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Antireflective Paraboloidal Microlens film for Boosting Power Conversion Efficiency of Solar Cells Chaolong Fang, Jun Zheng, Yaoju Zhang, Yijie Li, Siyuan Liu, Weiji Wang, Tao Jiang, Xuesong Zhao, and Zhihong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19743 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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ACS Applied Materials & Interfaces

Antireflective Paraboloidal Microlens Film for Boosting Power Conversion Efficiency of Solar Cells Chaolong Fang, Jun Zheng, Yaoju Zhang*, Yijie Li, Siyuan Liu, Weiji Wang, Tao Jiang, Xuesong Zhao and Zhihong Li College of Physics and Electronic Information Engineering, Wenzhou University, Wenzhou 325035, China KEYWORDS: paraboloidal microlens arrays; antireflective film; solar cell; soft imprint lithography; self-cleaning

ABSTRACT: Microlens arrays can improve light transmittance in optical devices or enhance the photoelectrical conversion efficiency of photovoltaic devices. Their surface morphology (aspect ratio and packed density) is vital to photon management in solar cells. Here, we report a 100%packed-density paraboloidal microlens array (PMLA), with a large aspect ratio, fabricated by direct-write UV laser photolithography coupled with soft imprint lithography. Optical characterization shows that the PMLA structure can remarkably decrease the front-side reflectance of solar cell device. The measured electrical parameters of the solar cell device clearly and consistently demonstrate that the PMLA film can considerably improve the

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photoelectrical conversion efficiency. In addition, the PMLA film has superhydrophobic properties, verified by measurement of a large water contact angle, and can enhance the selfcleaning capability of solar cell devices.

1. INTRODUCTION Photovoltaic (PV) cells generate electricity by activating electrons from lower energy states to higher energy states using the energy of captured photons from sunlight. Reflection loss from the front surface of PV cells leads to insufficient utilization of the incoming sunlight. In this regard, various antireflective structures have been designed to improve the proportion of captured incident light.1-5 A variety of micro/nanostructures, such as nanodomes,6 micro/nanocones,5,7-8 micro/nanopyramids,9-12 nanowire,4, 13 nanopillars14-15 and nanospheres,16-17 with broadband and omnidirectional light-harvesting capabilities have been proposed to decrease surface reflection and thus increase light trapping in PV materials. Although etching micro/nanostructures directly on the active material can significantly improve light-capturing efficiency, this generally requires expensive equipment and complicated processes that are not economically viable or are impractical for the fabrication of large-area devices. In addition, integrating nano/microstructures into active photovoltaic materials introduces defects and surface recombination increase, hence a delicate design should be carefully considered.18-19 Mounting additional micro/nanostructure antireflective (AR) films on the PV device can improve light-capturing properties without sabotaging the balance between the light absorption and photocarrier dynamics.20-22 Therefore, various AR micro/nanostructures have been present to suppress the front-surface reflection of PV device and thus improve photoelectrical conversion efficiency (PCE).23-24 Although all these structures demonstrate considerable AR properties,


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parts of them were fabricated using costly and low-efficient methods, such as masked wetting and dry etching.18,

25

Imprint lithography using a master mold is a good select to fabricate

textured polymer AR

films because of

high

efficiency with

low cost.26-27 The

micro/nanostructure molds can be generated by processes, such as, masked dry/wet etching,24, 2829

microsphere self-assembly30-31 and anodic oxidation together with a nanoimprint process.32-35

