FA0.8MA0.2SnxPb1–xI3 Hybrid Perovskite Solid Solution: Toward

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FA0.8MA0.2SnxPb1−xI3 Hybrid Perovskite Solid Solution: Toward Environmentally Friendly, Stable, and Near-IR Absorbing Materials Maddalena Patrini,† Paolo Quadrelli,‡ Chiara Milanese,‡ and Lorenzo Malavasi*,‡ †

University of Pavia and CNISM, Via Bassi 6, 27100 Pavia, Italy University of Pavia and INSTM, Viale Taramelli 16, 27100 Pavia, Italy

‡

S Supporting Information *

ABSTRACT: We report the first investigation addressing the synthesis and characterization of the FA0.8MA0.2SnxPb1−xI3 solid solution showing a complete solubility of Sn on the Pb-site leading to cubic single-phase materials. The explored composition shows excellent phase stability and absorbance in the near-IR spectral region.

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INTRODUCTION

thus making, in principle, possible a complete solubility at the perovskite B-site with small lattice distortions.15 Currently, there are still very few examples of Pb-Sn systems reported in the literature and most of them are related to MAbased Pb/Sn mixed materials.16−20 In general, the efficiencies reported for Sn-based PSCs are lower with respect to those reported for Pb-based devices, and this can be partially due to the very limited research efforts carried out until now in Sn/Pb mixed systems, starting from the phases synthesis and characterization to the device realization. However, a common result of the previous reports on MA-based Sn/Pb mixed systems is a clear extension of the light absorbance in the nearIR with band-gap (Eg) values reducing up to 1.2 eV in Sn-rich samples with respect to 1.55 eV found for MAPI.16,17 Considering that the most efficient PSCs are based on mixed FA/MA cations, we carried out an extensive investigation on the synthesis and characterization of the FA0.8MA0.2SnxPb1−xI3 solid solution. The choice of this stoichiometry for the amine cations has been done considering the results of the current literature for PSCs employing mixed perovskites and also because this is just the lowest MA content that can stabilize the perovskite α-phase over the time.8 To the best of our knowledge, this is the first work reporting the investigation of a Sn/Pb mixed FA-rich hybrid perovskite system.

In the past few years, the impressive achievements in the field of hybrid perovskites allowed one to reach unprecedented efficiencies in perovskite solar cell (PSCs) devices.1−3 Most of the absorbing layers are based on methylammonium lead iodide (MAPI), optionally mixed with Br and/or Cl ions to improve stability and performance.4−7 However, the most recent reports on record efficiencies in PSCs devices have been obtained with mixed formamidinium (FA)/methylammonium (MA) cations on the perovskite A-site.8−14 The interest in the use of FAPI-based perovskites is related to the fact that (i) the larger FA cation leads to more symmetric perovskites with respect to MAPbI3 (MAPI) phase, (ii) the smaller band gap of FAPI allows the near-IR absorption, and (iii) perovskites containing FA cations have an improved stability. However, pure formamidinium lead iodide is not stable in the perovskite structure and a small amount of MA is required to stabilize the “black” perovskite α-phase. As a matter of fact, the best device efficiencies have been obtained with an MA:FA ratio around 0.2:0.8.8,9 Parallel to the evolution of the hybrid perovskite field, there is a significant concern related to the presence of lead in these materials and the replacement of Pb with environmentally friendly metals in organohalide perovskites for photovoltaics applications seems to be a required step in order to develop this promising technology. This couples also to the possibility of a further modulation of the absorption spectrum toward the near-IR. Among possible candidates to replace for Pb, tin represents a natural choice even though the problems related to Sn2+ oxidation by air should be considered in device design. However, Sn2+ has an ionic radius very close to that of Pb2+, © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Samples of general formula FA0.8MA0.2SnxPb1−xI3 (with nominal x = 0, 0.2, 0.4, 0.6, 0.8, 1) were synthesized according to a general and original procedure we developed.15,21 In a typical synthesis, proper Received: August 25, 2016

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DOI: 10.1021/acs.inorgchem.6b02055 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. SEM images for the x = 0.2 and 0.8 samples of the FA0.8MA0.2SnxPb1−xI3 solid solution.

