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Article

Effects of NH4SCN Additive in the FAPbI3 Perovskite Films in a Sequential Deposition Method

by
Jorge Luis Miró-Zárate
1,
Miltón Carlos Elías-Espinosa
1,*,
Felipe Cervantes-Sodi
2 and
Carolina Janani Diliegros-Godines
3,*
1
Instituto Tecnologico y de Estudios Superiores de Monterrey, Campus Santa Fe, School of Engineering and Science, Mexico City 01389, Mexico
2
Physics and Mathematics Department, Universidad Iberoamericana Ciudad de Mexico, Prolongación Paseo de la Reforma 880, Mexico City 01219, Mexico
3
Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apdo. Post. J-48, Puebla 72570, Pue, Mexico
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(5), 795; https://doi.org/10.3390/cryst13050795
Submission received: 28 March 2023 / Revised: 6 May 2023 / Accepted: 8 May 2023 / Published: 10 May 2023
(This article belongs to the Section Crystal Engineering)
Browse Figures
Versions Notes

Abstract

:
This study determined the effect of introducing the NH 4 SCN additive in the precursor solution of PbI 2 of a sequential deposition method in an open atmosphere to form FAPbI 3 perovskite over a glass substrate. Adding NH 4 SCN leads to different intermediate layers according to the concentration of the additive. From an adequate concentration, an ionic substitution between I − and SCN − is promoted, providing a unique path for nucleation and growth of FAPbI 3 due to significant changes in morphology. The intermediate layer with a proper amount of NH 4 SCN enhanced the physical properties of FAPbI 3 . It contributed to understanding the crystallinity and morphological conditions for favorable growth of FAPbI 3 directly over a glass substrate. A concentration of 40% gave rise to the biggest grain size, homogeneous morphology, higher absorption, and prevalence of black phase in the α / δ phase coexistence. As a result, the perovskite with the NH 4 SCN additive showed a positive effect on the growth mechanisms and enhanced stability due to the mixed α / δ -phase and grain size ∼1350 nm. The preceding makes FAPbI 3 film with a concentration of 40% a good option for application as stable perovskite in solar cells.

1. Introduction

The demand for renewable energy has been increasing in recent years. This comes along with the need to develop new solar cell technologies. Among the generations of solar cells, the third generation has been the most promising approach to having solar cells with high theoretical solar efficiencies and relatively low environmental impacts. Hybrid organic-inorganic perovskites (HOIPs) are strong candidates for large-scale application of next-generation photovoltaic technologies due to their potential low cost, high performance, and solution processability. In addition, their outstanding optoelectronic properties and increased power conversion efficiency (PCE) from 3.8% to 23.7% in just a few years [1,2] have facilitated their integration into several applications such as tandem solar cells, building-integrated photovoltaics, photovoltaic-driven catalysis, among others.
One of the most used crystalline HOIPs is the methylammonium lead halide (MAPbI 3 ), with a direct bandgap and excellent light absorption. However, MAPbI 3 undergoes a phase transition to a product of PbI 2 at low temperatures, it is not stable above 100 °C, suffers light and thermal degradation and it is not stable against moisture [3]. To improve MAPbI 3 stability issues, the exchange of organic/inorganic cation CH 3 NH 3 + to a slightly larger CH(NH 2 ) 2 + has been introduced.
Another material of great interest is the formamididium lead halide (FAPbI 3 ). FAPbI 3 presents a symmetric perovskite structure with black trigonal photoactive α -FAPbI 3 phase, a time-enhanced operation stability, and awards higher thermal and photostability than MAPbI 3 [4]. However, FAPbI 3 crystallizes at room temperature as a yellow photoinactive hexagonal δ -FAPbI 3 phase that leads to unstable device operations accompanied by difficulty in achieving stable phase and quality films [5].
Several deposition methodologies are reported in the literature to improve the quality and stability of FAPbI 3 , such as additive-assisted deposition, solvent engineering and sequential deposition [6,7,8]. The sequential deposition, upon all other methods, yields a means of crystallization control by involving an intermediate layer of PbI 2 from which perovskite reacts with the predeposited architecture, and can serve as nucleation center for the perovskite film growth [7]. However, despite the ease of manufacture and reduced material cost, a PbI 2 incomplete conversion to FAPbI 3 , caused by formamamidinium iodide (FAI) reaction in the PbI 2 smooth surface, still faces the challenge of producing high quality FAPbI 3 films [9].
To overcome these issues, controlling the perovskite crystallization process, escaping defects of films, ensuring a complete surface coverage and reducing grain boundaries are essential [8]. Some strategies for better control and maintenance of the desired crystal phase include the use of ionic additives and hydrohalic acids [10]. Specifically, ammonium thiocyanate (NH 4 SCN) is a promising additive candidate due to its effects on perovskites, where NH 4 + slows down the crystal growth, reducing defects on the films [11] and SCN − promotes the formation of the α -phase of FAPbI 3 , which also improves the stability of the perovskite [12,13].
In this work, we present the combination of the most promising approaches to obtain stable perovskite growth in an open atmosphere: sequential deposition and the use of the pseudohalogen additive ammonium thiocyanate. We studied the introduction of NH 4 SCN at different molar concentrations (between 20% and 50%) into the PbI 2 precursor solution in a sequential deposition method. The study will focus on the growth of the films over a glass substrate and then the mechanisms to form the FAPbI 3 films, allowing us to understand how the FAPbI 3 perovskite grows naturally with the NH 4 SCN additive. A unique intermediate layer with different nucleation and growth pathways for FAPbI 3 was observed. At 40% concentration, it results in the highest absorption and grain size values, accompanied by a predominance of high crystallinity α -phase, homogeneous morphology, and a slightly lower bandgap than pure FAPbI 3 . The film with 40% additive concentration exhibits enhanced stability by showing an evident prevalence of α -phase above PbI 2 and δ -phase after 15 days of open atmosphere exposure. The optimized amount of additive is a molar concentration of 40% of NH 4 SCN to form FAPbI 3 films with optical and structural properties adequate to use in a perovskite solar cell.

