Low Threshold Amplified Spontaneous Emission from Efficient Energy Transfer in Blends of Conjugated Polymers

The concentration dependence of amplified spontaneous emission (ASE) and lasing characteristics in the blend of blue emitting poly(9,9-dioctylfluorene) (PFO) and ladder-type poly(indenofluorene) (2LF-BT) has been reported. A methodology for selecting the optimal blend composition in the blend systems is developed to enhance the optical gain characteristics. It is observed that as the PFO concentration increases, the ASE threshold rapidly decreases from the original 65 to 4.8 μJ cm^-2 (30% PFO). Subsequently, the ASE threshold value gradually increases by further increasing the PFO concentration. Under this circumstance, 30% PFO is the optimized concentration for PFO/2LF-BT binary blend films with the maximum gain of 93 cm^-1 and superior ASE stability upon annealing. According to atomic force microscopy (AFM) images, morphologies of blend samples are smooth with no phase separation regardless of the PFO concentration, demonstrating attractive advantages over other previously reported blend systems which usually suffer from phase separation. Distributed feedback (DFB) lasers based on the blend film with 30% PFO have low thresholds of 4.85 μJ cm^-2 (0.97 kW cm^-2). These optical and gain characteristics render the conjugated polymer blends attractive materials for organic lasing. It is worthwhile to mention that the strategy for judicious selection of the blend systems which comprise conjugated polymers with similar backbone structures has made a great contribution to the improvement of organic lasing performance in terms of ASE properties and optical gains.