Table 1. Nominal and Experimental x Values for the FA0.8MA0.2SnxPb1−xI3 System Together with the a Lattice Constant Determined from XRD and the Eg Value Determined by Diffuse Reflectance

stoichiometric amounts of Pb acetate and Sn acetate are dissolved in an HI excess under continuous mechanical stirring under a nitrogen atmosphere. Hypophosphoric acid is added to the solution and an inert atmosphere is maintained in the reaction environment in order to prevent Sn oxidation. Then, the solution is heated to 100 °C and the corresponding amines solutions (40 wt % in water) are added in the stoichiometric amount to achieve the MA:FA 0.2:0.8 ratio. The solution is then cooled down to 46 °C at 1 °C/min, until the formation of a precipitate, which is immediately filtered and dried under vacuum overnight. Finally, the samples are treated at 100 °C for 2 h before the characterization. The final products are handled and stored in a glovebox with oxygen and water contents lower than 1 ppm. All the reagents were purchased from Sigma-Aldrich in pure form and were used without any further purification. The crystal structure of the samples has been characterized by room-temperature Cu-radiation X-ray Powder Diffraction (XRD) in a Bruker D8 diffractometer by using a Bruker dome sealed in the glovebox avoiding air exposure. The optical diffuse reflectance spectra of the different perovskites were measured from 0.8 to 4.5 eV (250−1500 nm, with step of 1 nm) by a Varian Cary 6000i equipped with an integrating sphere. For this kind of measurements, polycrystalline powders were compacted into pellets of about 5 mm in diameter and reflectance spectra were calibrated using a standard reference disk. The morphology of the samples have been studied by scanning electron microscopy in a EvoMA10 instrument by Zeiss. The real elemental composition of the samples was determined by energy-dispersive X-ray spectroscopy using an INCA Energy 350 X Max detector from Oxford Instruments linked to the SEM. Cobalt standard was used for the calibration of the quantitative elementary analysis. All the samples manipulations were carried out under an inert atmosphere thanks to a homemade sample holder allowing the transport of the powders from the glovebox to the SEM chamber. Those analyses are essential to correlate the physical properties with the real stoichiometry instead of a nominal cations ratio. Differential scanning calorimetry measurements have been performed by heating the samples from −90 to 200 °C at 5 °C/min under N2 flux in Al sealed pans in a Q2000 equipment by TA Instruments.

nominal x value

experimental x value

a (Å)

Eg (eV)

0 0.20 0.40 0.60 0.80 1

0 0.17 0.39 0.63 0.82 1

6.3522(1) 6.3469(1) 6.3275(1) 6.3154(1) 6.3112(1) 6.3076(1)

1.49 1.27 1.20 1.20 1.28

that the employed synthetic procedure allowed a quite good control over the metal stoichiometries. Throughout the remainder of this paper, we are going to make use of the experimentally determined compositions shown in Table 1 in order to provide the most reliable correlation with the investigated properties. The phase purity and crystal structure of the samples have been investigated by means of laboratory XRD. Figure 2a reports the patterns for all the considered stoichiometries. It is possible to observe that the XRD patterns in Figure 2a are very similar for all the members of the solid solution and that the materials are single-phase without the presence of any impurities (compare the data with the reference pattern, dark red vertical bars, in the plot). Pure FAPI perovskite, previously considered to be trigonal (space group, s.g., P3m1), has been recently shown to be cubic with space group Pm3̅m, as determined by means of neutron diffraction.22 Doping of FAPI with MA on the perovskite A-site maintains the cubic structure up to very low MA-doping. In addition, 20% of MA on the FAPI perovskites stabilizes the perovskite phase, thus avoiding the time-dependent transformation to the “yellow” hexagonal δ-phase of FAPI (s.g. P63mc).12,23 On the other hand, the FASnI3 crystal structure at room temperature (α-phase) has been very recently found to be cubic with a lattice parameter of 6.321(4) Å.24 From the Rietveld refinement of all the samples of the investigated FA0.8MA0.2SnxPb1−xI3 solid solution, a very good agreement in the fit has been achieved by considering the cubic symmetry of FAPI perovskite (s.g. Pm3̅m). As an illustrative example, Figure 2b reports the Rietveld refined pattern for the FA0.8MA0.2SnI3 sample. The refined cubic lattice parameter is 6.3076(4) Å, which is slightly smaller than the lattice parameter found for FASnI3 (6.321(4) Å).24 This difference comes from the 20% doping of the smaller MA ion on the hybrid perovskite A-site (ionic radii for FA and MA have been estimated to be

