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Extended Photo-Conversion Spectrum in Lowtoxic Bismuth Halide Perovskite Solar Cells Malin B. Johansson, Huimin Zhu, and Erik M. J. Johansson J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01452 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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The Journal of Physical Chemistry Letters

Extended Photo-Conversion Spectrum in LowToxic Bismuth Halide Perovskite Solar Cells Malin B. Johansson,# Huimin Zhu,# and Erik M. J. Johansson*

Department of Chemistry- Ångström Laboratory, Div. Physical Chemistry, Box 523, SE-751 20 Uppsala, Sweden

AUTHOR INFORMATION

Corresponding Author

*[email protected]

# Both authors has contributed equally

ABSTRACT: Lead-based perovskites show very promising properties for use in solar cells, however, the toxicity of lead is a potential inhibitor for large-scale application of these solar cells. Here, a low-toxic bismuth halide, CsBi3I10, is synthesized from solution and the optical properties and crystal structure are compared with previously reported Cs3Bi2I9 perovskite, and the photovoltaic properties are also investigated. The XRD pattern suggests that the CsBi3I10 film has a layered structure with a different dominating crystal growth direction than the Cs3Bi2I9 perovskite. A band gap of 1.77 eV is obtained for the CsBi3I10 film, which is smaller than the band gap of Cs3Bi2I9 at 2.03 eV, and an extended visible light absorption spectrum is therefore obtained. The solar cell device with CsBi3I10 shows a photocurrent up to

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700 nm, and this work shows therefore the possibility for increased light absorption and higher photocurrents in solar cells based on bismuth halide perovskites.

TOC GRAPHICS

Lead halide perovskites have recently been shown to be very promising for use in solar cells and devices based on these materials have now reached the remarkable solar to electricity power conversion efficiency record of 22.1 % in only six years.1-3 Despite this great success in efficiency of these solar cells, the toxicity of lead may be an issue in particular for large scale use.4 It would therefore be advantageous to find a non-toxic perovskite material that can be used for solar cells. Tin (Sn) has been proposed as a replacement of lead for lead-free perovskite solar cells and the record efficiency is now over 6 %.5 However, it has been argued that the tin perovskites may be even more toxic than lead perovskites.4 Another metal that can replace lead and form perovskites with suitable band gap values is Germanium (Ge).6 The drawback with the use of the rare metal Ge is the high cost and expensive refinement techniques. An alternative metal with similar properties as lead is bismuth. Bismuth 2 ACS Paragon Plus Environment

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perovskite (Cs3Bi2I9) based solar cells was published in 2015 by Park et al.7 This paper reported on a comparison of bismuth perovskites with methyl ammonium and cesium cations, and promising solar cell results were obtained for the bismuth perovskites with cesium cations. Time-resolved room-temperature photoluminescence (PL) decay studies of bismuth perovskites have also shown that the majority carriers recombine through long-lived processes, which is important for solar cell devices.8-9 The electron diffusion length in bismuth tri-iodide (BiI3) has been reported to be 1.9 -4.9 µm, calculated from the electron mobility and effective mass of the electron, and the hole diffusion length is expected to be much shorter.10-13 BiI3 has mainly been used in other application areas, and it specifically possess promising properties for room temperature gamma-ray detectors because of the high bulk density and high atomic number.14-15 There is at least one report investigating BiI3 for use in solar cells as light absorbing material,16 and the results show that BiI3 is photoactive, although the efficiencies for the solar cells are rather low. BiI3 has also been investigated as a hole transport material for organic solar cells with promising results.17 In this work, we present the preparation of a bismuth halide with the chemical composition CsBi3I10 for the use as light absorber in solar cell devices. To understand the properties of this material we have investigated the optical absorption and material structure, which was also compared with Cs3Bi2I9 and BiI3 samples. The cation cesium (Cs+) has been chosen as a result of previous works by Park et al.,7 where it was found that more efficient solar cell devices was obtained with Cs+ cations compared to methylammonium cations in the bismuth perovskite. After spin-coating the solutions, all the samples Cs3Bi2I9, CsBi3I10 and BiI3 has an orange color which change for the CsBi3I10 and BiI3 samples during heating to dark blackbrownish color, see the samples after heat treatment in the inset of Figure 1. The results obtained here show that a broader light absorption spectrum and therefore a higher



