Studying the temperature-dependence of singlet exciton fission by using transient fluorescence technique

Abstract

During the past ten years, singlet exciton fission has been considered as an effective carrier-multiplication method to improve the energy-conversion efficiency of photovoltaic devices. However, the question of whether this process requires thermal activation is still under debate. Although different researchers have made a lot of effort, more explicit experimental results are still required in order to elucidate this basic question. In this work, highly efficient fission molecule, i.e. rubrene, was selected to be the research object. By using co-deposition technique, we fabricated four rubrene-doped films in which rubrene molecules were uniformly mixed with other inert organic molecules, forming amorphous composite solid. For each sample, steady-state photoluminescence spectra and their time-resolved fluorescence decay curves were measured at 300, 250, 200, 150, and 100 K, respectively. Upon cooling from 300 K down to 100 K, the emission features continuously sharpened, leading to a substantial increase in the signal around 575 nm. Decomposition of photoluminescence spectrum confirms the close relevance between radiative recombination of singlet excitons and molecular thermal vibration. Theoretically, based on a traditional three-state reaction model of “S 1 +S 0 ↔ 1 (TT) i ↔T 1 +T 1 ”, all measured fluorescence decay curves could be well fitted by using a set of coupled rate equations. Then the important rate constants involved in singlet exciton fission process were obtained by curve-fitting. It was found that the functional relationship between the transition rate of “S 1 +S 0 → 1 (TT) i ” process and temperature was accordant with the conventional Arrhenius law. This was in line with the endothermic nature of singlet exciton fission in rubrene material. Furthermore, according to the detailed balance condition of molecular reaction dynamics and Arrhenius laws, the experimentally determined energy difference in endothermic reaction was consistent with its theoretical value. These results are of significance for clarifying the microscopic picture and physical mechanism of exciton fission. Finally, we give some deep analysis about the variation of temperature-dependences of singlet fission processes in rubrene and tetracene materials with different morphology.