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Nanoengineering High-Performance Low-Cost Perovskite Solar Cells Utilising Singlet Fission Materials

Final Report Summary - 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 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 exciton fission is one such process by avoiding 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 aimed 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 aimed 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

Each of these objectives have, on the whole, been achieved. Objective 1 was completed during the outgoing phase (Period 1). 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. Working solar cells for Objective 2 (device implementation) were also constructed during period 1. Tetracene and pentacene have been shown to be an effective hole transporter.

Objective 3 (spectroscopic characterisation) was completed during period 2 and these results are helping to guide the materials and device fabrication. The time-resolved spectroscopy of the perovskite/fission interfaces were investigated in the presence of a magnetic field, allowing modulation of the fission process. This confirmed the charge transfer from the perovskite to the fission material but didn’t provide a clear signature for contributions to the device from singlet fission (triplet exciton dissociation).

The spectroscopy element has also led to three new unexpected but exciting breakthroughs about these materials. Specifically, these are:
1. The researcher has found a set of light-soaking treatments that can vastly improve the optical, emission and transport properties of these materials. The researcher also found a series of additives that can also improve the emission and stability. Both of these aspects will help achieve the stability and performance aspects of Objective 2.
2. The first visualization of ion migration in perovskite films under illumination
3. The first experimental observation of an indirect bandgap character of metal halide perovskites. 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.

The overall ambitious aim of a fission material boosting the performance of a perovskite solar cell was not achieved. This is partly because perovskite solar cells have improved so rapidly that the state-of-the-art benchmark has shifted, but mostly because the low bandgap perovskites required for the perovskite/fission devices are only now reaching a quality good enough (and tunable enough) for implementation into high performance devices. The researcher has laid the groundwork to continue this work and has a PhD student now working on the project. It is expected that the first demonstration of a perovskite solar cell utilizing singlet fission will be achieved in his newly-established group soon.

The broader output from the project has been tremendous, with a total of 22 peer-reviewed research papers (17 published, 5 under review) that have significantly advanced the state-of-the-art of this very exciting field. These works have provided the platform for solar cells to move toward and even beyond the conventional limits. Such a technology has the potential to dramatically reduce the price of solar, in turn helping emissions and renewable energy targets to be more comfortably met and opening up a variety of jobs in the energy sector.

During the fellowship, the researcher was awarded a number of prizes for his contributions to the field, including the IUPAP Young Scientist in Semiconductor Physics Prize (2016) and the European Physical Society Early Career Prize (2017). He has been awarded an ERC Grant and Royal Society University Research Fellowship and has now established his own research group as a PI at Cambridge University.