Dr Zhifang Du

Prof Mathieu M Mainville

Joachim Vollbrecht

Alana L Dixon

Nora Schopp

Max Schrock

Prof Zhengxing H Peng

Jianfei Huang

Prof Sangmin Chae

Harald Ade

Mario Leclerc

G N Manjunatha Reddy
email: gnm.reddy@univ-lille.fr

Thuc-Quyen Nguyen

T.-Q Nguyen

Prof. H H Ade

Leclerc

Prof G N M Reddy

Quyen Nguyen

H Zhao

H B Naveed

B Lin

X Zhou

J Yuan

K Zhou

H Wu

R Guo

M A Scheel

A Chumakov

S V Roth

P. Müller Z Tang

P Müller

J Zhao

Y Li

G Yang

K Jiang

H Lin

W Ma

H Yan

L Hong

H Yao

Z Wu

Y Cui

T Zhang

Y Xu

R Yu

Q Liao

B Gao

K Xian

Ye H Y Woo

Z Ge

J Hou

Z Woo

J Ge

Adv Mater Hou

L Ye

Y Xiong

Z You

H You

Adv Mater Ade

T L Nguyen

H Choi

S Yun

T J Shin

S Hwang

J Y Kim

Q Zhang

S Li

C Wang

Insights into Bulk Heterojunction Organic Solar Cells Processed from Green Solvent

Keywords: green solvents, organic photovoltaics, impedance spectroscopy, recombination dynamics, solid-state NMR green

The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so-called green solvents during the fabrication processes. However,

Introduction

The past five years have witnessed considerable research attention and rapid development of solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs), owing to their low cost, mechanical flexibility and potential for a wide range of clean energy applications. 1,2 In particular, significant improvement in power conversion efficiencies (PCEs) of over 18% is recently achieved, with this trend expected to continue. 3,4 High performance OSCs are commonly processed with halogenated solvents such as chlorobenzene (CB), o-dichlorobenzene and chloroform despite being considered highly hazardous to human health and the environment. 5 Accepted Article

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-hexyldecyloxy)phenylene) Accepted Article hexyldecyloxy)phenylene) [1,2,5]thiadiazole)] (PPDT2FBT) and [6,6] Accepted Article [1,2,5]thiadiazole)] (PPDT2FBT) and [6,6] which are

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which are processed with

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This article is protected by copyright. All rights reserved the conventional chlorinated solvent. Other green solvents such as 2-Methyl THF and (R)-(limonene could certainly be a possible focus for our future study. 10,22,23 The chemical structures and energy diagram of PPDT2FBT and PC 61 BM are shown in Figure 1. The efficient and cost-effective conjugated polymer, PPDT2FBT (M w =82 kg mol -1 ), is synthesized by a water-based method of direct (hetero)arylation polymerization (DHAP) with few steps, which can potentially facilitate the scaled-up production of organic solar cells. 24 The optimized PPDT2FBT:PC 61 BM devices show an increase of PCE from 7.0 ± 0.1% when processed from CB to 9.1 ± 0.1% when processed with o-XY. This is quite noteworthy, since in most cases either a similar or a reduced PCE are observed when solar cells are fabricated with green solvents compared to their counterparts processed with halogenated solvents. 9,[25][26][27][28] Analysis of photovoltaic properties and charge dynamics reveals that the high-performance system processed from o-XY benefits from more efficient charge transport and smaller recombination losses. This work provides insight into the mechanisms that lead to the improved performance of green-solvent-processed devices.

Results and discussions

PPDT2FBT

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PPDT2FBT

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The optoelectronic properties of organic solar cells are intimately related to the BHJ morphology and solid-state organization of donor-acceptor moieties, determining charge carrier dynamics including generation, recombination, and extraction processes. 29 By analyzing BHJ morphologies at different length scales, we can evaluate the impact of solid-state organization on the charge transport and device properties. First, we employ Atomic Force Microscopy (AFM) to study the surface morphology of the PPDT2FBT:PC 61 BM films cast from CB at room temperature and o-XY under hot processing conditions at 110 o C (Figure 2a, 2b). 30 Both blend films show relatively smooth surface with similar root-mean-square (RMS) values of 1.58 nm for CB and 2.13 nm for o-XY. With high-resolution scans (1 µm  1 µm), the height profile shows that the film cast from o-XY exhibits smaller topographic features. Next, we examine the molecular orientation and long-range molecular ordering in the BHJ films using Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). The 2D GIWAXS images of the blend films are shown in Figure 2c and 2d, and the corresponding linecuts are shown in Figure S3 of the Supporting Information. The detailed analysis of the pristine films can be found in the Supporting Information (Figure S1,S2). Both blend films cast from CB and o-XY display considerably similar molecular ordering characteristics based on the GIWAXS results. Each film contains the isotropic PC 61 BM producing a scattering halo near 1.3 Å 1 (d-spacing of 0.5 nm)

with coherence lengths values of L c ≈ 2 nm. In both BHJ films, the dominant features corresponding to the π-π stacking motif with face-on orientation are identified with scattered intensities at 1.64 Å 1 (d-spacing of 0.38 nm) in each direction with no significant difference in the π-π stacking L c values between them. This result suggests that charge transport can occur both in the in-plane and out-of-plane directions in the photoactive layers, which signifies that the utilization of a green solvent does not detrimentally affect the BHJ molecular packing. 31 The This article is protected by copyright. All rights reserved main differences between the two BHJ films are in the lamellar L c values. Lamellar stacking peaks are present near 0.3 Å 1 in both directions with a higher order reflection near 0.6 Å 1 . The lamellar L c values of the film vary from 6.9 nm (CB) to 3.7 nm (o-XY). The L c for in-plane lamellar stacking decreases by 3.1 nm and that of the higher order reflection for out-of-plane lamellar stacking decreases by 2.1 nm. However, the same peaks are present in both BHJ film,

showing that molecular ordering is largely maintained regardless of the processing solvents.

Thus, we employ more advanced techniques for insights on more nuanced morphological traits, such as the D:A interfacial area, the domain sizes, and the composition variations, which monotonically relate to the domain purity.

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lamellar stacking decreases by 3.1 nm and that of the higher order reflection for

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lamellar stacking decreases by 3.1 nm and that of the higher order reflection for lamellar stacking decreases by 2.1 nm. However, the same peaks are present in

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This article is protected by copyright. All rights reserved The Resonant-Soft X-ray Scattering (RSoXS) technique is employed to gain insights into the morphological features including the domain sizes as well as the D:A composition variation of the BHJ blends on a length scale ranging from 10 nm to over 500 nm. [32][33][34] The Lorentzcorrected and film thickness-normalized RSoXS profiles of both systems at a photon energy of 283.8 eV are shown in Figure S4 (Supporting Information), which show apparent differences in both the position and the intensity of the peak. The peak for the film cast from CB exists at q ≈ 0.069 nm 1 , whereas the blend prepared from o-XY shows a peak at q ≈ 0.138 nm 1 with a significantly smaller intensity. The corresponding long period of the o-XY cast film (~46 nm) is about half of the value of the CB processing film (~91 nm). Since the domain size can be roughly estimated as half of the long period, the green-solvent-processed films show smaller domains (~23 nm) than the CB processed films (~45.5 nm), which is consistent with the surface topography observed in AFM measurement. The average smaller domains predominantly existing in the o-XY processed devices could result in better charge generation due to larger D:A interfacial areas than those of CB processed devices. To further investigate the relative average purity of the domains, the RMS composition variation is obtained from the integration of the scattering intensities. The relatively lower value for the RMS composition variation (0.66) in the o-XY processed BHJ films indicates a lower average domain purity and a higher degree of molecular intermixing of the donor and acceptor, which would enable efficient charge generation in the BHJ layer. 35 Such differences in the average domain purity are likely resulted from kinetic

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The Resonant

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The Resonant-

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both the position and the intensity of the peak.

nm

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0.069 nm 

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1

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PPDT2FBT. These 2D ssNMR

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. These 2D ssNMR interactions between donor

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acceptor molecules with respect to the aromatic core of the donor polymer,

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acceptor molecules with respect to the aromatic core of the donor polymer, cessed from CB and o

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This article is protected by copyright. All rights reserved

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.

Solid

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Solid-

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) with 5

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This article is protected by copyright. All rights reserved compounds and blends using 2D To further understand the role of green solvent processing on the BHJ morphology and therefore the device performance, photophysical properties of solar cells fabricated with the inverted device architecture of ITO/ZnO/PPDT2FBT:PC 61 BM/MoO x /Ag are analyzed and compared. Under the background of solar cell study, we optimize the additive and the casting temperature for each solvent to compare the difference between green solvent-casted and CBcasted OSCs with best performance for each case. Details of the optimization can be found in Table S2. It is worth mentioning that an eco-compatible additive, panisaldehyde (AA), has been chosen as the green solvent additive regarding the health and environmental hazards of common chlorinated additives such as DIO and 1-chloronaphthalene (1-CN). 23 The current density-voltage (J-V) characteristics of the PPDT2FBT:PC

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.

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U

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Under the background of solar cell study, we optimize the additive and the casting

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nder the background of solar cell study, we optimize the additive and the casting temperature for each solvent to compare the difference between green sol

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temperature for each solvent to compare the difference between green sol casted OSCs with best performance for each case. Details of the optimization can be found in

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casted OSCs with best performance for each case. Details of the optimization can be found in

Table S2

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Table S2.

