Impact of Selective Contacts on Long-Term Stability of CH3NH3PbI3

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Impact of Selective Contacts on Long-Term Stability of CHNHPbI Perovskite Solar Cells 3

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Hui-Seon Kim, Ja-Young Seo, and Nam-Gyu Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09412 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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The Journal of Physical Chemistry

Impact of Selective Contacts on Long-Term Stability of CH3NH3PbI3 Perovskite Solar Cells Hui-Seon Kim, Ja-Young Seo and Nam-Gyu Park*

School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea

*Corresponding author N.-G. P.: E-mail: [email protected], Tel: +82-31-290-7241. Fax: +82-31-290-7272

Current address of Hui-Seon Kim: Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland

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Abstract Instability of organic-inorganic halide perovskite solar cells (PSCs) under continuous light illumination in ambient air atmosphere has been issued. Efforts have been made to improve the PSC stability by chemical engineering of perovskite or device encapsulation. However, most of attentions have been paid to perovskite material itself rather than its dynamic interaction with selective contacts. In this study we report on the impact of selective contacts on long-term stability. When the unencapsulated PSC with mesoscopic structure, bl-TiO2/mpTiO2/CH3NH3PbI3 (MAPbI3)/spiro-MeOTAD, was illuminated in ambient air atmosphere, photocurrent was rapidly declined despite less degraded absorbance. Impedance spectroscopic measurements confirmed that a new semicircle appeared at high frequency with exposure time, which was indicative of formation of a new layer at the MAPbI3/spiroMeOTAD interface inhibiting charge collection. The device prepared in ambient air atmosphere was encapsulated in N2 using UV-curable sealant, which demonstrated no deterioration in absorbance but continual decrease in photocurrent, photovoltage and fill factor with exposure time up to 300 h. This indicates that photo-induced barrier at MAPbI3/spiro-MeOTAD interface could not be effectively overcome by encapsulation. Replacement of HTM layer from spiro-MeOTAD to NiO was found to be an effect way not to create the barrier layer, which eventually improved long-term stability. Our results clearly suggest that the selective contacts play a critical role in long-term stability of PSCs.


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Introduction Since the perovskite solar cell demonstrated a high initial power conversion efficiency (PCE) of 9.7% with 500 h stability (ex-situ) in 2012,1 the efficiency has been dramatically increased reaching 22.1% in 2016.2 From the aspect of a practical use of the perovskite solar cells, their long-term stability should be established in parallel with high efficiency. In this regard, the stability problem has been recently raised as a critical issue and many efforts have been made to improve the stability of perovskite solar cells. However, the most of studies focused on the intrinsic stability of perovskite itself. The stability of perovskite under various condition (dark, light, moisture, vacuum, N2, O2, etc.) was investigated to understand the decomposition mechanism.3-10 Chemical engineering of perovskite material directly enhanced the interaction energy between atoms, resulting in a significant improvement of moisture- and photo-stability.11-13 Furthermore, passivation layers were also introduced to increase the moisture resistivity indirectly.14,15 The study on the intrinsic stability of perovskite is important in terms of understanding the material and suggesting the efficient way to improve stability. Nevertheless, the stability of device employing perovskite is even more important because the device is not only the most analogous form with a commercialization product but also capable of considering the interplay between perovskite and its adjacent component such as selective contacts. During solar cell operation the bias voltage at maximum power point should be applied, which encourages the ions in perovskite to migrates toward the selective contact to compensate the external electric field.16-18 The most common device structure is composed of fluorine-doped tin oxide (FTO)/blocking layer TiO2 (bl-TiO2)/mesoporous TiO2 (mp-TiO2)/CH3NH3PbI3 (MAPbI3)/2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene

(spiro-

MeOTAD)/Ag or Au. Besides, the best PCE was also obtained with this structure.19 The migrated ions were reactive to the particular selective contacts such as TiO2 and spiroMeOTAD leading to chemical reaction20,21 and I-V hysteresis,22-24 which are definitely unfavorable for long-term stability. In this study, we report on the impact of selective contacts on long-term stability of perovskite solar cell. We have investigated long-term stability of the unencapsulated and the encapsulated devices with FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD in ambient air. Degradation mechanism is monitored by impedance spectroscopy to understand the effect of selective contacts. Imperfect stability even from the encapsulated device is found to be 3 ACS Paragon Plus Environment


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related to interfacial charge transport changed with light exposure time, which is overcome by changing the selective contacts from mp-TiO2 and spiro-MeOTAD to NiO and PCBM.

