Solution-Processed "Silver-Bismuth-Iodine" Ternary Thin Films for Lead-Free Photovoltaic Absorbers

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.

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 ...

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.,...

Materials

Name	Company	Catalog Number	Comments
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