Metal halide perovskites are poised to lead the next generation of solar cells. The 3C project will help seed funding opportunities to understand static and dynamic disorder effects.
- Solar Power
- density functional theory
- renewable energy
The Department of Energy (DOE) has set the goal for making solar electricity cost-competitive to traditionally generated electricity by 2020, without subsidies. To achieve this, the price of solar electricity for residential rooftops needs to be lowered from the current price of 18 cents per kilowatt-hour (kWh) down to 9 cents per kWh. The current state-of-the-art solar cell materials, Si and CdTe, are limited by their high materials cost. Recently, metal halide perovskites (MHPs) have been promoted as the most promising next generation solar cell materials.[2,3] This is because MHPs can combine high efficiency (over 22 % which rivals that of conventional silicon solar cells) with low materials and manufacturing costs. This unique combination of high efficiency and low-cost enables MHP solar cells to be at 5 to 8 cents per kWh. What is also exciting about MHPs is that they can be deposited on various substrates at low-temperatures. This enables MHPs to be manufactured into lightweight and flexible solar cells that can be used in various space/aerospace applications such as satellites and drones.
Although promising, MHP solar cells have not yet been commercialized. A main challenge is the unreliability in device performance. This is largely due to lack of fundamental understanding on why MHPs result in high solar cell efficiency under certain conditions. For example, the observations so far have been that complex alloys of different MHPs perform the best as solar cells.[2,3] However, the microscopic structure-property-performance relationships in these complex MHP alloys are far from understood. The complexities involved with charactering and understanding the effects of structural disorders in these alloys have limited the research field to relying on trial-and-error approaches which are costly, slow and unreliable. Solving this challenge requires understanding of complex atomic structure dynamics and how they are related to optoelectronic properties and solar cell device performance.
To this end, a team of 3 Cavaliers consisting of two physicists and one engineer has been assembled. The team will combine complimentary techniques that can probe properties of MHPs from atomic to macroscopic scales: molecular structure and dynamics at the microscopic level, possible atomic segregation at the mesoscopic level, and solar cell efficiency at the macroscopic level. Louca (Department of Physics, CGAS) brings an expertise in neutron and x-ray scattering to characterize global and local structural distortions of the complex alloys of MHPs.[6,7] Lee (Department of Physics, CGAS) brings an expertise in neutron spectroscopy and first principles (Density Functional Theory: DFT) calculations to characterize the rotational and vibrational dynamics.[8-10] Choi (Department of Chemical Engineering, SEAS) brings an expertise in materials synthesis, optical spectroscopy, electrical transport measurements and solar cell fabrication and testing.[5,11-14]
By performing systematic neutron and x-ray scattering experiments on MHPalloys, such as alloys of formamidinium lead iodide and methylammonium lead iodide, as a function of different organic cation ratios, we will characterize the evolution of global crystal structures and local distortions as a function of the cation ratio. Inelastic neutron scattering will be performed to characterize and study how the rotational and vibrational (phonon) dynamics evolve in the alloys as the cation ratios change. The determined global crystal structure and local distortions will allow us to perform DFT calculations on the alloys to understand the evolution of the electronic band structures, vibrational dynamics and electron-phonon coupling in the alloys. The optical and electrical properties, such as photoluminescence lifetime, trap density and charge mobility, will be measured and compared with the DFT calculations. Solar cell device will be fabricated and tested to perform systematic studies of the relation between the cation ratio and device performance.
 The SunShot Initiative’s 2030 Goal. https://www.energy.gov/eere/solar/sunshot-2030.
 Brenner, T. M., Egger, D. A., Kronik, L., Hodes, G., Cahen, D. Hybrid organic—inorganic perovskites: low-cost semiconductors with intriguing charge-transport properties. Nature Reviews Materials 1, 15007 (2016).
 Stranks, S. D., Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices. Nature Nanotechnology 10, 391-402 (2015).
 Song, Z., McElvany, C. L., Phillips, A. B., Celik, I., Krantz, P. W., Watthage, S. C., Liyanage, G. K., Apul, D., Heben, M. J. A technoeconomic analysis of perovskite solar module manufacturing with low-cost materials and techniques. Energy Environmental Science 10, 1297-1305 (2017).
