  Carrier multiplication (CM), a photo-physical process to generate multiple
electron-hole pairs by exploiting excess energy of free carriers, is explored
for efficient photovoltaic conversion of photons from the blue solar band,
predominantly wasted as heat in standard solar cells. Current state-of-the-art
approaches with nanomaterials have demonstrated improved CM but are not
satisfactory due to high energy loss and inherent difficulties with carrier
extraction. Here, we report ultra-efficient CM in van der Waals (vdW) layered
materials that commences at the energy conservation limit and proceeds with
nearly 100% conversion efficiency. A small threshold energy, as low as twice
the bandgap, was achieved, marking an onset of quantum yield with enhanced
carrier generation. Strong Coulomb interactions between electrons confined
within vdW layers allow rapid electron-electron scattering to prevail over
electron-phonon scattering. Additionally, the presence of electron pockets
spread over momentum space could also contribute to the high CM efficiency.
Combining with high conductivity and optimal bandgap, these superior CM
characteristics identify vdW materials for third-generation solar cell.
