(501h) Development of Two Sequential Kmc Models to Describe Ligand Exchange and Charge Transport for CsPbBr3 Quantum Dots with Improved Stability and Charge Carrier Mobility

Inorganic halide perovskite quantum dots (QDs) such as CsPbBr₃ are in the spotlight as materials for semiconductors, photodetectors, LEDs, and solar cells due to their relatively easy synthesis method and excellent optoelectronic properties [1]. However, QDs have a fatal disadvantage where the efficiency is easily deteriorated by light, high temperature, and moisture. Hence, the stability of QDs must be increased for its widespread implementation. Ligands located on the outer side of the QDs are commonly utilized to increase the stability of QDs because ligands physically protect the QDs by preventing dispersion and aggregation [2]. During the QD synthesis, long-chain ligands (e.g., oleic acid) act as natural surfactants to facilitate synthesis and stabilize precursors. However, long-chain ligands may cap the surface and hinder efficient electron transport by increasing the gap between the QDs (i.e, working as an insulator) [3]. Therefore, the ligand-exchange procedure with short-chain ligands such as 1,2-ethanedithiol (EDT) or Mercaptopropanoic acid (MPA) is essential to reduce the gap and thereby increase conductivity. Furthermore, the type of ligands alters electronic structure of QDs confined exciton state, which significantly affects the surface, electrical, and optical properties of QDs [4]. While several experimental studies have been recently carried out, there is a lack of the fundamental understanding of the ligand exchange process.

First, we proposed a kinetic Monte Carlo (kMC) model to enhance our understanding of the ligand-exchange phenomenon. The developed kMC model describes the surface of QDs in the solution using a two-dimensional lattice of size N [5]. Four microscopic events were considered in the kMC simulation to describe the ligand-exchange process: (i) attachment of long-chain ligand, (ii) detachment of long-chain ligand, (iii) attachment of short-chain ligand, and (iv) detachment of short-chain ligand. Parameters such as the ligand concentration, the size of QDs, and the solution concentration were available from the relevant experimental data [6,7]. The activation energy of the attachment and detachment rate was derived from the difference in bandgap energy values before and after ligand exchange. The kMC model enables the prediction of ligand exchange ratio and morphology on the surface of QDs.

However, the direct measurement of surface characteristics of QDs in the solution (i.e., surface morphology and exchanged ligand ratio) is not readily available in the literature [8]. Therefore, a model (i.e., a charge-transport kMC model) was developed to quantify the state of ligand exchange. Since the activation energy values for the transport of exciton and charge carrier are affected by the QDs’ surface characteristics [9], the simulation results of the charge-transport kMC model can reasonably explain the ligand-exchange phenomena. Similar to the ligand-exchange kMC model, four events were considered in the kMC simulation for charge transport: (i) charge carrier generation by light absorption, (ii) diffusion by hopping, (iii) decay, and (iv) recombination. Additionally, the charge-transport rates of the QD core and short-chain ligands were calculated from Marcus formula. The long-chain ligand rate was computed from the Miller-Abrahams formula that is widely used in the charge-transport kMC of organic solar cells. The developed model that integrates the ligand-exchange kMC model and the charge-transport kMC model was validated by the Current-Voltage curve data. [10]

In conclusion, our study addressed the knowledge gap present in the interpretation and modeling of the ligand-exchange phenomenon. Specifically, a first-principled ligand-exchange kMC model and a charge-transport model were integrated to simulate the ligand-exchange phenomenon. Subsequently, this model was validated against a system of (long-chain) oleic acid ligands with (short-chain) MPA ligands. The developed model provided a foundation for optimizing the QD surface passivation, by enhancing stability and charge mobility.

Literature Cited

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