By contrast, maskless wetting etching is a low-cost and high-efficient method to fabricating AR structures36. This method is usually used to fabricating Si molds and is unfriendly to the environment. The rose petal as master mold is an almost costless method but the area of the master mold is limited to the size of the rose petal,37 which is incompatible with the requirement of large-area AR film in solar cells. Many studies have shown that a microlens array (MLA) with high aspect ratio and high packing density is of key importance for improving AR effectiveness.38-39 In this study, we propose a paraboloidal microlens array (PMLA) AR structure with high aspect ratio and 100% close-packed density using the UV laser writing method,40 which is straightforward and environment-friendly, and can produce large area devices. The PMLA structure be facilely transferred onto PDMS film and the reproduction quality is higher than that of nanometer-scale texture molds. This is due to high viscosity of PDMS causing resolution decrease of nanostructure replica.41 Optical measurements show the transferred convex PMLA can effective suppress the front-surface reflection loss of coverglass-packaged commercial Si solar cells verified by theoretical simulation using the finite-difference time-domain (FDTD) and the raytracing method. Comparative analysis using ray-tracing method indicates the AR property of the PMLA structure is superior to hemispherical MLAs and micropyramids. The electrical characteristics showed the antireflective PDMS PMLA can markedly improve the PCE of


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commercially available Si solar cell. In addition, the PMLA AR films have superhydrophobic characteristics with large water contact angle and can enhance self-cleaning capability of solar cells. 2. EXPERIMENTAL DETAILS Figure 1 schematically illustrates the procedure for preparation of close-packed PMLA PDMS film with high aspect ratio. First, a hexagonally arranged array of circles was designed, with circle diameter and gap between circles of 4 and 7 µm, respectively. This design was then irradiated onto a 10-µm-thick positive photoresist film on a fused silica substrate at a scanning velocity of 1 mm/s using a direct-write UV laser photolithography system (Heifei Chip Foundation Microelectronics Equipment Co. Ltd., ML-C900, 375 nm) (Figure 1(a)). After developed and baked, a paraboloidal concave pattern of photoresist on the fused silica substrate was obtained (Figure 1(b)). This patterned photoresist-on-substrate was modified using chlorotrimethylsilane to facilitate the detachment of PDMS film, to become a useable master mold. To form PMLAs, a mixture of PDMS prepolymer and its crosslinking agent with weight ratio of 10:1 were spin-coated onto the master mold surface with 500 r/min for 1 min and followed by thermally cured at 80 ˚C for 2 h (Figure 1(c)). Finally, PDMS MLA films of ca. 120-µm thick were acquired by directly detaching the PDMS from the photoresist master mold (Figure 1(d)), and subsequently mounted on a coverglass-packaged commercial Si solar cell (purchased from Taobao website) without any adhesive. The PDMS PMLA film closely and firmly sticks to the coverglass owing to the self-adhesive property of PDMS.42 Although the coverglass could further increase the front-side reflectance of a Si solar cell, this does not affect the investigation of the antireflective performance of the PDMS PMLA film. 3. RESULTS AND DISCUSSION


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3.1 Effect of Exposure Power on Structure Profile Figure 2(a) shows scanning electron microscopy (SEM) images of PMLAs which were obtained by duplicating photoresist molds fabricated using UV laser exposure powers from 14 to 24 mW. The SEM images show the geometric morphology fabricated the MLAs is paraboloidal. It is found that when exposure power varies from 14 to 16 mW, PMLA bump height increases with exposure power, while the interval distance between microlenses is not obviously changed, as shown in Figure 2(a)-(c). Figure 2(d)-(f) shows that both bump height and width of PMLA simultaneously increase as exposure power is changed from 20 mW to 24 mW. The fabricated PMLA structures were then measured using atomic force microscopy (AFM), with the measured parameters shown in Figure 3(a). It is clearly shown that the width of each microlens bump is constant at the beginning of the increase of exposure power and then gradually increases up to a maximum value of 7 µm at 24 mW, while the height of microlens keeps increasing in the range of 14-24 mW and reaches a maximum value of ca. 7.4 µm at 24 mW. Figure 3(b) shows the influence of exposure power on the aspect ratio. The aspect ratio keeps increasing to 1.07 at 24 mw, then decreases with the exposure power. These interesting phenomena can be explained according to the light intensity distribution of the UV exposure. The light intensity distribution Iz(r, θ) (r2=x2+y2, θ=atan(y/x)) along the Z-direction of penetration depth can be expressed as:43 Iz(r, θ)=Ip(r, θ)exp(-µz/ρ)

