Manufacture of Binary Nanofeatured Polymeric Films Using Nanosphere Lithography and Ultraviolet Roller Imprinting

materials

Article Manufacture of Binary Nanofeatured Polymeric Films Using Nanosphere Lithography and Ultraviolet Roller Imprinting

Demei Lee 1 , Ming-Yi Hsu 1,2,3, Ya-Ling Tang 1 and Shih-Jung Liu 1,4,*

1 Department of Mechanical Engineering, Chang Gung University, Taoyuan 33302, Taiwan; [email protected] (D.L.); [email protected] (M.-Y.H.); [email protected] (Y.-L.T.) 2 Department of Medical Imaging and Intervention, Chang Gung Memorial Hospital-Linkou, Taoyuan 33305, Taiwan 3 Department of Diagnostic Radiology, Chang Gung Memorial Hospital-Keelung, Keelung 20401, Taiwan 4 Bone and Joint Research Center, Department of Orthopedic Surgery, Chang Gung Memorial Hospital-Linkou, Taoyuan 33305, Taiwan * Correspondence: [email protected]; Tel.: +886-3-211-8166; Fax: +886-3-211-8558

Abstract: This paper describes the manufacture of binary nanostructured films utilizing nanosphere lithography and ultraviolet (UV) roller imprinting. To manufacture the binary nanofeatured template, polystyrene nanocolloids of two distinct dimensions (900 and 300 nm) were primarily self-assembly spun coated on a silicon substrate. A roller imprinting facility equipped with polydimethylsiloxane molds and ultraviolet radiation was employed. During the imprinting procedure, the roller was steered by a motor and compressed the ultraviolet-curable polymeric layer against the glass substrate, where the nanofeatured layer was cured by the UV light source. Binary nanofeatured films were thus obtained. The influence of distinct processing variables on the imprinting of nanofeatured films was investigated. The empirical data suggested that with appropriate processing conditions, binary nanofeatured plastic films can be satisfactorily manufactured. It also demonstrated that roller Citation: Lee, D.; Hsu, M.-Y.; Tang, imprinting combined with ultraviolet radiation can offer an easy yet effective method to prepare Y.-L.; Liu, S.-J. Manufacture of Binary binary nanofeatured films, with a miniatured processing time and enhanced part quality. Nanofeatured Polymeric Films Using Nanosphere Lithography and Ultraviolet Roller Imprinting. Keywords: nanosphere lithography; roller imprinting; binary nanofeatured film Materials 2021, 14, 1669. https:// doi.org/10.3390/ma14071669

Academic Editor: Andrea Sorrentino 1. Introduction Subsiding the sizes of a substance to the nanoscale generally leads to the variation of Received: 18 February 2021 physical/chemical properties. Nanotechnology permits the achievement of novel materi- Accepted: 26 March 2021 als/devices with essential structural element in nanoscale and is assessed via managing at Published: 29 March 2021 either the atomic, molecular, or supramolecular level. The unique characteristic of binary nanostructure (or micro/nanostructure) [1] has been used in photosensors (with modifica- Publisher’s Note: MDPI stays neutral tion of optical properties in absorption, reflection, and color), promoted Raman imaging [2], with regard to jurisdictional claims in and enhanced energy storage and transformation efficiency in photovoltaics [3,4]. The published maps and institutional affil- structure has also been employed for varying material surface’s wettability, thus provid- iations. ing advantages in applications comprising self-cleaning, anti-icing, fluidic control and drag reduction [5–8]. Meanwhile, material surface with managed topographic features at the micro and nanoscales has been demonstrated to influence the entire cell behav- ior as well as the ultimate cell/material integration [9,10]. Binary nanofeatured surface Copyright: © 2021 by the authors. can also be used to direct differentiation into a specific cell lineage in the nanoscale cir- Licensee MDPI, Basel, Switzerland. cumstance [11,12]. The development of manufacturing methods for binary nanofeatured This article is an open access article surface is thus highly desired. distributed under the terms and Owing to its advantages, the roller imprinting [13] has been swiftly exploited during conditions of the Creative Commons the past decade as a favorable option to traditional nanofabriction methods to satisfy the Attribution (CC BY) license (https:// needs resulting from the new progresses in the semiconductor and flexible electronics creativecommons.org/licenses/by/ industries, etc. It is also the most demanding technology, owing to the high yield for 4.0/).

