Evolution of Chemical Composition, Morphology, and Photovoltaic

Mater. , 2016, 28 (1), pp 303–311. DOI: 10.1021/acs.chemmater.5b04122. Publication Date (Web): December 11, 2015. Copyright © 2015 American Chemica...
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Evolution of Chemical Composition, Morphology, and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite under Ambient Conditions Weixin Huang,†,‡ Joseph S. Manser,†,§ Prashant V. Kamat,†,‡,§ and Sylwia Ptasinska*,†,⊥ †

Radiation Laboratory, ‡Department of Chemistry and Biochemistry, §Department of Chemical and Biomolecular Engineering, and Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States

⊥

S Supporting Information *

ABSTRACT: The surface composition and morphology of CH3NH3PbI3 perovskite films stored for several days under ambient conditions were investigated by X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction techniques. Chemical analysis revealed the loss of CH3 NH 3+ and I− species from CH3 NH 3PbI3 and its subsequent decomposition into lead carbonate, lead hydroxide, and lead oxide. After long-term storage under ambient conditions, morphological analysis revealed the transformation of randomly distributed defects and cracks, initially present in the densely packed crystalline structure, into relatively small grains. In contrast to PbI2 powder, CH3NH3PbI3 exhibited a different degradation trend under ambient conditions. Therefore, we propose a plausible CH3NH3PbI3 decomposition pathway that explains the changes in the chemical composition of CH3NH3PbI3 under ambient conditions. In addition, films stored under such conditions were incorporated into photovoltaic cells, and their performances were examined. The chemical changes in the decomposed films were found to cause a significant decrease in the photovoltaic efficiency of CH3NH3PbI3.

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INTRODUCTION

Although perovskite solar cells (PSCs) are highly efficient, they undergo rapid and undesirable degradation, which limits the practical application of PV devices.19−24 Degradation of CH3NH3PbI3 perovskite due to several independent factors has been systematically studied. For instance, exposures to high humidity under dark storage cause changes to the crystallization in CH3NH3PbI3 perovskite, which leads to the reduction (∼16fold) of the half-life of PSCs from 0 to 90% relative humidity (RH).25 Annealing at relatively high temperatures also strongly affects perovskite morphology.26 Furthermore, exposure to concentrated sunlight accelerates the degradation of CH3NH3PbI3 perovskites.27 Therefore, the exposure of perovskite films to environmental conditions (e.g., heat, light intensity, moisture) can result in their synergistic degradation. A recently proposed mechanism for the perovskite degradation indicated that CH3NH3PbI3 decomposes into PbI2 and CH3NH3I, involving a further decomposition of the latter product into CH3NH2, and I2.28 In addition, another study suggested a slightly different decomposition mechanism in which the presence of water in the air can result in the release of PbI2, CH 3 NH 2 , and HI. 6 Although the decomposition of CH3NH3PbI3 has been proposed as mentioned above, our grasp of the details of the degradation mechanism of perovskite

The shift toward the use of sunlight as an abundant source of renewable and sustainable energy has led to a recent surge in a new generation of thin-film solar cells.1−4 In particular, the rapid growth of research into organic−inorganic hybrid perovskites has led to promising developments in photovoltaic (PV) technology with power conversion efficiencies greater than 20%.5−10 Unlike conventional PV materials, hybrid perovskites can be fabricated easily using low-temperature solution preparation methods.11,12 This has led to rapid progress in perovskite PV efficiency, from the initial dye-sensitized architecture to thin film device stacks mirroring commercial CdTe and copper indium gallium selenide thin film technologies.13 The structures of organic−inorganic perovskites are typically represented by the general formula AMX3, where A is a small organic cation in the three-dimensional structures (e.g., CH3NH3+ or (HC(NH2)2+), M is a metal cation (e.g., Sn2+ or Pb2+), and X is a monovalent halide anion (e.g., Cl−, Br−, or I−). Varying the metal or halide components can influence the perovskite bandgap between 1.5 and 3.1 eV, providing tunable absorption across the visible spectrum.14−16 In addition to tunability, the large power conversion efficiencies, simple and low-cost solution preparation methods, and ease of incorporation into a wide variety of device architectures make hybrid perovskites promising light harvesters for solid-state solar cells.17,18 © 2015 American Chemical Society

