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Organic-Inorganic perovskite and organic semiconductor films with improved crystal properties via reel-to-reel solution coating; application to photovoltaics and field effect transistors

Periodic Reporting for period 2 - Crystal Solar (Organic-Inorganic perovskite and organic semiconductor films with improved crystal properties via reel-to-reel solution coating; application to photovoltaics and field effect transistors)

Reporting period: 2018-01-01 to 2018-12-31

Society's reliance on fossil fuels for energy is driving climate change, resulting in rising global temperatures and associated rising sea levels, damage to natural ecosystems, and increased risks of extreme natural disasters. The most abundant alternative source of energy comes from the sun; the light irradiated from the sun carries enough power to supply the world with its current energy needs many times over. While photovoltaic panels made of silicon are being rapidly adopted on both rooftops and in utility scale solar parks, there are applications where the rigidity, weight, and relatively low efficiency of existing silicon solar panels bar their use. In addition, factories that produce silicon solar cells are extremely expensive, limiting the ability to rapidly ramp up silicon solar cell production to meet the growing clean energy needs of the world economy.

This project develops alternative materials and devices based on them, to enable efficient, low cost, rapidly scalable, and lightweight solar panels. The project focuses on a relatively new class of photovoltaic semiconductors; metal halide perovskites. These benefit from an unusual array of properties: they exhibit optoelectronic properties rivaling those of carefully manufactured single crystal semiconductors even when made through rapid printing techniques from inexpensive precursors. The materials absorb light strongly, so only very thin layers (less than 1 micrometer) are necessary to absorb all of the incoming sunlight which in turn allows them to be used in very light and flexible solar cells. Finally, their bandgap can be tuned by simple tuning of the material's chemical composition. This allows for the development of solar cells consisting of multiple semiconductor layers with complimentary bandgaps, which can reach theoretically higher efficiencies than those made from just one active semiconductor layers. Effectively, this allows us to stack multiple solar cells on top of each other, which absorb complimentary parts of the solar spectrum in a way that raises the overall efficiency of the final solar cells. A challenge of metal halide perovskites is their susceptibility to degradation when exposed to humidity or oxygen.

This project aims to develop metal halide perovskite materials and methods of depositing them to make very efficient solar cells. Specifically, methods for tuning the bandgap are a subject of intense research, as are methods of deposition, which should allow the technology to be scaled and approach commercial readiness. This is accomplished by tuning the chemical composition, altering the bandgap by both directly influencing the energetics of the metal and halide orbitals and also indirectly by introducing structural distortions which change the metal-halide orbital overlap. Understanding of the structural properties can also be used to understand degradation mechanisms in these materials. The goal is to use newly developed materials with new bandgaps and deposition methods to make efficient tandem solar cells made of two perovskite absorber layers. specifically, focus is also on understanding mechanisms relating the chemical and structural composition of the materials to bandgap and stability.
The early stages of the project were focused on developing novel chemical compositions of metal halide perovskites to open up a new range in bandgap critical to making all-perovskite tandem solar cells. Specifically, small (<1.3 eV) bandgaps were targeted as no suitable perovskite materials had been previously demonstrated. We found that changing the chemical composition by including some Sn on the B site of the ABX3 crystal structure lowered the bandgap from 1.5 to 1.2eV. In addition, changing the lattice parameters by using the small Cs cation on the A site rather than the large formamidinium cation also allows tuning of the bandgap via a structural distortions. This was a new finding, and has allowed the community to gain an improved understanding on the role of structural distortions on electronic properties. These materials were characterized and their structural properties thoroughly analyzed. A new framework for understanding structural effects on the bandgaps of metal halide perovskite semiconductors was deduced. In addition, a new framework for understanding the mechanism of oxidation in Sn containing metal halide perovskite semiconductors has been developed. This has enabled us to overcome the largest shortcoming of the initially prepared small bandgap, Sn containing, perosvkite materials. We incorporated these new materials into a perovskite solar cell stack to demonstrate the first prototype of such a device. This required us to develop a new device architecture, to enable the deposition and coating of perovskite layers on top of each other.

The work so far has resulted in 9 published articles, one of them in Science and another in Nature Energy.
We demonstrated the first efficient solar cells based on small bandgap perovskites and used them to make the first perovskite multijunction solar cells. We have developed novel understandings of bandgap tuning and chemical stability in mixed metal perovskite semiconductors far exceeding the state of the art understanding before our work. These breakthroughs have the potential to deliver the highest performances of any solar cells of any technology at the lowest costs. Continued work will allow us to achieve solar cell efficiencies beyond those of single junction perovskite solar cells and beyond those of conventional solar cell technologies. We demonstrated the potential of our breakthroughs through thorough optical and electrical modeling using experimentally derived parameters; this work has been published and gives a clear roadmap for achieving >30% power conversion efficiencies.

Other than continuing to improve the performance of the multijunction solar cells, scale up of the technology is being attempted and will constitute a large fraction of the remaining work. Our initial attempts at coating these new perovskite materials using scalable coating techniques has been successful. Scaling full tandem solar cells will be a focus of coming work.
Dr. Tomas Leijtens and Prof. Michael McGehee study a perovskite solar cell