Properties of Plasma Enhanced Chemical Vapor Deposition Barrier Coatings and Encapsulated Polymer Solar Cells

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1 Properties of Plasma Enhanced Chemical Vapor Deposition Barrier Coatings and Encapsulated Polymer Solar Cells QI Lei ( ), ZHANG Chunmei ( ), CHEN Qiang ( ) Laboratory of Plasma Physics and Material, Beijing Institute of Graphic Communication, Beijing , China Abstract In this paper, we report silicon oxide coatings deposited by plasma enhanced chemical vapor deposition technology (PECVD) on 125 µm polyethyleneterephthalate (PET) surfaces for the purpose of the shelf lifetime extension of sealed polymer solar cells. After optimization of the processing parameters, we achieved a water vapor transmission rate (WVTR) of ca g/m 2 /day with the oxygen transmission rate (OTR) less than 0.05 cc/m 2 /day, and succeeded in extending the shelf lifetime to about 400 h in encapsulated solar cells. And then the chemical structure of coatings related to the properties of encapsulated cell was investigated in detail. Keywords: PECVD, polymer solar cells, encapsulation, SiO x PACS: j, jr DOI: / /16/1/10 (Some figures may appear in colour in the online journal) 1 Introduction With increasing depletion of the energy resources, there is a strong demand on renewable energy resources for human being. Organic solar cells (OSCs) have emerged as a promising alternative renewable energy resources due to their potential of low-cost processing on large areas, possible semi-transparency, mechanical flexibility and light weight [1,2]. In the past few decades, researchers utilized various methods to improve the efficiency of OSCs for the purpose of commercial application [3 8]. Recently, the efficiency up to 10.6% in novel conjugated polymer based photovoltaic devices by Yang s group [9] boosts the OSCs program remarkably. However, the stability and short lifetime blocking the OSCs commercialization are still existing challenges. The degradation propensities of organic semiconductor materials exposed to moisture and/or oxygen in the atmospheric environment influence the cell stability, the efficiency and electrical properties as well as the shelf lifetime. Besides, the low work function materials of cell electrodes are also susceptible to degradation when exposed to water vapor and/or oxygen environment [10]. In order to extend the lifetime for commercial applications, polymer solar cells, in particular flexible cells, require encapsulation with a highly transparent and low permeability barrier material to against atmosphere oxidizing agents, specifically water. In this work, silicon oxide coatings, deposited on both sides of a flexible and transparent polyethyleneterephthalate (PET) by plasma enhanced chemical vapor deposition (PECVD) technology, are utilized as the barrier layer to encapsulate polymer solar cells blended with poly-3-hexylthiophene (P3HT) and [6, 6]-phenyl C61 butyric acid methylester (PCBM), and to prevent degradation caused by ambient gases. 2 Experiments 2.1 Barrier coating deposition The encapsulation film used in this study is transparent PET substrate coated on both sides with a low permeability SiO x by PECVD. Silicon oxide coatings on both sides of the 125 µm PET were deposited separately under the same parameters. The experimental parameters for silicon oxide deposition are listed in Table 1. We use 40 khz pulsed power as the power source, hexamethyldisiloxane (HMDSO) (98%) as the precursor, oxygen (O 2 ) (99.999% in purity) as oxidant gas, and argon (99.99% in purity) as diluting and ionized gas to deposit silicon oxide coatings. Schematic diagram of the plasma setup is shown in Fig. 1 [11]. Table 1. The processing parameters of SiO x coating in PECVD Parameters Value Ratio of oxygen-to-hmdso Exposure time (min) 20 Applied power (W) 30 Working pressure (Pa) 14 Duty cycle 20 Ar flow rate (sccm) 5 supported by National Natural Science Foundation of China (No ), Beijing Natural Science Foundation (No ), and 2011BAD24B01, KM , KM , BIGC Key Project (No ) and PHR , PHR , Fujian Provincial Department of Science and Technology Key Project of China (No. 2012H0008) 45

