Scholarly article on topic 'Perovskite FA1-xMAxPbI3 for Solar Cells: Films Formation and Properties'

Perovskite FA1-xMAxPbI3 for Solar Cells: Films Formation and Properties Academic research paper on "Nano-technology"

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{"Organic-inorganic perovskites" / "Formamidinium Lead Iodide" / "planar heterojunction" / "Morphology control" / "Optical absorption" / Photoluminescence}

Abstract of research paper on Nano-technology, author of scientific article — B. Slimi, M. Mollar, I. Ben Assaker, I. Kriaa, R. Chtourou, et al.

Abstract Organic-inorganic hybrid perovskite formamidinium lead triiodide NH2CHNH2PbI3 (FAPbI3), methylammonium lead triiodide CH3NH3PbI3 (MAPbI3) and formamidinium methylammonium lead triiodide (NH2CHNH2)1-x(CH3NH2)xPbI3 (FA1-xMAxPbI3) thin films were synthesized and deposited on indium tin oxide glass substrates by spin coating process. Thin films of mixed FA1-xMAxPbI3 (x = 0-1) perovskites obtained by mixing FAPbI3 and MAPbI3 in different proportions. The morphological, structural and optical proprieties of all synthetized perovskites have been analyzed as a function of the MA/FA ratio. X-ray diffraction analyses indicated the formation a cubic perovskite phase with space group Pm-3m in the composition range 0 ≤ x ≤ 1. Mixed perovskites FAMAPbI3 showed a high absorbance in the infrared region 780-900nm. The band gap energy estimated from absorbance spectral measurements for FAMAPbI3 thin films ranges from 1.50eV for FAPbI3 to 1.56eV for MAPbI3, respectively. The overall PL emissions of mixed FA/MA perovskite thin films are located in intermediate values between 773nm and 810nm.

Academic research paper on topic "Perovskite FA1-xMAxPbI3 for Solar Cells: Films Formation and Properties"

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Energy Procedia 102 (2016) 87 - 95

E-MRS Spring Meeting 2016 Symposium T - Advanced materials and characterization techniques

for solar cells III, 2-6 May 2016, Lille, France

Perovskite FAi-xMAxPbl3 for solar cells: films formation and

properties

B. Slimiab c, M. Mollara, I. Ben Assakerb, I. Kriaad, R. Chtouroub, B. Maria *

a Institut de Disseny i Fabricacio, Universität Politècnica de Valencia, Cami de Vera s/n, 46022 Valencia, SPAIN bLaboratoire Photovoltaïques, Centre de Recherches et des Technologies de l'Energie Technopole Borj Cedria, Bp 95, Hammam Lif2050,

Tunisie

c Faculté des Sciences de Bizerte, Université de Carthage, Tunisie d Laboratoire de Chimie Moléculaire Organique, 5 Avenue Taha Houssein Monfleury, 1089 Tunis, Tunisie

Abstract

Organic-inorganic hybrid perovskite formamidinium lead triiodide NH2CHNH2Pbl3 (FAPM3), methylammonium lead triiodide CH3NH3Pbl3 (MAPM3) and formamidinium methylammonium lead triiodide (NH2CHNH2)i-x(CH3NH2)xPbl3 (FAi-xMAxPbl3) thin films were synthesized and deposited on indium tin oxide glass substrates by spin coating process. Thin films of mixed FAi_xMAxPbl3 (x = 0-1) perovskites obtained by mixing FAPbl3 and MAPbl3 in different proportions. The morphological, structural and optical proprieties of all synthetized perovskites have been analyzed as a function of the MA/FA ratio. X-ray diffraction analyses indicated the formation a cubic perovskite phase with space group Pm-3m in the composition range 0 < x < 1. Mixed perovskites FAMAPM3 showed a high absorbance in the infrared region 780-900 nm. The band gap energy estimated from absorbance spectral measurements for FAMAPM3 thin films ranges from 1.50 eV for FAPM3 to 1.56 eV for MAPM3, respectively. The overall PL emissions of mixed FA/MA perovskite thin films are located in intermediate values between 773 nm and 810 nm.

