Improvement of the self-cleaning capabilities and transparency of cover glasses for solar cell applications by modification with atmospheric pressure plasma , Duksun Han, Seoung Kyu Ahn, Sangho Park, and Se Youn Moon Citation: AIP Advances 6, 075218 (2016); doi: 10.1063/1.4960115 View online: http://dx.doi.org/10.1063/1.4960115 View Table of Contents: http://aip.scitation.org/toc/adv/6/7 Published by the American Institute of Physics AIP ADVANCES 6, 075218 (2016) Improvement of the self-cleaning capabilities and transparency of cover glasses for solar cell applications by modification with atmospheric pressure plasma Duksun Han,1 Seoung Kyu Ahn,2 Sangho Park,1 and Se Youn Moon1,3,a Department of Applied Plasma Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, 561-756, Republic of Korea Solar Energy Department, Korea Institute of Energy Research, 71-2 JangDong, YuseongGu, DaeJeon, 305-343, Korea Department of Quantum system Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, 561-756, Republic of Korea (Received May 2016; accepted 19 July 2016; published online 26 July 2016) Using a cover glass is indispensable for protecting solar cells in photovoltaic systems Herein, the surface of the cover glass was modified by atmospheric pressure plasma to enhance the self-cleaning effect without degrading the transmittance A lower surface energy was achieved by depositing fluorocarbon polymers, and a micro-nano multi-scale morphology was built on the cover glass within 50 s These two properties led to an increase in the hydrophobicity, which enhanced the self-cleaning effect of the surface The morphology of the surface also helped to improve the transparency by reducing reflections Both the enhanced self-cleaning effect and the improved transparency induced by the atmospheric pressure plasma treatment were confirmed by analyzing the total conversion efficiency of a solar cell by outdoor field testing C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4960115] I INTRODUCTION Photovoltaic systems, generally known as solar cells, represent a promising method to supply sustainable energy without the undesirable production of carbon dioxide To generate electrical power, photovoltaic systems are typically constructed outdoors due to their ability to obtain efficient light absorption from the sun The cover glass, which is the top layer of a solar cell, plays an important role in preventing both contamination and damage caused by a variety of sources including dust, acid rain, and hail.1–3 However, this indispensable cover glass can cause a conversion efficiency drop in the photovoltaic systems due to its limited transmittance.4 Moreover, cover glasses can be easily contaminated in various environments while protecting photovoltaic systems This is known as the soiling effect and is one of the major obstacles that should be overcome to maintain the initial conversion efficiency of solar cells during outdoor operation.5 For example, in moderate dust conditions, soiling can lead to a performance reduction in the photovoltaic system of 15% to 30%; this can increase to a 100% loss via cementation in regions that experience high dust and humidity levels.5 For these reasons, numerous studies have been conducted to reduce the accumulation of dust on the cover glass as well as to increase the efficiency of the solar cell module itself.6–8 For example, micro shell arrays on flexible polydimethylsiloxane have shown super-hydrophobicity with low hysteresis, leading to a significant self-cleaning effect.6 Moth-eye patterns on the protective glass have been used to decrease the reflection of incident light and have demonstrated desirable hydrophobicity with improved performance in photovoltaic systems.7,8 a Author to whom correspondence should be addressed: E-mail: symoon@jbnu.ac.kr 2158-3226/2016/6(7)/075218/7 6, 075218-1 © Author(s) 2016 075218-2 Han et al AIP Advances 6, 075218 (2016) However, most previous works have modified the protective layer before installing the photovoltaic system outdoors Furthermore, the use of additional patterning processes typically requires higher costs and longer production times In this paper, therefore, we developed a fast, one-step fabrication method to prepare a transparent and self-cleaning surface on the cover glass of a photovoltaic system In addition, we also found a light-trapping effect, which was induced from the depositing polymers with micro-nano multi-scale structures via atmospheric pressure plasma treatment To modify the cover glass, the plasma processing time required only 50 s and did not necessitate any additional processes Moreover, this method can be applied to photovoltaic systems that have already been built/installed or are already in operation A micro-nano multi-scale structure with a low surface energy was fabricated on the cover glass by plasma treatment at atmospheric pressure with a gas mixture of helium, methane (CH4), and octafluorocyclobutane (C4F8).9,10 Plasma is generally known as a state of ionized gas, which consists of electrons and ions Accelerated electrons impact the neutral gas, resulting in the production of reactive radicals and charged particles (e.g., electrons and ions) These species