Doping-Enhanced Visible-Light Absorption of CH3NH3PbBr3 by Bi3+

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Doping-Enhanced Visible-Light Absorption of CH3NH3PbBr3 by Bi3+-Induced Impurity Band without Sacrificing Bandgap Lipeng Han, Lili Wu, Cai Liu, and Jingquan Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12026 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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The Journal of Physical Chemistry

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Doping-Enhanced Visible-Light Absorption of CH3NH3PbBr3 by Bi3+-

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Induced Impurity Band without Sacrificing Bandgap Lipeng Han,† Lili Wu,*,† Cai Liu*,‡ and Jingquan Zhang†

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†College

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‡Shenzhen

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of Materials Science and Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China. Institute for Quantum Science and Engineering, and Department of Physics, Southern University of Science and Technology, 1088 Xueyuan Avenue, Nanshan District, Shenzhen 518055, China. * Corresponding author at: College of Materials Science and Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China. (L. Wu). E-mail addresses: [email protected] (Wu L.), [email protected] (Liu C.).

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ABSTRACT: Intrinsic organic−inorganic metal halide perovskites (OIHP) have

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shown widespread applications in optoelectronic devices. And controllable doping is a

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very convenient route to tailor the performance of perovskites and endow them with

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new properties. Here we add dopants into the perovskite precursor solution to achieve

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controllable incorporation of trivalent cations (Bi3+) into CH3NH3PbBr3 single crystal

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by using the inverse temperature crystallization method. A comprehensive

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spectroscopy study was performed. Although there is a significant red shift of the

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optical absorption after doping, the band gap of Bi-doped MAPbBr3 crystals does not

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change. Bi3+ incorporation increases the density of states in the band gap, which can act

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as a scaffold for photon absorption with below-bandgap light. This approach opens a

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promising route for the perovskite material design to obtain more light absorption

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without sacrificing bandgap, which gives a broad range of possibilities to photoelectric

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implications not only for in the preparation of intermediate band gap photovoltaic

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devices but also tunable LEDs and so on.


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INTRODUCTION

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Organic-inorganic metal halide perovskites (OIHPs), especially MAPbX3 (MA =

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CH3NH3+, X = Br− or I−) have received extensive concern since the first report on the

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perovskite-based solar cell in 2009.1 During the following years, the power conversion

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efficiency (PCE) of perovskite solar cells have rapidly increased with considerable

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durability. The present PCE of perovskite solar cell was certified to be more than 23%,

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which makes it promising for practical applications.2 The excellent performance

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benefits from the outstanding structural and photoelectric properties of perovskite

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materials associated with high optical absorption coefficient, long carrier lifetime and

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ambipolar conductivity.3,4

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Among many research domains of perovskite materials, one of the major concerns

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is the toxicity of lead (Pb). The use of Pb is particularly problematic because Pb-based

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perovskites tend to decompose under ambient conditions, releasing harmful

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compounds.5,6 In order to find novel nontoxic perovskites for photovoltaics, significant

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experimental and computational work has sought to identify metal cation replacements

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for Pb. Nevertheless, divalent metal cations (Ge2+, Sn2+, Mg2+, Ca2+, Sr2+, and Ba2+) are

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not suitable for replacing Pb2+ due to ion size mismatch or poor stability.7 In addition,

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theoretical studies have shown that the superior photovoltaic properties such as

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extremely high optical absorption coefficient, long carrier lifetime and diffusion length

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of Pb-based halide perovskites could be attributed to lone-pair 6s2 and inactive Pb 6p0

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states of Pb2+.8 In consideration of ion size matching and 6s26p0 electronic structure,

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heterovalent ion may be appropriate substitution to Pb2+. Trivalence bismuth ion (Bi3+),

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with much lower toxicity than Pb2+ and similar ionic radii (1.18 Å for Pb2+ and 1.02 Å

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for Bi3+), is isoelectronic (6s2), which presents the resemblance in chemical behavior 3 ACS Paragon Plus Environment


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for these cations.9,10 Besides, Bi−Br bond length (axial: 2.926 Å, equatorial: 2.849 Å

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and 2.826 Å) in the octahedron BiBr63− closely matches that of the Pb−Br bond (2.95

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Å) in the corresponding octahedral structure.11,12 The bismuth cation can form regular

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chains built of nearly regular octahedra with halide anions.13 Bi3+ can be a promising

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substitution to constitute perovskite materials.

