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Fengjiu Yang , Hong En Lim , Feijiu Wang , Masashi Ozaki , Ai Shimazaki , Jiewei Liu , Nur Baizura Mohamed , Keisuke Shinokita , Yuhei Miyauchi , Atsu...
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Highly Efficient and Stable Perovskite Solar Cells by Interfacial Engineering Using Solution-Processed Polymer Layer Feijiu Wang,†,‡ Ai Shimazaki,§ Fengjiu Yang,† Kaito Kanahashi,∥ Keiichiro Matsuki,∥ Yuhei Miyauchi,† Taishi Takenobu,∥,⊥ Atsushi Wakamiya,*,§ Yasujiro Murata,§ and Kazunari Matsuda*,† †

Institute of Advanced Energy and §Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Advanced Science and Engineering, Waseda University, Tokyo 169-8555, Japan ⊥ Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan ∥

S Supporting Information *

ABSTRACT: Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices.

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INTRODUCTION

Many methods have been proposed for improving the stability and active-layer coverage in perovskite solar cells.16,21−24 Although many efforts have been made, work is still ongoing in the search for exceptionally stable, environmentally friendly, and inexpensive materials for use in solving critical device problems through facile and cost-effective methods. Polymers, such as poly(methyl methacrylate) (PMMA), represent one type of candidate for meeting these requirements. PMMA can form a compact layer based on an inter-cross-linked network and has been widely used to protect devices from oxygen and moisture.25−27 Meanwhile, some reports have claimed that PMMA plays a role in passivating the surface trap states and suppressing the hysteresis in field-effect transistors and light-emitting devices.28,29 Few reports on the applications of polymer layers for improvements in solar cell performance have been published.25,27,30 Passivation and tunneling effects in perovskite solar cells have been reported.27,30 However, the detailed mechanism of polymers in solar cells is still not clear and requires further investigation. The grain boundaries and roughness of perovskite thin layers that induce many pinholes are difficult to remove by the solution-processing method even though related fabricating technology has been greatly improved. Some pinholes are deep enough to result in direct connection between the HTM and

Solar cells based on methylammonium lead halide pervoskites (CH3NH3PbX3, X = I, Br, Cl) have attracted much attention in recent years.1−8 Since 2010, the power conversion efficiency (PCE) of perovskite solar cells has been improved dramatically through improvements and engineering of solvents,2 interfaces,3,9 and materials.4,10 Several properties of methylammonium lead halide perovskites (CH3NH3PbX3) contribute to their superior performance as active layers in fabricated solar cells: a broad range of strong light absorption,5 small exciton binding energy,11,12 long charge diffusion length,7 and high charge-collection efficiency.13 However, some emergent issues with CH3NH3PbX3 materials remain to be solved for future industrial applications. Large crystallites have been observed to result in the formation of large voids (pinholes) and boundaries in the perovskite layer,14−16 which can produce shunt-leakage paths that reduce the photovoltaic performance. This occurs because such defects allow direct contact between the holetransport material (HTM) and the electron-transport layer (ETL), acting as a parallel diode in the device and deteriorating both the fill factor (FF) and the open-circuit voltage (Voc). Meanwhile, the surface traps in perovskite lead to nonradiative recombination losses, which also degrade the performance of solar cells.17,18 Moreover, the vulnerability of CH3NH3PbX3 to humidity and the escape of CH3NH2X from CH3NH3PbX3 are the main causes of instability in methylammonium lead halidebased solar cell devices.19,20 © XXXX American Chemical Society

Received: December 2, 2016 Revised: December 13, 2016

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DOI: 10.1021/acs.jpcc.6b12137 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic diagram showing the structure of a CH3NH3PbI3 solar cell with a PMMA layer. (b) Cross-sectional SEM image of a perovskite solar cell with a PMMA layer. m/c-TiO2 denotes the mesoporous layer of TiO2 and the compact layer of TiO2. The inset shows a topview SEM image of CH3NH3PbI3 covered by a PMMA layer. The scale bar in the image corresponds to 1 μm. (c) Current density−voltage curves of a perovskite solar cell containing a PMMA layer measured during forward and backward voltage sweeps. (d) IPCE spectra of perovskite solar cells with a PMMA layer.

