  A simple and efficient approximation scheme to study electronic transport
characteristics of strongly correlated nano devices, molecular junctions or
heterostructures out of equilibrium is provided by steady-state cluster
perturbation theory. In this work, we improve the starting point of this
perturbative, nonequilibrium Green's function based method. Specifically, we
employ an improved unperturbed (so-called reference) state $\hat{\rho}^S$,
constructed as the steady-state of a quantum master equation within the
Born-Markov approximation. This resulting hybrid method inherits beneficial
aspects of both, the quantum master equation as well as the nonequilibrium
Green's function technique. We benchmark the new scheme on two experimentally
relevant systems in the single-electron transistor regime: An electron-electron
interaction based quantum diode and a triple quantum dot ring junction, which
both feature negative differential conductance. The results of the new method
improve significantly with respect to the plain quantum maste equation
treatment at modest additional computational cost.