©2016 The Authors.Published byElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of The European Materials Research Society (E-MRS).

Keywords: Organic-inorganic perovskites; Formamidinium Lead Iodide; planar heterojunction; Morphology control; Optical absorption; Photoluminescence.

* Corresponding author. Tel.: +34-963.877-5250; fax: +34-963-887-7189. E-mail address: bmari@fis.upv.es

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of The European Materials Research Society (E-MRS). doi:10.1016/j.egypro.2016.11.322

1. Introduction

Hybrid organic-inorganic perovskite solar cells have become one of the most attractive photovoltaic technologies over the last five years due to their with simple synthesis from solution, high conversion efficiencies for devices and low costs fabrication [1-3]. First report on long-term durable 9.7% efficient perovskite solar cells based on methylammonium lead triiodide (CH3NH3Pbl3 or MAPM3) was released in 2012 [4] and the formamidinium lead triiodide (HC(NH2)2PbI3 or FAPbls) perovskite, with broader light absorption delivering power conversion efficiency (PCE) up to 13.8% were reported in 2015 [5-9]. Subsequent improvements on the fabrication [10-12] and the annealing processes [13-15] and structure engineering for mixed perovskites [16-18] resulted in PCE higher than 15% [19-20]. Recently, perovskite solar cells have been classified as a new type of solar cell and a record PCE of 16.2% have been certified [21]. Perovskite solar cells have been selected as one of the biggest scientific breakthroughs of2013 [22]. So far, the highest performance has overcome 20.1% [23].

Increasing the photocurrent by expanding the absorption spectra of ABX3 perovskites by means of chemical modification has been proposed as a method for further improving solar cell efficiency. For example, replacing the methylammonium cation (MA+) in MAPbl3 with a formamidinium cation (FA+), which has a larger ionic radius, results in an ABX3 perovskite with a smaller bandgap for broader-spectrum light harvesting [24-26]. And the combining MAI with FAI in a sequential deposition method, the efficiencies of 14.9% and 16.01% were obtained [28-30]. In comparison to MAPM3, the more recently developed FAPM3 perovskite is less studied, despite the importance of efficiency in solar cells for example 16.6% [22]. The preparation of the high-quality black a-FAPbl3 perovskite film is still a challenge, because the yellow 5-FAPbl3 non-perovskite polymorph phase formed from one-step solution processing at room temperature is very hard to remove.

The crystal structure of FAPM3 is attributed to the larger ion size of FA+ compared to MA+ (the ionic radii of FA and MA are 2.79 and 2.70 A, respectively [30]. Theoretically, the yellow 5-FAPbl3 non-perovskite polymorph phase can transform to the photoactive black a-FAPbl3 phase at high temperature (140 °C). Actually, it is not easy to fully transform 5-FAPbl3 into a-FAPbl3 when it fills the scaffold layer probably because of the stress from the substrate

In this paper, we examine the effect of annealing temperature on the properties of mixed FAi_xMAxPbl3 (x = 0-1) perovskites with different FA/MA ratios. Morphological, structural and optical properties of mixed FAi_xMAxPbl3 perovskite films were studied as a function of their composition.

2. Experimental

2.1. Synthesis of Formamidinium Iodide [HC(NH2)2I] (FAI)

Formamidine iodide (HC(NH2)2l or FAI) was synthesized by mixing 0.08 mol formamidine acetate (Sigma Aldrich) with 0.08 mol Hydriodic acid (HI) (57 wt. % in H20, distilled, stabilized, 99.95% from Sigma Aldrich). The mixture was stirred for 30 min in a round-bottom flask, which was kept in an ice bath (0 0C). The solvent was then evaporated using a rotary evaporator and then the solid was recrystallized in ethanol two times. The solid obtained was washed thoroughly with ether until a white crystalline powder was obtained. The powders were finally dried under vacuum for one night before use. The elemental analysis shows the weight ratio of C:N:H in the as-prepared FAI is 7.30:16.46:2.83, which is in very good agreement with theoretical atomic ratio. It was found that the mixture of Pbl2 and FAI with a mole ratio of 1:1 was soluble in DMF.