2. Materials and Methods

2.1. Materials and Synthesis Process of FAPbI 3 Perovskite

All experiments were performed in an open atmosphere following methodologies similar to those reported in [13,14]. Figure 1 exhibits the schematic diagram of the experiment. FAPbI 3 films were prepared via the sequential deposition method. In the first step, a precursor solution of 1 M of lead (II) iodide (PbI 2 , 99%) and an amount of ammonium thiocyanate (NH 4 SCN, ACS reagent, ≥97.5%) were mixed in 1 ml of n,n-dimethylformamide (DMF) (HCON(CH 3 ) 2 HPLC ≥99.9%) and spin-coated (4000 r.p.m., 60 s) onto a previously cleaned glass substrate and then annealed at 80 °C for 30 min. In the second step, the PbI 2 -coated substrate was immersed in a 12 mg/mL solution of formamidinium iodide (CH 5 IN 2 :FAI) with isopropanol (IPA) at 80 °C for 90 s, followed by an annealed treatment of 170 °C for 60 min to form FAPbI 3 films.
The films are identified as 20%, 30%, 40% and 50% according on the amount of NH 4 SCN added in the first step.

2.2. Characterization

The crystallographic properties of fresh perovskite films were measured by X-ray diffraction equipment (XRD) (D8 Advanced Eco from Bruker, Karlsruhe, Germany) with Cu-K α radiation of wavelength λ C u = 1.5418 Å, and scan rate of 6.66 deg/min over the Bragg angle range of 10° to 50°. Degradated perovskite films were measured with XRD equipment (PANALYTICAL EMPYREAN, Malvern Panalytical, Malvern, United Kingdom). The FAPbI 3 perovskite crystal size was calculated by taking the value of the full-width at half maximum (FWHM) of the 11.8°, 12.7° and 13.9° peaks. The absorption spectra of the perovskite were recorded by ultraviolet, visible, near-infrared (UV-VIS-NIR) absorption spectroscopy (VARIAN AGILENT CARY 5000, Santa Clara, United States) with a double beam, where the beam reference used was glass substrate. Fresh morphologies of the perovskite films and intermediate layer were characterized on a (Hitachi SU 3500, Tokyo, Japan) scanning electron microscope (SEM) with a low tension of 5.0 kV.

