Triiodide-Induced Band-Edge Reconstruction of a Lead-Free

Jun 1, 2018 - However, most of the known lead-free hybrids show wide band gaps (Eg > 1.9 ... adopting the zero-dimensional (0D) perovskite-like framew...
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Cite This: Chem. Mater. 2018, 30, 4081−4088

Triiodide-Induced Band-Edge Reconstruction of a Lead-Free Perovskite-Derivative Hybrid for Strong Light Absorption Weichuan Zhang, Xitao Liu, Lina Li, Zhihua Sun,* Shiguo Han, Zhenyue Wu, and Junhua Luo* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China

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ABSTRACT: Bismuth-based hybrid perovskites have recently attracted great attention for their environmentally friendly processing, chemical stability, and photoresponsive properties. However, most of the known lead-free hybrids show wide band gaps (Eg > 1.9 eV) that afford weak visible-light absorption. Here, we show a newly conceptual design strategy of intercalating triiodide to decrease the prototypic band gap by ca. ∼0.44 eV. A new hybrid semiconducting material of (4methylpiperidinium)4·I3·BiI6 (MP-T-BiI6), adopting the zerodimensional (0D) perovskite-like framework, was reconstructed from its prototype of (4-methylpiperidinium)3Bi2I9 (MP-Bi2I9). It is noteworthy that MP-T-BiI6 has a narrow band gap of 1.58 eV, almost comparable to that of CH3NH3PbI3 (∼1.5 eV), which reveals its potential as the highly efficient light absorber. Importantly, its semiconducting properties were solidly confirmed by notable hole mobility (∼12.8 cm2 V−1 s−1), charge-trap density (∼1.13 × 1010 cm−3), and photoconductive behaviors. Moreover, theoretical calculations further disclose that the I-5p orbitals of triiodide induce the band-edge reconstruction and behave as a new conduction-band minimum at the Brillouin zone center. This work paves a potential pathway for the large and diverse family of lead-free hybrids to compete with lead absorbers.

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applications.24−28 Although there has been great attention in Bi-based perovskites as light absorbers, these hybrids have so far displayed wide band gaps of Eg > 1.9 eV,29−35 which show inferior light absorption as compared to MAPbI3 (∼1.5 eV). To obtain the narrow band gap close to that of MAPbI3, one fascinating design strategy is to transmute the divalent metal ions into one trivalent ion Bi3+ and one monovalent ion M+, forming a double perovskite, for example, A2MBiX6. Although numerous efforts have been devoted to designing strong lightabsorbing lead-based perovskites, only limited success has been achieved to date.36−42 In this context, to rationally design new lead-free semiconducting hybrids with strong light absorption is of great importance to both basic research and commercialization. Alternatively, a more attractive branch of Bi-based perovskite hybrids is to incorporate other anions, such as triiodide43,44 or iodide molecule,45,46 to reconstruct the prototypic structures. In particular, the recent study shows that a promising method of introducing iodide moiety (i.e., I3−) into lead-based perovskite could dramatically influence the optical absorption and decrease the concentration of deep-level defects.47 This suggests that the tailoring of I3−-intercalated hybrid might act as an effective strategy to design new lead-free perovskite-like

INTRODUCTION Hybrid organic−inorganic halide perovskites of AMX3 (A = organic cations, Cs; M = metal; X = Cl, Br, I) have recently emerged as a new family of light-absorbers for optoelectronic and photovoltaic applications.1−5 Among them, lead perovskite of MAPbI3 (MA = CH3NH3) has been proven to be a remarkable optical absorbing material for solar cells,6−8 which achieved a certified power conversion efficiency up to 22.1%,9−12 catching up with the commercial silicon-based solar cells. Despite the remarkable performance in a variety of device architectures, the toxicity of lead remains the main obstacle for the commercial applications. Under this condition, the impressive progress of lead-based solar cells promotes the exploration of environmentally friendly light-absorbing materials. Substantial lead-free metal halides, based on tin, bismuth, manganese, palladium, and copper, have been intensively explored and considered as candidates in the large family of metal halide perovskites.13−18 Tin analogous perovskites, such as MASnI3 and H2NCHNH2SnI3, display a narrow band gap at the near-infrared region as compared to their lead hybrid counterparts.19−23 However, the majority of tin-based perovskites are quite unstable when exposed to air and moisture. Another promising group of materials for lead alternatives are the bismuth-based (Bi-based) hybrid organic−inorganic halides, which show much better stability under ambient atmosphere than lead perovskites and provide a series of promising environmentally friendly materials for optoelectronic © 2018 American Chemical Society

