Thioacetamide‐Assisted Crystallization of Lead‐Free Perovskite Solar Cells for Improved Efficiency and Stability

Perovskite solar cells (PSCs) have made great strides in recent years, operating inside the theoretical Scholkely–Quisser efficiency limit with a certified power conversion efficiency (PCE) of 26.1%. However, lead toxicity from Pb‐based PSCs can harm the environment. As a result, the search for nontoxic and environmentally friendly substances to replace Pb in perovskites is the need of the hour. Tin has emerged as the most viable choice to replace Pb, due to its favorable electronic properties and smaller bandgaps of Sn‐based perovskites between 1.1 and 1.4 eV, strong charge carrier mobility, and high theoretical efficiency of 32%. Sn vacancies and point defects, on the other hand, are easily produced in Sn perovskites, leading to nonradiative recombination. Furthermore, interfacial flaws and traps impede further performance improvement. In this research, to produce high‐quality Pb‐free perovskites for high‐performance PSCs, a Lewis‐base thioacetamide (TAA) is added to the simple FASnI3 perovskite solution. FASnI3 and TAA additive‐based films effectively control perovskite film crystallinity and grain size via Lewis acid–base reaction. The champion FASnI3 + TAA‐based PSC achieves a maximum PCE of 10.67% while paving a facile way for other compositional perovskite analogues to be integrated into highly efficient and operationally stable PSCs.


Introduction
Perovskite solar cells (PSCs) have been a revolutionary technology in the photovoltaics arena.Particularly in the last decade, [1,2] PSCs exhibit several desirable properties such as long charge-carrier diffusion lengths, [3] low exciton binding energies, [4] high absorption coefficients, [5] and controllable bandgap.Due to these factors, lead halide PSCs can now reach up to 26.1% power conversion efficiency (PCE) [best research cell efficiencies chart by NREL], [6][7][8][9][10] but those Pb-based PSCs can have adverse effects on the environment, including toxicity of Pb residues from perovskites. [11]As a result, the search for nontoxic and ecologically sound elements to substitute Pb in perovskites is very exciting.Other Pb-free PSCs have gained traction, [12,13] with Sn being among the most promising due to its high charge carrier mobility, [14] and the theoretical efficiency of Sn-based perovskites is predicted around 32% due to their narrow bandgap between 1.1 and 1.4 eV. [15]Sn-based PSCs have drawn much reorganization as a lightning rod of Pb-free PSCs, due to Sn 2þ 's comparable ionic radii and outermost electronic configuration with Pb 2þ .Because of the low standard redox potential of Sn 2þ /Sn 4þ which results in swift oxidation of Sn 2þ to Sn 4þ at room temperature ambience and formation of vacancies, Sn-based PSCs have shown poor PCE and operational stability.
Producing high-quality Sn-based perovskite films is still challenging owing to a lack of basic knowledge of the Sn 2þ compounds.Such problems obstruct the possible realization of matching film features to Pb-based PSCs such as surface characteristics, pinhole-free active layer morphology, and low crystallinity.To date, the most effective Sn-based PSCs used FASnI 3 as the photoactive layer.As noted, the reaction between A and B site cations within the FASnI 3 perovskite precursor was rapid.This quick interaction is induced by SnI 2 's increased reactivity as a Lewis acid, which leads in fast crystal growth with obvious pinholes derived by the uncontrolled nucleation and in turn the nonuniformity of thin films, poor crystallinity, and high defect density. [16]Incorporating Lewis bases as additions in the Sn-perovskite precursor solution, which might impair the interaction between precursor components, was one suggested method for preventing fast crystallization.[19] Sn-based PSCs (Sn-PSCs) have been claimed to have a PCE of up to 14%, [20,21] Highly efficient and stable PSCs in ambient air were obtained by including a Lewis base 2-pyridylthiourea addition into the perovskite precursor solution. [22]When added to the precursor solution, thioacetamide (TAA) (CH 3 CSNH 2 ) proved to be the most efficient regulator of methylammonium lead iodide (MAPbI 3 ) perovskite film formation. [23]Patil et al. discovered the employment of TAA in (MA x FA 1-x )-PbI 3 perovskite films led to the development of PSCs that were both highly efficient and operationally stable. [24]FASnI 3 and TAA additive-based films effectively controlled the crystallinity and grain size of the perovskite films.The champion FASnI 3 þ TAA-based PSC device demonstrated the highest PCE of10.67% and stability (up to what?).Furthermore, the TAA additive produced a larger grain with low-surface roughness, and high-quality perovskite film with enhanced short-circuit current density ( J SC ), open-circuit voltage (V OC ), fill factor (FF), and PCE.