participate in physical and chemical reactions on the surface of the cover glass Among the various techniques that can be used to prepare hydrophobic surfaces, atmospheric pressure plasma treatment is a promising method to achieve desirable surface modifications in open air The transmittance and hydrophobicity were controlled by changing process parameter such as flow ratio of the gas mixture Increasing the C4F8 gas flow leads to super-hydrophobicity but degrades the UV-visible transmittance To evaluate the efficiency of the photovoltaic system with plasma-modified cover glasses, both indoor dust-rain cycle testing and outdoor long-term testing were conducted II EXPERIMENTAL SETUP The transparent hydrophobic surfaces were prepared by atmospheric pressure plasma treatment with He, CH4 and C4F8 gases, as shown in Fig A 13.56 MHz radio frequency power (PTS PG0.313) was delivered to the atmospheric pressure glow discharge plasma source through an auto-matching network The power electrode was surrounded by a dielectric tube to prevent arcing and to sustain stable discharge.9 The cover glasses (Marienfeld, 76 mm × 26 mm × mm thickness) were moved left and right on a motorized plate to ensure uniform modification Helium was flowed at a fixed rate of liters per minute (lpm); otherwise, the flow rates of CH4 and C4F8 gas were independently controlled by mass flow controller (MFC, AtoVac AFC500) Gas mixture ratio R is defined as a percentage of flow rate of C4F8 over the fixed total flow ratio of 50 sccm (R = [C4F8/(CH4 + C4F8)%]) The cover glasses were cleaned with isopropyl alcohol and dried in open air before being exposed to the plasma for 50 s under different conditions III RESULTS AND DISCUSSION Images of water droplets on the bare glass and modified glasses are shown in Fig 2(a) As normally observed, water droplet was spread on the surface of bare glass rather than a shape of FIG Schematic illustration of the atmospheric pressure plasma processing system, which included a plasma source, mass flow controller, rf power generator, and auto-matcher 075218-3 Han et al AIP Advances 6, 075218 (2016) FIG (a) Photographic images of the water droplets on the bare glass, and modified glasses (b) Measured dynamic water contact angles of advancing and receding angles for the bare glass, and modified glasses (R=4, 8, and 16) (c) SEM images of the glass surfaces of bare glass, and modified glasses (d) Chemical compositions are presented using XPS analysis sphere Improvement of hydrophobicity is simply observed on the modified surfaces through the changes in shapes of water droplets This evolution of hydrophobicity was quantitatively measured by goniometer (Femtofab SmartDrop) Dynamic water contact angle (WCA) including advancing and receding was immediately measured by captive bubble method after the surface modification.11 Advancing WCA was initially about 30◦ and gradually increased to 178◦ during R was increased from to 16 as shown in Fig 2(b) Otherwise, WCA hysteresis defined as the difference between advancing and receding angle was decreased from 21.5◦ to 10◦ when R was and 16, respectively Physical morphology and chemical composition of the modified surfaces were presented in Fig 2(c) and 2(d), respectively Fig 2(c) shows the SEM images of the modified surfaces depending on a change of R including the bare glass The size of the morphological features gradually increased as the R was increased In case of bare glass, no specific morphology was found in the SEM image Alternatively, a hierarchical morphology showing a micro-nano multi-scale structure was built on the surface when R was larger than To examine the chemical compositions of the modified surface, XPS measurements were conducted and analyzed Fluorocarbons such as CF and CF2 are increased; otherwise, C-H bonding is decreased while R is changed from to 16 as shown in Fig 2(d) Since the surface tension energy of CFx is about 15–25 dynes/cm at 20 ◦C, which is much lower than that of C-H bonding, greater hydrophobicity is resulted with increasing R.12 From these results, the low surface tension energy and hierarchical structure contributed to the hydrophobicity 075218-4 Han et al AIP Advances 6, 075218 (2016) FIG (a) Transmittance of the cover glasses modified by various plasma treatment conditions (b) I-V characteristics of the solar cell covered with the bare glass and the plasma-modified glasses In enlarged graph, Isc of modified glass (R=4) is slightly higher than the bare glass Figure 3(a) shows the change in transmittance of the cover glasses modified by plasma treatment as R is changed from to 16 including bare glass The transparency of the cover glasses generally degraded as the flow rate of C4F8 increased This is the case when R is greater than over the whole wavelength region, with the exception of the longer wavelength region The transmittance of the glass with subwavelength structures on the surface was partially increased compared to the bare glass case, depending on the scale of the structures.13 All modified glasses showed higher transmittance than bare glass in the longer wavelength region except R of 16 However, the transmittance decreased relative to the bare glass between 700 and 800 nm