Invited Presentation: Understanding Energy Transfer Pathways in Semiconducting Carbon Nanotube Thin Films Using Two Dimensional White Light Spectroscopy

Carbon nanotubes are attractive materials for the light absorbing active layers of photovoltaic devices because of their tunable bandgaps, strong optical absorptivity, and ultrafast intra-tube charge and exciton transport. Also of critical importance in carbon nanotube devices are inter-tube charge and exciton transport processes.  However, these are generally less well understood.  We have developed two-dimensional white light spectroscopy (2D WL) as a novel probe of coupled thin films of semiconducting nanotubes and many other systems in the solar energy sciences. The advantage of exploiting 2D WL over typical 2D electronic spectroscopies is that the spectral bandwidth produced from supercontinuum generation is significantly broader than that accessible from an optical parametric amplifier. Thus, we are able to cover both S₂₂ and S₁₁ ranges with both pump and probe. By studying the evolution (as a function of waiting time) of crosspeaks between the S_ii states for different bandgap nanotubes coupled together in thin film networks, we are able to map out S_ii-S_jj inter-tube energy transfer pathways and rates and obtain maps of instantaneous photoexcitation distributions. Several surprising results will be reported.