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Abstract
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. The development of the SiGe cell under a GaAsP cell can lead to a 34% relative increase in efficiency over a silicon solar cell. The design, fabrication, characterization and analysis of low bandgap SiGe solar cells for operation in a GaAsP-SiGe dual junction structure is the objective of this thesis.
This work demonstrates the improvement of short circuit current (Jsc), open circuit voltage (Voc) and efficiency for the SiGe solar cell, operating underneath the GaAsP device. This involves a target Jsc of 21 mA/cm2 at 1 sun from photons with wavelength longer than 750 nm and with a 450 mV band gap-voltage offset (Woc=Eg/q-Voc) under 20 suns illumination. Numerical and analytical methods are introduced to demonstrate the SiGe solar cells’ performance limits and to examine the trade-offs between I-V performance, cell structure and material composition. Material compositions are confirmed with scanning electron microscope, energy-dispersive X-ray spectroscopy and electrochemical capacitance voltage profiling.
First principle analysis shows that a Si.15Ge.85 cell with a 5 µm base has potential to produce the expected current and an efficiency of 8.8%, when it operates beneath the GaAsP cell at 20 suns illumination. Experimentally, a Woc of 435 mV under 20 suns was measured with a Si.18Ge.82 cell. The best 1 sun short circuit current measured for a SiGe cell at wavelengths beyond 750 nm is 20.45 mA/cm2, with an optimized fabrication process and device structure. Adding a SiO2 back surface reflector to the device is demonstrated to improve the Jsc beyond 21 mA/cm2. The best efficiency is improved from 1.7% to 3.0% from the SiGe bottom cell filtered by GaAsP at 1 sun illumination. By optimizing the grid design and reducing the series resistance, the cell could achieve an efficiency of 4.63% operating beneath the GaAsP cell at 20 suns illumination.
Additionally, further analysis is conducted by comparing the performances of the SiGe bottom cell in a GaAsP-SiGe device with that of a SiGe cell working as a single junction device. Results from the devices are consistent, indicating the feasibility of predicting the SiGe performance in the GaAsP-SiGe structure, based on its performance when working as a single junction device.