Synthesis and optimization of properties of thin films of FAx(MA1-X)PbI3 grown by spin coating with perovskite structure to be used as active layer in hybrid solar cells

1. Introduction

Despite the few years of development of Perovskite based solar cells, huge progress has been made [1-5], achieving in a short time efficiencies greater than 22% [6]. The hybrid organic/inorganic perovskite materials have a generic structure ABX₃, where A is a monovalent cation (usually methylammonium (MA) CH₃NH₃+, formamidinium (FA) CH₃(NH₂)₂ ⁺), B is a divalent metal (Pb²⁺ or Sn²⁺) and an anion X (Cl. ; Br. ; I. ). Perovskite thin films can be processed using various techniques including spin coating [7], dip coating [8] and evaporation in vacuum ambient [9] making them one of the most versatile photovoltaic (PV) technologies.

High Perovskite solar cells performances have been attributed to exceptional material properties and a tunable band gap from 1.1 to 2.3 eV [10]. However, there are concerns with respect to the degradation of the ABX₃ compounds upon contact with moisture as well as light-induced trap-state formation [11]. In particular, the compound MAPbI3 (MAPI) tends to degrade when it is exposed to a humid environment and the compound FAPbI3 (FAPI) is structurally unstable at room temperature and grows in a non-photoactive phase (hexagonal d-FAPbI₃); however, this compound recrystallizes in the ideal phase α-FAPbI₃ (photoactive) at high temperatures [12 ,13].

This work reports results related to the effect that the incorporation of formamidinium in the structure of thin films of CH₃NH₃PbI₃ (MAPbI₃) deposited by spin coating has on its stability as well as on the morphological, structural and optical properties.

2. Methodology

Cs_xFA_1-xPbI₃ thin films were deposited in one step by spin-coating at 4000 rpm, using a solution 1M with composition MA_xFA_1-xPbI₃ ( 0<x<1 ) in γ- Butyrolactone (GBL), followed by antisolvent treatment consisting of the addition of a second solvent maintaining the sample in rotation and then annealed for 10 minutes at 110⁰ C. The antisolvent treatment was carried out using toluene, dichloromethane and ethyl acetate as second solvent, which were added during time intervals that varied between 5 and 40 seconds.

The solution used for the synthesis of MA_xFA_1-xPbI₃ films was prepared mixing lead iodide (PbI₂) with a mixture of metilamonium iodide (MAI) and formamidinium iodide (FAI). The PbI₂ used are commercial reagent (Sigma Aldrich) while the FAI and MAI precursors were synthesized in our laboratory following the procedure described in Ref [14].

The MA_xFA_1-xPbI₃ samples were characterized by means of diffuse reflectance measurements performed using a Varian Cary 5000 spectrophotometer, as well as by X-ray diffraction (XRD) measurements performed with a Philips X’Pert Pro PANalytical diffractometer, using the radiation Cu-Kα (1.540598 Å), an acceleration voltage of 40 KV and a current 40 mA; the atomic force microscopy (AFM) images were done with a PSI Auto- probe CP microscope in contact mode and the film thickness was determined using a Veeco Dektak 150 surface profiler.

3. Results and discussion

3.1 Identification of the phases present in the FAxMA1-xPbI3 films

The phases present in thin FA_xMA_1-xPbI₃ films prepared with different FA/MA ratios (0≤x≤1) and subjected to antisolvent treatment, were identified by means of XRD measurements. In Figure 1 are shown XRD spectra of FA_xMA_1-xPbI₃ films prepared varying the FA/MA ratio from x=0 to x=1.0. It is observed MAPbI₃ samples (x=0) exhibit diffraction peaks at 2 θ=14.14°, 20.02°, 24.51, 28.47° 31.89°, 33.7°, 40.61° and 43.15° corresponding to the reflection planes (110), (112), (211), (202), (220), (310), (224) and (330) that have been assigned to the black phase of perovskite with tetragonal crystalline structure (space group I4/mcm, a=8,896 Å, c=12,698 Å) [15,16]. The samples prepared with composition corresponding to 0.2≤x≤0.6 present an additional reflection at 2 θ=13.9⁰ which has been assigned to the α-FAPbI₃ phase [17], indicating that this type of samples grows with a mixture of the phases MAPbI₃ and α-FAPbI₃. It is also observed that samples prepared with composition corresponding to x≥0.8 present new reflections that have been assigned to the PbI₂ phase and to the yellow phase d-FAPbI₃ [18]. Theinset of figure 1 are shown scans of the diffractograms around the peak at 2 θ=14.14°.

3.2 Study of degradation of FAxMA1-xPbI3 films

The effect that the incorporation of FA on the structure of MAPbI3 films has on the stability of the FA_xMA_{1- x}PbI₃ films, was evaluated through measurements of XRD made after its deposition and after several days of being exposed to the environment. In figure 2 are displayed XRD spectra of typical MAPI, FAPI, and FA_0.4MA_0.6PbI₃ films, measured after being exposed to the environment for several days. A sweep was made between 2 θ=10⁰ and 2 θ=15⁰ considering that peaks associated with secondary phases appear in this region, as a result of the degradation of the samples.

