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 perovsk....

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 ....

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

List of materials used in this article

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