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The trenches themselves are not visible

but numerous, smaller vertically orient-

ed structures may be seen. These struc-

tures might indicate local variations in

the condition of the aluminum paste

that fills the trenches.

To learn more about these struc-

tures, the cell was imaged acoustically

with a Sonoscan C-SAM tool. This tool

takes advantage of the fact that the

interface between two materials will

reflect some portion of a VHF or UHF

ultrasound pulse that strikes the inter-

face. The tool’s transducer scans just

above the sample surface. Because

the ultrasonic frequencies used by the

transducer do not travel through air, a

small jet maintains a column of water

between the moving transducer and

the sample surface at all times.

As it moves at speeds that may

exceed 1 m/s, the transducer sends an

ultrasound pulse into the cell. When it

is moving through a homogeneous ma-

terial, the pulse is absorbed to some

degree, but the material sends back

no echoes. However, when the pulse

n solid state physics, an electron that

acquires sufficient energy may move

from the valence band to the conduc-

tionbandof anatom.When it does, it gen-

erates two separate entities—an electron

and an electron hole. Both are carriers of

electric charges. When the two entities

later move from the conduction band to

the valence band, they recombine and

lose their ability to carry a charge. This re-

combination greatly limits performance

in common screen-printed solar cells, but

is minimized by PERC (passivated emitter

and rear cell) solar cells.

In common screen printed solar

cells, recombination can occur wherev-

er monocrystalline silicon contacts with

metal. On the cell’s top surface, oppor-

tunities for recombination are limited.

The area covered by metal is typically

less than 3% of the total, and much of

the metal is separated from the silicon

by insulation. Conversely, the cell’s bot-

tom surface is bonded to aluminum.

This silicon-to-aluminum interface—the

back surface field, or BSF—is where

significant power is lost as the elec-

tron-hole pairs recombine in common

solar cells.

However, in PERC cells, the BSF

aluminum is separated from the die’s

silicon by an insulation layer. The in-

sulation is patterned and interrupted

at intervals by trenches that contain

an aluminum paste. It is only in these

trenches that there is contact between

aluminum and silicon, so the complete

cell experiences less power loss through

recombination.

PERCCELL EFFICIENCY

PERC cells are forecast to grow

from the current level (roughly 4%) to

a much larger percentage of the solar

cell market. Overall, a solar cell’s power

output depends on how much light the

cell reflects and on the energy level pro-

duced at a desired wavelength. By limit-

ing recombination, PERC cells can pro-

vide an absolute efficiency boost of 1%,

making their relative efficiency about

7% greater than the efficiency of other

widely used screen printing methods.

The highest PERC cell efficiency is

achieved when the trenches are com-

pletely filled with aluminum paste and

contain no air pockets. Sonoscan Inc.,

the maker of C-SAM* (C-mode scan-

ning acoustic microscope) tools, and

the Photovoltaic Manufacturing Con-

sortium are working together to deter-

mine the impact of incompletely filled

trenches on power output. This article

covers the early stages of this work—

the nondestructive and destructive

imaging performed to characterize the

trenches. Electroluminescence images,

acoustic images, and scanning electron

micrographs were used in this research.

IMAGINGRESULTS

The electroluminescence image of

one corner of a PERC cell featuring chan-

nels for aluminum paste cut into the sil-

icon is shown in Fig. 1. To produce this

image, an electric current was applied

to the cell and variations in light output

across the cell were recorded. Dark re-

gions indicate poor electrical output.

Metal structures such as the horizontal

structure have poor electrical output.

TECHNICAL SPOTLIGHT

BOOSTINGPERC

SOLARCELL OUTPUT

By limiting recombination, passivated emitter and rear cell (PERC) solar cells can

provide an absolute efficiency boost of 1%, making their relative efficiency about

7%greater than that of other screen printing methods.

Fig. 1 —

Electroluminescence imaging

shows dark vertical lines that might be

unfilled trenches.