Experimental methods Chemicals Methylammounium iodide (MAI) 30 mL of hydroiodic acid (57 wt% in H2O, Aldrich) was slowly added to 28 mL of methylamine (40% in methanol, TCI). The mixture was stirred for 2 h in an ice bath and evaporated to obtain the product. The product was stirred with ether for 15 min three times. The washed product was dissolved in methanol to make saturated solution which was added to ether drop by drop. The precipitates were filtered and dried in a vacuum oven overnight. Device fabrications Transparent conducting oxide (FTO, Pilkington, TEC-8, 8 Ω/sq or ITO, Buwon, 7 Ω/sq) substrates were washed using a neutral detergent and sonicated sequentially in ethanol, acetone and ethanol bath for 15 min. The washed substrates were treated with UV-ozone for 20 min before depositing materials. FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au structure. For the bl-TiO2, 0.15 M titanium diisopropoxide bis(acetylacetonate) (75wt% in 2-propanol, Aldrich) was prepared with 1buthanol (99.8%, Aldrich) as a solvent and deposited on the cleaned FTO substrates by spincoating method (WS-650-23, Laurell). The mp-TiO2 layer was subsequently deposited on the bl-TiO2 layer by spin-coating of mp-TiO2 solution where 0.1 g of TiO2 paste (50 nm-sized TiO2 particle:terpineol (Aldrich):ethyl cellulose (Aldrich):lauric acid (Fluka)=1.25:6:0.9:0.1) was dispersed in 1 mL of 1-buthanol. The TiO2-depositied films were annealed at 550 oC for 1 hour. 1.78 M MAPbI3 precursor solution was prepared with dimethylformamide (DMF, 99.8%, Aldrich) as a solvent where MAI:PbI2 (99.9985%, Alfa Aesar):dimethyl sulfoxide (DMSO, 99.8%, Aldrich) was 1:1:1 in molar ratio. The precursor solution was dropped on the TiO2 substrate and spun where diethyl ether was dropped during spinning, which was followed by heating at 65 oC for 1 min and 100 oC for 30 min sequentially. The HTM solution included 56.4 mM 2,2’,7,7’- spiro-MeOTAD (Feiming Chemical Limited), 29.9 mM bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI, 99.95%, Aldrich) and 188 mM 4-tertbutylpyridine (TBP, 96%, Aldrich). The HTM solution was dropped while spinning. Au (99.99%) was deposited on the top of spiro-MeOTAD layer using a thermal evaporator at ca. 10-6 torr. FTO/bl-TiO2/MAPbI3/spiro-MeOTAD/Au structure. The device was fabricated in a same manner with FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD except mp-TiO2 layer. 4 ACS Paragon Plus Environment

