Evidence of intra-band assisted escape in multi-quantum well solar cells

For solar cells where relatively shallow energy wells (i.e InGaAs in GaAs) are embedded in the intrinsic region of a solar cell, the thermionic escape dominates the carrier extraction process. In this case, it is justifiably believed that carriers escape directly from the fundamental hole or electron level to the continuum. Thus, one generally observes a decay of the photoluminescence signal and an increase of the carrier collection efficiency activated by energies corresponding to that of the electron to conduction band and hole to valence band energies. Here, we have undertaken a study of somewhat deeper type quantum well solar cells in the InAsP/InP material system. Within the framework of temperature dependant photoluminescence spectroscopy combined with theoretical analysis, we provide the first experimental evidence for the possibility of intra-band assisted, namely, heavy hole to light hole, escape. This process is found to significantly facilitate the escape probability leading to optimal carrier collection. This observation provides a vital insight whereby a critical engineering of the intra-band energy states may be undertaken to efficiently collect carriers from deep well quantum confined solar cells.