Design of multifunctional phosphorescent emitters with charge transport moieties


Asanga B. Padmaperuma1, Phillip K. Koech1, Niranjan Govind1, Eugene Polikarpov1, Jens T. Darsell1, Jonathan L Male2, and Daniel J. Gaspar1. (1) Energy and Efficiency Division, Pacific Northwest National Laboratory, 908 Battelle Blvd, MS K2-44, Richland, WA 99352, (2) Energy and Efficiency Division, U.S. Department of Energy, 1000 Independence Avenue, Washington, DC 20585
The development of organic solid state white lighting with 50% power conversion efficiency, a goal of the US DOE, would provide significant energy efficiency gains. However, the power efficiency and stability of blue OLEDs continue to be a major technical challenge. For OLEDs to meet this goal, they will require both high quantum efficiency, and low operating voltage. High efficiencies are achieved using organometallic phosphor doped OLEDs. The simplest and lowest cost method of generating white light is to convert part of the emission from a blue light source to white using a system of phosphors external to the light generating region. Blue dopants require a wide bandgap host material, however, poor hole injection in these hosts have limited achievable efficiencies. Furthermore, in most cases the emitter is involved in charge trapping. A direct consequence of emitter charge trapping is a decrease in the operating voltage with increased dopant concentration but also reduced device efficiency at higher current densities. Therefore, we have developed tailored phosphorescent emitter molecules with charge transporting groups to improve emitter charge transport. Computational analyses of proposed tailored phosphors were carried out using the NWChem computational package to evaluate the effect of functionalization on the emission energy. Reorganization energies were computed from the optimized geometries using analysis of the adiabatic potential energy surfaces of the neutral and charged states, in order to demonstrate the increased charge transport ability of the multifunctional emitter. Best candidates were synthesized and then characterized for thermal stability by differential scanning calorimetry coupled to thermal gravimetric analysis. The design concept and synthesis of emitters with charge transporting moieties will be discussed, along with electronic, thermal and physico-chemical properties.