Methylammonium iodide (CH3NH3I or MAI) from Sigma Aldrich was used as received.

2.2. Synthesis of thin film perovskites

FAPbl3 and MAPbl3 perovskites precursor solutions were prepared from an equimolar mixture of FAI/MAI and Pbl2 in dimethylformamide (DMF) solution at 40 %wt with ratios 1:1 (1:1 mol %) for FAI:PbI2 and MAI:PbI2and then stirred for 2 h at 70 °C. The mixture was deposited on indium tin oxide (ITO) covered glass by spin-coating at 3500 rpm for 11 s. A drop of toluene was added after 2-4 s before the end then dried at 5000 rpm for 30 s. The

resulting perovskite layers were then annealed at 140 and 150 °C for 20 min. ITO glass substrates were previously cleaned with ethanol, isopropanol and water for 15 min respectively and then dried with clean dry air. Bulk FAPbl3, MAPbl3 and FAi_xMAxPbl3 (x= 0-1) sample were prepared via drying DMF solution at different temperature on ITO substrates.

2.3. Characterization

Perovskite thin films were characterized by field-emission scanning electron microscopy (FE-SEM) (ZEISS UL TRA55) at an acceleration voltage of 10 kV, Energy Dispersive X-ray Spectrometer (EDX) mounted on the FE-SEM), X-ray diffraction is using a RIGAKU Ultima IV in the Bragg-Bentano configuration using CuKa radiation. Photoluminescence (PL) spectra were recorded at room temperature. The PL excitation source was a He-Cd laser at 325 nm. Photoluminescence data were recorded by a Si-based CCD detector Hamamatsu. Optical measurements were performed at room temperature using a spectrometer Ocean Optics HR4000 equipped with a Si-CCD detector. An integrating sphere was used to collect both direct and diffuse transmittance.

3. Results and discussion

3.1. X-ray analyses

Figure 1 shows the X-Ray diffractograms for FAPbL, MAPbL and FAMAPbL thin films. XRD peaks are located at 14.1, 20.0, 24.4, 28.4, 31.8, 40.6 and 43 degree for MAPbI3 and slightly shifted to smaller angles for FAPbI3. As an example the (0 0 1) peak is located at 13.8° for of a-FAPbI3 and at 14.1° for MAPbI3. The shift is due to the bigger size of FA cation with respect to MA cation that expands the crystal lattice. XRD peaks for composite FAi_xMAxPbl3 perovskite films are located between those of FAPbL and MAPbL films and shift their position proportionally to the MA content. Such a gradual shift in diffraction peaks position angle indicates that mixed FAi_ xMAxPbl3 perovskite are formed with both organic cations (FA and MA) are inserted in the same lattice.

The yellow 5-phase of FAPbL was produced at annealing temperatures below 150 °C, as indicated by the typical peak at 11.8° corresponding to non-perovskite structure. Increasing the annealing temperature to 170 °C the secondary phase (S-FAPbL) disappears similarly for the mixed FAi_xMAxPbl3 for x=0.2. The S-FAPbL phase in mixed perovskites disappears when the temperature increases and when FA content decreases, confirming that mixed FA/MA perovskites become more stable. The sharp diffraction peaks for the synthesized MAPbL, FAPbL and FAi_xMAxPbl3 films reveals the high crystallinity of the films. Further this fact is fairly consistent with the results found in literature [18].