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RESULTS AND DISCUSSION Figure 1 shows some representative secondary electrons scans for the x = 0.2 and 0.8 powders at 2000× magnification. As can be appreciated, the samples are made of relatively big aggregates composed of microsized grains. Images for all the samples investigated are reported in Figures S1−S4 of the Supporting Information (SI). The x values (Sn content) for the samples of the FA0.8MA0.2SnxPb1−xI3 solid solution determined by means of the EDX probe are reported in the first column of Table 1 and compared to the nominal values indicated above. From Table 1, it is possible to see that the real stoichiometries are very close to the nominal ones, confirming B

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Figure 2. (a) XRD patterns for the samples of the FA0.8MA0.2SnxPb1−xI3 solid solution. Patterns are vertically shifted to clarify viewing, together with the calculated pattern for the cubic symmetry Pm3̅m (dark red vertical bars). (b) Rietveld refinement of the FA0.8MA0.2SnI3 with the Pm3̅m space group.

Figure 3. (a) Lattice parameter and (b) cell volume for the FA0.8MA0.2SnxPb1−xI3 solid solution, respectively, as a function of x.

253 and 217 pm, respectively).25 From the refinement of all the samples, we obtained the lattice parameter and calculated the cell volume. The data, reported as a function of the experimentally determined Sn content (x), are shown in Figure 3a,b, and listed in Table 1. Overall, the value of the lattice parameter of the cubic cell (Figure 3a), and consequently of the cell volume (Figure 3b), progressively reduce by increasing the Sn content (x value) in the FA0.8MA0.2SnxPb1−xI3 solid solution. It is interesting to note that, even though the difference in the ionic radii between Pb2+ and Sn2+ in octahedral coordination is relevant, 1.20 and 0.93 Å, respectively, the contraction of the lattice parameter is only about 0.7% going from x = 0 (all Pb) to x = 1 (all Sn), suggesting that the overall size of the unit cell is less influenced by the cation substitution on the B-site of the hybrid perovskite structure with respect to all-inorganic perovskites. The diffraction investigation has shown that the crystal structure of the FA0.8MA0.2SnxPb1−xI3 solid solution is cubic and does not change along with the Pb/Sn cation substitution. The thermal stability of the solid solution samples has been

determined by means of differential scanning calorimetry (DSC) from −50 to 200 °C. The data are reported in Figure 4. As can be seen from Figure 4, no endothermic or exothermic peaks are present in the temperature interval explored for all the samples. In the inset of Figure 4, the DSC trace of pure FAPI is reported. In this case, a clear endothermic peak is evident at about 160 °C, which corresponds to the hexagonal to cubic phase transition which is absent in the samples of the FA0.8MA0.2SnxPb1−xI3 solid solution. This confirms that, also in the mixed Pb/Sn samples, the transition involving the hexagonal phase of parent FA-based perovskite is efficiently removed by the 20% MA-doping on the A-site of the hybrid perovskite, as it has been shown to occur for the FA0.8MA0.2PbI3 sample.26 In order to further verify the stabilization of the perovskite phase in this mixed system, XRD patterns have been acquired after 20 days of samples’ storage in a glovebox. Such kind of experiments resulted to be essential in FA-rich mixed protonated amines systems where the stabilization of the perovskite phase is only apparent and after some time there is a C

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stoichiometries. Figure 6a,b reports the vis−NIR diffuse reflectance spectra and the trend of band gaps as a function of x, respectively. The measurements have been carried out on both compacted disks and powder, leading to the same trend. The Eg values have been obtained from the extrapolation of the linear part of [F(R) hν]2, where F(R) is the Kubelka−Munk function F(R) = (1 − R)2/2R.26,27 From Figure 6a, it is possible to observe a progressive shift of the absorption edge toward higher wavelengths (lower energies) by replacing Sn on the Pb-site, together with a reduction of the overall reflectance of the samples and a broadening of the band-gap line shape. This has been previously observed in the MASnxPb1−xBr3 solid solution; the origin of such decrease in the samples reflectance is still not clear and could be due to a very small self-doping of Sn4+ ions even under the highly controlled conditions used to carry out all the manipulations and characterization of the sample.28 However, as it has been previously experimentally and theoretically determined in MASnxPb1−xI3 compounds, this effect can be partially due to the increased and broadened absorption coefficient spectrum with increasing Sn content, as well as to the variation of grain size and morphology.20,29 As for the Eg values, the value at x = 0 is in agreement with the previous literature while the progressive increase of the Sn content leads to a significant reduction up to about 1.18 eV for x = 0.82.21 Data for the FA0.8MA0.2SnI3 sample are not shown because it was not possible to obtain any clear absorbance in the spectra of this sample even on several batches coming from separated synthetic routes. From the results reported in the figures above, it is interesting to note the extension of the band gap of these low-Pb containing hybrid perovskites in the nearIR, which is a spectral region of great interest for optimal solar cell applications, yielding the maximum theoretical conversion efficiency17,30 Moreover, together with the extension of the band gap toward the near-IR region, the Sn-doping has been theoretically shown to significantly reduce the effective mass of the holes, while keeping the effective mass of the electrons unchanged.31 It is known that the effective mass plays a critical role in the exciton diffusion length of photoexcited charges, favoring the separation of electrons and holes and leading to longer diffusion lengths.