Materials 2021, 14, 1669. https://doi.org/10.3390/ma14071669 https://www.mdpi.com/journal/materials Materials 2021, 14, 1669 2 of 10

industrial fabrications. During the imprinting process, a pre-manufactured mold holding an opposite of the needed features is compressed against a resist-coated substrate to duplicate the features by deformation. Various duplications can be completed from one sole pre-manufactured mold employing this scheme. Additionally, roller ultraviolet (UV) imprinting technology [14], due to its advantages of low-cost, high-yield, and vast-area imitating, has received increasing attentions from both academia and industries for the successive manufacture of nanostructures such as optical lithography, deposition, and etching. The innovative UV roller imprinting technique allows the patterning of high- quality structured layers on glass/glass-based devices at lowest costs. With regard to the molding performance, distinct 2D/3D features with dimensions spanning from few micrometers to sub-nanometers have been successfully achieved [15]. Nanospheres lithography [16,17] is a fabrication process with reference to the self- assembly of nanocolloids. A lithography mask is first prepared by submerging the substrate in the nanocolloids suspension. After the vaporization of solvent, a self-assembled mono- layer is created on the substrate surface. This is followed by the staking of the aspired substance onto the lithography mask. After the removal of the template, a periodic array of nanocolloids is obtained. The process has received increasing attention in recent years, mainly owing to the feasibility of producing regular patterns onto large area with rational cost [18]. In this study, we detailed the manufacture of binary nanofeatured films via a soft-mold roller imprinting device with UV radiation capability. Binary nanofeatured template was first prepared by self-assembling polystyrene nanocolloids of two distinct dimensions (i.e., 900 and 300 nm) on silicon substrates. The soft mold [19] was acquired by pouring the polydimethylsiloxane (PDMS) solution onto the template so as to obtain a binary nano-cavity array for roller imprinting. In imprinting, the roller rotates and presses the UV-curable polymeric layer onto the glass substrate. Once cured, binary nanofeatured polymeric films were acquired and characterized. The impact of different variables on the imprinting of nanostructured films was also explored.

2. Materials and Method 2.1. Materials Colloidal nanospheres of polystyrene (PS) of 900 nm/300 nm were provided by micro-Particles GmbH (Berlin-Adlershof, Germany). Other materials employed for the experiments included surfactant Triton X-100 and ethanol acquired from Sigma-Aldrich (St. Louis, Mo, USA), polydimethylsiloxane (PDMS) SYLGARD 184 pre-polymer mixture and cross-linker from Dow Corning (Elizabethtown, KY, USA), DS-UPS-Aw dispersant provided by Golden Innovation (Taipei, Taiwan), FL171-10 ultraviolet curable epoxy resin, with a refractive index 1.45 at 365 nm wavelength and a viscosity of 320–470 cps at 25 ◦C, from Everwide Chem. (Taipei, Taiwan), and hexane purchased from JT-Baker (Phillipsburg, NJ, USA).

