Eksitonų dinamika bifluoreno kristaluose skirtuose lazerių taikymams

Organic optoelectronics as a field has grown immensely over the last few decades currently offering flexible devices at low production cost and changing the future of consumer electronics. Despite success in OLED display and photovoltaic applications, organic materials have yet not found major applications in lasers due to degradation under strong excitation conditions and low electrical conductivity. In this case, organic crystals offer best combination of optical and electrical properties for organic lasers. However, strong intermolecular interactions in densely packed molecular crystals lead to formation of excitons (a quasiparticle corresponding to strongly bound charge pair created by electrical or optical excitation) that induce high losses due to reabsorption and exciton annihilation in organic gain material. This work presents a thorough study of excitonic effects for lasing in organic crystals based on bifluorene derivatives. In an effort to avoid the negative effects and achieve low lasing threshold in crystals two strategies were devised aiming to control excitonic coupling via modification of molecular and crystal structure. First strategy was based on modification of molecular rigidity, where flexible fragments allowing for stronger molecular vibrations were found to diminish the effects of excitonic coupling and thus improve lasing performance. Another strategy was based on doping of crystal structure with lower concentration of highly emissive molecules. Here, strong excitonic coupling in crystals comprised of rigid bifluorene derivatives was found to mediate extremely efficient energy transfer to dopant, which allowed to eliminate reabsorption and exciton annihilation. Both strategies were proven to be successful for achieving one of the lowest amplified spontaneous emission threshold values recorded for organic crystals, and hence proved the potential of bifluorene crystals for laser applications