 Deng, X., Wilkes, G. C., Chen, A. Z., Prasad, N. S., Gupta, M. C., Choi, J. J. Room-Temperature Processing of TiOx Electron Transporting Layer for Perovskite Solar Cells. Journal of Physical Chemistry Letters 8, 3206-3210 (2017).
 Louca, D., Egami, T., Brosha, E. L., Röder, H., Bishop, A. R. Local Jahn-Teller distortion in La1−xSrxMnO3 observed by pulsed neutron diffraction, Physical Review B 56(14), R8475 (1997).
 Proffen, T., Billinge, S. J. L., Egami, T., Louca, D. Structural analysis of complex materials using the atomic pair distribution function — a practical guide. Zeitschrift für Kristallographie - Crystalline Materials 218, 132-143 (2009).
 Chen, T., Chen, W.-L., Foley, B. J., Lee, J., Ruff, J. P. C., Ko, J. Y. P., Brown, C. M., Harriger, L. W., Zhang, D., Park, C., Yoon, M., Chang, Y.-M., Choi, J. J., Lee, S.-H. Origin of long lifetime of band-edge charge carriers in organic–inorganic lead iodide perovskites. Proceedings of the National Academy of Sciences 114, 7519-7524 (2017).
 Chen, T., Foley, B. J., Ipek, B., Tyagi, M., Copley, J. R. D., Brown, C. M., Choi, J. J., Lee, S.-H. Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite. Phys. Chem. Chem. Phys. 17, 31278-31286 (2015).
 Chen, T., Foley, B. J., Park, C., Brown, C. M., Harriger, L. W., Lee, J., Ruff, J., Yoon, M., Choi, J. J., Lee, S.-H. Entropy-driven structural transition and kinetic trapping in formamidinium lead iodide perovskite. Science Advances 2, e1601650 (2016).
 Chen, A. Z., Shiu, M., Ma, J. H., Alpert, M. R., Zhang, D., Foley, B. J., Smilgies, D.-M., Lee, S.-H., Choi, J. J. Origin of vertical orientation in two-dimensional metal halide perovskites and its effect on photovoltaic performance. Nature Communications 9, 1336 (2018).
 Choi, J. J., Lim, Y.-F., Santiago-Berrios, M. B., Oh, M., Hyun, B.-R., Sun, L., Bartnik, A. C., Goedhart, A., Malliaras, G. G., Abruna, H. c. D., Wise, F. W., Hanrath, T. PbSe Nanocrystal Excitonic Solar Cells. Nano Letters 9, 3749-3755 (2009).
 Foley, B. J., Girard, J., Sorenson, B. A., Chen, A. Z., Niezgoda, J. S., Alpert, M. R., Harper, A. F., Smilgies, D.-M., Clancy, P., Saidi, W. A., Choi, J. J. Controlling nucleation, growth, and orientation of metal halide perovskite thin films with rationally selected additives. Journal of Materials Chemistry A 5, 113-123 (2017).
 Niezgoda, J. S., Foley, B. J., Chen, A. Z., Choi, J. J. Improved Charge Collection in Highly Efficient CsPbBrI2 Solar Cells with Light-Induced Dealloying. ACS Energy Letters 2, 1043-1049 (2017).
This research will lead to improved efficiency and reliability of MHP solar cells that are much needed for large scale production and commercialization. The proposed work will answer some of the key questions in this emerging research field. This is a very competitive field both in industry and academia and has the high potential for technology transfer. This interdisciplinary and large-scope topic is a good fit for an NSF MRSEC program. The 3 Cavalier funding will be used to obtain preliminary results in preparation for the next NSF MRSEC call. If the 3C funding runs out before the project is completed, funding will be ensured from startup funds.
This research will involve three graduate students, each of whom will be fully dedicated in the materials synthesis (1 student) and characterization using elastic neutron (1 student) and inelastic neutron (1 student) scattering techniques. We will also involve 2 undergraduate students for this research for solar cell fabrication and testing.