2.2. Prepare the Nanofeatured Template Figure1 shows schematically the procedure for assembly of binary nanosphere arrays. PS nanospheres of 300 nm were ﬁrst mixed with the surfactant at a ratio of 0.7:0.5 (v/v), while the colloidal nanospheres of 900 nm were compounded with the surfactant at a ratio of 0.3:0.5 (v/v). The solution of 900 nm nanospheres were spun coated onto the substrate, using a distilled (DI) water:ethanol ratio of 1:1, a surfactant:PS sphere ratio of 1:2, and 5% wt of dispersant. Three stages of spin speed (spin time) were employed, i.e., 500 rpm for 30 s, accompanied by 1500 rpm for 30 s, and 2000 rpm for 60 s. After the coating of 900 nm spheres, the solution of 300 nm nanospheres was then spun coated employing a DI water:ethanol ratio of 1:1, a surfactant:PS sphere ratio of 1:2, 10% wt of dispersant, and spin speed (spin time) of 3000 rpm (30 s). Table1 lists the parameters used in the sequential spin coating process. Materials 2021, 14, x FOR PEER REVIEW 3 of 10

ploying a DI water:ethanol ratio of 1:1, a surfactant:PS sphere ratio of 1:2, 10% wt of dis- Materials 2021, 14, 1669 persant, and spin speed (spin time) of 3000 rpm (30 s). Table 1 lists the parameters3 of 10 used in the sequential spin coating process.

Figure 1. The schematic experimental process for assembly of binary nanosphere arrays. Figure 1. The schematic experimental process for assembly of binary nanosphere arrays. Table 1. The operation conditions utilized for spin coating of binary 900 nm/300 nm nanosphere arrays. Table 1. The operation conditions utilized for spin coating of binary 900 nm/300 nm nanosphere Spin Speed Nanosphere Surfactant:PS Dispersant arrays. Step DI Water:Ethanol (Spin Time) Size (nm) Sphere (%) rpm (s) Spin Speed Nanosphere Size DI Surfactant:PS Dispersant500 (30) Step One 900 5 1500 (30)(Spin Time) (nm) Water:Ethanol Sphere (%) 1:1 1:2 2000 (60) rpm (s) Two 300 10 3000 (30) 500 (30) One 900 5 1500 (30) 2.3. Preparation of Soft Mold 1:1 1:2 2000 (60) Polydimethylsiloxane (PDMS) pre-polymer solution was ﬁrst mixed with the cross- Twolinker (Figure3002a), followed by the addition of hexane (Figure2b). Assembled10 binary3000 (30) array was the nanocolloid-patterned substrate, which was plasma sputtered (Figure2c) 2.3. Preparationvia a sputtering of Soft device Mold (PDC-001, Harrick Plasma, Ithaca, NY, USA) and Argon gas. The sputtering time was 5 min. The soft mold was prepared by casting the PDMS pre-polymer mixturePolydimethylsiloxane over the binary nanosphere (PDMS) array pre- thatpolymer acts as solution a template was (Figure first2d). mixed After being with the cross- linkerplaced (Figure in a vacuum2a), followed chamber by for the 5 min addition (Figure2e), of the hexane PDMS ( moldFigure was 2b). put inAssembled an isothermal binary array ◦ was oventhe nanocolloid at 60 C for 3 h-patterned (Figure2f). The substrate soft mold, which was trimmed was fromplasma the template sputtered post-curing (Figure 2c) via a (Figure2g), and was placed in acetonitrile for ultrasonication for 30 min to eliminate sputteringremaining device PS nanospheres (PDC-001, on Harrick the mold Plasma, (Figure2h). Ithaca, A PDMS NY mold, USA) with and nanocavities Argon wasgas. The sput- teringthus time obtained. was 5 min. The soft mold was prepared by casting the PDMS pre-polymer mixture over the binary nanosphere array that acts as a template (Figure 2d). After being placed in a vacuum chamber for 5 min (Figure 2e), the PDMS mold was put in an isothermal oven at 60 °C for 3 h (Figure 2f). The soft mold was trimmed from the template post- curing (Figure 2g), and was placed in acetonitrile for ultrasonication for 30 min to eliminate remaining PS nanospheres on the mold (Figure 2h). A PDMS mold with nanocavities was thus obtained.

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FigureFigure 2. Schematic 2. Schematic prepation prepation of soft of softmold. mold.

