Fluoride Derivative as Precursor for Enhancing the Efficiency of

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C: Energy Conversion and Storage; Energy and Charge Transport

New Tin (II) Fluoride Derivative as Precursor for Enhancing the Efficiency of Inverted Planar Tin/Lead Perovskite Solar Cells Teresa S. Ripolles, Daiki Yamasuso, Yaohong Zhang, Muhammad Akmal Kamarudin, Chao Ding, Daisuke Hirotani, Qing Shen, and Shuzi Hayase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09609 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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The Journal of Physical Chemistry

Figure 1. (a) XRD patterns and (b) relative lattice strain calculated by Williamson-Hall method of Pb/Sn perovskites with different concentrations of SnF2 complex as an additive. 102x47mm (300 x 300 DPI)

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Figure 2. (a) Device structure of the inverted perovskite solar cells and the thickness of each layer; (b) J-V curves of the devices without/with (5 mol %, 10 mol %, and 25 mol %) additive of SnF2 complex measured under simulated 1 sun; (c) IPCE measurement and the Jsc calculated from IPCE spectra. 308x103mm (150 x 150 DPI)


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Figure 3. J-V curves under simulated 1 sun illumination intensity and IPCE of solar cells pristine, SnF2 complex and SnF2 as additives into the Sn-Pb perovskite precursor. 101x49mm (300 x 300 DPI)



Figure 4. Energy level diagram of (a) control perovskite, and perovskite solved with either (b) SnF2 or (c) [SnF2(DMSO)]2 additives.29 The Eg was calculated by PA, and the HOMO level was extracted from PYS. 171x64mm (300 x 300 DPI)


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Figure 5. TA responses of Sn-Pb perovskite without additive (pristine), with additives [SnF2(DMSO)]2 and SnF2 on glass substrate. The solid line corresponds to the fit data with an exponential function. 100x99mm (300 x 300 DPI)



Figure 6. (a) Impedance spectra at short circuit conditions and (b) resistance at high frequency regime under constant AM 1.5 G light intensity from 0 V to open circuit voltage for the photovoltaic devices under study, without additive (pristine), SnF2 and [SnF2(DMSO)]2 additives into the Sn-Pb perovskite precursor solution. 96x46mm (300 x 300 DPI)


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New Tin (II) Fluoride Derivative as Precursor for Enhancing the Efficiency of Inverted Planar Tin/Lead Perovskite Solar Cells Teresa S. Ripolles,1 Daiki Yamasuso,1 Yaohong Zhang,2 Muhammad Akmal Kamarudin,1 Chao Ding,2 Daisuke Hirotani,1 Qing Shen,2 Shuzi Hayase1* 1

Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4

Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan 2

Department of Engineering Science, Faculty of Informatics and Engineering, The University of

Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan

*Corresponding author: S. H. E-mail: [email protected] Telephone: +81-93-695-6044


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ABSTRACT: Hybrid organic-inorganic lead halide perovskite has become one of the most attractive materials for future low-cost and high-efficiency solar technology. However, serious concern about the content of toxic lead in photovoltaic devices emphasizes the mixture of tin/lead. Here, we synthesized and fully characterized tin (II) fluoride with a molecule of dimethyl sulfoxide as [SnF2(DMSO)]2 complex to be used as an additive in the Sn-Pb perovskite precursor to photovoltaic applications. Once the concentration of the SnF2 complex had been optimized to be 10 mol %, the highest efficiency achieved in inverted architecture was 15.93 % with open circuit voltage of 0.77 V, short circuit current of 26.53 mA cm-2 and fill factor of 0.78. Transient absorption and impedance spectroscopy corroborated that this complex reduces the trap states and improves the transport recombination at interface compared with solar cells without additives or SnF2 additive which the latter is widely used. Here, we suggest an alternative method to control the Sn+2 reaction in Sn-Pb perovskite devices.


