Hot Carrier Injection into Molecular Crystals and Its Relevance to the Field Dependence of Photocurrents

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Abstract
Extrinsic photocurrents in anthracene and p‐chloranil crystals resulting from either electron photoemission from a metal contact or sensitized electron injection from excited adsorbed tetracene molecules are investigated. At field strengths, where space charge effects are unimportant, i.e. where currents usually are assumed to be saturated, their field and temperature behaviour can be quantitatively described assuming that both generation processes initially yield hot electrons bound to the positive countercharge by Coulombic attraction. Direct escape, determined by scattering in the Coulombic potential well, is a temperature independent process giving rise to an exponentially field dependent photocurrent component. The scattering mean free path in anthracene is found to be (11 ± 2) Å. Thermalized carriers can either undergo geminate recombination or can overcome the Coulombic potential barrier by thermally activated diffusion.