Experimental determination of the charge/neutral branching ratio η in the photoexcitation of π-conjugated polymers by broadband ultrafast spectroscopy

We demonstrate a long-sought reliable method for determining the important branching ratio eta between photogenerated charged polarons and neutral excitons in pi-conjugated polymer films and solutions, using femtosecond transient photomodulation spectroscopy with broad spectral range from 0.14 to 2.7 eV. We found that both excitons and polarons are instantaneously photogenerated, but eta critically depends on the film nanomorphology, which ultimately controls the interchain coupling strength. We also found that a correlation exists within each polymer family between the obtained eta value, photoluminescence quantum efficiency, and the transient polarization memory lifetime; where the interchain coupling strength in the film determines them all. We show that eta varies from less than 1% in solutions and glassy films of poly(p-phenylene-vinylene) derivatives, where the polymer chains are relatively isolated; to more than 30% in ordered films that contain lamellae, such as regio-regular poly(3-hexyl-thiophene). Our results may serve for matching polymers to specific device applications, where polymers with large eta values are good candidates for photodetector and photovoltaic applications, whereas those with small eta values are more suitable for active layers in organic light emitting devices.