Boosting Light Harvesting in Perovskite Solar Cells by Biomimetic

Subscriber access provided by TUFTS UNIV

Applications of Polymer, Composite, and Coating Materials

Boosting Light Harvesting in Perovskite Solar Cells by Biomimetic Inverted Hemispherical Architectured Polymer Layer with High Haze Factor as an Antireflective Layer Dong Hyun Kim, Bhaskar Dudem, Jae Woong Jung, and Jae Su Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Boosting Light Harvesting in Perovskite Solar Cells by

Biomimetic

Inverted

Hemispherical

Architectured Polymer Layer with High Haze Factor as an Antireflective Layer Dong Hyun Kima, Bhaskar Dudema, Jae Woong Jung b* and Jae Su Yua* a

Department of Electronic Engineering, Kyung Hee University, 1732 Deogyeong-daero,

Giheung-gu, Yongin-Si, Gyeonggi-do 446-701, South Korea. b

Department of Advanced Materials Engineering for Electronics & Information, Kyung Hee

University, 1732 Deogyeong-daero, Giheung-gu, Yongin-Si, Gyeonggi-do 446-701, South Korea. *Corresponding author. Email address: [email protected] (J. W. Jung), [email protected] (J. S. Yu)

Abstract Biomimetic micro architectured polymer layers such as inverted hemispherical architectured (IHSA)-polydimethylsiloxane (PDMS) and hemispherical architectured (HSA)-PDMS layers were developed by a simple and cost-effective soft imprinting lithography method via a hexagonal close-packed polystyrene microsphere arrays/silicon mold. However, the IHSAPDMS/glass possessed superior antireflection (AR) characteristics with the highest/lowest average transmittance/reflectance (Tavg/Ravg) values of approximately 89.2%/6.4% compared to

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the HSA-PDMS/glass, flat-PDMS/glass and bare glass (Tavg/Ravg ∼88.8%/7.5%, 87.5%/7.9% and 87.3%/8.8%, respectively). In addition, the IHSA-PDMS/glass also exhibited a relatively strong light scattering property with the higher average haze ratio (Havg) of ∼ 38% than those of the bare glass, flat-PDMS/glass, and HSA-PDMS/glass (i.e., Havg ∼ 1.1, 1.7 and 34.2%, respectively). At last, to demonstrate the practical feasibility in light control of the solar cells, the IHSA-PDMS was laminated onto the glass substrates of perovskite solar cells (PSCs) as an AR layer and their device performances were explored. Consequently, the short circuit current density of the PSCs integrated with the IHSA-PDMS AR was improved by ~17% when compared with the device without AR layer, resulting in promising power conversion efficiency (PCE) up to 19%. Therefore, the IHSA-PDMS is expected to be applied as an AR layer for solar cells to enhance their light absorption as well as the PCE.

Keywords: antireflection, polydimethylsiloxane, soft imprinting lithography, perovskite, solar cells.

Graphical abstract


Page 2 of 35

Page 3 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Nowadays, energy harvesting from renewable energy resources such as solar energy, wind energy, mechanical energy, hydro energy, etc. is one of the great concerns to overcome the energy crisis and environmental pollution by conventional power supplies.1-5 Especially, the research on energy harvesting utilizing solar energy which is clean and effectively infinite among renewable energies is getting more attention, and it is expected to grow rapidly as a future industry.6-7 In the case of crystalline silicon (Si) solar cells commercialized among solar cells, it is predicted that the supply of raw materials will become difficult due to the scarcity of the raw material of Si and the product prices will be continuously increased.8 Therefore, new types of solar cells including dye-sensitized solar cells (DSSCs) and aqueous-processed polymer and nanocrystal solar cells have developed. 9-14 In recent years, perovskite solar cells (PSCs) are strengthening its position as a next-generation solar cell to replace the Si solar cells with their advantages of intense and broadband absorption, long carrier lifetime, simple manufacturing process, low cost, and excellent photovoltaic performance.15,16 So far, extensive research has been done for the enhancement of power conversion efficiency (PCE) of the PSCs.17,18 For example, the formation of pin-hole free perovskite thin films19-21 and the design of new materials via compositional/optical engineering22-24 or process to fabricate high-quality perovskite thin films etc. have been suggested.17,18,25 Consequently, the state-of-the-art photovoltaic performance of the solid-state PSCs reached the PCE over 22% under 1 sun conditions.26 On the other hand, approaches to enhance the conversion efficiency of various solar cells or modules by employing the highly transparent antireflection (AR) layers on their external-facing surfaces of transparent substrates (e.g., glasses, plastics, etc.) have been also proposed.27-31 The use of AR layers is more practical and facile to enhance the light absorption in the active layer of



the solar cells by reducing the surface reflection losses over a broad range of light incident wavelengths and angles. Additionally, the AR layers can be also utilized to protect the solar cells from exposure to external impact, heat, ultraviolet (UV) radiation and corrosive acid rain in harsh outdoor environments. Recently, the AR layers with biomimetic architectures consisting of nano and microarchitectures have been widely studied in several solar cells. However, the production of this biomimetic nano and microarchitectures involves complicated and expensive fabrication techniques such as lithography/patterning and etching processes.32,33 In order to overcome the drawbacks of expensive and time-consuming fabrication techniques, we adopted a simple, cost-effective and fast soft imprint lithographic (SIL) patterning method to create a transparent AR layer with microarchitectures such as inverted hemispherical architectured (IHSA)-PDMS and hemispherical architectured (HSA)-PDMS layers. Moreover, in SIL method, once master molds are made, they can be repeatedly used for pattern transfer. In this work, we employed the hexagonal close-packed polystyrene (PS) microsphere arrays/Si as a master mold, which is prepared by a relatively simple and costeffective drop coating method. These micro-grating architectures (i.e., IHSAs and HSAs) on PDMS layer can efficiently enhance the light absorption in the solar cells by reducing the surface reflection owing to the linearly gradient-refractive-index (GRIN) profile and the extension of light path lengths or strong electromagnetic fields, showing high haze factor.34,35 Thus, the optical and photovoltaic performances, as well as the surface wetting properties of IHSA- and HSA-polymers were systematically studied. Moreover, the finite-difference-time-domain (FDTD) theoretical simulations were also performed to analyze the optical light scattering properties of IHSA-PDMS and HSA-PDMS layers. Finally, we investigated the effect of IHSA-PDMS and HSA-PDMS AR layers on the light control of the PSCs, and their photovoltaic performance was


Page 4 of 35

Page 5 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


compared.

