Design, fabrication, characterization and analysis of an efficient germanium: Silicon solar cell for a multi-junction solar cell system.

摘要

The design, fabrication, characterization and analysis of low band gap germanium silicon (Ge:Si) solar cells for operation with a silicon solar cell in a multi-junction concentrator system is the objective of this thesis. This is the first report of high Ge concentration Ge:Si solar cells on Si. We achieved a Ge:Si solar cell with an efficiency of 1.37%, an open circuit voltage (Voc) of 267mV and a fill factor (FF) of 63% below Si at 30 suns. This solar cell has a short circuit current density (Jsc) of 7.91mA/cm2 below Si at one sun after correcting for the application of an anti-reflection (AR) coating.;Optical properties of Ge:Si make it a good candidate to absorb long wavelength photons which are transmitted through high or medium band gap materials, like Si. Its spectral sensitivity can reach up to 1800 nm. First principles were used to design Ge:Si solar cells and predict their photovoltaic properties below Si. Simulations show that 88% Ge concentration Ge:Si solar cells can achieve 2.3% efficiency below Si at 30 suns.;High quality Ge:Si layers with high Ge concentration (above 85%) were achieved on Si substrates using reduced pressure chemical vapor deposition (RPCVD) technology. Scanning electron microscopy (SEM) was used to analyze the surface property. Secondary ion mass spectrometry (SIMS) and spreading resistance profiling (SRP) were used to monitor the germanium, impurity, and dopant concentrations.;First generation Ge:Si solar cells had P type 92%Ge concentration Ge:Si absorbers grown on Si. To minimize the misfit and threading dislocations, Ge:Si graded layers were grown on Si before the growth of high Ge concentration Ge:Si. N type Si caps were grown on top of the P type Ge:Si absorbers to form the PN junction and passivate the surface. The first generation Ge:Si solar cell achieved an efficiency of 0.57% below Si at 30 suns. Through analyzing the IV (current voltage) and QE (quantum efficiency) of fabricated Ge:Si solar cells, performance improvement plans for FF were designed using the predictive models. These led to a third generation Ge:Si solar cell. A maximum efficiency of 0.79% was obtained at 88%Ge concentration below Si at 30 suns.;Light trapping can increase the effective path length of photons in the solar cell. In this work, two light trapping configurations were considered. These configurations have texturing, AR coatings and back reflectors. Photon counting and ray tracing were used to evaluate their performance for Ge:Si solar cells below Si. Model indicates that the optical path length of photons with the energy near the band gap of 88% Ge concentration Ge:Si can reach 19 to 21 times that of the thickness of the Ge:Si absorber for two light trapping configurations. The fourth generation Ge:Si solar cell with the Al back reflector achieved 5.72mA/cm2 Jsc below Si at one sun which is 60% higher than that of the third generation cell. Furthermore, the fifth generation Ge:Si solar cell with a SiO2/Al reflector achieved a 7.91mA/cm2 Jsc below Si at one sun which demonstrates that the SiO2/Al reflector has better reflection than the Al mirror in our Ge:Si solar cells. The effective path length of photons in the fifth generation cell reached 17 times that of the thickness of the Ge:Si absorber. After increasing the Jsc of 88%Ge content Ge:Si solar cells by applying the light trapping, we achieved an efficiency of 1.3% below Si at 30 suns. This efficiency is 60% of the theoretical maximum. Moreover, the efficiency calculated by the product of best achieved Voc (373mV), Jsc (7.91mA/cm2) and FF (66%) indicates that 88%Ge content Ge:Si solar cell can reach 1.9% efficiency below Si at 30 suns which is 80% of the theoretical maximum.;This work develops the design rules that demonstrate the pathway to achieve 2.3% efficient Ge:Si solar cells below Si in a multi-junction concentrator system.