Over the past decade, the global cumulative installed photovoltaic (PV) capacity has grown exponentially, reaching 591 GW in 2019. Rapid progress was driven in large part by improvements in solar cell and module efficiencies, reduction in manufacturing costs and the realization of levelized costs of electricity that are now generally less than other energy sources and approaching similar costs with storage included. Given this success, it is a particularly fitting time to assess the state of the photovoltaics field and the technology milestones that must be achieved to maximize future impact and forward momentum. This roadmap outlines the critical areas of development in all of the major PV conversion technologies, advances needed to enable terawatt-scale PV installation, and cross-cutting topics on reliability, characterization, and applications. Each perspective provides a status update, summarizes the limiting immediate and long-term technical challenges and highlights breakthroughs that are needed to address them. In total, this roadmap is intended to guide researchers, funding agencies and industry in identifying the areas of development that will have the most impact on PV technology in the upcoming years.
Technology Computer Aided Design modeling is used to examine the performance under light concentration of a 4-J solar cell Ge-based that includes a 1-eV MBE-grown dilute nitride subcell. The 1-eV solar cell is modeled and examined by using material parameters extracted from detailed electro-optical characterization prior to be included into a multijunction structure. The modelling reveals the impact of the electric field-assisted collection in the performance of single junction solar cells and its effect when included in a 4-Junction solar cells. This effect is responsible for the lower FF (~15% lower) in the 4J when including the dilute nitride subcell, especially if it limits the photocurrent. Finally, an optimization procedure based on dilute nitrides with higher material quality is performed resulting in a 4-Junction solar cell with an efficiency of 47% for concentrations between 1000-2000 suns direct terrestrial spectrum.
Homoepitaxial GaN films doped with Mg were grown by rf-plasma molecular beam epitaxy on Ga-polarity (0001) MOCVD templates. Convergent beam electron diffraction analysis establishes that the film polarity changes from [0001] to [0001] when the Mg flux during growth is approximately one monolayer per second. Secondary ion mass spectrometry indicates a doping concentration of ~10 20 cm-3 in the film where the inversion occurs and shows a reduced Mg incorporation efficiency after further growth. Transmission electron microscopy shows that the inversion domain boundary is faceted predominantly along the {0001} and {h,h,-2h,l} planes, with l/h approximately equal to 3. Using first-principles total energy calculations we show that the {h,h,-2h,l} segments of the boundary are stabilized by the incorporation of Mg in threefold coordinated lattice sites.
Optically detected magnetic resonance measurements are carried out to study formation of Ga interstitial-related defects in Ga(In)NAs alloys. The defects, which are among dominant nonradiative recombination centers that control carrier lifetime in Ga(In)NAs, are unambiguously proven to be common grown-in defects in these alloys independent of the employed growth methods. The defects formation is suggested to become thermodynamically favorable because of the presence of nitrogen, possibly due to local strain compensation.
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