Trapping and decay of negative charge in silicon nitride films for photovoltaic applications

Abstract

Surface passivation is a major technology area requiring improvement in order to increase device efficiency for most commercial solar cells. As the thickness of solar cells continues to decrease, surface passivation becomes even more important as the efficiency loss due to poorly passivated surfaces becomes greater. This thesis aims to combine the excellent passivation properties of silicon nitride (SiN<subscript>x) with the application advantages offered by corona charging to embed negative charges, particularly for the rear surface of conventional solar cells. Normal SiN<subscript>x film contains positive charges, which is often beneficial as it results in field effect passivation of n type surfaces. However, for p type surfaces, the positive charge in SiN<subscript>x induces a depletion region which increases recombination, and results in a parasitic shunt [1]. Therefore, the ability to trap negative charge in SiN<subscript>x for long periods of time could be of interest for solar cell applications.

This thesis focuses on a simple technique to modify positive charges to negative charges in SiN<subscript>x films. The negative charge trapping behavior of silicon nitride thin films using various processing methods is studied. The effect of post-deposition annealing on the charge trapping ability and charge stability is investigated. Capacitance-Voltage measurements are used to estimate the injected negative charge density in silicon nitride films. It is observed that post-deposition annealing prior to charging can increase the charge trapping ability and improve the charge stability in silicon nitride films. Higher and more stable negative charge density can be achieved in nitrogen-rich silicon nitride films. Lifetime results also show that the negatively charged silicon nitride layers lead to an improvement in surface passivation. However, there is also some evidence that the process of negative charge injection can result in an increase in the interface defect density but still within acceptable levels.

The thesis shows that negatively charged LPCVD silicon nitride and PECVD silicon nitride are both effective for the surface passivation of p type surfaces. The lifetime has been improved after negative charge injection. A charge decay model is proposed to explain the observed charge decay with time at elevated storage temperatures in the silicon-oxide-nitride (ON) and silicon-oxide-nitride-oxide (ONO) structures. It is, therefore, concluded that these structures, together with the charge-injection technologies developed in this thesis are viable and promising options for the surface passivation of silicon solar cells.