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Energy Conversion and Storage; Plasmonics and Optoelectronics

Humidity-Induced Photoluminescence Hysteresis in Variable Cs/Br Ratio Hybrid Perovskites John M. Howard, Elizabeth M. Tennyson, Sabyasachi Barik, Rodrigo Szostak, Edo Waks, Michael F. Toney, Ana Flavia Nogueira, Bernardo R. A. Neves, and Marina S. Leite J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01357 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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The Journal of Physical Chemistry Letters

Humidity-Induced Photoluminescence Hysteresis in Variable Cs/Br Ratio Hybrid Perovskites John M. Howard1,2, Elizabeth M. Tennyson1,2, Sabyasachi Barik2,3, Rodrigo Szostak5,6, Edo Waks2,4, Michael F. Toney5, Ana F. Nogueira5,6, Bernardo R. A. Neves2,7, Marina S. Leite1,2* 1

Department of Materials Science and Engineering, University of Maryland, College Park, MD 20740, USA 2 Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20740, USA 3 Department of Physics, University of Maryland, College Park, MD 20740, USA 4 Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20740, USA 5 Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA 6 Institute of Chemistry, University of Campinas, Campinas - SP, 13083-970, Brazil 7 Department of Physics, Federal University of Minas Gerais, Belo Horizonte - MG, 31270-901, Brazil *Correspondence and requests for materials should be addressed to: [email protected]

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Abstract Hybrid organic-inorganic perovskites containing Cs are a promising new material for light absorbing and light emitting optoelectronics. However, the impact of environmental conditions on their optical properties is not fully understood. Here, we resolve and quantify the influence of distinct humidity levels on the charge carrier recombination in CsxFA1-xPb(IyBr1-y)3 perovskites. Using in situ environmental photoluminescence (PL), we temporally and spectrally resolve light emission within a loop of critical relative humidity levels. Our measurements show that exposure up to 35% rH increases the PL emission for all Cs (10-17%) and Br (17-38%) concentrations investigated here. Spectrally, samples with larger Br concentrations exhibit PL redshift at higher humidity levels, revealing water-driven halide segregation. The compositions considered present hysteresis in their PL intensity upon returning to a low moisture environment due to partially reversible hydration of the perovskites. Our findings demonstrate that the Cs-Br ratio strongly influences both the spectral stability and extent of light emission hysteresis. We expect our method to become standard when testing the stability of emerging perovskites, including lead-free options, as well as to be combined with other parameters known for affecting material degradation, e.g. oxygen and temperature.

