Temperature dependence of exciton and charge carrier dynamics in organic thin films

Abstract

We report on physical mechanisms behind the temperature-dependent optical absorption, photoluminescence (PL), and photoconductivity in spin-coated films of a functionalized anthradithiophene (ADT) derivative, ADT-triethylsilylethynyl (TES)-F, and its composites with C${}_{60}$ and another ADT derivative, ADT-TIPS-CN. Measurements of absorption and PL spectra, PL lifetimes, and transient photocurrent were performed at temperatures between 98 and 300 K as a function of applied electric field. In pristine ADT-TES-F films, absorptive and emissive species were identified to be disordered H aggregates whose properties are affected by static and dynamic disorder. The exciton bandwidths were \ensuremath{\le}0.06 and \ensuremath{\sim}0.115 eV for absorptive and emissive aggregates, respectively, indicative of higher disorder in the emissive species. The exciton in the latter was found to be delocalized over approximately four to five molecules. The PL properties were significantly modified upon adding a guest molecule to the ADT-TES-F host. In ADT-TES-F/C${}_{60}$ composites, the PL was considerably quenched due to photoinduced electron transfer from ADT-TES-F to C${}_{60}$, while in ADT-TES-F/ADT-TIPS-CN blends, the PL was dominated by emission from an exciplex formed between ADT-TES-F and ADT-TIPS-CN molecules. In all materials, the PL quantum yield dramatically decreased as the temperature increased due to thermally activated nonradiative recombination. Considerable electric-field-induced PL quenching was observed at low temperatures at electric fields above \ensuremath{\sim}10${}^{5}$ V/cm due to tunneling into dark states. No significant contribution of ADT-TES-F emissive exciton dissociation to transient photocurrent was observed. In all materials, charge carriers were photogenerated at sub-500-ps time scales, limited by the laser pulse width, with temperature- and electric-field-independent photogeneration efficiency. In ADT-TES-F/C${}_{60}$ (2$%$) composites, the photogeneration efficiency was a factor of 2--3 higher than that in pristine ADT-TES-F films. In ADT-TES-F/ADT-TIPS-CN (2$%$) blends, an additional charge carrier photogeneration component was observed at room temperature at time scales of \ensuremath{\sim}20 ns due to exciplex dissociation. At \ensuremath{\sim}0.5--5 ns after photoexcitation, the carriers propagated via thermally and electric-field-activated hopping with an activation energy of \ensuremath{\sim}0.025 eV. At time scales longer than \ensuremath{\sim}5 ns, charge transport of carriers that are not frozen in traps proceeded through tunneling via isoenergetic sites.