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.

Laser induced defects in laser doped solar cells

Progress on large area n‐type silicon solar cells with front laser doping and a rear emitter

High power lasers are attractive for low-cost solar cell fabrication. However, laser process can generate crystal lattice defects that would decrease the photovoltaic efficiency. This study examines the effect of long pulsed laser annealing for improving the cell efficiency and results are compared with the short pulsed laser and furnace annealing. Results show that long pulsed laser annealing is effective in increasing the short circuit current for laser processed back-contact solar cells. The short circuit current increases by 1.5 times indicating the removal of defects. Laser annealing would allow widespread use of laser processing method for low-cost solar cell manufacturing.

Laser induced defects in silicon solar cells and laser annealing

A Q-switched ruby laser has been used to alloy deposited layers of tin on (100) GaAs. The tin concentration in the GaAs substrates has been investigated by electrical measurements, electron probe microanalysis, Rutherford backscattering and transmission electron microscopy. The results show that a high concentration of tin diffuses into the GaAs for an energy density up to 0.6 J cm−2 and the electrical properties improve with increasing energy density. However, at high energy densities this leads to the introduction of damage near the GaAs surface. At the highest energy density of 2 J cm−2, very complex dislocation networks are produced and a cellular structure results along with microcracking of the surface. This produces high levels of residual strain in the surface.

Laser alloyed tin layers on GaAs

Progress in the effort to increase the end-of-life efficiency of solar cells for space use is assessed. Silicon solar cell efficiency, radiation effects, and gallium arsenide cells are emphasized.

Solar Cell High Efficiency and Radiation Damage, 1979

High quality silicon p‐n junctions have been prepared by alloying a vacuum evaporated Al film into n‐type Si and by epitaxial regrowth of an As doped Si amorphous layer onto p‐type (100) Si by a single short pulse of ruby laser radiation. Transmission electron microscopy investigations indicated that a defect‐free epitaxial layer was grown on the (100) Si surface; however, some polycrystalline structure in the very near‐surface region was observed, which does not seem to affect the junction characteristics. Laser assisted junction formation in GaAs was demonstrated by alloying Mg films into n‐type GaAs; however, diode characteristics show a large leakage current which may have been caused by the surface damage.

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J. Narayan

Papers

Transmission electron microscopy and electrical properties measurements of laser doped silicon and GaAs