Received: October 23, 2015 Revised: December 11, 2015 Published: December 11, 2015 303

DOI: 10.1021/acs.chemmater.5b04122 Chem. Mater. 2016, 28, 303−311

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Figure 1. (a) Photographs of CH3NH3PbI3 films deposited on fluorine-doped tin-oxide (FTO) and stored under ambient conditions for several days; evolutions of photoelectron spectra of (b) I 3d5/2 and (c) Pb 4f, and (d) the atomic ratios of I/Pb and N/Pb. The error bar in panel d represents the standard error of the mean of six samples. The spectral fittings of I 3d5/2 and Pb 4f are shown in Figures S2a,b. N/Pb and I/Pb ratios are presented in Table S1a.

Figure 2. Evolutions of (a) C 1s and (b) O 1s photoelectron spectra and ratios of (c) CO32−/Pb and (d) O529.3 eV/Pb of perovskites stored under ambient conditions. The error bars in panels c and d represent the standard error of the mean of six samples. The spectral fittings of C 1s and O 1s are shown in Figures S2c,d. CO32−/Pb and O529.3 eV/Pb ratios are presented in Table S1b.

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RESULTS AND DISCUSSION Following exposure to the laboratory environment (details in the Experimental Section), CH3NH3PbI3 perovskite films showed a gradual color conversion from dark gray to yellow with increasing storage time (Figure 1a). Therefore, to assess the chemical changes occurring over several days of storage, we recorded the high-resolution photoelectron spectra. The survey spectra are shown in Figure S1 in the Supporting Information. Several representative spectra for I 3d5/2 and Pb 4f are presented in Figure 1, panels b and c, respectively. The I 3d5/2 spectrum shows a strong peak at a binding energy (BE) of 619.4 eV for the pristine film, as has been previously reported for CH3NH3PbI3 perovskites.16 Interestingly, after several days of storage, the total area under the I 3d5/2 peak decreased gradually relative to the total area of both Pb 4f peaks. This attenuation of the relative intensity implies a decrease in the concentration of iodide ions within the perovskite samples. The atomic ratios of I/Pb and N/Pb are estimated from the integrated areas under the Pb 4f, I 3d5/2, and N 1s peaks, which

that has been exposed to ambient conditions is still in its infancy. Therefore, our ultimate aim is to obtain details on the evolution of hybrid perovskite composition and morphology under ambient conditions that have still remained an unsolved challenge. In this study, we examined the transformation of the composition and morphology of CH3NH3PbI3 films under ambient laboratory conditions. To elucidate both the chemical changes and the influence of these compositional changes on solar cell performance, we analyzed the results of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction (XRD) of perovskite films exposed to ambient conditions. An improved understanding of the degradation mechanisms and the factors that influence a perovskite stability can facilitate fabrication of PSCs with improved longevity and thus their practical applicability. 304

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Figure 3. Evolution of (a) O 1s photoelectron spectra and ratios of (b) O529.3 eV/Pb and CO32−/Pb and (c) I/Pb for PbI2 powder stored under ambient conditions. The error bars in panels b and c show the standard error of the mean of six samples. Pb 4f and C 1s spectra are shown in Figure S3. The spectral fittings of C 1s and O 1s are shown in Figures S4a,b. CO32−/Pb, O529.3 eV/Pb, and I/Pb ratios are presented in Table S2.