2. Experimental procedure and simulation modeling Fabrication of IHSA-PDMS and HSA-PDMS Figure 1 shows the schematic illustration of fabrication procedure for (i) IHSA-PDMS and (ii) HSA-PDMS layers by a SIL via the hexagonal close-packed PS microsphere arrays/Si master molds. Initially, a hexagonal close-packed monolayer of PS microsphere arrays on Si substrate was prepared by a drop-coating method. For that, firstly, a 2 × 2 cm2 of Si substrate was ultrasonically cleaned with acetone, methanol and de-ionized water, and then dried with the nitrogen gas flow. Furthermore, the cleaned Si substrate was exposed to the UV plasma for 3000 s, to modify its surface wetting behavior and remain hydrophilic. Besides, the PS colloidal solution consisting of the microspheres of 2 µm size was diluted by adding 1 wt% of the solution into 34.7 mM of sodium dodecyl sulfate (SDS) solution. And then, the diluted PS colloidal solution was ultrasonicated for 10 min to ensure the complete dissolution of PS solution into SDS. Next, a Teflon ring was placed on the UV plasma-treated Si substrate and firmly fixed on it, to prevent the leakage of diluted PS colloidal solution from the space between the Teflon ring and the Si substrate. At last, a 10 µl of diluted PS colloidal solution was dropped into Teflon ring fixed on Si substrate. Subsequently, the diluted PS colloidal solution dispersed on Si substrate was dried (i.e., casting) in an oven at 40 oC for 3 h. Eventually, the PS microspheres with twodimensional (2D) periodic hexagonal symmetry were realized on Si substrate and the sample was further utilized as a mold to fabricate IHSA-PDMS (Figure 1(i)). To fabricate the IHSA-PDMS layer, a hard PDMS (h-PDMS) consisting of a mixture of the trimethylsiloxyterminated vinylmethylsiloxane-dimethylsiloxane (VDT-731; Gelest, Inc.)



and methylhydrosiloxane-dimethylsiloxane (HMS-301; Gelest, Inc.) copolymers was spin-coated onto the PS microsphere arrays/Si master mold, and then cured at a temperature of 75 °C for 30 min in an oven. After curing, a soft PDMS (s-PDMS), prepared by the mixture of base resin and curing agent (Sylgard 184, Dow Corning Co.) with a 10:1 weight ratio, was poured on the hPDMS/mold, and subsequently cured at 75 °C for 2 h. At last, the cured PDMS layer was peeled off from the mold, thus producing the IHSA-PDMS layer with the thickness of approximately 150 µm. Afterwards, to fabricate the HSA-PDMS layer by the SIL technique (as shown in Figure 1(ii)), the IHSA-PDMS was utilized as a master mold. Thus, the surface of IHSA-PDMS was treated (or rinsed) with a mixture of 5 µl of trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane and 5 ml of n-hexane solution for 30 s and dried in an oven at 30 °C for 12 h. Such surface treatment can be utilized to enhance the hydrophobicity of the surface of IHSA-PDMS, and also used to prevent the new PDMS film from sticking to the IHSA-PDMS mold. Next, as mentioned above, the h- and s-PDMS solutions were subsequently coated on IHSA-PDMS, followed by the curing treatment. Therefore, the HSA-PDMS was also produced by peeling off it from the IHSA-PDMS layer. Preparation of PSCs: The PSCs were fabricated in normal-device architecture. Figure 1 also depicts the schematic diagram of device architecture. As shown in Figure 1, the PSC device is constructed on a fluorine-doped tin oxide glass substrate (FTO, Pilkington). Firstly, the patterned FTO glass substrates were cleaned by ultrasonication in ethanol, acetone, and isopropanol for 30 min, respectively. Titanium diisopropoxide bis(acetylacetonate) solution (75% in isopropanol, Aldrich) diluted in 1-butanol was spin-coated on the FTO substrate at 2000 rpm for 40 s, followed by heating at 500 °C for 30 min. The TiO2 paste (Dyesol) diluted in ethanol was spin-coated at 4000


Page 6 of 35

Page 7 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rpm for 30 s, followed by heating at 500 °C for 30 min. The perovskite solution containing 1 mmol of PbI2 (Alfar Aesar), 1 mmol of CH3NH3I (Dyesol), and 1 mmol dimethyl sulfoxide (DMSO, Aldrich) was dissolved in 600 mg of N, N-dimethylformamide (DMF, Alfa Aesar) and was spin-coated on the top of the TiO2 layer. The solution was spin-coated at 1000 rpm for 5 s and 4000 rpm for 15 s while dripping diethyl ether onto the substrate during second spinning step. The sample was then heated at 100 °C for 5 min, resulting in the formation of a dark perovskite film. The solution of 2,2′,7,7′-tetrakis(N,N-pdimethoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD, 56 mg), 4-tertbutylpyridine (30 mg) and bis(trifluoromethane)sulfonimide lithium salt (6 mg) in chlorobenzene (1 ml) was coated at a spin rate of 2500 rpm for 20 s to form the hole transport layer. Finally, an 80 nm-thick Au electrode was deposited by thermal evaporation using a shadow mask to form an active area of 10 mm2. At last, the AR layers were simply attached (or laminated) on the external face of the PSC devices using a plastic tweezer before every measurement (as shown in Figure 1). Characterization: The surface morphologies of as-fabricated PS microsphere arrays/Si mold, IHSA-PDMS, and HSA-PDMS layers were observed by using a field-emission scanning electron microscope (FE-SEM; LEO SUPRA 55, Carl Zeiss) and an atomic force microscope (AFM; XE150, PSIA). The optical characteristics of different PDMS layers and bare glass were measured by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere at near normal incidence. The incident angle-dependent optical characteristics (i.e., transmittance and reflectance) were also examined by using a spectroscopic ellipsometer (V-VASE, J.A.Woollam Co. Inc.) at various incident angles of 20-70° for non-polarized light. The surface wetting behavior of as-fabricated samples was measured by utilizing a contact angle measurement



system (Phoenix-300, SEO Co., Ltd.). The photovoltaic characteristics were measured by using a solar simulator (SUN 3000, ABET) with a 1000 W Xe short arc lamp and a source meter (Keithley 2400). The basic current density-voltage (J-V) characteristics of the devices were measured in reverse sweep at a rate of 1000 mV/s. For the hysteresis analysis, the rate scan rate was reduced to 100 mV/s. External quantum efficiency (EQE) spectra were obtained using a 300 W xenon arc lamp as the light source coupled to a monochromater (TLS-300x Xe light source, Newport) with an optical power meter (2935-c, Newport). After calibration using a silicon photodiode (818-UV, Newport), the EQE spectra were recorded by illuminating the PSCs with monochromatic light. Numerical modeling and simulations: For theoretical analysis of the effect of microarchitectures size on the optical properties of IHSA-PDMS/glass, optical modeling and simulations were carried out by the rigorous coupled-wave analysis (RCWA) method using a commercial software package (Diffract MOD, Rsoft Design Group). To design the theoretical model, the IHSAs on the surface of PDMS with a hexagonal symmetry is roughly represented in the Cartesian coordinate system by a scalarvalued function of three variables, f (x, y, z). Similarly, the theoretical analysis of the optical light scattering behavior of IHSA-PDMS and HSA-PDMS layers was performed by the finite difference time-domain (FDTD) method using a commercial software (full wave, Rsoft Design Group). To design the theoretical model, the hemispherical architectures and inverted hemispherical architectures on the PDMS film in the simulation were represented by the periodic geometry of the Cartesian coordinate system by a scalar function of two variables f(x, z), for simplicity. The y-polarized amplitude field (Ey) was calculated for an incident plane wave having a Gaussian beam profile normalized at λ = 550 nm. The period and depth of the hemispherical