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. It is worth mentioning that an eco

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It is worth mentioning that an eco

This article is protected by copyright. All rights reserved and o-XY in Figure 4a indicate an increase of the short-circuit current from 12.5 ± 0.3 mA cm 2 to 15.6 ± 0.2 mA cm 2 and an increased PCE from 7.0 ± 0.1% to 9.1 ± 0.1% using green solvent processing. The average PCE values from 20 devices under the optimized conditions are provided in Table 1. The increase in the J SC is in agreement with the morphology results discussed above, in which smaller domain sizes and higher D:A interfacial areas observed in o-XY processed blends should lead to better charge generation. Furthermore, the active layer of the o-XY processed device is thicker than that of the CB cast device (276 nm vs. 238 nm), thus there may be more photons absorbed, resulting in more photogenerated carriers and higher J SC . optical transfer-matrix based model. [49][50][51][START_REF] Laquai | Elementary Processes in Organic Photovoltaics[END_REF] The comparison shows that processing with o-XY is beneficial for a thick active layer, since at positions  200 nm from the bottom electrode the generation rate is higher in the o-XY processed film (G = 2.03.9 × 10 21 cm 3 s 1 ) than the CB processed film (G = 0.82.9 × 10

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). First,

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First, the

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, we focus on

This article is protected by copyright. All rights reserved linear relationship with a slope of s = kT/q (k is the Boltzmann-constant, T is the absolute temperature, and q is the elementary charge), the dominant loss mechanism is bimolecular recombination. 21,56 Deviations of the slope may indicate the presence of trap states (bulk traps:

s > kT/q; surface traps: s < kT/q). In both systems, the slope of the V OC vs. light intensity is equal to s = 1.2 kT/q (Figure 4b), which is a sign for the presence of trap-assisted Shockley-Read-Hall (SRH) recombination via deep traps in the bulk. A more advanced approach based on capacitance spectroscopy is required to complement the qualitative results obtained from the V OC vs. light intensity plots. Thus, we use a comprehensive analytical model based on a combination of Langevin and Shockley-Read-Hall theory to quantify the total non-geminate recombination losses, including bimolecular recombination as well as bulk and surface trap-assisted recombination, which can provide valuable information on the charge recombination and extraction dynamics. 42,[56][START_REF] Vollbrecht | A[END_REF][58][59][60][61][62] To get quantitative insights into the non-geminate recombination dynamics, capacitance spectroscopy is applied to determine the charge carrier density in the active layer under different operational biases and light intensities. 63,64 A small AC signal (40 mV) is applied to prevent its effect on the impedance and maintain the steady state properties of the system during the measurement under different illumination as well as DC bias and frequency. Considering the effect of the series resistance of the device as well as the parasitic inductance of the connecting cables, 65 the measured capacitance of the BHJ layer is corrected and can be described by the following Equation 1.

𝐶 ��� = - � � [ � �� � �� � (� � �� � ) � �(�"��� � ) � ], (1)
Here, 𝐿 � is the inductance of the connecting cables, 𝜔 = 2πν is the angular frequency of the AC signal, and 𝑍 � as well as 𝑍 �� are the real and imaginary components of impedance, respectively.

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; surface traps:

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; surface traps: Accepted Article extraction dynamics. 42

kT Accepted Article kT/ Accepted Article / kT/ kT Accepted Article kT/ kT q Accepted Article q ( Accepted Article (Figure

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42

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To get quantitative insights

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To get quantitative insights spectroscopy is applied to determine the charge carrier density in the active layer under different

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1.

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In the dark, the corrected capacitance measured under a large reverse bias (3 V) shows a horizontal line, which is equivalent to a frequency independent capacitance. From this, we can determine the geometrical capacitance (C g ) of the active layer. Voltage-dependent impedance spectroscopy is performed to yield the charge carrier density n (Figure 5), via integration of the chemical capacitance C chem using the following equations.

𝑛(𝑉

��� ) = 𝑛 ��� + � ��� ∫ 𝐶 ���� � ��� � ��� d𝑉 ��� , (2)
𝑛 ��� = � ��� 𝐶 ��� (𝑉 � -𝑉 ��� ), (3)
where V 0 is the forward bias at which the photocurrent is equal to zero, A is the device area (0.094 cm 2 ), L is the thickness of the active layer, V sat is the reverse bias at which the photocurrent saturates (3 V), and C sat is the internal capacitance, which is determined by the difference of the corrected capacitance in the dark and under illumination at V sat . 31,66,67 The obtained charge carrier density at V OC under 100 mW cm 2 illumination in devices using green solvent processing show a higher value (n = 5.9 × 10 16 cm 3 ) than that of the corresponding reference devices (n = 1.2 × 10 16 cm 3 ). To further understand the device performance, the effective mobility μ eff under 100 mW cm 2 illumination (Figure S14) is calculated with the following equation.

𝜇 ��� (𝑛, 𝑉 ��� ) = �(�)� ���(�)[����� ��� ] , (4)
where V cor is the corrected voltage considering the voltage drop over the series resistance (𝑉 ��� = 𝑉 ������� -𝐽(𝑉)𝑅 ������ ), and J(V) is the current density obtained from the J-V curves.

With the effective mobilities, the devices prepared from o-XY have a significantly higher value (μ eff = 2.16 × 10 3 cm 2 V 1 s 1 ) in comparison to their counterparts prepared from CB (μ eff = 4.45 × 10 4 cm 2 V 1 s 1 ). To further yield quantitative results for the recombination dynamics, we model the recombination current density J rec using the charge carrier density n and compare the model to the experimental values of the recombination current density J rec obtained from J-V curves. 42,56,68,69 Details of this modeling can be found in the Supporting Information.

The Langevin coefficient ξ, the density of deep traps in the bulk N t,b , and the density of surface traps N t,s are used as fitting parameters to compare the calculated J rec,sum over five different intensities with the experimental results. The fitted J rec,sum is in good agreement with the experimental recombination current density J rec for all tested illumination intensities without having to individually adjust the fitting parameters for each intensity (for green solvent cast devices: ξ = 0.0278, N t,b = 9.26 × 10 12 cm 3 ; For CB cast devices: ξ = 0.0061, N t,b = 2.81 × 10 14 cm 3 ; for both systems, N t,s is smaller than 10 9 cm 2 , which is negligible in the fitting). Moreover, the respective recombination coefficients are further calculated with the fitted trap densities.

Both blends show similar bimolecular recombination coefficients ranging from k bm = (2.586.59)

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.

The voltage

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The voltage processed from Accepted Article ,56

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6,

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,68,69

Accepted Article 68,69 Details of this modeling can be found in the

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Details of this modeling can be found in the

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The Langevin coefficient

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The

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This article is protected by copyright. All rights reserved × 10 12 cm 3 s 1 and bulk-trap assisted recombination coefficients ranging from k bulk = (1.106.56) × 10 4 s 1 across the voltage range from 2 V to V OC . Furthermore, the recombination dynamics discussed above can also be verified by fitting the light intensity dependent behavior of the V OC , which is already introduced in the scope of Figure 4b. In essence, the relevant recombination parameters that have been determined above can be gauged via the experimental V OC at different light intensities and a reasonable correspondence between the experimental and fitted V OC can be observed; a detailed description can be found in the SI (Figure S17). As the recombination coefficients between the blends are quite similar, this alone cannot explain the device performance differences. Therefore, we further quantify the average extraction time of charge carriers (τ ex ), as shown in Figure 6a, to obtain a comprehensive understanding of the charge carrier dynamics using the following equation:

𝜏 �� = ���(�) �(�) . ( 11
)
The o-XY processed devices have a much faster average extraction time τ ex = 0.090.74 µs from 2 V to V OC , compared with the devices processed from CB (τ ex = 0.265.6 µs), which is consistent with the higher effective mobility. Meanwhile, the charge recombination lifetime (τ rec ) can be further described by the following equation:

𝜏 ��� = 𝜏 �� + 𝜏 ���� = � � �� � � + � � ���� � = � � �� � + � � ���� . ( 12
)
The

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). As the recombination coefficients between the blends are quite

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As the recombination coefficients between the blends are quite cannot explain the device performance differences. Therefore, we further quantif

devices have a V Accepted Article V OC Accepted Article OC V OC V Accepted Article V OC V , compared

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, compared consistent with the higher effective mobility. Meanwhile, the Accepted Article consistent with the higher effective mobility. Meanwhile, the can be further described by the following equation

Accepted Article can be further described by the following equation

The devices prepared from

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The devices prepared from Based on our study, donors and acceptors with comparable good solubility in green solvent, and the corresponding film cast from green solvent shows a well-maintained, or even improvement of the favorable BHJ morphology can be suitable for green solvent processing.

Conclusions

In summary, we investigate differences in the BHJ morphology and the subsequent changes of the optoelectronic properties of PPDT2FBT:PC

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). The

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of the investigated devices.

Based on our study, donors and acceptors with comparable

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Based on our study, donors and acceptors with comparable the corresponding film cast from green solvent shows a well

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the corresponding film cast from green solvent shows a well of the favorable BHJ morphology can be suitable for green solvent processing.

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of the favorable BHJ morphology can be suitable for green solvent processing. (2 V to V OC ) for the green-solvent-processed devices, which correlates well with their higher

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. Ultimately, t

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Ultimately, t

This article is protected by copyright. All rights reserved

Experimental section

Material synthesis. The PPDT2FBT polymer was synthesized by direct (hetero)arylation polymerization (DHAP) according to previously recorded biphasic conditions. 24,72 In a 20 mL microwave vial, 211.0 mg (0.296 mmol, 1 eq) of Accepted Article solution mixture is stirred for 10 minutes and precipitated in a 10% HCl:methanol (1:9) solution.

The precipitate is filtered through a 0.45

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The precipitate is filtered through a 0.45 methanol and hexanes, then extracted by chloroform. The polymer solution is reduced to 20

Accepted Article This article is protected by copyright. All rights reserved EQE measurements were conducted with an EQE system, in which the monochromatic light intensity was calibrated using a Si photodiode. The spectral distributions of the real and imaginary components of the impedance of all devices were measured by an impedance analyzer (Solartron SI 1260A) in the dark and under different illumination intensities. To prevent the effect of the AC signal on the impedance during the measurement, a small amplitude AC signal (40 mV) was applied.

Topographic Characterization. All topographic measurements were obtained using an Asylum MFP-3D operating in closed loop mode mounted atop an Olympus inverted optical microscope under an inert atmosphere. Furthermore, n+ diamond coated silicon AFM tips with a resonant frequency of ~20 kHz and a force constant of ~0.5 N m 1 were used (NanoSensors). All images were obtained at a force of 2 nN. First order image flattening was performed on the morphology images and the measured morphology heights were offset to set the scale start point to 0 nm using Asylum Research AFM software version 14, programmed with IGOR Pro.