The formation of PbCO3 and PbO was also confirmed by the C 1s and O 1s spectra (Figure 2a,b). We attribute the appearance of a higher BE peak at 288.9 eV in the C 1s spectrum to PbCO3 for samples stored under ambient conditions. Moreover, we observed no signal at this BE in the C 1s spectra from the FTOcoated glass that was exposed to the same experimental conditions, which rules out any carbon-/oxygen-containing contamination of the substrate. We attributed the appearance of a signal at a BE of 529.3 eV in the O 1s spectrum (Figure 2b) to α-PbO.32,33 Figure 2, panels c and d reveal increases in both the atomic ratios of CO32−/Pb and O529.3 eV/Pb, indicating the transformation of CH3NH3PbI3 into PbCO3 and PbO under ambient conditions. The CO32−/Pb ratio was calculated from the total area under a peak corresponding to carbonate (288.9 eV) in the C 1s spectra, while O529.3 eV/Pb was calculated from the total area under the O 1s peak located at 529.3 eV. The CO32−/Pb ratio increased gradually over 21 days of storage, and the O529.3 eV/Pb ratio started to increase after 4 days of storage. The absence of α-PbO in samples stored within the first 4 days indicates that the α-PbO formation might result from some intermediate products rather than directly from the perovskite. In addition, we observed two other features at BEs of 531.1 and 532.4 eV in the O 1s spectra. The former feature originated from PbCO3, β-PbO, and the FTO glass,30,31,34 while the latter was derived from lead(II) hydroxide (Pb(OH)2).35 Note that only 50% of Pb was in the form of PbCO3 and α-PbO after 21 days of storage, and no signal was detected that corresponded to the other compounds in the photoelectron spectra. Therefore, the remainder of the chemical compounds in the stored film containing Pb were most likely Pb(OH)2 and β-PbO. Additionally, we recorded photoelectron spectra for PbI2 powder stored under the same ambient conditions to confirm or disprove the formation of PbI2 as a transition phase or a final degradation product, which has been suggested for thermally decomposed perovskite.29 Interestingly, a clear peak at a BE of 529.3 eV was observed in the O 1s spectra for the PbI2 samples

showed that both ratios decreased as a function of the storage time (Figure 1d). The declining ratio of I/Pb during ambient storage suggests the gradual loss of iodide ions in initially stoichiometric perovskite. A recent study in an ultrahigh vacuum environment demonstrated that I/Pb and N/Pb ratios also significantly decrease at 200 °C.29 The I/Pb and N/Pb ratios decreased to 2.0 and 0, respectively, at that temperature. The reduction of I/Pb to 2.0 suggests that the perovskite decomposed thermally to form PbI2 via the evaporation of CH3NH2 and HI.29 In our study, the I/Pb and N/Pb ratios were found to be 0.2 and 0 after 21 days, indicating the formation of alternative lead compounds rather than PbI2. Further analysis of the Pb 4f spectra assisted in the clarification of the perovskite degradation mechanism under laboratory ambient conditions. The Pb 4f spectra for the pristine perovskite exhibit symmetrical peaks at 138.5 and 143.4 eV, corresponding to the spin−orbit splitting of the Pb 4f7/2 and 4f5/2 components, respectively (Figure 1c). It is worth mentioning that a peak corresponding to the Pb−I bond in the Pb 4f spectra for PbI2 and CH3NH3PbI3 has the same BE. Rensmo and co-workers observed a BE shift of 0.3 eV between PbI2 and CH3NH3PbI316 since their photoelectron spectra were aligned on the BE scale with reference to the Fermi level. Thus, the BE shift between PbI2 and CH3NH3PbI3 was due to the shift of the valence band edge of PbI2. The Pb 4f peaks in our study broadened after several days of storage, extending to both lower and higher BEs. This broadening suggests the presence of lead carbonate (PbCO3) at a BE of 139.3 eV and lead(II) oxide (PbO) at a BE of 137.8 eV.30−32 As reported previously, thermal treatments above 100 °C, as well as hard X-rays, induced the reduction of Pb2+ to metallic lead (Pb0).29 However, we observed no significant amount of Pb0 in the photoelectron spectra. The difference between the two measurements most likely stems from the experimental conditions, that is, different temperatures and different powers of the X-ray sources. 305