Page 8 of 35

Page 9 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


architectures were considered as 2 µm and 1 µm, respectively (Figure 2). For both the simulations, the thicknesses of the PDMS and glass were fixed to 150 µm and 200 µm, respectively. And, the refractive indices of PDMS and glass were assumed to be 1.43 and 1.52, respectively.36,37

3. Results and discussion Figure 2 shows the top- and side-view SEM images of the (a) PS microsphere arrays/Si, (b) IHSA-PDMS, and (c) HSA-PDMS. Also, the 30°-tilted views of the SEM images of (d) IHSAPDMS (e) HSA-PDMS are shown in Figure 2. As shown in Figure 2a, a hexagonal close-packed monolayer of PS microspheres was formed on Si substrate by the facile and cost-effective dropcoating method. The period and diameter of these PS microspheres were noticed as ∼ 2 µm and ∼ 2 µm, respectively. Using the SIL method, these micro-spherical architectures with 2D periodic hexagonal pattern arrays on PS microsphere arrays/Si master molds were successfully and inversely transferred onto the surface of PDMS without any distinct distortion and deformation, creating the IHSA-PDMS, as can be seen in Figures 2b and 2d. The period and diameter of the IHSAs on the surface of PDMS are almost identical compared to the PS microspheres on the master mold. And, the height of IHSAs on PDMS was observed as approximately 1.1 µm. However, this IHSA-PDMS layer was further employed as a replication mold to develop the HSA-PDMS layer (Figure 2c). The period and height of the hemispherical architectures of HSAPDMS layer were noticed almost similar to those of the replication mold. But, the diameter was slightly decreased and observed as approximately 1.86 µm, instead of 2 µm. This may be attributed to the shrinkage of the cross-linked PDMS during the heat treatment. Resultantly, the HSAs and IHSAs with hexagonal periodic symmetry were formed on the surface of PDMS layer



by the SIL technique via the PS microspheres/Si master molds. Moreover, the antireflective characteristics of these both kinds of PDMS (i.e., IHSA-PDMS and HSA-PDMS) layers were analyzed to reduce the Fresnel surface reflectance losses of the transparent substrate such as bare glass. Figure 3 shows the 10 × 10 µm2 AFM scan images and depth profiles of the (a) PS microsphere arrays/Si, (b) IHSA-PDMS and (c) HSA-PDMS. The AFM image of PS microsphere arrays/Si master mold (Figure 3a) clearly revealed that the PS spheres with the diameter of 2 µm are uniformly distributed on Si substrate with 2D periodic hexagonal symmetry. As shown in the SEM image, using the SIL pattern transfer method, the IHSA-PDMS and HSAPDMS patterns were relatively well transferred onto the surface of PDMS from the PS microsphere arrays/Si master molds without obvious distortion. In particular, the AFM images can clearly indicate the pattern depth and the pattern period. For the IHSA-PDMS and HSAPDMS, the average depths were approximately 0.66 and 0.86 µm and the average periods were 1.75 and 1.87 µm, respectively. In addition, the diameters of IHSAs and HSAs on the surface of PDMS were observed to be approximately 1.8 and 1.96 µm, respectively, which are well matched with the SEM measurement results. Figures 4a and 4b show the measured total transmittance and reflectance spectra of the flat-PDMS, IHSA-PDMS and HSA-PDMS laminated on bare glass. For comparison, the total transmittance and reflectance spectra of the bare glass are also shown in Figures 4a and 4b. Inset of Figures 4a also depicts the schematic diagram of IHSA-PDMS laminated on bare glass, to investigate its total transmittance and reflectance characteristics. As can be seen in Figures 4a and 4b, by laminating the flat-PDMS layer on the glass surface, the optical total transmittance (or reflectance) is slightly enhanced (or reduced) as compared to the bare glass over a wide


Page 10 of 35

Page 11 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


wavelength

range

of

300-800

nm,

exhibiting

the

relatively

high/low

average

transmittance/reflectance (Tavg/Ravg) values of approximately 87.5%/7.9% compared to the bare glass (∼87.3%/8.8%). This is mainly attributed to the step refractive index distribution of the constituent materials (i.e., air/flat-PDMS/bare glass = 1/1.43/1.52). On the other hand, the lamination of both the micro architectured PDMS layers on the glass (i.e., IHSA-PDMS/glass and HSA-PDMS/glass) further enhanced (or reduced) the optical transmittance (or reflectance) of the glass. This is observed due to the linearly GRIN distribution between the air and PDMS via the IHS/HS architectures.36 Besides, these microarchitectures on PDMS layer can also extend the optical path lengths owing to the diffracted and rebound lights between the IHS/HS architectures,38 which can efficiently enhance the transmission of glasses by reducing their surface reflection. The light scattering characteristics will be discussed elaborately in succeeding text (Figure 5). However, by comparing these both micro architectured PDMS layers, the IHSAPDMS/glass exhibited a highest (or lowest) Tavg (or Ravg) value of 89.2% (or 6.4%) compared to that of the HSA-PDMS/glass (i.e., Tavg/Ravg = 88.8%/7.5%). This is caused by the difference in diffraction along the array direction of hemisphere architecture shape, as confirmed in the FDTD simulation (Figure 5c). However, to further investigate the effect of the size of IHSAs on the total transmittance of IHSA-PDMS, we performed a theoretical study through RCWA simulations. For this study, the period (or size) and depth of IHSAs were assumed to be in the ranges of 0-5 µm (with an interval of 0.5 µm) and 0-2.5 µm (with an interval of 0.25 µm), respectively. Figure 4c shows the contour plot of the variation of the calculated total transmittance spectra of IHSA-PDMS/glass as a function of wavelength and size of IHSAs. The inset of Figure 4c also depicts the calculated Tavg in the wavelength range of 300-800 nm for the IHSA-PDMS/glass at various sizes of IHSAs. These simulation results clearly indicate that the