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photocurrent is obtained for the CsBi3I10 based solar cell device in comparison to the Cs3Bi2I9 based device. To investigate the suitability of the CsBi3I10 bismuth perovskite for photovoltaic applications, the optical properties were determined from UV-vis measurements (Figure 1). The absorption coefficient was calculated by measuring the transmittance and reflectance of the sample (see supplementary information) and the resulting spectrum is shown in Figure 1 together with the spectra for Cs3Bi2I9 and BiI3. Partial density of states calculated for Cs3Bi2I9 and BiI3 has been reported previously and these calculations show that light induced excitation from the valence band to the conduction band occurs mainly from occupied I p with a small contribution of Bi s states into empty Bi p + I p states. 11

Figure 1. The natural logarithm of the absorption coefficient for the CsBi3I10, Cs3Bi2I9 and BiI3 samples. In the absorption spectra in Figure 1, an Urbach tail18 is discernible below the absorption edges, in particular for the BiI3 and the CsBi3I10 samples. By using the logarithm of the 4 ACS Paragon Plus Environment

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absorption coefficient α, a clear separation between the demarcation energy Ed and interband absorption is seen in Figure 1. The position of the change in slope, marked with a dotted line, denotes the position of Ed.19 The demarcation energy can be interpreted as the energy of the band gap and is approximated to Ed ≈ 2.03 eV (610 nm) for the Cs3Bi2I9 sample, which agree with the band gap energy.7-9 A large red shift of the band gap energy is obtained for the CsBi3I10 sample where the band gap is estimated to 1.77 eV (700 nm), which is similar to the BiI3 band gap. This lower band gap improves the light harvesting, which is an advantageous in solar cells, but is still higher than the optimal band gap for a single junction solar cell. On the other hand it may be very suitable for the top cell in a tandem solar cell device.20 An advantage is also that the top layer is made from solution which may simplify the production process technique. The inset of Figure 1 shows the CsBi3I10, Cs3Bi2I9 and BiI3 materials on glass substrates. The color is orange/red for the Cs3Bi2I9 sample and brown/black for the CsBi3I10 sample, and the BiI3 sample has a more brownish tone. Before heating all three samples have a more orange tone, and the black and brown colors start to appear in the CsBi3I10 and BiI3 samples when the substrates are heated and the materials start to crystallize. The absorption coefficients for CsBi3I10, Cs3Bi2I9 and BiI3 between 350-500 nm were estimated to 1.4·105, 7·104 and 2.2·105 (cm-1) respectively. The absorption coefficients for CsBi3I10 and BiI3 is therefore comparable to the absorption coefficient of the lead-based perovskites21 (around 1·105 (cm-1)), which enables the use of the thin films in solar cell devices. Grazing incidence x-ray diffraction (GIXRD) was used to determine the crystal structure of the CsBi3I10, Cs3Bi2I9 and BiI3 samples, and the results are shown in Figure 2. The GIXRD pattern reveals the crystal structure of the measured thin films, with a phase change from rhombohedra structure in the BiI3 sample with lattice constants of a = 7.52 Å and c = 20.72 Å,22 to the hexagonal phase in the Cs3Bi2I9 sample with lattice constants of a = 8.41Å and c = 5 ACS Paragon Plus Environment