2 measured by a Keithley 4200 source meter using xenon lamp (100 mw/cm 2 ). 3 Results and discussion 3.1 Coating properties and structure Fig.1 Schematic diagram of the plasma setup The composition and structure of silicon oxide coatings are obtained by the FTIR spectra (Thermo Scientific NICOLET 6700, USA). Oxygen transmission rate (OTR) of barrier coatings is acquired by commercial available MOCON instrument OX-TRAN MODEL 2121 with a sensitivity of cc/m 2 /day at 23 C at 0% relative humidity. Water vapor transmission rate (WVTR) is acquired by commercial available MOCON instrument AQUATRAN MODEL 1 with a sensitivity of g/m 2 /day at 37 C and 100% relative humidity. And the light transmission rate is gained by UV-visible spectrophotometer (UV-2501 pc). 2.2 Preparation of OSCs The encapsulated OSCs consist of barrier coating/ indium tin oxide (ITO) anode/poly (3,4-ethylenedioxy thiophene): poly-(styrenesulfonate)(pedot : PSS)/po ly-3 hexyl-thiophene- 2, 5- diyl (P3HT): 6, 6-phenyl- C61-butyric acid methyl ester PCBM)/Al cathode/barrier coating. The SiO x coated PETs are used to seal the front and back sides of the cell, where the gap between two barrier coatings is sealed with the epoxy glue. Patterned glasses coated with ITO (thickness of 120 nm) are cleaned ultrasonically in detergent, alcohol, acetone and alcohol for 10 min, respectively, then UV-ozone treats them for 5 min after drying by N 2 stream. The cleaned substrates are spin-coated with buffer layer PEDOT: PSS (thickness of 30 nm) at 2000 rpm for 50 s, and then annealing at 120 C for 20 min in the ambient oven. The active layer consisting of 1 wt.% P3HT and 0.8 wt.% PCBM in chlorobenzene solution is consequently spin-coated on PEDOT: PSScoated ITO substrate at 800 rpm, 1100 rpm, 1400 rpm, 1800 rpm or 2000 rpm for 50 s, respectively, in order to explore the influence of active layer thickness on the cell properties. The prepared samples in the covered glass Petri dishes are then baked at 70 C for 20 min. Al cathode is lastly deposited through a shadow mask on surface to define an active device area of 4 mm 2. The whole organic photovoltaic (OPVs) undergoes postannealing from 0 min to 60 min exactly at 150 C for exploring the role of annealing time. As comparison the un-encapsulated cell is prepared in the same process, and the un-encapsulated and encapsulated solar cells are stored in identical condition. The characteristics of current density-voltage (J-V ) are The coating properties deposited by PECVD process are investigated for different processing parameters, including O 2 /HMDSO ratio, exposure time, applied power, working pressure and Ar concentration. We find that the ratio of O 2 /HMDSO is one of the most important factors. Therefore we mainly concentrate on the correlations between WVTR and the ratio of O 2 /HMDSO in this work. As well known the HMDSO has a high evaporation pressure, the required amount of HMDSO for SiO x coating deposition can be achieved even without heating the bubble. Then we utilize the self-vaporized pressure to control HMDSO flow rate in this work, whereas the oxygen flow rate is varied based on the set ratio by mass flow control. The processing parameters are listed in Table 1. Fig. 2 reveals the FITR spectrum of SiO x coatings at various ratios of O 2 /HMDSO from 2 to From FTIR spectra, we see only the peaks at 1060 cm 1 and 813 cm 1, corresponding to Si-O-Si asymmetric stretching vibration and stretching vibration, respectively. There is no observable impurity peak in the FITR spectrum except a wide peak at cm 1 of Si-H bond which might arise from adsorption from the environment. We then conclude that coatings prepared under these parameters are of high purity. Fig.2 FTIR spectrum of silicon oxide coatings prepared at various ratios of O 2/HMDSO from 2 to 0.25 In order to illustrate the relationship of the coating structure with barrier property, we deconvolute the peak at 1060 cm 1 into three peaks located at 1023 cm 1, 1063 cm 1 and 1135 cm 1 by Gaussian fitting, respectively, as Table 2 and Fig. 3 show. As known the network structure is related to high barrier property [12], i.e. the more the network structure of SiO x, the higher the barrier property of coating. We find that the ratio of network structure is increased with increasing O 2 concentration. Then we consider that a proper O 2 concentration is necessary to achieve a low 46

