Influence of Very Efficient Back Reflectors on the Quantum Efficiency of Solar Cells
Very reflective back side reflectors directly below the active region of a solar cell alter the photon absorption and carrier collection dynamics in a manner that can be exploited for enhanced efficiency. The enlarged optical path increases the short circuit current, and the confinement and (re)absorption of the photoluminescence leads to a higher open circuit voltage through the photon recycling effect, which is critical in radiative limited solar cells. Furthermore, interesting features arise in the experimental external and internal quantum efficiencies (EQE and IQE) for solar cells with such reflective back side reflectors. This includes an increase in EQE magnitude compared to cells on absorptive substrates, and an extension of the IQE edge towards longer wavelengths. The former characteristic is due to an increased photon path length in the active region, which is a consequence of the high reflection occurring in the back side. The latter, however, is not a real feature of the interaction between the light and the solar cell materials, but a consequence of the way the IQE is defined and calculated. The resulting separation of the IQE and EQE near the bandedge of the solar cell thus questions the concurrent definitions for the IQE commonly referred to in the literature, which is clarified in this work. The influence of the definition of quantum efficiency in the material diffusion length extraction is also discussed.