Substantial manufacturing-cost reductions in mainstream silicon-wafer (c-Si) based solar cell technologies have recently been achieved mainly due to savings through economy of scale. Hence a recent forecast for the future large-scale use of photovoltaics predicts that solar energy will contribute nearly a third of newly-installed electricity generation capacity worldwide between now and 2030. To reach this goal however and to assure a widespread deployment of PV, the cost for PV-generated energy still needs to be further reduced. A large fraction of the cost of solar power is not the modules themselves, but the fixed costs of frames, inverters, installation and land, which is termed the balance of systems (BOS). The BOS is not reducing in price as fast as the module costs, hence the only sure means to continue the downward drive in the cost of PV is to enhance the absolute efficiency of the modules, without overtly increasing their cost. The key aim of this project is to realise highly efficient hybrid tandem solar cells with high stability. The specific target is to achieve a power conversion efficiency of over 25% when integrating a wide band gap perovskite solar cell with a crystalline silicon solar cell. A solar cell is composed of a light absorbing photoactive material as the main component which generates electrical current. But this layer is contacted by multiple further materials to ensure efficient charge extraction and high voltage generation in the solar cell. Our philosophy is to undertake an extremely focussed project, employ as many existing proven materials as possible, apart from the perovskite absorber layer, and integrate them judiciously within the perovskite-Silicon tandem solar cells. This will minimise the risk, and maximise the possibility of delivering an entirely stable tandem solar cell. In the process of doing so, and throughout the investigations, we will create highly efficient bifacial perovskite solar cells (which can receive light illumination from both sides) and enhance our understanding of the fundamental mechanisms occurring at the junctions between the perovskite and the charge collection layers. The project is extremely timely, since the perovskite solar cells are already at the appropriate efficiency to enhance existing PV in a tandem configuration, provided effective integration into a tandem structure can be achieved. In addition, much progress on the overall stability of the perovskite solar cells and large area processing has already been achieved, making it highly likely that the output of this project will be transferred directly into a commercial product.

Potential Impact:
The primary purpose of this project is to realise high efficiency perovskite hybrid tandem solar cells with efficiency exceeding that of single junction silicon PV, the current market leader. The market opportunity for advanced functional materials in energy related markets alone is predicted to be over $113Bn by 2018, the current OLED market is already over $12Bn and the global PV market is around $100Bn. The impact this project will have is to develop a new technology which in the first instance can capture a significant share of the PV market. The developments in perovskite materials and devices also have the prospect to spill over into other markets, both envisaged currently, and non-envisaged, by delivering enhanced operation and functionality of the materials and the electronic interfaces.

There is overwhelming evidence that our increasing consumption of fossil fuels and the associated emission of carbon dioxide is leading to climate change. This has brought new urgency to the development of clean, renewable sources of energy, and to reduction of our energy consumption by developing new low energy consumption devices to satisfy the growing demand. Photovoltaic devices that harvest the energy provided by the sun have great potential to contribute to the solution, but uptake of photovoltaic energy generation has been weakened by the cost of devices based on current technology. Although silicon PV continues to steadily drop in price, the key to creating a step reducing cost is the development of new photovoltaic materials offering either a step increase in efficiency or allowing easy, large-scale processing from solution or low-temperature evaporation that does not require costly purification and high-energy, slow deposition processes. The route we are undertaking here is to increase the absolute efficiency of terrestrial PV by combining the best of emerging PV technology with the most successful commercialised technology to date, i.e perovskite/Si tandem solar cells. This is not the "end goal" for ultimately low cost PV, but is a practical solution which could offer real and achievable near term advantages over incumbent technologies. This project will catapult the concept into a high level of technology readiness, and will subsequently fuel and enhance the prospects for the growing PV industry

Beyond commercial, economic, environmental and societal impact, the activities within this project will aid in the training and education of both scientists and the general public. The training of PDRAs and PhD students in this industrially relevant area will create an employment pool for jobs in research, R&D, energy sectors and other economic areas, and carry the knowledge and skills they acquire into those fields. Public outreach events, such as hands on experimental activities at schools, and lectures to the general public and professional societies, will be enhanced by the excitement of rapidly advancing research and technology.