total transmittance of IHSA-PDMS is strongly dependent upon the size of IHSAs. As the size of IHSAs is increased from 0 to 2.5 µm, the total transmittance or Tavg value of IHSA-PDMS/glass is gradually increased from 88.55 to 91.38%, respectively. By further increasing the size of IHSAs from 3 to 5 µm, the Tavg value of IHSA-PDMS/glass is almost identical (i.e., ∼91.43±0.05%). In particular, the IHSA-PDMS with the microarchitectures sizes higher than 1.5 µm clearly exhibits a high transmittance of >91% over the wavelength range of 350-800 nm. Therefore, from theoretical simulations, the IHSA-PDMS with efficient AR characteristics can be achieved only when the size of micro architectures is higher than 1.5 µm. Herein, we selected the IHSA-PDMS with the microarchitectures size of ∼2 µm to attain high transmittance, which is confirmed by the theoretical (Figure 4c) and experimental (Figure 4a) analyses. However, for solar cell applications, the solar-weighted transmittance (TSW) (solar-weighted reflectance, RSW), which is defined by the ratio of the usable photons transmitted (reflected) to the total usable photons,39 is very important to estimate in the wavelength range of 300-800 nm. As expected, the IHSA-PDMS/glass exhibited a higher (lower) TSW (RSW) value of ∼92.7% (∼6.4%) than those of the other samples (i.e., TSW ∼ 92.2, 91.4 and 91.1% and RSW ∼ 7.3, 8.0 and 8.8% for the HSAPDMS/glass, flat-PDMS/glass and bare glass, respectively). From the above results, it can be expected that the IHSA-PDMS can be employed as an effective AR cover layer, which enables to enhance the PCE of solar cells by suppressing the surface reflectance losses. For the bare glass, flat-PDMS/glass, IHSA-PDMS/glass, and HSA-PDMS/glass, the photon flux density (PFD; which is the number of photons transmitted through the sample),40,41 was also calculated to explore their transmittance properties at specific wavelengths of the solar spectrum (Figure 4c). The PFD also provides the information that the solar cell (i.e., PSC) could generate photocurrent at specific wavelengths in the solar spectrum. As shown in Figure 4c, the IHSA-PDMS/glass


Page 12 of 35

Page 13 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exhibited a higher PFD spectral distribution over the broad wavelength region of 350-800 nm than those of the other samples (i.e., bare glass, flat-PDMS/glass and HSA-PDMS/glass). The inset of Figure 4c also clarifies that, as compared to the bare glass, the IHSA-PDMS/glass also exhibited much higher PFD spectral distribution at wavelengths of 400-600 nm, which is a highintensity range at the AM 1.5G solar spectrum. Consequently, compared to all other samples, the IHSA-PDMS laminated on PSC can be expected to efficiently enhance its efficiency, owing to the larger photo-generated currents obtained from the cell absorption layer of PSC incorporated with the IHSA-PDMS.42 However, the PSCs are usually exposed in air with strong sunlight (or heat environment) for a long time. Hence, the thermal durability of polymer (or AR) layers laminated on PSCs is more essential to study.33 For this, we heat-treated the surface of flat-, HSA-, and IHSA-PDMS layers, by placing them into an oven for 10 h in a temperature range of 20-180°C. Furthermore, we explored the influence of temperature on its optical transmittance characteristics. Figure S1 (Supporting Information) shows the average TSW values of the flat-, HSA-, and IHSA-PDMS/glass as a function of temperature in the wavelength range of 300-800 nm. As shown in Figure S1, the TSW values of the flat-, HSA-, and IHSA-PDMS/glass are observed as ∼91.4±0.2%, 92.1±0.2%, and 92.7±0.2%, respectively, which are nearly invariant over a temperature range of 30-180°C. Therefore, the results clearly suggest that the above mentioned PDMS layers are relatively stable under an external environmental temperatures (20180°C), and can also be employed as a cover layer of PSCs to protect from heat exposure.” As described above, the diffracted and rebound lights between the IHS/HS architectures lead to a strong light scattering in the PDMS film. Such transmitted light scattering can result in the enhancement of light absorption in the active layer of solar cells.43 Figure 5a shows the diffuse transmittance spectra of the bare glass, and flat-PDMS, IHSA-PDMS and HSA-PDMS



laminated on bare glass. From Figure 5a, it can be clearly concluded that both the bare glass and flat-PDMS/glass is indicating almost no diffuse transmitted light (i.e., 35%) at the wavelengths of 350-700 nm. This is mainly due to the IHS or HS architectures with micro-scale (i.e., ∼2 µm) periods larger than incident light wavelengths, which can generate higher orders of diffracted waves in the transmission.38 Moreover, the haze factor (H), which is given by the ratio of the diffuse transmission (Td) to the total transmission (Tt) [i.e., H (%) = Td / Tt × 100], is often used to evaluate the light scattering property of the sample. For both the IHSA-PDMS/glass and HSAPDMS/glass, the highest average H (Havg) values of 38 and 34.2% were observed at wavelengths of 300-800 nm, respectively, as compared to the other samples (i.e., Havg ~ 1.1% for the bare glass and Havg ~ 1.7% for the flat-PDMS/glass). Thus, the calculated Havg values are also affirming that the IHSA-PDMS and HSA-PDMS can highly diffuse the light. To further demonstrate the strong light scattering property of both the IHSA-PDMS and HSA-PDMS, the green (532 nm), violet (455 nm), and red (650 nm) lasers were used to irradiate their surfaces, as shown in Figure 5b. For comparison, the surface of flat-PDMS was also irradiated with the laser. As shown in Figure 5c, the laser light passing through the flat-PDMS reached the screen without any scattering (or spreading). In contrast, the laser lights with various wavelengths (λ = 532, 455 and 650 nm) were transmitted through both the IHSA-PDMS and HSA-PDMS, exhibiting a strong scattering pattern on the screen. Moreover, the light scattering characteristics of IHSAPDMS and HSA-PDMS layers can be further confirmed from the FDTD simulations. The microhemisphere arrays can improve light absorption in the active layer of the solar cell by strong light scattering. Figure 5d shows the contour plots of the calculated Ey-field distribution by FDTD


Page 14 of 35

Page 15 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


simulation of incident light propagation from air to the bare glass, IHSA-PDMS and HSA-PDMS. For the bare glass, there is almost no light diffraction while the IHSA-PDMS and HSA-PDMS show a large diffracted light distribution at λ = 550 nm. In other words, the IHSA-PDMS and HSA-PDMS layers with a period of 2 µm exhibit strong light scattering with wide angular spreading and assist light propagation across the interface from air to the PDMS. However, when comparing IHSA-PDMS and HSA-PDMS, light diffraction spreads widely in IHSA-PDMS while light diffraction in HSA-PDMS spreads inward. Due to this phenomenon, diffusion of IHSA-PDMS is better at a distance of more than 20 µm after the light propagation through the pattern. It can be expected that the IHSA-PDMS will be better in the light absorption.44 From the optical simulation results, the IHSA-PDMS and HSA-PDMS can be expected to exhibit a strong scattering effect from the hemispherical architectures on the surface of PDMS. And, this strong light scattering effect could improve the light absorption in the solar cell. Additionally, the surface wetting behaviors of the flat-PDMS, IHSA-PDMS and HSAPDMS layers were also evaluated by measuring their water contact angles (θCA) and are shown in the inset of Figure 5a. As shown in the inset of Figure 5a, both the IHSA-PDMS and HSAPDMS revealed the higher θCA values of 121° and 106°, respectively, as compared to the flatPDMS (θCA ∼91°) indicating a hydrophobic surface. This is due to the relatively high surface roughness of PDMS, which is introduced by the IHSAs and HSAs distributed on the surface of PDMS layer.

