  Q. Ma et al.[1] recently reported a strong photocurrent associated with
charge neutrality in graphene devices with non-uniform geometries, which they
interpreted as an intrinsic photoresponse enhanced by the momentum non-relaxing
nature of electron-electron collisions at charge neutrality. Here we argue that
gradients in charge carrier density give rise to a photothermoelectric effect
(PTE) which is strongly peaked around charge neutrality, i.e. at p-n junctions,
and such p-n junctions naturally arise at the edges of graphene devices due to
fringing capacitance. Using known parameters, the PTE effect in the presence of
charge density gradients predicts the sign, spatial distribution, gate voltage
dependence, and temperature dependence of the photoresponse in non-uniform
graphene devices, including predicting the observed sign change of the signal
away from charge neutrality, and the non-monotonic temperature dependence,
neither of which is explained by the intrinsic photocurrents in graphene. We
propose future experiments which may disentangle the contributions of PTE and
intrinsic photocurrent in graphene devices.
