Study On The Dynamics Of Singlet Exciton Fission Between Rubrene Molecules

In recent years, the singlet exciton fission process(SF) occurring in polyacene and its derivatives(for instance, pentacene, tetracene and rubrene) has become a scientific hotspot in the field of organic electronics. Theoretically, the organic molecules with the property of singlet fission can be used as a new type of sensitizer, which is able to effectively enhance the quantum efficiency of organic photovoltaic devices. On one side, the SF is a carrier multiplication process. This is because each triplet exciton can, in principle, be dissociated into its component electron and hole, thereby enhancing the photocurrent of organic photovoltaic devices. One another side, the triplet excitons have much longer lifetime than singlet excitons, which facilitate the excitons diffusion to the interface and dissociation in devices. Whereas in practical applications, the possible quenching effect between the excitons(including the singlet and triplet excitons) and charges might lead to the failure of the above sensitizing mechanism. And researchers have different views of the carriers’ transportation and internal microscopic kinetics mechanisms in organic photovoltaic devices. This paper will focus on the multiple important problems which are related with the singlet fission and exciton quenching processes, carrying out the specific experimental studies. Combing with the steady state and magnetic field effect measurements of photoluminescence and photocurrent of devices, the influence of thermal excitation, and intermolecular coupling strength on the rate constant of singlet fission will be studied in detail. The research of this work will try to achieve the quantitative results and decisive conclusions, thoroughly analyzing the microscopic mechanisms of singlet fission, and promoting the progress in the aspects of theoretical explanation and device application of singlet fission. Therefore research microscopic mechanisms of singlet fission which will help to improve the photo-electric conversion capability in organic photovoltaic devices and increase its potential value.In this thesis, we have study the influence of thermal excitation and intermolecular coupling strength on the rate constant of singlet fission. For the research of intermolecular coupling strength influence on the rate constant of singlet fission, the adjacent rubrene inter-molecular were change by doping which would change the coupling strength between adjacent rubrene molecules. It was found that the intermolecular coupling strength had significantly affected on the rate constant of singlet fission. For the research of thermal excitation influence on the rate constant of singlet fission, we measured the photoluminescence and magneto-photoluminescence effect at different temperatures in rubrene-doped organic films(Alq3: rubrene(20%)). We found that the rate constant of singlet excitons fission is temperature-dependent within the temperature range of 50-300 K. The results clearly confirmed that the singlet exciton fission in rubrene-doped film was a thermally activated process. Our work reveals that temperature is an effective way to tune fission rate constant in rubrene molecules.Our results summarizes as the following:(1) Firstly, some theoretical knowledge about singlet exciton fission process was introduced and the research current situation of singlet fission process in the world. We also described the basic conditions for singlet exciton fission process and analyze its theoretical models, such as, excited-electron transfer model, excited state internal conversion model. In addition, by using experiments we qualitatively research the influence of thermal excitation and inter-molecular coupling strength on the rate constant of singlet fission. We also use magnetic field effects to study the microscopic dynamics mechanism of exciton fission in rubrene molecules.(2) The third charter mainly described the influence of intermolecular coupling strength on the rate constant of singlet fission in rubrene molecules. Experimentally, the compound photovoltaic device with structure of Alq3: rubrene(x%) were fabricated. The magnetic field effect of photoluminescence from rubrene-doped organic films was recorded at room temperature. It’s found that the amplitude of MPL exhibited a non-linear dependence on the averaged intermolecular distance. Such an observation implies that the intermolecular coupling(IMC) which is tuned by changing the intermolecular distance is able to significantly affect the intensity of fission process. Theoretically, the non-linear dependence of MPL as a function of intermolecular distance can be explained by using the Landau-Zener theory for non-adiabatic transition. Experimentally, investigating the variation of singlet fission with different IMC could be an important means to study the microscopic dynamics of fission process.(3) The fourth chapter mainly introduces the temperature influence on the excitons fission rate in rubrene doped films. The rubrene-doped film with structure of Alq3:rubrene(20%) were fabricated and measured the photoluminescence and magneto-photoluminescence effect at different temperature. When cooling the temperature from 300 K down to 50 K, the emission features continuously increased and the magnetic effect of spectra were decreased. This indicated that excitons fission process between rubrene molecules were temperature-dependent. That is to say, singlet exciton fission in rubrene- doped film was a thermally activated process.