Subsequently, the substrate was immersed in a solution of TiCl4 (440 μL, special grade) in 100 mL of water at 70 °C for 30 min and then rinsed twice with distilled water. The substrate was sintered at 500 °C for 20 min. The mesoporous TiO2 layer was formed by spin-coating an ethanol suspension of a TiO2 paste (PST-18NR, particle diameter ≈ 20 nm), followed by drying at 125 °C and annealing at 500 °C. Active Layer Fabrication.32 In an Ar-filled glovebox, a 1.3 M solution of PbI2 and CH3NH3I in dimethyl sulfoxide (DMSO) was deposited on the mesoporous TiO2 by spincoating at room temperature, and then 0.5 mL of toluene was dropped on the rotating substrate. The resulting transparent film was annealed on a hot plate at 100 °C for 20 min. Hole-Transport Layer and Electrode Fabrication. The prepared PMMA in chlorobenezene was spin-coated on the substrate at a rotation speed of 4000 rpm for 30 s and then dried for 30 min at 70 °C. A solution for spiro-OMeTAD coating was prepared by dissolving 72.3 mg of spiro-OMeTAD in 1 mL of chlorobenzene, to which 28.8 μL of 4-tertbutylpyridine, 9.1 mg of lithium bis(trifl uoromethanesulfonyl) imide (Li-TFSI), and 13.2 mg of Co(4-t-butylpyridyl-2-1Hpyrazole)3·3TFSI had been added. The spiro-OMeTAD solution was deposited at 4000 rpm for 30 s. Finally, a gold layer (80 nm) was thermally deposited on the hole-transport layer. Characterization and Measurements. The optical absorption spectra of the perovskite films were obtained using a UV/vis/NIR spectrophotometer (Hitachi, U-4100). To perform photovoltaic testing, the devices were irradiated using a solar simulator (San-Ei Electric, XES-40S1) under AM 1.5 conditions (100 mW cm−2), and the J−V data were recorded using a source meter (Keithley, 2400) with a 2 × 2 mm2 mask. The AM 1.5 conditions for the solar simulator were confirmed using a standard cell (Bunkoukeiki, BS-500BK). The solar cells were tested without light presoaking and with a voltage sweep rate of 50 mV/s. For the incident photon-to-current conversion efficiency (IPCE) measurements, the devices were tested using a monochromatic xenon arc light system (Bunkoukeiki, SMO250III). The microstructure of the perovskite film and the

ETL. To avoid the direct connection between the HTM and ETL, PMMA is expected to fill voids (pinholes) in the perovskite layer, which would contribute to improvements in the photovoltaic performance and stability of perovskite solar cells. Herein, we report the improvement of the photovoltaic performance and stability of organo-lead iodide perovskite (CH3NH3PbI3) solar cells through the insertion of a solutionprocessed PMMA layer between CH3NH3PbI3 and 2,2′,7,7′tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD). We achieved a high photovoltaic conversion efficiency of 18.1% in organo-lead iodide perovskite solar cells by using a PMMA layer (at a concentration of 10 mg/mL). Moreover, the photovoltaic performance of the perovskite solar cells containing a PMMA layer exhibited a very small decrease of only about 5% from the original value during exposure to ambient moisture over 20 days. The mechanisms that lead to improved photovoltaic performance and stability by a solutionprocessed polymer layer are discussed in detail.

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EXPERIMENTAL SECTION Chemicals and Materials. Fluorine-doped tin oxide (FTO) was purchased from Asahi Glass Co., Ltd., Tokyo, Japan. Purified PbI2 (L0279)31 and CH3NH3I (M2556) were kindly provided as a gift by Tokyo Chemical Industry Co., Ltd. Mesoporous titanium oxide (PST-18NR) was purchased from JGC Catalysts and Chemicals Ltd. PMMA (MW ≈ 15000, powder) was purchased from Sigma-Aldrich Co., Ltd. Dehydrated (H 2 O < 8 ppm) N,N-dimethylformamide (DMF), 2-propanol, and chlorobenzene were purchased from Wako Chemical Co., Ltd., and used after drying. All other chemicals and reagents were purchased from Wako Chemical Co., Ltd., and used as received unless otherwise noted. TiO2 Layer Fabrication. A patterned and cleaned transparent conducting oxide substrate (FTO, 25 × 25 mm2, Asahi Glass Co., Ltd.) was covered with a ∼25-nm-thick TiO2 compact layer by spray pyrolysis of a titanium diisopropoxide bis(acetylacetonate) (75 wt % in 2-propanol, Tokyo Chemical Industry Co., Ltd.) solution in ethanol (0.05 M) at 450 °C. B

