The objective of this work is to design, fabricate, characterize and analyze wide band gap gallium phosphide (GaP) solar cells which can be used in multi-junction solar cell systems as the top junction solar cell. The highest reported efficiency for a GaP solar cell has been achieved in this work. This solar cell has an open circuit voltage (Voc) of 1.53V, short circuit current density (Jsc) of 2.56mA/cm2, fill factor (FF) of 74.06% and efficiency of 2.90% compared to the previous best reported efficiency of 1.17%.
The wide band gap of GaP (2.26eV) makes it a very good candidate for the top junction solar cell in a multi-junction solar cell system. A wide band gap solar cell can increase the efficiency of the system by absorbing and converting the high energy photons more efficiently. A five-junction solar cell system that includes a GaP solar cell as the top junction has the potential to achieve over 50% efficiency. The novel system is based on the co-design of the optics, interconnects and solar cells that enables each individual solar cell to have separate electrical contacts. The separate contact design is important to the multi-junction system because it can eliminate the lattice and current match requirements. The absolute efficiency gain of using a GaP solar cell as the top junction in the five-junction solar cell system can be as high as 3.6%. In this thesis, the GaP solar cells have been designed, fabricated and improved using a systematic approach.
High quality GaP epitaxial layers were grown on GaP substrates using liquid phase epitaxy (LPE). The layer quality was analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD).
The baseline and the first generation GaP solar cells were fabricated using a p on n structure. The solar cells were analyzed using quantum efficiency and current-voltage measurement. Specific materials parameters were then improved using predictive models based on these results. The analysis identified that the diffusion length in the base ( n-type) limited the performance. Accordingly, a second generation GaP solar cell was designed, fabricated and tested using an improved n on p structure. The second generation GaP solar cell demonstrated significant improvement.
Four more generations of GaP solar cells have been designed and fabricated using the predictive model based on the second generation GaP solar cell’s analysis. The four new generations of GaP solar cells focused on first increasing the emitter region diffusion length; second, further improvements in the base region diffusion length; third, the emitter thickness and; finally, the front surface passivation. Systematic improvements have been achieved from these four new generations of GaP solar cells. The sixth generation GaP solar cell with AlGaP front surface passivation has achieved the highest efficiency GaP solar cell to date.
Optical analysis of the GaP as a transparent top solar cell in a five-junction solar cell system has been performed to determine the design rules for this solar cell to be used as a “transparent” top solar cell. A new five-junction solar cell system has been proposed based on this analysis.
With improved epitaxial layers growth conditions and solar cell designs, a pathway to 12.6% efficiency GaP top solar cells is described in this work.
The development of wide band gap solar cells will broaden the design space for multi-junction solar cells. This work demonstrates the design rules for wide bandgap solar cells. The result will be beneficial to the design of multi-junction solar cells and the improvement of the solar cell module conversion efficiency.