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21.24 Å.23 The most significant diffraction peaks in the BiI3 crystal pattern is from the (003) plane at 12.79o and the (300) plane at 41.56o in 2θ, while the Cs3Bi2I9 sample has the highest intensity from the (202), (203), (204) and (205) planes at 25.9o, 27.6o, 29.8o and 32.5o. Cs3Bi2I9 has a zero-dimensional crystal structure with two face-sharing metal-halide octahedrons, separated by cesium ions.7 The face-sharing metal-halide octahedrons are placed in layers, with a larger spacing to the next layer, which generate large crystal flakes,7 see Figure 3. The CsBi3I10 shows diffraction peaks at the same position as both BiI3 and Cs3Bi2I9, but with different dominating peaks in the XRD pattern and the most significant peaks in CsBi3I10 are (003), (006) and (300). This suggests that CsBi3I10 contains a layered crystal structure similar to BiI3 but that these layers are partly broken into the zero-dimensional structure like Cs3Bi2I9 in between the layers. The growth direction of CsBi3I10 is similar to BiI3. The suggested structures of Cs3Bi2I9 (a), CsBi3I10 (b) and BiI3 (c) can be seen in the inset of Figure 2.24 They are presented in the A-B plane (with the C-axis pointing towards the reader).


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Figure 2. GIXRD patterns of (a) Cs3Bi2I9, (b) CsBi3I10 and (c) BiI3. The red lines correspond to the Cs3Bi2I9 reference23 and black lines to the BiI3 reference.22The inset shows the suggested structures in the A-B plane from the given references for Cs3Bi2I9 and BiI3 and a suggestion of CsBi3I10.

Figure 3 shows SEM images of the Cs3Bi2I9 and CsBi3I10 samples. The morphology of the Cs3Bi2I9 sample (Figure 3a and b) shows hexagonal flakes with a size up to 2 µm which are vertically organized, penetrating the TiO2 surface. The TiO2 surface can be seen in the gaps between the large flakes.

Figure 3. SEM images of Cs3Bi2I9 (a and b), CsBi3I10 (c and d) and BiI3 (e and f) under low and high magnification, observe the different scale bars.

The CsBi3I10 sample instead shows a more uniform film with a smoother coverage of the TiO2 surface (Figure 3c and d). However, pinholes can be observed on the surface of the CsBi3I10 sample as well as BiI3 (Figure 3c, d, e and f). The reason for the formation of pinholes in the 7 ACS Paragon Plus Environment


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film is not clear, but it may be related to the solvent evaporation during the formation of the film. The more uniform coverage of CsBi3I10 compared to Cs3Bi2I9 is probably related to the difference in crystal structure observed in the GIXRD patterns, which show different dominant crystal directions. The crystal growth of these materials may be influenced by several factors. For example it has been observed that at low temperature, BiI3 microcrystals grow with the c axis parallel to the substrate, whereas at higher temperatures the crystals grow with the c axis perpendicular to the substrate.25

To evaluate the potential application of the CsBi3I10 in solar cells, devices based on a mesoscopic architecture were prepared and characterized.

Thin films of CsBi3I10 were

deposited from solution onto mesoporous TiO2 films under nitrogen gas flow protection and P3HT (poly-3-hexyl-thiophene) was used as hole transport material (HTM) on the CsBi3I10, a silver (Ag) contact was thereafter deposited on the P3HT layer resulting in the final device structure: glass / FTO / compact TiO2 / mesoporous TiO2 / CsBi3I10 / P3HT / Ag (see experimental section for complete description of device preparation). The device structure and SEM of the cross-section of the device is shown in Figure 4a-b. In this study P3HT is used as HTM instead of spiro-OMeTAD, which was previously used for the Cs3Bi2I9 perovskite,7 the reason for using P3HT in this study is that the addition of 4-tertbutyl-pyridine (used as additive in the spiro-OMeTAD HTM solution) dissolves the CsBi3I10 layer.


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Figure 4. (a) A schematic picture of the layers in the CsBi3I10 solar cell. (b) The SEM cross section of the CsBi3I10 solar cell (c) J-V curves of the BiI3, Cs3Bi2I9 and CsBi3I10 solar cell. (d) IPCE of the CsBi3I10, BiI3 and Cs3Bi2I9 device.

