Hybrid Perovskite, CH3NH3PbI3, for Solar Applications: An Experimental and Theoretical Analysis of Substitution in A and B Sites

Sánchez-Coronilla, Antonio; Navas, Javier; Gallardo, Juan Jesús; Martín, Elisa I.; De los Santos, Desireé; Hernández, Norge C.; Alcántara, Rodrigo; Toledo, José Hidalgo; Fernández-Lorenzo, Concha

doi:https://doi.org/10.1155/2017/9768918

Journal of Nanomaterials

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2017 | Article ID 9768918 | https://doi.org/10.1155/2017/9768918

Hybrid Perovskite, CH₃NH₃PbI₃, for Solar Applications: An Experimental and Theoretical Analysis of Substitution in A and B Sites

Antonio Sánchez-Coronilla,¹Javier Navas,²Juan Jesús Gallardo,²Elisa I. Martín,³Desireé De los Santos,²Norge C. Hernández,⁴Rodrigo Alcántara,²José Hidalgo Toledo,¹and Concha Fernández-Lorenzo²

Academic Editor: Stefano Bellucci

Received20 Dec 2016

Accepted16 Jan 2017

Published09 Feb 2017

Abstract

The effect of the incorporation of into the CH₃ sites of the tetragonal perovskite CH₃NH₃PbI₃ is analysed. Also, how it affects the introduction of Cd²⁺ cations into Pb²⁺ sites for a perovskite with 25 at.% of is addressed. The incorporation of into perovskite leads to a dramatic loss of crystallinity and to the presence of other phases. Moreover, the NH₄PbI₃ was not found. The less formation of perovskite when is incorporated is due to geometrical factors and not changes in the chemical state bonding of the ions. Also, the samples where perovskite is formed show similar band gap values. A slight increase is observed for samples with and 0.75. For the sample with , a drastic increase of the band gap is obtained. Periodic-DFT calculations agree with the experimental structural tendency when is incorporated and the density of states analysis confirmed the experimental band gap. The perovskite with 25 at.% of was selected for studying the effect of the concentration of Cd on the structural and electronic properties. The theoretical band gap values decreased with the Cd concentration where the narrowing of Cd s-states in the conduction band plays an important role.

1. Introduction

Organic-inorganic hybrid perovskite of the composition ABX₃ has recently attracted strong research interest because of its photovoltaic properties [1]. This simple structure consists of a network of corner-sharing BX₆ octahedra, where the B atom is a metal cation (usually Sn²⁺ or Pb²⁺) and X is a monovalent anion, such as Cl⁻, Br⁻, or I⁻. Also, the A cation is selected to neutralize the total charge and it can even be a molecule. In this case, the organic cation must fit into a rigid and relatively small cuboctahedral hole formed by the 12 nearest X atoms, thus, limiting the dimension of the selected molecule. The most common A cation is [CH₃NH₃]⁺ (MA). In turn, perovskite solar cells have achieved a power conversion efficiency record higher than 20% [2]. The different components of the perovskite solar cells affect their conversion efficiency. Also, the effect of using mixtures of ions in the three positions of the perovskite (A, B, and X) has been analysed previously [3, 4]. For example, the substitution of I^- with Cl^- or Br^- has been widely studied; and doping with inorganic cations such as Sn²⁺, Sr²⁺, or Ca²⁺ in Pb²⁺ sites has been analysed to understand its effect on optical and electronic properties [5–8].

