SPECTRAL AND PHOTOCARRIER DYNAMICS IN THIN-FILMS OF PRISTINE AND ALKALI-DOPED C-60

This article summarizes our contribution to three distinct areas of re search on thin films of fullerenes. In the first part we present resul ts of an in-situ optical absorption study on K-doped thin films of C-6 0 from which an unambiguous assignment of certain low energy electroni c transitions in C-60 and KxC60 was deduced. The second part of the ar ticle concerns transient and steady-state photoconductivity (PC) of pr istine and oxygen-exposed thin films of C-60 examined as a function of temperature and photon energy. We find that the presence of oxygen is responsible for the creation of deep traps which reduce the steady-st ate and transient PC by several orders of magnitude, particularly in t he 1.6-2.3 eV excitation region. The thermally activated transport in an oxygen-free C-60 sample is quenched upon exposure to oxygen so that both steady-state and transient PC become nearly temperature independ ent. In the last part we present two examples of utilizing C-60 thin f ilms for application in opto-electronics. The mechanism of light-induc ed charge transfer at the conducting polymer-C,, interface is describe d and the current versus voltage rectifying characteristics ( > 10(4)) of the MEH-PPV/C-60 thin film heterojunction is presented. In additio n, a non-linear transient photovoltaic response in a Al/C-60/Au sandwi ch-type device featuring control of polarity with optical bias is desc ribed.