3. Results and Discussion

3.1. FAPbI 3 Films

The XRD patterns on glass substrate are shown in Figure 2a. The 20% film exhibits the presence of PbI 2 and FAPbI 3 α and δ phases. The peaks at 11.8°, 20.5° and 41.7° belong to δ -FAPbI 3 , while the peak at 12.7° (001) can be assigned to PbI 2 (JCPDS card no. 79-0803). Peaks at 13.9° (111), 19.8° (012), 24.3° (021), 28.1° (222), 31.5° (123), 40.2° (024), 42.8° (333), and 49.7° are in agreement with α -FAPbI 3 [15]. The 30% film also presents FAPbI 3 α and δ phases and a reduced signal for PbI 2 . Additional peaks at 26°, 31.5° and 32.9° appear for the δ -FAPbI 3 phase. For the 40% film, a higher presence of α -phase can be noticed due to the intensity of the 34.5° (030) peak increases. Finally, a mixture of α and δ phases with the predominance of α -phase and the disappearance of the PbI 2 peak is shown in the 50% film.
Furthermore, it can be noted that as the NH 4 SCN concentration increases, the following can be observed: a few peaks of δ -FAPbI 3 start to decrease gradually until they disappear at 50%. The remaining δ -FAPbI 3 continues lowering its intensity. The intensity of the α -FAPbI 3 phase peaks at 13.9°, 28.1° and 42.8° increases, while the intensity of peaks at 19.8°, 24.3°, 34.5°, 40.2° and 49.7° decreases.
Figure 2b shows the FWHM of the characteristic peaks of each phase: 11.8°, 12.7° and 13.9° for δ -FAPbI 3 , PbI 2 , and α -FAPbI 3 , respectively. The estimated crystallite size (c.s.) calculated based on the Scherrer equation (D = k λ /( β cos θ ), where D is the crystallite size, k = 0.94, λ C u = 1.5418 Å, β is the FWHM and θ is the Bragg angle) are imprinted in Figure 2b for each crystalline phase. The 40% film shows the lowest FWHM and the highest estimated crystallite size of 52 nm for α -phase, indicating an improvement of crystallinity [11]. Similar crystallite sizes were reported by Zhu et al. [16].
The absorbance spectra are shown in Figure 3 for films from 20% to 50% of NH 4 SCN. The films without NH 4 SCN show an absorbance spectrum similar to the one reported by Mozaffari et al. [14], for films with δ -FAPbI 3 phase prevalence which is not beneficial for solar cell applications (Figure S1).
The absorbance spectra for all the films show the characteristic absorption edge of the α -phase around 800 nm, with a few variations depending on the concentration of NH 4 SCN. This is similar to the reported by other authors [10,17]. However, other absorption peaks and behaviors are present from ∼650 nm to ∼300 nm. Therefore, to understand the absorbance spectra, the spectrum of each sample will be discussed in two regions: one over 750 nm and the second from ∼650 nm to ∼300 nm.
It can be seen that the 40% film presents a higher absorption in the visible range associated with the prevalence of α -phase. Imran et al. also observed this; their study showed an improved absorption in the visible range for the photoactive perovskite phase [17]. Furthermore, the 40% film shows a flat profile in the region from 440 nm to 500 nm, indicating the presence of single crystals and a low presence of plumbates (indicated by the red box in Figure 3a). However, the presence of absorption bands in 387, 430, and 509 nm are in agreement with multiplumbate ions of PbI 3 S 3 − , PbI 4 S 2 2 − , PbI 6 4 − , respectively (marked with an asterisk in Figure 3a) [18], and are consistent with the peak at 12.7°.
As explained, the α -phase presence increases with the NH 4 SCN content, accompanied by a decrease or disappearance of the δ -FAPbI 3 phase and PbI 2 . Consequently, it can be expected that the absorbance spectrum for the α -phase ought to show a significant improvement in the absorption in the visible range. However, films with concentrations other than 40% show a drop of absorbance from ∼800 nm to ∼370 nm. For the 20% and 30% films, this behavior can be attributed to the DMF solvent and the incomplete adhesion of the FAPbI 3 film to the glass substrate, as will be shown in SEM micrographs and is in agreement to the previously reported by Kovalenko et al. [18]. The 50% film presents a behavior similar to 20% and 30% films but with better absorption due to the higher presence of α -phase.