(1)

where Ip is the light intensity distribution on the top of the photoresist, µ is the absorption coefficient of the photoresist, z is the penetration depth from the top surface of the photoresist, and ρ is the photoresist density. For 16 mW or less exposure power, the light intensity Ip outside


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the preset cell is lower than the exposure threshold of the photoresist in spite of increasing the exposure power while increasing Iz enhances the light penetration depth. Consequently, the bump height of microlens is increased and the width is almost constant. However, when the exposure power is changed from 16 to 24 mW, light intensity causes photochemical reaction outside the scheduled exposure cells. Ip outside the preset cell exceeds the exposure threshold of photoresist and Iz increases continuously, which leads to the increase of both the width and the height. When exposure power is 24 mW, the exposured area reaches its maximum value such that a 100%-packed-density pattern is formed, as shown in Figure 2(f), in which the height of PMLA bump is 7.4 µm. For exposure power beyond 24 mW, Ip becomes high enough to cause the photochemical reaction of the total surface of the photoresist and thus the height of PMLA is no longer increased. 3.2 Charateristics of PDMS PMLA Film The obtained PMLA PDMS film has many unique properties suitable for AR layers of PV devices. Figure 4(a) shows the PDMS PMLA film fabricated at 24 mW naturally adhered on polyethylene terephthalate (PET) substrate without any adhesive, where the thickness of the PDMS film and PET substrate is 120 and 200 µm, respectively. Note that the PMLA film shows a diffraction effect signifying excellently ordered distribution of the paraboloidal microlenses. In addition, it is obviously seen that the PMLA PDMS film blends synchronously with the bending of the PET film, which indicates the PDMS film is firmly adhered to the PET film, and further demonstrated by the SEM image of the cross-sectional PDMS film adhered on the PET film in the inset of Figure 4(a). In fact, the PDMS film can also adhere to other materials, such as glass, silicon and polymers. This self-attachable property indicates PDMS PMLA film can be conveniently mounted and replaced on a solar cell surface.


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Then, we examined the effectiveness of a PDMS PMLA AR film (fabricated under 24 mW UV exposure power) on a commercial coverglass-packaged Si solar cell. Figure 4(b) shows photos of two Si solar cells with 2 cm × 2 cm area. The left solar cell with the PDMS PMLA film appears completely black, while the right cell without the PDMS PMLA film appeared to be gray, and the finger electrodes of the solar cell are clearly visible. This signifies that the PDMS PMLA film can reduce the front-side reflectance of solar cells, which will be discussed in detail later. Furthermore, besides AR properties, the PDMS PMLAs possess superhydrophobic properties. Figure 4(c) shows a 2 µL water droplet on the top of the PMLA PDMS film, with a contact angle of 154.8°. By contrast, the contact angle of a water droplet on the flat PDMS film without PMLA structure was found to be 98.2°, as shown in Figure 4(d). The hydrophobicity of the PDMS PMLA film mainly originates from the enhanced surface roughness as proposed by the Cassie and Baxter model.44 In fact, hydrophobic materials have been extensively explored for self-cleaning surfaces.45-47 The liquid droplet shape for the inclination angle of 20° is shown in Figure S1. The advanced angle and receding angle are 154.2° and 142.7°, which indicate that water droplets would easily slip from such a PDMS PMLA AR film. The video in the supporting information clearly shows that water droplets didn’t stay on the surface of the PMLA film, which indicates the hydrophobicity of the solar cell is enhanced by the PMLA structure. 3.3 Effect of Packed Density and Bump Height on Antireflection To quantitatively analyze the AR properties of the PDMS PMLA films, reflectance spectra from 400 to 900 nm were measured. PDMS PMLA films fabricated under different UV exposure power were adhered on the top of the same glass-packaged Si solar cell device for comparison.


