Single-step post-production treatment of lead acetate precursor-based perovskite using alkylamine salts for reduced grain-boundary related film defects

Powered by the worldwide efforts of research groups experienced in dye-sensitized, and thin-film solar cells, perovskite solar cells (PSCs) reached a power conversion efficiency of 25.7% within 10 years. However, the presence of defects and trap density within the active layer’s grain boundaries commonly operates as non-radiative recombination centers. Hence, intensive efforts have been reported to passivate the inevitable bulk and interface defects of the active layer using additives or post-treatment processing to enhance the efficiency and stability of PSCs. Herein, a facile post-treatment strategy based on wet processing methylammonium lead triiodide, MAPbI 3 (prepared from lead acetate and methylammonium iodide precursors) films with organic amine salts (FABr and FAI) is demonstrated. As a result, high-quality films of mixed per-ovskites (FA x MA 1-x PbI 3-x Br x and FA x MA 1-x PbI 3 ) were obtained. The surface treatment has efficiently passivate the defects in the host film, suppressing the non-radiative carrier recombination. Compared to the control device, the increased open-circuit voltage (from 0.5 V to 1 V) and fill factor (FF) values of the optimized device based on FA x MA 1-x PbI 3 showed a PCE of 16.13%. And our findings revealed that post-treatment is possible on wet perovskite film aged for a few minutes prior to its post-treatment, which saved the energy used for pre-annealing.