DOI: 10.1021/acs.jpcc.6b12137 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (a) Current density−voltage curves with varying concentrations of PMMA solution. (b) Jsc, Voc, FF, and PCE values of perovskite cells as functions of PMMA concentration. (c) Ideality factor of perovskite solar cells as a function of PMMA concentration. The inset shows a plot of ln(I) versus voltage to extrapolate the ideality factor. (d) PMMA concentration dependence of Rsh and Rs for perovskite solar cells.

hundreds of nanometers, as shown in the inset of Figure 1b. Some voids and pinholes in perovskite layers on mesoporous TiO2 can originate from bubbles during the drop-casting and annealing process, which produces crystal boundaries in the perovskite layer.33 Typical photocurrent density−voltage (J−V) curves of a perovskite solar cell containing 10 mg/mL PMMA coated on CH3NH3PbI3 are shown in Figure 1c. The forward scan from the short-circuit voltage to the open-circuit voltage with a forward bias voltage (FS) and the reverse scan from the opencircuit voltage to the short-circuit voltage with a backward bias voltage (BS) were measured to investigate the cell hysteresis. The solar cell was tested under conditions of Air Mass 1.5 global sunlight at 100 mW/cm2 (AM 1.5 G), with illumination from the FTO side at a scan rate of 50 mV/s. The FS J−V curve of the solar cell exhibited a short-circuit current density (Jsc) of 22.0 mA/cm2, an open-circuit voltage (Voc) of 1.03 V, and a fill factor (FF) of 0.72, resulting in a PCE (η) of 16.4%, whereas the BS J−V curve presented a Jsc value of 21.9 mA/ cm2, a Voc value of 1.05 V, and an FF value of 0.77, resulting in an η value of 17.7%. We compared the device performances of cells with and without a PMMA layer. The typical PCEs of cells without PMMA were 14.1% in FS scans and 15.4% in BS scans in the J−V curves (Supporting Information, Figure S1). The PCEs of the solar cells were much improved by the addition of a thin PMMA layer coating between the perovskite and HTM layers. The results of systematic performance investigations are described below. The hysteresis of J−V curves in the perovskite solar cells with and without the PMMA layers was also investigated (Supporting Information, section S1). The evaluated hysteresis index (HI) was about 0.01 in the perovskite solar cells both with and without a PMMA layer, which further suggests the superior photovoltaic performance in the former perovskite solar cells. Figure 1d shows the incident photon-to-current efficiency (IPCE) spectrum for a perovskite solar cell containing a

cross-sectional structures of the devices were characterized by scanning electron microscopy (SEM; JEOL, JEM-6500F). The surface topography was measured by tapping-mode atomic force microscopy (AFM) using Nanscope Analysis software (version 1.50, Bruker). The AFM images were obtained for perovskite layers with a thickness of ∼300 nm cast on a cleaned quartz substrate. Capacitance (C)−voltage (V) measurements were carried out with a frequency response analyzer (Solartron Analytical) and a dielectric interface (Solartron) in an Ar glovebox. Preparation of the Samples for Optical Analysis. For photoluminescence (PL) analysis, a CH3NH3PbI3 thin film with a thickness of ∼100 ± 10 nm was directly fabricated on a glass substrate using the same substrate cleaning and CH3NH3PbI3 procedures as used for the solar cells. The excitation wavelength for PL spectroscopy was 532 nm. Steadystate PL spectroscopy measurements were performed under a weak excitation power density to prevent degradation of the perovskite film.

