Atomically Precise Graphene Nanoribbon Heterojunctions for Excitonic Solar Cells

By mixing pure precursor monomers and nitrogen-doped equivalents, atomically sharp wiggle-edged heterojunctions can be obtained via the combined action of Ullmann coupling followed by cyclodehydrogenation [Cai et al., <i>Nat. Nanotechnol.</i> <b>2014</b>, <i>9</i>, 896]. We used first-principles density functional theory and the many-body <i>GW</i> approach to establish the role of doping (boron and nitrogen) in a variety of graphene nanowiggles displaying a range of band gaps. The substitution of C atoms located at the edges of the structures does not significantly affect the magnitude of the band gaps, but leads to their relative upshift or downshift depending on the dopant. This shift is found to vary quasi-linearly as the concentration of dopant increases. Consequently, tunable type-II staggered band alignments are formed in graphene nanowiggle heterojunctions. We predict that these type-II heterojunctions can provide ultrathin solar cells with power conversion efficiencies up to 22.0%