TOC

Cs#17%/Br#17%* Cs#10%/Br#38%*


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Organic-inorganic perovskite films exhibit ideal properties for a wide range of optoelectronic devices, such as light emitting diodes (LEDs)1-4 and solar cells5-7. However, optical and electronic degradation under ambient operating conditions currently limits their implementation into reliable technologies8. The hybrid perovskite structure takes the form of ABX3, with an organic cation (typically formamidinium (FA) CH(NH2)2+ or methylammonium (MA) CH3NH3+) occupying the A-site, a divalent metal (Pb2+) for the B-site, and the halide anion (I-, Br-, or Cl-) located at the X-site. Partially replacing the MA or FA cations with small amounts of Cs was recently shown to enhance thermal stability and process parameter tolerances9-10. The inclusion of Cs compresses the lattice and reduces halide and cation mobility, permitting solar cells with over 1000 hours of stable power output under very specific operation conditions11. Additional optimization of both the Cs and halide concentrations within the perovskite compound is critical for further bandgap tunability, and will likely yield materials that pass accelerated aging tests12 and inhibit moisture degradation. Thus far, incorporating 10-17% of Cs bolsters the structural and environmental stability of FAPbI3 due to an increase in the Goldschmidt tolerance factor10, 13 . Moreover, tuning the Br concentration from 17 to 38% can potentially provide optimal bandgaps for tandem photovoltaics5-6. Therefore, this work focuses on analyzing the light emission properties and stability of four different perovskite samples, which encompass all combinations of these established Cs and Br concentration values. Because the perovskite light-absorbing layer is often composed of inhomogeneous grains, assessment of its variation at similar length scale is important for determining the degree of variability within the samples. High spatial resolution microscopy methods are indispensable in determining the underlying mechanisms of perovskite performance transiency from the nano- to the meso- and microscale14-15, as revealed by Kelvin probe force microscopy16-18 and conductive AFM19-20. Another approach based on optical microscopy, entitled micro-photoluminescence (micro-PL), has proven essential as it can spatially resolve radiative recombination in perovskite thin films and devices21-26. Researchers have used micro-PL on perovskites to quantify detrimental interfacial effects21, identify the role of oxygen absorption on carrier lifetime22, reveal environmental passivation of sub-bandgap defect states23, and show anisotropic diffusion of electronic charge carriers24. Yet, despite widespread use of this technique, the relationship between relative humidity (rH), an established degradation-driving parameter8, 12, 27-28 , and perovskite luminescence emission has not yet been quantified. Here we determine the spatial and spectral dependence of light emission upon exposing perovskites to a controlled relative humidity cycle. We investigate four Cs-containing mixed-halide perovskite films with variable chemical composition, selected based on their promising thermal and structural stability. By spatially resolving the PL signal, we find that while low levels of humidity (< 40% rH) boost the carrier radiative recombination rate, there is a threshold level that then suppresses PL. Our measurements also verify that moisture-induced PL enhancement depends on the halide concentration: samples with larger concentrations of Br have maximum PL emission at 35% rH, while those with lower Br content plateau at 55% rH. Spectral measurements reveal that increasing water vapor pressure content raises the mobility of halide ions in 38% Br films. Finally, we identify the composition (Cs0.1FA0.9Pb(I0.62Br0.38)3) with the largest emission hysteresis, resulting from partially reversible H2Odriven chemical reactions. Our in situ environmental PL technique can be expanded to other perovskite systems, and is an excellent tool for thin film stability diagnostics prior to full device development. We perform humidity-dependent loops and acquire PL in situ for Cs0.1FA0.9Pb(I0.83Br0.17)3, Cs0.17FA0.83Pb(I0.83Br0.17)3, Cs0.17FA0.83Pb(I0.62Br0.38)3, and Cs0.1FA0.9Pb(I0.62Br0.38)3 films to determine how moisture exposure affects the radiative recombination of charge carriers as a function of chemical


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composition. Here, we identify the samples by their Cs and Br concentrations (for example, Cs-10%/Br 17% refers to the Cs0.1FA0.9Pb(I0.83Br0.17)3). Figure 1a-d shows representative scanning electron microscope (SEM) micrographs of the perovskite films, where all samples are composed of ~100 nm grains. These samples did not undergo humidity cycling and, as such, do not exhibit moisture-induced morphological features. Figure 1e-h displays 20 × 20 µm2 PL images acquired at < 5% rH. While the average grain diameter is below the diffraction limit for the objective employed in our measurements, micro-PL is extremely useful as it provides statistics of the variation of the optical response heterogeneity arising from the films. Cs-10%/Br-17%

Cs-17%/Br-38%

Cs-10%/Br-38%

(b)

(c)

(d)

(f)

(g)

(h)

SEM

(a)

Cs-17%/Br-17%

1 μm (e)

1

PL maps

PL (norm.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 μm

0

Figure 1. Morphology and humidity-dependent PL imaging of different composition perovskites. (a-d) SEM micrographs and (e-h) normalized PL images at < 5% rH for Cs-10%/Br-17%, Cs-17%/Br-17%, Cs-17%/38%-Br, and 10%-Cs/38%-Br, respectively. All PL images are independently normalized by their individual minimum and maximum values.

PL at < 5% rH (Figure 1e-h) reveals substantial differences in the spatial features for all compositions investigated here. This indicates that exposure to higher water vapor concentrations has a nonuniform effect on the spatial distribution of luminescence emission. Optical micrographs obtained under < 5% rH and after PL imaging at 55% rH (Figure S2) show detectable morphological changes, with the formation of new structures at 55% rH not seen in the pristine state (i.e. unexposed to ambient environment) that also account for the differences observed in the PL data. There, the global gradients in PL intensity that occurs over the entire 20 × 20 µm2 scan region are due to the procession of moisturedriven chemical reactions that spur halide migration and reduce surface recombination22, 29. We contrast global gradients with the local gradients (typically 1-5 µm) that arise from differences in charge carrier recombination processes of groups of perovskite grains. Additionally, given the pixel dwell time of 0.015 s, light soaking effects are negligible6, 24. To understand the moisture-induced degradation threshold of different perovskite compositions, we quantify the average value of PL emission as a function of humidity, shown in Figure 2. The black curves represent the rH level as a function of time (right y-axis), demonstrating our reproducible and reliable control of the moisture level within the environmental chamber, and define the humidity loop. The relationship between local radiative recombination and humidity is determined by detecting the PL response of each sample across a 20 × 20 µm2 area at stabilized < 5%, 15%, 35%, and 55% rH levels.