45,46

However, the IHSA-PDMS layer with very high hydrophobic surface wetting

behavior is more beneficial for solar cell applications. Since, the dust particles and pollutants available in outdoor environments are very harmful to solar cells, which can easily stick on its surfaces and prevent the light absorption into its active layer. Resultantly, the output performance (or PCE) of solar cells can degrade. But, the IHSA-PDMS laminated on the surface



of solar cells help to remove (or wash out by rain droplets) these dust particles and pollutants (i.e., self-cleaning function), owing to a high hydrophobic wetting behavior. Therefore, the IHSA-PDMS with an additional self-cleaning function could enhance the practical feasibility of solar cells, in outdoor environments. Finally, the IHSA- and HSA-PDMS layers are laminated on the glass surface of PSCs as a light-harvesting tool and a protecting cover-layer to demonstrate its device feasibility. Figure 6a-6c shows the J-V curves, EQE spectra, and enhancement percentages of the photovoltaic parameters of the PSCs with flat-PDMS/glass, HSA-PDMS/glass and IHSA-PDMS/glass relative to the PSC with bare glass, under the illumination of 100 mW/cm2 (AM 1.5G). Here the thickness of all these AR layers was noted as ≈150 µm. Figure 6d depicts the cross-sectional view SEM image of a complete PSC device utilized for these measurements. As shown in Figure 6d, the tandem PSC consists a normal architecture of FTO/blocking TiO2 (bl-TiO2)/mesoporous TiO2 (mp-TiO2)/perovskite (CH3NH3PbI3) film /Spiro-OMeTAD/Au. Herein, the photoactive perovskite layer such as CH3NH3PbI3 thin film was spin-coated on the top of the mesoporous TiO2 layer. Moreover, the photovoltaic characteristics of PSC devices with and without AR layer were also summarized in Table 1. The reference PSC without any AR layer showed a PCE of 16.21% with an open-circuit voltage (VOC) of 1.10 V, a short-circuit current density (JSC) of 19.91 mA/cm2 and a fill factor (FF) of 0.74. Further, we examined the effect of AR layers on the device performance. Resultantly, the same device with HSA-PDMS/glass exhibited PCE to 17.72% with a JSC of 21.81 mA/cm2, a VOC of 1.10 V and a FF of 0.74, while the IHSA-PDMS led to the PCE of 19.00% with a JSC of 23.29 mA/cm2, a VOC of 1.10 V, and an FF of 0.74. The AR layers-employing devices obviously exhibited increased JSC values from 19.91 mA/cm2 to 21.81 mA/cm2 and 23.29 mA/cm2 for the HSA-PDMS/glass and IHSA-PDMS/glass, respectively,


Page 16 of 35

Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


while showing almost the same VOC and FF values as those of the device without any AR layer. For comparison, we also measured the photovoltaic characteristics of the device with bare PDMS/glass which has a flat surface, i.e., flat-PDMS/glass. As shown in Figure 6c, the flatPDMS did not result in evident improvement in JSC, indicating almost the same PCE value as compared to that without any AR layer. This result indicates that the improved device efficiency is only attributed to the improved JSC due to the reduced reflection and strong light scattering effect by the top AR layer. The PCE histogram of the devices with different AR layers is displayed in Figure S2 of the Supporting Information (SI). We noted here that all the marginal hysteresis behaviors of J-V curves were observed from the devices with different AR layers and they were very similar, which indicates that the AR layers did not affect the charge transport/collection properties of the PSC devices, but only increased the light absorption of the solar cell devices (Figure S3 and Table S1). However, by employing the IHAS-PDMS as an AR layer of PSC, the PCE is predominantly improved from 16.21% to 19.00%, exhibiting the enhancement percentage of ∼17.21%. This PCE enhancement percentage is similar or superior as compared to the results in solar cells with various AR layers (i.e., PCE enhancement percentage ∼4.86% for the CdTe solar cell with a 3D nanocone AR film,47 ∼10.38% for the perovskite solar cell with a nanocone PDMS layer,48 ∼4.32 or 10.49% for the organic or c-Si solar cells with a random-size inverted-pyramid structured PDMS,49 ∼15.66 or 13.45% for the organic or perovskite solar cells with a biomimetic haze film,50 ∼6.16% for the c-Si solar cell with a viola textures on NOA63 film51) reported in other previous literatures. Moreover, the microarchitectured Si mold (i.e., Hexagonal close-packed polystyrene microsphere arrays/Si) to develop the IHSA-PDMS was also fabricated by a simple drop-coating method, unlike a complex and time-consuming process such as nanoimprinting, multi-step anodization, and wet



etching process etc.47-49 Therefore, the IHSA-PDMS can be efficiently used as an AR layer for any type of solar cells to enhance its efficiency. We also applied these AR layers (flat-, HAS-, IHSA-PDMS) for DSSCs, to examine their universal applicability in solar cells. As described in the Supporting Information of Figure S4 and Table S2, the AR layer effectively improved the JSC of the DSSCs, leading to the enhanced PCE from 7.46 to 8.35%. Furthermore, in order to investigate the influence of AR layers thickness on the device performance, the flat-, HSA- and IHAS-PDMS layers with various thickness (i.e., ≈150, 580, 1050, and 2100 µm) were employed as AR layers of PSCs and examined the photovoltaic characteristics. The corresponding J-V curves of PSCs with bare glass and various AR layers such as flat-PDMS, HSA-PDMS, and IHSA-PDMS of different thickness were shown in Figure S5. The photovoltaic parameters such as VOC, JSC, FF, and PCE values of PSCs with AR layers of different thickness are shown in Table S3. The photovoltaic characteristics of PSCs with IHSA-PDMS layers are almost similar even though the PDMS layers thickness is changed from 150-2100 µm. And, it's also true with other AR layers such as flat-PDMS and HSA-PDMS. Therefore, these results clearly concluding that the thickness of AR layer can't predominately affect the device performance. Figure 6b shows the EQE spectra of the PSCs with or without AR layers. The maximum EQE value was increased in the order of bare glass device < flat-PDMS/glass device < HSA-PDMS/glass device < IHSA-PDMS/glass device, which is consistent with the JSC values of the devices with different AR layers under 1 sun illumination (Table 1). The calculated JSC values from the integration of EQE spectra were also mentioned in Table 1 and Figure S6. It should be noted here that the EQE at wavelengths less than 400 nm is almost the same for all the devices because of the strong absorption of UV light in the glass substrate. By considering that all the measured solar cell devices are exactly the same despite different AR layers, we concluded that the EQE