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ITO/PEDOT:PSS/MAPbI3/PC[60]BM/Au structure. PEDOT:PSS (Clevios P VP. AI 4083, Heraeus) solution was diluted in methanol with 1:1 volume ratio. The PEDOT:PSS solution was coated on the ITO using spin-coater, which was followed by heating at 150 oC for 30 min. MAPbI3 layer was prepared in a same manner. 20 mg of PCBM (1-matierial) was dissolved in 1 mL of chlorobenzene (99.8%, Aldrich). The PCBM solution was dropped while spinning. Au was evaporated in a same manner. FTO/NiO/MAPbI3/PC[60]BM/Au structure. 60 mM nickel acetate tetrahydrate (98%, TCI) in monoethanol amine (in methoxy ethanol, 99%, Aldrich) was coated on the FTO substrate and heated on 400 oC for 1 h. Heat treatment at 125 oC for 5 min was carried out between spincoating process and two times-coated substrate was finally heated at 400 oC for 30 min. MAPbI3, PCBM and Au layers were deposited on the NiO substrate in a same manner. Device encapsulation. Devices (24 mm·24 mm) were encapsulated with glass (20 mm·15 mm) using UV curing adhesive and sealant (3124L(MS), ThreeBond). UV curing sealant was deposited using a dispenser (Super Sigma CMII, Musashi Engineering) and exposed to UV for 90 s. Measurements Photocurrent-voltage (I-V) curves were obtained using a LED solar simulator (Verasol-2, Oriel Instruments, class AAA) equipped with LED (400~1000 nm) and a Keithly 2400 source meter. One sun intensity (1.5G 100 mW/cm2) was adjusted using a silicon solar cell (NREL-calibrated) with KG-5 filter. A metal aperture mask (0.125 cm2) was attached during the I-V curve measurement. 500 ms of voltage settling time was given between applying voltage and reading current. Absorbance of film was measured using Lambda-35 (Perkin Elmer). Steady state PL was measured with Quantaurus-Tau system (Hamamatsu) equipped with 464 nm laser (pulse duration: 53 ps, peak power: 231 mW) for excitation. Impedance spectroscopy measurements were carried out using PGSTAT 128N (Autolab, Eco-Chemie) with a small perturbation AC 20 mV sinusoidal signal on the DC voltage from 0.1 V to 0.8 V under one sun illumination (AM 1.5G, 100 mW/cm2). The frequency ranging from 1 MHz to 0.1 Hz was applied. The measured Nyquist plots were fit using Z-View software using an equivalent circuit composed of a series resistance (Rs) and two or three R-C components (resistance and capacitance in parallel) in series was used.

Results and discussion An unencapsulated perovskite solar cell with mesoscopic structure, FTO/bl-TiO2/mpTiO2/MAPbI3/spiro-MeOTAD/Au, was prepared and exposed to the continuous light 5 ACS Paragon Plus Environment


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illumination (100 mW/cm2) to investigate the effect of ambient air on the stability. Figure 1ad shows the photovoltaic parameters as a function of light exposure time, where the shortcircuit photocurrent density (JSC) is significantly decreased by 89% within 10 h but the opencircuit voltage (VOC) and the fill factor (FF) are relatively less decreased by 31% and 14%, respectively. The power conversion efficiency (PCE) is reduced by 94% within 10 h mainly due to the reduction of JSC.

Figure 1. Photovoltaic parameters of (a) short-circuit current density (Jsc), (b) open-circuit voltage (Voc), (c) fill factor (FF) and (d) power conversion efficiency (PCE) for an unencapsulated FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au device as a function of light exposure time under continuous illumination with LED light intensity of 100 mW/cm2 in air atmosphere with temperature of ca. 45 oC and relative humidity (RH) of about 40%. JSC is proportional to the light harvesting efficiency, charge separation efficiency and charge collection efficiency. Absorbance and photoluminescence (PL) spectra were measured to investigate the basis for the JSC reduction with light exposure time. Three kinds of layouts, glass/MAPbI3, FTO/bl-TiO2/mp-TiO2/MAPbI3 and FTO/bl-TiO2/mp-TiO2/MAPbI3/spiroMeOTAD, were prepared. Figure 2a represents the photos of the respective structures taken with light exposure time, where color changes are indicative of decomposition of perovskite layer. In the glass/MAPbI3 and the FTO/bl-TiO2/mp-TiO2/MAPbI3 structures, the MAPbI3 films directly exposed to ambient air turn yellow in 24 h. This is indicative of decomposition to PbI2, which is confirmed by a gradual development of PbI2 peak near 500 nm in absorbance spectra (Figures S1a and S1b in Supporting Information). On the other hand, the MAPbI3 film covered with the spiro-MeOTAD layer is relatively stable but decomposed in 155 h, which indicates that the spiro-MeOTAD layer protects temporarily the perovskite film 6 ACS Paragon Plus Environment