Received: March 21, 2018 Revised: May 17, 2018 Published: June 1, 2018 4081

DOI: 10.1021/acs.chemmater.8b01200 Chem. Mater. 2018, 30, 4081−4088

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determined by a cutoff energy of 310 eV for MP-Bi2I9 and 770 eV for MP-T-BiI6. To achieve the accurate density of the electronic states, the k-space integrations were done with Monkhorst−Pack grids with a 3 × 4 × 6 and a 5 × 4 × 3 k-point for MP-Bi2I9 and MP-T-BiI6, respectively. The other parameters and convergent criteria were the default values of CASTEP code. Photoelectric Measurement. The I−V characteristics were determined with a two-electrode configuration at room temperature (Figure S7). The crystal was sandwiched between the rectangular electrodes (0.75 mm × 1 mm) Ag (∼100 nm), deposited on both sides of the single crystal (Ag/MP-T-BiI6/Ag). The thickness of crystals was measured as 1.3 mm (Figure S7). Current−voltage curves of the device performed by a model 6517B electrometer with an ST102D Probe station. Single-crystal photodetectors based on large-size crystals of MP-T-BiI6 were assembled by depositing the silver stripe patterns on the surface of crystals. Photoelectric performance of singlecrystal devices was measured under an AM 1.5 simulated solar light. Photoluminescence (PL) and Time Decay. The luminescence and its decay curves were measured on an FLS980 spectrofluorometer. The time decay was extracted by triexponential fitting. Stability Studies. Freshly prepared powder samples of MP-T-BiI6 were placed on clean glass slides for this experiment. For the humidity study, a sample was placed in a plastic box with holes in a drawer to minimize light exposure, and the relative humidity was maintained at ∼55%. For the light stability study, the samples were irradiated with a broad spectrum lamp (AM 1.5 simulated solar light) at the same humidity environment. For the sunlight stability study, the samples were covered with a transparent glass and directly exposed by the sunshine (ultraviolet index, ∼7; humidity, >70%). Powder X-ray diffraction (PXRD) for MP-T-BiI6 was performed on a Miniflex600 Xray diffractometer at room temperature. The diffraction patterns were collected in the 2θ range of 5°−50° with a step size of 0.5°. The simulated PXRD patterns were acquired by the software of mercury.

materials with strong optical absorptions. In this work, we report a new Bi-based semiconducting material of (4methylpiperidinium)4·I3·BiI6 (MP-T-BiI6), which was achieved through intercalating I3− ions into the prototype of (4methylpiperidinium)3Bi2I9 (MP-Bi2I9). As expected, MP-TBiI6 shows a narrow band gap of 1.58 eV, being close to that of MAPbI3 (∼1.5 eV). Theoretical analyses of their electronic structures disclose that the intercalated triiodide ions behave as the crucial band gap determinant for the conduction-band minimum. Besides, large-size single crystals were successfully prepared, which exhibit superior semiconducting behaviors, including notable hole mobility of ∼12.8 cm2 V−1 s−1, chargetrap density of ∼1.13 × 1010 cm−3, and photoconductive behaviors. As far as we are aware, the combined advantages of high stability and environmentally friendly feature make MP-TBiI6 a promising candidate as a light-absorbing material for the photovoltaic and optoelectronic application. This finding paves a new way to the rational engineering of lead-free lightabsorbing materials.

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EXPERIMENTAL SECTION

Materials. A reaction mixture, containing 4-methylpiperidine (0.99 g, 10 mmol) and Bi2O3 (1.16 g, 2.5 mmol) in 30 mL of HI (47%) solution, was slowly evaporated at room temperature. After several days, red needle crystals of MP-Bi2I9 were obtained. For the growth of MP-T-BiI6 crystals, MP-Bi2I9 was redissolved in an oxidized hydroiodic acid solution, and brownish black bulk crystals were obtained after slow evaporation in a few days. Structure Determination. High-quality microcrystals were selected for single-crystal structure determination, which was performed on an Agilent SuperNova Dual diffractometer equipped with a graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) at 293 and 290 K, respectively. The collection of the intensity data, cell refinement, and data reduction were carried out with the program CrysAlisPro.48 Crytsal structures were solved by the direct method with program SHELXS and refined with the least-squares program SHELXL.49 The structures were verified using the ADDSYM algorithm from the program PLATON,50 and no higher symmetries were found. Details of crystal parameters, data collection, and structure refinement are summarized in Table S1. The selected bond distances and angles are presented in Table S2. UV−Vis−NIR Diffuse Reflectance Spectroscopy. The UV− vis−NIR diffuse reflection data were obtained with a scanning wavelength between 200 and 1200 nm on a LAMBDA 950 UV/vis spectrophotometer at room temperature. The BaSO4 powder sample was used as a standard (100% reflectance), and the optical band gap can be determined by the variant of the Tauc51 equation:

