Orientation-Controlled (h0l) PbI2 Crystallites Using a Novel Pb-Precursor for Facile and Quick Sequential MAPbI3 Perovskite Deposition.

Organic-inorganic hybrid lead halide perovskites have shown significant progress in the last few years having achieved efficiencies over 25% at the lab scale. The sequential deposition technique has provided a robust approach in the perovskite film fabrication. However, obtaining a reproducible and quality perovskite film has always been challenging because of the highly crystalline and ordered (001) oriented underlying PbI2 film. Here, we report a simple solution approach to fabricate a PbI2 residue-free, superior grade perovskite film by using a compositional engineered PbI2-precursor solution. We demonstrate that the Pb-precursor film crystallized into a R-centered Hexagonal metric lattice with (h0l), (hk0), and (00l) orientations provides a more efficient and quicker conversion into perovskites compared to conventional (001) oriented 2H-PbI2. A porous and multi-oriented PbI2 film is prepared by rationally incorporating a volumetric fraction of Pb(Ac)2·3H2O in the typical PbI2/dimethylformamide precursor solution, which significantly improves the surface features of PbI2 as well as the structural properties. As a result, a compact, smooth, and large grain perovskite can be obtained by accomplishing a full conversion with comparatively much less reaction time. Furthermore, a comprehensive mechanism of structural modification of PbI2 and the role of its orientation in ameliorating the reaction kinetics has been demonstrated.

Introduction

Perovskite solar cells have gained remarkable attention in the field of cost-effective photovoltaic (PV) technologies. The solution-processable abilities and exceptional optoelectronic qualities of the organic–inorganic lead halide films have strengthened this technology to stand over conventional PV technologies and drastically modulated the efficiencies from 3.8% to a record 25.2% in the last few years.[1,2] Perovskites, with a figurative ABX3 formula, where A represents a monovalent cation, B stands for divalent cation, and X include anions (generally, I–, Cl–, Br– or their combinations), have drawn significant attention in the field of PV research because of its outstanding photoconversion properties, including high optical absorption coefficient (1.5 × 104 cm–1 at 550 nm for CH3NH3PbI3) with tunable band gap,[3,4] long diffusion lengths for electron and holes (around 1069 nm for electron diffusion and 1213 nm for hole diffusion in CH3NH3PbI3–Cl perovskite),[5,6] low excitonic binding energy (14–25 meV),[7] high crystallinity, and process energy-efficient preparation methods.[8,9]

The PV performance of the perovskite solar cell is highly dependent on the quality of the solution-processed perovskite film. Numerous strategies have been adapted in improving the quality of the perovskite film using surface engineering,[10−12] solvent engineering,[13,14] changing the composition of the precursor solution,[4,15] incorporating more efficient and robust electron/hole transport layers, interface materials, and so forth.[16,17] The fabrication method of perovskites also plays a major role in determining the performance of the perovskite film. The sequential (two-step) deposition method offers the advantage of better control over morphology and fabrication robustness over other solution-processable methods.[18,19]