GIWAXS. All GIWAXS measurements were performed at the Advanced Light Source at

Lawrence Berkeley National Lab on the 7.3.3 beamline. The sample was scanned with an incidence angle of 0.12° and a photon energy of 10 keV (λ = 1.24 Å), while under a helium environment to minimize beam damage and reduce air scattering. The width of the incident Xray beam is about 1 mm, and silver behenate was used to calibrate the lengths in the reciprocal space. A 2D detector (PILATUS 2 M from Dectris) with a sample-to-detector distance of 276.9 mm was used to collect the images. The Nika software package for Igor (by Wavemetrics) and the Igor script WAXStools were used to process the figures. 73,74 RSoXS. All RSoXS measurements were performed at the Advanced Light Source at Lawrence Berkeley National Lab on the 11.0.1 beamline following the previously established

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(Solartron SI 1260A) in the dark and under

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(Solartron SI 1260A) in the dark and under effect of the AC signal on the impedance during the measurement, a small amplitude AC signal

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effect of the AC signal on the impedance during the measurement, a small amplitude AC signal (40 mV) was applied.

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(40 mV) was applied.

Topographic

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Topographic Characterization.

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Characterization.

Asylum MFP

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Asylum MFP-

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GIWAXS. All GIWAXS measurement

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All GIWAXS measurement Lawrence Berkeley National Lab on the 7.3.3 beamline. The sample was scanned with an

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Lawrence Berkeley National Lab on the 7. This article is protected by copyright. All rights reserved protocols. 25 The samples were performed in a transmission geometry with linearly polarized photons under high vacuum (1 × 10-7 torr) and two -dimensional scattering patterns were collected on a cooled (-4 5 °C) CCD with PS300 used for geometry calibration. The Nika software package for Igor (by Wavemetrics) were used for data processing. 73,74 Solid-state NMR spectroscopy. Spin This article is protected by copyright. All rights reserved 1 H-detected 2D 1 H- 13 C HETCOR spectra were acquired with 160 t 1 increments, each by coadding 32 transients in PC 61 BM, 64 transients in PPDT2FBT and 80 transients in the case of blends. The 1 H and 13 C experimental shifts were calibrated with respect to neat TMS using adamantane as an external reference (higher ppm 13 C resonance, 35.8 ppm, and the 1 H resonance, 1.8 ppm).

  systematic understanding of green requires the characterization of morphologies at different length scales Accepted Article requires the characterization of morphologies at different length scales a comprehensive Accepted Article a comprehensive study Accepted Article study as optical and electrical properties of the devices Accepted Article as optical and electrical properties of the devices -

Figure 1 .

 1 Figure 1. (a) Molecular structures and (b) energy level diagram of PPDT2FBT and PC 61 BM.

Figure 2 .

 2 Figure 2. 1 µm  1 µm AFM topography images of PPDT2FBT:PC 61 BM films cast from (a) CB

-

  both the position and the intensity of the peak.

Figure 3 .

 3 Figure 3. Solid-state 1 H-detected 2D 13 C-1 H heteronuclear correlation NMR spectra of

H

  MAS of blends in the top horizontal dimension Accepted Article H MAS of blends in the top horizontal dimension compounds and blends in the vertical dimensions. 2D Accepted Article compounds and blends in the vertical dimensions. 2D coded as depicted in the molecular structure. All spectra are acquired at 18.8 T ( Accepted Article coded as depicted in the molecular structure. All spectra are acquired at 18.

1 H- 1 H

 11 Double-Quantum-Single-Quantum (DQ-SQ) correlation NMR spectroscopy.1 H DQ signals can be excited for dipolar coupled 1 H-1 H pairs at the sub-nm (<0.5 nm) distances. In a 2D DQ-SQ spectrum, on-diagonal and off-diagonal signals correspond to the through-space dipole-dipole interactions between chemically equivalent and nonequivalent protons, respectively. The analysis of 1 H DQ-SQ spectra of neat donor and acceptor materials, and their blends is given in FigureS10of the Supporting Information. Although 1 H DQ-SQ spectra of blends contain overlapped 1 H signals from donor and acceptor molecules, subtle differences in the off-diagonal intensities, which originate from H-H proximities between aliphatic-and aromatic groups of donor and acceptor molecules, further corroborate the different donor-acceptor molecular orientations at the BHJ interfaces of OPV blends processed from CB and o-XY solvents. These differences in molecular orientations and intermolecular interactions at D:A interfaces are expected to influence the charge transport in BHJ blends, leading to different PCE values in their OSCs.

Figure 4 .

 4 Figure 4. (a) J-V curves of the PPDT2FBT:PC 61 BM solar cells processed from o-XY and CB under AM 1.5 G illumination at 100 mW cm -2 and (b) V OC vs. light intensity.

  illumination at 100 mW cm Accepted Article G illumination at 100 mW cm Average photovoltaic performances of 20 OSCs based on Accepted Article Average photovoltaic performances of 20 OSCs based on processed with o Accepted Article processed with o XY and CB under 100 Accepted Article XY and CB under 100 Accepted Article This article is protected by copyright. All rights reserved o-XY 15.6 ± 0.2 0.80 ± 0.005 0.72 ± 0.01 9.1 ± 0.1 To further corroborate the above analysis on the increase in the J SC , we calculate charge generation rate from the optical properties of the blend films processed with CB (238 nm) and o-XY (276 nm). First, the reflectance and transmittance spectra are measured (Figure S11 in the Supporting Information). While the extinction coefficient k remains unaltered, we find a slightly decreased refractive index n for the o-XY processed film (n = 1.15-1.30) compared to the CB processed film (n = 1.19-1.38) in the range of 300 nm to 700 nm, causing a reduced reflectance of the incident light. Next, the spatially dependent generation rate is simulated based on the device architecture and the optical properties of the CB and the o-XY processed film with an

Figure(

  Figure

  Accepted Articlespectroscopy is applied to determine the charge carrier density in the active layer under different operational biases and light intensities. effect of the series resistance of the device as well as the parasitic inductance of the connecting Accepted Article effect of the series resistance of the device as well as the parasitic inductance of the connecting

Figure 5 .

 5 Figure 5. The voltage-dependent charge carrier density of PPDT2FBT:PC 61 BM devices

  quantitative results for the recombination dynamics, we model Accepted Article To further yield quantitative results for the recombination dynamics, we model recombination current density Accepted Article recombination current density the experimental values of the recombination current density Accepted Article the experimental values of the recombination current density ,5

  Figure S1Accepted ArticleFigure S17 Accepted Article 7).

71 Figure 6 .

 716 Figure 6. (a) Charge carrier lifetime τ rec and extraction time τ ex , and (b) voltage-dependent

θ

  of the investigated devices.

.

  This article is protected by copyright. All rights reserved traditional (CB) and green (o-XY) solvents. Comparable good solubility of PPDT2FBT and PC 61 BM in green solvent, and the corresponding film cast from green solvent with a wellmaintained molecular ordering, and even improvement of the favorable BHJ morphology can be suitable for green solvent processing. The characterization of the BHJs at different length scales using AFM, GIWAXS, and RSoXS techniques indicate the presence of a well-mixed phase morphology with increased D:A interfacial areas and smaller domain sizes for the o-XY processed blend. 2D ssNMR spectroscopy results reveal that the PC 61 BM molecules are intercalated into the sidechain regions of the donor polymers, in particular, the green-solventprocessed BHJ morphology exhibits specific 2D correlations that favor better intermixing and orientation of PC 61 BM molecules in the vicinity of the PPDT2FBT aromatic core. These features of molecular packing facilitate the charge generation efficiency and transport in the greensolvent-processed devices. Hence, the optimized PPDT2FBT:PC 61 BM devices processed with o-XY solvent exhibit a higher charge mobility, a faster charge extraction, and a higher PCE (9.1 ± 0.1%) compared to the CB cast devices (7.0% ± 0.1%). Non-geminate recombination and extraction dynamics are determined by employing a quantitative analytical model based on capacitance spectroscopy. A higher charge carrier density and effective mobility are observed in the solar cells prepared from o-XY. As a result, smaller competition factors stemming from slow non-geminate recombination and fast charge extraction are shown over the studied voltage range

  methanol and hexanes, then extracted by chloroform. The polymer solution is reduced to20 Accepted Article mL and poured into methanol prior to filtration with a Accepted Article mL and poured into methanol prior to filtration with a solubilized in toluene and washed at reflux over Accepted Article solubilized in toluene and washed at reflux over trihydrate scavenger solution to remove the catalytic residues. The organic phase is washed 5Accepted Articletrihydrate scavenger solution to remove the catalytic residues. The organic phase is washed 5 times with boiling nanopore water to remove the remaining salts. The organic phase is reduced,Accepted Articletimes with boiling nanopore water to remove the remaining salts. The organic phase is reduced,Accepted Article poured into methanol, andAccepted Article poured into methanol, andThis article is protected by copyright. All rights reserved PL120 with a Styrene-DVB gel column. The eluent was 1, 2, 4-trichlorobenzene (TCB), heated at 110°C. The calibration was performed using a monodisperse polystyrene standard.Fabrication and characterization of organic solar cell devices.All the devices were fabricated /PPDT2FBT:PC 61 BM/MoO 3 /Ag. Photovoltaic devices were fabricated according to the following procedures. ITO substrates (purchased from Thin Film Devices, Inc.) were cleaned by detergent, then sequentially ultrasonicated in DI water, acetone and isopropanol for 30 min.Tetrahydrofuran (THF) and diethyl zinc were mixed in a volume ratio of 5:1 and spin-coated on the clean ITO substrates at 4000 rpm for 30s, followed by 15 min annealing at 150 °C to make the ZnO film. Solutions (total concentration of 40 mg/mL) of PPDT2FBT:PC 61 BM in a 1:1.5 weight ratio were prepared in either CB with 1 vol% DIO processing additive or o-XY, which was stirred and kept at elevated temperatures (CB at 60 °C, o-XY at 80 °C) over night. Prior to spin casting, 1 vol% p-anisaldehyde (AA, 98%) was added as the solvent additive to the stock o-XY solution and the solution was heated up to 110°C. On the top of the ZnO layer, the hot blend solution was spin-coated (CB: 1100 rpm, o-XY: 1400 rpm, 60 s) to form active layers, of which the thickness was measured by an Ambios XP-100 stylus profilometer. The MoO 3 /Ag (7 nm/120 nm) electrode with an active area of 0.094 cm 2 was then deposited on top of the active layer by thermal evaporation in high vacuum (<10 6 torr).Device measurements. All the photovoltaic and electrical measurements were conducted inside a glovebox under nitrogen atmosphere. Photovoltaic characteristics were measured with a high-quality optical fiber to guide the light from the solar simulator equipped with a Keithley 2635A source measurement unit. J-V curves were measured under AM 1.5G illumination (100 mW•cm-2

  under an inert atmosphere. Accepted Article microscope under an inert atmosphere. resonant frequency of ~20 kHz and a force constant of ~0.5 N m Accepted Article resonant frequency of ~20 kHz and a force constant of ~0.5 the measured morphology heights were offset to set the scale start point Accepted Article and the measured morphology heights were offset to set the scale start point to 0 nm using Accepted Article to 0 nm using Asylum Research AFM software version 14, programmed Accepted Article Asylum Research AFM software version 14, programmed Accepted Article GIWAXS.

