Effects of Small Polar Molecules (MA+ and H2O) on Degradation

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Effects of Small Polar Molecules (MA+ and H2O) on Degradation Processes of Perovskite Solar Cells Chunqing Ma,†,‡ Dong Shen,†,‡ Jian Qing,†,‡ Hrisheekesh Thachoth Chandran,†,‡ Ming-Fai Lo,*,†,‡ and Chun-Sing Lee*,†,‡ †

Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China ‡ City University of Hong Kong Shenzhen Research Institute, Shenzhen 518057 Guangdong, P. R. China S Supporting Information *

ABSTRACT: Degradation mechanisms of methylammonium lead halide perovskite solar cells (PSCs) have drawn much attention recently. Herein, the bulk and surface degradation processes of the perovskite were differentiated for the first time by employing combinational studies using electrochemical impedance spectroscopy (EIS), capacitance frequency (CF), and X-ray diffraction (XRD) studies with particular attention on the roles of small polar molecules (MA+ and H2O). CF study shows that short-circuit current density of the PSCs is increased by H2O at the beginning of the degradation process coupled with an increased surface capacitance. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSCs involves a lattice expansion process, which facilitates MA+ ion diffusion by creating more efficient channels. These results provide a better understanding of the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film. KEYWORDS: surface, bulk, degradation, polar molecules, perovskite solar cell

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and H2O are closely related to the degradation processes.20 However, there are so far few studies on the effects of the small polar molecules on PSC performances, and understanding of their roles in degradation is far from clear. Herein, the degradation process of PSCs of MAPbI3 prepared with different PbI2/MAI ratios (1:0.9, 1:1, and 1:1.1) is investigated. The PSC prepared with the 1:1 PbI2/ MAI ratio, which shows three distinct degradation phases, is chosen for detailed investigation with particular attention toward distinguishing surface and bulk degradations. The underlying degradation mechanisms are investigated with complementary characterizations, including scanning electron microscopy (SEM), X-ray diffraction (XRD), capacitance− voltage (CV), capacitance frequency (CF), and electrochemical impedance spectroscopy (EIS). Using these studies, we present clear experimental evidence showing the roles of the small polar molecules MA+ and H2O on the surface and bulk degradation.

INTRODUCTION Organic−inorganic perovskite materials, mostly based on methylammonium lead iodide (MAPbI3), have shown significant potential for photovoltaic applications.1−5 However, their limited environmental and photo stabilities under operating conditions have hindered their commercialization.6−10 Tremendous efforts have been dedicated to understanding their degradation mechanisms and developing appropriate solutions to improve their stabilities. Degradation of perovskite materials can be caused by humidity, oxygen, light, and heat.11−13 Seok et al. observed a remarkable color change of MAPbI3 after being exposed to a humidity of 55%, which suggests that the perovskite is quite sensitive to moisture.14 Katz et al. explored degradation testing of perovskite films under concentrated sunlight of 100 suns (1 sun = 100 mW cm−2) and found that the decomposition of perovskite is induced by illumination in the presence of water but not dominated by illumination.15 In nearly all studies, water has been considered as a main cause of the instability of the perovskite. It has been demonstrated that the degradation of the perovskite is caused by hydrolysis reactions in which hydrated perovskite phases, MAPbI3·H2O and MA4PbI6·2H2O, can be formed and is followed by the release of gas phase (MAI) upon illumination and heating.16,17 It was also reported that there are mobile MA+ ions in the perovskite film during degradation.18,19 It appears that polar molecules including MA+ © XXXX American Chemical Society

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RESULTS AND DISCUSSION Morphologies and structures of the perovskite films prepared at PbI2/MAI ratios of 1:0.9, 1:1, and 1:1.1 are investigated with SEM and XRD measurements. All of the films were prepared Received: January 26, 2017 Accepted: April 18, 2017

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DOI: 10.1021/acsami.7b01348 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a−c) Top-view SEM images of MAPbI3 prepared at different PbI2/MAI ratios: (a) 1:0.9, (b) 1:1, and (c) 1:1.1. (d) XRD patterns of the films.