The characteristic absorption edge of α -FAPbI 3 (Figure 3a) lies in 805, 802, 784, and 800 nm for the 20%, 30%, 40%, and 50% films, respectively. The 20%, 30% and 50% films show a redshift in the absorption edge, from 780 nm to 810 nm. This redshift is related to the increased crystallinity of the α -FAPbI 3 , as reported by Zhang et al. [19]. Furthermore, these films show a bandgap (E g ) of E g = 1.40 eV, while the 40% has an E g value of 1.42 eV, as shown in Figure 3b. These values are slightly lower than the reported for pure α -FAPbI 3 (E g = 1.43 eV) [20]. The decrease in the E g for 20%, 30%, and 50% films is also associated with the increase in α -FAPbI 3 phase crystallinity [21].
Figure 4 shows the SEM micrographs of perovskite films. The 20% film is shown in Figure 4a and exhibits an island-like morphology conformed by grains size of ∼950 nm corresponding to the α -FAPbI 3 phase [22]. In addition, this can be observed through small precipitates and a large bar-shape of size ∼7 μ m (pointed with arrows and a circle, respectively, in Figure 4a) which is in agreement with the EDS, corresponding to PbI 2 (Table S1), confirming the presence of PbI 2 already shown in the XRD of Figure 2a by the 12.7° peak, already reported by Yu et al. [23]. Its noteworthy that what is observed between islands corresponds to the glass substraste, supporting the decrease behavior of the absorbance observed in Figure 3a.
The 30% film is shown in Figure 4b. A non-uniform growth of grains of the size of ∼350 nm and small darker regions corresponding to the glass substrate can be seen. Various studies demonstrated that as NH 4 SCN additive increases the grain size, too [24]. However, the mixture of three phases with the predominance of δ -phase reduces grain size [21], and results in the low absorbance behavior discussed earlier. The precipitates associated with the PbI 2 phase changed to a radial finger-shaped (marked with a circle in Figure 4b) of different dimensions, followed by the appearance of PbI 2 points distributed on the film’s surface (indicated with arrows in the inset of Figure 4b). The 30% film shows a better adherence which may be associated with an increase in absorbance, relative to 20% film, in the region between 680 and 900 nm.
An improvement in the morphology for 40% film is shown in Figure 4c, as the consequence of homogeneous surface coverage with significantly smaller glass substrate area without film (less than 5%) and larger grains of ∼1350 nm. This change in morphology may be due the higher presence and high crystallinity of α -phase, in addition to the increase in FWHM of δ -phase and intensity reduction of the PbI 2 characteristic peak (Figure 2). Furthermore, the low intensity of the 12.7° peak is in concordance with the size reduction of PbI 2 precipitates. Based on the foregoing, the surface morphology and crystallinity improve the absorbance shown in Figure 3a. Even when the samples show an uneven coverage over glass substrates, it is well known that an appropiate election of the electron transport layer leads to a homogeneous perovskite layer and a reduction of pinholes.
The morphology of the 50% film is shown in Figure 4d. The reduction of surface coverage accompanied by an increment of glass substrate area reduces the absorbance (Figure 3a). It can be observed that the precipitates associated with the PbI 2 phase have no crystalline shape anymore. Consequently, there is no PbI 2 peak in XRD, but the UV-VIS absorption bands confirm the presence of multiplumbates precipitates in the film. However, despite low absorbance, the increased α -phase in 50% film showed a better absorbance regarding 20% and 30%.
It is evident in Figure 4 that adding a proper amount of NH 4 SCN to the precursor solution of PbI 2 is related to the formation of an intermediate layer, which leads to a significant change in the morphology of the perovskite films.

