Teng Kho

Superacid Passivation of Crystalline Silicon Surfaces

The reduction of parasitic recombination processes commonly occurring within the silicon crystal and at its surfaces is of primary importance in crystalline silicon devices, particularly in photovoltaics. Here we explore a simple, room temperature treatment, involving a nonaqueous solution of the superacid bis(trifluoromethane)sulfonimide, to temporarily deactivate recombination centers at the surface. We show that this treatment leads to a significant enhancement in optoelectronic properties of the silicon wafer, attaining a level of surface passivation in line with state-of-the-art dielectric passivation films. Finally, we demonstrate its advantage as a bulk lifetime and process cleanliness monitor, establishing its compatibility with large area photoluminescence imaging in the process.

Towards industrial advanced front-junction n-type silicon solar cells

Recent progress in the development of advanced front-junction n-type monocrystalline solar cells for potential industrial fabrication is presented. The textured, boron-diffused front surface is passivated with a stack of Atmospheric Pressure Chemical Vapor Deposited (APCVD) Al2O3 and Plasma-Enhanced Chemical Vapor Deposited (PECVD) SiNx. A champion cell with an in-house measured efficiency of 21.6% is obtained for small-area cells (i.e., 2×2 cm2) fabricated at the ANU. The high open-circuit voltage of 664 mV demonstrates the excellent passivation of both the front and rear surfaces. The cell design and process have demonstrated a good tolerance to substrate resistivity variations, with an average cell efficiencies close to 21% for resistivity varying between 3 and 10 Ω·cm. Moreover, with an adaption of the process developed at the ANU, large-area cells (i.e., 12.5×12.5 cm2) are fabricated at Trina Solar on n-type Cz substrates with a resistivity of 2.5 Ω·cm. A champion cell with an in-house measured efficiency of 20.5% is obtained, demonstrating a high potential in commercializing the advanced cells developed in this work. Finally, simulations reveal that further improvements in cell efficiency are to be mainly achieved through further optimisations of the rear side contact geometry and rear surface passivation.

Rounded rear pyramidal texture for high efficiency silicon solar cells

Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies in the range 24.4%-25.6% As high as these efficiencies are, there are opportunities to increase them further by improving on the light trapping. Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping than cells with texture on just the front. One of the principle losses of double-sided pyramidal texture is the light that escapes after a second pass through the cell when the facet angles are the same on the front and rear. This contribution investigates how this loss might be reduced by changing the facet angle of the rear pyramids. A textured pyramid rounding is introduced to improve the light trapping. The reduction in surface recombination that rounding the facets introduces is also evaluated. With confocal microscopy, spectrophotometry and ray tracing, the rounding etch time required to yield the best light trapping is investigated. With photoconductance lifetime measurements, the surface recombination is found to continue to decrease as the rounding time increases. The spectrophotometry and ray tracing suggests that the double sided textured samples featuring rounded rear pyramids have superior light trapping to the sample with a planar rear surface. The high-efficiency potential of rounded textured pyramids in silicon solar cells is demonstrated by the fabrication of 24% efficient back-contact silicon solar cells.

Etch-back simplifies interdigitated back contact solar cells

The process of making Interdigitated Back Contact (IBC) solar cell is implemented by a novel simplified etch-back technique, while aiming for no compromise on high-efficiency potentials. Simplified etch-back creates localized heavy and light phosphorus and boron diffusions simultaneously. This process also leaves localised heavy diffusions to be approximately a micron higher than neighbouring light diffusion regions. In comparison to the IBC solar cells that ANU developed to date [1], key advantages of this technique feature reduction in cell process steps; requires only two diffusions to create p, p+, n and n+ diffusions; no high-temperature oxidation masking steps required as diffusion barriers; independent optimization of contact recombination, lateral carriers transport and surface passivation; and potential higher silicon bulk lifetime and reduced contamination due to low thermal budget. Based on the etch-back technique, the total saturation current density deduced from the test structures for the IBC cell is below 30 fA/cm2.