Cesium Bismuth Iodide Solar Cells from Systematic Molar

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Cesium Bismuth Iodide Solar Cells from Systematic Molar Ratio Variation of CsI and BiI3 Malin B. Johansson,*,† Bertrand Philippe,‡ Amitava Banerjee,§ Dibya Phuyal,‡ Soham Mukherjee,‡ Sudip Chakraborty,§ Mathis Cameau,‡ Huimin Zhu,† Rajeev Ahuja,§,∥ Gerrit Boschloo,† Håkan Rensmo,‡ and Erik M. J. Johansson*,†

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†

Ångström Laboratory, Division of Physical Chemistry, Department of Chemistry, Uppsala University, Box 523, SE-75120 Uppsala, Sweden ‡ Division of Molecular and Condensed Matter Physics, Department of Physics and Astronomy, Uppsala University, SE-75120 Uppsala, Sweden § Condensed Matter Theory Group, Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala, Sweden ∥ Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden S Supporting Information *

ABSTRACT: Metal halide compounds with photovoltaic properties prepared from solution have received increased attention for utilization in solar cells. In this work, low-toxicity cesium bismuth iodides are synthesized from solution, and their photovoltaic and optical properties as well as electronic and crystal structures are investigated. The X-ray diffraction patterns reveal that a CsI/BiI3 precursor ratio of 1.5:1 can convert pure rhombohedral BiI3 to pure hexagonal Cs3Bi2I9, but any ratio intermediate of this stoichiometry and pure BiI3 yields a mixture containing the two crystalline phases Cs3Bi2I9 and BiI3, with their relative fraction depending on the CsI/BiI3 ratio. Solar cells from the series of compounds are characterized, showing the highest efficiency for the compounds with a mixture of the two structures. The energies of the valence band edge were estimated using hard and soft X-ray photoelectron spectroscopy for more bulk and surface electronic properties, respectively. On the basis of these measurements, together with UV−vis−near-IR spectrophotometry, measuring the band gap, and Kelvin probe measurements for estimating the work function, an approximate energy diagram has been compiled clarifying the relationship between the positions of the valence and conduction band edges and the Fermi level.

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INTRODUCTION The progress of low-cost and solution-based deposition of perovskite materials for solar cells has been impressive over the past few years and with record power conversion efficiency (PCE) now reaching 23.3%.1−4 Many improvements in material properties like stability and band gap have been realized to increase the open-circuit voltage (Voc) and longterm performance.5,6 Because of their simple fabrication process and high efficiency, perovskite photovoltaic materials have now almost reached the benchmarks of silicon (Si) solar cells and likely will have a significant influence on the future solar cell market. However, because of the toxicity of lead (Pb), a current research trend is to investigate alternative lightharvesting materials with optoelectronic properties similar to those of high-efficiency lead halide based perovskites. Bismuth (Bi)-based halides could be a promising alternative for the replacement of Pb because Bi3+ is a stable and nontoxic cation with an electronic configuration, [Xe]4f145d106s26p0, isoelectronic with that of Pb2+.7−17 The electronic structure consists of vacant 6p orbitals that constitute the conduction band (CB) and a filled 6s orbital mixed with the iodine (I) 5p © XXXX American Chemical Society

orbitals in the valence band (VB). The composition of the antibonding VB character contributes to shallow defect states and long carrier lifetimes in the material, while the high density of states of p orbitals in the conduction band minimum (CBM) can generate strong absorption.18 Previous work has shown that strong light absorption can be obtained for Bi compounds.9,19,20 The stability is also better for CsBi2I95 and (MA)3Bi2I921 compared to the usual Pb perovskites. There are several structural, optical, and electronic characterization reports for BiI3 for photovoltaic applications.22−24 The charge-transport properties and recombination lifetime were shown to have time constants up to 1.3−1.5 ns with recombination lifetimes of 180−240 ps.24 The charge transport and lifetime are closely related to the tolerance of defects and where those defects occur, that is, if they are deep states or shallow and close to the CB or VB.25 In Bi-based double perovskite nanocrystals, it was found that two fast-trapping processes are dominating the charge processes, intrinsic selfReceived: April 26, 2019

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DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

properties, with the aim of finding an efficient and energetically suitable bismuth halide based material for future solar cell devices.

