Abstract
The transient photoconductivity of C 70 single crystal excited by 800 ns pulsed N 2 laser pulse was studied. The mobility of positive and negative free charge carriers was determined (≈1cm 2/Vs) which is found to be similar to that determined for C 60 single crystal. 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. Dark 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 phenomenon was shown to be connected with triplet excitons which detrap injected electrons or holes just producing photoenhanced current. Triplet exciton annihilation may follow. Studying the decay of transient photocurrent within hundreds of microseconds time domain has permitted us to characterize the trapping sites as being 0.1 eV deep and having a size of about 25 Å. The trapping sites are speculated to be caused by structural imperfections which cause local increase of π-electron density and polarizability. A prominent electrical field effect (of Poole-Frenkel type) was revealed on the thermal detrapping rate that appears to be a rare example of the electrical field effect on the neutral trapping sites.
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