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Summary

Herein, we present detailed protocols for solution-processed silver-bismuth-iodine (Ag-Bi-I) ternary semiconductor thin films fabricated on TiO₂-coated transparent electrodes and their potential application as air-stable and lead-free optoelectronic devices.

Abstract

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. However, poor surface morphologies and relatively high bandgap energies have limited their potential. Silver-bismuth-iodine (Ag-Bi-I) is a promising semiconductor for optoelectronic devices. Therefore, we demonstrate the fabrication of Ag-Bi-I ternary thin films using material solution processing. The resulting thin films exhibit controlled surface morphologies and optical bandgaps according to their thermal annealing temperatures. In addition, it has been reported that Ag-Bi-I ternary systems crystallize to AgBi₂I₇, Ag₂BiI₅, etc. according to the ratio of the precursor chemicals. The solution-processed AgBi₂I₇ thin films exhibit a cubic-phase crystal structure, dense, pinhole-free surface morphologies with grains ranging in size from 200 to 800 nm, and an indirect bandgap of 1.87 eV. The resultant AgBi₂I₇ thin films show good air stability and energy band diagrams, as well as surface morphologies and optical bandgaps suitable for lead-free and air-stable single-junction solar cells. Very recently, a solar cell with 4.3% power conversion efficiency was obtained by optimizing the Ag-Bi-I crystal compositions and solar cell device architectures.

Introduction

Solution-processed inorganic thin-film solar cells have been widely studied by many researchers seeking to convert sunlight directly into electricity^1,2,3,4,5. With the development of material synthesis and device architecture, lead halide-based perovskites have been reported to be the best solar cell absorbers with a power conversion efficiency (PCE) greater than 22%⁵. However, there are growing concerns about the use of toxic lead, as well as stability issues of lead-halide perovskite itself.

It has recently been reported that bismuth-based hybrid perovskites can be formed by incorporating monovalent cations into a bismuth iodide complex unit and that these can be used as photovoltaic absorbers in mesoscopic solar cell architectures^6,7,8. The lead in the perovskites can be replaced with bismuth, which has the 6s² outer lone pair; however, so far only conventional lead halide methodologies have been used for bismuth-based hybrid perovskites with complex crystal structures, despite the fact that they have different oxidation states and chemical properties⁹. In addition, these perovskites have poor surface morphologies and produce relatively thick films in the context of thin-film device applications; therefore, they have a poor photovoltaic performance with high band-gap energy (> 2 eV)^6,7,8. Thus, we sought to find a new method to produce bismuth-based thin-film semiconductors, which are environmentally friendly, air-stable, and have low band-gap energy (< 2 eV), considering the material design and methodology.

We present solution-processed Ag-Bi-I ternary thin films, which can be crystallized to AgBi₂I₇ and Ag₂BiI₅, for lead-free and air-stable semiconductors^10,11. In this study for the AgBi₂I₇ composition, n-butylamine is used as a solvent to simultaneously dissolve the silver iodide (AgI) and bismuth iodide (BiI₃) precursors. The mixture is spin-cast and annealed at 150 °C for 30 min in an N₂-filled glove box; subsequently, the films are quenched to room temperature. The resultant thin films are brown-black in color. In addition, the surface morphology and crystal composition of the Ag-Bi-I ternary systems are controlled by the annealing temperatures and precursor ratio of AgI/BiI₃. The resulting AgBi₂I₇ thin films exhibit a cubic phase crystalline structure, dense and smooth surface morphologies with large grains of 200 – 800 nm in size, and an optical band gap of 1.87 eV starting to absorb light from a wavelength of 740 nm. It has recently been reported that by optimizing the crystal compositions and device architecture, Ag-Bi-I ternary thin-film solar cells can achieve a PCE of 4.3%.

