The search for renewable sources of electric power has become very intense in recent years, as the world demand for power grows, and traditional sources of fossil fuels deplete or become difficult to reach. Therefore, producing electrical power directly from the light of the Sun becomes increasingly attractive. The central challenge for solar energy production is a design of efficient, inexpensive and environmentally compatible solar cells. While the design of traditional Si-based cells seems to be reaching its maximal efficiency at a relatively high cost production, great promise lies in alternative, novel absorber materials, such as organic solar cells. Being cost-effective and environmentally friendly, organic solar cells possess favourable mechanical properties relevant in practical applications.
From the physical point of view, a key feature that distinguishes organic solar cells from traditional ones is the fact that while in a traditional cell absorption of a photon immediately creates free charge carriers (an electron and a hole), in an organic solar cell the electron and the hole remain bound to each other, creating an exciton. To obtain electrical current, the exciton must first dissociate into a free electron and hole. The ultimate aim is to find materials where excitons split promptly and where the charge carriers rapidly travel to the electric contacts, to prevent their recombination, so they would not be lost for conduction.
Nowadays, computational materials science is capable to accurately characterize a variety of complex, heterogeneous materials, discover new materials and predict their properties before they are even synthesized in the lab. However, a full theoretical and computational description of excitons and their dynamics in organic photovoltaic mateirals is still missing.
Time-dependent density functional theory (TDDFT) is a theoretical framework suitable for the task. It is a rigorous reformulation of time-dependent quantum mechanics, which can fully describe the dynamics of a many-electron system by focusing on its time-dependent density. Particularly, it can describe how the electrons in the system intetacts with electromagnetic radiation -- most relevant to excitons creation. Development of the appropriate methodology for exciton detection and characterization in TDDFT and its application to photovoltaic materials is a subject on which our group plans to focus efforts in the near future.
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