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Electronic Supplementary Material

Organic–inorganic bismuth (III)-based material: A lead-free, air-stable and solution-processable light-absorber beyond organolead perovskites

Miaoqiang Lyu1, Jung-Ho Yun1, Molang Cai2, Yalong Jiao2, Paul V. Bernhardt3, Meng Zhang1, Qiong Wang1,

Aijun Du2, Hongxia Wang2, Gang Liu4, and Lianzhou Wang1 ()

1 Nanomaterials Centre, School of Chemical Engineering and AIBN, The University of Queensland, St Lucia, Brisbane, QLD 4072,

Australia 2 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology,

Brisbane, QLD 4001, Australia 3 School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane 4072, Australia 4 Advanced Carbon Division, Institute of Metal Research Chinese Academy of Sciences (IMR CAS), 72 Wenhua Road, Shenyang 110016,

China

Supporting information to DOI 10.1007/s12274-015-0948-y

Part S1 Detailed experimental details All experimental processes were performed in ambient air unless otherwise mentioned. All chemicals were

used as received in this work. Abbreviations: MAI = methylammonium iodide (CH3NH3I), MBI = methylammonium

bismuth iodide ((CH3NH3)3Bi2I9), P3HT = Poly(3-hexylthiophene-2,5-diyl), DMF = N,N-dimethylformamide, TiO2

blocking layer = TiO2 BL.

Preparation of MAI

Typically, 24 mL of methylamine (Sigma-Aldrich, 33 wt.% in absolute ethanol), 10 mL of hydroiodic acid

(Sigma-Aldrich, 55 wt.% in water), and 100 mL of absolute ethanol were mixed at 0 °C and magnetically stirred

for 2 h. MAI was crystallized by a rotary evaporator at 50 °C. A yellowish powder was finally achieved via

rotary evaporation, which was further purified to white colour after re-crystallization in the absolute ethanol

and diethyl ether. Finally, the MAI powder was dried in a vacuum oven overnight at 60 °C and stored in a

glove-box (O2 level < 0.1 ppm, H2O level < 0.1 ppm).

Deposition of BiI3 films

Bismuth iodide (0.5897 g) was added into 1 mL anhydrous DMF to form 1 mol·L–1 solution. After sonication for

1 h, the solution was filtered using PTFE syringe filters (0.45 μm). The substrates used for BiI3 film deposition

Address correspondence to l.wang@uq.edu.au

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were preheated at 100 °C before spin-coating. The spin-coating conditions were 2,000 rpm for 30 s and the

ramping rate was 200 rpm·s–1. After spin-coating, the film was annealed at 150 °C for 10 min. The BiI3 solution

in DMF shows reddish colour. The as-spin-coated film showed red colour, which changed to black upon heat

treatment.

Preparation of (CH3NH3)3Bi2I9 solution

A (CH3NH3)3Bi2I9 solution (1 mol·L–1 in DMF) was prepared by mixing 0.2384 g of MAI, 0.5897 g of BiI3 and

1 mL of anhydrous DMF. The mixed solution was treated by ultra-sonication for 30 min and finally filtered by

PTFE syringe filters (0.45 μm), finally achieving a clear dark reddish solution.

Growth of MBI single crystals

To prepare MBI single crystals, 0.2384 g of MAI, 0.5897 g of BiI3 were added to 2 mL of anhydrous methanol and

the solution was mixed by ultra-sonication for ~0.5 h. (CH3NH3)3Bi2I9 is less soluble in methanol than that in the

DMF. However, the boiling point of methanol is much lower than that of DMF. Clear (CH3NH3)3Bi2I9 solution

(0.5 mL) in methanol was transferred to another clean vial and the solvent was allowed to evaporate slowly

overnight in the fume-hood at room temperature, leading to the formation of MBI single crystals on the inner

wall of the vial.

Fabrication of TiO2 mesoporous layers

A TiO2 BL was deposited firstly on the cleaned FTO substrate by an aerosol spray pyrolysis method at 500 °C.

