Crystalline-Silicon (c-Si) solar cells continue to be a leader amongst the various competing solar celltechnologies. Reasons for this include stable supply of silicon, well-developedmanufacturing processes, and, of course, the high and growing conversionefficiencies that can be achieved. Here, we report on Laser Doped SelectiveEmitters (LDSE) – a relatively straightforward, laser-based manufacturing processthat has been demonstrated that it can increase cell efficiency by up to 1–2%points over conventional cells.

A conventional p-type wafer c-Si solar cell has a thin but heavily-doped n++ regionof silicon on the front surface. This region, generated via high-temperaturephosphorus gas furnace diffusion, forms a p-n junction, directing current flow intoa grid pattern of thin conducting strips on the cell surface. These are referred to asfinger lines, and consist of a metallic paste material that is screen printed onto

Laser-Doped SelectiveEmitters (LDSE) with aGreen CW Laser

The mantra of “better-faster-cheaper” continues to drive the photovoltaicindustry. To meet the demands of higher throughput and better quality – at alower cost – today’s industrial tool providers are feeling the challenge. Forthin film scribing processes, such as those used in the manufacture ofamorphous silicon thin film photovoltaic devices (a-Si TFPV), the goal is toincrease throughput and lower cost per panel. Laser scribing at a higherspeed is one way to achieve the goal.

We used Spectra Physics’ Mosaic™ 532-11 laser to achieve a high-speed,single-pass scribing process using a scanning galvanometer. The laserbeam was focused on to the surface of the material to create 50 mmablation spots, which created 50-mm wide scribe lines when scanning athigh speeds from 2 to 8 m/s. The results using the Mosaic 532-11 laserwere then compared to those achieved using a typical 532 nm DPSSQ-switched laser.

When comparing the Mosaic’s output power to that of a typical DPSS Q-switched laser over a wide range of operating conditions (0–500 kHz), theadvantage becomes clear. Average power for Mosaic 532-11 remains highthroughout the entire range of pulse repetition frequencies (PRFs). At thesame time, the output pulse energy is stable, with variation typically below3% (1�σ). This capability of Mosaic to deliver precise energy pulses at ahigher repetition rate is exactly what is required for a high speedscribing process.

For P2 layer a-Si TFPV scribing, the benefit of the Mosaic’s stable output athigh PRFs is readily apparent. With a typical DPSS Q-switched 532 nm lasersource, increasing the PRF quickly ushers in severe pulse energy instability,

0 50 100 150

Pulse Repitition Frequencies (kHz)

PPS

200 250 300 350

14

12

10

8

6

4

2

0

Pow

er(W

) / P

PS (%

1o)

}

Typical GreenDPSS QSW Laser

Mosaic 532-11

resulting in scribe width variation. Note too that the increasing number ofintermittent “hot” pulses could cause unwanted damage to the underlyingfilm/substrate. At the highest PRFs, the pulse energy is too low to createa contiguous scribe, ultimately resulting in poorly performing solar panel.

In contrast, the Mosaic generates a continuous stream of equal-energypulses at even the highest PRFs – as evidenced by the clearly uniformscribe width – for a wide range of PRF. And furthermore, the high outputpower at the highest PRFs allow for maximal scribe speed to be achieved.Results of this test shows that accelerating to scribe speeds of >4 m/sand operating laser at >100 kHz while still maintaining continuousisolation scribes requires very stable PPS and having <3% PPS at >100 kHz clearly helps.

Application FocusApplication FocusIndustrial Laser

Applications Lab

No. 10

High-Speed Thin FilmPhotovoltaic Scribing

Mosaic 532-11 output power and stability compared to typical DPSS Q-switchedlasers, with increasing pulse repetition rate

a-Si TFPV P2-level scribing with typical DPSS Q-switched laser (top) and withthe new Mosaic 532-11 laser (bottom)

50 kHz - 2 m/s 100 kHz - 4 m/s 200 kHz - 8 m/s

50 µm

High-Speed Thin Film Photovoltaic Scribing

q10DS-071103

Spectra-Physics® Lasers Sales1-800-775-5273 sales@spectra-physics.comwww.newport.com/spectra-physics

P r o d u c t : M o s a i c ™ 5 3 2 - 1 1

Typically, PV scribe tool designers opt for one of two options for maximizingthroughput: (1) beam-splitting for simultaneous multi-beam scribing atrelatively low speeds; or (2) single-beam, high-speed scanning (usingscanning galvanometers, for example) at the lasers highest operable PRF.Now, with the unique combination of high output power and stable pulseoutput – both at very high PRFs – the Mosaic may open the door to a newregime for tool design: split beam + high-speed scanning at the same time.

The Spectra-Physics’ Mosaic family of lasers was introduced in 2011. TheMosaic 532-11 produces consistent power and low pulse-to-pulse stabilityover a wide range of operating pulse repetition frequencies (0–500 kHz). Forhigh-speed and high-quality laser processing, the Mosaic laser offersunparalleled performance, at a price ideally suited for high-volumeproduction tooling.

Model Wavelength Peak PowerAveragePower Pulse Width Repetition Rate

Mosaic 532-11 532 nm 11 kW at 50 kHz22 kW at 30 kHz

>11 Wat 50 kHz

<15 ns 0–500 kHz

© 2011 Newport Corporation. All rights reserved. Spectra-Physics, the Spectra-Physics logo and the Newport logo are registeredtrademarks of Newport Corporation. Mosaic is a trademark of Newport Corporation.