Protocol

1. Preparation of Bare-glass, Fluorine-doped Tin Oxide (SnO₂:F) Substrates

1. To clean the bare-glass, fluorine-doped tin oxide (FTO) substrates, sonicate them sequentially in an aqueous solution containing 2% Triton, deionized (DI) water, acetone, and isopropyl alcohol (IPA), each for 15 min.
2. Put the cleaned substrates in the heating oven at 70 °C for 1 h to remove the residual IPA.

2. Preparation of Compact TiO₂ Layers (c-TiO₂) to Block the Electrons

1. For the preparation of a c-TiO₂ precursor solution, drop 0.74 mL of titanium isopropoxide (TTIP) slowly into 8 mL of anhydrous ethanol (EtOH) while stirring vigorously, and then rapidly inject 0.06 mL of hydrochloric acid (HCl) into the solution. Stir the resulting solution overnight at room temperature.
   NOTE: Use a 20 mL glass vial, a 35 – 37% concentration of HCl, and a magnetic stirrer.
2. Filter the prepared c-TiO₂ precursor solution using a syringe and a 0.2 µm-pore-size filter, drop it onto the cleaned FTO substrate, and then spin-cast the substrate at 3,000 rpm for 30 s.
3. Thermally-anneal the substrates by heating them in an oven at 500 °C for 1 h and then allow them to cool to room temperature.
4. Soak the substrates in a 0.12 M titanium tetrachloride (TiCl₄) aqueous solution at 70 °C for 30 min and then wash them thoroughly using DI water to remove any residual TiCl₄.
5. Thermally-anneal the substrates at 500 °C for 1 h and then allow them to cool to room temperature for an interfacial improvement of the c-TiO₂ layer. Store the resulting c-TiO₂-coated substrates in N₂-filled conditions until use.

3. Preparation of Mesoporous TiO₂ Layers (m-TiO₂) to Improve Electron Extraction

1. For the preparation of an m-TiO₂ precursor solution, add 1 g of 50 nm-sized TiO₂ nanoparticle paste (SC-HT040) to a 10 mL glass vial with 3.5 g of 2-propanol and 1 g of terpineol and then stir everything until the paste has perfectly dissolved.
   NOTE: The 50 nm-sized TiO₂ nanoparticle paste is highly viscous and must be carefully handled using a spatula.
2. Spin-cast 200 µL of the prepared 50 nm-sized TiO₂ nanoparticle paste solution at 5,000 rpm for 30 s onto the c-TiO₂-coated FTO substrates.
3. Thermally-anneal the resulting substrates in an oven at 500 °C for 1 h and then allow them to cool to room temperature.
4. Soak the substrates in the 0.12 M TiCl₄ aqueous solution at 70 °C for 30 min and then wash them completely using DI water to remove any residual TiCl₄.
5. Thermally-anneal the substrates at 500 °C for 1 h and then allow them to cool to room temperature for an interfacial improvement of the m-TiO₂ layer. Store the resulting c-TiO₂– and m-TiO₂-coated substrates in N₂-filled conditions until used.

4. Fabrication of AgBi₂I₇ Thin Films

1. Treat the bare glass substrates under an ultraviolet (UV) lamp with an intensity of 45 mA/cm² with a UV ozone cleaner for 10 min to ensure that the substrates are clean and hydrophilic. Do not treat the c- and m-TiO₂-coated FTO substrates with the UV ozone cleaner.
   NOTE: X-ray diffraction (XRD), absorbance, and Fourier-transform infrared (FT-IR) spectra were investigated using Ag-Bi-I thin films fabricated on bare glass substrates. The c- and m-TiO₂-coated FTO substrates were used for solar cell devices.
2. Vigorously vortex 0.3 g of BiI₃ (0.5087 mmol), 0.06 g of AgI (0.2544 mmol), and 3 mL of n-butylamine until everything is completely dissolved and then syringe-filter the mixture using a 0.2 µm-pore-size polytetrafluoroethylene (PTFE) filter.
3. Drop 200 µL of the precursor solution onto the substrates and then spin-cast them at 6,000 rpm for 30 s with a controlled humidity below 20%. Immediately transfer the resultant yellowish-red film to an N₂-filled glove box ready for thermal annealing.
4. Begin the thermal annealing of the resulting film at room temperature, then heat the film to 150 °C, and maintain a temperature of 150 °C for 30 min. Quickly quench the annealed film to room temperature. The final film will have a shiny and brown-black color. To quickly quench the annealed substrate, remove it from the hot plate which was set to 150 °C.
5. For Ag-Bi-I ternary thin films of a different composition, such as Ag₂BiI₅, change the precursor molar ratio of AgI to BiI₃ from 1:2 to 2:1 and use the same volume of the n-butylamine solvent. Anneal the resulting film using the method described above.
6. To investigate the temperature-dependent Ag-Bi-I formation using XRD patterns, FT-IR spectra, surface morphologies, and absorbance spectra, use thermal annealing temperatures of 90, 110, and 150 °C for the Ag-Bi-I ternary thin films.

