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Core/Shell Structured TiO2/CdS Electrode to Enhance the Light-stability of Perovskite Solar Cells Insung Hwang, Minki Baek, and Kijung Yong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09442 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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ACS Applied Materials & Interfaces

Core/Shell Structured TiO2/CdS Electrode to Enhance the Light-stability of Perovskite Solar Cells Insung Hwang, Minki Baek, and Kijung Yong* Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Hyoja-Dong, Pohang-Si, 790-784, Republic of Korea E-mail: [email protected]. Tel: +82-54-279-2278. Fax: +82-54-279-8619.

Abstract In this work, enhanced light stability of perovskite solar cell (PSC) achieved by the introduction of a core/shell-structured CdS/TiO2 electrode and the related mechanism are reported. By a simple solution-based process (SILAR), a uniform CdS shell was coated onto the surface of a TiO2 layer, suppressing the activation of intrinsic trap sites originating from the oxygen vacancies of the TiO2 layer. As a result, the proposed CdS-PSC exhibited highly improved light stability, maintaining nearly 80% of the initial efficiency after 12 hours of full sunlight illumination. From the XRD analyses, it is suggested that the degradation of the efficiency of PSC during illumination occurs regardless of the decomposition of the perovskite absorber. Considering the light-soaking profiles of the encapsulated cells and the OCVD characteristics, it is likely that the CdS shell had efficiently suppressed the undesirable electron kinetics, such as trapping at the surface defects of the TiO2 and preventing the resultant charge losses by recombination. This study suggests that further complementary research on various effective methods for passivation of the TiO2 layer would be highly meaningful, leading to insight into the fabrication of PSCs stable to UV-light for a long time. Keywords Perovskite solar cell, light-stability, surface defects, core/shell structure, light-soaking profile

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Introduction In the field of solar cell research, it is irrefutable that the organic-inorganic hybrid perovskite solar cell has been one of the hottest issues in the field over the past few years. Although methylammonium lead halide perovskite was first reported in the 1980’s,1 the first application of perovskite in a photovoltaic device was recently reported in 2009 by Miyasaka et al.2 After that, an enhancement in power conversion efficiency3 and the introduction of a solid-state electrolyte4 to perovskite solar cells have been reported, while a new reproducible two-step synthesis process for perovskite was introduced by M. Graetzel.5 On the basis of these pioneering studies, research on perovskite solar cells has been vigorously activated, resulting in a great increase of power conversion efficiencies to over 20%.6 However, it is so difficult to separate the impressive advances in the field of perovskite solar cells from the accompanying critical problems. Although perovskite solar cells have exhibited outstanding power conversion efficiencies on the basis of the intrinsic advantages of perovskite, such as the superior light absorption coefficient,3 easily tunable band gap energy,7 and excellent charge carrier mobilities,8 they have some critical weak points at the current stage. As M. Graetzel has commented,9 they are unstable (to moisture, UV-light, and heat), toxic due to the presence of lead, and exhibit hysteresis in J-V measurements. Without solving these critical issues, it is impractical for perovskite solar cells to be utilized in the solar cell industry. Attempts to resolve the toxicity problem by replacing Pb with Sn (which is in the same group on the periodic table as Pb) are underway.10-12 The hysteresis behaviors have been studied by physical and electrical approaches, though the exact reason for them and their true nature are still subject to debate.13-15 The instability of perovskite solar cells is the main topic of this study (especially the lightinstability). Perovskite solar cells are known to be especially susceptible to moisture and UV-light, showing drastic decreases of power conversion efficiency when exposed to these conditions.16-17 There have been some previous reports on enhancing the moisture-stability of the cells by controlling the composition of the perovskite,7 introducing a hydrophobic hole-transport material (HTM) or an