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Figure 5(a) shows reflected spectra of the glass-packaged Si device with and without PDMS PMLA films, in which the exposure power of the fabricated PMLA films were 14, 18, and 24 mW. The integrated reflectivity is defined as:

ܴ=

ഊ భ

‫׬‬ഊ మ ௥ሺఒሻௗఒ ఒమ ିఒభ

(2)

where λ1 and λ2 are the start and end wavelengths in the measurement or simulation, and r(λ) is the reflectivity of the corresponding wavelength. The calculated integrated reflectivity of the solar cell device without PDMS PMLA AR film is ca. 7.03% for the given wavelength range. The integrated reflectivities of the device with PDMS PMLA AR film were 5.94% for 14 mW, 5.12% for 18 mW, and 2.96% for 24 mW UV exposure power. Obviously, the reflectivities of the device with the PMLA AR films are lower than that of the device without PMLA film. Figure 5(b) shows the influence of the UV exposure power on the integrated reflectivity of the solar cell device with PMLA AR film. Obviously, the AR effectiveness of PMLA is closely related to the exposure power used. Notably, the optimal AR effectiveness exactly corresponds to the PMLA film fabricated under 24 mW exposure power, which has the maximum aspect ratio of 1.06 and packed density of 100%. In order to analyze AR effectiveness of the proposed PMLA structure fabricated under different exposure power, the reflected spectra of Si solar cells with different PDMS PMLA films from 400-900 nm with an interval of 5 nm were simulated by using the FDTD method. Integrated reflectivity of the solar cell with PDMS PMLA film can be calculated using Eq. (2). Figure 5(c) shows the integrated reflectivity of the device as a function of the packed density of the PMLA, where the microlens bump height is 3.5 µm. It is found that the reflectivity of the solar cell device decreases with increasing packed density and reaches its lowest value at 100%


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packed density. In fact, it has been reported that high packed density is of importance to improve the light transmittance for hemispherical MLAs.38-39,

48

Figure 5(d) shows the integrated

reflectivity decreases with increasing bump height when the microlens width and packed density are fixed at 7 µm and 0.907, respectively. Obviously, the reflectivity of the device rapidly decreases with increasing microlens height. According to grating equation, for a grating with 7 µm pitch, interference between the diffracted orders from neighboring microlenses does not occur, which indicates that antireflective property is primarily due to capturing the reflecting and scattering light.49 In this case, the light-capturing performance of the microstructure can be analyzed by ray-tracing. Figure S2 shows ray-tracing simulations of PMLA structures under different packed density. Note that the width and the height of both PMLAs are the same and incident light areas with 5 rays are the same. Obviously, high optical trapping capability is corresponding to high packed density. In addition, the light-capturing properties of PMLA structures with different aspect ratios were also analyzed using the ray tracing method (Figure S3). Packed density and width of both PMLA structures kept same and incident light areas with 5 rays were same for the sake of comparison. It is observed that the PMLA array with a large height can capture the reflected light rays. These results further demonstrate that high packed density and aspect ratio can facilitate light transmission. In addition, comparison of the simulated reflectivity in the given wavelength of the Si solar cell with hemispheric and paraboloidal MLA films are shown in Figure S4, where the packed density and aspect ratio are 0.907(Hexagonally closed-packed arrangement) and 0.5 (hemisphere), respectively, for two solar cells. It can be found that the AR performance of the paraboloidal MLA is superior over that of the hemispheric MLA. To further demonstrate the superiority of the paraboloidal profile, a visual analysis was performed using the TracePro ray-


