Structure Tunable Organic–Inorganic Bismuth Halides for an

Jun 23, 2017 - Lead-Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too? Lusheng Liang , Peng Gao. Advanced S...
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Structure Tunable Organic−Inorganic Bismuth Halides for an Enhanced Two-Dimensional Lead-Free Light-Harvesting Material Mu-Qing Li,†,# Yue-Qiao Hu,†,# Le-Yu Bi,† Hao-Lan Zhang,† Yanyan Wang,† and Yan-Zhen Zheng*,† †

Frontier Institute of Science and Technology (FIST), State Key Laboratory of Mechanical Behavior for Materials, MOE Key Laboratory for Nonequilibrium Synthesis of Condensed Matter and School of Science, Xi’an Jiaotong University, Xi’an 710054, China S Supporting Information *

S

emiconducting organic−inorganic halides are a promising class of materials for optoelectronic applications such as light emitting diodes, field-effect transistors and photovoltaics.1−8 This renewed interest was agitated by the discovery of a family of lead-based hybrid perovskites APbI3 (A = CH3NH3+, (H2N)2CH+) for solar cell applications, which show rapidly increasing power conversion efficiency (from 4% to more than 22%) in just several years.9 Comparison studies revealed that both halogen and dimensionality of inorganic anions in lead perovskites affect the semiconductor properties and their performance as light absorbers.1,10−12 However, the toxicity of lead is a primary concern for the large-scale use of these materials, especially given the materials’ solubility in water.13−16 As bismuth(III) has the same 6s26p0 electronic configuration as that of lead(II) but low toxicity and can also form diverse organic−inorganic hybrids,17,18 it is widely expected as an environmentally friendly alternative for lead. Some bismuthbased hybrids have been recently investigated for solar energy conversion, but the best power conversion efficiency is only 1.09%.19,20 In addition to the poor surface morphologies in cell fabrication, the low dimensionality of the inorganic anions, which limits carrier concentration and mobility, may be a major limitation for photovoltaic efficiency of organic−inorganic bismuth halides (OIBHs). Unlike Pb2+, Bi3+ tends to form low dimensional connectivity with halogens17,18 and there are only two OIBHs with 2D inorganic anions reported. One is the [Bi2Br9]n3n− with methylammonium as counteraction.21 The other is the [Bi2/3I4]n2n− with metal-deficient layered-perovskite structure template by 5,5‴-bis(aminoethyl)-2,2′:5′,2″:5″,2‴quaterthiophene.22 Therefore, it is still a challenge to find a method to synthesize high dimensional halobismuthate frameworks. Herein, we found the mixed-halogen approach is able to improve the connectivity in a series of OIBHs (TMP)[BiI5] 1, (TMP)[BiBr5] 2, (TMP)[BiCl5] 3 and (TMP)1.5[Bi2I7Cl2] 4 (TMP = N,N,N′,N′-tetramethylpiperazine). Single crystal X-ray diffraction revealed that the pure halide compounds (1−3) have only normal 1D inorganic components, while 4 with mixed halogens has a unique 2D inorganic anion structure (Figures 1 and 2, Tables S1 and S2). The optical band gaps of 1−4 are 2.02 eV, 2.67 eV, 3.21 and 2.10 eV, respectively. Electrical conductivity (σ) measurements show that 4 has the highest conductivity (2.37 × 10−6 S/cm at room temperature) and a simple device using 4 as light absorber shows efficient © 2017 American Chemical Society

Figure 1. Crystal structures of 1 (a), 2 (b) and 3 (c) and the in situ formation of TMP2+ (d). Bi, orange; I, purple; Br, red; Cl, green; C, gray; N, blue. H atoms are omitted for clarity.

photoconductivity response and very high stability under longtime irradiation. All the syntheses were carried out in solvothermal condition, and the TMP2+ dication in 1−4 is from in situ methylization of piperazine (Figure 1d). The structure of 1 consists of inorganic anions of quasi-1D zigzag chains of vertexes-sharing [BiI6] octahedrons, separated by TMPs (Figures 1a, S1 and S2). The inorganic chain structure for 1 is derived from one and a half crystallographically independent [BiI6] octahedrons, with Bi−I bond lengths ranging from 2.9189(3) to 3.4703(4) Å. Specially, I(6) and I(7) bridge the adjacent Bi atoms to support the backbone of the inorganic chain with the long Bi−I distances ranging from 3.2485(3) to 3.4703(4) Å. I(5), I(8), I(9) and I(10) are in the trans-position with the shortest terminal Bi−I distances ranging from 2.9189(3) to 2.9652(3) Å, whereas I(1), I(2), I(3) and I(4) are in the cis-positon with intermediate lengths ranging from 3.0292(3) to 3.1263(3) Å. In addition to the bond length differences, the I−Bi−I bond angles deviate slightly from 90°, with the biggest difference 95.34(1)° Received: March 12, 2017 Revised: June 23, 2017 Published: June 23, 2017 5463

