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Carrier multiplication is a phenomenon in which a single, high-energy excited electron can promote multiple additional electrons into the conduction band of a semiconductor. In conventional photocells, an absorbed photon can only excite one electron. However, by utilizing the flexibility afforded by atomic layer materials, we can engineer electronic energy scales that allow carrier multiplication to take place.
Figure 1: Schematic of energy levels in a MoSe2-WSe2 system that can give rise to carrier multiplication.
Highly efficient carrier multiplication has been demonstrated in several important nanoscale systems, including nanocrystal quantum dots, carbon nanotubes and graphene. However, such e–h pair multiplication has not been observed in 2D devices. In our research we studied electron–hole (e–h) pair multiplication in 2D heterostructures composed of monolayer MoSe2 and bilayer WSe2 integrated into field-effect heterojunction devices.
In plain language we can describe an energy diagram of the carrier multiplication in WSe2-MoSe2 device like so: when a photon strikes the WSe2 layer, it knocks loose an electron, freeing it to conduct through the WSe2. At the junction between the two materials, the electron drops down into MoSe2. The energy given off in the drop catapults a second electron from the WSe2 into the MoSe2, where both electrons are free to move and generate electricity.
Speaking more technically, in the e–h pair excitation process requiring the lowest excess energy, an electron in WSe2 gains the combined energy of the interlayer potential energy ΔEc and the kinetic energy of the source–drain electric field to create a low-energy electron in MoSe2 plus an interlayer e–h pair (shown in Figure 1), satisfying this energy relation:
eW +Ke(VSD) = eMo +(eMo +hW)
An optically excited eW electron in the conduction band of WSe2 undergoes efficient multiplication into an eMo electron and additional e–h pairs. By exploiting this highly efficient interlayer e–h pair multiplication process, we demonstrate near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels.
For more info, see our brief video summary: