Encapsulation for improving the lifetime of flexible perovskite solar cells

Encapsulation for improving the lifetime of flexible perovskite solar cells

Nano Energy (]]]]) ], ]]]–]]] 1 Available online at www.sciencedirect.com 3 5 journal homepage: www.elsevier.com/locate/nanoenergy 7 9 COMMUNICAT...

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Nano Energy (]]]]) ], ]]]–]]]

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Available online at www.sciencedirect.com

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COMMUNICATION

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Encapsulation for improving the lifetime of flexible perovskite solar cells

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Hasitha C. Weerasinghea,n, Yasmina Dkhissib, Andrew D. Scullya, Rachel A. Carusoa,b, Yi-Bing Chengc a

Commonwealth Scientific and Industrial Research Organization (CSIRO), Manufacturing Flagship, Clayton, Victoria 3168, Australia b PFPC, School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia c Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia Received 12 June 2015; received in revised form 14 September 2015; accepted 9 October 2015

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KEYWORDS Q3 Perovskite solar cells;

Encapsulation; Barrier films; Stability; Lifetime

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Abstract The effect of encapsulation on improving the operational lifetime of flexible perovskite-based solar cells prepared on polymer substrates is presented. The devices were fabricated on polyethylene terephthalate films coated with indium-doped zinc oxide substrates. Mesoporous TiO2 nanoparticles were used as the electron-transport layer and 2,20 ,7,70 -tetrakis-(N,N-di-pmethoxyphenylamino)-9,90 -spirobifluorene as the hole-transport layer. The stability of nonencapsulated devices and devices encapsulated using two different architectures, referred to in the present work as ‘partial’ and ‘complete’ encapsulation, were evaluated on exposure to ambient conditions. The lifetime of the encapsulated flexible perovskite solar cell devices was extended significantly compared with that of the non-encapsulated devices. Permeation testing revealed that the post-encapsulation ingress of moisture through the adhesive layers and around electrical contacts constitutes a significant lifetime-limiting factor. Impedance spectroscopy indicates a gradual increase in the charge-transfer resistance at one of the device interfaces during degradation. These findings highlight the importance of continued development of encapsulation architectures to further prolong device lifetime. & 2015 Published by Elsevier Ltd.

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Introduction

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Corresponding author. Tel.: +61 3 95457829. E-mail address: [email protected] (H.C. Weerasinghe).

Organic–inorganic hybrid perovskite compounds have recently attracted great attention in the field of photovoltaic research due to their superior light-harvesting characteristics. After the

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http://dx.doi.org/10.1016/j.nanoen.2015.10.006 2211-2855/& 2015 Published by Elsevier Ltd.

Please cite this article as: H.C. Weerasinghe, et al., Encapsulation for improving the lifetime of flexible perovskite solar cells, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.006

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first perovskite-based solar cell (PSC) device reported by Miyasaka et al. in 2009 [1], tremendous progress in the performance of PSCs has been made [2–4]. Owing to the recently reported power conversion efficiency (PCE) values over 18% for laboratory-scale devices fabricated on rigid glass substrates by several research groups, perovskite-based photovoltaic devices are now considered as a highly promising nextgeneration low-cost photovoltaic technology [5–7]. High efficiencies reported for this type of device have been attributed to the strong light absorption of the perovskite layer and the dissociation of weakly-bound excitons into free carriers having large diffusion length [8,9]. Perovskite-based solar cells are usually fabricated on a rigid glass substrate with a TiO2 layer processed at high temperature (4450 1C) used as an electron-transporting (hole-blocking) layer. Although such devices have shown higher performance, fabrication of PSCs on polymer-based flexible substrates using lower-temperature processing techniques is considered to be more appealing for high-throughput manufacture of PSC modules in a roll-to-roll system [10,11]. Flexible PSC devices displaying PCEs of over 10% have been reported by several groups [12,13]. Despite the substantial progress in efficiency and effective transfer of the technology onto flexible substrates, the major challenges of PSCs that hinder commercialization of this technology in the near future are the toxicity of the water-soluble lead compounds and the poor device lifetime. Recently, the lead-containing perovskite layer has been replaced by lead-free materials to produce PSCs having reasonably high efficiencies [14,15]. However, the poor stability of the highly efficient lead-based PSC devices, and that of the leadfree PSCs, has not yet been fully addressed. If lead free materials suffer the same stability issues as lead based perovskites, then understanding the effect of encapsulation on the stability of the lead-based system will be helpful for future encapsulation of the lead-free system. The performance of PSC devices is known to be highly susceptible to deterioration upon exposure to ambient atmospheric conditions; thus the preparation of PSC modules having adequately long operating lifetimes for their intended end-use remains a major challenge. Developing encapsulation technologies to limit the exposure of PSCs to moisture and oxygen is imperative. In the present work, various commercially available flexible plastic barrier materials and new encapsulation protocols were investigated in order to inhibit the ingress of moisture and oxygen. The lifetime of PSCs was improved substantially through encapsulation using films having good moisture-barrier properties. The shelf-life of encapsulated flexible PSC devices prepared using an indiumdoped zinc oxide-coated polyethylene terephthalate (IZO-PET) substrate could be extended to more than 500 h under ambient conditions. Thin films of deposited metallic Ca were employed to identify possible moisture/oxygen ingress paths in the encapsulated systems.