INTRODUCTION
As the result of intensive research progress made by researchers worldwide since 2009, metal halide perovskite solar cells (PSCs) have reached a high potential for becoming the next alternative in the photovoltaics industry. [1]ith the advancement of PSCs, the typical device structure (n-i-p) has experienced a remarkable boost in power conversion efficiency (PCE) from 3.8% [2] to a recently validated figure of 25.7%, [3] a value close to that of crystalline silicon solar cells.The inverted structure (p-i-n) of PSCs, on the other hand, demonstrated a recent report of 25.0% PCE. [4]Due to its simple and low-energy construction technique, material cost-effectiveness, and low hysteresis properties, the inverted structure of PSC has become increasingly appealing. [5]The improvement in performance for both PSC structures is due to many studies focusing on compositional and stoichiometric, [6] crystallization process optimizations, [7] selective charge transport layer development, [8] and mainly due to bulk and interface passivation engineering of the perovskite films. [9]6b,10] A homogeneous perovskite film with complete coverage, even surface, and high-quality crystallization is crucial to guaranteeing sufficient light harvesting with decreasing non-radiative recombination, resulting in better efficiency and stability. [10]9b] As a result, present efforts are focused on overcoming the difficulty of coating and growing uniform, large-grain, high-quality perovskite thin films.Hence, several thin-film crystallization procedures, such as anti-solvent, hot-casting, vacuum quenching, gas blowing, additives, and various post-treatments were used. [11]he addition of organic halide salts (as additives or passivators) altered the A-site cations and/or X-site anions of ABX 3 (where A is an organic cation MA + , FA + , and B is a metal cation, Pb 2+ or Sn 2+ , and X is a halide anion, I, Cl, and Br), resulting in mixed types of perovskites.6b] Furthermore, the conditions during solution preparation and perovskite film fabrication have been reported to influence the final perovskite film quality. [12]In this regard, it is reported that, the solution fabrication procedure at low temperatures and quick precursor crystallization causes most defects in perovskite films, leading to mainly lower open-circuit voltage (V OC ) and fill factor (FF) values for inverted PSCs. [13]hang et al. [14] for example, investigated the crystallization nature of perovskite films prepared from three lead precursor sources, including lead acetate (PbAc 2 ), lead iodide (PbI 2 ), and lead chloride (PbCl 2 ).The authors found that the crystallization process of the perovskite film prepared from PbAc 2 was the most rapid in nature.
The development of additive and post-growth treatments of deficient perovskite films is a more attractive method to boost the PCE of PSCs, and there have been some efforts in this area. [15]Cheng et al. [16] reported that dropping chlorobenzene on a perovskite precursor film during the spinning process generates a homogeneous perovskite film with good reproducibility.Excess organic amine salts, such as methylammonium iodide (MAI), methylammonium chloride (MACl), ammonium chloride (NH 4 Cl), and methylammonium bromide (MABr), were used as additives in the perovskite precursor solution to improve crystalline properties and surface morphology. [1,17]This kind of additive improves the morphology while also causing the creation of mixed cation or mixed anion perovskite films and tuning of the band gap of the perovskite active layer, resulting in an increase in V OC . [18]18b,19] Apart from the additive role, it has been reported that organic amine salts can also be used to passivate and remove such entanglements and flaws, and transform a low-quality perovskite layer into a smooth and compact film. [20]For instance, Zhang et al. [21] utilized an antisolvent followed by post-treatment with a formamidinium iodide (FAI) precursor solution on a pre-annealed methylammonium triiodide (MAPbI 3 ) film (made from PbI 2 and MAI) and obtained a defect-free mixed FA x MA 1-x PbI 3 film. And, so far, to our best information, there is no report about post-treatment performed on wet perovskite host film with a suitable time gap before the post-treatment is carried out.
Herein, we introduced an effective post-treatment on a wet MAPbI 3 host film and tried to decipher how posttreatment affected the crystallization and morphology of the final perovskite film as a function of the wet MAPbI 3 film's idle time.Idle time is the optimal waiting time used before the host film was post-treated.This present study also combines the compositional engineering and defect passivation approaches in a single-step post-treatment procedure, aided by the anti-solvent role of isopropanol (IPA) solvent.The idea of a solution-induced Ostwald ripening approach is used in the post-treatment phase [23] utilizing formamidinium bromide (FABr) and formamidinium iodide (FAI) dissolved in IPA.This was accomplished by spin-coating FABr and FAI solutions (concentrations of 5 mg mL -1 each) onto an aged wet MAPbI 3 thin film (made from PbAc 2 and MAI).As a result, the pristine MAPbI 3 thin-film was changed into high-quality large grain size crystals of mixed perovskite thin-films of FA x MA 1-x PbI 3-x Br x and FA x MA 1-x PbI 3 respectively.Film quality, V OC , FF, and PCE of produced inverted PSCs have improved as a result of the post-treatment production.The champion device based on FA x MA 1-x PbI 3 has an efficiency of around 16 %, which is much higher than the pristine MAPbI 3 -based PSCs' efficiency of 5%.
To prepare the organic precursor used for post-treatment, FABr, and FAI (5 mg mL -1 each) were dissolved in IPA.PC 60 BM was dissolved in chlorobenzene with a concentration of 40 mg mL -1 .All Solutions were stirred for at least 24 hours at 70 • C. As cathode contacts, pre-patterned indium doped tin oxide (ITO) substrates (Xinyan Technology Limited, 10/cm 2 ) were used.We have fabricated an inverted structure of PSCs with a device configuration of ITO/PEDOT:PSS/MAPI/PC 60 BM/TiOx/Al.

Device fabrication
First, the patterned ITO substrates were coded and cleaned in a series of steps using detergent, deionized water, toluene, and IPA, each for 15 minutes in an ultrasonic bath at 40 • C followed by drying with a nitrogen stream.PEDOT:PSS Clevios PH solution was spin-coated on top of the dry and cleaned ITO substrate as a hole transport layer (HTL) at 3000 rpm for 60 seconds.While the PEDOT:PSS thin films were still hot from being patterned and annealed at 160 • C in the air for 20 minutes, they were immediately transferred to a nitrogen-filled glovebox and allowed to cool to room temperature.The host perovskite precursor solution (110 µL) was spin-coated on top of the PEDOT:PSS thin film at 3000 rpm for 120 seconds and kept at room temperature.After 3 minutes waiting time, the wet MAPbI 3 thin film was then evenly drip-coated separately with 100 µL of either FABr or FAI and spin-coated after 10 seconds of loading time at 3000 rpm for 45 seconds.Both the control film, and post-treated films were annealed on a hot plate at 130 • C for 5 minutes resulting in a crystalline perovskite absorber of MAPbI 3 (control), FA x MA 1-x PbI 3-x Br x (FABr-treated) , and FA x MA 1-x PbI 3 (FAI-treated).The electron transport layer was deposited by spin-coating PC 60 BM solution in a two-step program at 1000 rpm for 85 seconds and at 6000 rpm for 5 seconds.To prevent Al electrode reactivity with the perovskite layer due to potential PC 60 BM flaws, a titanium oxide (TiOx) solution (1.5% in IPA) was spin-coated on top of the non-annealed PC 60 BM layer at 3000 rpm for 35 seconds.Finally, devices were completed with the evaporation of ∼200 nm Al as cathode contact electrodes through a shadow mask in a high vacuum (∼3 × 10 -6 mbar pressure).In the same method as the films used to build inverted PSCs, pristine and mixed cation/anion perovskite films were generated for optical and morphological characterization measurements.