Thus, this article shows the synthesis of organic-inorganic hybrid perovskite starting from MAPbI₃ and substituting the CH₃ cation by . Perovskites with the formula (A)_x(MA)_1−xPbI₃ (; 0.50; 0.75; 1.00) were synthesized. These perovskites were characterized using X-ray fluorescence and CHNS technique for the elemental analysis, X-ray diffraction to determine the presence of the perovskite structure, X-ray photoelectron spectroscopy in order to analyse the chemical state bonding of the elements in the samples, and UV-vis spectroscopy for determining the optical properties and the band gap energy. Consequently, the presence of affected the formation of the perovskite structure because distortions were produced in the structure. In turn, periodic density functional theory (DFT-periodic) calculations were performed to rationalize the experimental information for (A)_x(MA)_1−xPbI₃ (; 0.50; 0.75; 1.00). The theoretical results indicate that the (A)_0.25(MA)_0.75PbI₃ structure has an appropriate percentage of A⁺ and MA⁺ ions for being effective for its application in solar cells and was selected for the introduction of Cd²⁺ cations into Pb²⁺ sites. Thus, the increased concentration of Cd on the structural and electronic properties of (A)_0.25(MA)_0.75Pb_1−xCd_xI₃ with , 0.25, 0.50, 0.75, and 1.00 was studied. In this regard, the theoretical results show that the theoretical band gap values decreased with the Cd concentration. In this sense, the narrowing of Cd s-states in the conduction band plays an important role.

2. Materials and Methods

2.1. Experimental Section

The synthesis of the perovskite phases was performed by the reaction of commercial PbI₂ and nominal amounts of CH₃NH₃I and commercial NH₄I. First, CH₃NH₃I was synthesized using the following procedure [6, 9, 10]: HI (57 wt% in water, 10 mL, 0.076 mol) and CH₃NH₂ (33 wt% in ethanol, 11.33 mL, 0.091 mol) were stirred in an ice bath for 2 h. Next, the mixture was evaporated at 50°C for 1 h, and CH₃NH₃I was obtained. Then, the solid was cleaned three times using Et₂O (purity ≥ 99.8%). On the other hand, for the synthesis of the perovskite phase PbI₂ (purity 99%, 1.153 g) was mixed with the corresponding stoichiometric amount of CH₃NH₃I and NH₄I to obtain nominal concentrations ()/[() + (CH₃)] of 0, 25, 50, 75, and 100 at.% in γ-butyrolactone (purity ≥ 99%, 2 mL) at 60°C for 18 h.

On the other hand, the elemental analysis was performed using two instrumental techniques. X-ray fluorescence (XRF) was performed to determine the amount of Pb and I using a sequential XRF spectrometer supplied by Bruker®, model Pioneer S4. The CHNS technique was used to obtain the amount of C, H, and N, using an elemental analyser supplied by Leco®, model CHNS932. To determine the crystalline phases in the samples, X-ray diffraction (XRD) was used. The patterns were recorded using a D8 Discover diffractometer supplied by Bruker with Cu-Kα radiation. The scan conditions were from 10 to 70° in 2θ with a resolution of 0.02°, 40 kV, and 40 mA. Moreover, X-ray photoelectron spectroscopy (XPS) was used to study the chemical bonding states and the chemical compositions of the samples. The spectra were obtained by using a Kratos® Axis UltraDLD spectrometer, with monochromated Al-Kα radiation (1486.6 eV) and a 20 eV pass energy. The binding energy scale was given with an accuracy of 0.01 eV. Finally, the optical band gap () values were determined from diffuse reflectance UV-Vis (DR-UV-Vis) measurements. The spectra were collected by using a system assembled in our laboratory composed of an integrating sphere supplied by Spectra Tech®, a USB2000+ spectrometer supplied by Ocean Optics®, and a Xe lamp, model ASB-XE-175, supplied by Spectral Products®, as the illumination source.

2.2. Computational Framework

DFT-periodic calculations were performed using the Vienna Ab Initio Simulation Package (VASP) [11–14] with the projector-augmented wave (PAW) method [15, 16]. The number of plane waves in VASP was controlled by a cut-off energy, set in our calculations to to satisfactorily describe the system [11–16]. The electron exchange and correlation were treated within the generalized gradient approximation (GGA) [17]. In the case of GGA, Perdew-Burke-Ernzerhof (PBE) [17] functionals were used. Both the cell shape and atomic positions were optimized using a conjugate-gradient algorithm, where the iterative relaxation of atomic positions was stopped when the forces on the atoms were less than 0.01 eV/Å. Also, a Gaussian smearing with was applied.