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Introduction Since the seminal paper published by Miyasaka et al. on the use of metal halide perovskite for solar cell application, the number of research paper on the topic has increased significantly.1 The valuable properties of perovskite semiconductors have attracted great attention due to their numerous applications on electronics and photovoltaic applications. One of the most common used material for thin film photovoltaics is CH3NH3PbI3 (MAPbI3) due to achieve high photoconversion efficiencies,2-3 stable crystal phase at room temperature,4 high absorption coefficients,5 and high charge carrier mobility6 and diffusion length.7-8 However, despite these unique properties, the use of lead limit the large-scale production as lead is known to be toxic.9 Thus, arises the need to replace lead with non-toxic materials. One candidate is tin due to its abundance in nature and does not pose a threat to human and the surrounding. The first attempt to prepare tin halide perovskite was reported by Mitzi et al. using a low temperature process to produce polycrystalline tin perovskite.10 The same group also prepared 2D tin perovskite, however no photovoltaic properties were reported in the paper.11 The first tinbased lead-free perovskite solar cells was reported by Snaith et al. in 2014, achieving a maximum efficiency of 6 %.12 However, encapsulation was necessary as the material is unstable in ambient atmosphere. Hayase et al. reported the performance of mixed Sn and Pb perovskite solar cell with 4 % efficiency.13 They cited the role of Pb to reduce the oxidation of Sn in addition to the improvement in the light absorption of up to 900 nm. Since then, the efficiency of Sn-Pb perovskite solar cells has increased up to ~17 %.14-15 However, tin perovskite has been shown to be instable in air where it readily oxidises from Sn2+ to Sn4+, meaning that the solar cell must be encapsulated before undergoing measurement in air.16 The oxidation of Sn2+ left vacancies into the perovskite film which act as recombination


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traps by capturing free charge carriers and reduces the cell performance. To address this drawback, reducing agent such as tin (II) fluoride (SnF2) has been added as precursor to the perovskite solution.17 SnF2 is found to be effectively suppress the formation of Sn4+ cations and help to decrease the conductivity of the perovskite film. Yan et al. has found that the optimum concentration is approximately 10 mol % SnF2 with respect to the molar concentration of Sn where higher concentration will result in phase separation.18 Other alternatives of SnF2 have been studied which are beneficial in terms of crystal growth and highly uniform and flat perovskite layers. In particular, PbI2 when dissolved in dimethlyformamide, DMF, or dimethyl sulfoxide, DMSO, forming its counterparts in complex.19 These complexes are responsible for grain boundary healing process.20 Due to surface morphology improvements, devices prepared with complex showed higher photovoltaic performance and with less hysteresis.21 Seok et al. has explored the use of SnF2-pyrazine complex to prepare tin-based perovskite solar cells in which resulted the formation of dense and uniform perovskite layer with low Sn4+ concentration.22 Recently, Wakamiya et al. has reported the synthesis of tin halide complexes without impurities such as Sn4+ through sublimation process.23 However, tin-based complexes solar cells were not reported. These additives present a route for improving the performance of tin-based perovskite solar cells. Herein, we suggest a new SnF2 derivative coordinated with DMSO molecules such as [SnF2(DMSO)]2 complex to avoid the facile reaction of Sn2+ oxidation into the light absorber precursor in inverted planar Sn-Pb mixture perovskite solar cells. The SnF2 complex was synthesized by a simple method already reported and then characterized by numerous techniques. The photovoltaic applications were carried out in inverted planar mixture Sn-Pb perovskite solar cells which the SnF2 complex was added into the perovskite Sn precursor


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solution with different concentrations, being the optimum 10 mol %. Comparable performances were obtained to SnF2 additive but device characterization suggested that the new additive showed better electrical processes. Experimental section Materials. Materials purchased by TCI were formamidinium iodide (HC(NH2)2I, FAI, >98.0%), methylammonium iodide (CH3NH3I, MAI, >98.0%), and methylammonium bromide (CH3NH3Br, MABr, >98.0%). The other materials were purchased by Sigma-Aldrich, tin (II) iodide (SnI2, 99.99%, trace metals basis), lead (II) iodide (PbI2, 99.99%, trace metals basis), lead (II) bromide (PbBr2, 99.99%, trace metals basis), tin (II) fluoride (SnF2, 99%), dimethyl sulfoxide (DMSO, ≥99.9%, anhydrous), and N,N-dimethylformamide (DMF, 99.8%). All materials were used as received. The hole transport material PEDOT:PSS pursed by Heraeus. [SnF2(DMSO)] 2 complex synthesis. The additive was synthetized by a method previously reported.24 SnF2 power was dissolved by DMSO and heated at 120 ºC until complete dissolution. Then, a cool bath precipitated a white solid, which was washed with CH2Cl2 and dried in vacuum. Undoped and Sn-Doped Precursor Solutions. (FASnI3)0.6(MAPbI3)0.3(MAPbBr3)0.1 perovskite solution was prepared according to a previously reported procedure.25 Three perovskite solutions, FASnI3, MAPbI3, MAPbBr3, were synthetized into glove box at 40 ºC stirring for over 3 h. 447 mg/mL SnI2 and 206.4 mg/mL FAI dissolved in DMSO:DMF (1:4 by vol:vol) synthetized FASnI3 solution. MAPbI3 perovskite was prepared by mixing 553.2 mg/mL and 190.8 mg/mL in DMSO:DMF (1:4 by vol:vol). MAPbBr3 solution was synthetized by 220.2 mg/mL and 67.2 mg/mL in DMSO:DMF (1:4 by vol:vol). The mixture by volume of 0.6:0.3:0.1 of each (FASnI3):(MAPbI3):(MAPbBr3) define the stock undoped perovskite solution. The