CHARACTERIZATIONS
To analyze the morphology and crystal development information of our perovskite thin films, scanning electron microscopy (SEM) imaging was performed with a Sigma VP Field Emission SEM (Carl-Zeiss AG, Germany) utilizing the InLens detector with an accelerating voltage of 6 kV.X-ray diffraction (XRD) was performed with the diffractometer Panalytical X'Pert Pro MPD.The X-ray source is a copper line fine focus tube (k a1 + k a2 radiation), incident beam optics parallel mirror, 1/32

Post-treatment and morphology of perovskite films
To passivate the flaws (defects) on the control perovskite thin film (MAPbI 3 ), a post-treatment approach employing the Ostwald ripening effect [23] was used on the wet MAPbI 3 film.Figure 1A shows the step-by-step sequential procedure followed for the post-treatment process, along with intermediate outcomes, and the inverted perovskite solar cell (PSC) device layer stack is depicted in Figure 1B.The host perovskite solution (PbAc 2 + MAI) was first dropcast and spin-coated on a PEDOT:PSS coated ITO (for the device) or on a bare glass substrate (for the film).After the idle time (3 minutes), the organic salt solutions were cast onto the wet MAPbI 3 film, followed by spin coating after a loading time of 10 seconds.During the loading time, the wet and pretty yellow MAPbI 3 film starts to change its color into gray and black due to the intermixing of precursors (mixed perovskite intermediate products).The effects of the FABr and FAI precursor solutions on the crystal growth kinetics, film morphology, defect passivation, optical, and photovoltaic performance of the final mixed perovskite thin films have been thoroughly investigated.The wet FA x MA 1-x PbI 3-x Br x and FA x MA 1-x PbI 3 films were turned into darker gray and light black during the spin coating time.They all become compact black mixed perovskite thin films after annealing (Figure 1A).
The main focus of this study is on the combined influence of compositional engineering and passivating defects associated with the grain boundaries (GBs) and the surface of the host perovskite film using the post-treatment procedure.It is reported that the majority of flaws in polycrystalline perovskite films are found at GBs. [23] And these GBs are the primary cause of non-radiative recombination and create channels for oxygen and moisture infiltration, resulting in the film's degradation. [25]Hence, in order to investigate the surface morphology, particularly microstructural changes, of the perovskite films before and after post-treatment, a scanning electron microscope (SEM) measurement was used.
First, in order to find the optimal idle (or aging) time before the wet host film was post-treated and for further insights into the corresponding perovskite film crystallization, we have systematically varied the idle time between 0, 3, 5, 10, and 20 minutes.FAI solution was selected for the post-treatment and assigned as 0 min-FAI, 3 min-FAI, 5 min-FAI, 10 min-FAI, and 20 min-FAI with respect to the idle times producing mixed cation FA 1-x MA x PbI 3 film.Directly after the spin-coating process of the host film, there is obviously some remaining solvent inside the wet film.The rate of solvent evaporation ultimately determines how long this wet film stage exists. [26]However, there is no report available regarding the aging time required for wet film (based on the lead acetate precursor perovskite) prior to its post-treatment.Nevertheless, there are reports on precursor solution aging, [27] pre-annealing film aging, [28] and perovskite solar cell device aging. [29]The top views of SEM images, showing the morphological and  2A-E.With increasing idle time, the following effects became apparent: (i) the size distribution of the perovskite crystallites become larger (Figure 2C-E), and ii) the number of grain boundary defects also increased (Figure 2D,E).
While increasing crystal size is beneficial for device performance, a higher number of grain boundary defects may increase non-radiative recombination and shunting.The optical data shows basically no variation in the absorptance, except for the 0 min-FAI, which shows a somewhat lower absorption (Figure 2F).Interestingly, the PL data shows an initially very steep rise in signal up to the 5 minutes idle time, followed by a decay of the same (Figure 2G).This indicates that opposing processes do occur over idle time.The slight blue shift in PL for the 3 and 5 minutes idle times originates from the reduction of spontaneous nonradiative recombination due to the trap states, which provides an explanation for the improvement in V OC and FF in PSC. [23]Both the SEM and optical results are consistent with each other.Our findings show that post-treatment is possible on wet perovskite films that have been aged for an optimal idle time of 3 minutes, saving energy from the pre-annealing procedure.6a] Based on the optimized idle time, the effect of posttreatment on the wet MAPbI 3 host film using the organic precursor solutions (FABr, and FAI in IPA) has been investigated.The film morphology and grain size distribution of the control film and the mixed perovskite films generated by post-treatments are compared in Figure 3.In comparison to the post-treated films, the control (MAPbI 3 ) film shows rough and low-quality grains with a noticeable number of pinholes and grain boundaries, as shown in the top view of the SEM image in Figure 3A.This is because, during the annealing step, the by-product methylammonium acetate (MAAc) has been reported to be unstable as it is easily evaporated from the film, leaving a void area behind.The counter by-products MACl and MAI from PbCl 2 and PbI 2 precursors took a little longer to be removed. [14]On the other hand, the FAI-treated (FA x MA 1-x PbI 3 ), and FABr-treated (FA 1-x MA x PbI 3-x Br x ) films have demonstrated better crystallization and morphology (Figure 3B,C).Additionally, as compared to the control, both FABr and FAI-treated films have a larger mean grain size (Figure 3D), with the order of MAPbI 3 < FABr-treated < FAI-treated.Low-quality host MAPbI 3 films are transformed into compact perovskite films with reduced surface flaws due to the combined effects of FABr And FAI solution post-treatment.A former comparison of one-step and two-step perovskite film deposition revealed that the deposition in two-steps produces significantly better film shapes and crystal domain sizes than the one in one-step deposition. [30]Hence, our post-treatment also includes a two-step deposition effect, resulting in improved crystal growth.
Gwyddion software was also used to investigate and compare the surface roughness of the control and posttreated perovskite films (Figure 4A).The root mean square (RMS) surface roughness of the FABr and FAI modified J SC [mA cm -2  perovskite films is 68.5 nm and 67.7 nm, respectively, which is significantly less than the control film's 78.1 nm.This shows that the post-treatment effectively modified the control film and resulted in smoother perovskite films.The lower RMS suggests that the GBs and voids have been decreased, resulting in improved carrier extraction, which is consistent with the SEM images (Figure 3) and J-V results (see section 4.3, Table 1).

