Dissociation of Frenkel Excitons at the Interface between a Molecular Crystal and a Metal

Abstract

The first part of the paper deals with the non-radiative decay modes of n x* triplet excitons a t the interface between a p-chloranil crystal and an aluminum contact studied by means of extrinsic photocurrents. Utilizing the technique of inserting fatty acid monolayers between crystal and metal, which allows variation of the interaction distance, it is shown (i) that the dominant exciton decay process is a fast charge transfer reaction and (ii) that its rate constant decreases exponentially with increasing reaction distance. I n the second part of the paper a model is presented to treat exciton dissociation processes a t a metal surface in general. It is based on the concept of configuration interaction between the bound molecular exciton state and a continuum of ionized states.Der erste Teil der Arbeit beschaftigt sich mit Photostrom-Messungen zur Untersuchung der strahlungslosen Zerfallsmoglichkeiten von n n*-Triplett-Exzitonen an der Grenzflache zwischen einem p-Chloranil-Kristall und Aluminium. Der Wechselwirkungsabstand wird mit Hilfe zwischengezogener monomolekularer Fettsaureschichten variiert. Es zeigt sich erstens, da13 der Exzitonenzerfall im wesentlichen durch eine schnelle Charge-transferReaktion bestimmt wird und zweitens, da13 ihre Geschwindigkeitskonntante exponentiell mit wachsendem Reaktionsabstand abflllt. I m zweiten Teil der Arbeit wird ein Modell zur Behandlung von Exzitonen-Dissoziationsprozessen an Metalloberflachen im allgemeinen vorgelegt. Es griindet sich auf das Konzept der Konfigurationswechselwirkung zwischen dem gebundenen molekularen Exzitonenzustand und einem Kontinuum von ionisierten Zustanden.

Extrinsic and intrinsic dc photoconductivity in a conjugated polymer

“…10 It is well established that, upon exciting a molecular crystal through a semitransparent electrode, the onset of photoconductivity coincides with the absorption edge because excitons diffusing to the interface may find it energetically favorable to dissociate by transferring one of the constituent charges to an acceptor level inside the electrode, e.g., the Fermi level in the case of a metal. 11,12 Its twin inside the crystal can easily escape the attractive image potential and contribute to photoconductivity. Organic molecules with appropriately located molecular orbitals, present either at the interface or in the bulk, may act in a similar way, thus giving rise to sensitized photoconductivity.…”

Currents and photocurrents in organic materials determined by the interface phenomena

“…E ex and E g denote exciton energy and band gap energy in organic material, respectively. Hole injection into organic material is possible by tunneling of an electron from distance x [31,32]. After electron tunneling to free states in metal electrode or to some acceptors in nonmetallic electrode (eq.…”

“…Usually, it is assumed that injection and recombination currents at metal -organic material interface follow from tunneling processes [31,52,53]. According to that assumption, Eq.…”

Energy Transfer in Competition to Charge Transfer as a Method to Determine the Rate Constant for Charge Carrier Injection in the Sandwich System: Cyanine Dye Monolayer/p-Chloranil Crystal

“…than at the metal electrode where for a distance of 27 Ä an efficiency of only 10~2 has been found [24]. The peak at 495 nm with a shoulder at 465 nm resembles the absorbance of a dye monolayer on a glass sliee (Fig.…”