skite-Derivative Hybrid for Strong Light Absorption - ACS Publications

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Triiodide-Induced Band-Edge Reconstruction of a LeadFree Perovskite-Derivative Hybrid for Strong Light Absorption Weichuan Zhang, Xitao Liu, Lina Li, Zhihua Sun, Shiguo Han, Zhenyue Wu, and Junhua Luo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01200 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Chemistry of Materials

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, Junhua Luo* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China

ABSTRACT: Bismuth-based hybrid perovskites have recently attracted great attention for their environmentally friendly processing, chemical stability, and photo-responsive properties. However, most of the known lead-free hybrids show wide bandgaps (Eg > 1.9 eV) that afford weak visible-light absorption. Here, we show a newly-conceptual design strategy of intercalating triiodide to decreases the prototypic bandgap by ca. ~0.44 eV. A new hybrid semiconducting material of (4methylpiperidinium)4·I3·BiI6 (MP-T-BiI6), adopting the zero-dimensional (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 bandgap of 1.58 eV, almost comparable with 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.

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 siliconbased 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 leadbased solar cells promotes the exploration of environmentally friendly light–absorbing materials. Substantial leadfree 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 H2NCHNHSnI3, display a narrow bandgap at near infrared region compared to their lead hybrid counterparts. 19-23 However, the majority of tin-based perovskites are quite unstable when exposed to air and mois-

ture. 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 series of promising environmentally friendly materials for optoelectronic applications.24-28 Although there has been great attention in Bi-based perovskites as light absorbers, these hybrids have so far displayed wide bandgaps of Eg > 1.9 eV,29-35 which show inferior light absorption compared to MAPbI3 (~1.5 eV). To obtain the narrow bandgap 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, e.g., A2MBiX6. Although numerous efforts have been devoted to designing strong light-absorbing lead-based perovskites, only limited success has been achieved to date. 36-42 In this context, to rationally design new leadfree 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 molecule45, 46 to reconstruct the

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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 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 achieve through intercalating I3− ions into the prototype of (4-methylpiperidinium)3Bi2I9 (MP-Bi2I9). As expected, MP-T-BiI6 shows a narrow bandgap 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, charge-trap 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-T-BiI6 a promising candidate as light-absorbing material for the photovoltaic and optoelectronic application. This finding paves a new way to the rational engineering of lead–free light–absorbing materials.

EXPERIMENTAL SECTION Materials. A reaction mixture, containing 4methylpiperidine (0.99 g, 10 mmol) and Bi2O3 (1.16 g, 2.5 mmol) in 30 mL 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-TBiI6 crystals, MP-Bi2I9 was re-dissolved 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 nm and 1200 nm on a

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LAMBDA 950 UV/Vis Spectrophotometer at room temperature. The BaSO4 powder sample was used as a standard (100% reflectance) and the optical bandgap can be determined by the variant of the Tauc 51 equation: (hv·F(R∞))1/n = A(hv - Eg) where h is the Planck’s constant, n is the frequency of vibration, n = 1/2 for a direct allowed transition, while n = 2 for indirect allowed transition. F(R∞) is the KubelkaMunk function, Eg is the band gap and A is the proportional constant. Computational Methods. First-principles densityfunctional theory (DFT) calculations were performed with the Cambridge Sequential Total Energy Package (CASTEP). 52, 53 The 4-methylpiperidinium cations of MPBi2I9 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 were used for MP-TBiI6. The exchange-correlation 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 5d10 6s2 6p3; I 5s2 5p5; C 2s2 2p2; N 2s22p3 and H 1s1. The numbers of plane waves included in the basis sets were determined by a cutoff energy 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 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) (Figure S7). Currentvoltage curves of the device performed by a Model 6517B Electrometer with an ST-102D Probe station. Singlecrystal photodetectors based on large-size crystals of MPT-BiI6 were assembled by depositing the silver stripe patterns on the surface of crystals. Photoelectric performance of single crystal devices were 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 tri-exponential fitting.


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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 (the ultraviolet index: ~7; the humidity: >70%). Powder Xray diffraction (PXRD) for MP-T-BiI6 was performed on a Miniflex600 X-ray 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.

freshly prepared powders were 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 calorimeter (DSC) indicates no phase transitions within this temperature range, which will be greatly favorable for its long-term usage (Figure S5).