Perovskite tetragonal phase was characterized by the I4/mcm space group [17]. The tag KSPACING determined the number of k-points in the mesh. A value of 0.4 for sampling the Brillouin zone for perovskite bulk was enough to obtain negligible changes in the optimized cell parameters and energy. The resulting cell parameters were and for the tetragonal phase, in agreement with the experimental values (, ) [17].

The (MA)₄Pb₄I₁₂ cell was optimized as a model for the tetragonal phase of perovskite. The A⁺ ion substitutes the MA⁺ ion and the most stable structures have been reported. Thus, the structures (A)(MA)₃Pb₄I₁₂, (A)₂(MA)₂Pb₄I₁₂, (A)₃(MA)₁Pb₄I₁₂, and A₄Pb₄I₁₂ were optimized. The incorporation of each proportion of Cd into the (A)(MA)₃Pb₄I₁₂ structure was performed in substitutional positions. The Cd atom substitutes the Pb atom in the perovskite structure taking into account the equivalent sites. Close attention was paid to the orientational disorder of MA within the tetragonal structure, reporting the most stable configuration for each doped structure, as reported elsewhere [6, 18, 19].

The density of states (DOS) and projected density of states (PDOS) for the relaxed structures were obtained using the tetrahedron method with Blöchl et al. corrections [19] and a KSPACING of 0.3. For all the systems, DFT + U [20] calculations were performed to take into account the on-site Coulomb interaction. A U value of 5 eV was used for d orbitals [21]. Electron localization function (ELF) [22–26] images and structure images were obtained using the Vaspview software and ChemCraft 1.6 [27], respectively.

3. Results and Discussion

3.1. Elemental Analysis

The elemental composition of the samples was determined using X-ray fluorescence spectroscopy (XRF) to measure I and Pb and the CHNS technique to determine C, H, and N. The results of the weight percentages obtained are shown in Table 1, which also shows the atomic percentage calculated for each element. It is possible to observe only small deviations with regard to the nominal composition (see Experimental Section). To enhance understanding, in the discussion of the results the samples will be identified by the nominal value of x.

3.2. X-Ray Diffraction

Figure 1 shows the XRD patterns for the samples synthesized. The diffractogram for the sample with , without , shows the typical peaks of the reflections of the planes of a tetragonal structure (I4/mcm space group), which has been reported for MAPbI₃ perovskite [28–32]. In addition, the typical peaks for this perovskite structure show the most intense reflections at approximately 14°, 28°, and 31°, these being two double peaks. A peak at about 23.5° is assigned to the reflection plane (211) of the tetragonal perovskite structure mentioned and is the main difference with the cubic perovskite structure (Pm3m space group) that is also found in this kind of hybrid perovskites [28]. Thus, the presence of perovskite cubic phase is negligible in our samples. Also, Figure 1 shows that the typical tetragonal structure is maintained in the patterns for the samples with up to 0.75, but also in these cases a crystallinity loss is observed in the XRD patterns. Not only is the presence of tetragonal perovskite observed in the samples where some amount of ammonium iodide is added, but also other peaks are observed. So, the incorporation of ammonium groups into the tetragonal perovskite structure leads to a dramatic loss of the crystallinity of the perovskite phase and to the presence of other phases.

