Characterization of n-type Silicon Oxide for Use in Thin Film Solar Cells

This thesis studied the characteristics and applications of n-type silicon oxide (SiOx) produced through plasma enhanced chemical vapor deposition were studied through. n-type SiOx has shown excellent results in various applications of solar cell technology including back reflectors and intermediate reflecting layers. These results can aid in making amorphous silicon cells thinner. Thinner cells are not only cheaper and less resource intensive but are known to be much less susceptible to degradation, which is a major research goal in amorphous silicon solar cells. This thesis used three experiments to characterize and utilize n-type SiOx. First, the effects of varying deposition parameters of SiOx films were analyzed. SiOx layers were then applied as back reflectors of p-i-n amorphous silicon solar cells. Finally, SiOx was integrated into a two-period, one-dimensional photonic crystal. Forward power, pressure, CO2 flow and PH3 flow during deposition were varied to study their effects on SiOx films. Optical and electrical characteristics, as well as the deposition rate, of the films were measured as each deposition parameter was varied. Raman spectroscopy was also used to analyze the crystalline silicon structures in the material. These measurements showed that an increase in the oxygen content of SiOx enhanced its optical characteristics, while decreasing its electrical characteristics. CO2 flow as well as power and pressure had significant an impact on the oxygen content. Furthermore, it was found that increasing PH3/SiH4 flow ratio above 0.024 decreased the doping concentration of the SiOx film and had adverse effects on both the conductivity and activation energy of the films. Raman spectroscopy results suggested that the SiOx contained 5-7 nm crystalline silicon features. SiOx layers were deposited on amorphous silicon cells using both a p-i-n-SiOx configuration, as well as a p-i-SiOx configuration that contained no n-type amorphous silicon layer. In addition to varying the SiOx deposition parameters, the thickness of the n-type amorphous silicon layer was also varied. These cells were measured in a solar simulator and an external quantum effficiency setup. The p-i-n-SiOx solar cells achieved an initial efficiency of 10.8 % and a JSC of 16.65 ma/cm2. This was a relative efficiency enhancement of 15.3 % and a relative JSC enhancement of 17.5 % over standard p-i-n solar cells. p-i-SiOx cells achieved an initial efficency of 9.85 % and JSC of 16.55 mA/cm2. Degradation tests showed stabilized efficiencies of 8.2 % for p-i-n-SiOx cells and 7.9 % for p-i-SiOx cells. The external quantum efficiency results showed significant improvement of p-i-n-SiOx cells over p-i-n cells in the red spectra, which are attributed to enhanced reflection. There was also significant improvement in the blue spectra, which is attributed to enhanced electron collection at the back of the solar cell. SiOx was also integrated into a photonic crystal using n-type amorphous silicon as the second material. The photonic crystal was designed through careful tuning using the Advanced Semiconductor Analysis (ASA) simulation package. The results were very promising and showed how photonic crystals can be finely tuned for specific solar cell applications such as an intermediate reflecting layer for a micromorph cell.