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 MPBi2I9 (Figure 1a). Using this strategy, other hybrids of the same family containing I3- were also obtained as the lightabsorbing materials (i.e. A4•I3•BiI6, Figure S1), which paves a potential pathway for the large and diverse family of Bibased 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- octahedral 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 MP-T-BiI6 belongs to the triclinic system with a space group of P1 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 iso− lated 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 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, stability of

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



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and their bandgap energies were extrapolated by assuming an indirect band gap from a Tauc plot of (F(R∞)hv)1/2. As shown in Figure 2b, the absorption spectrum of MPBi2I9 shows an absorption cutoff wavelength of 635 nm (2.02 eV), similar to most of the known binuclear Bibased hybrids.40 For MP-T-BiI6, however, the triiodide ion intercalation leads to a clear redshift to 780 nm, and its bandgap energy drops sharply from 2.02 eV to 1.58 eV. For a direct comparison, the band gap energies of several typical light-absorbing hybrid materials have been shown in the Figure 2c. It is distinct that most of the Bi-based hybrid perovskites display wide bandgaps of Eg > 1.9 eV, inferior to that of MAPbI3 (~1.5 eV). Conversely, the triiodide-intercalated hybrid of MP-T-BiI6 shows one of the lowest band gap among Bi-based light-absorbing materials, almost close to MA3PbI3, which will be advantageous for the light absorption.

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) Bandgap energy of MP-T-BiI6 compared with some hybrid perovskites. Details on the band gap energy values are shown in Supporting Information Table S3.

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

To evaluate the potential of MP-T-BiI6 as lightabsorbing candidate, diffuse reflectance spectra of MP-TBiI6 and MP-Bi2I9 were determined and converted to absorbance spectra by the Kubelka−Munk transformation,

To further study the contribution of I3− ion, band structures and partial density of states (pDOS) of MP-Bi2I9 and MP-T-BiI6 were calculated using density functional theory (DFT) method. For MP-Bi2I9, its band structure shows


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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 agree with the experimental trends (Figure 3b). These calculation results also further confirm the significance of I3− ion for the bandgap 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 inorganic framework of MP-Bi2I9 is responsible to its bandgap. In contrast, the pDOS of MP-T-BiI6 displays drastic changes after the triiodide ion intercalated. That is, both the upper part of valence-band and the bottom of conduction-band consist of the hybridization of I-5p orbitals (Figure S12). Compared with 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 bandgap changes for organicinorganic hybrids. Apparently, both DFT and pDOS have further shown that I3−-ion intercalation has a huge effect on improving its optical absorption.

Figure 4. Charge densities in the frontier molecular orbitals of MP-Bi2I9 (a) and MP-T-BiI6 (b).

Figure 4 intuitively shows charge densities in the frontier molecular orbitals (the conduction band minimum and 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 maximum are highly delocalized along the whole inorganic metal halide framework (Figure 4a). For MP-T-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 with most of reported metalhalide perovskites (Figure 4b). 38, 62 Obviously, the intercalated triiodide plays an important role in band-edge reconstruction, and such an unusual frontier molecular orbitals between the conduction-band minimum and valence-band maximum could greatly support triiodideintercalated 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 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, i.e. a short-lived process (τ1 = 0.426 ns, 51%), a moderatelifetime 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 (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-T-BiI6 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 Ohmic region to trap filling region (VTFL = 8.2 V). Onset voltage for the trap-filled limit (TFL) voltage was used for the traps density in the perovskite-derivative crystal (VTFL = entrapd2/2εε0, where d is the thickness of signal 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 limits 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. Thin-film photovoltaic devices show enormous potential for the research of clean renewable energy, and the quality of thin-film has direct influence on the application of hybrid perovskite light absorber. 68 Hence, the photoelectric behavior of MP-T-BiI6 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



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Figure 5. Semiconducting performance of MP-T-BiI6. (a) the PL spectrum of crystals. (b) Time-resolved PL and fits for the PL decay time (τ) in single-crystal 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 . −2

and under an AM 1.5 simulated solar light at 6.3 mW cm . Inset: Diagram of the single-crystal photodetector device.

(Figure 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 future.

hybrids, which provide a new design strategy for exploring lead-free light-absorbing materials.

ASSOCIATED CONTENT Supporting Information

CONCLUSION In 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 bandgap of 1.58 eV, which is almost close to that of MAPbI3 (~1.5 eV). Theoretical calculations indicate that the I-5p orbits 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

The Supporting Information is available free of charge on the ACS Publications website at DOI: Single-crystal x-ray structures, differential scanning calorimeter measurements, thermogravimetric, charge densities, current-voltage measurements, topographical AFM and SEM image, XRD pattern, absorption spectra, photocurrent response, and table of crystal data, selected bond lengths and band gap of Bi-perovskites. CCDC contains the supplementary crystallographic data for this paper. 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.

AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected]. [email protected]. ORCID Junhua Luo: 0000-0002-7673-7979

Notes The authors declare no competing financial interest.

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

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