Geminate recombination of charge carriers in organic solids

One of the most dramatic differences in the dynamics of charge transport between high and low mobility materials is the kinetics of carrier (electron-hole or ion pair) recombination. For example, in germanium where the electron and hole mobilities are around 103 cm²/v sec, the second order recombination coefficient¹ is around 10⁻¹³ cm³/sec, while in anthracene the recombination coefficient² is about 10⁻⁶ cm³/sec. In anthracene the electrons and holes have mobilities close to 1 cm²/V sec and, therefore, the mean free path of the carriers must be on the order of a few angstroms, whether the transport is described in terms of a band model or a hopping model. In pure semiconductors, on the other hand, the carrier mean free path can be thousands of angstroms and we believe that the large difference in recombination cross section is primarily due to the large difference in mean free path. Geminate recombination (i. e., recombination of the original members of a carrier pair) is a phenomenon closely related to volume recombination, and in this paper we will present experimental data which strongly supports the contention that geminate recombination is an important process in low mobility solids; geminate recombination has never been observed in high mobility substances and the experimental indicators are conspicuously absent in at least one organic liquid (neo pentane) where the electron mobility is around 50 cm²/v sec.