Figure 2 illustrates that in samples of MAPI, the peak at 2θ=12.6⁰ which is characteristic of the PbI₂ phase, appears after 15 days; in samples of FAPI two peaks in 2 θ=12.6 and 2 θ=11.7 associated to the PbI₂ and d- FAPbI₃ phases appears a few hours after the deposition. These results indicate that both MAPI and FAPI samples are degraded when exposed to the environment, with FAPI films being much more unstable than those of MAPI. On the other hand, it is observed that in samples of FA_0.4MA_0.6 PbI₃, the peak associated with the PbI₂ phase appears after 6 days of exposure to the environment, indicating that this type of sample is more stable than those of FAPI but less stable than the MAPI samples. The better stability against moisture exposure of MAPI is related to the fact that MAPI has a better tolerance factor (TF=9.0) than FAPI (TF=10) [19], associated with the greater ionic radius of the FA.

The main degradation mechanism of MAPI and FAPI compounds when were exposed to moisture and oxygen from the environment, has been attributed to a catalytic reduction of these compounds in PbI₂, CH₃NH₂ (or CH₃(NH)₂) and HI [20].

In contrast to MAPbI₃, which, upon exposure to humidity, decomposes to PbI₂ and volatile organic phases, FA-based perovskite exhibits a change in color from dark to yellow due to an undesirable phase transition from the α to d-phase [21]. This transition is highly undesirable for photovoltaic applications, since the non-perovskite d-phase has a significantly larger bandgap and inferior charge-transport properties.

3.3 Effect of incorporating the cation FA in the structure of MAPbI3 films on the microstructure of FA_xMA_1-xPbI₃ films

It has found the full width at half maximum (FWHM) value of FA_xMA_1-xPbI₃ films is affected by the amount of MA cations substituted by FA cations, indicating that this parameter affects its crystallinity. On the other hand, considering that the peak broadening is affected by crystallite size D and lattice strain Ɛ (relative change in size with respect to the size before experiencing an external force) induced by structural defects [22], these two parameters were determined by the X-ray line broadening method using the Williamson-Hall equations [23] given by:

Ecuación 1

where D is the crystallite size, λ is the wavelength of the CuKα radiation, k is a constant equal to 0.94, βhkl is the peak width at a half-maximum intensity and θhkl is the peak position.

Plotting the term βcosθ with respect to 4sinθ for the preferred orientation peaks, the strain and crystallite size can be obtained from the slope and y-intersect of the fitted line. The values of D and ε estimated from the UDM model are summarized in Table 1.

The results of table 1 show that an increase in the proportion of ions of MA that are replaced by ions of FA, gives rise to a slight increase of the crystallite size of the FA_xMA_1-xPbI₃ films; these results also show that the lower values of microstrain ε are obtained by replacing the ion MA with the ion FA in proportions between 40% and 60%, indicating that the substitution of MA by FA in the above-mentioned proportions results in the growth of FAMAPbI₃ samples with improved crystallinity.

3.4 Effect of incorporating the cation FA in the structure of MAPbI₃ films on the optical properties of FA_xMA_1-xPbI₃ films

The effect of the amount of FA incorporated in the structure of MAPbI₃ films on the optical gap of FA_xMA_1-xPbI₃ films was evaluated through diffuse reflectance measurements. For this, it used a method described by A. B. Morphy based on the Kubelka, Munk theory [24], which allows determining the absorption coefficient (α) and energy band gap (Eg) of materials thatscatter light, from measurements of diffuse reflectance [25]. Initially, the absorption coefficient is calculated from diffuse reflectance measurements and then, taking the values of α found and considering that for direct gap materials the relationship (F(R)hν)² = A (hν-Eg) is fulfilled, the value of Eg can be determined from the intercept with the hν axis that results from extrapolating the linear region of the curve (F(R)hν)² vs hν. In figure 3 are displayed curves of diffuse reflectance and (F(R)hν)² vs hν obtained for FA_xMA_{1- x}PbI₃ films, prepared varying the FA content between x=0 and x=1.0.

As shown in figure 3, an increase in the proportion FA:MA gives rise to an Eg decrease, from a value of 1.61 eV for the sample of composition MAPbI₃ until a value of 1.47 eV for the sample of composition FAPbI₃, indicating that the incorporation of FA in the structure of the FAPbI₃ leads to a change in the structure of bands, apparently induced by the expansion produced in the lattice of the FA_xMA_1-xPbI₃ as a consequence of the larger size of the ion FA compared with that of the ion MA. These results agree with those reported by other authors [26], who also observed a decrease in the value of Eg by increasing the proportion FA:MA.

It is well known that the presence of structural defects generates states of tails of bands that participate in photons absorption processes which can be evaluated through the calculation of the absorption coefficient near the band edge (αυ) that has an exponential dependence on photon energy and Urbach energy [26] given by:

Ecuación 2

Where E_u is the Urbach energy, Ei and α₀ are constant. Thus, a plot of lnα vs hν should be linear and Urbach energy can be obtained from the slope. In Table 2 are the values of the Urbach energy of FA_xMA_1-xPbI₃ films, prepared varying the FA:MA ratio.