The formation of the perovskite structure is estimated on the basis of two factors: Goldschmidt’s tolerance factor and the “octahedral factor” . Goldschmidt’s factor is commonly used for the analysis of the formation of perovskite structures and is estimated for the ABX₃ system according to , where , , and are the effective ionic radii of A, X, and B ions, that is [CH₃NH₃]⁺ or , I^-, and Pb²⁺ in our case [33]. The “octahedral factor” is calculated according to [34, 35]. Thus, according to Goldschmidt’s factor, the perovskite structure is formed for values in the 0.75–1.00 range. If , the ideal cubic perovskite structure would be formed, while for values below 1 octahedral distortions are obtained [35]. For halide-based perovskites, the usual values are in the 0.813–1.107 range, while the “octahedral factor” is greater than 0.442 in these cases [35]. These two factors were used to analyse the structures of MAPbI₃ and APbI₃. The effective ionic radius values of , I^-, and Pb²⁺ were taken from the literature [36, 37]. The values are 1.5, 2.2, and 1.19 Å for , I^-, and Pb²⁺, respectively. In turn, the effective ionic radius of [CH₃NH₃]⁺ was calculated according to a rigid sphere model with free rotation of its centre of mass, as reported in the literature [38]. The value obtained is 1.74 Å. Thus, the values for and are 0.822 and 0.541 for MAPbI₃ and 0.772 and 0.541 for APbI₃. Thus, the MAPbI₃ system fulfils both conditions, which is in accordance with the formation of tetragonal perovskite structure observed in the XRD patterns for this sample. On the other hand, the APbI₃ system shows a suitable “octahedral factor” because of the fact that these ions are the same as that for MAPbI₃. But, Goldschmidt’s factor for APbI₃ is lower than the usual limit value for halide-based perovskites . This is in line with the results obtained using XRD, which show how the formation of perovskite structure decreases when the proportion of ammonium ion in the samples increases.

3.3. X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectra for sample with were acquired to analyse the oxidation states and the bonding states of the elements in the samples. Figure 2(a) shows the general spectra of the sample with a perovskite structure and the basic assignment of the peaks found. Different zones in the spectra were recorded in detail for analysis. Figure 2(b) shows the Pb 4f spectra of the samples. The binding energy (BE) for the Pb 4f_7/2 was around 138.0 eV, which is coherent with results reported previously [39, 40]. Also, the spectra for Pb 4f show spin-orbit components well-separated. This separation was around 4.9 eV, which is typical for Pb²⁺. On the other hand, Figure 2(c) shows the I 3d spectrum. Also, the spectra show well-separated spin-orbit components with a separation of about 11.5 eV, which is coherent with the results reported for I^- [39]. Also, Figure 2(d) shows the N 1s spectrum. The signal for N 1s appears at about 402 eV, and the signal shows low signal/noise ratio probably due to the lower amount of N in the sample with .

(a)

(b)

(c)

(d)

Therefore, the results from XPS show the typical spectra of Pb 4f, I 3d, and N 1s for tetragonal perovskite.

3.4. UV-vis Spectroscopy

The optical band gap for the samples synthesized was estimated from UV-vis spectra in diffuse reflectance mode. Figure 3 shows UV-vis spectra obtained for the samples synthesized. From these spectra, the presence of tetragonal phase is observed up to values of of 0.75, which is coherent with XRD results. The sample with is clearly different, probably because of the fact that tetragonal perovskite is not obtained. This sample has an absorption band at shorter wavelengths than the sample with , that is, MAPbI₃. The spectrum for the sample with can be compared with the spectrum obtained for commercial NH₄I used as reagent in the synthesis. It is possible to observe the presence of NH₄I without reacting. So, this is coherent with XRD results.

The band gap of the samples can be estimated from the spectra registered. The Kubelka-Munk formulism from the diffuse reflectance measurements is calculated according to [41], where is the percentage of reflected light and and are the absorption and the scattering coefficients. The Tauc plot relates the band gap and the incident photon energy, and for direct transitions it is given by [42, 43], where is the incident photon energy, is a constant depending on the transition probability, and is the band gap. So, the band gap values were estimated and are shown in Table 2. It is possible to observe that the samples where tetragonal perovskite is formed show similar band gap values. A slight increase is observed for samples with and 0.75. For the sample with , a drastic increase of the band gap is obtained.

3.5. Theoretical Analysis of (A)_x(MA)_1−xPbI₃

From the results of the experimental characterization, theoretical calculations were performed to gain a deeper understanding of the effect of varying the A cation in the ABX₃ structure of MAPbI₃ perovskites. Thus, a study was performed in the tetragonal structure of (A)_x(MA)_1−xPbI₃ with , 0.25, 0.50, 0.75, and 1.00.

