High efficiency IBC solar cell with oxide-nitride-oxide passivation

Abstract

This thesis considers the optimisation of an SiO2-SiNx-SiOx (ONO) dielectric coating for silicon solar cells comprising thermal SiO2, plasma-enhanced chemical vapour deposition PECVD SiNx and SiOx, together with the development and fabrication of high efficiency interdigitated back contact (IBC) solar cells.

The three-layer ONO dielectric stack provides excellent surface passivation and anti-reflection properties. The initial thermal SiO2 layer provides chemical surface passivation. The second layer of the PECVD SiNx hydrogenates the underlying thermal SiO2 and provides a field-effect passivation with positive charges. The third layer of the PECVD SiOx improves the overall anti-reflection coating (ARC) of an ONO stack in air. Positive corona charging of the ONO stack further improves the surface passivation, and annealing at 400 C traps the charges and renders it stable for at least two years. An optimised ONO stack for surface passivation has achieved lifetimes above previous parameterised Auger limits. The n-type wafers with resistivities of 0.5, 1.07, 1.77 and 100 ohms.cm achieved lifetimes of 3.7, 15.1, 25.5 and 170 ms, respectively.

The surface passivation of an ONO stack was fine-tuned for both phosphorus and boron diffusions for integration into IBC solar cells. A large contribution of positive charges within the stack was mainly from the thermal SiO2 and PECVD SiNx. Post-oxidation annealing (POA) of the thermal SiO2 and high SiNx refractive index reduced the overall positive charge of the ONO stack. A lower positive charge improved the passivation on the boron diffused surface while retaining excellent passivation on the phosphorus diffused surface.

The cell design and fabrication processes for IBC solar cells were improved from previous fabrication processes at the Australian National University (ANU) by utilising ONO surface passivation. The refinement for IBC solar cell design was based on detailed simulations. The improvements to the cell processing included permanently eliminating low bulk lifetime defects on high resistivity float zone (FZ) wafers, gettering of contaminants, and consistent texturing results. Implementing optimised ONO stacks for surface passivation and an ARC onto the IBC solar cell achieved a certified efficiency of 25.0%. Non-ideal recombination contributed to a relatively low fill factor for the champion cell, and disregarding the non-ideal recombination in the simulation results revealed that an efficiency of 25.2% is achievable.