In‐plane heterostructure engineering provides unique opportunities to control device properties. Here, a single‐sheet solar cell made of a graphene sheet functionalized into 1D channels is explored. Compared to vertical heterostructure architectures based on 2D materials, the single‐sheet solar cell shows potential for improved robustness against defects, enhancement of polaron dissociation, extra freedom for functionalization, and coverage of the entire solar spectrum. The partition width, device length, and functionalizations can be tuned independently to optimize the key optoelectronic properties for photovoltaic performance.