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Then bismuth-based and Pb-free perovskites have been investigated by a large

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number of researchers. Tang et al. synthesized MA3Bi2X9 (X = Cl, Br, I) perovskite

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quantum dots with photoluminescence quantum yield up to 12% by a collaborative

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solvent ligand-assisted re-precipitation method.14 Tong et al. developed a sensitive red-

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light photodetector based on CsBi3I10 perovskite thin film by a simple spin-coating

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method, which was very sensitive to 650 nm light, with an on/off ratio as high as 105.15

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Ma et al. used Cs3Bi2I9 as the photoactive layer in solution-processed heterojunction

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solar cell devices, which is a conceptual beneficial exploration and yield power

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conversion efficiencies of ~ 0.2%.16 Jain et al. prepared (MA)3Bi2I9 perovskite films as

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the absorbed layer of solar cell that yielded hysteresis-free efficiencies upto 3.17%.17

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Taken all together, properties of Bi-based perovskites are significantly attenuated.

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Ghosh et al. revealed that the electronic bandstructure of Bi-based perovskite materials

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possessed the high carrier effective masses along with large indirect bandgap, which

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would exclude its use in a single junction solar cell. And they also suggested that the

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presence of deep level defects is another major issue for Bi-based ternary halide

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perovskites, which are not applicable in the optoelectronic field.18 As a result, replacing

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partial Pb2+ of Pb-based perovskites is a compromise strategy. Furthermore, performing

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incorporation of Bi3+ on metal cation site can be an approach not only to reduce toxicity

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of Pb-based perovskites, but also to tailor their performance or endow them with new

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properties due to its matched radius and 6s26p0 electronic structure. An efficient in situ 4 ACS Paragon Plus Environment

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chemical route achieved the controlled incorporation of Bi3+ into perovskite, which has

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a significant effect on optoelectronic properties of MAPbBr3 and MAPbCl3 single

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crystals.19−22 In order to investigate the intrinsic effects of Bi3+ incorporation on Pb-

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based perovskite materials, high quality perovskite single crystals are ideally suited. In

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this paper, we prepared a series of MAPbBr3 and MAPb1-xBixBr3 single crystals. A very

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comprehensive spectroscopy study applying wide range of electromagnetic wave from

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X-ray to radio frequency was conducted here to obtain the information about their

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structure, optical and electronic properties. And a comparative investigation was given

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to elucidate the ambiguous role of Bi3+ incorporation into Pb-based perovskite materials

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and provide more insights for the important domain of doping perovskites.

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EXPERIMENTAL SECTION

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Chemicals and reagents. Lead bromide (PbBr2, 99.999%), methylammonium

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bromide (MABr, 99.5%) and N,N-dimethylformamide (DMF, 99.9%) were purchased

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from Youxuan Advanced Election Tech Co. Ltd. (Yingkou, China). Bismuth bromide

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(BiBr3, 99%) was purchased from Sigma-Aldrich. All salts and solvents were used as

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received without any further purification.

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Single-Crystal Preparation. MAPbBr3 and MAPb1-xBixBr3 perovskite single

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crystals were grown by inverse temperature crystallization method as reported

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previously with a slight modification.23 A 1.25 M solution of MAPbBr3 was prepared

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by dissolving equimolar amounts of MABr and PbBr2 in 2 mL of DMF at room

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temperature. For Bi-doped solutions, x% (molar ratio) PbBr2 was replaced by

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equimolar BiBr3 with 10%, 20%, and 30%. The solutions were then filtered using PTFE

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filters with a 0.22 μm pore size, and the filtrate was transferred into a 5-mL vial. The

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vial was kept in an oil bath undisturbed between 70 °C and 90 °C for 6 h. When these 5 ACS Paragon Plus Environment


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single crystals were taken out from the feed solution, they were washed quickly by

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DMF to dissolve the solute residue and dried with N2. All procedures were carried out

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under ambient conditions and humidity of 55-57%.

