Experimental determination of the charge/neutral branching ratioηin the photoexcitation ofπ-conjugated polymers by broadband ultrafast spectroscopy

Abstract

We demonstrate a long-sought reliable method for determining the important branching ratio $\ensuremath{\eta}$ between photogenerated charged polarons and neutral excitons in $\ensuremath{\pi}$-conjugated polymer films and solutions, using femtosecond transient photomodulation spectroscopy with broad spectral range from 0.14 to $2.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We found that both excitons and polarons are instantaneously photogenerated, but $\ensuremath{\eta}$ 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 $\ensuremath{\eta}$ value, photoluminescence quantum efficiency, and the transient polarization memory lifetime; where the interchain coupling strength in the film determines them all. We show that $\ensuremath{\eta}$ 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 $\ensuremath{\eta}$ values are good candidates for photodetector and photovoltaic applications, whereas those with small $\ensuremath{\eta}$ values are more suitable for active layers in organic light emitting devices.