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from decomposition by ambient air. The normalized absorbance at 750 nm is plotted as a function of the light exposure time in Figure 2b. Since absorbance near 750 nm corresponds to the absorption edge of MAPbI3, change in absorbance near 750 nm enables us to readily notice the collapse of MAPbI3. Absorbance of the MAPbI3 films directly exposed to air starts to decline rapidly from 9 h, whereas little decrease in absorbance is observed for the MAPbI3 film covered with the spiro-MeOTAD layer. It is noted that the absorbance is not significantly decreased in 9 h for both with and without spiro-MeOTAD, which underlines that the light harvesting is not a major factor for the substantial decrease in JSC within 9 h. The 750-nm normalized absorbance approaches zero in 79 h for the spiro-MeOTAD contained structure. However, its color at 79 h in Figure 2a is different from those for the layouts without spiroMeOTAD, which indicates that decomposition products and/or decomposition mechanisms may depend on the selective contact material.

Figure 2. (a) Photos of the perovskite films in glass/MAPbI3 (top), FTO/bl-TiO2/mpTiO2/MAPbI3 (middle) and FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD (bottom) as a function of light exposure time. (b) Normalized absorbance at 750 nm as a function of light exposure time for glass/MAPbI3 (black), FTO/bl-TiO2/mp-TiO2/MAPbI3 (red) and FTO/blTiO2/mp-TiO2/MAPbI3/spiro-MeOTAD (green). Inset shows absorbance spectra with extended exposure time scale. All films were not encapsulated and exposed to ambient atmosphere under one sun light illumination with temperature of ca. 45 oC and RH of ca. 40%.



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Steady state PL measurements were performed for the films exposed for 0, 4 and 9 h (Figure 3). PL intensity is unexpectedly increased for the glass/MAPbI3 film with light exposure time as show in Figure 3a, which is likely to be attributed to the presence of the decomposed product PbI2 that can suppress the non-radiative charge carrier recombination of MAPbI3 as expected from PbI2-excess MAPbI3.25,26 Unlike the PL increase in the glass/MAPbI3 sample, the MAPbI3 film employing TiO2 (FTO/bl-TiO2/mp-TiO2/MAPbI3) shows relatively lower PL intensity and furthermore PL intensity hardly changes with exposure time (apart from a slight increase in PL with exposure time) despite the identical condition of direct exposure of MAPbI3 to air as can be seen in Figure 3b. This indicates that the charge injection into TiO2 layer is mostly maintained. Interestingly, the initial effective PL quenching at the MAPbI3/spiro-MeOTAD interface gradually is suppressed, resulting in an increase in PL intensity with exposure time (Figure 3c). A pronounced PL peak appeared after 24 h in the FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD underlies that the hole injection into spiro-MeOTAD layer is significantly inhibited, which is responsible for the significant decrease in JSC.

Figure 3. Photoluminescence (PL) spectra of (a) glass/MAPbI3, (b) FTO/bl-TiO2/mpTiO2/MAPbI3 and (c) FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD films with light exposure time. Inset in (c) shows PL for 0h, 4h and 9h. Excitation wavelength was 464 nm. All films were not encapsulated and exposed to ambient atmosphere (ca. 45 oC and RH 40%) under one sun illumination. Lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI) is normally employed as a dopant with tert-butylpyridine (tBP) for spiro-MeOTAD due to its poor hole mobility.27 Further experimental was carried out to clarify the effect of additives (Li-TFSI and tBP) in spiro-MeOTAD on the decreased charge injection. The pristine spiro-MeOTAD without additives demonstrates an efficient charge separation at the beginning of exposure as shown 8 ACS Paragon Plus Environment

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in Figure 4a, while the PL intensity for the spiro-MeOTAD with additives is gradually increased with light exposure time, maximized at exposure for 13.7 h and then decreases again (Figure 4b). This indicates that the additives in spiro-MeOTAD play a role in inhibiting the charge separation. The PL intensity starts to be reduced again after 13.7 h, which is likely to be ascribed to the reduced number of photo-generated charges due to the decomposition of MAPbI3, as confirmed by reduced absorbance shown in Figure 2b. In Figure 4c, The intensities of PL peaks are plotted as a function of light exposure time depending on presence and absences of additives. It is noted that the PL intensity increases abruptly for the MAPbI3 film in contact with the spiro-MeOTAD employing additives compared to the spiro-MeOTAD without additives, which obviously suggests that the additives in spiro-MeOTAD enervate charge separation under illumination in ambient air.