Results and Discussion
We investigated various properties of TAA additive-assisted perovskite precursor to investigate its role on FASnI 3 perovskite films.Figure 1 depicts the growth mechanism of FASnI 3 perovskite film formation by TAA additive as well as the crystal structure of perovskite (experimental details are explained in supplementary information).Initially, a FASnI 3 -based perovskite precursor solution was produced by dissolving FAI, SnI 2 , SnF 2 , and TAA with varying concentrations in dimethyl sulfoxide (DMSO) solvent.The solution without and with TAA are stirred inside an N 2 -filled glove box at room temperature to produce the intermediate FAI-SnX 2 -(DMSO)-TAA phase.The solvent evaporates after annealing the perovskite film, and the mesophase transforms the perovskite precursor into FASnI 3 .The mesophase crystallized into FASnI 3 perovskite in a highly crystalline phase. [25]During the crystallization of FASnI 3 perovskite materials, without TAA, FAI and SnI 2 simply react, resulting in small particles.TAA promotes the dissolution of SnI 2 , by which produces FA þ and Sn 2þ at the same time and then gradually separates to form the perovskite layer's uniform and expanded grain size.TAA addition to the perovskite solution is critical to the perovskite nucleation and crystal growth.Crystallization of TAA-based perovskite has a higher Gibbs free energy and results in fewer nucleation centers formed around TAA. [26,27] This mediates the crystal growth, resulting in more uniform and compact crystalline grainsand closely packed grain boundaries. [26]o investigate intermolecular interactions, post-TAA addition, fourier transform infrared spectroscopy (FTIR) spectra of FASnI 3 and FASnI 3 þ TAA thin films were captured.1020 cm À1 , as shown in Figure 2a.Pure TAA powder exhibits a C=S stretch at 1031 cm À1 , shifting to 1018 cm À1 for FAI:SnI 2 : DMSO:TAA, illustrated in Figure 2b.The C=S stretch at 715 cm À1 for TAA shifts to 713 cm À1 for FAI:SnI 2 :DMSO: TAA, as indicated in Figure 2c.As TAA shares Sn 2þ Lewis acidity with DMSO, its C=S stretch slightly shifts to 713 cm À1 in the presence of FAI:SnI 2 :DMSO due to interaction.This could also result from the interaction between TAA's C=S and DMSO's S=O.The findings highlight robust TAA interaction with Sn 2þ and reinforcement of intermolecular interactions with FAI:SnI 2 :DMSO:TAA adducts.Stretching of the S=O and C=S spectra demonstrated the existence of coordination between Lewis base and the perovskite lattice. [24,28]The use of Lewis acidbase adducts (Lewis acid-Sn 2þ and Lewis base-sulfur) to switch the morphology of perovskite and slow the crystallization rate results in high-grade, large uniform grains.It has the dynamic properties of producing perovskite films with improved crystalline grain growth.Sulfur (S) from TAA may inhibit charge carrier recombination at interfaces, block ion migrations, mitigate interfacial defects, and increase PSC stability. [29]igure 3a-d shows the results of a morphological investigation through field emission scanning electron microscopy (FE-SEM) to understand the effect of TAA addition.It was found that adding TAA to the perovskite precursor solution enhanced the perovskite film's quality and enlarged grain sizes.The formation of more nucleation sights in an unannealed control perovskite film, as shown in Figure 3a, leads to smaller grain size after annealing, [24,30] TAA-added devices (Figure 3c) cause slow nucleation, which is advantageous for larger grains.After annealing, the average grain size of the control perovskite film without TAA addition is %291 nm calculated from FE-SEM images using the ImageJ software (Figure 3b-e).With the addition of a minute TAA, the grain size increased (Figure S1, Supporting Information), and the optimal amount of 0.5 wt% produced a smooth and homogeneous perovskite layer with an average grain size of %520 nm (Figure 3d-e).However, increasing the   TAA can potentially alter the crystallization process and enhance the morphology of perovskite films.TAA forms an adduct with the solute in the precursor solution, and ion exchange with SnI 2 happens during the solvent removal step through antisolvent treatment and annealing. [30,31]hen TAA is added to the precursor solution, S from the Lewis base strongly interacts with Sn 2þ (Lewis acid-base reaction), resulting in the production of an adduct, as shown by FTIR analysis (Figure 1 and 2).As a consequence, the presence of TAA slowed the crystallization process, resulting in larger grains inside the perovskite bulk. [32]The Lewis base character of the TAA may also contribute to enhanced connectivity of neighboring perovskite grains through mediated crystallization, with lesser grain boundaries. [33]However, excessive TAA content may cause additional intermediate production, which can stymie the perovskite film development process and produce low-quality film with smaller grains and increased surface roughness.