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RESULTS AND DISCUSSION Bulk black crystals of MP-T-BiI6 were prepared as a potential light-absorbing material, which was rationally engineered by intercalating I3− ions into its prototype of MP-Bi2I9 (Figure 1a). Using this strategy, other hybrids of the same family containing I3− were also obtained as the light-absorbing materials (i.e., A4·I3·BiI6, Figure S1), which paves a potential pathway for the large and diverse family of Bi-based hybrids to compete with lead absorbers. As shown in Figure 1b, MP-Bi2I9 adopts a zero-dimensional (0D) perovskite-like structure with the centrosymmetric space group of Pnma, described as the isolated face-sharing octahedra of [Bi2I9]3−. It is noteworthy that organic cations are bonded to adjacent anionic clusters of [Bi2I9]3− octahedron through weak N−H···I hydrogen bonds (Figure S3). That is, the corner-sharing [Bi2I9]3− octahedra are connected by the organic cations along the b-axis, which conforms to the growth morphology (Figures 1a and S2). After intercalating I3− ions, the crystal structure and morphology change greatly (Figure 1c). Structural analyses reveal that MPT-BiI6 belongs to the triclinic system with a space group of P1̅ at 290 K (Table S1). It displays the 0D lead-free structure with the individual metal halide ions of BiI6, completely isolated from each mononuclear cluster and separated by triiodide ion and organic amine cations. This individual 0D structure might influence the electron transport.47,56 Further analysis on crystal structure discloses that organic cations are bonded to the isolated BiI6 octahedron and the intercalated I3− ions through N1−H1A···I1, N1−H1B···I2, N2−H2C···I2, N2−H2C···I3, and N2−H2D···I2 hydrogen bonds, respectively (Figure S4). It is worth pointing out that each I3− ion behaves as the bridge to bond two adjacent organic cations, which could greatly

(hν·F(R ∞))1/ n = A(hν − Eg ) where h is Planck’s constant, and n is the frequency of vibration, where n = 1/2 for a direct allowed transition, while n = 2 for indirect allowed transition. F(R∞) is the Kubelka−Munk function, Eg is the band gap, and A is the proportional constant. Computational Methods. First-principles density-functional theory (DFT) calculations were performed with the Cambridge Sequential Total Energy Package (CASTEP).52,53 The 4-methylpiperidinium cations of MP-Bi2I9 are disordered at room temperature. For the calculation, a proposed structure with all 4-methylpiperidinium cations aligned along the same direction was used for MP-Bi2I9, and single-crystal structure was used for MP-T-BiI6. The exchangecorrelation functional was described by a generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof functional for solids (PBEsol) scheme.54 The interactions between the ionic cores and the electrons were described by the norm-conserving pseudopotential.55 The following orbital electrons were treated as valence electrons: Bi, 5d106s26p3; I, 5s25p5; C, 2s22p2; N, 2s22p3; and H, 1s1. The numbers of plane waves included in the basis sets were 4082

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Figure 1. Crystal photographs of MP-Bi2I9 and MP-T-BiI6 (a), and their packing diagrams, respectively (b and c). Hydrogen atoms were omitted for clarity.

support triiodide-intercalated hybrids to grow a large-size block single crystal for various applications.57,58 It is known that the light and thermal stability are important factors for hybrid organic−inorganic halide perovskites,59,60 while the classic hybrid perovskite of MAPbI3 is unstable to moisture.16,39 For MP-T-BiI6, the stability of freshly prepared powders was evaluated under three different conditions, including a ∼55% relative humidity environment with shading treatment for 270 days, a ∼70% relative humidity environment with direct sunlight (ultraviolet index = ∼7) for 30 days, and irradiated with a broad spectrum halogen lamp under ∼55% relative humidity for 7 days. PXRD patterns obtained after these experiments clearly show that MP-T-BiI6 has excellent stability (Figure 2a). In addition, thermogravimetric analysis (TGA) shows that MP-T-BiI6 is stable up to 426 K, and differential scanning calorimetry (DSC) indicates no phase transitions within this temperature range, which will be greatly favorable for its long-term usage (Figure S5).

Figure 2. (a) PXRD patterns of MP-T-BiI6 after exposure in different conditions. The arrow labels the impurity peak. (b) Absorbance spectra of MP-T-BiI6 (black) and MP-Bi2I9 (red). Inset: Tauc plot showing the characteristic of an indirect band gap. (c) Band-gap energy of MP-T-BiI6 as compared to some hybrid perovskites. Details on the band gap energy values are shown in Table S3.