Figure 4. Photoluminescence (PL) spectra as a function of light exposure time for (a) glass/MAPbI3/spiro-MeOTAD without additives and (b) glass/MAPbI3/spiro-MeOTAD with additives (Li-TFSI and tBP) depending on light exposure time. Excitation wavelength was 464 nm. (c) PL peak intensity as a function of time for the MAPbI3 in contact with spiroMeOTAD with (filled circles) and without (filled squares) additives. All films were not encapsulated and exposed to ambient atmosphere under light. Since the malfunction of charge separation observed when exposed to light in ambient air implies a problem at the MAPbI3/spiro-MeOTAD interface, impedance spectra were measured with light exposure time in order to elucidate the degradation mechanism for the unencapsulated FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD with Li-TFSI and tBP/Au device. As exposure time increases, new arc appears at 4641.5 Hz as shown in Figure 5a. The measured Nyquist plots are fit to an equivalent circuit having three R-C components in series as shown in the inset of Figure 5b, where Rh1 represents an initial high frequency resistance at 366,525.6 Hz and Rh2 the newly developed one at 4,641.5 Hz. The frequency9 ACS Paragon Plus Environment


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capacitance curves corresponding to the Nyquist plots (Figure 5) can be seen in Figure S1, which shows developing plateau near 4641.5 Hz as expected. The resistance in high frequency is closely related to the charge transfer resistance in selective contact.28-30 As can be seen in Figure 5b, Rh (= Rh1 + Rh2) increases with increasing the light exposure time, which is indicative of poor charge collection. The newly developed R-C suggests a new interface and/or a new phase formed during light exposure, especially related to the MAPbI3 layer because the R-C arc at 4,641.5 Hz cannot be observed from a device excluding the MAPbI3 layer such as the FTO/bl-TiO2/mp-TiO2/spiro-MeOTAD/Au layout measured in a same manner (Figure S2). Therefore, we suggest here that a very thin PbI2 layer or an ionmigration induced barrier layer could be formed at the MAPbI3/spiro-MeOTAD interface. The underlying barrier layer probably hinders the charge separation and consequently the charge transfer resistance increases.

Figure 5. (a) Nyquist plots (applied bias voltage = 0.5 V) of a device (FTO/bl-TiO2/mpTiO2/MAPbI3/spiro-MeOTAD/Au) depending on light exposure time. (b) Resistance at high frequency (Rh) (Rh = Rh1 + Rh2) as a function of light exposure time, where inset represents the equivalent circuit used to fit the Nyquist plots. Rh increases gradually with light exposure time. Impedance spectroscopic measurements were carried out for the same device used for the photovoltaic characterizations. In the absence of spiro-MeOTAD (glass/MAPbI3 or FTO/bl-TiO2/mp-TiO2/MAPbI3), MAPbI3 was found to be gradually converted into PbI2 as confirmed by its absorbance peak near 500 nm (Figures S3a and S3b), leading to the full conversion into PbI2 as shown in the X-ray diffraction (XRD) results (Figure S4). On the other hands, MAPbI3 with spiro10 ACS Paragon Plus Environment

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MeOTAD (FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD) showed no distinct absorbance peak corresponding to PbI2 as increasing exposure time (see Figure S3c), which is further confirmed by XRD in which PbI2 is not observed (Figure S4). This indicates that the decomposition mechanism of MAPbI3 is apparently dependent on the existence of spiroMeOTAD. When considering the fact that MAPbI3 in the presence of spiro-MeOTAD results in the distinctive differences in optical characteristics, charge-transfer behavior and decomposition mechanism. To exclude the effect of ambient air on the stability under continuous light exposure, an encapsulated device was prepared and its stability is compared with the previous unencapsulated device. The encapsulation was carried out in N2-filled glove box with UVcurable epoxy and its schematic structure along with photos of the encapsulated devices is shown in Figure 6a. Figure 6b shows photos of the encapsulated film (FTO/bl-TiO2/mpTiO2/MAPbI3/spiro-MeOTAD) with light exposure time. Black color is hardly changed with exposure time for over 300 h. In the normalized absorbance at 750 nm shown in Figure 6c, the absorbance of the encapsulated film is constantly maintained, whereas the absorbance of film without encapsulation is decayed by 84.4% in 150 h.

