In weakly or non‐luminescing molecular crystals like p‐chloranil, information concerning the exciton dynamics can be derived from injection currents when charge carriers are formed during electron transfer between excitons and an appropriate electron donor‐ or acceptor species in aqueous electrolyte. Due to the slow orientation polarization of water molecules as compared to the hopping frequency of injected charge carriers, image forces can be neglected when aqueous electrodes are applied. Consequently, at external field strengths of 10+ V/cm, steady state saturation currents and space‐charge‐free transients are observed which may yield information as to the kinetics of the electron transfer reaction and the diffusion of excitons (lifetime, diffusion length). In the same field strength region, photocurrents originating from exciton decay at a metal contact are still determined by the action of image forces and by surface recombination of charges. Secondary effects like enhanced trapping of charge carriers within the image potential barrier influence the current rise in space‐charge‐free transients.
Photocurrents in anthracene crystals originating from exciton decay a t silver and aqueous electrolyte electrodes were measured in steady state as well as in space-charge-free transients. In both experiments metal and electrolyte electrodes show markedly different behaviour at field strengths in the range lo3-lo5 V/cm. This is attributed to the influence of the image force on charge separation at metal electrodes contrary to electrolyte contacts where the image force is negligible due to the slow orientation polarization of water as compared to the hopping frequency of injected charge carriers. From the steady state current-voltage plot the surface recombination rate of holes at the silver electrode is estimated to be of the order of lo3 cmlsec.
In weakly or non‐luminescing molecular crystals like p‐chloranil, information concerning the exciton dynamics can be derived from injection currents when charge carriers are formed during electron transfer between excitons and an appropriate electron donor‐ or acceptor species in aqueous electrolyte. Due to the slow orientation polarization of water molecules as compared to the hopping frequency of injected charge carriers, image forces can be neglected when aqueous electrodes are applied. Consequently, at external field strengths of 10+ V/cm, steady state saturation currents and space‐charge‐free transients are observed which may yield information as to the kinetics of the electron transfer reaction and the diffusion of excitons (lifetime, diffusion length). In the same field strength region, photocurrents originating from exciton decay at a metal contact are still determined by the action of image forces and by surface recombination of charges. Secondary effects like enhanced trapping of charge carriers within the image potential barrier influence the current rise in space‐charge‐free transients.
Ideal space-charge-limited current transients are observed in anthracene crystals under the condition of a continuous charge injection during transient duration. This is achieved through electron transfer between ruby laser created triplet excitons and molecular oxygen in an aqueous electrolyte electrode. For thin crystals of a-perylene a thermally activated (0.1 eV) drift mobility of holes ph,Ile = (0.4 f 0.05) cm2/Vs has been measured. Unter der Bedingung einer kontinuierlichen Ladungstragerinjektion wahrend des Laufzeitsignals werden in Anthrazenkristallen ideale raumladungsbegrenzte Stromimpulse gemessen. Die kontinuierliche Ladungstrlgerinjektion wird uber den Zerfall von Triplett-Exzitonen an der Kristall-Oberflache bewirkt, die durch Rubin-Laser-Anregung erzeugt wurden. Fur a-Perylen-Kristalle wurde eine thermisch aktivierte (0,l eV) Driftbeweglichkcit von Lochern ph, lie = (0,4 f 0,05) cm2/Vs gemessen.
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