Interplay between Iodide and Tin Vacancies in CsSnI3 Perovskite

Jul 10, 2017 - C , 2017, 121 (30), pp 16447–16453 .... Korea Basic Science Instrumentation Center, Busan, using a pulsed 30 keV Bi+ ..... We have ob...
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Interplay between Iodide and Tin Vacancies in CsSnI3 Perovskite Solar Cells G. Rajendra Kumar,† Hee-Je Kim,† Senthil Karupannan,§ and Kandasamy Prabakar*,† †

Department of Electrical and Computer Engineering, Pusan National University, San 30, Jangjeong-Dong, Gumjeong-Ku, Busan-46241, South Korea § Department of Physics, Bannari Amman Institute of Technology, Sathyamangalam 638 401, Tamil Nadu India S Supporting Information *

ABSTRACT: Tin-based perovskite materials have the drawbacks of high density of Sn vacancies, structural deformations of SnI6− octahedra, and oxidation of unstable Sn2+ states, resulting in poor chemical stability processed at inert and open atmosphere. In this work, we demonstrate the temperature effects on reduction of Sn vacancies (Vsn) in polymorphic CsSnI3 perovskite solar cells. Evidence of light-induced I− ion diffusion and an interplay between iodide and Sn vacancies were briefly presented. We have observed by X-ray photoelectron spectroscopy that the formation of iodide vacancies (VI) are highly activated at 60 °C, contributing to the reduction of acceptor defects, mainly Sn vacancies (Vsn). The formation of SnO− and SnO2− at high temperature mitigates the Sn−I interaction and eventually increases the trap density at Au/CsSnI3 interfaces. We have observed by time-of-flight secondary ion mass spectrometry that the delocalized I− ions are accumulated near the metal contacts and form AuI− ions which diffuse through the material, inhibiting the exciton transport.

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INTRODUCTION Recently, the organic−inorganic perovskite solar cells (PSCs) have brought a promising material to photovoltaic technology. Especially, the methylammonium lead iodide (CH3NH3PbI3) PSCs power conversion efficiency (PCE) exceeded 22% within a short span of time and has become an attractive candidate for the solar cells.1−3 Despite the interesting properties of CH3NH3PbI3 and its multitude of fabrication techniques, the instability remains unsolved. Moreover, the toxicity of lead (Pb) limits its application and demand to find an alternative material for large-scale applications.4,5 The organic−inorganic Sn halide perovskite CH3NH3SnI3 perovskite was proposed to replace Pb2+ with less toxic Sn2+ divalent cation and achieved PCE of 5.23%.6,7 However, tin-based perovskites have a high density of Sn cation vacancy defects which serve as p-type dopants and recombination centers in addition to natural oxidation of Sn2+ to Sn4+ when exposed to air and even in glovebox with small quantity of oxygen.8,9 Currently, the mixture triple cation materials such as FA0.75Cs0.25Sn0.5Pb0.5I3 (FA = CH(NH2)2) were demonstrated for stabilizing Sn2+ cations.10 Then, the inorganic halide perovskite (ABX3, where A is an inorganic cation (Cs+), B is usually Sn2+, Bi2+, or Pb2+, and X is a halide anion I−, Br−, Cl−) were introduced to overcome the stability and hysteresis issues caused by unstable CH3NH3+ cation.11,12 Each inorganic counterpart brings new challenges for optimizing its superior performance in solar cell applications. The cesium-based inorganic (CsPbX3) perovskite compounds showed a parallel © 2017 American Chemical Society

