Novel CdS Hole-Blocking Layer for Photostable Perovskite Solar Cells

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A Novel CdS Hole-Blocking Layer for Photo-Stable Perovskite Solar Cells Insung Hwang , and Kijung Yong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12336 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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ACS Applied Materials & Interfaces

A Novel CdS Hole-Blocking Layer for Photo-Stable Perovskite Solar Cells Insung Hwang and Kijung Yong* Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea E-mail: [email protected], Tel) +82-54-279-2278, Fax) +82-54-279-8619

Abstract Currently, the stability issue of the perovskite solar cells (PSCs) is one of the most critical obstacles in the commercialization of PSCs. Although the incredible advances in the photovoltaic efficiencies of PSCs have been achieved during the last few years, researches on the stability of PSCs have been relatively less explored. In this study, a new kind of CdS hole-blocking layer replacing the traditional compact TiO2 layer is developed to improve the photo-stability of PSCs, because the intrinsic oxygen vacancies of TiO2 surface are suspected of the main reason causing photo-induced degradation of PSCs. As a result, PSCs with the CdS layer exhibit considerably improved photostability, maintaining over 90% of the initial efficiency after the continuous sunlight illumination for 12 h, while the TiO2-PSC retains only 18% of the initial efficiency under the same condition. Charge transfer characteristics related to the photo-degradation are investigated by various analyses including the electrochemical impedance spectroscopy, open-circuit voltage decay, and time-resolved photoluminescence decay measurement. CdS-PSC exhibits negligible degradations in the charge carrier dynamics, while the TiO2-PSC suffers from severely damaged characteristics like increased charge recombination rate, charge transfer resistance, and reduced charge extraction rate. Keywords CH3NH3PbI3, perovskite solar cells, photo-stability, hole-blocking layer, CdS

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Introduction Since the first report of organic-inorganic hybrid perovskite solar cells by Kojima et al.,1 perovskite solar cells (PSCs) have attracted tremendous attention of researchers in the photovoltaic fields. Although the power conversion efficiency (PCE) of the PSC in the first report was 3.8%, recent PCE of PSCs has been incredibly increased to 20.1% in the last few years.2 This outstanding advance of the PSC was possible on the basis of various advantages of the perovskite as a light absorber, such as the high light absorption coefficient,3 excellent charge carrier mobilities for both electrons and holes,4 easily tunable band gap energy,5 and various facile synthetic processes for the perovskites.6-8 However, as M. Graetzel has commented,9 current PSCs have some critical limitations which are needed to be overcome before its practical usages as the photovoltaic devices. The major issues of current PSCs are toxicity of the perovskite, insufficient stability of the devices, and hysteric behavior during the efficiency measurements. The toxicity of perovskites is originated from the presence of Pb element in CH3NH3PbX3 (MAPbX3, X = Cl, Br, or I) perovskites which are the most widely used kinds of perovskite. There have been some efforts to replace the Pb element with a non-toxic element such as Sn,10-12 although the current results are required to be improved in further studies. The hysteresis of PCEs of PSCs is well-known phenomena, which means the disaccord between the PCEs obtained from the same PSC but different measuring conditions.13 The hysteresis of PCE occurs depending on the scan directions and scan rates in the J-V measurements. To get rid of the unreliability, H. Snaith suggested the concept of stabilized efficiency which can be obtained by measuring the photocurrent during a certain time at the bias potential of the maximum power point.13 Although the exact reasons for the hysteresis have not been completely revealed yet, many studies have been reported to understand the mechanism of hysteresis or to relieve the hysteresis phenomena.13-16 Insufficient stability of PSC originates from the vulnerability of conventional PSCs to moisture or continuous sunlight illumination. Exposure of PSCs to moisture or constant sunlight causes severe degradations in the performances of PSCs.17-18 Vulnerability of perovskite to moisture is inevitable