In the process of perovskite film fabrication using a two-step method the lead iodide film is deposited as the first step, and the perovskite is formed in the second step via the intercalation of methylammonium iodide (MAI) molecules. The process of intercalation involves a change in the density and volume because of the conversion of PbI2 into MAPbI3. The major challenge involved in this process is improper and nonuniform diffusion of MAI molecules, leading to incomplete conversion to perovskites.[20,21] Although, a full conversion may be achieved by lengthening the reaction time or by increasing the MAI concentration, but it may affect the performance of the perovskite. A longer dipping time may cause the de-intercalation of MAI molecules from MAPbI3, and a higher MAI concentration may give rise to residual MAI on the grain boundaries.[20] Another issue reported in this fabrication technique is the poor morphology of the perovskite. Several reports have been published to achieve a quality perovskite by modifying the conversion process or by engineering the lead iodide precursor film. Grätzel’s group published a report to form a high-quality pinhole-free PbI2 film from a homogenous precursor solution of PbI2/dimethylformamide (DMF) prepared by adding a small amount of water in the solution.[22] Wu and co-workers[23] used potassium iodide and water in the PbI2/DMF precursor solution to obtain a densely packed lead iodide morphology. In another report, the ACN additive was used in the PbI2 precursor solution to improve the perovskite morphology.[24] Some reports have used the strategy to form a porous PbI2 film to improve the conversion process. For example, Cao et al.[25] and Zhang et at.[26] used MAI and tri-n-butyl phosphate as the additive in a lead precursor solution to obtain the porous PbI2 film that leads to complete conversion into the perovskite. Duan et al.[27] added a low boiling point polymerized additive s-MMA in the PbI2 precursor solution that facilitates a mesoporous PbI2 film with tridimentionally scaffolds of sMMA, which resulted in a compact and crystalline MAPbI3 film. Yi et al.[28] used a PbX2 solution containing PbI2 and PbBr2 salts in the DMF/dimethyl sulfoxide solution in certain molar ratios to improve the morphology of the lead halide layer and achieve full conversion into the perovskite. All these reports focused on complete conversion by improving the morphology of the underlying PbI2 layer. However, the morphology is not the only culprit behind the incomplete and inefficient conversion of PbI2 into MAPbI3. One of the major reasons is the crystallization of the PbI2 film in a layered and highly ordered (001) crystallographic orientation that does not facilitate enough space for volume expansion during conversion reaction and results in poor perovskite morphology with PbI2 residues. Hence, a more robust approach is much needed to fabricate a less ordered and randomly oriented structure of lead iodide that can provide a fast and complete perovskite conversion with improved performance, ideal for large area perovskites. In contrast to these reports, our work paves the way to crystallize the PbI2 film in a more random fashion having multiple crystal orientations with a porous morphology and high surface area, thus addressing the fundamental issue the in conversion process.

In this article, we develop a simple and very effective approach to readily fabricate a perovskite film with superior optoelectronic qualities and having no PbI2 residues. Herein, for the first time (to the best of our knowledge), we demonstrate a facile way to crystallize the PbI2 film in an R-centered hexagonal crystal lattice system with (hk0), (h0l) orientations rather than its standard (001)—2H polytype phase for the fabrication of the perovskite via a two-step deposition approach. The formation of multiple crystal orientations in the PbI2 film has been achieved by a Pb-I2/(Ac)2 precursor solution, wherein Pb(Ac)2·3H2O is added in the PbI2/DMF solution in a volumetric ratio to an extent that it facilitates the desired structural and morphological advancement in the Pb–precursor film. On the one hand, incorporation of Pb(Ac)2·3H2O in the PbI2/DMF solution results in structural improvements by impelling multiple crystallographic orientations. On the other hand, it engenders morphological enhancement in the PbI2 film, providing porous surface features. We found that the judicious and optimized amount of Pb(Ac)2·3H2O in the PbI2/DMF solution tends to reduce its standard (00l) orientation proportion and drive the crystallization of PbI2 into (h0l) and (hk0) plane orientations. Incorporation of lead acetate trihydrate along with HI may release the acetate and hydrogen ions through a dissociation reaction (below) in a weakly acidic medium.

This weak acid environment reduces the surface energy of the (h0l) orientation and favors its crystallization; however; the crystallization in the (00l) orientation reduces because of higher surface energy.[29,30] We reveal that these (h0l) and (hk0) plane orientations of the PbI2 film help in faster and full conversion into the perovskite along with enhanced photophysical and surface properties. Furthermore, we have analyzed the impact of Pb–precursor composition on the surface features and morphology of the PbI2 film and final perovskite film quality. A thorough investigation has been demonstrated on how the multiple orientated crystallites of the PbI2 film help in quicker and efficient conversion. This work provides an easy and proficient solution to the conventional issues like incomplete conversion and reproducibility in the two-step perovskite deposition. Also, our method reports a reduction in the annealing temperature of the PbI2 film that will be beneficial in large-scale applications.