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solution was kept in a sealed glass vial and stirred for about 2 h at 40 °C and homogeneous yellow solution was obtained. The doped solutions were prepared by adding to FASnI3 solution either 10 mol % of SnF2 or x mol % of [SnF2(DMSO)]2, being x 5, 10, and 25 %. Device fabrication. The pre-etched indium tin oxide (ITO) substrates (10-15 ohm, Nippon Sheet Glass, Japan) were cleaned ultrasonically with deionized water, acetone, and isopropanol for 10 min, respectively. The hole transport layer PEDOT:PSS (60 nm) was deposited onto ITO substrates in two sequential steps, 500 rpm for 5 s, and 4000 rpm for 60 s, by spin coater in air atmosphere and an annealing treatment at 175 ºC for 30 min was carried out. Then, the substrates were transferred into glove box under nitrogen ambient until end of the device fabrication. The as-prepared perovskite precursor solutions were filtered using 0.45 μm PVDF syringe filter and coated onto the ITO/PEDOT:PSS. The absorber layer (250 nm) was deposited at room temperature by one step and anti-solvent method by spin coater as 4000 rpm for 60 s and during the first 10 s of the spinning process, the substrate was treated by drop-casting toluene solvent 1 mL. Finally, a 20 nm C60, 5 nm BCP, and 70 nm Ag were thermal evaporated through a shadow mask to define the cell area of 0.1 cm2. [SnF2(DMSO)] 2 complex characterization. The thermal stability of the perovskite was analyzed by TG and DTA). The DTG-60 Shimadzu measured simultaneously TGA and DTA at a heating rate of 10 ºC/min under N2 atmosphere from room temperature to 500 ºC. FT-IR spectra were registered by using a FP-6500 JASCO spectrofluorometer in KBr pellets. The spectra were the result of averaging out 32 scans obtained at ambient temperature in wavelength ranging from 4000 to 500 cm-1. H-NMR measurements were carried out using a Bruker Avance400 NMR spectrometer. The crystal structure of the films (FASnI3)0.6:(MAPbI3)0.3:(MAPbBr3)0.1 with x mol % [SnF2(DMSO)]2, being x 5, 10 and 25, as additive was examined using a Rigaku Smartlab x-


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ray diffractometer from 10 to 60 deg at a scanning step of 0.01/s with monochromatic Cu K radiation (= 1.54056 Å, 45 kV/200 mA). The HOMO levels were measured through PYS (KV205-HK, Bunkoukeiki Co., Ltd, Japan). Characterization of perovskite solar cells. J-V measurements were carried out in a solar simulator (CEP- 2000SRR, Bunkoukeiki Inc) under AM1.5G light intensity (100 mWcm-2) from short circuit to open circuit direction, forward direction, and the opposite direction which is reverse direction. The light intensity was calibrated with a silicon solar cell. All solar cells were measured using a mask with active area of 0.1 cm2. The photovoltaic measurements were carried out at a scan rate of 0.1 Vs-1 with 100 ms delay time and 10 mV voltage step. The measurement of IPCE was performed through CEP-2000SRR, Bunko Keiki with 300 W Xe lamp. The monochromator was adjusted to 1×1016 mWcm-2 and was monitored by Si photodiode. In the fs-TA setup,26-27 the laser source was a titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs. The light was separated into two parts. One part was used as a probe pulse. The other part was used to pump an optical parametric amplifier (OPA) (a TOAPS from Quantronix) to generate light pulses with a wavelength tunable from 290 nm to 3 µm. This was used as the pump light to excite the sample. In this study, the pump light wavelength was 470 nm and the probe beam wavelength was 920 nm. For all measurements, the pump and probe light were irradiated from the glass side and the TA measurements were carried out in a N2 atmosphere. The IS measurements were performed using an Autolab PGSTAT204 equipped with a frequency analyzer module FRA32M. All measurements were carried out under light from short circuit conditions to close open voltage. The range of the frequencies was between 10 Hz to 1 MHz in an amplitude of 0.01.


