Moisture-Resistant Electrospun Polymer Membranes for Efficient and

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Moisture-Resistant Electrospun Polymer Membranes for Efficient and Stable Fully Printable Perovskite Solar Cells Prepared in Humid Air Pongthep Prajongtat, Chakrit Sriprachuabwong, Ratchada Wongkanya, Decha Dechtrirat, Jutarat Sudchanham, Nirachawadee Srisamran, Winyoo Sangthong, Piyachat Chuysinuan, Adisorn Tuantranont, Supa Hannongbua, and Nattaporn Chattham ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05032 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Moisture-Resistant Electrospun Polymer Membranes for Efficient and Stable Fully Printable Perovskite Solar Cells Prepared in Humid Air Pongthep Prajongtat,*,† Chakrit Sriprachuabwong,‡ Ratchada Wongkanya,† Decha Dechtrirat,† Jutarat Sudchanham,‡ Nirachawadee Srisamran,‡ Winyoo Sangthong,§ Piyachat Chuysinuan,║ Adisorn Tuantranont,‡ Supa Hannongbua,┴ and Nattaporn Chattham¶ † Department

of Materials Science, Faculty of Science, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok, 10900 Thailand

‡ Graphene

and Printed Electronics for Dual-Use Applications Research Division (GPERD),

National Science and Technology Development Agency, 111 Thailand Science Park, Phahonyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani, 12120 Thailand §

Center for Advanced Studies in Nanotechnology for Chemical, Food, and Agricultural

Industries, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok, 10900 Thailand ║ Laboratory ┴ Department

of Organic Synthesis, Chulabhorn Research Institute, Bangkok, 10210 Thailand

of Chemistry, Faculty of Science, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok, 10900 Thailand

¶ Department

of Physics, Faculty of Science, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok, 10900 Thailand

Keywords: organolead trihalide perovskite, fully printable perovskite solar cells, electrospinning, moisture stability, hydrophobic polymer membranes Corresponding Author * Email: [email protected]. Phone: +662 562 5555 ext 1517. Fax: +662 942 8036 1 ACS Paragon Plus Environment

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

ABSTRACT Fully printable perovskite solar cells (PPSCs) attract attention in the photovoltaic industry and research owing to their controllable and scalable production with reduced material waste during manufacturing. However, the commercialization of PPSCs has been impeded by their inherent vulnerability to ambient moisture, leading to a rapid loss of device efficiency and lifetime. Here, we propose a novel idea to enhance the photovoltaic performance and stability of PPSCs in humid air (relative humidity exceeding 80%) by using electrospun hydrophobic polymer membranes, i.e. polylactic acid (PLA), polycaprolactone (PCL), and PLA/PCL blends, as moisture-resistant layers for PPSCs. After optimizing the morphologies, hydrophobicity, and thermal properties of the electrospun membranes by varying the contents of the polymer components in the membranes, the unencapsulated devices with these membranes demonstrated power conversion efficiencies of up to 8.2%, which was significantly higher than for devices without the membranes (6.8%). Moreover, devices with the optimum electrospun membrane retained over 85% of their original efficiency after being stored in humid air for over 35 days. In comparison, devices without the electrospun membranes lost about 50% of their initial efficiency over the same time. Our work is very useful for the development of highly efficient and stable commercial PPSCs.

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1. INTRODUCTION Perovskite solar cells (PSCs) using organolead trihalide perovskite layers—for example, methylammonium lead iodide (CH3NH3PbI3, MAPbI3)—as light absorbers have emerged as the next generation of photovoltaic devices because of their cost-effectiveness, simple preparation based on low-temperature solution processing, and high power conversion efficiency (PCE).1-3 Solution-processed PSCs can be fabricated using various techniques, including spin coating,4 dip coating,5 and vacuum flash-assisted deposition6. Printing techniques, such as screen and inkjet printing, attract significant attention in the scientific community and photovoltaic markets since they allow for controllable and scalable production as well as for reduced material waste during manufacturing.7-10 The fabrication of fully printable PSCs (PPSCs) was first reported by Ku et al.9. Their devices were composed of a three-layer mesoporous stack, including TiO2, ZrO2, and carbon, serving as an electron collector, insulating spacer, and conductive back electrode, respectively. To form perovskite absorbing layers, perovskite precursor solutions were infiltrated into the mesoporous stack through the top of the carbon electrode. Although PPSCs have been extensively investigated, to date the commercialization of PPSCs is still restricted by their inherent vulnerability to ambient moisture, leading to a rapid loss of device efficiency and lifetime.11-13 Therefore, enhancing the long-term stability of PPSCs by reducing the degradation of perovskite absorbers in humid environments is an important issue that urgently needs to be solved. Several novel strategies have been employed to obtain stable and efficient PSCs, including additive addition,10,14 halogen doping,15 and perovskite self-assembly in polymer scaffolds16,17. Recently, layers of hydrophobic molecules and polymers were inserted between perovskite and hole transport or electron collection layers to prevent degradation of the 3 ACS Paragon Plus Environment