Result and Discussion

Structural and Morphological Advancement in PbI2

X-ray diffraction (XRD) patterns of the reference (controlled/pure) PbI2 film and lead acetate trihydrate mixed PbI2 films (PbI2/PbAc2) are recorded and shown in Figure . The diffraction pattern of pure PbI2 (100:0), as shown in Figure a, clearly represents a predominant orientation of the crystallites in (001) lattice arrangement at around 12.98° (JCPDS card no.: PbI2 73-0591). The controlled film exhibits a hexagonal crystallographic system (2H polytype) of the hexagonal family with a one–sixfold rotational symmetry. The crystal structure of hexagonal PbI2 with plane designations is shown in Figure b. Lattice parameters are calculated from diffraction data, as shown in Figure a, and confirm the hexagonal lattice system with a = b = 4.55 Å, c = 6.97 Å, and cell volume of 125.36. We found that the addition of lead acetate trihydrate for the first two stages, that is, PbI2/PbAc2 volume ratio of (90:10) and (80:20), does not lead to any detectable change in the diffraction peaks (Figure c) but causes a significant decrease in the crystallinity of the PbI2 film, as indicated by reduced peak intensity and improved full width at half-maximum (fwhm), as shown in Figure e. The fwhm values calculated using Gaussian fitting for (001) reflections indicate a drastic increase from 0.272° (for pure PbI2) to 0.919° (for 80:20 film). Interestingly, a further increase in lead acetate volume affects the crystal lattice of the lead iodide film as seen from the diffraction patterns. In the case of the 70:30 ratio film, the signature hexagonal oriented (001) plane of lead iodide has diminished and another parallel plane, oriented in (003), has appeared at 1/3 of d001. The most intense peak is positioned at 28.76° representing the (104) orientation along with other less intense diffractions at 40.02° (110), 23.6° (101), 31.6° (105), and 41.95° (108) [JCPDS card no.: PbI2 73-1753].[31] A few traces of lead acetate has been detected with negligible intensity and indicated with a “star” mark in the diffraction patterns. Peak analysis and lattice parameter calculations (a = b = 4.49 & c = 20.57 with a cell volume of 449.19) confirms a trigonal crystal system for the 70:30 ratio film with a R-centered hexagonal metric lattice system (as shown in Figure d). Further increase in lead acetate concentration causes the vanishing of lead iodide diffraction peaks, and a strong reflection pertaining to lead acetate has been noticed. Further addition of lead acetate was found to have an adverse effect on morphology as explained in subsequent sections.

Figure 1

(a,c) XRD diffraction patterns of pure PbI2 and PbI2/PbAc2 (b,d) structural representation of hexagonal and trigonal structures with plane arrangements and (e) fwhm and peak intensity graphs for PbI2/Pb(Ac2) ratio films.

This change in the crystallization orientation of the (70:30) ratio lead halide precursor film can be an outcome of Pb(Ac)2·3H2O. DMF solution has a pH value close to neutral.[32] Because of the hydrolysis mechanism of DMF, it dissociates into formic acid and dimethylamine. Because DMA (dimethylamine) has a strong basic character with respect to the weak acidic nature of formic acid, the pH of the medium shift toward an alkaline regime.[33−35] Although the dissociation reaction of DMF is very slow, heating the solution to dissolve PbI2 may speed up the process.[32] Another problem arising because of DMF hydrolysis is that the disintegrated product DMA undergoes a reaction with MAI during the intercalation process, which leads to the formation of a nonperovskite DMAPbI3 phase reducing the overall performance.[36−39] The addition of Pb(Ac)2·3H2O/DMF solution (slightly acidic) and HI (to dissolve the lead salts) shifts the medium of the solution toward weak acid. The acidic nature of the solution or availability of H+ ions in the solution may reduce the surface energy of the (h0l) orientation and facilitates more crystallization in this particular orientation.[29,30]