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Results and discussion [SnF 2 (DMSO)] 2 complex characterization The complex [SnF2(DMSO)]2 synthetized was characterized by several techniques. Firstly, using thermogravimetric analysis, the material was subjected to thermal analysis to observe the weight change over time and temperature. The thermogravimetry curves of [SnF2(DMSO)]2 complex featured series of decompositions at different temperatures (Figure SI 1). At 100 ºC, an initial weight loss was observed which could be ascribed to evaporation of water about 2% of total weight of the sample, the second weight reduction caused by the solvent DMSO volatility, and finally, the third decrease was because of material decomposition. From room temperature to 500 ºC, the [SnF2(DMSO)]2 complex was decomposed about 12 % of the total weight at room temperature. Same analysis was carried out for SnF2 additive which represents the material commonly used. Pure SnF2 gain weight about 1.5 % at higher temperatures over 500 ºC which means that SnF2 has been oxidized into SnF4 and SnO2. The decomposition temperature of each material was corroborated by differential thermal analysis (DTA) (Figure SI 1). The decomposition for SnF2 powder started at 280 ºC and for the [SnF2(DMSO)]2 complex at 307 ºC. Secondly, Fourier-transform infrared (FTIR) spectroscopy was used to confirm the newly synthesized compound where the change in dipole moment upon infrared irradiation could give the information about the molecule (Figure SI 2). The SnF2 pristine and SnF2 complex indicated similar peaks related to reference sample of potassium bromide KBr such as 3423 cm-1, 3273 cm1

, and 1635 cm-1, corresponded to the vibrations of 3 asymmetric stretch, 1 symmetric stretch,

and 2 bend, respectively. In particular, [SnF2(DMSO)]2 complex showed additional peaks at


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1424 cm-1 which corresponds to the asymmetric stretch of the S-O bond and at 996 cm-1, 934 cm1

, and 557 cm-1 that match with the ester group (S-OR).

Finally, proton nuclear magnetic resonance spectroscopy (H-NMR) was performed to determine the structure of the synthesized material. Both spectra (Figure SI 3) showed two main peaks at 7.26 ppm and 1.5 ppm that corresponds to the solvent deuterated chloroform, CDCl3, and moisture, respectively. Additional peak depicted at 2.61 ppm corresponds to SnF2 complex sample. Photovoltaic characteristics of [SnF 2 (DMSO)] 2 -based solar cells Films of Pb/Sn perovskite (FASnI3)0.6:(MAPbI3)0.3:(MAPbBr3)0.1 prepared with different molar concentrations of [SnF2(DMSO)]2 deposited on glass substrates were analyzed by x-ray diffraction XRD. As shown in Figure 1a, the main peaks observed in all samples are located at 14.16 º and 28.47 º, that correspond to (110) and (220) lattice planes. The addition of 5 mol %, 10 mol % and 25 mol % SnF2 complex into the Sn-Pb perovskite solution did not resulted in additional peaks but the intensity of the peaks increased with higher concentration of SnF2 complex (Figure SI 4). Therefore, it can be assumed that the complex did not distort the crystal structure of the perovskite. Similar diffraction patterns were reported for these type of perovskites.15, 25


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Figure 1. (a) XRD patterns and (b) relative lattice strain calculated by Williamson-Hall method of Pb/Sn perovskites with different concentrations of SnF2 complex as an additive.

