Non-aliphatic Protic Ammonia for Post Healing of Formamidinium-containing Perovskite Films

Shuping Pang (  pangsp@qibebt.ac.cn ) Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences https://orcid.org/0000-0002-2526-4104 Zhipeng Li Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Xiao Wang Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Zaiwei Wang École Polytechnique Fédérale de Lausanne https://orcid.org/0000-0001-9725-0206 Zhipeng Shao Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Lianzheng Hao Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Xiaofan Du Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Yi Rao Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Chen Chen Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Dachang Liu Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Li Wang Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences Guanglei Cui Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences


Main Text
Organic-inorganic hybrid halide perovskite materials with the general formula ABX 3 , where A is methylammonium (MA) or formamidinium (FA), B is Pb or Sn, and X is I, Br or Cl, have emerged as a class of promising light-harvesting materials since 2009 1 , reaching a certi ed power conversion e ciencies (PCE) as high as 25.5% 2 , which is comparable to commercialized thin-lm solar cell technologies.The crystal structure of ABX 3 perovskite can be regarded as a [MX 6 ] 4-octahedron in a three-dimensional (3D) space with a common apex angle connecting, where the A site ions ll up the gap of the octahedral and the structure is stabilized by Van der Waals forces 3 .These special structural characteristics provide us with a variety of solution processing methods to prepare high-quality perovskite lms [4][5][6] .
The high boiling point polar dimethylformamide (DMF), γ-butyrolactone (GBL), dimethyl sulfoxide (DMSO) and N,N-dimethylacetamide (DMA) are the common solvents employed to dissolve the perovskite precursors with the forming of solvated iodoplumbate complexes related to the coordination between O-donor ligands (OL) and Pb(II) 4,[7][8][9] .The Lewis basicity of solvents is thought to correlate with "coordinating ability" with lead halide salt which can be predicted by their Gutmann's Donor Number (D N ) with a trend of DMSO>DMA>DMF>GBL [10][11] .The higher D N solvents coordinate more strongly with the Pb(II) center, which in turn results in the formation of intermediate OL-PbI 2 -RAI (R refers to MA or FA, etc.) prior to perovskite crystallization during the lm fabrication process.The anisotropic growth nature of the intermediate phase leads to one-dimensional (1D) ber-like structures 12 .In this regard, it is de nitely e cacious to introduce stronger coordinating additives (such as Thiourea 13 and Pyridine 14 ) and the fast nucleation scenarios (such as antisolvent extraction, gas-quenching, and vacuum-assisted drying) to modify the crystal nucleation/growth nature of the intermediate phase 7,[15][16] .
Among of them, the polar protic aliphatic methylamine gas (MA g ) featured with the presence of hydrogen bonding and low boiling point is becoming an impressive coordinating agent to regulate the crystallization process of thin pure MAPbI 3 lms [17][18][19] 18 , and the adoption of MA g as a volatile solvent system has become a commercially viable technology for the MAPbI 3 devices with excellent device reproducibility [20][21][22][23][24][25][26][27][28][29] .In comparison with MA-rich perovskite lms, formamidinium lead iodide (FAPbI 3 ) has more complex crystallization behavior with narrow processing window, which makes it more di cult to achieve the high-quality layers through the normal slot-die coating and spraying technologies.There are already many attempts to introduce MA grelated methods for the fabrication of FA-based perovskite layer, unfortunately, resulting in degradation of the 3D perovskite phase 21 .The underlying reason of the irreversible transformation of FAPbI 3 with protic aliphatic amines treatment is still not fully clear, let alone make efforts to solve this problem.
Here, we have systematically studied the underlying chemical reactions between amines (R-NH 2 )/FA gases and MAI/FAI salts used in perovskite precursor under different conditions, and elucidated the condensation reaction between FAI and protic aliphatic amines.On the bases of the interaction of solvent-solute and solute-solute, protic non-aliphatic ammonia (NH 3 ) was selected to avoid the degradation of the perovskite phase during the post healing process.Particularly, it is demonstrated that low processing temperature is crucial for perovskite layer to enhance NH 3 molecules uptake reaching a owable state, and the presence of hydrogen bonding enable a homogenous crystallization to pure perovskite phase.As a result, a rough FA-containing perovskite lm was effectively healed to a highly uniform, compact one, reaching an e ciency of 23.3%.This demonstrates the high advantage of the NH 3 gas post healing technology, and it is easy for industrial transition for upscaling the fabrication of highly e cient perovskite solar cells.