2.4. 2.4.Roller Roller Imprinting Imprinting of Nanofeatured of Nanofeatured Films Films ImprintingImprinting tests tests were were completed completed utilizing utilizing a lab a- lab-developeddeveloped UV UVroller roller imprinting imprinting ap- ap- paratusparatus (Figure (Figure 2i) 2[20],i) [ 20 which], which includes includes a UV a UVlight light source source,, a roller a roller PDMS PDMS mold mold,, a speed a speed-- controllablecontrollable motor motor-steered-steered table table,, and and a reservoir a reservoir that that holds holds the thelight light curable curable polymer. polymer. The The 2 UV UVsource’s source’s utmost utmost power power (ByWell (ByWell Mater., Mater., New New Taipei Taipei City, City, Taiwan Taiwan)) is 39 is00 3900 mW/cm mW/cm2, , possessingpossessing a 365 a nm 365 wavelength. nm wavelength. Two distinct Two distinct amounts amounts of light of radiations light radiations,, 530 and 5303900 and 2 mW3900/cm2 mW/cm, were utilized., were The utilized. travel The rate travel of the rate table of thewas table set at was 5.2 set, 13.1 at, 5.2, or 13.1,20.9 mm/s. or 20.9 The mm/s. imprintingThe imprinting pressures pressures were modified were modified by altering by the altering roller/glass the roller/glass substrate clearance substrate, clearance,which werewhich adjusted were via adjusted two Z- viastages two situated Z-stages above situated the table above. Three the table. clearances Three, clearances, −200, 0 and− 100200, 0 μm,and were 100 adoptedµm, were for adoptedthe imprinting for the proce imprintingdure. The procedure. negative The clearance negative implies clearance interven- implies tionintervention of the roller ofand the glass roller substrate. and glass substrate. DuringDuring roller roller imprinting imprinting,, the theUV- UV-curablecurable resin resin was was initially initially retained retained in the in reservoir the reservoir.. TheThe PDMS PDMS mold mold contacted contacted the resin the resinonce the once roller the stamp roller stamprotated. rotated. The photopolymer The photopolymer mix- turemixture was compressed was compressed against againstthe nano thecavit nanocavitiesies on the PDMS on the roller. PDMS Upon roller. the Upon roller the con- roller contacting the glass substrate, polymeric films with binary nanostructures were created tacting the glass substrate, polymeric films with binary nanostructures were created on on the glass substrate following the UV irradiation. Once peeled off from the substrate, the glass substrate following the UV irradiation. Once peeled off from the substrate, nanofeatured film was acquired. nanofeatured film was acquired. 3. Results and Discussion 3. Results and Discussion Figure3A,B show the scanning electron microscopy (SEM) images of self-assembled nanosphereFigure 3A,B array show and the replicatedscanning electron soft mold, microscop respectively.y (SEM The) images arrays of were self-assembled analyzed uti- nanospherelizing an array atomic and force replicated microscope soft mold, (AFM). respectively. Figure4A,B The display arrays the were evaluated analyzed imaged utiliz- of ing thean atomic self-assembled force microscope binary 900 (AFM). nm/300 Figure nm nanocolloid 4A,B display array the andevaluated replicated imag softed of mold, the respectively. The average surface roughnesses (Ra) thus obtained were 51.3 nm and 36.3 nm,