Further analysis of the XRD data gives the information about the relative strain within the material. The strain is estimated using Williamson-Hall plot in which Gaussian and Lorentzian fitting were used.28 Detailed discussion on the determination of the relative strain from Williamson-Hall plot is given in the Supporting Information. It can be observed that the relative strain in Figure 1b is smallest at 5 mol % [SnF2(DMSO)]2 and is larger than pure sample after doping with more than 10 mol %. From the XRD plot, no peak shift was observed, however peak broadening was detected. This peak broadening suggests that the crystals are inhomogeneously strained, induced by vacancies, lattice dislocations or any other structural defects.29-30 Photovoltaic solar cells showed in Figure 2a with an inverted configuration of ITO/PEDOT:PSS/Sn-Pb Perovskite/C60/BCP/Ag were prepared with different doping level concentrations of SnF2 complex, such as 0, 5, 10, 25 mol %.


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Figure 2. (a) Device structure of the inverted perovskite solar cells and the thickness of each layer; (b) J-V curves of the devices without/with (5 mol %, 10 mol %, and 25 mol %) additive of SnF 2 complex measured under simulated 1 sun; (c) IPCE measurement and the Jsc calculated from IPCE spectra.

The current density-voltage (J-V) characteristics of the champion solar cells are shown in Figure 2b and the data is summarized in Table 1. The data revealed that, as expected, the addition of SnF2 complex into the solution precursor improve all photovoltaic characteristics respect to the control device. In particular, the short circuit current Jsc increased from 13.92 mA cm-2 to 24.66 mA cm-2, fill factor FF from 0.63 to 0.69 and open circuit voltage Voc from 0.46 V to 0.58 V with a small amount of SnF2 complex 5 mol % into Sn-Pb perovskite compared to pristine solar cell. Further increasing of the complex, increased the Voc up to 0.70 V, however, the Jsc reduced almost 3 mA cm-2 due to the additive affect the charge extraction. Therefore, 10 mol % of SnF2 complex was the optimum condition to achieve the highest efficiency of 12.36 % with Jsc of 24.78 mA cm-2, FF of 0.72, and Voc of 0.69 V. It is important to note that the Incident Photon-tocurrent Conversion Efficiency (IPCE) showed in Figure 2c was in agreement with the Jsc results. By integrating the IPCE spectra, the Jsc can be calculated and was well-fitted with those obtained at simulated 1 sun illumination intensity (Table 1). As indicated above, small amount of the additive increases the charge extraction.


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Table 1. Photovoltaic parameters under AM 1.5G simulated illumination from a solar simulator of x mol % [SnF2(DMSO)]2, being x 0, 5, 10, and 25, as additive in Sn-Pb perovskite solar cells and the Jsc calculated by the integration of the IPCE spectra.

[SnF2(DMSO)]2 (mol %) 0 5 10 25

Jsc (mA/cm2)

Voc (V)

FF

Eff (%)

Rs (Ωcm2)

Rsh (Ωcm2)

Jsc cal. (mA/cm2)

13.92 24.66 24.78 21.87

0.46 0.58 0.69 0.70

0.63 0.69 0.72 0.67

4.00 9.89 12.36 10.25

3.57 2.03 2.03 2.60

370.30 497.10 636.40 459.10

13.32 23.27 23.63 22.06

Once the concentration of [SnF2(DMSO)]2 complex was optimized to 10 mol %, same experimental conditions were used to compare devices fabricated with pure SnF2 and [SnF2(DMSO)]2 complex. J-V curves measured under AM 1.5 G illumination, 100 mW cm-2, plotted in Figure 3a indicates that the complex showed slightly higher Voc, while the other parameters remain constant (see Table 2 and Figure 3a). Figure SI 5 analyzed the forward and reverse scan directions of J-V curves under 1 sun. Slightly hysteresis performance was observed for pristine and SnF2 solar cells, and non-hysteresis effect was demonstrated for SnF2 complex device. With regard to IPCE results showed in Figure 3b, the Jsc calculated from the IPCE spectra were in agreement with the current measured from the J-V curves under simulated 1 sun illumination.


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Figure 3. J-V curves under simulated 1 sun illumination intensity and IPCE of solar cells pristine, SnF2 complex and SnF2 as additives into the Sn-Pb perovskite precursor.

