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SiGe materials on Si substrate are promising candidates to act as the bottom cell in a tandem structure, due to its high mobility, good process compatibility, adjustable lattice constant, and capability of absorbing light of wavelengths up to 1800 nm. This work demonstrates the improvement of short circuit current and efficiency for the SiGe solar cell, operating in a GaAsP-SiGe two-terminal configuration tandem device. A numerical model is proposed to predict the short circuit current of the SiGe cell. Experimentally, the highest short circuit current density is increased from 12.9 mA/cm 2 to 19.4 mA/cm 2 , with optimized fabrication process and device structure. The efficiency of the SiGe bottom cell is improved from 1.7% to 3.0% filtered by GaAsP. It is demonstrated that the cell has potential to achieve an efficiency of 4.6% at 20 suns illumination. Different mole fractions of Ge in the structure are confirmed from the X-ray diffraction line scanning image. [ABSTRACT FROM AUTHOR]

A compositionally graded silicon germanium (Si 1−x Ge x ) buffer layer is a leverage technology that grows III–V materials on silicon (Si) substrates with low threading dislocation density. This study determined average optical absorption coefficients of Si 1−x Ge x graded buffer layers at wavelengths beyond the band gap of Si by measuring the transmittances of Si 1−x Ge x on Si wafers. The results show that assuming linear changes of absorption coefficients of Si 1−x Ge x with the change of germanium (Ge) composition will significantly overestimates the absorption in the Si 1−x Ge x graded buffer layers. Using determined average absorption coefficients of graded buffer layers, an accurate optical model was developed for a fabricated planar Si 0.15 Ge 0.85 on Si solar cell. This model shows that significant amounts of light at the band edge of Si are absorbed by the Si substrate. However, the majority of the light absorbed by the Si substrate cannot contribute to the short-circuit current of the solar cell, as the generated electron-hole carriers are too far away from the p-n junction and thus have a very low probability of being collected by the p-n junction. By thinning the Si substrate from 675 µm to 200 µm, a 0.5 mA/cm 2 short-circuit current density ( J SC ) improvement under AM1.5 G spectrum was observed on the Si 0.15 Ge 0.85 solar cell. Further thinning the Si substrate to 50 µm should lead to a total J SC gain of 1.6 mA/cm 2 . This study provides a practical path for achieving a highly efficient two-terminal current matched GaAsP/SiGe tandem solar cell on Si substrate. [ABSTRACT FROM AUTHOR]

Silicon germanium (Si 1− x Ge x ) is a material with high mobility and relatively low bandgap making it an attractive candidate for the bottom subcell in a multi-junction solar cell. Optical constants of Si 1− x Ge x films grown on silicon substrates with germanium (Ge) compositions of 77%, 82%, 85% and 88% from wavelength of 400 nm to 1450 nm at room temperature are reported. Spectroscopic ellipsometry is used to obtain the real part of refractive index for the whole wavelength range and the extinction coefficients for short wavelengths. Optical transmittance ratio of two structures being different only in the thicknesses of the Si 1− x Ge x films is used to extract the extinction coefficients for long wavelengths. We demonstrate that the optical constants of Si 1− x Ge x films grown on silicon substrates with the aid of graded buffer layers are similar to that of their corresponding bulk materials. However, the absorption coefficients of Si 1− x Ge x with 88% Ge determined by us at around 1000 nm is three times higher than previously interpolated results. The main reason for this discrepancy may be due to the inaccuracy of interpolation in the wavelength range where the material's bandgap changes very quickly. In addition, it should be noted that the Ge composition of the Si 1− x Ge x films we are able to obtain are determined to be ±1% which is much more accurate than the previous experimentally determined results which have an accuracy of ±5%. [ABSTRACT FROM AUTHOR]

Low bandgap Si (1− x ) Ge ( x ) solar cells are designed, fabricated, characterized and analyzed for the purpose of acting as the bottom cell in a GaAsP–SiGe tandem solar cell. The development of the SiGe cell under a GaAsP cell can lead to a 34% relative increase in efficiency over that of a silicon solar cell. This work focuses on making a SiGe bottom cell that can generate 21 mA/cm 2 of short circuit current ( J sc ) at 1 Sun from photons beyond 780 nm and with a 450 mV band gap–voltage offset ( W oc ) under 20 Suns illumination. Numerical and analytical methods are introduced to demonstrate the SiGe solar cells׳ performance limits and to examine the trade-offs among I – V performance, cell structure and material composition. Material compositions are confirmed with energy-dispersive X-ray spectroscopy (EDS) and electrochemical capacitance voltage profiling (ECV). First principles analysis shows that a Si .15 Ge .85 cell with a 5 µm base can produce the expected current and an efficiency of 8.8% under 20 Suns. By increasing base doping, we achieve a W oc of 435 mV under 20 Suns with a Si .18 Ge .82 cell. The best 1 Sun short circuit current measured from a Si .12 Ge .88 cell at wavelengths beyond 780 nm without anti-reflection coating, back surface reflector, or back texturing is 12.9 mA/cm 2 . Adding a back surface field to the structure will lead to a higher V oc and lower W oc . Implementing light trapping with a higher base angle of pyramids can lead to the target J sc of 21 mA/cm 2 . [ABSTRACT FROM AUTHOR]

GaAsP/SiGe dual-junction solar cells have been epitaxially grown on silicon substrates which have the potential of achieving 1-sun tandem efficiencies of 40%. With the addition of light trapping this lattice-matched two-terminal structure can be current-matched and facilitates high performance from the III–V top cell while maintaining the cost advantages of silicon solar cells. The SiGe graded buffer allows for lattice matching of the top and bottom cell while providing a low dislocation interface between the silicon substrate and the device layers. This two-terminal dual-junction structure design is presented and demonstrates a 10.4% relative improvement in J SC and a 1.7% absolute improvement in efficiency over previous best devices. These initial structures have reached an efficiency of 18.9% under 1-sun. The devices suffer from high series resistance and exhibit reduced fill factors. A near term pathway to efficiencies approaching 25% is described. [ABSTRACT FROM AUTHOR]

Lattice matched and current matched GaAsP/SiGe tandem solar cells on Si have the potential of 40% conversion efficiency [8] . A corrected three-terminal tandem efficiency of 20.6% under 1× illumination has been achieved, a relative improvement of approximately 9% compared to the 18.9% efficiency reported previously. Current matching is realized under 2.1× illumination. This is achieved by improving the bottom cell structure, manipulating the bottom cell active area and adjusting the light spectrum during light IV measurements. This paper describes the design of the improved structure, a simplified fabrication process and a detailed current matching analysis of this tandem solar cell. Pathways to achieve the realistic efficiency of over 25% under 1× illumination and near 30% under 20× illumination are also discussed. [ABSTRACT FROM AUTHOR]