We started with the study of the underlying chemical reactions between R-NH 2 /FA gases and FAI salts (Table 1).The gases involved included methylamine (MA g ), ethylamine (EA g ), propylamine (PA g ), butylamine (BA g ), formamidine (FA g ), and ammonia (NH 3 ).The preparation of relevant gases and the details of experimental operation are provided in the supporting information.Among of them, the synthesis of pure FA g is extremely tricky because of its instability at room temperature (RT), which is decisive for its storage and healing treatment for FA-based perovskite lms.
First, deuterated DMSO (DMSO-d 6 ) solvent was used to collect FA g gas as illustrated in Figure S1(A), where mixed FACl and NaOH powders were kept at RT.The use of DMSO-d 6 is to facilitate subsequent direct nuclear magnetic resonance (NMR) measurement.Unfortunately, only NH 3 signal was detected in the NMR spectrum (Table1(No.1) and Figure S2).The possible reason is that the boiling point of FA g is too high to diffuse into the DMSO-d 6 solvent.The formation of the NH 3 is indicative of side reactions in the process of gas preparation (Scheme S1).Then the temperature of the reactor and the gas pipeline was increased to 150 ℃ and DMSO-d 6 solvent was still kept at RT.In this case, four main compounds were detected as follows: s-triazine > formamide > FA > NH 3 (Table1(No.2) and Figure S3).The s-triazine with a boiling point of 114 ℃ should be the product of condensation reaction of three FA molecules with formation of NH 3 (Scheme S1(b)).In comparison, the relatively low NH 3 content is owing to its volatile property and most of NH 3 molecules were evaporated from DMSO-d 6 solution before NMR characterization.This side reaction was also detected when the FAI powder sample was heated at 150 ℃ 30 .The by-product formamide should be the hydrolyzation product of FA g in the presence of H 2 O which was generated from the neutralization between the FACl and NaOH (Scheme S1(a)).
Considering that there is long time interval from gas collection to NMR measurement, then acetic acid (HOAc) was employed to capture FA g .By this way, the as-prepared FA g could be quickly converted to FAAc before their condensation reaction.As expected, NMR spectrum of the reaction product is exactly FAAc without any other impurity components (Table1(No.3) and Figure S4).The above results strongly indicate that FA g molecule could be produced, but it only stable in a short time.
Based on this method, the interaction between FAI and FA g was further studied.The FAI powder was then adopted to capture the as-produced FA g prepared from FACl and NaOH at 150 ℃.Besides the FAI, there is also formamide signal in the NMR spectrum (Table 1(No.4), Figure S5 and Scheme S1(a)).Now it's clear that the spontaneous decomposition and addition-elimination reaction among the FA g molecules highly limited their healing effect on FAPbI 3 perovskite lms.In comparison with FA g , other amines are much more stable.When FAI powder was treated with MA g , EA g , PA g and BA g , the powder rstly transformed to a liquid state, and then the powder was put in vacuum to degas and obtain the nal powder (Figure S1B).It is worth noting that there is no FAI signal in NMR spectrum of the nal powder sample (Table 1 (No.5-8) and Figure S6).
Taking MA g as an example (Table 1(No.5)), the signals founded in NMR spectrum belong to N-methyl formamidinium iodide (MFAI) and N,N'-dimethyl formamidinium iodide (DMFAI) with the mole ratio of about 6:1 (Figure S6(a)).The related chemical reaction mechanism is illustrated in Scheme 1.The MA g molecule is nucleophilic and the lone-pair electrons of N atom could attack electrophilic group imine bond in FAI to form MFAI with an addition-elimination process.There is also an imine bond in the formed MFAI, which can carry out the second time addition-elimination reaction with MA g to form DMFAI 31 .Meanwhile, ion exchange reactions between FAI, MFAI or DMFAI with MA will also occur, and FA g , MFA g and DMFA g will be generated, respectively.In the whole reaction process, the addition-elimination reaction is irreversible, while the ion exchange reaction is reversible.In the condition with excessive MA g , the nal solid powder is mainly composed of DMFAI.