  3.3 beamline. The sample was scanned with an incidence angle of 0.12° and a photon energy of 10 keV (λ = 1.24 Å), while under a helium Accepted Article incidence angle of 0.12° and a photon energy of 10 keV (λ = 1.24 Å), while under a helium environment to minimize beam damage an Accepted Article environment to minimize beam damage an ray beam is about 1 mm, and silver behenate was used to calibrate the lengths in the reciprocal Accepted Article ray beam is about 1 mm, and silver behenate was used to calibrate the lengths in the reciprocal space. A 2D detector (PILATUS 2 M from Dectris) with a sample Accepted Article space. A 2D detector (PILATUS 2 M from Dectris) with a sample mm was used to Accepted Article mm was used to collect the images. The Nika software package for Igor (by Wavemetrics) and Accepted Article collect the images. The Nika software package for Igor (by Wavemetrics) and

  Accepted ArticleNMR experiments were carried out by co for the neat PPDT2FBT material and 5 s and PPDT2FBT:PCAccepted Articlefor the neat PPDT2FBT material and 5 s and PPDT2FBT:PC Accepted Article XY, respectively. In the case of PC Accepted Article XY, respectively. In the case of PC H relaxation delays were of 6 s

Table 1 .

 1 Average photovoltaic performances of 20 OSCs based on PPDT2FBT:PC 61 BM and processed with o-XY and CB under 100 mW•cm -2 AM 1.5 G illumination.

	Accepted Article provided in Accepted Article provided in
	Solvent	J SC (mA cm 2 )	V OC (V)	FF	PCE (%)
	CB	12.5 ± 0.3	0.79 ± 0.002	0.70 ± 0.01	7.0 ± 0.1

Table 1

 1

Table 1 .

 1 Accepted Article

	Accepted Article . discussed above Accepted Article discussed above, Accepted Article , in which smaller domain sizes Accepted Article in which smaller domain sizes processed Accepted Article processed blend Accepted Article blends Accepted Article s should Accepted Article should processed Accepted Article

processed device is thicker than that of the CB cast device (276 nm vs. 238 nm), thus there Accepted Article device is thicker than that of the CB cast device (276 nm vs. 238 nm), thus there more photons absorbed Accepted Article more photons absorbed Accepted Article . Accepted Article . ( Accepted Article

  21 cm 3 s 1 ). Next, we simulate the maximum, theoretical shortcircuit current density J SC,theo based on the generation rate (Figure S12, Supporting Information) and compare it with the experimentally determined saturated photocurrent density J ph,sat , to obtain information on geminate recombination losses.[START_REF] Baran | [END_REF][54][55] A ratio of J ph,sat /J SC,theo = 0.995-1.018 is found for both films, which indicates the absence of geminate recombination losses in both

	films. generation rate Subsequently, we focus on understanding the losses caused by non-geminate Accepted Article Accepted Article generation rate from the optical properties of the blend films processed with CB (238 Accepted Article from the optical properties of the blend films processed with CB (238 nm Accepted Article nm).
	recombination in OSCs processed from CB and o-XY. The dominant non-geminate

recombination mechanism in the solar cells is qualitatively determined by analyzing the opencircuit voltage (V OC ) dependence on the light intensity (I). Generally, if the V OC -lnI plot shows a

  61 BM systems processed from

	Accepted Article  Accepted Article 4.1×10 Accepted Article 4.1×10  Accepted Article 4 Accepted Article 4 , while the devices prepared from CB show significantly large Accepted Article , while the devices prepared from CB show significantly large 1.34 Accepted Article 1.34). T

J

  SC and FF values. Ultimately, this work demonstrates that it is possible to obtain high PCEs for devices processed from green solvent, which are expected to pave the way towards cleaner commercialization and industrial-scale fabrication of OSCs.

	Accepted Article suitable for green solvent processing Accepted Article suitable for green solvent processing AFM, GI Accepted Article AFM, GIW Accepted Article WAXS, and Accepted Article AXS, and morphology Accepted Article morphology with Accepted Article with increased D:A interfacial area Accepted Article increased D:A interfacial area processed Accepted Article processed blend Accepted Article blend. Accepted Article . 2D ssNMR spectroscopy results reveal that the Accepted Article 2D ssNMR spectroscopy results reveal that the intercalated into the sidechain regions of the donor polymers Accepted Article intercalated into the sidechain regions of the donor polymers processed BHJ morphology exhibit Accepted Article processed BHJ morphology exhibit orientation of Accepted Article orientation of PC Accepted Article PC 61 Accepted Article 61 BM Accepted Article BM of molecular packing Accepted Article of molecular packing processed Accepted Article processed devices Accepted Article devices solvent Accepted Article solvent exhibit a Accepted Article exhibit a higher char Accepted Article higher char ) compared to Accepted Article ) compared to the Accepted Article the extraction dynamics Accepted Article extraction dynamics ar Accepted Article ar capacitance spectroscopy. Accepted Article capacitance spectroscopy. the solar cells prepared from o Accepted Article the solar cells prepared from o geminate recombination and fast Accepted Article geminate recombination and fast V Accepted Article V OC Accepted Article OC V OC V Accepted Article V OC V ) for the green Accepted Article ) for the green FF Accepted Article FF values Accepted Article values.

  ) are then added to the mixture and the vial is sealed. The reaction is stirred for 15 minutes at room temperature, then heated at 100 °C for 20 h with vigorous stirring. The reaction is then cooled down to 70 °C and the resulting polymer is diluted with 10 mL of xylene. The polymer ) are then added to the mixture and the vial is sealed. The reaction is stirred for 15 minutes Accepted Article [0.2]) are then added to the mixture and the vial is sealed. The reaction is stirred for 15 minutes at room temperature, then heated at 100 °C for 20 h with vigorous stirring. The reaction is then

	1,4-dibromo-2,5-bis[(2-hexyldecyl)oxy]-benzene, 5,6-difluoro-4,7-di-2-thienyl-2,1,3-benzothiadiazole, Pd(OAc) 2 , 16.6 mg (0.0472 mmol, 0.16 eq) of P(o-OMePh) 3 , 1.635 g (11.8 mmol, 40 eq) of 99.5 mg (0.296 mmol, 1 eq) of 2.6 mg (0.0118 mmol, 0.04 eq) of K 2 CO 3 and 30.2 mg (0.296 mmol, 1 eq) of pivalic acid are added. The vial is then purged under vacuum and filled with argon. Degassed water (1.5 mL, [0.2]) and degassed toluene (1.5 mL, [0.2]solution mixture is stirred for 10 minutes and precipitated in a 10% HCl:methanol (1:9) solution. The precipitate is filtered through a 0.45 µm nylon filter and washed on a Soxhlet apparatus with methanol and hexanes, then extracted by chloroform. The polymer solution is reduced to 20-30 mL and poured into methanol prior to filtration with a 0.45 µm nylon filter. The polymer is solubilized in toluene and washed at reflux overnight in a sodium diethyldithiocarbamate trihydrate scavenger solution to remove the catalytic residues. The organic phase is washed 5 times with boiling nanopore water to remove the remaining salts. The organic phase is reduced, poured into methanol, and filtered, as previously mentioned. The polymer is vacuum-dried to yield a purple solid. (Yield 79%) M n =39 kg mol -1 ; M w =82 kg mol -1 ; Đ=2.1; λ max absorption=648 Accepted Article microwave vial, 211.0 Accepted Article microwave vial, 211.0 1,4-dibromo-2,5-bis[(2-hexyldecyl)oxy]-benzene, 99.5 Accepted Article 1,4-dibromo-2,5-bis[(2-hexyldecyl)oxy]-benzene, 99.5 5,6-difluoro-4,7-di-2-thienyl-2,1,3-benzothiadiazole, Accepted Article 5,6-difluoro-4,7-di-2-thienyl-2,1,3-benzothiadiazole, 2 Accepted Article 2 , 16.6 Accepted Article , 16.6 mg (0.0472 mmol, 0.16 Accepted Article mg (0.0472 mmol, 0.16 and 30.2 Accepted Article and 30.2 mg (0.296 Accepted Article mg (0.296 vacuum and filled with argon. Degassed wa Accepted Article vacuum and filled with argon. Degassed wa [0.2]Accepted Article at room temperature, then heated at 100 °C for 20 h with vigorous stirring. The reaction is then cooled do Accepted Article cooled down to 70 °C and the resulting polymer is diluted with 10 wn to 70 °C and the resulting polymer is diluted with 10 Accepted Article solution mixture is stirred for 10 minutes and precipitated in a 10% HCl:methanol (1:9) solution.
	nm. Size exclusion chromatography measurements were performed using a Varian Instrument

Solid state NMR spectroscopy.

