Given the increasing energy demands of the world and the threat of carbon-dioxide driven global warming, it is increasingly apparent that there is an urgent need for an energy source that is abundant, doesn’t produce carbon dioxide, and can supply a large fraction of the world’s demands. A promising option for this is solar photovoltaics (PVs), devices converting photons directly to usable electrical power. Current state of the art crystalline silicon PV modules have power conversion efficiencies (PCEs) of above 20% and cost around 0.3-0.5 $/W. These modules are generally rated to operate for 25 years, with payback time for domestic use generally 5-10 years. This long-term financial consideration and initial expense limits uptake, meaning that in order to install PVs rapidly enough to limit the impending energy crisis, a new approach is needed. Panels must have a better power to cost ratio to reduce payback time.
Solution-processed PVs have recently attracted significant interest for their potential to offer lower-cost processing, with organic photovoltaics, dye-sensitized solar cells and quantum dot solar cells showing promise in this area. However, with PCEs of 10-12%, they are still not cost-competitive with c-Si. More recently, hybrid organic-inorganic perovskites have attracted a great deal of interest. These materials are named for their ABX3 crystal structure, where A=Cs+, CH3NH3+, H2NCHNH3+, B=Pb2+, Sn2+, X=Cl-, Br-, I-. These materials are cheap, earth-abundant, solution-processable semiconductors and ideal for incorporation into photovoltaics. Their high material quality and versatility has enabled a meteoric rise in their efficiency, making them the fastest developing photovoltaic technology yet and a prime candidate for a low-cost, high efficiency photovoltaic technology. In less than four years of intensive research, lab-scale device efficiencies are reaching above 20% PCE (the maximum theoretical PCE from these devices is 28%), and rough estimates indicate that they could generate power at ~0.2$/W. However, even this is not sufficiently superior to the low costs of Si to warrant the expenses of scaling up fabrication. Possibly, the most promising incarnation of the perovskite solar cell is as a ‘tandem’ device, employing two materials absorbing different parts of the solar spectrum to achieve even higher efficiencies while keeping costs low. Theoretical predictions show that such tandem devices could achieve up to 36% PCE, making them ultimately more promising than the single-junction perovskite devices.
Tandem perovskite devices so far have been limited by the quality of the perovskite films – they contain lots of small crystal grains, and the grain boundaries between these are thought to be detrimental to charge transport, limiting performance. This project aims to produce high efficiency, low cost tandem perovskite devices by fabricating and characterizing single-crystal thin films of perovskites and stacking them in tandem architectures. This will be done by using solution chemistry known from nanocrystal research to control crystal growth and will eliminate the problem of grain boundaries within the devices, allowing very high performance devices that can be fabricated at low costs, providing a potential solution to help mitigate the impact of climate change.