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Chemistry of Materials stored over 2 days, corresponding with those for α-PbO (Figure 3a). In this case, both the CO32−/Pb and O529.3 eV/Pb ratios showed a similar trend within 8 days of storage (Figure 3b), indicating that PbCO3 and α-PbO were formed at the same rate. In contrast, PbCO3 and α-PbO were formed at different rates in the CH3NH3PbI3 films, as shown in Figure 2, panels c and d. Moreover, the I/Pb ratio (Figure 3c) for PbI2 powder decreased more rapidly than it did for the CH3NH3PbI3 perovskite film. The differences in the decomposition rate of PbI2 and CH3NH3PbI3 indicate that PbI2 might not be a transient compound or a final degradation product under these conditions. As recently proposed, perovskite degrades in the presence of water into the compounds used for its synthesis (CH3NH3I and PbI2).6 Water molecules trigger the decomposition, and CH3NH3I undergoes subsequent transformation to form HI, which can be dissolved and then removed in an aqueous environment, resulting in a loss of iodide (CH3NH3PbI3 → PbI2 + CH 3 NH 2 ↑ + HI↑). Another proposed pathway of CH3NH3PbI3 decomposition suggests the formation of PbI2 and CH3NH3I with further dissociation of CH3NH3I into methyl amine (CH3NH2), iodine (I2), and H2O.28 Furthermore, the formation of a monohydrated phase (CH3NH3PbI3·H2O), suggested as the first decomposition step, is confirmed by elipsometry and XRD.36 Additional exposure to moisture subsequently leads to the formation of a dihydrate characterized by isolated [PbI6]4− units charge-balanced by associated CH3NH3+.25,36,37 In our study, we also performed XRD measurements to better understand the effects of ambient conditions on perovskite structures and to reveal the factors that cause changes in the chemical composition of these films. Figure 4, panel a shows the

Rather than a diffraction peak at 12.58°, which would correspond to that of PbI2 crystals, a shift of 0.22° to a higher 2θ value was observed, resulting in a new diffraction peak at 12.80°. A similar peak shift toward a higher value (13.02°) was previously interpreted as the formation of a transient phase, that is, PbI2+xx−, during perovskite synthesis.41 Such a phase was observed before the completion of CH3NH3PbI3 synthesis in a solution with CH3NH3I:PbI2 < 1. Because of the deficiency of CH3NH3+ and I−, the presence of this transient state may be a consequence of a variation in the coordination environments of the lead ions.41 In our work, because of the decomposition of perovskite into CH3NH3+ and I− species under ambient conditions, Pb atoms cannot coordinate with sufficient amounts of CH3NH3+ and I− during degradation and therefore most likely form a similar transient state. Because the reaction pathways for synthetic and degradation processes are relatively different, the transient state in our study appeared at 12.80°, which was slightly lower than the value reported by Yang et al.41 In addition to changes in the stoichiometry of CH3NH3PbI3 films upon degradation, a peak shift can also occur due to lattice strain.42,43 Lead iodide may be formed with different lattice parameters, and the coherency strain between domains can cause a peak shift in XRD patterns. Therefore, in our study, the peak shift suggests that the degradation of CH3NH3PbI3 under ambient conditions is associated with the formation of a transient structure, that is, PbI2+xx− (0 ≤ x < 1). Moreover, the absence of peaks in the XRD patterns, corresponding to PbCO3, Pb(OH)2, and PbO, indicates that other products are amorphous domain constituents in the ambient stored films. Thus, on the basis of our XPS and XRD results, we deduce the formation of the transient phase PbI2+xx− and suggest that the following reactions occur upon CH3NH3PbI3 degradation under ambient conditions: 4CH3NH3PbI3 + 8H 2O → (CH3NH3)4 PbI6 ·2H 2O + 3Pb(OH)2 + 6HI↑

(1)

(CH3NH3)4 PbI6 ·2H 2O → (CH3NH3)x PbI 2 + x + (4 − x)CH3NH 2 ↑ + (4 − x)HI + 2H 2O

(2)

2(CH3NH3)x PbI 2 + x + 2CO2 + O2 → 2PbCO3 + 2I 2 ↑ + 2xCH3NH 2 ↑ + 2x HI↑

(3)

(CH3NH3)x PbI 2 + x + H 2O + 1/2O2 → Pb(OH)2 + I 2 ↑ + x HI ↑ + xCH3NH 2↑

Figure 4. (a) XRD patterns of PbI2 powder, and CH3NH3PbI3 films stored under ambient conditions for 0, 4, 12, and 21 days, (b) expanded view of the XRD patterns in the region of 2θ between 12° and 15°. Characteristic XRD peaks are indicated by brown squares for PbI2, gray squares for CH3NH3PbI3, pink circles for PbI2+xx−, and orange stars for FTO substrate.