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Experimental

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Device fabrication and encapsulation

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Perovskite solar cell devices having efficiencies of more than 10% were fabricated on IZO-PET substrates using recently published methods [13,16]. The precursor preparation for the perovskite and hole-transporting layers, and the

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spin-coating steps of the perovskite layer and the holetransport layer (2,20 ,7,70 -tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene, spiro-OMeTAD) were carried out in a dry-box to minimize any interaction with moisture/oxygen during the device preparation. The perovskite was deposited by spin coating 25 mL of a 40 wt% solution of CH3NH3I and PbI2 (molar ratio 1:1) in DMF at 6500 rpm for 30 s. A 60 psi dry N2 gas stream was blown onto the film for 10 s from the third second of spinning. The films were subsequently annealed on a hot plate at 100 1C for 10 min in the dry box. After letting the films cool for 5 min, 20 mL of a spiro-MeOTAD solution in chlorobenzene (68 mM spiro-MeOTAD, 150 mM t-bp and 25 mM lithium bis (trifluoromethanesulphonyl)imide) (Li-TFSI) was spin coated at 3000 rpm for 30 s. An 80 nm-thick gold layer was evaporated under high vacuum onto the spiro-OMeTAD layer to complete the device fabrication. Devices were stored in a N2-filled glovebox maintained at moisture and oxygen levels below 0.1 ppm until encapsulation. Viewbarriers (Mitsubishi Plastic, Inc) was used as the plastic barrier encapsulant film for the metal-electrode side of the device. According to the manufacturer, the water vapor transmission rate(WVTR), overall thickness and transparency in the visible spectrum of this material are 5  10  3 g m  2 day  1, 85 mm, and 89%, respectively. Typically, commercially-available plastic barrier encapsulant films are not supplied with an integrated adhesive layer. Therefore, sealing materials were required to complete the encapsulation process by bonding the barrier materials onto the device. A transfer adhesive pre-coated on a paper liner (467 MP 3M™ Adhesive Transfer Tape, is a transparent 60 mm thick acrylic adhesive on polycoated Kraft paper liner) was used as the sealing material for this work and was firstly laminated onto the barrier film, which was then cut to the required sizes. Barrier films with the laminated adhesive layer were then pre-conditioned as described previously to minimize entrapped moisture and oxygen contained within the encapsulation materials [17]. After 12 h of drying process under vacuum, the moisture content of the encapsulation materials was evaluated using an Arizona Instruments moisture-content analyzer (Computrac Vapor Pro) and was bellow 1 pm. Following the pre-conditioning step, and after removing the paper liner, the barrier film integrated with the adhesive was laminated at 100 1C onto the flexible PSC devices using an office-type laminator (Peach 3500). The encapsulation process was carried out in a N2filled glovebox at moisture and oxygen levels below 1 ppm. The two encapsulation architectures adopted in this work are illustrated in Figure 1. For the ‘partially’-encapsulated devices the electrical contacts are made through direct contact with the devices, whereas the electrical connections for the ‘completely’-encapsulated devices are made via thin copper wires soldered onto the modules. Device performance was measured before and after encapsulation, and encapsulated devices showed no noticeable change in performance due to the lamination process.