Figure 6. (a) Cross-sectional device layout with encapsulation and plane view of the real devices with and without encapsulation after light exposure. (b) Photos of the encapsulated FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD film taken after relevant light exposure time. (c) Normalized 750 nm-absorbance for the FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD films with and without encapsulation. Figure 7a-d compares the normalized photovoltaic parameters of the encapsulated and the unencapsulated devices. Although encapsulation significantly improves long-term stability, JSC, VOC, FF and PCE still steadily decrease with exposure time and the light 11 ACS Paragon Plus Environment


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exposure for 300 h declines JSC, VOC, FF and PCE by 28%, 16%, 30% and 57%, respectively. As we discovered the new interfacial resistance that was newly developed by light exposure, large decreases in JSC and FF for the encapsulated FTO/bl-TiO2/mp-TiO2/MAPbI3/spiroMeOTAD/Au device are related to such an interfacial issue.

Figure 7. Light exposure time dependent normalized photovoltaic parameters of (a) shortcircuit current density (Jsc), (b) open-circuit voltage (Voc), (c) fill factor (FF) and (d) power conversion efficiency (PCE) for the FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au devices with (black) and without (red) encapsulation. The data were obtained at temperature of ca. 45 oC and RH of ca. 40% under one sun (100 mW/cm2) illumination. Since the poor stability of the encapsulated FTO/bl-TiO2/mp-TiO2/MAPbI3/spiroMeOTAD/Au is expected to be related to the selective contacts, we further investigate longterm stability of perovskite solar cell with different structures. Long-term stability of planar structure (FTO/bl-TiO2/MAPbI3/spiro-MeOTAD/Au) without mp-TiO2 layer shows similar tendency with that of the mesoscopic structure with mp-TiO2 (Figures S5 and S6), which indicates that spiro-MeOTAD plays critical role in degradation. Long-term stability of invert structures without spiro-MeOTAD has been investigated. The well-known invert structure with the ITO/PEDOT:PSS/MAPbI3/PCBM/Au configuration demonstrates rapid degradation mostly due to the considerable decrease in JSC (Figure S7), which is attributed to acidic and hygroscopic properties of PEDOT:PSS.31 The damage of ITO/PEDOT:PSS with exposure time is indicated by gradual increase in a series resistance (RS) of the device (Figure S8d) because RS is related to the ohmic conductance in device.32 Since PEDOT:PSS can be replaced with other oxide p-type materials such as NiO. We have tested long-term stability of the NiO-based invert layout (FTO/NiO/MAPbI3/PCBM/Au), which is compared with the 12 ACS Paragon Plus Environment

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mesoscopic structure (FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au). In Figure 8a-d, the encapsulated device employing NiO and PCBM demonstrates improved stability compared to the mesoscopic structure, showing a decrease by 21% for JSC, 8% for VOC, 12% for FF and 36% for PCE (the non-normalized photovoltaic parameters of the NiO-based invert layout is shown in Figure S9). The initial decrease is pronounced in the mesoscopic structure with spiro-MeOTAD, which is however significantly reduced for the NiO-based device. Here the effect of moisture cannot be ruled out because the devices were prepared in ambient air and the sealant might be permeable to moisture to some extent. However, the comparison of two structures enables us to distinguish the effect of selective contact.

Figure 8. Normalized photovoltaic parameters of (a) short-circuit current density (Jsc), (b) open-circuit voltage (Voc), (c) fill factor (FF) and (d) power conversion efficiency (PCE) as a function of light exposure time for the devices with FTO/bl-TiO2/mp-TiO2/MAPbI3/spiroMeOTAD/Au (black) and FTO/NiO/MAPbI3/PCBM/Au (green). All devices were encapsulated in N2 atmosphere. The data were obtained at temperature of ca. 45 oC and RH of ca. 40% under one sun (100 mW/cm2) illumination. Figure 9a-d compares the decay rate of photovoltaic parameters of the NiO-based and spiro-MeOTAD-based devices. The decay rate was obtained by calculating d(normalized photovoltaic parameter)/dt. It is noted that a significant degradation occurs in the mesoscopic structure within 5 h. However, similar degradation rates for both devices are observed for the prolonged exposure after 5 h. The average decay rate of normalized JSC, VOC, FF and PCE for the mesoscopic structure for 300 h is -0.00097/h, -0.00067/h, -0.00092/h and -0.00213/h, respectively, while the device employing NiO and PCBM shows lower decay rate of 0.00074/h, -0.00016 V/h, -0.00025/h and -0.00087/h. In case of mesoscopic structure, the degradation in early phase is detrimental to long-term stability. 13 ACS Paragon Plus Environment


