Microscopic picture of charge generation in conjugated polymers

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Semiconducting conjugated polymers are an emerging class of materials that bear great promise for various applications in optoelectronics, photonics, and photovoltaics. In the present project, we will study charge generation in conjugated polymers from a microscopic theoretical perspective. This includes both studies of (i) charge generation through molecular doping, important for optoelectronic applications, and (ii) charge photo-generation after optical excitation, the process at the heart of organic photovoltaics. Our study will be focussed on technologically relevant co-polymers with built-in donor-acceptor structure. The benefits of these co-polymers lie (among other things) in an increased charge separation efficiency. On the other hand, it was recently found that the doping efficiency in these more complex systems is affected by geometrical disorder in the random positioning of the small molecules used for doping relative to the conjugated chains. In the first part of the present project, we will revisit the different electronic mechanisms such as charge-transfer complex formation and integer electron transfer contributing to doping of conjugated polymers. We will develop a detailed microscopic understanding of the physical/chemical processes at work which in the long run will allow us to develop strategies to achieve more efficient doping. A possible route that will be explored in the present project is the use of dopant molecules that are large enough to interpolate between donor and acceptor entities of the conjugated chains. The second part of the present project will be concerned with charge generation through dissociation of the primary photo-induced excitations (excitons) in conjugated chains into separated or loosely bound charge carriers. Based on ab-initio calculations of the non-adiabatic molecular dynamics in the excited-state manifold after photo-exitation, we will directly analyze the influence of excess excitation energy on dissociation processes and explicitly study the role of intra- and inter-molecular relaxation channels. An important ingredient to the present project also is our work together with our experimental partners. Optical spectroscopy of electronic excitations and vibrational modes will reveal characteristic fingerprints of the relevant electronic mechanisms and will be directly comparable to the spectral properties obtained theoretically. Based on the underlying fundamental physical processes that will be studied in the present project, general conclusions will be drawn that are applicable to a large class of conjugated materials and will contribute to improving performance potential of these materials in the future.