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weakness of perovskite, caused by the presence of polar organic cations in the perovskites (methylammonium, or formamidinium). There have been various studies to enhance the stability of PSCs against moisture by the control of perovskite composition,5 introduction of a hydrophobic carbon/polymer-composited hole transport material (HTM),19 doping of HTMs,20 and encapsulation of PSCs with a hydrophobic polymer.21 Meanwhile, the vulnerability of PSCs to sunlight is another critical problem of PSCs. Especially, UV-light is considered to be the main reason for the degradation of performances of PSCs. H. Snaith et al. have suggested that the degradation of PCEs of PSCs is caused by charge traps of the oxygen vacancies in the TiO2 electrode, which are activated by UVlight.22 Some approaches have been suggested to overcome the photo-induced degradation problem of PSCs, including the passive scaffold of Al2O3 replacing TiO2,22 use of an UV-downshifting nanophosphor layer,23 or a surface passivation process of the TiO2 electrode.24 In this work, we suggest a new solution to improve the photo-instability of PSCs by applying a non-oxide material to replace the TiO2. Thermally evaporated CdS layer was utilized for the role of hole-blocking layer (HBL) instead of the conventional compact TiO2 layer, to exclude the negative effects of the oxygen vacancies. CdS was expected to be a suitable electron-selective transporting layer (at the same time, HBL) because CdS is a n-type semiconductor with a good electron conductivity and appropriate conduction band level between those of perovskite and FTO (F-doped tin oxide, which is the transparent conducting oxide on the glass substrate). The idea was inspired by the fact that CdS has been the most promising and widely-used material in quantum dot-sensitized solar cell fields, which implies the potential of CdS in the photovoltaic applications.25-27 Due to its proper characteristics as an alternative HBL, CdS-PSCs developed in this work exhibited considerably improved photo-stability to the constant light illumination. Fairly well-maintained cell performances and consistently efficient charge transfer characteristics were observed in the CdS-PSC, although its initial PCE was slightly lower than that of the conventional PSCs with the TiO2 compact layers.



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Experimental Section Preparation of chemicals. For the device fabrication and analyses, following chemical products were purchased and used without further purification: PbI2 (powder, 99.9985%, Alfa Aesar), methylammonium iodide (MAI, powder, Dyesol), Spiro-OMeTAD (Merck), and all the other salts and anhydrous solvents from Sigma-Aldrich (CdS powder, lithium bis(trifluoromethylsyfonyl)imide salt (Li-TFSI), titanium diisopropoxide bis(acetylacetonate) (75 wt% in IPA), N,N-dimethylformamide, tert-butylpyridine (tBP), chlorobenzene, acetonitrile, 1-butanol, diethyl ether, and dimethyl sulfoxide (DMSO)) Device fabrication. The overall processes for the device fabrication are lucidly illustrated in Scheme 1. To fabricate the reference TiO2 PSC, compact TiO2 layer was deposited on the cleaned FTO glass by the spin-coating and calcination processes (0.3 M Ti(acac)2OiPr2 solution diluted with 1-butanol, 5500 RPM for 30 seconds, and 500 °C calcination for 30 minutes). Meanwhile, for the fabrication of CdS-PSC the CdS HBL was deposited by the thermal evaporation with the thickness sensor of quartz crystal microbalance (QCM). Like the previous literature,28 CdS powder was loaded in the crucible and heated at the enough vacuum condition. The thickness of CdS was varied from 20 to 50 nm. Except for the HBL deposition process, all the other fabrication processes were equally applied for the both devices. Perovskite absorber layer was deposited on the each HBL by the 1-step spin-coating method using the Lewis-base adduct.29 461 mg of PbI2, 159 mg of MAI, and 78 mg of DMSO were dissolved in 600 mg of DMF and stirred for 1 hour at room temperature. The resultant MAI•PbI2•DMSO adduct solution was dropped onto each HBL and spun at 4000 RPM for 25 seconds. Before the rotating substrate become opaque, 0.4 ml of diethyl ether was quickly dropped onto the rotating substrates (in this study, dropped at 9 seconds after the rotation started). The adduct became the MAPbI3 perovskite after the drying processes at 60 °C for 1 minute and 100 °C for 2 minutes. 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). Spiro-OMeTAD was deposited onto the perovskite layer by spin-coating of this solution at 3000 RPM for 30 seconds.


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Finally, Au with the thickness of 80 nm was deposited by a thermal evaporation. Characterization. J-V measurement, electrochemical impedance spectroscopy (EIS), opencircuit voltage decay (OCVD), and stabilized efficiencies were obtained with a solar simulator (Sun 3000, ABET Tech.) and a potentiostat (Iviumstat, IVIUM Tech.). The output power of light from the solar simulator was fixed at AM 1.5 G (100 mW/cm2). UV-vis absorption spectra were taken by Optizen POP (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 a potentiostat (IviumStat, IVIUM Tech.). Transient PL decay was measured using an invertedtype scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 20x objective (at KBSI, Daegu center, South Korea). A 470 nm single-mode pulsed diode laser (~ 100 ps pulse width at 5~20 MHz repetition rate) was used as an excitation source.