--AJ --1 ---- L ni i LAA_H

L .. » FA06MA04Pbl3

FA04MA06Pb,3

FA02MA08Pbl3

'------------------ M™ Jjl^ _ _ MAPbl3

^ • * ' UU-A__^ ™Pbl3

T---1-1-1->-1-1-1->-1->-1->-r

10 15 20 25 30 35 40 45 50 2 0 (degree)

Figure 1. X-Ray diffractograms of the FAi_xMAxPbI3 thin-films perovskites with different molarratio ofFA/MA

(x= 0, 0.2, 0.4, 0.6, 0.8, 1).

Figure 2 shows a zoom in the region corresponding to (0 0 1) and (0 0 2) diffraction peaks. The position of the both peaks shifts to higher angles when the MA content in the thin film increases. (0 0 1) diffraction peaks are located at 13.915, 13.934, 13.966, 13.995, 14.025 and 14.106degreeforx = 0, 0.2,0.4,0.6,0.8and l,respectively.

Diffraction peaks located at 21.26, 30.25, 35.16, 37.34 and 45.20 degree corresponding to ITO substrate is also observed. Since the XRD measurement system ensures an uncertainty for 2 9 lower than + 0.02, the observed peak shifts are not due to experimental errors. The gradual diffraction angle shift indicates that the two cations are both inserted in the same lattice and the shift of peak maximum toward lower angle for increasing MA/FA ratios is indicative of an increase in lattice parameter. The changes in the lattice parameter are due to the incorporation of the larger FA+ cation instead of the smaller organic MA+ cation (the ionic radii of FA+and MA+ are 2.79 and 2.70 A, respectively) [32,33].

(001) (002)

FAPbl3 JA

FA08MA02Pbl3 Jl.

FA06MA04Pbl3

FA04MAo6Pbl3

FA02MA08Pbl3 -J N

MAPbl3

2 0 (degree)

Figure 2. X-Ray diffractograms of the FAi_xMAxPbI3 thin-films magnified view ofthe region 12-15° and 27-30°.

FAPbl3 perovskite crystallizes in a cubic phase for temperatures higher than 150 °C. On the other hand, MAPbl3 thin films crystallize and stabilize in a cubic phase Pm-3m at room temperature [34-38]. Mixed FAMAPbl3 perovskites crystallize in the same cubic structure corresponding to the spatial group labeled as P432 (Pm-3m). The lattice parameters of FAPbl3 are calculated to be a=6.352 A, which is in good agreement with previously reported phase for FAPbI3 [39, 28].

Table 1: Lattice parameters of mixed FAi_xMAxPbI3 perovskite films as a function of MAI composition (x), whenx varies in the range 0-1.

MAI concentration (x) in FAIi.xMAIxPbI3 0 0.2 0.4 0.6 0.8 1

a(A) 6.352 6.346 6.339 6.323 6.309 6.270

The lattice parameters of mixed FAi_xMAxPbl3 thin film perovskites are shown in Table 1. The lattice parameters of MAPbl3 are calculated to be a = 6.270 and the FAPbl3 a=6.352 A, exhibits the lattice parameter (a) of the FAi_ xMAxPbl3 phases indexed by cubic as a function of MA content (x). The lattice parameters increase with the decrease of MA content. Despite of the phase of FAPbl3, the lattice parameter of FAi_xMAxPbl3 (0 < x > 1) phases exhibits a linear relationship with the FA content in each region. Therefore, the linear trend indicates the formation of the FAi_xMAxPbl3 with change oflattice parameters rather than phase ofMAPbl3.

3.2. SEM and EDX analyses

FE-SEM studies were conducted to examine the influence of MA incorporation in the morphology of thin films. Figure 3 presents' top views FE-SEM images of the FAPbl3, MAPbl3 and FAi_xMAxPbl3 perovskite thin films deposited on ITO substrates with different MA fractions. The FAi_xMAxPbl3 film has a dense and a homogenous morphology with fiber-like crystals with the presence of void due to solvent evaporation and some crevices between

the crystal boundaries. In case of FAi_xMAxPbl3, it consists of aggregate crystals with some cracks. The obtained films with different MA content have a different shape, morphology and size. Indeed, as the composition x increases, the aggregation of gains decreased, due to the volume between the two cations of FA and MA.

