Abstract: We consider a simple model which allows to describe either light emitting diodes (LED) or solar cells. The model is based on a Jaynes-Cummings model which is coupled to leads. The aim is to calculate the electron current as well as the photon production rate using an abstract Landauer-BŁttiker formula. Basic facts on the Landauer-BŁttiker formula are recalled. In detail different situations and their influence on the electron and photon currents are analyzed. The talks is based on a common paper with Horia Cornean (Aalborg University), Lukas Wilhelm (WIAS Berlin) and Valentin A. Zagrebnov (CPT, Marseille).