5. Fabrication of Solar Cell Devises Using AgBi₂I₇ Thin Films

1. Use poly(3-hexylthiophene) (P3HT) as a hole-transporting material in the AgBi₂I₇ thin-film solar cells. Add 10 mg of P3HT to 1 mL of chlorobenzene and then stir the mixture at 50 °C for 30 min until the P3HT has perfectly dissolved. Filter using a 0.2 µm-pore-size PTFE filter. Prepare and store the P3HT in an N₂-filled glove box.
2. Drop 100 µL of the P3HT dissolved in chlorobenzene onto the AgBi₂I₇ thin films fabricated on the c- and m-TiO₂-coated FTO substrates, and then spin-cast the substrates at 4,000 rpm for 30 s in an N₂-filled glove box. Thermally-anneal the P3HT film at 130 °C for 10 min for the structural orientation of P3HT.
3. Use a thermal evaporator with a deposition rate of 0.5 Å/s and a bar pattern shadow mask to deposit 100 nm-thick gold (Au) electrodes as a top metal contact in the AgBi₂I₇ thin-film solar cells.

Representative Results

It has been reported that the Ag-Bi-I ternary systems, which are regarded as promising semiconductors, are crystallized in various compositions, such as AgBi₂I₇, AgBiI₄, and Ag₂BiI₅¹⁰, according to the molar ratio of AgI to BiI₃. Earlier studies have shown that bulk crystal forms with various compositions of Ag-Bi-I ternary systems can be experimentally synthesized by changing the molar ratio of AgI and BiI₃ and that each composition has a different XRD pattern¹⁰.

Unlike bulk crystals, we sought to develop solution-processed Ag-Bi-I ternary thin films, which can be used directly as an active layer in optoelectronic devices. In this study, n-butylamine was used as a solvent to simultaneously dissolve AgI and BiI₃ and then prepared each Ag-Bi-I thin film with a different molar ratio of AgI to BiI₃ (1:2, 1:1, and 2:1). First, we carried out XRD measurements on each film (Figure 1). The XRD patterns for the Ag-Bi-I thin film prepared with a molar ratio of 1:2 (AgI:BiI₃) showed a single peak at 2θ ~ 42°; this indicates that AgBi₂I₇ has a crystalline composition with a cubic structure (space group Fd3m, a = b = c = 12.223 Å). However, peak splitting was apparent in the region of 2θ ~ 42° when the molar ratio of AgI:BiI₃ exceeded 1:1, and the film with a molar ratio of 2:1 showed that Ag₂BiI₅ has a hexagonal structure (space group R3m, a = b = 4.350 Å, c = 20.820 Å)^10,12.