Table 2 reveals that the value of Eu decreases by increasing the FA:MA ratio, indicating that an increase in the amount of FA incorporated in the structure of the compound MAPbI₃ yields an increase of the structural defects.

Recently Yang Li et al. made a publication [27] in which they emphasize systematically describing the methodologies for manufacturing Perovskite materials based on FA that have been more successful, such as additive engineering, intermolecular exchange, interfacial engineering in solution processing, and chemical vapor deposition and presented a detailed discussion of the relationship between fabrication methods, film properties, and device efficiency. In particular, this paper reported the band gap value of FAPbI₃ films prepared by different methods varies between 1.40 and 1.47 eV, which agrees with the one obtained in this work. In addition, we have found that the FA-perovskite has a narrower bandgap compared with pure MAPbI₃, a result that also agrees with that reported by Yang Li et al.

The main difference between the results obtained from this work and those reported in the paper by Yang Li et al., is about this report put special emphasis on the effect that the substitution of the MA cation with the FA cation causes on the properties of FA_x (MA_1-x) PbI₃ films synthesized by spin coating, while in the paper by Yang Li et al., it focuses on making a discussion of the effect that the synthesis method of FAPbI₃ films creates on the film properties and device efficiency.

3.5 Effect of FA incorporation in the MAPbI₃ structure on the morphology of FA_xMA_1-xPbI₃ thin films

The effect of incorporating FA in the MAPbI₃ structure on the FA_xMA_1-xPbI₃ morphology films prepared with different FA:MA ratios and subjected to antisolvent treatment with toluene, was studied through AFM measurements. Figure 4 shows AFM images of typical FA_xMA_1-xPbI₃ thin films prepared with different FA/MA ratios and the grain size of the samples is shown in Table 3. These results reveal in general, the FA_xMA_1-xPbI₃ films grow with a sub-micrometric size compact grains structure and that their grain size increases by increasing the FA:MA ratio.

4. Conclusions

The effect that the substitution of the ion MA by the ion FA causes on the optical, structural and morphological properties of FA_xMA_1-xPbI₃ films prepared by spin coating in one step, was studied through diffuse reflectance, AFM and XRD measurements. From these studies, it was found that the samples with composition MAPbI₃ (x=0) grow in the CH₃(NH₂)₂PbI₃ phase with tetragonal perovskite-type structure, whereas the samples prepared replacing the ion MA by the ion FA in a proportion less than 20% grow with a mixture of the tetragonal MAPbI₃ and the *-FAPbI₃ phases. When the substitution is greater than 80%, the samples grow with a mixture of the PbI₂ phase and the yellow phase δ- FAPbI₃. Evaluation of the degradation through XRD measurements made after the deposition and after several days of exposure to the environment, indicated that samples with a composition corresponding to x=0 (MAPI) are significantly more stable than samples whom the MA cation is replaced by the FA cation. In particular, thin films of FAPI (x=1) initiate their degradation only a few hours after being deposited.

Estimation of the microstrain Ɛ value of FA_xMA_1-xPbI₃ films, made through XRD measurements, indicated that in general samples prepared with composition x between 0.4 and 0.6 grew with improved crystallinity. It was also found that an increase in the proportion FA:MA gives rise to a Eg decrease, from a value of 1.61 eV for samples of composition MAPbI₃ (x=0) until a value of 1.47 eV for samples with composition x=1 (FAPbl3).

AFM measurements revealed that the morphology of FA_xMA_1-xPbI₃ thin filmsis significantly improved through anti-solvent treatment. These samples exhibited in general formation of compact grains that resulted in a good degree of substrate coverage. This study also revealed that an increase of the FA:MA proportion gives rise to an increase in the grain size.

Acknowledgments

This work was supported by Colciencias (Contract #184/2016) and Universidad Nacional de Colombia, sede Bogotá, Facultad de Ciencias, Departamento de Física, Grupo GMS&ES, Bogotá D.C, Colombia.

References

Additional information

How to cite: G. Gordillo-Guzmán, O. Virgüez-Amaya, C. Otálora-Bastidas, C. Calderón-Triana, C. Quiñones-Segura, “Synthesis and optimization of properties of thin films of FAx(MA1-X)PbI3 grown by spin coating with perovskite structure to be used as active layer in hybrid solar cells,” Rev. UIS Ing., vol. 19, no. 1, pp. 87-94, 2020. doi: 10.18273/revuin.v19n1-2020008

Revista UIS Ingenierías
ISSN: 1657-4583
Vol. 19
Num. 1
Año. 2020

Synthesis and optimization of properties of thin films of FAx(MA1-X)PbI3 grown by spin coating with perovskite structure to be used as active layer in hybrid solar cells

GerardoOphyrCamiloClaraCésar Gordillo-GuzmánVirgüez-AmayaOtálora-BastidasCalderón-TrianaQuiñones-Segura
Universidad Nacional de ColombiaFundación Universidad de AméricaUniversidad Industrial de SantanderUniversidad Nacional de ColombiaInstitución Universitaria Politécnico Grancolombiano,ColombiaColombiaColombiaColombiaColombia