Mixed Dimensional Heterostructures
People: Chen Wang, Chengmei Zhong, Jack Olding, Suyog Padgaonkar
Sunlight provides more energy in 1 hour than all humans use in a year, eclipsing all other renewable and nonrenewable energy sources combined in terms of theoretical, extractable, and technical potential. However, the combined output of all current solar photovoltaics caps at 227 gigawatts, which only meets 1.5% of global energy demand at full capacity. Furthermore, the economic incentive of solar photovoltaics lags far behind fossil fuels, necessitating strong efforts to lower the cost of solar energy.
One promising approach lies in the use of mixed dimensional (MD) van der Waals (vdW) heterostructure materials to replace the current market leader in solar cell materials, crystalline silicon, which inherently has weak absorption near its band edge in the near-IR region that limits its efficiency, especially in thin films. MD vdW heterostructures are comprised of a layered 2D material that interacts through vdW forces with a material of a different dimensionality. MD vdW heterostructures can be incorporated into p-n semiconductor heterojunctions, which show promise for photovoltaic applications when materials are carefully selected to optimize the charge transfer processes.
In collaboration with the Hersam and Lauhon groups, we are interested in the synthesis and study of the photophysics of mixed dimensional heterostructures.
We recently studied devices with pentacene, a p-type 0D organic molecule electron donor, and MoS2, an n-type 2D transition metal dichalcogenide (TMD) acceptor, demonstrate ultrafast charge transfer along with long-lived charge separation of the electron-hole pairs. Using ultrafast transient absorption spectroscopy, we discovered that this combination is limited by the tendency of pentacene to devolve into two low energy triplet states on a sub-picosecond scale through singlet fission. Different 0D p-type materials, such as QDs or organic polymers, that do not engage in this ultrafast charge carrier dissipation process may be more promising in creating a longer-lived charge-separated state to increase the overall photovoltaic efficiency of the device. Furthermore, MoS2 has been shown to lose efficiency with increasing thickness, which encourages exploration of new 2D materials.