Optical characterization of perovskite films
To further understand the crystal phase properties and composition of the perovskite films before and after posttreatments with FABr and FAI solutions, X-ray diffraction (XRD) measurements (2theta-omega-scan) were carried out on the prepared perovskite films; MAPbI 3 , FA x MA 1-x PbI 3-x Br x and FA x MA 1-x PbI 3 .Figure 4B compares the main characteristic XRD peak of the three perovskite films on the glass substrate.There is a small peak around 12.58 • , which is the (003) peak of hexagonal lead iodide (PbI 2 ) residue.This is due to the loss of MA + as MAAc resulted in a less stoichiometric equilibrium between MAI and PbAc 2 . [14]And this residue (PbI 2 ) was consumed during the post-treatment.The main perovskite (110) diffraction peak for MAPbI 3 appears at 14.08 • .And this diffraction peak shifts to 14.24 • for FA x MA 1-x PbI 3-x Br x , indicating the presence of bromine (Br), in agreement with the previous report. [31]This shift towards a higher degree angle is a result of the decreased lattice spacing induced by the substitution of larger iodine atoms with that smaller bromine atoms.The peak shift from 14.08 • for MAPbI 3 to a smaller angle of 13.97 • for the FAI-treated film indicates the formation of the mixed-cation of FA x MA 1-x PbI 3.
[6a] The shift is due to the partial replacement of smaller size MA with larger size FA which increases the lattice spacing in the octahedral cage.While the perovskite thin films are highly textured with (110) orientation perpendicular to the surface, rocking-curve measurements revealed a random distribution of the crystallites parallel to the surface, as expected for amorphous glass substrates.
The optical properties (absorptance, and photoluminescence) of the post-treated perovskite films in comparison to the control counterpart, were also evaluated, as shown in Figure 4C,D.When comparing the FABr-treated film (FA x MA 1-x PbI 3-x Br x ) to the control film, the calculated absorptance spectra reveal a blue shift of about 35 nm at the onset of absorptance, indicating a larger bandgap due to Br incorporation.The same blue shift trend was seen in the PL spectra (Figure 4D) as well.It was shown before that adding the correct amount of Br to the MAPbI 3 system causes a blue shift, and a substantially widened bandgap, which play a role in increasing the open-circuit voltage [17] but sacrificing the absorption results in reduced current density. [32]n contrast, the absorptance edge and PL spectrum of the FAI-treated film (FA x MA 1-x PbI 3 ), reveal a red shift (Figure 4C,D).6a] In summary, the post-treatment procedure allows tuning the bandgap of the host MAPbI 3 perovskite system by producing mixed cation/anion perovskite films, of which the FAItreated film shows higher absorptance (Figure 4C).The calculated optical band gaps (1.58 eV for MAPbI 3 , 1.56 eV for FA x MA 1-x PbI 3 , and 1.68 eV for FA x MA 1-x PbI 3-x Br x ) and the peak positions of both the absorptance and PL results revealed the formation of mixed perovskites after posttreatment, which is in agreement with the XRD results (Figure 4B) and the literature report. [33]