3.2. Intermediate Layer

Based on the strong interaction between the atoms due to the similarity of the ionic radius of the cation NH 4 + with Pb 2 + and the anion SCN − (2.15–2.22 Å) with I − (2.22 Å), respectively [11], it can be inferred that adding NH 4 SCN in the precursor solution of PbI 2 would lead to an intermediate layer that may improve the nucleation and growth of the FAPbI 3 perovskite. In order to study this phenomenon, XRD patterns of PbI 2 with NH 4 SCN films, after being annealed at the first step of the sequential deposition method, were recorded as shown in Figure 5. These films will be named p-20%, p-30%, p-40% and p-50%. An additional percentage of NH 4 SCN was included (p-80%) to observe if, at a higher concentration of NH 4 SCN, a favorable growth of the final perovskite film is promoted.
P-20% and p-30% films exhibit the characteristic peaks at 12.7° (001), 25.1° (002), and 38.6° (003) of PbI 2 . From p-40% onwards, an ionic substitution of the I − to the SCN − anions are promoted. This can be observed with the rise of the diffraction peak intensity at 21.5°, corresponding to Pb(SCN) 2 . This ionic substitution was to be expected based on the chemical similarity of the pseudohalogen (SCN) with the halogen (I) [25].
As the NH 4 SCN concentration increases, the intensity of the peaks corresponding to the PbI 2 phase shows a reduction, the estimated crystallite size increases from 42 nm for p-20% to 44 nm for p-50% while the Pb(SCN) 2 peak becomes more intense than the p-20%. Finally, it is interesting that for p-80%, additional peaks appear at 25.5° (011) and 34.3° (102), corresponding to PbI 2 and another one at 44.1° (110) which is associated with NH 4 SCN [26]. This suggests that NH 4 SCN, at higher concentrations, neither evaporates completely, nor fully reacts with PbI 2 , resulting in remanents on the film. (More information of 80% films can be found in Supplementary Materials).
Despite the intensity increment of the Pb(SCN) 2 peaks observed in the intermediate layer, in the FAPbI 3 XRD patterns, illustrated in Figure 3a, there is no trace of Pb(SCN) 2 . It has been previously reported that at the first annealing process part of NH 4 SCN evaporates by separating in NH 3 and HSCN due to the low stability of the NH 4 SCN molecule [27], leaving only a trace of S, C, and N distributed in the surface. This phenomenon was also observed by Lin and Cai et al. [24,28] and corroborated in this work by EDS (Table S2). At the EDS analysis in the FAPbI 3 film, the S signal was weaker than the signal showed in the PbI 2 intermediate layer, confirming that most of the S could evaporate during the second annealing process (Table S3). This reveals the importance of NH 4 SCN in the change in morphology.
Figure 6a–d presents the SEM micrographs of the intermediate layer according to NH 4 SCN concentration. The P-20% film exhibits irregular porous morphology with nearly complete coverage. Needle-like precipitates are barely present on the surface, and the film seems composed of multilayers. The p-30% film is more homogeneous with a smoother surface and better coverage than p-20%. Some holes with an elongate shape can be observed. The intensity reduction of PbI 2 peaks with the increasing Pb(SCN) 2 peaks discussed earlier gives rise to more significant morphological changes from p-30% to p-40% and above films.
The p-40% film shows a flake morphology, interconnected by porous branches that seem in upper and lower layers with higher substrate area. At p-50%, the wafer-shapes are entirely defined, surrounded by crystalline aligned precipitates that generate a star-shape morphology. A fibrous and porous morphology is also present in the film, covering the remaining area of the glass substrate.
Ke et al. reported that with only 5% of Pb(SCN) 2 added to the perovskite precursor, grains can reach sizes of 2 μ m [29]. In this study, the concentration of NH 4 SCN provokes significant morphological changes and raises the intensity of Pb(SCN) 2 peak at the intermediate layer but not in the final perovskite film. the morphology changed from irregular porous to fiber-like as a higher amount of NH 4 SCN and Pb(SCN) 2 was added.
