Since the first demonstration of lasing in organic thin films, the field of organic solid state lasers (OSL) has seen impressive progress. Broad optical gain spectra combined with relatively simple processing seed the prospect for a cost effective, easy to integrate and widely tunable laser source. While electrical operation has not yet been demonstrated, highly efficient organic gain materials combined with high-Q resonator geometries (distributed feedback (DFB), VCSEL, etc.) now allow for OSLs, optically pumped by simple inorganic laser diodes or even LEDs. In spite of this success, continuous wave (cw) operation of OSLs remains a challenge. Typically, the emission of an OSL terminates within 10 ns even when pumped with substantially longer pulses. To understand the underlying photo-physical mechanisms that impede cw operation, and to ultimately find strategies to overcome them, careful assessment of losses related to the generation of triplet excitons and polarons in the organic gain medium is required. We will show that both triplet state absorption and triplet singlet annihilation are indispensable to fully explain the dynamics of OSLs. We will discuss case studies of low threshold organic lasers based on neat and dye doped polyfluorene gain media. We present measurements of the density, the spectral absorption and the lifetime of triplet excitons in these systems at room temperature. We evidence that especially in low-threshold guest-host systems, the lifetime of triplets may be significantly increased due to a reduced mobility as a result of trapping on the dopant molecules. To overcome triplet related losses, various concepts of triplet management can be considered. Alternatively, we show that there are polyfluorene based gain media with separated optical gain and triplet-related absorption spectra, which pave the way towards cw organic lasers.
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