PHOTOVOLTAICS: Laser processing yields solar cells with 22% efficiency

Solar cells made from crystalline silicon (Si) dominate the solar photovoltaic market.

Feb 1st, 2007
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Solar cells made from crystalline silicon (Si) dominate the solar photovoltaic market. Typical conversion efficiencies of industrial crystalline Si solar cells are 14% to 16%, but novel laser-processing techniques can improve the photon-to-electron conversion process. Researchers at the Institut für Solarenergieforschung Hameln (ISFH; Emmerthal, Germany) have developed a solar-cell manufacturing sequence called rear-interdigitated single evaporation (RISE) for Si back-contacted solar cells based on laser processes that has pushed conversion efficiency up to 22%.1

Today, many companies use laser processing in the production of Si solar cells. BP Solar (Frederick, MD) uses a laser-grooved buried-grid technique in which a laser groove is machined into the Si surface and filled with metal to act as a front electrical-contact grid. The advantage of this process is reduced shadowing loss compared to standard front metallization. Advent Solar (Albuquerque, NM) uses a different method called emitter wrap-through in which highly doped walls of laser-drilled through-holes in the Si wafer conduct the current from the front side of the emitter to metal contacts placed on the back surface, further reducing shadowing losses.

Noncontact process

In the production process for the RISE solar cell, laser processing defines a patterned emitter and base region on the back side of the solar cell and laser ablation also allows reliable self-aligned contact separation after metallization (see figure). The noncontact process (important for reducing wafer breakage) first involves ablation of regions of a thin silicon nitride or silicon oxide layer placed over the back side of the crystalline Si wafer into an interdigitated pattern of emitter and base regions by a Q-switched, frequency-tripled (355 nm) Nd:YVO4 (vanadate) laser. Any crystal damage reduces the carrier lifetime and likewise reduces conversion efficiency. A potassium hydroxide (KOH) etching process removes this laser-induced damage. After etching, phosphorus diffusion creates the emitter region, leaving the raised silicon nitride and oxide areas as the base region. Next, a self-aligning contact-separation process of metal evaporation followed by wet chemical etching is used to separate the emitter and base regions.


Laser processing of the back side of a solar cell creates an emitter and base region (inset shows a scanning-electron micrograph of a structured silicon surface with separated contact levels). A wet chemical-etching step removes silicon crystal damage (due to laser ablation) that can reduce carrier lifetime, resulting in a highly efficient solar cell with greater than 22% conversion efficiency.
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To optimize the solar-cell conversion efficiency, the researchers of ISFH and of the Laser Zentrum Hannover (Hannover, Germany) explored carrier lifetime as a function of crystal damage by evaluating different laser sources and KOH etch depths.2 It was found that the depth of laser-induced damage for a frequency-tripled 355 nm laser was 3 µm, 4 µm for a frequency-doubled 532 nm laser, and more than 20 µm for a Nd:YAG 1064 nm laser.3 As long as the damage is removed to these depths, more than 20% conversion efficiency for the solar cell is not jeopardized; however, the etch depths need to be considered in terms of the required source-laser costs and the thickness of the Si wafers being used in the production process.

“The RISE concept uses laser processing as a key technology,” says solar-cell group head Rüdiger Meyer. “Since we use noncontact processes that are already known from other industrial applications in most cases, the production of the RISE solar cell is industrially feasible for thin, large-area crystalline Si wafers. This will result in costs that at least will be comparable to those of today’s standard solar-cell production.”

Gail Overton

REFERENCES

1. P. Engelhart et al., Proc. 21st EUPVSEC 2006, Dresden, Germany, 773.

2. A. Schoonderbeek et al., paper M704, ICALEO 2006, Scottsdale, AZ.

3. P. Engelhart et al., paper M703, ICALEO 2006, Scottsdale, AZ.

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