Engineering Photophysical Properties in Doped Perovskite Nanocrystals: Synthesis Strategies and Control of Energy Transfer Processes

Abstract

Perovskite nanocrystals (NCs) have emerged as a family of revolutionary materials for energy conversion and light-emitting devices, offering high photoluminescence (PL) efficiency and a tunable band gap. State-of-the-art perovskite NCs show near-unity PL quantum yield (QY) in the entire visible range and reasonable long-term stability. The ionic nature of chemical bonds in perovskite NCs is favorable for doping them with transition and post-transition metals (e.g., Cd, Mn, Cr, Al, Sn) as well as rare-earth elements (e.g., Ce, Yb, Tm, Er), which allows us to tune their PL across a much wider spectral range (400 to 1500 nm). Moreover, lead-based CsPbX₃ (X = Cl, Br, I) NCs doped with Yb³⁺ ions demonstrate the existence of so-called quantum cutting effect, when one excited photon generates two emission photons, resulting in a PL QY of up to 200%. Despite all the advantages, further practical implementation of CsPbX₃ NCs is hindered by the presence of toxic Pb²⁺ ions. Thereby, search for more environmentally friendly perovskite-family nanomaterials is currently ongoing. Lead-free double perovskite (DP) NCs with a general formula A₂M(I)M(III)X₆ represent the next generation of halide perovskite materials, offering a versatile platform for compositional engineering. Their crystal structure enables a broader range of doping and alloying possibilities compared to conventional lead-based CsPbX₃ perovskites, allowing for the controlled incorporation of various metal cations and precise tuning of optical and electronic properties. This enhanced adaptability renders lead-free DP NCs promising candidates for environmentally sustainable and high-performance optoelectronic applications. Among different DP materials, Cs₂NaM(III)Cl₆ perovskite stands out due to its high defect tolerance, excellent stability, and substantial doping capacity. Besides, the absence of Ag⁺ ions in its composition allows us to avoid formation of Ag(0) defects that are often found at the NC surface. The PL band position of Cs₂NaM(III)Cl₆ could be widely tuned through doping and alloying with various trivalent cations (e.g., Cr, Sb, Sm, Tb).

For the further advancement of both lead-based and lead-free perovskite NCs with tunable optical properties, a thorough understanding of the underlying physical mechanisms governing energy transfer in doped and alloyed systems is essential. To address this challenge, the present PhD thesis focuses on the development of novel synthesis approaches that enable efficient doping, as well as on the investigation of fundamental photosensitization mechanisms that define optical performance of these NCs.

Chapter 1 provides a comprehensive literature review on energy transfer processes in doped and alloyed perovskite NCs. It discusses the synthetic strategies that enable incorporation of dopants into both lead-based and lead-free NCs, with particular emphasis on the resulting modifications of their photophysical properties. Furthermore, this chapter explores the formation of alloyed perovskite systems and highlights their potential as a versatile platform for rational design and precise engineering of the NCs’ optical response.

Chapter 2 describes the experimental details of the conducted research related to this thesis. It includes the main synthetic approaches used for the preparation of doped lead halide perovskite nanocrystals and lead-free DP nanocrystals, along with the instrumental and analytical methods applied to study their chemical composition, morphology, crystal structure, and photophysical characteristics.

Chapter 3 is aimed at understanding the photosensitization mechanisms in Yb³⁺-doped perovskite NCs which is essential for their application in photonics. Near-infrared (NIR) PL of Yb³⁺-doped mixed-halide CsPbCl_xBr3_−x NCs is studied as a function of temperature. At the same time, the NC emission is also found to be strongly influenced by the stoichiometry of the host perovskite matrix. To explain the observed experimental trends, a theoretical model is developed in which energy transfer from the perovskite matrix to Yb³⁺ ions occurs through the intermediate trap states situated beneath the conduction band (CB) of the host. The described model provides an excellent agreement with experimental results and is further validated through the measurements of emission saturation at high excitation powers and NIR PL QY as a function of the anion composition. These findings establish trap-mediated energy transfer as the dominant photosensitization mechanism in Yb³⁺-doped CsPbCl_xBr_3−x NCs and reveal new ways of engineering their optical properties for light-emitting and light-harvesting applications.

Chapter 4 focuses on lead-free vacancy-ordered DP crystals and describes the difference in optical performance of their bulk phase and the NC form. Specifically, Cs₂ZrCl₆:Te⁴⁺ DP NCs are synthesized, and their optical properties are compared to the bulk powders with the same composition. Temperature-dependent spectroscopy reveals that the NCs sustain a thermal sensitization of the intermediate trap state which is located between the self-trapped state of the host (Cs₂ZrCl₆) and the triplet states of the dopant (Te⁴⁺). This creates a pathway for the non-radiative recombination and thus decreases the energy transfer efficiency from host to dopant. Importantly, this pathway is suppressed in larger (40 nm) Cs₂ZrCl₆:Te⁴⁺ NCs, resulting in higher PL QY of 24%, as compared to 7% for the 22 nm NCs. Furthermore, the emission spectral range of these DP NCs can be extended into NIR by incorporating rare-earth ions as additional dopants. This study has established a crucial relation between the optical properties and the size effect in lead-free vacancy-ordered DPs and thus lays a foundation for further improvement of their optical performance.

Chapter 5 explores band gap engineering in lead halide perovskites through the lead-site doping as a promising strategy to achieve blue-shifted emission in NCs without relying on quantum confinement or halide mixing. Here, the structure and photophysical properties of CsPb_1−xCd_xBr₃ NCs with a varied amount (3, 8, and 15%) of Cd(II) doping are considered. The incorporation of the increasing amount of Cd²⁺ ions results in an up to 5 nm decrease of the average NC size, while the emission is blue-shifted from 515 to 485 nm. Applying ultrafast transient absorption (TA) spectroscopy, a significant enhancement is observed in the absorption oscillator strength of CsPb_1−xCd_xBr₃ NCs along with an almost threefold increase in the hot carrier temperature, which indicates more efficient population of the band edge compared to pristine CsPbBr₃. Furthermore, it is demonstrated that CsPb_1−xCd_xBr₃ NCs follow their own volume scaling law for the exciton-exciton annihilation threshold and rate. Specifically,
Cd(II)-doped CsPbBr₃ NCs with a smaller size exhibit a higher Auger threshold than the larger pristine CsPbBr₃ NCs, which makes them potentially useful for light-emitting and lasing applications. The insights gained into the excited carrier dynamics in CsPb_1−xCd_xBr₃ NCs open new pathways for the development of efficient nanoscale emitters in the blue spectral range.

Chapter 6 presents a general low-temperature synthesis strategy for lead-free DP NCs with the chemical composition of Cs₂NaMe(III)Cl₆, establishing a versatile pathway to obtain different materials of this class. The developed approach enables the controlled incorporation of trivalent metal cations of different Lewis hardness. By unifying the synthesis process across a broad compositional range, this study paves the way for the design of environmentally benign, high-quality perovskite NCs suitable for next-generation optoelectronic applications.

Conclusion and Outlook is the final section of this thesis which provides a comprehensive summary of the major findings concerning energy transfer mechanisms and doping strategies in perovskite NCs. Furthermore, it offers a critical outlook on the persisting challenges and prospective research directions that may facilitate broader implementation of doped perovskite NCs in advanced optoelectronic devices.

Date of Award	17 Aug 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Andrey ROGACH (Supervisor)