Cell stability is other main parameter to take into account. The solar cells were kept into the glove box for 13 days and under dark conditions. The J-V curves are plotted in the Supporting Information (Figure SI 7). It is interesting to note that the device with SnF2 complex suffers less degradation processes than the other two devices under study, without additive or SnF2 counterpart. The coordination of [SnF2(DMSO)]2 molecules with the perovskite structure belong longer time stable. It is clearly observed that after 13 days, the perovskite without additive or with SnF2, decreased all photovoltaic parameters drastically. However, the device with [SnF2(DMSO)]2 complex into the perovskite remained the Jsc, and the Voc and the FF decreased slightly. Table 2. Photovoltaic characteristics of devices without additive (pristine) and with 10 mol % of [SnF2(DMSO)]2 and 10 mol % of SnF2 as additives in Sn-Pb perovskite solar cells. The Jsc calculated from the integration of IPCE spectra is also included.

Additive Pristine SnF2

Jsc (mA/cm2)

Voc (V)

FF

Eff (%)

Rs (Ωcm2)

Rsh (Ωcm2)

21.64 26.55

0.50 0.74

0.66 0.77

7.04 15.23

2.55 1.84

665.30 2330.00


Jsc cal. (mA/cm2) 20.59 24.96

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[SnF2(DMSO)]2

26.53

0.77

0.78

15.93

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1.68

2326.00

25.03

Optical and Electrical properties of [SnF 2 (DMSO)] 2 -based solar cells The optical absorption properties of the samples were studied using a gas-microphone photoacoustic (PA) technique.25 A 300 W xenon arc lamp was used as the light source. A monochromatic light beam was obtained by passing the light through a monochromator. This beam was modulated with a mechanical chopper and focused onto the surface of a sample placed inside a sealed PA cell with nitrogen gas. PA spectrum measurements were carried out within the wavelength range of 500-1200 nm with a modulation frequency of 33 Hz at room temperature. The PA signal was measured by first passing the microphone output through a preamplifier and then to a lock-in amplifier. The PA spectra were normalized using the PA spectrum from a carbon black sheet. Table 3. Photoacoustic data energy band gap E g, Urbach energy Eu, and steepness factor .

Additive Pristine SnF2 [SnF2(DMSO)]2

Eg (eV)

Eu (meV)



1.40 1.36 1.33

49 25 20

0.51 1.0 1.25

Figure SI 6 showed the optical absorption spectra of the pristine, and with additives SnF2 derivatives samples on glass measured using the photoacoustic technique at room temperature. Using these spectra, the bandgap energies Eg were calculated for each sample (Table 3), being 1.40 eV for the control Sn-Pb perovskite and Eg reduced with the addition of SnF2 complex to 1.33 eV and SnF2 to 1.36 eV. Taking into account Eg data and the Photoemission Yield Spectroscopy (PYS) (Figure SI 8) to measure the HOMO levels, the energy level diagram is plotted in Figure 4.


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Figure 4. Energy level diagram of (a) control perovskite, and perovskite solved with either (b) SnF 2 or (c) [SnF2(DMSO)]2 additives.31 The Eg was calculated by PA, and the HOMO level was extracted from PYS.

Other important parameter to estimate from PA spectra is the Urbach energy (Eu) that was obtained from the exponential slope of the absorption spectra (Figure SI 6). This exponential tail offers information about band structure, disorder, defects, impurities, and electron-phonon interactions in semiconductor materials.32 It is considered that Eu reflects the reflection of the disorder and/or defects in the semiconductor crystal. The defect states were reduced with the addition of SnF2 complex, lower Eu showed in Table 3, into the perovskite solution, and increased with SnF2 and without additive. Correspondingly, the steepness factor  (kBT/Eu, where kB and T are Boltzmann’s constant and the absolute temperature, respectively) increases as the addition of SnF2 complex was used. As a consequence, control Sn-Pb perovskite showed faster recombination of photoexcited carriers and performed lower Voc and FF. These conclusions are in concordance with the TA and IS results explained below. To explore detailed mechanisms of carrier dynamics, femtosecond transient absorption (fs-TA) measurements were carried out.26-27 The fs-TA measurements were performed with a pump light wavelength of 470 nm and the probe beam wavelength of 920 nm. More details of the experiments are presented in experimental section. Figure 5 shows the TA response of the Sn-Pb perovskite absorbers without additives (pristine) and with additives of SnF2 and [SnF2(DMSO)]2,


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after photoexcitation. It is observed that the decay of the TA response becomes much slower when a small amount of additive was added as a precursor into the perovskite solution.