To evaluate the potential of MP-T-BiI6 as light-absorbing candidate, diffuse reflectance spectra of MP-T-BiI6 and MPBi2I9 were determined and converted to absorbance spectra by the Kubelka−Munk transformation,61 and their band-gap energies were extrapolated by assuming an indirect band gap from a Tauc plot of (F(R∞)hν)1/2. As shown in Figure 2b, the absorption spectrum of MP-Bi2I9 shows an absorption cutoff wavelength of 635 nm (2.02 eV), similar to most of the known 4083

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calculation results also further confirm the significance of I3− ion for the band-gap reconstruction. The right side of Figure 3 shows their partial density of states projected on the constitutional atoms. For MP-Bi2I9, the valence-band maximum mainly results from the nonbonding states of I-5p, while the Bi-6p and I-5p orbitals account for the in-plane dispersion of the conduction-band minimum. It indicates that the inorganic framework of MP-Bi2I9 is responsible for its band gap. In contrast, the pDOS of MP-T-BiI6 displays drastic changes after the triiodide ion intercalated. That is, both the upper part of the valence band and the bottom of the conduction band consist of the hybridization of I-5p orbitals (Figure S12). As compared to prior reports,38,62 most of the pDOS calculations of hybrid perovskites have shown that the conduction-band minimum is predominantly determined by the p orbitals of metal and halide. Here, the results disclose that the intercalated I3− ions can cause both structure and band gap changes for organic−inorganic hybrids. Apparently, both DFT and pDOS have further shown that I3−-ion intercalation has a huge effect on improving its optical absorption. Figure 4 intuitively shows charge densities in the frontier molecular orbitals (the conduction-band minimum and

binuclear Bi-based hybrids.40 For MP-T-BiI6, however, the triiodide ion intercalation leads to a clear redshift to 780 nm, and its band-gap energy drops sharply from 2.02 to 1.58 eV. For a direct comparison, the band gap energies of several typical light-absorbing hybrid materials have been shown in Figure 2c. It is distinct that most of the Bi-based hybrid perovskites display wide band gaps of Eg > 1.9 eV, inferior to that of MAPbI3 (∼1.5 eV). Conversely, the triiodideintercalated hybrid of MP-T-BiI6 shows one of the lowest band gaps among Bi-based light-absorbing materials, almost close to MA3PbI3, which will be advantageous for the light absorption. To further study the contribution of I3− ion, band structures and partial density of states (pDOS) of MP-Bi2I9 and MP-TBiI6 were calculated using density functional theory (DFT) method. For MP-Bi2I9, its band structure shows an indirect band gap at the Brillouin zone center with an energy value of 2.66 eV, which is slightly higher than the experimental value of 2.05 eV (Figure 3a), due to the neglecting of spin−orbit coupling effects during the calculations.62,63 After intercalating I3− ions, the band structure of MP-T-BiI6 displays an unusual flat in-plane dispersion of the valence band. The calculated decrease in energy value upon intercalation is 480 meV, which well agrees with the experimental trends (Figure 3b). These

Figure 4. Charge densities in the frontier molecular orbitals of MPBi2I9 (a) and MP-T-BiI6 (b).

valence-band maximum), obtained from the optimized geometry of molecular cluster. In the prototype of MP-Bi2I9, the charge densities at the valence-band maximum and conduction-band minimum are highly delocalized along the whole inorganic metal halide framework (Figure 4a). For MPT-BiI6, the secondary and the highest occupied molecular orbital are contributed by individual metal halide ions of the BiI6 octahedron (Figure S6). However, in the conduction-band minimum, the charge densities are regularly distributed along the intercalated triiodide part in the lowest occupied molecular orbital, which is a great difference from most of the reported metal-halide perovskites (Figure 4b).38,62 Obviously, the intercalated triiodide plays an important role in band-edge reconstruction, and such an unusual frontier molecular orbital between the conduction-band minimum and valence-band maximum could greatly support triiodide-intercalated hybrids for further application as the light absorber. It is known that dopants can reconstruct the trap state that changes semiconductor performance.47 Therefore, we further measured the PL and the free-carrier lifetime of MP-T-BiI6. The PL spectrum of MP-T-BiI6 was determined under the 680

Figure 3. Calculated band structure and projected density of states (pDOS, right) for MP-Bi2I9 (a) and MP-T-BiI6 (b). 4084

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Figure 5. Semiconducting performance of MP-T-BiI6. (a) PL spectrum of crystals. (b) Time-resolved PL and fits for the PL decay time (τ) in singlecrystal samples at room temperature. (c) I−V trace of perovskite crystals exhibiting different regions obtained from the log I versus log V plots. Inset: Diagram of the device structure for I−V measurements with SCLC model. (d) I−V curves in the dark and under an AM 1.5 simulated solar light at 6.3 mW cm−2. Inset: Diagram of the single-crystal photodetector device.