Morphology and surface properties of pure PbI2 and lead acetate mixed PbI2 films has been studied using microscopic tools. A Figure panel (a–d) shows the scanning electron microscopy (SEM) images of pure PbI2 and PbI2/PbAc2 ratio films. The surface of the pure PbI2 film shows inhomogeneity with various irregular chunk formation of crystallites and some pinholes in the film. As the volume ratio of lead acetate trihydrate increases, it reduces the chunk formation in the film and improves the homogeneous character in the film, yet quite a few pinholes can still be seen in the film. For a volume ratio of 30%, we observed a homogeneous fully covered film with a compact arrangement of grains. Although we found that the addition of a certain amount of Pb(Ac)2 in the PbI2/DMF solution reduces the grain size, it improves the surface characteristics of the film providing grainy features and a porous morphology that can help in better infiltration of MAI with ample space for perovskite grain growth. For a (50:50) ratio film, morphology deteriorates and forms the needle-shaped chunks.

Figure 2

SEM images of (a) PbI2 and (b–d) lead acetate mixed PbI2 films.

A more detailed investigation of Pb(Ac)2 addition on the surface properties of the lead iodide film has been studied using atomic force microscopic images, as shown in Figure .

Figure 3

AFM) 2D (left) and 3D images (right) of (a) PbI2 and lead acetate mixed PbI2 films.

AFM images taken on 5 μm × 5 μm window size samples follow the trend derived in SEM analysis. A 3D view of the pristine PbI2 sample shows a multitude of surface undulation with a substantial number of pinholes that can be seen as the dark spots in the 2D image. Furthermore, with the 10% addition of lead acetate trihydrate, surface undulations have reduced but numerous chunks can still be seen in a 3D view in the form of spikes. By increasing the ratio to 30% (70:30 film), we found homogeneous and grainy features with a compact arrangement as inferred from the SEM analysis. Besides this, we observed an increase in the surface roughness from 0.349 ± 0.11 nm (for pure PbI2) to 5.66 ± 0.58 nm (for 70:30 ratio film) and an increase in the surface area from 25.0 to 25.30 μm2, respectively. The higher values of surface roughness and surface area facilitate more sites for MAI molecules to interact with and enhance the infiltration capability, leading to better perovskite conversion efficiency.[40] Further increasing the lead acetate trihydrate concentration (for 50:50 ratio) gives a PbI2 look-alike surface with more undulations and inhomogeneities. The improvement in surface characteristics has been tabulated in Table .

Table 1

Surface Property Analysis from AFM Data for PbI2 and Lead Acetate Trihydrate Mixed PbI2 Films

sample	surface roughness (nm)	max. height of profile (nm)	surface area (μm2)
PbI2	0.349 ± 0.11	1.69 ± 0.5	25.0
PbI2/Pb(Ac2) [90:10]	2.64 ± 0.94	18.09 ± 8.2	25.13
PbI2/Pb(Ac2) [70:30]	5.66 ± 0.58	28.14 ± 4.28	25.30
PbI2/Pb(Ac2) [50:50]	0.36 ± 0.08	1.81 ± 0.4	25.0

Perovskite is prepared from the pure PbI2 and lead acetate trihydrate mixed PbI2 [PbI2/Pb(Ac)2 ratio] film by spin-coating the MAI/isopropyl alcohol (IPA) solution as explained in the Experimental Section.

