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Metamorphic solar cells can have optimal bandgap combinations through the use of compositionally graded buffers, where the lattice constant is slowly varied over several microns of growth. Bragg reflectors consist of several microns of alternating layers with refractive index contrast and provide a useful internal reflection to multijunction solar cells with optically thin subcells. In this work, we implement distributed Bragg reflectors within the compositionally graded buffers of inverted metamorphic solar cells to add functionality to the buffer. The reflectance of this AlGaInAs “graded buffer Bragg reflector” is very similar to the reflectance of a similar AlGaAs Bragg reflector external to a buffer as well as the reflectance predicted by the transfer matrix model, indicating that the roughness of the buffer does not drastically reduce the reflection. Reflectance of 72%, 91%, and 98% is achieved in 2, 4, and 8 μ m buffers using AlGaInAs layers that alternate between 30% and 70% aluminum content. Using a 2 μ m graded buffer Bragg reflector, the 1.0-eV mismatched subcell of a GaAs/GaInAs tandem has a minor increase in threading dislocation density compared to a standard graded buffer and a small, 20 mV, loss in voltage. As the buffer is thickened, the voltage loss is recuperated and excellent subcell voltages are achieved, indicating that the Bragg reflector is not severely hindering dislocation glide. We demonstrate that the benefits of the graded buffer Bragg reflector for optically thin subcells and subcells containing quantum wells, and conclude that Bragg reflectors can effectively be implemented within graded buffers, adding functionality without adding cost.

The development of compositionally graded buffer layers (CGBs) with enhanced transparency would enable novel five and six junction solar cells, with efficiencies approaching 50% under high concentration. We demonstrate highly transparent grades between the GaAs and InP lattice constants on both A- and B-miscut GaAs substrates, employing Al x Ga y In1- x - y As and highly Se-doped Burstein–Moss (BM) shifted Ga x In 1- x P. Transparency to >810 and >890 nm wavelengths is demonstrated with BM-shifted Ga x In1- x P on B-miscut substrates and Al x Ga y In1- x - y As/Ga x In1- x P(Se) combined grades on A-miscut substrates, respectively. 0.74 eV GaInAs solar cells grown on these transparent CGBs exhibit ${{W}}_{{\rm{OC}}}= {\text{0.41}}\, {\text{V}}$ at ${\text{15}}\ {\text{mA/ cm}}^{2}$ , performance comparable with the state-of-the-art Ga x In1- x P grade employed in the four-junction-inverted metamorphic multijunction (IMM) cell. A ${\text{GaAs/ 0.74 eV}}$ GaInAs tandem cell was grown with a transparent BM-shifted Ga x In1- x P CGB to verify the CGB performance in a multijunction device structure. Quantum efficiency measurements indicate that the CGB is completely transparent to photons below the GaAs bandedge, validating its use in 4–6 junction IMM devices with a single-graded buffer. This tandem represents a highly efficient two-junction band gap combination, achieving 29.6% ± 1.2% efficiency under the AM1.5 global spectrum, demonstrating how the additional transparency enables new device structures.

To advance the state-of-the-art in III-V multijunction solar cells towards high concentration efficiencies approaching 50%, development of a high-quality ∼1.7 eV second junction solar cell is of key interest for integration in five or more junction devices. Quaternary GalnAsP solar cells grown lattice-matched on GaAs allows bandgap tunability in the range from 1.42 to 1.92 eV and offers an attractive Al-free alternative to conventional AlGaAs solar cells. In this work, we investigate the role of growth temperature towards understanding the optimal growth window for realizing high-quality GalnAsP alloys. We demonstrate bandgap tunability from 1.6 to 1.8 eV in GalnAsP alloys for compositions close to the miscibility gap, while still maintaining lattice-matched condition to GaAs. We perform an in-depth investigation to understand the impact of varying base thickness and doping concentration on the carrier collection and performance of these 1.7 eV GalnAsP solar cells. The photo-response of these cells is found to be very sensitive to p-type zinc dopant incorporation in the base layer. We demonstrate prototype 1.7 eV GalnAsP solar cell designs that leverage enhanced depletion width as an effective method to overcome this issue and boost long-wavelength carrier collection. Short-circuit current density (J sc ) measured in field-aided devices were as high as 17.25 m A/cm2. The best GalnAsP solar cell in this study achieved an efficiency of 17.2% with a J sc of 17 m A/cm2 and a fill-factor of 86.4%. The corresponding open-circuit voltage (V oc ) 1.7 eV measured on this cell represents the highest V oc reported for a 1.7 eV GalnAsP solar cell. These initial cell results are encouraging and highlight the potential of Al-free GalnAsP solar cells for integration in the next generation of III-V multijunction solar cells.

Multi-junction technology is rapidly advancing, which puts increasing demands on IV characterization resources. We report on a tool and procedure for fast turn-around of IV data under the reference conditions, but also under controlled variations from the reference conditions. This enhanced data set can improve further iterations of device optimization.