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Measurement and Characterization. Flame atomic absorption spectroscopy

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(FAAS, SpectrAA 220FS, Varian) was used to quantify Bi content in crystals. A field

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emission scanning electron microscope (SEM) (S-4300, Hitachi) was used to acquire

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SEM images. X-ray diffraction (XRD) spectra of crystals were measured using an X-

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ray diffractometer (Empyrean, PANalytical B.V.) with Cu Kα radiation source (λ =

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1.5406 Å). X-ray photoelectron spectroscopy data were collected using an X-ray

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Photoelectron Spectrometer (XSAM800, Kratos) with a monochromatic Al Kα X-ray

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source. Fitting procedures to extract peak positions and relative stoichiometries from

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the XPS data were carried out using the Casa XPS software suite. Optical diffuse-

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reflectance spectra of single crystal powders were recorded with a UV-vis-NIR

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spectrophotometer (UV-3600, Shimadzu) operating from 1000 nm − 400 nm at room

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temperature. Raman measurements were conducted on a Raman spectrometer

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(LabRAM HR, HORIBA) with excitation lines of 633 nm. Infrared (IR) spectra were

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recorded on a spectrometer (Nicolet 6700, Thermo Fisher Scientific). Solid-state

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nuclear magnetic resonance (NMR) spectra were acquired on a spectrometer (Bruker-

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Avance, 500 MHz) using a 2.5 mm MAS probe with samples fully packed inside

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zirconium oxide rotors. The photoluminescence (PL) spectra of bulk crystals were

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performed with a spectrofluorometer (FLS980, Edinburgh) and a 375-nm laser was

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used as the excitation source.

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RESULTS AND DISCUSSION

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Motivated by this remarkable resemblance of Bi3+ and Pb2+, we prepared Bi-doped 6 ACS Paragon Plus Environment

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MAPbBr3 single crystals to explore the effects of bismuth incorporation on structure

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and properties of perovskites. Bi-doped bulk crystals entirely lacking grain boundaries

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(Figure S1) had a cuboid shape with a typical lateral size of 5 mm and 3 mm thickness.

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Their shape was the same as undoped MAPbBr3 (parallelepiped shape), although the

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growth of some crystals could be confined by the wall of the vial (Figure 1a).

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Furthermore, the color of Bi-doped single crystals was readily tuned by adding Bi3+ in

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the feed solution. An obvious color change, from orange to black, was observed. Since

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the perovskite crystal is fragile, it is prone to fragmentation at the edge during the

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removal of crystals from the feed solution and surface cleaning. It is noteworthy that

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broken places at the edge of the Bi-doped MAPbBr3 single crystals presented dark red,

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marking with green circles in Figure 1a. The light irradiated on perovskite single

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crystals was fully absorbed due to their high absorption coefficients. However, light

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inside single crystals can hardly go out from the smooth surface due to the internal total

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reflection. Therefore, the intact surface appeared black. The color difference between

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broken surface and intact surface was quite big. It may indicate that the intrinsic color

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of Bi-doped MAPbBr3 crystals is not black, as we see in eyes. In consideration of the

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photon recycling effect of OIHP, re-absorption of the emitted light occurs efficiently in

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thick perovskite single crystals.24,25 Hence, the thickness of Bi-doped MAPbBr3 crystals

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may be responsible for this color change. About 1-mm thick Bi-doped crystals were

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then prepared from feed solutions with the same Bi3+ concentration by decreasing

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growth time (Figure 1b). And their dark red colors indeed confirmed the important

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effect of thickness on the color change of Bi-doped crystals.



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Figure 1. (a) Photographs of undoped and Bi-doped MAPbBr3 single crystals with ~ 3-

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mm thickness. (b) Photographs of thinner undoped and Bi-doped MAPbBr3 single

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crystals (~ 1-mm thickness).

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The accurate Bi molar ratio of Bi-doped single crystals obtained via FAAS has

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been shown in Table 1. It is clear that the Bi content in the crystal increased with

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increasing Bi concentration in feed solution. For the nominal 20% Bi doping (20%

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Bisolution), the amount of Bi in the doped crystal was just about 0.55%. The Bi content

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was found to be significantly lower than the nominal Bi3+ amount in the feed solution,

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indicating superior difficulty of doping perovskites. Furthermore, when the Bi

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concentration increased from nominal 20% to 30% in feed solution, the increase range

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of the actual Bi content in doped crystal was negligible and the doping amount of Bi

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still maintained around 0.55%. The size of the doped ions did not meet the requirements

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of the perovskite structure perfectly. Their forced entry into perovskite lattice increased

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the energy of the system and was suppressed seriously. Dopant incorporation can be

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only achieved in a small concentration. The rest of Bi3+ ions were still in the precursor

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solution. Although the dopant content in the feed solution was continuously increasing, 8 ACS Paragon Plus Environment

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dopant incorporation level in crystals was more and more difficult to promote.26