trapping and surface-defect trapping.26 The trap states can be passivated by adding oleic acid in Bi nanocrystals, which increases the photoluminescence quantum efficiency.27 A higher charge mobility in the ab plane has been calculated for the Cs3Bi2I9 and MA3Bi2I9 structures, compared to the direction of the c axis. The effective electronic masses in the ab plane are *me = 1 for Cs3Bi2I9 and *me = 3.66 for MA3Bi2I9.28 In the c-axis direction, the effective electronic mass is *me = 119 for MA3Bi2I9 compared with *me = 3.46 for Cs3Bi2I9.28 This is a large difference despite nearly identical lattice constants of the two compounds. These results lead to interest in the growth direction and fine-tuning of the synthesis process for optimal charge transport.28 The solvent- and solution-based deposition processes are crucial for morphology.29−31 The Lewis basicity of the solvents that are used during deposition can affect the film morphology and surface coverage.29 Previous reports have shown that the mixture of solvents dimethyl sulfoxide and N,N-dimethylformamide (DMF) can affect the growth, shape, and size of the grains from methylammonium bismuth iodide.30 Cs3Bi2I9 crystallized within 5 s, and the rapid nucleation suppressed the grain growth. Also, lower-dimensional (two-dimensional, 2D) systems have merits of their own for use in optoelectronic devices.32 Bi can be combined with other metal ions to obtain doubleperovskite-structured materials.13,33,34 These double perovskite materials show promising photoluminescence lifetimes; for example, the double perovskite structure Cs2AgBiBr6 measured 660 ns.13 The indirect band gap of Cs2AgBiBr6 also affects the lifetime of the charges. Solar cells based on silver bismuth iodide in other crystal structures have shown promising PCEs.8,16,35 The highest PCE of a bismuth halide solar cell is from Ag3BiI6, with 4.3% PCE. The structure is named a rudorffite, after the discoverer of their prototype oxide NaVO2.35 The record is still far away from the high efficiencies obtained for lead halides mentioned earlier. Another notable bismuth halide solar cell reaching 1.22% is based on the AgBi2I7 cubic-phased structure.16 Bismuth halides have a suitable band gap for both tandem devices and a single-junction planar solar cell. We have previously shown by changing the molar ratio of the precursors and increasing the amount of BiI3 that the band gap can be shifted from 2.03 to 1.77 eV.9 In a recent study, we investigated three types of dopant-free polymers as hole conductors, P3HT, P3TI, and TQ1, for bismuth halide solar cells and found that TQ1 could improve the efficiency in the red-wavelength region.10 The VB properties of Cs3Bi2I9 and its related precursor BiI3 were studied carefully using X-ray techniques,12 and it was shown that the primary difference between the two materials was the positions of the I states. To undergo the same development as that for lead halide perovskites, the chemical composition of bismuth halides needs to be refined, the crystallinity and grain growth need to be investigated more, and an energetically suitable hole- and electron-transport material needs to be found. The bismuth halides have a rich structural diversity including distortion, vacancies, and various modes of aggregation equal to that of lead halides.23 In this paper, bismuth halides are synthesized from solution and the photovoltaic, optical, and electronic properties as well as crystal structures are analyzed and compared. Different CsI/ BiI3 molar ratios of the precursor solutions are tested that enable manipulation of both the overall optical and electrical

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EXPERIMENTAL SECTION

Materials. BiI3 (Sigma-Aldrich, 99.99%), CsI (Sigma-Aldrich, 99.9%), and poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]2,5-thiophenediyl] (TQ1; Sigma-Aldrich, 99.995%) were obtained. The hole-transport materials (HTMs) were dissolved in chlorobenzene (Sigma-Aldrich, 99.8%), and the concentration of TQ1 was 10 mg/mL. For precursor solution preparation, the processes and synthesis conditions are the same as those used in our previous work.9 Fabrication of Solar Cells. The procedures of etching of the Fdoped SnO2 (FTO) and formation of the TiO2 blocking and scaffold TiO2 layers have carefully been explained in a previous work.9 The precursor solution was spin-coated on the TiO2 substrate at 3000 rpm for 30 s under the protection of nitrogen. After heating at 125 °C for 30 min in a drybox (the relative humidity is around 20%), perovskite films were formed. Then, HTM solutions were spin-coated on the top of the perovskite films separately under 2000 rpm for 30 s. Last, 80nm-thick gold electrodes were evaporated on the HTM layer by thermal evaporation at ∼10−5 mbar. UV−Vis Spectra. Optical reflectance and transmittance of the samples were measured with a PerkinElmer Lambda 900 double-beam UV−vis−near-IR spectrophotometer equipped with an integrating sphere and a Spectralon reflectance standard. X-ray Diffraction (XRD). The structures of the films were determined by grazing-incidence X-ray diffraction (GIXRD), using a Siemens D5000 θ−2θ goniometer with Cu Kα (λ = 1.54051 Å) radiation and a 0.4° Soller slit collimator, which had a resolution of 2θ = 0.3° (Bruker AXS, Karlsruhe, Germany). Scans were recorded in the range from 2θ = 10 to 80°. Scanning Electron Microscopy (SEM). The surface morphology and grain size of the films were characterized with SEM, employing a LEO 1550 FEG instrument with an in-lens and secondary electron detector operating at 3 kV. Power Conversion Efficiency (PCE). The photovoltaic performances of the cells were recorded by using a Keithley 2400 source meter with a scan rate of 50 mV s−1 under air mass (AM) 1.5 G (1000 W m−2) illumination with a solar simulator (model 91160), which was calibrated with a standard Si solar cell (Fraunhofer ISE), and the power supplier was a Newport Oriel (model 69911). The solar cells were masked during the measurement, and the active area was defined as 0.125 cm2. Incident Photon-to-Current Conversion Efficiency (IPCE). The IPCE spectra were recorded using a Keithley multimeter (model 2700) as a function of the wavelength of 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 0.125 cm2 (circular-shaped mask). Kelvin Probe Measurements. A vibrating Kelvin probe with a probe diameter of 2 mm (KP Technology, UK) was used to determine the surface potential. Contact potential difference measurements were carried out in a N2 atmosphere. For each category, the measurements were obtained from different spots and different samples, and the average value is reported. The work function of the Kelvin probe was calibrated by a freshly cleaned highly ordered gold (Au) surface. The Kelvin method is an indirect technique because electrons are not extracted from the surface; instead, the vibrating tip acts as a counter electrode, and the studied surface becomes one of the plates in a parallel-plate capacitor. The probe setup has a tracking system to avoid the effects of other surfaces that are in capacitive coupling with the tip/sample. Hard X-ray Photoelectron Spectroscopy (HAXPES). HAXPES was carried out at BESSY II (Helmholtz Zentrum Berlin, Germany) at the KMC-1 beamline36 using the HIKE end station,37 provided with a usable photon energy range from 2 to 12 keV. In this work, a photon B

DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Picture of the different CsI/BiI3 samples prepared in this work together with the pure BiI3 precursor. (b) Crystal structures of Cs3Bi2I9 and BiI3. Green balls represent Cs and purple balls Bi, and I atoms are minimized to simplify the structural view. energy of 4000 eV was used by selecting the first-order light from a Si(311) crystal of a double-crystal monochromator (OxfordDanfysik). The pressure in the analysis chamber was ∼10−8 mbar. Overview spectra were measured with a pass energy (Ep) of 500 eV, while 200 eV was used for core-level peaks and VB spectra. The HAXPES spectra presented in this work were energy calibrated versus the Fermi level at zero binding energy, which was determined by measuring a Au plate in the electric contact with the sample and setting the Au 4f7/2 core-level peak to 84.0 eV after curve fitting. The quantification and intensity ratios presented between different core levels were calculated from the experimental results after correcting the intensity by the photoionization cross section for each element at their specific photon energy, using database values.38,39 Soft X-ray Photoelectron Spectroscopy (SOXPES). SOXPES was carried out at Beamline I-411 at a MAX II accelerator for synchrotron radiation in Lund, Sweden.40 A photon energy of 758 eV was selected using a modified Zeiss SX-700 monochromator, and the photoelectron kinetic energies (KEs) were measured using a Scienta R4000 WAL analyzer. The films investigated by SOXPES had a lower surface coverage, enabling detection of the TiO2 substrate as well. The SOXPES spectra were energy calibrated versus TiO2 by setting the Ti 2p core-level peak at 459.45 eV. Computational Methodology. The structural properties of experimentally synthesized BiI3 and Cs3Bi2I9 have been investigated based on a density functional theory formalism, as implemented in Vienna Ab initio Simulation Package (VASP) program,41,42 and the results are correlated with the experimental results of the work function. In order to validate the experimental outcome from the perspective of work function analysis, we have modeled four surface structures, namely, BiI3 (001), BiI3 (110), Cs3Bi2I9 (001), and Cs3Bi2I9 (110). All of the surface structures have been optimized until the Hellman−Feynman forces are smaller than 0.01 eV/Å. For simulating the surfaces of BiI3 (001) and Cs3Bi2I9 (001), we have used a large vacuum of 15 Å in order to consider the surface periodicity throughout all of the calculations. A projector-augmented-wave method has been used to describe the core electron behavior and the interaction between valence electrons and the ion. The valence electrons are described by a plane-wave basis set with an energy cutoff of 500 eV for all four surface systems after the energy convergence was considered with different cutoff energies. The Perdew−Burke− Ernzerhof form of the generalized gradient approximation has been employed as the exchange-correlation functional43 to obtain the optimized configuration and, consequently, the respective optical

absorption cross section. The Brillouin zone has been sampled using 5 × 5 × 1 Monkhorst−Pack-type k points, which has been found to be reasonable for all four surface systems.

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RESULTS AND DISCUSSION Material Characterization. Cesium bismuth iodide samples prepared with various CsI/BiI3 molar precursor ratios of 1.5:1 Cs3Bi2I9, 1:1, 1:2, 1:3, and 1:9 as well as BiI3 were solution-processed using DMF as the solvent. The solution was spin-coated on a scaffold layer of TiO2/blocking TiO2/FTO glass. Figure 1a shows the samples after they have been annealed at 125 °C for 30 min. The color varies from orange for the Cs3Bi2I9 sample (1.5:1 ratio) to totally black for the 1:3 molar ratio and then to gray-black in the BiI3 sample, as illustrated in Figure 1a. The crystal structures of the extremes are Cs3Bi2I9 and BiI3, which are shown in Figure 1b. The zerodimensional (0D) Cs3Bi2I9 consists of Cs+ with isolated facesharing [Bi2I9]3− polyanions, whereas BiI3 has a 2D structure consisting of Bi3+ on the octahedral sites of every other layer of hcp iodide in an edge-sharing honeycomb ordering. When CsI is added to BiI3, the 2D structure splits up to form the 0D structure. Figure 2 shows SEM images of the surface of the samples investigated in this study, and a higher magnification is presented in the inset for each sample. It can be observed that the morphology is very different for the entire set. For the 1.5:1 Cs3Bi2I9 sample (Figure 2a), there are thin flakes (∼50 nm) that stand perpendicular to the TiO2 film surface. The grains formed like flakes create an inhomogeneous and rough surface, and between these flakes, there might occur some pinholes or small open areas with mesoporous TiO2. These flakes are still observed at a 1:1 molar ratio (Figure 2b), but they are considerably thicker than ∼200 nm and are randomly oriented on the surface. The morphology changes completely for the 1:2 sample (Figure 2c), showing grains that have become more like connected spheres. At higher molar concentration of BiI3, the grains become very small with an increase of the overall porosity in the 1:9 sample. Finally, the BiI3 sample has again better observable grains. The mesoporous TiO2 substrate was C

DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX

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does not cover the FTO glass homogeneously, as seen at higher magnification. Figure 3a presents the XRD patterns of the films as a function of different molar ratios of CsI/BiI3. Pure BiI3 crystallizes in a trigonal crystal system (space group R3̅) with lattice parameters a = 7.516(3) Å and c = 20.718(2) Å.44 The structure can be described as a hexagonal-close-packed (hcp) array of I− ions, with Bi3+ occupying 1/3 of the octahedral sites and between adjacent layers of I− ions. A slight addition of CsI (CsI:BiI3 = 1:9) hardly affects the global symmetry of the parent BiI3 crystal. With increased addition of CsI (CsI:BiI3 = 1:3 to 1:1), additional reflection peaks become visible, suggesting growth of a new phase. Complete transformation to this new phase occurs upon further addition of CsI (CsI/ BiI3 ratio of 1.5:1), which could be identified as a 0D structure of Cs3Bi2I9, where the cesium (Cs) and Bi are not electronically well connected, whereas BiI3 has a 2D-layered structure.28 The Cs3Bi2I9 structure has a hexagonal structure45 with space group P63/mmc and lattice parameters a = 8.409 Å and c = 21.243 Å. Locally, the transformation is mediated by the rearrangement of adjacent BiI6 octahedra changing from face-sharing (Cs3Bi2I9) to corner-sharing (BiI3) along the a and b directions, thereby allowing the system to attain a 2D structure. It is important to note here that the systems exhibit strong nonmonotonic changes in the relative intensities of different Bragg peaks, suggesting significant preferred orientations. Here we focus on a few diffraction planes, outlined and numbered as 1−5 in Figure 3a. Peak 1 has a low intensity for Cs3Bi2I9 and 1:1 and then the highest intensity for 1:2 and 1:3 and then decreases again for 1:9 and BiI3. It can be referred to as the (010) plane at 2θ = 12.1° in Cs3Bi2I9 and a combination