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RESULTS AND DISCUSSION

Methylammonium lead iodide perovskite (CH3NH3PbI3) was fabricated on compact TiO2 layers by a one-step method. The procedures for the fabrication of the perovskite solar cells are described in the Experimental Section. Figure 1a shows the device architecture of a perovskite solar cell with a thin PMMA layer on the perovskite layer. A cross-sectional image of the device and a top-view image of the perovskite layer are shown in Figure 1b and the inset, respectively. The device architecture and SEM images of a perovskite solar cell without a PMMA layer are also presented in the Supporting Information (Figure S1). The typical thicknesses of the compact TiO2, mesoporous TiO2, and CH3NH3PbI3 layers were approximately 25, 100, and 300 nm, respectively, and those of the PMMA and spiroOMeTAD layers were approximately 10 and 220 nm, respectively, according to the cross-sectional SEM images. The perovskite layer was microcrystalline with a length scale of C

DOI: 10.1021/acs.jpcc.6b12137 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Table 1. Photovoltaic Parameters Extracted from Current Density−Voltage Curves of CH3NH3PbI3 Perovskite Solar Cells with Different PMMA Concentrations CPMMA (mg/mL)

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Rsh (kΩ cm2)

Rs (Ω cm2)

n

0 5 10 15 30

21.5 22.0 22.0 21.3 19.7

1.01 1.02 1.03 1.00 1.01

0.67 0.68 0.72 0.67 0.60

14.6 15.3 16.4 14.2 12.0

0.67 0.90 1.19 1.04 0.38

6.79 7.28 7.47 6.61 9.16

2.53 2.16 2.20 2.17 2.62

PMMA layer. The IPCE responses from ∼300 to ∼780 nm suggest that the perovskite layer plays a role in both solar-light absorption and photocarrier generation in the solar cell. The integrated current was also calculated using the overlap between the external quantum efficiency (EQE) spectrum and the AM 1.5 G solar photon flux for the cell. Note that the calculated integrated photocurrent yield for the current density was 21.4 mA/cm2, which is consistent with the photocurrent density (∼22.0 mA/cm2) determined from the J−V curves in Figure 1c. We investigated the influence of the PMMA layer thickness on the photovoltaic performance by changing the PMMA concentration in the fabrication process. The thickness of PMMA was verified using cross-sectional SEM images of the cells with different PMMA concentrations (5, 10, 15, and 30 mg/mL), as described in section S2 of the Supporting Information. Figure 2a shows typical J−V curves for perovskite solar cells with different PMMA concentrations. Table 1 reports the values of Jsc, Voc, FF, PCE, shunt resistance (Rsh), series resistance (Rs), and ideality factor (n) obtained from analyses of the J−V curves. The values for Jsc, Voc, FF, and PCE were 21.5 mA/cm2, 1.01 V, 0.67, and 14.6%, respectively, for a solar cell without PMMA (0 mg/mL). This PCE value is typical for perovskite (CH3NH3PbI3) solar cells fabricated by a one-pot method. The improved values of Jsc = 22.0 mA/cm2, Voc = 1.02 V, and FF = 0.68 result in a PCE of 15.3% for the perovskite solar cells using a PMMA concentration of 5 mg/mL. A further enhancement in PCE to 16.4% results from values of Jsc = 22.0 mA/cm2, Voc = 1.03 V, and FF = 0.72 obtained by increasing the concentration of PMMA to 10 mg/mL. The FF increased because Rsh improved from 0.67 kΩ cm2 (0 mg/mL) to 1.19 kΩ cm2 (10 mg/mL). In contrast, the PCEs of perovskite solar cells decreased to 14.2% and 12.0% at increased PMMA concentrations of 15 and 30 mg/mL, respectively. These results indicate that thick PMMA layers reduce the quantum tunneling of holes to the anode, resulting in low carrier collection efficiencies. Figure 2b displays the changes observed in photovoltaic performance (Jsc, Voc, FF, and PCE) as a function of PMMA concentration. The PCE exhibited a maximum at an intermediate PMMA concentration (10 mg/mL). The change in PCE is related to the enhancements in Jsc and FF. Figure 2d shows the PMMA concentration dependence of Rsh and Rs evaluated from the analysis of J−V curves. Both the Rsh and Rs values determine the FF, which strongly affects the PCE of solar cells. The Rs value was found to increase continuously with increasing PMMA concentration, whereas the Rsh value reached a maximum at an intermediate concentration of PMMA (10 mg/mL) and decreased with further increases in the PMMA concentration. The balance between Rsh and Rs indicates that the optimum conditions correspond to an intermediate PMMA concentration (10 mg/mL), which results in a maximum FF value of the cells.