photovoltaic performance with their organic counterpart, thus offering new potential platform to fabricate stable PSCs. However, the cubic α-CsPbI3 films with a bandgap of 1.77 eV are formed at a temperature of 300 °C and undergo immediate phase transition to orthorhombic phase when exposed to ambient conditions. However, the stable yellow phase of CsPbI3 obtained at RT with orthorhombic structure is nonphotoactive. Further, the unstable phase was stabilized by alloying with Br− (CsPbIBr2) and showed orthorhombic to cubic phase transition temperature at 100 °C at the cost of increased bandgap.13,14 On the other hand, CsSnI3 perovskite has an ideal bandgap of 1.4 eV for solar cell applications with higher theoretical short-circuit photocurrent (Jsc) density of 34.3 mA cm−2 compared to 25.9 mA cm−2 of CH3NH3PbI3 perovskite (1.55 eV) due to extended light harvesting in the infrared region.7 The CsSnI3 (black phase) has highest hole mobility (∼585 cm2 V−1 s−1) among p-type materials with conductivity (∼200 S cm−1).15 Considering the above facts, CsSnI3 can be used as both light absorber and hole conductor in solar cell applications. The carrier concentration and conductivity of CsSnI3 depend on the concentration of Sn, I, and Cs vacancies, which is important to optimize the solar cell performance.16 CsSnI3 was used as a hole conductor in solid-state dyeReceived: June 27, 2017 Revised: July 8, 2017 Published: July 10, 2017 16447

DOI: 10.1021/acs.jpcc.7b06278 J. Phys. Chem. C 2017, 121, 16447−16453

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The Journal of Physical Chemistry C

cells. The spin-coated substrates were preheated at 40 °C for 5 min followed by solvent treated with chlorobenzene and then annealed at 60 and 150 °C for 15 min in ambient atmosphere. Finally, an 80 nm thick gold electrode was deposited by thermal evaporation system (Korea Vacuum Tech) through a shadow mask at a pressure of 3 × 10−6 Torr. The active area of the device was 0.09 cm2. Solar cells were fabricated under the open atmosphere, and the humidity was about 50%. 2.2. Crystal Structure and Morphological Analysis. The phase identification of the spin-coated CsSnI3 films was analyzed by X-ray diffraction (XRD; Bruker D8-Advance) with Cu Kα radiation (λ = 1.540 Å) source operated at 40 kV and 30 mA in the range 20−70. 2.3. Surface and Chemical State Analysis. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher Scientific (U.K.) ESCALAB 250 system with monochromatic Al Kα radiation of 1486.6 eV and with an electron takeoff angle of 45°. The measurement was conducted without ion etching, to avoid reconfiguration of the bonds, and the pressure of the chamber was kept at 10−10 Torr during measurement. The survey spectrum was scanned in the binding energy (BE) range 100−1200 eV in scan steps of 1 eV with spot size 500 μm. All the obtained binding energy was compensated with the core level peak of adventitious carbon (C 1s) at 284.6 eV fixed as a reference. The uncertainty in binding energy position was within 0.05 eV for an element. Peak fitting and quantitative analysis were done using Casa XPS program (Casa Software Ltd.), and the results were justified by average matrix relative sensitivity factor (AM-RSF) on peak area and an atomic sensitivity factor of the identified components. The binding energy of the identified components was assigned from the NIST X-ray Photoelectron Spectroscopy Database (NIST Standard Reference Database 20, Version 4.1). 2.4. Current−Voltage (J−V) Characteristics. The current−voltage (J−V) characteristics of PSCs were examined under one sun illumination (AM 1.5G, 100 mW/cm2) using an ABET Technologies (USA) solar simulator with an irradiance uniformity of ±3%. 2.5. TOF-SIMS Characterization. TOF-SIMS experiments were performed with a TOF-SIMS (ION-TOF GmbH, Münster, Germany) at Korea Basic Science Instrumentation Center, Busan, using a pulsed 30 keV Bi+ primary beam with a current of 1.01 pA. Negative ion spectra were internally calibrated using H−, C−, C2−, C3−, and C4− peaks normalized to the respective total secondary ion yields. The chemical images of the analyzed area were recorded with 128 × 128 pixel resolution during the data acquisition. The depth profile is a square of 500 μm × 500 μm using Cs beam 1.0 keV.