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inorganic HTM,18-19 surface passivation of the perovskite with an interlayer,20 and hydrophobic encapsulation of the entire cell.21 Meanwhile, the cause of the instability of perovskite solar cells to UV-light is currently not clearly defined. Leijtens et al. have suggested that the decreased efficiency of perovskite solar cells under continuous illumination is because of the activation of surface traps in the mesoporous TiO2 layer,22 while Seigo et al. argued that it is because the decomposition of the perovskite absorbing layer is accelerated by the photo-catalytic properties of the TiO2.23 To date, the dominant reason for the decreasing performance of perovskite solar cells under UV-light illumination remains unclear. In this study, the core/shell structure of TiO2/cadmium sulfide (CdS) was used as an electron transporter for perovskite solar cells to passivate the surface trap sites of the TiO2 layer. CdS has been the most widely used inorganic sensitizer in the quantum dot-sensitized solar cell field,24-27 which ensures its photo-electronic suitability to the application. Considerably enhanced stability to continuous illumination at 1 sun was observed in the perovskite solar cell with the CdS shell (CdSPSC). On the basis of some simple analytic results, it could be concluded that this enhanced stability to light resulted from the effect of passivation on the surface traps at TiO2, rather than the prevention of perovskite decomposition. From this new approach, it is suggested that light-stable PSC may be achieved by effective surface modification.

Experimental section Materials and device fabrication All chemicals in the experiments were purchased and used as received. Titanium dioxide paste (18NR-T) and CH3NH3I (MAI) were purchased from Dyesol. Cadmium sulfate (CdSO4), sodium

sulfide

nonahydrate

(Na2S·9H2O),

lead(II)

iodide

(PbI2),

lithium

bis(trifluoromethylsyfonyl)imide salt (Li-TFSI), titanium diisopropoxide bis(acetylacetonate) (75 wt% in IPA), N,N-dimethylformamide, tert-butylpyridine (tBP), chlorobenzene, and acetonitrile were all



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bought from Sigma-Aldrich. Spiro-OMeTAD was purchased from Merck to be used as the HTM. For the fabrication of PSCs, a diluted Ti(acac)2OiPr2 solution was spin-coated for 30 s at 5500 RPM onto cleaned FTO glass. The glasses were dried at 120°C for 10 minutes followed by additional spin-coating of the diluted TiO2 paste at 3500 RPM for 30 s. The deposited samples were calcinated at 500°C for 30 minutes to create an anatase-phase thin TiO2 electron transport layer. For passivation of the TiO2 surface traps, a thin layer of CdS was coated onto the surface of TiO2 by the successive ionic layer adsorption reaction (SILAR) method (this step was omitted in the fabrication of reference bare PSCs).25 The electrode was alternatively immersed in 50 mM precursor solutions (a mixture of distilled water and methanol in volume ratio of 1:3) of Cd2+ and S2- for 1 minute each and washed with distilled water to remove weakly adsorbed ions and particles. The number of repeated cycles was controlled from 2 to 10 cycles to vary the thickness of the CdS layer and the corresponding cell performances. CH3NH3PbI3 perovskite was deposited onto the bare TiO2 electrode and the TiO2/CdS electrode via a conventional two-step spin-coating method.5 1 M of PbI2 in DMF was first spin-coated at 6000 RPM for 30 s, and then followed by spin-coating of 10 mM MAI in IPA for 2000 RPM of 20 s after 20 s of waiting time. Each step was accompanied by a drying step at 100 °C for 30 minutes on the hot plate. HTM was then deposited along with the well-known coating conditions and mixture composition of Spiro-OMeTAD.28 80 mg of Spiro-OMeTAD was dissolved in 1 ml of chlorobenzene with the additives of 28.8 µl of tBP and 17.5 µl of Li-TFSI solution (520 mg of Li-TFSI in 1 ml of acetonitrile). The solution was spin-coated onto the perovskite film at 2000 RPM for 20 s. On the HTM-deposited cells, an Au top electrode with a thickness of 70 nm was thermally evaporated (active area: 0.1 cm2). Characterizations Electronic measurements such as the J-V measurement, electrochemical impedance spectroscopy (EIS), open-circuit voltage decay (OCVD), and stabilized efficiencies were conducted


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with a solar simulator (Sun 3000, ABET Tech.) and a potentiostat (Iviumstat, IVIUM Tech.). To ensure the consistency of the results, the output power of light from the solar simulator was maintained at AM 1.5 G (100 mW/cm2). Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) images were obtained from NINT in POSTECH (JEM-2200FS, JOEL). X-ray diffraction (XRD) patterns were measured by a D/MAX-2500/PC, while UV-vis absorption spectra were taken on an OptizenPOP (Mecasys Co.). Incident photon to current efficiency (IPCE) was measured with a power source (150 W Xenon lamp 13014, ABET), monochromator (MonoRa-500i, Dongwoo Optron Co.), and potentiostat (IviumStat, IVIUM Tech.).

