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Energy‐Dependent Excitonic Injection of Charge into Anthracene Crystals
Author(s) -
Strzelecka B.,
Godlewski J.,
Kalinowski J.
Publication year - 1992
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.2221690223
Subject(s) - exciton , excited state , excitation , atomic physics , absorption (acoustics) , electric field , anthracene , condensed matter physics , charge carrier , diffusion , chemistry , singlet state , molecular physics , materials science , physics , optics , thermodynamics , organic chemistry , quantum mechanics
Steady‐state exciton‐injection currents (EIC) as a function of electric field and temperature are measured in anthracene crystals upon energy‐selective excitation within the S 0 → S 1 transition. The current for each given absorption coefficient decreases as the excitation energy (ℏω) passes through the high‐energy wing and is swept downward the low‐energy wing (Urbach tail) of the absorption band, demonstrating that the flux of excitons reaching the injecting contact is energy dependent. The maximum absorption at the S 0 → S 1 transition establishes a demarcation energy ℏω 0 . Above ℏω 0 only diffusion of mobile (band) excitons (S) is an injection‐determining process, below ℏω 0 thermal activation of localized states (S t ) becomes an unavoidable step in the S‐exciton creation process, diminishing the effective flux of S‐excitons reaching the contact. An analysis of the dependence of the activation energy on the localization energy ℏω 0 — ℏω of primarily excited S t states indicates that EIC are the sum of a singlet exciton‐injection contribution and a triplet exciton‐injection contribution, the triplets being produced via iniersystem crossing transitions from S and S t states. A theoretical description of excitonic processes underlying charge injection is presented and applied to the experimental results. Agreement between theoretical predictions and experiment is good if the frequency factor for the thermally activated S t → S excitation is assumed to be ≈ 10 9 s –1 .