sensitized solar cells, and highest PCEs of 10.2% with SnF2 doping and 3.72% for pure CsSnI3 were reported .17 However, it has been claimed that B-γ-CsSnI3 is not energetically favorable to act as hole conductor with N719 dye, despite being a good hole-conducting material.18 Moreover, CsSnI3, used as a light absorber by evaporation, exhibited very low PCE of 0.9%19 due to its high p-type electrical conductivity which causes the solar cells to have low shunt resistance and low efficiency. CsSnBr3 planar solar cells prepared by vapor deposition exhibited a PCE of 0.55% under ambient conditions while solution processed with SnF2 additives exhibited a PCE of 2.1%.17,20 Unfortunately, the addition of SnF2 into CsSnI3 results in poor film morphology and generates high Jsc but fails to increase the Voc and PCE. The Voc of CsSnI3 (0−0.4 V) PSCs is limited by the existence of Sn vacancies, and hence SnF2 was added into CsSnI3 as a passivating dopant to reduce recombination and control the metallic conductivity of CsSnI3 perovskite. The highest PCE reported in CsSnI3 perovskite is less than 2% with SnF2 dopants.21 Hence, a comprehensive knowledge about the formation of Sn vacancies and CsSnI3 degradation mechanism is highly essential to overcome the fundamental limitations behind the CsSnI3 PSCs. Primarily, it is vital to understand the polymorphic behavior of CsSnI3 perovskite prepared at ambient atmosphere to explore the suitability as a light harvester in PSCs. Therefore, the solution-processed CsSnI3 films were annealed at different phase transition temperatures of 60 and 150 °C for determining its polymorphic behavior at ambient atmosphere. Further, new strategies to reduce the rate of oxidation of Sn-based perovskites prepared in an open atmosphere are highly essential. Here, we have demonstrated the effect of Sn−I interplay and formation of iodide vacancies on reduction of Sn vacancies under heat energy in CsSnI3 PSCs. We have further correlated the dynamics of diffused I− ions at different phase transition temperatures to understand the covalence in Sn−I interaction using time-of-flight secondary ion mass spectrometry (TOF-SIMS).

2. EXPERIMENTAL SECTION 2.1. Solar Cell Fabrication and Characterizations. Fluorine-doped tin oxide (FTO) glass substrate (Hartford Glass) was patterned by Zn metal powder and 1 M HCl in aqueous solution. The patterned substrates were cleaned by ultrasonication with detergent, deionized water, acetone, and ethanol and dried using N2 gas. First, TiO2 compact layer was spin-coated on FTO substrate at 2000 rpm for 30 s using titanium diisopropoxide bis(acetylacetonate) (75% in 2propanol, Aldrich) diluted in 1-butanol (99.8%, Aldrich) solution. The substrates were preheated to 120 °C for 10 min and treated with a 40 mM TiCl4 aqueous solution at 60 °C, followed by annealing at 450 °C for 30 min. After cooling to RT, the mesoporous TiO2 layer was spin coated at 5000 rpm for 30 s using a commercial TiO2 paste (18NRT, Dyesol) diluted in ethanol, followed by annealing at 500 °C for 30 min. Equimolar (0.25 M) mixture of anhydrous cesium iodide (CsI) and tin iodide (SnI2) (Sigma-Aldrich) was dissolved in dimethylformamide (DMF) containing dimethyl sulfoxide (DMSO) solvent of about 5 vol % under continuous stirring at 60 °C for 12 h. The obtained yellow-orange solution was filtered by 0.45 μm poly(tetrafluoroethylene) (PTFE) filter and spin coated several times on FTO/compact TiO2/mesoporous TiO2 layers at 3000 r.p.m for 30 s to fabricate mesoscopic solar

3. RESULTS AND DISCUSSION 3.1. Phase Transition Kinetics under Temperature. Diffraction patterns of CsSnI3 polymorphs (B-γ, B-β, and B-α phase) at different phase transition temperatures are shown in Figure 1. At RT, CsSnI3 shows a black phase (B-γ) threedimensional orthorhombic crystal structure with a space group of 62 (Pnma) having lattice constants a = 8.688 Å, b = 8.643 Å, c = 12.378 Å (ICDD No: 01-071-1898).18−22 It is important to note that the B-γ phase undergoes a phase transition to Y-phase when exposed to air. The diffraction pattern of B-γ-phase shows an unindexed peak (*) at 39.37° and 51.49° attributed to the Yphase, typically described as Cs2SnI6 phase.23,24 The prominent reflected planes of (131), (240), and (320) at 25.25°, 26.44°, and 27.61°(2θ) are the characteristics peaks of CsSnI3 16448

