Metal-oxide carrier-selective contacts for on-chip embedded photovoltaics [electronic resource]

In this thesis, a systematic study of transition metal oxide integration to thin crystalline silicon (c-Si) solar cells as carrier-selective contacts is presented. Thin c-Si solar cell technology is suitable for on-chip embedded photovoltaics to harvest energy from ambient light powering internet-of-things (IoT) systems for environmental, medical and agricultural monitoring. It has also the potential to be integrated with a perovskite cell to result in high efficiency (> 30\%) tandem cell on flexible substrate. Unlike the conventional Si photovoltaic (PV) cells, thin solar cells face loss of absorption due to thinner absorber which can be mitigated by adopting a light trapping technique. However, our simulation suggests that contact selectivity limits the performance significantly when the absorber gets thinner even if light trapping is adopted. Contact selectivity implies the ability of a contact to allow one type of carrier while blocking the other type, and p-n junction provides this in a conventional PV cell. It is observed that when the absorber thickness gets smaller, significant performance improvement can be achieved by improving the contact selectivity beyond what is obtained from a p-n junction. In this thesis, transition metal oxides are introduced as carrier-selective contacts to thin Si solar cells. Using oxides having asymmetric band offsets to Si, the contact selectivity of p-n junction solar cell can be improved resulting in a significant improvement in solar cell efficiency. Next, a heterojunction structure of solar cell is introduced using these oxides that does not require any high doped junctions inside the Si absorber. Removing high doped regions from the area and thickness constrained on-chip PV cell and replacing them with the transparent oxides utilizes the absorber material efficiently. Detailed simulation study of the heterojunction structure is presented where the impact of metal Fermi level pinning on oxides and the doping of the oxides on device performance is studied. It is shown that characterizing the Fermi level pinning properties of these oxides is important. Therefore, the Fermi level pinning properties of nickel oxide contacts are studied, a promising hole selective material, on Si experimentally. We also observe that doping plays a critical role in determining the performance of the oxides as selective contacts. A novel technique of introducing p-doping into nickel oxide using ultra-violet (UV)/ozone treatment technique is shown. Both spectroscopic and electrical characterization suggests that UV/ozone treatment introduces Ni vacancy defects in nickel oxide, which dopes it p-type. Finally, the integration of nickel oxide and titanium oxide as hole selective and electron selective contacts respectively in an ultra-thin Si solar cell (2 um) is demonstrated showing 10.8% efficiency without any light trapping scheme used. This represents a ~13% relative improvement from a control cell without any oxide contact. Detailed physical characterization of the contact heterostructures is provided to understand the mechanism of performance improvement. The thesis is concluded with an outlook for the future works regarding double heterojunction solar cell replacing highly doped homojunction.

Keywords: Electrical Engineering.
Advisor: Saraswat Krishna
Harris J. S. (James Stewart)
Pop Eric advisor. Thesis advisor