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After the exposure to 55% rH, the films are returned to the initial low humidity level to measure the PL emission again. Clearly, relative humidity conditions up to 35% increase the radiative recombination of the perovskites for all concentrations of Cs and Br. Additionally, the standard deviations of the PL images (errors bars in Figure 2) show the sample-independent trend of increasing spatial heterogeneity in the luminescence emission for relative humidity levels up to 35%. When raised to 55% rH, the radiative recombination efficiency of the Br-17% samples (Figure 2a-b) plateaus while that of the Br-38% (Figure 2c-d) decreases, which will be further discussed later. The average PL emission of all four films as a function of both humidity and time is shown together in Figure S3.

Cs'10%/Br'17%$

Cs'17%/Br'17%$ (b)$

(a)$

Cs'10%/Br'38%$

Cs'17%/Br'38%$

(d)$

(c)$

(a)$

Figure 2. PL emission and relative humidity as a function of time. Clockwise: (a) Cs-10%/Br-17%, (b) Cs-17%/Br17%, (c) Cs-17%/38%-Br, and (d) 10%-Cs/38%-Br perovskites. The PL data (dashed lines) error bars indicate the standard deviation of the respective PL images and the shaded areas in the relative humidity data (solid lines) represent the rH sensor uncertainty.

Further insight into the effect of relative humidity on luminescence emission is gained by examining the time dependent distribution of PL intensities. Histograms, acquired by binning the pixels of the 20 × 20 µm2 PL images (displayed in Figure 3a-d), show notable differences in their spatial heterogeneity. Here, the broadness of the histograms is indicative of the overall temporal response. All histograms have the same bin widths and number of bins, allowing for direct comparison of all samples regardless of humidity level or composition. While higher Br concentrations lead to less stability against ambient moisture, the histograms reveal Cs content to be the key parameter controlling the emission stability; the Cs-17% films show a broader distribution of PL intensity upon completing the humidity cycle.


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Cs-10%/Br-17% (a)

Cs-17%/Br-17% (b)

Cs-17%/Br-38% (c)

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Cs-10%/Br-38% (d)

rH < 5% 15% 35% 55% < 5%

(e)

(f)

(g)

(h) < 5% 15% 35% 55% < 5%

Figure 3. Spatial and spectral evolution of humidity-dependent PL. Normalized histograms of PL images (log scale) and normalized PL spectra for perovskites containing (a, e) Cs-10%/Br-17%, (b, f) Cs-17%/Br-17%, (c, g) Cs17%/Br-38%, and (d, h) Cs-10%/Br-38% throughout a relative humidity loop. The arrows indicate the order of the measurements.

The humidity-dependent spectra (Figure 3e-h) confirms that moisture affects the phase stability of the perovskite films, with 55% rH leading to the most pronounced redshift, which begins to reverse upon exposure to dry air. In the case of the Cs-10%/Br-17% sample (Figure 3e), the PL peak only redshifts 0.02-0.04 eV. The Cs-17%/Br-17% perovskite shows the highest spectral stability, broadening slightly with increasing moisture exposure while remaining centered at 1.65 eV. By contrast, Br-38% films exhibit substantial redshift at higher relative humidity. When kept below 5% rH, the Cs-17%/Br-38% material displays a PL peak centered at 1.76 eV. As the humidity is raised, the peak redshifts, reaching 1.73 eV at 15% rH, 1.7 eV at 35% rH, and, finally, 1.64 eV at 55% rH. Upon returning to the < 5% rH condition, the peak blueshifts back up to 1.7 eV. Similar behavior is observed for the Cs-10%/Br-38% perovskites. These results on Br-38% films imply moisture-induced halide segregation30, with domains enriched in either iodide or bromide where the bandgap is lower or higher in energy, respectively. For instance, the Cs-10%/Br-38% spectrum acquired at 35% rH contains an intermediate peak centered at 1.66 eV, indicative of a 30% Br region. We infer that the activation energy for halide migration is lower in the case of Br-38%, permitting a higher degree of halide vacancy mobility. Our measurements are in agreement with prior reports showing PL peak instability for higher concentrations of Br in MAPb(IxBr131 x) 3 . Figure 4 shows the humidity loops of the average PL intensity acquired for each sample (the arrows indicate the procession of the humidity cycle in time). Here, the hysteresis refers to the difference between the states of a sample upon return to the same initial conditions, after being exposed to various


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settings (in this case rH levels). Thus, the difference between the initial and final low humidity PL measurements determines the extent of luminescence hysteresis due to dry air and moisture exposure. We establish that the hysteresis is not due to simple material degradation (which can cause a substantial decrease in PL) by performing an equivalent series of PL measurements under dry air without humidity (see Figure S5), confirming that the average PL intensity increases monotonically under such conditions. The Cs-10%/Br-17% sample exhibits the largest increase in PL intensity when exposed to 15% rH and a moderate amount of hysteresis. The Cs-17%/Br-17% behaves similarly, though experiences its most substantial luminesce enhancement at 35% rH. For 38% Br samples, the decrease in emission after completing the cycle is still evident, but the enclosed area broadens at higher humidity due to the intolerance to 55% rH in comparison to the 17% Br films. Interestingly, the Cs-17%/Br-38% sample has negligible hysteresis despite the reduction in PL intensity at 55% rH.

(a)+ Cs#10%/Br#17%+

(b)+ Cs#17%/Br#17%+

(d)+ Cs#10%/Br#38%+

(c)+ Cs#17%/Br#38%+

Figure 4. Effect of humidity cycling on PL emission. Clockwise: PL vs. relative humidity for (a) Cs-10%/Br-17%, (b) Cs-17%/Br-17%, (c) Cs-17%/38%-Br, and (d) 10%-Cs/38%-Br perovskite thin films. The dashed lines and arrows are guides for the eye, indicating the pathway for PL evolution as a function of humidity.

In our measurements, we flow dry air composed of 80% N2 and 20% O2 and the absorption of O2 occurs through diffusion into the perovskite’s iodide and bromide vacancies32-34. Dry air was chosen over pure N2 to mimic conventional device processing under ambient conditions, as the current main challenge towards perovskites’ commercialization is related to materials’ stability. Further, the PL signal is known for being higher under dry air conditions than under N222, which enables us to distinguish small and significant differences between the samples and between the different stages of rH. We expose each perovskite thin-film to dry air for eight minutes prior to PL microscopy and spectroscopy data acquisition and assume complete O2 uptake, as supported by isothermal gravimetric analysis on MAPbI332. The


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interaction of the excitation laser and the absorbed O2 leads to superoxide formation and this passivates surface states that reduce internal PL quantum yield in pristine films22. While not the focus of this work, note that carefully controlled durations of full-sample illumination may increase the PL quantum yield further22. The exposure of some perovskites to water vapor is known to cause material degradation8, 27, 35-36. In this sense, the relative humidity level, if sufficiently high, controls the speed and, therefore, the extent of the chemical reactions governing decomposition. When H2O is introduced into the system, it begins to intercalate the perovskite structure. At 55% rH, sufficient water exists to form a thin layer of a monohydrate phase (Equation 1) at the surface, as previously observed for other perovskite compositions27-28, 36-40. Once enough water has been absorbed, the dihydrate phase forms and leads to the perovskite decomposition into PbI2, PbBr2, and CsxFA1-xBryI1-y27-28, 39, as indicated by Equations 2-3.

(

Csx FA1−x Pb Iy Br1−y

)

3

4 #$Csx FA1−x Pb Iy Br1−y

)

3

• H2O%& ↔ Csx FA1−x

Pb Iy Br1−y

)

6

(

(Cs FA ) x

1−x 4

(

+ H2O ↔ Csx FA1−x Pb Iy Br1−y

(

3

• H2O

) Pb (I Br ) ↔ Pb ( X ) + 4 (Cs FA (

• 2H2O

)

4

2

y

1−y 6

x

(1)

( )

• 2H2O + 3Pb X

)

I Br1−y + 2H2O

1−x y

2

+ 2H2O

(2) (3)