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tracing software (Figure S5). Obviously, the sharp profile of the paraboloidal MLA leads to reflected light easily being captured by neighboring microlenses, which indicates higher antireflective performance. Figure 6 shows the external quantum efficiency measurement of a Si solar cell with and without PDMS PMLA film, which provides direct evidence to the reduction of the front-side reflectance of the solar cell. In order to further comprehend the antireflection performance, reflectance and transmittance spectra of the PDMS PMLA film fabricated under 24mW were measured (Figure S6). The PDMS PMLA film has high transmittance and low reflectance properties. These results indicate the PDMS PMLA film can effectively reduce reflectance loss of Si solar cell and thus improve the performance. 3.4 Electrical Characteristics The purpose of fabricating the close-packed PMLA is to facilitate improvement of the PCE of solar cells. Therefore, we used a solar simulator to measure the electrical characteristics of a Si solar cell with and without the PDMS PMLA AR film (fabricated under 24 mW exposure power) under 1 Sun illumination. The measured current density-voltage (J-V) curves and electrical parameters are shown in Figure 7(a). Table 1 of Figure 7(a) shows that the PDMS PMLA AR film causes the short-circuit current density Jsc to increase from 35.6 to 37.2 mA/cm2, while the open-circuit voltage Voc and fill factor FF show a marginal increase. Consequently, the PCE of the device is improved from 16.7% without the PMLA film to 17.7% with the PMLA film, which indicates an absolute PCE improvement of 1%. The measured results demonstrate the PDMS PMLA AR film improves the photon-capturing capability of the Si solar cell without any damage.


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To confirm the reliability of the improvement of the PCE, we measured electrical characteristics of three different solar cells with the same PDMS PMLA AR film. Figure S7(a)(c) show the J-V curves of three solar cells with and without the same PDMS PMLA AR film. And the measured parameters summarized in Table S1 consistently show that the PDMS PMLA AR film can improve PCE. This PCE improvement is ascribed to the light capturing effects of the PDMS PMLA film, as mentioned above. The designed PMLA AR film can improve the absolute PCE of the silicon solar cell by 1%, which is lower than the 1.7% absolute PCE improvement produced by a previously reported PDMS inverted pyramid AR film.50 This is mainly due to the geometric parameter (period and depth-to-width ratio) of the structure which is closely related to AR properties. To facilitate a comparison, the absorptance of a silicon substrate with hexagonal close-packed PMLA and close-packed inverted pyramid AR films, and without any film, was simulated using TracePro commercial ray-tracing software. For simulations, the height of the PMLA and the depth of the inverted pyramid were chosen to be 7 µm, and the bottom diameter of the paraboloidal microlens and the side length of bottom square of micropyramid were 7 µm, indicating both their depth-towidth ratios are equal to 1. The thickness of silicon substrate and the thickness of both microstructure AR films were 200 µm and 20 µm, respectively. 100 rays were irradiated uniformly on the unit cell of microstructure at normal incidence over the wavelength range of 300 to 1100 nm. Any reflected ray with power lower to 0.01of an incident ray was neglected. Figure 8 shows the simulated absorptance spectra of Si substrate with two AR films of PMLA and micropyramid. As comparison, the absorptance of Si substrate without any AR film was simulated. Obviously, both microstructures facilitated increased light absorption of the overall


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substrate. However, PMLA film demonstrated superior AR performance. As a result, the PMLA structure is a good select for reducing reflectance of Si solar cell. It should be noted that the above optical measurements and electrical characterizations of solar cells were performed under normal incidence. In reality, solar cells are irradiated by light incident over a wide range of angles, due to atmosphere scattering and the relative motion of the sun. Therefore, the incident-angle-dependent performance should be characterized. Figure S8 shows a schematic diagram of the rotary table for PCE measurement under different incident light angles. The solar cell was mounted on the sample stage, which was mounted on a rotation shaft connected with an angle sensor. When the rotation shaft was revolved, the height between light source of solar simulator and the center of the solar cell was constant and the rotated angle can be obtained through the angle sensor. Using the rotary table, the PCEs of the solar cell with different incident angle can be obtained. Figure 9 shows that as the incident angle is increased, the PCEs of Si solar cells with and without the PDMS PMLA AR film are dramatically decreased. This PCE decrease is mainly due to the increase of Fresnel reflection losses and the reduction of the solar cell area irradiated by the incident light. However, the device with the PMLA AR film exhibits superior solar power generation over a wide incident angle range of 060°. The minimum improvement was 5.6% at an incident angle of 10°, with the maximum improvement of 21% being recorded at 60°. The trend of the results suggests that even higher performance improvement might be achieved at higher incident angles. 4. CONCLUSION Preparation of close-packed PMLAs with a large aspect ratio has been demonstrated by directwrite UV laser photolithography coupled with soft imprint lithography. The fabricated PDMS