  -coated PPDT2FBT and PC 61 BM neat materials and PPDT2FBT:PC 60 BM blends processed from CB and o-XY solvents were scratched off from the glass substrates and packed into either 1.3 mm (outer diameter) zirconia rotors fitted with Vespel ® caps. All fast MAS (50 kHz) 1D 1 H,13 C, and 2D 1 H-13 C NMR experiments were carried out on a Bruker Avance Neo (18.8 T, Larmor frequencies of 1 H and 13 C were 800.1 MHz and 201.2 MHz, respectively) spectrometer with 1.3 mm H-X probehead. Single-pulse 1 H MAS NMR experiments were carried out by co-adding 32 transients.1 H relaxation delays were of 2 s for the neat PPDT2FBT material and 5 s and PPDT2FBT:PC 61 BM blends processed from CB and o-XY, respectively. In the case of PC 61 BM acceptor molecules processed from CB and o-XY, the 1 H relaxation delays were of 6 s and 30 s, as determined from saturation recovery measurements and analyses. All 1D 13 C{ 1 H} cross polarization (CP)-MAS experiments of neat materials and blends were carried out by co-adding 2048 transients. 2D 1 H-1 H double-quantumsingle-quantum (DQ-SQ) spectra of neat compounds and PPDT2FBT:PC 60 BM blends were acquired using a one rotor period Back-to-Back (BaBa) sequence.75,76 2D 1 H-1 H DQ-SQ spectra of PPDT2FBT neat materials and PPDT2FBT:PC 61 BM blends were acquired using 200 t 1 increments, each by co-adding 16 transients, using the States method to achieve sign discrimination in the vertical indirect dimension with a rotor-synchronized t 1 increment of 40 µs corresponds to two rotor periods 2  r . 2D 1 H-1 H DQ-SQ spectra of PC 61 BM materials processed from CB and o-XY were acquired using 80 t 1 increments, each by co-adding 16 transients. The Bruker Avance Neo (18.8 T, Larmor frequencies of Accepted Article out on a Bruker Avance Neo (18.8 T, Larmor frequencies of 201.2 MHz, respectively) spectrometer with 1.3 mm H Accepted Article 201.2 MHz, respectively) spectrometer with 1.3 mm H NMR experiments were carried out by co

	Accepted Article software package for Igor (by Wavemetrics) Accepted Article software package for Igor (by Wavemetrics) Solid Accepted Article Solid-Accepted Article -Solid-Solid Accepted Article Solid-Accepted Article state NMR spectroscopy. PPDT2FBT:PC Accepted Article PPDT2FBT:PC 60 Accepted Article 60 BM blends processed from Accepted Article BM blends processed from substrates and Accepted Article substrates and packed into either 1.3 mm (outer diameter) zirconia rotors Accepted Article packed into either 1.3 mm (outer diameter) zirconia rotors caps Accepted Article caps. Accepted Article . All fast MAS (50 All fast MAS (50 Accepted Article out on a

Acknowledgements

Z. D, J. V., A. L. D., N. S., M. S., J. H., S. C. and T.-Q. N. acknowledge funding from the Air Force Office of Scientific Research (AFOSR) Grant #FA9550-19-1-0348. M.M. and M.L. acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for their support and Brilliant Matters for providing monomers. M. M. thanks Sentinelle Nord (APOGÉE) for the scholarship. J.V. acknowledges primary funding by the Alexander-von-Stiftung. G. N. M. R. gratefully acknowledges the Financial support from the IR-RMN-THC FR-3050 CNRS France for conducting solid-state NMR measurements, and the European Union's Horizon 2020 research and innovation programme under the Marie Skł odowska -Curie grant no. 795091. Z.P. and H.A. were supported by ONR grants N000141712204 and N000142012155.

GIWAXS and RSoXS research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract No. DE-AC02-05CH11231. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We thank Ziyue Zhu and Dr. Jaewon Lee for valuable suggestions as well as Dr. Akchheta Karki, Dr. David Xi Cao, Sangcheol Yoon, Dr. Brett Yurash, Dr. Tung Dang Nguyen, Dr. Álvaro Daniel Romero-Borja and Alexander Lill for useful discussions.

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Acknowledgements

Accepted Article Acknowledgements Z. D, J. V.,

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Accepted Article

adamantane as an external reference (higher ppm Accepted Article adamantane as an external reference (higher ppm

Supporting Information

Accepted Article

Supporting Information

pporting Information is available from the Wiley Online Library or from the author.

Accepted Article

pporting Information is available from the Wiley Online Library or from the author.


Parth Raval

Rhiannon M Kennard

Eugenia Vasileiadou

Clayton J Dahlman

Ioannis Spanopoulos

Michael L Chabinyc

Mercouri Kanatzidis

G Manjunatha Reddy
email: gnm.reddy@univ-lille.fr

Eugenia S Vasileiadou #

G N Manjunatha Reddy

Understanding instability in formamidinium lead halide perovskites: kinetics of transformative reactions at grain and sub-grain boundaries

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

long-term stability with respect to the rigorous standards of the damp heat test (International Electrotechnical Commission IEC-61646 stability assessment at 85 °C and 85% relative humidity in the air for over 1000 h) is at the forefront of hybrid perovskites research. 21,[START_REF] Cheacharoen | Encapsulating Perovskite Solar Cells to Withstand Damp Heat and Thermal Cycling[END_REF] To date, these tests have been done on FA-based alloy compositions used in solar cells: methylammoniumformamidinium lead bromide-iodide (MA, FA)Pb(Br, I)3, (Cs, FA)Pb(Br, I)3 and (Cs, MA, FA)PbI3. [START_REF] Cheacharoen | Design and Understanding of Encapsulated Perovskite Solar Cells to Withstand Temperature Cycling[END_REF][START_REF] Bush | 6%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability[END_REF][START_REF] Nair | A Study on Optoelectronic Performance of Perovskite Solar Cell under Different Stress Testing Conditions[END_REF] Within the recent past, much efforts have been expended to the stabilization of black()-FAPbI3 phase using additive and interfacial engineering strategies that lead to stable and efficient photovoltaic cells. [START_REF] Masi | Stabilization of Black Perovskite Phase in FAPbI3 and CsPbI3[END_REF][START_REF] Fan | Review of Stability Enhancement for Formamidinium-Based Perovskites[END_REF][START_REF] Jeong | Pseudo-Halide Anion Engineering for α-FAPbI3 Perovskite Solar Cells[END_REF][START_REF] Min | Perovskite Solar Cells with Atomically Coherent Interlayers on SnO2 Electrodes[END_REF][START_REF] Hui | Stabilizing Black-Phase Formamidinium Perovskite Formation at Room Temperature and High Humidity[END_REF][START_REF] Park | Stabilization of Formamidinium Lead Triiodide α-Phase with Isopropylammonium Chloride for Perovskite Solar Cells[END_REF][START_REF] Liu | Stabilization of Highly Efficient and Stable Phase-Pure FAPbI3 Perovskite Solar Cells by Molecularly Tailored 2D-Overlayers[END_REF][START_REF] Li | Formamidinium-Based Lead Halide Perovskites: Structure, Properties, and Fabrication Methodologies[END_REF][START_REF] Doherty | Stabilized Tilted-Octahedra Halide Perovskites Inhibit Local Formation of Performance-Limiting Phases[END_REF] However, in order to understand how to further improve the stability of such alloys and neat formulations, we must first understand the transformative reactions and degradation of the reference compounds with single-cation/single-anion compositions, FAPbI3 and FAPbBr3. We, therefore, turn our attention to these two compositions, with particular attention paid to FAPbI3, as this composition is known to be less stable at near ambient temperatures. [START_REF] Binek | Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide[END_REF][START_REF] Kim | Thermodynamics of Multicomponent Perovskites: A Guide to Highly Efficient and Stable Solar Cell Materials[END_REF] The dynamic interface between corner-sharing lead halide octahedra and organic cations is thought to affect the instability in perovskites, which further depends on compositions and other external stimuli that lead to structural deterioration and point defects. For FAPbI3, the metastable black cubic phase () converts to the yellow hexagonal phase () at ambient temperatures. [START_REF] Qiu | An Interdiffusion Method for Highly Performing Cesium/Formamidinium Double Cation Perovskites[END_REF][START_REF] Chen | Entropy-Driven Structural Transition and Kinetic Trapping in Formamidinium Lead Iodide Perovskite[END_REF] The transition → FAPbI3 occurs through various intermediate stages, separated by a free energy barrier of the order of hundreds of meV. This barrier results in a large thermal hysteresis between the → transition occurring at 350 K, whereas the → transition occurring at 290 K, and explains at least in part, how the -FAPbI3 can be kinetically trapped at ambient temperature. [START_REF] Chen | Entropy-Driven Structural Transition and Kinetic Trapping in Formamidinium Lead Iodide Perovskite[END_REF] Several in situ and ex-situ characterization techniques have been used to study phase stability in FA-based and FA-rich perovskites. [START_REF] Kundu | In Situ Studies of the Degradation Mechanisms of Perovskite Solar Cells[END_REF][START_REF] Li | Formamidinium-Based Lead Halide Perovskites: Structure, Properties, and Fabrication Methodologies[END_REF][START_REF] Cordero | Stability of Cubic FAPbI3 from X-Ray Diffraction, Anelastic, and Dielectric Measurements[END_REF] It has been suggested that the grain boundaries are susceptible to water-ingress, leading to → phase transformation. [START_REF] Cordero | Influence of Temperature, Pressure, and Humidity on the Stabilities and Transition Kinetics of the Various Polymorphs of FAPbI3[END_REF] However, this reaction is reported to endure between hours to months to complete, as different studies report the different synthetic, fabrication, and aging conditions. [START_REF] Han | Single Crystal Formamidinium Lead Iodide (FAPbI3): Insight into the Structural, Optical, and Electrical Properties[END_REF][START_REF] Steele | Direct Laser Writing of δ-To α-Phase Transformation in Formamidinium Lead Iodide[END_REF][START_REF] Zheng | Synthesis of Formamidinium Lead Iodide Perovskite Bulk Single Crystal and Its Optical Properties[END_REF] In addition, photo thermal induced resonance (PTIR with a spatial resolution of ~100 nm) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) techniques have been employed to characterize the humidity-induced phase transformations and to probe surface and sub-surface compositions in FA-rich perovskites. [START_REF] Ho | Grain Transformation and Degradation Mechanism of Formamidinium and Cesium Lead Iodide Perovskite under Humidity and Light[END_REF] Solidstate NMR (ssNMR) spectroscopy is a local probe that provides information on the structures and dynamics in hybrid perovskites with site-specificity: for example, ssNMR techniques have been used to gain insight into chemical doping, cation-ordering and dynamics, interfacial engineering, phase stability, and degradation products. [START_REF] Kubicki | NMR Spectroscopy Probes Microstructure, Dynamics and Doping of Metal Halide Perovskites[END_REF][START_REF] Dahlman | Interfaces in Metal Halide Perovskites Probed by Solid-State NMR Spectroscopy[END_REF][START_REF] Askar | Multinuclear Magnetic Resonance Tracking of Hydro, Thermal, and Hydrothermal Decomposition of CH3NH3PbI3[END_REF][START_REF] Franssen | Solid-State NMR of Hybrid Halide Perovskites[END_REF][START_REF] Grüninger | Microscopic (Dis)Order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites[END_REF][START_REF] Kazemi | Molecular-Level Insight into Correlation between Surface Defects and Stability of Methylammonium Lead Halide Perovskite Under Controlled Small Methods[END_REF][START_REF] Rosales | Persistent Dopants and Phase Segregation in Organolead Mixed-Halide Perovskites[END_REF] However, a precise understanding of how water molecules induce a "catalysis-like" transformative reaction, i.e., how water reacts without directly integrating into the initial perovskite () or the final non-perovskite () FAPbI3 structures, and insight into the nature of the intermediate phase(s) is lacking, which necessitates further investigation. Because this reaction contributes to the FAPbI3 instability that rigorously affects the performance of solar cells, it is crucially important to understand the molecular origins of the phase transition, kinetic and thermodynamic factors, which influence the degradation of hybrid perovskites. [START_REF] Ali | Interfacial and Structural Modifications in Perovskite Solar Cells[END_REF] Here, we examine the → FAPbI3 transformative reaction triggered by moisture as a function of particle size, concentration of water vapor in the air, and laboratory light illumination.

