31 research outputs found
Hot photocarrier dynamics in organic solar cells
Photocurrent in an organic solar cell is generated by a charge transfer reaction between electron donors and acceptors. Charge transfer is expected to proceed from thermalized states, but this picture has been challenged by recent studies that have investigated the role of hot excitons. Here we show a direct link between excess excitation energy and photocarrier mobility. Charge transfer from excited donor molecules generates hot photocarriers with excess energy coming from the 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
Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss
The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells
A typical signature of charge extraction in disordered organic systems is
dispersive transport, which implies a distribution of charge carrier mobilities
that negatively impact on device performance. Dispersive transport has been
commonly understood to originate from a time-dependent mobility of hot charge
carriers that reduces as excess energy is lost during relaxation in the density
of states. In contrast, we show via photon energy, electric field and film
thickness independence of carrier mobilities that the dispersive photocurrent
in organic solar cells originates not from the loss of excess energy during hot
carrier thermalization, but rather from the loss of carrier density to trap
states during transport. Our results emphasize that further efforts should be
directed to minimizing the density of trap states, rather than controlling
energetic relaxation of hot carriers within the density of states