Crystallographic and Morphological Analysis of the Perovskite

Figure shows the XRD patterns of the perovskite film. All the films exhibit the tetragonal crystal systems with diffraction at 2theta values of around 14.47, 24.87, 28.74, 32.18, and 41.04° corresponding to plane orientations of (110), (211), (220), (114), and (224).[41] In the diffraction pattern of the perovskite from pure PbI2, we can see the strong reflection at around 12.85° and 38.98° and can be assigned to unreacted PbI2.[9] The addition of Pb(Ac)2 in the pristine PbI2 solution helps in improved conversion of the underlying layer into the perovskite and substantially reduces the intensity of the PbI2 signature diffraction peak. As inferred from the diffraction pattern, for a (70:30) ratio film, compete conversion has been achieved as no traces of remnant lead iodide can be spotted. The dynamics behind the complete conversion has been discussed in the subsequent section.

Figure 4

(a) XRD patterns of the perovskite film converted from PbI2 and PbI2/Pb(Ac)2 ratio films and (b) distribution of the perovskite grain size and (c–f) SEM images of perovskite films as prepared from pure PbI2 and PbI2/Pb(Ac)2 films.

Figure c–f panels show the SEM images of MAPbI3 films prepared from PbI2/Pb(Ac)2 films after spin-coating of the MAI/IPA solution. It can be deduced from the images that the perovskite film converted from pure PbI2 has numerous pinholes, irregular surface morphology, and loose packing of grains, whereas, for MAPbI3 converted from acetate mixed PbI2 films has fewer number of pinholes and shows a uniform morphology throughout the film. Especially, in the case of the 70:30 volume % ratio, MAPbI3 has full coverage with no pinholes and represents a dense and tightly packed surface. For further increase in the lead acetate ratio, we observed that the film has developed some intergranular gaps and are not favorable for a quality perovskite. The amount of lead acetate used in the precursor solution significantly affects the grain size distribution of the as-converted perovskite film (Figure b). For the pristine PbI2 film, the average grain size was around 260 nm with a narrow distribution of the grains. For 30 vol % mixed acetate film, grains were found to be very broad with a significant number of grains falling in the range of 500–750 nm. Although the average grain size for the film is close to 300 nm, the distribution follows a broad range. As shown in the SEM image in Figure e, there are many large grains in the film, and several smaller crystallites have formed in between the large grains to fill up the intergranular spaces. Because the (70:30) ratio film has shown a uniform porous morphology with a large surface area and higher surface roughness (as explained in SEM and AFM sections), it provides more flexibility for the expansion of perovskite crystals during conversion.[30] The results of diffraction data and microscopic analysis indicate that the addition of Pb(Ac)2 in PbI2 (to some extent) provides better control over morphology and better conversion. From energy-dispersive X-ray spectrometry (EDXS) measurements of samples (Figure S1), we found that a further increase in the acetate vol % leads to iodide deficiency in the final MAPbI3 film. For the 60:40 film, we noted that the Pb/I atomic wt % ratio was around 1:2. This iodine deficiency in the perovskite film can lead to surface vacancies that act as mid-band trap states and reduce the charge collection efficiency of solar cells.[42]

Photophysical Properties of the Perovskite

Figure a presents the UV–vis absorption spectra of MAPbI3 prepared from controlled PbI2 and lead acetate mixed films. Clearly, all the perovskite films show absorption in almost the entire visible region with an absorption onset at around 770 nm. MAPbI3 from the controlled PbI2 film exhibits a very low absorption that might be due to incomplete conversion of PbI2 into the perovskite. High crystallinity and strong orientation of crystallites in one plane (001) may hinder the intercalation of MAI molecules into the PbI2 structure. From the absorption graph, it is evident that absorption intensity increases with an increase in the Pb(Ac)2 concentration in the PbI2/DMF solution. The MAPbI3 film from the (70:30) ratio film shows the strongest absorption in the entire visible spectrum and maybe accounted because of the complete transformation of PbI2 into MAPbI3 and fully covered, tightly packed crystallites of the perovskite.[43,44] For the lead acetate concentration ratio above 30 volume %, absorption in the lower wavelength region starts reducing. The photoluminescence (PL) emission spectra of the MAPbI3 films are shown in Figure b. Apparently, MAPbI3@(70:30) shows the strongest emission among all perovskite films. The perovskite film from bare PbI2 exhibits a poor surface morphology and is associated with a larger number of pinholes that lead to nonradiative charge recombination and shows low PL.[45,46] On the other hand, a pinhole-free and densely packed morphology (as shown in the SEM images, Figure e) of the perovskite@(70:30) minimizes the shunting paths and allows charges to recombine radiatively.[46] Low emission from MAPbI3 films prepared from the (60:40) film can also be justified with a comparatively smaller grain size and iodine deficit surface vacancies. Device result analysis is discussed in Figure S3 and Table S1 These enhanced physical properties of the perovskite from the PbI2/Pb(Ac)2 film shows significant improvement in the device performance, as shown in the Figure S3 and Table S1.

