In 1970, Esaki and Tsu proposed using heterostructures for applications in optoelectronics and the first suggestion to use intersubband transitions in order to create a laser was made by Kazarinov and Suris. Over the past several years, Quantum Cascade Lasers (QCL) have proved to be very promising candidates for practical sources of radiation, particularly in the mid-infrared region. Transport modeling of charged particles in semiconductor devices is done by means of boundary value problems. In order to model situations with net current flows and obtain current-voltage characteristics of a quantum device, one has to devise boundary conditions that allow for current carrying states instead of using inappropriate ones, such as the homogeneous Neumann or Dirichlet boundary conditions, which yield to a self-adjoint Hamiltonian. The system described by such a Hamiltonian is closed. Hence, there is no interaction with the environment and the current density is equivalent to zero. Therefore, we must consider open quantum systems with non-selfadjoint boundary conditions. We propose boundary conditions that yield current carrying states as solutions of the Schrödinger equation. The theoretical development is based on a Robin boundary condition approach. Within this scheme, we calculate the current density from the wave functions satisfying these boundary conditions. Comparing the results obtained with the proposed approach to other simulations based on the Tsu-Esaki model, we find that the concept of non-selfadjoint boundary conditions for the Schrödinger equation is satisfactory to QCL simulations.