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Letter
Optimization of Stable Quasi-Cubic FAxMA1-xPbI3 Perovskite Structure for Solar Cells with Efficiency beyond 20% Yi Zhang, Giulia Grancini, Yaqing Feng, Abdullah M. Asiri, and Mohammad Khaja Nazeeruddin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00112 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017
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ACS Energy Letters
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Optimization of Stable Quasi-Cubic FAxMA1-xPbI3
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Perovskite Structure for Solar Cells with Efficiency
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beyond 20%
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Yi Zhang,1,2 Giulia Grancini,1* Yaqing Feng,2 Abdullah M. Asiri,3 and Mohammad Khaja Nazee-
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ruddin1*
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1
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Switzerland.
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2
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
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3
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University,
Group for Molecular Engineering of Functional Materials, EPFL Valais Wallis, CH-1951 Sion,
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Jeddah, Saudi Arabia
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Corresponding Author
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*
[email protected],
[email protected] 13
ABSTRACT. Complex compositional engineering of mixed halides/ mixed cations perovskite
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has recently fostered a rapid progress in perovskite solar cell technology. Here we demonstrate
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that by simply adding 10% of Formamidinium (FA+) into methylammonium lead iodide
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(MAPbI3) a highly crystalline and compositionally uniform perovskite is formed, self-organizing 1
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into a stable “quasi-cubic” phase at room temperature. We reached power conversion efficiency
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of
over
20.2%,
the
highest
reported
so
far
for
FAMAPbI3
perovskite.
19
20
TOC GRAPHICS
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Hybrid perovskite technology with astonishing power con-version efficiency (PCE) beyond 22%
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promises to be an ideal candidate for near future solar power generation
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excellent optoelectronic properties
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techniques 7, the ability of the perovskite to accommodate different cations and anions within the
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lattice gives room for a myriad of different material combinations
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methylammonium (MA+), formamidinium (FA+), cesium (Cs+) and Rubidium (Rb+) along with a
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mixture of Cl, Br and I halides enabled the fabrication of high efficiency solar cells with, in some
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cases, improved stability 3,7. Additionally, ion intermixing enables a fine tuning of the material
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bandgap with direct impact in tandem solar cell engineering 2. However, as a draw-back,
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compositional engineering induces severe changes in the crystal structure and film morphology
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associated with halide phase segregation 7,8 and light/field-induce ion movement 9–12 altering the
5,6
1–4
. Beyond their
and ease, versatility and low cost in the processing
2,3
. The incorporation of
2
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device function. Interestingly, the inclusion of Cs+ has been shown to enhance device stability 4,9.
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However, the added complexity of the material structure and the difficulties in reproduce the
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performances casts the doubts on the ease of reproducibility and scaling-up of this route. On the
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other side, a much simpler structure where only FA is introduced with MA as the organic
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compound leaving intact the inorganic backbone, have attracted a lot of interest 10–13. FA-based
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perovskite has shown a superior stability compared to the methylammonium, longer charge
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diffusion length and band gap closer to the ideal value 2,7,7,9,11,11,13–16. However, the larger radii of
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the FA cation disturbs the formation of a stable α phase and high quality FAMAPbI3 film,
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especially when large percentage of FA is used (note that pure FAPbI3 undergoes to phase
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transition into yellow non-perovskite δ-FAPbI3
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device performances of MAFAPbI3 to 15-16% due to the poor crystallization of the resulting
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film
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MAPbI3 perovskite processed by a one-step deposition method followed by solvent engineering
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step, a high crystal quality film can be obtained. By means of structural and optical
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characterization we found that the FA+ intercalates with the MA+ within the voids of the PbI6
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octahedra stabilizing the perovskite structure into a “quasi-cubic” phase at room temperature. We
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fabricated solar devices where the perovskite is sandwiched between the 2,2’,7,7’-tetrakis(N,N-
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di-pmethoxyphenylamine)- 9,9’-spirobifluorene (spiro-OMeTAD) hole-transporting material and
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the mesoscopic TiO2 scaffold (m-TiO2) electron transporter in a standard architecture 3,20. The
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results show a significant step forward in the device efficiency, with champion device showing
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PCE>20.2% accompanied by a very good statistics on device performances and hysteresis-free
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behavior. Such performances, not far from what obtained with much more complicated structures
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which incorporate many different compound, can be obtained with a much simpler and highly re3
17–19
12 7,10
)
. This, in turns, have limited so far the
. In this study we found that using a small amount of FA (between 10-30%) into the
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producible protocol. This is utmost importance for the scaling-up and future commercialization
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of this technology.