perovskite layer.18-20 The stability of PSCs can be enhanced significantly by using this approach. However, a major drawback of this approach is that the band energy alignment of the inserted hydrophobic layers does not match compatibility with that of the perovskite and hole transport or electron collection layers, resulting in inefficient charge extraction and low device efficiency. Therefore, only ultra-thin hydrophobic layers were useful for charge tunneling processes.20 Moreover, deposition of the hydrophobic layers onto the perovskite layers can cause a significant decrease in interfacial interactions between the perovskite and hole transport or electron collection layers due to the non-wetting surfaces of the perovskite layers. Therefore, to solve these issues, one can move the hydrophobic layers to the top of the back electrodes in the devices. This is not only highly compatible with the electronic structures of perovskite interfaces but is effective for improving the stability of PSCs. To fabricate a PPSC with a hydrophobic layer on the back electrode, a hydrophobic layer with a porous structure is required for perovskite infiltration. Fortunately, electrospun polymer membranes are appropriate for this purpose since they have porous structures that exhibit porefilling properties and allow the infiltration of materials.21,22 Furthermore, the physical and thermal properties of electrospun polymer membranes, such as hydrophobicity, porosity, and thermal stability, can be easily modified by blending different polymers together.23 It should be noted that electrospun polymer membranes are prepared by an electrospinning technique that applies a strong electric field to polymer solutions in order to create a polymer jet.24,25 Electrospun membranes have been used in a wide variety of applications, including tissue engineering,21 photocatalysis,26 and hydrogen energy generation27. Recently, electrospun membranes have been successfully incorporated into optoelectronic devices and, in most cases,


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devices based on electrospun membranes demonstrated improved performance compared with those based on nanoparticles.28-30 Here, for the first time, we fabricate PPSCs with electrospun hydrophobic polymer membranes as moisture-resistant layers for use in humid air (RH exceeding 80%). The electrospun membranes were prepared from polylactic acid (PLA), polycaprolactone (PCL), and PLA/PCL blends, which were used as model hydrophobic polymers. In comparison to PPSCs without the electrospun membranes, devices with the electrospun membranes demonstrated improved photovoltaic performance by achieving PCEs of up to 8.2% under standard AM 1.5 illumination. Moreover, excellent moisture stability was also obtained for unencapsulated devices with the electrospun membranes, which could endure moisture attack for over 35 days. This suggests that the electrospun membranes act as highly efficient moisture resistant layers for PPSCs. Nevertheless, the photovoltaic performance and stability of the devices depend strongly on the morphologies, hydrophobicity, and thermal properties of the electrospun membranes, which can be adjusted simply by varying the PLA and PCL content in the electrospun membranes. 2. EXPERIMENTAL 2.1 Preparation of electrospun polymer membranes To fabricate electrospun polymer membranes, various polymer solutions, including PLA, PCL, and PLA/PCL blends, were prepared. For the pure polymers, 8% w/v PLA (Mw ~60,000 g/mol, Sigma-Aldrich) and PCL (Mw ~80,000 g/mol, Sigma-Aldrich) solutions were prepared in a mixed chloroform (99.8%, RCI Labscan) and methanol (99.9%, RCI Labscan) solvent with a volume ratio of 3:1. For the blends, 8% w/v PLA/PCL mixtures, containing 20 (80), 40 (60), 60



(40), and 80 (20) wt% PCL (PLA), were also prepared in the mixed chloroform and methanol (3:1 v/v) solvent. The polymer solutions were stirred overnight at room temperature to achieve homogeneous solutions. The electrospinning setup consists of a syringe pump (New Era Pump Systems, Inc., NE-1000), high voltage power supply (Gamma High Voltage Research, Inc., ES30P-5W220), and metal collector, as shown in Figure 1(a). During electrospinning, the polymer solutions were pumped at 2 mL/h from a 10 mL glass syringe, through a stainless steel needle (inner diameter 0.603  0.019 mm), onto F-doped SnO2 (FTO) coated glass substrates (15 Ω/sq., Solaronix) attached to the metal collector covered with aluminum foil. The distance from the needle to the collector was set at 13 cm. To eject the polymer solutions towards the FTO substrates, a voltage of 25 kV was applied to the needle and the collector. The electrospinning experiments were performed at room temperature and an RH of about 80%.