Similarly, when FAI powder was treated with EA g , PA g and BA g gases, the addition-elimination reaction, ion exchange reaction and the main reaction product are presented in Scheme 1 and Table 1(No.6-8).It should be also stressed in particular that the above chemical reaction mechanism is only applicable to dry condition.When the gas resource contains H 2 O molecules (Figure S1C), the nal product is RAI powder rather than RFAI and DRFAI when FAI powder was treated by RA (R is referred to H, Me, Et etc.) gas formed from its aqueous solution.(Table 1 (No.9-11) and Figure S7).If the FAI powder was treated only by H 2 O, there is no other by-products detected in the nal product (Table 1 (No.12) and Figure S8).It is speculated that the water molecules can decompose the deprotonated RFA and DRFA to RA and HCOOH (Scheme 1).In comparison, the optical photos and XRD patterns of δ-FAPbI 3 , MFAPbI 3 , DMFAPbI 3 lms, and the α-FAPbI 3 lms before and after MA g healing treatment were shown in Figure S9.
As we can see, except α-FAPbI 3 , the other lms are all non-perovskite phases.Conceivably, it is because the size of RFA and DRFA cations (R refer to alkyl moiety in MA, EA, PA and BA) are too large to ll the gap of PbI 6 octahedrons for the formation of a stable 3D perovskite phase.This is why aliphatic amines cannot be employed as the solvents or healing gases for fabrication of FA-containing perovskite materials.
Inspired by the above addition-elimination reaction, dry protic NH 3 gas was selected to avoid changes in composition after the gas treatment.As expected, there is no composition change of FAI powder after treating with dry NH 3 gas (Table 1(No.13)and Figure S10) and the corresponding reaction process is presented in Scheme 2. The addition-elimination reaction will still occur, but the actual composition of the FAI powder will not change.Moreover, the H 2 O should be avoided during the NH 3 gas treating because it will cause the decomposition of FAI to NH 4 I (Figure S7).Here the question is how to enhance the x value in FAPbI 3 •xNH 3 .We found that the absorbed amount of NH 3 molecules is seriously associated with the processing temperature.When the temperature was reduced to -5 ℃, a white powder was formed with the x value of ~7.When the temperature was reduced to -15 ℃, the white solid gradually becomes slurry with certain uidity as presented in right vial of Figure 3A.It was calculated that the x value is ~12 in the liquid state.
The kinetic process of degassing NH 3 gas from FAPbI The chemical interactions between FAPbI 3 and NH 3 molecules were calculated with density functional theory (DFT) method as presented in Figure 2C.As a Lewis base, NH 3 is a coordinating molecule with D N number (Gutmann's donor number) of 59 Kcal/mol 33 , which is strong enough to be as an electron donor with the formation of strong coordination bond with Pb 2+ .Therefore, the adsorption of one molecule of NH 3 to form FAPbI 3• 1NH 3 should be owing to the strong coordination bond.In addition to coordination bond, there is the presence of hydrogen bonding, which is necessary for the formation of liquid FAPbI 3 •xNH 3 adduct.The N atom is electronegative because of the presence of lone pair of electrons, which has certain interaction with H atom of FA cations or other NH 3 molecules.It is worth emphasizing that the NH 3 amount absorbed by FAPbI 3 is highly related to the temperature and pressure, and here we only calculated the adsorption of 6 NH 3 molecules.It was calculated that there are three different kinds of hydrogen bond lengths between 1 FAPbI 3 and 6 NH 3 molecules, 1.73 Å, 2.13 Å, 2.70 Å, respectively.It was also found that the existence of I -could also form weak hydrogen bond with NH 3 and shorten the hydrogen bond lengths surrounding it (Figure 2C).Therefore, it can be concluded that, in the liquid FAPbI 3 •xNH 3 adduct, there is relatively strong coordination bond between Pb 2+ and NH 3 , medium strength hydrogen bond between FA + and NH 3 , and much weak hydrogen bond between NH 3 and NH 3 (Figure 2D).