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Device analysis

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All devices were stored under ambient conditions. The average ambient temperature was 22.570.2 1C, and the

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Please cite this article as: H.C. Weerasinghe, et al., Encapsulation for improving the lifetime of flexible perovskite solar cells, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.006

Encapsulation for improving the lifetime

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Flexible Perovskite Solar Cell Device

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Flexible Barrier Encapsulant Film with integrated Adhesive

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Figure 1 Schematic representation of (a) ‘partial’ and (b) ‘complete’ encapsulation architectures.

relative humidity in the laboratory ranged from 30% to 80%. Current–voltage (J–V) characteristics of the devices were measured using a Keithley 2400 source meter under illumination of simulated sunlight provided by an Oriel solar simulator equipped with an AM 1.5 G filter. Devices were covered by a black metal mask having an aperture of 0.16 cm2. As the device efficiency of the PSC devices increased upon initial light exposure, device performance was recorded after exposing the device to light for 5–10 min and carrying out several scans until a constant efficiency was observed. J–V measurements were carried out in the 0– 1.2 V range with a 100 mV s  1 scan rate from forward bias to short-circuit. Post-mortem analyses of devices were performed at the end of the lifetime study. Cross-sectional images were obtained using a Nova Dual Beam FIB-SEM and cystallography data was obtained using a Philips vertical diffractometer with Cu Kα radiation. The electrical properties of the PSC devices were characterized using Impedance spectroscopy (IS). IS measurements were carried out under dark conditions using an electrochemical workstation (RST5200, Zhengzhou Shiruisi Instrument Co., Ltd.) with no DC-biased voltages over the frequency range of 0.1 Hz to 1 MHz, with a 10 mV perturbation voltage. The IS experimental data was analyzed using EC-Labs software.

Results and discussion

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The photovoltaic properties of the flexible PSC devices (IZOPET/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au) were analyzed by measuring the J–V characteristics under a light intensity of 100 mW cm  2 (1.5 AM). Typical J–V curves obtained when scanning from short-circuit to forward bias (SC-FB), and from forward bias to short-circuit (FB-SC), are shown in Figure 2. The performance of the flexible PSC devices was comparable with PSC devices fabricated on glass substrates, with mean values of PCE, short-circuit current (JSC), open-circuit voltage (VOC), and fill-factor (FF) of 1272%, 16.870.8 mA cm  2, 1020730 mV, and 0.6970.09, respectively. Significant hysteretic behavior

79 Figure 2 Current–voltage curves of a perovskite-based solar cell devices under 1-sun illumination scanned from short-circuit to forward-bias (SC-FB, a) and from forward-bias to shortcircuit (FB-SC, b) at a scan rate of 100 mV s  1. Inset lists the photovoltaic parameters for the device.

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was observed in the J–V curves, and the device performance metrics derived from J–V measurements were dependent on the scan direction. Although the short-circuit current and the opencircuit voltage were relatively unaffected by the scan direction, the fill factor was lower when scanning in the SC-FB direction. This hysteretic behavior can be interpreted as a consequence of the presence of defects acting as traps for charge carriers, polarization effects upon applied bias, or build-up of charges due to mobile cations [18,19]. Since first reported, the hysteretic phenomenon in perovskite solar cells has been a subject of debate and a consensus on its origins has yet to be met. More recently, the J–V scan hysteresis was found to be accompanied by a time-dependent photocurrent response, which was attributed to the capacitive characteristic of CH3NH3PbI3 [20]. The ferroelectric polarization of CH3NH3PbI3 induced by CH3NH3+ reorientation and lattice distortion effects is also strongly suspected to be correlated with the reported transient photocurrent [21,22]. The primary objective of the present study was to investigate the effect of encapsulation on the durability of flexible PSC devices. Accordingly, all devices were stored under ambient laboratory conditions, and their performance was monitored by periodically measuring their J–V characteristics. J–V analysis of each device immediately after encapsulation showed no significant change in the J–V characteristics due to the encapsulation process. Figure 3 shows the key device parameters for the non-encapsulated, ‘partially’ and ‘completely’ encapsulated devices normalized to their initial values and plotted as a function of the storage time under ambient conditions. Upon exposure to ambient humidity at room temperature, non-encapsulated devices exhibited a rapid deterioration in JSC, VOC and fillfactor, and had limited effectiveness after 100 h. The degradation of CH3NH3PbI3 on exposure to atmospheric moisture is well known [3,23], although the details of the degradation mechanism of PSC devices are yet to be fully elucidated. Through detailed experiments, Niu et al. have shown that CH3NH3PbI3 can degrade in the presence of