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Figure 9. Decay rate of normalized photovoltaic parameters of (a) short-circuit current density (Jsc), (b) open-circuit voltage (Voc), (c) fill factor (FF) and (d) power conversion efficiency (PCE) as a function of light exposure time for the devices with FTO/bl-TiO2/mpTiO2/MAPbI3/spiro-MeOTAD/Au (black) and FTO/NiO/MAPbI3/PCBM/Au (green). All devices were encapsulated in N2 atmosphere. The data were obtained at temperature of ca. 45 o C and RH of ca. 40% under one sun (100 mW/cm2) illumination. In terms of light harvesting and charge separation for the encapsulated devices, the selective contacts seem to have an insignificant effect because absorbance and PL are hardly changed with exposure time. The normalized absorbance at 750 nm (near absorption edge) shows very stable and constant irrespective of selective contacts (Figure S10). PL quenching remains also unchanged regardless of selective contacts, which indicates that charge separation occurs efficiently after long-term exposure to light (Figure S11). It was reported that the metal (Au) penetrated into the active layer in N2 atmosphere, which is one of bases for degradation.21,33 However, Au penetration is not observed even after 300-h light exposure in the encapsulated devices because RS values (except for the PEDOT:PSS based device) are hardly changed with exposure time (Figures S8a-c). Cross-sectional SEM images (Figures S12-S14) after 300-h light exposure shows no evidence on Au penetration into spiroMeOTAD or PCBM. Impedance spectroscopic measurement was performed with the encapsulated devices to investigate the charge transport and recombination behavior with respect to the exposure time. Surprisingly, the devices with different selective contacts result in completely different tendency, emphasizing the impact of selective contact on stability. Figures 10a and 10b shows Rh of the devices with FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au and FTO/NiO/MAPbI3/PCBM/Au, respectively.

In Figure 10a, Rh is substantially increased 14

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with exposure time except for 0 h. The sudden decrease in Rh after 0 h might be related to the improved charge conductivity by initial light soaking34 but the effect was not observed in current-voltage measurement because impedance spectroscopic measurement was carried out prior to the current-voltage measurement. The increase in Rh with exposure time implies that the charge collection is steadily hindered, which is responsible for a continual decrease in FF and JSC as well. Compared to the high applied voltage, the Rh is more steeply increased in low applied voltage, which is indicative of more decrease in photocurrent at low bias voltage region including JSC. On the other hand, Rh of the invert structure employing NiO and PCBM in Figure 10b shows totally different tendency with that of mesoscopic structure. Rh at low applied voltage region is even decreased with exposure time, while Rh at high applied voltage region is slightly but negligibly increased (Figure 10b), which is indicative of more stable charge collection all over the voltage range compared to the mesoscopic structure. Recombination resistance (Rrec) obtained from low frequency range is monitored with exposure time (Figures 10c and 10d). Rrec is considerably decreased in the beginning and reaches saturation afterward for the mesoscopic structure as can be seen in Figure 10c. The relatively slow decrease in Rrec at low applied voltage implies a gradual decrease in charge collection at low applied voltage due to the increased recombination and thus results in lowered FF while rapid decrease in Rrec at high applied voltage is directly related to VOC, which is well consistent with the tendency of FF and VOC observed in Figure 8. On the contrary, little dependence of Rrec on the exposure time is observed for the device with NiO and PCBM in Figure 10d, which can explain the more stable VOC. Significant decrease in FF with exposure time for the mesoscopic structure is originated from the increased Rh and the decreased Rrec with exposure time, whereas more stable Rh and Rrec are responsible for more stable FF for the device with NiO and PCBM.