DOI: 10.1021/acs.chemmater.7b01017 Chem. Mater. 2017, 29, 5463−5467

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The close Cl···Cl interactions in the adjacent chains are 3.532(1) Å for Cl(1)···Cl(2) and 3.984(2) Å for Cl(4)···Cl(4)a, which are also close to twice of the ionic radius for the chlorine ion (1.81 Å). From the above structural analyses, we can see that various bismuth halide chains can be incorporated into OIBHs by varying the halogens. It is a natural thought that the mixing of different halogens will introduce competition in constructing OIBHs, which may lead to interesting novel structures. In this regards, we mixed two kinds of halogens as starting materials and found that a new phase 4 can be isolated when the molar ratio of HI: HCl is in the range of 3:7 to 8:2. The structure of 4 composes of a unique 2D honeycomb-like [Bi2I7Cl2]n3n−anionic framework decorated by discrete organic TMP2+ cations (Figure 2). There are two crystallographically independent Bi atoms, seven I atoms and two Cl atoms. Bi1 and Bi2 are situated in slightly distorted octahedral [BiI5Cl] and [BiI4Cl2] coordination environments, respectively (Figure 2a). The honeycomb-like 2D anionic framework consists of tenmembered Bi cycles, in which the metal centers are linked by eight μ-I and two μ-Cl2 bridges. Alternatively, the anionic framework can be viewed as constructed by 1D [BiI5Cl]∞ chain running along b axis bridged by the [Bi2I8Cl2] dimers via sharing I(3) to form a 2D structure (Figure 2b,c). The organic TMP2+ cations are accommodated in the honeycomb without classic hydrogen bonding interactions (Figure 2d). The I···I interactions in the adjacent layers are ranging from 3.904(2) Å to 4.271(2) Å. It should be noted that OIBHs with 2D inorganic components are rarely observed (see Table 1).18 This is very

Figure 2. (a) Subunit of 4. (b) Layer constructed by Bi2I8Cl2 dimer and [BiI4Cl]∞ chain. (c) 2D layer of 4. (d) 3D packing diagram of 4.

occurring for the I(8)−Bi(1)−I(9) angle, which incorporates the two shortest Bi−I bonds. The two Bi−I−Bi angles along the chain are 178.40(1)° and 160.40(1)°. The later one deviating substantially from 180° occurs for Bi(1)−I(7)−Bi(2). The inorganic [BiI5]n2n− chains and (TMP)2+ cations form alternating layers along the b axis (Figure S2b), with inorganic layers consisting of a pseudo-2D array of [BiI5]n2n− chains that extend along the [011] direction of the structure (Figure S2c). The closest I···I interaction in the alternating layers is 3.834(1) Å[I(2)−I(3)], slightly shorter than the I···I interactions in the pseudoinorganic layers (4.083(1) Å for [I(10)−I(10)a] and 3.913(1) Å for [I(9)−I(9)a]). These distances are very close to twice the ionic radius for the iodide ion (2.20 Å), indicating that the chains are in close contact. Using hydrobromic and hydrochloric acids to replace hydroiodic acid, complexes 2 and 3 were isolated. Like (TMP)[BiI5], the (TMP)[BiBr5] structure can be viewed as [BiBr5]n2n− chains being separated by TMP2+ cations (Figures 1b, S3 and S4). Different to the [BiI5]n2n− zigzag chain in 1, the [BiBr5]n2n− chain in 2 features a twisted zigzag chain with vertex-sharing [BiBr6] octahedral units (Figure S3). It is worth noting that the replacement of the halogen caused significant structure change, rather than a simple halogen substitution reaction in perovskite type complexes. The Bi−Br lengths are ranging from 2.693(2) Å to 3.003(2) Å, which are obviously shorter than Bi−I distances in 1. The Br−Bi−Br bond angles in [BiBr6] octahedron are ranging from 83.69(6)° to 96.72(2)°. The close Br···Br interactions in the adjacent chains are 3.620(3) Å for Br(2)···Br(3) and 4.070(3) Å for Br(4) ··· Br(4)a, indicating that the chains are in close contact (Figure S4). The organic TMP2+ cations are located in the channels and contact with the 1-D [BiBr5]n2n− anions through Coulombic interactions. (TMP)[BiCl5] is isomorphous with (TMP)[BiBr5] (Figures 1c, S5 and S6) that consists of [BiCl5]n2n− chains and TMP2+ cations. The Bi−Cl lengths are further reduced and ranging from 2.5678(8) Å to 2.8824(3) Å. The Cl−Bi−Cl bond angles in [BiCl6] octahedron are ranging from 82.28(2)° to 96.94(2)°.

Table 1. Dimension and Band Gap of Selected OIBHs compound

dimension

bandgap

ref.