Thin-film photovoltaic devices show enormous potential for the research of clean renewable energy, and the quality of thin film has a direct influence on the application of hybrid perovskite light absorber.68 Hence, the photoelectric behavior of MP-TBiI6 thin film was investigated using an Au device architecture with adjacent intervals about 180 um under AM 1.5 simulated solar light at 6.3 mW cm−2 (Figures S7−S11). It is found that the thin film of MP-T-BiI6 displays clear photoconductive behavior, suggesting that our device performance can be further optimized in the future. These relevant properties indicate that the I3−-intercalated semiconducting materials are promising for optoelectronic applications.

nm excitation at room temperature. A narrow and weak emission centered at 795 nm has been observed in Figure 5a. For its time-resolved trace (Figure 5a), the PL intensity shows a fast initial drop followed by a slower decay, which can be divided into three processes, that is, a short-lived process (τ1 = 0.426 ns, 51%), a moderate-lifetime process (τ2 = 13.3 ns, 19%), and a long-lived component (τ3 = 57.9 ns, 30%). The measured carrier lifetime is significantly higher than those of some other Bi-based perovskites, such as (MA)3Bi2I9 (760 ps),28 and (MA)3Bi2Br9 (τ1 = 1.96 ns, 98%; τ2 = 7.99 ns, 2%),26 which indicates that MP-T-BiI6 possesses enormous potential for optoelectronic application. The carrier mobilities for MP-TBiI6 single crystals were determined by using space charge limited current (SCLC) analysis.64−66 As shown in Figure 5c, the typical logarithmic I−V trace of displays a clear transition from the ohmic region to the trap filling region (VTFL = 8.2 V). Onset voltage for the trap-filled limit (TFL) voltage was used for the trap density in the perovskite-derivative crystal (VTFL = entrapd2/2εε0, where d is the thickness of single crystal, ε = 13.2 is the relative dielectric constant of MP-T-BiI6, which is determined by impedance analyzer, and ε0 is the vacuum permittivity). The trap density ntrap of MP-T-BiI6 was calculated to be 1.13 × 1010 cm−3. Further, a trap-free space charge limit current regime (Vb = 29.2 V) has been observed (red line), which is well fitted by the Mott−Gurney law: JD = 9εε0μVb2/8d3 (where Vb is applied voltage). The curve fitting was well derived with a hole mobility of ∼12.8 cm2 V−1 s−1. In addition, single-crystal photodetectors based on large-area individual crystals have been fabricated with silver stripe patterns.67 Figure 5d depicts a typical photoresponse to an AM 1.5 simulated solar light at 6.3 mW cm−2, which reveals the potential of MP-T-BiI6 for the optoelectronic applications.

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CONCLUSION We have demonstrated an effective strategy to intercalate I3− ions into the Bi-based hybrid, which could be used to design new lead-free hybrid perovskites with strong light absorption. Black large-size single crystals of MP-T-BiI6 exhibit a hole mobility of ∼12.8 cm2 V−1 s−1 and a charge-trap density of ∼1.13 × 1010 cm−3. The Tauc plot from the reflectance spectrum shows a narrow band gap of 1.58 eV, which is almost close to that of MAPbI3 (∼1.5 eV). Theoretical calculations indicate that the I-5p orbitals of I3− groups act as the critical factor to induce the band-edge reconstruction and behave as a new conduction-band minimum at the Brillouin zone center. Additionally, the photoconductive behavior further confirms that the I3−-intercalated hybrids are promising for optoelectronic applications. This result charts an efficient path to intercalating I3− group into a wide range of organic−inorganic hybrids, which provide a new design strategy for exploring leadfree light-absorbing materials. 4085

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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.chemmater.8b01200. Single-crystal X-ray structures, differential scanning calorimetry measurements, thermogravimetric, charge densities, current−voltage measurements, topographical AFM and SEM image, XRD pattern, absorption spectrum, photocurrent response, and table of crystal data, selected bond lengths, and band gap of Biperovskites (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Junhua Luo: 0000-0002-7673-7979 Notes

The authors declare no competing financial interest. CCDC contains the supplementary crystallographic data for this Article. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif. The corresponding CCDC numbers are 1562211 and 1819342.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21622108, 21525104, and 21601188), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), and the Youth Innovation Promotion of CAS (2014262 and 2015240).

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REFERENCES

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