Results and Discussions The traditional TiO2 compact layer was successfully replaced by the thermally evaporated CdS thin layer. After putting the CdS powder into the crucible, the target was heated at the vacuum condition to deposit the evaporated CdS onto the cleaned FTO glasses. The thickness of evaporated CdS film was measured by the QCM equipped in the thermal evaporator. Considering that the conventional thickness of TiO2 compact layer used in PSCs is ~ 50 nm, the thickness of CdS HBL was controlled from 20 to 50 nm. Morphology of the evaporated CdS film with the optimum thickness of 20 nm (which will be discussed later) was observed by the scanning electron microscope (SEM) and shown in Figure 1. For the comparison, there are also shown the images of the bare FTO glass, TiO2 compact layer with the thickness of ~30 nm, and the MAPbI3 perovskite film deposited on the CdS thin layer. The TiO2 compact layer was deposited by the conventional spin-coating and calcination method,30 while the perovskite film was deposited by the recently reported Lewis-base adduct method.29 In the CdS sample, the uniformly deposited CdS film covering the rough FTO surface was



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observed. The film was seemed to be deposited in the form of aggregated CdS nanoparticles, making the surface of sharp FTO crystals smooth and round-shaped. On the other hand, in the TiO2 compact layer negligible difference in the surface shapes was detected. Interesting point was that because of the different electron conductivities of TiO2 and CdS, much clearer SEM image of the CdS layer was obtained. According to the references,31-32 CdS has a higher electron mobility (~ 103 cm2/V·s) than TiO2 (~ 101 cm2/V·s), which implies CdS can be a promising alternative HBL material to TiO2. To be used as the HBL, an acceptable degree of transparency should be satisfied because the external light is injected to the perovskite layer after penetrating the HBL. Excessive light absorption by the HBL causes the decreased intensity of incident light to perovskite, resulting in the degradation of photocurrent generation. To figure out the optical property of the CdS HBL, the transmittances of the CdS HBL films deposited on FTO glass with various thicknesses were measured and shown in Figure 2 (a). Because CdS has the band gap energy of ~ 2.3 eV,33 the absorption onsets at the wavelength of ~ 550 nm were identically observed in all CdS HBL films. Also, as the CdS HBL becomes thicker, the lower transmittance was observed. Actually, CdS has a considerably high absorption coefficient of ~ 104 cm-1,34 which is one of the reasons why CdS quantum dot has been the most widely used sensitizer in quantum dot-sensitized solar cells. The advantageous optical property of CdS, however, became a disadvantage at this case by absorbing some part of the incident light. Therefore, the optimization of the thickness of CdS HBL is essentially required before the device fabrication. Meanwhile, the reference TiO2 HBL exhibited nearly the same transmittance (except for the slight absorption below the wavelength of ~ 350 nm) with the FTO glass in spite of the similar thickness with the CdS HBLs (Figure S1). The transmittance results are exactly coincident with the incident photon-to-current efficiency (IPCE) measurements of the PSCs with various CdS HBL thicknesses. Compared to the reference TiO2-PSC, there were observed in all the CdS-PSCs slightly decreased external quantum efficiencies (EQEs) in the overall region, and especially decreased below the wavelength of ~ 550 nm (Figure 2 (b)).