We also measured the UV-Vis absorption of the AgBi₂I₇ and Ag₂BiI₅ thin films which were prepared on the glass substrates (Figure 2a). Once the absorption spectra were normalized, the AgBi₂I₇ thin film absorbed longer wavelengths, up to ~740 nm, than the Ag₂BiI₅ thin film. Figure 2 displays the top-view scanning electron microscopy (SEM) images of each film. The surface morphology of the AgBi₂I₇ thin film can be seen clearly, with large grains and a dark brown color (Figure 2b). However, the Ag₂BiI₅ thin film shows light particles on the grains, which result from the excess AgI^13,14,15,16, and a light brown color (Figure 2c). We, therefore, chose to use the AgBi₂I₇ composition for further study, as it is more suitable for thin film-based optoelectronics in terms of light absorption and surface morphologies than the Ag₂BiI₅ composition.

Figure 3a shows that the experimental XRD pattern of the solution-processed AgBi₂I₇ thin film is consistent with the reported and calculated XRD patterns of AgBi₂I₇ crystals without the formation of secondary phases. As mentioned previously, we confirmed that the AgBi₂I₇ thin film has a cubic structure (space group Fd3m, a = b = c = 12.223 Å). Additionally, the AgBi₂I₇ film is highly humidity- and air-stable with no structural changes when stored in air for 10 d; this is understandable since AgI is highly stable in an aqueous medium (Figure 3b)^13,14,15,16.

Figure 4a shows a series of XRD patterns for Ag-Bi-I thin films as a function of the annealing temperature in N₂-filled conditions. We confirmed that Ag-Bi-I begins to crystallize above 90 °C in the form of the cubic phase, as shown by the (111), (400), and (440) peaks at 13°, 29°, and 42°, respectively (i.e., those corresponding to the asterisks in Figure 4a). The XRD peaks in the small angle regions (2θ < 10°) significantly reduced as the temperature increased, and finally disappeared at 150 °C with the gradual increase of cubic phase diffractions; this indicates that the AgBi₂I₇ film was fully crystallized in the cubic phase¹⁷. The FTIR spectra were measured in order to investigate the formation of the Ag-Bi-I systems in detail (Figure 4b). The as-prepared and non-annealed film showed the FTIR signals for N-H stretching (3200 – 3600 cm^-1), C-H stretching (2850 – 2980 cm^-1), and N-H bending (1450 – 1650 cm^-1) which resulted from the n-butylamine¹⁸. Although the as-prepared film was annealed at 90 °C, above the boiling point of n-butylamine (77 – 79 °C), the FTIR spectra still showed the associated peaks, although they were significantly decreased. This indicates that the remaining n-butylamine was weakly bound to the BiI₃ and AgI in the form of a metal halide-amine complex, suppressing the formation of the Ag-Bi-I building blocks by edge-, vertex-, or face-sharing¹⁹. These FTIR signals disappeared as the temperature increased further; this is explained by the removal of the n-butylamine which was bound to the BiI₃ and AgI complexes and which is closely related to the crystallization of AgBi₂I₇. We also examined the surface morphologies of the Ag-Bi-I films annealed at each temperature as shown in Figure 4c. As the temperature increases above 110 °C, the Ag-Bi-I films gradually begin to crystallize in the cubic phase with small grains, and fully crystallize with dense and uniform surface morphologies including large grains with the size of 200 – 800 nm (i.e., the crystallization number per unit area was 4.08 x 10⁸ #/cm²) at 150 °C.

