The use of an energy funnel or hierarchy to efficiently transfer photoexcitations is not new. In photosynthetic systems, whether in plants, algae, or photosynthetic bacteria, to achieve high efficiency, special light-harvesting protein complexes convert sunlight to excitons. High efficiency is achieved by temporarily storing excitons in these complexes before shuttling them to the reaction center where the conversion to usable energy takes place. For added efficiency, photosynthetic systems have adopted an energy hierarchy that funnels excitations from the light-harvesting complexes to the reaction center.

The use of an energy funnel to increase the efficiency of photovoltaic devices, however, has not been widely adopted, due in part to limited materials compatibility and energy landscape. An energy funnel can absorb over a broad solar spectrum while minimizing the volume of material needed to construct a device, which can reduce the dark leakage current. Carbon nanotubes (CNTs) provide an ideal system to explore this new feature since a single material system can provide a wide range of energy levels. Depending on the diameter, excitonic levels in CNTs span from the visible to near-infrared spectrum. Furthermore, the device design can take advantage of the high aspect ratio of CNTs, to efficiently transport both free carriers and excitons along the nanotube.

Here, we describe our efforts to develop photovoltaic p-n diodes using energy hierarchy. We use a network of CNTs in a p-n junction configuration where the transport is mostly along the CNTs. To form the p-n junction, we use buried split gates that are biased independently to form the p- and n-doped regions. The use of split gates is an extension of our earlier studies on single CNT p-n diodes.

We use different chirality CNTs to implement the energy hierarchy. Specifically, the network is structured to funnel excitons by placing larger bandgap semiconducting nanotubes on top of smaller bandgap CNTs. We discuss features that show enhanced exciton to photocurrent generation in these devices compared to devices without the energy hierarchy.