposited sequentially on the active layer through a thermal evaporation. The area of the final device specified as an anode area was 0.13 cm2. 2.3 Fabrication of c-Si solar cells: To fabricate the c-Si solar cells, an emitter and a backsurface-field (BSF) layers were formed using Czochralski (CZ) n-type Si wafers (resistivity of 1-3 Ω cm, 400 µm-thick) by the spin-on-doping (SOD) process. For the BSF layer, a phosphorus dopant source (P509, Filmtronics, Inc.) was spin-coated on a dummy Si wafer at the speed of 2000 rpm for 30 s, and then the dummy wafer was baked on a hot plate at 200 ℃ for 20 min. The diffusion process was carried out in a tube furnace under a mixed ambient atmosphere of O2 (125 sccm) and N2 (500 sccm) at 900 ℃. We positioned the Si substrate facing the phosphorus-coated dummy wafer for the conformal doping. To remove the silicate glass formed after the diffusion process, diluted hydrofluoric acid (HF (50%): DI water = 1: 10) solution was used. Subsequently, the emitter layer was formed using a Boron dopant source (B155, Filmtronics, Inc.) on the front side. The boron dopant source was spin-coated on a dummy Si wafer at the speed of 2000 rpm for 30 s, and then the dummy wafer was baked on a hotplate at 200 ℃ for 20 min. Also, we positioned the Si substrate so that it faced the boron-coated dummy wafer. The doping process was conducted in the tube furnace under a N2 atmosphere 880 ℃. Next, boron silicate glass was removed by a diluted HF solution. To passivate the emitter surface, a 10 nm-thick aluminum oxide (Al2O3) layer was deposited by atomic layer deposition (Lucida D100, NCD) and 60 nm-thick silicon nitride (SiNx) layer was formed by plasma-enhanced chemical vapor deposition (PEH-600, SORONA). And then annealing process at 450 ℃ was followed. To form front and rear metal electrode, 500 nmthick Al were deposited by using a thermal evaporator.

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2.4 Optical characterization of RSIPS-PDMS sticker: The surface of RSIPS-PDMS sticker were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Optical transmittance and reflectance measurements were performed via an UV-visNIR spectrophotometer (Cary 5000, Agilent) equipped with a 110 mm integrating sphere. 2.5 Photovoltaic characterization of solar cells: The photovoltaic performances of solar cells were measured using an Oriel Sol3A Class AAA solar simulator (Newport) under AM 1.5G illumination. The incident flux was confirmed with a calibrated power meter, and double-checked using a NREL-calibrated solar cell (PV Measurements, Inc.) EQE measurement is carried out using a Xe light source and a monochromator in a wavelength range of 300 to 1100 nm. 2.6 Finite-difference time domain simulations (FDTD): The simulations were carried out using the commercial FDTD software package Lumerical® FDTD Solutions 8.15. The geometries of the RSIPS-PDMS sticker were estimated according to the SEM images. For description of light propagation and reflectance of the RSIPS-PDMS sticker in wavelength range of λ from 300 nm to 1 µm, inverted pyramid structures of different depths (1–5 µm) and locations were randomly generated on the top surface of the PDMS layer for purposes of describing light propagation and reflectance of the RSIPS-PDMS sticker in the wavelength range of 300 nm ≤ λ ≤ 1 µm. Forty-six inverted pyramid structures were generated at the periodic boundary of 21 × 21 µm2 in the x and y directions. The refractive index of PDMS was assumed to be 1.4 in the simulation.

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3. RESULTS AND DISCUSSION

Figure 1. (a) A schematic of the soft-lithography process for random-size inverted-pyramidstructured polydimethylsiloxane (RSIPS-PDMS) sticker. Cross-sectional SEM images of (b) c-Si mold and (c) RSIPS-PDMS sticker. Insets of (b) and (c) are tilted SEM images. Images of attaching and detaching of RSIPS-PDMS sticker on (d) the glass substrate and (e) the c-Si substrate.

