Light-harvesting supra-molecular triad: a TDDFT study

First generation solar-cells, mostly based on the doped semiconductors single-junction concept, achieved laboratory efficiency not larger than 15%-20%, and a cost within 200 and 500 USD per squared meter. Many improvements, both in design and manufacturing processes, have made possible to lower the final cost below the 100 USD per squared meter limit in second generation devices. However the overall efficiency for the end-user at standard operating conditions has not improved much, and this fact has made the exploitation of sun light as an energy source not fully competitive with non-renewable sources. The problem with these classes of devices is mainly due to the intrinsic limitations of the operating mechanism for junctions in semiconductors, which impose strict limits on the freedom in band design.

An entirely new generation of cells was born when it became possible to construct nano-structured based units and assemblies. Nanometric systems greatly enhance the opportunity of tuning their electronic properties by varying the size and strength of the confining potential, and offer a variety of possible assemblies, also in combination with well-established semiconductor structures.

Some proposed designs within the class of the so called “third-generation“ cells include a dye-based component as the photo-reaction center, to be connected to one or more other units as electron donor or acceptor. We have studied the structural, electronic, and optical properties of a supra-molecular compound in this class, namely a beta-carotenoid-diaryl-porphyrin-pyrrol-fullerene triad using Time-Dependent Density-Functional Theory (TDDFT). The triad system shows longer life-time of the final charge-separated state with respect to its dyadic counterparts, and the accurate description of its properties represents a challenge both from the theoretical and the computational point of view.

Building on previous geometry-optimization work [1] we have performed an accurate calculation of the electronic structure for the ground state, and we have compared it to the structure of the separated component moieties. We have identified the relevant levels and orbitals involved in the photo-excitation process, and we have compared the calculated optical absorption spectrum with the experimentally available one. Our calculations [2] demonstrate that the main features of the TDDFT spectrum are in good agreement with the experimental data, and the analysis of the total absorption in terms of the absorption of the isolated moieties indicates that even at the TDDFT level (at least in the weak field limit) the component molecules in the triad do not appear to strongly interact. This fact indicates that the design of solar devices based on similar assemblies can be performed by individually addressing the active components, and then chemically joining them into a unit.

[1] T. Baruah and M. R. Pederson, J. Chem. Phys.125, 164706 (2006)
[2] N. Spallanzani, C.A. Rozzi, D. Varsano, T. Baruah, M. R. Pederson, F. Manghi and A. Rubio, J. Phys. Chem., in print

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