Pb(OH)2 → PbO + H 2O

(4) (5)

Steps 1 and 2 are the hydration of CH3NH3PbI3 and dehydration of (CH3NH3)4PbI6·2H2O.36 As suggested previously, water molecules in air are able to combine to the perovskite, forming a complex hydrate product.25,36,37 In the hydration process, the [PbI6]4− in the 3D network of CH3NH3PbI3 decays to a 0D framework of isolated octahedral in CH3NH3PbI3 dehydrate. To be stoichiometrically equivalent, other lead compositions should simultaneously form during the hydration. XRD patterns in the previous study show the absence of PbI2 peaks in the hydrated perovskite samples.36 In contrast, the formation of crystalline PbI2 occurs during a thermal degradation process of CH3NH3PbI3 that is confirmed by other

XRD patterns of pristine CH3NH3PbI3 as well as CH3NH3PbI3 films and PbI2 powder stored under ambient conditions. The two major diffraction peaks observed at the 2θ values of 12.58° and 14.21° of the (001) and (110) crystal planes are characteristic of PbI2 and pristine CH3NH3PbI3 perovskite, respectively.26,38−40 There was a significant difference between the XRD pattern of the pristine CH3NH3PbI3 film and films stored only for 2 days. 306

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Chemistry of Materials XRD studies.26,44 Therefore, it is possible for amorphous Pb(OH)2 or PbI(OH) to form when CH3NH3PbI3 is broken into the 0D structure (Step 1). As the hydrate product is only stable in high humidity, the repulsion of water molecules and the dissolution of CH3NH3+ from the perovskite lattices result in the formation of the transient phase, PbI2+xx− (Step 2). The resulting PbI2+xx− is reactive under further exposure to ambient conditions and decomposes into other lead compounds (Steps 3−5). As indicated upon our observation, the formation of PbI2+xx− from PbI2 has a different reaction rate than the corresponding transition states of CH3NH3PbI3 upon degradation in Steps 3−5. The standard Gibbs free energy changes of Steps 3−5 were calculated and are displayed in the Supporting Information. Using SEM, we examined the influence of exposure to ambient conditions on the morphology of the perovskite film on FTO glass. Figure 5 shows top-down SEM images of films following

mesoporous TiO2 (Supporting Information) followed by reaction with CH3NH3I dissolved in 2-propanol to form CH3NH3PbI3. Full details of the device fabrication are included in the Experimental Section. Note that films were stored under ambient conditions for the specified duration prior to depositions of the spiro-OMeTAD hole transport layer and gold counter electrode. Figure 6, panel a shows the current

Figure 5. SEM images of CH3NH3PbI3 perovskites stored under laboratory ambient conditions. Images a−d were recorded after 0 (pristine sample), 4, 12, and 21 days of storage, respectively.

Figure 6. (a) J−V curves of the best performing perovskite solar cells prepared from films stored under ambient conditions for the indicated duration. Both the forward (JSC to VOC) and reverse (VOC to JSC) scan directions are shown for each storage time. Voltage sweep rates were 25 mV/s for forward scans (FS) and 50 mV/s for reverse scans (RS). No light soaking or cell preconditioning was employed. (b) External quantum efficiency of devices representing the average photocurrent output of each series of degraded perovskite films.

their storage under ambient conditions for 0, 4, 12, and 21 days. As demonstrated previously, in humid conditions, the perovskite films undergo a recrystallization process and become smooth and highly ordered.25 In addition, annealing can result in the aggregation of the perovskite crystallites to form islands with discernible internal gaps.26 In our work, the SEM images revealed that storage under ambient conditions also changed film morphology significantly. The pristine perovskite films had a compact surface, and individual crystallite shapes were welldefined (Figure 5a). However, after being kept under ambient conditions for 4 days (Figure 5b), the perovskite film exhibited a porous structure across its entire surface. The perovskite films underwent further degradation after longer-term storage and showed more severe morphological changes such as the appearance of coarse surfaces. Given the distinct chemical and morphological transformations associated with perovskite degradation in ambient conditions, we sought to understand the effects of these changes on the solar cell characteristics of CH3NH3PbI3. We fabricated a series of solar cells that incorporated CH3NH3PbI3 as the lightabsorbing component according to a previously reported procedure.45,46 Briefly, PbI2 was deposited on a ∼150 nm