Figure 5

(a) UV–vis absorption and (b) PL emission spectra of the perovskite film prepared from bare PbI2 and PbI2/Pb(Ac)2 films.

From the discussions above, we identified that a 70:30 vol % ratio of PbI2 & Pb(Ac)2 gives the best suited structural and morphological characteristics of the Pb–precursor film and leads to complete conversion of the underlying layer into the perovskite with improved performance. We also noticed that this precursor film (70:30 ratio) leads to a much faster conversion into the perovskite, as compared to the pure PbI2 film. An in-detail investigation on the conversion rate and role of different crystal orientations in fast transformation has been elucidated in the following section.

Enhanced Perovskite Conversion Rate

Panel (a) in Figure depicts the absorption spectra of perovskite evolution from the controlled lead iodide (100:0 ratio) film. As indicated, the PbI2 film after 1 min of dipping time exhibits a very broad characteristic onset of the perovskite near 770 nm. The edge becomes steeper and absorption increases with an increase in the dipping time of the PbI2 film in the MAI/IPA solution. After 10–15 min of dipping time, absorption does not increase; however, we observed a drop in the absorption intensity. This indicates that the best possible conversion of PbI2 into the perovskite has occurred, and a further increase in dipping time might lead to deterioration of the already converted MAPbI3 film. Prolongation of the reaction time (dipping time) will not add to any further significant conversion of PbI2 into MAPbI3, despite it causing the back extraction of MAI from the as-converted perovskite film and lowering the absorption capabilities, as indicated in Figure a.[20] Also, for the 20 min dipped film, the absorption shoulder of MAPbI3 shows a shallow slope similar to the absorption graph after 1 min of dipping time.

Figure 6

(a) UV–vis absorption graph and (b) XRD patterns for the perovskite converted from the pure PbI2 film through varying the dipping time in the MAI/IPA solution. (c) XRD pattern and (d) UV–vis absorption graph for the perovskite conversion from the (70:30) precursor film with different dipping times and (e) remnant PbI2 content in the PbI2 film with respect to dipping time.