The current density-voltage (J-V) trace of the CsBi3I10 device was measured and the results are shown in Figure 4c. Comparing the parameters for solar cells with CsBi3I10 obtained here and for the previously reported Cs3Bi2I9 perovskite, the best device with CsBi3I10 shows a higher short circuit current (JSC) 3.4 mA/cm2, compared to previously reported 2.2 mA/cm2 for the Cs3Bi2I9 perovskite,7 but with a lower open circuit voltage (Voc) and fill factor (FF). The higher photocurrent for the solar cell with CsBi3I10 is obtained due to a broader light absorption spectrum with an extended absorption of light at longer 9 ACS Paragon Plus Environment


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wavelengths in comparison to the solar cell with Cs3Bi2I9, (see Figure 1), which results in increased light harvesting. The CsBi3I10 also gives a more uniform coating of the TiO2 film compared to the Cs3Bi2I9 perovskite (see Figure 3c-d), which may decrease the recombination between electrons in TiO2 and holes in the P3HT layer.

However, the rather low VOC

obtained for the solar cell with CsBi3I10 is possibly due to the use of HTM or electron transport materials with disadvantageous energy levels, which may reduce the maximum voltage in the device. In further optimization, different HTM materials will be investigated for use together with this perovskite, and the preparation parameters for the CsBi3I10 material will be optimized to reduce the pinholes observed in the layer and to reduce possible defect states. The Incident Photo to Current Conversion Efficiency (IPCE) for the device with CsBi3I10 is shown in Figure 4d. The IPCE spectra of CsBi3I10 covers the visible spectrum up to around 700 nm, which is in a good agreement with UV-vis spectrum of CsBi3I10, which shows a band gap of 1.77eV, see Figure 1. The results therefore show that CsBi3I10 has an extended light harvesting compared to Cs3Bi2I9, and that the extended light absorption also results in higher photocurrent compared to solar cells based on Cs3Bi2I9.7 Below, in Table 1, the results for the different solar cell devices are shown. All the solar cells are made with the same fabrication conditions, see the experimental part. The CsBi3I10 solar cell (1:3) shows much more promising results than the others. The CsBi3I10 film has been tested for stability in ambient air during 17h and the reproducibility of the solar cells is rather high ( see Figure S3 and S4 in supporting information).

Table 1: Devices with different molar ratio of CsI and BiI3 CsI: BiI3 (Molar Ratio)

Voc (V)

2

Jsc (mA/cm )


FF (%)

PCE (%)

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1: 3

0.31

3.40

38

0.40

1.5:1

0.26

0.18

37

0.02

0: 1

0.17

1.39

30

0.07

In summary, we have investigated a new low-toxic material, CsBi3I10, for use in solar cell devices. From UV-vis measurements the band gap was estimated to 1.77 eV, which gives a considerably (almost 100 nm) red-shifted absorption spectrum compared to Cs3Bi2I9. This shows the possibility to obtain a broader light harvesting spectrum for bismuth perovskite solar cells, and an IPCE spectrum with photocurrent response up to 700 nm was obtained. The GIXRD suggests that CsBi3I10 sample has a layered structure similar to Cs3Bi2I9 and BiI3, but with different dominating crystal directions. The SEM pictures show that the surface morphology is smoother than for the Cs3Bi2I9 sample, but contains pinholes. Based on the results obtained here, we can therefore conclude that CsBi3I10 is a very promising low-toxic material for solar cells, which also largely can harvest the photons in the visible region of the solar spectrum.

EXPERIMENTAL METHODS Materials Preparation. All chemicals were purchased and used as supplied: BiI3 (SigmaAldrich 99.99%) and CsI (Sigma-Aldrich 99.9%). The solvents were used in a mixture: N, Ndimethylformamide (DMF) / dimethyl sulfoxide (DMSO) is 13/1 by volume. Cs3Bi2I9 was prepared from CsI (1.5 M) and BiI3 (1 M) by using the mixed solvents above. CsBi3I10 was prepared from CsI (1 M) and BiI3 (3 M) by using the same solvent mixture. The precursor solution for the compact TiO2 layer was prepared by mixing titanium (IV) isopropoxide 11 ACS Paragon Plus Environment


