  Increasing demands for renewable sources of energy has been a major driving
force for developing efficient thermoelectric materials. Two-dimensional (2D)
transition-metal dichalcogenides (TMDC) have emerged as promising candidates
for thermoelectric applications due to their large effective mass and low
thermal conductivity. In this article, we study the thermoelectric performance
of lateral TMDC heterostructures within a multiscale quantum transport
framework. Both $n$-type and $p$-type lateral heterostructures are considered
for all possible combinations of semiconducting TMDCs: MoS$_2$, MoSe$_2$,
WS$_2$, and WSe$_2$. The band alignment between these materials is found to
play a crucial in enhancing the thermoelectric figure-of-merit ($ZT$) and power
factor far beyond those of pristine TMDCs. In particular, we show that the
room-temperature $ZT$ value of $n$-type WS$_2$ with WSe$_2$ triangular
inclusions, is five times larger than the pristine WS$_2$ monolayer. $p$-type
MoSe$_2$ with WSe$_2$ inclusions is also shown to have a room-temperature $ZT$
value about two times larger than the pristine MoSe$_2$ monolayer. The peak
power factor values calculated here, are the highest reported amongst gapped 2D
monolayers at room temperature. Hence, 2D lateral TMDC heterostructures open
new avenues to develop ultra-efficient, planar thermoelectric devices.