We measured the optical absorption of the Ag-Bi-I thin films using UV-Vis spectroscopy in order to investigate the changes in the optical properties as a function of the annealing temperature. Figure 5a shows a considerable difference in the absorption before and after the thermal annealing of the film. The as-prepared film showed a yellowish color and exhibited an absorption spectrum with a clear and sharp exciton peak at 474 nm²⁰. The absorption spectra of the films were dramatically red-shifted as the annealing temperature increased and, finally, we obtained an absorption spectrum sufficiently absorbent in the range of visible light (350 – 740 nm). The optical band gap (E_g) of the AgBi₂I₇ thin film annealed at 150 °C was obtained from the Tauc plot using the equation αhv ~ (hv–E_g)^1/2, where α is the absorption coefficient and hv is the photon energy. Here, E_g was calculated to be 1.87 eV (Figure 5b). We also used UV photoelectron spectroscopy (UPS) with He I (21.22 eV) photon lines from a discharge lamp to investigate the Fermi energy (E_f) and the valence band energy (E_v) level of the resultant AgBi₂I₇ film (Figure 5c). For this UPS measurement, the film was prepared on a gold substrate. The E_f was determined using the cutoff energy (E_cutoff) as shown in Figure 5c and was calculated to be 5.05 eV using the equation: E_f = 21.22 eV (He I) –E_cutoff. Linear extrapolation in the low binding-energy region gives E_v−E_f and, therefore, E_v was determined to be 6.2 eV. The conduction band energy (E_c) was evaluated using the optical band gap obtained from the Tauc plot, which made it possible to draw a schematic energy level diagram of the AgBi₂I₇ film, as shown in Figure 5d.

Figure 1: Different crystalline compositions of solution-processed Ag-Bi-I ternary thin films. This panel shows XRD patterns of Ag-Bi-I thin films fabricated using different molar ratios of AgI to BiI₃ after the thermal annealing at 150 °C: (1) 1:2, (2) 1:1, and (3) 2:1. The reference XRD patterns of AgBi₂I₇ and Ag₂BiI₅ were obtained from PDF Card No. 00-034-1372 and PDF Card No. 00-035-1025, respectively. The dashed box indicates the main XRD pattern used to identify the different crystallizations of the Ag-Bi-I ternary thin films. This figure has been modified from the work by Kim et al.¹. Please click here to view a larger version of this figure.

Figure 2: Comparison of solution-processed AgBi₂I₇ and Ag₂BiI₅ thin films. (a) This panel shows normalized UV-Vis absorption spectra of AgBi₂I₇ and Ag₂BiI₅ thin films. The other two panels are top-view SEM images of (b) AgBi₂I₇ and (c) Ag₂BiI₅ thin films, prepared on glass substrates with different molar ratios of precursors AgI to BiI₃. The insets in panels b and c show the photo images of each thin film. Please click here to view a larger version of this figure.

Figure 3: Crystal structure and air stability of solution-processed AgBi₂I₇ thin films. (a) This panel shows the experimental XRD peak data of an AgBi₂I₇ thin film. The reference and calculated XRD data for AgBi₂I₇ are obtained from PDF Card No. 00-034-1372 and computer program VESTA, respectively. (b) This panel shows the results of an investigation of the air stability of AgBi₂I₇ thin films using XRD measurement. The XRD of AgBi₂I₇ was measured before and after the sample was stored in air for 10 d. This figure has been modified from the work by Kim et al.¹. Please click here to view a larger version of this figure.

Figure 4: Structural change of Ag-Bi-I ternary thin films with a different thermal annealing temperature. These panels show (a) XRD spectra, (b) FTIR spectra, and (c) top-view SEM images of solution-processed Ag-Bi-I thin films as a function of the thermal annealing temperature. The asterisks in panel a indicate the main crystallized XRD peaks of AgBi₂I₇. This figure has been modified from the work by Kim et al.¹. Please click here to view a larger version of this figure.

Figure 5: Optical bandgap and energy band diagrams of AgBi₂I₇ thin films. The upper two panels show (a) UV-Vis spectra and (b) Tauc plots of Ag-Bi-I ternary thin films with a different annealing temperature. (c) This panel shows the UPS data in a high binding-energy region of an AgBi₂I₇ thin film annealed at 150 °C. (d) This is a representation of an energy band diagram of an AgBi₂I₇ thin film calculated using the Tauc plot and UPS. This figure has been modified from the work by Kim et al.¹. Please click here to view a larger version of this figure.