Notably, a rise in the baseline of absorption spectra, as shown in Figure a, was spotted after 10 min of dipping time, indicating scattering of the light because of increased roughness of the converted perovskite.[47] From Figure a, it appears that the film with 10 min soaking time shows the highest absorption, but the XRD pattern of the same (Figure b) exhibits a strong residual PbI2 peak implying the incomplete conversion. Even for the dipping time longer than 10 min, the diffraction patterns of MAPbI3 are associated with PbI2 peaks. This shows that either PbI2 is not fully converted into the perovskite even after 20 min of dipping or it may be the remnant PbI2 due to back extraction of MAI after prolonged dipping time. A similar conversion process was followed on the treated PbI2/Pb(Ac)2 (70:30) ratio film, and we found that this precursor film leads to complete conversion with much faster conversion time. From Figure d, the intensity in absorption spectra increases significantly with the dipping time, while a very small increase was noticed between 5 and 7 min, signifying the best possible conversion. At the same time, looking into the XRD pattern of the 5 min film (Figure c), almost all the PbI2 has been transformed into the perovskite. Further lengthening of reaction time to 7 min leads to the complete conversion with no trace of unreacted PbI2 in the film. Dipping time of more than 7 min might cause leaching of MAI from the MAPbI3 and lead to PbI2 residues, as indicated in the diffraction pattern of 10 min dipping time. Moreover, no significant increase in the baseline of absorption spectra was observed resembling the absence of light scattering effects and smoother surface morphology of the as-converted MAPbI3 film.[47]Figure e shows the remnant PbI2 content for both (perovskite from pure PbI2 and 70:30 mix ratio) samples calculated using the integration of X-ray reflections for lead iodide and perovskite by the following formula[48,49], where, IPbI and Iperovskite are the integrals of the respective diffraction peaks. As inferred from Figure e, the perovskite film from pure PbI2 shows around 54% residual content after 5 min of dipping time, whereas, only 11% remnant PbI2 was noticed for the perovskite from the 70:30 mix ratio film. For the controlled case, the best possible conversion still exhibits around 43% of unreacted lead iodide after 15 min of reaction time. Further lengthening of reaction time increases PbI2 content to 60%. In the counterpart, the perovskite from the 70:30 mix ratio, PbI2 content almost touched zero within 7 min of reaction time.

The above observations indicate that for the pure PbI2 film, even 20 min of reaction time was not good enough for complete conversion and causes MAI back extraction, whereas, for the (70:30) ratio Pb–precursor film, the conversion process is way faster and results in full conversion within 5–7 min. Although the advancement in the lead iodide film morphology and improved surface features contribute to the acceleration of the conversion process (as discussed in an earlier section), the first and foremost reason identified for this quicker conversion process lies in the crystallographic orientation of the precursor films. The pure PbI2 film shows strong crystal arrangements in only (001) orientations leading to high crystallinity. On the other hand, its mixed counterpart, the (70:30) ratio precursor film, exhibits multiple crystal orientations in (h0l), (hk0), and (00l) planes that facilitates an easier way for MAI molecules to intercalate much deeper in the lead iodide film. As discussed in the XRD explanation, as shown in Figure a, the 70:30 ratio film crystallizes with multiple crystal orientations identified as (104), (101), and (110) along with the standard (003) arrangement. The planar intersection angle between signature (003)/(001) and (104), (101), and (110) planes is found out to be 52.9, 79.2, and 90°, as shown in Figure a. The arrangement of these planes as per the planar intersection angle is shown in Figure b. On the other hand, the pure PbI2 film has a strong crystal orientation along the (001) plane along with a few crystals oriented in parallel planes (002) and (003), as shown in Figure c.

Figure 7

Schematic for plane orientations (a) plane intersection angles in the (70:30) film, (b) illustration for the orientation as per planer intersection angles, and (c) illustration of parallel planes in the pure PbI2 case.

Schematic illustrations for pure PbI2 and 70:30 ratio films are made on the substrate as per their plane intersection angle and represented in Figure a,b, respectively. It is comprehensive from illustrations that pure PbI2 film has limited scope for MAI penetration because of the tight arrangement of PbI2 crystals in one particular orientation, whereas, the counterpart, due to randomness in the orientation, facilitates a greater number of attacking sites as well as more space for MAI molecules to intercalate toward the substrate. This randomness and amorphous nature of the 70:30 ratio film attribute to a faster and full conversion of the perovskite. Another reason for this speedy conversion process can be the morphological improvement in the precursor layer with an improved surface area and cell volume. Diffraction patterns of the 70:30 ratio film, as shown in Figure a, confirm the presence of lead acetate in the film. This has also been confirmed by measuring the Fourier transform infrared (FTIR) spectra of pure PbI2 and 70:30 ratio film, as shown in Figure S2. During the conversion process, the reaction of MAI with lead acetate forms a very volatile byproduct (methyl acetate) and leads to a quicker crystallization process.