Page 18 of 35

Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


improvement is attributed to the improved light harvesting made by the AR layer because the EQE is a product of light harvesting efficiency, charge separation efficiency and charge collection efficiency. Figures 7a and 7b show the device parameter (such as VOC, FF, ISC, and PCE) variations of the PSCs with bare glass and various PDMS layers/glass as a function of the tilted angle of the incident light. The corresponding J-V curves of PSCs are shown in Figure S7. As shown in Figure 7a, the VOC and FF values of PSCs with various PDMS layers/glass as a function of the angle of incident light are almost similar to that of the PSC with bare glass. In contrary, the decrease in the JSC and PCE values of the PSCs with the HSA-PDMS/glass and IHSAPDMS/glass is relatively smaller than those of the PSCs with the bare glass and flat-PDMS/glass as the tilted angle of the device increased (Figure 7b). In addition, the enhancement percentage values of the PSC with IHSA-PDMS/glass against one with the bare glass in both the JSC and the PCE are higher than 16% at each tilted angle (Figure 7c and 7d), exhibiting the average enhancement percentage values of ~47% for the JSC and ~39% for the PCE, over the tilted angle range of 0-60°. Such an enhancement might be attributed to the enhanced light absorption into the active layer of the PSC device owing to the increased effective optical path lengths (i.e., strong haze factor) caused by the IHSAs on PDMS film.52 Consequently, the well-aligned microstructures with omnidirectional property can effectively suppress the surface reflection losses, resulting in an improved device performance under the oblique incident light. Therefore, the PSCs with these AR layers can effectively generate electricity under the circumstance of varying the incident angle of light, which is an advantage for the solar cells to harvest the sunlight incident in any directions (or angle) in a day, seasons, and low/high latitude areas. In addition, we also investigated the stability of the PSC devices under an ambient (or external)



environment with or without different AR layers. As shown in Figure S8, the devices retained 96, 95.9, 96.3, and 96.6% of the initial PCEs for the bare device, flat PDMS, HSA-PDMS, and IHSA-PDMS, respectively, after storage in ambient environment condition for 80 h. Thus, we can also conclude that the AR layer is beneficial to improve the ambient stability of the PSCs due to its hydrophobic nature.

4. Conclusions In summary, we developed the IHSA-PDMS and HSA-PDMS layers by a simple, cost-effective and fast SIL method via the hexagonal close-packed PS microsphere arrays/silicon master molds. Of both the PDMS layers, the IHSA-PDMS layer laminated on bare glass revealed superior AR (i.e., TSW ∼ 92.7% and RSW ∼ 6.4%) and light scattering properties with the highest average haze ratio of ∼38% over the wide wavelength range of 350-800 nm. In addition, the IHSA-PDMS layer also exhibited a hydrophobic property with the θCA value of 121°. With the introduction of the AR layers, the PCE of the PSCs were improved from 16.21 to 19.00% as the IHSA-PDMS was employed in the devices with a significant enhancement of the JSC from 19.91 to 23.29 mA/cm2. We thus believe that the AR layer with a well-aligned microstructure is a simple/easy/effective approach to increase the photon harvesting in the PSCs and thus to improve the photovoltaic performance. Moreover, the IHSA-PDMS as the most effective AR layer to suppress reflection and scattering of the light could be used to any types of solar cells, which would be applicable in universal AR layer for solar cell applications.

Acknowledgements


Page 20 of 35

Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (2017R1A2B4011998 and 2017H1D8A2031138).

References 1.

Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Coaxial Silicon Nanowires as Solar Cells and Nanoelectronic Power Sources. Nature 2007, 449, 885-890.

2.

Herbert, G. M. J.; Iniyan, S.; Sreevalsan, E.; Fajapandian, S. A Review of Wind Energy Technologies. Renewable Sustainable Energy Rev. 2007, 11, 1117-1145.

3.

Zhang, H.; Yang, Y.; Hou, T. C.; Su, Y.; Hu, C.; Wang, Z. L. Triboelectric Nanogenerator Built Inside Clothes for Self-Powered Glucose Biosensors. Nano Energy 2013, 2, 10191024.

4.

Williams, C. H.; Schwarz, A. M.; Reid, V. Patterns of Aquatic Weed Regrowth Following Mechanical Harvesting in New Zealand Hydro-Lakes. Hydrobiologia 1996, 340, 229-234.

5.

Dudem, B.; Ko, Y. H.; Leem, J. W.; Lim, J. H.; Yu, J. S. Hybrid Energy Cell with Hierarchical Nano/Micro-Architectured Polymer Film to Harvest Mechanical, Solar, and Wind Energies Individually/Simultaneously. ACS Appl. Mater. Interfaces 2016, 8, 3016530175.

6.

Yum, J. H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.; Bessho, T.; Marchioro, A.; Ghadiri, E.; Moser, J. E.; Yi, C.; Nazeeruddin, M. K.; Gratzel, M. A Cobalt Complex Redox Shuttle for Dye-Sensitized Solar Cells with High Open-Circuit Potentials. Nat. Commu. 2012, 1655, 1-7.



7.

Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C. S.; Chang, T. A.; Lee, Y. H.; Kim, H. J.; Sarkar, A.; Nazeeruddin, M. K.; Gratzel, M.; Seok, S. I. Efficient Inorganic-Organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors. Nat. Photonics 2013, 7, 486-491.

8.

Branker, K.; Pathak, M. J. M.; Pearce, J. M. A Review of Solar Photovoltaic Levelized Cost of Electricity. Renewable Sustainable Energy Rev. 2011, 5, 4470-4482.

9.

Duan, Y.; Tang, Q.; Liu, J.; He, B.; Yu, L. A Transparent Metal Selenide Alloy Counter Electrodes for High-Efficiency Bifacial Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2014, 53, 14569-14574.

10. He, M.; Zheng, D.; Wang, M.; Lin, C.; Lin, Z. High Efficiency Perovskite Solar Cells: From Complex Nanostructure to Planar Heterojunction. J. Mater. Chem. A. 2014, 2, 59946003. 11. Zeng, Q.; Hu, L.; Cui, J.; Feng, T.; Du, X.; Jin, G.; Liu, F.; Ji, T.; Li, F.; Zhang, H.; Yang, B. HighEfficiency Aqueous-Processed Polymer/CdTe Nanocrystals Planar Heterojunction Solar Cells with Optimized Band Alignment and Reduced Interfacial Charge Recombination. ACS Appl. Mater. Interfaces 2017, 9, 31345-31351.

12. Zeng, Q.; Chen, Z.; Zhao, Y.; Du, X.; Liu, F.; Jin, G.; Dong, F.; Zhang, H.; Yang, B. AqueousProcessed Inorganic Thin-Film Solar Cells Based on CdSexTe1−x Nanocrystals: The Impact of Composition on Photovoltaic Performance. ACS Appl. Mater. Interfaces 2015, 7, 23223-23230.

13. Zeng, Q.; Chen, Z.; Liu, F.; Jin, G.; Du, X.; Ji, T.; Zhao, Y.; Yue, Y.; Wang, H.; Meng, D.; Xie, T.; Zhang, H.; Yang, B. Aqueous-Processed Polymer/Nanocrystals Hybrid Solar Cells: The Effects of Chlorine on the Synthesis of CdTe Nanocrystals, Crystal Growth, Defect Passivation, Photocarrier Dynamics, and Device Performance. Sol. RRL 2017, 1, 1600020.


Page 22 of 35

Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14. Zhao, Y.; Zeng, Q.; Liu, X.; Jiao, S.; Pang, G.; Du, X.; Zhang, K.; Yang, B. Highly Efficient Aqueous-Processed Polymer/Nanocrystal Hybrid Solar Cells with an Aqueous-Processed TiO2 Electron Extraction Layer. J. Mater. Chem. A 2016, 4, 11738-11746.

15. Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Baker, R. H.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. 16. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. 17. Chueh, C. C.; Li, C. Z.; Jen, A. K.Y. Recent Progress and Perspective in SolutionProcessed Interfacial Materials for Efficient and Stable Polymer and Organometal Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 1160-1189. 18. Seo, J.; Noh, J. H.; Seok, S. I. Rational Strategies for Efficient Perovskite Solar Cells. Acc. Chem. Res. 2016, 49, 562–572. 19. Chen, C.; Zhai, Y.; Li, F.; Tan, F.; Yue, G.; Zhang, W.; Wang, M. High Efficiency CH3NH3PbI3:CdS Perovskite Solar Cells with CuInS2 as the Hole Transporting Layer. J. Power Sources 2017, 341, 396-403. 20. Xia., Z.; Bi, C.; Shao, Y.; Dong, Q.; Wang, Q.; Yuan, Y.; Wang, C.; Gao, Y.; Huang, J. Efficient, High Yield Perovskite Photovoltaic Devices Grown by Interdiffusion of SolutionProcessed Precursor Stacking Layers. Energy Environ. Sci. 2014, 7, 2619-2623. 21. Deng, Y.; Peng, E.; Shao, Y.; Xiao, Z.; Dong, Q.; Huang, J. Scalable Fabrication of Efficient Organolead Trihalide Perovskite Solar Cells with Doctor-Bladed Active Layers. Energy Environ. Sci. 2015, 8, 1544-1550.



22. Long, M.; Zhang, T.; Chai, Y.; Ng, C. -F.; Mak, T. C. W.; Xu, J.; Yan, K. Nonstoichiometric Acid–Base Reaction as Reliable Synthetic Route to Highly Stable CH3NH3PbI3 Perovskite Film. Nat. Commun. 2016, 7, 13503. 23. Yan, K.; Qiu, Y.; Xiao, S.; Gong, J.; Zhao, S.; Xu, J.; Meng, X.; Yang, S.; Xu, J. SelfDriven Hematite-based Photoelectrochemical Water Splitting Cells with ThreeDimensional Nanobowl Heterojunction and High-Photovoltage Perovskite Solar Cells. Mater. Today Energy 2017, 6, 128-135. 24. Long, M.; Chen, Z.; Zhang, T.; Xiao, Y.; Zeng, X.; Chen, J.; Yan, K.; Xu, J. Ultrathin Efficient Perovskite Solar Cells Employing a Periodic Structure of a Composite Hole Conductor for Elevated Plasmonic Light Harvesting and Hole Collection. Nanoscale 2016, 8, 6290-6299. 25. Jung, J. W.; Williams, S. T.; Jen, A. K. Y. Low-Temperature Processed High-Performance Flexible Perovskite Solar Cells via Rationally Optimized Solvent Washing Treatments. RSC Adv. 2014, 4, 62971-62977. 26. Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide Management in Formamidinium-Lead-Halide– Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376. 27. Wang, K. X.; Yu, Z.; Liu, V.; Cui, Y.; Fan, S. Absorption Enhancement in Ultrathin Crystalline Silicon Solar Cells with Antireflection and Light-Trapping Nanocone Gratings. Nano Lett. 2012, 12, 1616-1619. 28. Ingenito, A.; Isabella, O.; Zeman, M. Nano-Cones on Micro-Pyramids: Modulated Surface Textures for Maximal Spectral Response and High Efficiency Solar Cells. Prog. Photovolt: Res. Appl. 2015, 23, 1649-1659.


Page 24 of 35

Page 25 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29. Dudem, B.; Leem, J. W.; Lim, J. H.; Lee, S. H.; Yu, J. S. Multifunctional Polymers with Biomimetic Compound Architectures via Nanoporous AAO Films for Efficient Solar Energy Harvesting in Dye-Sensitized Solar Cells. RSC Adv. 2015, 5, 90103-90110. 30. Leem, J. W.; Choi, M.; Dudem, B.; Yu, J. S. Hierarchical Structured Polymers for LightAbsorption Enhancement of Silicon-Based Solar Power Systems. RSC Adv. 2016, 6, 5515955166. 31. Leem, J. W.; Choi, M.; Yu, J. S. Multifunctional Microstructured Polymer Films for Boosting Solar Power Generation of Silicon-Based Photovoltaic Modules. ACS Appl. Mater. Interfaces 2015, 7, 2349-2358. 32. Soderstrom, K.; Escarre, J.; Cubero, O.; Haug, F. J.; Perregaux, S.; Ballif, C. UV-NanoImprint Lithography Technique for the Replication of Back Reflectors for N-I-P Thin Film Silicon Solar Cells. Prog. Photovolt: Res. Appl. 2011, 19, 202-210. 33. Leem, J. W.; Dudem, B.; Yu, J. S. Thermal-Tolerant Polymers with Antireflective and Hydrophobic Grooved Subwavelength Grating Surfaces for High-Performance Optics. RSC Adv. 2016, 6, 79755-79762. 34. Heng, L.; Wang, X.; Yang, N.; Zhai, J.; Wan, M.; Jiang, L. P–N-Junction-Based Flexible Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2010, 20, 266-271. 35. Leem, J. W.; Guan, X. Y.; Choi, M. K.; Yu, J. S. Broadband and Omnidirectional HighlyTransparent Coverglasses Coated with Biomimetic Moth-Eye Nanopatterned Polymer Films for Solar Photovoltaic System Applications. Sol. Energy Mater. Sol. Cells 2015, 134, 45-53.



36. Maawali, S. A.; Bemis, J. E.; Akhremitchev, B. B.; Leecharoen, R.; Janesko, B. G.; Walker, G. C. Study of The Polydispersity of Grafted Poly(Dimethylsiloxane) Surfaces Using Single-Molecule Atomic Force Microscopy. J. Phys. Chem. B 2001, 105, 3965-3971. 37. Maruyama, T.; Enomoto, A.; Shirasawa, K. Solar Cell Module Colored with Fluorescent Plate. Sol. Energy Mater. Sol. Cells 2000, 64, 269-278. 38. Eom, S. H.; Wrzesniewski, E.; Xue, J. Close-Packed Hemispherical Microlens Arrays for Light Extraction Enhancement in Organic Light-Emitting Devices. Org. Electron. 2011, 12, 472-746. 39. Cui, H.; Pillai, S.; Campbell, P.; Green, M. A Novel Silver Nanoparticle Assisted Texture as Broadband Antireflection Coating for Solar Cell Applications. Sol. Energy Mater. Sol. Cells 2013, 109, 233-239. 40. Boden, S. A.; Bagnall, D. M. Sunrise to Sunset Optimization of Thin Film Antireflective Coatings for Encapsulated, Planar Silicon Solar Cells. Prog. Photovolt: Res. Appl. 2009, 17, 241-252. 41. Martel, A.; Briones, F. C.; Rodriguez, R. C.; Gamboa, J. M.; Romeo, N.; Bosio, A.; Pena, J. L. Physical Properties of Transparent Conducting Cd-Te-In-O Thin Films Outlining a Thermodynamic System for Transparent Conducting Oxides. Thin Solid Films 2009, 518, 413-418. 42. Shin, J. H.; Kim, Y. D.; Choi, H. J.; Ryu, S. W.; Lee, H. Multi-Functional SiO2 Moth-Eye Pattern for Photovoltaic Applications. Sol. Energy Mater. Sol. Cells 2014, 126, 1-5. 43. Leung, S. F.; Yu, M.; Lin, Q.; Kwon, K.; Ching, K. L.; Gu, L.; Yu, K.; Fan, Z. Efficient Photon Capturing with Ordered Three-Dimensional Nanowell Arrays. Nano Lett. 2012, 12, 3682-3689.