when R is between and 12 As seen in Figs 2(c), a micro-nano structure with a 200-nm-sized structure was observed when the R was 4; this increased the transmittance Since the changes in morphology reduced the reflection of light, an increase in the transmittance within a specific wavelength range was observed.14–16 This physical characteristic is related to light trapping, which influences the initial conversion efficiency of a photovoltaic system It is believed that the hydrophobicity enhances the self-cleaning effect when the cover glass is exposed to outdoor environments Additionally, increasing the flow rate of C4F8 caused a severe degradation in the transmittance, when R is larger than 16 Therefore, 075218-5 Han et al AIP Advances 6, 075218 (2016) TABLE I The open circuit voltage (Voc), short circuit current (Isc), fill factor, and total conversion efficiency obtained from the measured I-V curves using a solar cell simulator Condition Voc(V) Isc(mA) Fill factor (%) Efficiency (%) Bare glass R=4 R=8 R=16 7.0096 7.1376 7.1554 7.1171 14.32 14.37 14.14 13.20 60.55 60.89 61.30 60.48 13.28 13.84 13.75 12.6 obtaining adequate hydrophobicity without degrading the transparency is important for photovoltaic system applications The current (I)-voltage (V) characteristics and conversion efficiencies of a commercial solar cell covered by bare glass and plasma-modified glasses were measured Figure 3(b) shows the I-V curves for the solar cell with different cover glasses As seen in Fig 3(b), modified glass (R=4) showed slightly higher current density than that of bare glass The short circuit current (Isc) is usually influenced by the optical properties of solar cell These values between bare glass and modified glasses (R=4, 8) are not much different, which Isc of modified glass (R=4) is slightly higher than that of bare glass The open circuit voltage (Voc), short circuit current (Isc), fill factor, and total conversion efficiency are presented in Table I As described in Table I, the cell efficiencies for bare glass, and modified glass (R=4) were 13.28%, and 13.84%, respectively This improved initial efficiency was due to light trapping on the glass surface caused by the micro-nano multi-scale morphology The results from the I-V curves, transmittance values, suggest that the modified cover glass (R=4) improves the initial conversion efficiency of the photovoltaic system In addition, the improved hydrophobicity is believed to contribute to the self-cleaning effect; the hydrophobicity is improved as the flow rate of C4F8 is increased Self-cleaning surfaces can support natural cleaning by rain or snow through washing away dust and preventing soiling; this can restore the cell efficiency back near its original performance.5 Therefore, artificial dust-rain cycle testing and outdoor testing were conducted with the bare glass, and modified glasses in order to examine the self-cleaning effect To verify the self-cleaning effect, 0.03 g of carbon powder was used to imitate a dust environment The size of carbon power was from 20 –50 µm (VC006015, Goodfellow Cambridge Ltd., UK) to simulate typical outside dust The correlation between powder size and size of the micro-nano structure was not examined because the distribution of powder size was not controlled precisely in this study The water drops with 30 µL were used four times to clean the contaminated glasses which were tilted at 45 degrees Even when four water droplets were dropped onto the contaminated bare glass, the flow of water was obstructed by the carbon powder agglomerates Alternatively, the water droplets left a clear trace on the surface of modified glasses (R=4, and 8) On the contrary, the water droplets were bounced on the surface, and the self-cleaning effect was less efficient when the R was 16 The total conversion efficiencies of the glasses were measured at each step during two cycles of dust-rain testing, as presented in Fig 4(a) The initial efficiencies of bare glass, and modified glasses (R=4, 8, and 16) were 13.28, 13.84, 13.75, and 12.6% respectively After dust contamination of the bare glass sample, the cell efficiency was drastically decreased to 4.15%; this was slightly recovered to about 5% after water cleansing Alternatively, modified glass (R=4) showed a lower cell reduction after contamination (9.6%) and higher recovery after cleaning (11.56%) The amounts of efficiency recovery for the solar cell with bare glass were 0.82%p and 0.47%p after two successive artificial rain treatments, respectively However, when the solar cell was covered with the modified glass (R=4), the increments in efficiency were 1.63%p and 3.09%p, respectively as shown in Fig 4(b) In addition, the self-cleaning effect was clearly observed by the naked eye in the solar cell with the modified glasses (R=4 and 8) To evaluate the outdoor performance of the plasma-modified cover glasses, the total conversion efficiency was measured once a day for 20 days, as shown in Fig 4(c) The samples were tilted at 45 degrees outdoors condition and brought indoors once a day to measure the total conversion efficiency using a solar simulator During outdoor testing, the samples were exposed to rainfall three times Interestingly, after the