Periodic Reporting for period 1 - Crystal Tandem Solar (Single-Crystal Perovskite Tandem Solar Cells For High Efficiency and Low Cost)
Berichtszeitraum: 2017-01-01 bis 2018-12-31
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.
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.
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.