Figure 1. Schematic illustration of (a) the electrospinning setup and (b) cross-sectional structure of PPSCs with electrospun polymer membranes. 2.2 Fabrication of PPSCs with electrospun polymer membranes The structure of PPSCs with electrospun polymer membranes is shown in Figure 1(b). FTO substrates were patterned by etching with Zn powder (TCI chemicals) and 2M HCl (RCI 6 ACS Paragon Plus Environment

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Labscan) before being sequentially cleaned with detergent, deionized water, ethanol, and acetone for 15 min each in an ultrasonic bath. A 200 nm compact TiO2 (cp-TiO2) layer was coated on the patterned substrates by screen-printing with titania paste (Ti-Nanoxide BL/SP, Solaronix), and then sintered at 500°C for 30 min. Subsequentially, a 700 nm mesoporous TiO2 (mp-TiO2) layer was screen-printed on the top of the cp-TiO2 layer using titania paste (Ti-Nanoxide T165/SP, Solaronix), followed by sintering at 500°C for 30 min. Then, a 1 μm mp-ZrO2 layer was screenprinted on top of the mp-TiO2 layer using zirconia paste (Zr-Nanoxide ZT/SP, Solaronix), and sintered at 500°C for 30 min. Afterward, a 10 μm porous carbon counter electrode was screenprinted on top of the ZrO2 layer using carbon paste (Elcocarb B/SP, Solaronix), followed by sintering at 400°C for 30 min. After cooling down to room temperature, a PLA, PCL, or PLA/PCL blend membrane was electrospun on top of the carbon electrode using the electrospinning technique described in the previous section. Finally, a 40 wt% perovskite precursor solution containing CH3NH3I (Solaronix) and PbI2 (Sigma-Aldrich) with a molar ratio of 1:1 in anhydrous N, N-dimethylformamide (DMF, Sigma-Aldrich) was infiltrated into the mesoporous stack through the top of the electrospun membranes by an inkjet-printing technique.7 The inkjet-printing was conducted using a Dimatix Fujifilm DMP-2831 printer with a printing frequency, maximum voltage, and pulse width of 5 kHz, 22 V, and 8.5 μs, respectively. After the perovskite was dried at 90°C for 10 min, PPSCs with electrospun polymer membranes were obtained. Two batches of devices were fabricated for each membrane, resulting in a statistic of 8 cells for analysis of standard deviations. All fabrication steps were carried out in ambient air (~80% RH, 30°C). 2.3 Characterization



The sample morphologies were examined using a scanning electron microscope (SEM, Quanta 450 FEI) at 20 kV acceleration voltage. The electrospun membrane samples were sputter-coated with gold in a vacuum before the SEM observations, and the perovskite samples were not coated with gold. Image analysis software (Image J, Wayne Rasband, National Institute of Health, USA) was used to measure the average diameters of the electrospun nanofibers (100 different nanofibers were measured for each sample). The phase composition of the samples was determined by grazing incidence X-ray diffraction (GIXRD, Bruker D8 Advance) with a Cu Kα source using a step size and step time of 0.02° and 10 s, respectively. The thermal stabilities of the samples (sample weight ~10 mg) were investigated using a thermogravimetric analyzer (PerkinElmer, Pyris 1 TGA) in the range of 35–600°C at a heating rate of 5°C/min in an air atmosphere. The functional groups of PLA and PCL and interactions between PLA and PCL in electrospun membranes were studied using an attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrometer (Bruker, Vertex 70). The ATR-FTIR spectra were scanned from 400–4000 cm-1 with a spectral resolution of 2 cm-1. Contact angle measurements were performed with a contact angle meter (Dataphysics OCA 20) by dropping 5 μL water or perovskite precursor solution droplets at ten different positions on each electrospun membrane. The photocurrent density-voltage (J-V) characteristics of PPSCs were measured using a Wacom SSS Keithley 236 source meter under AM 1.5 G illumination. Light intensity generated by the light source, a 1000 W collimated xenon lamp (Xenon Lamp KXO-1000HSFD), was calibrated to 100 mW/cm2 with a certified silicon solar cell. A black mask with a 0.3 cm2 area was used to define the active area of the solar cells during the J-V measurements. For the stability tests, the photovoltaic parameters of the unencapsulated devices exposed to ambient air (~80% RH, 30°C) were collected for over 35 days. 8 ACS Paragon Plus Environment