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(TTIP) (Sigma-Aldrich 1 mL) and ethanol (9 mL). The TiO2 paste was prepared by diluting Dyesol paste (30 NR-T, particle size is 30 nm) with ethanol in a 2/7 weight ratio. Fabrication of solar cells: Zn powder and 5 M HCl in water were used to etch fluorine tin oxide (FTO) coated glass (Pilkington TEC 15, 15 Ω/ sq). Compact TiO2 layers (thickness ~20 nm) was deposited on the FTO glass by spray pyrolysis method on a hot plate at 500℃ for 30 min. Mesoporous TiO2 films (thickness ~370 nm) were prepared by spin-coating at 4000 rpm for 30 sec. Then the films were annealed onto a hot plate at 500℃ for 30 min. After that, the bismuth perovskite precursor solutions were spin coated onto the substrates at 3000 rpm for 30 sec, followed by heating on a hot plate at 125℃ for 30 min in a dry air box (relative humidity less than 20%). The hole transport material (HTM) P3HT (poly-3-hexyl-thiophene) was dissolved in chlorobenzene (15 mg P3HT, Sigma-Aldrich, in 1 mL chlorobenzene), and this solution was spin-coated on top of the perovskite films at 2000 rpm for 20 sec. Finally, 200 nm thick silver electrodes were evaporated on HTM layer by thermal evaporation at around 10-5 mbar.

Characterization UV-vis spectra: The absorption coefficient α was calculated from transmittance and reflectance spectra employing a Lambda 900 double-beam UV/Vis/NIR spectrophotometer (Perkin-Elmer) equipped with an integrating sphere, and a Spectralon reflectance standard. The absorption coefficient, α, was calculated from the special absorption according to26

α (λ ) =

1  1− R (λ )  ln   d  T ( λ ) 

where d is the thickness of the thin film. To obtain the absorption coefficient for the thin film (αf), the absorption from the glass was subtracted from the total absorption of the sample according to


(1)

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α f d f = α tot dtot − α glass d glass X-ray diffraction (XRD): The structure of the films were determined by grazing incidence x-ray diffraction (GIXRD), using a Siemens D5000 θ–2θ goniometer with CuKα (λ = 1.54051 Å) radiation and 0.4° Soller-slit collimator which had a resolution of 0.3o (2θ) (Bruker AXS, Karlsruhe, Germany). Scans were recorded in the range from 10 to 80° (2θ). Scanning electron microscopy (SEM): The surface morphology and grain sizes of the films were characterized with scanning electron microscopy (SEM) employing a LEO 1550 FEG instrument with in-lens and secondary electron detector operating at 3 kV. Power Conversion Efficiency (PCE): The photovoltaic performance of cells were recorded by using a Keithley 2400 source meter with a scan rate of 50 mV/s under one sun AM 1.5 G (1000 Wm-2) illumination with a solar simulator (Model: 91160) which calibrated with a standard Si solar cell (Fraunhofer ISE). The solar cells were masked during the measurement and the active area was defined as 0.124 cm2. Incident Photo to Current Conversion Efficiency (IPCE): The IPCE spectra were recorded using a Keithley multimeter (Model 2700) as a function of wavelength of the light from 350 to 900 nm. A monochromator (Spectral Products, CM 110) was used to obtain monochromatic light. The setup was calibrated with a standard Si solar cell (Fraunhofer ISE) prior to measurements. All solar cells were illuminated from the working electrode (glass substrate) side with an active area of 0.124 cm2 (circular shaped mask).

ACKNOWLEDGMENT We acknowledge the financial support obtained from the Swedish Energy Agency, Swedish Research Council (VR), Swedish Research Council (FORMAS), ÅForsk and the Göran


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Gustafsson Foundation. Huimin Zhu gratefully acknowledges the China Scholarship Council (CSC) for a PhD study fellowship.

Supporting Information Available: The supporting information includes optical data for the samples, structural models, statistics for several solar cell devices and SEM pictures of the precursor solution.