Figure 1a shows a schematic of the manufacturing process for the RSIPS-PDMS stickers; they were manufactured using a soft lithography method. A planar silicon substrate was chemically wet-etched to produce a random-size pyramid-structured crystalline-silicon (c-Si) master mold. To facilitate the peel-off process of the structured PDMS layer, we deposited a self-assembled monolayer (SAM) onto the c-Si master mold. After the deposition of the SAM layer, a PDMS solution was poured onto the Si master mold and subsequently cured; the cured PDMS layer was then detached from the mold. We then performed a scanning electron microscopy (SEM) analysis to examine the surface morphology of both the 8


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Si master mold and the fabricated RSIPS-PDMS layer, respectively. Figure 1b shows a crosssectional SEM image of the Si master mold showing a random-size pyramid structure with an average height and a base size of 8 µm and 7 µm, respectively. We confirmed that the Si master mold could be reused because there were no structural changes or residue after the PDMS layer had been detached. We also found that pyramidal structures were successfully transferred onto the surface of the PDMS using the soft lithography method. As shown in Figure 1c, the PDMS surface shows an inverted pyramid structure that is the inverse of the surface structure of the Si master mold. Figure 1d and e provide a demonstration of the “sticker” characteristic of the RSIPS-PDMS layer; we repeatedly tested for this characteristic by attaching/detaching the layer onto both the glass and c-Si substrates (supplementary video 1). Consequently, we confirmed that the fabricated RSIPS-PDMS film could be applied onto the surfaces of various substrates as a sticker. To analyze the optical characteristics of the RSIPS-PDMS stickers on both the glass and c-Si substrates, we measured their total transmittance and total reflectance spectra using a UV–Vis–NIR spectrophotometer (Cary 5000, Agilent Technologies, US) equipped with an integrating sphere (diameter = 110 mm), as shown in Figure S1 a–c. The reflectance spectra of the flat-PDMS/glass decreased to 8.2% at a 550-nm wavelength, which was 1% lower than the value obtained for bare glass of 9.2%. The reflectance of the flat-PDMS/c-Si was found to be much lower (26.3%) than that of bare c-Si (38%) at 550 nm. Because c-Si has a larger refractive index (4) than glass (1.53), the flat-PDMS layer significantly mitigates the large difference between the refractive indices of air and c-Si. The RSIPS-PDMS sticker, however, still further reduced the reflectance; RSIPS-PDMS/glass and RSIPS-PDMS/c-Si showed surface reflectance values of 6.3% and 16.4%, respectively, at the 550-nm wavelength. This

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can be explained using the effective refractive index (neff), which is estimated by the following equation:22 n = × + 1 − .

(1) In this equation, nair and nPDMS are the refractive indices of air and the PDMS sticker, respectively, and f is the filling fraction, which is the space proportion of PDMS compared to air. Since the filing fraction of the prepared RSIPS-PDMS stickers gradually increases from air to PDMS, neff gradually increases. Accordingly, the RSIPS-PDMS stickers were found to exhibit less reflectance owing to the gradual increase of neff between air and the PDMS. In photovoltaic applications, measuring solar-weighted reflectance (Rsw) is useful for evaluating an AR layer; Rsw is the ratio of reflected photons to total incoming photons, and it is calculated using the following equation:

=

!" , "

(2) where Is(#) and R(#) are the spectral irradiance and total reflectance, respectively. We calculated Rsw by using the experimentally obtained reflectance spectra and a standard air mass 1.5 global (AM 1.5G) solar spectral photon flux in a wavelength range of 300–800 nm for the glass substrate and 300–1100 nm for the c-Si substrate. The RSIPS-PDMS stickers on the glass and c-Si substrates were found to produce Rsw values of 6.88% and 17.2%, which were lower than those for both the bare glass and c-Si substrates (8.66% and 39.2%, respectively).

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Figure 2. (a) Simulated reflectance spectra of the flat-PDMS sticker and the RSIPS-PDMS sticker. The simulation scheme is illustrated in the inset. (b) Vertical cross sections of the time-averaged Poynting vector calculated at λ = 550 nm for the flat-PDMS sticker (left) and the RSIPS-PDMS sticker (right). (c) Horizontal cross sections of longitudinal and lateral components of the real part of the time-averaged Poynting vector (Re()) calculated at λ = 550 nm for the same RSIPS-PDMS sticker.