Please cite this article as: H.C. Weerasinghe, et al., Encapsulation for improving the lifetime of flexible perovskite solar cells, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.006

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Figure 3 Normalized current-voltage parameter dependence of non-encapsulated, ‘partially’-encapsulated, and ‘completely’encapsulated perovskite solar cell devices as a function of storage time under ambient conditions.

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moisture and sunlight; with PbI2 and I2 identified as degradation products [24]. Thus, to improve the lifetime of PSC devices, either the intrinsic stability of the perovskite devices needs to be improved by introducing alternative air-stable (i.e. less moisture sensitive) lightabsorbing perovskite derivatives, or extrinsic approaches such as device encapsulation using moisture barrier films are required to restrict the exposure of the perovskite material to permeating water vapor. Although devices fabricated using alternative perovskite absorbing layers, such as CH3NH3PbI2Br and CH3NH3PbBr3, have been reported recently [25] to display improved atmospheric stability, their efficiencies were much lower than devices incorporating the widely studied CH3NH3PbI3 (MAI) perovskite. Since high efficiencies have already been routinely demonstrated for CH3NH3PbI3 PSC devices, a question of significant interest is the extent to which applying flexible barrier encapsulation can improve the lifetime of MAI PSC devices fabricated on a flexible substrate without compromising their efficiency. As illustrated in Figure 3, significant enhancement in the device lifetime is observed for the ‘partially’-encapsulated and ‘completely’-encapsulated devices stored under the ambient humidity and temperature conditions compared with non-encapsulated control devices. ‘Partially’ encapsulated devices retained more than 80% of their initial PCE for over 400 h, with a rapid performance loss observed after 400 h. In the ‘partial’-encapsulation architecture, the devices are well protected by the barrier films against moisture permeation through the front and back sides of the devices, but continuous ingress of moisture is expected via unprotected edges of the PET device substrate and transfer adhesive layers. The devices encapsulated using the ‘complete’-encapsulation architecture were stable over

the duration of the entire storage period in the present work (500 h).

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Evaluation of encapsulation performance

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In order to investigate the routes for moisture ingress into the encapsulated devices, evaporated films of Ca on PET substrates were encapsulated using the ‘partial’ and ‘complete’ encapsulation architectures and the appearance of the encapsulated Ca sensors were monitored during exposure to ambient conditions for around 4 weeks. The pristine metallic Ca film has a mirror-like appearance that became transparent in those regions through which moisture and/or oxygen could have permeated due to the reaction of the Ca with moisture and oxygen to form CaO and Ca(OH)2. The resulting pattern of transparency of the Ca film reveals the pathways for moisture ingress. The results of the Ca tests carried out using the two encapsulation architectures utilized in this work are shown in Figure 4. As seen clearly in Figure 4, the primary path of moisture ingress for the ‘partially’-encapsulated Ca film was via the edges, presumably mainly through the adhesive layer. A thin copper wire was co-encapsulated with the ‘completely’ encapsulated Ca film in order to emulate the copper wire connections used in the ‘completely’-encapsulated devices. The rate at which moisture reaches the Ca film in this case was much slower than for the ‘partial’-encapsulation system. However, evidence for the onset of moisture/oxygen ingress to the Ca film via the region around the copper wire was observed after a storage period of around 600 h (Figure 4), implying incomplete contact between the adhesive layer and the wire. Ca tests for the ‘completely’-encapsulated Ca film without co-encapsulating the copper wire indicated the onset

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Please cite this article as: H.C. Weerasinghe, et al., Encapsulation for improving the lifetime of flexible perovskite solar cells, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.006

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Figure 4 Optical transmission photographs showing the loss of Ca film (dark area) of ‘partially’-encapsulated (upper panel) and ‘completely’-encapsulated (lower panel) Ca films as a function of storage time at ambient conditions.