The diode ideality factor, n, was also evaluated to understand the carrier-transport mechanisms in the solar cells.34 n is given by

k T d ln Idark 1 = B n q dV

(1)

where kB is the Boltzmann constant, q is the electronic charge, V is the applied voltage, and Idark is the dark current. The diode ideality factor was derived from the slope of the plot in the inset of Figure 2c and is shown as a function of the PMMA concentration in Figure 2c. The ideality factor without a PMMA layer was found to be 2.53, which suggests the occurrence of nonlinear recombination losses in the device.35 However, the ideality factor of 2.2 obtained at PMMA concentration of 10 mg/mL indicates a reduction in carrier recombination losses.36 Perovskite solar cells without an HTM were also investigated to further understand the effects of PMMA layers (Supporting Information, section S3). Atomic force microscopy (AFM) measurements were conducted to obtain information about the surface roughness in the perovskite layer. Figure 3a,b shows AFM images of perovskite layers with and without a PMMA layer (10 mg/mL). The height profiles indicated by the dotted lines in the AFM images are shown at lower part of Figure 3a,b. The perovskite layer without a PMMA layer exhibited greater roughness than that with a PMMA layer. The AFM image and the corresponding height profile of the perovskite layer without PMMA clearly shows grain boundaries in the microcrystalline perovskite with a typical size of about 250 nm. Moreover, the maximum height variation of ∼268 ± 15 nm is close to the thickness of the perovskite layer (∼300 nm), which suggests a strong possibility for connections to occur between the HTM and the ETL in the perovskite solar cell. The root-mean-square values of roughness were found to be 17.6 ± 1.6 and 28.0 ± 1.4 nm in perovskite layers with and without a PMMA layer, respectively, which also demonstrates the reduction in surface roughness achieved by the PMMA layer. Moreover, analysis of the AFM images of perovskite layers with and without a PMMA layer (Supporting Information, section S4) indicated surface coverage values of 99.8% and 93.2%, respectively. This suggests that the solution-processed PMMA covering the perovskite layer also preferentially filled the pinholes and boundaries in the perovskite layer. It is well-known that grain boundaries and pinholes in the perovskite layer strongly affect the performance of solar cells through shunt-current leak paths.14,16 The PMMA layer reduces the shunt-current paths between the HTM and the ETL, which improves the photovoltaic performance through carrier selection transport in solar cells.37 We also investigated the passivation effects of a PMMA layer on the perovskite layer using photoluminescence (PL) spectroscopy (Supporting Information, section S5). Figure 3c,d shows the results of spatial mapping of the integrated PL D

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intensity from 610 to 950 nm for perovskite layers with and without a PMMA layer. Perovskite with a PMMA layer exhibited a stronger PL intensity caused by effective passivation on the surface and in pinholes by the PMMA layer. These results suggest that a PMMA layer can successfully achieve surface and boundary passivation of perovskite layers. Figure 3e,f shows schematic diagrams of a carrier dynamics model in perovskite solar cells with and without a PMMA layer based on the experimental results. According to the model, the HTM composed of spiro-OMeTAD covering the perovskite layer penetrates and fills a large number of pinholes and grain boundaries in the perovskite layer of the cell without a PMMA layer (Supporting Information, section S6). This increases the possibility of partial connection between the spiro-OMeTAD filling the pinholes and grain boundaries in the perovskite and the ETL composed of TiO2, which reduces the built-in voltage of the solar cells (Supporting Information, section S7). Photogenerated holes conducted through spiro-OMeTAD in the pinholes and grain boundaries can recombine with electrons in the ETL to form shunt-leak current paths that produce carrier recombination losses. Moreover, surface and boundary trap states in the perovskite layer reduce the electron extraction efficiency through nonradiative carrier recombination processes. In contrast, the solution-processed PMMA covers the perovskite layer and penetrates and fills the pinholes and grain boundaries in the perovskite layer. PMMA provides insulation that prevents connections between the ETL and the HTM and passivates the surface trap states in the perovskite layer. These functions reduce the shunt-leak current paths and carrier recombination losses in the corresponding perovskite solar cell, which is also confirmed by the Rsh value, as shown in Figure 2d. A thin PMMA layer on perovskite, therefore, contributes to increases in FF, Voc, and Jsc, resulting in a higher PCE than for solar cells without a PMMA layer.