Results and discussions Scheme 1 illustrates the main idea of this research. In typical PSCs, UV light causes excitation of the electrons in TiO2, leading to desorption of the adsorbed oxygen and hence activation of surface traps.22 These traps disturb the normal photovoltaic process, resulting in the degradation of cell performance. To solve this problem, two possible fundamental solutions can be suggested: removal of the TiO2, or surface modification of the TiO2. The latter approach is reported in this study, in the form of a CdS shell-coated TiO2 layer. With the CdS shell, the activation of surface traps was considerably suppressed and hence the light-stability of the PSC was enhanced in some degree. Performance profiles of the proposed PSC and the proposal of the relevant mechanism will be covered in the following text. To verify whether the surface coating of the CdS layer on the TiO2 particle’s surface was successfully completed, transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) were used to analyze the CdS-TiO2 structure, as shown in Figure 1. TiO2 nanoparticles with a CdS layer deposited by 5 cycles of the SILAR method were dispersed in ethanol before they were loaded on the grid for TEM observation. From the TEM measurement, a fairly uniform CdS thin layer with an average thickness of ~2 nm was observed on the surface of the TiO2 particles. This uniform deposition was possible because of the nature of the SILAR method, that is,



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layer-by-layer growth of the product during the alternative provision of cationic and anionic precursors.29 Although no distinct lattice pattern of CdS was observed in the TEM images because of the thinness of the layer and the background noise, the presence of cadmium and sulfur was proven by EELS mapping. In the EELS mapping, Cd and S were uniformly spread along with the structure of the TiO2 nanoparticles. From the TEM and EELS, it was concluded that the CdS thin layer was successfully deposited on the TiO2 surface with good uniformity. In the Supporting Information (Figure S1), a TEM image with higher magnification and EELS mapping in another spot with a lower magnification are provided as additional proof of uniform CdS layer deposition. Performances of bare PSC and CdS-PSCs were investigated by controlling the thickness of the CdS shell. The thickness was easily controlled by changing the number of SILAR cycles because the thickness has a linear relationship with the number of SILAR cycles.30 CdS-PSCs with different number (n) of CdS SILAR cycles are denoted as n CdS-PSC. The results of J-V and EIS measurements conducted on the bare PSC and n CdS-PSCs are plotted in Figure 2. As Figure 2 a demonstrates, increased CdS thickness caused a slight decrease in the power conversion efficiency (PCE) of the PSCs, mainly due to the drop in the photocurrent generation (the distribution histograms of each device are provided in Figure S9). While similar ranges of photovoltaic parameters such as Voc and Fill factor (FF) were observed in the J-V curves (Voc of 0.91, 0.93, 0.93, and 0.91 V for bare PSC, 2, 5, and 10 CdS-PSCs, respectively and FF values of 0.69, 0.67, 0.70, and 0.67 in the same order), a pronounced decrease in Jsc was observed (19.0, 18.0, 17.6, and 16.6 mA/cm2) as the CdS layer became thicker. Additionally note that, to be clear about the hysteresis issue for the PSC, the hysteresis measurement and stabilized efficiency observation were conducted on both the bare PSC and CdS-PSC, as provided in Figure S2 of the Supporting Information. The scan rate for the hysteresis measurement (and all the J-V measurements in the main results) was 100 mV/s. No significant hysteresis was observed for either cell, encouraging us to fix the measuring conditions of backward scan at a rate of 100 mV/s for all subsequent measurements. To clarify the reason for the decrease in photocurrent generation in CdS, an EIS


