Start Date: September 2013
Michael McGehee, Department of Materials Science and Engineering, and Hemamala Karunadasa, Department of Chemistry, Stanford University
This project focuses on the design, synthesis and characterization of new families of inorganic-organic perovskites for low-cost, hybrid tandem photovoltaics. Researchers will develop new materials and device architectures with the potential to be printed at large scale atop conventional silicon or CIGS (copper indium, gallium selenide) solar cells. The ultimate goal is to improve the efficiency of these multi-junction cells to 25% or higher.
The cost of solar cells has decreased substantially over the last five years. However, for solar-generated electricity to compete with fossil-fuel power plants will require reducing costs by an additional factor of two or three. Since more than half of the cost of a solar system is associated with racking material and installation, it is important to develop higher-efficiency solar cells in order to reduce the area needed to generate electricity. Hybrid-tandem solar cells have the potential to decrease the cost-per-watt of solar cells, accelerating the worldwide adoption of solar energy and reducing greenhouse gases.
The most promising strategy for improving solar-cell efficiency well above 20% is to stack cells with varying bandgaps on top of each other so that the structure can absorb a broader portion of the solar spectrum and still use high-energy photons to generate a high voltage. This strategy has been used to set a world-record solar cell efficiency of 43%,1 but the technology is prohibitively expensive.
An even more attractive candidate for printing high bandgap solar cells has recently emerged: perovskites2-6. Two research teams have successfully replaced conventional organic dyes in dye-sensitized solar cells with organic-inorganic perovskites as the light-absorbing material. A schematic of one of the cells is shown in Figure 1.
The research team will use synthetic chemistry to assemble materials based on organo-metal-halide perovskites that can be fabricated and printed at low cost using high-throughput solution processing. The stoichiometry of the inorganic constituents in the 3D perovskites will be varied, and a layered (2D) perovskite will be developed to study the effect of the new structure on the bandgap and electrical properties of the material. The goal is to fully characterize solar cell properties, such as the exciton-binding energy, minority-carrier diffusion length and lifetime, background carrier concentration, and charge-carrier mobility. The long-term stability of perovskite solar cells will also be determined. Knowledge gained from these studies will enable a structural redesign of the devices so that they are more efficient and robust.
Once the efficiency and stability of perovskite solar cells is demonstrated, a prototype will be fabricated on top of conventional photovoltaic devices, such as crystalline and multi-crystalline silicon or CIGS, to create hybrid tandem solar cells (Figure 2). Preliminary optical calculations show that stacking a perovskite cell atop a conventional silicon cell can increase the overall efficiency from 20% to 25% or higher. This hybrid-tandem solar cell architecture leverages the success of commercial silicon solar cells and will remain commercially attractive as silicon cells continue to improve in efficiency and cost.
 Green, M. A., et al., Solar cell efficiency tables (version 39). Progress in Photovoltaics: Research and Applications 2012, 20, (1), 12-20
 Lee, M. M., et al., Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, (6107), 643-647
 Kojima, A., et al., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. Journal of the American Chemical Society 2009, 131, (17), 6050-6051
 Etgar, L., et al., Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells. Journal of the American Chemical Society 2012, 134, (42), 17396-17399
 Chung, I., et al., All-solid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485, (7399), 486-489
 Kim, H.-S., et al., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2