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3. RESULTS AND DISCUSSION With respect to the proposed device structure shown in Figure 1(b), electrospun hydrophobic polymer membranes were deposited onto the mesoporous stack prior to perovskite infiltration. Therefore, the morphologies of the electrospun membranes are one of the key parameters affecting perovskite loading and device performance. The morphologies of electrospun PLA membranes prepared at different electrospinning times, 20, 30, 40, and 50 s, on FTO substrates were studied using SEM (Figures S1 and S2 in Supporting Information), which showed that the electrospun membranes were constructed from PLA nanofibers with different nanofiber densities depending on the electrospinning time. The nanofiber densities (defined as the number of nanofibers per unit area) and the thicknesses of the electrospun membranes increased as the electrospinning time increased, indicating an increase in the total pore volumes of the membranes. It can be expected that increasing the total pore volume of the membranes could enhance the perovskite loading and device efficiency. Therefore, we fabricated PPSCs with electrospun PLA membranes prepared at different electrospinning times. Counterintuitively, the PCEs of the devices decreased with increasing electrospinning time and membrane thicknesses, and the devices prepared at 20 s electrospinning time demonstrated the highest PCEs (Figure S3 in Supporting Information). This is probably because the thick electrospun membrane exhibited a non-wetting property, leading to low perovskite loading and device performance. However, the stability of the devices prepared at different electrospinning times was not significantly different (a slight degradation, ~5%, after the devices were stored in ambient air for 2 weeks). Thus, the electrospinning time of 20 s was considered to be the optimum time and, in the following, our discussion on the electrospun membranes will be based on preparation at a 20 s electrospinning time.



3.1 Morphologies, hydrophobicity, and thermal properties of electrospun membranes Figure 2 shows the SEM images of electrospun membranes prepared from PLA/PCL blends containing PLA and PCL with different contents. The SEM images show that the electrospun membranes were built up from uniform, bead-free polymer nanofibers whose diameters varied depending on the amount of PLA and PCL in the blends. The diameters of the nanofibers prepared from 0 (pure PLA), 20, 40, 60, 80, and 100 (pure PCL) % PCL were observed as 832  283, 608  136, 603  171, 692  141, 657  201, and 996  303 nm, respectively. The diameters of the nanofibers prepared from pure polymers were noticeably larger than those prepared from polymer blends, while the diameters of nanofibers prepared from polymer blends were nearly equal. This suggests that the degree of chain entanglement in the polymer blend nanofibers was greater than in the pure polymer nanofibers.31 Furthermore, due to the constant electrospinning time, the nanofiber densities and the thicknesses of all electrospun membranes were very similar and about 54 nanofibers/5000 μm2 and 5 μm, respectively.


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Figure 2. SEM images of electrospun polymer membranes prepared from PLA/PCL blends containing (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, and (f) 100 wt% PCL. The insets present the diameter distribution of the nanofibers in the electrospun membranes. The wetting of electrospun membranes is the other key parameter greatly influencing the perovskite loading and the moisture-resistance of solar cells. It has been reported that the moisture-resistance of solar cells could be enhanced by increasing the hydrophobicity of MAPbI3 surfaces.32 However, based on our device structure, the inefficient infiltration of perovskite precursor solutions into highly hydrophobic membranes could occur because of the non-wetting characteristics of the membranes. Thus, optimization of the hydrophobicity of electrospun membranes, which can be indirectly determined by considering the wetting of the membranes, is



necessary to achieve optimum device efficiency and stability. The wetting of the electrospun membranes can be adjusted by varying the contents of the blend components, i.e. PLA and PCL, in the membranes. Typically, the wetting of material surfaces is examined using water contact angle measurements.33 Figure 3(a) presents the contact angles of water droplets on electrospun membranes prepared from PLA/PCL blends with different PCL contents, which were measured at room temperature. All membranes had contact angles greater than 90°, corresponding to low wettability. Moreover, the contact angles increased with increasing PCL content in the membranes and, consequently, the smallest and greatest contact angle values were observed as 129.7° and 144.3° for membranes prepared from pure PLA and pure PCL, respectively. This implies that the electrospun membranes with higher PCL contents exhibited more hydrophobicity. Furthermore, to investigate the effect of the operating temperature of solar cells on the hydrophobicity of the membranes, the 20% PCL membrane was heated at 60 oC (assumed to be the operating temperature of solar cells) for 10 min and then the water contact angles on the heated membrane were measured immediately. It was found that the contact angle on the heated membrane (138.1o  1.3o) was nearly equal to that on the non-heated membrane (137.9o  1.3o) (see Figure S4 in Supporting Information). This means that the hydrophobicity of the membranes was not influenced by the operating temperature of solar cells. Figure 3(b) shows the GIXRD patterns of electrospun membranes prepared from PLA/PCL blends containing PCL at different contents. There are no peaks observed for the electrospun membranes prepared from 0% PCL (pure PLA) and 20% PCL, suggesting that the PLA solidified in amorphous forms. However, when the PCL content in the membranes was equal to or greater than 40%, two diffraction peaks appeared at 2Ɵ = 21.15° and 23.53°, which can be assigned to the (110) and (200) planes of PCL, respectively34. This indicates that the PCL 12 ACS Paragon Plus Environment