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Fig. 1a shows the absorption spectra of the mixed cation lead iodide perovskites MAxFAx1-
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xPbI3
at different FA percentage (from 0 to 30%). Increasing the amount of FA% the absorption
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onset experiences a red-shift of around 25 nm as shown in the inset. The normalized
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photoluminescence (PL) spectra are reported in Fig. 1b. In agreement, a significant red-shift in
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the emission peak of MAxFAx1-xPbI3 when increasing x is observed. On top, a reduction of the
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broadening of the peak is measured (see the inset of Fig.1b). The gradual shift derives from the
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intermixing of FA and MA in the perovskite lattice, while the reduced inhomogeneous
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broadening can suggest a higher crystallinity increasing the FA% associated to lower energetic
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disorder with respect to the pure MAPbI3
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dynamical radiative decay is here monitored to better understand the influence of the perovskite
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composition, i.e. the addition of FA, on the carrier dynamics.
7,21
.
Fig. 1c reports the time-resolved PL. The
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Measurements are performed using time-correlated single photon counting (TCSPC) under
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low-intensity pulsed excitation at 460 nm at 20% the films shows the presence of large darker region possibly
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due to thinner areas or pinholes resulting from a non-uniform growth. Fig. 2 shows the X-Ray-
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diffraction (XRD) patterns of the mixed cation perovskites developed. With respect to the pure
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MAPbI3, the introduction of the large FA+ cation with the smaller MA+ decreases the tolerance
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factor and induces the formation of a stable cubic perovskite phase
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depending on film morphology and deposition conditions adding a small amount of FA (such as
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10%) might also lead to a tetragonal phase as observed when the film is formed by precipitation
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from aqueous hydroiodic acid 24. In our case, we observe that the peak position of the diffraction
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peaks at 14.1 (see also the zoom), 20.0, 24.4, 28.4 and 31.8 decrease with increasing FA%,
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which enlarges the crystal lattice going to a cubic or “quasi-cubic” crystal phase. The gradual
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shift in the diffraction angle further confirms that in the mixed MAxFA1-xPbI3 the two cations are
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co-present in the perovskite lattice, stabilizing the perovskite in the cubic phase. Fig. 2a further
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shows the zoomed (100) diffraction peak at 2θ ≈ 14°, which continuously shifts to 13.86° as the
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FA% increases. Note also that the peaks related to pure FAPbI3 or MAPbI3 phases are not
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present. This further support the fine intermixing of the cations in the mixed perovskite without
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inducing the phase segregation within the film. Fig. 2b shows the Raman spectrum of the mixed
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MAxFA1-xPbI3 perovskite films, here reported for the first time. The Raman spectra consist of
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four main peaks (see additional data reported in Table S1). Similarly to the pristine MAPbI3
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perovskite (see also Fig. S3), the peaks below 150 cm-1 reflects the Pb-I vibrations representing a
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combination of the Pb-I stretching and bending modes along with a minor contribution of the 5
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. Note that,
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organic cations previously identified at 129 cm-1 8,25–27. On the other side, the peak at higher
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wavenumber relates to the torsion of the organic cations 8. In particular, this peak has been
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measured and calculated around 230 cm-1 for the pure FA torsional mode, being on the other side
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shifted to 290 cm-1 for the pure MA torsion
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spectral region for the mixed MAxFA1-xPbI3 we observe that only one peak is observed that
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continuously red shifts increasing the amount of FA%. Note that in the case of halide
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segregation, as recently demonstrated by few of us 8, two distinguishable peaks from the FA+ and
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MA+ torsions can be identified, while in this case only one mode is identified related to the cubic
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perovskite structure where FA and MA are combined together and do not form segregated
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perovskite phases.