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measurement was conducted on the bare PSC and three types of CdS-PSCs, as shown in Figure 2 b in the form of a Nyquist plot. A bias voltage of -0.9 V, which is near from Voc values of these samples, was identically imposed during the measurement. As clearly observed, the diameters of the second semicircles in the plot became larger as the thickness of CdS layer increased. The equivalent circuit model and the resultant impedance values obtained from data-fitting are provided in the Supporting Information (Figure S3). In usual PSC, three characteristic resistances and impedances are notable: the series resistance of the TiO2 electrode (Rs), the interfacial impedance between TiO2 and perovskite (R1), and the interfacial impedance between perovskite and HTM (R2). Because the variable in this control experiment is the interfacial property of the TiO2/perovskite interface, the other resistances (Rs and R2) were expected to have similar values. From the data fitting analyses, Rs values from 15.1 to 15.8 Ω and R2 values from 4.5 to 9.5 Ω were obtained, showing insignificant differences, as expected (inconsistent variation of R2 is attributed to error from data fitting caused by the too dominantly large semicircles of R1). On the other hand, the interfacial resistance at the TiO2/perovskite interface (R1) exhibited a significant increase as the CdS layer became thicker (80.7, 105.8, 122, and 165.9 Ω, respectively). From this result, it could be suggested that CdS interlayer has increased interfacial resistance between the TiO2 and perovskite, resulting in the decrease of Jsc in the J-V measurement. Therefore, 2 CdS-PSC was selected as the control sample in further experiments and compared with bare PSC in various ways (because PSCs with thicker CdS layers exhibited the similar light-stability with the 2 CdS-PSC, but lower initial PCEs. Also, note that CdS layers thinner than 2 CdS-PSC had exhibited no distinct enhancement of stability according to our further experiments not described here. ). Because CdS is also a light absorber with a direct band gap energy of ~2.5 eV,31 investigation was required to determine whether the CdS interlayer in this experiment actually participated in absorbing the incident light. To demonstrate this interaction, the optical properties of the CdS interlayer were analyzed by UV-vis absorption spectroscopy and an IPCE measurement, as shown in Figure 3. For the light absorption measurement, films composed of FTO/TiO2/perovskite/HTM and FTO/TiO2/CdS/perovskite/HTM were fabricated in the same way as those used in the real device,



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except for the Au top electrode. The absorption spectra of both films showed the same features, including the onset at ~800 nm where the perovskite started to absorb the light. No distinguishable onset was observed near ~500 nm where CdS would start to absorb the light in the film with the CdS layer. This observation could also be confirmed when the absorption spectra were measured for the bare TiO2 and TiO2/CdS samples (inset). Because CdS layer was extremely thin, with a thickness of less than 2 nm, it did not affect the light absorption of the film. For more specific characterization, along with the wavelength, IPCE of both the bare PSC and CdS-PSC was measured in the wavelength range from 300 to 900 nm (Figure 3 b). IPCE curves for both cells exhibited an onset near ~800 nm, where the perovskite started to absorb the incident light. As expected from absorption spectroscopy, no distinct onset was observed near ~500 nm to correspond to the band gap energy of CdS. Instead, the overall IPCE values of CdS-PSC showed slightly decreased values compared with those of the bare PSC. Jsc values obtained by integrating the IPCE curves of both cells were calculated as 17.5 and 16.0 mA/cm2 for the bare PSC and CdS-PSC, respectively. Because the same absorber, perovskite, was used in both cells, there might be no difference in light harvesting, indicating that the difference in the charge transport step was responsible for the decrease of photocurrent generation. This deduction is consistent with the previous result of the EIS measurement. To confirm the effect of the CdS layer on the stability of PSC, a light-soaking measurement was conducted on both bare PSC and CdS-PSC. Instead of simply measuring Jsc changes with time, as in some other reports,23 sweeping measurements of the J-V curves of the cells were conducted at 3 minute intervals while the cells were exposed to constant illumination at the condition of 1 sun. In each J-V curve, the four photovoltaic parameters (efficiency, Voc, Jsc, and fill factor (FF)) were estimated, and their changes were plotted against time for 12 hours (Figure 4). The CdS passivation layer proved its effect on the stability by showing 77% maintenance of the initial efficiency after 12 h of illumination (9.9% to 7.6%), while the bare PSC exhibited a severe decrease of 58% of the initial efficiency (10.2% to 4.3%), leaving only 42% of the initial efficiency. The definite differences