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Table 1. The amount of Bi in the feed solution and obtained crystals. Bi mol% in feed solution 0 10 20 30

Bi wt% in obtained crystals 0 0.14 ± 0.01 0.24 ± 0.02 0.25 ± 0.01

Bi mol% in obtained crystals 0 0.32 ± 0.03 0.55 ± 0.02 0.57 ± 0.04

~ number of Bi atoms per cm3 of crystal − ~ 2 × 1019 ~ 4 × 1019 ~ 4 × 1019

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Besides, less and less Pb2+ impeded the formation of supersaturated MAPbBr3

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solutions. Nucleation was difficult in the feed solution with 30% Bi concentration only

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after a long heating time. We have tried to dope the Bi content with higher level (40%)

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under the same experimental conditions, and no single crystals were crystallized out

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from the solution. For the feed solution of 30% Bi concentration, canary yellow

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powdery precipitate gradually separated out at the later period of crystal growth (Figure

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S2), which was speculate as the Bi-based perovskite (MA3Bi2Br9). According to

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nucleation theory, nucleation should satisfy not only the thermodynamic requirements,

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but also the following kinetic requirements of structure fluctuation and energy

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fluctuation. When Bi-doped crystals continued to grow, the amount of Pb2+ in the feed

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solution became less and less. Structure fluctuation for MA3Bi2Br9 nucleation could

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generate more easily, then new Bi-based perovskite phase started to grow.27 To some

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extent, this phenomenon also interpreted the reason why Bi3+ ions can only substitute a

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very small part of Pb2+ in the MAPbBr3 crystal lattice.

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To investigate optical absorption difference, undoped and Bi-doped MAPbBr3

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crystals were ground into powders to conduct the solid-state diffuse-reflectance

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spectroscopy measurement. The undoped MAPbBr3 crystal powder was orange, and

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the Bi-doped crystal powder was dark red (inset in Figure 2), which was very close to

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the color of aforementioned broken area. In addition, the color difference between 9 ACS Paragon Plus Environment


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crystal powder with various Bi content was very subtle and cannot be distinguished by

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naked eyes. As depicted in Figure 2, the undoped MAPbBr3 single crystal demonstrated

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strong light absorption up to around 570 nm and corresponds to a band gap of ~ 2.17

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eV, which was consistent with previous reports.20,23 The sharp band edge in the

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absorbance spectrum suggested very few in-gap defect states in the crystal. Compared

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with the undoped counterpart, obvious red shifts of band edge were observed in the

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absorption spectra of Bi-doped MAPbBr3 single crystals. The absorption range was

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constantly widening along with the increasing amounts of Bi3+. In the case of the 0.55%

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Bi content, the band edge shifted from 570 nm to 700 nm. Due to the similar Bi amount

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of crystals grew from feed solutions with 20% and 30% Bi3+, their absorption spectra

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were very close. It should be noted that Bi3+ incorporation did not narrow the band gap

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of MAPbBr3 single crystal, which has been proven by the means of PL and

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spectroscopic ellipsometry.21,22 After doping Bi3+, the absorption spectra of the Bi-

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doped MAPbBr3 single crystals almost covered the entire visible spectrum. Along with

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the high optical absorption coefficient and large thickness, so they appeared black.

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While the light with a wavelength above 570 nm could not be absorbed by the undoped

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crystal, causing appearance orange color even if it was of thickness of several

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millimeters. The distinct broadening of absorption spectrum by Bi3+ incorporation

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accounted for the color change of Bi-doped MAPbBr3 single crystals, implying their

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great promise for optoelectronic applications (eg. the preparation of intermediate band

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photovoltaic devices and tunable LEDs).


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Figure 2. Normalized absorption spectra of undoped and Bi-doped MAPbBr3 crystals.

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Inset: corresponding photographs of powders of MAPbBr3 single crystals prepared

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from solution containing (a) 0 (b) 10% (c) 20% (d) 30% Bi.

