  Vertical integration of dissimilar layered materials in a so-called van der
Waals (vdW) heterostructure (HS) has emerged as a useful tool to engineer band
alignments and interfaces. In this paper, we investigate thermoelectric
currents in a phosphorene/graphene vdW nanoribbon consisting of an armchair
graphene nanoribbon (AGNR) stacked on an armchair phosphorene nanoribbon
(APNR). We focus on the currents driven by a temperature difference between the
leads in a two-probe junction. In contrast to pristine AGNRs and APNRs, such an
armchair-edged HS can provide several nano-amperes of the current at room
temperature, without any external field. External electric fields modify the
electronic band structure and are able to induce a type-I to type-II band
alignment transition and a direct-to-indirect band gap transition. Biasing the
APNR/AGNR by an external electric field is found to strongly increase the
thermally induced current and also control the direction of current flow at
moderate temperatures. These results are important for potential applications
of the APNR/AGNR vdW HS in flexible electronics and thermoelectric devices.