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solidified in crystal forms. It has been shown that the hydrophobicity of polymers increases as their crystallinity increases.35 Therefore, according to our GIXRD and contact angle results, the membranes based on crystalline PCL exhibited more hydrophobicity than the membranes based on amorphous PLA, consistent with previous reports in the literature35. The dependence of the ratio of the (110) and (200) peaks on the amount of PCL in the membranes is shown in the inset of Figure 3(b). Interestingly, the intensity ratios of the peaks increased with increasing amount of PCL and the highest intensity ratio was observed at 80 and 100% PCL (I(110)/I(200) ~3.2). This suggests that the PLA effectively controlled the growth of the crystalline PCL and that an increase in the amount of PLA in the electrospun membranes led to the reduction of the (110) plane and the formation of the (200) plane of PCL.

Figure 3. (a) Dependence of water contact angles on electrospun membranes on the PCL content in the membranes and (b) GIXRD patterns of electrospun membranes prepared from PLA/PCL blends containing PCL with different contents. The inset shows the dependence of the ratio of the (110) and (200) peaks of PCL on the amount of PCL in the membranes.



To examine the chemical interactions between PLA and PCL in electrospun membranes, ATR-FTIR experiments were performed. Figure 4 presents the ATR-FTIR spectra of electrospun membranes prepared from PLA/PCL blends. All membranes show similar patterns of ATR-FTIR spectra, consisting of the vibrational peaks of, for example, C-H (2941 cm-1), C=O (1726 and 1759 cm-1), C-O (1171 cm-1), and C-C (958 cm-1) bonds (see Figure 4(a)). However, the wavenumber of the C=O stretching peak of pure PLA (0% PCL) observed at 1759 cm-1 was shifted from that of pure PCL (100% PCL) observed at 1726 cm-1 because of their different structural properties36 (see Figure 4(b)). For the polymer blends, the C=O stretching bands of PLA and PCL still appeared at the same wavenumbers observed for pure PLA and pure PCL, and their intensities increased with increasing amounts of PLA and PCL in the membranes, respectively (see Figure 4(b)), implying that no chemical reaction occurred between PLA and PCL in the blend nanofibers and the structural properties of both polymers remained the same after blending.


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Figure 4. ATR-FTIR spectra of electrospun membranes prepared from PLA/PCL blends with different PCL contents; (a) wide spectral range (500–3500 cm-1) and (b) narrow spectral range (1300–1900 cm-1). The thermal properties of electrospun membranes were expected to strongly affect the long-term stability of PPSCs, especially during illumination, and they can be examined directly using thermogravimetric analysis (TGA). Figure 5 illustrates the TGA and derivative thermogram (DTG) curves of electrospun membranes prepared from PLA/PCL blends with different PCL contents. The characteristic thermal parameters of the electrospun membranes, i.e. the initial weight loss temperature (Ti) where 5% weight loss of the membranes occurs and the highest thermal degradation rate temperature (Tmax), were determined from the TGA and DTG curves. As shown in Figure 5(a), electrospun membranes prepared from the blends with 0, 20, 40, 60, 80, and 100% PCL started to degrade at Ti = 269, 274, 276, 278, 280, and 299°C, respectively. After that the membranes degraded rapidly and complete degradation was observed at 392, 445, 475, 485, 485, and 485°C, respectively. This indicates that increasing the amount of PCL in the membranes could enhance the thermal stability of the membranes. However, their thermal stability reached the maximum at the amount of PCL of 60% and remained the same at 80 and 100% PCL. Additionally, the membrane prepared from pure PLA (0% PCL) demonstrated a singlestep decomposition (Tmax = 311°C), corresponding to thermal scissions of C-C bonds in the polymer backbone (see Figure 5(b)). On the other hand, the membranes prepared from pure PCL and all the blends exhibited multi-step decomposition, involving the decomposition of amorphous PLA (zone I, Tmax = 304–311°C ), low-crystallinity PCL (zone II, Tmax = 362– 370°C), and high-crystallinity PCL (zone III, Tmax = 434–473°C) (see Figure 5(b)). Surprisingly, 15 ACS Paragon Plus Environment


the decomposition of low-crystallinity PCL (zone II) was not observed for the membranes prepared from the blends with a large amount of PLA, e.g. 20% PCL, whereas this decomposition was clearly observed for the membranes prepared from pure PCL and blends with a small amount of PLA, e.g. 40, 60, and 80% PCL. This confirms the XRD results that the amount of PLA in the membranes plays a critical role in the growth of the crystalline PCL.