In terms of device fabrication, cesium (Cs) doped FAPbI 3 material system was selected to eliminate the formation of the undesired δ-FAPbI 3 7 .Firstly, FA 0.9 Cs 0.1 PbI 3 (FACsPbI 3 ) raw lm was deposited on an FTO substrate and then heated at 140 ℃ for 20 min to remove the solvent.The lm morphology is shown in scanning electron microscope (SEM) image in Figure 3A with a dull surface.The growth of dendrite-like perovskite crystals, and voids between them is typical of one-step-processed perovskite lms using DMF solvent.The size of voids in the starting raw lm can reach up to several micrometers.Then the lm was transferred into a home-made chamber with a semiconductor chilling plate controlling chamber temperature at -15 ℃.Subsequently, NH 3 gas was introduced into the chamber and the lm bleached promptly, with the formation of a colorless intermediate liquid phase FACsPbI 3 •xNH 3 .Once removed from the NH 3 gas atmosphere, the lm was quickly transferred on a hot plate and heated at 140 ℃ for 20 min.Instead, the dull lm changed to a shiny lm.All the dendrite-like crystals and voids totally disappeared, and a dense, smooth FA-based perovskite lm emerged as displayed in Figure 3B, which is responsible for the visual evolution of lm from dull to shiny.It is worth noting that the NH 3 gas absorption and degassing rate of perovskite lms take place much faster than powder samples because of their thin thickness.The corresponding atomic force microscope (AFM) image indicated that NH 3 healed perovskite lm has a root mean square (RMS) roughness of only 9 nm over a 20 × 20 µm 2 area (Figure 3C), which is lower than the lm prepared by the traditional antisolvent method with an RMS of 24 nm (Figure S13).
Figure 3D showed X-ray diffraction (XRD) patterns of spin-coated FACsPbI 3 and NH 3 healed FACsPbI 3 perovskite lms on FTO substrates.After NH 3 gas healing, the diffraction peak intensity of the pure phase (110) preferred orientation perovskite lm has increased by more than 100 times.The enhancement of crystallization was also evidenced by a decreased full width at half-maximum (FWHM) of the (110) peaks from 0.203 to 0.114 as compared in Figure 3E. Figure 3F demonstrated ultraviolet-visible (UV-vis) optical absorption and normalized photoluminescence (PL) spectra.Both the raw and healed FACsPbI 3 perovskite lms presented the same absorption edge at around 815 nm, but NH 3 healed FACsPbI 3 perovskite lm exhibited signi cantly increased absorbance in short wavelength region because of the improved lm quality and uniformity.The slight blue shift of the PL peak of healed FACsPbI 3 perovskite lm is attributed to the reduction of shallow level defects as have been previously reported by performing surface passivation 34,35 .
Based on NH 3 gas healing method, FACsPbI 3 perovskite lms were prepared and studied as the light absorber layers in normal structured perovskite solar cells with SnO 2 as electron transport layer and Spiro-OMeTAD as hole transport layer, respectively.The champion solar cell (Figure 4A) based on NH 3 gas healing method displayed an e ciency of 23.31%, with open-circuit voltage (V OC ) of 1.14 V, shortcircuit current density (J SC ) of 25.36 mA/cm 2 , ll factor (FF) of 80.61% and with negligible hysteresis for the device (Figure S14 and Table S1).The steady output for the best device presented a quasi-steady output of 22.32% (Figure S15).The J SC of NH 3 healed FACsPbI 3 solar cell is comparable with the integrated J SC from EQE results (24.41 mA/cm 2 ) in Figure 4B.The distribution histogram of 50 devices PCE at reverse scan direction is presented in Figure 4C, showing excellent reproducibility.Figure S16 is a typical cross-sectional SEM of solar cell device with 500 nm thick high dense and uniform perovskite layer.Bene ting from the high uniformity, there is no big different of the PCEs of perovskite solar cells along with increasing active areas from 0.16 cm 2 to 1.00 cm 2 (Figure 4D and Table S2).The slight decrease of FF is attributed to the increase of series resistance (Rs) from FTO substrate.A prototype PSC module (PSM) with an area of 25 cm 2 consisting 5 cells in series connection was also fabricated reaching an e ciency over 15% (Figure S17).