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Materials 2021, 14, 1669 5 of 10 selfself-assembled-assembled binary binary 900 900 nm/300 nm/300 nm nm nanocolloid nanocolloid array array and and replicated replicated soft soft mold, mold, respec- respec- tively.tively. The The average average surface surface roughnesses roughnesses (Ra) (Ra) thus thus obtained obtained were were 51.3 51.3 nm nm and and 36.3 36.3 nm, nm, respectively. The experimental data show that the spin coating technology can satisfacto- respectively.respectively. The The experimental experimental datadata show show that that the the spin spin coating coating technology technology can satisfactorilycan satisfactorily self-assemble the 900 nm/300 nm nanocolloids on the substrate with consistent distri- rilyself-assemble self-assemble the the 900 900 nm/300 nm/300 nm nm nanocolloids nanocolloids on theon the substrate substrate with with consistent consistent distribu- distri- butions. Furthermore, the binary nanostructured arrays were effectively duplicated onto butions.tions. Furthermore, Furthermore, the the binary binary nanostructured nanostructured arrays arrays were were effectively effectively duplicated duplicated onto the onto thethePDMS PDMS PDMS mold mold mold with with with well well well distributions. distributions. distributions.

Figure 3. SEM images of (A) assembled 300 nm/900 nm binary array, (B) replicated soft mold, (C) imprinted nanofeatured FigureFigure 3. SEM 3. SEM images images of of(A ()A assembled) assembled 300 300 nm/900 nm/900 nm nm binary binary array, array, ( (BB)) replicated replicated softsoft mold,mold, ( C(C)) imprinted imprinted nanofeatured nanofeatured film. (Scale bar: 1 μm). film.ﬁlm. (Scale (Scale bar: bar: 1 μ 1m)µm)..

Figure 4. AFM surface morphologies of (A) assembled binary nanosphere array, (B) replicated soft mold, (C) imprinted FigureFigure 4. AFM 4. AFM surface surface morphologies morphologies of of (A (A) )assembled assembled binary binary nanosphere nanosphere array, ((BB)) replicated replicated soft soft mold, mold, (C ()C imprinted) imprinted nanofeatured film. nanofeaturednanofeatured film. film. The imprinting device equipped with the soft mold was used to mold the binary TheThe imprinting imprinting device device equipped equipped with with the the soft soft mold mold was was used used to to mold mold the the binary binary nanofeaturednanofeatured films. films. The The impact impact of of the the clearance clearance between between the the roller and thethe glassglass substrate substrate nanofeaturedwas investigated. films. TheThe empiricalimpact of outcome the clearance (Figure between5A) indicates the roller that and a clearance the glass of substrate 0 µm waswas investigated investigated. .The The empirical empirical outcome outcome ( Figure(Figure 5 5AA) )indicates indicates that that a aclearance clearance of of 0 0μ μmm imprintedimprinted films films possess possesseded the the utmost superior superior duplicability. duplicability The. The conformation conforma oftion polymeric of poly- imprintedfilm to the films soft mold’spossess nanofeaturesed the utmost is thesuperior main concern duplicability for roller. The imprinted conforma films.tion During of polymeric film to the soft mold’s nanofeatures is the main concern for roller imprinted films. mericthe imprintingfilm to the procedure, soft mold’s a pressurenanofeatures is imposed is the on main the softconcern mold for on roller the roller imprinted to steer films the . During the imprinting procedure, a pressure is imposed on the soft mold on the roller to Duringphotopolymer the imprinting solution procedure, to flow into a the pressure nanocavities. is imposed The applied on the pressure soft mold can on be the enhanced roller to steer the photopolymer solution to flow into the nanocavities. The applied pressure can steerby reducingthe photopolymer the clearance solution between to theflow roller into andthe thenano substrate.cavities. WhenThe applied the pressure pressure is too can bebe low,enhanced enhanced not enough by by reducing reducing polymer the liquidthe clearance clearance is compressed between between into the thethe roller nanocavities.roller and and the the Imprinted substrate. substrate. film When When quality the the

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Materials 2021, 14, 1669 6 of 10 pressure is too low, not enough polymer liquid is compressed into the nanocavities. Im- printed film quality deteriorates accordingly. Nevertheless, when the imposed pressure isdeteriorates too high, the accordingly. PDMS mold Nevertheless, may be distort whened, the pressing imposed abundant pressure photopolymer is too high, the solution PDMS intomold the may cavities. be distorted, As soon pressing as the pressure abundant is removed, photopolymer the rubbery solution soft into may the restore cavities. its ge- As ometrysoon as and the pressurepush out isthe removed, overflowing the rubberyliquid photopolymer. soft may restore Replicated its geometry film quality and push is thus out decreased.the overﬂowing liquid photopolymer. Replicated ﬁlm quality is thus decreased.

