Advanced III-V Solar Cells with Back Reflectors

III-V semiconductor solar cells have proven their exceptional performance in terms of conversion efficiency, setting the record to date by exploiting multijunction solar cell architectures. Further advances require deployment of even newer designs, combining emerging materials with optical management of the solar radiation. In addition to the demand to increase the efficiency, beneficial functionalities, for example in space applications, are required such as flexibility, high power-to-weight ratio, or ability to reuse the III-V substrate for decreasing the manufacturing costs. To this end, this thesis concentrates on the development of thin-film III-V solar cell architectures, utilizing back reflectors for increasing the effective absorption length. The methods employed in this study include fabrication and characterization of the back reflectors and the solar cells using standard and in-house developed procedures. In terms of advanced III-V materials enabling more efficient spectral harvesting, this thesis is focused on dilute nitride materials, i.e., GaInNAs and InAs/GaAs quantum dot solar cells. Both approaches aim to increase the absorption beyond the 1 eV spectral range and benefit from the deployment of the back reflectors.

The key enabling result is the development of a planar Ag/Cu back reflector with high reflectance and suitability to be used as a back contact. As an extension of this, a reflector with a polymeric pyramid grating was demonstrated, providing enhanced diffraction of light and resilience to electron irradiation. In terms of demonstrating advanced solar cell architectures, dilute nitride solar cells with back reflector, having bandgap energies of 1 eV and 0.8 eV were fabricated. As a key result, short-circuit current densities enabling current matching in solar cells with three or more junctions were demonstrated. Furthermore, for thin-film InAs/GaAs quantum dot solar cells, the use of a planar back reflector led to two times higher current generation when compared to a standard wafer-based configuration. Although this is an expected result in terms of current generation, the key observation is that the solar cell exhibited an open circuit voltage of 0.884 V, which is one of the highest values reported for quantum dot solar cells grown by molecular beam epitaxy. Finally, a parametrization model enabling simulation of the effect of the reflectors with the pyramid grating was validated on the InAs/GaAs quantum dot solar cell, revealing 12 times higher photocurrent generation in the quantum dot layers.