Discussion

We have provided a detailed protocol for the solution fabrication of Ag-Bi-I ternary semiconductors, which are to be exploited as lead-free photovoltaic absorbers in thin-film solar cells with mesoscopic device architectures. c-TiO₂ layers were formed on FTO substrates to avoid electron leakage flowing into the FTO electrodes. m-TiO₂ layers were sequentially formed on c-TiO₂-coated FTO substrates to improve the electron extractions generated from the photovoltaic absorbers (i.e., the Ag-Bi-I thin films). Both c-TiO₂ and m-TiO₂ were treated with TiCl₄ aqueous solutions in order to passivate the TiO₂ surface traps; this leads to the interfacial improvement of each TiO₂ layer. The Ag-Bi-I precursor solution was spin-coated with the humidity maintained below 20%; this was because butylamine solvent has a low boiling point and is highly reactive with moisture in the air, which could strongly influence the surface morphology. The resultant yellowish-red thin films were thermally-annealed in an N₂-filled glove box in order to obtain the resultant black-brown and shiny thin films of AgBi₂I₇. When annealed in ambient conditions, the Ag-Bi-I thin films showed reddish colors and hazy morphologies, resulting from the oxidation of bismuth iodide. To complete the device fabrication, P3HT was spin-cast onto AgBi₂I₇ thin films, followed by a gold (Au) deposition, to function as a hole-transporting layer and top electrode, respectively.

As shown in Figure 1 and Figure 2, Ag-Bi-I ternary systems were crystallized in various compositions, such as AgBi₂I₇ and Ag₂BiI₅, according to the different precursor ratios of AgI and BiI₃. The thermal annealing conditions influence the absorptions, grain sizes, and surface morphology of the as-prepared Ag-Bi-I thin films. Previous studies on Ag-Bi-I ternary systems focused on the synthesis and analysis of bulk crystals; however, we have reported for the first time that AgBi₂I₇ thin films can be prepared using a spin-coating-based solution process and then used successfully as a lead-free solar cell absorber¹¹. Recently, many researchers have followed this work in order to further develop the material quality itself, as well as the solar cell performance^21,22.

There is still room for the further development of solution-processed Ag-Bi-I ternary thin-film solar cells in terms of material quality and device architecture engineering. Many papers related to Ag-Bi-I ternary materials have recently been published in peer-reviewed journals and, therefore, we believe that further research into Ag-Bi-I ternary systems will make great strides in the field of solution-processed and environment-friendly thin-film solar cells.

Materials

Bismuth(III) iodide, Puratronic, 99.999% (metals basis)	Afa Aesar	7787-64-6	stored in N2-filled condition
Silver iodide, Premion, 99.999% (metals basis)	Afa Aesar	7783-96-2	stored in N2-filled condition
Butylamine 99.5%	Sigma-Aldrich	109-73-9
Triton X-100	Sigma-Aldrich	9002-93-1
Isopropyl alcohol (IPA)	Duksan	67-63-0	Electric High Purity GRADE
Titanium(IV) isopropoxide	Sigma-Aldrich	546-68-9	≥97.0%
Ethyl alcohol	Sigma-Aldrich	64-17-5	200 proof, ACS reagent, ≥99.5%
Hydrochloric acid	SAMCHUN	7647-01-0	Extra pure
Titanium tetrachloride (TiCl4)	sharechem
50nm-sized TiO2 nanoparticle paste	sharechem
2-propanol	Sigma-Aldrich	67-63-0	anhydrous, 99.5%
Terpineol	Merck	8000-41-7
Heating oven	WiseTherm
Oxygen (O2) plasma	AHTECH
X-ray diffraction (XRD)	Rigaku		Rigaku Miniflex 600 diffractometer with a NaI scintillation counter and using monochromatized Cu-Kα radiation (1.5406 Å wavelength).
Fourier transform infrared (FTIR)	Bruker		Bruker Tensor 27
field-emission scanning electron microscope (FE-SEM)	Hitachi		Hitachi SU8230
UV-Vis spectra	PerkinElmer		PerkinElmer LAMBDA 950 Spectrophotometer
Ultraviolet photoelectron spectroscopy (UPS)	RBD Instruments		PHI5500 Multi-Technique system