density−voltage (J−V) curves of the best performing devices prepared from perovskite films stored under ambient conditions for 0 (pristine), 3, 7, and 18 days. Both the forward (short-circuit current, JSC, to open-circuit potential, VOC) and reverse (VOC to JSC) scan directions are displayed to highlight the well-known hysteresis associated with this perovskite device architecture.47,48 In Figure S5, a statistical analysis is provided for the relevant device parameters (JSC, VOC, fill factor, and efficiency) across the entire sample set for each exposure period. The pristine films and those stored under ambient conditions for 3, 7, and 18 days exhibited an average power conversion efficiency of 13.4 ± 0.6, 12.6 ± 1.0, 2.2 ± 1.1, and 0.24 ± 0.02%, respectively. Considering the loss of halogen and organic species from the perovskite structure after several days of storage (Figure 1), the sustained efficiency of the devices prepared from films that were kept for 3 days revealed that these devices are relatively insensitive to slight chemical and structural variations. However, a sharp drop in 307

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experimental approaches. We demonstrated that the absence of iodide and the formation of other lead compounds in films after exposure for up to 21 days were due to the decomposition of the hybrid perovskite material. The XRD patterns showed that PbI2+xx− appeared as a transient phase upon CH3NH3PbI3 degradation. Furthermore, the different reaction pathways and faster degradation in PbI2 powder under the same experimental conditions also confirmed the distinct crystalline structure of PbI2+xx−. Devices that incorporated CH3NH3PbI3 films stored under ambient conditions revealed that photovoltaic parameters and anomalous perovskite J−V hysteresis were dependent upon degradation. Our results constitute a basis for more detailed investigations to gain a deeper understanding of the chemical structure and degradation mechanisms of organic−inorganic perovskites. The results here provide crucial information to better understand the composition and morphology changes in CH3NH3PbI3 perovskite that occur under ambient conditions, thereby facilitating the development of techniques for the synthesis and postconditioning of perovskite thin films.

efficiency occurred after 7 days of storage, resulting primarily from decreased JSC and fill factor values (Figure S5). As evident from the external quantum efficiency (EQE) of the various devices in Figure 6, panel b, degradation results in the preferential loss of light harvesting at wavelengths greater than ∼525 nm, which is consistent with previous reports,25 and resulted in the change in film color from dark gray to yellow. Considering the bulk bandgap of PbI2 (Eg = 2.3 eV),49 the higher energy absorption of films stored for 18 days may result from semiconducting PbI2 domains that form upon CH3NH3PbI3 degradation. However, as indicated by the chemical and structural characterization presented above, PbI2+xx− formed upon the degradation of CH3NH3PbI3. An altered coordination environment around Pb could explain the slight discrepancy between the photocurrent onset (∼525 nm, Figure 6b) and the bulk PbI2 band gap. An examination of the J−V characteristics of the perovskite films revealed that, in addition to the standard device parameters, the degree of J−V hysteresis was also a function of CH3NH3PbI3 degradation. This was most apparent from the discrepancy in fill factor (Pmax/(JSC × VOC)) between the forward (FS) and reverse (RS) scans in pristine films and those stored under ambient conditions for 3 days (Figure S5). As proposed previously,48 J−V hysteresis can be quantified using the dimensionless hysteresis index described by Hysteresis index =