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PMLA film exhibits excellent flexibility and self-attachable properties, which can be firmly attached onto coverglass-encapsulated Si solar cells. The PMLA AR film is superhydrophobic with a large water contact angle, which can improve the self-cleaning capabilities of solar cells. More importantly, the experimental investigation and theoretical simulation consistently shows that the PMLA structure can significantly suppress the reflectance loss of Si solar cells. Electrical characterizations demonstrate that the PMLA structure can improve short-circuit current density, resulting in considerable enhancement of PCE. These results demonstrate the promising potential of multifunctional PDMS PMLA film for use in PV devices. ASSOCIATED CONTENT Supporting Information. The calculated reflectance of a Si PV device with spherical and paraboloidal microlens arrays of the same height, width, packed density. The measured J-V curves, electrical parameters of three different devices with and without PDMS PMLA film. Experiment video of self-cleaning property of PMLA structure. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION *Tel.: +86-577-8668-9016. Fax: +86-577-8668-9016. E-mail: [email protected]. Funding Sources This work was supported by Natural National Science Foundation of China (NSFC) (61377021). Notes The authors declare no competing financial interest.


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Figure 1. Preparation procedure of PDMS convex PMLA film. (a) Radiated exposure of photoresist film by direct-write UV laser photolithography system in a hexagonally-arranged style. (b) The photoresist master mold after developing and baking. (c) Premixed PDMS poured on photoresist mold, followed by a degassing and curing process. (d) Regular convex PDMS PMLA after peeling from the photoresist mold.


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Figure 2. (a)-(f) SEM images of PDMS MLAs obtained by duplicating the photoresist mold fabricated under different UV exposure energy of 14, 15, 16, 20, 22 and 24 mW. Inset: SEM images of samples placed at tilting angle of 60°. Scale bar 10 µm.


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Figure 3. (a) Measured width and height of PMLA bumps as a function of exposure power and (b) calculated aspect ratio as a function of exposure power.


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Figure 4. (a) Flexible PDMS PMLA film fabricated under 24 mW exposure power. (b) Photos of coverglass-packed Si solar cell devices with and without a PMLA film at 24 mW. (c) A water drop of 2 µL on the PMLA PDMS film showing a large contact angle of 154.8°. (d) A drop of water on the flat PDMS film showing a contact angle of 98.2°.


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Figure 5. Measured reflection spectra of Si solar cell device with and without PMLA films fabricated at UV exposure powers of 14 mW, 18 mW, and 24 mW. (b) Integrated reflectivity of the device with PMLA structure versus exposure power. (c) Calculated integrated reflectivity as a function of packed density for a fixed bump height of 3.5 µm. (d) Calculated integrated reflectivity as a function of bump height when packed density and microlens width are kept unchanged at 91% and 7 µm, respectively.


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Figure 6. Measurement of external quantum efficiency for a Si solar cell with and without PDMS PMLA film.


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Figure 7. Measured J-V curves of coverglass-packed Si PV device with and without PMLA film at normal incidence. Inset Table 1: Electrical parameters of the devices.


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Figure 8. Simulated absorptance spectra of Si substrate without AR film (Naked Si), and with PMLA and micropyramid AR films.


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Figure 9. Dependence of solar cell PCEs on incident angle with and without the PDMS PMLA film, and the relative PCE improvement of the solar cell with the PDMS PMLA film.


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