Figure 2. Degradation process of devices prepared with different PbI2/MAI ratios. Normalized (a) Jsc, (b) Voc, (c) FF, and (d) PCE.

representing the MAPbI3 crystallographic planes (110), (220), and (310), respectively, in all of the films.23 For investigation of the degradation process, devices with a configuration of ITO/PEDOT:PSS/perovskite/C60/BCP/Ag were prepared with different PbI2/MAI ratios. Current density−voltage (J−V) characteristics of the devices right after fabrication are summarized in Figure S1 and Table S1. Different photovoltaic performances can be observed in the PSCs prepared with different PbI2/MAI ratios. We associate

on ITO/PEDOT:PSS substrates. SEM images (Figure 1) clearly show that the three films are compact and uniform.21 Unlike the other two samples (Figure 1b and c), the sample prepared with a 1:0.9 PbI2/MAI ratio (Figure 1a) is decorated with rodlike particles with bright contrast, which is speculated to be PbI2.22 This is consistent with the observation that the XRD peak from the PbI2 phase at around 12.7° increases as the PbI2 ratio increases (Figure 1d). Three dominant planes are observed at diffraction angles of 14.13, 28.41, and 31.82°, B


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Figure 3. CF plots of PSCs prepared with a 1:1 PbI2/MAI ratio during stages (a) 1, (b) 2, and (c) 3. (d) Time evolution of surface capacitance determined at a frequency of 1 Hz.

Figure 4. (a) Normalized Jsc time plot of devices with different PbI2/MAI ratios under 2 sun illumination. (b) Carrier density profile of devices with different PbI2/MAI ratios calculated from Mott−Schottky analysis as a function of distance x (nm) from the MAPbI3/C60 interface. (c) CF plots of the devices with different PbI2/MAI ratios, and (d) schematic diagram showing the effects of H2O absorption on the contact.

MAPbI3 perovskites indicated that MAI-terminated surfaces are more sensitive to moisture,25 whereas the stronger Pb−I bond endows PbI-terminated surfaces with better resistance against moisture-induced degradation. Normalized Jsc plots (Figure 2a) suggest that the samples prepared with different PbI2/MAI ratios have different degradation processes. The 1:0.9 PbI2/MAI sample experiences a gradual degradation, and the degradation process of the 1:0.9 sample begins from the edge of the device toward the center as shown in Figure S3. In contrast, for the MAI-rich sample (1:1.1), the Jsc drops quickly after a small initial increase, and the degradation process begins from both the edge and the

this to a self-doping effect of the residual MAI and PbI2, which can influence the electrical and optical properties of perovskite.24 Short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) of the unpackaged devices were measured every 60 min at room temperature with a relative humidity (RH) of 80 ± 5% (Figure 2). Degradation processes measured under moist nitrogen and dry air are shown in Figure S2, and results indicate that the degradation is largely related to the moisture. As shown in Figure 2, the stability decreases with increasing MAI concentration. This result can be explained by the moisturesensitive nature of MAI. Recent dynamic simulations of C


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Figure 5. (a) XRD patterns of the MAPbI3 (1:1) sample at the beginning of stages 1 and 3. (b, c) Time evolution characterization of the XRD patterns. (d) Nyquist plots of the sample at different stages. (e) Bulk capacitance (as measured at 106 Hz) and (f) ion diffusion coefficient as functions of time during stage 3.

center of the device (Figure S3). Noting that the Ag back contact is dense and impervious to water, the route for water ingress could be along the interface from the edge of PSCs. The degradation route of the PSC prepared with a 1:1.1 PbI2/MAI ratio of indicates that the perovskite surface is moisturesensitive, which is consistent with the moisture-sensitive property of MAI. Similar to the external quantum efficiency (EQE) degradation observed by Song et al.,26 Jsc in the 1:1 sample shows three distinct phases of degradation. During stage 1, a slight increase of Jsc from 1 to 1.13 is observed in the first 8 h. Stage 2 features a fast initial drop in Jsc, which is then stabilized. After the slow degradation at the end of stage 2, stage 3 is again characterized by rapid degradation. These results are highly reproducible. The normalized Jsc for eight devices prepared with 1:1 PbI2/MAI and EQE spectra of perovskite devices during the degradation process are shown in Figures S4 and S5. For the mechanism underlying each stage to be better understand, it is important to deconvolute surface and bulk degradations of the perovskite. EIS is one possible way to distinguish between the characteristic impedance signals from the bulk and those arising from the surface because of the different response frequencies.27,28 EIS measurements were carried out over a frequency range of 1−106 Hz and an AC amplitude of 10 mV at room temperature. CF plots are just a rephrasing of the impedance data. Complex capacitance is defined as

