▎ 摘　　要

Multiple exciton generation has the promise of improving photovoltaic device efficiency. However, occurrence of this phenomenon in semiconductor nanocrystals is controversial. Using the Pariser-Parr-Pople Hamiltonian and an impact ionization mechanism, we examined over 800 finite graphene nanostructures and have identified cove-periphery hexagons, armchair-periphery hexagons, zigzag parallelograms, rectangular, and cove-periphery rectangular structures with efficient low-threshold biexciton generation. Using the experimental exciton cooling rate, we estimate that absorbed photons can generate between 1.3 and 1.8 biexcitons per absorbed photon in optimal structures.