8,26,27
(see also Fig. S3). From the analysis of this
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Solar cells using 0-30% addition of FA have been fabricated and optimized. The current (J)–
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voltage (V) characteristics of the solar cells under simulated air mass 1.5 global standard sunlight
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(AM1.5 G) are reported in Fig. 3a along with the device parameters. Champion devices show for
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the pure MAPbI3 power-conversion efficiency (PCE) of 18.59%, in line with what reported in
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literature for the pristine compound
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higher open circuit voltage (Voc) and, for 10% FA, a significant improvement in the device PCE.
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The most efficient FA0.1MA0.9PbI3-based device provides a PCE of 20.26%, outperforming the
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single-cation compositions MAPbI3. Device statistics showing the parameters averaged on 30
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devices are reported in Fig. S4 along with the photovoltaic parameters in Table S2. This further
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confirms the statistically higher performances we could obtain with the addition of
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FA0.1MA0.9PbI3. In Table 1 we report the PV characteristics of a series of cells with varying the
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FA%. Fig. 3b shows the JV- hysteresis of the cell (the corresponding photovoltaic parameters are
20
. Solar cells with the addition of FA generally show a
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listed in Table S3). The FA0.1MA0.9PbI3 shows the lowest hysteresis factor suggesting that
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adding 10%FA strongly stabilize the structure limiting any light or field induced ion movement
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and impeding the halide segregation. Note also that a stable device behavior is obtained after few
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seconds of illumination, upon which stable device parameters could be registered (see details in
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Fig. S5 and in Table S4). This overalls favors a stable device operation. Further details on the
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long term stability of the device are reported in Fig. S6. Similarly to the MAPbI3 an initial de-
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cay is observed, possibly due to the inter diffusion of the Gold top electrode 28. However, on
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longer time, an improved long term stability is registered at least for the first 300 h time lapse.
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The incident photon- to-current conversion efficiency (IPCE), or external quantum efficiency,
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across the visible spectrum is presented in Fig. 3c. The photocurrent onset is shifted in the red for
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the FA0.1MA0.9PbI3 device from 780 to 800 nm providing a gain in the photocurrent. The higher
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Voc can correlate with the slightly larger band gap upon the introduction of FA. Integration of
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the IPCE spectra (340–850 nm) over the air mass 1.5 global (AM1.5G) solar emission spectrum
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yielded short-circuit photocurrent densities of 22.57 mA·cm-2 and 21.77 mA·cm-2 for
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FA0.1MA0.9PbI3 and pure MAPbI3, respectively. The statistics of the photovoltaic parameters
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obtain are reported in Fig. 3d, showing that the results are statistically significant and
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reproducible, providing a strong proof of the concept here presented.
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In conclusion we have here reported for the first time a simple and highly reproducible method to
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obtain high efficient hybrid perovskite solar cells by incorporating 10% of FA into the MAPbI3
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structure. The formation of a stable “quasi-cubic” phase at room temperature accompanied by
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longer charge diffusion length and improved crystal quality is the reason behind the high
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performances obtained. We envisage that, although a lot of work with promising results is 7
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ongoing on mixed halide perovskite including mixture of Cs+, Rb+ as cat-ions and Iodine and
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Bromide as anion, a much simpler and robust method, as the one here presented, can sustain the
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realization of >20% efficient device and, more importantly, open the way for the upscaling of the
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hybrid perovskite technology.
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Figures
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Figure 1. a. Absorption Spectra of MAxFA1-xPbI3 films (x=0; 10%; 20%, 30%). In the inset band
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edge position as a function of FA%. b. PL spectra of MAxFA1-xPbI3 films (x=0; 10%; 20%, 30%)
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excitation at 460nm. In the inset (top) FWHM of the PL spectra retrieved from single peak fitting
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and (bottom) Peak position as a function of FA%. c. PL decay upon excitation at 460 nm at
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excitation density of