Figure 5. InfluenceInﬂuence of of ( (AA)) distance distance between between roller roller stamp stamp and and glass glass substrate substrate;; (A (A11): ):+100 +100 μm,µm, (A (2A2): 0): μ 0m,µ m,(A3 (A3): −2):00− 200μm,µ (mB), rolling speed; (B1): 5.23 mm/s, (B2): 13.08 mm/s, (B3): 20.93 mm/s, (C) UV amount; (C1):3900 mW/cm2, (C2): 530 mW/cm2, (B) rolling speed; (B1): 5.23 mm/s, (B2): 13.08 mm/s, (B3): 20.93 mm/s, (C) UV amount; (C1): 3900 mW/cm2, on the manufacture of 300 nm/900 nm binary nanostructured films. (Scale bar: 1 μm). (C2): 530 mW/cm2, on the manufacture of 300 nm/900 nm binary nanostructured ﬁlms. (Scale bar: 1 µm).

Figure5 5BB displays displays the the impact impact of of the the table’s table’ shiftings shifting speed speed on theon the reproducibility reproducibility of the of thenanofeatures. nanofeatures The. The SEM SEM images images show show that that the the quality quality of reproducedof reproduced nanofeatures nanofeatures lessens less- ensas the as the moving moving speed speed is increased.is increased This. This can can be be explained explained by by the thefact fact thatthat aa pressurepressure is enforced on the glass substrate oppose opposedd to the PDMS mold during roller imprinting for certain period inin orderorder toto press press the the photopolymer photopolymer mixture mixture to to flow flow into into the the nanocavities nanocavities for fornanofeature nanofeature conformity. conformity. As As the the table table is shifted is shifted too too fast, fast the, the photopolymer photopolymer has has no potentno po- tenttime time to entirely to entire fulfillly ful intofill in theto cavities. the cavities. Replicated Replicated nanofeatures nanofeatures thus thus deteriorate. deteriorate. The effect effect of of UV UV radiation radiation doses doses on on imprinted imprinted polymer polymer film film quality quality was also was assessed. also as- sessTheed. microimages The microimages in Figure 5inC Figure indicate 5C that indicate the duplicability that the duplicability of the imprinted of the nanofeatures imprinted nanofeaturesraises as the UV raises dose as isthe increased. UV dose Inis increased roller imprinting,. In roller after imprinting the binary, after nanosphere the binary array nan- osphereis created array on theis created substrate, on the the substrate, nanofeatures the nanofeature demand UVs demand irradiation UV ir toradiation photocure to pho- the topolymericcure the polymer mixtureic into mixture solid. into However, solid. However as the employed, as the employed dose of dose UV irradiationof UV irradiation is too islow, too the low replicated, the replicated nanosphere nanosphere array array may may not take not take in sufficient in sufficient energy energy to photocure to photocure the thepolymers. polymers Consequently,. Consequently, the liquidthe liquid mixture mixture slumps slumps and and the duplicabilitythe duplicability reduces. reduces. By employing employing the the appropriate appropriate processing processing parameters, parameters, polymeric polymeric films films with with 900 nm/ 900 nm/300300 nm nm binary binary nanosphere nanosphere array array could could be be satisfactorily satisfactorily prepared prepared (Figures(Figures3 3CC and and4 C).4C). The angles of water contact for the self-assembledself-assembled nanocolloid arrays, duplicated PDMS molds, and imprinted nanostructured films were measured. The measured results in

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molds, and imprinted nanostructured films were measured. The measured results in Fig- ureFigure 6 illu6 illustratestrate that that the the water water contact contact angles angles for for the the silicon silicon substrate, substrate, assembled binarybinary nanospherenanosphere array, array, replicated soft mold, and imprinted nanofeatured film ﬁlm were 34.95°◦,, 126.75°◦,, 123.26 123.26°,◦, and and 106.74° 106.74◦,, respectively. respectively. Imprinted Imprinted binary binary nanofeatured nanofeatured films ﬁlms exhibited tthehe expected expected highly hydrophobic characteristic.