DOI: 10.1021/acs.jpcc.7b06278 J. Phys. Chem. C 2017, 121, 16447−16453

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atomic fractions of Sn2+, Sn4+, I−, and I3− (%) are tabulated in Table S1 (Supporting Information). The high-resolution XPS spectra of Cs 3d, I 3d, and Sn 3d doublets are shown in Figure 2a−c. These doublets were fitted with an area ratio of 3:2 (d5/2:d3/2) due to the degenerate spin states with spin−orbit splitting (SOS) of 14, 11.50, and 8.40 eV respectively.27 Cs 3d core-level spectra were fitted with a single peak corresponding to Cs+ charge state at 726.04 and 739.92 eV for Cs 3d5/2 and Cs 3d3/2 peaks, respectively.28 The minor chemical shift (0.5 eV) found at high temperatures is due to the large ionic radius, and high electropositive monovalent of Cs+ cation forms intrinsic ionic complexes with high electronegative I− anions during annealing. The shift in binding energy of Cs−I complexes may lead to bond polarization and autocompensation at the surface. Here, Cs+ cation can act as a charge compensation center, and its concentration was not affected by temperature. The surface oxidation of CsSnI3 directly affects its phase transformation especially from B-γ phase to Y-phase in air. The I 3d doublets were deconvoluted into two distinct oxidation states associated with I− at 619.13 and 630.65 and triiodide I3− at 620.03 and 631.51 eV at RT respectively. The atomic fractions of I−/ I3−, Sn2+/I−, and Sn2+/I3− are tabulated in Table S2 (Supporting Information). The concentration of VI increased with temperature due to its lower formation energy (0.3 eV) which might decrease the hole concentration.16,29 The Sn 3d doublets were deconvoluted into Sn4+ charge states at 485.6 and 494.1 eV and Sn2+ at 485.2 and 493.7 eV respectively. The high electronegative I− ions form a strong covalent bond with Sn cations as dihedral (I−Sn−I) interfaces for Sn2+ charge states. The area ratio of Sn2+/Sn4+ was used to estimate the density of Sn vacancies at different temperatures in CsSnI3 perovskite and was 0.26 and 0.45 respectively for RT and 150 °C while the fraction of Sn2+ has been increased to 1.98 at 60 °C as shown in Table S2 (Supporting Information). This problem should be addressed in thermodynamic aspect; the increase in internal energy of CsSnI3 with temperature resulted a phase transition due to change in covalency and intermolecular force of SnI− octahedra (tilting).25 The formation enthalpy (ΔH) of CsSnI3 (0.3 eV) is usually very small and hence exhibits a long Sn−I bond length with weak covalency.30 So, we found that the B-γ phase was not dynamically stable due to the enormous Sn vacancy traps at RT. The high concentration of Sn2+ at 60 °C suggests a decrease in Vsn defects and generation of more donor vacancies in CsSnI3. The hygroscopic CsSnI3 underwent oxidation at 150 °C, because annealing was performed in open atmosphere and

Figure 1. XRD spectra of CsSnI3 polymorphs at various phase transition temperatures: orthorhombic (B-γ) at RT, tetragonal (B-β) at 60 °C, and cubic (B-α) phase at 150 °C.