Figure 2. SEM images of the surfaces of the samples (a) 1.5:1 Cs3Bi2I9, (b) 1:1, (c) 1:2, (d) 1:3, (e) 1:9, (f) BiI3, (g) mesoporous TiO2, and (h) CsI. A higher magnification is presented as an inset with a scale bar equivalent to 200 nm for parts a−g and 1 μm for part h.

also analyzed, and we can observe particles of approximately 30 nm diameter in Figure 2g. The second precursor, i.e., CsI, was also deposited on FTO glass by spin coating (Figure 2h), and after elimination of the solvent, a fractal pattern is formed that

Figure 3. (a) GIXRD patterns of 1.5:1 Cs3Bi2I9, 1:1, 1:2, 1:3, 1:9, and BiI3 samples. The red lines correspond to ref 45, and the black lines correspond to ref 44. (b) Fraction of the 2D BiI3 phase as a function of the CsI/BiI3 precursor ratio. (c) Absorbance as a function of the wavelength of the same samples in the range 300−1200 nm. D

DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. HAXPES spectra of the Cs 4d, I 4d, and Bi 5d core-level peaks curve-fitted with the different CsI/BiI3 samples and precursors BiI3 and CsI. Spectra were recorded with a photon energy of 4000 eV, and their intensities were normalized to 1 for a clearer representation.

circles) as a function of the changing CsI/BiI3 ratio. Moreover, the Bragg peaks are quite broad and, in some cases, quite asymmetric in nature, rendering proper identification of the background a little ambiguous. The broadening of the Bragg peaks can be ascribed to small crystallite sizes of both phases, as calculated in Figure S4b. While Cs2Bi2I9 forms smaller crystallites compared to BiI3, increasing the Cs content, increases the difference between the crystallite sizes of Cs3Bi2I9 and BiI3, driving the system toward a heterogeneous mixed phase instead of a single phase. This is possibly due to large differences in the ionic size, charge, and bonding tendencies of Cs+ and Bi3+, thereby making solid solubility of the two cations extremely unfavorable. Figure 3c shows the absorbance, A(λ), of the samples calculated from the measured total transmittance, T(λ), and total reflectance, R(λ) [A(λ) = 100 − T(λ) − R(λ)]. The thicknesses of the thin films vary with the grains on the micrometer scale, in particular for the 1.5:1 and 1:1 Cs3Bi2I9 samples, where the grains are standing up vertical from the TiO2 surface (see the SEM images in Figure 2). The band gap of 1.5:1 Cs3Bi2I9 is approximately 2.07 eV (600 nm). For BiI3rich systems (CsI:BiI3 = 1:2, 1:3, and 1:9), the band-gap shifts

of the (011) plane in Cs3Bi2I9 and the (00−3) plane at 2θ = 12.8° in BiI3. This is the preferred orientation for the 1:2 and 1:3 films. Peak 2 at 2θ = 21.1°, the (−120) diffraction plane, exists clearly in the Cs3Bi2I9 film and slightly in the 1:1 sample. The highest-intensity peak of BiI3, labeled as 3 at 2θ = 27°, corresponds to the (113) diffraction plane, which must be the preferred orientation for this film. The (024) diffraction at 2θ = 29.7° (peak 4) is clear in the 1:1, 1:2, and 1:3 samples, coming from the Cs3Bi2I9 phase. Peak 5 at 2θ = 41.6° corresponds to the (030) plane in pure BiI3, and those at 2θ = 41.5° in Cs3Bi2I9 occur in the c−f samples, with the highest in the BiI3 sample. From XRD measurements, we can therefore conclude high orientation from the normal to the (010) plane in the 1:2 and 1:3 films. Also, the samples formed using intermediate precursor ratios (1:1, 1:2, 1:3, and 1:9) contain mixtures of the 0D Cs3Bi2I9 and 2D BiI3 structures. This prompted us to model the measured diffraction patterns for the intermediate precursor ratios (1:1, 1:2, 1:3, and 1:9) in terms of mixtures of the 0D Cs3Bi2I9 and 2D BiI3 structures. The details of the methodology are given in Figure S3. Figure 3b compares the estimated fraction of BiI3 (red up-triangles) in the mixture compared to the nominal fraction of BiI3 (blue E

DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX

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and the 1:2, 1:3, and 1:9 samples at ∼75.6 eV. Bi 5d core-level peaks (d5/2 and d3/2 components separated by 3.05 eV), also shown in Figure 4, are more complex because of the presence of metallic Bi (Bi0 components in gray at 24.4 eV) and the presence of Cs 5s at 24.0 eV, which was also observed in a previous work by Phuyal et al.12 Previous publications measuring in situ photoelectron spectroscopy on perovskite with Pb2+ demonstrated both the presence47 of Pb0 in the samples and reduction48 to Pb0 under X-ray photoelectron spectroscopy (XPS)/ultraviolet photoelectron spectroscopy (UPS) irradiation. We can therefore conclude that, in the samples studied here, Bi0 components are observed for similar reasons as those of the Pb0 observed for Pb-based perovskites. The peaks from the 1:9, 1:3, and 1:2 samples have binding energies similar to that of pure BiI3 (∼25.8 eV). The Bi 5d5/2 peak of the 1.5:1 sample is shifted toward higher energy to ∼26.3 eV. The most interesting sample is 1:1 because two different core-level peaks (26.0 and 26.7 eV) can be detected, suggesting that Bi is found in two different chemical environments in this sample. This can be compared with the absorbance of the 1:1 sample (Figure 3b) showing two bands possibly reflected by the inhomogeneous color of this sample (Figure 1a). Overall, the 1:2, 1:3, 1:9, and BiI3 samples are very similar concerning the XPS, XRD, and UV−vis measurements. A preferred rhombohedral structure is already visible in the 1:2 sample, which then intensifies with the decrease of Cs. Insight into the chemical composition of the samples is presented in Table 1, where the Cs/Bi, I/Bi, and I/Cs ratios are reported. We also report in Figure 5 the relative intensity of

to approximately 1.77 eV (700 nm), which correlates with the previously calculated band-gap values for Cs3Bi2I9 and BiI3.9,29,46 The light absorption is in line with our calculated phase fractions (Figure 3b) of 0D Cs3Bi2I9 and 2D BiI3, which shows significantly high concentrations of BiI3 in the mixture. For the 1:1 sample, the phase fractions of Cs3Bi2I9 and BiI3 are roughly similar, which explains the presence of two absorption shoulders, one at ∼600 nm originating from higher-band-gap Cs3Bi2I9 and one at ∼700 nm originating from the lower-bandgap BiI3. The broad peak at around ∼550 nm seen in Cs3Bi2I9 and the 1:1 molar ratio might be the excitonic contribution, in agreement with the previously calculated rather high effective mass of charges connected to high-binding-energy excitons, which match the expectation of a 0D structure.28 HAXPES measurements were also carried out on the different CsI/BiI3 mixtures as well as the precursors BiI3 and CsI deposited on a TiO2/FTO substrate. The overview spectra of these samples are presented in Figure S3, and all of the main core-level peaks detected are annotated. In Figure S9 is also a presentation of the low-binding-energy range (+85 to −2 eV). The five different CsI/BiI3 mixtures are perfectly covering the substrate because no characteristic peaks from TiO2 can be observed, as is also confirmed by the SEM images shown in Figure 1. Stoichiometry quantification and intensity ratios are calculated for all of these samples by using the Cs 4d, I 4d, and Bi 5d core levels. The curve fitting of these core-level peaks is presented in Figure 4, and the quantifications are summarized in Table 1. HAXPES measurements of these core levels detect Table 1. Cs/Bi, I/Cs, and I/Bi Ratios Calculated from the Experimental Results Presented in Figure 4a sample 1.5:1 1:1 1:2 1:3 1:9 BiI3 CsI

Cs/Bi ratio exptl (intended) 1.5 (1.5) 1.1 (1.0) 0.2 (0.5) 0.4 (0.33) 0.07 (0.11)

I/Bi ratio exptl (intended) 3.6 3.2 2.8 3.1 2.7 2.7

(4.5) (4.0) (3.5) (3.33) (3.11) (3.0)

I/Cs ratio exptl (intended) 2.3 (3.0) 2.9 (4.0) 17.5 (7.0) 7.2 (10.0) 36.0 (28.0) 0.8 (1.0)

a

The theoretical values given in these tables are based on the amounts of CsI and BiI3 initially introduced as precursors in the solution.

very similar KEs and, in turn, similar probe depths, and these core levels have been selected rather than the classical Cs 3d, I 3d, and Bi 4f. In addition, the Bi 4f7/2 core level (∼159.5− 159.0 eV) is strongly overlapping with that of Cs 4p (∼160 eV), as observed in the overview spectra of the CsI and BiI3 in Figure S3. Cs 4d, I 4d, and Bi 5d are presented in Figure 4. The Cs 4d and I 4d (d5/2 and d3/2 components separated by 2.8 and 1.71 eV) peaks can all be fitted using a single peak doublet, and the main differences are their binding energy positions. Regarding I 4d, two vertical lines were placed to show the I 4d5/2 positions for the CsI and BiI3 precursors at 49.7 and 49.2 eV, respectively. We can see that the samples rich in BiI3 (1:9, 1:3, and 1:2) have a similar binding energies compared to pure BiI3 (i.e., ∼49.2 eV) and a chemical shift is observed for the 1:1 and 1.5:1 samples to ∼49.3 and ∼49.6 eV, respectively, toward pure CsI. A similar behavior can be observed on the Cs 4d spectra with a Cs 4d5/2 core level of pure CsI at ∼76.3 eV. The 1.5:1 and 1:1 samples have their peaks at ∼76.1 and ∼75.8 eV

Figure 5. Relative ratio (%) of metallic Bi (Bi0) detected with a photon energy of 2100 and 4000 eV for the various stoichiometries investigated.

metallic Bi detected by using two different energies, 2100 and 4000 eV. All of the samples containing Bi have traces of Bi0 (1−5%), and we can observe that more Bi0 is detected at 2100 eV compared to 4000 eV, suggesting that metallic Bi is mainly located toward the surfaces of the samples in all cases. Note that the amount of metal was observed to slightly increase during long X-ray exposure. As a consequence, the presented spectra were recorded on a fresh spot, and thus metallic Bi either was related to the Bi0 already present in the BiI3 precursors or was formed during the annealing step. The HAXPES spectra and quantification suggested that the 1:2, 1:3, and 1:9 samples are relatively close to the values expected for the BiI3 sample and the addition of CsI is acting more as a doping while the 1.5:1 and 1:1 samples present more F

DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX

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The sample with the highest current density is 1:3, reaching 3.18 mA cm−2. The sample with the lowest voltage was BiI3 with 0.35 V. The samples therefore show a stepwise decrease in voltage with increasing molar ratio of BiI3 and a stepwise increase in the photocurrent until a maximum at the molar ratio of 1:3 (Table 2). Voc for the solar cells is a function of the

peculiar composition, surface morphology, and crystal and electronic structures, as observed on SEM, XRD, and absorption data. Device Performance. The different solutions with BiI3 and CsI were spin-coated on a FTO/TiO2 blocking/TiO2 meso substrate; a layer of TQ1 was deposited on the bismuth halides as a HTM, and Au was used as the counter electrode. The current density−voltage (J−V) curves of the different devices are presented in Figure 6, and the measurements were

Table 2. Performance Parameters of Different Fabricated Solar-Cell Devicesa sample

PCE (%)

Voc (V)

Jsc (mA cm−2)

FF (%)

1.5:1f 1.5:1b 1:1f 1:1b 1:2f 1:2b 1:3f 1:3b 1:9f 1:9b BiI3f BiI3b

0.07 0.07 0.62 0.57 0.50 0.49 0.47 0.44 0.49 0.48 0.36 0.34

0.70 0.68 0.57 0.55 0.43 0.44 0.37 0.37 0.43 0.43 0.33 0.35

0.29 0.29 2.22 2.19 2.79 2.73 3.18 3.17 2.72 2.70 2.76 2.71

34 33 49 47 42 41 40 38 42 42 39 37

a

All of the reported samples were prepared under the same conditions. The “f” and “b” stand for the forward and backward scans, respectively.

band gap, quasi-Fermi-level splitting, doping, and rate of recombination. It is concluded from the SEM image (Figure 2) that the surface coverage varies with crystal formation and the 1.5:1 sample can have higher probability for recombination when TiO2 is in direct contact with the HTM, which then lowers the efficiency. The defect density with deeper trap states in Bi materials may also be a source for the rather low performance compared to the Pb-based perovskites.25 The main results extracted from these J−V curves are summarized in Table 2. The IPCE was measured for the series of samples seen in Figure 7a. The first shoulder starting from 700 nm correlates

Figure 6. J−V measurements for the samples with the molar concentrations (a) 1.5:1 Cs3Bi2I9, (b) 1:1, (c) 1:2, (d) 1:3, (e) 1:9, and (f) BiI3. The solid line is the forward scan and the dashed line corresponds to the backward scan, which are respectively labeled as f and b.

performed under simulated AM 1.5 irradiance of 100 mW cm−2. A circular-shaped mask was used (0.125 cm2) to confine the illuminated active area and to avoid edge effects. Sample 1.5:1 has comparably high voltage, 0.7 V, with a low current, 0.29 mA cm−2. The relatively low current, compared to our previously published results for this material, might be due to less efficient band matching of the VB position, approximately −6.9 eV, and the highest occupied molecular orbital (HOMO) level of the HTM TQ1, −5.7 eV.10 Another reason for the low current may be the specific interaction between the HTM and surface of this material. The choice of the HTM was based on previous work, where it was found that TQ1 improved the photon-to-current conversion in the red-wavelength region for the BiI3-rich materials.10 Furthermore, the HOMO level of TQ1 can work for all of the samples even if the energy difference is smaller than the VB in some cases, discussed further below, at the energy-level mapping. In sample 1:1, the current increased to 2.22 mA cm−2, whereas the voltage decreased to 0.57 V. As seen in Figure 3b, there is a decrease of the band gap from 2.07 to 1.77 eV between the 1.5:1 and 1:1 samples, which will increase light absorption in the film and also the photocurrent. The 1:1 sample has the highest efficiency of the different materials, 0.62%, and as discussed above, the film is most probably a mixture of Cs3Bi2I9 and BiI3.

Figure 7. (a) IPCE and (b) charge-carrier lifetime measurements of the different 1.5:1 Cs3Bi2I9, 1:1, 1:2, 1:3, 1:9, and BiI3 samples.

well with the result from the absorbance of sample 1:1−9 and BiI3 seen in Figure 3b. The second increase of IPCE starts at 500 nm and is specifically significant for the 1.5:1 sample. The other samples have structures similar to that of 1.5:1, with a systematic increase in 300−500 nm, which seems to be connected to how much hexagonal structure occurs mixed with the rhombohedral structure. The carrier lifetime, τe, was estimated from the Voc transient decay using different light intensities, from 1 to 0.1 sun (Figure 7b). The 1.5:1 sample has higher Voc and electron lifetime, τe = G

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Figure 8. Zoom-in of the upper VB (+4.5 to −0.5 eV energy range) of the different CsI/BiI3 mixtures, the precursors, and the TiO2 substrate measured by HAXPES with a photon energy of 4000 eV. The VBM was determined using a logarithmic intensity scale by linear extrapolation.

Figure 9. VB spectra of the CsI/BiI3 mixtures, the precursors, and the TiO2 substrate measured by SOXPES with a photon energy of 758 eV. The VBM was determined using a logarithmic intensity scale by linear extrapolation.