Photovoltaic properties
Current density-voltage (J-V) curves were measured inside the glovebox using a Keithley 2400 source-measure unit to evaluate the photovoltaic performance of our devices.Figure 5A,B display the dark and light current densityvoltage curves respectively.And the corresponding photovoltaic parameters are summarized in Table 1.The device produced from the control MAPbI 3 films have a deficient photovoltaic performance.However, the devices resulting from post-treatment by FABr and FAI showed a significant improvement in performance.The V OC improves from 0.5 V of the control device to 0.989 V and 0.992 V for FABr and FAI treated films, respectively.Nevertheless, due to some lingering defects and the energy bandgap mismatch with PEODT:PSS (used as HTL), the V OC values for all devices are still less than 1.0 V. [34] The FF values for the post-treated devices are way higher than the control devices (Table 1).
In the dark, a conventional solar cell operates like a diode, with a different amount of current flowing under forward bias (V > 0) versus reverse bias (V < 0).The diode's J-V characteristic shifts by the short-circuit current when exposed to light.The reverse current of the device with the post-treated films is smaller than that of the device with the control MAPbI 3 film, showing that the post-treatment reduces the leakage current running over the defect states. [35]In addition, the devices constructed from post-treated films have better diode characteristics at a forward bias compared to the control (Figure 5A).The overall increased device performance of PSCs made from post-treated films is commensurate with a reduction in defect states. [17]o extract information regarding the charge carrier collection properties of the solar cells, the EQE was measured outside the glovebox.It can be seen that FAI-treated devices exhibit a larger EQE across a wide spectral range (Figure 6A), indicating more charge collection, which is in agreement with its J-V results.Moreover, in agree-ment with the optical properties reported before, there is a modest spectral blue shift near the EQE onset around 800 nm for the FABr-treated device and a slight spectral redshift away from the EQE onset around 800 nm for the FAI-treated device (Figure 6B).
For statistical comparison, we include the multiple battery photovoltaic parameters distribution (V OC , J SC , FF, and PCE) obtained under one sun simulated illumination, computed and plotted for 20 cells each (Figure 7).The trend observed for the best devices is basically confirmed for a more considerable number of them.And except for J SC , the photovoltaic parameters for the solar cells made from the post-treatment are much higher than their control counterpart.
It should be highlighted that the high V OC and FF values obtained for the FABr and FAI-treated devices are mostly responsible for the final rise in PCE.Because the carrier diffusion length is frequently greater than the absorber thickness (generally around 500 nm), parasitic nonradiative recombination occurs at the absorber/charge transport layers interface (major recombination) and within the charge transport layers that limits the V OC value in PSC. [34]The increase in V OC is owed not only F I G U R E 7 Statistical comparison of device performance between devices made from MAPI 3 , FABr, and FAI-treated perovskite films and their control counterparts measured under simulated AM 1.5 sunlight of 100 mW cm -2 irradiance to the post-treatment passivation impact but also to the active layer's band gap tuning.For each perovskite film we built, we calculated the shift in optical band gap energy (E G, opt ).The E G, opt value was increased from 1.58 eV for the control film (MAPbI 3 ) to 1.68 eV for the FABr-treated FA x M 1-x PbI 3-x Br x film, indicating the presence of Br, which is in agreement with previous reports. [31]However, due to the higher ionic radius size of the FA + ion as compared to the MA + ion, a minor decrease in the band gap was seen for FA x MA 1-x PbI 3 .This leads to a reduction in E G, opt of 1.56 eV, while the E G, opt of pure FAPbI 3 is reported to be 1.53 eV. [33]Despite the fact that the Br -content of FABrtreated PSCs helps to widen the band gap, the devices did not yield higher V OC than FAI-treated devices.The photoinduced halide segregation in mixed-halide PSCs could be a potential cause for the relative loss in V OC . [36]The formation of FA x MA 1-x PbI 3 , and FA x M 1-x PbI 3-x Br x films as a result of the post-treatment is also confirmed by the XRD analysis.

