Abstract
Metal halide perovskites (MHPs), a subclass of perovskite materials, have emerged as one of the most promising candidates for the next generation optoelectronic technologies due to their exceptional electronic and optical characteristics. The applications of MHPs have been broadened considerably from their initial deployment in photovoltaic cells (PVs) to multiple optoelectronic devices such as light emitting diodes (LEDs), photodetectors, scintillators, and photolytic systems. Despite the wide range of applications, there persists an urgent need for the development of highly stable and efficient MHP-based optoelectronic devices, a prerequisite to meet the rigorous standards of industry scale mass production. However, the exact origins of the inefficiency and instability issues in perovskite materials remain inadequately understood, thus impeding the comprehensive solutions to enhance their efficiency and operational stability. This thesis, therefore, begins with the objective to elucidate the fundamental mechanisms underlying the inefficiency and instability issues in perovskite materials and devices, and proposes efficacious solutions for the development of highly stable and efficient optoelectronic devices, including LEDs and PVs.Perovskite light emitting diodes (PeLEDs) are fabricated via the quantum confinement of conventional bulk MHPs, which improves the exciton binding energy and facilitates the exciton recombination process. Three strategies have been proposed to improve exciton binding and device efficiency: nano three-dimensional (nano 3D) perovskite fabrication, perovskite nanocrystal synthesis, and quasi-two-dimensional (quasi-2D) perovskite formation. Of all three strategies, quasi-2D perovskite formation has demonstrated unique advantages due to its well-confined excitons within multi-quantum-well structures, simple fabrication processes, and broad functionalization potentials. As a result, quasi-2D PeLEDs have achieved the highest reported external quantum efficiency (EQE) among all PeLEDs to date. However, intrinsic issues related to EQE, and stability remain to be addressed, particularly with regards to the operational stability. Therefore, it is necessary to conduct a comprehensive analysis of the causes of instability and inefficiency in quasi-2D PeLEDs, while corresponding solutions should be proposed to address these issues, as listed below.
1. Phase dimensions. Quasi-2D perovskites exhibit unique multi-quantum-well structure with mixed phase dimensions, where the thickness of a single quantum well can be described with the n value (n represents the number of the lead-centred octahedra along the thickness direction in a single quantum well). In quasi-2D perovskites, excitons can transfer from large band gap low n components to small band gap larger n phases. Therefore, the control of phase dimensions has become eminently critical for efficient inter-phase energy transfer and consequent high luminescent efficiency. To resolve the phase dimensions, in-depth characterizations have figured out that the low n phases were the origin of perovskite instability and deficiency, and a simple and novel mixed solvent post-treatment, “solvent sieve” method, was developed to selectively remove the low n phases and facilitate the energy transfer process. As a result, highly stable and efficient PeLEDs were fabricated with the solvent sieve method, demonstrating not only high EQE and half-lifetime T50 of 29.5% and 18.67 h at 12,000 cd/m2 (equivalent over 50,317 h or 5.7 years at 100 cd/m2), respectively, but also extraordinary resistance to air and moisture, maintaining over 75% of film photoluminescence quantum yield and 80% of device EQE after stored in the ambient for 100 days. The simple solvent sieve method confirmed the feasibility of MHPs for luminescence applications and unleashes the efficiency and stability potentials of PeLEDs for future commercial applications.
2. Defects. Defects inside MHPs trigger non-radiative exciton recombination and lead to higher reactivity and severe ion transportation, which deteriorate the efficiency and stability of PeLEDs. Therefore, effective control of defects should be conducted to improve the efficiency and stability of PeLEDs. In this study, a sweet coordination strategy was proposed to optimize the perovskite growth process and significantly suppress the defect formation. Therefore, highly efficient, and stable PeLEDs were fabricated with a maximum EQE of 31.3% and an operating half-life over 76.9 hours at an initial luminance of 10,000 cd/m2 (equivalent over 211,800 hours or 24.2 years at 100 cd/m2). This work paves the road towards future applications of PeLEDs in display and lighting industries.
3. Ion migration. Ion migration effect originating from the soft deformable lattice of MHPs has been considered as one of the main causes for the limited lifetime of PeLEDs. However, due to the complexity of the perovskite components, simple methods to spontaneously suppress the migration of all perovskite ions are still challenging. Therefore, a highly fluorinated molecule, 1H,1H-perfluorohexylamine (PFHA), was selected to spontaneously form strong coordination with all perovskite ions and significantly suppress the ion migration process. As a result, ultra-stable and efficient green PeLEDs was fabricated with a champion EQE of 27.1% and a T50 lifetime of 103.17 hours at 10,000 cd/m2 (equivalent to over 326,252 hours or 37.2 years at 100 cd/m2), setting a record for PeLEDs operational stability. This work confirms the stability potential of PeLEDs and sheds light on the possibility that PeLEDs can be commercialized in the future display and lighting industries.
Quasi-2D perovskites are also used as perovskite photovoltaic cells (PePVs), where the ligands inside the quasi-2D perovskites can stabilise the perovskite structures and improve the stability of the PePVs. However, problems still hinder the progress of quasi-2D PePVs, including severe interfacial problems between the perovskite and the charge transporting layers. Therefore, modification of the interface should be conducted to improve the performance of quasi-2D PePVs.
4. Interface. The interfaces between the perovskite and the charge transporting layers determine the exciton transportation process within PePVs. The buried interface between the perovskite and the bottom electron transporting layer in conventional PePVs is also critical for perovskite growth and film quality. Therefore, a multifunctional passivator ammonium thiocyanate (NH4SCN) was used to treat the SnO2/perovskite interface of quasi-2D perovskite solar cells. This interfacial modification method effectively passivated the interfacial defects and regulated the energy levels, with significant improvement in the wettability of perovskite precursors on tin oxide films and the quality of the perovskite films. This strategy of NH4SCN interfacial modification achieved high efficiency and stable quasi-2D PePVs with a maximum power conversion efficiency (PCE) of 21.96% and an impressive stability of maintaining 90% of the initial PCE after 3,000 h in the ambient atmosphere.
In summary, this thesis has introduced four critical issues for both PeLEDs and PePVs and four corresponding solutions to resolve the issues. As a result, highly stable and efficient PeLEDs and PePVs have been developed. All these solutions have demonstrated a bright future for the industrialisation of perovskite optoelectronic devices.
| Date of Award | Oct 2024 |
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| Original language | English |
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| Supervisor | Hao Chen (Supervisor), Xinyu Zhang (Supervisor) & Chaoyu Xiang (Supervisor) |