Community Research and Development Information Service - CORDIS

Periodic Report Summary 1 - SOLAR BEYOND SILICON (Nanoengineering High-Performance Low-Cost Perovskite Solar Cells Utilising Singlet Fission Materials)

Metal halide perovskites are generating enormous attention for their use in high performance but potentially low-cost optoelectronic applications such as solar cells and light-emitting diodes (LEDs). Since 2012, the power conversion efficiencies (PCEs) of perovskite solar cells has increased from 3% to over 22%, and their bandgap tunability lends them to a variety of novel device applications.

Efficiencies and hence costs cannot improve indefinitely because the PCE of single-junction solar cells is fundamentally constrained by the Schockley-Queisser (SQ) limit (~30%). This limit primarily arises because low-energy photons are not absorbed, while photons with energies higher than the bandgap are absorbed but the energy in excess of the bandgap is rapidly lost by thermal relaxation. Singlet fission exciton fission is one process that can help to avoid these relaxation losses. In this process, photoexcitation of a material capable of fission with a high-energy photon produces a spin-singlet exciton which can undergo a spin-conserving process to generate two triplet excitons. When incorporated with a second material that absorbs the lower energy photons and can dissociate the triplet excitons, the device is able to exceed the SQ limit and in principle reach theoretical efficiencies approaching 50%.

This project aims to combine the process of singlet fission in an organic semiconductor such as pentacene or tetracene with a low-bandgap perovskite system to demonstrate a low-cost, high-efficiency photovoltaic device. There are three objectives, each of which aims to significantly improve on the state-of-the-art while encapsulating a viable training program:
1. Synthesise and deposit organic semiconductors and new perovskites
2. Fabricate and characterise PV devices with increased efficiencies and enhanced stability
3. Use ultrafast spectroscopy to understand the device photophysics and improve performance

The project is on track to achieve its ambitious aims. Objective 1 has been completed. The researcher was able to fabricate low bandgap perovskites (<1.3 eV) which are required for matching with singlet fission materials such as tetracene. The researcher confirmed the compatibility of these perovskites with the organic materials both in terms of deposition and in terms of electronic compatibility. Tasks for Objective 2 (device implementation) are now well under way and working solar cells have been constructed. Tetracene and pentacene have been shown to be an effective hole transporter. However, no significant contribution from singlet fission has yet been detected. The researcher is currently pursuing different architectures and tuning the perovskite further to lower bandgap to attempt to realise contributions of triplets to the photocurrent. The researcher is confident that this will be better facilitated upon return to the home institution where additional novel low bandgap materials are being generated, allowing for an excellent opportunity for knowledge exchange and completion of the project.

Tasks for Objective 3 (spectroscopic characterisation) were started earlier than expected as this is helping guide the materials and device fabrication. The spectroscopy element has also led to three new unexpected but exciting breakthroughs. Specifically, these are:
1. The researcher has found a set of treatments that can vastly improve the optical, emission and transport properties of these materials. This will help achieve the stability aspects of Objective 2.
2. The first experimental observation of an indirect bandgap character of metal halide perovskites. This was published in Nature Materials in October 2016 and the researcher is a corresponding author. This work has generated a great deal of excitement, opening up new and interesting areas of the field. This will impact the final device design for a fission-boosted perovskite solar cell.
3. Through a collaboration with CSIRO in Australia, he has performed measurements at the ALS synchrotron in Berkeley to study the local structural properties of metal halide perovskites and how these directly correlate with emission properties. This work has the potential to revolutionize how the community considers the microscale properties of these perovskites.

It is expected that the first demonstration of a fission-boosted perovskite solar cell will be achieved by the end of the project. This will provide the platform for solar cells beyond the conventional limits. This first demonstration have enormous ramifications, with the potential to dramatically reduce the price of solar, in turn helping countries to more comfortably meet their emissions and renewable energy targets and potentially opening up a variety of jobs in the energy sector.

Reported by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
United Kingdom

Subjects

Life Sciences
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