PhD defense - Jeppe Holm
Nanowire Growth for Photovoltaics
Solar cells commercial success is based on an efficiency/cost calculation. Nanowire solar cells is one of the foremost candidates to implement both very efficient and cheap third generation solar cells.
Resonance effects between the light and nanowire causes an inherent concentration of the sunlight into the nanowires, and means that a sparse array of nanowires (less than 5% of the area) can absorb all the incoming light. This means that single junction nanowire solar cells have a higher theoretical maximum efficiency than equivalent planar solar cells. We have demonstrated the built-in light concentration of nanowires, by growing, contacting and characterizing a solar cell consisting of a single, vertical, gallium arsenide(GaAs) nanowire grown on silicon with a radial p-i-n-junction. The average concentration was ~8, and the peak concentration was ~12.
By increasing the number of junctions in solar cells, more energy per absorbed photon can be extracted. Current multi-junction solar cells efficiency is hampered by the fact combining the most complimentary materials, from an absorption standpoint, is impossible due to mismatches in the crystal structure. Nanowires solve this problem, since the small footprint of grown nanowires relaxes the crystal matching constraint. 1.7eV is the ideal bandgap for a top junction in a dual junction solar cell, where silicon is the bottom junction. This can be obtained with GaAs0.8P0.2. We have demonstrated how to incorporate phosphorous(P) into Ga-catalyzed nanowire growth, and grown GaAs1-xPx nanowires with different inclusions of P(x ) directly on silicon. We have demonstrated GaAs0.8P0.2 nanowires with a lattice matched shell surrounding the core with the same composition. The GaAsP core-shell nanowires were doped to produce radial p-i-n junctions in each of the nanowires. In order to improve the efficiency, a surface passivating shell consisting of highly doped, wide bandgap indium gallium phosphide was grown. Some of the nanowires were removed from their growth substrate and turned into single nanowire solar cells (SNWSC). The best device showed a conversion efficiency of 10.2% under 1.5AMG 1-sun illumination.
The thesis can be downloaded from http://qdev.nbi.ku.dk/student_theses/
Professor Jesper Nygård, Niels Bohr Institute, Faculty of Science, University of Copenhagen, Denmark
Associate professor Søren Stobbe (chair), Niels Bohr Institute, Faculty of Science, University of Copenhagen, Denmark
Professor Helge Weman, Department of Electronics and Telecommunications, Norwegian University of Science and Technology, Trondheim, Norway
Associate Professor Magnus Borgström, The Nanometer Structure Consortium, Lund University, Sweden