perovskite. The peak splitting observed at 48.9° and 49.1°(2θ) associated with the (004) plane and the transformation from orthorhombic B-γ to tetragonal B-β phase with space group 127 (P4/mbm) was found at 60 °C. The crystallographic symmetry of SnI6− octahedra was shifted during the phase transition with a decrease in the unit cell about two times along the c axis (a = 8.772 and c = 6.261).25 The absence of diffraction peak at 39.37° (2θ) and change in the characteristic peak shape confirm the presence of tetragonal B-β phase at 60 °C. The tetragonal B-β phase was transformed to the B-α phase cubic structure with space group 221 (Pm3m) at 150 °C. In cubic phase, the Sn−I bond length of SnI6− octahedra is equal to one-half of cubic lattice constant (a = 6.219) along the x-axis. The temperature-driven phase transition of B-α phase was confirmed by the formation of new crystal plane (044) at 47.95° while the B-γ phase exhibits two crystal planes of (004) and (321) at the similar diffraction range.15,26 The highsymmetry B-α phase makes a minimum atomic distortion between iodine-bonded tin atoms and low-symmetry B-γ phase in SnI6− octahedra. The mutual rotation of SnI6− octahedra and lattice strain driven by temperature are the key factors for phase transition in CsSnI3 perovskite. 3.2. Surface Chemical States and Interplay of Tin and Iodide Vacancies in CsSnI3. XPS analysis was used to quantify the surface compositions of CsSnI3 and to study their chemical states. The survey spectra, C 1s and Ti 2p core-level spectra of CsSnI3 perovskite at RT, 60, and 150 °C are shown in Figures S1, S2, and S3 (Supporting Information). The

Figure 2. HRXPS of (a) Cs 3d, (b) I 3d, and (c) Sn 3d core peaks performed on CsSnI3 films at different temperatures. 16449

DOI: 10.1021/acs.jpcc.7b06278 J. Phys. Chem. C 2017, 121, 16447−16453

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The Journal of Physical Chemistry C hence high electronegative oxygen more easily interacted with Sn2+ than low electronegative I− ions, forming Sn−O/SnO2 species. However, it is difficult to distinguish Sn−O and SnO2 species due to the small binding energy difference of Sn 3d core levels spectra. The incorporation of oxygen species restricted the formation of Sn−I and eventually increased the VI trap density at high temperature.31,32

4. TOF-SIMS ANALYSIS 4.1. Secondary Ion Mass Spectra Analysis. Before ToFSIMS analysis, 60 and 150 °C annealed films were subjected to the light irradiation (Xenon Arc lamp, 1 kW/m2 under AM 1.5) to investigate the interplay of photons with atomic/molecular fragments of CsSnI3.33,34 Figure 3 presents the SIMS of

Figure 4. ToF-SIMS depth profiles of annealed CsSnI3 films at 60 and 150 °C. The sputtering was performed from top layer in a square area of 500 μm × 500 μm.

Figure 3. ToF-SIMS negative ion mass spectra of CsSnI3 perovskite samples at 60 and 150 °C annealing temperatures.

photon-irradiated CsSnI3 films annealed at 60 and 150 °C. The major signal was observed from I− ions at lower mass region (m/z = 126.90) while [IO]− and [I2]− ions were found at m/z of 142.89 [IO]− and 253.80 [I2]− for 60 and 150 °C annealed films. Importantly, the characteristic signals for CsSnI 3 perovskite were found at m/z 246.80 [SnI]−, 373.71 [SnI2]−, and 627.54 [SnI4]−. The as-identified characteristic molecular fragments are the primary evidence of Sn−I interface in CsSnI3 perovskite. The surface oxidation of CsSnI 3 was confirmed from the predominant m/z at 135.89 of [SnO]− and [SnO2]− in the lower mass region for 150 °C annealed films. The gold molecular fragments appear at m/z 323.87 of [AuI]− and 590.91 [Au3]− due to the light-induced I− diffusion at the CsSnI3 interfaces. The mechanism of light-induced I− diffusion could not be completely elucidated from the mass analysis, and hence ToF-SIMS depth profile was used to study the molecular, interface, and compositional properties of CsSnI3 PSCs.35 4.2. Depth Profile of CsSnI3. Figure 4 shows the ToFSIMS depth profiles of photon-irradiated CsSnI3 at 60 and 150 °C. The Sn cations interact with diffused I− ions and eventually form characteristic SnI−, SnI2−, and SnI4− ions. The distribution of characteristic ions was severely distorted upon annealing at 60 and 150 °C, and their concentrations were increased in the order SnI−> SnI2−> SnI4− due to change in activation energies of diffused I− ions under light illumination. The formation of SnI2− inhibited the Sn4+ oxidation, leading to reduced trap