Figure 6. WaterWater contactcontact angles angles of of (A ()A silicon) silicon substrate, substrate, (B )( self-assembledB) self-assembled binary binary nanocolloid nanocolloid array, array, (C) replicated (C) replicated soft mold, soft mold, (D) imprinted nanofeatured film. (D) imprinted nanofeatured ﬁlm.

Finally,Finally, to simply demonstra demonstratete the capability capability of of imprinted imprinted binary binary nanofeatured nanofeatured films, films, photocurrentphotocurrent-voltage-voltage tests tests were were carried out out,, utilizing a lab lab-made-made device that consists of polycrystalline silicon panel, Xenon lamp that possesses a power supply of 2400 mV mV (Titan (Titan ElectroElectro-Optics,-Optics, Taipei, Taipei, Taiwan), Taiwan), and and simulation simulation code. code. The Thedistance distance between between the light the source light sourceand solar and cell solar was cell maintained was maintained at 120 mm, at 120 and mm, all measurementsand all measurements were carried were outcarried at 25 out◦C. atFigure 25 °C.7 illustrates Figure 7 illustrates the estimated the estimated current-voltage current profiles-voltage of profiles solar cell of coveredsolar cell with covered flat withfilm andflat f nanofeaturedilm and nanofeatured film on top film of on it. top Table of 2it. shows Table the2 shows measured the measured open circuit open voltage circuit voltage(Voc), fill (Voc), factor fill (FF), factor short (FF), circuit short currentcircuit current (Isc), and (Isc), efficiency and efficiency of energy of energy transformation transformation(Eff). Binary (Eff). Binary nanofeatured nanofeatured films demonstrated films demonstrated greater greater energy energy transformation transformatio efficiencyn efficiency(6.50%) (6.50%) than flat than films flat (5.38%).films (5.38%). The Isc The and Isc FF and increased, FF increased mainly, mainly due due to the to factthe fact that that the thecell cell absorbs absorbs more more light light because because ofthe of the nanostructure. nanostructure. Meanwhile, Meanwhile, the the light light captured captured by bythe the integrated integrated cell/flat cell/flat film film and and cell/nanofeatured cell/nanofeatured film film systems systems might might be be slightly slightly different differ- entduring during measurement, measurement, which which in turn in turn led to led tiny to variationtiny variation of the of Voc the values. Voc values. Additionally, Addition- the ally,binary the nanofeatured binary nanofeatured films developed films in developed this work in exhibited this work superior exhibited energy superior transformation energy transformationefficiency to unitary efficiency nanofeatured to unitary films nanofeatured of either 300 films nm of (6.02%) either or300 900 nm nm (6.02%) (5.96%) or [90021]. nmWith (5.96%) a binary [21]. nanofeature, With a binary the nanofeature, surface absorbs the surface most of absorbs the incident most of light, the thusincident reducing light, thusreflection, reducing especially reflection, in 300–1000 especially nm in wavelength 300–1000 regimenm wavelength [22–24]. Theregime binary [22 nanotextured–24]. The bi- narysurface nanotextured also increases surface the path also lengthincreases of lightthe path as it l travelsength of through light as theit travels cell, which through in turn the enhances energy transformation [25]. Furthermore, compared to unitary structure, hybrid

