Principal Investigator Moungi Bawendi
By using small-bandgap semiconductors (Eg ~ 0.4 eV), we are able to synthesize quantum dots that absorb photons all the way from the UV to the visible and into the infrared regions of the solar spectrum. These materials are solution-processable and can function on a flexible electrode, so the final solar cell product could be printed like newspaper, which would be much less expensive than the fabrication of solar cells on the market today. While quantum dot solar cells have been successfully made in a number of labs, the field of quantum dot solar cells is relatively new, and the fundamental mechanisms for charge transport and photoconductivity are not fully understood. Much of our work focuses on understanding how current flows through a solid-state quantum dot system and how quantum dots interface with other materials to absorb sunlight and output electricity.
Characterization of charge transport in NC films -- The ability to develop NC based devices depends on a strong understanding of the nature of charge transport in the NC films. Carrier transport occurs through hopping between localized states in the semiconductor nanocrystal core. The levels of trap states, type of charge doping, interdot barriers and exciton lifetimes vary not only with the type of NC used, but also with the ligand, annealing temperature and exposure to air and light. For example, by replacing the long surface ligand (a crucial component of the nanocrystal synthesis and the reason the nanocrystals can be suspended in solution) with a shorter ligand, the nanocrystals can get closer together in the film. An electron moving between nanocrystals can do so much more easily when the ligand barrier is shorter. In a solar cell that translates to a higher current at a given voltage, or equivalently, a higher power.
We study these fundamental properties both in the lab and in collaboration with Kastner group in the physics department at MIT.
Quantum dots in photovoltaic architectures -- The behavior of a photovoltaic cell as a whole depends on each of the materials in the architecture as well as the interfaces between them. We fabricate solar cells with various architectures and materials to investigate how the energy levels and transport properties affect the overall solar cell performance.
Semiconductor nanocrystals have features of both molecules and bulk semiconductors, which gives them versatility in device architecture. They can be mixed into a solution with a polymer in a bulk-heterojunction architecture as a carrier separation and transport network, or deposited into a nanocrystal-only film to create a junction with an adjacent device layer.
We fabricate and study these devices both in the lab and in collaboration with the Bulovic group in the electrical engineering department at MIT.