The effect of current collection at active
grain boundaries of multicrystalline
Silicon photovoltaic devices
(from doctoral thesis of Rita Ebner, June 2003)
The grain boundaries in mc-Silicon solar cells are regions of enhanced recombination for minority carriers and pose potential barriers to majority carriers. Together with the higher concentration of in-grain defects in mc-Silicon, this is the main reason for lower conversion efficiency of multi- compared to mono-crystalline Silicon solar cells. The usual way of countering these effects is to passivate the grain boundaries.

In the present work another method has been investigated. The metallic front contact pattern of mc Silicon solar cells were put over and along grain boundaries (grain boundary oriented finger grid, GBOF). The front contact patterns were applied galvanically using nickel as a diffusion barrier and silver as the conductor, in view of a possible application in laser grooved buried contact cells.

The study was done on Baysix 100 x 100 mm2 wafers of about 270 µm thickness, which were boron doped between 0.5 and 2.0 Ohm.cm. Several batches of up to 80 wafers each were processed. Four different kinds of front contact patterns were investigated: ON, OFF, STD and GRID. There were four different studies: - Triplets study: ON - OFF - STD; - Quartets study: ON - OFF- STD - GRID; - Pairs study: ON - OFF; STD - GRID and ON - GRID.


"ON-cell" and "OFF-cell"

"STD-cell" and "GRID-cell"
The first wafer received a grain boundary orientated finger grid ("ON"). It was tried to contact directly along the grain boundaries. The second wafer ("OFF") got the same grid as the first wafer, but was rotated by 90°, so that the grid passed over grain boundaries only accidentally. The standard pattern ("STD"), which was optimised for the given sheet resistance covering the same area as the grain boundary orientated grid was galvanized on the third wafer. The fourth wafer was contacted with a grid-pattern ("GRID"). The shading on the GRID-pattern had to be the same as the shading of the ON- or STD patterns.

The typical fraction of grid length on grain boundaries was about 77 % for ON-cells whereas it was only between 25 % and 30 % for OFF- and GRID-cells. For the STD-cells it was about 25 %. The detection of the grain boundaries was done by optical contour tracing methods.

The cells received no passivation of surfaces and grain boundaries and no antireflection coating.

Results:

- current-voltage measurements were taken in the dark and under illumination:

Under AM1.5 conditions the ON-cells showed an efficiency increase of over 5 % relative to the OFF- and STD-cells and an increase of about 3 % relative to the GRID-cells. They also had a higher fill factor, sometimes exceeding 80 %, a higher short circuit current and a lower series resistance.

The next figure shows quite clearly that the ON-cells outperform the other types in terms of power output.

As the main cause for this difference the lower series resistance losses in the emitter sheet and between emitter and metal of the GBOF-grid could be established, because the difference in power output became smaller with increasing temperature, as expected theoretically, and it also became smaller with lower illumination levels.