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The Bi content of the single crystal grew in the feed solution containing 20% BiBr3

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tended to the limit value, and there was no obvious difference of optical absorption

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spectra between 20% Bisolution and 30% Bisolution crystals. Hence, the 20% Bisolution

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crystal, as the representative of Bi-doped crystals, was further characterized along with

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the pristine MAPbBr3 single crystal to comparatively identify the impact of Bi3+

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incorporation. To characterize quality of crystalline materials and analysis the change

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of crystal structure, we performed XRD measurement for undoped and 20% Bisolution

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single crystals (Figure 3). Strong peaks of the undoped crystal at 15.2°, 30.4°, and 46.1°

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corresponded to the (001), (002) and (003) lattice planes, which belonged to the cubic

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structure and perfectly assigned Pm3m space group. For 20% Bisolution single crystal, it

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also exhibited the cubic perovskite structure without characteristic peaks of other phase,

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e.g., BiBr3. Compared to the XRD diffraction pattern of undoped single crystal, peaks

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of the 20% Bisolution crystal slightly shifted to larger 2θ values, indicating a lattice

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shrinkage. Careful XRD analysis revealed slight lattice parameter decrease between 11 ACS Paragon Plus Environment


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undoped (5.93 Å) and 20% Bisolution (5.89 Å) single crystals, implying the possibility of

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substitutional doping of Pb2+ by Bi3+ due to its smaller ion radius.28 It is worth noting

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that the diffraction peak intensity of 20% Bisolution crystal was reduced by nearly two

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orders of magnitude, suggesting a reduced crystalline order by Bi3+ incorporation.

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Similarly, there was a little broadening in the full width at half maxima (FWHM) before

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and after doping, which also indicated the microstrain across the crystals. The XRD

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results demonstrated that trace amounts of Bi3+ incorporation will slightly affect the

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structure of crystal. Usually, the changes of crystal structure can in turn affect properties

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of OIHPs.29,30

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Figure 3. X-ray diffraction spectra of undoped and 20% Bisolution single crystals. The

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inset shows the enlarged view of a small angle region.

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Next, the high-resolution XPS measurement was performed on the freshly cleaved

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crystal surfaces of undoped and 20% Bisolution single crystals. For the undoped MAPbBr3

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single crystal, the Pb 4f and Br 3d doublet binding energies were observed at 138.26

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eV (Pb 4f7/2), 143.12 eV (Pb 4f5/2), 68.02 eV (Br 3d5/2) and 69.06 eV (Br 3d3/2),

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respectively (Figure 4a). The oxidation state of Pb can be assumed to Pb(II) according

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to the previous reported literature.31 Bi 4f doublet was clearly resolved at the 159.16 eV 12 ACS Paragon Plus Environment

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and 164.44 eV binding energies, confirming the presence of Bi again (Table S1). These

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two peaks with the separation of 5.28 eV were characteristic of Bi3+ 4f7/2 and 4f5/2, as

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reported in the literatures, and can be assigned to Bi3+.32−34 In high-resolution XPS

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spectra of the 20% Bisolution single crystal, the expected ratio of 4/3 for the spin−orbit

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split f-levels was in good agreement with the experimental data (Figure 4e and f). The

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Pb 4f doublet and Br 3d binding energies were slightly higher than the values for

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undoped MAPbBr3 single crystal (Table S2). This implied that the electron density

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around Pb2+ and Br− could be reduced due to the stronger electronegativity of Bi3+,

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which weaken the shielding effect for inner electrons.35,36 The inner electrons combined

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with their atomic nucleus more closely. Therefore, the Pb 4f doublet and Br 3d binding

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energies were slightly increased.



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Figure 4. High-resolution XPS spectra of (a) all elements (b) N 1s, (c) C 1s, (d) Br 3d,

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(e) Pb 4f and (f) Bi 4f of freshly cleaved undoped and 20% Bisolution single crystal

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surfaces.

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It is noteworthy that there was obvious difference of XPS spectra (N 1s and C 1s)

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after Bi3+ doping. Figure 5 shows these spectra which were performed de-convoluted

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into a summation of Gaussian–Lorentzian curves. Compared with the undoped single 14 ACS Paragon Plus Environment

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crystal, the N-H (red curve in Figure 5b) peak shifted toward the low binding energy

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direction, which suggested that shielding effect was slightly enhanced due to the

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increasing electron density outside the N atomic nucleus by the Bi3+ incorporation. This

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phenomenon might be an indicator of strength attenuation of N+−H···Br hydrogen

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bonding in the cubic phase of MAPbBr3 after doping. The electronegativity of N atom

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might become stronger to decrease electron density outside the C atomic nucleus.

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Hence, the peak of C-N (blue curve in Figure 5d) became more obvious and shifted to

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higher binding energy. Because the C-H bond was not significantly affected, there was

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no change about it.