(CH3NH3)3Bi2I9 (H2AETH)BiI5 ((CH3)2S(CH2)2NH3)BiI5 (HO2C(C6H4)CH2NH3)BiI4 [BiI2(EO5)]Bi2I7 [(H2TMDP)(H2O)]Bi3I11 [(DB18C6)Na(MeCN)2]2[Bi6I20] 1 (CH3NH3)3Bi2Br9 (H2AEQT)Bi2/3I4 4

0D 1D 1D 1D 1D 1D 1D 1D 2D 2D 2D

2.1 eV

19 23 24 24 25 26 27 this work 21 22 this work

1.9 eV 2.1 eV

1.8 eV 2.02 eV 2.5 eV 2.12 eV

different from the case of lead-based organic−inorganic hybrids.17,18 The [Bi2I7Cl2]n3n− anionic framework reported here is the first 2D, mixed-halide anion of bismuth. Inspired by some recent solar cell applications of layered perovskites11,12 we explored the properties related to photovoltaic application of 4 and compared with other compounds with the same organic cation (see below). Optical spectra of powder samples 1−4 were measured in diffuse reflection mode by UV−vis-NIR spectrophotometer (Figure 3). The as-recorded reflectance was transformed into absorption using the Kubelka−Munk function to give the band gaps of 1−4, which were determined as 2.02, 2.67, 3.21 and 2.10 eV, respectively. For comparison, the band gap of BiI3 at room temperature was also determined as 1.66 eV. Considering the major contribution of inorganic components, band gaps of the OIBHs are comprehensive results of the kind of halogens, the connectivity of the inorganic components (nuclearity, 5464

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concentrations. Though 4 has some chlorine atoms, which makes its optical band gap larger than that of 1, it shows much higher conductivity. This result strongly supports the conclusion that high dimension of inorganic component is beneficial to enhance the charge transport in OIBHs. Photoconductivity of 4 was also measured using a pellet sample (Figure 5). Under the photoexcitation of a 450 W

Figure 3. Optical band gaps of 1−4 and BiI3 derived from UV−vis diffuse reflectance spectroscopy.

dimension, I···I contacts etc.) and even some unintuitive factors like special orbital interaction.24 Nevertheless, the broadening band gaps of 1−3 demonstrate the tunable feature by halogens. Compared to 1, compound 4 with mixed halogen (I and Cl) and higher dimension shows wider band gap indicating halogen have more significant influence on band gaps than dimensionality. The band gaps of 1 and 4 are close to the double layered perovskite complex (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 (1.99 eV)11 and the double perovskite complex Cs2AgBiBr6 (2.1 eV),28 which should be suitable for light-harvesting applications. Electrical conductivities of 1−4 were measured using a twoprobe direct current (DC) method with pressed pellets of the powdered samples and single crystal samples at 297 K (Figure 4). The conductivities of 1, 2, 3 and 4 pellets are 1.54 × 10−7,

Figure 5. Photocurrent of a pellet of 4 achieved by periodic switching of a light source.

xenon lamp, the enhanced currents prove the light-response of 4. At a voltage of 25 V, the photocurrent increased from 0.20 μA (dark) to 0.68 μA (light). Notably, the repeatable switching in 4 (achieved by turning light on/off) showed little temporal change in photocurrent (Figure 5, inset), indicating that the light-response is reversible. The photosensitive conductivity further demonstrates the semiconductor nature of 4 and indicates the possibility of being a light-harvesting and lightdetecting material. The instability of perovskite APbI3 to moisture significantly hampers the application of this material.31,32 However, 4 is very stable under light and moisture. As-prepared powders of 4 were stored either in the dark at 55% relative humidity or irradiated at 50 °C with a 350 W xenon lamp (∼1 Sun) for 1 week. PXRD patterns of 4 after moisture or light exposure showed no evidence of material decomposition (Figure S7). In conclusion, four new OIBHs were synthesized by facile one-pot solvothermal reaction and their optical and electrical properties were studied. Though the organic cations in 1−3 are exactly same, different halogens result in markedly different configurations in the bismuth halide chains. More importantly, by using mixed halogens, a rare case of 2D OIBH, 4, has been isolated. The observed optical properties confirmed that compounds 1−4 are semiconductors with optical band gaps of 2.02−3.21 eV. The band gap of 2.10 eV, electrical conductivity of 2.37 × 10−6 S/cm, efficient photoconductivity and high stability under moisture or light exposure reveal that 4 is a potential lead-free light-harvesting material.

Figure 4. Plots of current density versus electric field strength (J−E curves) for 1−4 at 297 K.

2.64 × 10−9, 3.98 × 10−11 and 1.59 × 10−6 S/cm, respectively. The conductivities of the single crystals of 1 and 4 are 2.29 × 10−7 and 2.37 × 10−6 S/cm, respectively. Hence, the conductivities of 1 and 4 fall in the range of typical semiconducting materials and are comparable to other crystalline inorganic−organic hybrid compounds.29,30 With similar anionic structure (chain), electrical conductivities of 1, 2 and 3 present negative correlation with the optical band gaps. This can be mainly attributed to decreasing of carrier

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01017. 5465

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Experimental details including syntheses, crystallographic data, XRD patterns (PDF) Crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

*Y.-Z. Zheng. E-mail: [email protected]. ORCID

Yan-Zhen Zheng: 0000-0003-4056-097X Author Contributions #

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21473129 and 21620102002), “National Young-1000-Plan” program, and the Fundamental Research Funds for the Central Universities.

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