The transformative reactions are characterized at different length scales using X-Ray Diffraction (XRD), optical microscopy, photoluminescence, and ssNMR spectroscopy techniques. Specifically, local chemical environments of organic cations and lead octahedra in black (), yellow (), and intermediate () phases are analyzed by 1D 1 H, 2 H, 207 Pb, and 2D 1 H-1 H and 1 H-207 Pb ssNMR spectroscopy. The reaction kinetics, rate constants and half-lives were measured and compared for the black FAPbI3 particles exposed to moisture at different conditions and durations.

We then introduce on-the-fly deuteration approach of NH2 sites in FA + cations to capture the different dynamic behaviors of FA + cations in the intermediate phases. The -FAPbI3 exhibits excellent air stability for several months at ambient temperatures and at 40% RH. However, it transforms into -FAPbI3 within a few days upon exposure to moisture at 80% RH, whereby the laboratory illumination further accelerates this transformation. In contrast to FAPbI3, FAPbBr3 particles show enhanced moisture stability over a range of moisture exposure conditions, which indicates choice of halogen atom plays a significant role in governing the environmental stability of hybrid perovskites.

Figure 1 shows the cubic () to hexagonal () phase transition in bulk crystals as observed by powder X-Ray Diffraction (XRD), photoluminescence (PL), and solid-state 1 H and 207 Pb NMR spectroscopy. For XRD, the close match in peak locations and intensities between the simulated and experimental XRD patterns (Supporting Information, SI, Figure S1) indicates that the powders of the  and  phases are phase-pure in bulk. [START_REF] Stoumpos | Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and near-Infrared Photoluminescent Properties[END_REF][START_REF] Weller | Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α-[HC(NH2)2]PbI3, at 298 K[END_REF] For the FAPbI3 after 8h of hydration (100% RH), the characteristic (100) peak of the delta phase appears near 12°, and the transformation is complete after 21h (Figure 1a). To examine the moisture stability required for the IEC 61646 damp heat test, we then acquired XRD patterns of the same FAPbI3 material as a function of exposure time to moisture at 85% RH (SI, Figure S2), which indicates that the complete → FAPbI3 transformation takes longer than 2 days to complete. These two stability tests confirm that the water vapor in the air has a detrimental effect on the ambient stability -FAPbI3 phase. These results are corroborated by the PL studies (Figure 1b) of neat -FAPbI3 phase that shows a strong PL intensity peak at 790 nm and a weaker PL intensity (non-normalized) for the intermediate phase (85% RH, 15h of exposure) and no PL intensity for the hexagonal phase after 45h of exposure to moisture.

To examine the cubic () to hexagonal () phase transition more closely, we employed high-field 1 H and 207 Pb ssNMR spectroscopy. Specifically, the 207 Pb chemical shift is sensitive to Pb-I distances, apical Pb-I-Pb bond angles, and distortions in lead iodide octahedra. 48,51-53 1D 207 Pb magic-angle spinning (MAS) spectra of black, intermediate, and yellow FAPbI3 crystals (Figure 1c) exhibit different signals, whereby the signal centered at ~1556 ppm in the fresh FAPbI3 corresponds to Pb atoms in cubic phase (-FAPbI3). The same material upon exposure to moisture (85±5% RH, 20h) exhibited two well-resolved 207 Pb signals at ~1556 and ~1164 ppm, which are attributable to two distinct 207 Pb local environments in the cubic and hexagonal phases. After exposure to 85±5% humidity for 45h, only the peak near ~1164 ppm is present, which confirms that this peak corresponds to the hexagonal phase. The low-frequency shift of 207 Pb peak near 1164 ppm indicates octahedral tilting, consistently with previous 207 Pb assignments. [START_REF] Lee | Layer-by-Layer Structural Identification of 2D Ruddlesden-Popper Hybrid Lead Iodide Perovskites by Solid-State NMR Spectroscopy[END_REF][START_REF] Hanrahan | Sensitivity-Enhanced 207 Pb Solid-State NMR Spectroscopy for the Rapid, Non-Destructive Characterization of Organolead Halide Perovskites[END_REF][START_REF] Febriansyah | Hybrid 2D [Pb(CH3NH2)I2]n Coordination Polymer Precursor for Scalable Perovskite Deposition[END_REF] FAPbI3 particles were estimated by analyzing electron micrographs (SI, Figure S3). The → FAPbI3 transition visualized by optical microscopy (SI, Figure S4) suggests that the particle boundaries populated with the -FAPbI3 phase (i.e., large surface area of black phase is open for the interaction with water molecules) trigger the transformative reactions at the surface, which then penetrates into the sub-surfaces and to the bulk. Due to different distributions of particle sizes, there is a large heterogeneity in how fast -FAPbI3 convert into -FAPbI3 phase. It is plausible that this heterogeneity in reaction rate depends on the concentration of reactive interfaces at the grain boundaries as well as reactive intra-grain interfaces. [START_REF] Li | The Critical Role of Composition-Dependent Intragrain Planar Defects in the Performance of MA1-xFAxPbI3 Perovskite Solar Cells[END_REF] Nonetheless, the smaller FAPbI3 particles react with water faster than the larger particles (SI, Figures S5, andS6), due to the high surface area associated with the former, allowing greater interaction with water. Kinetics plots of → phase transformation measured by means of the 1 H signal (-CH site of FA + , 8.1 ppm) intensity decay in the  phase and the simultaneous buildup of the 1 H signal intensity of the same sites for the  phase (8.5 ppm) as a function of moisture exposure time can be approximated to the exponential build-up/decay function. A good agreement was observed between the experimental intensity buildup and first-order kinetics model with a co-efficient of determination R 2 =0.995 (SI, section 5, Figure S7), and data fitted against this model led to poor agreement. The (pseudo)firstorder reaction kinetics suggests that the phase transformation is mediated by "liquid-like" transient species (vide infra), indicating the surface-induced dissolution of the black phase followed by a precipitation of the yellow phase. We reasoned that the reaction rate depends on the surface wettability (water vapor concertation) and/or the particle exposure area. This trend is observed for different humidity points and particle sizes, following the (pseudo)first-order kinetics (Figure 2, and Figures S8b,d,S9), whereby a rapid evolution of → phase transformation occurs at an early hydration period. This sudden increase in the formation of the -FAPbI3 can be attributed to the susceptible water ingress at the particle boundaries of -FAPbI3, [START_REF] Yun | Humidity-Induced Degradation via Grain Boundaries of HC(NH2)2PbI3 Planar Perovskite Solar Cells[END_REF][START_REF] Park | Inorganic Rubidium Cation as an Enhancer for Photovoltaic Performance and Moisture Stability of HC(NH2)2PbI3 Perovskite Solar Cells[END_REF][START_REF] Ono | Progress on Perovskite Materials and Solar Cells with Mixed Cations and Halide Anions[END_REF] which is consistent with the optical microscopy analysis (SI, Figures S5, andS6). The rate at which the → FAPbI3 transformative reaction occurs can be modeled by the (pseudo)first-order kinetics, which leads to the estimation of rate constants and half-lives of -FAPbI3 particles (Table 1). When large particles (40-100 m) are exposed to moisture (85% RH), ~50% of the -FAPbI3 is formed in less than 15h, which then rises to over 90% in ~2 days and the complete transformation occurs after several days. Based on the analysis of kinetics plots, the rate constant for this reaction is estimated to be 0.052 h -1 with a half-life decay time of ≈13.3h (SI, Table S1). By comparison, smaller -FAPbI3 particles (20-40 m) exhibited rapid transformation kinetics (Figure S8c,d) upon exposure to moisture (85% RH) due to the increased water/particle interfacial interaction that accelerates the → FAPbI3 transformation. In this latter case, the rate of transformative reaction was found to be 0.061 h -1 and the half-life, t1/2 (85% RH, 20-40 m) is ≈11.4h. It should be noted that the difference between the half-lives (t1/2) for large and small particles is 2h (SI, Figure S9) which is relatively short, due to the non-uniformity in particle sizes, shapes, and overlap between the particle size distributions. Further insight into the role of external stimuli such as water vapor and light illumination on the reaction kinetics of → transformation can be obtained by analyzing kinetic plots of FAPbI3 obtained from 1 H MAS NMR spectroscopy. For small particles (20-40 m), Figure 2 presents the kinetics of → FAPbI3 phase transition examined at different concentrations of water molecules in the air (85% and 40% RH). Although continuous exposure to water vapor at 85% RH leads to a → FAPbI3 phase transformation in a few days with a rate constant of 0.042h -1 (Figure 2a), the same material upon exposure to 40% RH leads to relatively slow transformation with a half-life decay time of 3327 h (~133 days) and the complete transformation is estimated to take up to a year by extrapolating the kinetics plots (Table 1). These results are in line with the optical microscopy analysis (SI, Figures S4-S6), which indeed explains that it would take ≈16.5h of exposure at 85% RH to convert nearly one-half of the -FAPbI3 into -FAPbI3, however, the complete transformation into -FAPbI3 takes several days. This is likely due to the much lower water ingression through the particle boundaries that are covered by the yellow phase, thus leading to a sluggish phase transformation of black particles trapped in the sub-surfaces or reactive intragrain interfaces in FAPbI3 particles. For the large particles of over 100 m exposed to moisture (40% RH), a complete → FAPbI3 phase transformation is expected to take over a year due to relatively low concentration of water vapor in the air. Trace amounts of surface preabsorbed chemical species such as trapped water, dioxygen, or precursors used in the synthesis process may also contribute to the degradation process. [START_REF] Yun | Humidity-Induced Degradation via Grain Boundaries of HC(NH2)2PbI3 Planar Perovskite Solar Cells[END_REF] However, the species were not detected either in NMR or XRD experiments, suggesting that any such chemical residues are at minuscule concentrations if present. S2). We highlight that, despite the excellent thermal stability of the FAPbI3 black phase, [START_REF] Smecca | Stability of Solution-Processed MAPbI3 and FAPbI3 Layers[END_REF] the large variation in the kinetics of → FAPbI3 transformation is due to the deleterious effect of water molecules and light illumination combined. The most important takeaway from this study is that the impact of concentration of water vapor, and light illumination on the kinetics of transformative reactions can be disentangled and compared, among which relative humidity in the air seems to play a vital role in governing the environmental stability of optoelectronic devices. For example, it has been shown the FAPbI3-based solar cell efficiency reduces to one-half of its initial value within a few hours upon exposure to moisture (40 °C, 70% RH, 5h, AM 1.5G UV-light illumination), consistently with this study. [START_REF] Zhang | Profiling the Organic Cation-Dependent Degradation of Organolead Halide Perovskite Solar Cells[END_REF] However, the stability of FAPbI3 and FA-rich thin films depends on various factors such as deposition and annealing conditions, and the encapsulation and passivation strategies further exacerbate the situation. Therefore, an extrapolation of the kinetics plots of the crystals to the thin films in solar cells and other optoelectronic devices is less straightforward.