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Figure 5. High-resolution XPS spectra of N 1s and C 1s of cleaved undoped and 20%

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Bisolution single crystal surfaces after processing in Casa XPS software suite and

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deconvolution.



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The valence electron spectrum is sensitive to the valence bond structure of

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organics, which often becomes the unique fingerprint spectrum of organics. A slight

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change of binding energy peak among 2.5 ~ 4.3 eV could be found by Bi3+ doping

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(Figure 6a). For MAPbBr3, the upper edge of valence band (VB) was dominated by Br

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4p orbit (with minor antibonding contributions from Pb 6s). When Pb2+ ions in the

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crystal lattice were replaced after Bi3+ incorporation, trace amounts of interstitial Br−

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could be generated, which may cause this slight change. Valence band maximum (VBM)

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energies relative to the Fermi energy (EF=0) of single crystals can be obtained from the

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onset energy values of spectra. As shown in Figure 6b, VBM values are nearly

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invariable considering the measurement uncertainty (±0.05 eV), in line with previous

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reports,37,38 which indicated that the VB structure hardly changed by Bi3+ doping. In

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addition, the signal intensity of N 2s and C 2s orbital was weaken after Bi3+

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incorporation, which may be also caused by the weaker hydrogen bond. The decreased

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strength of the hydrogen bond made the bond force constant of N+−H larger, and the N

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atom with stronger electronegativity also enlarged the bond force constant of N−C, so

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that the electrons of N 2s and C 2s were not easy to be excited.

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Figure 6. (a) XPS valence band spectra and (b) VBM of cleaved surfaces of undoped

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and 20% Bisolution single crystals.


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290

Raman spectra of undoped MAPbBr3 and 20% Bisolution single crystals were shown

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in Figure 7. Main peaks were assigned as follow (Table S3). The sharp and intense

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bands at 970 and 1479 cm−1 were attributed to the C-N stretching and the NH3

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asymmetric (asym.) bending modes, respectively. The signals at 922 and 1257 cm−1

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corresponded to the rocking modes of MA+. The peak at 1596 cm−1 was assigned to the

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twisting mode of NH3. The high frequency peaks at 2829 cm−1 and 2970 cm−1

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corresponded to NH3 stretching and CH3 asym. stretching, respectively. The bands at

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3039, 3130 and 3175 cm−1 were constituted of the splitting of the NH3 symmetric (sym.)

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stretching. Xie et al. revealed that the vibrational modes of the MA+ cation were highly

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sensitive to the microenvironment and the chemical interactions between the organic

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cation and the inorganic framework mainly took place through the NH3 end of the

301

MA+.39 The aforementioned splitting was due to the hydrogen bonding with the halides

302

in the form of N+–H···Br. By contrast, all vibration models associated with the NH3

303

shifted slightly towards low wavenumber after doping, implying that PbBr64− inorganic

304

framework has been affected by Bi3+ incorporation. Furthermore, the changes in

305

frequency of Raman peaks indicated that there was tension in the crystal, which can

306

also serve as an indication of crystal lattice shrinkage caused by the substitution of Pb2+

307

with smaller Bi3+. And the broadening of Raman peaks suggested a decrease in crystal

308

quality, which was also in line with the result of XRD.



309 310

Figure 7. Raman spectra evolution before and after Bi3+ incorporation.

311

The hydrogen bonding plays an important role in the interaction of the organic

312

MA+ with the inorganic lead halide host structure.40−42 IR spectroscopic was used to

313

understand the interplay between the organic MA+ and inorganic host structures (Figure

314

8). Assignments and comparisons between IR bands from MAPbBr3 single crystals

315

before and after doping were summarized in Table 2. When Pb2+ was substituted by

316

Bi3+, significant blueshift as large as 4 cm−1, 3 cm−1 and 3 cm−1 were observed for the

317

asym. NH3 bending, sym. NH3 stretching and asym. NH3 stretching modes, respectively.

318

These observations also indicated that the Bi3+ incorporation changed the PbBr64−

319

framework significantly due to the high sensitivity of NH3 to the microenvironment.

