  Thermoelectric junctions are often made of components of different materials
characterized by distinct transport properties. Single material junctions, with
the same type of charge carriers, have also been considered to investigate
various classical and quantum effects on the thermoelectric properties of
nanostructured materials. We here introduce the concept of defect-induced
thermoelectric voltage, namely, {\it thermodefect voltage}, in graphene
nanoribbon (GNR) junctions under a temperature gradient. Our thermodefect
junction is formed by two GNRs with identical properties except the existence
of defects in one of the nanoribbons. At room temperature the thermodefect
voltage is highly sensitive to the types of defects, their locations, as well
as the width and edge configurations of the GNRs. We demonstrate that the
thermodefect voltage can be as high as $1.7\,$mV/K for $555$-$777$ defects in
semiconducting armchair GNRs. We further investigate the Seebeck coefficient,
electrical conductance, and electronic thermal conductance, and also the power
factor of the individual junction components to explain the thermodefect
effect. Taken together, our study presents a new pathway to enhance the
thermoelectric properties of nanomaterials.
