48 research outputs found
Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites
Solar cells based on the organic-inorganic tri-halide perovskite family of
materials have shown remarkable progress recently, offering the prospect of
low-cost solar energy from devices that are very simple to process. Fundamental
to understanding the operation of these devices is the exciton binding energy,
which has proved both difficult to measure directly and controversial. We
demonstrate that by using very high magnetic fields it is possible to make an
accurate and direct spectroscopic measurement of the exciton binding energy,
which we find to be only 16 meV at low temperatures, over three times smaller
than has been previously assumed. In the room temperature phase we show that
the binding energy falls to even smaller values of only a few
millielectronvolts, which explains their excellent device performance due to
spontaneous free carrier generation following light absorption. Additionally,
we determine the excitonic reduced effective mass to be 0.104me (where me is
the electron mass), significantly smaller than previously estimated
experimentally but in good agreement with recent calculations. Our work
provides crucial information about the photophysics of these materials, which
will in turn allow improved optoelectronic device operation and better
understanding of their electronic properties
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