3 QI Lei et al.: Properties of Plasma Enhanced Chemical Vapor Deposition Barrier Coatings content of organic silicon for high barrier property. After optimization of process parameters, we achieved the lowest WVTR value of g/m 2 /day on a single-layer silicon oxide coated PET. Table 2. Ratio of linear, network, cage structure of the SiO x derived from Fig. 3 O 2:HMDSO Linear Network Cage WVTR structure structure structure (g/m 2 (1023 cm 1 ) (1063 cm 1 ) (1135 cm 1 ) /day) 2: : : Fig.4 SEM of silicon oxide coating on PET substrate Thus in order to reduce the voids and to further block the moisture permeation path, we deposited silicon oxide coatings on both sides of the plastic substrate. As expected, after coating on both sides of PET, a WVTR value of ca g/m 2 /day and OTR value of less than 0.05 cc/m 2 /day (it is the limitation of the machine) were achieved, i.e. the barrier properties were indeed significantly improved with coating on both sides of PET. Since the permeation of water in barrier coatings is dominated by defects and permeation paths in inorganic barrier coatings, it is not surprising that a remarkable reduction of WVTR is achieved on PET with SiO x coating on both sides. Additionally, Fig. 5 shows that the light transmission rate of silicon oxide coated PET is ca. 90% in the entire visible spectrum. It is similar to the original plastic. The reason is that pure SiO x has a low diffraction index, it shall not affect the PET transparency. So we can predict that SiO x is perspective transparent coating to be used in encapsulation of organic electronic device, like OLED, OPV, and organic electrons. (a) O 2:HMDSO=2, (b) O 2:HMDSO=1:2, (c) O 2:HMDSO=1:4 Fig.3 Gaussian fitting of FTIR spectrum of silicon oxide coating at 1060 cm 1 Fig. 4 is the SEM image of SiO x on PET substrate prepared under optimization condition. The dense aggregated particles are visualized on the plastic surface, which is responsible for the high barrier property. With a close investigation, however, we find that the coating formed by particle accumulations has nano-scale voids on the surface, which might serve as the path for moisture to permeate through the coatings. Fig.5 PET The light transmission rate of silicon oxide coated 3.2 OSCs properties For comparison of the cell efficiency, we built cells by adopting the spin speed of 800 rpm, 1100 rpm, 1400 rpm, 1800 rpm and 2000 rpm, respectively, in the active layer deposition. The obtained parameters of cells are shown in Fig. 6(a) and Table 3. We find that the thickness of active layer built at spin speed of 2000 rpm is much thinnest than others, and the cell manifested a lowest efficiency. The highest efficiency of device is achieved in the sample prepared at spin 47

4 speed of 1800 rpm. We believe that it is because at this spin speed, an active layer of appropriate thickness is formed. As a result, the lower resistance of the device leads to a higher efficiency of solar cell. Then we vary the annealing time from 0 min to 60 min, the parameters of solar cells are shown in Fig. 6(b) and Table 3. It is found that the highest efficiency is achieved at annealing time of 30 min. The explanation is that a proper annealing treatment benefits the phase separation in the device, which enhances the absorption of light. An excess annealing time, on the other hand, causes a larger phase separation degree, which results in a low efficiency. of the encapsulated solar cell for different storage times in dark and in illumination conditions. From these figures we find that with the increase of storage time V oc reamins almost unchanged, whereas F F has a little bit decline but J sc is greatly reduced, which causes the efficiency to decrease in the same trend. In illumination condition, in Fig. 7(b), one can see that the series resistance R s is always increasing with storage time. As indicated in the literature [13], J sc is strongly influenced by series resistance. This can rationally explain the reverse trend in J sc and R s. As known R s is mainly determined by the deterioration of charge transport in the layers (e.g. bulk material degradation), the