To clarify enhanced light trapping of the RSIPS-PDMS sticker, we investigated the light propagation and reflectance of the RSIPS-PDMS sticker in a wavelength range of 300 nm–1 µm with the three-dimensional finite-difference time-domain (FDTD, Lumerical) simulation. For description of the RSIPS-PDMS sticker, we generated randomly inverted 11



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pyramid structures of different etch depths (1–5 µm) and at different locations on the top surface of the PDMS sticker in the simulation. The pyramidal angles were set to 55° for all of the inverted pyramidal structures. PDMS is a transparent material (n = 1.4) with negligible propagation and scattering losses in the visible range. Assuming the thickness of the bottom PDMS layer underneath the inverted pyramid structures would not be critical in redirection of the power propagation through the sticker, the total thickness of the RSIPS-PDMS sticker is minimized to 9 µm in the simulation. Figure 2a shows a comparison of the calculated reflectance spectra of RSIPSPDMS and flat-PDMS stickers. Unwanted oscillations, which are shown in the reflectance spectrum of the flat-PDMS sticker as a result of overestimated Fabry–Perot interferences between flat-PDMS surfaces, are filtered for comparison with the reflectance spectrum of the RSIPS-PDMS sticker. Each reflectance spectrum was averaged over all wavelengths of interest and the RSIPS-PDMS sticker shows a λ-averaged reflectance (Rλ-avg) of 5.4%, which is slightly lower than that of the flat-PDMS sticker (6.2%). Reflection from the roughed front surface could be reduced through a gradual increase of the effective index of the surface structure similarly to well-known AR behaviors observed for the textured structures. However, it is likely that the reflection from the smooth bottom interface of the RSIPSPDMS sticker and the air was not significantly affected by the structure of the front surface; thus, the total reflection of the RSIPS-PDMS sticker would not be significantly reduced than that of the flat-PDMS sticker. To qualitatively analyze the incident-light diffusion induced by the RSIPS-PDMS stickers, we analyzed the power diffusion by the RSIPS-PDMS sticker with the timeaveraged Poynting vector () which represents the power flow near the structure. The power, incident normally on the structure, would be partially redirected and diffused by the structured front surface and this power diffusion can be visualized through a comparison of 12


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the vertical power distributions of the flat and textured surfaces, as shown in Figure 2b. The longitudinal and lateral components of the Poynting vector are compared with each other as shown in Figure 2c, indicating significant power redirections to randomized paths as well. Each inverted pyramidal structure has randomized sizes and locations in the simulation. However, all of the inverted pyramidal structures have same orientation and their surfaces have same inclined angle of 35° to the incident power. Interferences, resulted from structural regularity, could be significantly suppressed by the randomized locations of the inverted pyramidal structures. This simple assumption allows that Fresnel refractions through air-PDMS and PDMS-air interfaces of individual inverted pyramidal structures could be critical in the redirection of the incident power through the RSIPS-PDMS sticker. Redirection of the incident light and change in the optical path can be described using Fresnel refractions on the interfaces of each inverted pyramid structure and can be numerically analyzed using a far-field projection of the transmitted light, as shown in Figure S2, which reveals substantial redirection and diffusion of the incident light through the RSIPS-PDMS sticker compared with those through the flat-PDMS sticker.

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Figure 3. (a) Optical images depending on the distance between a RSIPS-PDMS sticker and a background image. (b) Haze ratio of a bare glass, a flat-PDMS/glass and a RSIPSPDMS/glass. (c) A schematic of the light scattering effect using a laser beam (λ=532 nm) passing through the RSIPS-PDMS sticker. Images of laser beam through (d) a flat-PDMS sticker and (e) a RSIPS-PDMS sticker.