C(ω) =

1 iωZ(ω)

(1)

. In the CF plots shown in Figure 3, the low- and highfrequency capacitances are mainly contributed by the surface and bulk properties of the perovskite layer, respectively.29 Time evolution characterization of the surface capacitance (value measured at 1 Hz) is shown in Figure 3d. The surface capacitance first shows a mild increase in stage 1, sharply increases in stage 2, and finally decreases rapidly in stage 3. Song et al. have proposed that EQE changes in stage 1 are likely to due to adsorption of water molecules on the surface of the perovskite film.26,30 It is interesting that adsorption of water molecules on the surface can indeed explain the mild increases in Jsc and surface capacitance observed here. It was reported that water molecules can passivate recombination centers on the perovskite surface, leading to reduced surface trap states and better carrier extraction.31−33 Thus, the increased Jsc can be explained by the passivation effect of water molecules. For comparison, the Jsc and surface capacitance measurements were repeated under illumination with a solar simulator using AM1.5G at an intensity of 2 sun. Corresponding results are shown in Figure 4a. An initial increase in Jsc can be observed in the samples prepared with PbI2/MAI ratios of 1:1 and 1:1.1 but not in the 1:0.9 sample. To understand this difference, we carried out Mott−Schottky analysis (Figure 4b) and CF D


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ACS Applied Materials & Interfaces measurements (Figure 4c) for the three devices. The carrier density N(x) in the perovskite as a function of distance x from the MAPbI3/C60 interface can be calculated from the CV using the equation N (x ) = −

D=

(3)

.where LD is the effective ion diffusion length, which is assumed to be equal to the thickness of the perovskite film as shown in Figure S6c. The ion diffusion coefficient follows the order of the bulk capacitance in stage 3. Enhancement of the ion diffusion coefficient in stage 3 (Figure 5f) is a consequence of the formation of more efficient diffusion channels due to water incorporation in the active layer film, as supported by the XRD results. With diffusion of the MA+ ions, the degradation of perovskite crystals leads to the formation of PbI2 crystals, which will block the channels. As a consequence, the ion diffusion coefficient decreases with the formation of PbI2 crystals. With these results, we conclude that the H2O incorporation and MA+ ion diffusion are the main causes of the bulk degradation.

−2 ⎞−1

2 ⎛ dC(x) ⎟ ⎜ qε0εr A2 ⎝ dV ⎠

L D2 TW

(2)