We find that exposure to 35% rH does not noticeably degrade the Cs-containing perovskite films, but, instead, improves the luminescence efficiency (i.e. optical properties) of the material, in agreement with prior results on FAPbI341. The decomposition of the perovskite layer into precursors initially removes surface defect states not already passivated by O2 (see Figure S4a-c). This reduction in trap states leads to higher internal quantum efficiency and long charge carrier lifetimes, explaining the increase in PL intensity42. However, the decomposition chemical reactions occur more pronouncedly at 55% rH, with noticeable degradation and new surface trap states after ~1 hr at this humidity condition (Figure S4d). These recently formed surface trap states remain even after the sample is returned to a low humidity environment (Figure S4e). The histograms of PL intensity in Figure 3a-d reveal the instability in radiative recombination upon reaching 55% rH. Here, an exponentially decreasing ‘tail’ in PL emerges for all compositions. However, only the Cs-17% perovskites sustain this behavior upon returning to < 5% rH, possibly indicating that larger Cs content, (i.e. more lattice compression), slows the dehydration of the film. X-ray diffraction (XRD) measurements (Figure S6) indicate that these water-perovskite interactions are limited to the surface region of the films for the humidity levels tested here, as the measurements before, during, and after the treatment show no significant variations. Note, however, that for extremely high humidity lebels (rH of 80%) the Cs-containing perovskites investigated showed changes in the bulk material, as presented in Figure S7. Humidity also influences the mobility of the halide ions, with the strength of the effect depending on their relative concentrations, as previously reported29. While moisture initially increases the intensity of luminescence for all samples, the instability of the PL peak is only present for the films incorporating 38%-Br, as revealed by spectrally-resolved PL in Figure 3e-h. Light-induced halide segregation, an established phenomenon in MAPb(IxBr1-x)331, is not observed at < 5% rH for any composition considered here, as in accordance with previous studies5-6. Additionally, in between spectra acquisitions, 8-10 minutes elapsed with the samples held under dark conditions, sufficient time for the reversal of any photoinduced trap formation31. Instead, we find that moisture is the primary driver of halide segregation. This result is supported by the fact that the PL peak in Br-38% films increasingly redshifts into the vicinity of the Br-17% as the relative humidity increases. Other researchers have established that the


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halide concentration permits bandgap tuning43, thus local segregation would result in spectral heterogeneity and is captured with high-resolution microscopy methods as shown here. Additionally, recent DFT calculations indicate that intercalation of H2O into the perovskite lattice lowers the vacancymediated halide ion migration activation energy29, supporting the moisture-driven segregation effect. Partial reversibility of the redshift in Br-38% films can be attributed to a higher activation energy for halide movement once the hydrate phase forms29. We conjecture that lower concentrations of Cs yield more reversible halide migration due to a less constrained perovskite lattice. The hysteresis and humidity loop areas, obtained from Figure 4, provide insight into the tolerance of the perovskite layers towards humidity cycling. Notably, the enclosed areas suggest that the ratio of Cs and Br concentrations strongly influence the hysteretic behavior of the films. Figure 4 reveals that the area enclosed within the hysteresis loop is smallest for Cs-17%/Br-17%. The area is 2× larger for both the Cs-10%/Br-17% and the Cs-17%/Br-38% samples. Finally, the Cs-10%/Br-38% perovskite presents the maximum area, which is 4× larger than of the Cs-17%/Br-17% film. We highlight that the largest area occurs at the maximal ratio of Br/Cs concentration. These measurements on Cs-containing perovskites imply that additional compositional tuning could produce high-quality films without emission hysteresis and minimal hysteresis loop areas. To summarize, we performed humidity loop micro-PL on four different compositions of perovskite films, quantifying the extent of emission enhancement due to moisture exposure. Our measurements reveal that rH levels up to 35% increase the overall PL yield for perovskite layers containing 10-17% Cs and 17-38% Br. Uniquely, samples containing 38% Br exhibited a decrease in luminescence under 55% rH, while those with 17% Br sustain their PL enhancement. For all compositions, exposure to 55% rH changes the spatial distribution of radiative recombination. Humiditydependent spectra show that the magnitude of PL peak redshift of both thin films containing 38% Br is correlated with the ambient moisture level. Finally, we established that humidity cycling leads to hysteresis in the PL emission for all compositions except for Cs-10%/Br-38%. Our work underscores the importance of environmentally controlled PL in elucidating fundamental causes of perovskite instability. Understanding and controlling the effect of moisture on radiative recombination, as shown by our environmental micro-PL measurements, will help inform the optimal environmental conditions and compositions of future fabrication efforts, leading to perovskite-based optoelectronic devices with increased stability. Further, we foresee our environmental PL platform to be combined with temperature dependent measurements, allowing for the deconvolution of each parameter in material degradation. Acknowledgements The authors thank N. Ballew and T. Weimar for technical assistance. M.S.L. thanks funding support from the National Science Foundation (ECCS, award 16-10833), the Ovshinsky Sustainable Energy Fellowship from APS, and SPRINT FAPESP-UMD. M.S.L. and E.M.T. thank the 2017-20178 Hulka Energy Research Fellowship and the UMD All-S.T.A.R. award. R. S. thanks the São Paulo Research Foundation (FAPESP, grant 2017/12582-5). B.R.A.N. received financial support from CAPESBrazil. A.F.N. thanks the São Paulo Research Foundation (FAPESP, Process number 2015/50450-8) and SPRINT FAPESP-UMD. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.