3.8 × 10−3 s, compared to the rest of the samples, τe = ∼0.5 × 10−3 s, which explains the higher photovoltage for this device. However, still 1.5:1 Cs3Bi2I9 is the least functional solar cell because of its low photocurrent. When the photoelectrical measurements for the devices are summarized, it can be concluded that the samples with a mixture of Cs3Bi2I9 and BiI3 show the best solar cell performances. For Cs3Bi2I9-based solar cells, the photocurrent is low, and it has been suggested that the rather high carrier effective masses along with large indirect band gap will lower the efficiency of Cs3Bi2I9.49 However, the low photocurrent may also be related to charge transfer at the interfaces with TiO2 and the HTM because a higher photocurrent was previously observed for this material using a different HTM. The different morphology might also affect the overall

efficiency of the solar cell. Small grains create more interfaces for the charge carriers to pass through to reach the electron/ hole conducting materials, which affect the charge-carrier lifetime. Between the small vertical flakes in the Cs3Bi2I9 sample, the mesoporous TiO2 is in direct contact with the HTM and gives rise to recombination centra, which decrease the photocurrent. In the solar cells based on the mixed compounds, the photovoltage is higher for solar cells based on more BiI3 like the 1:3 and 1:9 samples but lower for solar cells based on only Cs3Bi2I9, and the photocurrent is still as high as that for BiI3-based solar cell. The increase in the photovoltage compared to that of BiI3 may be explained by the longer carrier lifetime for the mixed compounds, and this therefore results in an overall better photovoltaic performance. However, more work on the morphology, crystal structure, and interfaces has H

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Table 3. Summary of the Band Gap (Eg), VB Edge Position (VB Edge), and Work Function (WF) Experimentally Determined via UV−Vis, Photoelectron Spectroscopy, and Kelvin Probe Measurements aample

Eg(eV) ± 0.1

VB edge (eV) vs Efa ± 0.1 log bulk

VB edge (eV) vs Ef ± 0.1 log surface

WF (eV) ± 0.1

VB edge (eV) vs Evacb ± 0.1 log bulk

VB edge (eV) vs Evac ± 0.1 log surface

1.5:1 1:1 1:2 1:3 1:9 BiI3 TiO2

2.01 1.78/1.90 1.77 1.77 1.79 1.80 3.2

1.15 0.85 0.65 0.85 0.65 0.85 2.85

2.15 1.92 1.65 1.80 0.75 1.18 3.37

−4.75 −4.60 to −5.23 −5.19 −5.00 −5.25 −5.04 −4.6

−5.9 −5.45 to −6.08 −5.84 −5.85 −5.9 −5.89 −7.45

−6.9 −6.52 to −7.15 −6.84 −6.8 −6.0 −6.22 −7.97

a

Ef = Fermi energy, bEvac = work function energy

Figure 10. Work function measurements over a surface area of 650 × 650 μm of the (a) 1.5:1 Cs3Bi2I9 and (b) BiI3 samples. (c−f) Theoretical calculations of the work function of the four surface planes of (c) Cs3Bi2I9 (001), (d) BiI3 (001), (e) Cs3Bi2I9 (110), and (f) BiI3 (110).

to be done to draw conclusions on the reasons for the different charge-carrier lifetimes. Larger crystal grains seen in sample 1:1

(Figure 2b) also reduce the problems with conduction of charges over grain boundaries, and therefore the transport of I

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Figure 11. Energy diagram based on the results from Table 3 showing the energy levels of the absorbing bulk material, the interface, and the TiO2 material.

charges within that film may be enhanced and recombination decreased. The crystallite size might also affect the charge mobility. Interestingly, the mixing of two crystal structures may therefore be advantageous for solar cell performance. This is in a way similar to the bulk heterojunction organic solar cells, where two organic materials are mixed to form efficient devices.50 However, proving such bulk heterojunction effects would require extensive investigations, which is out of the scope of this paper. Energy-Level Mapping. The VBs of the different samples and precursors were investigated by SOXPES and HAXPES using photon energies of 758 and 4000 eV, respectively. As discussed above, it was not possible to detect any signals from the TiO2 substrate in the HAXPES measurements, and the resulting VBs are thus representative from the bulk overlayer on top of the scaffold oxide. Thinner films were used for the SOXPES measurements where the TiO2 scaffold substrate was detected, giving experimental insight into the energetics of the interface between the light-absorbing compounds and TiO2. The variations in valence band maximum (VBM) versus the Fermi level were estimated using a procedure based on extrapolation on a logarithmic intensity scale to the background signal, as shown in the HAXPES and SOXPES VB spectra presented in Figures 8 and 9. The VBM values obtained are presented in Table 3. The VBM measured using the more conventional approach with linear intensity scale is presented in Figures S4 and S5. For similar materials, the use of a linear scale has led to an overestimation of the VB edge, while the use of a logarithmic scale emphasizes the tail of the VB spectrum. A Kelvin probe setup was used to estimate the work function level for the different busmuth halide compounds on top of the TiO2 substrate. The work function value is a sensitive indicator of the bulk and surface properties, where the former include information of doping, while the latter include surface dipole

effects linked to the surface structure, stoichiometric imperfections, and absorbed compounds on the surface.51,52 BiI3 has a random arrangement of grains with a polycrystalline structure, and this is also the case for all of the samples except Cs3Bi2I9 and 1:1. where the SEM pictures suggest that the ab plane of the crystals is standing vertically up from the surface (see also Figures 2 and 3). A surface scan (650 × 650 μm) gave the work function values of the different surface planes. Figure 10 shows data of the work function of 1.5:1 and BiI3 over a 650 × 650 μm surface and a comparison to theoretical calculations. The experimental fluctuation over the surface is 0.025 eV for 1.5:1 and 0.012 eV for BiI3. The theoretical calculations of the work function of the four surface planes in Figure 10c−f reveal that the work functions are rather different for different surface planes (see the Supporting Information for a further detailed discussion). The averages of the work function values for the different materials are summarized in Table 3, together with the VB edges for surface and bulk measurements as well as Eg values measured from UV−vis spectroscopy. The calculated energy levels of BiI3 correlate rather well with the previously calculated values by Lehner et al.23 The calculated energy levels for Cs3Bi2I9 and TiO2 were slightly higher in the work by Lehner et al.; on the other hand, the experimental results correlate well with the values determined in this report.23 The HOMO energy level for the HTM TQ1 is −5.7 eV, which can be compared with the last two columns.10 The energy level of the VB of the surface, seen in the last column, is not that close to the TQ1 value of −5.7 eV. However, all of the energy levels of the VB are lower than the HOMO level, which makes hole transport from the light-absorbing layer to the HTM possible. With the combined results from UV−vis (band gap), Kelvin probe (work function), and XPS (VB edge), we can estimate the energy-level structure, and from the results presented above, a tentative energy diagram for the complete interface is J

