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Controlling Wavefunction Overlap for Triplet Energy Transfer in Organic/Nanocrystal Quantum Dots Hybrids

Periodic Reporting for period 1 - TRITON (Controlling Wavefunction Overlap for Triplet Energy Transfer in Organic/Nanocrystal Quantum Dots Hybrids)

Reporting period: 2019-04-01 to 2021-03-31

This project aims to develop the fundamental science for a new platform for optoelectronics and photochemistry based on coupling triplet excitons from organic semiconductors (OSCs) to semiconductor quantum dots (QDs) such as lead chalcogenides. The knowledge gained will overcome many current limitations of triplet exciton coupling from organic to inorganic semiconductors for emerging optoelectronic applications.

This targets for this project are of great importance to society, as many emerging technologies under development are based on OSCs and QDs, and recently the hybrid of them are found to have some very unique properties that can potentially overcome many problems that the materials possess standalone. The findings from this project would provide guidance in the field of solar energy conversion, lighting, bioimaging and other optoelectronic devices.

The candidate and the host group aim to conduct systematic studies on the factors that govern triplet energy transfer (TET) within the OSCs/QDs hybrid systems. Throughout the project, the candidate has identified some key factors dominates the TET efficiency, and developed a series of chemical processes that enable one to control these factors, in order to fabricate the materials with the targeted properties.

It is concluded that very delicate control is needed to fabricate a hybrid material with efficiency TET transfer. Some of the important factors for TET can be well controlled, while some others are more difficult. For example, it is relatively simple to reduce the distance between the QDs and OSCs, which can lead to ~100% TET efficiency, however when doing so the distance between the QDs would also decrease, leading to quenching of the photoluminescence and hence lower overall emission yield. Such kind of problems are likely the next main targets to be tackled in the future, in order to fabricate materials with high overall energy conversion efficiency.
The candidate has conducted the following work through the project:

(1) Synthesized/fabricated high-quality QDs that are suitable for being used as part of the hybrid materials. The photoluminescence quantum yield (PLQY) for the materials have been improved relatively 30-50% for QDs in all band gap ranges, compared with what the group could achieve previously. This was achieved by new development in the synthesis and purification methods.

(2) Fabricated a series of OSCs/QDs hybrid materials. The candidate has identify several material combinations that can achieve ~100% TET efficiency. The overall PLQYs of the materials have also been improved progressively during the projects, due to the improved processing methods.

(3) The performance of the hybrid materials have been characterized by a series of steady-state techniques, in order to confirm the PLQYs, TET efficiency, material morphology and composition, and the relevance between these have been related, hence some key factors to the overall performance have been identified.

(4) The TET processes in the hybrid materials have been studied in details, using time-resolved spectroscopy set-ups. The processes of the generation of triplets in the OSCs, transfer to the QDs, and re-emission from the QDs have been observed in details, and now the timescale for each to occur are clear in each hybrid material.

(5) Prototype photon-multiplying films based on the hybrid materials have been fabricated. The TET efficiency between the materials can be close to 100%, and the multiplication factor can now achieve ~1.3. However, the baseline emission yield of QDs are still low, due to problems of energy level mismatch, PL quenching due to aggregations, and likely some other compatibility problems between the materials, so the overall PLQY of the system is still relatively low.

(6) The candidate has also discovered some unusually fast exciton transport behaviour in pure QD solid films at the very early timescales. This can potentially provide important guidance to both the hybrid materials and the general society that study nano-materials with excitonic features.

For future work, the main targets would be to improve the baseline PLQY of the QDs, solve the compatibility problems between the materials, and search for more OSCs that have higher energy levels to match with QDs that have higher PLQYs.
Some of the findings have been included in the group’s publications. Several manuscripts are also currently under-review or close to submission to scientific journals. The delay was mainly caused by the COVID-19 pandemic and the lockdown/lab restrictions from Mar 2020.
The beneficiary has conducted close collaboration with Cambridge Photon Technology, a spin-off company of the university. The candidate has provided important technology advices to the company, which has helped them in securing several patents. The major product is to fabricate photon-multiplying films that can be used in solar panels, which can potentially increase the energy conversion efficiency by relatively 10~20% given the same panel area. This technology can have great potential in renewable energy technologies, as photovoltaics is one of the major players in the field. In the further future the findings from this project can contribute to reduce greenhouse gas emission and tackle climate change.

The findings from this project also provide fundamental guidance to a wider research community that study OSCs, QDs and the hybrids for other applications such as lighting and bioimaging.

For education, the Cavendish Laboratory is very conscious of its role in communicating the importance and excitement of contemporary physics and its cognate disciplines to young people and the general public. The Laboratory supports a full-time officer who runs the Education Outreach Office. One of the key roles of the Educational Outreach Office is to stimulate interest and encourage wider participation in science amongst 11-19 year olds. The flagship event organised by the Laboratory each year is the annual "Physics at Work" (P@W) exhibition and Cambridge Science Festival (CSF). Although this was cancelled in 2020 and the work was not presented in 2021, the beneficiary will seek the chance to join this event in 2022.
Summary for publication