first rainfall, the efficiency increased by 0.516%p and 0.612%p, and 0.44%p for R=4, 8, and 16, 075218-6 Han et al AIP Advances 6, 075218 (2016) FIG (a) The change in the conversion efficiency caused by the contamination and cleaning process cycles and (b) recovered efficiency during the artificial rain-dust test (c) The measured conversion efficiency of the solar cell covered by bare glass (closed square) and plasma-modified glasses (open circle for R=4, closed triangle for R=8, and open diamond for R=16) Rain fell three times during the 20-day outdoor field test (d) Averaged conversion efficiency of modified glass showed the best performance at R=8 during the outdoor field test respectively Otherwise, the efficiency of the untreated solar cell decreased from 12.36% to 12.99% During the 20-day field test, the averaged efficiency was most high in the case of R=8 as shown in Fig 4(d) The efficiencies also increased with all cases after the second and third rainfalls A slight decrease in the efficiency was observed for all samples under a clear sky (0–3, 6–10, and 12–20 days), as shown in Fig 4(d) From these results, the atmospheric pressure plasma modification showed better initial conversion efficiency due to the improved transparency and also maintained better performance during a 20-day outdoor test under rainy and clear environmental conditions IV SUMMARY In summary, we developed a simple and fast process to modify the cover glasses of photovoltaic systems by atmospheric pressure plasma treatment The development of a micro-nano structure on the surface of the glass and the lower surface energy due to the presence of fluorocarbon polymers led to improved hydrophobicity and transparency The hydrophobicity contributed to the self-cleaning effect and the micro-nano structure also enhanced the transmittance via light trapping These two properties were varied by changing the flow rates between CH4 and C4F8 gases to increase the solar cell efficiency Plasma treatment time also can be an important factor for practical applications besides the effects of gas mixture In previous work, it was investigated the effect of plasma treatment time in the surface modification when we used He and CH4 In that case, the contact angle was saturated over 50 sec.17 In this work, also the 50 sec was the minimum time to develop the superhydrophobic surface However, since the thickness of deposited polymer was 075218-7 Han et al AIP Advances 6, 075218 (2016) increased with the treatment time, we fixed the treatment time as 50 sec to avoid the polymer thickness effects on the glass transparency This technique can be applied to photovoltaic systems at any time during their operation During a 20-day outdoor test, the average total conversion efficiency of the bare glass-covered cell was 12.79%; this value increased to12.96% when the plasma-modified glass was used to cover the cell ACKNOWLEDGMENTS This research was partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1A6A3A01 008332), and R&D Program ‘Plasma Advanced Technology for Agriculture and Food (Plasma Farming) through the National Fusion Research Institute of Korea (NFRI) funded by government, and Chonbuk National University 2016 G.J Jorgensen, K.M Terwilliger, J.A DelCueto, S.H Glick, M.D Kempe, J.W Pankow, F.J Pern, and T.J McMahon, Sol Energy Mater Sol Cells 90, 2739 (2006) J.S Son, S Kundu, L.K Verma, A.J Sakhuja Danner, C.S Bhatia, and H.S Yang, Sol Energy Mater Sol Cells 98, 46 (2012) A.A Hegazy, Renewable Energy 22, 525 (2001) M Mani and R Pillai, Renewable and Sustainable Energy Rev 14, 3124 (2010) S Travis, A.-Q Ali, and L K Lawrence, Renewable and Sustainable Energy Rev 22, 698 (2013) J.H Shin, K.S Han, and H Lee, Photovoltaics Res Appl 19, 339 (2011) Y.B Park, H Im, M Im, and Y.K Chio, J Mater Chem 21, 633 (2011) S.H Lee, K.S Han, J.H Shin, S.Y Hwang, and H Lee, Prog Photovoltaics Res Appl 21, 1056 (2013) D Han and S.Y Moon, Plasma Processes Polym 12, 172 (2015) 10 D Han and S.Y Moon, Thin Solid Films 587, 34 (2015) 11 J.D Andrade, R.N King, D Gregonis, and D.L Coleman, J Polym Sci Polym Symp 66, 313 (1979) 12 A.W Adamson and A.P Gast, Physical Chemistry of Surfaces, 6th ed (Wiley and Sons, New York, 1997) 13 Y.M Song, H.J Choi, and Y.T Lee, Opt Express 12, 13063 (2010) 14 L.S Roman, L Inganäs, O Granlund, T Nyberg, M Svensson, M.R Andersson, and J.C Hummelen, Adv Mater 12, 189 (2000) 15 H Sai, Y Kanamori, K Arafune, Y Ohshita, and M Yamaguchi, Photovoltaics Res Appl 15, 415 (2007) 16 P Campbell and M.A Green, J.Appl Phys 62, 243 (1987) 17 S Noh and S Y Moon, J Appl Phys 115, 043307 (2014) ... ADVANCES 6, 075218 (2016) Improvement of the self- cleaning capabilities and transparency of cover glasses for solar cell applications by modification with atmospheric pressure plasma Duksun Han,1 Seoung... Transmittance of the cover glasses modified by various plasma treatment conditions (b) I-V characteristics of the solar cell covered with the bare glass and the plasma- modified glasses In enlarged... effect was clearly observed by the naked eye in the solar cell with the modified glasses (R=4 and 8) To evaluate the outdoor performance of the plasma- modified cover glasses, the total conversion efficiency