320

The obvious wavenumber blueshift of asym. NH3 stretching vibration presented here

321

meant the increasing force constant and weaker electron cloud density average degree

322

of N+-H, which suggested a decrease in the hydrogen bond strength of N+−H···Br after

323

Bi3+ doping.43,44 An intimate structure-property relationship exists in halide perovskites,

324

whereby a certain cooperative structural distortion, known as octahedral tilts.45,46 Lee

325

et al. found that while steric effects dominate the tilt magnitude in inorganic halides,

326

hydrogen bonding between an organic A-cation and the halide frame plays a significant

327

role in hybrids. These excellent photoelectronic properties of hybrid perovskites highly 18 ACS Paragon Plus Environment

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328

correlate with the hydrogen bonding, associated with order−disorder behaviors of the

329

MA+ cations and numerous tilting patterns of the corner-connected inorganic PbX64−

330

octahedra.47−49 Tiny octahedral tilting of the halide frame might be caused by the

331

weaker hydrogen bonding, which in turn can affect the spatial symmetry of perovskite

332

structure and photoelectronic properties. Besides, a spectral broadening can be

333

observed, also implying a higher disorder in the doped sample.

334 335

Figure 8. IR spectra of the undoped MAPbBr3 single crystal and 20% Bisolution doped

336

MAPbBr3 one.

337

Table 2. Overview of typical functional groups and their corresponding peak positions

338

of MAPbBr3 single crystals before and after doping. Band assignment CH3 rocking C-N stretching NH3 rocking C-N stretching sym. NH3 bending asym. NH3 bending sym. NH3 stretching asym. NH3 stretching

339

Wavenumber (cm-1) Undoped 20% Bisolution 908 909 962 963 1243 1245 1387 1388 1469 1470 1571 1575 3104 3107 3173 3176

Shift after doping (cm-1) +1 +1 +2 +1 +1 +4 +3 +3

In order to obtain information about the uniformity of the chemical environment 19 ACS Paragon Plus Environment


340

of MA+, we performed solid-state 1H and 13C NMR of pristine and Bi-doped MAPbBr3

341

crystals. As shown in Figure 9, it can be observed that both of the 1H and 13C NMR

342

spectra broadened for the 20% Bisolution single crystal, which may indicate a slight

343

environment change of around MA+ ion. This broadening was probably due to the

344

increased disorder in the crystal since the chemical shift of nuclei was sensitive to the

345

local structural distortions.50

346 347

Figure 9. (a) Solid-state 1H NMR and (b) 13C NMR spectra of pristine and Bi-doped

348

MAPbBr3 single crystals.

349

Further evidence of the impact of Bi3+ incorporation on the energetic disorder and

350

microstrain was obtained from PL. As shown in Figure 10a, a sharp Peak 4, with a

351

FWHM of 90 meV, can be assigned as bimolecular recombination of photocarriers,

352

implying the low trap-state density and high crystal quality of the undoped MAPbBr3

353

single crystal.11,51 Peak 3, with the FWHM of 30 meV, was the result of filtered

354

photocarrier recombination emission PL signals leaking out from the top surface and

355

edge of the crystal after multiple reflection.52 The additional broad lower energy Peak

356

1 (1.95 eV) and Peak 2 (2.00 eV) were from excitons trapped in native or surface defects

357

(defect-related PL).53,54 In the PL of Bi-doped MAPbBr3 crystal, Bi3+ incorporation led

358

to the severe suppression of the photocarrier recombination emission, whose signal 20 ACS Paragon Plus Environment

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359

intensity was almost quenched by >99%, in accordance with previous reports.26,27,38

360

The degree of lattice deformation was greatly increased after Bi3+ doping. A Coulomb

361

potential field could be formed around the Bi3+ impurity ions, which locally destroyed

362

the periodic potential field near the impurity. Photocarriers could be also scattered when

363

they moved to the vicinity of the impurity ions. Although the PL peak of photocarriers

364

became extremely weak, it can serve as the indication of the unchanged bandgap (inset

365

of Figure 10b). It is noteworthy that PL centered at ~ 2.00 eV (peak of native defects)

366

was still obvious, which further suggested that the bandgap did not change. As can be

367

seen in Figure 10b, a broad PL ranging from 1.50 to 2.00 eV, peaking at 1.78 eV was

368

distinct. The emission intensity was much stronger than those of other peaks, indicating

369

the existence of impurity band by Bi3+ dopant.

370 371

Figure 10. Steady-state PL emission spectra of (a) undoped crystal, which were

372

decomposed into four emission bands. The fitting is based on multipeak Lorentzian

373

functions, (b) 20% Bisolution MAPbBr3 single crystals, and inset: the enlarged view of

374

the region between 2.1 eV and 2.5 eV.