The disappearance of the yellow 5-FAPbl3 phase can also be monitored via the SEM micrographs. Here the needle-like crystals obtained for low MA contents can be correlated to the existence of 5-FAPbl3 phase. Needle-like morphologies (low MA contents) are connected with the disappearance of XRD reflections corresponding to the cubic structure. Higher amounts of MA result in the stabilization of the a-FAPbl3 phase.

Fig 3. FESEM images of FAi_xMAxPbI3 thin films arrays grown over large surface areas on ITO substrates.

Table 2 displays the EDX results of all synthesized samples. FA, MA, Pb and I elements are homogeneously distributed in the crystals perovskite, suggesting that Pb and I are uniformly incorporated in the FAPbl3, MAPM3 and FAi_xMAxPbl3 in grain perovskite rather than existing in separate chemical phases. The results analysis of EDX for all films shows that the between Pb and I are very close to the theoretical values, which are 75% and ~25% for I and Pb, respectively.

Table 2: EDX analysis for Iodine and Lead content in mixed FAi_xMAxPbI3 thin films (x=0-l).

Perovskites Pb (Atomic %) I (Atomic %)

FAPbI3 24.25 75.75

FAO.8MAO.2PM3 24.09 75.91

FAO.6MAO.4PM3 23.97 76.03

FAO.4MAO.6PM3 23.59 76.41

FA0.2MA0.8PbI 23.31 76.69

MAPbI3 23.84 76.16

3.3. UV-vis spectra

The absorption spectra of FAi_xMAxPbl3 thin films on ITO are presented in Figure 4. The trend is lower the MA content the lower the cutoff wavelength. The MAPbl3 exhibits the lowest cutoff wavelength while FAPbl3 has the highest and cutoff wavelengths for mixed FAi_xMAxPbl3 are between. A systematic shift of the absorption band edge to longer wavelengths when the MA content increases is observed. Therefore perovskite band gap can be tuned by varying MA percentage in the loading solution.

The onset of bandgap absorption for FAPbl3 and MAPbl3 thin films is around 825 nm, 793nm (corresponding to energy of 1.50 and 1.56 eV, respectively) is similar to previous observation [26]. The narrower band gap of FAPbl3 makes it feasible to absorb light over ^800 nm and switches a greater inverse proportion of the sun energy to electricity.

Figure 4. The UV-visible absorption spectra from 650 nm to 900 nm for FAi_xMA xPbI3 (x = 0-1) thin films.

Figure 5 shows the evolution of the interplanar spacing depending on percentage of MA, decrease in the amount of MA distance increases is well explains the effect of the cation size so it confirms both cations are well mixed. As the MA content increases, the absorbance edge shifts to short wavelength values, indicating the increase of the band gap energy of the prepared perovskite thin films. The onset band gap of mixed FAi_xMAxPbl3 perovskite thin films are located in intermediate values between 1.563 eV (MAPM3) and 1.502 eV (FAPM3) meaning that the band gap can be tuned by varying the composition of the ratio FA/MA (See Table 3)[4-6]. The results are in good agreement with theory, which predicts that the higher the content of MA cation the higher the band gap energy. In addition band gap energy for mixed FAi_xMAxPbl3 perovskites is between the band gap energies of MAPM3 and FAPM3 which further support this claim a new phase synthesized. It is also worthy to notice that for absorption measurements we used an integrating sphere to collect the direct and diffuse transmittance in order to remove the effect of light scattering originating from refraction and reflection phenomenon inside the perovskite crystals.

■0 ' 6.30-

6.26-1—!-

-1-1-1-1-1-1-1—

—^—t b fU)l

1-1.61 • 1.60 -1.59 • 1.58

-1.56 S -1.55 -1.54 -1.53

Figure 5. Interplanar spacing (d) and band gap energy (Eg) as a function of [FA/MA] ratio.

