Engineering purple bacterial photovoltaic complexes for device applications

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This project is concerned with the first attempts to convert a protein that powers biological photosynthesis into a material that can be used at the heart of an efficient solar cell. The project comes at a time when there is growing interest in alternative technologies for providing mankind with increasing amounts of energy in ways that will not exacerbate environment change. Mankind is facing an energy crisis. It is estimated that our global energy demand will double by the middle of the 21st century, and could more than treble by 2100. For well-understood reasons increased burning of fossil fuels will not be able to meet this soaring energy demand, and although many of the alternative energy sources currently being considered may address need in the short term, they suffer from limited capacity, uneven geographical distribution, adverse environmental impact and political/security problems. The only source of alternative energy freely available across the whole planet is solar energy. Biological photosynthesis consumes around 250 terawatts of the sun's output annually, dwarfing the current 16 terawatt energy demand of the global population, but still only a tiny fraction of the 120,000 terawatts of solar energy incident on the Earth every year. Humans have always been reliant on the products of photosynthesis for the bulk of their energy, in a variety of ways, but there is now increasing interest in clean technological approaches to harvesting this free and largely benign source of energy. Photovoltaics, the direct generation of electric current from light energy, has become a familiar concept with the public through the advent of silicon-based solar panels. However this technology has its limitations, and there is a pressing need to develop alternative photovoltaic materials that are lighter, cheaper to produce, less reliant on rare elements, and are more efficient under non-optimal conditions. It is likely that in the future solar energy will be harvested through a wide range of technologies, each suited to particular conditions. One approach that has seen a rapid growth in interest over the last ten years is the use of natural photovoltaic proteins as a photovoltaic material. Plants, algae and bacteria exploit light energy through nanometer-sized proteins called reaction centres. These proteins capture light energy and convert it into a form that the organism can use, and in the key event light energy is used to move an electron along the length of the reaction centre, leaving a positive charge at one end and a negative charge at the other. Just as with the terminals of a battery, or a silicon solar panel, this difference in charge can be used as a source of power. It is already known that if reaction centres are carefully removed from a plant or bacterium, and attached in a particular way to an electrode and illuminated, an electric current can be generated. The next stage in the evolution of this technology is to adapt natural reaction centres as more effective photovoltaic materials by altering their structures and various aspects of how they deal with light energy and electrons. This project is a first step in this direction, seeking to convert the reaction centre from a photosynthetic bacterium into a material that will produce larger photocurrents and is better suited to operation inside a solar cell rather than inside a bacterium. The attractive aspect of exploiting natural reaction centres in this way is that they convert harvested light energy into an electrical potential difference with an efficiency that is close to 100 %, they can be synthesised in large quantities through natural processes, and they have capacity for self-assembly and self-protection/repair. The longer term goal of the research is to develop new materials for harvesting solar energy, either through direct adaptation of natural proteins, or by providing new insights that will inspire new generations of photovoltaic materials.