This question is mainly to understand the general practice to do EQE correctly. In one of the prof's lab I share where they have the setup to do EQE measurements and it is custom made (not a commercial setup). The beam diameter is 0.6 mm and they use Si as the calibration source. However, for EQE it is important that light is shinning on the active area uniformly so that extracted EQE values are correct after calibration. Currently, I am using an open loop type contact where the middle 1.5 mm x 1.5 mm square is completely open for the light spot to shine on the active area. But I want to go for a finger/grid type top contact. So I want to know what are the design principles used in general so that I can have light uniformly shining on the active area with 0.6 mm spot size so that it will not be like in some part light is reflected more that the other part.
Thanks
Dear Natis,
The metal grid pattern is designed to minimize the power losses by the lateral emitter resistance and the metallic stripes. The parameters of the metal grid is the line spacing S, the line width w and the line thicness t in addition to the the sheet resistance of the the thin emitter layer and the the conductivity of the metal. In addition there is shadowing loss due to materialization. The overall resistance of these objects makes out the series resistance of the solar cell.
For sizing of the metal grid for solar cells, there is good references such as that given in the LINK:http://www.pveducation.org/pvcdrom/design/metal-grid-pattern
In my openion for measuring the external quantum efficiency, the solar cell is shortcircuit. In this mode of operation the the shortcircuit current is not sensitive to the series resistance RS and hence to the metal grid so long as the series resistance is much smaller than the diode resistance. This means that so long as the current in the series resistance is much larger than that of the dark current in the diode of the solar cell.
So i would propose that your coarse grid will function so good as the the intended fine grid that you want to make. This is what i expect from my experience. There is no problem at all for measuring the the external quantum efficiency using .6x.6 mm^2 window regions. The results will be the same.
Best wishes.
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