The precursor solution for the spray pyrolysis is 0.2 M isopropanol solution of titanium diisopropoxide

bis(acetylacetonate) and the thickness of the TiO2 blocking layer was controlled to be 80–100 nm. A TiO2 mesoporous

layer was deposited onto the FTO/TiO2 BL by doctor blading. Typically, the TiO2 paste was prepared by ultra-

sonicating a mixture of commercial TiO2 nanopac paste (TiO2 paste, TP-18AT, NANOPAC Co., Ltd.) and

anhydrous ethanol (weight ratio 1:0.5). The substrate was covered by 3 M scotch tape to define the thickness

and protect the unwanted area. After doctor blading, the samples were annealed in a furnace at 500 °C for 60 min

with a ramping rate of 5 °C·min–1.

Device fabrication

Fluorine-doped tin oxide (FTO) coated glass slides (FTO22-7, Yingkou OPV Tech New Energy Co. Ltd) were

firstly pre-patterned by removing part of the conductive layer with 2 M HCl aqueous solution and zinc powder.

The etched substrates were cleaned in acetone, ethanol and isopropanol, subsequently. After oxygen plasma

treatment (RIE, ATX-600, 30 W, 10 min), a thin TiO2 block layer was deposited by spray pyrolysis at 500 °C

using 0.2 M isopropanol solution of titanium diisopropoxide bis(acetylacetonate) as the precursor solution.

One-step spin-coating method was used to deposit the MBI film on the TiO2 mesoporous layers, in which

50 μL of (CH3NH3)3Bi2I9 solution (1 mol·L–1 in DMF) was dispensed onto the substrate and spin-coated at

4,000 rpm for 30 s with a ramping rate of 4,000 rpm·s–1, followed by heating at 100 °C for 10 min on a hotplate. A

reddish film was achieved after heating treatment. Pristine P3HT (MW 54,000–75,000, Sigma-Aldrich, 15 mg·mL–1

in 1,2-dichlorobenzene) was used as the hole-transporting layer, which was spin-coated at 2,000 rpm for 30 s

after 60 soaking. Then, the coated substrates were further annealed at 100 °C for 15 min to remove the remaining

solvent. The devices were left in the glove box overnight. Finally, a ~60 nm gold contact electrode was deposited

by e-beam evaporation (Temescal FC-2000). The cell area was defined by a metal mask with an aperture area of

7.068 mm2.

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Characterizations

The current–voltage (I–V) plots were recorded using a solar simulator (AM 1.5, 100 mW·cm–2, Oriel) equipped

with a Keithley model 2420 digital source meter. Powder X-ray diffraction (XRD) measurement was performed

on a Bruker Advanced X-ray Diffractometer with Cu Kα from 10° to 70° (2θ). Scanning electron microscope

(SEM) images were recorded by a Field-Emission Scanning Electron Microscope (FE-SEM, JEOL 7100). Optical

image of single MBI crystals were carried out on an optical microscope (Leica). Reflectance spectra of the MBI

films were performed by a UV–visible light spectrometer (Shimadzu UV2450). For the stability test of the MBI

films, the as-deposited MBI films on the bare FTO substrates were placed in the ambient air with a relative

humidity level ranging from 30% to 80%. Then, XRD spectra were analysed periodically to confirm the crystal

structure. X-ray photoelectron spectroscopy (XPS, Kratos Ultra) was performed using mono Al X-ray gun. The

theoretical XRD spectrum of MBI was calculated by Mercury 3.3 software (The Cambridge Crystallographic

Data Centre (CCDC)) based on the results of single crystal X-ray diffraction measurement. The Conductivity of

the MBI films were characterized by a Van der Pauw four-point probe equipped with a computer controlled

source meter (KeithLink LRS4-T, Taiwan) at room temperature. Hall effect measurement was performed by a

cryogen free high field measurement system (Cryogenics Ltd., UK) equipped with Keithley 2400, in which

a source current of 1 mA was used and the magnetic field was scanned from –5 to 5 Tesla. The incident

photon-to-current conversion efficiency (IPCE) was measured via a Newport 1918-c power meter equipped

with a 300 W Oriel xenon light source and an Oriel Cornerstone 260¼ m monochromators in DC mode. A

standard Si reference was used before the IPCE measurement.