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The performances of the various CdS-PSCs and the reference TiO2-PSC were investigated by the J-V measurement and described in Figure 3 (a). For the reliable reproducibility, PCEs of the 36 identical devices at each condition were measured and expressed as the PCE distribution histogram (Figure S2). The J-V curves in Figure 3 (a) are the representative curves, which are the most frequently observed during the measurements. The reference TiO2-PSC exhibited the PCE of 15.4%, with the photovoltaic parameters of Voc (1.02 V), Jsc (22.3 mA/cm2), and F.F (0.68). This reference PCE performance is similar to the previously reported PCEs of the planar-structured PSCs.8, 35 On the other hand, slightly decreased PCEs were observed in the CdS-PSCs with various thicknesses. As the CdS HBL became thicker, the devices exhibited gradually decreasing PCEs of 12.2%, 11.5%, and 10.1%, respectively, because of the extended length of electron pathways to the bottom FTO electrode, and the decreased incident light to the perovskite layer. The related photovoltaic parameters of the devices are listed in Table 1. It can be concluded that the CdS HBL should be thinner as much as possible. However, when the CdS film was too thin (below ~ 20 nm), the severe decrease of PCE was observed because of the diminished hole-blocking effects by the CdS HBL (the J-V curves are provided in Figure S3). Therefore, 20 nm was considered to be the appropriate thickness of CdS HBL in this study. In further analyses, CdS-PSC with the CdS HBL of 20 nm was prepared and investigated. To acquire the more reliable PCE information, stabilized efficiencies of the both TiO2-PSC and CdSPSC (20 nm) were measured at the constant bias voltage of 0.8 V (Figure 3 (b)). This stabilized efficiency measurement is a commonly used measuring method to obtain the reliable PCE of PSCs, by avoiding the hysteresis phenomena of PSCs.13 To get the stabilized efficiency, the output current from the cell under the light illumination is measured at a fixed bias potential where the maximum output power was detected in the J-V curve of the cell. After a certain time for the stabilization of the output current, the stabilized efficiency is estimated by the product of bias potential and the stabilized photocurrent. As revealed in the graph, the TiO2-PSC and CdS-PSC generated the photocurrent density of 19 mA/cm2 and 15.2 mA/cm2 respectively, which can be converted into the stabilized efficiencies of 15.2% and 12.1% respectively. The obtained stabilized efficiencies are coincident with the previously acquired PCEs from the J-V measurement, indicating there was no significant



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hysteresis in the both PSCs. The hysteresis can be also checked with the J-V curves depending on the scan directions (Figure S4). To investigate the photo-stability characteristics of both PSCs, the J-V curves of the TiO2-PSC and CdS-PSC before and after the light illumination for 12 hours were measured and plotted in Figure 4. The high power conversion efficiency of 15.4% generated by the initial TiO2-PSC was strikingly reduced to 3.1% after the light illumination. On the contrary, although the initial efficiency of the CdS-PSC was slightly lower than that of TiO2-PSC, it demonstrated highly maintained PCE after the light illumination, exhibiting the insignificant decrease of PCE from 12.2% to 11.4%. Considering the severe photo-induced degradation of TiO2-PSC, it can be proposed that CdS-PSC of this work has a higher potential than the TiO2-PSC in the future practical usage. The related photovoltaic parameters were further investigated by the continuous light-soaking measurement. To get the photo-stability profile of a solar cell, two kinds of measuring methods are widely used with the potentiostat. One of them is measuring the photocurrent generated by the cell under the continuous light illumination, and displaying the photocurrent density as a function of illuminated time.23, 36 In the other way, the usual J-V characteristic of the device is measured at a certain interval time while the device is exposed to the continuous light illumination. The photostability of devices in this study was investigated with the latter method. With the time interval of one hour, J-V profiles of the TiO2-PSC and CdS-PSC were measured under the continuous light illumination, and the resultant photovoltaic parameters are exhibited in Figure 5 with the error bars. The mean value of PCEs of the reference TiO2-PSC exhibited the severe degradation during the constant light illumination of 12 hours, retaining only 18% of the initial PCE. On the other hand, highly improved photo-stability was observed in the CdS-PSC where the final PCE mean value was 91% of the initial PCE. This well-retained PCE of the CdS-PSC was an integrated result of the highlymaintained photovoltaic parameters of Jsc, Voc, and FF, as shown in Figure 5 (b), (c), and (d). On the contrary, the all photovoltaic parameters of the reference TiO2-PSC exhibited gradually degrading tendencies, resulting in the severe degradation of PCE. This result suggests that previously reported


