Exciton and Charge Dynamics in Molecular Semiconductors

Photon to charge conversion is a key process underlying both photosynthesis and solar cells. In organic light-harvesting systems photon absorption leads to the formation of a tightly bound electron-hole pair, the exciton. The elegant photosynthetic apparatus evolved by nature employs cascading charge and energy transfer steps to dissociate the exciton and achieve long-range charge separation. In contrast, a new class of organic photovoltaic cells (OPVs), based on molecular semiconductors, use only a single heterojunction to dissociate excitons and achieve charge separation. How the electron and hole overcome their mutual attraction in these systems remains an unanswered question. In this talk I will highlight some of the key aspects of the dynamics of excitons and charges in molecular semiconductors, with a specific focus on their application in photovoltaics. Ultrafast spectroscopic techniques are used to elucidate the key photophysical processes within these systems and demonstrate that charge separation is facilitated by quantum mechanical charge delocalisation rather than energy-gradient–driven intermolecular hopping1. I will also briefly discuss the processes of exciton fission2, wherein a spin-singlet exciton splits to yield two spin-triplet excitons. This processes allows for the extraction of two electrons for each photon absorbed and could enable solar cells to overcome the Shockley–Queisser limit on power conversion efficiency.  

1	Bakulin, A. A. et al. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science, 1217745 (2012).
2	Rao, A. et al. Exciton Fission and Charge Generation via Triplet Excitons in Pentacene/C60 Bilayers. J. Am. Chem. Soc. 132, 12698-12703, (2010).