Single-crystal X-ray diffraction measurement

Single-crystal X-ray diffraction data of MBI single crystals were recorded by an Oxford Diffraction Gemini

Ultra S CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å). The crystal was cooled to 190 K with an

Oxford Cryosystems Desktop Cooler. The structure was solved by direct methods with SHELXS [S1] and refined

by full matrix least-squares analysis with SHELXL97 within the WinGX package [S2].

First-principles electronic band structure calculations

All the calculations were performed based on the density functional theory (DFT) using VASP code [S3, S4]. The

electron exchange and correlation was treated with generalized gradient approximation (GGA)/Perdew, Burke,

and Ernzerhof (PBE) functional [S5]. The cut-off energy for plane-wave basis set was 500 eV. For the first

Brillouin zone, the k-point grid [S6] was set to be 5 × 5 × 2. The optimized structure contains 6 C, 36 H, 6 N, 4 Bi

and 18 I atoms, respectively. All atoms were full relaxed and force and energy were converged to 0.01 eV·Å–1

and 10–5 eV, respectively.

The band alignments were derived from surface calculations using of vacuum potential as a common

reference and aligning the average electrostatic potential in crystals to the vacuum potential. Valence- and

conduction-band positions relative to the average electrostatic potential are separately determined from bulk

calculations. For the surface calculations we use a repeated-slab geometry and choose (CH3NH3)3Bi2I9 (001)

surface with each slab containing 4 layers. A vacuum layer with a thickness at least 15 Å was set to minimize

artificial interactions between two neighboring slabs. All atoms near the surface were full relaxed and residual

force and energy were less than 0.01 eV·Å–1 and 10–5 eV, respectively.

CCDC No. 1059280 includes the supplementary crystallographic data for methylammonium bismuth iodide

(MBI), which can be available free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the

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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1 EZ, United Kingdom; or by email:

deposit@ccdc.cam.ac.uk.

Part S2 Additional Tables, Figures and discussions

Table S1 Crystallographic data for MBI ((CH3NH3)3Bi2I9)a

Empirical formula C3H18Bi2I9N3

Formula weight 1,656.26

Temperature (K) 190(2)

Wavelength (Å) 0.71073

Crystal system Hexagonal

Space group P63/mmc

a, c (Å) 8.4668(6), 21.614(2)

Volume (Å3) 1,341.9(2)

Z, calculated density (g·cm–3) 2, 4.099

Absorption coefficient (mm–1) 23.441

F(000) 1,400

Crystal size (mm3) 0.36 × 0.14 × 0.091

Theta range for data collection (°) 2.92 to 29.01

–10 ≤ h ≤ 11

–10 ≤ k ≤ 8 Index ranges

–29 ≤ l ≤ 26

Reflections collected 3,788

Independent reflections 694 [R(int) = 0.1235]

Completeness to theta = 25.00° (%) 99.4

Absorption correction Analytical

Max. and min. transmission 0.18 and 0.036

Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 495/2/294

Goodness-of-fit on F 2 1.214

Final R indices [I > 2sigma(I)] b R1 = 0.0689, wR2 = 0.1672

R indices (all data) b R1 = 0.0752, wR2 = 0.1723

Largest diff. peak and hole (e·Å–3) 1.9 and –1.8 a Obtained by monochromated Mo Kα (λ = 0.71073 Å) radiation.

b 1/ 22 22 2 2

1 o c o 2 o c o/ , /R F F F wR w F F F .

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Table S2 Atomic coordinates (× 104) and equivalent isotropic displacement parameters (Å2 × 103). U(eq) is defined as one third of the trace of the orthogonalized Uij tensor

x y z U(eq)

Bi(1) 6667 3333 1555(1) 38(1)

I(1) 10038(3) 5019(1) 2500 53(1)

I(2) 3311(2) 1656(1) 812(1) 72(1)

N(1) 13333 6667 1110(30) 160(30)

C(1) 12890(50) 5780(100) 490(30) 100(40)

N(2) 10000 10000 2500 290(130)

C(2) 9250(60) 8500(110) 2040(40) 30(30)