increase of R s with storage time means that the active layer in the cell is degraded during the storage, which may be the reason of the cell efficiency degradation. However, more than one mechanism may be involved in the degradation processes as discussed below. Fig.6 (a) The J-V curves of devices with different spin speeds for active layer deposition, (b) the J-V curves of the devices with different annealing times Table 3. Values of performance parameters of devices with different spin speeds for active layer formation and various annealing times Spin speed Annealing time J sc V oc F F E ff (rpm) (min) (ma/cm 2 ) (V) (%) (%) Eventually with the optimal process parameters in cell building we obtained cells with the performances of J sc =7.25 ma/cm 2 ; V oc =0.65 V; F F =0.63; E ff =2.97%. Based on the optimized cell, we explore the lifetime of the sealed cell by PET with SiO x coating on both sides. Fig. 7(a) and (b) are respectively the J-V curves Fig.7 (a) The J-V curves of the encapsulated solar cell for different storage times in dark, (b) The J-V curves of the encapsulated solar cell for different storage times in illumination We calculated time evolutions of the normalized performance parameters of the encapsulated solar cell, as shown in Fig. 8. One can see that both V oc and F F are more stable for a long period compared with J sc. It implies that the moisture-induced degradation is the main factor in the deterioration of cell efficiency because the moisture-induced degradation affects differently the open circuit voltage and the short circuit current density: to the open circuit voltage the moistureinduced degradation is a localized failure in functional area, while to the other it is a uniform degradation of the whole device [1]. 48

5 QI Lei et al.: Properties of Plasma Enhanced Chemical Vapor Deposition Barrier Coatings 4 Conclusions The silicon oxide (SiO x ) coatings, deposited on both sides of a flexible and transparent PET by PECVD technology, has been utilized as primary barrier layer to encapsulate polymer solar cells based on blended poly- 3-hexylthiophene (P3HT) and [6, 6]-phenyl C61 butyric acid methylester (PCBM). We conclude that SiO x coatings can efficiently prevent degradation arising from the permeated ambient gases. After optimizing the processing parameters of SiO x deposition, an over 400 h shelf lifetimes was achieved in the encapsulated solar cells. Fig.8 The temporal evolution of the open circuit voltage (V oc), fill factor (F F ), short circuit current density (J sc) and efficiency (E ff ) of the encapsulated solar cell Fig. 9 compares aging property of the encapsulated and un-encapsulated solar cells. For un-encapsulated solar cell, E ff is decreased by 50% after ca. 50 h storage period; for encapsulated solar cell, on the other hand, the lifetime is greatly extended to 400 h (50% the initial efficiency). It means that the SiO x coatings can indeed effectively block water and oxygen permeation. Fig.9 Normalized efficiency of encapsulated and unencapsulated solar cells versus storage time in the dark condition under ambient air References 1 Wang Xizu, Zhao Xinxin, Xu Gu. 2012, Solar Energy Materials and Solar Cells, 104: 1 2 Lungenschmieda Christoph, Dennler Gilles, Neugebauer Helmut. 2007, Solar Energy Materials and Solar Cells, 91: Kim Jinyoung, Lee Kwanghee, E Coates Nelson. 2007, Science, 222: Hu Jia, Wu Zhongwei, Wei Huaixin. 2012, Organic Electronics, 13: Vanlaeke P, Swinnenb A, Haeldermans I. 2006, Solar Energy Materials and Solar Cells, 90: Peet Jeffrey, Senatore Michelle L, Heeger Alan J. 2009, Adv. Mater., 21: 2 7 Li Gang, Yao Yan, Yang Hoichang. 2007, Adv. Funct. Mater., 17: Cheng Yenju, Yang Shenghsiung, Hsu Chainshu. 2009, Chem. Rev., 109: You Jingbi, Dou Letian, Yoshimura Ken, et al. 2013, Nature Communications, 2411: 1 10 Kima Namsu, Potscavage Jr. William J. 2012, Solar Energy Materials and Solar Cells, 101: Fei Fei, Wang Zhengduo, Chen Qiang. 2013, Surface and Coatings Technology, 228: Grilla Alfred. 2003, J. Appl. Phys., 94: Grossiord Nadia. 2012, Organic Electronics, 13: 438 (Manuscript received 21 August 2013) (Manuscript accepted 31 October 2013) address of corresponding author CHEN Qiang: chenqiang@bigc.edu.cn 49