Cambridge-AMOLF Collaboration on Photonic and Optoelectronic Control of Thin-Film LEDs and Solar Cells

This Centre-to-Centre collaboration addresses a set of research opportunities that require the close integration of optoelectronic materials and device engineering with state-of-the-art light management for large area solar cells and LEDs. The collaboration brings the AMOLF LMPV group, recognised for its work in 'light management' for solar cells, to work closely with the Cambridge research programme on thin-film perovskite and organic solar cells and LEDs. The AMOLF activity is centred in the very strong 'national laboratory' framework of solar cell research in the Netherlands, and brings strengths that have not been systematically developed in the UK.

Thin-film diodes made with lead halide perovskites now support solar cells and LEDs with excellent electronic properties, but challenges with light in/outcoupling can limit performance. This is a particular challenge for perovskite LEDs for which light outcoupling is currently limited to 20%. By harnessing the special luminescent properties of perovskites, including photon recycling, with engineered optical structures, the outcoupling in the forward direction will be raised towards 100%.

One way to improve a solar cell beyond the single-junction limit is to harvest the high-energy part of the solar spectrum with an organic material capable of singlet fission, a process by which the energy from one high-energy photon is shared between two lower-energy triplet exciton states. Cambridge has pioneered the science of singlet fission and developed the concept of the Photon Multiplier. In this all-optical thin-film device, incident high-energy photons (<540nm) will be converted into two low-energy photons, each at around 1000 nm, which can then be absorbed by a silicon solar cell underneath. The challenge to be undertaken here is to develop suitable photonic designs that direct the emission of these IR photons towards the Si solar cell without introducing optical losses in the module.

This collaborative project will lead to an improvement in the performance of thin-film LEDs and solar cells, achieved through integrated design of the electronic and optical operation. The potential for improved performance covers perovskite solar cells and perovskite LEDs. The LEDs are currently limited by poor light out-coupling, but there is the potential to raise their external quantum efficiencies above that of other thin film LED technologies. A further component of the project is the design and implementation of optimised optical coupling between a light-absorbing, 'photon multiplier' film above a silicon solar cell, so that infra-red photons generated in the film are directed to and absorbed in the silicon below. All these projects are grounded in basic science, but have very considerable industrial and commercial consequence:

Improvement in the efficiency of solar cells is now considered critical to reducing the cost of solar electricity, since the balance of systems costs now outweigh the costs of the solar cell itself. Our work on solar cells will provide significant efficiency enhancements, and hence will accelerate the deployment of photovoltaic systems with attendant economic and environmental benefits. The consortium is well-placed through our interactions with UK and European companies and national laboratories to ensure that these benefits are realised locally. Cambridge already has a strong IP position in the 'photon multiplier' area, which will be further enhanced through this project and will allow direct economic benefit in the UK via IP licensing. There is a real potential for UK manufacturing of the 'photon multiplier' films that can be supplied to global PV module manufacturers.

Perovskite LEDs were pioneered in Cambridge, and their rapid improvement in performance has lifted them to commercial significance. Further improvements in performance, including those delivered by this project, will accelerate their commercialisation. IP will build on the substantial base already established. In partnership with collaborators at the University of Oxford, Cambridge has founded a start-up company, Heliochrome, as a vehicle for IP in this area, and to address the initial challenges of commercialisation.