Figure 5. (a) TGA and (b) DTG curves of electrospun membranes prepared from PLA/PCL blends with different PCL contents. The highlighted zones, I, II, and III in (b), represent the decomposition of amorphous PLA, low-crystallinity PCL, and high-crystallinity PCL, respectively. Next, perovskite layers were inkjet-printed on the mesoporous stack through the top of various electrospun membranes. The morphologies of perovskite layers are shown in Figure 6. Perovskite layers with flower-like structures were observed for the samples with electrospun membranes containing 0 (pure PLA), 20, 40, 60, and 80% PCL (Figures 6(a)–(e), respectively), while perovskite layers with square-shaped structures were observed for samples with an electrospun membrane containing 100% PCL (pure PCL) (Figure 6(f)). Obviously, the sizes of 16 ACS Paragon Plus Environment

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the flower-like and square-shaped structures were strongly correlated with the amount of PCL in the membranes. When the amount of PCL in the membranes increased from 0, 20, 40, 60, 80 to 100%, the sizes of the flower-like structures were gradually reduced, and finally, the flower-like structures were transformed into square-shaped structures with relatively smaller sizes (14.34  1.56 μm), leading to a reduction of perovskite layers surface coverage on the electrospun membranes to 82, 78, 72, 69, 62, and 44%, respectively. As shown in Figure S5 in Supporting Information, the degradation of the samples with high surface coverage of the capped perovskite layers (e.g. sample with the 20 and 40% PCL membranes) was significantly lower than that of the samples with low surface coverage of the capped perovskite layers (e.g. sample with the 80% PCL membrane), even the hydrophobicity of the membranes with small amounts of PCL was lower than that of the membranes with high amounts of PCL (see Figure 3a). This suggests that the capped perovskite layers were useful for improving the stability of the perovskite films underneath. It is speculated that the capped perovskite layers react with ambient moisture and change to PbI211, and the created PbI2 acts as very thin blocking layers to prevent the penetration of moisture into the devices, which can reduce the degradation of the perovskite films underneath and enhance the long-term stability of the devices.



Figure 6. SEM images of perovskite layers inkjet-printed on the TiO2/ZrO2/C mesoporous stack with electrospun membranes prepared from PLA/PCL blends with (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, and (f) 100% PCL. 3.2 Photovoltaic performance of PPSCs We explored electrospun membranes with the structure shown in Figure 1(b) as moistureresistant layers for PPSCs. The amount of infiltrated perovskite is expected to be directly proportional to the wetting of the membranes, whereas the moisture-resistance of the membranes is inversely proportional to their wetting37. Therefore, the optimum wetting of the membranes is required for efficient and stable PPSCs, and it can be determined by considering the PCEs and long-term stability of the devices prepared with different membranes. Figure 7(a) illustrates photocurrent density-voltage (J-V) characteristics for PPSCs with and without electrospun 18 ACS Paragon Plus Environment

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membranes measured under simulated AM 1.5 illumination of 100 mW/cm2. The shapes of the J-V plots for the devices prepared with different membranes were totally different, caused by the different photovoltaic parameters. The highest PCE of 8.20% with an open circuit voltage (VOC) of 0.928 V, short-circuit current density (JSC) of 20.63 mA/cm2, and fill factor (FF) of 0.428 was observed for the devices based on the 20% PCL membrane, while the lowest PCE of 2.63% with a VOC of 0.658 V, JSC of 14.43 mA/cm2, and FF of 0.277 was observed for the devices based on the 100% PCL membrane. As shown in Figure 7(b), the PCEs of the devices tended to decrease with the amount of PCL in the membranes or as the water contact angles of the membranes increased, implying that the wettability of the membranes strongly affected the performance of the devices. According to the SEM images (Figure 6) and the wetting of the membranes by perovskite precursor solution (Figure S6 in Supporting Information), the amount of the MAPbI3 absorber loaded into the low hydrophobic membranes, such as 0 and 20% PCL membranes, was considerably larger than that into the highly hydrophobic membranes, such as the 80 and 100% PCL membranes. This resulted in an increase in the number of photogenerated charge carriers and JSC,37 which was the main reason for the improved performance of the devices prepared with low hydrophobic membranes. Moreover, compared with the devices prepared using the 20% PCL membrane, the devices prepared without electrospun membranes showed a considerably lower PCE of 6.83% with a JSC of 19.97 mA/cm2, VOC of 0.902 V, and FF of 0.379. The JSC values of both devices were very close, whereas the reduced VOC and FF values of the devices without electrospun membranes can be ascribed to the increased defect concentration originating from moisture attack on the MAPbI3 absorber during device fabrication,38 as a result of having no moisture-protection layer. Furthermore, hysteresis effects in PPSCs prepared with and without the 20% PCL membrane were examined, as shown in Figure S7 in Supporting



