Long-Range Charge and Energy Transfer at Heterojunctions for Photovoltaics Beyond the Shockley-Queisser Limit

The development of high-efficiency low-cost renewable energy sources is one of the most pressing research challenges today. Two promising technologies in this area are photovoltaics (PV) and Solar Fuel generation systems. PV work by absorbing sunlight to generate electrical charges that are then collected in an external circuit. Solar Fuel systems work by absorbing sunlight and then using the charges produced to drive redox chemistry to produce chemical fuels from readily available starting materials, for example splitting water to produce H2, which is a powerful fuel.

But the cost to efficiency ratio of both these technologies is too high currently. In order to drive the price of these technologies down to match fossil fuels, fundamental breakthroughs are required in the way these systems harness solar energy. This project seeks to tackle this challenge by building on recent insights into quantum mechanical processes in organic semiconductors to improve the efficiency both of current and future PV systems as well as put in place new design ruled for high-efficiency solar fuel generation systems.

At the heart of many kinds of PV and Solar Fuel systems are interfaces between organic and inorganic semiconductors. The role of these interfaces, known as heterojunctions, is to separate opposite charges, hole and electrons, from each other and prevent their recombination. We will use the latest breakthroughs in ultrafast laser spectroscopy to study these interfaces and develop novel structure that efficiently separate charges.

The biggest energy loss in PV is a process known as thermalization. This refers to the fact that the absorption of a high-energy photon generates one electron-hole pair just as the absorption of a low-energy photon does. The extra energy of high-energy photons above the bandgap is lost as heat. This problem affects all commercially deployed PV today and has long been considered a fundamental loss. Indeed it leads to what is known as the Shockley-Queisser limit on efficiency, which is 33% for an idea PV of bandgap 1.1eV. Here we will use a unique quantum mechanical process in organic semiconductors called Singlet Exciton Fission, to overcome this loss. Singlet Fission allows two electron-hole pairs to be generated in certain organic materials when a photon is absorbed. We will design new ways by which these electron-hole pairs can be harvested at the organic/inorganic interface, leading to improved efficiencies. The methods and structures we will develop using this process would be compatible both with current and future PV technologies, allowing them to over come the Shockley-Queisser limit on efficiency. This could dramatically improve the efficiency of PV and help bring about their wide scale deployment.

The development of new high-efficiency photovoltaics (PV) and photocatalysis technologies is crucial to long term environmental sustainability, allowing for decarbonisation of the global economy. This will in future enable renewable energy to meet the price point of conventional fossil fuels, which is required for large-scale adoption of these technologies.

Key to this effort is the development of new technologies that can surpass existing paradigms of performance. This project seeks to establish such technologies by harnessing the quantum mechanical properties of organic semiconductors to create a new generation of organic/inorganic heterojunctions. The results are likely to find broad application in the areas of PV and photocatalysis, as well as opening new avenues in spintronics and quantum technologies.

The UK has no manufacturing base in PV today. On the other hand, the UK is a leading player in the organic and printed electronics area. This project could allow this strength to be capitalised on, via the production of solution processable thin organic down-converters that would be compatible with both current and future PV technologies, allowing the UK to gain a share of the PV market in the near to medium term. This technology could allow all single junction PV cells to better harness solar energy and approach the Shockley-Queisser limit. Moreover, the technology would be compatible with materials and manufacturing processes for current PV technologies, which could enable easy adoption. Thus the project is likely to be of wide interest to both the PV and printed electronics industries. Technologies resulting from the project will be taken forward via collaboration with industrial partners and engagement with the wider PV industry through forums such as the EPSRC Supergen SuperSolar Hub.