Summary and discussion
We investigated the effect of post-treatment with organic salts without pre-annealing of the perovskite host film.The resultant perovskite film's morphology improved over the control perovskite film.This is because there is inter-calation between the organic precursors, MAI/FABr and MAI/FAI that affects the formation of perovskite crystals and corresponding morphology. [37]Small MAPbI 3 crystals can be swiftly re-dissolved and regrown into more giant crystals by the post-treatment-induced Ostwaldtype ripening process at the appropriate concentration of the FABr or FAI solution (for example, 5 mg mL -1 ).And this leads to the simultaneous process of compositional engineering and defect passivation approaches in a single-step post-treatment procedure, aided by the anti-solvent role of the isopropanol solvent.This in turn allows us to fine-tune the band gap of the perovskite films between 1.56 and 1.68 eV, with the control film (MAPbI 3 ) at 1.58 eV and FA x M 1-x PbI 3-x Br x at 1.68 eV.However, due to the larger ionic radius size of the FA + ion as compared to the MA + ion, a minor decrease in the bandgap was seen for FA x MA 1-x PbI 3 at 1.56 eV.It can be clearly seen form the J-V results, the band gap values did not have significant contribution to the device performance.For example, MAPbI 3 has higher band gap (1.58 eV) than FA x MA 1-x PbI 3 (1.56 eV), but did not show better efficiency.As a result, the mixed perovskite films (FA x M 1-x PbI 3-x Br x and FA x M 1-x PbI 3 ) with better crystallization and morphology obtained from the post-treatment shows improved charge collection with higher values of V OC , FF, and PCE compared to the control devices (Figure 7).
Aside from the benefits of defect passivation and band gap tuning by post-treatment, the selection of a proper electron transport layer (ETL) and hole transport layer (HTL) is also important.For example, PC 60 BM as HTL has the role of passivating the grain boundaries of perovskite films in combination with the anti-solvent role of its solvent chlorobenzene, reducing shunting routes and improving device performance. [38]Shunting channels in the inverted structure of PSC have previously been reported as a result of the interaction between PEDOT:PSS (used as HTL) and PC 60 BM via the void area of the perovskite layer, which affects not only the diode characteristics but also the V OC loss of PSCs. [39]This is why our control device, with noticeable pinholes in the perovskite film, has an extremely low V OC .Thus, the fabrication of defect-reduced perovskite layers with a smooth morphology, as performed in this study, inhibits unwanted contact between PEDOT:PSS and PC 60 BM and reduces shunting.