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EXPERIMENTAL SECTION

Synthesis of Perovskites and Film Preparation. FTO coated glass slides were cleaned by ultrasonication for 30 min in a detergent solution, followed by rinses with DI H2O, EtOH, and cleaned in air plasma for 5 min. The CH3NH3PbI3 precursor was composed of an equimolar solution of PbI2 (Ultradry grade, 99.999%, Alfa) and methylammonium iodide (Dyesol) N,N-dimethylformamide (DMF) stirred for 2 h at 70 °C. This solution was then spin-cast onto the FTO glass at 2000 rpm for 30 s and annealed at 100 °C for 5 min to form a CH3NH3PbI3 film structure, as indicated by a color change from yellow to dark gray. All steps were carried out under a dry nitrogen glovebox. Storage of Perovskite and PbI2 Powder under Ambient Conditions. The exposure environment can be described as laboratory ambient conditions. The perovskite films were kept in a lidded glass vessel to prevent dust from entering the box. The vessel was not sealed, and small holes were made in the lid to ensure that air could flow in and out of the container. The vessel was placed on a benchtop at a constant room temperature of 298 ± 1 K and relative humidity of 24 ± 2%. The perovskite films were left in a container under constant light from fluorescent tube bulbs (GE 40805- F34CW/RS/WM/ECO/CVG). The optical emission spectrum of the fluorescent tube bulbs was measured by an Ocean Optics USB2000+ and is presented in Figure S6. The light intensity was between 0.25 and 0.5 mW/cm2, as measured by a Thorlabs S302C thermal power sensor. The perovskite films were kept for certain time periods before characterization. PbI2 powder was pressed and mounted on copper foil. Before measurement, the PbI2 samples were placed in the same environment as the perovskite films. Characterization. The perovskite samples were characterized by XPS, a surface sensitive technique, which was carried out under vacuum conditions with a PHI VersaProbe II spectrometer (Al Kα, 1486.6 eV). All elemental spectra were collected at the same analyzer pass energy. Quantitative analysis was achieved from the following equation: (N1/ N2) = ((I1S2)/(I2S1)), where N is the concentration of atoms, I is the peak intensity of photoelectrons, and S is the atomic sensitivity factor provided by the instrument manufacturer. All spectra were calibrated to their corresponding C 1s, which is 285.3 eV,53 and binding energy values for all fitted and assigned spectral peaks are stated with an accuracy of 0.2 eV (Figures S2 and S4). Analyses of XPS peaks were performed with a Casa program. The images of film morphologies were obtained using an FEI Magellan-400 field emission scanning electron microscope (FESEM) operated at 5 keV. XRD measurements were conducted with a Bruker D8 Advanced Davinci Powder X-ray Diffractometer using a Cu Kα source, a standard step size of 0.025 deg, and an acquisition time of 2 s deg−1. The perovskite films on FTO substrates were mounted without further modification. Perovskite Solar Cell Fabrication. A portion of the conducting FTO surface was etched by depositing a thin layer of Zn powder and

JRS (@80% VOC) − JFS (@80% VOC) JRS (@80% VOC)

where JRS(@80% VOC) and JFS(@80% VOC) are the current densities determined from reverse and forward J−V scans, respectively, at a potential that corresponds to 80% of the VOC. A higher hysteresis index indicates a greater discrepancy between the J−V characteristics obtained from reverse and forward scans and thus a greater degree of hysteresis. The average hysteresis index for each exposure interval is given in Table S3 in the Supporting Information. The films kept under ambient conditions for 3 days showed a 43% increase in the hysteresis index relative to the pristine films. The largest degree of hysteresis was observed in films stored for 7 days. Interestingly, the average hysteresis index of the 18-day samples was similar to that of pristine films. This lower value was likely due to the fact that most of the photovoltaic activity of the film had diminished within 18 days of storage. Although the origin of J−V hysteresis in perovskite solar cells is subject to ongoing debate, one potential explanation for the degradation-induced hysteretic change observed in our study is the alteration in microstructure that occurs upon sample storage (Figure 5). As demonstrated previously, larger perovskite grain sizes can mitigate J−V hysteresis,50 possibly due to a reduction of surface trap states at the grain boundaries.51 Therefore, we expected that the morphological evolution of perovskite films under the ambient conditions outlined in Figure 5 (i.e., formation of pinholes and coarsening of the crystalline surface) would increase the number of surface states and thus the degree of hysteresis. The mechanism for reduced hysteresis with extended degradation (18-day films) is less clear but may result from changes in inherent mobile ion concentrations as iodine and CH3NH3I are expelled from the film.52

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CONCLUSIONS In this work, we examined the stability of CH 3NH3PbI3 perovskite films under ambient conditions using various 308