Information. It was found that the devices with the electrospun membrane demonstrated less hysteresis than those without the membrane. Moreover, the large-area PPSCs (active area of 1 cm2) with the membrane clearly exhibited low hysteresis. This might be due to the suppressed decomposition of perovskite by the membrane, resulting in efficient charge transport in the devices.39,40

Figure 7. (a) Current density-voltage (J-V) characteristics under AM 1.5 G illumination for PPSCs with and without electrospun membranes prepared from PLA/PCL blends containing 20, 60, and 100% PCL. The inset of (a) shows the photovoltaic parameters obtained from the champion device with the 20% PCL membrane and (b) dependence of the device PCEs on the amount of PCL in the membranes (blue square) and on the average water contact angles of the membranes (red circle). 3.3 Long-term stability of PPSCs To investigate the influence of moisture-resistant electrospun membranes on the longterm stability of PPSCs, the PCEs of unencapsulated devices with and without electrospun membranes stored under highly humid conditions (RH ~80%, 30°C) were collected for over 35


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days. It should be noted that, after the devices were fabricated successfully, the devices were kept in plastic sample boxes that were placed in humid air during the stability test. As shown in Figure 8(a), the PCEs of the devices with electrospun membranes gradually decreased as the storage time increased, whereas the devices without electrospun membranes degraded quickly during the same storage time. Moreover, the devices based on the 20% PCL membrane demonstrated the highest stability towards moisture, the PCE was retained at 87% of the initial value after being stored for 35 days. In comparison, the PCE of the devices without electrospun membranes retained 53% of their initial value over the same time. This indicates that the optimum electrospun membranes (20% PCL and 80% PLA) not only greatly enhanced the device performance, but also considerably improved the long-term stability of the devices exposed in humid air due to their strong nonhygroscopic properties. The significant reduction of the PCEs of the devices without electrospun membranes might be caused by the rapid decomposition of the MAPbI3 absorber in ambient moisture. Therefore, the decomposition of MAPbI3 layers with and without the 20% PCL membrane as the moisture-resistant layer was investigated using GIXRD. Figure 8(b) illustrates the GIXRD patterns of MAPbI3 layers inkjet-printed on the mesoporous stack with and without electrospun membranes obtained before and after storage in humid air for 14 days. Most of the diffraction peaks matched very well with the characteristics of MAPbI3, while the only weak characteristics of PbI2 were observed.41 Naturally, the decomposition of MAPbI3 caused by moisture attack results in generation of PbI2.11 Therefore, comparing the intensity ratios of the PbI2 and (110) MAPbI3 peaks (hereafter, denoted as IPbI2 / IMAPbI3) between the samples before and after being stored in humid air could be a reasonable approach to determine the decomposition of MAPbI3.42



Before storing the samples in humid air, the IPbI2 / IMAPbI3 ratios of the perovskite layers inkjet-printed on the mesoporous stack with and without the 20% PCL membrane were 0.05 and 0.08 respectively. The value obtained from the sample with the membrane was slightly lower than that obtained from the sample without the membrane since the decomposition of the MAPbI3 layer during its preparation processes was limited by the membrane. After storing the samples in humid air for 2 weeks, the samples with the 20 % PCL membrane demonstrated only a slight increase in the IPbI2 / IMAPbI3 ratio (IPbI2 / IMAPbI3 = 0.13). On the other hand, the perovskite sample prepared without the electrospun membrane showed a dramatic increase in the IPbI2 / IMAPbI3 ratio (IPbI2 / IMAPbI3 = 0.40). This strongly suggests that the electrospun membranes acted as highly efficient moisture-resistant layers for preventing MAPbI3 degradation.

