Singlet Exciton Fission And Its Magnetic Field Effect In Rubrene-doped Amorphous Films

Over the past few years, singlet exciton fission has attracted a broad interest in the field of physics, chemistry, and organic electronics. Generally, singlet fission in organic solid is a process in which a high energy singlet exciton shares its half of energy with another molecular in the ground state, and subsequently both of them convert into a pair of low energy triplet excitons. In theory and experiments, many researchers have proved it can be used to improve the quantum efficiency of the organic photovoltaic device to use the organic material with properties of singlet fission as a new type of sensitizer. However, there are still many unsolved issues in this process of actual application. This paper will elaborated for the following two aspects.First, for all the photogenerated singlet excitons, fission process is just one of the decay pathways. In a real device, other two process including the radiative recombination and excitons dissociation also play important roles in some cases. Basically, for many fission materials, singlet fission, radiation, and dissociation constitute all the decay channels which might seriously compete with each other. In order to effectively utilize the advantages of singlet fission, it obviously requires that the fission rate should be faster than the rate of the other two decay channels. Although there has been a significant progress in fission sensitized photovoltaic device, an explicit analysis about the competition between these three decay routes, i.e., fission, radiation, and dissociation of singlet excitons, has not been given.Second, the researchers found that intermolecular distance and stacking geomettries can also affect the speed of the exciton fission rate. Experimental studies showed that monocrystalline organic material, due to its fixed intermolecular distance and orientation, have a much faster exciton fission rate. However, exciton fission process can only be very few on molecules in amorphous film. Actually, in the actual large scale manufacturing process, the organic films by thermal evaporation or spin coating is substantially amorphous, which force us to ponder a problem:whether disordered intermolecular arrangement will hinder the exciton fission process? Or whether exciton fission process can occur in disordered arrangement of organic molecules?Based on the above two issues, the organic fissile material rubrene was doped to the host material forming composite films. Due to the exciton fission process can be modulated by the applied magnetic field, the film samples’magnetic field effects on the photoluminescence and photocurrent was measured. Combined with time-resolved fluorescence decay, these two issues could be analyzed and resolved.The main works in this Dissertation are listed as following:(1) In the first chart, the physical mechanism of the exciton fission process was discussed, including two kinds of theoretical models of charge-transfer and internal conversion. Secondly, the satisfied conditions of the exciton fission was described. Through some examples, we further understand the impact on the exciton fission rate in the two case in which singlet exciton energy level is greater or less than twice the triplet exciton energy levels. Finally, the singlet fission research status and some presence problem was discussed to reveal the purpose and significance of this paper.(2) In singlet fission sensitized photovoltaic devices, the photoexcited singlet excitons might undergo three different decay pathways including fission, radiation, and dissociation. For rubrene-doped amorphous films, we analyzed the competition between these three decay channels by using transient and steady-state measurements of photoluminescence and photocurrent, as well as their magnetic field effects. The experimental results and theoretical calculations show that the internal quantum efficiency of our device can reach about 124%.(3) In this section, the problem whether or not efficient fission process can occur in amorphous thin films was settled. In this experiment, we prepare the doped films with different concentrations rubrene. The transient fluorescence decay and magnetic field effect of steady-state photoluminescence of rubrene-doped organic films were measured at room temperature. Then, the magnetic field effect of singlet fission rate between doped rubrene molecules was determined. Based on the Merrifield’s phenomenological theory, the difference between the B-field effects of fission rates in ordered and disordered systems were analyzed in detail. Comparing the experimental results and theoretical curves showed that singlet fission process mainly took place on those randomly arranged rubrene molecules in doped films. This experimentally confirmed the occurrence of efficient singlet fission in amorphous organic solid. Additionally, the fission rate was expected to be related with the charge mobility of material.