Direct Observation of Carrier Cooling in Organic Semiconductors
Edwin J. Heilweil, Paul A. Lane, Paul D. Cunningham, Joseph S. Melinger
Photocurrent in an organic solar cell is generated by a charge transfer reaction between the electron donor and acceptor. The effect of excess excitation energy and the relative contributions of electron and hole transfer are the subject of intense scrutiny. Here we directly study photoconductivity on the sub-picosecond time scale by time-resolved terahertz (TRTS) and transient absorption (TA) spectroscopies. Charge transfer from an excited electron donor generates a hot electron that localizes within a picosecond, demonstrating a direct link between excess excitation energy and photocarrier mobility. On the other hand, charge transfer after excitation of the electron acceptorgenerates a localized carrier with terahertz photoconductivity dynamics consisting of an instrument limited rise to a long-lived signal. The long-lived photoconductivity is not sensitive to the hole mobility of the electron donor, leading us to assign it to electrons (C60 anions).