To experimentally confirm the light-scattering effect of the RSIPS-PDMS stickers, we carefully investigated the change of optical image by light diffusion depending on the distance between the RSIPS-PDMS sticker and a background image. As shown in Figure 3a, we observed a relatively clear image when the RSIPS-PDMS sticker was attached onto the image; however, the image became more blurred as the RSIPS-PDMS sticker was lifted off from the image because the transmitted light through the sticker was highly diffused. To quantitatively evaluate the light diffused through the RSIPS-PDMS stickers, we calculated the optical haze using the following equation:

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$% =

&' &(

× 100, (3)

where Td and Tt are the diffuse transmittance and total transmittance, respectively. As shown in Figure S3, Td was measured using a transparent glass substrate. The RSIPS-PDMS sticker showed a much greater average haze ratio value (97.4%) than the bare-glass and flatPDMS/glass (0.85% and 1.07%, respectively) in Figure 3b. We were, therefore, able to confirm that the RSIPS-PDMS stickers would cause the light to be highly diffused. A 532-nm green laser was used to irradiate the surface of a RSIPS-PDMS sticker, as shown in Figure 3c, in order to demonstrate the diffused light and the corresponding increased optical path length. The light passing through the flat-PDMS sticker reached the screen without spreading, whereas the light transmitted through the RSIPS-PDMS sticker spread widely on the screen (Figure 3d and e). Using the projected image, we calculated the increased optical path length of the light. When the distance between the PDMS sticker and the screen was 8 cm, the horizontal axis length of the diffused light was 9 cm. Based on our experimental results, we estimated that the angle of the diffused light passing through the RSIPS structure was 29.4° (Figure S4). This implies that the optical path length of the light through the RSIPS-PDMS sticker was 14.7% longer than that induced by the flat-PDMS. Thus, it seems that this greater optical path length, in addition to surface reflection characteristics, is one of the critical factors underpinning the absorption enhancement produced by the RSIPS-PDMS stickers.

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Figure 4. (a) J-V curves and (b) EQE of organic solar cell (OSC) with and without RSIPSPDMS sticker. (c) J-V curves and (d) EQE of c-Si solar cell with and without RSIPS-PDMS sticker.

To investigate the enhanced light-absorption of photovoltaic devices that use RSIPSPDMS stickers, we investigated the effect of these stickers on the photovoltaic performance of organic solar cells (OSCs) and c-Si solar cells. The performances of all 10 OSCs and c-Si solar cells with and without the RSIPS-PDMS sticker are summarized in Table S1. The best performance of the solar cells with the RSIPS-PDMS sticker is shown in Figure 4. Figure 4a shows the current density–voltage (J–V) curves of an OSC both before and after an RSIPSPDMS sticker was attached to it. The reference cell shows a power-conversion efficiency (PCE) of 8.57%, an open-circuit voltage (Voc) of 0.759 V, short-circuit current density (Jsc) of

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16.6 mA cm−2, and a fill factor (FF) of 68.0%. With the RSIPS-PDMS sticker, the OSC exhibited the following: PCE = 8.94 %, Voc = 0.759 V, Jsc = 17.2 mA cm−2, and FF = 68.5%. Only Jsc increased significantly, which implies that the RSIPS-PDMS sticker efficiently enhanced light absorption without causing any damage. In Figure 4b, the external quantum efficiency (EQE) was measured in order to verify the increased light absorption induced by the RSIPS-PDMS sticker. We found that the EQE increased slightly over the wavelength range of 400–800 nm. The EQE of the OSC with the RSIPS-PDMS sticker in the wavelength region from 300 to 400 nm was not changed because of severe ultraviolet (UV)-light absorption by the glass substrate, as shown in Figure S5. In addition, the c-Si solar cell with the RSIPS-PDMS sticker showed a Jsc of 38.4 mA cm−2, which was greater than that of the bare c-Si solar, i.e., 35.3 mA cm−2, whereas Voc and FF did not change significantly; this can be seen in Figure 4c and Table 1. Owing to this greater Jsc, the efficiency of the c-Si solar cell increased from 16.2% to 17.9%. As observed for the OSCs, the RSIPS-PDMS sticker improved the quantum efficiency of the c-Si solar cells over the entire wavelength range; this can be observed in Figure 4d. However, the c-Si solar cell with the RSIPS-PDMS sticker showed a slightly decreased EQE compared to the c-Si solar cell without the RSIPS-PDMS sticker in the wavelength range from 500 to 600 nm because of the slightly increased surface reflectance of the c-Si solar cell with the RSIPS-PDMS sticker, as shown in Figure S6. To confirm the mechanical stability of the RSIPS-PDMS sticker, we measured the change in the efficiency of the solar cells upon repeated attachment–detachment of the RSIPS-PDMS sticker onto the devices (Figure S7). The results indicated that the device efficiency remained almost constant during multiple attachment–detachment processes. In addition, we confirmed via SEM that the surface morphology of the RSIPS-PDMS sticker did not change after 100 attachment–detachment processes as shown in Figure S7.