For the precipitates described in p-50% film, the additive increment causes an increment of S and a decline of I (Table S4). It can be assumed that the major presence of sulfur generates multiplumbate ions that correspond to the precipitates. The PbI 2 atomic weight seems to correspond to the irregular morphology of p-20% and p-30%, flakes of p-40%, and wafers of p-50% (Table S5).
The temporary ionic substitution at the first step makes SCN ion highly suitable for forming hydrogen bonds with CH(NH 2 ) 2 [30]. Furthermore, the higher enthalpy and electronegativity of SCN than iodine [31] promotes the reaction of the SCN with FAI solution at the second step of deposition. Accompanied by a constant reduction of PbI 2 peaks in the intermediate layer, avoiding secondary phases and encouraging a quick conversion for the FAPbI 3 film [32].
The intermediate layer acts as a nucleation structure for the FAPbI 3 perovskite, in which it forms and grows through the architecture of each additive concentration. The morphology, the crystallinity and the phases in perovskite FAPbI 3 , depend on the immersion time, r.p.m. of the spin coating at first step, and the quantity of NH 4 SCN.
Dong et al. found that by increasing the immersion time, a gradual grain size elongation can be seen in the FAPbI 3 film [27]. In this work, it can be observed that the concentration of NH 4 SCN generates an increment in grain size. Moreover, the higher immersion time results in an excessive elongation of the film. If to the foregoing high-r.p.m. spin coating is added at the first step, a low cover of the substrate area is to be expected. This phenomenon was also observed by Tai et al., concluding that with higher than 4000 r.p.m., a full coverage of the surface will not be obtained [12].
In the same way, immersion with a surplus of time could extract an excess of organic cation CH(NH 2 ) 2 + , causing its dissolution in the IPA solution and leading to incomplete conversion of PbI 2 to perovskite [33]. This may explain the presence of the δ -phase even with the high concentrations of NH 4 SCN that promote the α -phase.
It can be observed that the seeming multilayers, the porous in the irregular morphology, and the flakes morphology as well as in the branches of the p-20% and p-40% films, respectively, promote the nucleation and growth of FAPbI 3 perovskite film by allowing the diffusion of FAI solution in a larger quantity of vacant spaces. This results in the nucleation and growth of a more homogeneous FAPbI 3 perovskite film with larger grain size. Similarly, Moreno et al. [34] observed that a porous layer of PbI 2 promotes the diffusion of methylammonium iodide (MAI) solution into the film, leading to a better reaction with PbI 2 .
Apart from that, p-30% film has a smooth morphology on most of its surface, and p-50% has wafers with smooth surfaces. These intermediate smooth morphologies will not allow the diffusion of FAI solution, enabling only to react with the PbI 2 on the surface, resulting in a non-uniform growth and the prevalence of PbI 2 and δ -phase due to an incomplete conversion to FAPbI 3 . Fu et al. also observed this phenomenon, proving the initial formation of MAPbI 3 on the surface, obstructing the diffusion of MAI for a complete reaction with PbI 2 [35]. The fiber-like morphology of the p-50% film is too thin, resulting in a dilution of the fiber-like intermediate architecture during the immersion in FAI, leaving a poor intermediate layer for perovskite nucleation.
It must be noted that the 40% film has the biggest grain size and more homogeneous film compared to the rest. This may be due to a preferential balance between the DMF, NH 4 SCN and PbI 2 enhancing the ionic substitution of SCN − in the intermediate layer accompanied by and optimum immersion time that, in addition, promotes the α -phase. The prior properties enhance not only a good quality film, but a more stable perovskite film.