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between the cells were observed especially in Voc and FF. The value of Voc for CdS-PSC was nearly constant during the light-soaking measurement (940 mV to 900 mV), while that of bare PSC was reduced from 900 mV to 760 mV. Likewise, the FF of CdS-PSC fluctuated from 0.62 to 0.57, while that of bare PSC showed a large decline from 0.61 to 0.40. To be clear with the moisture-stability issue, additional control experiments were conducted and exhibited in Figure S8. The devices exposed to moisture but no light exhibited negligible degradations in PCEs, which indicates moisture-induced degradation of PSC can be ignored in this study. We suggest that the reason for this difference in the stability can be ascribed to the surface traps of the TiO2 mesoporous layer, as previously reported.22 Without the CdS passivation layer, many surface trap sites of the TiO2 became activated, creating some energy levels lower than the ordinary conduction band (CB) level of TiO2. These defect energy levels were responsible for the decrease of Voc because Voc is theoretically the energy level difference between the anode (FTO or TiO2) and the cathode (Au). Additionally, these defect sites trap the generated electrons and provide additional recombination pathways for them, resulting in the decline of Jsc and FF, as well as Voc. Compared with the CdS-PSC, a more severe increase of charge transport resistance of the bare PSC was observed (as shown in Figure S4 in the Supporting Information) explaining the overall decrease in Jsc and the serious decrease in FF of the bare PSC. On the contrary, the thin CdS layer seemed to considerably passivate the trap sites of the TiO2, showing a relatively moderate decrease in these photovoltaic parameters. The suggested mechanism and some supporting evidence will be described in the following paragraphs. Because there is another possible cause of degradation of cell efficiency, namely, the decomposition of the perovskite absorber by UV-light,23 we tried to verify the dominant cause in this research. XRD pattern analysis was the appropriate experimental technique to clarify this issue; through the XRD result, it could be clearly determined whether the perovskite absorber decomposed during illumination. For the XRD analysis, all the components of PSC were deposited on the FTO glasses,

creating

sample

geometries

of

FTO/TiO2/perovskite/HTM/Au


and


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FTO/TiO2/CdS/perovskite/HTM/Au. XRD patterns of each sample were measured before and after the samples were exposed to the illumination for 12 h (the Au top electrodes were peeled off with adhesive polyimide tape right before the XRD measurement). Figure 5 shows the XRD results for the perovskite films (a) without the CdS shell and (b) with the CdS shell. In both results, the XRD peaks of the initial perovskite crystals were clearly observed at 14, 20, 23.5, 24.5, 25.4, 28.5, 31.9, 35, 40.7, 43.1, and 50.4°, showing exact coincidence with previously reported XRD patterns of the perovskite absorber.32-33 The peaks marked with * correspond to the FTO layer on the glass, according to the reference (JCPDS #77-0447). In the film with the CdS shell, no distinct peak corresponding to CdS was observed because the extremely low amount of CdS was insufficient to be visualized in the XRD pattern. Interestingly, in both films after 12-h illumination, the all peaks from the as-deposited perovskite layers were observed with nothing missed. In the previous study, XRD patterns of decomposed perovskite CH3NH3PbI3 had been measured, showing some side-peaks at 12.7, 25.7, 39, and 52.4° corresponding to PbI2.21 However, none of these PbI2 peaks were observed in either sample after 12 h-illumination, indicating that degradation of PSC’s efficiency occurs regardless of the decomposition of the perovskite absorber. This fact can also be verified easily with the naked eye, because no color change is observed during the illumination, as shown in Figure S6 of the Supporting Information; it is well known that perovskite is very dark brown and PbI2 is bright yellow. Therefore, the light-sensitivity of PSC should be explained by a factor other than perovskite’s decomposition, as covered in the following analyses. Another persuasive suggestion explaining the efficiency decrease of PSC during illumination is trap site activation at the surface of mesoporous TiO2. According to previous research,22 TiO2 has intrinsic oxygen vacancies that act as trap sites for electrons. When the device is not operating, some of the oxygen vacancies are passivated by adsorbed oxygen molecules, keeping the reversible adsorption-desorption process in equilibrium. When the cell is exposed to light, electrons in the valence band (VB) of TiO2 are excited by UV ray and then play a critical role in desorbing the adsorbed oxygen at the oxygen vacancies (namely, activation of the trap sites). As a result, some of