Overall, our study indicates that the large grains are likely to be beneficial for the environmental stability of FAPbI3 and FA-rich perovskite formulations.

The fundamental pertinent question is how the phase transformation occurs at the particle boundaries, and how to resolve the interfacial structure of |-FAPbI3 phase. We sought to understand if there is a significant mixing of cubic and hexagonal phases during the transformation and any amorphous phases, as XRD shows only the reflections corresponding to  and  phases, and as optical microscopy suggests lack of intermixed  and  phases (Figures 3a,S6). Previous studies suggested that the thermal transformations in lead halide perovskites occur through liquidlike interfaces. [START_REF] Bischak | Liquid-like Interfaces Mediate Structural Phase Transitions in Lead Halide Perovskites[END_REF] The liquid-like behavior indeed facilitates surface reactions in order to modulate hybrid perovskite structures and properties. [START_REF] Xue | Crystalline Liquid-like Behavior: Surface-Induced Secondary Grain Growth of Photovoltaic Perovskite Thin Film[END_REF] In a 2D 1 H-1 H Double-Quantum-Single-Quantum (DQ-SQ) correlation NMR experiment, 2D peaks are detected for through-space dipolar coupled 1 H-1 H pairs within 5 Å distance. A detailed analysis of DQ-SQ spectra of black, partially transformed and yellow phases is given in SI (Figure S11). For the moisture exposed -FAPbI3 sample (Figure 3d), a set of on-and offdiagonal DQ peaks at 14.8 and 15.6 ppm (gray box) corresponding to the 1 H sites in NH2 and intra/intermolecular dipolar-coupled CH↔NH2 sites of -FAPbI3 (intra-and intermolecular H-H distances are ≈4.54 Å and ≈2.28 Å) are detected. In addition, on-and off-diagonal DQ peaks at 17.0 and 16.1 ppm (yellow box) corresponding to the same moieties in -FAPbI3 are observed. If FA + cations in the black and yellow phases were intermingled, the DQ-SQ spectrum would show a signal at 8.2+8.5 = 16.7 ppm, however, there is no DQ peak at this frequency (Figure 3d, dashed oval). This is due to fast reorientational motions of FA + cations in -FAPbI3 phase that partially average out dipole-dipole interactions or due to the flexible heterogenous interface between the phase-separated black and yellow regions or both of these combined.

Extending beyond the 1 H DQ-SQ NMR length scales (5 Å), 2D 1 H-1 H spin-diffusion (SD) experiment allows the magnetization exchange to occur between dipolar coupled 1 H-1 H sites within and beyond a nanometer distance to be probed. Specifically, the spin magnetization exchange between chemically inequivalent and dipolar coupled protons manifests as off-diagonal peaks. A detailed analysis of 2D 1 H-1 H SD spectra of FAPbI3 before and after exposure to moisture acquired with various mixing times, mix, is presented in SI (Figure S12). For the -FAPbI3, there are no off-diagonal peaks between -CH (-FAPbI3) and -CH (-FAPbI3) signals in the 2D spin diffusion spectrum (Figure 3e, dashed burgundy circles), indicating that there is no magnetization exchange between FA + cations in the black and yellow phases. This corroborates the above ssNMR analyses and confirms that the  and  FAPbI3 domains are locally phase segregated (micrograph, Figure 3a), indicating that the structural transition occurs through a flexible and heterogeneous interface. [START_REF] Yun | Humidity-Induced Degradation via Grain Boundaries of HC(NH2)2PbI3 Planar Perovskite Solar Cells[END_REF] This leads us to investigate the dynamic behavior of organic cations in the -FAPbI3 phase. For example, site-specific dynamics of organic cations have been previously studied by ssNMR and other complementary techniques. [START_REF] Kubicki | NMR Spectroscopy Probes Microstructure, Dynamics and Doping of Metal Halide Perovskites[END_REF][START_REF] Grüninger | Microscopic (Dis)Order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites[END_REF][START_REF] Piveteau | Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials[END_REF][START_REF] Mozur | Cesium Substitution Disrupts Concerted Cation Dynamics in Formamidinium Hybrid Perovskites[END_REF][START_REF] Drużbicki | Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites[END_REF][START_REF] Fabini | Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites[END_REF][START_REF] Dahlman | Dynamic Motion of Organic Spacer Cations in Ruddlesden-Popper Lead Iodide Perovskites Probed by Solid-State NMR Spectroscopy[END_REF] We introduce an in situ deuteration strategy that enables deuteration of FA + cations during the phase transformation. Upon exposure of -FAPbI3 to D2O, the hydrogen atoms of NH2 groups in FA cations exchange with deuterium sites, which can be achieved on-the-fly during the exposure to moisture (see Experimental section). The degree of deuteration in the converted -FAPbI3 phase is estimated to be ~70% based on the quantitative 1D 1 H NMR measurements and analysis (SI, Figures S13, andS14). Insight into the dynamics of FA + cations in perovskite and non-perovskite phases were then obtained by analyzing 2 H NMR spectra of fresh and aged FAPbI3 materials (Figure 4), whereby 2 H lineshape is sensitive to the local mobility and dynamic nature of the local ND2 groups in black and yellow phases, and surface adsorbed D2O molecules. [START_REF] Knop | Alkylammonium Lead Halides. Part 2. CH3NH3PbX3 (X = C1, Br, I) Perovskites: Cuboctahedral Halide Cages with Isotropic Cation Reorientation[END_REF][START_REF] Kubicki | Cation Dynamics in Mixed-Cation (MA)x(FA)1-xPbI3 Hybrid Perovskites from Solid-State NMR[END_REF][START_REF] Roiland | Multinuclear NMR as a Tool for Studying Local Order and Dynamics in CH3NH3PbX3 (X = Cl, Br, I) Hybrid Perovskites[END_REF] 1D 2 H spectrum of FAPbI3 treated with D2O (6 h, 85% RH) shows a peak at ~7.4 ppm corresponding to ND2 sites in the black phase of FA + cations as well as minuscule concentration of D2O molecules ingress at the grain boundaries, both of these undergo fast re-orientational motions at the NMR time scales leading to a narrow signal. To test this, a 2 H spectrum of liquid D2O is acquired and compared, which showed a narrow signal at ~4.8 ppm due to the fast reorientational motion of D2O molecules.

Particularly interesting feature is that the same material after exposure to D2O for 20 h displays different quadrupolar splittings (detected at much higher intensities for the FAPbI3 after 68 h exposure to D2O), which are simulated to obtain the quadrupolar constants (CQ) of 53 and 81 kHz. This is in accord with the 2 H NMR spectra of control samples: a neat -FAPbI3 material after exposure to D2O displays identical lineshape and quadrupolar splitting (Figure 4, Figure S15), and a neat cubic FAPbBr3 as a control material (since it does not undergo a moisture induced phase transition) exhibits a narrow 2 H NMR feature (SI, Figure S16). It can be reasoned that these two different quadrupolar splittings correspond to relatively slow re-orientational motion of ND2 sites of FA + cations in the yellow phase; one that corresponds to ND2 sites facing toward the face-shared lead octahedra at the organic-inorganic interfaces, and the other corresponds to the ND2 sites at the organic-organic (FA + /FA + ) interface in between the chain-like lead octahedra. This suggests that the phase transformation occurs at the grain boundaries of the black phase triggered by the D2O ingression that transforms into the yellow phase, whereby the trapped water molecules (if any) and FA + cations in the black FAPbI3 particles at sub-surface layers undergo relatively fast reorientational motion than the NMR timescales and are likely to exhibit a flexible "liquid-like" interface. This is also consisted with the first-order reaction kinetics. While further investigation is required to quantify the prevalence and impact of moisture on the → phase transformation at sub-grain boundaries, the results presented here show that a multinuclear ( 1 H, 2 H, 207 Pb) NMR spectroscopy can provide a powerful and non-destructive way of probing phase transformations and characterizing the -FAPbI3 phase. In contrast to FAPbI3, FAPbBr3 exhibits enhanced moisture stability for several weeks, as revealed by combined XRD and ssNMR analysis. The experimental and simulated XRD powder patterns, shown in Figure 5a, indicate the phase purity in bulk. The formability and stability of hybrid perovskites depend on the Goldschmidt tolerance factor (t), with t values between 0.8 and 1.0 favoring the formation of cubic perovskite structures: FAPbI3 and FAPbBr3 exhibited t values of ~0.88 and ~0.90, leading to different phase stabilities. [START_REF] Li | Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys[END_REF][START_REF] Fan | Perovskites for Photovoltaics: A Combined Review of Organic-Inorganic Halide Perovskites and Ferroelectric Oxide Perovskites[END_REF][START_REF] Bartel | New Tolerance Factor to Predict the Stability of Perovskite Oxides and Halides[END_REF] A theoretical study corroborates that the FAPbBr3 shows a greater extent of hydrogen bonding interactions than FAPbI3. [START_REF] Svane | How Strong Is the Hydrogen Bond in Hybrid Perovskites?[END_REF] The high Several aspects of the molecular-level understanding of transformative and reconstructive reactions in FAPbX3 (X=I, Br) have been elucidated, including the kinetics of degradation reactions at grain/sub-grain boundaries as functions of water vapor concentration, particle size, and light illumination, as well as the chemical nature of the -FAPbI3 phase. Using combined time-resolved XRD and ssNMR spectroscopy, we unraveled the reaction kinetics of moistureassisted structural transformations. Our results indicate that the cubic phase of FAPbI3 is kinetically stable, whilst the cubic phase of FAPbBr3 is thermodynamically stable at ambient temperatures with a wide range of humidity levels and exposure times in agreement with the recent study. [START_REF] Vasileiadou | Shedding Light on the Stability and Structure-Property Relationships of Two-Dimensional Hybrid Lead Bromide Perovskites[END_REF] The half-lives of -FAPbI3, intra-grain morphology and chemical nature of intermediate species characterized at different length scales provide rich mechanistic insights. The key takeaway from this study is that the ambient stability of the black FAPbI3 phase strongly depends on the external stimuli and kinetics of degradation reactions at the grain and sub-grain boundaries; a combination of these effects leads to variation of the transformation time from days to several months, or even a year. The reaction process that enables the transformation from the black phase to the yellow phase involves the surface-initiated local dissolution and precipitation mechanism, with water being the solvent. The synergy of multi-scale characterization techniques that are complementary with respect to each other and on-the-fly deuteration of organic cations demonstrated in this study is likely to be an essential feature of the strategies to understand the transformative reactions in hybrid halide perovskites and develop interfacial engineering strategies in the future. This study also suggests that the single crystal FAPbI3 and FA-rich materials provide a path forward for stable and efficient optoelectronics.

