UC San Diego

The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs)^1-7. According to a simple model based on spin statistics, the injected charges form spin-singlet (S₁) excitons and spin-triplet (T₁) excitons in a 1:3 ratio^2-4. After the first report of a highly efficient OLED² based on phosphorescence, which is produced by the decay of T₁ excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage^2-6. Because T₁ excitons have lower energy than S₁ excitons (owing to the exchange interaction), use of the energy difference could-in principle-enable exclusive production of T₁ excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yet been established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope^8-20 and demonstrate a simple method of selective formation of T₁ excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule^21-25 adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T₁ excitons without creating their S₁ counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T₁ excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage.