Strained Layer Superlattice Solar Cells
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Date
2007-05-11
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Abstract
For several years, photovoltaic researchers have searched for a material to extend solar cell absorption to wavelengths beyond the GaAs cut-off to increase efficiency of multi-junction solar cells. The best record efficiency of any solar cell is currently near 40% for a 3-Junction GaInP⁄GaAs⁄Ge cell at high solar concentration manufactured by Spectro-Lab. Higher efficiency can be realized with a 3-Junction GaInP⁄GaAs⁄1eV or 4-Junction GaInP⁄GaAs⁄1eV⁄Ge configuration if a high quality, lattice matched 1eV material can be found. The best material candidate for several years has been InGaAsN due to a very large bandgap reduction with small N concentrations, but quality and performance remains low despite considerable effort by many researchers to improve the material to adequate device quality. In particular, the carrier diffusion lengths are greatly reduced compared to GaAs due to poorly understood defects. For very low N compositions, below about 1%, device quality is maintained.
Here a novel solution is proposed to develop a high quality material that is both lattice matched to GaAs and has a bandgap around 1eV. By inserting a strained-layer-superlattice of In[subscript 0.28]Ga[subscript 0.72]As⁄GaAs[subscript 0.25]P[subscript 0.75] into the i-region of a GaAs p-i-n diode, effective bandgaps near 1eV have been demonstrated. Bandgap reduction is realized by increasing In composition in the well layer. Compressively strained InGaAs well layers are grown below the critical layer thickness to prevent formation of misfit dislocations. Strain is balanced via subsequent growth of a tensile strained GaAsP layer, which is also below the critical layer thickness. This structure is then repeated for several periods to build up total InGaAs thickness for increased photon absorption. Thin GaAsP barriers are required for effective carrier collection which imposes a high P composition for strain balance. Cells with thick barriers exhibited low currents, indicating that carrier tunneling is critical for proper device performance. Solar cells demonstrated in this research effort are at least equal in performance to initial InGaAsN based cells and show potential for efficient operation.
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Keywords
photovoltaic, solar cell, efficiency, multi quantum well, strain balance, superlattice
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Degree
PhD
Discipline
Materials Science and Engineering
