Advanced Processing of III‒V Multijunction Solar Cells

Photovoltaics (PV) is a considerable means for providing sustainable energy solutions. The specific PV technology addressed within this thesis is lattice-matched III–V multijunction solar cells (MJSCs) that incorporate dilute nitride bottom junctions, grown by molecular beam epitaxy. Such solar cells are the solution of choice for meeting high-efficiency demands especially in space industry and terrestrial concentrator photovoltaics. To obtain high-quality devices, every aspect of the device fabrication needs to be optimized. This accounts not only for the device design, but also for the microfabrication steps, which are often determinant in fulfilling the high-efficiency potential of MJSCs.

The research focuses on two thematic areas concerning microfabrication processes: i) wet etching of III–V heterostructures including dilute nitrides, and ii) development of advanced front contact grid for high-efficiency operation. The research methods include conventional semiconductor device microfabrication and characterization methods, which were adapted for various optimization routines. These were applied for microfabrication of GaInP/GaAs/GaInNAsSb triple- junction solar cells.

The most important achievements concerning wet etching include developing suitable etching chemistries for dilute nitride compounds GaInNAs, GaInNAsSb, and GaNAsSb – namely NH3:H2O2:H2O, H2SO4:H2O2:H2O, and H3PO4:H2O2:H2O in different compositions, and defining the related etching characteristics. In addition, a nonselective HIO3:HCl:H2O wet etchant was developed for mesa formation of III–V multijunction solar cells, providing good photovoltaic performance. The etching characteristics were defined and conditions causing unwanted selectivity within III–V heterostructures were identified.

The work included microfabrication development of an advanced grid concept where the busbar is located outside the active area. The study revealed the need to avoid plasma-induced damage; this was mitigated by introducing an additional wet etching step into the process. In general, the developments demonstrated are of general use for advancing the device fabrication of III–V multijunction solar cells and in wider perspective, of other III–V semiconductor components as well.