REFERENCES (1) Habibi, M.; Zabihi, F.; Ahmadian-Yazdi, M. R.; Eslamian, M. Progree in emerging solution-processed thin film solar cells-Part II: Perovskite solar cells. Renew. Sustainable Energy Rev. 2016, 62, 1012. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012, 338, 643-647. (3) Kim, H. S.; Lee, C.; Im, J.; Lee, K.; Moehl, T.; Marchioro, A.; Moon, S.-J.; HumphryBaker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591. (4) Babayigit, A.; Ethirajan, A.; Muller, M.; Conings, B. Toxicity of organometal halide perovskite solar cells. Nat. Mater. 2016, 15, 247-251. (5) Noel, N. K.; Strank, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A. A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M.B.; et al. Lead-free organicinorganic tin halide perovskites for photovoltaic applications. Energy Environ. Sci. 2014, 7, 3061. (6) Krishnamoorthy, T.; Ding, H.; Yan, C.; Leong, W. L.; Baikie, T.; Zhang, Z.; Sherburne, M.; Li, S.; Asta, M.; Mathews, N.; et al. Lead-free germanium iodide perovskite materials for photovoltaic applications. J. Mater. Chem. A 2015, 3, 23829. (7) Park, B.-W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo, G.; Johansson, E. M. J. Bismuth based hybrid perovskites A3Bi2I9 (A:Methylammonium or Cesium) for Solar Cell Application. Adv. Mater. 2015, 27, 6806-6813. (8) Slavney, A. H.; Hu, T.; Lindenberg, A. M.; Karunadasa, H. I. A bismuth-halide double perovskite with long carrier recombination lifetime for photovoltaic applications. J. Am. Chem. Soc. 2016, 138, 2138-2141. (9) Hoye, R. L. Z.; Brandt, R. E.; Osherov, A.; Stecvanovic´, V.; Strank, S. D.; Wilson, M. W. B.; Kim, H.; Akey, A. J.; Perkins, J. D.; Kurchin, R. C.; et al. Methylammonium bismuth iodide as a lead-free, stable hybrid organic-inorganic solar absorber. Chem. Eur. J. 2016, 22. (10) Fornaro, L.; Saucedo, E.; Mussio, L.; Gancharov, A.; Cuña, A. Bismuth tri-iodide polycrystalline films for digital X-ray radiography applications. IEEE Trans. Nucl. Sci. 2004, 51, 96-100. (11) Dmitriev, Y. N.; Bennett, P. R.; Cirignano, L. J.; Klugerman, M. B.; Shah, K. S. Bismuth iodide crystal as a detector material: some optical and electrical properties. Proc. SPIE 1999, 3768, 521-529. 14 ACS Paragon Plus Environment

