Unravelling halide segregation in hybrid perovskites for Si tandem photovoltaics

Renewable energy sources offer exciting opportunities to address challenges caused by energy security and climate change. Photovoltaic (PV) cells in particular can enable sustainable generation of electricity on a large scale: the solar energy incident on the surface of the earth in one hour is enough to provide the whole world's current yearly energy requirements. As an exciting newcomer to the PV landscape, organic-inorganic metal halide perovskites now show certified power conversion efficiencies for single-junctions thin film solar cells in excess of 22%. The best performing single-junction cells are currently all based on lead iodide perovskites with A-PbI3 formula, where the cation A is typically methylammonium (MA), formamidinium (FA), Caesium (Cs) or a mixture thereof.

Many analysts in the renewable energy sector believe that the most effective commercialisation of these novel perovskites is in combination with existing, well-established silicon technology. Here, a perovskite thin-film cell is combined with a silicon cell in a 2- or 4-terminal tandem cell, boosting efficiency at small additional cost. For optimised tandem architectures, the photocurrents created by each cell need to be balanced, which requires a perovskite with band gap near 1.75eV, significantly above the typical bandgap of ~1.5eV displayed by the established A-PbI3 materials. To date, the only high-performance perovskite thin-film materials ideally matched for tandem applications with silicon are based on the A-Pb(Br_x I_(1-x))3 system, which allows band gap tunability from ~1.5 to ~2.2eV when the bromide content is varied between x=0 (iodide only) and x=1 (bromide only).

However, the mixed halide perovskites are affected by an instability whose origin mystifies researchers. When illuminated with visible light, the material segregates spontaneously into iodide-rich and bromide-rich domains. This effect is transient, and recovers in the dark over the timescale of minutes. For photovoltaic applications, the potential voltage shifts and charge trapping associated with this effect are highly detrimental to the aim of stable PV operation. Recent research at Oxford and in the international research community has shown that materials can sometimes be stabilized through choice of A-cation and enhanced crystallinity. However, photo-stability was found to depend sensitively on processing conditions, with instability recurring when protocols or environmental conditions were varied. These incipient studies suggest that the photo-induced halide segregation is not as such intrinsic and therefore can be remedied, but a global picture of how this can be done remains elusive.

Our programme will identify the causes underlying this effect and pioneer new materials that are photo-stable over projected solar cell life spans. We will achieve these aims through a novel programme that brings together a team of world-leading investigators with complementary skills in photovoltaic materials and devices, advanced spectroscopy and high-resolution electron microscopy, and in-situ crystal structure analysis. The outcomes of this programme will enable the development of long-term photo-stable, fully optimized materials for use in tandem cells with established silicon photovoltaic technology.

This project will establish the origin of photo-instability for what is currently the only promising hybrid perovskite material system capable of delivering the 1.75eV bandgap required for fully optimized tandem cells with silicon PV. The resulting long-term photo-stable materials to be developed as a result of this project will therefore enable the disruptive combination of hybrid perovskite PV with the currently established silicon technology. This technology will deliver additional efficiency gains without the need to challenge the current major incumbent (silicon PV), and is compatible with existing infrastructure and processes already used in that industry. Therefore, any outcomes from this project can be immediately transferred to the manufacturing industry and will be highly relevant for large area terrestrial solar power generation.

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 a step increase in efficiency and/or allow easy, large-scale processing from solution or low-temperature evaporation that does not require costly purification and high-energy, slow deposition processes. Very high efficiency solar cells have been realised in group-III-V materials using tandem and multi-junction architectures, however these devices are currently too expensive to produce for practical large-area deployment. Adding an additional PV layer in tandem to existing silicon technology is arguably one of the lowest hanging fruit given that the cost base for silicon is already well established and the additional perovskite layer offers a significant boost in efficiency at little additional cost.

The global PV market is currently close to $100bn pa with 90% of the market presently met by c-Si and only 10% by thin-film technologies. Production of stable, high-efficiency perovskites optimized to go in tandem with silicon hence has the potential for high impact through strong market coverage, delivering a premium product for the entire PV market. This could then access the majority of a $100bn market, and accelerate the growth of this PV market.
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 postdoctoral researcher associates 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.