Table S3 Selected bond distances (Å) for (CH3NH3)3Bi2I9

Bi(1)–I(2) 2.9391(18)

Bi(1)–I(1) 3.2061(19)

N(1)–C(1) 1.489(10)

N(2)–C(2) 1.489(10)

Symmetry transformations used to generate equivalent atoms:

#1 –y+1, x–y, z #2 –x+y+1, –x+1, z #3 x, y, –z+1/2

#4 –y+2, x–y, z #5 –x+y+2, –x+2, z #6 –x+y+1, –x+2, z

#7 –y+2, x–y+1, z #8 –y+2, x–y+1, –z+1/2 #9 –x+y+1, –x+2, –z+1/2

Table S4 Selected bond angles (°) for (CH3NH3)3Bi2I9

Bond Angle (°)

I(2)–Bi(1)–I(2)#2 92.93(7)

I(2)#1–Bi(1)–I(1) 91.46(4)

I(2)–Bi(1)–I(1) 173.61(7)

I(1)–Bi(1)–I(1)#2 83.79(5)

I(2)#1–Bi(1)–I(1)#1 173.61(7)

Bi(1)–I(1)–Bi(1)#3 79.10(6)

Symmetry transformations used to generate equivalent atoms:

#1 –y+1, x–y, z #2 –x+y+1, –x+1, z

Figure S1 (a) MBI solution in anhydrous DMF solution, showing a deep reddish colour; (b) a photo of a MBI film deposited onto a TiO2 mesoporous layer coated FTO substrate.

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Figure S2 XPS of the (CH3NH3)3Bi2I9 film on a FTO glass.

Figure S3 UPS spectrum of (CH3NH3)3Bi2I9 film on a FTO substrate. The valence band maximum (VBM) is estimated to be around 5.90 eV (inset graph) and hence the valence band energy is ~ –5.90 eV with respect to the vacuum level.

Figure S4 Three-dimensional AFM image of the (CH3NH3)3Bi2I9 crystal on a platinum coated glass substrate and this crystal was used for KPFM measurement.

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Figure S5 SEM images of a (CH3NH3)3Bi2I9 infiltrated TiO2 mesoporous film. (a) Low-magnification surface morphology; (b) high- magnification surface morphology; (c) surface morphology of the film showing the comparison of the white and dark areas; (d) back- scattering SEM image of the same area shown in (c), in which the white area can be confirmed to be occupied with higher concentration of Bi due to a larger atomic number of Bi compared with that of Ti.

Figure S6 (a) SEM images of a (CH3NH3)3Bi2I9 infiltrated TiO2 mesoporous film; (b) energy dispersive spectrum of the MBI infiltrated TiO2 mesoporous film collected on the marked point.

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Table S5 Quantified element distribution of the EDS result for the sample presented in Fig S6

Element Mass% Atom%

C 4.56 12.40

N 4.87 11.37

O 23.76 48.51

Ti 27.05 18.45

Sn 6.62 1.82

I 22.47 5.78

Bi 10.67 1.67

Total 100.00 100.00

Figure S7 (a) A cross-sectional SEM image of the planar structured device; (b) the I–V curve of the assembled device measured under irradiation of simulated AM 1.5 sunlight with a light intensity of 100 mW·cm–2.

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Figure S8 A representative I–V curve of the device with a structure of FTO/TiO2 BL/TiO2mpl+P3HT/Au, under irradiation of simulated AM 1.5 sunlight with a light intensity of 100 mW·cm–2.

Figure S9 The IPCE results for solar cells using P3HT only (red curve) and MBI+P3HT (black curve).

Figure S10 The time-dependent efficiencies for the best-performing device.

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Table S6 Current–voltage summary of the time-dependent performance of the best-performing device

Test time Voc (V) Jsc (mA·cm–2) Fill factor Efficiency%

Day 1 0.283 0.566 0.495 0.080

Day 2 0.328 0.594 0.528 0.103

Day 3 0.359 1.005 0.508 0.186

Day 9 0.323 1.118 0.536 0.194

Day 14 0.362 0.976 0.487 0.174

Day 21 0.370 0.976 0.493 0.181

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[S5] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865–3868.

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