3.3. Stability of Perovskite Films

In order to explore the perovskite films stability, perovskites were aged for 15 days prior to XRD measurements. The XRD patterns (Figure S2) show that all peaks have a slight displacement to higher angles due to a distortion in the crystalline lattice [36]. There is also an increase in δ -phase at the expense of a reduction in α -phase. This, along with the appearance of peaks associated with PbI 2 , Pb(SCN) 2 and FAI are related to the films’ degradation.
The highest prevalence of α -phase in 20% and 40% films are attributed to the morphology of both films with less grain boundaries and homogeneity, which enhanced the stability against moisture and increased the lifetime of the films. Otherwise, the higher intensity of PbI 2 and δ -phase in the 30% and 50% films are in agreement with the morphology with more grain boundaries and with small grain size, which allows the infiltration of moisture, as was observed by another author [37].
Finally, it is known that mixed α / δ -phases make the films thermodynamically stable due to the mixing of both entropies [38], which in agreement with the results of this work, the presence of mixed α / δ -phase in the fresh perovskite and degradated perovskite films, result in a favorable change in the entropy to reduce phase conversion. The films in this work present a higher stability compared with recent work where a complete degradation of δ -phase to α -phase was reported [39].

4. Conclusions

In this work, an experimental method to obtain FAPbI 3 over a glass substrate in a sequential deposition method in an open atmosphere was reported. The impact of an intermediate layer fabricated with additive NH 4 SCN introduced in the precursor solution of PbI 2 was studied. The intermediate layer acts as a nucleation structure for the FAPbI 3 in which it forms and grows through the architecture of each additive concentration. In 40% of NH 4 SCN concentration film, the perovskite shows a slightly lower bandgap than pristine FAPbI 3 , predominance of α -phase with enhanced crystallinity, more significant absorption in visible range and homogenous granular coverage with a grain size of ∼1350 nm. It appears that the multilayers and the porous structure in the irregular morphology in p-40% allowed the perovskite film to grow favorably. Consequently, the 40% film shows enhanced stability due the morphology and the entropy change by the mixed α / δ -phase. The presence of SCN in the films is fundamental to raising the decomposition energy of the FAPbI 3 , and the open atmosphere enhanced the stability against atmosphere factors. It can be concluded that adding an adequate concentration of NH 4 SCN in the precursor solution of a sequential deposition method leads to a favorable path for growing FAPbI 3 for use in solar cells.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst13050795/s1, Table S1. Elemental analysis by energy-dispersive X-ray on bar-shaped morphology in 20% film; Table S2. Elemental Analysis by energy-dispersive X-ray of p-40% film after first annealing; Table S3. Elemental Analysis by energy-dispersive X-ray of 40% film after second annealing; Table S4. Elemental Analysis by energy-dispersive X-ray of p-50% film over a precipitate; Table S5. Elemental Analysis by energy-dispersive X-ray of p-50% film over a waffle morphology; Figure S1. Absorbance spectrum of pristine FAPbI 3 (0% of NH 4 SCN); Figure S2. Comparative XRD patterns between fresh FAPbI 3 and degradated FAPbI 3 with different concentration of NH 4 SCN; Figure S3. Top-view SEM micrograph of p-80% film. The white scale bar indicates 10 μ m. Figure S4. Top-view SEM micrograph of FAPbI 3 perovskite film with 80% concentration of NH 4 SCN. White scale bar indicates 10 μ m; Figure S5. Absorbance UV-Vis light spectra of FAPbI 3 perovskite films with different concentration of NH 4 SCN including 80%; Figure S6. XRD patterns of the FAPbI 3 perovskite film with 80% concentration of NH 4 SCN.

Author Contributions

Conceptualization, J.L.M.-Z., M.C.E.-E. and C.J.D.-G.; methodology, J.L.M.-Z., M.C.E.-E., F.C.-S. and C.J.D.-G.; validation, J.L.M.-Z., M.C.E.-E. and C.J.D.-G.; formal analysis, J.L.M.-Z. and C.J.D.-G.; investigation, J.L.M.-Z., M.C.E.-E., F.C.-S. and C.J.D.-G.; resources, M.C.E.-E., F.C.-S. and C.J.D.-G.; data curation, J.L.M.-Z., M.C.E.-E., F.C.-S. and C.J.D.-G.; writing—original draft preparation, J.L.M.-Z. and C.J.D.-G.; writing—review and editing, J.L.M.-Z., M.C.E.-E., F.C.-S. and C.J.D.-G.; supervision, M.C.E.-E. and C.J.D.-G.; project administration, J.L.M.-Z., M.C.E.-E. and C.J.D.-G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received scholarship from the Mexican government institution CONACyT for the doctoral studies of J.L.M.-Z.; F.C.-S. acknowledges funding from CONACYT Synergy project 1564464; F.C.-S. also received funding from FICSAC and FISMAT at Universidad Iberoamericana Ciudad de Mexico; C.J.D.-G. acknowledges funding from VIEP-BUAP 2023.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors give thanks to Ulises Salazar Kuri for his help in the measurement of degradated XRD perovskite films. José Adrián Díaz Hernández, Josué Hernández and Alma Cuellar for his support and knowledge to conduct all the experiments. Jorge Luis Miró Zárate thanks to Mexican government institution CONACyt and Instituto Tecnológico y de Estudios Superiores de Monterrey for its support through a doctoral studies scholarship.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
NH 4 SCNAmmonium Thiocyanate
PBI 2 Lead (II) Iodide
FAPbI 3 Formamidinium Lead Halide
IIodine
SCNThiocyanate
HOIPHybrid Organic-Inorganic Perovskite
PCEPower Conversion Efficiency
MAPbI 3         Methylammonium Lead Halide
FAIFormamidinium Iodide
NH 4 Ammonium
DMFDimethylformamide
r.p.m.Revolution Per Minute
IPAIsopropanol
UV-VIS-NIRUltraviolet Visible Near Infrared
SEMScanning Electron Microscope
XRDX Ray Difraction
FWHMFull Width at Half Maximum
E g Bandgap
Pb(SCN) 2 Lead (II) Thiocyanate
EDSEnergy Dispersive Espectroscopy
MAIMethylammonium Iodide