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(12) Brandt, R. E.; Kurchin, R. C.; Hoye, R. L. Z.; Poindexter, J. R.; Wilson, M. W. B.; Sulekar, S.; Lenahan, F.; Yen, P. X. T.; Stevanovic, V.; Nino, J. C. et al. Investigating of bismuth triiodide (BiI3) for photovoltaic applications. J. Phys. Chem. Lett. 2015, 6, 42974302. (13) Brandt, R. E.; Stevanovic, V.; Ginley, D. S.; Buonassisi, T. Identifying defect-tolerant semiconductors with high minority-carrier lifetimes: beyond hybride lead halide perovskites. MRS commun. 2015, 5, 265-275. (14) Han, H.; Hong, M.; Gokhale, S. S.; Sinnott, S. B.; Jordan, K.; Baciak, J. E.; Nino, J. C. Defect engineering of BiI3 single crystals: Enhanced electrical and radiation performance for room temperature gamma-ray detection. J. Phys. Chem. C 2014, 118, 3244-3250. (15) Matsumoto, M.; Hitomi, K.; Shoji, T.; Hiratate, Y. Bismuth tri-iodide crystal for nuclear radiation detectors. IEEE Trans. Nucl. Sci. 2002, 49, 2517-2520. (16) Lehner, A. J.; Wang, H.-H.; Fabini, D. H.; Liman, C. D.; Hébert, C.-A.; Perry, E. E.; Wang, M.; Bazan, G. C.; Chabinyc, M. L.; Seshadri, R. Electronic structure and photovoltaic application of BiI3. Appl. Phys. Lett. 2015, 107. (17) Boopathi, K. M.; Raman, S.; Mohanraman, R.; Chou, F.-C.; Chen, Y.-Y.; Lee, C.-H.; Chang, F.-C.; Chu, C.-W. Solution-processable bismuth iodide nanosheets as hole transport layers for organic solar cells. Sol. Energy Mater. Sol. Cells 2014, 121, 35-41. (18) Urbach, F. The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids. Phys. Rev. 1953, 92, 1324-1324. (19) Ferlauto, A. S.; Ferreira, G. M.; Pearce, J. M.; Wronski, C. R.; Collins, R. W. Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultraviolet: Applications in thin film photovoltaics. J. Appl. Phys 2002, 92, 2424-2436. (20) Ameri, T.; Dennier, G.; Lungenschmeid, C.; Brabec, C. J. Organic tandem solar cells: A review. Enery Environ. Sci. 2009, 2, 347-363. (21) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photon. 2014, 8, 506-514. (22) Trotter, J.; Zobel, T. The crystal structure of SbI3 and BiI3. Z. Kristallogr. 1966, 123, 67-72. (23) Arakcheeva, A. V.; Bonin, M.; Chapuis, G.; Zaitsev, A. I. The phases of Cs3Bi2I9 between RT and 190 K. Kristallogr. 1999, 214, 279. (24) Momma, K.; Izumi, F. VESTA 3 for three-dimentional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272-1276. (25) Cuña, A.; Aguiar, I.; Gancharov, A.; Pérez, M.; Fornaro, L. Correlation between growth orientation and growth temperature for bismuth tri-iodide films. Cryst.Res.Technol. 2004, 39, 899-905. (26) Hong, W. Q. Extraction of extinction coefficient of weak absorbing thin films from spectral absorption. J. Phys. D:Appl. Phys. 1989, 22, 1384-1385.



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Abstract Image 205x143mm (150 x 150 DPI)

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Figure 1. The natural logarithm of the absorption coefficient for the CsBi3I10, Cs3Bi2I9 and BiI3 samples 230x168mm (150 x 150 DPI)



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Figure 3. SEM images of Cs3Bi2I9 (a and b), CsBi3I10 (c and d) and BiI3 (e and f) under low and high magnification, observe the different scale bars. 128x135mm (150 x 150 DPI)


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Figure 2. GIXRD patterns of (a) Cs3Bi2I9, (b) CsBi3I10 and (c) BiI3. The red lines correspond to the Cs3Bi2I9 reference23 and black lines to the BiI3 reference.22The inset shows the suggested structures in the A-B plane from the given references for Cs3Bi2I9 and BiI3 and a suggestion of CsBi3I10. 251x172mm (150 x 150 DPI)



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Figure 4. (a) A schematic picture of the layers in the CsBi3I10solar cell (b) The SEM cross section of the CsBi3I10 solar cell device (c) J-V curve of the CsBi3I10 solar cell(d) IPCE of the CsBi3I10 solar cell. 242x77mm (150 x 150 DPI)


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Figure 4. (a) A schematic picture of the layers in the CsBi3I10 solar cell. (b) The SEM cross section of the CsBi3I10 solar cell (c) J-V curves of the BiI3, Cs3Bi2I9 and CsBi3I10 solar cell. (d) IPCE of the CsBi3I10, BiI3 and Cs3Bi2I9 device. 272x208mm (300 x 300 DPI)



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Figure 4. (a) A schematic picture of the layers in the CsBi3I10 solar cell. (b) The SEM cross section of the CsBi3I10 solar cell (c) J-V curves of the BiI3, Cs3Bi2I9 and CsBi3I10 solar cell. (d) IPCE of the CsBi3I10, BiI3 and Cs3Bi2I9 device. 272x208mm (300 x 300 DPI)


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Extended Photo-Conversion Spectrum in Low ... - ACS Publications

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