Title: Some Like It Hot: Photocarrier Dynamics in Organic Solar Cells

Abstract: Organic semiconductors are attractive materials for optoelectronic devices such as light-emitting diodes and solar cells. For all their advantages, there is a fundamental problem with making efficient organic solar cells. Light absorption by these materials creates neutral, bound states (akin to excitons in conventional semiconductors) rather than free electron-hole pairs. This problem has been overcome by initiating a charge transfer reaction between two materials, electron donors and acceptors. The resulting photoexcitation dynamics are complicated and fascinating. We are particularly interested in the role of excess energy in charge transfer and carrier mobility. We combined ultrafast terahertz photoconductivity with transient absorption to disentangle carrier mobility and excitation dynamics over a timescale from picoseconds to milliseconds. Charge transfer from excited donor molecules generates hot photocarriers with excess energy arising from the energetic offset between the lowest unoccupied molecular orbital of the donor and that of the acceptor. Hot photocarriers manifest themselves through a short-lived spike in terahertz photoconductivity that decays on a picosecond timescale as carriers thermalize. Different dynamics are observed when exciting the acceptor at its absorption edge to a thermalized state. Charge transfer in this case generates thermalized carriers described by terahertz photoconductivity dynamics consisting of an instrument-limited rise to a long-lived signal.