Figure 4 shows the local geometry for the most stable configuration of the optimized perovskite structures when MA⁺ were substituted by A⁺. For the purpose of comparison, Figure 4 also includes the MAPbI₃ structure in tetragonal phase. As it can be seen in Figure 4 the substitution of MA⁺ by A⁺ ions caused distortions in the structure. The most striking changes are produced for (A)_0.75(MA)_0.25PbI₃ and APbI₃ structures. As an example, the structural distortion is clearly reflected in the variation in the Pb-I-Pb angles of the central plane view shown in Figure 4 when compared with the MAPbI₃ structure (Table 3). As shown in Table 3 the α, β, γ, and δ angles decrease when A⁺ is included in the structure and substitutes the MA⁺. The (A)_0.25(MA)_0.75PbI₃ structure is the one that remains the most similar to the MAPbI₃ perovskite followed by (A)_0.5(MA)_0.5PbI₃ (Table 3). Thus, it is understandable that (A)_0.25(MA)_0.75PbI₃ and (A)_0.50(MA)_0.50PbI₃ were formed experimentally maintaining the tetragonal phase, while the (A)_0.75(MA)_0.25PbI₃ and APbI₃ structures present variation of c.a. 9–15° in the angles when compared to MAPbI₃ perovskite (Table 3), which means a structural tension that makes its experimental formation difficult. Therefore our results are concordant with those obtained experimentally.

The changes in the band gap for the (A)_x(MA)_1−xPbI₃ (, 0.25, 0.50, 0.75, and 1.00) structures were analysed by means of DOS (Figure 5). As can be seen in Figure 5 the bad gap increases with the proportion of A⁺ ions when compared with MAPbI₃ perovskite. In turn, the gap for (A)_0.25(MA)_0.75PbI₃ structure is slightly greater than the gap for MAPbI₃ perovskite. Thus, both the geometrical study and the DOS analysis indicate that (A)_0.25(MA)_0.75PbI₃ structure has an appropriate percentage of A⁺ and MA⁺ ions for being effective for its application in solar cells.

3.6. Theoretical Analysis of A_0.25MA_0.75Pb_1−xCd_xI₃

From the results expounded above, the A_0.25MA_0.75PbI₃ perovskite was selected for exploring theoretically the changes in the structural and electronic properties of the material when the Pb is substituted by Cd. This metal was selected because it is known to decrease the band gap in MAPbI₃ perovskites [6, 38] when it is used in B position as an alternative to Pb. Thus, an analysis was performed into the effect of increasing the concentration of Cd on the structural and electronic properties of (A)_0.25(MA)_0.75Pb_1−xCd_xI₃ with , 0.25, 0.50, 0.75, and 1.00.

The theoretical results show that the incorporation of Cd²⁺ ions produced changes when compared with the (A)_0.25(MA)_0.75PbI₃ perovskite. The local geometry for the most stable configuration of the optimized structures is shown in Figure 6. As an example, the Pb-I-Cd and Cd-I-Cd angles of the cut corresponding with the front face of the tetragonal structures as indicated in Figure 6 are shown in Table 4. As Table 4 shows, η and μ angles increase in accordance with the concentration of Cd, reaching their highest values in (A)_0.25(MA)_0.75CdI₃. The χ angle hardly changes when compared with the same angle for A_0.25MA_0.75PbI₃ while σ decreases when the concentration of Cd increases. In addition, it is also observed that, generally, the Cd-I distances are shorter compared with the Pb-I ones (c.a. 5–7%). Thus, the ELF plot in 2D corresponding to the Cd-I and Pb-I interactions for the (A)_0.25(MA)_0.75Pb_0.75Cd_0.25I₃ geometry is represented in Figure 6 as an example of how the contour plot for Pb is different from that of Cd. Figure 6 shows that the outlines of the ELF for I and Pb almost overlap by means of slight electron localization (sky blue colour) that provides more stability to the Pb-I interaction, while the introduction of Cd produces directionality in the outlines of the electron localization of I towards Cd, which explains the shorter Cd-I distances. These results lead us to believe that the structural tensions in the theoretical structures (A)_0.25(MA)_0.75Pb_0.25Cd_0.75I₃ and (A)_0.25(MA)_0.75CdI₃ are greater than those in the structures with low concentrations of Cd²⁺ ions.