3.4. Luminescence properties

Figure 6 shows the normalized PL spectra of the mixed FAi_xMAxPbl3 thin films. A significant red shift (about 37 nm) in the emission peak from MAPbL (2max=773 nm) to FAPbL (2max=810 nm) was observed consistent with the absorption spectrum along with a noticeable broadening of the emission profile [40]. The gradual shift in emission indicates the formation of a solid solution of FA/MA in the perovskite lattice. Here, we also observe a gradual transition from MAPbL to FAPbL in PL spectra, in good agreement with the XRD and absorbance spectra again indicating that the transition between perovskites occurs via fully crystalline mixed cation phases. The similarity of the observed trends suggests that the ratio of intercalated cations is similar to that of the dissolved cations in the precursor solution. The nonlinearity of the emission shift reflects interactions between FA and MA cations.

Table 3 shows the energy position of the absorption edge and the position of the PL peak at room temperature for the mixed perovskite FAi_xMAxPbl3 (x=0-l) thin films synthesized in this work. The wavelength position of the maximum of emission is shifted to lower wavelengths when compared with the onset of the absorption edge of FAPbL (810 nm) up to MAPbL (773 nm). The component of PL spectra with the highest energy corresponds to transitions from the conduction band to the valence band and the lower energy components of the PL spectra involve energy levels inside the band gap, which is in agreement with the sub band gap absorption mentioned before. The wavelength position of the maximum of the photoluminescence emission is shifted to the lowest wavelengths when compared with the inset of the absorption edge, but with a small displacement related to the concentration of MA.

The high intensity of PL emission suggests that most decay transitions are radiative and nonradiative decay is negligible. Since the radiative recombination dominates one can speculate that most energy levels inside the band gap correspond to shallow levels and electron-hole pairs formed during optical excitation recombine radiatively emitting photons. The amount of deep levels inside the bandgap is very low and as a result both pure and mixed perovskites exhibit high luminescent efficiency.

- FAPbl3

- MAPbl,

650 675 700 725 750 775 800 825 850 875

Fig 5. Normalized PL at room temperature for mixed FAi.xMAxPbI3 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) perovskite thin films.

Table 3: The estimated band gap for thePL and absorption with the variation of FA/MA ratio.

Perovskites

Eg (Abs) (eV)

Eg (PL) (eV)

FAPbI3

1.502 1.507 1.514 1.530 1.554 1.563

1.531 1.547 1.569 1.572 1.584 1.604

FAo.8MAo.2PbI3 FA0.6MA04Pbl3 FA0.4MA0.6PbL FA0.2MA0.8PbL MAPbI3

4. Conclusion

The mixed FAi_xMAxPbl3 perovskites prepared by mixing FAPbl3 and MAPbl3 in the desired proportions (x=0-1), were synthetized and deposited as thin films onto ITO substrates by spin coating in one step. All synthetized FAi_xMAxPbl3 (x=0-l) perovskites reported in this paper crystallized in the same cubic phase irrespective of the x the value and a unique spatial group Pm-3m (P432) was observed. The evolution of the angle 20 and increases for all samples and directly related to the increase in MA. Absorbance measurement shows that the perovskite films exhibit a very high absorbance in the infrared region. The absorption edge of the thin films can be tuned along the visible spectrum from 650 nm to 900 nm. The band gap energy of perovskite thin film was estimated from absorbance spectral measurement, it was found that the onset of the absorption edge for FAi_xMAxPbl3 thin films reaching intermediate values ranges from 1.50 (FAPbL) to 1.56 eV (MAPbls). In the PL study, the incorporation of MA into FAMAPbl3 shifts the PL emission to shorter wavelengths, the overall PL emission of mixed FA/MA perovskite thin films are located in intermediate values between 810 nm (FAPbL) and 773 nm (MAPbL). Finally the results obtained confirmed that the mixed perovskites were synthesized.

Acknowledgements

This work was supported by Ministerio de Economía y Competitividad (ENE2013-46624-C4-4-R) and Generalitat valenciana (Prometeus 2014/044).

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