Figure 3. (a,b) AFM images of the perovskite layer surface (a) without and (b) with a PMMA layer, and the lower part shows height profiles of (a) and (b) along the dashed white line cut, respectively. (c,d) PL images of perovskite (c) without and (d) with a PMMA layer. (e,f) Schematics of the carrier dynamics model in perovskite solar cells (e) without and (f) with a PMMA layer.

Figure 4. (a) Current density−voltage curves of perovskite solar cells without and with a PMMA layer. (b) IPCE spectra of solar cells without and with a PMMA layer. The inset shows the normalized IPCE spectra without and with a PMMA layer. (c) PCE histograms of perovskite solar cells for 45 devices without a PMMA layer and 60 devices with a PMMA layer. (d) Stability of photovoltaic performance in perovskite solar cells during 20 days under ambient conditions (humidity ≈ 70%) at room temperature. E

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over, the reproducibility and variability of the devices were improved because of the reduced number of shunt-current paths in the perovskite solar cells containing the PMMA layer. This inexpensive and environmentally friendly polymer layer contributes to an improved PCE, stability, and reproducibility of perovskite solar cells and shows high potential for application in various film-based electronic devices.

We next optimized the photovoltaic performance of perovskite solar cells containing a PMMA layer. Figure 4a shows the J−V curves of the perovskite solar cells with and without a PMMA layer that exhibited the best performances. The Jsc, Voc, and FF values of perovskite solar cell with a PMMA layer were 22.3 mA/cm2, 1.06 V, and 0.77, respectively, which produced a high PCE of 18.1% in comparison with a PCE of 16.1% for cells without a PMMA layer. The corresponding IPCE spectra of solar cells containing PMMA were found to have broad plateaus with a maximum value of 90.1% over the entire visible-light range, as shown in Figure 4b. This indicates a high charge-collection efficiency of the perovskite solar cells. The integrated Jsc values calculated from the IPCE spectra are 21.3 and 19.5 mA/cm2 in cells with and without a PMMA layer, respectively, which match well with the values measured from the J−V curves in Figure 4a. The normalized IPCE spectra shown in the inset of Figure 4b display an improvement in EQE from 500 to 750 nm resulting from insertion of the PMMA layer. This result is important for further improving the solar cell performance because most cells fabricated using CH3NH3PbI3 show low IPCEs in the long-wavelength region. We tested the photovoltaic performances of many perovskite solar cells (45 cells without and 60 cells with a PMMA layer) to investigate the reproducibility of the devices. As seen in Figure 4c, the histograms of PCE clearly demonstrate better performance in perovskite solar cells containing a PMMA layer, with a small standard deviation. The average PCE value of 17.34% ± 0.49% for perovskite solar cells with a PMMA layer is much better than the value of of 16.05% ± 0.74% obtained for cells without PMMA, demonstrating the high reproducibility and performance of perovskite solar cells containing a PMMA layer. Additionally, the polymer layer in perovskite solar cells effectively works to protect the layer from moisture and oxygen in the air and to limit the escape of CH3NH2I from the active layer.9,27 Figure 4d shows the time evolution of the PCE values, that is, the stabilities, of solar cells with and without a PMMA layer stored under ambient humidity conditions at room temperature (humidity, ∼70%; temperature, 298 K). The PCE of the perovskite solar cell without a PMMA layer degraded to 175 μm in SolutionGrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970.

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CONCLUSIONS In summary, we have described an improvement in the photovoltaic performance and stability of organo-lead perovskite solar cells obtained through the addition of a solutionprocessed PMMA layer. Perovskite solar cells containing a PMMA layer exhibited high photovoltaic efficiency above 18%. The PMMA layer effectively separates the HTM from the ETL, which eliminates shunt-current paths and passivates the perovskite layer, thereby improving the PCE. Simultaneously, PMMA also acts as a protective layer that shields perovskite from oxygen and moisture under ambient conditions. MoreF

DOI: 10.1021/acs.jpcc.6b12137 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b12137 J. Phys. Chem. C XXXX, XXX, XXX−XXX