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insufficient photo-stability of PSCs might not be attributed to the intrinsic characteristic of perovskite absorber. Rather, it can be attributed to the external factors like the device architecture or compositing materials. CdS is considered to be one of the solutions for photo-stable PSCs because there is no oxygen vacancy in CdS, which means charge trapping effect by the HBL might be reduced. Additional analyses were conducted to reveal the change of charge carrier dynamics in PSCs by the light illumination. At first, the electrochemical impedance spectroscopy (EIS) measurement and open-circuit voltage decay (OCVD) analyses were conducted for the both TiO2-PSC and CdS-PSC, before and after the continuous light illumination of 12 hours (Figure 6). EIS measurements of the PSCs were conducted with the bias potentials of open circuit voltages of each device, and plotted in the form of Nyquist plot. Unlike some common Nyquist plots of DSSCs showing three semicircles,37 only one dominant semicircle was observed in each measurement. It can be postulated that the semicircle comes from the interfacial impedance at the interface of HBL/perovskite, because it is known that the carrier conductivity of hole is much higher than that of electron,9 and hence charge separation at the HBL/perovskite interface is much slower than at the perovskite/HTM interface.16 Therefore, the dominant impedance was considered to be the interfacial impedance at HBL/perovskite, and its value was calculated by fitting the Nyquist plots with the equivalent circuit (Figure S5). Values of the charge transfer resistance (Rct) of the initial devices were calculated as 108 Ω for the TiO2-PSC and 120 Ω for the CdS-PSC, respectively. Rct of the CdS-PSC slightly increased to 130 Ω after the light illumination for 12 hours. On the other hand, significant increase of Rct was observed in the TiO2-PSC, showing the Rct value of 283 Ω after the same light illumination for 12 hours. The considerable increase of Rct in the TiO2-PSC indicates severe degradation in the charge transfer dynamics occurs during the continuous light illumination. The coincident result was also found in the OCVD measurement (Figure 6 (b)). The Voc decay of the both initial devices exhibited similarly decreasing curves, indicating that charge recombination in the reference PSC and CdS-PSC occurs at the similar rate. However, after the illumination strongly accelerated decaying rate was observed in the TiO2-PSC, while almost no change was found in the decay curve of the CdS-PSC. The accelerated



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recombination rate of the TiO2 PSC after the light illumination can be explained by the presence of electron trap sites, as proposed in the previous literature.22 Trapped electrons in the oxygen vacancies at the TiO2 surfaces (which are activated by the constant UV exposure) are likely to recombine with the holes in the perovskite absorber, resulting in the accelerated recombination rate. On the other hand, because there is no oxygen vacancy in the CdS HBL, it has lower concentration of surface trap density, which explains why the less degradation was observed in the CdS-PSC after the light illumination. The photo-induced degradation mechanism of the both devices was additionally investigated by the time-resolved photoluminescence (PL) decay measurement. To investigate the charge extraction dynamics at the HBL/perovskite interface exclusively, HTM was not deposited onto the perovskite layer. The both TiO2/perovskite film and CdS/perovskite film before and after the light illumination were prepared and their PL decay was observed in the time scale of nanosecond (Figure 7 (a)). Interestingly, the similar charge extraction rate was observed in the both initial films (estimated average PL lifetime by a data-fitting was 7.74 ns in the TiO2/perovskite film, and 7.99 ns in the CdS/perovskite film. The raw data before the data-fitting is provided in Figure S6). However, a considerably decreased charge extraction rate was observed in the TiO2/perovskite film after the light illumination (PL lifetime of 15.07 ns), while a negligible difference was found in the aged CdS/perovskite film (PL lifetime of 8.33 ns). This huge difference in the charge extraction rates once again suggests that the photo-induced degradation of PSCs is closely related to the charge transfer dynamics, rather than the chemical degradation of perovskite absorbers. The suggested photo-induced degradation mechanism of both PSCs is illustrated in Figure 7 (b). As previously reported,22 surface trap sites of the TiO2 layer are considered to be activated by the continuous light illumination. The activation of these trap sites are known to be originated from the adsorption and desorption reaction of oxygen atoms at the oxygen vacancies. Trapped electrons in these trap sites are easy to recombine with the holes in the perovskite and HTM, as described in the left side of the schematic illustration. On the contrary, the CdS HBL seems not to form such a


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significant deep trap energy level, demonstrating highly maintained photovoltaic performances during the constant light exposure.

Conclusion In conclusion, a new kind of hole-blocking layer (HBL) made of CdS for perovskite solar cell (PSC) is reported in this study, to improve the photo-stability of PSCs. The CdS HBL was deposited by the thermal evaporation, and compared with the traditional HBL (TiO2 compact layer) in the various analyses. As revealed in the result of absorption spectrum and IPCE measurement, some portion of incident light intensity to the perovskite layer was reduced by the light absorption of CdS HBL, resulting in the slight decrease of power conversion efficiency (15.4% of the TiO2 PSC, and 12.2% of the CdS-PSC). In spite of the slightly decreased photovoltaic efficiency, CdS-PSC could be an attractive PSC because it demonstrated considerably enhanced photo-stability under the continuous sunlight illumination. While the traditional planar TiO2 PSC lost 82% of its initial efficiency after 12 hours of light illumination, CdS-PSC in this work maintained 91% of its initial efficiency under the same condition. From the additional analyses of EIS, OCVD, and transient PL decay for the mechanism study, the improved photo-stability of the CdS-PSC is considered to be the advantageous characteristic of oxygen vacancy-free CdS HBL. Unlike the TiO2 with many oxygen vacancies and surface traps activated by the light illumination, CdS has less such defects and hence less degrading photovoltaic performances under the light exposure. On the basis of this study, it is expected to explore more ideal HBL materials for PSCs to achieve the two major goals: high power conversion efficiency and excellent photo-stability of PSCs.