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Chemistry of Materials Notes

reacting with 2 M HCl for 5 min. The FTO surface was then cleaned by ultrasonication in ethanol for 20 min, followed by oxygen plasma treatment for 5 min. The compact TiO2 layer was deposited by spincasting a 0.15 M solution of titanium diisopropoxide bis(acetylacetonate) (Aldrich) in anhydrous 1-butanol at 700 rpm for 10 s, 1000 rpm for 10 s, and 2000 rpm for 30 s. The film was then dried at 125 °C for 10 min. The mesoporous TiO2 layer was deposited by spincasting a diluted paste of 40 nm TiO2 particles using the same spin program detailed earlier (paste synthesis is described elsewhere54). Mesoporous films were annealed at 550 °C for 1 h followed by controlled cooling to room temperature. Electrodes were then treated with 20 mM aqueous TiCl4 at 90 °C for 10 min followed by rinsing and annealing at 500 °C for 30 min. CH3NH3PbI3 was deposited by two-step solution processing. First, 1 M PbI2 (ultradry grade, 99.999%, Alfa) was dissolved in anhydrous N,Ndimethylformamide by stirring at 70 °C for 1 h. The solution was then passed through a 0.2 μm syringe filter. A 50 μL aliquot was spread evenly across a TiO2 electrode and spin-cast at 5000 rpm for 20 s, followed by drying at 40 °C for 3 min and 100 °C for 5 min. Next, 100 μL of 8 mg/ mL CH3NH3I (Dyesol) in anhydrous 2-propanol was spread evenly across the PbI2 film followed by two additional aliquots of 100 μL. The precursors were allowed to react for 20 s before spin-casting at 2000 rpm for 20 s and drying at 100 °C for 5 min to form the final CH3NH3PbI3. At this point, the films were exposed to ambient conditions in the same manner as described for the samples used in our XPS study prior to hole transport layer and counter electrode deposition. The hole transport layer was composed of 72.3 mg/mL of spiroOMeTAD (EMD Millipore), 28.8 μL/mL of 4-tert-butylpyridine (Aldrich), 17.5 μL/mL of bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, Aldrich), and 10 mol % FK 102 Co(II) PF6 Salt (Dyesol) relative to spiro-OMeTAD in anhydrous chlorobenzene. LiTFSI and FK 102 Co(II) PF6 Salt were added from 520 and 300 mg/mL of acetonitrile stock solutions, respectively. The hole transport layer was spin-cast at 5000 rpm for 30 s. After overnight storage in a dry air desiccator, a 100 nm Au counter electrode was deposited by thermal evaporation through a shadow mask at 10−6 Torr and 10 Å/s deposition rate. Hybrid perovskite and hole transport layer preparation were carried out under nitrogen atmosphere with less than 5 ppm water. Characterization of Perovskite Solar Cells. Photocurrent− voltage measurements were carried out under 100 mW/cm2 simulated solar irradiation generated from a 300 W Xe lamp with AM 15G filter using a Princeton Applied Research 2273 (PARstat) potentiostat. The incident power density was measured using a Thorlabs S302C thermal power sensor. The voltage sweep was maintained at 50 mV/s for reverse scans (VOC to JSC) and 25 mV/s for forward scans (JSC to VOC). No preconditioning or light soaking was utilized. EQE measurements were carried out in 10 nm increments using a Newport Oriel QE/IPCE measurement kit with a silicon photodiode detector.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award No. DE-FC02-04ER15533. This is contribution number NDRL 5076 from the Notre Dame Radiation Laboratory. The authors thank the cSEND Materials Characterization Facility for the use of the PHI VersaProbe II XPS and the use of the Bruker pXRD. J.S.M. acknowledges the support of King Abdullah University of Science and Technology (KAUST) through the award OCRF-2014-CRG3-2268.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04122. Survey spectra and spectral fitting for CH3NH3PbI3 perovskite films and for PbI2 powder; statistical analysis of the relevant device parameters; optical emission spectrum of fluorescent tube bulbs; different atomic ratios obtained from photoelectron spectra for CH3NH3PbI3 and PbI2; average hysteresis index for each exposure interval; calculated Gibbs free energy changes for reaction steps 3−5 (PDF)

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DOI: 10.1021/acs.chemmater.5b04122 Chem. Mater. 2016, 28, 303−311

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Chemistry of Materials

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