Page 83 - Applied Photovoltaics

x large area devices
x large contact areas
x processes compatible with textured surfaces.
Typical commercially mass-produced solar cells use the following processing
sequence:

1. Texturing of surfaces to form pyramids, which decrease the percentage of
incident light reflected from the cell from about 33 to 11%, by causing
reflected light from the pyramid face to strike at least one other pyramid face
before escaping from the surface.
2. Phosphorus diffusion of the top surface, to provide a thin but heavily doped n-
type layer.
3. Screen-printing and firing of aluminium or aluminium-doped silver paste onto
the rear of the cell, to produce a back surface field and a rear metal contact.
4. Chemical cleaning.
5. Screen-printing and firing of front silver metal contact.
6. Edge junction isolation to destroy the conducting path between the front and
rear metal contacts.

4.6 THE LASER GROOVED, BURIED CONTACT SOLAR CELL

A novel metallisation scheme, whereby laser grooves define the location and cross-
sectional shape of the top surface metal conductors, has been developed at the
University of New South Wales’ Centre for Photovoltaic Engineering. It is now in
large scale commercial production by BP Solar in Spain, and marketed as the ‘Saturn’
cell. The new cell structure is called a ‘Laser Grooved, Buried Contact Solar Cell’
(BCSC), and is illustrated in cross section in Fig. 4.16.
The benefits of the BCSC over conventional cell manufacturing processes are:
x large metal aspect ratios (contact thickness / width)
x very fine top contact grid lines (20 ȝm wide)
x reduction of shading losses on large area devices from 10–15% in screen
printed cells to 2–3%
x excellent fill factors owing to low resistive losses in the metallisation and low
contact resistance
x increased metal cross-sections, without increasing shading, by increasing
groove depth with the same width
x device sizes can be increased without performance loss
x no photolithography, antireflection coatings, polished or lapped surfaces or
expensive materials such as Ti-Pd-Ag metallisation are required
x very simple process
x generate electricity at significantly lower cost than standard screen-printed
sequence (Jordan & Nagle, 1994)

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