Associated Content

Materials and methods, powder XRD patterns of FAPbI3, SEM micrograms, HR optical microscopy images, 1D 1 H MAS, kinetics plots, data fitting and 2D

Present address

¤ Department of Chemistry, University of South Florida, Tampa, FL, 33620

  While 1 H MAS NMR spectrum (Figure 1d) of the black phase displays signals associated with -CH (8.1 ppm) and -(NH2)2 groups (7. 4 ppm) of FA + cations, additional signals corresponding to -CH (8.5 ppm) and -(NH2)2 groups (7.6 ppm) of FA + cations in the yellow phase have emerged after 5h of exposure to moisture and complete conversion into the yellow phase occurs after 45h (~2 days). Notably, high-field 1 H MAS NMR (21 T, 900 MHz) improved our resolution abilities to examine reaction kinetics of the → phase transformation, as discussed below.

Figure 1 .

 1 Figure 1. (a) Ex-situ XRD patterns of FAPbI3 as a function of moisture (100% RH in the air) exposure time. Peaks near 16.6 degrees correspond to quartz substrates denoted by asterisk and (b) photoluminescence spectra of FAPbI3 as a function of moisture (85% RH) exposure time. 1D (c) 207 Pb and (d) 1 H MAS NMR spectra of fresh (-FAPbI3) and aged (-FAPbI3, -FAPbI3) materials upon exposure to moisture (85% RH). Signals associated with  and  polymorphs are indicated and color-coded as depicted in the schematic structure figure in (d). Crystal structures of  and  polymorphs are shown in the inset of (a). All 1 H (900.2 MHz) and 207 Pb (167.6 MHz) spectra were acquired with 50 kHz MAS.

Figure 2 .

 2 Figure 2. Kinetics plots of 1 H signal intensity buildup of -CH sites of FA + cations in -FAPbI3 and the simultaneous 1 H signal intensity decay for the same sites in -FAPbI3 as a function of exposure time to moisture in the dark at relative humidity levels (a) 85% RH and (b) 40% RH.

  another factor that could contribute to the →-FAPbI3 phase transformation. To test this, we studied the moisture-induced (85% RH) degradation kinetics of small particles (20-40 m) under dark and laboratory light illumination. The presence of light illumination (500-750 Lux) accelerates → FAPbI3 phase transformation, further reducing the half-lives from 16.5h to 11.4h (SI, Figure S10, and Table

  Understanding the chemical nature of an intermediate phase at short-length scales (< 1 nm) is expected to cast light on the surface/interfacial reactions that cause perovskite degradation. A schematic of → transformation with plausible intermediate products based on the optical microscopy analysis is shown in Figure3a,b, and 2D1 H-207 Pb, and 1 H-1 H correlation spectra of partially transformed FAPbI3 particles (aged for 24 h at 85% RH) are presented in Figure3c-e. In particular, optical microscopy images of -FAPbI3 particles (Figure3a) formed during the phase transition show distinct morphological features indicating the presence of yellow and black FAPbI3 domains. By analyzing the 2D 207 Pb-1 H correlation spectrum of the same -FAPbI3 phase (Figure3c), through-space sub-nanometer207 Pb-1 H proximities between FA + and PbI6 octahedra at the organic-inorganic interface are distinguished and identified: 2D peaks at 1556 ppm ( 207 Pb) and 7.4 and 8.1 ppm ( 1 H) correspond to the organic-inorganic interface in -FAPbI3, and low-intensity signals between 207 Pb (1556 ppm) and 1 H (7.6 and 8.5 ppm) sites indicate that the through-space proximities between and -FAPbI3 moieties. It can be hypothesized that the minuscule concentrations of [PbI6] 4-, [Pb(H2O)6] 2+ , [PbI2.4H2O] [PbI3.3H2O] -[PbI4.2H2O] 2-complexes may exist at the interface during the phase transformation, although undetected in both XRD and ssNMR experiments. Insight into the organic-organic interface can be obtained by analyzing 1 H-1 H correlation spectra (Figure 3d,e).

Figure 3 .

 3 Figure 3. (a) High-resolution optical microscopy images of fresh and aged (after exposure to moisture) FAPbI3 particles. (b) Schematic of → FAPbI3 transformation upon exposure to moisture. Solid-state 2D ssNMR spectra of -FAPbI3 phase obtained after exposure to moisture (24 h, 85% RH): (c) 1 H-207 Pb heteronuclear correlation acquired with 6 ms of cross-polarization contact time, (d) 1 H-1 H DQ-SQ correlation and (e) 1 H-1 H spin-diffusion, acquired at room temperature and at 21 T ( 1 H 900.2 MHz) with 50 kHz MAS. The corresponding skyline projections are shown along the top 1 H SQ horizontal axis. Correlated signal intensities originating from dipolar-coupled 1 H-1 H pairs are depicted by colored circles. In (c-e), 2D peaks depicted in gray and yellow boxes correspond to the black and yellow FAPbI3 phases, and the dashed red circles indicate the absence of dipolar interactions between the FA + cations in these two phases.

Figure 4 .

 4 Figure 4. Solid-state static 1D 2 H NMR spectra of FAPbI3 acquired at room temperature as a function of exposure time to D2O vapor (85% RH) together with the 1D 2 H NMR spectrum of liquid D2O acquired under the same conditions. A static 2 H NMR spectrum of control -FAPbI3 material obtained after exposure to D2O for 48h is acquired and compared. All spectra were acquired at 18.8 T ( 2 H = 122.8 MHz) and at room temperature. Inset on top is an optical microscopy image of a partially transformed FAPbI3 crystal displaying the black and yellow regions at intra-grain interfaces.

field 1 H

 1 MAS NMR data of FAPbI3 and FAPbBr3 confirms this trend: for FAPbBr3 the 1 H peaks are observed at higher frequencies (Figure5b: NH2 and CH peaks at 7.7 and 8.3 ppm) indicating stronger hydrogen bonding interactions than the -FAPbI3 that displays peaks at lower frequencies (Figure1d: NH2 and CH peaks at 7.4 and 8.1 ppm). 1D 207 Pb MAS spectrum (Figure5c) of fresh FAPbBr3 shows a207 Pb signal centered at ~551 ppm corresponds to Pb atoms in a cubic phase,[START_REF] Aebli | Lead-Halide Scalar Couplings in 207 Pb NMR of APbX3 Perovskites (A = Cs, Methylammonium[END_REF] and this signal after exposure to moisture (85% RH, 7 days) retained its position, suggesting that the moisture has little impact on the local structures of lead octahedra. In addition, a static 2 H NMR spectrum of FAPbBr3 after exposure to D2O (SI, FigureS15) shows a narrow feature centered at ~7.7 ppm, suggesting the fast reorientational motion of FA + cations in the cubic phase. Identical 1D 1 H and 2D 1 H-1 H DQ-SQ (Figures5b, S17) spectral patterns of fresh and aged (85% RH, 7 days) powders further confirmed the enhanced environmental stability of FAPbBr3 material.

Figure 5 .

 5 Figure 5. (a) Powder X-ray diffraction patterns of experimental and simulated FAPbBr3. Solidstate (b) 1D 207 Pb (c) 1 H NMR spectra of fresh and aged FAPbBr3 acquired at 50 kHz MAS frequency together with photographs taken before and after exposure to moisture for 7 days (85% RH).

Table 1 .

 1 Half-life decay time associated with → phase FAPbI3 transformation in the dark with varying relative humidity. The particle size distribution is 20-40 m.

  [START_REF] Eperon | Metal Halide Perovskite Tandem and Multiple-Junction Photovoltaics[END_REF] H-1 H DQ-SQ and 2D 1 H spin diffusion NMR spectra, 2 H and 1 H lineshape analyses of FAPbI3. 1D static 2 H and 1 H lineshape analyses, 1D 1 H, 2 H, and 2D 1 H-1 H MAS NMR spectra of FAPbBr3.

Acknowledgements

At Northwestern University and UCSB, this work is mainly supported by the Department of Energy, Office of Science, Basic Energy Sciences, under Grant No. SC0012541 (synthesis, structure, and characterization of physical properties). The research reported here also made use the shared facilities of the UCSB MRSEC (National Science Foundation DMR 1720256), a member of the Materials Research Facilities Network (www.mrfn.org). R.M.K. gratefully acknowledges the National Defense Science and Engineering Graduate fellowship for financial support. G. N. M. R. and P. R. acknowledge the funding from University of Lille and region Hautsde-France (HDF), and the support from the IR-RMN-THC FR-3050 CNRS France for conducting ssNMR measurements. G. N. M. R. acknowledges the funding from EU-H2020 research and innovation programme under the Marie Skłodowska-Curie grant no. 795091.

Conflict of Interest

Authors declare no conflict of interest.