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Matteo Cococcioni: Computational design and optimization of phosphorescent sensitizers for efficient, low-cost organic solar cells

The design of more efficient phosphor sensitizers able to couple with broadly absorb- ing organic donors will be based on the investigation of the microscopic factors controlling electronic excitation energies, and on the screening of possible modifications to their molecular structure to im- prove their properties. The computational design of phosphor sensitizer will parallel the experimental measurement of their properties in Holmes group. The first stage of the computational modeling ac- tivity will focus on Ir(ppy)3 and will aim at reproducing the electronic excitation energies measured by Holmes to set calculations to a proper level of accuracy. This study involves comparing the ground state of the system with its excited ones both in the singlet and triplet configurations, characterizing the redistribution of electrons in the excited states and analyzing different possible factors (as, e.g., the crystal field splitting of the Ir d levels) that are responsible for the electronic spectrum of the molecule.

Preliminary calculations on this system show that the most dramatic changes in the electronic structure of the complex upon excitation involve the Ir center and the bonds with atoms in its first coordination shell. Next step will thus assess the role of the ligand groups (structure, symmetry and chemical composition) in determining the energy splitting between Ir electronic states. Figure 2: Electronic charge- density difference between the singlet excited state and the ground state of Ir(ppy)3.

Electronic charge- density difference between the singlet excited state and the ground state of Ir(ppy)3.

Electronic charge- density difference between the singlet excited state and the ground state of Ir(ppy)3.

Important insight will be gained comparing the results obtained with different Ir-based phosphor sensitizers (as, e.g., commercially available iridium(III)bis(2-phenylbenzothiozolato-N,C2') (Bt2Ir(acac))) and, possi- bly, from analogous complexes with different metal centers. Based on the knowledge we mature on the electronic structure of the above-mentioned systems, possible modifications to their molecular structure will be at- tempted to lower the singlet-triplet splitting and characteristic excitation energies towards the red region so that phosphor triplet excited states can be “nested” in those of most typical fluorescent host materials, result “ex- citable” by visible radiation, and be able to efficiently revert to the donor host. In the long term, the study will be also extended to the coupling between donor and phosphor molecules and, in particular, to the electron transfer process between the two systems both in the singlet and triplet states. This step, in fact, controls not only the conversion efficiency of the singlet exciton into a triplet one, but also the way phosphor complexes respond (e.g., how their structure relaxes) to the exciton and its optimization is crucial to improve cell efficiency.

The computational study of candidate phosphor sensitizers in the donor layer will be carried out with fully quantistic techniques based on Density Functional Theory (DFT). Corrective schemes to DFT functionals will be necessary to capture the “strongly correlated” behavior of valence electrons in localized d states. In particular, we will use the so-called LDA+U+V approach that is designed to describe correlated electrons on hybridized (e.g., molecular) states with minimal computational cost. Spin-orbit interactions will be implemented in the corrective functional to allow for the accurate description of singlet-triplet transitions. The precise calculation of excited electronic levels will also take advantage from a Koopmans-corrected DFT functional3 that significantly improves the agreement between the computed spectrum and photo-emission experiments. The relatively low computational cost of these approaches (compared to more sophisticated, e.g., quantum chemistry methods) will be crucial for the efficient screening of candidate phosphor complexes.

Funded by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013

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