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samples, inducing a more p-doped material. The photovoltaic performances of the devices made with these different samples were also characterized. The results show that the samples based on mixtures of the two crystal structures have the highest photovoltaic performances. Measurements of the electron lifetime show that the samples with a mixture of the two crystal structures have electron lifetimes between the lifetime for the 1.5:1 molar ratio Cs3Bi2I9 and BiI3 samples, which explains the intermediate voltage of the samples with a mixture of the two crystal structures. Therefore, we conclude that a mixture of two crystal structures can result in an improved photovoltaic performance compared to the pure crystal phases. The results therefore suggest that a system with a mixture of two structures may be interesting for further studies for future solar cell devices.

drawn in Figure 11. In the diagram, the Fermi level has been set to zero energy. All samples show the same trend, indicating a band bending to higher binding energies for the absorber energy levels (measured using HAXPES) at the absorber/ substrate interface (measured using SOXPES). The trend also indicates a decrease in band bending at the highest concentration of BiI3, with the smallest difference between the bulk and interface in the 1:9 sample. The Fermi levels of the thick 1:2 and 1:9 molar ratio samples are shifted closer to the VB, 0.24 and 0.25 eV, respectively, from the intrinsic value. Also for the thick 1:1, 1:3, and BiI3 samples, the Fermi levels are shifted slightly against the VB, ∼0.04 eV. The only sample that has the Fermi level closer to the CB than the VB is the thick 1.5:1 molar ratio sample. This sample has a Fermi level shifted slightly toward the CB. The energy levels for the bismuth halides on the TiO2 layer are therefore generally in accordance with a classical p−n junction (or an i−n junction for Cs3Bi2I9), where the bismuth halide is, in general, p-type and the TiO2 layer is n-type. When the different samples are compared, it can also be noted that the VB edge is rather similar for the thick samples, except for the 1.5:1 sample. This is in agreement with the results from the XRD and UV−vis measurements above, suggesting a mixture of the 0D Cs3Bi2I9 and 2D BiI3 structures for the samples with ratios 1:1, 1:2, 1:3, 1:9, and BiI3 because a similar VB edge (corresponding to BiI3) can be expected for these samples. Although in this investigation we focus on the bismuth halide/TiO2 junction, the bismuth halide/HTM system must then in the future also be considered for optimal solar cell performance. The detailed interaction between the bismuth halides and HTM is of large importance for the charge transfer and collection of photogenerated charges, which we have shown in a previous work.10

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01233.

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Optical transmittance and reflectance, structure description, tables of energy levels, overview XPS spectra, and XPS zoom-in of the upper VB (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.B.J.). *E-mail: [email protected] (E.M.J.J.). ORCID

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Malin B. Johansson: 0000-0003-2046-1229 Bertrand Philippe: 0000-0003-2412-8503 Amitava Banerjee: 0000-0002-3548-133X Dibya Phuyal: 0000-0003-0351-3138 Sudip Chakraborty: 0000-0002-6765-2084 Rajeev Ahuja: 0000-0003-1231-9994 Gerrit Boschloo: 0000-0002-8249-1469 Erik M. J. Johansson: 0000-0001-9358-8277

CONCLUSION In this work, cesium bismuth iodide solar cells are investigated. Different ratios of the precursors CsI and BiI3 were used to prepare a number of samples, and the samples were characterized by a number of techniques. The surface morphology of the bismuth halide compounds was characterized with SEM, showing great distribution of grain formation for the different compounds, from flakes standing up from the TiO2 surface for the 1.5:1 molar ratio Cs3Bi2I9 to smaller cubic grains in BiI3. The bismuth halide compounds were also analyzed optically with UV−vis spectrophotometry to estimate the band-gap values, from 2.07 eV for the 1.5:1 Cs3Bi2I9 sample to 1.77 eV for the rest of the samples, as well as transmittance and reflectance characteristics. The structures were analyzed with XRD, and a hexagonal 0D structure was observed for the 1.5:1 molar ratio Cs3Bi2I9 sample and a rhombohedral 2D structure for the BiI3 sample. In the SEM, XRD, and UV−vis results for the samples with molar ratios between Cs3Bi2I9 and BiI3, a mixture of the two (0D Cs3Bi2I9 and 2D BiI3) crystal structures is observed. On the basis of the UV−vis, HAXPES, and Kelvin probe measurements, an approximate picture of the energy-level diagrams for the different solar cells was also suggested. All samples showed band bending, which decreased with an increase of BiI3, showing the smallest difference between the bulk and interface in the 1:9 sample. The Fermi levels of the thick 1:2 and 1:9 molar ratio samples are shifted closer to the VB, 0.24 and 0.25 eV, respectively, from the intrinsic value, which can be connected to the high amount of metallic Bi for these two

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support obtained from the Swedish Energy Agency, Swedish Research Council, Swedish Research Council (FORMAS), Swedish Foundation for Strategic Research, and Carl Tryggers Stiftelse for Vetenskaplig Forskning. H.Z. gratefully acknowledges the China Scholarship Council for a Ph.D. study fellowship. A.B. is thankful to Erasmus Mundus for a doctoral fellowship. HZB is acknowledged for the allocation of synchrotron radiation beamtime. SNIC and HPC2N are also acknowledged for providing computing time.

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REFERENCES

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K

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DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01233 Inorg. Chem. XXXX, XXX, XXX−XXX