Electron bombardment technique for the study of electrical transport properties in organic semiconductors

Abstract

This paper describes in some detail the electron bombardment technique used for the study of electrical transport properties in organic semiconductors. A platelet specimen is fitted with electrodes on opposite sides and charge carriers are generated near the top electrode by a short bombarding pulse. Both light and electron pulses have been used for irradiating the crystal specimen and the particular advantages of electron beam excitation are discussed. A steady or pulsed applied field draws one type of carriers across the specimen and the transit time ‘T’ is determined by charge integration. This leads directly to the drift mobility. However, in the case ofp-n junction solar cells, the samples were bombarded with electron beam under reverse-biased conditions and the lifetime of minority charge carriers was determined from the decay of the concentration of excess minority charge carriers. Lifetime of minority charge carriers in silicon solar cells was found to be ∼25 μs, whereas in silicon specimens the lifetime of charge carriers was ∼8 μs. Measurement of carrier lifetimes, drift mobilities and carrier yields in organic semiconductors with the electron bombardment technique provides valuable information regarding the generation and recombination mechanisms, and the evaluation of the perfection of organic semiconductors which are to be used later for the fabrication of a variety of devices. Details of a space charge avoidance technique are given.