Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Crystal Tandem Solar (Single-Crystal Perovskite Tandem Solar Cells For High Efficiency and Low Cost)

Teaser

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...

Summary

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.

Work performed

The project has progressed well. The initial plan was to focus on perovskite-silicon tandem structures, but a shift in focus was made to take advantage of the opportunity for achieving the overall project objectives via an alternative and more efficient route. The researcher was part of a collaboration that made a relevant breakthrough just before the start of this project. This breakthrough, published in Science, was realizing all-perovskite tandem solar cells, by discovery and engineering of a perovskite material having a low band gap that was suitable for the low gap cell in a tandem. Based on our initial demonstration and success with this architecture, the low gap perovskite was deemed to be a more promising material than Silicon for use in a tandem, as it enables full solution processing, lower costs, and the use of flexible, lightweight substrates while theoretically having the same efficiency limits as Silicon. Therefore, the goal of this project was switched to making highly efficient all-perovskite tandems, rather than perovskite-on-silicon.
Towards engineering large-grain perovskite films, the researcher was able to fabricate perovskite nanocrystals with varying shape and size, including large, mm-scale, sheet-like nanocrystals. Tests were carried out on applying the same technique to low bandgap perovskite nanocrystals in order to move towards growth of large crystals of these, which had never been reported. The researcher found that while direct growth of low bandgap nanocrystals and single crystals was not easily attainable, a novel cation exchange process could be used to transform the wide gap nanocrystals into the low bandgap nanocrystals, providing a facile route to morphology-independent composition control.
The researcher characterized the nanocrystals fabricated using ultrafast time-resolved photoluminescence spectroscopy, and made the important observation that Auger recombination rates were different in perovskite nanocrystals incorporating different cations. This will help guide compositional engineering to attain the best photovoltaic materials in the future.
The researcher developed a process for effectively turning multi-crystalline films into something more closely approaching single crystal films. This involved a treatment that effectively melted the crystal grains together and re-crystallized them in larger crystalline grains. This allowed elimination of grain boundaries throughout the film, which enabled much higher performance of photovoltaic devices when characterized. Based on this breakthrough, all-perovskite tandem solar cells were fabricated, a new efficiency record for all-perovskite tandems (19.4%) was attained.

Final results

The attainment of a new efficiency record for all-perovskite tandems is a significant milestone that leads the way towards the project having a real-world impact. This is still far from the efficiency limit for these structures and suggests that they will indeed be able to surpass efficiencies for Silicon solar panels, especially when advances such as the knowledge gained on controlling crystal growth are incorporated fully into the devices. This will provide a low-cost, flexible, lightweight and high efficiency solar technology that can easily be produced at high volume worldwide and will provide clean power to help mitigate fossil fuel-driven climate change.
The researcher will continue to work on this tandem technology beyond the end of the project; he has co-founded a startup company, Swift Solar, (www.swiftsolar.com) that aims to commercialize the latest iterations of the perovskite tandem technology and drive it to having a real world impact.