Invited Paper EM1-ThM10
High Efficiency Si Cells and the Challenges to Integrate the Light Management

Thursday, November 13, 2014, 11:00 am, Room 311

In this talk, we discuss the stringent requirements that enable the high efficiency, industrially relevant, Si solar cells, and show that the light management approaches need to be developed integral to the whole device. The highest efficiencies known today are obtained with PERL (UNSW), IBC (SunPower) and IBC Heterojunction (Panasonic) device architectures. The challenge is to balance the excellent bulk, surface, and contact passivation leading to high Voc, with light management features leading to high Jsc. For example, pyramid-texturing the front of the cell leads to an excellent visible range response and near-Lambertian light trapping in near-infrared, but the associated 1.7x increase in the front surface area increases the recombination current prefactor. The IBC approaches significantly restrict any light management structures on the back of the cell (e.g. plasmonics, gratings). Novel passivated contact approaches currently pursued by NREL [1] and others [2] show promise to significantly enhance cell efficiencies in an industrially relevant way. In particular, we have developed [1] high performance tunneling passivated full area back contact for the n-CZ based cell. Because of the high-low junction induced by a deposited n+ poly-Si contact layer, there is no P-doping involved in BSF formation. The contact itself provides better BSF passivation than currently used nitride-passivated BSF surface, has low contact resistance, and does not need patterning as in PERT or PERL structures. We then discuss of how advanced optics and light trapping can fit into this device architecture and are there viable alternatives to the near-Lambertian pyramid texturing. With near-perfect bulk and surface/contact passivation, the importance of light management increases. However, it becomes increasingly more challenging to justify much thinner wafer cells even with perfect Lambertian light trapping. For high efficiency, passivated contact wafer cells, additive light management structures provide potential efficiency improvements and will be discussed. Finally, we will briefly discuss numerous light management approaches examined in the course of our previous Si program that focused on <10 micron thin c-Si devices (small pyramids, black Si, dielectric nanoparticle backscatterers), and their potential to high efficiency Si cells.

1.D. L. Young et al., SiliconPV conference proceedings 2014.

2.F. Feldmann et al, Sol. En. Mat. Sol. Cells (2014) 120, pp. 270-274.