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Table 1. The best device performance of OSC and c-Si solar cell with and without the RSIPS-PDMS sticker. Jsc [mA cm-2]

Voc [V]

FF [%]

PCE [%]

Without RSIPS-PDMS sticker

16.6

0.759

68.0

8.57

With RSIPS-PDMS sticker

17.2

0.759

68.5

8.94

Without RSIPS-PDMS sticker

35.3

0.598

76.7

16.2

With RSIPS-PDMS sticker

38.4

0.608

76.5

17.9

Device

OSC

c-Si solar cell

*10 devices performance of OSCs and c-Si solar cells with and without the RSIPS-PDMS sticker are displayed in the Table S1.

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4. CONCLUSIONS In conclusion, we developed RSIPS-PDMS stickers using a soft-lithography process and used a pyramid-structured c-Si substrate as its master mold. When the RSIPS-PDMS stickers were laminated onto glass and c-Si substrates, the solar-weighted reflectances were lowered to 6.88% and 17.2%, respectively; the values for bare-glass and c-Si substrates were in comparison 8.66% and 39.2%, respectively. The light-diffusion effect caused by the RSIPS-PDMS stickers was investigated using an FDTD simulation and an experimental estimation of the optical path length. The optical path length was found to increase by up to 14.7% when the stickers were used. Owing to enhanced light absorption by the sticker, the maximum power conversion efficiency of the OSC and c-Si solar cells were 8.94% and 17.9%, respectively, which represents an increase of 4.2% and 10.5% compared to the values from the reference devices, respectively. Thus, we believe that the RSIPS-PDMS stickers proposed in this paper prove to be promising candidates for use as a universal platform for enhancing the light-absorption capacity of highly efficient solar cells.

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ASSOCIATED CONTENT Supporting Information. Total transmittance and reflectance of flat-PDMS and RSIPS-PDMS sticker on bare glass and c-Si substrates, Diffuse transmittance of flat-PDMS and RSIPS-PDMS on a bare glass, A schematic diagram showing a method of calculating angle of diffused light, Far-field projections of the transmitted light through the RSIPS-PDMS sticker of the simulation period, A schematic showing redirections of the incident power by Fresnel refractions through PDMS–air interfaces of the RSIPS-PDMS sticker, Normalized power integrations of the transmitted light through the RSIPS-PDMS and flat-PDMS stickers over given half angles, Light absorption spectra of the glass substrate and the RSIPS-PDMS sticker, Surface reflectance spectra of the c-Si solar cell without the RSIPS-PDMS sticker and the c-Si solar cell with the RSIPS-PDMS sticker, SEM images of the RSIPS-PDMS sticker, The change in the device efficiency of the organic and c-Si solar cells upon repeated detachment–attachment of the RSIPS-PDMS sticker, Devices performance of OSCs and c-Si solar cells with and without RSIPS-PDMS sticker, and Summary of recent progress in the textured polymer films. (PDF) Video of a demonstration for RSIPS-PDMS sticker characteristic by repeatedly attaching/ detaching the sticker onto both glass and c-Si substrates. (AVI)

AUTHOR INFORMATION †

These authors contributed equally to this work.

Corresponding Author

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*E-mail: [email protected], [email protected], [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (NRF-2017R1A2B4002738). It was also supported by research and development program of the Korea Institute of Energy Research (B7-2426). I. Hwang was supported in part by Global Research Laboratory Program (Grant No 200900439) and by Max Planck POSTECH/KOREA Research Initiative Program (Grant No 2016K1A4A4A01922028) through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning. I. Yoon was supported by the CNU research fund of Chungnam National University.

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