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the generated photo-electrons in the device are trapped at the sites, leading to abnormal recombination and hence the decline of the photovoltaic performance of the cell. This proposed degradation mechanism has been confirmed once again in this study by a simple light-soaking measurement with the encapsulation of devices. The J-V characteristics of both the bare PSC and CdS-PSC with/without the encapsulation were measured at 3 minute intervals. The resultant efficiency and changes in Voc values are shown in Figure 6 (Jsc and FF values from these measurements are also provided in the Supporting Information, Figure S7). Interestingly, CdS-PSC exhibited negligible differences in both efficiency and Voc degradation curves, while the encapsulated bare PSC showed more severe degradation than the unsealed bare PSC. This result can be explainable as follows. i) Encapsulation blocks the access of oxygen outside the cell. ii) Because the oxygen adsorption-desorption process at the TiO2 surface is reversible, a lack of oxygen concentration around the cell would accelerate the desorption of oxygen. iii) Therefore, for the bare PSC case, encapsulation lowers the oxygen concentration of the cell, resulting in more severe degradation of the cell performance. iv) On the contrary, the CdS shell has already coated the surface of the TiO2 uniformly, originally passivating the oxygen adsorption-desorption reaction. In that case, the degradation of cell performance is considerably suppressed because of the restrained activation of trap sites by the CdS shell. Moreover, it is not affected by encapsulation because of the pre-encapsulated CdS/TiO2 electrode. Although the CdS/TiO2 structured electrode is introduced in this study for the first time, the encapsulation effect on the bare PSC observed here coincides well with the previous research.22 Deduced from these results and references, it is proposed that the degradation of efficiency of PSC can be attributed to TiO2 surface traps, and it can be considerably suppressed by the deposition of a CdS shell on the mesoporousTiO2. Open-circuit voltage decay (OCVD) of both the bare PSC and the CdS-PSC before and after illumination for 12 h was measured to investigate the electron dynamics in the devices (Figure 7). The cells were exposed to the light of 1 sun for 1 s, and then the light was turned off after the Voc values of the cell stabilized. The instantaneous decrease in Voc values of the cells were measured and plotted



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with time in Figure 7 a. As clearly observed, the fresh devices of bare PSC and the CdS-PSC exhibited similar decay curves of the Voc values, indicating that the electron recombination rates of both initial cells were not so different. However, a significant difference was observed when the OCVD curves of both aged cells were measured. A reduced initial value of Voc and drastically accelerated decay behavior were observed in the aged bare PSC, while an initial Voc value similar to the fresh sample and a less pronounced decay curve were measured for the aged CdS-PSC. Both issues of decreased initial Voc and accelerated decay rate of the aged bare PSC can be explained by the activated surface trap sites facilitating the recombination of the generated electrons. It is supposed that the Voc value of the aged bare PSC declined because the activated surface traps of TiO2 form some defect energy levels below the conduction band of TiO2. Additionally, trapped electrons in these defect sites easily recombine with surrounding holes, resulting in the accelerated Voc decay. On the other hand, the activation of traps in CdS-PSC was suppressed by the CdS shell to some degree, as proved by the OCVD results. According to the literature, electron lifetime (߬௘ ) can be estimated by the equation:

߬௘ = −

௞ಳ ் ௗ௏೚೎ ିଵ ( ) , ௘ ௗ௧

where ݇஻ is Boltzmann constant, e is the charge of the electron, and T is the temperature.34 The derivatives of Voc over time are shown in Figure 7 a and translated into electron lifetimes in Figure 7 b. The electron lifetimes of the aged CdS-PSC were significantly longer than those of the aged bare PSC, confirming again the effect of the CdS shell on the recombination kinetics.