states, and formed the Sn−I interface that might improve conductivity. The oxygen species were from the surface oxidation of CsSnI3 in ambient atmosphere. The SnO− and SnO2− formed by the interaction of Sn2+ and Sn4+ states with oxygen species from air were lower in concentration at 60 °C than 150 °C. Here, the competition between oxygen and diffused I− ions to interact with Sn2+ and Sn4+ states would have been decided by their electronegativity and ionic radius. Accordingly, the low ionic radius with high electronegative oxygen species more efficiently interacted with Sn2+ and Sn4+ states than the less electronegative diffused I− ions. Hence, the formation of SnO− and SnO2− mitigated the Sn−I interaction and eventually increased the density of traps and created an immobile ionic species at CsSnI3 surface. Meanwhile, the excess drifted mobile I− ions were accumulated at the metal surface, creating an electric field which further restricted the I− ions diffusion. Moreover, the accumulated I− ions interact with gold forms AuI− due to the lower formation energy of I− ions, and the AuI− ion trace was higher at 150 °C than 60 °C annealed samples due to the excess delocalized I− ions in perovskite− metal interface as shown in Figure 4. The Au3− is a representative signal of gold, and its depth profile up to 1000 s sputtering time indicates the diffusion of Au3− ions due to the absence of the hole-conducting layer. The diffused Au3− ions act as metallic traps which further suppressed the exciton transport at the perovskite interface. ToF-SIMS depth profiles 16450

DOI: 10.1021/acs.jpcc.7b06278 J. Phys. Chem. C 2017, 121, 16447−16453

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The Journal of Physical Chemistry C of I2−, IO−, TiO2−, and TiO3− in CsSnI3 at 60 and 150 °C are shown in Figure S4 (Supporting Information). The TiO2− and TiO3− ions were due to the thick mesoporous TiO2 underneath layer in CsSnI3 layer. CsSnI3 signals found up to 2000 s etching proves the efficient infiltration of CsSnI3 into the mesoporous TiO2 layer. The intense I− signals were observed at the top surface of the sample while its derivatives I2− and IO− were detected at the perovskite−mesoporous interface, confirming the distribution of I− ions while the diffusion of its derivatives was not observed at the metal−perovskite interfaces for both 60 and 150 °C annealed samples. 4.3. 3D Secondary Ion Images of CsSnI 3. The reconstructed 3D from the depth profile secondary ion images of CsSnI3 perovskite molecular fragments and its spatial distribution at 60 °C (A, C) and 150 °C (B, D) are shown in Figure 5. It illustrates that the I− ions were uniformly

accumulated at the top surface of the sample at 150 °C than 60 °C due to the higher mobility, and gold derivatives AuI− are formed at the top surface. The 2D and 3D images of remaining species and total ions are shown in Figure S5 and S6 respectively (Supporting Information).