375

Yamada et al. roughly ascribed absorption shift in Bi-doped MAPbBr3 to the

376

Urbach tail induced by lattice distortion without more explanation.21 The Urbach tail is

377

only a simply description of light absorption shift. Here, an intermediate band model 21 ACS Paragon Plus Environment


378

based on the band theory was given to further explain the optical property change after

379

Bi3+ doping. Bi3+ incorporation resulted in localized perturbations in the periodic lattice

380

potential extending around the impurity. Although the feed solution contained 1% Bi,

381

the number of Bi atoms per cm3 of obtained Bi-doped MAPbBr3 single crystal was

382

almost 1018. In the case of heavy doping, the average distance between Bi impurities

383

rapidly diminished. The localized impurity states interact with each other, causing the

384

electron wave functions between impurity atoms to overlap. The isolated impurity level

385

expands into an energy band, which is generally called an impurity band. Carriers can

386

transport in the impurity band through the communization movement between the

387

impurities, which in turn suppress the Shockley-Hall-Read non-radiative recombination.

388

During the continuous growth of perovskite crystal, it is easy for Bi3+ to replace Pb2+

389

duo to their similar radii and very high calculated formation energy (1.1 eV) of Bi3+

390

interstitial.23 The substitution resulted in BiPb defects in the crystal lattice. Yan et al.

391

predicted that the defects of BiPb could generate energy levels below the conduction

392

band (CB) minimum.55 Previous works also reported that, in doped materials, the

393

structure stabilizes through the formation of vacancies.56,57 The introduction of the

394

heterovalent Bi3+ could also made the crystal lattice more prone to local vacancies and

395

form defect states. When Bi substitutionals were mixed with vacancies, they could

396

couple efficiently to produce a shallow defect band appeared at 1.78 eV above VBM of

397

host MAPbBr3 (Figure 11). The experimentally observed absorption onset red-shift was

398

due to the absorbance related to the defect states rather than a decrease of the perovskite

399

band gap. Gap states were expected that the absorption of sub-bandgap photons

400

generated electron–hole pairs by pumping electrons from the VB to the CB via the gap

401

states, as was seen in intermediate-band solar cells.58,59 We anticipated that the defect

402

states within the bandgap acted as a scaffold for photon absorption with below-bandgap 22 ACS Paragon Plus Environment

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403


light.

404 405 406

Figure 11. Band diagram structure of 20% Bisolution MAPbBr3 single crystal.

CONCLUSIONS

407

Pristine and Bi-doped MAPbBr3 single crystals were grown by inverse

408

temperature crystallization method. A limit amount (~ 0.55%) of Bi3+ ions can be

409

introduced into the host crystal structure. For Bi-doped crystals, the significant red-shift

410

of optical absorption edge could be observed. And we demonstrated herein reasons of

411

changes about structure and optical property. Although preserving the structure of the

412

host MAPbBr3, slight lattice shrinkage and distortion were caused by the incorporation

413

of trivalent Bi ions, implying the substitution of Pb2+ by Bi3+. In addition, tiny tilting of

414

the PbBr64− octahedral frame derived from weaker hydrogen bonding should also

415

account for the change of spatial symmetry of perovskite structure and optical

416

properties. PL spectra suggested the apparent color change was due to a significant

417

increase of sub-band gap state density (impurity band), acting as a scaffold for photon

418

absorption with below-bandgap light. The previous debate about whether Bi3+

419

incorporation narrows the band gap of MAPbBr3 was further clarified by elaborating

420

the effect of the impurity band. And this gap-state engineering can be a feasible way to

421

enhance photoabsorption, and therefore providing a promising route to the perovskite 23 ACS Paragon Plus Environment


422

material design in optoelectronic field.

423

ASSOCIATED CONTENT

424

Supporting Information

425

SEM of cleaved crystals and element mapping images of 20% Bisolution MAPbBr3

426

single crystal, Photograph of precipitate in the feed solution with 30% Bi3+, Elemental

427

composition as determined by XPS analysis, Fitting information of the high resolution

428

XPS spectra, and Raman shift of vibration modes before and after doping.

429

ACKNOWLEDGMENTS

430

This work was supported by the National Natural Science Foundation of China

431

(Grant No. 61704117), and the Key Research and Development Program of Sichuan

432

Province of China (2017GZ0052).

433

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Doping-Enhanced Visible-Light Absorption of CH3NH3PbBr3 by Bi3+

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