Decreased Synthesis Costs and Waste Product Toxicity for Lead Sulfide Quantum Dot Ink Photovoltaics

Abstract

the photoactive QD films, leading to poor QD PV device performance. [5] It has been shown that a significant improvement in QD PV performance can be achieved if the deposited PbS QD films are chemically treated to undergo ligand exchange, replacing OA with shorter ligands, such as halides and short-chain thiols, which leads to high charge conductivities within QD films, boosting the power conversion efficiency of QD PVs. [5] Solution-phase ligand exchange is currently the leading method of preparing the active layers in top-performing PbS QD PVs. In a separate synthetic step, OAcapped PbS QDs are exchanged with short ligands in solution and then suspended at a high concentration in a solvent to form an ink. [1][2][3][4] This ink can then be deposited on a substrate to form the QD PV device active layer, [1][2][3][4] with recently reported PbS QD PVs achieving 12% power conversion efficiency (PCE). [1] With this advancement the fabrication of the active layer is now more facile, however, the scalability of PbS QD solar cells is still uncertain. A recent report by Jean et al. revealed that current solutionphase QD ligand exchange methods create an added cost of $6.30 g −1 of QDs, an expense that negatively impacts the commercial viability of QD PV modules. [8] In addition to this added cost, the toxicological properties, environmental impact, and end-of-life disposal of Pb-based PVs, including perovskites, is of critical importance and has received only minimal attention. [9][10][11] For example, the leading PbS QD ligand exchange procedures employing lead halides such as lead(II) bromide [1][2][3] and lead(II) iodide [1][2][3][4] have yet to be evaluated with regards to U.S. Environmental Protection Agency and European Union regulatory limits.In this study, we present a solution-phase ligand exchange method that reduces the amount of utilized lead by employing tetrabutylammonium iodide (TBAI) as the source of iodide ligands. The resulting PbS QD PVs have a 10% PCE under AM1.5 illumination, with more than 1000 h of storage stability when stored unpackaged in ambient conditions. Importantly, this synthetic methodology is evaluated with regards to its ability to lower costs and toxicity compared to the current champion lead halide (PbX 2 ) based ligand exchange methods. [1] This work provides a potential pathway toward improved compatibility with industrial scale thin-film PV device production as well as a framework to test the toxicity of new device fabrication procedures for a wide range of PVs. Use of lead sulfide (PbS) colloidal quantum dot (QD) films as photoactive layers in photovoltaic (PV) devices typically requires replacement of nativeQD ligands with lead-based capping ligands (i.e., PbX 2 , X = Br, I) for the best-performing QD PVs. This ligand replacement process often requires additional solvents and toxic reagents. In the present study, an alternative PbS QD PV fabrication method with a lead-free tetrabutylammonium iodide (TBAI) ligand source and lower material requirements and toxicity is demonstrated,...

PbS Quantum Dots with Inorganic Ligands: Physical Modeling of the Charge and Excitation Transport in Photovoltaic Cells

“…The particle size was determined from the absorption spectrum of PbS CQDs with initial OA ligands in a toluene solution (Supporting Information, Figure S1) using the equation E abs (eV) = 0.41 + 0.96/r 2 + 0.85/r from ref 19, where E abs is the first excitonic peak energy and r is the CQD radius. Substitution of the initial oleic acid ligands with TBAI was carried out in a solution in the open air within 15 min in accordance with the procedure described in ref 16. The solution of TBAI (360 mg) in 1.8 mL of ethanol and the colloid solution of PbS (124.8 mg) in 2.08 mL of octane were mixed, and the resulting mixture was vigorously stirred for 30 s and then centrifuged.…”

“…As a result, films of PbS quantum dots (QDs) passivated with iodine atoms turned out to be a promising material, on the basis of which PV cells with an efficiency exceeding 10% and a continuous operation period of more than 1000 h were obtained. 16 The technology for producing films is based on the procedure of the replacement of the initial ligand (oleic acid), which is chemisorbed on the CQD surface during its synthesis, on TBAI. By 2019, a relatively simple and very effective technique for replacing ligands in a CQD solution was developed, which significantly improves the quality of the subsequently produced film.…”

“…By 2019, a relatively simple and very effective technique for replacing ligands in a CQD solution was developed, which significantly improves the quality of the subsequently produced film. 16 mechanism, which is not only efficient but also very unique from a physical point of view. Further progress in the development of photovoltaics based on these films is closely tied to the understanding of the physical mechanism of chargecarrier transport, which is still not entirely clear.…”

“…This not only significantly reduces the distance between neighboring CQDs but also increases the efficiency of the electron transport of CQD films. As a result, films of PbS quantum dots (QDs) passivated with iodine atoms turned out to be a promising material, on the basis of which PV cells with an efficiency exceeding 10% and a continuous operation period of more than 1000 h were obtained . The technology for producing films is based on the procedure of the replacement of the initial ligand (oleic acid), which is chemisorbed on the CQD surface during its synthesis, on TBAI.…”

Passivation Strategy of Reducing Both Electron and Hole Trap States for Achieving High-Efficiency PbS Quantum-Dot Solar Cells with Power Conversion Efficiency over 12%

“…Notably, almost all single-junction PbS CQDSCs with a PCE of more than 12% have been prepared based on the solution-phase ligand exchange method ,,, because of their low surface exposure during the ligand exchange process. Some recent explorations with synthetic methods and ligand engineering have also provided new ideas for reducing the large consumption of lead precursors and amines that are currently ubiquitous in solution-phase ligand exchange methods and further reduce their costs. Recently, another simpler and industry-suitable procedure for PbS QD film deposition was reported, in which widely used methanol (MeOH) was replaced with acetonitrile (ACN) as the rinsing solvent and the ACN “solvent-curing” effect (i.e., the reassembly of QDs in the film during the solid-state ligand exchange (SSE) approach) is utilized to greatly simplify the deposition process while improving the device efficiency.…”