5. LIGHT-INDUCED I− ION DIFFUSION AND FORMATION MECHANISM OF SN−I INTERFACE IN CSSNI3 PSCS The mobile I− ions in the CsSnI3 perovskite materials were associated with the vacancy defects density of adjacent atoms which were controlled by external temperature and light. The CsSnI3 perovskite might have an ionic conduction in addition to electronic conduction due to lower activation energy of I− ion (0.1−0.6 eV) than Cs and Sn species 36 which spontaneously dissociate even at RT. During light interaction with mobile I− ion, three major phenomena occurred: (1) exciton formation; (2) conventional hopping process of I− ion; (3) light-induced I− ion diffusion process as shown in Figure 6.35,37 The light-induced I− ions diffused to the octahedral edge of the adjacent atoms in B-γ phase.38,39 The formation of Sn−I could be beneficial to minimize the Sn4+ charge states at the metal−perovskite interface. The change in crystal orientation caused by temperature forms covalent bond comprising two or four I− ions with Sn atoms under light.40 There may be a lesser number of traps due to the formation of Sn−I at the CsSnI3 interface. At 60 °C, the mesoscopic CsSnI3 perovskite yields the PCEs of 0.90% with high Voc of 0.66 V, the highest to date. The highly distorted SnI6 octahedra found in B-α phase at 150 °C yielded Voc of 0.59 V due to formation of [SnO]− or [SnO2]− deep trap states.41 At higher temperatures, the diffused I− ions interacted with gold to form AuI− and accumulated at the metal−interface which further increased the diffusion of Au through the material. We have observed highest Voc of 0.66 V for hole conductor free mesoscopic CsSnI3 PSCs is much higher than for devices prepared in glovebox. The increase in Voc can be attributed to the reduction of Sn vacancies in CsSnI3 PSCs. The current density−voltage curve of mesoscopic CsSnI3 PSCs is shown in Figure S7 (Supporting Information). Our findings concluded that the diffused high electronegative I− ions inhibit the carrier recombination caused by Sn vacancies and will enhance the photovoltaic characteristics in CsSnI3 PSCs.

Figure 5. ToF-SIMS 3D images for [SnI]−, [SnI2]−, [SnI4]−, [I]−, [SnO]−, [SnO2]−, [AuI]−, and [Au3]− ions in CsSnI3 at 60 °C (A, C) and 150 °C (B, D) annealing temperatures.

distributed in CsSnI3 device. However, uneven distribution was found for SnI−, SnI2−, and SnI4− ions in 60 and 150 °C annealed films. The figures show diffusion of Au3− ions, and the diffusion length is more for 150 °C than 60 °C annealed films and ensures the detrimental effects on the photovoltaic performance by reducing fill factor (FF) and current density (Jsc) of the CsSnI3 PSCs. Meanwhile, the increased density of [SnO2]− and SnO− ions at 150 °C acts as recombination centers which concomitantly prevents the formation of SnI−, SnI2−, and SnI4−. Moreover, higher density of I− ions was

6. CONCLUSION This work emphasizes the photoinduced I− ions diffusion and an interplay between iodide and tin vacancies and their

Figure 6. Mechanism of light-induced [I]− ion diffusion in CsSnI3 polymorph: (a) B-γ phase, (b) B-β phase, (c) B-α phase. 16451

DOI: 10.1021/acs.jpcc.7b06278 J. Phys. Chem. C 2017, 121, 16447−16453

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The Journal of Physical Chemistry C

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collective impact on reduction of Sn vacancies in CsSnI3 PSCs. We have systematically shown the formation of Sn−I interface and correlated the dynamics of I− ion diffusion with temperature by using TOF-SIMS and XPS analysis. Moreover, the light-induced I− ions were accumulated near the metal surface at 150 °C due to the presence of additional SnO2− traps. The core-level spectra revealed that the concentration of Sn vacancy defects decreases, whereas the donor defects concentration (VI) increases at 60 °C. Since the Sn−I formation reduces an accumulation of iodide ions at the metal surface, the accumulated I− ions react with gold to form AuI− ions which diffused through the perovskite interface, minimizing the FF and PCE. Our findings suggested that the Bβ phase of CsSnI3 is more suitable for active light absorber than the other two phases. The intrinsic issues of CsSnI3 PSCs need to be solved for getting its superior photovoltaic performance in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06278. Experimental and characterization section; survey spectra and HRXPS C 1s and Ti 2p spectra of CsSnI3; average matrix RSFs method; TOF-SIMS depth profiling; 2D and 3D analysis; J−V curve of mesoscopic CsSnI3 solar cells (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Tel.: 051- 510-7334. ORCID

Kandasamy Prabakar: 0000-0001-7582-0765 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through a National Research Foundation of Korea (NRF) grant funded by the Korean government (No. 2014005051).

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