Light Amplification in an Organic Solid-State Film with the Aid of Triplet-to-Singlet Upconversion

Organic laser dyes can be optically excited to achieve light amplification. The buildup of an excessive population of triplets is generally believed to limit the duration of the light amplification because of optical losses through excited-state absorption, so triplet excitons are usually eliminated by using a triplet quencher. However, destroying the triplets limits the electroluminescence efficiency of organic materials under electrical pumping and is counterproductive to realizing electrically pumped organic laser devices. Herein, light amplification is reported which constructively uses triplet states in an optically pumped organic film. In this system, the triplets are converted into singlets by reverse intersystem crossing in a "triplet harvester," and then the singlets are resonantly transferred to the singlet state of the laser dye. Since this approach permits the constructive use of triplets, not only gain-narrowed emission but also enhanced electroluminescence efficiency was observed, indicating that the threshold current density for lasing might be reduced.