Figure 4. DSC measurements for the samples of the FA0.8MA0.2SnxPb1−xI3 solid solution. Inset: DSC measurement for the FAPI sample.

clear conversion to the hexagonal δ-phase (unpublished results). The samples reported in Figure 2 have been reanalyzed after 3 weeks of storage in a glovebox. Figure 5 shows the XRD

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CONCLUSIONS

In this paper, we reported the first investigation addressing the synthesis and characterization of the FA0.8MA0.2SnxPb1−xI3 solid solution. This peculiar mixed composition on the perovskite Asite provided the best performance in the actual PSCs. The used synthetic approach allowed us to prepare single-phase materials from x = 0 to x = 1, thus confirming the relative solubility of Pb and Sn within the perovskite lattice. An accurate Sn/Pb cation ratio has been determined by means of EDX probe, thus allowing a reliable correlation between measured properties and actual stoichiometry. The replacement of Pb with Sn showed to preserve the cubic symmetry of the pristine materials throughout the solid solution with a progressive slight reduction of the unit cell as a function of the Sn content (x). The perovskite phase has been shown to be stable within the period of time considered in the present work without any transition to the hexagonal phase. Through DSC measurements, we confirmed as well the absence of any phase transition between −90 and 200 °C. The optical properties showed a progressive shift of the absorbance edge toward longer

Figure 5. XRD patterns for the samples of the FA0.8MA0.2SnxPb1−xI3 solid solution after 21 days of storage. Red and blue vertical bars refer to the reference patterns of the hexagonal and cubic structures of FAPI, respectively.

patterns for all the samples of the solid solution in the 10−20° range. In this range, the two most intense peaks of the α-phase (cubic, Pm3̅m) and of the δ-phase (hexagonal, P63mc) are present, which are highlighted by means of black and dark yellow vertical bars, respectively. The results presented in Figure 5 clearly indicated that the phases of the whole solid solution are stable, at least in the time window explored, with respect to a possible time-dependent transition of the perovskite phase toward the hexagonal phase. The samples of the FA0.8MA0.2SnxPb1−xI3 solid solution underwent optical measurements in order to define the fundamental band-gap values as a function of (real) D

DOI: 10.1021/acs.inorgchem.6b02055 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) Diffuse reflectance spectra for the FA0.8MA0.2SnxPb1−xI3 solid solution. (b) Energy gap trend as a function of x for the FA0.8MA0.2SnxPb1−xI3 solid solution.

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wavelengths by increasing x up to the near-IR with the band gap reducing up to 1.18 eV for x = 0.83. The present results provide a new series of samples which present a reduced (or no) Pb content, a stable cubic perovskite unit cell, and band-gap values around 1.25−1.18 eV, thus extending the absorbance to the near-IR spectrum, which is of great interest for advanced applications of the perovskite solar cells. As a matter of fact, during the paper preparation, it has been reported the successful preparation of a PSC with a (FASnI3)0.6(MAPbI3)0.4 absorber having an absorption edge of ∼1.2 eV and achieving a power conversion efficiency (PCE) of about 15%, thus demonstrating the effective power of similar mixed compositions in solar cell devices.32

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02055. SEM images for the samples of the FA0.8MA0.2SnxPb1−xI3 solid solution (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paolo Quadrelli: 0000-0001-5369-9140 Lorenzo Malavasi: 0000-0003-4724-2376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Silvia Carpino and Matteo Cresci are acknowledged for their contribution to the experimental part. The authors gratefully acknowledge the project PERSEO-“PERrovskite-based Solar cells: Towards high Efficiency and lOng-term stability” (Bando PRIN 2015-Italian Ministry of University and Scientific Research (MIUR) Decreto Direttoriale 4 novembre 2015 n. 2488, project number 20155LECAJ) for funding. E

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DOI: 10.1021/acs.inorgchem.6b02055 Inorg. Chem. XXXX, XXX, XXX−XXX