Figure 5. TA responses of Sn-Pb perovskite without additive (pristine), with additives [SnF 2(DMSO)]2 and SnF2 on glass substrate. The solid line corresponds to the fit data with an exponential function.

All of the TA responses can be fitted very well by an exponential equation as shown in Figure 5 where the solid line corresponds to fitting results, and the decay time constants correspond to the lifetimes of the photoexcited carriers in the Sn-Pb perovskites which are summarized in Table 4. The photoexcited carrier lifetime of the pristine Sn-Pb perovskite layer is as short as 32 ps, mainly due to high unoccupied trap states of impurities into absorber from Sn4+. However, longer lifetimes of the photoexcited carriers are detected with the additives because reduced the defects. It should be emphasized that the SnF2 complex showed the longest photoexcited carrier lifetimes close to 960 ps, and shorter lifetime was detected for the well-known SnF2 additive of 305 ps approximately. These data are also in good agreement with the Eu which reflects the defect density in the Sn-Pb perovskite layer prepared under different conditions (Table 3). Therefore,


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these results indicate that the preparation method with the additives [SnF2(DMSO)]2 could largely reduce the defects in Sn-Pb perovskite. Table 4. TA decay time constant at 920 nm.

Additive Pristine SnF2 [SnF2(DMSO)]2

 (ps) 32±1 304±8 957±17

To corroborate the photovoltaic and fs-TA results, impedance spectroscopy IS analysis was carried out under light to corroborate that the new additive SnF2 complex showed better electrical properties compared to the (i) widely used SnF2 additive into the precursor solution and (ii) without additive Sn-Pb perovskite solar cells. The IS measurement was performed under constant simulated 1 sun light intensity from short circuit to open circuit conditions every 100 mV. Each bias applied, the measurement was carried out in a frequency range from 1 MHz to 100 mHz using a potentiostat. An AC voltage with a perturbation amplitude of 20 mV was applied on the device.


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Figure 6. (a) Impedance spectra at short circuit conditions and (b) resistance at high frequency regime under constant AM 1.5 G light intensity from 0 V to open circuit voltage for the photovoltaic devices under study, without additive (pristine), SnF2 and [SnF2(DMSO)]2 additives into the Sn-Pb perovskite precursor solution.

The impedance spectra showed two semicircles in Figure 6a and were well-fitted with the equivalent circuit model reported previously.33 In particular, we focused in the resistance at high frequency range because corresponds to the transport resistance of electron along the perovskite layer interface which is plotted in Figure 6b.34 As expected, the pristine device showed poor transport resistance that is reflected in the lowest Voc (see Table 2). Sn-Pb perovskite film require an additive to improve the charge transport and avoid the Sn2+ oxidation that easily occurs in atmosphere or nitrogen environments. To that end, the additives that are under study in this manuscript, improved the electron transport at the interface as shown in Figure 6b. At same voltage applied, the solar cell with [SnF2(DMSO)]2 as an additive showed slight increase resistance compared to SnF2 counterpart, that indicates less-trap states related to the recombination processes at interface. This conclusion is in agreement with the density-of-states


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(DOS) measured from PYS measurement (Figure SI 9), which a narrow DOS of SnF2 complex and width bands were observed for pristine and SnF2-based solar cells. Conclusions To summarize, SnF2 coordinate with DMSO molecule as [SnF2(DMSO)]2 complex has been demonstrated to be an alternative in Sn-Pb perovskite solar cells, achieving high photoconversion efficiencies. The concentration of 10 mol % [SnF2(DMSO)]2 showed comparable photovoltaic response but slightly improvements in the Voc to SnF2 additive which is the common additive used in Pb-free solar cells. However, the new additive showed reduction in trap states, larger recombination lifetime and improvements in the charge transport recombination at the interface concluded from photoacoustic transient absorption and impedance spectroscopy measurements. The results suggested that this additive is a great candidate to replace the common additive SnF2 in Sn-based solar cells. Acknowledgments This work has been supported by KAKENHI from the Japan Society for the Promotion of Science (JSPS) under the Grant-in-Aid-for Young Scientist B (Grant Number JP16K17947). Supporting Information Tg, DTA data, FT-IR data, H-NMR data, and XRD data of [SnF2(DMSO)]2 complex, Williamson-Hall plot, and hysteresis, photoacoustic spectra, cell stability and PYS of perovskite films. References


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