Enhancement of EQE for MBE grown InAs/GaAs Quantum Dot Solar Cell with Back Reflector
Timo Aho1, Antti Tukiainen1, Sanna Ranta1, Farid Elsehrawy2, Marianna Raappana1, Riku Isoaho1, Arto Aho1, Federica Cappelluti2, Mircea Guina1
1Optoelectronics Research Centre, Tampere University, Tampere, Finland
/2Department of Electronics and Telecommunications, Politecnico di Torino, Torino, Italy

Quantum dot (QD) heterostructures provide an attractive choice to extend the solar spectral response of single-junction GaAs solar cells. In addition, QDs are applicable for tuning the bandgap of sub-junctions in multijunction solar cells. To achieve reasonable photon absorption, a high number of QD sheets and a high QD density are required. Fabrication of such devices is challenging due to defects formed in the crystal structure during the epitaxial growth. In addition, open circuit voltage of QD solar cells tends to be lower compared to GaAs solar cells without QDs. One solution to alleviate these issues is to grow fewer QD sheets and simultaneously apply a reflector on the backside. With highly reflective planar back surface reflectors, the length of the optical path in the photogeneration layers can be effectively doubled. This configuration requires thin-film design employing substrate removal. Such thin-film solar cells provide additional benefits in applications where flexibility and high power-to-weight ratio are needed.
We report on molecular beam epitaxy grown (MBE) InAs/GaAs quantum dot solar cells utilizing thin-film configuration with back surface reflectors. For the implementation of the back reflector, contacts based on Ti/Au, Au, and Ag, and their suitability to offer high reflectivity and good conductivity was assessed. External quantum efficiency measurements reveal two times higher current generation for the quantum dots with the thin-film solar cell with the back reflector compared to an unthinned substrate solar cell. An open-circuit voltage of 0.884 V is demonstrated for the thin-film QD solar cell with the back reflector, which is amongst the highest values reported for MBE-grown QDSCs. Future developments concerning the benefits in using a novel design employing pyramidal diffraction gratings as part of the back reflector are also discussed.