Synergistic Passivation of AgBiS₂ Ternary Colloidal Quantum Dots: A Strategy for Enhancing Stability and Performance Investigated with First Principles Calculations

Solution-processed semiconductors, including perovskites, colloidal quantum dots(CQDs), and conjugated polymers, have gained significant attention for their potential applications in energy conversion and optoelectronics. Such semiconductors are characterized by their printability, low cost, and large form factors. CQDs, in particular, exhibit unique physical properties, such as quantum confinement effects, which allow for tuning of optical bandgaps across the visible to infrared spectrum. The lead-chalcogenide (PbS) CQDs have the ability to tune the bandgap to the infrared (~1800 nm) range. This unique feature allows for the realization of low bandgap solar cells when used as back cells in multi-junction structures. The best outcomes were achieved by introducing heavy metals, such as lead (Pb), into CQD compounds. However, due to the toxicity of these metals, the commercialization of related devices was limited.<br/>AgBiS₂, which is a ternary compound, has attracted significant attention owing to its high absorption coefficients, tunable bandgap, and environmental stability. Various strategies have been proposed to enhance the device performance of thin-film AgBiS₂ CQD solar cells, resulting in a promising current density even under ultrathin conditions. Despite these efforts, AgBiS₂ CQD solar cells demonstrate a relatively low power conversion efficiency (PCE) compared to lead-based CQD solar cells. The main drawback of solution-processed AgBiS₂ is attributed to the use of long-chain hydrocarbon ligands during the synthesis process, which impedes charge transport in practical applications. Consequently, the exchange of surface ligands emerges as a promising strategy to overcome this limitation. Nevertheless, the theoretical understanding of the surface properties of AgBiS₂ CQDs is still lacking, which can potentially influence the exchange kinetics in the synthesis process.<br/>In this presentation, an effective passivation of AgBiS₂ CQD surface is proposed by designing a synergistic passivation strategy with [AgI₂]^-, [AgBr₂]^-, and Na⁺ based on density functional theory (DFT) calculations. Utilizing (111) and (100) facets which are expected to be the primary components of CQDs, single-ligand passivation calculations on each surface demonstrated that individual cations do not effectively adhere to the (111) surface. On the other hand, the simulation revealed that the synergistic ligand approach was shown to be complementary to selective surface passivation. It was found that the initial covering of the metal halide anion reconfigures the surface, leading to subsequent cation passivation. Moreover, it was also demonstrated that the synergistic exchange can easily passivate both (111) and (100), providing a uniform passivate strategy for multiple facets. Furthermore, the experiments have demonstrated the feasibility of the theoretically suggested synergistic passivation and the improvement in performance. In particular, XPS measurements indicated that stable AgBiS₂ CQD ink were realized through synergistic passivation mechanism as suggested by the simulation. Finally, the solar cell that was fabricated based on our CQD ink achieved a power conversion efficiency (PCE) of 8.14%, which is 11% higher than the previous best report.<br/>In conclusion, our synergistic uniform passivation strategy successfully mitigates the issue of selective surface passivation of a single ligand. By uniformly passivating the CQD surface, our strategy provides a route to significantly enhance the stability and suppress the trap states. Based on this high-quality CQD solids, we also experimentally demonstrated that ink stability, short response time, and PCE of AgBiS₂ CQD-based optoelectronic devices were significantly improved. Additionally, we anticipate that such a synergistic ligand strategy will serve as a universal approach for the development of high-performance CQD-based optoelectronic devices.