In Figure 4E, we also evaluated the storage stability of NH 3 -FACsPbI 3 , anti-MAPbI 3 , anti-FACsPbI 3 , anti-FA 0.9 Cs 0.05 MA 0.05 PbI 3 (FMCsPbI 3 ) devices in the air with humidity around 30%.Obviously, compared to MAPbI 3 system, FACsPbI 3 system devices exhibited excellent stability in general.More speci c, the device based on NH 3 gas healing method was almost no e ciency decrease under 300 days storage in the air, but the MA-rich PSCs degraded rapidly.Their phase composition, optical properties and morphologies of the 300-day-aged PSCs with different kinds of perovskite materials were studied by XRD, UV-vis and cross-section SEM (Figure S18-21).From these results, we can see that FACsPbI 3 devices still maintained decent cross-sectional morphology with clear grain boundaries, strong optical properties, and almost few PbI 2 .However, there is a large amount of PbI 2 formed in the 300-day-aged MA-rich PSCs, resulting in the lm color signi cantly lighter, which is because of the much lower sublimation temperature of MA than FA unit [36][37] .
In closing, we have developed a low temperature non-aliphatic protic NH 3 gas post healing of FAcontaining perovskite lms, which provides an unprecedented capability for processing of high-quality, uniform perovskite lms for high-performance photovoltaic devices and beyond.It is demonstrated that the chemical reaction between FAI and NH 3 cannot change the lm composition, and in particular, the low

a
Scheme 1. (see Supplementary Files) The addition-elimination reaction and the ion exchange reaction between FAI and R-NH 2 molecules (R is referred to H, Me, Et, n-Pr or n-Bu etc.), and the hydrolysis reaction of FA, RFA and DRFA.

Scheme 2 .
Scheme 2. (see Supplementary Files) The reaction between FAI and NH 3 molecules without change in the composition.
3 •xNH 3 complex was measured in detail by monitoring its weight change when it was kept in an open condition.The degassed NH 3 number per FAPbI 3 as the function of time is illustrated in Figure2Bwith the temperature increased step-by-step from 20 ℃ to 120 ℃.When we placed the glass vial containing the FAPbI 3 •xNH 3 slurry on a balance and opened its lid, its weight dropped rapidly at initial stage and then gradually became slower.After 30 min and 60 min, the sample was heated to 40 ℃ and 60 ℃, and maintained for 30 min, respectively.The sample was weighed at regular intervals, showing continuous slow weight loss.When the sample was heated to 80 ℃, the sample quickly reached a relatively stable state then almost no weight change in the maintaining 30 min, which was calculated about 1 NH 3 molecule per FAPbI 3 .Finally, the temperature was increased to 120 ℃, the x value is closed to 0, which means that all absorbed NH 3 molecules have been removed without any residue. Figures

Figure 2 The
Figure 2

Figure 3 Properties
Figure 3

Figure 4 Solar
Figure 4 . In this case, the formed intermediate is a novel metastable (PbI 2 -MAI)•xMA complex, instead of simple coordinating bound dominated OL-PbI 2 -MAI adduct.The perfect conversion from intermediate (PbI 2 -MAI)•xMA to perovskite phase is guaranteed by the disordered nature of hydrogen bonds in (PbI 2 -MAI)•xMA intermediate phase and the ultrafast evaporation of MA g .This healing behavior of MA g on MAPbI 3 perovskite lm is rstly reported by Zhou et al. in 2015

Table 1 .
The synthesis of FA g and chemical reactions between amines/formamidine gases and FAI salts