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Figure 5 FIB-SEM cross-sectional images, XRD spectra, and photos (inset) of (a) pristine non-encapsulated, (b) aged nonencapsulated, (c) aged ‘partially’-encapsulated, and (d) aged ‘completely’-encapsulated perovskite solar cell devices fabricated on IZO-PET substrates. Post-mortem FIB/SEM and XRD analyses of the aged samples were carried out after 1500 h of storage.

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of ingress of moisture/oxygen via the edges after storage for around 1000 h (results not shown).

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Post-mortem device analysis The cross-sectional SEM image and X-ray diffraction (XRD) pattern of a pristine non-encapsulated device are shown in Figure 5(a). As depicted in Figure 5(a), the perovskite film in a fresh non-encapsulated device exhibited the peaks characteristic of the tetragonal phase of CH3NH3PbI3 on an IZO-PET substrate. As seen in the photographic image and the XRD pattern in Figure 5(b), the perovskite film in the nonencapsulated device lost its characteristic dark color and its crystallinity after 500 h storage. The only crystalline peaks evident for the aged non-encapsulated device were those associated with the IZO-PET substrate and Au electrode. In the aged (4500 h) encapsulated devices, given the nature of the encapsulation, the Au layer was peeled off upon the removal of the encapsulation film that integrated with the adhesive layer during the preparation of samples for the XRD and SEM analysis. For the ‘partially’-encapsulated device, the perovskite degradation was evident from the SEM image of the device cross-section, and XRD showed that the perovskite layer contained CH3NH3PbI3 and PbI2. The initially 80-nm thick TiO2 layer swelled significantly upon storage for 4500 h at ambient conditions for the non-encapsulated and ‘partially’encapsulated devices. Such swelling was not observed in the ‘completely’-encapsulated device. The cross-sectional SEM and photographic images of the aged ‘completely’-encapsulated device shown in Figure 5(d) show no sign of degradation after storage for 1500 h. Consistent with these images, the 110 peak associated with the CH3NH3PbI3 layer also remained clearly evident in the XRD pattern. These results indicate that the lifetime of the flexible PSC devices can be increased considerably by using simple barrier encapsulation protocols. In order to investigate the effect of spiro-MeOTAD on the device degradation, devices were constructed with and without spiro-MeOTAD in the absence of any additives, and the stability of these two types of devices was compared with the devices with spito-MeOTAD and Li-TFSI dopant. All these three types of devices were encapsulated using ‘partial’ encapsulation architectures and found completely degraded after 500 h under ambient conditions. This suggests that for the partially-encapsulated devices stored under ambient conditions, device degradation could primarily be due to the degradation of the perovskite layer and/or due to the interfacial changes associated with the perovskite layer.

‘completely’-encapsulated devices exhibited a clear drop in the efficiency after 1500 h, no distinguishable changes such as delamination or swelling of the deposited layers were evident in the SEM images of the aged ‘completely’encapsulated devices ( Figure 5(d)) compared to the pristine devices ( Figure 5(a)). Thus, it can be postulated that the primary device degradation may occur due to subtle changes in the interfaces. Thus, impedance spectroscopy was carried out to track device degradation through changes in the interfacial electrical properties. IS measurements of encapsulated PSC devices showed no change after storage under ambient conditions for 100 h, and were not analyzed further. The Nyquist plots of nonencapsulated perovskite devices obtained initially and after storage are shown in Figure 6. The Nyquist plot consists of a combination of a lower frequency semi-circle and a semi-circle at higher frequencies having a smaller radius, and can be represented by resistive (R) and constant-phase (CPE) equiv alent-circuit elements at low (LF) and high (HF) frequencies, RLFCPELF and RHFCPEHF, respectively. These two semi-circles possibly represent two different interfaces in the device having two different characteristic response times given by their RCPE time constant. The constant-phase elements (CPEs) relate to the non-ideal behavior of the capacitors and are often used instead of an ideal capacitor in IS analysis of photovoltaic devices due to the inhomogeneity of the interfaces [28,29]. The CPEs can be defined by a capacitive contribution, CPE-T, and the quality factor, CPE-P, where the value of the latter is for an ideal capacitor. An ideal fit was observed for the equivalent circuits shown in the inset of Figure 6. In these circuits RS represents the Ohmic resistance and is largely related to the sheet resistance of the substrate. The R and CPE-T values obtained from fitting of the IS data before and after storage for 2 h and 20 h under ambient conditions are listed in Table 1. The magnitude of RLF increased considerably after storage, whereas the magnitudes of RHF, CPE-THF and CPETLF remained essentially unchanged. Therefore, it is speculated that the observed increase in the magnitude of RLF upon storage is could be due to the increase of interfacial