References

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Crystals 13 00795 g001 550
Figure 1. Schematic diagram of perovskite FAPbI 3 fabrication by sequential deposition method.
Figure 1. Schematic diagram of perovskite FAPbI 3 fabrication by sequential deposition method.
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Figure 2. (a) XRD patterns and (b) FWHM and estimated crystallite size (c.s.) of 11.8°, 12.7°, and 13.9° characteristic peaks of δ -FAPbI 3 , PbI 2 , and α -FAPbI 3 phase, respectively, of the perovskite films with different concentration of NH 4 SCN.
Figure 2. (a) XRD patterns and (b) FWHM and estimated crystallite size (c.s.) of 11.8°, 12.7°, and 13.9° characteristic peaks of δ -FAPbI 3 , PbI 2 , and α -FAPbI 3 phase, respectively, of the perovskite films with different concentration of NH 4 SCN.
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Figure 3. (a) Absorbance UV-Vis light spectra indicating the flat profile and the multiplumbate ions of PbI 2 and (b) bandgap calculation for the FAPbI 3 perovskite films with different concentrations of NH 4 SCN.
Figure 3. (a) Absorbance UV-Vis light spectra indicating the flat profile and the multiplumbate ions of PbI 2 and (b) bandgap calculation for the FAPbI 3 perovskite films with different concentrations of NH 4 SCN.
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Figure 4. Top-view SEM micrographs of FAPbI 3 perovskite films, from (a–d) 20–50% concentration of NH 4 SCN. The small precipitates and the bar-shaped in 20% film are indicated with arrows and a circle, respectively. The radial finger-shaped is marked with a circle in 30% film, and PbI 2 points are highlighted with arrows on the inset of 30% film. The scale bar indicates 10 μ m.
Figure 4. Top-view SEM micrographs of FAPbI 3 perovskite films, from (a–d) 20–50% concentration of NH 4 SCN. The small precipitates and the bar-shaped in 20% film are indicated with arrows and a circle, respectively. The radial finger-shaped is marked with a circle in 30% film, and PbI 2 points are highlighted with arrows on the inset of 30% film. The scale bar indicates 10 μ m.
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Figure 5. XRD patterns of intermediate layer from p-20% to p-80% concentration of NH 4 SCN.
Figure 5. XRD patterns of intermediate layer from p-20% to p-80% concentration of NH 4 SCN.
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Figure 6. Top-view SEM micrographs of intermediate layers from (a–d) p-20% to p-50% concentration of NH 4 SCN. The scale bar indicates 10 μ m.
Figure 6. Top-view SEM micrographs of intermediate layers from (a–d) p-20% to p-50% concentration of NH 4 SCN. The scale bar indicates 10 μ m.
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Miró-Zárate, J.L.; Elías-Espinosa, M.C.; Cervantes-Sodi, F.; Diliegros-Godines, C.J. Effects of NH4SCN Additive in the FAPbI3 Perovskite Films in a Sequential Deposition Method. Crystals 2023, 13, 795. https://doi.org/10.3390/cryst13050795

AMA Style

Miró-Zárate JL, Elías-Espinosa MC, Cervantes-Sodi F, Diliegros-Godines CJ. Effects of NH4SCN Additive in the FAPbI3 Perovskite Films in a Sequential Deposition Method. Crystals. 2023; 13(5):795. https://doi.org/10.3390/cryst13050795

Chicago/Turabian Style

Miró-Zárate, Jorge Luis, Miltón Carlos Elías-Espinosa, Felipe Cervantes-Sodi, and Carolina Janani Diliegros-Godines. 2023. "Effects of NH4SCN Additive in the FAPbI3 Perovskite Films in a Sequential Deposition Method" Crystals 13, no. 5: 795. https://doi.org/10.3390/cryst13050795

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