The differences in grain sizes were confirmed through atomic force microscopy (AFM) topography images (Figure 4a,b).The total surface current level was higher for the TAA additiveassisted perovskite than for the control samples, as shown in Figure 4c,d, and the average surface current increased from 9.22 to 9.77 nA.The variation in the current sensing atomic force microscopy (CS-AFM) data is appropriately categorized by three parameters for an in-depth investigation of optoelectronic performance: 1) surface current variation between grains and at grain boundaries; 2) surface current variation inside the perovskite grain interior; and 3) surface current variation between grains within the scanned area.In order to ascertain the primary mechanisms driving performance enhancement, we substantiated these factors by superimposing line profiles, topography, and surface current mapping for comparison (Figure 4e,f ).The evidence indicated that grain boundaries yield lower current compared to the grain interiors.Historically, grain boundaries have been considered as recombination hotspots.Owing to the heightened density of defects, surface current at grain boundaries in CS-AFM images ought to be lower than that at grain interiors.Surface current behavior near grain boundaries may be affected by a variety of factors such as grain compactness, film composition, device design, and other factors. [34,35]Additionally, the ion migration led J-V hysteresis at grain boundaries can impact the surface current in CS-AFM. [34]In perovskites without and with TAA, we explored and established a consistent link between the grain boundary and surface current.The line profiles in Figure 4e-f show that when the precursor was treated with TAA, the hysteresis effect at the grain boundaries reduced.
X-Ray diffraction (XRD) analyses were carried out to determine the effect of TAA on the crystallinity of FASnI 3 perovskite films.The XRD curves of the perovskite films without (0.00 wt%) and with TAA additive (0.5 wt%) are shown in Figure 5a.The (100), (200) diffractions can be assigned to the peaks observed at two values of 14.10 0 , 28.41 0 , for all FASnI 3 films. [36,37]hen compared to control film, the relatively higher intensity of the (100) diffraction peak is attributed to the TAA addition (up to 0.5 wt%).Figure 5b shows how the intensity of the 100 peaks and their full width half maxima (FWHM) changed in perovskite films with various thioacetamide additive concentrations.The lowest FWHM and maximum intensity for 0.5 wt% TAA determined by XRD analysis showed a minute quantity of TAA might increase the crystallinity of perovskite films. [38]he crystallinity of the perovskite layer is generally established to be a crucial component in determining PSC performance.
Crystallite size of perovskite was calculated using Scherrer formula (Equation ( 1)), [39] where D p is the average crystallite size, β line broadening in radians, θ Bragg angle, and λ is X-Ray wavelength.The crystallite size of the perovskite was calculated to be 52.93 nm without TAA additive and 69.12 nm with TAA additive.Significant improvement in crystallite size resulted in increased domain size, as shown in top-view SEM morphology images.Because of the inferior-quality perovskite films with small grains and aggregates, the FWHM intensities of the (100) peaks were weaker at higher TAA concentration (0.75 wt%) (Figure 5b).
Figure 5c depicts the UV-vis spectra of FASnI 3 perovskite films with varying concentration of TAA addition.The TAA adduct approach increased the absorption edge intensities of perovskite films due to the formation of high-quality perovskite films with large grains and improved crystallinity. [40]The inset in Figure 5c shows the Tauc plot from which we estimated the bandgap of the Sn-based perovskite material (1.39 eV).The photoluminescence (PL) experiment is particularly valuable for determining the perovskite layer's recombination characteristics. [41]o investigate the PL emission spectra of FASnI 3 , perovskite is deposited on a glass substrate without any electron or hole transport layer.Figure 5d shows that the FASnI 3 perovskite film prepared with 0.5 wt% TAA has higher emission intensity than the control perovskite film, indicating that radiative recombination of the carriers has been enhanced, which is beneficial for effective charge carrier transport and also suggests suppression of nonradiative recombination, which could be due to lower defect density and superior film quality of the perovskite film with TAA as evident from XRD and SEM measurements. [42,43]his reduction of nonradiative recombination with enhanced charge transport to electron and hole transport layers might lead to increased open circuit voltage (V OC ) of PSCs due to less band bending of the perovskite impacted by trap states.
Figure 6a presents the planar inverted (pin) PSC device architecture, synthesized in this work.Figure 6b depicts the forward and reverse J-V characteristics of control and 0.5 wt% of TAA.Table 1 summarizes the corresponding photovoltaic performance parameters.The best efficiency for FASnI 3 perovskite without any TAA additive (Control) was 6.75%, with a shortcircuit current density ( J SC ) of 18.91 mAcm À2 , an open-circuit voltage (V OC ) of 0.592 V, and a FF of 0.60.After the addition of TAA with optimized concentration, the performance of PSC steadily increased, with 0.5 wt% TAA attaining the champion efficiency of 10.67% ( J SC = 24.38 mAcm À2 , V OC = 0.626 V, FF = 0.70).Figure S3 (Supporting Information) presents the detailed J-V scan curves and photvoltaic parameters for PSC devices against varied concentration of TAA.The increased PCE can be attributed to the enhanced quality of the perovskite film.However, the introduction of trap states between the perovskite film and the hole and electron transport layers triggers recombination, consequently leading to a relatively lower V OC along with a noteworthy impact.PSC performance has increased principally because of an increase in J SC values, which may be ascribed to the superior quality of perovskite film obtained for 0.5 wt% TAA, which has large grains, better crystallinity, and lower trap states.However, as expected, PSCs with 0.75 wt% TAA or more performed badly owing to the production of micrograins and aggregates on the perovskite films.PSCs with 0.5 wt% TAA were explored further because of their higher performance, and their properties were compared to control devices produced using pure FASnI 3 film.Table 1 demonstrates minimal hysteresis behavior in both PSCs without and with 0.5 wt% TAA (photovoltaic parameters for different concentrations of TAA were presented in Table S1, Supporting Information).The typical external quantum efficiency (EQE) curves of PSCs without and with TAA (0.5 wt%) are shown in Figure 6c.