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Impedance spectroscopy (IS) has been used to study the resistive and capacitive components of the PSC devices in more detail [17,26,27]. In these measurements, a small amplitude AC voltage is applied to the device and the current response is measured as a function of AC frequency, under steady-state conditions. A quantitative evaluation of the resistive and capacitive elements in the device requires the use of equivalent-circuit analysis for a given device structure. The relevant equivalent circuits applied in the present work are shown in Figure 6. Although the

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117 Figure 6 Impedance spectra of non-encapsulated perovskite solar cell devices measured initially, and after storage for 2 h and 20 h. The triangles indicate the experimental data, and the lines are the semi-circles recovered from analysis using ECLabs software with the equivalent circuits shown in the inset. Spectra were measured under dark conditions with 0 V DC-bias voltage.

Please cite this article as: H.C. Weerasinghe, et al., Encapsulation for improving the lifetime of flexible perovskite solar cells, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.10.006

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Table 1 Best-fit parameter values obtained from analysis of impedance spectra for the non-encapsulated perovskite solar cell devices as a function of storage time.

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13 resistance of a certain interface of the device during degradation. Any increase in interfacial-resistance can restrict the intrinsic charge flow of the device and hinder the device performance. Further studies are currently underway to elucidate the mechanism of aging that causes an increase in the LF impedance and identify the affected interface/s in order to better understand the exact physical origins of degradation of the PSC devices.

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Encapsulation of PSC devices using high-quality barrier materials and simple encapsulation architectures can significantly extend their lifetime under ambient storage conditions. Ca film tests demonstrated the significant role of moisture/oxygen ingress via adhesive layers and around embedded electrical wire contacts, highlighting the need for further improvements in encapsulation architectures to achieve PSC lifetimes that meet commercial end-use requirements. Impedance spectroscopy measurements on nonencapsulated PSC devices indicate that their internal resistance increases during exposure to ambient moisture and oxygen conditions suggesting that the loss of device performance is associated with the formation of more resistive interfaces within the device. These preliminary results illustrate the potential utility of impedance spectroscopy in elucidating the degradation mechanism of perovskite devices, and further studies are underway to better understand the degradation processes of PSC devices. The results of this study illustrate the technical feasibility of substantially extending the lifetime of PSC devices by employing barrier encapsulant materials and appropriate barrier encapsulation architectures. This finding is of considerable practical importance for the design of large-scale, roll-to-roll encapsulation systems and processes for the manufacture of printed perovskite-based solar cell modules in the future.

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Acknowledgments

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This research was supported by funding through a Sustainable Energy Research and Development Grant and a Victoria's Science Agenda Grant from the Victorian Government Department of Primary Industries and Department of Business and Innovation Victoria State Government, Australia, respectively, together with financial support from the Australian Renewable Energy Agency Australian Government, Australia (ARENA)

through the Australian Centre for Advanced Photovoltaics (ACAP). HW also gratefully acknowledges a Postdoctoral Fellowship from ARENA. The authors are grateful to the Melbourne Advanced Microscopy Facility at The University of Melbourne for providing access to electron microscopy facilities and support from a Melbourne Research Grant Support Scheme.

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