Conclusion In conclusion, a novel approach to enhance the light-stability of PSC by the introduction of a core/shell-structured electrode is reported here. With the facile solution-based synthetic process SILAR, a thin CdS shell layer was deposited onto the surface of TiO2, resulting in the effective surface passivation of PSC, rendering it stable to the light. After some optimization experiments


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regarding the thickness of the CdS shell, light-soaking profiles of bare PSC and CdS-PSC were observed for 12 hours of continuous illumination. After illumination, nearly 80% of the initial efficiency was observed in the CdS-PSC, while only 40% of the initial efficiency was retained by the bare PSC. On the basis of XRD, the encapsulation effect, and OCVD analyses, the passivation effect of the CdS shell on the surface traps of TiO2 was proposed as the main reason for the enhanced lightstability. Although perfect stability without any loss of efficiency could not be achieved yet, this study is meaningful from the point of view of presenting another possibility of surface modification for light-stable PSC, instead of insulating scaffolds like Al2O3.

Acknowledgement This work was supported by the National Research Foundation of Korea (2013-R1A2A2A05-005344).

Supporting Information More data including additional SEM and TEM images for the CdS/TiO2 electrodes, hysteresis and stabilized efficiencies of the initial devices, fitting of EIS data of the initial devices, Nyquist plots of the bare PSC and CdS PSC before and after illumination, raw data of J-V curves of the bare PSC and CdS PSC during the light illumination, digital photographs of the perovskite film with and without the CdS shell before and after the illumination, light-soaking parameter profiles, control-experiment result for the moisture influence, and the initial PCE distribution histogram of the various devices. The Supporting Information is available free of charge on the ACS Publications website at DOI:



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Figures

Scheme 1. Schematic illustration explaining the mechanism of enhanced light-stability of PSC by the CdS shell on the TiO2 electrode.



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Figure 1. TEM and EELS mapping of the core/shell structured CdS/TiO2 electrode. Uniform coating of the CdS thin layer with a thickness of ~2 nm was observed at the surface of TiO2. EELS mapping shows the presence of elemental Ti, O, Cd, and S.


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Figure 2. (a) J-V curves of the bare PSC and CdS-PSCs with various thicknesses of CdS shells, controlled by the number of SILAR cycles. (b) Nyquist plots of the bare PSC and various CdS-PSCs obtained from the EIS measurement.



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Figure 3. (a) UV-vis absorption spectra of the films with TiO2/perovskite/HTM and TiO2/CdS/perovskite/HTM on FTO glasses. Inset shows the spectra of bare TiO2 and CdS/ TiO2 on FTO glasses. (b) IPCE data of the bare PSC and CdS-PSC in a wavelength range from 300 to 900 nm.


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Figure 4. Profiles of the photovoltaic parameters of bare PSC and CdS-PSC obtained from J-V curves measured at 3 minute intervals during illumination for 12 hours. For clear visualization, data points were selected with an interval of 15 minutes. The raw data of the J-V curves of the devices are provided in the Supporting Information (Figure S5).



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Figure 5. XRD patterns of the actual devices of bare PSC (a), and CdS-PSC (b), with the Au top contact peeled off. On each graph, the patterns of the as-prepared film (black line), and the film illuminated for 12 h (red line) are visualized.


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Figure 6. Light-soaking profiles of the bare PSC and the CdS-PSC with and without encapsulation. The effect of encapsulation on the cell stability can be investigated here. The rest of the parameters (Jsc, and FF) are given in the Supporting Information (Figure S7).



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Figure 7. OCVD analyses of the bare PSC and CdS-PSC before and after illumination for 12 h. (a) Voc decay curves of the cells over time. Although time interval of raw measurements was 10 ms, data points with an interval of 200 ms are shown here to allow for clear visualization. (b) Calculated electron lifetimes of the devices are plotted with the Voc values.


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