Controlling triplet\u2013triplet upconversion and singlet-triplet annihilation in organic light-emitting diodes for injection lasing

Abstract

Significant progress has recently been made in the field of organic solid-state lasers. However, achieving lasing action from organic semiconductors under electrical excitation remains challenging due to losses introduced by triplet excitons. Here, we report experimental and theoretical results that confirm a positive contribution of triplet excitons for electrically-driven organic lasing via a bimolecular triplet-triplet upconversion (TTU) mechanism. We study a model fluorescent material, 9-(9-phenylcarbazole-3-yl)-10-(naphthalene-1-yl)anthracene, revealing that TTU can lower the threshold current densities required to achieve lasing under current injection. However, to achieve the best performance, the singlet-triplet annihilation (STA) must be simultaneously minimized. Hence, an experimental strategy to simultaneously obtain high TTU with low STA is demonstrated in host-guest system with coumarin 545T as the guest laser dye. This system has a low amplified spontaneous emission threshold of 1.7 µJ cm−2 under nanosecond optical pumping, and a more than three orders of magnitude improvement in J50 in organic light-emitting diodes as compared to a reference blend.