As a result of enhanced charge carrier transfer, collection, and absorption, PSCs with TAA exhibited improved percentage EQE values in the 300-900 nm wavelength region as compared to the control cells.Calculated from the EQE spectra, the integrated J SC values for control and TAA devices are 18.52 and 23.61 mAcm À2 , respectively.Further validation of the data was accomplished by measuring the steady-state PCE at a potential close to the maximum power point for PSCs with and without TAA additive.Both devices exhibited steady-state PCE values of around 6.27 and 10.35%, respectively (Figure 6d).These PCE values in steady state are compatible with the results taken from J-V scans (Figure 6b). Figure 6e displays the statistical distribution of PSC performance based on a set of 30 best performing devices for control and TAA-modified devices.The average PCE of pure FASnI 3 -based PSCs was 5.43% AE 0.72%, whereas the average PCE of TAA-induced perovskite film growth was 9.86% AE 0.81%.
The electrochemical impedance spectroscopy (EIS) may be used to assess the kinetics of interfacial charge carrier transfer and the recombination characteristics of solar devices.Figure 7a shows the Nyquist plots of the control and TAA-modified PSCs at frequencies ranging from 1 Hz to 1 MHz with a bias of 0.6 V in the dark.Table S2 (Supporting Information) shows the fitted values of spectra for series resistance (R s ), recombination resistance (R ct ), and capacitance (C PE ) obtained from EIS curves after fitting the Nyquist plot with an equivalent circuit model in the inset of Figure 7a.Both devices have a single semicircle that represents charge transfer at the ITO/PEDOT:PSS/FASnI 3 /C 60 interfaces and corresponds to the R ct recombination resistance. [32]The R ct calculated from the Nyquist plot for TAA-based devices is 93 kΩ, which is much higher than for control PSCs, suggesting that TAA addition in perovskite contributed to defect passivation or recombination suppression.
As a result, TAA-modified device facilitates charge carriers transport to the respective carrier selective layers while enhancing the photovoltaic yields. [44]The value of C PE represents the amount of charge stored at defects. [45]The reduced C PE value of 3.70 nF found verifies the TAA-based device's capacity to passivate defects.Figure 7b illustrates a Mott-Schottky analyses of PSC with and without TAA.The TAA-assisted device exhibited the greatest built-in potential (V bi ) of 0.43 V (the x-intercept in the Mott-Schottky plot), suggesting excellent charge carrier transport, separation, and collection.
[48] TPC curves demonstrate that the photocurrent in the TAA-based device declined quicker than in the pristine device when the minimum resistance (50) route was applied as shown in Figure 7c.By fitting the TPC curves with a monoexponential decay function (A.exp Àt/τ ), the charge transport time (τ t ) for the TAA-based device was determined to be 1.94 μs, whereas the to the control device, the TAA-based device transient photovoltage decayed much more slowly as shown in Figure 7d.The charge recombination lifetime (τ r ) in TAA-based devices estimated from the TPV decay curves was 4.34 μs longer than the control device's 4.27 μs, [45,49] indicating the slower carrier recombination in the TAA-based device.This is primarily due to the PSC's lower trap density, which matches the higher V OC obtained from the TAA-FASnI 3 devices.
X-Ray photoelectron spectroscopy (XPS) measurements were performed to further clarify the change of the FASnI 3 film after TAA additive and are shown in Figure 8a,b.The two peaks deconvoluted from the Sn 3d 5/2 peak at 486.7 and 487.6 eV in the Sn 3d XPS spectra of the FASnI 3 films, [50,51] without and with TAA additive (Figure 8a,b), are attributed to Sn 2þ and Sn 4þ , respectively.As per the data fitting shown in Table S3 (Supporting Information), the TAA additive significantly reduces the Sn 4þ composition from 28.2 to 6.5%, implying that the TAA additive prevents the oxidation of Sn 2þ in the film.We postulated that the Sn-rich environment in the perovskite synthesis caused the decreased oxidation of Sn 2þ .The addition of TAA considerably raised the chemical potential of elemental Sn, as shown by a minor change in the binding energy after film formation, which raises the formation energy of Sn vacancies and hence retards the oxidation process from Sn 2þ to Sn 4þ and Sn vacancies. [52,53]The stability of the encapsulated devices was evaluated in Figure 8c by tracking the evolution of PCE over time while the devices were stored in atmospheric conditions.As shown in Figure 8c, the TAA additive devices retained 81% of the initial PCE after 1000 h of storage in nitrogen atmosphere, whereas the reference device retained only 53% of the initial PCE.The water droplet contact angle images and values of the FASnI 3 /glass samples are shown in Figure S6 (Supporting Information) and Figure 8d.Contact angle values for FASnI 3 /glass samples follow the same trend as refractive index values (crystallinity) of FASnI 3 thin films, as shown in Figure 3b and 5a; contact angle values are also compared with AFM images in Figure S4 (Supporting Information). [54]The FASnI 3 thin film with the greatest surface roughness has the smallest contact angle of 37°, whereas the 0.5 wt% TAA added perovskite has the greatest contact angle of 58°with the least surface roughness.The smooth surface of FASnI 3 thin films, which led to their partial hydrophilicity, may thus be utilized to explain the stability of TAA adductassisted perovskite.