Page 26 of 35

Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44. Zhu, X.; Zhu, L.; Chen, H.; Yang, L.; Zhang, W. Micro-Ball Lens Structure Fabrication Based on Drop on Demand Printing the Liquid Mold. Appl. Surf. Sci. 2016, 361, 80-89. 45. Hong, D.; Ryu, I.; Kwon, H.; Lee, J. J.; Yim, S. Preparation of Superhydrophobic, LongNeck Vase-Like Polymer Surfaces. Phys. Chem. Chem. Phys. 2013, 15, 11862-11867. 46. Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday. Soc. 1944, 40, 546-551. 47. Tsui, K. -H.; Lin, Q.; Chou, H.; Zhang, Q.; Fu, H.; Qi, P.; Fan, Z. Low-Cost, Flexible, and Self-Cleaning 3D Nanocone Anti-Reflection Films for High-Efficiency Photovoltaics. Adv. Mater. 2014, 26, 2805-2811. 48. Tavakoli, M. M.; Tsui, K. -H.; Zhang, Q.; He, J.; Yao, Y.; Li, D.; Fan, Z. Highly Efficient Flexible Perovskite Solar Cells with Antireflection and Self-Cleaning Nanostructures. ACS Nano 2015, 9, 10287-10295. 49. Hwang, I.; Choi, D.; Lee, S.; Seo, J. H.; Kim, K. -H.; Yoon, I.; Seo, K. Enhancement of Light Absorption in Photovoltaic Devices using Textured Polydimethylsiloxane Stickers. ACS Appl. Mater. Interfaces 2017, 9, 21276-21282. 50. Li, K.; Zhang, Y.; Zhen, H.; Wang, H.; Liu, S.; Yan, F.; Zheng, Z. Versatile Biomimetic Haze Films for Efficiency Enhancement of Photovoltaic Devices. J. Mater. Chem. A 2017, 5, 969-974. 51. Schmager, R.; Fritz, B.; Hunig, R.; Ding, K.; Lemmer, U.; Richards, B. S.; Gomard, G.; Paetzold, U. W. Texture of the Viola Flower for Light Harvesting in Photovoltaics. ACS Photonics 2017, 4, 2687-2692.



Page 28 of 35

52. Dudem, B.; Heo, J. H.; Leem, J. W.; Yu, J. S.; Im, S. H. CH3NH3PbI3 Planar Perovskite Solar Cells with Antireflection and Self-Cleaning Function Layers, J. Mater. Chem. A 2016, 4, 7573-7579.

TABLE TABLE 1. Photovoltaic characteristics of the PSCs studied in this work under standard AM 1.5G illumination (intensity = 100 mW/cm2).a

No AR

1.10 (1.09)a 19.91 (19.54)

19.85

74.03 (73.90)

16.21 (16.01)

EQEMax [%] 81.88

flat-PDMS

1.10 (1.09) 20.02 (19.58)

20.11

74.18 (73.88)

16.34 (15.94)

82.20

21.64 23.32

73.88 (73.50) 74.15 (73.81)

17.72 (17.16) 19.00 (18.55)

88.23

AR layer

VOC [V]

HSA-PDMS 1.10 (1.09) IHSA-PDMS 1.10 (1.09)

JSC [mA/cm2]b

21.81 (21.42) 23.29 (22.83)

JSC [mA/cm2]c

FF [%]

PCE [%]

a

Values in brackets are the average parameters calculated from more than 14 devices. Measured JSC values under 100 mW/cm2 illumination. c Integrated JSC values from EQE measurement of the champion device in each condition.

b


90.86

Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic diagram of fabrication procedure for (i) IHSA-PDMS and (ii) HSA-PDMS layers by a SIL via the hexagonal close-packed polystyrene microsphere arrays/Si master molds. The schematic diagram of the PSC device architecture and the lamination of IHSA-PDMS and HSA-PDMS on the device is also shown.



Figure 2. Top-view and cross-sectional SEM images of (a) hexagonal close-packed PS microsphere arrays/Si, (b) IHSA-PDMS and (c) HSA-PDMS layers. The 30°-tilted views of the SEM images of (d) IHSA-PDMS and (e) HSA-PDMS layers.


Page 30 of 35

Page 31 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. 10 µm × 10 µm scan AFM images and depth profiles of the fabricated (a) PS microsphere arrays/Si, (b) IHSA-PDMS and (c) HSA-PDMS layers.



Figure 4. Measured total (a) transmittance and (b) reflectance spectra of bare glass, flat-PDMS, IHSA-PDMS and HSA-PDMS laminated on bare glass. (c) Contour plot for the variation of calculated total transmittance spectra of IHSA-PDMS/glass as a function of wavelength and size of IHSAs. (d) Estimated PFD of the bare glass, and flat-PDMS, IHSA-PDMS and HSA-PDMS laminated on bare glass. The schematic diagram of the IHSA-PDMS laminated on the bare glass to enhance its total transmittance, and calculated Tavg in the wavelength range of 300-800 nm for IHSA-PDMS as a function of IHSAs size are also shown in the inset of (a), and (c), respectively.


Page 32 of 35

Page 33 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) Measured diffused transmittance spectra of bare glass, and flat-PDMS, IHSAPDMS and HSA-PDMS laminated on bare glass. (b) Schematic diagram of the light scattering effect using a laser beam (λ= 532 nm) passing through various PDMS layers. (c) Photographic images of laser beams with the various wavelengths (i.e., 532, 455 and 650 nm) passed through the flat-PDMS, IHSA-PDMS, and HSA-PDMS layers. Contour plots of the Ey distributions for the incident light propagating from air to the (d) bare glass, (e) IHSA-PDMS and (f) HSA-PDMS layers at the wavelength of 550 nm. Photographs of a water droplet on the surface of flat-PDMS, IHSA-PDMS and HSA-PDMS layers are shown in the inset of (a).



Figure 6. (a) J-V curves, (b) EQE spectra, and (c) enhancement percentages of the photovoltaic parameters of the PSCs with flat-PDMS/glass, HSA-PDMS/glass and IHSA-PDMS/glass relative to the PSC with bare glass. (d) Cross-sectional SEM image of a complete PSC device utilized for these measurements.


Page 34 of 35

Page 35 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Measured (a) VOC and FF, and (b) JSC and PCE of the PSCs with and without various PDMS layers as a function of the tilted angle of the incident light. The enhancement percentage in (c) JSC and (d) PCE for the PSC with the various PDMS layers/glass relative to the reference PSC with the bare glass.


Boosting Light Harvesting in Perovskite Solar Cells by Biomimetic

Recommend Documents