Energy Transfer in Carbon Nanotubes

One of the most active areas or research today is the discovery and understanding of next-generation solar cell materials. Quantum dots, perovskites, conducting polymers and many other materials are being studied in order to understand and harness their energy transfer properties. A promising new class of materials is semi-conducting carbon nanotubes where the nanotubes are used in the photoabsorbing layer, similar to molecular dyes in dye-sensitized solar cells. Isolated carbon nanotubes have extremely large exciton diffusion lengths, large carrier mobilities, low trap densities, high optical cross sections, and excellent air stability (compared to organic photovoltaic dyes) which are all critical properties for solar cells. These purified thin films represent an entirely new class of materials, whose properties need to be understood. Their ultrafast dynamics are especially important because fast exciton transfer is directly linked to solar cell performance.

Figure 1: Qualitative example of charge transfer in carbon nanotubes.

Using transient absorption spectroscopy, we are able to measure the time scale for energy transfer within a film of carbon nanotubes. Figure 2 shows the transient absorption spectra for nanotubes in solution (a,c,e) and thin film (b,d,f) to study uncoupled and coupled systems, respectively. From these measurements, we can see that there is a fast component to the exciton hopping followed by a slower pathway. 

Figure 2: Transient absorption spectra for nanotubes in solution (a,c,e) and thin films (b, d, f).

By studying the anistropy of our transient signals, we can gain insights into how the energy transfer is distributed spatially. Anistropy measurements (Figure 3) show that for the uncoupled nanotbues, the excitons do not spatially reorient, while in the film, excitons that hop undergo a rapid change in orientation, indicating that hopping between nanotube bundles happens rapidly.

Figure 3: Anistropy measurements of carbon nanotubes in solution (left) and in thin film (right).

Further experiments using 2D ES are underway to gain a more complete understanding of the photophysics in these thin films.