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cell, which in turn enhances energy transformation [25]. Furthermore, compared to uni- Materials 2021, 14, 1669 tary structure, hybrid structures are beneficial for absorbing the infrared spectrum8 of 10of solar radiation, from which a solar cell can absorb more thermal energy [26]. The binary featured surface may also trap the weakly absorbed light reflected from the back surface by totalstructures internal arereflection beneficial at for the absorbing front surface/air the infrared interface. spectrum All of solar these radiation, promote from the which efficiency of thea solar solar cell cell can acco absorbrdingly. more thermalFinally, energy the experimental [26]. The binary results featured in this surface work may suggested also trap that the thebinary weakly nanostructured absorbed light reflectedfilms can from effectively the back surfaceenhance by totalthe energy internal transformation reflection at the effi- front surface/air interface. All these promote the efficiency of the solar cell accordingly. ciencyFinally, of solar the experimentalcells, which further results intestified this work the suggested efficacy of that nanosphere the binary lithography nanostructured and UV rollerfilms imprinting can effectively for the enhance fabrication the energy of binary transformation nanofeatured efficiency surfaces. of solar cells, which further testified the efficacy of nanosphere lithography and UV roller imprinting for the fabrication of binary nanofeatured surfaces.

Figure 7. Effect of nanofeatured films on the current-voltage (I-V) characteristics of solar cell. Figure 7. Effect of nanofeatured films on the current-voltage (I-V) characteristics of solar cell. Table 2. I-V properties of solar cells with flat film and binary nanofeatured film. Table 2. I-V properties of solar cells with flat film and binary nanofeatured film. 2 2 Solar Cell Vmax (V) Imax(mA/cm ) VOC (V) ISC (mA/cm ) FF (%) Eff (%) 2 2 SolarFlat Cell film Vmax 0.69(V) Imax(mA/cm 1.128) 0.710VOC (V) ISC 1.323 (mA/cm ) 0.57FF (%) 5.38Eff (%) 900/300 nm Flat film 0.690.62 1.128 1.008 0.7410.710 1.3961.323 0.630.57 6.50 5.38 900/300nanofeature nm 0.62 1.008 0.741 1.396 0.63 6.50 nanofeature 4. Conclusions This paper prepared binary nanofeatured films using nanocolloid lithography and 4. Conclusions ultraviolet roller imprinting. A lab-developed PDMS mold roller imprinting apparatus preparedThis paper with prepared UV radiation binary capability nanofeatured was adopted. films The using impacts nanocolloid of distinct lithography imprinting and ultravioletvariables roller on the imprinting. duplicability A of lab binary-developed nanofeatures PDMS were mold investigated. roller imprinting By utilizing apparatus the preparedsuitable with processing UV radiation parameters, capability polymeric was films adopted. with binary The 900impacts nm/300 of nmdistinct nanosphere imprinting variablesarrays on can the be satisfactorilyduplicability manufactured. of binary nanofeatures The results inwere this workinvestiga showted. that By UV utilizing roller the imprinting can offer an easy yet potent method to fabricate binary nanostructured film at suitable processing parameters, polymeric films with binary 900 nm/300 nm nanosphere an ambient temperature with low pressures. This will offer important merits with regard arraysto a can minimized be satisfactorily fabrication timemanufactured. and improved The product results quality. in this work show that UV roller imprinting can offer an easy yet potent method to fabricate binary nanostructured film at an ambient temperature with low pressures. This will offer important merits with regard to a minimized fabrication time and improved product quality.

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Author Contributions: Conceptualization, S.-J.L.; Data curation, D.L. and Y.-L.T.; Funding acquisi- tion, M.-Y.H. and S.-J.L.; Investigation, D.L. and Y.-L.T.; Methodology, D.L., M.-Y.H. and S.-J.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was sponsored by the Ministry of Science and Technology, Taiwan (Contract No. 109-2221-E-182-058-MY2), and Chang Gung Memorial Hospital (Contract No. CRRPD2K0011 and CRRPD2K0021). Data Availability Statement: Data is contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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