| 02 July 2012
A multidisciplinary team of physicists, chemists and mathematicians at the Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts (US), and a computer scientist at the Universidad Autónoma de Madrid in Spain has found a new mathematical approach to simulating the electronic behavior of noncrystalline materials that lack an orderly crystal structure, which could expedite the search for better solar cell materials.
Typically, researchers base their predictions about the behavior of a candidate material on calculations using the energies of electrons and the interactions between electrons. In a “disordered” material, these values, which normally arise from the way molecules are arranged, are unknowns. “The lack of perfect regularity greatly complicates the theoretical modeling of the electronic structure of these materials,” says Jiahao Chen, a postdoctoral associate in MIT’s Department of Chemistry and lead author of the paper, "Error analysis of free probability approximations to the density of states of disordered systems". “In other words, it is difficult to predict characteristics such as the absorption spectrum, absorption efficiency and transport rates when the material is not crystalline, but rather all jumbled up on the molecular scale.” His electronic structure models take this disorder in the spatial arrangement of molecules into account, giving researchers a pretty good idea just how disordered the molecular structure of a material is.
“Our current work represents the first steps towards understanding how disorder affects the device characteristics of solar cells,” says Dr. Chen. “We hope that this will ultimately lead to better theoretical models of things like absorption efficiencies and transport rates in solar devices, which will facilitate further R&D in prototyping new devices that can exploit the unique physics enabled by the presence of disorder.”
Dr. Chen explains that in the quest for new solar materials, Eigenvalues of Hamiltonian matrices give researchers information like the density of states and absorption spectra of these materials. “In this paper, we explore how accurately we can approximate these eigenvalues using free probability theory.” This new method in principle would allow researchers to greatly speed up the calculation of device characteristics by bypassing the eigenvalue calculations. “We have found that while the answers are no longer exactly correct, they can nevertheless be very accurate. Plotting these results on top of the exact answer, it is often difficult to tell the difference to the naked eye regardless of how strong or weak the disorder is in the model,” the MIT scientist says. “To our knowledge, this is the first application of free probability theory to study organic semiconductors or any other material used in solar energy devices.”
Now the team wants to develop more sophisticated models that can lead to even higher accuracies. Ultimately, MIT’s new method could lead to reducing the cost of computational modeling of next-generation solar materials, thus help catalyze future solar technologies.
Written by Sandra Henderson, Research Editor, Solar Novus Today
Image: wikimedia commons/Asad856