. Panels b and c in Figure 4 show that the increased MAI concentration leads to a high surface carrier density (at small x) and surface capacitance. In Figure 4c, the only difference in the three samples is the PbI2/MAI ratio. The higher surface capacitance in the 1:1.1 sample is attributed to the higher concentration of polar MA+ molecules. Cheng et al. have reported that carrier density in the perovskite device increases with the relative amount of MAI used during fabrication of the perovskite layer.34,35 Therefore, the surface composition is directly related to the carrier density and surface capacitance. Consequently, in the 1:1.1 sample, the increased surface carrier density and capacitance is consistent with the assumption of a higher MA+ concentration on the perovskite surface. The moisture-sensitive property of MAI will facilitate the adsorption of water molecules, and the water molecules can passivate recombination centers at the interface, leading to an increased Jsc. We now proceed to consider the Jsc decay during stage 2 (Figure 2a). This process is attributed to the hydrogen bond formed between MA+ and H2O, which supposedly weakens the bond between MA+ and the PbI6 octahedron.36 This hypothesis is supported by the degradation process of the 1:1.1 sample when exposed to 2 sun illumination: a large decay in Jsc following the Jsc peak can be observed (Figure 4a). The high MAI concentration on the film surface has two effects: (a) uncoordinated MA+ ions can be released from the perovskite film easily, and (b) the moisture-sensitive property of the MAI facilitates H2O absorption. Thus, degradation of the surface structure is the main cause of stage 2 degradation. For stage 3, a detailed analysis with XRD, CF, and ion diffusion calculations was carried out. The XRD results (Figure 5a−c) reveal that the Jsc drop during stage 3 is related to bulk degradation of the perovskite. The MAPbI3 perovskite (110), (220), and (310) peaks actually move toward the lower 2θ side and broaden (fwhm increases from 0.128 to 0.247) at the beginning of stage 3. These results suggest the incorporation of water molecules into the perovskite lattice, leading to a lattice expansion and crystal breakage. Water incorporation can also be verified by the bulk capacitance increase, as shown in Figure 5e. Perovskite peaks then split after the water incorporated into the perovskite crystal (Figure 5b and c).37 Under prolonged exposure, the overall peaks of the perovskite disappear (Figure 5b and c) with the increased PbI2 peak. The degradation process in stage 3 also includes an ion diffusion process, which releases the MA+ and the I− ion from the inorganic scaffold and forms PbI2 crystals. As shown in Figure 5d, the EIS of stage 3 shows a linear region, which is a feature of the ion diffusion. It has been reported that the mobile ion corresponding to the diffusion is the MA+ ion.18,38,39 We fit the impedance spectra in Figure S6a and obtained the Warburg time constant TW. All other Nyquist plots in stage 3 are shown in Figure S6b. From the value of TW, we have calculated the effective chemical diffusion coefficient (D) using the equation

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CONCLUSIONS In this work, electrochemical impedance spectroscopy is applied for the first time to different the bulk and surface degradation processes in the methylammonium lead iodide perovskite layer. It was demonstrated that the adsorption of water, which is believed to be the first step of the perovskite degradation, will lead to an initial increase in the Jsc. The bulk degradation of perovskite induced by the H2O incorporation is also verified, which accompanies an ion diffusion process. These results highlight the important role of small polar molecules on the degradation processes of PSCs, which are also the main cause of the bulk degradation. The present results suggest that approaches such as mixing MA+ with Cs+ or FA+, for controlling the influences of the small polar molecules and inhibition of ion diffusion, should be important for improving the stability of PSCs.

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EXPERIMENTAL METHODS

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ASSOCIATED CONTENT

Perovskite Precursor Solution Preparation and Device Fabrication. The solutions were first prepared by mixing different amounts of MAI (Lumtech) with 1 mmol PbI2 (Sigma-Aldrich) in 1 mL of anhydrous DMF (RCI Labscan) and 1 mmol DMSO (SigmaAldrich). The solutions were then stirred at room temperature overnight. Solar cells with a configuration of ITO/PEDOT:PSS/ perovskite(300 nm)/C60 (20 nm)/BCP(8 nm)/Ag were then fabricated as we reported previously with the perovskite layer prepared by one-step spin-coating and the organic layer prepared by thermal evaporation.40,41 Characterization. J−V curves of the PSCs were measured at 100 mW cm−2 using an Oriel 150 W solar simulator. Surface morphologies of samples were characterized with SEM (Philips XL30 FEG SEM), and XRD measurements were recorded with a Philips X’ Pert diffractometer using Cu Kα radiation. The CV, CF, and EIS measurements were conducted with a ZAHNER IM6 workstation. EIS measurements were carried out over a frequency range of 1−106 Hz and an AC amplitude of 10 mV at room temperature. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01348. J−V characteristics of PSCs at different ratios; photos of the devices during degradation; degradation process of PSCs in moist nitrogen and dry air; degradation process of a PSC with a 1:1 PbI2/MAI ratio; EQE spectra of perovskite device degradation process at stages 1−3; EIS plot and fitting results of PSCs (1:1 sample) at stage 3; E


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Nyquist plots of PSCs (1:1 sample) measured during stage 3; cross-section SEM of PSCs (1:1 sample) right after fabrication (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: mingfl[email protected]. *E-mail: [email protected]. ORCID

Chun-Sing Lee: 0000-0001-6557-453X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Research Grants Council of the Hong Kong Special Administrative Region, China (Project CityU 11304115), and the National Natural Science Foundation of China (51473138).

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Effects of Small Polar Molecules (MA+ and H2O) on Degradation

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