Figure 8. (a) PCE evolution as a function of the storage time in humid air (RH ~80%) of unencapsulated PPSCs with and without electrospun membranes. (b) Grazing incidence X-ray diffraction patterns for perovskite layers inkjet-printed on the TiO2/ZrO2/C mesoporous stack with the 20% PCL membrane (left) and without electrospun membranes (right), which were


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obtained on the first day of sample preparation and after storing the samples in humid air for 2 weeks. 4. CONCLUSIONS In summary, we have proposed a novel idea to enhance the performance and long-term stability of PPSCs in humid air by using electrospun hydrophobic polymer membranes, i.e. PLA, PCL, and PLA/PCL blends, as moisture-resistant layers for the devices. Our device structure was adapted from the structure reported by Ku et al.9 by depositing the electrospun membranes on the top of the carbon electrode in the devices prior to perovskite infiltration. The morphologies, wettability, and thermal properties of the prepared electrospun membranes were systematically investigated using various techniques, including SEM, water contact angle measurement, GIXRD, FTIR, and TGA. After optimizing the properties of the membranes by varying the contents of the polymer components in the membranes, the PCEs and moisture stability of the devices with the optimum membranes (20% PCL and 80% PLA) were considerably enhanced compared with those of the devices without the membranes. The considerable enhancement of the performance and moisture stability of the devices with the electrospun membranes was mainly caused by the stabilization of perovskite absorbing layers with the moisture-resistant membranes. Our work demonstrated that the use of electrospun hydrophobic polymer membranes as moisture-resistant layers for PPSCs is an effective strategy for the fabrication of efficient and stable solar cells in humid air, which could pave the way for commercial photovoltaic technologies. AUTHOR INFORMATION Corresponding Author



* E-mail: [email protected]. Notes The authors declare no competing financial interest. Acknowledgment PP is grateful to the Development and Promotion of Science and Technology Talent Project (DPST, Thailand) and the Thailand Research Fund (MRG5980013) for their support. AT gratefully acknowledges the Thailand Research Fund for TRF Research Team Promotion Grant (RTA 6180004). SUPPORTING INFORMATION Top-view and cross-sectional SEM images of electrospun PLA membranes prepared at different electrospinning times. PCE Evolution as a function of the storage time of devices with PLA membranes prepared at electrospinning times. Water contact angles on the 20% PCL membrane obtained before and after heating the membrane at 60 oC for 10 min. XRD patterns of samples with different capped perovskite layers. Contact angles of perovskite precursor solution on electrospun membranes. REFERENCES (1) Kojima, A.; Techima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050–6051.


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TOC Graphic



Figure 1. Schematic illustration of (a) the electrospinning setup and (b) cross-sectional structure of PPSCs with electrospun polymer membranes. 277x98mm (150 x 150 DPI)

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Figure 2. SEM images of electrospun polymer membranes prepared from PLA/PCL blends containing (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, and (f) 100 wt% PCL. The insets present the diameter distribution of the nanofibers in the electrospun membranes. 95x123mm (150 x 150 DPI)



Figure 3. (a) Dependence of water contact angles on electrospun membranes on the PCL content in the membranes and (b) GIXRD patterns of electrospun membranes prepared from PLA/PCL blends containing PCL with different contents. The inset shows the dependence of the ratio of the (110) and (200) peaks of PCL on the amount of PCL in the membranes. 309x127mm (150 x 150 DPI)


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Figure 4. ATR-FTIR spectra of electrospun membranes prepared from PLA/PCL blends with different PCL contents; (a) wide spectral range (500–3500 cm-1) and (b) narrow spectral range (1300–1900 cm-1). 113x94mm (150 x 150 DPI)



Figure 5. (a) TGA and (b) DTG curves of electrospun membranes prepared from PLA/PCL blends with different PCL contents. The highlighted zones, I, II, and III in (b), represent the decomposition of amorphous PLA, low-crystallinity PCL, and high-crystallinity PCL, respectively. 415x172mm (150 x 150 DPI)


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Figure 6. SEM images of perovskite layers inkjet-printed on the TiO2/ZrO2/C mesoporous stack with electrospun membranes prepared from PLA/PCL blends with (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, and (f) 100% PCL. 165x96mm (150 x 150 DPI)



Figure 7. (a) Current density-voltage (J-V) characteristics under AM 1.5 G illumination for PPSCs with and without electrospun membranes prepared from PLA/PCL blends containing 20, 60, and 100% PCL. The inset of (a) shows the photovoltaic parameters obtained from the champion device with the 20% PCL membrane and (b) dependence of the device PCEs on the amount of PCL in the membranes (blue square) and on the average water contact angles of the membranes (red circle). 464x170mm (150 x 150 DPI)


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Figure 8. (a) PCE evolution as a function of the storage time in humid air (RH ~80%) of unencapsulated PPSCs with and without electrospun membranes. (b) Grazing incidence X-ray diffraction patterns for perovskite layers inkjet-printed on the TiO2/ZrO2/C mesoporous stack with the 20% PCL membrane (left) and without electrospun membranes (right), which were obtained on the first day of sample preparation and after storing the samples in humid air for 2 weeks. 352x147mm (150 x 150 DPI)


Moisture-Resistant Electrospun Polymer Membranes for Efficient and

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