CONCLUSION
Our simple, energy saver, and manageable post-treatment method of defect passivation on wet MAPbI 3 host film (aged for 3 minutes) enormously lowers grain-boundary related film defects and associated non-radiative recombination losses.And this causes a remarkable rise in V OC , and FF, leading to a rise in PCE as compared to the control devices.The XRD, optical (absorptance and PL), and morphological analysis revealed that the use of FABr and FAI solutions for post-treatment were able to normalize the surface chemical environment of the wet host MAPbI 3 film.Hence, as compared to the control film, a very smooth morphology, and larger grain-sized mixed FA x MA 1-x PbI 3-x Br x and FA x MA 1-x PbI 3 films were obtained.We believe that in a single post-production step, this strategy has played three roles; precursor compensation, compositional engineering, and defect passivation, as confirmed by the results obtained from optical, XRD, and SEM measurements.On top of this, our findings have confirmed that post-treatment is possible on wet perovskite film aged for optimal idle time of 3 minutes.That saves the energy of the pre-annealing step.And the films produced had the same crystal quality as those made from pre-annealed MAPbI 3 and treated with FAI in the same way.

C O N F L I C T O F I N T E R E S T
We have no conflicts of interest to disclose.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.

F I G U R E 1 A
, Schematic flow chart depicting (A) the post-treatment procedure using organic precursor salts that result in mixed perovskite films and (B) the inverted architecture layer stack of a perovskite solar cell with a laser diode emitting at 405 nm.For calculated absorption, thin-film transmission and reflection spectra were recorded with two Avantes AvaSpec-ULS3648-USB2-UA-25 fiber spectrometers sequentially and reassembled to the thin film absorptance via the equation: A = 1 -T -R (where; T-transmittance and R-reflectance).The current density-voltage (J-V) characteristics of the solar cell devices were recorded with a computer-controlled source-measure unit, Keithley 2400, under one sun AM1.5 illumination with a class A solar simulator (LED-based) and in the dark, respectively.In order to see the spectral response of the solar cells, external quantum efficiency (EQE) was measured with a Bentham PVE 300 using a lock-in amplifier under short-circuit conditions after illuminating the cells with a monochromator laser.The light intensity was calibrated with a standard silicon solar cell.

F I G U R E 2
Top view SEM images of FA x MA 1-x PbI 3 films fabricated by FAI post-treatment on (A) 0 minutes, (B) 3 minutes, (C) 5 minutes, (D) 10 minutes, and (E) 20 minutes aged MAPbI 3 films, and the (F) absorptance, and (G) photoluminescence spectra of all the corresponding films microstructural changes of FAI-treated wet MAPbI 3 host film as a function of the idle times, are presented in Figures

F I G U R E 3
Top view of the SEM images of (A) control MAPbI 3 film, (B) FABr-treated, (C) FAI-treated MAPbI 3 films, and (D) histogram showing the grain size distribution of all films F I G U R E 4 A, Root mean square roughness, (B), XRD patterns, (C), Absorptance, and (D), photoluminescence spectra of control MAPbI 3 , FABr-treated, and FAI-treated MAPbI 3 films TA B L E 1 Photovoltaic performance parameters extracted from the J-V characteristics measured at 100 mW cm -2 (AM 1.

F I G U R E 5
(A) dark and (B) illuminated current-voltage characteristics for planar perovskite solar cells (PSCs) based on control MAPbI 3 thin film and with FABr and FAI treatment F I G U R E 6 (A) external quantum efficiency, EQE (B) magnified EQE spectra for planar perovskite solar cells (PSCs) based on control (MAPbI 3 ), FABr, and FAI treatment Z. T. G. gratefully acknowledges financial support from the DAAD (Deutscher Akademischer Austauschdienst), Funding programme/-ID: 57299294.H. H. and U. S. S. are grateful to the Thüringer Ministerium für Wirtschaft, Wissenschaft und Digitale Geselschaft (TMWWDG) for funding the CEEC Jena (RIS3 Innovation Center).The SEM facilities of the Jena Center for Soft Matter (JCSM) were established with a grant from the Deutsche Forschungsgemeinschaft (DFG).Open Access funding enabled and organized by Projekt DEAL.