Figure 8

Schematic illustration for MAI attack (a) in the case of pure PbI2 less number of available sites and (b) in the (70:30) film, randomization in orientation lead to easy intercalation.

Thus, the composition change in the lead iodide solution led to enhanced morphology and random orientation of lead iodide paving the way to quickly fabricate a high-performance perovskite film.

Conclusions

We developed a facile Pb–precursor composition of PbI2/Pb(Ac)2·3H2O with the lead acetate concentration varying from 10 to 40 vol % that can provide a complete and very quick conversion of an underlying PbI2 film into a MAPbI3 perovskite using a sequential deposition approach. The advancement in the method reported herein for the Pb–precursor film results in improved device performance and facilitates a quicker and complete conversion of PbI2 into the perovskite. The new precursor composition utilizes PbI2/Pb(Ac)2·3H2O volume concentrations in 70:30 vol % ratio and results in the PbI2 film with multiple crystal planes in (104), (003), (101), and (110) orientations. This new precursor composition also facilitates a porous Pb–precursor film with high surface area and roughness. Altogether, this new precursor composition enables more active sites and space for the MAI reaction and volume expansion contributing to the PbI2-free perovskite film with enhanced photophysical properties. The use of the 70:30 volume ratio Pb–I2/Ac2 film results in a much faster conversion of PbI2 into MAPbI3 and took only 5–7 min of reaction time for full conversion as compared to the conventional case that shows significant residual PbI2 traces even after 20 min of dipping time. Moreover, we have schematically demonstrated the underlying mechanism for the role of crystal orientation in efficient and fast conversion of PbI2 into the MAPbI3 perovskite. The methodology developed herein put forward a novel approach in a sequential deposition process and makes it more efficient for obtaining high-performance perovskite films.

Experimental Section

Materials

Lead iodide (PbI2, Sigma-Aldrich, 99.995%), lead acetate tri-hydrate Pb(Ac)2·3H2O, (Alfa Aesar, 99%), MAI (Great Cell Solar), DMF (anhydrous Sigma-Aldrich), and IPA (anhydrous, Alfa Aesar). All the received chemicals were used as it is without further purification.

Perovskite Film Fabrication

Substrates were cleaned using a Hellmanex soap solution (2%), followed by rinsing by DI water, acetone, and IPA. Then, substrates were sonicated in acetone and IPA each for 15 min, followed by nitrogen flush drying. Pure PbI2 solution was prepared by dissolving the PbI2 powder in DMF (0.7 M). Lead acetate trihydrate solution of similar molarity in DMF was prepared in another vial. Both the solution were mixed appropriately by volume in order to make (90:10), (80:20), (70:30), and (60:40) ratio mixtures. These solutions were spin-coated on cleaned substrates at 3000 rpm for 30 s followed by annealing to make the PbI2 film. As-prepared lead iodide precursor films were converted into a perovskite by spin-coating 20 mg/mL of the MAI/IPA solution at 2000 rpm for 60 s, followed by annealing at 100 °C for 30 min.

Perovskite Conversion Rate Test

The solution (20 mg/mL) was prepared in a beaker. The substrate coated with lead halide films were dipped into the solution for the desired experimental time followed by quick rinsing with IPA. Films were then annealed at 100 °C for 30 min.

Characterizations

The crystal structure and plane orientation were characterized with a Rigaku ultima IV X-ray diffractometer with a Cu Kα radiation source at a scan rate of 5°/min and step size of 0.02°. Morphological studies were performed by a Zeiss EVO 18 scanning electron microscope. Surface studies were performed by a Bruker—Dimension Icon atomic force microscopy instrument. Absorption and emission spectroscopies were performed by PerkinElmer LAMBDA 1050 and Shimadzu RF6000 spectrophotometers.