Conclusions
In conclusion, incorporating TAA into the perovskite precursor solution resulted in FASnI 3 perovskite film with compact surface morphology, bigger crystalline grains, reduced grain boundaries, and mitigated defects.As a result of the incorporation of TAA at its optimal concentration, the PCE increased significantly from 6.75% to 10.67%, and the stability of the PSCs was also improved.The TAA-modified PSC maintained 81% of its original PCE after 1000 h of storage, whereas the PCE for the control device dropped to 53% over the same time span.The reactivity of "S" from TAA's Lewis base with Lewis acid Sn 2þ ions, as well as the NH 2 bonding interaction of TAA's amine group with iodide and FAþ ions, contributed to the enhancement of device PCE stability.This work provides a facile strategy for making lead-free perovskite films with reduced traps and structural disorders, for highly efficient and operationally stable PSCs.

Figure 1 .
Figure 1.Schematics of interactions and grain growth mechanism of FASnI 3 perovskite with thioacetamide.

Figure 3 .
Figure 3. a) SEM imaging of unannealed reference and b) FASnI 3 þ TAA perovskite films; c) SEM imaging of annealed perovskite film for reference and d) FASnI 3 þ TAA perovskite films, and e) histogram representing grain size of FASnI 3 perovskite without and with TAA.
concentration beyond 0.5 wt% resulted in nonhomogeneous grains distribution with aggregates observed within perovskite active layer.

