LASER-PULSE INDUCED TRANSIENT PHOTOCONDUCTIVITY OF C-70 SINGLE-CRYSTAL

The transient photoconductivity of C-70 Single crystal excited by 800 ns pulsed N-2 laser pulse was studied. The mobility of positive and ne gative free charge carriers was determined (approximate to 1 cm(2)/V s ) which is found to be similar to that determined for C-60 Single crys tal. Trapping of free carriers in C-70 was revealed to be much faster than in C-60 leaving for the lifetime of free carriers only 16 ns. Dar k charge injection was shown to occur from gold or aluminum electrodes , and excitation in these conditions resulted in delayed generation of the photocurrent within the time domain of microseconds. This phenome non was shown to be connected with triplet excitons which detrap injec ted electrons or holes just producing photoenhanced current. Triplet e xciton annihilation may follow. Studying the decay of transient photoc urrent within hundreds of microseconds time domain has permitted us to characterize the trapping sites as being 0.1 eV deep and having a siz e of about 25 Angstrom. The trapping sites are speculated to be caused by structural imperfections which cause local increase of pi-electron density and polarizability. A prominent electrical field effect (of P oole-Frenkel type) was revealed on the thermal detrapping rate that ap pears to be a rare example of the electrical field effect on the neutr al trapping sites.