Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10784−10791
Improved Passivation of PbS Quantum Dots for Solar Cells Using Triethylamine Hydroiodide Dasom Park,† Randi Azmi,† Youngho Cho,† Hyung Min Kim,† Sung-Yeon Jang,*,‡ and Sanggyu Yim*,† †
Department of Chemistry, Kookmin University, 77 Jeongneung-ro, Seoul 02707, South Korea Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea
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ABSTRACT: Surface passivation of colloidal quantum dot (CQD) is one of the most crucial factors used to determine the power conversion efficiency (PCE) of CQD based solar cell (CQDSC) devices. In this work, we developed novel alkylammonium iodide based ligands, which can achieve more effective passivation of iodide for PbS based CQD than the conventionally used tetra-nbutylammonium iodide (TBAI). Sufficient ion dissociation property and higher acidity of triethylamine hydroiodide (tri-EAHI) led to the enhanced oleate ligand removal and iodide passivation compared to TBAI. Owing to the improved iodide passivation by tri-EAHI, the sub-bandgap trap density was successfully reduced, which offered lower doping density and higher electron mobility than TBAI. As a result, the depletion region in CQD active layers was widened, while the charge recombination in CQDSC was significantly reduced. The CQDSCs with the tri-EAHI achieved significantly higher PCE (9.84%) than that obtained using conventional TBAI (9.20%). KEYWORDS: Colloidal quantum dot, Solar cell, Lead sulfide, Triethylamine hydroiodide, Ligand exchange
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INTRODUCTION Colloidal quantum dots (CQDs) have attracted increasing attention as promising photoactive materials for the nextgeneration solar cells due to their size-dependent bandgap tunability, high absorption coefficient, solution processability, and multiple-exciton generation.1−5 During the past decade, the performance of PbS-based CQD solar cells (CQDSCs) has soared, and power conversion efficiency (PCE) > 12% was reported.6,7 The PCE improvement has been achieved mainly by the development of high-quality CQD active materials and the optimization of device architecture.8,9 The surface of the as-synthesized CQDs is generally covered by long-chain organic ligands such as oleic acid and oleylamine for the colloidal dispersion in organic solvents; however, these long-chain ligands hinder charge carrier transport. The replacement of the long-chain ligands to shorter organic or inorganic linkers such as 1,2-ethanedithiol (EDT), 1.3propanedithiol (PDT), 3-mercaptopropionic acid (MPA), and tetra-n-butylammonium iodide (TBAI) has been critical for achieving the desired optoelectronic properties.10−14 Typically, the ligand exchange is carried out in the solid state after the deposition of the CQD layers. In particular, the bilayers of n-type TBAI-exchanged CQD/p-type EDTexchanged CQD were used as the layers for high-efficiency devices.13−15 © 2019 American Chemical Society
While TBAI is the current state-of-the-art ligand for the preparation of iodide-capped n-type CQDs, the steric hindrance around the nitrogen atom may not be optimal for the efficient replacement of organic long-chain ligands to iodide.8 Other iodide-based ligands such as metal iodides16 and methylammonium lead iodide (CH3NH3PbI3)17 have been used to explore various strategies for further optimization of iodide passivation; however, the performance of these PbSCQDs was far inferior to those using TBAI. Furthermore, the passivation with metal iodides often caused cationic exchange of Pb at the surface of PbS-CQDs, which changes the Pb/S stoichiometry.18,19 However, thus far, few studies have been conducted to reveal the effects of the chemical structure of ligands on the efficiency of solid-state exchange (SSE). More detailed understanding of the factors that can control the efficiency of SSE is of significant fundamental and technological interest. The development of iodide-based ligands that can outperform TBAI is a critical step for further optimization. In this work, we developed molecularly engineered alkylammonium iodide (AMI) based ligands, which can outperform TBAI in terms of iodide passivation. Four different Received: March 18, 2019 Revised: May 14, 2019 Published: May 27, 2019 10784
DOI: 10.1021/acssuschemeng.9b01542 ACS Sustainable Chem. Eng. 2019, 7, 10784−10791
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic illustration of the ligand exchange of PbS QDs using TBAI and tri-EAHI ligands. Synthesis of Triethylamine Hydroiodide. Hydroiodic acid (HI) solution (57 wt %, distilled, 99.999% trace metals basis) and triethylamine (≥99%) were purchased from Sigma-Aldrich. An HI solution (17 g, corresponding to 0.07 mol HI) was diluted in water (100 mL) and cooled down to 4 °C. Triethylamine (0.07 mol) was added into the solution and stirred for 30 min. After the solvent was evaporated, the remaining salts were recrystallized using a mixed solvent of water/ethyl acetate and then were filtered. The filtered salts were dried in a vacuum oven at 60 °C overnight. MAI (powder, Dyesol), TEAI (98%, Aldrich), and TBAI (reagent grade, 98%, Aldrich) were used as purchased without further purification. Fabrication of PbS CQDSCs. An indium−tin-oxide (ITO)coated glass substrate (sheet resistance
