Laser induced defects in laser doped solar cells

Abstract

Laser doping offers a promising method to define selective emitters for solar cells. Its main advantage is the localised nature of the laser beam, which allows melting of the surface area without heating the bulk. The ability to perform this process over a dielectric film offers further benefits, such as the possibility of creating self‐aligned metallisation patterns simultaneously with the selective emitter formation. However, laser induced defects, contaminations and discontinuities in the selective emitter can reduce solar cell performance. In this work the influence of different dielectric films on defect formation is investigated. It was found that a thin oxide beneath the SiNx improves the implied open circuit voltage of the solar cells for a wide range of laser output powers. Fewer defects were observed when using this SiO2/SiNx stack compared to the standard single SiNx anti‐reflection coating layer. It was also found that the recrystallised silicon layer grows epitaxially according the substrate orientation. No dislocation or stacking faults were observed in deeper areas using transmission electron microscopy, although some defects were observed near the surface. Electron beam induced current images revealed discontinuities in junctions formed with high laser powers. We conclude that micro‐cracks create these discontinuities, which can potentially induce shunts. Finally, laser doped solar cells with a standard SiNx and with a double SiO2/SiNx stack layer as anti‐reflection coating were compared. An efficiency of 18.4% on a large area commercial grade p‐type CZ substrate was achieved. Copyright © 2010 John Wiley & Sons, Ltd.

Edge isolation of solar cells using laser doping

“…Groups B and C performed the 20 µm 20 µm 20 µm best in terms of FF and pFF, and Group B exhibited the lowest J 02 values indicating that laser doping prior to SiN x deposition introduced the least amount of recombination in the depletion region. This is consistent with what has been previously reported in the literature, where it was hypothesised that when laser doping through a dielectric layer, defects in the silicon structure are formed along the edge of the laser doped lines due to the difference in thermal expansion coefficients of silicon and the dielectric layer [23][24][25]. In addition, the laser doped surface region itself was passivated when performing the SiN x deposition after the laser doping R sh above 1000 Ω cm 2 was achieved for Groups A, B and C, with most exceeding 10,000 Ω cm 2 .…”

Large area p-type PERL cells featuring local p+ BSF formed by laser processing of ALD Al2O3 layers

“…One advantage of performing laser doping using Al as a dopant is the high diffusion coefficient of aluminum in molten silicon (6.8 Á 10 À 8 m 2 /s) compared to other p-type dopants such as boron (1.2 Á 10 À 8 m 2 /s) or gallium (3.6 Á 10 À 8 m 2 /s) [26]. In addition, besides process simplification, avoiding the use of a boronbased dopant source may assist in limiting the generation of detrimental boron-oxygen defects [27] by avoiding localized regions with high concentrations of both boron and oxygen, which can be incorporated into the silicon from boron based spin-on dopant sources [28,29].…”

Boron Implanted, Laser Annealed p+ Emitter for n-type Interdigitated Back-contact Solar Cells

“…5). Moreover, the laser induced defects (mainly point defects) during the high speed recrystallization [14] may also increase the J oe , which could be improved by heating the substrate during the annealing [15]. Compared with the diffused emitters, a relative high surface concentration and shallow junction depth is observed for as-obtained emitters.…”