| Summary: | Metal halide perovskites (MHPs) have garnered extensive attention from the scientific community since its inception as a light harvester in 2009. With a general formula of ABX3, where the monovalent A-site cation is surrounded by four corner-sharing BX6 metal halide octahedra, solar cells based on this material (PSCs) have shown remarkable efficiencies owing to their superior optoelectronic properties. Research on MHP-based LEDs (PeLEDs) has made tremendous progress following early reports on light emission in 2014, with current state-of-the-art devices exhibiting external quantum efficiencies exceeding 20%. However, industrialization of PeLED is not without challenges, with performance, reliability, and scalability; all contingent towards advancing the fundamental understanding of these materials and their light-emitting characteristics.
Ideally, for effective light emission, high radiative recombination rate is required and can be achieved through either (a) suppression of non-radiative recombination or (b) improvements to radiative recombination via charge carrier confinement. The tendency for non-radiative recombination to precede radiative recombination leads to the mediocre efficiencies seen at low driving voltages, underscoring the importance of remedying defects. Radiative recombination can be enhanced through employment of an energetic landscape, where structural modulation of the emitter facilitates efficient light generation via a process termed as energy cascade. Such a landscape is obtained through the templated formation of quasi-2D MHPs, where on injection of charge carriers into higher band gap domains, the charge transfer to the energetically favoured lowest band gap domains ensures increased charge carrier concentration and efficient emission.
This thesis is focused on enhancing radiative recombination through materials processing techniques for defect alleviation as well as via engineering an energetic landscape for mitigation of non-radiative losses and efficient bimolecular radiative recombination through charge carrier localization. Though necessary for solvent removal, annealing also accelerates film degradation due to the formation of under-coordinated surface atoms; and a post-deposition treatment that results in an enhancement to the device performance, was developed. Reduced grain size is an effective way to increase spatial confinement of charge carriers and boost radiative recombination; however, the accompanying effect of increasing grain boundary density needs to be contained. An additive engineering technique that introduces a small organic molecule during processing, forming small passivated grains thus promoting radiative recombination, was effectively devised.
Efforts to improve efficiency of PeLEDs have been hampered by low exciton binding energies, moisture instability and tendency for light degradation. Layered (2D) MHPs, offering superior ambient stability and high exciton binding energies were introduced to form mixed-dimensional MHPs. The notable increase in performance of these MHPs stem from the formation of multiple domains of varying band gaps, creating an energetic landscape, whereby charge carriers undergo an energy cascade from larger band gap domains to the energetically favoured lower band gap domains. The rapid energy transfer process circumvents non-radiative recombination in the larger band gap domains, resulting in confinement of charge carriers in the lowest band gap domains which not only overwhelm the defect states, but also facilitate highly efficient photon generation. Translating these design principles into practice necessitates the implementation of new processes and rational selection of molecular moieties that engineer mixed-dimensionality in the MHPs. While the energy cascade phenomenon has been demonstrated for red and green emissions, this challenge is accentuated for blue emission, where charge carrier funneling to the smallest band gap domains needs to be countered. A protocol to influence the crystallization kinetics and to regulate the formation of pure 2D domains was developed to yield record efficiencies and spectrally stable blue emission.
To summarize, the work presented in this thesis is centered on managing defects and designing an energetic landscape that favours radiative recombination. A combination of prudent molecular additive selection, process design, and fine tuning of a mono-halide material system enable both defect passivation and effective energy cascade to be achieved. This mitigation of non-radiative recombination process and improved photon generation due to charge carrier localization boosted emission; highlight the importance of defect management and process design for high performance PeLEDs.
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