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Supporting Information Thin film fabrication description, details and schematic of experimental setup for environmental photoluminescence, optical micrographs of studied regions at < 5% and 55% rH conditions, aggregated average PL intensity of samples as a function of humidity and time, average PL under just light and dry air, and XRD measurements.

References (1) Xiao, Z.; Kerner, R. A.; Zhao, L.; Tran, N. L.; Lee, K. M.; Koh, T.-W.; Scholes, G. D.; Rand, B. P. Efficient Perovskite Light-Emitting Diodes Featuring Nanometre-Sized Crystallites. Nat. Photonics 2017, 11, 108. (2) Colella, S.; Mazzeo, M.; Rizzo, A.; Gigli, G.; Listorti, A. The Bright Side of Perovskites. J. Phys. Chem. Lett. 2016, 7, 4322-4334. (3) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; et al. Perovskite Energy Funnels for Efficient Light-Emitting Diodes. Nat. Nanotechnology 2016, 11, 872. (4) Zou, S.; Liu, Y.; Li, J.; Liu, C.; Feng, R.; Jiang, F.; Li, Y.; Song, J.; Zeng, H.; Hong, M.; et al. Stabilizing Cesium Lead Halide Perovskite Lattice through Mn(II) Substitution for Air-Stable Light-Emitting Diodes. J. Am. Chem. Soc. 2017, 139, 11443-11450. (5) Bush, K. A.; Palmstrom, A. F.; Yu, Z. J.; Boccard, M.; Cheacharoen, R.; Mailoa, J. P.; McMeekin, D. P.; Hoye, R. L. Z.; Bailie, C. D.; Leijtens, T.; et al. 23.6%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2017, 2, 17009. (6) McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.; Saliba, M.; Horantner, M. T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B.; et al. A Mixed-Cation Lead Mixed-Halide Perovskite Absorber for Tandem Solar Cells. Science 2016, 351, 151-5. (7) Son, D. Y.; Kim, S. G.; Seo, J. Y.; Lee, S. H.; Shin, H.; Lee, D.; Park, N. G. Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering. J. Am. Chem. Soc. 2018, 140, 1358-1364. (8) Huang, W.; Manser, J. S.; Kamat, P. V.; Ptasinska, S. Evolution of Chemical Composition, Morphology, and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite under Ambient Conditions. Chem. Mater. 2015, 28, 303311. (9) Saliba, M.; Matsui, T.; Seo, J. Y.; Domanski, K.; Correa-Baena, J. P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; et al. Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ. Sci. 2016, 9, 1989-1997. (10) Li, Z.; Yang, M.; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K. Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys. Chem. Mater. 2015, 28, 284-292. (11) Christians, J. A.; Schulz, P.; Tinkham, J. S.; Schloemer, T. H.; Harvey, S. P.; Tremolet de Villers, B. J.; Sellinger, A.; Berry, J. J.; Luther, J. M. Tailored Interfaces of Unencapsulated Perovskite Solar Cells for >1,000 Hour Operational Stability. Nat. Energy 2018, 3, 68-74. (12) Domanski, K.; Alharbi, E. A.; Hagfeldt, A.; Grätzel, M.; Tress, W. Systematic Investigation of the Impact of Operation Conditions on the Degradation Behaviour of Perovskite Solar Cells. Nat. Energy 2018, 3, 61-67.


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