Supporting Information Optical transmittance of the compact TiO2 layer on the FTO glass, distribution histograms of the photovoltaic performances of the TiO2 PSC and various CdS PSCs, J-V curves of the various



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PSCs adopting the CdS HBLs with the thicknesses below 20 nm, J-V curves of the TiO2 PSC and CdS PSC obtained with the both forward and reverse scan directions, equivalent circuit models used in the EIS analyses, and raw data of the time-resolved PL decay measurements.

Acknowledgement This work was supported by the National Research Foundation of Korea (2013-R1A2A2A05005344).

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Figures

Scheme 1. Schematic illustration describing the overall processes of the device fabrication. The reference TiO2 PSC was prepared along with the upper process line, while CdS PSC was fabricated by the lower processes.


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Figure 1. Scanning electron microscope (SEM) images of (a) the bare cleaned FTO glass, (b) compact TiO2 layer with the thickness of ~30 nm deposited onto FTO glass, (c) evaporated 20 nm CdS thin layer on the FTO glass, and (d) the MAPbI3 perovskite film deposited on the CdS layer. All the images are taken at the same magnification, with the scale bar of 1 um.



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Figure 2. (a) Optical transmittance measurement results of the bare FTO substrate and FTO/CdS layers with various thicknesses from 20 to 50 nm. Transmittance spectrum of the compact TiO2 layer was nearly same with the base line, as provided in Figure S1. (b) Incident photon-to-current efficiency (IPCE) measurement of the reference TiO2 PSC and various CdS PSCs with different CdS thicknesses.


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Figure 3. (a) Representative J-V curves of the reference TiO2-PSC and the CdS-PSCs with various CdS thicknesses, measured under the 1 sun condition (light illumination with the power of 100 mW/cm2). (b) Stabilized efficiency measurement of the TiO2-PSC and 20-CdS PSC, measured at the fixed bias potential of 800 mV.



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Figure 4. J-V curves of the both PSCs measured before and after the continuous light illumination for 12 hours. PCE of the CdS-PSC was almost maintained, while the severe PCE degradation from 15.4% to 3.1% was observed in the TiO2 PSC.


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Figure 5. Time-dependent degradation profiles of photovoltaic parameters of the TiO2-PSC and 20-CdS PSC. For the clear comparison of photo-stability, the normalized values of (a) PCE, (b) short-circuit current density, (c) open-circuit voltage, and (d) fill factor are provided. Three identical devices are fabricated for each type of PSC, and their average values are indicated with the error bars.



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Figure 6. (a) Nyquist plots of the reference TiO2 PSC and CdS PSC before and after the light illumination for 12 hours, obtained from the electrochemical impedance spectroscopy (EIS) measurements. (b) normalized opencircuit voltage decay (OCVD) curves of the both devices before and after the light illumination for 12 hours.


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Figure 7. (a) Time-resolved photoluminescence (PL) decay curves of the both devices, before and after the light illumination for 12 hours. For the clarity, the all intensities are normalized and plotted in the log scale. (b) proposed charge transfer mechanisms explaining the different photo-stabilities of the TiO2 PSC and CdS PSC.



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Table 1. Photovoltaic parameters of the reference TiO2-PSC and the CdS-PSCs with various CdS thicknesses. Cell ID

Jsc (mA/cm2)

Voc (mV)

Fill factor

PCE (%)

TiO2-PSC

22.35

1016

0.68

15.4

20-CdS PSC

17.54

977

0.71

12.2

30-CdS PSC

16.74

981

0.70

11.5

50-CdS PSC

16.11

973

0.65

10.1


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Table Of Contents (TOC)


Novel CdS Hole-Blocking Layer for Photostable Perovskite Solar Cells

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