Figure 4 .
Figure 4. a) Topography images pf perovskite layer without and b) with TAA additive; c) CS-AFM images without and d) with TAA.The white line in the AFM images indicates the extracted heights and photocurrents.The pink dashed lines depict some typical placements of various grain boundaries of perovskite e) without and f ) with TAA.

Figure 5 .
Figure 5. a) XRD patterns of FASnI 3 perovskite films with and without TAA.b) FASnI 3 film FWHM and intensity calculated from 100 XRD peak.c) UV-vis absorption spectra, with an inset Tauc plot indicating the Sn-based perovskite material's bandgap.d) PL emission spectra of FASnI 3 perovskite films with 0.00 to 0.75 wt% TAA additive.

Figure 6 .
Figure 6.a) FASnI 3 perovskite layer schematics, b) J-V curves, c) incident photon to converted electron (IPCE) spectra of PSCs without and with 0.5 wt% TAA additive, d) steady-state PCE obtained at constant bias for PSCs, and e) statistics of PCE for 30 PSCs devices each, fabricated without and with TAA.

Figure 8 .
Figure 8. a) XPS spectra of Sn 3d 3/2 and Sn 5d 5/2 of PSC without TAA and b) with TAA additive, c) stability of devices without and with TAA additive, and d) surface roughness versus contact angle comparison with different concentrations of TAA additive.

Table 1 .
Best-performing photovoltaic parameters for control and 0.5 wt% TAA devices with both forward and reverse sweeps.