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Figure 10. Resistance at high frequency (Rh) of (a) FTO/bl-TiO2/mp-TiO2/MAPbI3/spiroMeOTAD/Au and (b) FTO/NiO/MAPbI3/PCBM/Au as a function of light exposure time. Recombination resistance (Rrec) at low frequency of (c) FTO/bl-TiO2/mp-TiO2/MAPbI3/spiroMeOTAD/Au and (d) FTO/NiO/MAPbI3/PCBM/Au as a function of light exposure time. Impedance spectroscopy measurements were carried out with the same devices used in the photovoltaic parameter measurements under the same condition. All devices were encapsulated in N2 atmosphere. The comparably lowered stability of the mesoscopic structure could be originated from the intrinsic instability of TiO2 and spiro-MeOTAD layer or the interplay at the MAPbI3 interface. In case of spiro-MeOTAD, it was reported that its symmetry structure easily induced crystallization by the aid of additive.31 Otherwise, the migrated ions of MAPbI3 with biasing might be reactive to the particular contact,21,22 which impede charge transfer in a different way with existence of moisture. In terms of interaction between selective contacts and MAPbI3, the systematic investigation is highly required for further understanding and development of stability. Besides, all devices were fabricated in ambient air prior to encapsulation in N2 in this work, which cannot completely exclude the effect of moisture inclusion in fabrication process.

Conclusion In conclusion, we studied the long-term stability of perovskite solar cells under 16 ACS Paragon Plus Environment

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continuous one sun (100 mW/cm2) light exposure for about 300 h. For the unencapsulated mesoscopic

structured

device,

FTO/bl-TiO2/mp-TiO2/MAPbI3/spiro-MeOTAD/Au,

a

decrease in JSC was mainly responsible for the degradation of photovoltaic performance (94% decrease in PCE after 10 h) in ambient air. According to detection of a new interfacial impedance component appeared with exposure time, the inhibited charge injection into spiroMeOTAD with poor charge transport, rather than the reduced absorbance of perovskite, was found to dominate the stability. Encapsulation in N2 atmosphere significantly enhanced the stability of device; however, still considerable decrease in performance was observed (57% decrease in PCE after 300 h). Little change in absorbance of perovskite but significant decrease in JSC with exposure time observed from the encapsulated device strongly evidenced charge collection problem. The barrier layer formed inbetween MAPbI3/spiro-MeOTAD was hardly overcome by encapsulation. Such a barrier layer was overcome by the replacement of selective

contacts

with

NiO

and

PCBM,

comprising

the

invert

structure

of

FTO/NiO/MAPbI3/PCBM/Au, which eventually improved long-term stability (36% decrease in PCE after 300 h) due to the mostly invariant resistances at selective contacts with exposure time. Our results clearly indicate that the selective contacts play a critical role in long-term stability of perovskite solar cells.

Supporting Information Supporting Information is available free of charge on the ACS Publications website

Figures providing impedance spectroscopic data, absorbance spectra and XRD patterns as a function of light exposure time for the unencapsulated devices with mesoporous TiO2 (mp-TiO2) layer, Photovoltaic parameters as a function of light exposure time for the encapsulated devices with planar structure (bl-TiO2 or PEDOT:PSS) and mesoscopic structure, series resistance as a function of light exposure time for planar structure (bl-TiO2 or NiO or PEDOT:PSS) and mesoscopic structure. Photovoltaic parameters as a function of light exposure time for the encapsulated device with planar structure with NiO. SEM images for the aged devices.

Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts No. 17 ACS Paragon Plus Environment


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NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System), NRF-2015M1A2A2053004 (Climate Change Management Program), and NRF2012M3A7B4049986 (Nano Material Technology Development Program). This was also supported in part by NRF-2016M3D1A1027663 and NRF-2016M3D1A1027664 (Future Materials Discovery Program). We thank In-Hyuk Jang for programing the long-term stability measurement.


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Impact of Selective Contacts on Long-Term Stability of CH3NH3PbI3

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