The electronic structure in terms of DOS associated with (A)_0.25(MA)_0.75Pb_1−xCd_xI₃ (, 0.25, 0.50, 0.75, and 1.00) structures is shown in Figure 7. The DOS analysis of Figure 7 shows the band gap decreases with the concentration of Cd. For clarity purposes in Figure 7 dotted lines showing the narrowing of the band gap between the valence band (VB) and the conduction band (CB) upon increasing the concentration of Cd have been included. Previous studies with MAPbI₃ perovskites have shown that Cd d-states contribute to the edge of the valence band while the s-states of Cd contribute to the origin of the conduction band [6, 38]. Thus, the contribution of Cd s-states is also included in Figure 7 and it is shown that the contribution of the s-states of the Cd to the origin of the CB plays a fundamental role in the decrease in the band gap, as reported elsewhere [38]. In addition, our theoretical results let us to conclude that structures with a lower proportion of Cd, such as (A)_0.25(MA)_0.75Pb_0.75Cd_0.25I₃ and (A)_0.25(MA)_0.75Pb_0.50Cd_0.50I₃, with lesser structural tension than those with higher proportion would be of interest for being used in photovoltaic devices.

(a)

(b)

4. Conclusions

In this work, the synthesis of (A)_x(MA)_1−xPbI₃ (; 0.25; 0.50; 0.75; 1.00) tetragonal perovskite was performed. From the experimental characterization, the presence of perovskite cubic phase is negligible in our samples. Also, a crystallinity loss in the samples when cations were incorporated was observed, and the APbI₃ perovskite was not found. So, the incorporation of ammonium groups into the tetragonal perovskite structure lead to a dramatic loss of the crystallinity of the perovskite phase and to the presence of other phases. Also, Goldschmidt’s factor for APbI₃ (t = 0.772) is lower than the usual limit value for halide-based perovskites (t = 0.813). This is in line with the results obtained using XRD, which show how the formation of perovskite structure decreases when the proportion of ammonium ion in the samples increases. On the other hand, the results from XPS show the typical spectra of Pb 4f, I 3d, and N 1s for tetragonal perovskite. So, the less formation of perovskite phase when is incorporated is due to geometrical factors and not from changes in the chemical state bonding of the ions in the structure. Also, UV-vis spectra are coherent with XRD results. The spectra for the sample with are similar to that of commercial NH₄I, so APbI₃ was not formed. In turn, the band gap energy was estimated. The samples where tetragonal perovskite is formed show similar band gap values. A slight increase is observed for samples with and 0.75. For the sample with , a drastic increase of the band gap is obtained.

DFT calculations show that the tendency followed by the band gap values calculated by means of DOS for A_x(MA)_1−xPbI₃ with , 0.25, 0.50, 0.75, and 1.00 tetragonal perovskite was in agreement with that obtained experimentally. In this sense, the DOS and structural analysis indicate that (A)_0.25(MA)_0.75PbI₃ structure has an appropriate percentage of A⁺ and MA⁺ ions for being effective for its application in solar cells. (A)_0.25(MA)_0.75PbI₃ was selected for the substitution of Pb by Cd and structural tensions were observed in the structures with high concentrations of Cd²⁺ ions. The band gap decreases when the concentration of Cd increases. The projected density of states focused on the Cd s-states shows the important role played by s-states of Cd in narrowing the band gap between the VB and CB. Finally, from the DOS and structural analysis it can be concluded that the structures with the lowest Cd proportion with lesser structural tensions, such as (A)_0.25(MA)_0.75Pb_0.75Cd_0.25I₃ and (A)_0.25(MA)_0.75Pb_0.50Cd_0.50I₃, would be of interest for their application in solar cells.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank the Ministerio de Economia y Competitividad (MINECO) of the Spanish Government for funding under Grant no. ENE2014-58085-R. Calculations were made through Centro Informático Científico de Andalucía (CICA) (Spain). Antonio Sánchez-Coronilla thanks VPPI-US for the financial support.

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Copyright © 2017 Antonio Sánchez-Coronilla et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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